Download Danon disease - Journal of Cell Science

© 2016. Published by The Company of Biologists Ltd | Journal of Cell Science (2016) 129, 2135-2143 doi:10.1242/jcs.184770
COMMENTARY
ARTICLE SERIES: CELL BIOLOGY AND DISEASE
Danon disease – dysregulation of autophagy in a multisystem
disorder with cardiomyopathy
Teisha J. Rowland, Mary E. Sweet, Luisa Mestroni and Matthew R. G. Taylor*
Danon disease is a rare, severe X-linked form of cardiomyopathy
caused by deficiency of lysosome-associated membrane protein 2
(LAMP-2). Other clinical manifestations include skeletal myopathy,
cognitive defects and visual problems. Although individuals with Danon
disease have been clinically described since the early 1980s, the
underlying molecular mechanisms involved in pathological
progression remain poorly understood. LAMP-2 is known to be
involved in autophagy, and a characteristic accumulation of
autophagic vacuoles in the affected tissues further supports the idea
that autophagy is disrupted in this disease. The LAMP2 gene is
alternatively spliced to form three splice isoforms, which are thought to
play different autophagy-related cellular roles. This Commentary
explores findings from genetic, histological, functional and tissue
expression studies that suggest that the specific loss of the LAMP-2B
isoform, which is likely to be involved in macroautophagy, plays a
crucial role in causing the Danon phenotype. We also compare findings
from mouse and cellular models, which have allowed for further
molecular characterization but have also shown phenotypic differences
that warrant attention. Overall, there is a need to better functionally
characterize the LAMP-2B isoform in order to rationally explore more
effective therapeutic options for individuals with Danon disease.
KEY WORDS: Danon disease, Autophagy, Lysosome-associated
membrane protein 2, LAMP-2, Cardiomyopathy, Medical genetics,
Macroautophagy
Introduction
Danon disease is a rare but severe X-linked form of cardiomyopathy
that usually leads to profound hypertrophic cardiomyopathy (HCM;
a thickening of the heart muscle) and causes death or a need for
cardiac transplantation in males by the third decade (Fig. 1A)
(D’souza et al., 2014; Taylor et al., 2007). When Danon disease was
first described in 1981, ultrastructural analysis of muscle biopsies
from two boys with the disease showed that their cells contained
abundant glycogen particles, primarily within lysosomal vacuoles
(Danon et al., 1981). Because of these observations, the disease was
originally postulated to be due to a glycogen storage defect.
However, although glycogen-containing autophagic vacuoles have
continued to be reported in affected individuals (Cheng and Fang,
2012), genetic, histological and ultrastructural analyses have
revealed that disruption of autophagy – a process by which cells
disassemble dysfunctional or unneeded molecular and cellular
components for reuse – is the probable underlying mechanism of
Danon disease.
Danon disease is caused by mutations in the lysosome-associated
membrane protein 2 (LAMP2) gene, usually leading to deficiency of
Cardiovascular Institute and Adult Medical Genetics Program, University of
Colorado Denver, Aurora, CO 80045, USA.
*Author for correspondence ([email protected])
the LAMP-2 protein (Eskelinen, 2005; Nishino et al., 2000).
LAMP-2 is an abundant constituent of the membrane of the
lysosome, which is a eukaryotic organelle containing digestive
enzymes that break down its contents (Konecki et al., 1994).
Unsurprisingly, based on its location, LAMP-2 is involved in
autophagy, with its three different splice isoforms (LAMP-2A,
LAMP-2B, and LAMP-2C) each playing different autophagyrelated roles. There are three main types of autophagy:
macroautophagy, chaperone-mediated autophagy (CMA) and
microautophagy (Mrschtik and Ryan, 2015). The LAMP-2A
isoform has been clearly shown to be involved in CMA
(Eskelinen et al., 2005; Schneider and Cuervo, 2014), whereas the
role of LAMP-2B has been characterized to a much lesser extent,
but it is likely to be involved in macroautophagy (Bandyopadhyay
et al., 2008; Tanaka et al., 2000; Nishino et al., 2000).
Macroautophagy is the most prevalent form of autophagy and is
thought to be the type of autophagy that is most affected in Danon
disease (Endo et al., 2015; Tanaka et al., 2000).
In this Commentary, following a brief summary of clinical
manifestations, the molecular and cellular aspects of Danon disease
will be presented; these findings suggest that loss of the LAMP-2B
isoform has a crucial role in causing Danon disease through
mediating the disruption of macroautophagy and potentially other
vesicle-trafficking processes. We will then discuss the
characterizations conducted using mouse models separately owing
to the phenotypic differences observed when these models are
compared to the human disease. Based on these findings, we argue
that a better understanding of the functions of the LAMP-2B
isoform is needed before more effective therapeutics for Danon
disease individuals can be developed.
Clinical manifestations of Danon disease
Males with Danon disease first show symptoms at 12 years of age,
on average, and the majority (88%) develop HCM. By the time they
are approximately 18 years old, the disease has led to cardiac
transplantation or death (Boucek et al., 2011). Severe HCM,
arrhythmias and a need for cardiac transplantation are the most
morbid features. Although it is an X-linked disease, heterozygous
females (i.e. carriers) are also affected but develop a more dilated
cardiac phenotype and a progression of disease that occurs
approximately 15 years later than in male counterparts (Prall
et al., 2006; Boucek et al., 2011). Postmortem and explanted
hearts display substantial fibrosis (D’souza et al., 2014) (Fig. 1B),
and many affected individuals (males and females) develop
arrhythmias that lead to sudden cardiac death. Specifically,
ventricular (Hedberg Oldfors et al., 2015; Miani et al., 2012;
Maron et al., 2009; Lobrinus et al., 2005) and atrial (Charron et al.,
2004) arrhythmias have both been reported and are often severe,
even in women (Miani et al., 2012). Additionally, individuals with
Danon disease can also develop (usually relatively mild) skeletal
myopathy (80–90% of males), primarily mild cognitive defects and
2135
Journal of Cell Science
ABSTRACT
COMMENTARY
Journal of Cell Science (2016) 129, 2135-2143 doi:10.1242/jcs.184770
learning disabilities (70–100% of males), and visual problems (69%
of males) (Thiadens et al., 2012; Schorderet et al., 2007; Prall et al.,
2006; D’souza et al., 2014). For a complete and recent review of
clinical descriptions of affected individuals, see D’souza et al.
(2014). For reviews discussing other diseases that share clinical
similarities with Danon disease, including Pompe disease,
Niemann-Pick type C and other lysosomal storage diseases, see
Ruivo et al. (2009), D’souza et al. (2014) and Pastores and Hughes
(2015). Although Danon disease shares clinical manifestations with
other diseases, the primary distinguishing feature of Danon disease
is a probable disruption of autophagy, as evidenced by the
molecular phenotype, which will be explored next.
Molecular phenotype of Danon disease
Since its original description in 1981, subsequent findings have
revealed that disrupted autophagy is the probable underlying
mechanism of Danon disease, caused by mutations in the LAMP2
gene that usually lead to a loss of expression of LAMP-2. Given the
roles of LAMP-2 in autophagy – which will be discussed in detail
later – disruption of autophagy is not surprising. Accordingly, in
cardiac tissue of individuals with Danon disease, dramatic increases
in cytoplasmic vacuolation are apparent (Endo et al., 2015; He et al.,
2014) (Fig. 1C). In these cells, expression of microtubule-associated
protein 1 light chain 3 proteins (encoded by MAP1LC3A,
MAP1LC3B, MAP1LC3C; collectively referred to here as LC3),
markers of autophagy that are discussed more later, as well as
ubiquitin, which comprises the ubiquitin–proteasome system, are
increased in myocardium autophagic vacuoles, suggesting that the
maturation and clearance of autophagosomes is stunted (Endo et al.,
2015). The number of lipid droplets in these cells is also increased.
In addition, the diseased cardiomyocytes are hypertrophic, and the
myocardium undergoes degeneration, as evidenced by lipofuscin
accumulation, which can be indicative of damage to cellular
organelles, including lysosomes and disarray of myofibrils (He
et al., 2014; Endo et al., 2015).
Recently, fibroblast biopsies taken from two males with Danon
disease were reprogrammed into induced pluripotent stem cells
(iPSCs) and differentiated into cardiomyocytes (iPSC-CMs); this
has allowed for further phenotypic and functional studies of the
disease on a controlled molecular level (Hashem et al., 2015). These
derived cardiomyocytes displayed molecular and cellular
phenotypes that were similar to those observed in Danon disease
tissues and included accumulation of intracytoplasmic vacuoles,
hypertrophy and impaired autophagic flux with a nearly complete
deficiency of mature autophagic vacuoles, as assessed using an LC3
reporter (discussed further in the Autophagy section) (Hashem
et al., 2015). Interestingly, both skin fibroblasts and iPSC-CMs from
the Danon disease individuals had significant accumulations of
2136
early autophagic vesicles (compared to wild-type cells), and
starvation (which induces autophagy) increased the number of
such vesicles in the iPSC-CMs, but not in the fibroblasts; this might
be related to the metabolically active nature of cardiac cells. Longer
Ca2+-decay times were also observed in the Danon iPSC-CMs,
which is consistent with a decline in systolic and diastolic function
(seen in heart failure), and in other iPSC-CMs used to model heart
failure phenotypes. Furthermore, the iPSC-CMs displayed
mitochondrial defects, including increased levels of mitochondrial
oxidative stress, and increased levels of apoptosis (Hashem et al.,
2015). These effects could potentially be related because
mitochondria normally produce reactive oxygen species (ROS) as
oxidative phosphorylation by-products; therefore, dysfunctional
mitochondria can produce excessive amounts of ROS and might be
more prone to activate apoptosis (Shires and Gustafsson, 2015).
Histological examinations of skeletal muscle biopsies from
Danon individuals, which have been more frequently reported
owing to the relative ease of obtaining these tissues, have resulted in
similar findings and are also indicative of autophagy disruption. For
instance, in skeletal muscle biopsies, an accumulation of vacuolar
structures are consistently found; these contain acetylcholine
esterase, proteins of the sarcolemma (the muscle fiber cell
membrane) and extracellular matrix proteins (typically those of
the basal lamina, such as dystrophin, laminin and spectrin) (Dougu
et al., 2009; Endo et al., 2015; Yang and Vatta, 2007), and are
known as autophagic vacuoles with unique sarcolemmal features
(AVSFs). AVSFs should be located near the peripheral sarcolemma
if they are sarcolemma-derived but, because they are instead found
scattered throughout the cytoplasm, it is thought that these AVSFs
are generated through a poorly understood de novo biogenesis
process (Endo et al., 2015). Disease-associated skeletal muscle
vacuoles have also been reported to accumulate glycogen (Bertini
et al., 2005; He et al., 2014; Yang and Vatta, 2007), degenerating
mitochondria (Yang and Vatta, 2007), lipids (Bertini et al., 2005)
and basophilic granules (implicating buildup of lysosomal
organelles in the myofibers) (He et al., 2014; Sugie et al., 2005;
Endo et al., 2015; Dougu et al., 2009), and to express increased
levels of LC3 and ubiquitin (Endo et al., 2015).
Pathological studies of brain tissue from individuals with Danon
disease have been limited, although an autopsy study on one
individual with intellectual disability has been reported (Furuta
et al., 2013). Here, histological and microscopic analysis of the brain
tissue revealed similar findings to those reported for skeletal and
cardiac muscle biopsies, namely the striking presence of lipofuscinlike granules containing glycogen, as well as lysosomal storage
abnormalities as revealed by electron microscopy analyses and,
moreover, a lack of normal autophagic vacuoles (Furuta et al., 2013;
Endo et al., 2015). However, unlike the observations made in
Journal of Cell Science
Fig. 1. Explanted cardiac muscle from a 14-year-old male with Danon disease. Images show (A) a cross-section of explanted heart with biventricular
hypertrophy and fibrosis, (B) a photomicrograph (with trichrome stain) of cardiomyocytes (red) and extensive fibrosis (blue) and (C) a photomicrograph (with
hematoxylin and eosin stain) revealing extensive vacuolization. Scale bars: 100 μm. Reprinted from Taylor et al. (2007) with permission. Copyright 2007, Nature
Publishing Group.
skeletal and cardiac muscle biopsies, the studied neurons did not
appear to express LC3 and exhibited only low levels of ubiquitin.
Clearly, further investigation is needed to better understand the
pathological phenotype in the brain.
Some clinical studies have described the ophthalmic
manifestations of individuals with Danon disease (Thiadens et al.,
2012; Prall et al., 2006; Lobrinus et al., 2005). Both male
individuals with Danon disease and female carriers have been
found to have retinopathy, including peripheral pigmentary
retinopathy (Prall et al., 2006) and peripheral ‘salt-and-pepper’
retinopathy (Schorderet et al., 2007; Thiadens et al., 2012), although
sometimes the condition is milder in females. In one study, both
enrolled males had a near-complete loss of pigmentation of the
retinal pigment epithelium (RPE) in the macula (Prall et al., 2006),
whereas a boy from another study had RPE hypopigmentation
(Schorderet et al., 2007). Another study of a family found that two
out of four male members with Danon disease developed cone-rod
dystrophy, with thinning of the RPE and overlying photoreceptor
layers (Thiadens et al., 2012). Photoreceptor cells (comprised of
rods and cones) absorb photons of light through components called
photoreceptor outer segments (POSs). Each day, at least 2000–4000
POSs are shed by photoreceptors and broken down by the
underlying RPE layer through phagocytosis (Thiadens et al.,
2012). Unlike autophagy, phagocytosis requires import of its
cargo, but similar to autophagy, its cargo must be degraded through
localization to, and fusion with, the lysosome (Nandrot, 2014).
Some key phagocytic factors have been identified, including
myosin VIIA – a mechanochemical protein thought to transport
nascent POS-based vesicles (i.e. phagosomes) into the cell (Gibbs
et al., 2003) – as well as proteins and proteases (e.g. βA3-crystallin,
A1-crystallin and cathepsin D) that are involved in lysosomal
degradation following fusion of the phagosome with the lysosome
(Nandrot, 2014; Valapala et al., 2016). However, it is likely that
other factors with an active role in retinal phagocytosis are yet to be
identified, particularly because these molecular mechanisms have
only been characterized using nocturnal rodent models, and
phagocytotic cycles are different in diurnal animals, such as
humans (Nandrot, 2014). Interestingly, disruption of retinal
phagocytosis leads to lipofuscin accumulation (Nandrot, 2014),
which is also seen in Danon individuals, as described above.
LAMP-2 is highly expressed in RPE lysosomes (Schorderet et al.,
2007), indicative of a role in the retinal phagocytotic process.
Indeed, mutation of the LAMP2 gene could ultimately lead to the
loss of RPE and photoreceptors, and thus to a loss of visual acuity
(Thiadens et al., 2012).
LAMP-2 structure and isoforms
Understanding the structure of LAMP-2 and its splice isoforms is
important for determining their varying roles in autophagy, and how
these might be disrupted in Danon disease. The human LAMP-2
protein contains nine exons, with exons 1 to 8 and part of exon 9
making up the lysosome lumenal domain. In this large lumenal
domain, four loops are predicted to form through disulfide bridges
and flank a proline-rich ‘hinge’ region (Eskelinen et al., 2005). The
hinge region acts as a linker for two similar glycosylated domains,
each of which contains two of the disulfide bridges (Wilke et al.,
2012). The lumenal domain of LAMP-2 is heavily glycosylated to
avoid degradation due to the low pH of the lysosome (Mrschtik and
Ryan, 2015). For additional details on the recently solved crystal
structure of the LAMP-2 protein – with a focus on the lumenal
domain – see Wilke et al. (2012). The remaining part of exon 9
comprises both a single transmembrane domain (24 amino acids
Journal of Cell Science (2016) 129, 2135-2143 doi:10.1242/jcs.184770
long) and a short cytoplasmic tail (11 amino acids long) at the Cterminal end (Eskelinen et al., 2005; Konecki et al., 1994, 1995).
The cytoplasmic tail is likely to function as a receptor for the uptake
of proteins and, occasionally, other molecules (such as RNA and
DNA) into the lysosome for their degradation (D’souza et al., 2014;
Nishino et al., 2000). It is worth noting that although LAMP-1,
which belongs to the same family as LAMP-2, shares 37% amino
acid sequence homology with LAMP-2, these proteins have been
found to be distinct and to have largely separate functions
(Eskelinen, 2006). Overall, the LAMP1-knockout mouse
phenotype is relatively mild (Eskelinen, 2006), and although there
might be shared functions between LAMP-1 and LAMP-2, LAMP1 levels have not been found to increase to compensate for blockage
of CMA, which is the best characterized function of LAMP-2
(Massey et al., 2006), nor does transient expression of LAMP-1
alleviate cholesterol accumulation (Schneede et al., 2011). For a
detailed comparison of LAMP-1 and LAMP-2 functions, see
Eskelinen (2006).
As mentioned previously, the LAMP2 gene can be alternatively
spliced to give rise to three splice isoforms, LAMP-2A, LAMP-2B
and LAMP-2C. For a thorough review of the isoform nomenclature
and standardization across different species, see Eskelinen et al.
(2005). Exon 9 of LAMP-2 pre-messenger RNA (mRNA) is
alternatively spliced to create the three splice isoforms, which are
identical in their lysosome lumenal domains but differ in the
transmembrane domain and cytoplasmic tail (Fig. 2C). LAMP-2A
plays a role in CMA, whereas the other isoforms do not appear to be
involved in CMA (Eskelinen et al., 2005). Studies suggest that
LAMP-2B could instead be involved in macroautophagy, primarily
based on the impaired fusion seen in individuals with mutations in
LAMP-2B only (Bandyopadhyay et al., 2008; Tanaka et al., 2000;
Nishino et al., 2000). Both CMA and macroautophagy will be
discussed in detail later. LAMP-2C is less characterized but is
thought to be primarily, and potentially solely, involved in the
degradation of RNA and DNA (RNautophagy and DNautophagy,
respectively) based on pull-down assays identifying interacting
proteins as being RNA- or DNA-binding proteins (Fujiwara et al.,
2013, 2014). Owing to these differences in function and tissue
expression, which will be explored more in the following sections,
LAMP-2A and LAMP-2B are thought to be more relevant to Danon
disease and, consequently, these isoforms will receive more
attention than LAMP-2C in this Commentary.
In their cytoplasmic tails, LAMP-2A and LAMP-2B share 27%
(3 out of 11) identical amino acids, and an additional 27% (3 out of
11) of their amino acid residues are similar, with 45% (5 out of 11)
of residues not showing any similarity. Such differences in the
cytoplasmic tail might enable the isoforms to perform separate
receptor-mediated functions (i.e. CMA and macroautophagy) and
make this region, possibly, of most relevance to Danon disease
(D’souza et al., 2014; Nishino et al., 2000).
Role of LAMP-2B in Danon disease
Although most LAMP2 mutations leading to Danon disease are
predicted to result in deficiency of all three of the LAMP-2
isoforms, isoform-specific Danon disease mutations have only so
far been found in the LAMP-2B isoform. This suggests that LAMP2B deficiency is potentially sufficient (owing to the reporting of
isoform-specific mutations) and necessary (all mutations affect
LAMP-2B) to cause Danon disease (D’souza et al., 2014; Boucek
et al., 2011; Nishino et al., 2000) (Table 1). For extensive lists of
reported LAMP-2 mutations in individuals with Danon disease, see
Cheng and Fang (2012), Dougu et al. (2009) and D’souza et al.,
2137
Journal of Cell Science
COMMENTARY
COMMENTARY
Journal of Cell Science (2016) 129, 2135-2143 doi:10.1242/jcs.184770
Key
Identical residues
Similar residues
Cysteine residues in
disulfide bridges
Fig. 2. Amino acid sequence alignments of the LAMP-2A and LAMP-2B isoforms in human and mouse. Alignments show (A) human and mouse LAMP-2
(exons 1–8), (B) human and mouse LAMP-2A and LAMP-2B isoforms (exon 9 only), which are the LAMP-2 isoforms most relevant to Danon disease, and
(C) human LAMP-2A and LAMP-2B isoforms (exon 9 only). Blue and yellow backgrounds indicate identical and similar amino acids, respectively. Gray
backgrounds indicate cysteine residues thought to be involved in disulfide bridges. Black, red and green font colors indicate lumenal, transmembrane and
cytoplasmic residues, respectively (Eskelinen et al., 2005). Amino acid sequences were obtained from Ensembl, and alignments were generated using the
Bioinformatics Organization Multiple Align Show tool.
(2014). Accordingly, deficiency of LAMP-2B alone has been found
to cause ‘the full Danon syndrome’, as stated by the authors,
including HCM, skeletal myopathy and the cognitive defects that
have been detected in one individual (Nishino et al., 2000). In one of
the individuals that we have studied, LAMP-2B deficiency was also
found to result in a typical case of Danon disease, requiring cardiac
transplantation (Boucek et al., 2011). However, in another
individual, LAMP-2B deficiency alone was found to only cause a
milder presentation Danon syndrome, with mild left ventricular
diastolic dysfunction, limb-girdle muscle weakness and sub-clinical
neuropathy (Hong et al., 2012). It is also worth noting that a LAMP2B missense mutation that results in apparently normal levels (but
irregular distribution) of LAMP-2B expression in skeletal muscle
biopsies from multiple members of one family has been associated
Table 1. Evidence for the involvement of the LAMP-2A and LAMP-2B isoforms in the development of Danon disease
Findings related to the
LAMP-2A isoform
Findings related to the LAMP-2B
isoform
References
Genotype
Usually deficient in all
three LAMP-2 isoforms
Deficiency of this isoform
alone has not been
reported in Danon
disease individuals
Deficiency of this isoform alone
has been associated with full
Danon syndrome or a milder
phenotype
Missense mutation associated
with mild cardiac phenotype and
severe skeletal myopathy
D’souza et al., 2014; Boucek et al.,
2011; Nishino et al., 2000; Hong
et al., 2012; van der Kooi et al.,
2008
Protein expression
levels in affected
tissues
Cardiac, skeletal muscle,
retinal tissue and
cognitive functions
most impaired
Low levels in skeletal
muscle and brain tissue
Similar levels to LAMP-2B
in the heart
High levels in unaffected
tissues: placenta, lung,
liver, kidney, pancreas,
prostate
High levels in skeletal muscle and
brain tissue
Similar levels to LAMP-2A in the
heart
Konecki et al., 1995; Furuta et al.,
1999
Autophagy
association
Accumulation of
autophagic vesicles
Plays a role in CMA
Not involved in CMA
Thought to be involved in
macroautophagy
Tanaka et al., 2000; Nishino et al.,
2000; Eskelinen et al., 2005;
Bandyopadhyay et al., 2008;
Endo et al., 2015
2138
Journal of Cell Science
Characteristics
General findings in
individuals with Danon
disease
COMMENTARY
Journal of Cell Science (2016) 129, 2135-2143 doi:10.1242/jcs.184770
with different Danon-disease-like symptoms, including a
potentially much milder cardiac phenotype (although there was
sudden cardiac death at the age of 28 of one female family member),
potentially more severe and progressive muscle weakness (which is
uncommon in Danon disease individuals), decreased visual acuity
and normal intellectual abilities (van der Kooi et al., 2008).
Interestingly, upon ultrastructural examination, accumulation of
autophagic vacuoles containing glycogen, lipid droplets and
lipofuscin were also observed in this family (van der Kooi et al.,
2008). It is possible that compared to loss of expression of all three
isoforms, LAMP-2B deficiency or dysfunction alone might
typically result in milder Danon-disease-like cardiac and cognitive
phenotypes but still give rise to similar or more severe skeletal
myopathy. This could be explained by the tissue-specific expression
levels of the normal LAMP-2 isoforms, their roles in these tissues,
and/or cross-talk and compensatory mechanisms occurring between
the different forms of autophagy (Kaushik et al., 2008).
Supporting the idea of a key role of LAMP-2B in the causality of
Danon disease is the tissue-specific expression of the different
LAMP-2 isoforms; LAMP-2B appears to be more highly expressed
in the most affected tissues compared to LAMP-2A and LAMP-2C
under normal conditions (Table 1). Human LAMP-2B has
specifically been found to be more abundantly expressed at the
mRNA level in skeletal muscle and brain tissue than the LAMP-2A
isoform, which is present at only low levels in these tissues
(Konecki et al., 1995; Furuta et al., 1999; Rothaug et al., 2015).
LAMP-2B is most highly expressed in skeletal muscle (Konecki
et al., 1995), which could be the reason why the most striking
disease symptom reported in members of a family with a LAMP-2B
missense mutation is severe and progressive skeletal muscle
weakness (van der Kooi et al., 2008). Human LAMP-2A, by
contrast, is more ubiquitously expressed; it is expressed in placenta,
lung, liver, kidney, pancreas and prostate tissues (Konecki et al.,
1995; Furuta et al., 1999). Both LAMP-2A and LAMP-2B are
thought to be expressed at similar levels in the human heart,
although this is based on limited studies (Konecki et al., 1995; Yang
and Vatta, 2007). Investigations in human tissues have not been
reported; however, in 2013, a tissue expression study of LAMP-2C
mRNA in mice found this isoform to be most highly expressed in
brain tissue and present at lower levels in other tissues, including
detectable levels in skeletal muscle and the heart (Fujiwara et al.,
Isolation
membrane
(phagophore)
2013), although a later study detected no LAMP-2C mRNA in the
mouse brain (Rothaug et al., 2015). In agreement with some of these
findings, we have conducted RNA sequencing (RNA-Seq) analysis
of normal human iPSC-CMs and normal explanted human hearts,
and found that expression levels of the LAMP-2A and LAMP-2B
isoforms are similar in these cells and tissues, with both isoforms
significantly more highly expressed than LAMP-2C (unpublished
data; T.J.R., M.E.S., L.M., M.R.G.T., Eric Adler, Sherin Hashem
and Carmen Sucharov). It is possible that the relative expression
levels of the isoforms are altered depending on certain metabolic
conditions, such as starvation (which induces autophagy); this could
affect the relevance of the different isoforms to the Danon disease
phenotype. Additional expression studies have been conducted in
mice, and these will be explored separately later owing to
phenotypic differences observed between individuals with Danon
disease and mouse models. The molecular phenotype of individuals
with Danon disease described above, wherein macroautophagy
specifically appears to be impacted, is also supportive of a key role
for LAMP-2B (Table 1).
Autophagy
Macroautophagy is thought to be the type of autophagy most
affected in Danon disease, owing to the accumulation of autophagic
vacuoles in muscle biopsies and the idea that the disease is largely
caused by loss of expression of the LAMP-2B isoform, which is
thought to be involved in macroautophagy (Table 1) (Endo et al.,
2015; Tanaka et al., 2000). In macroautophagy, cargo is engulfed by
double-membraned vesicles called autophagosomes, which fuse
with the lysosome to degrade their contents; this typically results in
free fatty acids and amino acids for the cell to reuse (Nishida et al.,
2015; Mrschtik and Ryan, 2015; Massey et al., 2008) (Fig. 3).
Macroautophagy can be selective or non-selective. CMA, by
contrast, is receptor mediated and generally a more selective process
where vesicles are not needed for shuttling cargo to the lysosome
(Schneider and Cuervo, 2014). The cytoplasmic tail of LAMP-2A is
thought to function as a receptor in CMA (Endo et al., 2015). In the
recently discovered degradation processes of RNautophagy and
DNautophagy, which are mediated by LAMP-2C, this isoform acts
as a receptor by interacting with RNA- or DNA-binding proteins,
respectively, to allow RNA or DNA to be directly taken up into
lysosomes (Fujiwara et al., 2013, 2014). Because nucleic acids have
Autophagosome
Lysosome
Chaperone–substrate
complex
Autolysosome
Endosome
Chaperone-mediated
autophagy
Macroautophagy
Key
Mitochondrion
Ribosome
Membrane
Peroxisome
Glycogen
Lipid granules
LAMP-2A
LAMP-2B
Hsc70
Protein
Fig. 3. Mammalian pathways of macroautophagy and chaperone-mediated autophagy (CMA). In macroautophagy, a variety of cargo can be engulfed by an
autophagosome, which fuses with a lysosome to form an autolysosome to then degrade its contents. Alternatively, an autophagosome can fuse with an
endosome to form an amphisome, which then fuses with the lysosome. The processes of autophagosome and amphisome fusion with the lysosome are thought
to require LAMP-2B, although any direct binding partners of LAMP-2B, which are important for these processes, are unknown. In CMA, target proteins are
recognized and bound by heat shock cognate protein 70 (Hsc70; also known as HSPA8), which forms a chaperone–substrate complex that is transported to the
lysosome, where interaction with LAMP-2A transport the target protein across the lysosome membrane.
2139
Journal of Cell Science
Amphisome
not been reported in the autophagic vacuoles that are characteristic
of Danon disease, these degradation processes will not be discussed;
please see Fujiwara et al. (2013, 2014) for details. Macroautophagy
and CMA are explored in more detail below with a focus on
macroautophagy because it appears to be the most affected in Danon
disease (Endo et al., 2015; Tanaka et al., 2000).
In macroautophagy, initially a small flat membrane sack of
cisterna, called an isolation membrane (or phagophore), forms in the
cytoplasm (Hamasaki et al., 2013; Nishida et al., 2015). Several
autophagy-related (Atg) proteins are involved in forming, expanding
and elongating the isolation membrane. (For in-depth discussions of
these Atg proteins, see Nemchenko et al., 2011; Mizushima et al.,
2011; Wesselborg and Stork, 2015; Nishida et al., 2015.) The
membrane becomes enclosed around its cargo to create a doublemembraned autophagosome (Nishida et al., 2015; Nemchenko et al.,
2011; Eskelinen et al., 2005). Material degraded through
macroautophagy includes defective mitochondria, ribosomes,
peroxisomes, endoplasmic reticulum and additional membrane
structures, glycogen, lipid granules and other cytosolic aggregates
(Kovsan et al., 2010; Kundu et al., 2008; Kuma and Mizushima,
2010; Schneider and Cuervo, 2014; Shires and Gustafsson, 2015;
Nishida et al., 2015). The targeted material might be identified by
cargo-recognizing proteins, which bind to the cargo and a membrane
component of the developing autophagosome such as LC3
(Schneider and Cuervo, 2014; Wild et al., 2014). For example,
PTEN-induced kinase 1 (PINK1), Parkin and p62 (also known as
SQSTM1) are involved in identifying mitochondria for degradation,
whereas p62 is additionally involved in recognizing lipid droplets
(Jin and Youle, 2012; Schneider and Cuervo, 2014). Mutations in
PINK1 and Parkin are associated with Parkinson’s disease, in which
cells exhibit defective mitochondrial turnover (Jin and Youle, 2012;
Schneider and Cuervo, 2014). LC3, a molecule frequently used as a
marker of autophagy (and the mammalian homolog of Atg8), has
been thought to be required for expansion of the isolation membrane
and autophagosome formation, although exceptions have been
found (Kishi-Itakura et al., 2014; Nishida et al., 2009). During
autophagy, LC3 is typically converted from the soluble form LC3-I
to the lipidated form LC3-II (Klionsky et al., 2007). However,
certain Atg-knockout cells (ATG5 and ATG3 knockouts) can
accumulate expanded isolation membranes without converting
LC3-I to LC3-II (Kishi-Itakura et al., 2014), and it has also been
found that in some alternative macroautophagy processes the LC3-I
to LC3-II conversion does not necessarily occur (Nishida et al.,
2009). Additionally, findings arising from measurement of the
conversion of LC3-I to LC3-II to determine autophagy activity and
rates have been controversial (Klionsky et al., 2007), altogether
making LC3 conversion a questionable marker of autophagy.
Following this, the outer membrane of an autophagosome fuses
with a lysosome membrane in a LAMP-2-dependent manner to form
an autolysosome (Endo et al., 2015); therein, the cargo of the inner
membrane is degraded by the digestive enzymes of the lysosome,
generating amino acids and lipids that are transported to the cytosol
to be reused (Nishida et al., 2015). Alternatively, an autophagosome
might fuse with an endosome (a vesicle formed from the plasma
membrane of the cell) in a LAMP-2-dependent manner to form an
amphisome, which then fuses with a lysosome. Because the LAMP2B isoform is thought to be involved in macroautophagy, these
lysosome fusion processes – which ensure proper autophagosome
function and maturation – are likely to be perturbed upon loss of
function of LAMP-2B. It is worth noting, however, that the presence
of excessive autophagic vacuoles in individuals with Danon disease
does not necessarily indicate that LAMP-2B deficiency is truly
2140
Journal of Cell Science (2016) 129, 2135-2143 doi:10.1242/jcs.184770
causal. Although LAMP-2 is primarily localized to the lysosome
membrane, it is additionally found in the membranes of early
endosomes, late endosomes and amphisomes, as well as in plasma
membranes (Endo et al., 2015). The presence of LAMP-2 on the
inner surface of lysosome membranes might also help to maintain an
acidic environment in the lysosome and prevent lysosomal digestive
enzymes from auto-digesting cellular cytoplasmic components
(Endo et al., 2015; Saftig et al., 2001). Interestingly, through its
cytoplasmic tail, LAMP-2B (but not LAMP-2A) has been found to
interact with the transporter associated with antigen processing like
(TAPL; also known as ABCB9) protein, which transports peptides
in the cytosol to the lysosomal lumen (Demirel et al., 2012).
Although this interaction does not appear to affect the subcellular
localization or transportation of the peptides, LAMP-2B might
instead act to increase the half-life of TAPL (Demirel et al., 2012).
Other major players in the fusion of autophagosomes to lysosomes
are the soluble NSF attachment protein receptor (SNARE) proteins;
these are not known to interact with LAMP-2. In particular, the
autophagosomal SNARE protein syntaxin 17 (STX17) has a key
role in this fusion process and localizes to the outer membrane of
autophagosomes (but is not found in isolation membranes);
for details on these and other fusion factors whose discussion is
beyond the scope of this Commentary, see Nishida et al. (2015),
Itakura et al. (2012), Schneider and Cuervo (2014), Shen and
Mizushima (2014). It is possible that the ubiquitin–proteasome
system and microautophagy are upregulated to compensate for
macroautophagy and/or CMA blockage, which could be a potential
therapeutic option to explore for Danon disease individuals (Massey
et al., 2008).
Like macroautophagy, CMA can be activated by cellular stress,
such as oxidative stress, starvation and UV exposure, but CMA is
much more selective than macroautophagy because it only targets
specific proteins (Massey et al., 2008; Mrschtik and Ryan, 2015;
Schneider and Cuervo, 2014). CMA uses the cytosolic chaperone
heat shock cognate protein 70 (Hsc70; also known as HSPA8),
which recognizes proteins containing a specific amino acid
targeting motif (KFERQ-like). Hsc70 binds to these proteins, and
the chaperone–substrate complex is transported to the lysosome,
where it interacts with LAMP-2A on the lysosomal membrane to
transport the target protein across the membrane and into the
lysosomal lumen for degradation. For more details on CMA that are
beyond the scope of this Commentary, see Kaushik and Cuervo
(2012), Schneider and Cuervo (2014).
Mouse models of Danon disease – similarities and
differences
A LAMP2 knockout mouse model of Danon disease has been used
in multiple studies with the aim to better characterize the disease;
however, although these mice display many features similar to those
seen in humans with Danon disease, clear differences have also
been reported. Based on amino acid sequence homology, the short
cytoplasmic tail, which might be involved in receptor-mediated
functions (i.e. CMA or macroautophagy), is highly conserved
between humans and mice for the LAMP-2A and LAMP-2B
isoforms [with 82% (9 out of 11) residues being identical for the
LAMP-2A isoforms, and 91% (10 out of 11) residues being
identical for the LAMP-2B isoforms] (Fig. 2B). However, the
lumenal portion of LAMP-2 (exons 1–8) is less conserved between
species [with 61% (226 out of 368) identical and 75% (276 out of
368) similar or identical residues, and 2% (10 out of 368) gaps]
(Fig. 2A). Like humans, mice also have a LAMP-2C isoform
(Eskelinen et al., 2005).
Journal of Cell Science
COMMENTARY
COMMENTARY
(Eskelinen, 2006), which reflect the dysfunctional mitochondrial
behavior observed in human iPSC-CMs derived from individuals
with Danon disease (Hashem et al., 2015). In the accumulated
autophagosomes in pathogenic mouse hepatocytes, fusion between
the autophagosomes and lysosomes appears to be impaired, whereas
fusion between endosomes and autophagosomes does not appear to
be altered (Eskelinen, 2005; Tanaka et al., 2000). Of note, Lamp2deficient mice accumulate cholesterol in their liver (Schneede et al.,
2011) and brain (Rothaug et al., 2015). This cholesterol defect has
been further characterized in Lamp2-deficient mouse embryonic
fibroblasts (MEFs), which accumulate unesterified cholesterol in
late endosomes and/or lysosomes (Eskelinen et al., 2004; Eskelinen,
2006). This accumulation could be rescued by the lumenal domain
and membrane-proximal part of LAMP-2 (in a mechanism
independent of CMA), suggesting that these regions of LAMP-2
might have a crucial role in the transport of unesterified cholesterol.
Specifically, unesterified cholesterol is generated by endogenous
synthesis in the cytosol or through low-density lipoprotein (LDL)
uptake, and becomes esterified in the endoplasmic reticulum.
LAMP-2 might be important for cholesterol transportation from the
cytosol to the endoplasmic reticulum (Schneede et al., 2011).
Concluding remarks
Although the progression of Danon disease in individuals has
become better characterized clinically, our understanding of the
underlying pathological molecular mechanisms remains limited. In
particular, improving our knowledge of the role of LAMP-2 in
macroautophagy, and potentially in other intracellular vesicle
transport mechanisms that the LAMP-2B isoform might be
involved in, might help us to better understand the disease and,
consequently, to develop more targeted and effective treatment plans.
Lamp2-deficient mice have been valuable tools in investigating this
disease, although as discussed here, caution should be taken when
drawing parallels between these models and the human pathology
because there are clear differences, such as a potentially more severe
cardiac phenotype in humans. The recent development of humanderived iPSC-CMs offers a promising way to investigate the
molecular mechanisms under controlled conditions, although this
cell-based system is clearly limited in recapitulating whole-animal
physiological conditions. Moving forward, researchers are likely to
need to utilize both animal and cellular models in order to best
understand the molecular aspects of Danon disease and rationally
develop effective treatments based on the knowledge gained.
Acknowledgements
The authors would like to thank Andrew J. Bonham (Metropolitan State University of
Denver, Denver, CO), for critical reading of the manuscript. The authors would like to
also thank Carmen C. Sucharov (University of Colorado Anschutz Medical Campus,
Aurora, CO) and Eric D. Adler and Sherin I. Hashem (University of California San
Diego, San Diego, CA) for their unpublished data contributions.
Competing interests
The authors declare no competing or financial interests.
Funding
Supported by National Institutes of Health [grant number 1R01HL109209-01A1]; and a
Trans-Atlantic Network of Excellence grant from Fondation Leducq (Leducq
Foundation) [grant number 14-CVD 03]. Deposited in PMC for release after 12 months.
References
Bandyopadhyay, U., Kaushik, S., Varticovski, L. and Cuervo, A. M. (2008). The
chaperone-mediated autophagy receptor organizes in dynamic protein
complexes at the lysosomal membrane. Mol. Cell. Biol. 28, 5747-5763.
Beersten, W., Willenborg, M., Everts, V., Zirogianni, A., Podschun, R.,
Schroder, B., Eskelinen, E.-L. and Saftig, P. (2007). Impaired phagosomal
2141
Journal of Cell Science
Phenotypically, both humans with Danon disease and Lamp2knockout mice have increased mortality rates, with 50% of mice
dying between the age of 20 and 40 days (Saftig et al., 2001; Tanaka
et al., 2000); all mice show an accumulation of autophagic vacuoles
in cardiac and skeletal muscle cells (Tanaka et al., 2000; Stypmann
et al., 2006; Saftig et al., 2001; Yang and Vatta, 2007; Eskelinen,
2006). Also, similar to the human pathology, Lamp2-knockout mice
develop severely reduced cardiac contractility, with a reduced
ejection fraction and cardiac output, as well as increased ratios of
heart weight to total body weight, important features for
recapitulating the hypertrophic cardiac phenotype characteristic of
Danon disease (Tanaka et al., 2000; Stypmann et al., 2006;
Eskelinen, 2006). However, the cardiac phenotype in humans seems
more severe overall; the vacuolation in human cardiomyocytes
appears to be more pronounced than it is in Lamp2-knockout mice.
This is relevant because cardiomyopathy is usually the major
defining characteristic of the human disease (Saftig et al., 2001). By
contrast, skeletal muscle physiologically appears to be affected to
similar extents in the mouse model and human disease; myofibers in
both systems are degenerative, with proximal muscles being more
severely impacted than distal ones (Saftig et al., 2001). Similar to
individuals with Danon disease who develop vision problems and
mild cognitive defects, some neuropathological studies of Lamp2knockout mice have also suggested that the central nervous system
is impacted, specifically manifesting as impaired learning (likely to
be caused by hippocampal dysfunction due to disrupted lysosomal
activity) and motor deficits (Rothaug et al., 2015). However, earlier
mouse studies have not reported abnormalities of the central
nervous system (Saftig et al., 2001; Endo et al., 2015).
Beyond the cardiac and skeletal muscle phenotypes, there are
potential differences in other organs affected. Lamp2-knockout
mice have been reported to have excessive accumulations of
autophagic vacuoles in organs that are not typically affected in
individuals with Danon disease, specifically the liver, pancreas,
spleen, thymus and kidneys (Saftig et al., 2001; Tanaka et al., 2000;
Yang and Vatta, 2007). Lamp2-knockout mice also display
periodontitis due to impaired neutrophils being unable to
effectively eliminate bacterial pathogens, which is likely to be
caused by reduced fusion of late endosomes and/or lysosomes with
phagosomes (Beersten et al., 2007). Although in some Danon
disease individuals, increases in the activity of liver enzymes and
autophagosome accumulations in hepatocytes have been reported,
the autophagic function of hepatocytes is much more severely
impacted in mice (Saftig et al., 2001; Yang and Vatta, 2007; Tanaka
et al., 2000). Overall, the mouse model might represent a more
severe pathology than that seen in individuals with Danon disease in
terms of the number of different organs affected, although the
cardiac phenotype in humans appears to be more severe, which is
noteworthy because this is the primary cause of mortality in people
(Saftig et al., 2001; Endo et al., 2015). However, it has been
proposed that defects in other tissues of individuals with Danon
disease might have been overlooked as the focus was on examining
the severe cardiac dysfunction (Endo et al., 2015).
In terms of molecular and cellular characterization, affected
tissues of the Lamp2-knockout mice share many similarities with
those of individuals with Danon disease, and examinations of cell
lines derived from mice have afforded insights into the molecular
pathogenic mechanisms of Danon disease. As seen in humans,
glycogen accumulates within the affected mouse model cells, both
inside and outside of autophagic vacuoles (Tanaka et al., 2000).
Affected mouse cardiac cells have been reported to frequently
contain a single mitochondrion in their autophagic vacuoles
Journal of Cell Science (2016) 129, 2135-2143 doi:10.1242/jcs.184770
maturation in neutrophils leads to periodontitis in lysosomal-associated
membrane protein-2 knockout mice. J. Immunol. 180, 475-482.
Bertini, E., Donati, M. A., Broda, P., Cassandrini, D., Petrini, S., Dionisi-Vici, C.,
Ballerini, L., D’Amico, A., Pasquini, E., Minetti, C. et al. (2005). Phenotypic
heterogeneity in two unrelated Danon patients associated with the same LAMP-2
gene mutation. Neuropediatrics 36, 309-313.
Boucek, D., Jirikowic, J. and Taylor, M. (2011). Natural history of Danon disease.
Gen. Med. Off. J. Am. Coll. Med. Gen. 13, 563-568.
Charron, P., Villard, E., Sebillon, P., Laforet, P., Maisonobe, T., Duboscq-Bidot,
L., Romero, N., Drouin-Garraud, V., Frebourg, T., Richard, P. et al. (2004).
Danon’s disease as a cause of hypertrophic cardiomyopathy: a systematic survey.
Heart 90, 842-846.
Cheng, Z. and Fang, Q. (2012). Danon disease: focusing on heart. J. Hum. Genet.
57, 407-410.
Danon, M. J., Oh, S. J., DiMauro, S., Manaligod, J. R., Eastwood, A., Naidu, S.
and Schliselfeld, L. H. (1981). Lysosomal glycogen storage disease with normal
acid maltase. Neurology 31, 51-51.
Demirel, O., Jan, I., Wolters, D., Blanz, J., Saftig, P., Tempe, R. and Abele, R.
(2012). The lysosomal polypeptide transporter TAPL is stabilized by interaction
with LAMP-1 and LAMP-2. J. Cell Sci. 125, 4230-4240.
Dougu, N., Joho, S., Shan, L., Shida, T., Matsuki, A., Uese, K., Hirono, K., Ichida,
F., Tanaka, K., Nishino, I. et al. (2009). Novel LAMP-2 mutation in a family with
Danon disease presenting with hypertrophic cardiomyopathy. Circ. J. Off. J. Jpn.
Circ. Soc. 73, 376-380.
D’souza, R. S., Levandowski, C., Slavov, D., Graw, S. L., Allen, L. A., Adler, E.,
Mestroni, L. and Taylor, M. R. G. (2014). Danon disease: clinical features,
evaluation, and management. Circ. Heart Fail. 7, 843-849.
Endo, Y., Furuta, A. and Nishino, I. (2015). Danon disease: a phenotypic
expression of LAMP-2 deficiency. Acta Neuropathol. 129, 391-398.
Eskelinen, E.-L. (2005). Maturation of autophagic vacuoles in mammalian cells.
Autophagy 1, 1-10.
Eskelinen, E.-L. (2006). Roles of LAMP-1 and LAMP-2 in lysosome biogenesis and
autophagy. Mol. Asp. Med. 27, 495-502.
Eskelinen, E.-L., Schmidt, C. K., Neu, S., Willenborg, M., Fuertes, G., Salvador,
N., Tanaka, Y., Lullmann-Rauch, R., Hartmann, D., Heeren, J. et al. (2004).
Disturbed cholesterol traffic but normal proteolytic function in LAMP-1/LAMP-2
double-deficient fibroblasts. Mol. Biol. Cell 15, 3132-3145.
Eskelinen, E.-L., Cuervo, A. M., Taylor, M. R. G., Nishino, I., Blum, J. S., Dice,
J. F., Sandoval, I. V., Lippincott-Schwartz, J., August, J. T. and Saftig, P.
(2005). Unifying nomenclature for the isoforms of the lysosomal membrane
protein LAMP-2. Traffic 6, 1058-1061.
Fujiwara, Y., Furuta, A., Kikuchi, H., Aizawa, S., Hatanaka, Y., Konya, C.,
Uchida, K., Yoshimura, A., Tamai, Y., Wada, K. et al. (2013). Discovery of a
novel type of autophagy targeting RNA. Autophagy 9, 403-409.
Fujiwara, Y., Kikuchi, H., Aizawa, S., Furuta, A., Hatanaka, Y., Konya, C.,
Uchida, K., Wada, K. and Kabuta, T. (2014). Direct uptake and degradation of
DNA by lysosomes. Autophagy 9, 1167-1171.
Furuta, K., Yang, X.-L., Chen, J.-S., Hamilton, S. R. and August, J. T. (1999).
Differential expression of the lysosome-associated membrane proteins in normal
human tissues. Arch. Biochem. Biophys. 365, 75-82.
Furuta, A., Wakabayashi, K., Haratake, J., Kikuchi, H., Kabuta, T., Mori, F.,
Tokonami, F., Katsumi, Y., Tanioka, F., Uchiyama, Y. et al. (2013). Lysosomal
storage and advanced senescence in the brain of LAMP-2-deficient Danon
disease. Acta Neuropathol. 125, 459-461.
Gibbs, D., Kitamoto, J. and Williams, D. S. (2003). Abnormal phagocytosis by
retinal pigmented epithelium that lacks myosin VIIa, the Usher syndrome 1B
protein. Proc. Natl. Acad. Sci. USA 100, 6481-6486.
Hamasaki, M., Furuta, N., Matsuda, A., Nezu, A., Yamamoto, A., Fujita, N.,
Oomori, H., Noda, T., Haraguchi, T., Hiraoka, Y. et al. (2013). Autophagosomes
form at ER-mitochondria contact sites. Nature 495, 389-393.
Hashem, S. I., Perry, C. N., Bauer, M., Han, S., Clegg, S. D., Ouyang, K., Deacon,
D. C., Spinharney, M., Panopoulos, A. D., Izpisua Belmonte, J. C. et al. (2015).
Brief report: oxidative stress mediates cardiomyocyte apoptosis in a human model
of Danon disease and heart failure. Stem Cells 33, 2343-2350.
He, J., Wang, Y. and Jiang, T. (2014). Danon disease. Herz 39, 877-879.
Hedberg Oldfors, C., Má thé , G., Thomson, K., Tulinius, M., Karason, K.,
Ö stman-Smith, I. and Oldfors, A. (2015). Early onset cardiomyopathy in females
with Danon disease. Neuromusc. Disord. 25, 493-501.
Hong, D., Shi, Z., Zhang, W., Wang, Z. and Yuan, Y. (2012). Danon disease
caused by two novel mutations of the LAMP2 gene: implications for two ends of
the clinical spectrum. Clin. Neuropathol. 31, 224-231.
Itakura, E., Kishi-Itakura, C. and Mizushima, N. (2012). The hairpin-type tailanchored SNARE syntaxin 17 targets to autophagosomes for fusion with
endosomes/lysosomes. Cell 151, 1256-1269.
Jin, S. M. and Youle, R. J. (2012). PINK1- and Parkin-mediated mitophagy at a
glance. J. Cell Sci. 125, 795-799.
Kaushik, S. and Cuervo, A. M. (2012). Chaperone-mediated autophagy: a unique
way to enter the lysosome world. Trends Cell Biol. 22, 407-417.
2142
Journal of Cell Science (2016) 129, 2135-2143 doi:10.1242/jcs.184770
Kaushik, S., Massey, A. C., Mizushima, N. and Cuervo, A. M. (2008). Constitutive
activation of chaperone-mediated autophagy in cells with impaired
macroautophagy. Mol. Biol. Cell 19, 2179-2192.
Kishi-Itakura, C., Koyama-Honda, I., Itakura, E. and Mizushima, N. (2014).
Ultrastructural analysis of autophagosome organization using mammalian
autophagy-deficient cells. J. Cell. Sci. 127, 4089-4102.
Klionsky, D. J., Cuvero, A. M. and Seglen, P. O. (2007). Methods for monitoring
autophagy from yeast to humans. Autophagy 3, 181-206.
Konecki, D. S., Foetisch, K., Schlotter, M. and Lichter-Konecki, U. (1994).
Complete cDNA sequence of human lysosome-associated membrane protein-2.
Biochem. Biophys. Res. Comm. 205, 1-5.
Konecki, D. S., Foetisch, K., Zimmer, K. P., Schlotter, M. and Lichter-Konecki,
U. (1995). An alternatively spliced form of the human lysosome-associated
membrane protein-2 gene is expressed in a tissue-specific manner. Biochem.
Biophys. Res. Comm. 215, 757-767.
Kovsan, J., Bashan, N., Greenberg, A. S. and Rudich, A. (2010). Potential role of
autophagy in modulation of lipid metabolism. Am. J. Physiol. Endocrinol. Metab.
298, E1-E7.
Kuma, A. and Mizushima, N. (2010). Physiological role of autophagy as an
intracellular recycling system: with an emphasis on nutrient metabolism. Sem. Cell
Dev. Biol. 21, 683-690.
Kundu, M., Lindsten, T., Yang, C.-Y., Wu, J., Zhao, F., Zhang, J., Selak, M. A.,
Ney, P. A. and Thompson, C. B. (2008). Ulk1 plays a critical role in the
autophagic clearance of mitochondria and ribosomes during reticulocyte
maturation. Blood 112, 1493-1502.
Lobrinus, J. A., Schorderet, D. F., Payot, M., Jeanrenaud, X., Bottani, A.,
Superti-Furga, A., Schlaepfer, J., Fromer, M. and Jeannet, P.-Y. (2005).
Morphological, clinical and genetic aspects in a family with a novel LAMP-2 gene
mutation (Danon disease). Neuromuscul. Disord. 15, 293-298.
Maron, B. J., Roberts, W. C., Arad, M., Haas, T. S., Spirito, P., Wright, G. B.,
Almquist, A. K., Baffa, J. M., Saul, J. P., Ho, C. Y. et al. (2009). Clinical outcome
and phenotypic expression in LAMP2 Cardiomyopathy. JAMA 301, 1253-1259.
Massey, A. C., Kaushik, S., Sovak, G., Kiffin, R. and Cuervo, A. M. (2006).
Consequences of the selective blockage of chaperone-mediated autophagy.
Proc. Natl. Acad. Sci. USA 103, 5805-5810.
Massey, A. C., Follenzi, A., Kiffin, R., Zhang, C. and Cuervo, A. M. (2008). Early
cellular changes after blockage of chaperone-mediated autophagy. Autophagy
4, 442-456.
Miani, D., Taylor, M., Mestroni, L., D’Aurizio, F., Finato, N., Fanin, M., Brigido, S.
and Proclemer, A. (2012). Sudden death associated with danon disease in
women. Am. J. Cardiol. 109, 406-411.
Mizushima, N., Yoshimori, T. and Ohsumi, Y. (2011). The role of ATG proteins in
autophagosome formation. Annu. Rev. Cell Dev. Biol. 27, 107-132.
Mrschtik, M. and Ryan, K. M. (2015). Lysosomal proteins in cell death and
autophagy. FEBS J. 282, 1858-1870.
Nandrot, E. F. (2014). Animal models, in “The Quest to Decipher RPE
Phagocytosis”. In Retinal Degenerative Diseases (ed. J. D. Ash, C. Grimm,
J. G. Hollyfield, R. E. Anderson, M. M. LaVail and C. B. Rickman), pp. 77-83.
New York: Springer New York.
Nemchenko, A., Chiong, M., Turer, A., Lavandero, S. and Hill, J. A. (2011).
Autophagy as a therapeutic target in cardiovascular disease. J. Mol. Cell. Cardiol.
51, 584-593.
Nishida, Y., Arakawa, S., Fujitani, K., Yamaguchi, H., Mizuta, T., Kanaseki, T.,
Komatsu, M., Otsu, K., Tsujimoto, Y. and Shimizu, S. (2009). Discovery of
Atg5/Atg7-independent alternative macroautophagy. Nature 461, 654-658.
Nishida, K., Kyoi, S., Yamaguchi, O., Sadoshima, J. and Otsu, K. (2015). The
role of autophagy in the heart. Cell Death Differ. 16, 31-38.
Nishino, I., Fu, J., Tanji, K., Yamada, T., Shimojo, S., Koori, T., Mora, M., Riggs,
J. E., Oh, S. J., Koga, Y. et al. (2000). Primary LAMP-2 deficiency causes Xlinked vacuolar cardiomyopathy and myopathy (Danon disease). Nature 406,
906-910.
Pastores, G. M. and Hughes, D. A. (2015). Non-neuropathic lysosomal storage
disorders: Disease spectrum and treatments. Best Pract. Res. Cl. En. 29,
173-182.
Prall, F. R., Drack, A., Taylor, M., Ku, L., Olson, J. L., Gregory, D., Mestroni, L.
and Mandava, N. (2006). Ophthalmic manifestations of Danon disease.
Ophthalmology 113, 1010-1013.
Rothaug, M., Stroobants, S., Schweizer, M., Peters, J., Zunke, F., Allerding, M.,
D’Hooge, R., Saftig, P. and Blanz, J. (2015). LAMP-2 deficiency leads to
hippocampal dysfunction but normal clearance of neuronal substrates of
chaperone-mediated autophagy in a mouse model for Danon disease. Acta
Neuropathol. Commun. 3, 6.
Ruivo, R., Anne, C., Sagne, C. and Gasnier, B. (2009). Molecular and cellular
basis of lysosomal transmembrane protein dysfunction. BBA-Mol. Cell Res. 1793,
636-649.
Saftig, P., von Figura, K., Tanaka, Y. and Lü llmann-Rauch, R. (2001). Disease
model: LAMP-2 enlightens Danon disease. Trends Mol. Med. 7, 37-39.
Schneede, A., Schmidt, C. K., Hö lttä -Vuori, M., Heeren, J., Willenborg, M.,
Blanz, J., Domanskyy, M., Breiden, B., Brodesser, S., Landgrebe, J. et al.
Journal of Cell Science
COMMENTARY
(2011). Role for LAMP-2 in endosomal cholesterol transport. J. Cell. Mol. Med. 15,
280-295.
Schneider, J. L. and Cuervo, A. M. (2014). Autophagy and human disease:
emerging themes. Curr. Opin. Genet. Dev. 26, 16-23.
Schorderet, D. F., Cottet, S., Lobrinus, J. A., Borruat, F.-X., Balmer, A. and
Munier, F. L. (2007). Retinopathy in Danon disease. JAMA Ophthal. 125,
231-236.
Shen, H.-M. and Mizushima, N. (2014). At the end of the autophagic road: an
emerging understanding of lysosomal functions in autophagy. Trends Biochem.
Sci. 39, 61-71.
Shires, S. E. and Gustafsson, A. B. (2015). Mitophagy and heart failure. J. Mol.
Med. 93, 253-262.
Stypmann, J., Janssen, P. M. L., Prestle, J., Engelen, M. A., Kö gler, H., Lü llmannRauch, R., Eckardt, L., von Figura, K., Langrebe, J., Mleczko, A. et al. (2006).
LAMP-2 deficient mice show depressed cardiac contractile function without
significant changes in calcium handling. Basic Res. Cardiol. 101, 281-291.
Sugie, K., Noguchi, S., Kozuka, Y., Arikawa-Hirasawa, E., Tanaka, M., Yan, C.,
Saftig, P., von Figura, K., Hirano, M., Ueno, S. et al. (2005). Autophagic
vacuoles with sarcolemmal features delineate Danon disease and related
myopathies. J. Neuropathol. Exp. Neurol. 64, 513-522.
Tanaka, Y., Guhde, G., Suter, A., Eskelinen, E.-L., Hartmann, D., LullmannRauch, R., Janssen, P. M. L., Blanz, J., von Figura, K. and Saftig, P. (2000).
Accumulation of autophagic vacuoles and cardiomyopathy in LAMP-2-deficient
mice. Nature 406, 902-906.
Journal of Cell Science (2016) 129, 2135-2143 doi:10.1242/jcs.184770
Taylor, M. R. G., Ku, L., Slavov, D., Cavanaugh, J., Boucek, M., Zhu, X., Graw, S.,
Carniel, E., Barnes, C., Quan, D. et al. (2007). Danon disease presenting with
dilated cardiomyopathy and a complex phenotype. J. Hum. Gen. 52, 830-835.
Thiadens, A. A. H. J., Slingerland, N. W. R., Florijn, R. J., Visser, G. H.,
Riemslag, F. C. and Klaver, C. C. W. (2012). Cone-rod dystrophy can be a
manifestation of Danon disease. Graefes Arch. Clin. Exp. Ophthalmol. 250,
769-774.
Valapala, M., Sergeev, U., Wawrousek, E., Hose, E., Zigler, J. S. and Sinha, D.
(2016). Modulation of V-ATPase by βA3/A1-Crystallin in retinal pigment epithelial
cells. Adv. Exp. Med. Biol. 854, 779-784.
Van der Kooi, A. J., van Langen, I. M., Aronica, E., van Doorn, P. A., Wokke,
J. H. J., Brusse, E., Langerhorst, C. T., Bergin, P., Dekker, L. R. C., Lekanne
dit Deprez, R. H. et al. (2008). Extension of the clinical spectrum of Danon
disease. Neurology 70, 1358-1359.
Wesselborg, S. and Stork, B. (2015). Autophagy signal transduction by ATG
proteins: from hierarchies to networks. Cell. Mol. Life Sci. 72, 4721-4757.
Wild, P., McEwan, D. G. and Dikic, I. (2014). The LC3 interactome at a glance.
J. Cell Sci. 127, 3-9.
Wilke, S., Krausze, J. and Bü ssow, K. (2012). Crystal structure of the conserved
domain of the DC lysosomal associated membrane protein: implications for the
lysosomal glycocalyx. BMC Biol. 10, 62.
Yang, Z. and Vatta, M. (2007). Danon disease as a cause of autophagic vacuolar
myopathy. Congenit. Heart Dis. 2, 404-409.
Journal of Cell Science
COMMENTARY
2143