Otoferlin: a multi C2 domain protein essential for hearing Otof_080812... · Molecular Biology of...
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Otoferlin: a multi‐C2 domain protein essential for hearing
T. Pangršič1,*, E. Reisinger2,* & T. Moser1,3,4,#
1InnerEarLab, Department of Otolaryngology and Collaborative Research Center 889,
University Medical Center Göttingen, Göttingen, Germany
2Molecular Biology of Cochlear Neurotransmission group, Dept. of Otolaryngology and
Collaborative Research Center 889, University Medical Center Göttingen, Göttingen,
Germany
3Bernstein Center for Computational Neuroscience, University of Göttingen, Göttingen,
Germany
4Center for Molecular Physiology of the Brain, University of Göttingen, Göttingen,
Germany
*equal contribution
#corresponding author
Key words: hair cell, calcium, priming, fusion, synaptic vesicle, ferlins
Correspondence:
Tobias Moser
InnerEarLab, Dept. of Otolaryngology and Collaborative Research Center 889, University
of Göttingen Medical Center, 37099 Göttingen, Germany
Tel.: 49‐551‐39‐8968
Fax.: 49‐551‐3912950, [email protected]
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Abstract
Sound is encoded at synapses between cochlear inner hair cells and the auditory nerve.
These synapses are anatomically and functionally specialized to transmit acoustic
information with high fidelity likely over a lifetime. The molecular mechanisms of hair
cell transmitter release have recently attracted substantial interest. Here we review
progress toward understanding otoferlin, a multi‐C2 domain protein identified a decade
ago by genetic analysis of human deafness. Otoferlin functions in hair cell exocytosis.
Several otoferlin C2 domains bind to Ca2+, phospholipids, and proteins. Current research
reveals requirements for otoferlin in priming and fusion of synaptic vesicles during
sound encoding. Understanding the molecular mechanisms through which otoferlin
functions also has important implications for understanding disease mechanisms that
lead to deafness.
Introduction
The inner hair cells (IHCs) of the mammalian inner ear transduce mechanical stimuli into
electrical signals and transmit them to auditory neurons via ribbon‐type synapses.
Physiologically, the IHC synapse features submillisecond precision and unprecedented
high rates of release and replenishment of vesicles. Its morphological feature is the
synaptic ribbon, a nanoscale proteinaceous complex tethering a halo of synaptic vesicles
close to the active zone (AZ; see Glossary). Except for the presence of the ribbon and the
use of L‐type Ca2+‐channels for stimulus‐secretion coupling, IHCs were thought to
employ the same set of synaptic proteins as glutamatergic synapses of the central
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nervous system (CNS; Box 1). However, already in 1999 immunohistochemistry and RT‐
PCR reported that synaptotagmin 1‐3, synaptophysin and synapsin are absent from this
synapse [1]. Subsequently, physiological and immunohistochemical studies indicated
that it also lacks complexins [2,3] and seems to operate without neuronal soluble N‐
ethylmaleimide‐sensitive factor attachment receptor (SNARE) proteins [4]. Instead, IHCs
use other proteins such as the C2‐domain protein otoferlin for exocytosis. We expect
that future studies will further demonstrate that this synapse does not use the same
molecules thought to be mandatory for docking, priming and fusion of vesicles in brain
synapses. Such molecular differences allow for studies of disorders of the auditory
synapse in an otherwise unaffected mouse model or in human patients. To date,
mutations in otoferlin, the L‐type voltage‐dependent Ca2+‐channel CaV1.3 and the
vesicular glutamate transporter 3 (Vglut3) have been identified to cause human
deafness [5–7] as a consequence of defective synaptic transmission at the hair cell
synapse [6,8–12]. These hearing disorders can therefore be referred to as
“synaptopathies” [13–15].
Otoferlin, similarly to many presynaptic proteins like synaptotagmins (Syts),
Munc13s and Rab interacting molecules (RIMs), contains several C2 domains (Fig. 1). It
belongs to the family of ferlin proteins, which includes dysferlin and myoferlin of
vertebrate myocytes, Fer‐1 of sperm in invertebrates, and misfire in Drosophila. Ferlins
have been generally implicated in membrane‐membrane fusion events (e.g. [16]), but
compared to other C2‐domain proteins involved in synaptic vesicle exocytosis, we have
just begun to understand their precise functions.
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Otoferlin is required for synaptic transmission at the IHC ribbon synapse
The analysis of otoferlin knockout mice gave first insights into the function of otoferlin
[8]. Otoferlin‐deficient IHCs display a severe reduction in Ca2+‐dependent exocytosis
with normal numbers of ribbon‐associated and docked vesicles [8] (Fig. 2, Box 2).
Therefore, it was concluded that otoferlin is essential for a late step of exocytosis (e.g.
vesicle priming and/or fusion [8]). Subsequently, otoferlin was shown to be required for
synaptic exocytosis of immature outer hair cells and type I vestibular hair cells [17,18].
The function of the vestibular system was partially impaired when assessed by
vestibular evoked potentials [18], while no gross vestibular deficit was detected in
behavioral assays [8]. This is in line with the less severe exocytosis phenotype found in
otoferlin‐deficient vestibular hair cells [18].
In contrast to otoferlin deletion, abrogation of similar CNS synaptic proteins like
Munc13‐1 or Syt1 is lethal. The disruption of the priming factor Munc13‐1 leads to an
almost complete block of evoked and spontaneous transmitter release at most neuronal
synapses [19,20]. Similarly, deletion of the Ca2+‐sensor protein Syt1 abolishes the
synchronous component of evoked response [21], while the effects on asynchronous
and spontaneous transmitter release vary among preparations [21–25]. How does this
compare to the observed effects in IHCs lacking otoferlin? These cells show no
detectable synchronous exocytosis [8,9] but slowly release transmitter regardless of
stimulation [9]. Similarly to Syt, otoferlin is mainly found on synaptic vesicles but also on
the plasma membrane [8,9] (Fig. 2b).
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Current hypotheses of otoferlin function in exocytosis
The observed association between otoferlin and synaptic vesicles, the structural and
biochemical similarities with Syt1, and the impairment of exocytosis despite abundant
vesicles at the AZ, led to the hypothesis that otoferlin functions as a Syt1‐like Ca2+‐
sensor for fusion in IHCs [8]. A subsequent investigation of an Otof missense mutation
indicated a role for otoferlin in vesicle replenishment and suggested priming as another
site of action [9].
Otoferlin: a Ca2+‐sensor for synaptic vesicle fusion?
The direct interaction of otoferlin with the neuronal SNAREs, Syntaxin 1 and SNAP25,
[8,26,27] supported the hypothesis that otoferlin may act as a Syt1‐like Ca2+‐sensor for
fusion. A reconstituted SNARE‐mediated membrane fusion assay showed that, similar to
Syt1, individual C2 domains of otoferlin (except C2A) and also otoferlin fragments
containing 3 consecutive C2 domains could stimulate SNARE‐mediated liposome fusion
in a Ca2+‐dependent manner [27]. An interaction of otoferlin and neuronal SNAREs in the
organ of Corti was indicated [8] and the presence of SNAREs in hair cells was reported
based on immunostaining and Western blotting (e.g. [1,3]). However, a recent study
cast doubt on the physiological relevance of these interactions by indicating that
neuronal SNAREs are not required for exocytosis in IHCs [4]. Obviously, otoferlin could
still interact with as yet unidentified SNAREs or SNARE‐like proteins mediating
exocytosis in IHCs.
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Ca2+‐ and phospholipid binding by several of the C2 domains [8,26,27] further
supports the Ca2+‐sensor hypothesis, though, the biochemical properties of these C2
domains are still under debate. Predicting Ca2+‐ and phospholipid binding sites is
challenging as otoferlin and non‐ferlin C2 domains share only low sequence similarity
(e.g. 31% amino acid identity between PKCα C2 and otoferlin C2D is one of the highest).
Further, except for the C2A domain, providing the first crystal structure of an otoferlin C2
domain [28] (Fig. 1), purification of otoferlin C2 domains expressed in E. coli is difficult.
In some studies, protein aggregates were dissolved under denaturing conditions and
proper protein folding after re‐naturation was not ascertained [8,26]. Moreover,
uncertainty about the extent of the C2 domains led to the design of different constructs
for expression [9,26,27], possibly affecting the results of binding assays. In a recent
comprehensive study on recombinant otoferlin C2 domains, the binding of Ca2+ was
indicated for all C2 domains with the exception of C2A [27]. This is consistent with other
results on C2A [28], C2D [8], and C2F domains [26], while a lack of Ca2+‐binding was
reported for C2F in another study [9]. Similarly, phospholipid binding has been indicated
for all otoferlin C2 domains in [27], but not for C2A in [28] and C2F in [9]. Future studies
solving the structure of the otoferlin C2 domains will ultimately reveal their borders and
help resolve controversy about Ca2+‐ and phospholipid binding sites.
The Ca2+‐sensor hypothesis has also been addressed in a study expressing Syt1 in
otoferlin‐deficient mouse IHCs [29]. Despite high viral transduction rates and proper
Syt1 targeting to synaptic vesicles, hearing and IHC exocytosis (Box 2) could not be
restored [29]. Further, otoferlin expressed in Syt1‐deficient neuroendocrine cells and
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hippocampal neurons failed to restore synchronous exocytosis. In the latter, a small
increase in asynchronous response was found, which may be related to the slightly
larger miniature excitatory postsynaptic potentials (EPSCs), potentially reflecting a
subtle increase in the number of postsynaptic AMPA receptors. Together, the results
argued against a simple functional equivalence of these structurally and biochemically
related proteins. However, potential pitfalls need to be considered. First, otoferlin may
not have been sufficiently targeted to the presynaptic terminals of hippocampal
neurons. Second, the fusion with eGFP might have impaired its function. Third, failure of
cross‐rescue may have resulted from a lack of the relevant interaction partners of
otoferlin and Syt1 in the respective cells.
Perhaps the strongest argument for the Ca2+‐sensor hypothesis comes from the
observation that otoferlin‐knockout IHCs never show synchronous exocytosis, not even
after a long period of exocytic arrest such as long hyperpolarization [8,9]. Specifically,
neither short Ca2+‐influx typically used to discharge the readily releasable pool of
vesicles (RRP; Fig. 2a, Box 2) nor Ca2+‐uncaging (Fig. 2c) elicited fast exocytic responses,
although small but significant exocytosis could be documented by capacitance changes
during long depolarizations of IHCs and by postsynaptic recordings in the absence and
presence of IHC stimulation. Therefore, although otoferlin is critical for vesicle
replenishment (see below), impaired vesicle replenishment is unlikely to account for the
complete lack of RRP exocytosis in otoferlin‐deficient IHCs. Instead, together with the
above‐mentioned evidence, this finding supports a role of otoferlin in vesicle fusion.
Future studies combining mutagenesis directed towards Ca2+‐binding sites in otoferlin C2
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domains ‐ ideally sparing vesicle replenishment ‐ together with flash photolysis of caged
Ca2+ (to probe for changes in the intrinsic Ca2+‐dependence of fusion, Box 2) will be
required to ultimately test the Ca2+‐sensor of fusion hypothesis for otoferlin.
Otoferlin: a priming factor enabling fast vesicle replenishment?
A function of otoferlin in IHC synaptic vesicle priming was uncovered when analyzing the
deaf pachanga mouse mutant [9] that carries a missense mutation in the C2F domain of
otoferlin elicited by random mutagenesis [30]. Mutant IHCs displayed normal synaptic
Ca2+‐influx and exocytosis of the RRP (Fig. 2a), a small but fast exocytic response to Ca2+‐
uncaging (Fig. 2c) and an unaltered intrinsic Ca2+‐dependence of exocytosis [9],
supporting the view that this mutation does not impair the mechanism of Ca2+‐triggered
fusion. However, a marked reduction of sustained exocytosis (Fig. 2a) and a slowed
recovery of the RRP from depletion indicated an impaired vesicle replenishment of the
release sites. This, together with an unchanged anatomical estimate of the synaptic
vesicle complement, and normal endocytic membrane retrieval, suggested that otoferlin
is involved in vesicle priming [9]. However, as these authors noted, a function of
otoferlin in clearing exocytic material from the release site towards the perisynaptic
sites of endocytosis [31] could not be excluded. Indeed, a role in coupling exocytosis to
endocytosis has been suggested for of Syt1 [32–34] and, hence, potentially, another
interesting parallel between otoferlin and Syt1 might exist. Vesicle replenishment in hair
cells is Ca2+‐dependent [35–39] and it is tempting to speculate that otoferlin is a key
effector of this process.
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The finding of normal RRP exocytosis in vitro (Fig. 2a, Fig. 3a) seemed hard to
reconcile with the profound hearing impairment in pachanga mice in vivo [9]. However,
in vivo recordings from single auditory nerve fibers and brainstem neurons showed that
lowering stimulus rates improved spiking at sound onset in pachanga mice [9] (Fig. 3b).
It was then hypothesized that, in vivo, with given evoked and spontaneous vesicle
release the impaired vesicle replenishment does not support the maintenance of a
sufficient standing RRP (i.e. the AZ is chronically depleted of primed, release‐ready
vesicles or perhaps of reusable synaptic machinery; Fig. 3c). Longer interstimulus
intervals partially restore a standing RRP (Fig. 3a), enabling some as yet limited neuronal
spiking upon presentation of sounds. In contrast, during in vitro experiments, cells are
kept at hyperpolarized potentials in‐between stimuli, so that “spontaneous” transmitter
release is blocked (see [40]). As a consequence, even the reduced rate of vesicle
replenishment can establish a normal‐sized RRP for exocytosis in IHCs of pachanga mice
upon stimulation at the low rates of repetition (0.01‐0.03 Hz) used in [9]. Figure 3
illustrates this hypothesis with the product of standing RRP and release probability
defining the transmitter release (Fig. 3a) and ensuing spike rate (Fig. 3b).
Why might cochlear IHCs require otoferlin as a special priming factor? The
answer may be related to the demands of synaptic encoding of sound, where each spiral
ganglion neuron (SGN) can spike at hundreds of Hz during ongoing acoustic stimulation.
Different from a bipolar cell of the retina or a vestibular ganglion neuron, each SGN is
driven by a single IHC AZ [41], reviewed in [42]). Moreover, SGNs seem to only spike
once for each presynaptic release event [43]. Therefore, in response to ongoing acoustic
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stimulation, the SGN spike rate is probably limited by the rate of vesicle replenishment
to the AZ. Assuming an adapted spike rate of 200 Hz, and one or more vesicles per
release event, the vesicle replenishment rate must be 200 Hz and likely greater. If the
average release event involves four [44] or seven [40] vesicles, then a replenishment of
at least 800‐1400 vesicle‐quanta would be required per second per AZ. IHCs in vitro can
sustain exocytic rates of 700 synaptic vesicles per second per AZ for at least 100 ms of
strong stimulation [9]. Assuming approximately 10 release sites per AZ, this indicates
that each ‘RRP slot’ can be replenished 70 times per second. These rates surpass
estimates in other synapses that lack otoferlin. For example, the IHC vesicle supply rate
exceeds that of rat bipolar cells by approximately an order of magnitude, despite
comparable synaptic structure and only slightly different RRP size (70 vesicles/s/AZ or 7
vesicles/s/RRP slot [45]). On the other hand, vestibular hair cells express otoferlin too,
but show lower replenishment rates, which are less strongly affected by disruption of
otoferlin [18].
The search for interaction partners of otoferlin is an active topic of research. In
addition to the above mentioned SNAREs, the unconventional Myosin VI [46,47], the
small G‐protein Rab8b [48], Ergic2 [49] and the CaV1.3 channel [26] were described to
interact with otoferlin. However, three of the studies [47–49] used a yeast‐two‐hybrid
bait covering half of the C2D domain (amino acids 904 to 1025 while C2D extends from
962 to 1095), which might reveal unspecific protein interactions due to misfolding of the
incomplete domain. Further functional studies to demonstrate the relevance of these
protein interactions would therefore be desirable.
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How can the priming and Ca2+‐sensing functions (Fig. 3c, d) be selectively
affected by different mutations of the otoferlin gene? We consider two possible
scenarios. While no significant biochemical difference between the mutant and the wild
type C2F domain has been described so far [9], the pachanga mutation might alter
binding site(s) for interaction(s) involved in priming but leave the molecular events
leading to fusion intact. Alternatively, priming and fusion may require different amounts
of proteins. For example, 30% of the normal levels of otoferlin, as detected in pachanga
IHCs, might suffice for intact fusion but not for proper priming. On the other hand, in
the complete absence of otoferlin, fusion events might be so rare that priming is no
longer the rate‐limiting step, although it, too, is affected by the lack of otoferlin (see Fig.
3c). However, the dependence of priming and fusion on otoferlin levels has yet to be
established. It is likely that the pachanga mutation renders the protein product less
stable, consequently only very little protein is detected by immunofluorescence (Fig. 2b).
Interestingly, cross‐breeding of pachanga with knockout mice resulted in a further
reduction of otoferlin protein levels in IHCs, leading to even less sustained exocytosis [9].
Human missense mutations in OTOF, including the mutations that confer enhanced
temperature‐sensitivity to otoferlin (see below), might also act by destabilizing the
protein. Research on mouse models and in vitro biochemical studies of otoferlin and
exocytosis at the IHC synapse will help to understand clinical findings described in
patients with mutations in otoferlin.
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Potential molecular mechanisms involving otoferlin
Compared to the putative Ca2+‐sensor of fusion (Syt1) and the putative priming factor
(Munc13‐1) acting at conventional synapses, little is known about the structure and
function of otoferlin. Moreover, direct comparison is hampered by the unconventional
exocytic machinery found in hair cells. Nevertheless, we provide some speculations on
potential molecular mechanisms of otoferlin function: Considering vesicle
replenishment and fusion, possible molecular mechanisms include (i) tethering of
opposing membranes, potentially involving otoferlin oligomerization, (ii) promotion of
the assembly of trans‐SNARE complexes, (iii) Ca2+‐dependent approximation of vesicular
and plasma membranes, (iv) interaction with other proteins mediating Ca2+‐triggered
fusion, and (v) interaction with endocytic proteins to facilitate the clearance of the
release site. Otoferlin's potential involvement in tethering synaptic vesicles to the
plasma membrane is an attractive hypothesis for its function in vesicle replenishment.
Indeed, vesicles at IHC synapses are tethered to the synaptic ribbon and/or to the
plasma membrane (e.g. [50]), but the molecular identity of the tethers remains
unknown. Ca2+‐dependent phospholipid binding has been indicated for several otoferlin
C2 domains [26,27]. Therefore, it seems conceivable that otoferlin inserts into the
cytosolic leaflet of the plasma membrane, acting as a cone or wedge that promotes
membrane bending and brings the opposing membranes in closer apposition, as has
been suggested for Munc13 C2B [51] and Syt C2 domains [52,53]; reviewed in [54–57]).
Alternatively, in analogy to predictions for Syt1, otoferlin might act to stabilize the yet
unidentified SNARE complex (as reported using liposome fusion assays [27]) or help to
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reduce electrical repulsion between the opposing membranes by way of the positive
charges of bound Ca2+‐ions. Finally, the coiled‐coil domain of otoferlin might form a
superhelix either with itself or with other proteins, and the “zippering‐together” of the
helices could help to overcome the energy barrier for fusion of the two membranes.
Human mutations and clinical findings
Pathogenic mutations in OTOF underlie the nonsyndromic autosomal recessive deafness
DFNB9 [5] and together with the non‐pathogenic mutations [58–65] provide some
indication for which protein domains are important for otoferlin function (Fig. 4). Non‐
pathogenic sequence variants are mostly found in the C2A domain or in the linker
regions between C2 domains. So far, more than 60 pathogenic mutations have been
reported. Among those are 15 pathogenic missense mutations of which 10 are found in
C2 domains C‐F (Fig. 4, see also [63]). In addition to disabling the function of the protein
at the synapse, missense mutations might lead to enhanced protein degradation, as
reported for the deaf5 [47,66] and pachanga [9] mutations in mice.
The hearing impairment varies among subjects with different mutations in OTOF.
In most patients, auditory brainstem responses are absent or highly abnormal despite
the presence of otoacoustic emissions and/or cochlear microphonics [60–62,64,65,67]
as also found in auditory neuropathy [68]. Together with the results of animal research
this indicates impaired SGN activation by IHC transmitter release (synaptopathy) despite
intact mechanoelectrical transduction and cochlear amplification [67]. Otoacoustic
emissions can also disappear [60], presumably due to subsequent loss of outer hair cells.
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Most pathogenic OTOF mutations lead to profound prelingual deafness. The
predominant one, the so‐called “Spanish mutation” (Q829X), causes a premature stop
codon and was found in 8% of the congenital non‐syndromic deaf population with
autosomal recessive inheritance of Spanish descent [58,63]. Missense mutations (Fig. 4)
produce heterogeneous phenotypes. Some cause profound prelingual deafness, as do
premature stop codons or frameshift‐inducing deletions or insertions [63]. Others, like
the L1011P mutation [69], leave residual high‐frequency hearing, which suggests partial
functionality of the mutated protein.
For some hearing‐impaired individuals, their phenotypes are exacerbated at
elevated body temperature. In these cases, an in‐frame deletion or missense mutations
in OTOF have been identified and proposed to mediate the temperature‐dependent
hearing loss. To date, four such point mutations have been described which affect
different sites of the protein (violet in Fig. 4), indicating a general mechanism like
protein destabilization and subsequent degradation leading to a reduction in protein
levels. Interestingly, when afebrile the individuals with the I515T, the G541S and the
E1804del mutations have almost normal hearing thresholds in pure tone audiograms
albeit with impaired speech recognition particularly in background noise [62,65,70]. In
contrast, at 38.1°C they are profoundly deaf for low frequencies, severely hearing
impaired for high frequencies, and they reported tinnitus. Case studies such as these
have provided launch pads for the investigation of specific neurobiological mechanisms
of hearing and deafness in the past decade.
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Concluding remarks and future directions
Interdisciplinary studies over the past decade have characterized otoferlin as a hair cell
specific protein essential for synaptic sound encoding. Following the discovery of OTOF
as a deafness gene, its expression, clinical genotype‐phenotype relationship, subcellular
localization, protein biochemistry and structure, functional role in synaptic transmission,
and disease mechanism(s) have been addressed. Otoferlin has become an important
bait for fishing other members of the unconventional hair cell release machinery, but
this search has just begun to yield results.
Recent studies have revealed the crystal structure of its C2A domain and
demonstrated Ca2+‐ and phospholipid binding of several other C2 domains. However, the
precise topology of Ca2+‐binding to the structure of full‐length otoferlin and the
functional consequences are not clear (see Box 3). In vitro work in knockout mice and
point mutants has played a major role in unraveling the function(s) of otoferlin. So far, it
has helped to pinpoint the function of otoferlin to hair cell exocytosis. Hypothetical roles
in Ca2+‐dependent vesicle priming and vesicle fusion are not mutually exclusive and will
guide future morphological and physiological analyses. Mouse models with missense
mutations in otoferlin will likely provide insights into disease mechanisms of moderate
hearing phenotypes, including temperature‐sensitive hearing impairment. The defective
vesicle replenishment in IHCs of pachanga mice, which is possibly related to the reduced
levels of otoferlin protein, could well be a common denominator of weaker otoferlin‐
related human hearing impairments. Establishing efficient gene transfer into IHCs will
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enable detailed structure‐function analysis in the otoferlin knockout background and
also lay the foundation for future gene replacement therapy.
Acknowledgment
We would like to thank M. Rutherford, C. Dean, R. Jahn and S. Hallermann for critical
reading of the manuscript. We thank Linda Hsu for preparing the artwork. This work was
supported by a grant from the Deutsche Forschungsgemeinschaft (DFG) through the
Collaborative Research Center 889 “Cellular Mechanisms of Sensory Processing” (to T. M.
and E. R.).
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Figure 1: Structure and function of various C2‐domain proteins
Schematic diagrams of the protein structure of a selection of ferlins (dysferlin, myoferlin
and otoferlin) and other C2‐domain proteins involved in synaptic exocytosis
(synaptotagmin, Munc13, RIM; for reviews see [55,71,72]), along with their putative
functions, are illustrated. Among the C2‐domain proteins, ferlins possess the highest
number of C2 domains known (up to seven). In addition, they contain a DysF and/or Fer
domain (60‐70 residue conserved motif), both specific for this family of proteins.
Proteins involved in membrane fusion events are often membrane anchored proteins
and contain either an amphiphatic α‐helix, which forms a superhelix with α‐helices of
interaction partners (as the SNARE motif in the SNARE complex), or domains which bind
to phospholipid membranes in a Ca2+‐dependent manner. Otoferlin contains both types
of domains: in addition to six C2 domains known for Ca2+‐ and phospholipid binding (ie.
C2A‐F), a coiled‐coil domain (CC) is present in one region otherwise predicted to be
unstructured. This CC domain might work in a similar manner as the SNARE motif,
although the other players forming a superhelix like in the SNARE complex still await
17
identification. In otoferlin, myoferlin and dysferlin, a seventh C2 domain is predicted to
fold between the C2D and the C2E domains (ie. C2de [73]). However, the sequence
similarity between these ferlin C2de domains and any other C2 domain is much lower
than the similarity amongst other ferlin C2 domains. The tertiary structure of the C2A
domain (PDB ID: 3L9B, upper right; reproduced, with permission, from [28]) is shown.
The crystal structure of otoferlin C2A shows a similar folding as the Munc‐13 C2B domain,
but has a shorter top loop [28]. Abbreviations: DUF, domain of unknown function; DysF,
dysferlin domain; Fer, Ferlin‐specific motif; MHD, Munc13 homology domain; PDZ,
postsynaptic density‐95/Discs large/Zona occludens‐1 domain; ;; TM, transmembrane
domain.
Figure 2: Exocytosis and otoferlin expression in IHCs of wild‐type mice and otoferlin
mutant mice
The schematic diagrams (insets to parts a and c) depict an IHC. Between 5 and 20
ribbon‐type active zones are typically seen, although only 3 are depicted here.
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Exocytosis was evoked by depolarizations of varying duration (in the case of part a) or by
flash photolysis of caged Ca2+ (part b, for a fast and global elevation of cytosolic [Ca2+])
measured as membrane capacitance increase (∆Cm).
a) Two components of ∆Cm in response to depolarization of IHCs in wild‐type mice (black
line): readily‐releasable pool (RRP; gray line) and the sustained component (black
dashed line). While RRP exocytosis is preserved, the sustained component of release is
strongly reduced in IHCs of pachanga mice (C2F missense mutants, green line). In
otoferlin knockout (KO) mice, exocytosis is almost absent (violet) [9]. b)
Immunofluorescence of otoferlin (otof: green) and vesicular glutamate transporter 3
(Vglut3: violet) expressed in IHCs of wild‐type (top), pachanga (middle) and otoferlin KO
mice (bottom). Pachanga IHCs show lower expression levels of otoferlin than the wild‐
type animals [9]. In the wild‐type IHCs the immunofluorescence signal for otoferlin and
Vglut3 largely overlap, while otoferlin shows a stronger staining than Vglut3 on the
plasma membrane. Scale bar: 5 µm. c) Upon Ca2+‐uncaging, a large pool of vesicles is
released within a few ms in wild‐type IHCs (black). In IHCs of pachanga mice (green line),
this fast component is strongly reduced (arrow); however, its time constant is
comparable to the fast component of the wild‐type response [9]. The strong reduction
in amplitude is currently not understood. It might reflect a reduction in the total number
of synaptic vesicles or experimental conditions that do not allow sufficient priming in
the mutant IHCs. In otoferlin‐KO mice, only some residual slow release can be observed
(violet) [9]. Adapted, with permission, from [9].
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Figure 3: Proposed roles of otoferlin in IHC vesicle priming and fusion.
a) Schematic drawing of the rate of exocytosis upon depolarization in IHCs of wild‐type
(black) and pachanga (green) mice roughly to scale (assuming a vesicle capacitance of
45 attofarad [74]): the rate of sustained release is strongly reduced in mutant IHCs [9].
b) The spike rate of auditory nerve fibers and bushy cells over time (post‐stimulus time
histogram) upon presentation of suprathreshold acoustic stimuli (gray bar) in wild‐type
and pachanga mice [9]. The poor sound encoding in pachanga mice was proposed to be
due to slowed vesicle replenishment, which was supported by in vitro data (a) and the
finding that decreasing the rate of stimulus repetition (10 versus 0.5 Hz: green versus
violet trace) improved sound encoding in mutant mice. In vitro, the mutant IHCs
20
sufficiently replenished the RRP of vesicles because synaptic release was inhibited for
several seconds between consecutive stimuli. The rate of spiking of auditory fibers is
driven by the rate of transmitter release from IHCs. In order to illustrate the
replenishment hypothesis we relate spike rate to the rate of transmitter release from
IHCs that is determined by the size of the standing RRP and the probability of
neurotransmitter release (Pr). For simplicity, we neglected the effects of refractoriness
and large bin‐width that lead to underestimation of the actual rate of release. The
standing RRP differs between the resting (Rest) and stimulated (Stim) conditions, but is
always much smaller at the AZ of the mutant IHCs. c) Schematic en‐face view of the AZ
with synaptic vesicles and Ca2+‐nanodomains in wild‐type, pachanga and otoferlin‐KO
IHCs in vivo. Normal numbers of docked vesicles were reported in both mutants. The
pachanga mutation spares fusion but impairs the priming of synaptic vesicles [9]. The
loss of otoferlin in KO mice severely affects fusion and probably priming [8,9]. Because
fusion events are extremely rare in otoferlin‐KO IHCs, enough time is available for
vesicles to reach fusion competence despite defective priming. d) Schematic model of
the proposed role of otoferlin in vesicle priming and fusion; Ca2+ is critical for both steps.
21
Figure 4: Single amino acid mutations in OTOF
Protein domain structure of otoferlin with pathogenic missense mutations and in‐frame
deletions (top), and sequence variants (bottom), according to [58–65,69,70,75,76]. The
numbers are based on the amino acid sequence of human otoferlin. C2 domains (C2A‐F)
are depicted according to analysis in silico [77] or the crystal structure [28]. A seventh
putative C2 domain is also depicted [73] (see Fig 1 for details).). Five potentially
pathogenic mutations that occurred in the heterozygous state in hearing impaired
patients ‐ and for which the mutation in the second allele has not been identified ‐ are
labeled by §. The P490Q* mutation was found in cis with I515T [59]. Mutations in violet
have been associated with cases of temperature‐sensitive hearing loss [62,64,65,70].
Abbreviations: CC, coiled‐coil domain; FerB, Ferlin‐specific motif ; TM, transmembrane
domain.
Glossary:
Active zone (AZ): the site of the presynaptic plasma membrane where neurotransmitter
is released opposite to the postsynaptic density.
22
C2 domains: β‐sandwich domains consisting of a pair of four‐stranded β‐sheets (see Fig.
1). They are found in protein kinase C, phospholipases, and in many presynaptic proteins
like synaptotagmins, Munc‐13s, and RIMs. C2 domains generally bind Ca2+ and
phospholipids and/or interact with other proteins. They are involved in a plethora of
cellular functions (i.e. exocytosis, regulation of GTPase activity, modification of lipids,
protein phosphorylation etc). In C2 domains within non‐ferlin proteins (such as
Synaptotagmins, Munc13s etc.), Ca2+ is coordinated by three to five negatively‐charged
aspartate residues located at specific positions in top loops 1 and 3. Phospholipid
binding occurs either simultaneously with that of Ca2+ to the top loops or independently
of Ca2+ to the β‐groove (reviewed in [78]).
Pathogenic mutations: mutations in the genomic DNA that grossly affect the length,
stability or function of the encoded protein. Deletions or insertions of nucleotides in the
coding sequence that lead to a shift in the reading frame are always considered to be
pathogenic. The same is true for point mutations leading to premature stop codons and
therefore to truncated, mostly nonfunctional proteins, which are termed nonsense
mutations. Mutations that interfere with the splice donor or acceptor site in the pre‐
mRNA are also mostly pathogenic. The exchange of one amino acid by another might be
deleterious for the protein function or stability, in which case those are called “missense
mutations”, or they do not impair the protein and are therefore termed “sequence
variants”.
23
Profound hearing loss: a very severe form of hearing impairment (hearing loss greater
than 90 decibels). Patients with such hearing impairment usually do not benefit from
hearing aids, but can be aided by cochlear implantation if the auditory pathway is intact.
Readily‐releasable pool of vesicles (RRP): population of vesicles that can immediately
fuse upon increase in intracellular [Ca2+] without any further preparatory step. In most
synapses these vesicles appear to be morphologically docked.
Ribbon synapse: a specialized type of synapse, where the AZ contains a presynaptic
electron‐dense body, the ribbon, which is composed of ribeye and other scaffold
proteins. Ribbon synapses are found in vertebrate hair cells, synaptic terminals of
photoreceptors, bipolar cells, pineal glands and electroreceptors. Structurally similar
synapses, known as T‐bar synapses, exist in Drosophila, however, such synapses lack
ribeye.
24
Box 1: Comparison of the excitatory CNS synapse and the IHC ribbon synapse
The conventional neuronal synapse and the IHC ribbon synapse differ in their molecular
anatomy and physiology (Figure I). While the AZs of neurons reside in the presynaptic
terminal remote from the soma, the AZs of IHCs are part of the epithelial soma. AZs
commonly contain a mesh of scaffold proteins, termed the cytomatrix of the active zone
(CAZ), that tether vesicles and position Ca2+‐channels at the sites of vesicle fusion
(reviewed in [79–81]). Some scaffold proteins are conserved between CNS synapses and
IHC ribbon synapses. However, the CNS AZs are usually small, and on average zero to
two vesicles are released by a brief stimulus (an action potential). Knowledge on several
proteins involved in vesicle maturation, docking, priming and release is quite detailed.
The large IHC AZ is equipped with a specialized molecular anatomy, which enables the
release of dozens to hundreds of vesicles per second in response to graded (receptor)
potentials. Most prominently, it contains the synaptic ribbon, primarily composed of
ribeye [82,83] tethering approximately 70‐200 synaptic vesicles with approximately 10‐
20 of them also tethered to the plasma membrane. The scaffold protein bassoon and
possibly other scaffolds link the ribbon to the AZ membrane. The ribbon and/or bassoon
stabilize a large complement of Ca2+‐channels and a large RRP (10‐20 vesicles) and
contribute to rapid vesicle replenishment [50,83,84].
25
Box 1 Figure I legend: Schematic diagram
illustrating some of the main anatomical and molecular differences between
conventional synapses of the mammalian CNS (left) and the IHC ribbon synapse (right).
Docked vesicles are shown in yellow, vesicles tethered to the CAZ or the ribbon are
shown in blue. Several synaptic proteins important for vesicle docking, priming and
fusion at CNS synapses seem to be absent from the IHC ribbon synapse, which instead
uses proteins that are either specific to hair cells or found in only few other cell types.
The table lists references for either reviews or first report (in mammals) and functional
characterization within the CNS (preference to mouse mutants) and IHC ribbon synapses.
“None” indicates that a protein is likely absent; ”?” indicates lack of published data for
mature IHCs (or conflicting results). Otoferlin is found on synaptic vesicles and in the
plasma membrane of hair cells (top right) and functions in vesicle priming and fusion.
26
Box 2: Properties of exocytosis at the IHC ribbon synapse
The presynaptic function of IHCs is often studied using patch‐clamp measurements of
Ca2+‐currents and membrane capacitance (Cm) (see Fig. 2a). Depolarization opens CaV1.3
channels [10,11,119] and consequently triggers exocytosis. The ensuing increase in
surface area of the IHC plasma membrane can be detected as a Cm increment (Cm).
Measurements of Cm reveal at least two components (fast and sustained) of exocytosis
upon depolarization and the corresponding transmitter release has been revealed by
paired pre‐ and postsynaptic recordings [40]. Paired recordings from frog hair cells and
connecting afferent fibers showed a good correspondence between the ∆Cm and the
postsynaptic EPSC charge transfer for short and long depolarizations [120], indicating
that most if not all exocytosis occurs at synapses. The fast component of the exocytic Cm
increase saturates with a time constant of a few milliseconds [35]. When expressed in
units of fused vesicles it agrees well with counts of membrane‐proximal or AZ‐tethered
vesicles by electron microscopy [9,38,50,83] and has been interpreted as the exocytosis
of a limited RRP tethered near the Ca2+‐channels [35,38,50,121,122]. The rate of sound‐
evoked spikes in afferent fibers can be predicted from in vitro RRP fusion rates,
depletion kinetics, and RRP replenishment rates [50,84,123]. This strongly suggests that
the RRP mediates sound encoding. The slower component of the Cm rise largely reflects
the re‐supply of vesicles into the RRP and their subsequent fusion with the plasma
membrane. The slow component scales in amplitude with the number of synapses [122]
and predicts auditory nerve fiber spiking with the same scale factor as needed for RRP
exocytosis [84]. IHC exocytosis evoked by fast and spatially homogenous elevations of
27
intracellular [Ca2+] via UV‐flash photolysis of caged Ca2+ also shows two kinetically
distinct phases (Fig. 2c). However, the total flash‐evoked Cm increment is very large
amounting to a plasma membrane surface increase of approximately 15% and the fast
Cm component exceeds the size of the depolarization‐evoked RRP exocytosis by 100‐
times (see Fig. 2a and c). This discrepancy is currently not understood, but likely involves
extrasynaptic fusion of vesicles during homogenous elevations of intracellular [Ca2+]
[124,125].
Box 3. Outstanding Questions
Can deafness in otoferlin mutants be rescued by expression of wild‐type
otoferlin in IHCs?
How is otoferlin distributed between various membranous organelles such as the
various populations of synaptic vesicles and endosomes and the plasma
membrane? To answer this question, further immunogold electron microscopy
and super‐resolution light microscopy are needed.
Which interacting protein(s) of otoferlin are relevant to its function in priming
and fusion?
Does otoferlin act as the Ca2+‐sensor of fusion in hair cells? How precisely does it
promote fusion?
What is the precise molecular mechanism underlying the promotion of vesicle
priming by otoferlin?
28
Do ferlins employ a common mechanism in mediating membrane fusion
reactions?
How do the six (or seven) C2 domains co‐operate in ferlin function?
How precisely does Ca2+ concentration affect ferlin function?
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