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THE ANATOMICAL RECORD 295:1877–1895 (2012)
The Trk A, B, C’s of Neurotrophins in
the Cochlea
Department of Biology, University of Iowa, Iowa City, Iowa
Department of Otolaryngology, University of Iowa, Iowa City, Iowa
Department of Cell Biology and Neuroscience, Rutgers University, Piscataway,
New Jersey
The spiral ganglion neurons (SGNs) are the afferent neurons of the
cochlea, connecting the auditory sensory cells—hair cells—to the brainstem
cochlear nuclei. The neurotrophins neurotrophin-3 (NT-3) and brainderived neurotrophic factor (BDNF) are expressed in the cochlea and both
support SGN survival during development. These neurotrophins remain
expressed in the postnatal cochlea and continue to play additional roles for
SGNs, contributing to maintenance of hair cell-SGN synapses and regulating expression of ion channels, presynaptic and postsynaptic proteins, and
SGN membrane electrical properties in a physiologically important spatial
pattern. Remarkably, NT-3 and BDNF have different, even opposing,
effects on SGN physiology despite the close similarity of their receptors
TrkB and TrkC. Recent studies have also raised the possibility that precursor proneurotrophin forms of the neurotrophins play a role in responses to
trauma in the cochlea, signaling through the proneurotrophin receptor
p75NTR. Here, we review expression and function of neurotrophins and
their p75NTR and Trk-family receptors in the cochlea. We focus, in particular, on neurotrophin functions other than support of SGN survival, including regulation of SGN neurite growth, synaptic and membrane physiology.
These functions, unlike survival, are ones for which BDNF and NT-3 substantially differ in their effects. Signal transduction mechanisms of p75NTR
and of Trk-family receptors are discussed, indicating how these lead to different responses, and we speculate on how BDNF and NT-3 can cause different phenotypic changes in SGNs. Because these complex signaling
interactions remain incompletely understood, use of neurotrophins as therapeutic agents in the cochlea should be approached with caution. Anat
C 2012 Wiley Periodicals, Inc.
Rec, 295:1877–1895, 2012. V
words: neurotrophins;
phenotypic changes; neurotrophic factors;
signal transduction; neurotrophin receptors;
expression patterns; hearing; cochlea
During neuronal development, neurotrophic factors
(NTFs) play an essential role in establishment of neuron
number through their control of cell survival/death
(Huang and Reichardt, 2001). This is certainly the case
in the developing peripheral auditory system: the cochlea. The afferent neurons of the cochlea, termed spiral
ganglion neurons (SGNs), require neurotrophic support
for their survival during development. This subject
has been extensively reviewed (Fritzsch et al., 2004),
Grant sponsor: NIH/NIDCD; Grant numbers: RO1 DC01856
(to R.L.D.), R01 DC002961 (to S.H.G.), R01 DC009405 (to
S.H.G.), P30 DC010362, F31 DC011680 (to E.B.); Grant
sponsor: American Hearing Research Foundation (to Q.W.).
*Correspondence to: Steven Green, Department of Biology,
University of Iowa, 143 Biology Building, Iowa City, IA
52242-1324. E-mail: [email protected]
Received 24 July 2012; Accepted 24 July 2012.
DOI 10.1002/ar.22587
Published online 8 October 2012 in Wiley Online Library
particularly with regard to one family of NTFs, the neurotrophins, and their receptors. Here, we review roles
for neurotrophins other than control of developmental
programmed cell death, with a focus on the postnatal
cochlea and distinct functions of BDNF and NT-3.
The anatomy and physiology of the cochlea have been
described elsewhere in detail (Slepecky, 1996) and is
summarized here and in Fig. 1. The cochlea is structured so that tuning to sound frequency is graded from
the cochlea base, which responds to the tones at the
high end of the organism’s frequency range, to the apex,
which responds to the lowest tones of the frequency
range. This defines a ‘‘tonotopic axis’’ of the cochlea. The
middle of the cochlea responds to midrange tones and is
also the region with the lowest threshold (highest sensitivity) for sound. This physiological gradient along the
cochlea largely results from base to apex changes in
physical properties (e.g., width and stiffness) of the ‘‘basilar membrane,’’ that is, the structure that conducts
sound vibrations in the cochlea. They are also reflected
in physiological characteristics of the neural elements,
as has been previously reviewed (Davis and Liu, 2011).
The sensory elements of the cochlea reside in the
‘‘organ of Corti,’’ a long narrow structure lying on the
basilar membrane that extends from the base of the
cochlea to its apex. Parallel to the length of the organ of
Corti are one row of inner hair cells (IHCs) and three
rows of outer hair cells (OHCs), with the fluid-filled tunnel of Corti between the IHCs and OHCs. In the rat
cochlea, there are 1,000 IHCs and 3,400 OHCs
(Keithley and Feldman, 1982). The cochlear hair cells
are the auditory sensory cells. The OHCs act primarily,
as mechanical amplifiers and the IHCs are the primary
sensory transducers. Correspondingly, the IHCs are
densely innervated by SGNs and the OHCs only
sparsely innervated.
In addition to the hair cells, the organ of Corti contains well-defined rows of stereotyped specialized
‘‘supporting cells,’’ which are essential to the structure
and function of the organ. Bordering the row of IHCs on
the inner or modiolar side of the organ of Corti is a row
of ‘‘border cells’’; on the other side of the IHC row is a
row of ‘‘inner pillar cells,’’ which are also the inner side
of the tunnel of Corti. The OHCs are bordered on the
modiolar side by the ‘‘outer pillar cells,’’ which are also
the outer side of the tunnel of Corti; on the outer side of
the OHC rows are ‘‘Hensen’s cells.’’ Directly associated
with these hair cells are the ‘‘inner and outer phalangeal
cells’’ (the latter usually called ‘‘Deiter’s cells’’), with a
row of inner phalangeal cells in contact with the IHC
and three rows of Deiter’s cells supporting the OHCs.
The spiral ganglion extends along the organ of Corti
and is modiolar to it. The SGNs are bipolar neurons.
Their central axons enter the modiolus (the central core
of the cochlear spiral) and collect to form the auditory
portion of the VIIIth nerve that projects to the cochlear
nuclei in the brainstem. The peripheral axons of the
SGNs extend radially to the organ of Corti. The type I
SGNs that innervate the IHCs comprise 95% of the
total SGN population. Each type I SGN makes a synaptic contact with just a single IHC. Because there are
>18,000 SGNs in the rat (Rueda et al., 1987), this
means that each IHC is presynaptic to many type I
SGNs. In the rat and mouse, there are 20 afferent synapses per IHC in the middle of the cochlea, the region of
lowest auditory threshold (greatest sensitivity), but the
number of synapses/IHC declines toward the apex and
base (Francis et al., 2004, 2006; Kujawa and Liberman,
2009; Meyer et al., 2009). The synapses are excitatory
and glutamatergic. Type I SGNs are myelinated except
at the distal end of the peripheral axon near the synapse
with the hair cell. The type II SGN axons, which are
entirely unmyelinated, extend past the IHC row and
then turn in a basal direction to run parallel to an OHC
row and make en passant synapses with many OHCs.
The neurotrophin family (Fig. 2) has four members,
nerve growth factor (NGF), brain-derived neurotrophic
factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin4/5 (NT-4/5; not shown in the figure). These are encoded,
respectively, by the Ngf, Bdnf, Ntf3, and Ntf4 genes in
the rat; Ngf, Bdnf, Ntf3, and Ntf5 genes in the mouse;
NGF, BDNF, NTF3, and NTF4 genes in human genomic
nomenclature. Neurotrophins assemble into tightly associated dimers, a structural feature crucial for their
function, as will be discussed later.
As is the case for other peptide growth factors and
hormones, the neurotrophins are proteolytically cleaved
from larger precursors. Neurotrophins are distinctive in
that the proneurotrophin precursors are released and
bind to their own receptor providing a signaling pathway
divergent from that of the processed neurotrophins. Conversion from proneurotrophins to mature neurotrophins
can be carried out after release by proteases in the
extracellular matrix, and this can happen in an activitydependent manner (Nagappan et al., 2009; Yang et al.,
2009). Another distinctive aspect of neurotrophin signaling is that release of neurotrophins can occur
constitutively, as is typical for peptide growth factors
and hormones or can occur in an activity-dependent
manner (Kohara et al., 2001). Thus, regulation of neurotrophin signaling can occur at any or all the levels of
transcription, translation, intracellular trafficking, intracellular
extracellular processing.
As summarized in Fig. 2, neurotrophins and proneurotrophins signal through two different types of receptors
(Reichardt, 2006). Trk family receptor protein-tyrosine
kinases are high-affinity receptors for neurotrophins.
There are three members of the Trk family: TrkA, TrkB,
and TrkC, encoded, respectively, by the genes Ntrk1,
Ntrk2, and Ntrk3 in rat or mouse genomic nomenclature, by NTRK1, NTRK2, and NTRK3 in human
genomic nomenclature. TrkA is the high-affinity receptor
for NGF, TrkB for BDNF and NT-4, and TrkC is the
high-affinity receptor for NT-3. Also, NT-3 binds with a
lower affinity to both TrkA and TrkB. Signaling through
these receptor protein-tyrosine kinases is generally hypertrophic
specifically, promoting neuronal survival, stimulating
neurite growth, promoting synaptogenesis, and potentiating synaptic strength (Huang and Reichardt, 2003;
Reichardt, 2006). The neurotrophin receptor p75NTR
binds all neurotrophins with low affinity but, in complex
Fig. 1. Basic neuroanatomy of the cochlea. A. Image showing
innervation pattern of hair cells. This is the middle region of a neonatal
rat cochlea dissected and placed in culture (Wang and Green, 2011).
The hair cells (blue) are labeled with anti-myosin VI antibody. The
SGNs (red) are labeled with anti-high molecular weight neurofilament
antibody. The three rows of OHCs and the single row of IHCs are
shown. The former are innervated by spiral bundles of type II SGN
axons that turn to run parallel to the organ of Corti spiral. The IHCs
are densely innervated by radial SGN axons. Scale bar ¼ 50 lm. B.
Diagrammatic representation of cochlear innervation: the pattern of radial and spiral innervation (red) of, respectively, IHCs and OHCs (blue)
is qualitatively represented, as is the increased density of innervation
in the middle of the cochlea. The representation is not intended to be
quantitatively accurate. C. Image showing innervation of hair cells.
This is similar to (A) but at higher magnification and with synapses
(green) labeled with anti-PSD95 antibody. The dense innervation of
IHCs by type I SGN axons can be visualized by the number of radial
fibers and number of synapses on each IHC. Type II fibers, which
form the spiral bundles, can be seen extending past the IHC row.
Scale bar ¼ 20 lm. D. Diagrammatic representation of cochlear innervation (red) and synapses (green) on IHCs and OHCs (respectively,
dark blue and light blue). As for (B), the representation is intended to
be accurate qualitatively but not quantitatively.
with the ubiquitous protein sortilin, makes a high-affinity receptor for proneurotrophins (Teng et al., 2010).
p75NTR is not a receptor protein-tyrosine kinase and
recruits intracellular signaling different from that activated by Trks (Teng et al., 2010). p75NTR signaling is
generally atrophic, promoting apoptosis, inhibiting neurite growth, and depressing synaptic strength (Teng
et al., 2010). Unlike Trks, p75NTR is expressed on glial
cells as well as on neurons. In the peripheral nervous
system, p75NTR is expressed on Schwann cells after axotomy (Johnson et al., 1988). Note that because
proneurotrophin cleavage and conversion from proneurotrophin to neurotrophin is regulated by activity and,
possibly, other stimuli, this amounts to an activity-dependent regulation of the ratio of Trk to p75NTR
signaling. Because these receptors elicit dramatically
Fig. 2. Neurotrophins and their receptors. The diagram summarizes relationships among proneurotrophins, neurotrophins, and their respective high-affinity signaling receptors p75NTR and Trks. Expression of
BDNF, NT-3, and, after trauma, proBDNF, has been shown in the cochlea. SGNs express TrkB and TrkC.
Schwann cells or organ of Corti supporting cells appear to express p75NTR after trauma. See text for
different responses, this regulation has crucially important consequences for the cell.
As summarized in Fig. 2, neither NGF nor TrkA is
expressed in the cochlea during development (Schecterson and Bothwell, 1994) and is not expressed in the
cochlea postnatally (Qiong Wang, Erin Bailey, Steven H.
Green, unpublished observations). All SGNs do express
both TrkB and TrkC (Ylikoski et al., 1993; Pirvola et al.,
1994; Mou et al., 1997), and SGN survival can be supported in vivo and in vitro by either BDNF or NT-3
(Lefebvre et al., 1994; Malgrange et al., 1996; Hegarty
et al., 1997; Mou et al., 1997). Thus, we focus here on
the roles of NT-3 and BDNF in the cochlea. Expression
of NT-3 and BDNF in the organ of Corti is summarized
in Fig. 3.
NT-3 expression has been investigated by use of in
situ hybridization in rats and mice (Pirvola et al., 1992b,
1994; Ylikoski et al., 1993; Schecterson and Bothwell,
1994; Wheeler et al., 1994) as well as by b-galactosidase
labeling in mice in which the LacZ coding sequence
replaces that of Ntf3 (Fari~
nas et al., 2001; Sugawara
et al., 2005, 2007). During development, NT-3 expression
appears to be primarily in supporting cells of the cochlear sensory epithelium rather than in the hair cells
themselves (Fari~
nas et al., 2001). Postnatally, NT-3
expression is highest in the hair cells, but it is also
abundantly expressed in adjacent supporting cells, for
example, phalangeal cells, in the organ of Corti (Sugawara et al., 2007). These studies identified two spatial
gradients of NT-3 expression in the neonatal and postnatal organ of Corti. The first is along the tonotopic
axis: NT-3 expression is highest in the cochlear apex and
lowest in the base. Second, NT-3 expression is higher on
the modiolar side of the organ of Corti, that is, higher in
the IHC region than in the OHC region. As described
‘‘BDNF and NT-3 have distinct actions in regulation of
SGN electrophysiological and synaptic phenotype’’ section, the spatial gradient of NT-3 expression plays an
important role in specifying position-dependent features
of SGNs. For example, exposure to NT-3 in vitro causes
SGNs to adopt an apical phenotype (Adamson et al.,
NT-3 expression declines postnatally, but some expression remains in hair cells and immediately adjacent
sensory cells even in older mice, primarily in the IHCs.
There is also evidence that NT-3 is expressed in SGNs
and glia in the postnatal cochlea (Hansen et al.,
2001a,b), but there does not appear to be a spatial gradient in the spiral ganglion (Bailey et al., 2012).
The spatial expression pattern of BDNF has been analyzed parallel to that of NT-3, in rats and mice, using
the same methods: in situ hybridization and LacZ
‘‘knock-in’’ (Pirvola et al., 1992b, 1994; Ylikoski et al.,
1993; Schecterson and Bothwell, 1994; Fari~
nas et al.,
2001). These studies have shown that, during embryonic
development, BDNF is expressed in the organ of Corti
Fig. 3. Correlation of neurotrophin expression and SGN physiological phenotype. NT-3 is expressed in
an apex to base gradient in the organ of Corti, whereas there is an opposite base to apex gradient of
cochlear BDNF expression. This is correlated with and causal to apex to base differences in SGN membrane electrical properties and expression of ion channels and synaptic proteins, summarized in the text.
and, like NT-3, in an apex to base gradient, highest in
the apex and lowest in the base. Unlike NT-3, BDNF
expression is restricted to hair cells in the cochlear base
with expression at approximately equal levels in IHCs
and OHCs. While always higher in the apex, BDNF
expression declines in the prenatal period throughout
the organ of Corti, so that BDNF mRNA is at very low
levels by postnatal day 1 (P1). BDNF expression transiently increases in the rodent organ of Corti postnatally
between approximately P4 and P9 (Wiechers et al.,
1999) in a base to apex gradient (Tan and Shepherd,
2006; Flores-Otero and Davis, 2011). Although BDNF
mRNA may be at low levels (Wiechers et al., 1999; Bailey et al., 2011), BDNF immunoreactivity has been
detected in organ of Corti hair cells in the adult (Tan
and Shepherd, 2006; Flores-Otero and Davis, 2011).
The spatially patterned BDNF expression observed in
the developing and postnatal organ of Corti plays important roles in position-specific SGN differentiation (see
‘‘BDNF and NT-3 have distinct actions in regulation of
SGN electrophysiological and synaptic phenotype’’ section), for example, basal portions of the organ of Corti
cocultured with SGNs induce a basal SGN phenotype in
a BDNF-dependent manner (Flores-Otero et al., 2007).
Moreover, BDNF is expressed in the postnatal spiral
ganglion in rats and mice (Wiechers et al., 1999; Hansen
et al., 2001b; Zha et al., 2001; Schimmang et al., 2003;
Ruttiger et al., 2007; Singer et al., 2008), and one study
has shown this expression to be in a base to apex gradient (Schimmang et al., 2003).
Quantitative real-time PCR (qPCR) analysis (Stankovic and Corfas, 2003) has been used to quantify
postnatal expression of BDNF and NT-3 in the cochlea
and vestibule although, because the entire organs were
assayed, localization of expression to specific cell types
within the inner ear could not be determined. The study
does confirm that both NT-3 and BDNF are expressed in
the mature inner ear, although BDNF levels are very
low in the cochlea relative to the vestibule.
NT-3 expression in the cochlea appears to require the
neuregulin signaling pathway. The neuregulins are a
family of intercellular signals, related to epidermal
growth factor, that signal via receptor protein-tyrosine
kinases of the ErbB family (Burden and Yarden, 1997).
In some cell types, the active receptor is an ErbB4 homodimer; in others, it is an ErbB2–ErbB3 heterodimer
(Burden and Yarden, 1997). In the cochlea, organ of
Corti supporting cells (Stankovic et al., 2004) and spiral
ganglion Schwann cells (Hansen et al., 2001a) express
ErbB2 and ErbB3, so are capable of responding to neuregulin. SGNs themselves express neuregulin (Hansen
et al., 2001a; Stankovic et al., 2004) so are, reciprocally,
capable of signaling to these Erb2/3-expressing cells. In
developing sympathetic ganglia, Verdi et al. (1996)
showed a reciprocal relationship between neuroblasts
and non-neuronal cells in which the former express
TrkC and are supported by NT-3 produced by non-neuronal cells. The latter express both ErbB2 and ErbB3 and
are supported by neuroblast-derived neuregulin. A similar reciprocal trophic relationship may exist between
SGNs and organ of Corti supporting cells in the cochlea.
Inhibition of ErbB function in supporting cells reduces
NT-3 expression in the cochlea and results in death of
most SGNs. Therefore, SGNs appear to be supported by
NT-3 derived from the organ of Corti and, in turn,
induce NT-3 expression in these cells via neuregulin signaling (Stankovic et al., 2004). Moreover, synaptogenesis
in the vestibule appears to depend on BDNF produced
in supporting cells, again in an ErbB-dependent manner
(Gomez-Casati et al., 2010).
In summary, both NT-3 and BDNF are expressed in
the developing cochlea in apex to base gradients. NT-3
continues to be expressed in an apex to base gradient in
the postnatal organ of Corti, although at lower levels
than prenatally. BDNF expression is low in the postnatal organ of Corti. Both BDNF and NT-3 are
expressed postnatally in the spiral ganglion in neurons
and/or glial cells. Thus, BDNF and NT-3 can mediate
trophic or tropic signaling from the organ of Corti to
SGNs and can provide autocrine or paracrine trophic or
tropic signaling within the spiral ganglion. Consistent
with this, either BDNF or NT-3 can maintain survival of
SGNs in culture in the absence of other trophic factors
(Malgrange et al., 1996; Hegarty et al., 1997; Mou et al.,
1997). Interactions among neurons and non-neuronal
cells are likely to be important in inducing expression of
neurotrophins, although it is not known how the neurotrophin expression gradients along the tonotopic axis are
generated and maintained.
There is evidence for NTFs other than neurotrophins
in the postnatal cochlea. Studies of rat cochleae (Ylikoski
et al., 1998) show glial cell line-derived neurotrophic factor (GDNF) mRNA present starting at postnatal day 7
(P7), just prior to hearing onset. In situ hybridization
shows that GDNF expression is in hair cells, becoming
restricted to IHCs over time (Ylikoski et al., 1998).
Quantitation with qPCR shows GDNF levels greater
than those of NT-3 in the cochlea of the mature mouse
(Stankovic and Corfas, 2003). Transcripts of other members of the GDNF family—neurturin, artemin, and
persephin—have also been found by reverse transcriptase-polymerase chain reaction (RT-PCR) in the cochlea
in a tissue isolate containing the organ of Corti and in a
modiolar tissue isolate containing the spiral ganglion
(St€over et al., 2000), although localization on the cellular
level within these regions has not yet been reported.
GDNF supports survival of cultured SGNs (Ylikoski
et al., 1998) indicating that SGNs must express a functional GDNF receptor complex. The canonical highaffinity receptor for GDNF family members contains the
receptor protein-tyrosine kinase Ret and one of the
members of the GDNF family receptor alpha (GFRa)
family. The former appears to be the signal-transducing
receptor, while the GFRa family members, which consist
only of an extracellular domain glycosylphosphatidylinositol (GPI)-linked to the plasma membrane, serve to
confer high-affinity binding to Ret and not to be signal
transducers (Airaksinen and Saarma, 2002). With regard
to the latter, several studies (Ylikoski et al., 1998; St€over
et al., 2000, 2001) have shown expression of GFRa1 in
the cochlea, specifically localized to SGNs (St€over et al.,
2001). St€over et al. (2000) have provided evidence for
expression of the other two members of the GFRa family,
GFRa2 and GFRa3, in the cochlea. Microarray gene
expression profiling of P32 and P60 rat spiral ganglia
(Bailey et al., 2012) confirms expression of GFRa1 and
GFRa2, although GFRa3 was not detected. Thus, coreceptors are potentially available for high-affinity binding
of the members of the GDNF family of NTFs.
The presence of the signal-transducing receptor Ret in
the spiral ganglion is still uncertain. Although two studies raised a possibility of Ret expression (St€over et al.,
2000, 2001), others have failed to detect Ret expression
(Ylikoski et al., 1998; Hashino et al., 1999). The previously mentioned microarray gene expression profiling
indicated only a low level of Ret expression in the
mature rat spiral ganglion, but this same study did
show expression of neural cell adhesion molecule
(NCAM) in the spiral ganglion. Because NCAM can sub-
stitute for Ret in GDNF signal transduction (Paratcha
et al., 2003), the fact that SGN survival and neurite
growth are promoted by GDNF (see later) is not incompatible with absence of Ret.
Finally, there is evidence that ciliary neurotrophic factor (CNTF) and its receptors are expressed in the cochlea
(Malgrange et al., 1998). Microarray gene expression
profiling (Bailey et al., 2012) has confirmed expression of
CNTF and CNTF receptors in the spiral ganglion. CNTF
promotes survival of cultured SGNs (Hartnick et al.,
1996; Whitlon et al., 2007) and has a strong synergistic
survival-promoting effect in combination with NT-3
(Staecker et al., 1995; Hartnick et al., 1996).
The effects of knockouts of NT-3, of BDNF, or of their
receptors have been previously reviewed (Fritzsch et al.,
2004). Briefly, mice with knockouts (KOs) of the NT-3
gene or of the BDNF gene show that either of these
NTFs can support SGN survival during development but
that presence of at least one is necessary for SGN survival. Double knockout of both BDNF and NT-3 results
in complete loss of SGNs (Ernfors et al., 1995). Similarly,
double knockout of TrkB and TrkC results in complete
loss of SGNs (Fritzsch et al., 1995). Evidently, any other
NTFs expressed in the developing cochlea do not suffice
to support survival.
Knockout of BDNF alone has a relatively modest
effect on cochlear innervation. In contrast, vestibular
neuron survival in BDNF KO mice is severely impaired
(Ernfors et al., 1994, 1995; Jones et al., 1994) indicating
that BDNF is an essential NTF for vestibular neurons.
This is consistent with the mature expression pattern in
which BDNF is expressed at much higher levels in the
vestibule than in the cochlea (Stankovic and Corfas,
2003). In the cochlea, TrkB or BDNF KO reduces innervation of the OHCs (Ernfors et al., 1995; Fritzsch et al.,
1998). This may be a consequence of lower levels of NT-3
expression in OHCs relative to IHCs, which could make
OHC innervation more dependent on BDNF. NT-3 or
TrkC KO results in loss of most SGNs (Ernfors et al.,
1995; Fritzsch et al., 1997, 1998; Fari~
nas et al., 2001)
although SGNs do survive in the apex. SGN death in
NT-3 KOs occurs during the period embryonic day (E)
13.5–15.5, just after innervation of hair cells. At this
time, BDNF expression is declining in the organ of
Corti, and BDNF is present in the apex but not in the
base (Fari~
nas et al., 2001). Therefore, survival of SGNs
in the apex in NT-3 KO mice is likely to be due to support of SGN survival by BDNF. Indeed, in a combined
BDNF and NT-3 double KO or a combined TrkB and
TrkC double KO, neither SGN survival is observed in
the cochlea nor vestibular neurons survival is observed
in Scarpa’s ganglion (Ernfors et al., 1995; Fritzsch et al.,
1995; Silos-Santiago et al., 1997). However, genetic
replacement of NT-3 by BDNF completely rescues SGN
survival in mice (Fari~
nas et al., 2001). These data imply
that BDNF and NT-3 are essentially equivalent in their
trophic action on SGNs during development. Further
support for this comes from observations of cultured
SGNs, which show that SGN survival can be supported
by adding either BDNF or NT-3 or NT-4/5 to the culture
medium (Zheng et al., 1995; Malgrange et al., 1996;
Hegarty et al., 1997; Mou et al., 1997). Although NT-4/5
is not expressed in the cochlea, it is a TrkB ligand comparable with BDNF in ability to signal through TrkB.
If hair cells are killed by certain means, for example,
by exposure to aminoglycoside antibiotics, SGNs gradually die but this SGN death can be reduced by
intracochlear infusion of NTFs or of viral vectors that
drive expression of NTFs; a subject that has been previously reviewed (Roehm and Hansen, 2005; Green et al.,
2008) and recently reviewed in detail in Budenz et al.
(in this issue). Briefly, in a paradigm that has been
applied with comparable results using cats, guinea pigs,
or rats, the animals are deafened by injection of an ototoxic aminoglycoside, in some cases combined with a
loop diuretic, that rapidly kills hair cells. SGNs die after
deafferentation although, unlike the very rapid death
that occurs after NT-3 KO in embryonic mice, the death
of SGNs in adult animals is slow. In deafened rats and
guinea pigs, SGN death occurs over a period of a few
months (Webster and Webster, 1981; Koitchev et al.,
1982; Bichler et al., 1983; Alam et al., 2007); in deafened
cats, SGN death occurs over a period of months to years
(Leake and Hradek, 1988). Observations of human postmortem tissue indicate that SGNs are capable of
surviving for many years in the absence of hair cells
(Nadol et al., 1989; Nadol, 1990, 1997).
If hair cell-derived NTFs were the sole source of neurotrophic support to SGNs, one would expect the SGNs
to die rapidly after loss of hair cells. However, the loss of
SGNs postdeafferentation is slow and gradual. Moreover,
elimination of hair cells by some means, for example,
thiamine deprivation, does not result in SGN death.
These data suggest, rather, that NTFs from sources
other than hair cells can provide neurotrophic support to
SGNs. Such sources include organ of Corti supporting
cells (Sugawara et al., 2005), neuronal and glial sources
within the spiral ganglion (Hansen et al., 2001a,b; Bailey et al., 2011), and the cochlear nuclei, which produce
NTFs (Bailey et al., 2011) and are necessary for SGN
survival (Spoendlin, 1971). The neurotrophic support
provided by these sources is not restricted to neurotrophins, BDNF and NT-3, but includes other NTFs,
including those summarized earlier (Bailey et al., 2011).
Nevertheless, as described by Budenz et al. (in this
issue), when SGN death would occur following hair cell
loss, it is reduced by supplying NTFs to the cochlea during the period when SGNs are dying. To take just a few
of many examples, intracochlear infusion of NT-3 from
an implanted minipump and cannula has been shown to
reduce SGN death in deafened guinea pigs (Ernfors
et al., 1996; Staecker et al., 1996). Similarly, intracochlear infusion of BDNF has been shown to reduce SGN
death in deafened guinea pigs (Staecker et al., 1996;
Miller et al., 1997; Shinohara et al., 2002), rats
(McGuinness and Shepherd, 2005), or cats (Leake et al.,
2011). Reduced SGN death was also observed in deafened guinea pigs with intracochlear expression of BDNF
from intracochlear injection of viral expression vectors
(Nakaizumi et al., 2004) or fibroblasts genetically engineered to express BDNF (Rejali et al., 2007). Aside from
similar effects on survival of SGNs, experiments using
cultured SGNs show that NT-3 and BDNF are both able
to elicit neurite growth from SGNs (Malgrange et al.,
1996; Hegarty et al., 1997).
NTFs other than neurotrophins that promote SGN
survival in vitro also reduce SGN death in deafened animals. Intracochlear infusion of GDNF (Ylikoski et al.,
1998), or artemin (Warnecke et al., 2010) reduces SGN
death in deafened guinea pigs. Also, intracochlear injection of GDNF (Yagi et al., 2000; Kanzaki et al., 2002) or
CNTF (Nakaizumi et al., 2004) viral expression vectors
into deafened guinea pigs reduces SGN death.
While these experiments show that BDNF, NT-3, and
other NTFs are sufficient to maintain survival of SGNs
after deafferentation, it is important to note that these
experiments do not show that NT-3 or any particular
NTF is necessary for survival of SGNs in the postnatal
cochlea. In contrast to the situation in NT-3 KO mice, in
which SGNs die rapidly, SGN death after deafferentation is slow. Thus, it is possible that NT-3 from sources
other than hair cells can support SGNs after hair cell
death. Organ of Corti supporting cells have been suggested as a source of NT-3 crucial to SGN survival
(Sugawara et al., 2005). NT-3 and BDNF may be
expressed in the ganglion itself (Hansen et al., 2001a,b;
Schimmang et al., 2003; Bailey et al., 2011) and provide
autocrine or paracrine trophic support to SGNs (Hansen
et al., 2001b).
Another possibility is that SGNs may become independent of support by NT-3 alone during postnatal
maturation and acquire the ability to have their survival
supported by other NTFs in the cochlea. (For example,
as noted above, GDNF is expressed in the postnatal
cochlea, can support SGNs in vitro, and can support
SGN survival after hair cell death.) These possibilities
cannot be distinguished without experiments in which
NT-3 signaling is normal during development but disrupted postnatally in hair cells or supporting cells or
cells in the spiral ganglion.
Because type I neurons comprise approximately 95%
of the ganglion, we will examine spiral ganglion morphological phenotype in the context of this class of neurons.
Similar to olfactory neurons and in distinction to those
in the dorsal root ganglion, most type I SGNs elaborate
a classical bipolar morphology. Each cell soma is surrounded by its axonal-like distal and proximal
myelinated processes that convey electrical signals into
the CNS. Thus, once the electrical signal is generated in
the initial segment (IS) in the cochlea, extending from
the postsynaptic contact to the hair cell to the heminode
adjacent to the foramina nervosa (Hossain et al., 2005),
it then travels via saltatory conduction through a distal
peripheral (DP) axonal process, through the cell soma,
and into a proximal peripheral (PP) axonal process. The
entire extent of the peripheral regions of the neuron,
aside from the IS, is myelinated by Schwann cells; compact myelin ensheaths the axonal processes, whereas a
unique form of loose myelin surrounds the cell soma
(Rosenbluth, 1962). The axon then enters the CNS
through the internal auditory meatus, assuming the features of a central process (CP), myelinated by
oligodendrocytes before bifurcating and extending to
form distinct tonotopic maps within the cochlear nucleus
(Lorente de No, 1933; Luo et al., 2009).
Neurite Outgrowth
The axonal details elaborated above are important to
keep in mind, because the lengths of specific regions of
the peripheral and central axonal compartments, like
the cell soma, vary tonotopically. Although the total
length of SGNs from their synapse on a hair cell to their
prominent bifurcation in the CNS is relatively uniform,
distinct axonal regions vary in length along the tonotopic axis (Fekete et al., 1984). Because the basilar
membrane is narrow in the high-frequency region and
the somata of basal neurons are closer to the internal
auditory meatus, the basal neuron IS, DP, and PP are
shorter compared to those of apical neurons (Ryugo,
1992). And because the high-frequency regions of the
cochlear nucleus tonotopic maps are further from the internal auditory meatus than are the low frequency
regions, the CP is longer in basal neurons compared to
apical neurons (Lorente de No, 1933).
Observations of neurite outgrowth are central to the
original observations of neurotrophin effects on the primary auditory afferents and may require precise
controls because of the tonotopically associated differences in neurite length. In addition to other factors that
play a role in neurite regeneration, such as FGF-1, FGF2, LIF, and depolarization (Hegarty et al., 1997; Gillespie
et al., 2001; Hansen et al., 2001b; Hossain et al., 2002;
Aletsee et al., 2003; Whitlon et al., 2007), both BDNF
and NT-3 have also been shown to enhance spiral ganglion neurite outgrowth in vitro (Malgrange et al., 1996;
Hegarty et al., 1997). A comparison of the effects of
BDNF and NT-3 on early embryonic (E11) spiral ganglia
showed that, at the concentrations used, BDNF was
more efficacious in promoting spiral ganglion neurite
outgrowth than was NT-3 (Pirvola et al., 1992a). Observations were made on total explants, thus, only an
overall assessment, rather than specific evaluations, can
be made from these data.
When examined in vivo, the effects of BDNF and NT-3
on the cochlea are clearly complex (Yang et al., 2011). In
accord with in vitro studies, addition of BDNF, either
alone or in combination with other trophic factors, has
been shown to increase SGN sprouting in vivo (Wise
et al., 2005; Glueckert et al., 2008; Leake et al., 2011).
In addition to promoting neuritogenesis, BDNF has also
been shown to have the potential to provide a cue for
targeting. In a study in which adenoviral vectors
expressing either BDNF or NT-3 were injected into the
scala tympani or scala media (Wise et al., 2010), SGNs
showed more organized outgrowth toward the injection
site. This approach could be highly useful when attempting to distinguish neurotrophin effects on neuronal
survival from a role in axon guidance and synapse
Soma Size
Another aspect of the phenotypic profile of SGNs that
varies along the tonotopic contour is soma size, with
neuron somata in the apex being smaller than those in
the base (Liberman and Oliver, 1984; Nadol et al., 1990;
Echteler and Nofsinger, 2000). Because the soma of
these bipolar neurons is interposed within the electrical
trajectory from sensory receptor to postsynaptic target,
soma size, which affects the amount of membrane to
charge and the internal resistance, can have a profound
impact on the timing and filtering characteristics of signal transmission (Robertson, 1976). Thus, in a system in
which action potential timing is critical to convey accurate information about sound frequency, one might
hypothesize that the feature of soma size must also be
highly regulated. Further, because neurotrophins are
tonotopically expressed and have the ability to alter
aspects of the electrophysiological phenotype of SGNs,
this opens the possibility that this morphological feature
is regulated in tandem with the electrophysiological phenotype. Studies examining the effects of neurotrophins
infused in damaged cochleae in vivo, clearly show that
in addition to promoting increased survival BDNF can
maintain (Leake et al., 2011) or increase the original
soma area, particularly in the basal region of the cochlea
(McGuinness and Shepherd, 2005; Shepherd et al., 2005;
Agterberg et al., 2008). A possible explanation for this
observation is that the site of BNDF delivery is the
round window, but based on the concentrations needed
to produce an effect on survival, it has been argued that
this is unlikely (Agterberg et al., 2008). An alternative
explanation is that the basal cochlea is more BDNF-responsive than the apex, a result consistent with other
actions of this neurotrophin on the properties of the
A complication to the interpretation of the observations described earlier is that regulated enlargement of
neuronal soma may be difficult to distinguish from the
possibility that a cell with appropriate trophic support
may be larger and, as a result, ‘‘healthier’’ than one
devoid of it. One way to examine this is to determine
whether a neurotrophin can predictably decrease soma
area while simultaneously promoting survival. This type
of mechanism might be predicted to regulate the smaller
size of apical spiral ganglion. Interestingly, this has been
observed in experiments carried out in vitro. In an NT-3
concentration series that was found to increase the survival of apical SGNs, significant decreases in soma size
were noted (Smith and Davis, 2012). Taken together,
from the studies carried out to date, there are strong
indications that soma size, in addition to electrophysiological phenotype, is likely to be differentially affected
by BDNF and NT-3. Additional experiments, however,
are required to determine how neurotrophin regulation
of soma size compares to that of ion channel or synaptic
protein regulatory mechanisms.
Additional investigation is required to explain the signaling mechanisms that underlie observations of
differential BDNF and NT-3 actions and how they intersect with other processes and factors to control the
overall characteristics of SGNs. Our current understanding does, however, indicate that, like other neurons in
the peripheral and central nervous systems, the primary
auditory afferents require complex and overlapping neurotrophin signaling mechanisms to attain their final
form. Furthermore, understanding how the tonotopically
distributed morphology, synaptic connections, and
electrical transmission properties are achieved presents
a future challenge required to define the unique neurons
that compose the spiral ganglion.
The above experiments imply that BDNF and NT-3
are approximately equivalent in their ability to promote
SGN survival or elicit neurite growth from SGNs.
Indeed, experiments in which the NT-3 gene is replaced
by a BDNF gene show that BDNF can rescue SGN survival in the absence of NT-3 (Coppola et al., 2001).
However, these same experiments show that BDNF and
NT-3 have distinctive actions in directing innervation of
the cochlea. While SGN number is normal or nearly so
in mice in which BDNF has replaced NT-3, the innervation of the organ of Corti is abnormal (Coppola et al.,
2001; Tessarollo et al., 2004; Yang et al., 2011). This is
due, in part, to invasion of the cochlea base by vestibular
neuron fibers (Tessarollo et al., 2004), which are BDNFdependent, but the spiral ganglion projection itself is
abnormal, particularly the projection to the OHCs,
which is diminished (Yang et al., 2011).
Synapses between IHCs and SGNs are susceptible to
excitotoxic trauma: exposure to agonists of non-Nmethyl-D-aspartate (NMDA)-type ionotropic glutamate
receptors results in destruction of these synapses in vivo
(Puel et al., 1995) and in vitro (Wang and Green, 2011).
There is some limited synapse regeneration after excitotoxic trauma and the number of regenerated synapses is
increased by addition of either BDNF or NT-3, with the
two neurotrophins being equally effective (Wang and
Green, 2011). However, blockade of NT-3 signaling significantly reduces synapse regeneration, even in the
presence of added BDNF (Wang and Green, 2011). One
interpretation of these results is that NT-3 and BDNF
are not equivalent in their effect on synapse regeneration in this system and that BDNF cannot completely
replace NT-3.
Primary auditory afferents possess both TrkB and
TrkC receptors, making them capable of responding to
multiple neurotrophins, and thus enabling a more complex and rich response than neurons that possess only a
single Trk receptor type. Fortunately, the functional
effects of both BDNF and NT-3 can be effectively
explored within the spiral ganglion because of its simple
and highly ordered organization. As mentioned earlier,
the ganglion is composed of a relatively uniform population of cells, 95% of which are classified as type I
neurons, which are predominately bipolar neurons that
make one-to-one synaptic connections with IHCs
(Spoendlin, 1973). Further, peripheral innervation patterns of type I neurons are organized into an array that
is systematically graded from the base to the apex of the
cochlea reiterating the tonotopic map of sound extending
from high to low frequencies, respectively. Thus, the spiral ganglion, with its highly regular peripheral synaptic
connections, follows the tonotopic map of sound that
originates from mechanical and cellular cochlea specializations (Rubel and Fritzsch, 2002). Commensurate with
this, the intrinsic membrane properties of early postnatal SGNs separated from their synaptic connections in
vitro display tonotopic kinetic features (summarized in
Fig. 3). Neurons that innervate high-frequency regions
display fast firing properties, in response to constant
current injection, such as rapid accommodation and
abbreviated action potential duration and latency. Conversely, neurons that innervate low-frequency regions
display slower firing properties, such as rapid to slow
accommodation and prolonged action potential duration
and latency (Adamson et al., 2002b).
Voltage-Gated Ion Channels
The graded kinetic membrane properties of SGNs are
largely orchestrated by the relative density and types of
voltage-gated ion channels, although structural features
of the neurons (see earlier) may also have a significant
impact. The specific ion channel types studied in this
context have been Kv1.1, Kv3.1, Kv3.3, Kv4.2, and large
conductance Ca2þ-activated Kþ (BK) channels (Adamson
et al., 2002b; Chen and Davis, 2006). Neurons capable of
rapid firing in the basal cochlea would be expected to
possess rapidly activating, high-voltage activated Kþ
channels, such as Kv3.1 and Kv3.3, voltage and Ca2þactivated Kþ channels, such as BK channels, and lowvoltage activated Kþ channels, such as Kv1.1, because,
in combination, these would limit action potential duration and increase accommodation (Wang et al., 1998;
Brew et al., 2003; Edgerton and Reinhart, 2003). Interestingly, as a consequence of the presence of these Kþ
channels, there would also be an elevation of action
potential firing threshold, but this is offset by the high
density of 2-amino-3-(5-methyl-3-oxo-1,2-oxazol-4-yl)propanoic acid (AMPA)-type receptors also found in this
neuronal population (see later). In contrast, neurons
with slow firing features in the apical cochlea would be
expected to possess low-voltage activated, inactivating
ion channels, such as Kv4.2, which are capable of delaying action potential responses to stimulus onset
(Baranauskas, 2007). Thus, the distribution pattern of
these ion channel types within the ganglion is consistent
with their physiological role enabling rapid changes in
membrane potential for high-frequency sounds and following slower changes in voltage for low-frequency
sounds. Expectedly, ion channel types that contribute to
fast firing characteristics (Kv1.1, Kv3.1, and BK) are
found in greater abundance in basal, high-frequency coding SGNs, whereas the ion channel type that contributes
to slow firing characteristics (Kv4.2) is enriched in the
apex, low-frequency coding SGNs (Adamson et al.,
One might hypothesize that these features must be
precisely regulated to achieve the frequency-associated
kinetic differences that have been described earlier.
Indeed, ion channel composition and firing features,
evaluated with immunocytochemistry and whole-cell
patch clamp electrophysiology, can be altered by both
BDNF and NT-3 and, in some cases, these two neurotrophins have opposing actions. This is the case for the
assessment of two ion channel types responsible for
high-frequency firing: Kv3.1 and BK. BDNF increases
the abundance of both ion channel types in apical
neurons, whereas NT-3 decreases their abundance in basal neurons (Adamson et al., 2002a). Interestingly, a
channel type that has a major impact on neuronal sensitivity, Kv1.1, that is enriched in the basal neurons
(Adamson et al., 2002b), is not oppositely regulated by
BDNF and NT-3. Its level is enhanced by BDNF, with
NT-3 having little or no effect. Similarly, yet in distinction to this, the ion channel associated with lowfrequency firing, Kv4.2, shows increased levels following
exposure to NT-3, with exposure to BDNF having little
or no effect (Adamson et al., 2002a).
These observations demonstrate two broad mechanisms of regulation within the classes of ion channels
identified in the spiral ganglion. First, BDNF and NT-3
have opposite regulatory effects on a specific set of ion
channel types; second, each neurotrophin acts separately
to up-regulate defined ion channels. If one assumes in
each of these examples that BDNF and NT-3 are working primarily through their cognate high-affinity
receptors, TrkB and TrkC, respectively, then these complex patterns of ion channel regulation indicate that
their signaling pathways are necessarily different. In
the first example cited above for Kv3.1 and BK, one hypothesis could be that a common TrkB/TrkC signaling
pathway may be activated, yet in one Trk-evoked pathway the sign of the response is inverted to mediate a
reversed outcome. In the second example cited above for
Kv1.1 and Kv4.2, one hypothesis might be that distinct
TrkB/TrkC signaling pathways are utilized, each having
exclusive regulatory effects on different ion channel
types. Although such fine distinctions have not yet been
revealed in different Trk signaling pathways, these types
of observations are valuable for formulating models that
can be tested in appropriate systems, such as the spiral
ganglion (Huang and Reichardt, 2003).
Synaptic Proteins
Because the function of primary afferents is to deliver
reliably the electrical output of sensory receptor generator potentials to higher order neural centers, it is
reasonable to suppose that the synaptic proteins that
comprise both the presynaptic and postsynaptic machinery of SGNs may also be highly regulated in a manner
commensurate with that seen for voltage-gated ion channels. Indeed, studies have shown that this is the case.
Superimposed upon the fine structure of the tonotopic
distribution of synapse number (Meyer et al., 2009) and
their structural complexities characterized around each
IHC circumference (Liberman et al., 2011), observations
of overall synaptic proteins levels throughout the ganglion have also been shown to be neurotrophindependent.
It is already known that apical SGNs form notably
larger end bulbs of Held with bushy cells in the cochlear
nucleus (Rouiller et al., 1986) and also show graded levels of presynaptic proteins compared to their basal
counterparts, as assessed by immunocytochemical analysis of anti-SNAP-25 and antisynaptophysin antibody
immunolabeling (Flores-Otero and Davis, 2011). In contrast to this, basal SGNs, which have substantially
higher voltage thresholds and lower resting membrane
potentials than apical neurons, show higher levels of
postsynaptic AMPA receptors compared to their apical
counterparts, as assessed by immunocytochemical analy-
sis of anti-GluR2, anti-GluR3, and anti-GluR2/3
antibody immunolabeling (Flores-Otero and Davis,
2011). What is notable about both of these distributions
is that the assessed presynaptic and postsynaptic proteins are regulated by neurotrophins similar to that
described for Kv3.1 and BK, such that BDNF and NT-3
are capable of working together to regulate synaptic proteins reciprocally. AMPA receptors are up-regulated by
BDNF and down-regulated by NT-3. In contrast, SNAP25 and synaptophysin are down-regulated by BDNF and
up-regulated by NT-3. Thus, the presynaptic proteins
assessed in the spiral ganglion were oppositely regulated
by BDNF and NT-3, a mechanism that was also
observed for postsynaptic proteins, but as a mirror
image (Flores-Otero et al., 2007).
These results are supported by experiments using an
in vitro preparation in which microisolates removed
from different tonotopic areas of the organ of Corti were
cocultured with apical or basal SGNs (Flores-Otero
et al., 2007). Basal SGNs were altered to a more apicallike phenotype when cocultured with apical organ of
Corti microisolates, and this effect was inhibited by an
anti-NT-3 function-blocking antibody. Conversely, apical
SGNs were altered to a more basal-like phenotype when
cocultured with basal microisolates; these characteristics
were significantly decreased by the additional of an antiBDNF function-blocking antibody. This is consistent
with the hypothesis that a high ratio of NT-3 to BDNF
in the apex results in enhanced NT-3 secretion from the
apical regions of the organ of Corti relative to the base,
and this induces apical characteristics in SGNs. The
inverse is the case in the base, a low ratio of NT-3 to
BDNF results in enhanced BDNF secretion from basal
regions of the organ of Corti compared to apical regions
and BDNF derived from the basal organ of Corti induces
basal characteristics in SGNs.
The observations above indicate that the overall electrophysiological signature of a SGN is the outcome of
complex regulatory mechanisms, in which distinctive
signaling by BDNF and NT-3 plays a crucial role. This
unique pattern of regulation provides a platform to
explore interactions between TrkB and TrkC signaling
pathways that appear to be substantially different, yet
highly interdependent. Observations from other neuronal systems and functional contexts suggest that yin
and yang interactions are a general principle of neurotrophin signaling (Lu et al., 2005). For example, it has
recently been observed that phenotypes as disparate as
survival/apoptosis (Hempstead, 2006) and long-term
potentiation (LTP) and long-term depression (LTD; Minichiello, 2009) are regulated by BDNF by novel
mechanisms associated with differential signaling
through p75NTR versus TrkB. Depending on the extent
to which proBDNF is cleaved to mature BDNF, respectively, p75NTR or BDNF will be preferentially bound,
with very different intracellular signaling pathways and
outcomes (see later) as a result.
Although one observes a yin and yang pattern of voltage-gated ion channel and synaptic protein regulation in
SGNs (Davis and Liu, 2011), the mechanism is unlikely
to involve p75NTR, which does not appear to be
expressed on SGNs. Rather, two similar receptor types,
TrkB and TrkC, are preferentially bound by two different ligands. From studies evaluating neurotrophin levels
reviewed earlier, NT-3 concentrations are expected to be
higher in the apical compared to the basal regions of the
cochlea, which is consistent with its phenotypic actions
on the SGNs. This poses an interesting issue, whether
at the highest concentrations NT-3, with its known promiscuous binding, also interacts with TrkB receptors,
thus increasing the potential complexity of neurotrophin
interactions within the spiral ganglion. Observations
consistent with this type of interaction have been
reported (Hansen et al., 2001b; Zhou et al., 2005), however, it is currently unclear whether this mechanism
contributes to the final functional regulation within the
spiral ganglion. A significant limitation to our ability to
explain the complex pattern of divergence and convergence in neurotrophin effects on SGNs is that kinetics of
neurotrophin binding to SGNs has never been assessed.
Thus, the actual affinities of SGNs for NT-3 and BDNF
are not known.
The role of BDNF in this process is even less clear. In
addition to the open question of whether or not this neurotrophin is produced and secreted by cochlear hair cells
postnatally, it is currently unresolved as to whether, as
would be consistent with functional data, BDNF concentrations are higher in basal regions of the cochlea (see
earlier). Furthermore, the presence of BDNF in the spiral ganglion, which is higher in basal than apical
neurons (Schimmang et al., 2003) opens up the question
as to whether BDNF has an autocrine or paracrine role
in phenotypic regulation, and how that compares with
neuronal survival (Hansen et al., 2001b). In this case,
the mechanisms of secretion and neurotrophin processing may be key factors to consider.
Thus, we anticipate that novel processes may underlie
the multiple regulatory mechanisms that have been
observed in SGNs. One might imagine that in addition
to specificity of neurotrophin concentrations and secretion mechanisms, that processing of the neurotrophin
proforms, novel receptor binding, or alternative signaling pathways, are also involved.
Neurons that express p75NTR also express at least one
of the Trks and Trk-p75NTR association increases specificity of the Trk for its corresponding neurotrophin(s)
and increases binding affinity. However, p75NTR also has
its own distinct signaling very different from that of Trk.
Signaling by p75NTR has been recently reviewed in detail
(Reichardt, 2006; Schweigreiter, 2006; Teng et al., 2010)
so will be only briefly summarized here. p75NTR is
expressed in neurons with at least one member of the
Trk family but may also be expressed in non-neuronal
cells, including central and peripheral glia. As diagrammed in Fig. 2, p75NTR alone binds neurotrophins
with low affinity but, with the ubiquitous protein sortilin
as a coreceptor, is a high-affinity receptor for proneurotrophins (Teng and Hempstead, 2004; Schweigreiter,
2006). In either case, p75NTR signaling is generally,
although not necessarily, proapoptotic (Reichardt, 2006;
Teng et al., 2010) and, in neurons, also inhibits neurite
growth (Teng et al., 2010) and depresses synaptic activity (Lu, 2003). Thus, for a cellular decision of survival
versus apoptosis, the presence of specific receptors and
ligands coordinate distinct functional outcomes. Cells
can be preferentially triggered to undergo apoptosis in
the presence of pro-BDNF, which can bind with high affinity to a p75NTR/sortilin receptor complex. Yet, these
same cells preferentially survive when mature BDNF
binds to TrkB (Lee et al., 2001). In another example,
LTP and LTD are differentially induced in hippocampal
neurons via, respectively, TrkB or p75NTR, depending on
whether proBDNF is processed to BDNF (Figurov et al.,
1996; Woo et al., 2005).
Proapoptotic signaling by p75NTR is due, in part, to
activation of the proapoptotic mitogen-activated protein
(MAP) kinase c-Jun N-terminal kinase (JNK) signaling
pathway (Harrington et al., 2002; Bhakar et al., 2003;
Kenchappa et al., 2010) and activation of the tumor necrosis
(Kenchappa et al., 2010). At least one other p75NTR-associated protein, neurotrophin receptor-interacting MAGE
homolog (NRAGE), may contribute to p75NTR-induced
apoptosis via a separate signaling pathway (Bertrand
et al., 2008) and also contribute to inhibition of neurite
growth (Feng et al., 2010).
The intracellular domain (ICD) of p75NTR can be
cleaved and released by the protease c-secretase, which
allows translocation of the ICD to the nucleus (Parkhurst et al., 2010). This results in increased
transcription of at least one gene, cyclin E1 (Parkhurst
et al., 2010). In addition, nuclear translocation of the
p75NTR ICD brings p75NTR-associated proteins including
the zinc-finger transcription factor neurotrophin receptor
interacting factor (NRIF) (Linggi et al., 2005) to the nucleus. This event also contributes to apoptosis
(Kenchappa et al., 2006), although the specific genes so
regulated have not yet been identified. Other intracellular signals activated by this versatile receptor include
RhoA, which contributes to antagonism of axon growth
by p75NTR (Yamashita et al., 1999), and NF-jB, which
contributes to cell differentiation and cell survival
responses to p75NTR observed in some contexts (Hamanoue et al., 1999). p75NTR signaling can be antagonistic
to neurotrophic signaling without necessarily being
proapoptotic (Greferath et al., 2012).
In the cochlea, p75NTR is clearly expressed in the
embryo (Abe et al., 1991; Despres et al., 1991), but
p75NTR expression declines to undetectable levels in the
first postnatal week and remains undetectable in the
postnatal and mature cochlea. In the organ of Corti,
p75NTR expression is restricted to the inner pillar cells
in the embryo and early neonate, but this expression disappears postnatally (von Bartheld et al., 1991; Sato
et al., 2006). In the spiral ganglion, p75NTR expression is
evident in embryonic and neonatal SGNs (von Bartheld
et al., 1991; Schecterson and Bothwell, 1994; Sato et al.,
2006) but, as in the organ of Corti, p75NTR expression in
the spiral ganglion disappears after birth. Although Sato
et al. (2006) showed p75NTR immunoreactivity in SGNs
in 1-month-old C57BL/6 mice, several studies (von Bartheld et al., 1991; Tan and Shepherd, 2006; Provenzano
et al., 2011) showed no p75NTR immunoreactivity in
SGN somata in 1-month-old normal hearing rats. In this
respect, SGNs are highly unusual neurons in expressing
Trks (specifically, TrkB and TrkC) but not p75NTR. Given
the known functions of p75NTR in Trk-expressing neurons, this suggests that affinity of SGNs for
neurotrophins may be lower than in most neurons, that
is, relatively high concentrations of neurotrophins may
be needed to elicit trophic or tropic effects on SGNs.
Nearly all published studies in which neurotrophins
have been used to support SGN survival in vitro or in
vivo have used >1 nM concentrations that greatly
exceed the high-affinity Kd of neurotrophin binding,
0.01 nM (Hempstead et al., 1991), so it is not known
whether SGNs can bind or respond to neurotrophins
with high-affinity. As noted earlier, neurotrophin binding
kinetics have never been assessed for SGNs. Moreover,
because of the lack of p75NTR on SGNs, it is conceivable
that TrkB and TrkC receptors on SGNs may bind neurotrophins with less specificity than these receptors on
most other neurons, which could allow greater cross-talk
between neurotrophins and Trks.
Remarkably, in deafened rats, p75NTR and its high-affinity ligand proBDNF are expressed in the spiral
ganglion (Tan and Shepherd, 2006; Provenzano et al.,
2011), although the expression is largely restricted to
spiral ganglion satellite and Schwann cells and not the
SGNs themselves (Provenzano et al., 2011). p75NTR
knockout mice have normal hearing onset and young
p75NTR knockout mice have normal hearing thresholds
(Sato et al., 2006; Brors et al., 2008), implying that
p75NTR is not necessary for normal hearing development. However, Sato et al. (2006) show that p75NTR KO
mice have an accelerated loss of SGNs in aging and
SGN death after deafening is accelerated in p75NTR KO
mice (Tan et al., 2010).
These studies imply a protective role for p75NTR in
the cochlea. This is remarkable given that p75NTR is typically thought of as proapoptotic. Nevertheless, the
available data suggest that, after trauma, p75NTR, which
is otherwise not expressed in the mature cochlea, is upregulated in non-neuronal cells and this, in turn,
reduces the death of SGNs. While the mechanism is not
known, Provenzano et al. (2011) have shown that nuclear translocation of the p75NTR ICD occurs in spiral
ganglion glia after deafening and that this is associated
with entry to the cell cycle. Because spiral ganglion glia
produce NTFs, including NT-3 (Hansen et al., 2001a),
p75NTR may be exerting a protective effect on neurons
indirectly, by maintaining glial number in the degenerating ganglion, allowing the glia to provide trophic
support to the neurons. More direct evidence for this has
come from use of a previously mentioned model for study
of trauma in the cochlea, that of excitotoxic trauma to
IHC-SGN synapses in vitro (Wang and Green, 2011).
Following trauma, p75NTR is expressed in the organ of
Corti supporting cells, and this has a protective role in
that it promotes synapse regeneration in a NT-3-dependent manner (Wang et al., 2009).
While signal transduction by Trk (summarized in Fig.
4) has been extensively studied and reviewed in detail
(Huang and Reichardt, 2003; Reichardt, 2006; Schweigreiter, 2006), most studies have been of TrkA, the firstdiscovered member of the family; TrkB and TrkC have
not been extensively investigated. Nevertheless, similarity of key sequences in the ICD of all three Trks
suggests that the mechanisms revealed by study of TrkA
and other receptor protein-tyrosine kinases apply also to
TrkB and TrkC. For any receptor protein-tyrosine kinase, ligands are divalent or multivalent. In the case of
neurotrophin signaling, all neurotrophins are noncovalently but tightly linked dimers. Binding of ligand to the
receptor extracellularly results in receptor dimerization,
cross-phosphorylation on certain tyrosine residues in the
intracellular portion of the dimerized protein-tyrosine kinases. This ‘‘autophosphorylation’’ by protein-tyrosine
kinases is crucial, as it causes recruitment to the receptor of effector or adaptor proteins that contain
phosphotyrosine-binding motifs. The effectors then initiate intracellular signaling pathways; adaptor proteins
recruit other effectors that initiate intracellular signaling
simultaneously activate several intracellular signaling
pathways after binding ligand.
Multiple effector and adaptor proteins bind Trk receptors, including phosphatidylinositol-3-kinase, Shc, Grb2,
c-Abl, Frs2, phospholipase C-c, and ARMS (Reichardt,
2006). These adaptors, in turn, recruit and activate protein kinases such as protein kinase B (PKB/Akt) and
MAP kinases including extracellular signal-regulated
kinase (ERK)1/2, ERK5, p38, Src-family protein-tyrosine
kinases, and others. By targeting cytoplasmic proteins
and nuclear transcription factors, these kinases promote
survival, neuronal differentiation, neurite growth, synapse potentiation, and most of the other Trk-mediated
effects of neurotrophins. Small GTPases, Ras and Rap1,
participate in linking Trk to activation of the ERK MAP
kinases but Trk also activates other small GTPases—
Rho family members Rac and Cdc42—that directly affect
organization of the actin cytoskeleton and so affect membrane, cell, and growth cone motility and guidance of
axon growth.
Neuronal Survival
The ability of trophic factors to promote survival via
protein-tyrosine kinase signaling depends crucially on
the protein kinase PKB/Akt (Matheny and Adamo,
2009). This general concept also applies specifically to
SGNs: inhibition of Akt prevents BDNF or NT-3 from
promoting SGN survival (Hansen et al., 2001b). Akt is a
multifunctional protein kinase, targeting multiple effectors to inhibit apoptosis. For example, Akt
phosphorylates and inactivates proapoptotic transcription factors of the Forkhead/FoxO family (Brunet et al.,
1999) and p53 (Ogawara et al., 2002) and Akt phosphorylates and inactivates the proapoptotic Bcl-2 family
protein Bad (Downward, 1999). Activation of the MAP
kinase JNK—which mediates proapoptotic effects of
NTF withdrawal (Maroney et al., 1999), cellular stress
(Lin, 2003) and of p75NTR (Teng et al., 2010)—is also
inhibited by Akt (Barthwal et al., 2003; Widenmaier
et al., 2009) thereby providing a mechanism by which
Trk signaling can abrogate proapoptotic p75NTR signaling. Suppression of JNK signaling may be relevant also
to SGN survival in vivo, because SGN apoptosis following loss of hair cells is accompanied by JNK activation
(Alam et al., 2007).
Neurite Growth
Neurotrophins can stimulate neurite growth via Trk.
With all of the factors and mechanisms that underlie
neurite outgrowth, it is not surprising to find that multiple, yet specific, signaling mechanisms, including ERK
Fig. 4. Signal transduction by Trk-family receptor protein-tyrosine
kinases. Trks are dimerized by divalent neurotrophins allowing autophosphorylation of certain tyrosine residues, some of which are diagrammatically illustrated by small blue circles in the intracellular
portion of Trk. On the left side, some effectors activated by Trk after
ligand binding are shown. The phosphotyrosine in the juxtamembrane
domain (Tyr515 in mouse TrkB and Tyr516 in mouse TrkC) is the primary site for recruitment of ERK MAP kinases and PKB/Akt. Trk
shares with other protein-tyrosine kinases the ability to assemble an
adaptor protein complex that strongly but transiently activate ERK
MAP kinases and other effectors at the plasma membrane. In addition, Trk has an atypical ability to assemble, on the juxtamembrane
MAP kinases, Akt, and small GTPases, underlie this
process. Interestingly, different aspects of axon growth
may be modulated by different signaling pathways with,
for example, ERK signaling increasing length and Akt
signaling increasing axon caliber and branching in sensory neurons (Markus et al., 2002). NT-3-evoked
extension of SGN neurites requires the Ras/ERK pathway (Aletsee et al., 2001) and JNK activity (Bodmer
et al., 2002; Atkinson et al., 2011) but is independent of
p38 (Aletsee et al., 2001) and is inhibited by Rho kinase
(Lie et al., 2010). Of depolarization-activated signals,
CaMKII, but not CaMKIV, inhibits SGN neurite growth,
although both kinases converge to promote survival in
these same cells (Hansen et al., 2003). To date, signaling
mechanisms involved specifically in BDNF-evoked neurite extension are unknown, limiting our ability to make
comparisons. Nevertheless, studies reported to date have
shown that inroads are being made to understand better
the diverse signaling contributions to spiral ganglion
neurite outgrowth.
domain phosphotyrosine, an adaptor protein complex that causes a
sustained activation of ERK MAP kinases. This allows retrograde signaling from axon terminals to the soma, crucial for neurotrophic gene
regulation. As indicated on the right side, phosphorylation of this juxtamembrane tyrosine is necessary for full enzymatic activation and autophosphorylation in TrkB, but it is not necessary for this in TrkC
(Postigo et al., 2002). Additional phosphotyrosine residues within the
kinase domain contribute to increased enzymatic activity and may
serve as alternative sites for effector recruitment. Finally, a phosphotyrosine residue at the C-terminal recruits the signaling enzyme phospholipase C-c.
Distinctive Temporal and Spatial Signaling by
Trk Receptors
The ERK family of MAP kinases (Johnson and Lapadat, 2002) are crucial effectors of receptor proteintyrosine kinases and are largely responsible for the mitogenic effect of these receptors. In neurons, which do not
undergo cell division, ERKs function in neuronal differentiation, neurite growth, neuronal plasticity, and other
roles. This involves a distinctive mode of ERK activation
by Trk, not shared by mitogenic signal transduction. Trk
engages two distinct mechanisms for ERK activation
with different temporal and spatial characteristics. The
adaptor proteins Shc and Grb2 recruit a Ras activator,
and activated Ras initiates a kinase cascade that results
in a rapid, strong, but transient Erk activation at the
plasma membrane. This mechanism shared by all receptor protein-tyrosine kinases and is a canonical
mechanism for triggering cell cycle entry and mitosis
(Davis, 1993).
A second mechanism, requiring adaptor proteins
Frs2 or ARMS, leads instead to recruitment of the
small GTPase Rap1 and prolonged Erk activity (Kao
et al., 2001; Arevalo et al., 2004). This mechanism is
distinctive of Trk and, possibly, other NTF receptors,
and is not used by mitogenic receptor protein-tyrosine
kinases. This latter mechanism occurs in neurons that
are postmitotic and, rather than triggering mitosis, the
prolonged Erk activity promotes neuronal differentiation (Qiu and Green, 1992). Moreover, this mechanism
operates on endosomes, as opposed to the plasma membrane, the site of Ras-mediated ERK activation (Wu
et al., 2001). Activation of Trk by neurotrophin binding
causes Trk to be internalized in endosomes. Endosomes
can be retrogradely transported from axon terminals,
where neurons encounter target-derived neurotrophins,
to the soma. This allows activated internalized Trk to
convey persistent Erk signaling from the terminal to
the soma to effect cell survival and regulate gene
expression (Ginty and Segal, 2002). Even this property
of Trk receptors may vary with different ligand–receptor interactions. For example, although TrkA can bind
both NGF and NT-3, only NGF can elicit TrkA internalization and retrograde signaling (Kuruvilla et al.,
Thus, Trk can activate ERK via two different signaling pathways. The Ras-dependent pathway, common to
all receptor protein-tyrosine kinases, is transient and
activates ERKs locally at the plasma membrane and is
important for mitogenic actions of ERKs as well as for
local effects of ERKs on cell metabolism and motility,
including promotion of growth cone motility and axon
growth. The Rap1-dependent pathway is persistent and
activates ERKs on endosomes allowing retrograde signaling to the neuronal soma for promotion of cell
survival and regulation of gene expression.
This brief summary glosses over some critical details
of Trk signal transduction, reviewed at length by Reichardt (2006) that may be crucial to consideration of how
TrkB and TrkC might elicit distinct responses. One such
detail, previously mentioned, is that TrkC cannot bind
BDNF but TrkB can bind NT-3, albeit at reduced affinity. Thus, NT-3 can elicit signaling through TrkB as well
as TrkC and may consequently elicit a higher level of
Trk signaling in SGNs than can BDNF.
Another such detail is that there is generally not an
exclusive relationship between a particular adaptorbinding site on Trk and a protein kinase. Rather, the
same or different adaptors binding to different sites on
activated Trk are able to activate the protein kinases
Erks and/or Akt, although the level or duration of activity may vary. Thus, because of such cross-talk, deletion
of a single adaptor-binding site on Trk may have subtle
quantitative but not necessarily drastic qualitative
effects on responses to NTFs. Similarly, subtle differences among Trks in relative affinity of specific adaptorbinding sites for their respective adaptors could result in
quantitative differences in intracellular signaling generated by the Trks. Even quantitative differences in
intracellular signaling may yield substantial differences
in outcome on the cellular level. For example, difference
in duration of activity of a particular intracellular signal, the Erk MAP kinase, means a difference between
mitogenic signaling and induction of neuronal
As discussed earlier, SGN express TrkB and TrkC so
are capable of responding to both BDNF and NT-3.
Indeed, BDNF and NT-3 can elicit many of the same
responses in SGNs in vivo or in vitro, for example, both
promote survival and neurite growth to approximately
the same degree. However, there are also instances
where BDNF and NT-3 elicit different or opposing effects
in neurons, for example, different turning responses in
spinal neuron growth cones (Song and Poo, 1999) and
different or opposing effects on dendrite growth in cortical pyramidal neurons (McAllister et al., 1997). In
SGNs, BDNF and NT-3 clearly can elicit different or
opposing effects, for example, regulation of membrane
physiology and expression of channels, receptors and
other synaptic proteins (Adamson et al., 2002a), as discussed earlier. This implies that TrkB and TrkC
intracellular signaling share some similarities but also
differ in important respects.
Although TrkA, TrkB, and TrkC appear to activate
the same major intracellular signals—Erk1/2 and Erk5
MAP kinases, PKB/Akt, phospholipase C-c, p38, Rac,
and so forth—and similarly promote neuronal differentiation, survival, neurite growth, synaptogenesis and
increased synaptic activity (Reichardt, 2006), it is possible that they can, nevertheless, direct some dissimilar
neuronal responses via nuanced quantitative differences
in the signaling generated. Such differences in signaling
might underlie different responses of SGNs to BDNF
and to NT-3. For example, as noted above, NT-3 is better
able than BDNF to promote proper synaptogenesis on
hair cells. This is evidenced by reduced synapse regeneration on IHCs after excitotoxic disruption in vitro when
NT-3 signaling is blocked, even in the presence of added
BDNF (Wang and Green, 2011). Moreover, genetic
replacement of NT-3 by BDNF in vivo (Tessarollo et al.,
2004) results in no significant reduction in SGN number,
consistent with similar ability of BDNF and NT-3 to promote SGN survival, but innervation of the organ of Corti
is disorganized. Thus, despite being equivalent in ability
to promote SGN survival, BDNF is not equivalent to NT3 in ability to promote and organize innervation of the
organ of Corti. As noted earlier, distinctive effects of
BDNF vs. NT-3 have been observed in CNS neurons
that express both TrkB and TrkC. However, the relative
simplicity of cochlear innervation and well-defined
effects of the neurotrophins on SGNs makes these neurons a particularly favorable system for studying the
mechanisms by which TrkB and TrkC can elicit different
An analysis by Postigo et al. (2002) of the role of the
important juxtamembrane domain adaptor binding
site—tyrosine 516 in TrkC—is consistent with a hypothesis that differences in the relative significance of such
sites in Trk proteins could underlie differences in the
ability of NT-3 and BDNF to support innervation of the
cochlea. Mutation of this site prevents association of critical adaptors, including Shc and Frs2, with activated
TrkC. This should and does compromise activation of
Erks and PKB/Akt—key effectors of neuronal survival
and neurite growth—by TrkC. However, because other
adaptors that can activate these kinase signaling pathways are able to bind TrkC at other sites, complete
blockade of kinase activation would be unlikely and, in
fact, was not observed (Postigo et al., 2002). Consistent
with this, mice homozygous for this mutation of the
TrkC Shc-binding site (trkCshc/shc mice) are viable postnatally, unlike trkC knockout (trkC/) mice. trkCshc/shc
mice had a decreased number of SGNs but the decrease,
only 25%, was modest compared to that in trkC–/–
mice. The corresponding mutation in TrkB had similar
consequences for vestibular neurons.
However, while the trkBshc mutation significantly
affected peripheral innervation by vestibular neurons,
the trkCshc mutation had little apparent effect on peripheral innervation by SGNs. These data imply that
TrkB and TrkC are similar in their reliance on this
adaptor site for promoting survival but differ significantly their reliance on this adaptor site for directing
axon growth and synaptogenesis. What can account for
this on the molecular level? Postigo et al. observed that
mutation of tyrosine 516 in the TrkC juxtamembrane domain, while markedly reducing activation of ERKs and
PKB, does not reduce TrkC autophosphorylation. That
is, TrkC remains enzymatically active and the remaining
phosphotyrosine residues are, presumably, capable of
some recruitment of intracellular signaling. In contrast,
mutation of the corresponding juxtamembrane phosphorylatable tyrosine in TrkB (Tyr515) results in greatly
diminished autophosphorylation. Presumably, in TrkB,
full enzymatic activation requires phosphorylation of
Tyr515 while in TrkC enzymatic activity is largely independent. While it is not clear how this mechanistic
difference between TrkB and TrkC can account for different responses to BDNF and NT-3 in SGNs, it is
consistent with a hypothesis that lack of equivalence in
TrkB and TrkC signal transduction can account for these
different responses.
Two types of receptors, p75NTR and Trk-family receptor protein-tyrosine kinases, can bind neurotrophins and
mediate responses to these important NTFs. The highaffinity proneurotrophin receptor p75NTR is expressed in
the developing cochlea and, after trauma, in the mature
cochlea, where it may have a protective role in response
to the trauma. SGNs express TrkB and TrkC so can
respond to both BDNF and NT-3. Both of these neurotrophins can promote SGN survival and stimulate neurite
growth. Nevertheless, there are significant differences in
SGN responses to BDNF and NT-3. For example, NT-3
may promote and properly organize synapse formation
in the organ of Corti in a way that BDNF is unable to
accomplish. BDNF induces physiological properties in
SGNs characteristic of SGNs in the basal cochlea,
whereas NT-3 induces physiological properties in SGNs
characteristic of those in the apical cochlea. These differences presumably derive from differences in association
of BDNF and NT-3 with TrkB and TrkC and differences
in the association of TrkB and TrkC with diverse intracellular signaling pathways. However, such differences
are largely unknown and may involve relatively subtle
quantitative aspects of signal transduction.
As described by Budenz et al. (in this issue), significant effort has been expended in determining the extent
to which NTFs, especially BDNF, NT-3, and GDNF, can
maintain SGN survival in vitro or in vivo in the absence
of normal afferent input. Because these factors are
indeed able to promote SGN survival in animal studies,
they have been proposed as therapeutic agents in
humans with sensorineural deafness due to loss of hair
cells for the purpose of maintaining SGN survival to
allow long-term efficacy of cochlear implants and for the
purpose of attracting SGN neurite growth toward electrodes to allow reduced stimulating current (Roehm and
Hansen, 2005; Shibata et al., 2011). Should it become
possible to regenerate lost hair cells, presumably there
will be proposals to use NTFs to maintain SGN survival
and promote synaptogenesis on regenerated hair cells.
While these are potentially beneficial uses of NTFs, they
should be approached with caution. As discussed earlier,
NTFs have many diverse effects on SGNs and non-neuronal cells of the cochlea and may use multiple signal
transduction mechanisms in rather subtle ways to
achieve these effects. Some of these effects may be desirable therapeutic outcomes and some not. Considerable
investigation remains to be done on the effects of NTFs
on SGNs before we can reliably predict the outcome of
NTF therapy on SGNs and choose an appropriate regimen of NTFs to optimize physiological and structural
characteristics of surviving SGNs.
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