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The neurotrophic hypothesis:
Targets of innervation secrete limiting amounts of
survival factors to generate a balance between the
size of the target organ and the number of innervating
Note: This hypothesis is mainly concerned with
peripheral neurons and peripheral target tissues.
Effect of Removing or
Augmenting Neural Targets on
the Survival of Related Neurons
• PN23091.JPG
1934: Victor Hamburger discovered
that removal of a limb bud resulted
in reduced numbers of sensory and
motor neurons in the spinal cord.
Effect of Removing or
Augmenting Neural Targets on
the Survival of Related Neurons
• PN23092.JPG
1939: Victor Hamburger showed that
transplantation of a supernumerary
limb resulted in increased numbers of
sensory and motor neurons in the
spinal cord.
Based on his limb-bud experiments, V. Hamburger
hypothesized that the targets of innervating neurons
provide signals that recruit undifferentiated cells to
develop into sensory or motor neurons.
(he was wrong)
In 1942, Levi-Montalcini and Levi proposed that target
derived signals maintain survival of differentiating neurons. In
1949, Hamburger and Levi-Montalcini repeated the limb bud
experiments and found that their results supported the
neurotrophic hypothesis.
Hamburger, V. and Levi-Montalcini, R. (1949) J. Exp. Zool. 111: 457-502.
1954: neurite outgrowth assay
1960: NGF purified
1969: NGF purified to
– extract
Stanley Cohen
+ extract
Rita Levi-Montalcini
1986: Levi-Montalcini and
Cohen win the Nobel prize
for Physiology or Medicine
“for their discovery of growth factors”
sympathetic neurons and some sensory
(CNS neurons do not require NGF for survival)
NGF-related factor purified in 1982 from pig
brain (shares ~50% homolog with NGF)
NT-3 and NT-4/5: were obtained by PCR cloning
All these factors are synthesized as ~250 aa precursors
that are processed into 120 aa proteins
Neurotrophin Evolution
Neurotrophins have only been isolated from chordates
Hallbook (1999) Curr Opin Neurobiol 9: 616-21
Structure of NGF bound to its receptor
The Trk Family of Receptor
Tyrosine Kinases for the
• PN23160.JPG
p75NTR: purified and cloned 1st, homology to TNFR
Trk: tropomyosin-related kinase, originally known as orphan receptors
(Found only in fish)
The effect of NT/NTR knockouts on neurons in the DRG
Fariñas et al. (2002) Brain Res Bull 57:809-816
Trk receptor signaling
When a neurotrophin binds to a trk receptor, the
kinase domain is activated resulting in
Autophoshorylation results in further
activation of the kinase domain, leading
to activation of three potential signaling
Our axons can be >1 m in length---how does the
neurotrophin/receptor complex signal to the neuronal
cell body?
Miller and Kaplan (2001) Neuron 32:767-770
Campenot, RB (1977) Local control of neurite development by nerve
growth factor. Proc Natl Acad Sci U S A. 74(10):4516-9.
(A method that can be used to study how NTs added to distal axons signal retrogradely)
Miller and Kaplan (2001) Neuron 32:767-770
In vitro assays have shown that NTs enhance
both axonal and dendritic growth
In vivo, the situation is more difficult to study
Why? In standard knockouts, it is difficult to separate the
survival effects of NTs from their effects on the
morphology of neurons.
This problem has begun to be addressed by using
conditional knockouts, or by crossing NT knockouts with
mouse mutants lacking pro-apoptotic genes.
Recent evidence from these kinds of experiments
suggests that long distance peripheral sensory axon
growth in vivo is NT-dependent.
NT’s roles in neuronal development and function
• NTs are expressed in regions of the
developing embryo that are traversed by
sensory axons en route to their targets.
• NTs affect the proliferation and
differentiation of CNS neuroepithelial
progenitors, neural crest cells, and
progenitors of enteric neurons in vitro (and
in some cases also confirmed in vivo).
• In the CNS, BDNF/TrkB signaling is
implicated in the development and
maintenance of cortical circuits.
What happens to neurons in the
absence of neurotrophic factors?
ap·o·pto·sis Pronunciation: "a-p&p-'tO-s&s,
-p&-'tOFunction: noun Inflected Form(s): plural ap·o·pto·ses /-"sEz
/Etymology: New Latin, from Greek apoptOsis a falling off
first use in Kerr et al. (1972) “Apoptosis: a basic biological
phenomenon with wide-ranging implications in tissue kinetics.” Br.
J. Cancer 26: 239-257
Apoptosis is a mechanism contributing to programmed cell death (PCD)
Stages during neuronal development where PCD occurs
Morphological types of cell death
Apoptosis: originally defined according to a set of characteristic
ultrastructural features that include nuclear and cytoplasmic
condensation, cell fragmentation and phagocytosis.
Necrosis: cell death as the result of injury, disease, or
pathological state (usually involves large numbers of cells and is
associated with inflammation). Chromatin condenses in multiple
small clumps and at later stages, cell membranes and organelles
Autophagy: (from the Greek, self-eating) Cytoplasm is destroyed
by lysosomal enzymes before any nuclear changes become visible.
A characteristic feature is the appearance of large autophagic
vacuoles in the cytoplasm. At later stages, chromatin condenses,
DNA laddering is evident and phagocytosis occurs.
Morphology of cell death
Ultrastructure of cell death
PCD also occurs in the glial lineage