Identification and cloning of site C splice variants of plasma membrane Ca-ATPase in the gerbil cochlea

PMCA2 pump mutations and hereditary deafness

Hair cells of the inner ear detect sound stimuli, inertial or gravitational forces by deflection of their apical stereocilia. A small number of stereociliary cation-selective mechanotransduction (MET) channels admit K⁺ and Ca²⁺ ions into the cytoplasm promoting hair cell membrane depolarization and, consequently, neurotransmitter release at the cell basolateral pole. Ca²⁺ influx into the stereocilia compartment is counteracted by the unusual w/a splicing variant of plasma-membrane calcium-pump isoform 2 (PMCA2) which, unlike other PMCA2 variants, increases only marginally its activity in response to a rapid variation of the cytoplasmic free Ca²⁺ concentration ([Ca²⁺]_c). Missense mutations of PMCA2w/a cause deafness and loss of balance in humans. Mouse models in which the pump is genetically ablated or mutated show hearing and balance impairment, which correlates with defects in homeostatic regulation of stereociliary [Ca²⁺]_c, decreased sensitivity of mechanotransduction channels to hair bundle displacement and progressive degeneration of the organ of Corti. These results highlight a critical role played by the PMCA2w/a pump in the control of hair cell function and survival, and provide mechanistic insight into the etiology of deafness and vestibular disorders.

Mechanisms of otoconia and otolith development

BACKGROUND

Otoconia are bio-crystals that couple mechanic forces to the sensory hair cells in the utricle and saccule, a process essential for us to sense linear acceleration and gravity for the purpose of maintaining bodily balance. In fish, structurally similar bio-crystals called otoliths mediate both balance and hearing. Otoconia abnormalities are common and can cause vertigo and imbalance in humans. However, the molecular etiology of these illnesses is unknown, as investigators have only begun to identify genes important for otoconia formation in recent years.

RESULTS

To date, in-depth studies of selected mouse otoconial proteins have been performed, and about 75 zebrafish genes have been identified to be important for otolith development.

CONCLUSIONS

This review will summarize recent findings as well as compare otoconia and otolith development. It will provide an updated brief review of otoconial proteins along with an overview of the cells and cellular processes involved. While continued efforts are needed to thoroughly understand the molecular mechanisms underlying otoconia and otolith development, it is clear that the process involves a series of temporally and spatially specific events that are tightly coordinated by numerous proteins. Such knowledge will serve as the foundation to uncover the molecular causes of human otoconia-related disorders.

Mechanisms of otoconia and otolith development

BACKGROUND

Otoconia are bio-crystals that couple mechanic forces to the sensory hair cells in the utricle and saccule, a process essential for us to sense linear acceleration and gravity for the purpose of maintaining bodily balance. In fish, structurally similar bio-crystals called otoliths mediate both balance and hearing. Otoconia abnormalities are common and can cause vertigo and imbalance in humans. However, the molecular etiology of these illnesses is unknown, as investigators have only begun to identify genes important for otoconia formation in recent years.

RESULTS

To date, in-depth studies of selected mouse otoconial proteins have been performed, and about 75 zebrafish genes have been identified to be important for otolith development.

CONCLUSIONS

This review will summarize recent findings as well as compare otoconia and otolith development. It will provide an updated brief review of otoconial proteins along with an overview of the cells and cellular processes involved. While continued efforts are needed to thoroughly understand the molecular mechanisms underlying otoconia and otolith development, it is clear that the process involves a series of temporally and spatially specific events that are tightly coordinated by numerous proteins. Such knowledge will serve as the foundation to uncover the molecular causes of human otoconia-related disorders.

The expression of plasma membrane Ca(2+)-ATPase isoform 2 and its splice variants at sites A and C in the neonatal rat cochlea

OBJECTIVE

To study the expression of plasma membrane Ca(2+)-ATPase isoform 2 (PMCA2) and its alternative splicing at sites A (the first intracellular loop) and C (the C-terminal region) in the neonatal rat cochlea.

METHODS

The cochleae from rats postnatal day 3 to postnatal day 4 (P3-P4) were dissected, fixed, embedded, and sectioned. Meanwhile, the cochlear coils from neonatal rats were isolated and fixed. Using immunofluorescence staining, the expression of PMCA2 was respectively examined in the cochlear sections and cochlear coils. In addition, the total RNAs of basilar membrane (BM, including the organ of corti, the same below), spiral ganglion (SG), spiral ligament (SL, including SV, the same below), and the whole cochlea from neonatal rats were respectively extracted and reverse transcribed to cDNAs, then subjected to primers flanking site A or C in the PMCA2 gene using reverse transcription polymerase chain reaction (RT-PCR). Western blot was also applied to detect the expression of PMCA2 isoforms in the cochlear tissues.

RESULTS

We found that PMCA2 is strongly expressed in outer hair cell (OHC) bundles, SG, and stria vascularis (SV), weakly expressed in Reissner's membrane (RM), and occasionally expressed in inner hair cell (IHC) bundles. Moreover, w/a is the major splice form of PMCA2 present in hair cell bundles, z/b and z/c are the major splice forms of PMCA2 present in SG, and w/a and w/c are the major splice forms of PMCA2 present in SV. In the whole cochlea, variants w, y, and z were detected at site A, and variants a, b, and c were detected at site C. Using Western blot, variant a or b was also detectable in the same cochlear tissues mentioned above.

CONCLUSIONS

PMCA2 and its splice variants at sites A and C are differentially expressed in cochlear tissues of neonatal rat.

The novel mouse mutation Oblivion inactivates the PMCA2 pump and causes progressive hearing loss

Progressive hearing loss is common in the human population, but we have few clues to the molecular basis. Mouse mutants with progressive hearing loss offer valuable insights, and ENU (N-ethyl-N-nitrosourea) mutagenesis is a useful way of generating models. We have characterised a new ENU-induced mouse mutant, Oblivion (allele symbol Obl), showing semi-dominant inheritance of hearing impairment. Obl/+ mutants showed increasing hearing impairment from post-natal day (P)20 to P90, and loss of auditory function was followed by a corresponding base to apex progression of hair cell degeneration. Obl/Obl mutants were small, showed severe vestibular dysfunction by 2 weeks of age, and were completely deaf from birth; sensory hair cells were completely degenerate in the basal turn of the cochlea, although hair cells appeared normal in the apex. We mapped the mutation to Chromosome 6. Mutation analysis of Atp2b2 showed a missense mutation (2630C-->T) in exon 15, causing a serine to phenylalanine substitution (S877F) in transmembrane domain 6 of the PMCA2 pump, the resident Ca(2+) pump of hair cell stereocilia. Transmembrane domain mutations in these pumps generally are believed to be incompatible with normal targeting of the protein to the plasma membrane. However, analyses of hair cells in cultured utricular maculae of Obl/Obl mice and of the mutant Obl pump in model cells showed that the protein was correctly targeted to the plasma membrane. Biochemical and biophysical characterisation showed that the pump had lost a significant portion of its non-stimulated Ca(2+) exporting ability. These findings can explain the progressive loss of auditory function, and indicate the limits in our ability to predict mechanism from sequence alone.

Plasma membrane Ca2+-ATPase isoforms in frog crista ampullaris: identification of PMCA1 and PMCA2 specific splice variants

Ca2+ ions play a pivotal role in inner ear hair cells as they are involved from the mechano-electrical transduction to the transmitter release. Most of the Ca2+ that enters into hair cells via mechano-transduction and voltage-gated channels is extruded by the plasma membrane Ca2+-ATPases (PMCAs) that operate in both apical and basal cellular compartments. Here, we determined the identity and distribution of PMCA isoforms in frog crista ampullaris: we showed that PMCA1, PMCA2 and PMCA3 are expressed, while PMCA4 appears to be negligible. We also identify PMCA1bx, PMCA2av and PMCA2bv as the major splice variants produced from PMCA1 and PMCA2 genes. PMCA2av appears to be the major Ca2+-pump operating at the apical pole of the cell, even if PMCA1b is also expressed in the stereocilia. PMCA1bx is, instead, the principal PMCA of hair cell basolateral compartment, where it is expressed together with PMCA2 (probably PMCA2bv) and PMCA3. Frog crista ampullaris hair cells lack a Na/Ca exchanger, therefore PMCAs are the only mechanism of Ca2+ extrusion. The coexpression of specific isozymes in the different cellular compartments responds to the need of a fine regulation of both basal and dynamic Ca2+ levels at the apical and basal pole of the cell.

A functional study of plasma-membrane calcium-pump isoform 2 mutants causing digenic deafness

Ca2+ enters the stereocilia of hair cells through mechanoelectrical transduction channels opened by the deflection of the hair bundle and is exported back to endolymph by an unusual splicing isoform (w/a) of plasma-membrane calcium-pump isoform 2 (PMCA2). Ablation or missense mutations of the pump cause deafness, as described for the G283S mutation in the deafwaddler (dfw) mouse. A deafness-inducing missense mutation of PMCA2 (G293S) has been identified in a human family. The family also was screened for mutations in cadherin 23, which accentuated hearing loss in a previously described human family with a PMCA2 mutation. A T1999S substitution was detected in the cadherin 23 gene of the healthy father and affected son but not in that of the unaffected mother, who presented instead the PMCA2 mutation. The w/a isoform was overexpressed in CHO cells. At variance with the other PMCA2 isoforms, it became activated only marginally when exposed to a Ca2+ pulse. The G293S and G283S mutations delayed the dissipation of Ca2+ transients induced in CHO cells by InsP3. In organotypic cultures, Ca2+ imaging of vestibular hair cells showed that the dissipation of stereociliary Ca2+ transients induced by Ca2+ uncaging was compromised in the dfw and PMCA2 knockout mice, as was the sensitivity of the mechanoelectrical transduction channels to hair bundle displacement in cochlear hair cells.

Splice-site A choice targets plasma-membrane Ca2+-ATPase isoform 2 to hair bundles

Localization of mechanotransduction in sensory hair cells to hair bundles requires selective targeting of essential proteins to specific locations. Isoform 2 of the plasma-membrane Ca2+-ATPase (PMCA2), required for hearing and balance, is found exclusively in hair bundles. We determined the contribution of splicing at the two major splicing sites (A and C) to hair-cell targeting of PMCA2. When PMCA2 isoforms were immunoprecipitated from purified hair bundles of rat utricle, 2w was the only site A variant detected; moreover, immunocytochemistry for 2w in rat vestibular and cochlear tissues indicated that this splice form was located solely in bundles. To demonstrate the necessity of the 2w sequence, we transfected hair cells with PMCA2 containing different variants at splice sites A and C. Although native hair bundles exclusively use the 2a form at splice-site C, epitope-tagged PMCA2w/a and PMCA2w/b were both concentrated in bundles, indicating that site C is not involved in bundle targeting. In contrast, PMCA2z/a was excluded from bundles and was instead targeted to the basolateral plasma membrane. Bundle-specific targeting of PMCA2w/a tagged with green fluorescent protein (GFP) was diminished, suggesting that GFP interfered with splice-site A. Together, these data demonstrate that PMCA2w/a is the hair-bundle isoform of PMCA in rat hair cells and that 2w targets PMCA2 to bundles. The 2w sequence is thus the first targeting signal identified for a hair-bundle membrane protein; moreover, the striking distribution of inner-ear PMCA isoforms dictated by selective targeting suggests a critical functional role for segregated pathways of Ca2+ transport.

Assembly of the otoconia complex to the macular sensory epithelium of the vestibule

In the inner ear, specificity of stimulus perception is achieved by associating the sensory epithelia of the three mechanoreceptor organs, the utricle/saccule, cristae, and cochlea, with distinct types of acellular matrices. Only the utricle and saccule have an extremely dense matrix, the otoconial complex, which overlies the sensory epithelium (macula) and provides inertial mass to generate shearing forces essential for the mechanoreceptors to sense gravity and linear acceleration. Such sensation is necessary for spatial orientation and balance. The importance of otoconia is clearly demonstrated by the impact of balance disorders upon the elderly population that involve otoconia degeneration, as well as by canalithiasis and cupulolithiasis, in which otoconia are dislocated. This underscores the need to understand how otoconia are formed and maintained and how to prevent their degeneration. To date, a number of otoconia-related proteins have been identified mostly in mice and bony fish. Although most of these proteins are also present in other structures of the inner ear, a distinct collection of proteins in the macula plus the unique ionic microenvironment of the endolymph near its epithelium likely contribute to the site-specific calcification of otoconia. Based on the current literature and ongoing research, this mini-review postulates a working model of how the otoconia complex is assembled specifically above the macular sensory epithelium of the vestibule. The central hypothesis of this model is that proteins are critical in sequestering calcium for crystallization in the calcium-poor endolymph. The review also sets forth some issues that need to be resolved in the future.

Low endolymph calcium concentrations in deafwaddler2J mice suggest that PMCA2 contributes to endolymph calcium maintenance

In vertebrates, transduction of sound into an electrochemical signal is carried out by hair cells that rely on calcium to perform specialized functions. The apical surfaces of hair cells are surrounded by endolymphatic fluid containing calcium at concentrations that must be maintained by active transport. The mechanism of this transport is unknown, but an ATP-dependent pump is believed to participate. Mutation of the Atp2b2 gene that encodes plasma membrane calcium ATPase type 2 (PMCA2) produces the deaf, ataxic mouse: deafwaddler2J (dfw2J). We hypothesized that PMCA2 might transport calcium into the endolymph and that dfw2J mice would have low endolymph calcium concentrations, possibly contributing to their deafness and ataxia. First, using immunocytochemistry, we demonstrated that PMCA2 is present in control mice inner and outer hair cell stereocilia where it could pump calcium into the endolymph and that PMCA2 is absent in dfw2J stereocilia. Second, using an aspirating microelectrode and calcium-sensitive fluorescent dye, we found that dfw2J mice endolymph calcium concentrations are significantly lower than those of control mice. These findings suggest that PMCA2, located in hair cell stereocilia, contributes significantly to endolymph calcium maintenance.

Plasma membrane Ca2+-ATPase isoform 2a is the PMCA of hair bundles

Mechanoelectrical transduction channels of hair cells allow for the entry of appreciable amounts of Ca(2+), which regulates adaptation and triggers the mechanical activity of hair bundles. Most Ca(2+) that enters transduction channels is extruded by the plasma membrane Ca(2+)-ATPase (PMCA), a Ca(2+) pump that is highly concentrated in hair bundles and may be essential for normal hair cell function. Because PMCA isozymes and splice forms are regulated differentially and have distinct biochemical properties, we determined the identity of hair bundle PMCA in frog and rat hair cells. By screening a bullfrog saccular cDNA library, we identified abundant PMCA1b and PMCA2a clones as well as rare PMCA2b and PMCA2c clones. Using immunocytochemistry and immunoprecipitation experiments, we showed in bullfrog sacculus that PMCA1b is the major isozyme of hair cell and supporting cell basolateral membranes and that PMCA2a is the only PMCA present in hair bundles. This complete segregation of PMCA1 and PMCA2 isozymes holds for rat auditory and vestibular hair cells; PMCA2a is the only PMCA isoform in hair bundles of outer hair cells and vestibular hair cells and is the predominant PMCA of hair bundles of inner hair cells. Our data suggest that hair cells control plasma membrane Ca(2+)-pumping activity by targeting specific PMCA isozymes to distinct subcellular locations. Because PMCA2a is the only Ca(2+) pump present at appreciable levels in hair bundles, the biochemical properties of this pump must account fully for the physiological features of transmembrane Ca(2+) pumping in bundles.

Role of alternative splicing in generating isoform diversity among plasma membrane calcium pumps

Calcium pumps of the plasma membrane (also known as plasma membrane Ca(2+)-ATPases or PMCAs) are responsible for the expulsion of Ca(2+) from the cytosol of all eukaryotic cells. Together with Na(+)/Ca(2+) exchangers, they are the major plasma membrane transport system responsible for the long-term regulation of the resting intracellular Ca(2+) concentration. Like the Ca(2+) pumps of the sarco/endoplasmic reticulum (SERCAs), which pump Ca(2+) from the cytosol into the endoplasmic reticulum, the PMCAs belong to the family of P-type primary ion transport ATPases characterized by the formation of an aspartyl phosphate intermediate during the reaction cycle. Mammalian PMCAs are encoded by four separate genes, and additional isoform variants are generated via alternative RNA splicing of the primary gene transcripts. The expression of different PMCA isoforms and splice variants is regulated in a developmental, tissue- and cell type-specific manner, suggesting that these pumps are functionally adapted to the physiological needs of particular cells and tissues. PMCAs 1 and 4 are found in virtually all tissues in the adult, whereas PMCAs 2 and 3 are primarily expressed in excitable cells of the nervous system and muscles. During mouse embryonic development, PMCA1 is ubiquitously detected from the earliest time points, and all isoforms show spatially overlapping but distinct expression patterns with dynamic temporal changes occurring during late fetal development. Alternative splicing affects two major locations in the plasma membrane Ca(2+) pump protein: the first intracellular loop and the COOH-terminal tail. These two regions correspond to major regulatory domains of the pumps. In the first cytosolic loop, the affected region is embedded between a putative G protein binding sequence and the site of phospholipid sensitivity, and in the COOH-terminal tail, splicing affects pump regulation by calmodulin, phosphorylation, and differential interaction with PDZ domain-containing anchoring and signaling proteins. Recent evidence demonstrating differential distribution, dynamic regulation of expression, and major functional differences between alternative splice variants suggests that these transporters play a more dynamic role than hitherto assumed in the spatial and temporal control of Ca(2+) signaling. The identification of mice carrying PMCA mutations that lead to diseases such as hearing loss and ataxia, as well as the corresponding phenotypes of genetically engineered PMCA "knockout" mice further support the concept of specific, nonredundant roles for each Ca(2+) pump isoform in cellular Ca(2+) regulation.

Presence of plasma membrane-bound Ca(2+)-ATPase in the secretory epithelia of the inner ear

The Ca2+ concentration of the endolymph is low, around 0.023 mM. Yet, because of the positive endocohlear potential, Ca2+ must be actively transported into the endolymphatic space. The mechanisms responsible for the active Ca2+ transport into the endolymph are not known. In this study, the presence of plasma membrane-bound Ca(2+)-ATPase (PMCA ATPase) in the endolymph-producing, secretory epithelia of the inner ear from guinea pig was investigated with immunoblotting and immunohistochemistry. The antibody used was a monoclonal antibody which recognizes an epitope shared by all four known isoforms of PMCA ATPase. With immunoblotting, a band corresponding to PMCA ATPase was found in the stria vascularis, the ampullary tissue, the utricle and the endolymphatic sac in assays from at least three different batches of tissue. With immunohistochemistry, a strong positive staining reaction for PMCA ATPase could be seen in the stria vascularis and the dark cells of the ampullary tissue and the utricle. The epithelial cells in the endolymphatic sac showed a moderate positive staining reaction. Accordingly, in this study the presence of PMCA ATPase was shown in all the endolymph-producing, secretory epithelia of the inner ear. These results indicate that PMCA ATPase plays a role in the regulation of the Ca2+ concentration in the endolymph.

The plasma membrane calcium pump, its role and regulation: new complexities and possibilities

Significant progress has been achieved in elucidating the role of the plasma membrane Ca2(+)-ATPase in cellular Ca2+ homeostasis and physiology since the enzyme was first purified and physiology since the enzyme was first purified and cloned a number of years ago. The simple notion that the PM Ca2(+)-ATPase controls resting levels of [Ca2+]CYT has been challenged by the complexity arising from the finding of four major isoforms and splice variants of the Ca2+ pump, and the finding that these are differentially localized in various organs and subcellular regions. Furthermore, the isoforms exhibit differential sensitivities to Ca2+, calmodulin, ATP, and kinase-mediated phosphorylation. The latter pathways of regulation can give rise to activation or inhibition of the Ca2+ pump activity, depending on the kinase and the particular Ca2+ pump isoform. Significant progress is being made in elucidating subtle and more profound roles of the PM Ca2(+)-ATPase in the control of cellular function. Further understanding of these roles awaits new studies in both transfected cells and intact organelles, a process that will be greatly aided by the development of new and selective Ca2+ pump inhibitors.

Evidence for differential regulation of calcium by outer versus inner hair cells: plasma membrane Ca-ATPase gene expression

The expression of mRNA encoding plasma membrane calcium ATPase (PMCA) subunit isoforms (1-4) and splice variants was examined in the adult and developing rat cochlea by PCR and in situ hybridization. High levels of PMCA mRNA expression were observed in the neurons of the spiral ganglion, and in hair cells. Spiral ganglion neurons expressed PMCA 1-3 beginning in embryonic development, reaching high levels shortly after birth, and continuing into adulthood. Inner hair cells expressed PMCA 1 at moderate levels from birth to the time of onset of cochlear function on postnatal day 12, and strongly from then until adulthood. Outer hair cells expressed PMCA 2 at high levels from shortly after birth through adulthood. The data suggest that the calcium clearance requirements of inner and outer hair cells are distinct. PMCA 2 is the isoform with the highest affinity for calmodulin, and has also been associated with high levels of inositol triphosphate. Its presence in outer hair cells suggests that regulation of the enzyme by calmodulin may be particularly important for this hair cell type. It further suggests that inositol phosphate may play a unique role in the outer hair cell.

Mutations in a plasma membrane Ca2+-ATPase gene cause deafness in deafwaddler mice

Hearing loss is the most common sensory deficit in humans. Because the auditory systems of mice and humans are conserved, studies on mouse models have predicted several human deafness genes and identified new genes involved in hearing. The deafwaddler (dfw) mouse mutant is deaf and displays vestibular/motor imbalance. Here we report that the gene encoding a plasma membrane Ca2+-ATPase type 2 pump (Atp2b2, also known as Pmca2) is mutated in dfw. An A-->G nucleotide transition in dfw DNA causes a glycine-to-serine substitution at a highly conserved amino-acid position, whereas in a second allele, dfw2J, a 2-base-pair deletion causes a frameshift that predicts a truncated protein. In the cochlea, the protein Atp2b2 is localized to stereocilia and the basolateral wall of hair cells in wild-type mice, but is not detected in dfw2J mice. This indicates that mutation of Atp2b2 may cause deafness and imbalance by affecting sensory transduction in stereocilia as well as neurotransmitter release from the basolateral membrane. These mutations affecting Atp2b2 in dfw and dfw2J are the first to be found in a mammalian plasma membrane calcium pump and define a new class of deafness genes that directly affect hair-cell physiology.

Plasma membrane Ca2+-ATPase extrudes Ca2+ from hair cell stereocilia

Mechanically sensitive hair cells of the auditory and vestibular systems use Ca2+ to control adaptation of mechanical transduction, to effect frequency tuning, to trigger neurotransmitter release, and to mediate efferent synaptic signaling. To determine the role that pumps play in regulation of Ca2+ in the hair bundle, the organelle responsible for mechanoelectrical transduction, we localized and quantified the plasma membrane Ca2+-ATPase (PMCA) of the bundle. We found that each hair bundle contains approximately 10(6) PMCA molecules or approximately 2000 per square micrometer of bundle membrane and that PMCA is the principal calmodulin binding protein of the bundle. Consistent with biochemical estimates of PMCA density, we measured with extracellular Ca2+-selective electrodes a substantial Ca2+ efflux from bundles. The number of bundle Ca2+ pumps and magnitude of resting Ca2+ efflux suggested that PMCA should generate a substantial membrane current as bundles expel Ca2+. Measurement of whole-cell currents revealed a transduction-dependent outward current that was consistent with the activity of PMCA. Finally, dialysis of hair cells with PMCA inhibitors led to a large increase in the concentration of Ca2+ in bundles, which suggests that PMCA plays a major role in regulating bundle Ca2+ concentration. Our data further indicate that PMCA could elevate the extracellular Ca2+ concentration close to hair bundles above the low level found in bulk endolymph.

Immunohistochemical localization of the Na-K-Cl co-transporter (NKCC1) in the gerbil inner ear

We mapped the cellular and subcellular distribution of the Na-K-Cl co-transporter (NKCC) in the adult gerbil inner ear by immunostaining with a monoclonal antibody (MAb T4) generated against human colon NKCC. Heavy immunolabeling was seen in the basolateral plasma membrane of marginal cells in the stria vascularis and dark cells in the vestibular system. Subpopulations of fibrocytes in the cochlear spiral ligament and limbus and underlying the vestibular neurosensory epithelium also stained with moderate to strong intensity, apparently along their entire plasmalemma. Because MAb T4 recognizes both the basolateral secretory (NKCC1) and the apical absorptive (NKCC2) isoforms of the co-transporter, we employed reverse transcription and the polymerase chain reaction (RT-PCR) to explore isoform diversity in inner ear tissues. Using NKCC1 and NKCC2 isoform-specific PCR primers based on mouse and human sequences, only transcripts for NKCC1 were detected in the gerbil inner ear. The presence of abundant NKCC1 in the basolateral plasmalemma of strial marginal and vestibular dark cells confirms conclusions drawn from pharmacological and physiological data. The co-expression of NKCC1 and Na,K-ATPase in highly specialized subpopulations of cochlear and vestibular fibrocytes provides further evidence for their role in recycling K+ leaked or effluxed through hair cells into perilymph back to endolymph, as postulated in current models of inner ear ion homeostasis.