Usherin is required for maintenance of retinal
photoreceptors and normal development
of cochlear hair cells
Xiaoqing Liu*, Oleg V. Bulgakov*, Keith N. Darrow†, Basil Pawlyk*, Michael Adamian*, M. Charles Liberman†,
and Tiansen Li*‡
*Berman–Gund Laboratory for the Study of Retinal Degenerations and†Eaton–Peabody Laboratory, Harvard Medical School, Massachusetts
Eye and Ear Infirmary, Boston, MA 02114
Edited by Jeremy Nathans, Johns Hopkins University School of Medicine, Baltimore, MD, and approved January 18, 2007 (received for review
December 11, 2006)
Usher syndrome type IIA (USH2A), characterized by progressive pho-
most common subtype of Usher syndrome. In this article, we show
that the USH2A protein, also known as usherin, is an exceptionally
large (?600-kDa) matrix protein expressed specifically in retinal
photoreceptors and developing cochlear hair cells. In mammalian
photoreceptors, usherin is localized to a spatially restricted mem-
brane microdomain at the apical inner segment recess that wraps
around the connecting cilia, corresponding to the periciliary ridge
postnatal development. Targeted disruption of the Ush2a gene in
mice leads to progressive photoreceptor degeneration and a moder-
hearing deficits in USH2A patients. These data suggest that usherin
is required for the long-term maintenance of retinal photoreceptors
and for the development of cochlear hair cells. We propose a model
plasma membrane and its large extracellular domain projecting into
the periciliary matrix, where they may interact with the connecting
cilium to fulfill important structural or signaling roles.
photoreceptor degeneration ? retina ? retinitis pigmentosa
hearing defect and retinitis pigmentosa (RP), a progressive degen-
eration of the retinal photoreceptors (1, 2). Based on clinical
features of the hearing defect, Usher syndrome is classified into
three types: types I, II, and III (3). Usher syndrome type I (USH1)
is the most severe form with profound congenital deafness and
vestibular dysfunction. USH2 is characterized by moderate non-
progressive hearing loss without vestibular dysfunction and a ten-
dency for more pronounced hearing loss for higher-frequency
sounds. USH3 is distinguished from USH2 by the progressive
nature of its hearing loss and occasional vestibular dysfunction. At
least 10 mapped chromosomal loci and 8 genes now are known to
be involved in the development of Usher syndrome (refs. 3 and 4;
USH2 accounts for well over one-half of all Usher cases (3, 5),
and mutations in the USH2A gene are responsible for the majority
of USH2 cases (5–9). In addition to typical USH2, a certain mutant
with little or no hearing defects (10). USH2A mutations were
estimated to underlie ?7% of all RP cases in North America, on a
par with other major RP genes such as RPGR and rhodopsin (11).
The product of the USH2A gene was designated usherin (8).
Usherin transcript was originally reported to be 5 kb (5), encoding
a putative protein of 170 kDa. This 170-kDa protein was predicted
to be entirely extracellular, with motifs resembling common extra-
cellular matrix proteins, and was described in several earlier studies
(5, 12–14). A more recent study identified additional exons for the
sher syndrome is the most frequent cause of combined deaf-
ness and blindness. Usher patients exhibit both sensorineural
sequence to 15 kb (15), encoding a putative 600-kDa protein. In
addition to the greatly expanded size, the recently identified exons
also were predicted to encode a membrane-spanning segment
There have been conflicting reports about usherin tissue distri-
bution and subcellular localization (4, 13, 16–18). The putative
600-kDa full-length protein never was confirmed from native
tissues, and the function of usherin was poorly understood. It is
proteins specifically are required in both photoreceptor and co-
chlear hair cells, suggesting key functional or structural features
shared between these two cell types. Indeed, both are ciliated
sensory neurons. Photoreceptors possess a specialized photosensi-
tive organelle known as the outer segment, a large modified cilium
a thin bridge called the connecting cilium. As the outer segment
continuously is renewed, the connecting cilium has the vital role of
trafficking nascent proteins to the outer segment. The analogous
kinocilium of a cochlear hair cell appears transiently during devel-
opment, when it is required for aligning the hair bundles (stereo-
cilia) into the highly organized adult formation (3, 19). Stereocilia,
also known as hair bundles, are mechanosensitive organelles re-
quired for displacement detection on a nanometer scale (20).
Mutations affecting stereocilia formation and maintenance lead to
deafness or Usher syndrome (16, 21–26). In addition, both types of
sensory neurons also are characterized by ribbon synapses.
To investigate the molecular mechanism of usherin function in
vivo, we carried out targeted disruption of the Ush2a gene in mice.
Based on phenotype analysis of the Ush2a-null mice and analysis of
usherin transcripts and proteins, we conclude that the dominant
form of usherin in photoreceptors is the 600-kDa polypeptide as
previously predicted (15). We propose that usherin function is
related to the true cilia of the retinal photoreceptors and cochlear
Author contributions: X.L. and T.L. designed research; X.L., O.V.B., K.N.D., B.P., M.A., and
M.C.L. performed research; X.L., M.C.L., and T.L. analyzed data; and X.L. and T.L. wrote the
The authors declare no conflict of interest.
This article is a PNAS direct submission.
Abbreviations: USH2A, Usher syndrome type II A; RP, retinitis pigmentosa; RPE, retinal
pigment epithelium; immunoEM, immunoelectron microscopy; ERG, electroretinogram;
GFAP, glial fibrillary acidic protein; DPOAE, distortion product otoacoustic emission.
Data deposition: The assembled murine usherin long variant sequence has been deposited
in the GenBank database (accession no. DQ073638).
‡To whom correspondence should be addressed at: Massachusetts Eye and Ear Infirmary,
243 Charles Street, Boston, MA 02114. E-mail: email@example.com.
This article contains supporting information online at www.pnas.org/cgi/content/full/
© 2007 by The National Academy of Sciences of the USA
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Murine and Human Usherins Are Highly Conserved. By PCR ampli-
the long and short usherin variants previously reported were
identified. The long usherin variant sequence was assembled from
kb in length, with an ORF of 5,193 aa codons (GenBank accession
human usherin variant (5,202 aa) (Fig. 1). Murine and human
extracellular domain consisting of LamNT, EGF-Lam, LamGL,
LamG, and FN3 motif repeats, a single membrane-spanning seg-
ment and a PDZ-binding C terminus predicted to reside intracel-
lularly. Therefore, murine and human usherins are structurally
highly conserved and likely to be functionally conserved as well.
The 600-kDa Long Usherin Variant Is the Predominant Form in Pho-
toreceptor Cells. We ablated Ush2a gene expression in mice by
targeted disruption (Fig. 2). We generated two antibodies against
the N and C termini of the long usherin sequence. The N-terminal
whereas the C-terminal antibody was specific for the long variant
(Fig. 1). With these antibodies, immunoblotting revealed a protein
that was higher than dystrophin (427 kDa). This protein was
completely ablated in Ush2a?/?retinas (Fig. 2a), identifying this
protein as usherin. Its apparent molecular mass was consistent with
the predicted size of 600 kDa for the usherin long variant. The
170-kDa short variant, however, was not detected by immunoblot-
ting using the N-terminal antibody (Fig. 2a), which was generated
This result does not rule out the presence of the shorter variant in
the retina but does indicate that the long usherin variant is more
By RT-PCR analysis, usherin transcript was found expressed in
neural retina or from the retinal pigment epithelium (RPE), we
performed RT-PCR analysis on dissected neural retinas and the
posterior eye cup from which the neural retina had been removed.
The latter tissue retained RPE. As shown in Fig. 2b, only the retina
gave the expected usherin amplification product, indicating that
usherin transcript was expressed primarily in the neural retina.
By using immunofluorescence, usherin was found in a region
between the outer and inner segments of the photoreceptors (Fig.
2c). This is the region where the connecting cilia, which link the
inner and outer segments, are aligned. The N- and C-terminal
usherin antibodies gave an identical staining pattern, which was
absent in the Ush2a?/?photoreceptors. Because the C-terminal
antibody would recognize the long variant only, the identical
staining patterns from the N- and C-terminal antibodies reinforce
our conclusion that the long variant predominates in the retina.
it found in the RPE or the photoreceptor synapses as others have
reported. These data suggest that usherin is expressed specifically
in retinal photoreceptor cells.
Usherin Marks the Apical Inner Segment Recess That Wraps Around
the Connecting Cilia. To determine usherin localization in photore-
ceptor cells more precisely, we carried out double-labeling immu-
nofluorescence with well defined photoreceptor proteins that lo-
RPGR is localized within the connecting cilia proper (27, 28).
Rootletin, the structural constituent of the ciliary rootlet and
separated from the connecting cilia only by a basal body, marks the
proximal boundary of the connecting cilia (29). RP1, on the other
hand, associates with the axonemal microtubules in the basal outer
31). Double labeling with these markers determined that usherin
was distal to rootletin, proximal to RP1, and overlapped with
RPGR (Fig. 3a).
Because usherin was predicted to be mostly extracellular, the
to the plasma membrane of the connecting cilia with its large
extracellular domain projecting into the periciliary matrix. Equally
plausible, however, was that usherin was tethered to the plasma
shares 82% sequence identity and the same motif arrangement with its
human ortholog. The lines (?-Usherin-N and ?-Usherin-C) above the murine
usherin sequence indicate the regions that were selected for making recom-
binant protein antigens. The two lines (170 kDa and 600 kDa) beneath the
murine usherin sequence denote the two predicted usherin variants that had
been reported. The site of targeted disruption encompassing exon 5 is indi-
cated. Symbols representing different motifs are given at the bottom of the
diagram. LamGL, LamG-like jellyroll fold domain; EGF-Lam, laminin-type epi-
dermal growth factor-like domain; FN3, fibronectin type 3; LamG, laminin G
Motif alignment of the murine and human usherins. Murine usherin
Eye cup (-retina)
Eye cup (-retina)
ting run on agarose gels and probed by either ?-Usherin-N and ?-Usherin-C
antibodies showed complete ablation of the long usherin variant (estimated
shown as a reference for molecular mass. The same samples run on a poly-
acrylamide gel and probed ?-tubulin served as a loading control. (b) RT-PCR
analysis of WT tissues showed that usherin transcripts originated from the
(green) pattern between the inner and outer segment, indicative of the
missing. Nuclei were counterstained blue with Hoechst dye 33342. IS, inner
segment; OS, outer segment; ONL, outer nuclear layer.
Ablation of the Ush2a gene expression in mice. For details of
www.pnas.org?cgi?doi?10.1073?pnas.0610950104 Liu et al.
membrane in a specialized region at the apical inner segment that
encircles the connecting cilia. The connecting cilia of photorecep-
tors sit in a recess formed by the invagination of apical inner
segment plasma membrane. The narrow band of periciliary matrix
materials separates the plasma membranes of the inner segment
and of the connecting cilium. Usherin could be anchored via its C
terminus to the plasma membrane of the cilium, the apical inner
segment recess, or both. Immunofluorescence microscopy would
not have sufficient resolution to distinguish between the two
patterns of usherin localization. We therefore carried out immu-
in photoreceptors. With the usherin C-terminal antibody, we found
that usherin was associated exclusively with the plasma membrane
of the apical inner segment recess (Fig. 3b). Based on these data,
we propose a model in which usherin is tethered to the plasma
come into contact and interact with the surface of the connecting
cilium (Fig. 3c).
Usherin Is Essential for the Long-Term Maintenance of Photorecep-
tors. The postnatal development of retinal photoreceptors in the
Ush2a?/?mice was indistinguishable from that of WT littermate
controls as judged by histology and electroretinograms (ERGs) in
younger mice. Up until 10 months of age, photoreceptor morphol-
ogy, such as nuclear layer thicknesses or inner/outer segment
lengths, was comparable to that of the controls as revealed by light
in the Ush2a?/?mice. That photoreceptors in the mutant were
under stress was indicated by an up-regulation of glial fibrillary
acidic protein (GFAP) in Mu ¨ller cells in the predegenerating
retinas. GFAP up-regulation is a nonspecific indicator of photore-
ceptor degeneration and typically precedes overt cell loss (32, 33).
began to emerge in the mutant retinas. These signs included the
gradual thinning of the photoreceptor nuclear layer and shortening
of the inner/outer segments. By 20 months of age, more than
one-half of the photoreceptors were lost, and the outer segments
became very short and disorganized (Fig. 4a). Along with the loss
of photoreceptors, ERG amplitudes declined so that by 20 months
the a- and b-waves were reduced by ?60% compared with those of
age-matched WT controls (Fig. 4b). By immunofluorescence stain-
ing for cone opsins, we found that cone outer segments were
severely shortened with cone opsins ectopically localizing to cell
bodies (Fig. 4c), suggesting that cone photoreceptors also were
undergoing degeneration. GFAP was found to be up-regulated
from as early as 2 months of age (Fig. 4c) and remained so at the
older ages (data not shown).
Usherin Deficiency Leads to Loss of Hair Cells and High-Frequency
Threshold Elevation. To investigate the role of usherin in the inner
studied hair-cell morphology by scanning electron microscopy, and
assessed cochlear function by measurement of distortion product
animals revealed expression in stereociliary bundles of both inner
and outer hair cells but was absent in Ush2a?/?mice (Fig. 5a). The
staining was strong at postnatal days 1–5 but was no longer
detectable at 2 months of age (data not shown).
In common laboratory strains of mice, hearing is strongly
Usherin Usherin/ /RootletinRootletin
Usherin Usherin/ /RPGRRPGR
UsherinUsherin/ /RP1 RP1
connecting cilia. By using confocal microscopy, usherin was located just distal to the ciliary rootlet (Top), overlapping with RPGR (Middle), and proximal to RP1
photoreceptor (cross-sectional view), no gold labels were found. (c) A schematic representation of usherin localization in photoreceptors. A longitudinal profile
of a photoreceptor in its entirety (Left), a cross-sectional view through the connecting cilium (Upper Right), and a 3D rendition of the photoreceptor inner
segment in relation to the connecting cilium (Lower Right) are shown. Usherin is represented by the green color in all images. A PDZ domain protein (orange)
is thought to interact with the C terminus (PDZ-binding motif) of usherin on the cytoplasmic side.
Subcellular localization of usherin in the photoreceptors. (a) Double labeling of usherin (green) with markers (red) known to localize in or close to the
Liu et al. PNAS ?
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influenced by allelic variations in the cadherin 23 gene (Cdh23),
particularly as they age (34). Our original mouse colony was
that leads to progressive, high-frequency hearing loss beyond 3
months of age (34). In our own preliminary hearing tests, we also
saw effects from this allele, as indicated by subnormal response
in a control group (data not shown). Thus, we embarked on a
breeding scheme to remove the Cdh23 hypomorphic allele
Ush2a?/?mutant with the CBA/CaJ strain homozygous for the
were Ush2a?/?and Cdh23753A/753G. F1sibling mating produced
several allelic combinations among the F2 littermates. PCR
amplification followed by DNA sequencing identified mice that
were Ush2a?/?; Cdh23753G/753Gand mice that were Ush2a?/?;
Cdh23753G/753G. These two groups of mice were enrolled for
hearing tests as the mutant and control, respectively.
DPOAE measurements at 4 months of age showed normal
cochlear function in the mutants at low frequency but a clear
threshold elevation at higher test frequencies (Fig. 5b). Such
observations are consistent with loss of outer hair cells in the
cochlear basal turn. Threshold elevation had not progressed when
the same cohort of mice was retested at 7 months of age (Fig. 5b).
No head tilt, circling, or other obvious signs of vestibular dysfunc-
tion were observed in the Ush2a?/?mice. Thus, the Ush2a?/?
phenotype is a nonprogressive moderate hearing loss resembling
that seen in human USH2A patients.
Upon completion of the hearing tests, cochleas were processed
for morphological examination. Scanning electron microscopy of
the Ush2a?/?mice revealed normal hair-cell populations and
graphs from an Ush2a?/?retina at the age of 20 months and an age-matched
WT control are shown . The photoreceptor cell layer in the mutant was much
thinner than that of the control. (b) Representative ERG tracings from a
ERG data from a cohort of mutant and control mice are summarized (Right).
Both the a- and b-wave amplitudes from the Ush2a?/?mutant (n ? 8) were
reduced significantly compared with the control (n ? 9) (P ? 0.001). OS, outer
segment; IS, inner segment; ONL, outer nuclear layer; INL, inner nuclear layer;
GC, retinal ganglion cells. (c) Immunofluorescence for cone opsins and GFAP
in mutant and control mice. Cone opsin antibodies (mixed blue and green
retina but highlighted cell bodies and little outer segments in the mutant (at
20 months; Left). GFAP was up-regulated in the mutant (at 2 months; Right).
Immunostaining signals are shown in orange. Cell nuclei are counterstained
blue with Hoechst dye 33342.
Retinal degeneration in the Ush2a?/?mutant. (a) Light photomicro-
DPOAE Thresholds (dB SPL)
Usherin in the cochlear
4 months7 months
f2 Frequency (KHz)
6 8 10
f2 Frequency (KHz)
cochlea by immunofluorescence. Usherin staining in the WT (at postnatal day 1)
rows of outer hair cells (OHC1, OHC2, and OHC3). Usherin was absent in the
mutant. (b) Mean cochlear thresholds (?SEM) measured at 4 and 7 months by
measurement of DPOAEs are shown. In each group, 20–24 ears (both ears from
statistically significant (P ? 0.001) for high-frequency sounds. Little difference
by scanning electron microscopy. Basal and apical turns are shown. Many outer
hair cells in the basal portion of the Ush2a?/?inner ear were missing, whereas
those in the middle turn appeared normal, consistent with the observation that
cochlear threshold shifts were more severe for high frequencies than for low
frequencies. Inner hair cells were present throughout the cochlear spiral.
The role of usherin in the inner ear. (a) Localization of usherin in the
www.pnas.org?cgi?doi?10.1073?pnas.0610950104 Liu et al.
normal-appearing stereocilia bundles on both inner and outer hair
cells throughout the apical half of the cochlea. In the basal turn,
cells and their stereociliary bundles appeared normal (Fig. 5c).
In this study, we generated and analyzed a mouse model in which
all known variants of usherin were ablated. Usherin-null mice
developed a spectrum of retinal and hearing defects closely resem-
bling those of USH2A human patients, which include progressive
no genetic mouse model of Usher syndrome has been shown to
develop overt photoreceptor degeneration. At least six mouse
models with mutations in Usher syndrome genes, both engineered
USH1B (myosin VIIA) (35–37), USH1C (harmonin) (38), USH1D
(sans) (40), and USH2C (very large G protein-coupled receptor 1)
(41). Hearing defects in these models resembled those of the
respective human conditions. However, none of these mouse mod-
els exhibited photoreceptor cell loss (42). The reason for the lack
of photoreceptor degeneration in genetic murine models of Usher
in murine Usher genes or a very slow disease progression in the
retina that makes it difficult to characterize. A recent study on
mouse Pcdh15 mutants shows that most of the mutant alleles affect
exons that are alternatively spliced, suggesting removal of the
affected exons could preserve some functional isoform of the gene
product in the retina (43). It is not clear to what extent alternative
murine models of Usher syndrome. The Ush2a?/?mutant de-
scribed here demonstrates overt photoreceptor degeneration in a
mouse model of Usher syndrome. Photoreceptor degeneration in
this model is slowly progressive, similar to RP in human patients.
These data establish the Ush2a?/?mutant mouse as a valid model
for Usher syndrome in terms of disease phenotype.
Two variants of usherin have been described in the literature
based on genetic and biochemical studies (5, 13, 15). The short
variant is predicted to be a secreted, extracellular protein, whereas
the long variant is anchored on the cell membrane with a large
extracellular domain and a short C-terminal PDZ-binding motif.
Our study finds that the long variant of usherin, an exceptionally
large protein of ?600 kDa, is the predominant form in the retina.
We further show that usherin localizes to a distinct region in the
apical inner segment that forms a semiencircled recess and wraps
around the connecting cilium. In frog photoreceptors, the plasma
ridges visible by scanning EM and is termed the periciliary ridge
with the nine microtubule arrangements of the ciliary axoneme.
Filaments were seen to bridge the top edges of the ridges. It is
ridge complex is a docking site for transport of proteins destined to
the outer segments (44). In mammalian photoreceptors, a pericili-
ary ridge complex is not apparent, perhaps because of the smaller
size of the cell. It can be assumed, however, that a functional
equivalent may exist. The spatially restricted distribution of usherin
at this patch of plasma membrane marks it as a specialized
microdomain and supports the view that a functional equivalent
periciliary ridge complex does exist in mammals. Usherin is likely
to be tethered via its C-terminal PDZ-binding motif to this mem-
brane microdomain through protein–protein interaction with PDZ
domain proteins (16–18) (see Fig. 3c) (unpublished observations).
cilium are ?0.1 ?m apart. Because of the large size of its extra-
membrane protrusions and/or with the surface of the connecting
cilium. These interactions likely form the basis of usherin function
in photoreceptor cells. Given that the initial development of
Ush2a?/?retinas apparently is normal and that the photoreceptor
phenotype is slowly progressive, we propose that usherin function
is required in the long-term maintenance of photoreceptor cells.
Usherin also is localized to the stereocilia of cochlear hair cells
but only in developing animals. In this study, we detected usherin
expression in mouse pups at postnatal days 1–5 but not in young
adults. Others have reported that usherin disappears from the
cochlear hair cells around postnatal day 15 (16). This transient
expression pattern of usherin is correlated temporally with the
maturation of stereocilia and the disappearance of the kinocilia
usherin mutant mice is nonprogressive. Both the transient
expression and the nonprogressive phenotype are consistent
with usherin’s having a role in the postnatal maturation of
cochlear hair cells. Kinocilium in a hair cell is a true cilium and
is the analogous structure to photoreceptor connecting cilium.
Stereocilia, on the other hand, are not true cilia but are actin
filament-based membrane protrusions. To some extent, they
resemble the membranous protrusions constituting the pericili-
ary ridge complex in photoreceptors. Kinocilia provide an
important positional cue for the rotational alignment of stereo-
ciliary bundles. Once maturation of stereocilia is complete, the
that usherin participates in the interactions between the kino-
cilium and stereocilia and among stereocilia themselves during
the period of dynamic changes in membrane topology on the
hair-cell apical surface. The proposal that usherin is a compo-
nent of the ankle links (16), which also appear transiently during
postnatal development, is consistent with this view. As the
kinocilium disappears, a functional requirement for usherin
the hearing loss is moderate and confined to the higher fre-
quency range and hair-cell degeneration is apparent only in the
basal turn of the cochlea. In this regard, it is similar to VLGR1,
another component of the ankle links (41). The loss of VLGR1
also leads to a hearing loss more pronounced in the higher-
frequency range and disruption of cochlear hair cells confined to
the basal half (41).
recently have proposed that usherin localizes and functions at the
photoreceptor and hair-cell synapses, in conjunction with other
Usher proteins (4, 17, 18). Our present study finds no evidence for
the presence of usherin at the photoreceptor synapse or a defective
synaptic transmission from photoreceptors to second-order retinal
usherin at the ribbon synapses. Usherin localizes to homologous
membrane microdomains (stereocilia in hair cells and the mam-
malian equivalent of a periciliary ridge complex in photoreceptors)
and putatively mediates the interaction between those structures
and a true cilium. These interactions appear to fulfill important
structural and signaling roles.
Materials and Methods
Generation of the Ush2a Knockout Mice.ThemurineUsh2agenewas
homozygotes we have generated (Ush2a?/?) were homozygous for
a cadherin 23 hypomorphic allele (Cdh23753A) from their ancestral
strains (C57BL/6 and 129 Sv). Because this hypomorphic allele
would interfere with the hearing evaluation (34), the Ush2a mutant
mice were crossed with CBA/CaJ, a strain with ‘‘gold standard’’
normal hearing (46), to generate Ush2a WT and knockout mice
homozygous for the WT allele of Cdh23 (Cdh23753G). For detailed
amplification, see SI Fig. 6 and SI Methods.
Liu et al. PNAS ?
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Light and Electron Microscopy. Tissue processing and embed- Download full-text
ding into Epon resin for light microscopy were performed as
described in ref. 29. ImmunoEM was performed with a preem-
bedding method that used gold-conjugated secondary antibodies
and silver enhancement. For detailed methods on immunoEM
and scanning EM of the cochleas, see SI Methods.
Functional Tests for the Retina and Cochlea. ERG was performed to
assess retinal function as described in ref. 47. DPOAE measure-
ments were performed for evaluation of cochlear function as
described in ref. 48.
1.2% agarose gels for separation because of its exceptionally large
region, generation of polyclonal usherin antibodies, and immuno-
fluorescence were performed following previously described pro-
tocols (33). The chicken ?-Usherin-N antibody and the rabbit
?-Usherin-C antibody were used for this study. For detailed meth-
ods, see SI Methods.
We thank Norman Michaud and Akella Sreedevi for histology and
confocal microscopy support; Leslie Liberman for dissection of inner ear
cochleas; Scott Adams for DNA sequencing; Xun Sun for GFAP
immunostaining; Eric Pierce for RP1 antibody; and Drs. Eliot Berson,
Thaddeus Dryja, and Michael Sandberg for helpful discussions. This
work was supported by National Institutes of Health Grants R01
EY10309, R01 DC00188, and P30 DC05209; the Foundation Fighting
Blindness; and National Institutes of Health Core Grant for Vision
Research P30 EY14104.
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