Tonotopy in calcium homeostasis and vulnerability of cochlear hair cells.

Ototoxicity, noise overstimulation, or aging, can all produce hearing loss with similar properties, in which outer hair cells (OHCs), principally those at the high-frequency base of the cochlea, are preferentially affected. We suggest that the differential vulnerability may partly arise from differences in Ca2+ balance among cochlear locations. Homeostasis is determined by three factors: Ca2+ influx mainly via mechanotransducer (MET) channels; buffering by calcium-binding proteins and organelles like mitochondria; and extrusion by the plasma membrane CaATPase pump. We review quantification of these parameters and use our experimentally-determined values to model changes in cytoplasmic and mitochondrial Ca2+ during Ca2+ influx through the MET channels. We suggest that, in OHCs, there are two distinct micro-compartments for Ca2+ handling, one in the hair bundle and the other in the cell soma. One conclusion of the modeling is that there is a tonotopic gradient in the ability of OHCs to handle the Ca2+ load, which correlates with their vulnerability to environmental challenges. High-frequency basal OHCs are the most susceptible because they have much larger MET currents and have smaller dimensions than low-frequency apical OHCs.

Reduced secretion of parathyroid hormone and hypocalcemia in systemic heterozygous ATP2B1-null hypertensive mice.

The ATP2B1 gene is associated with hypertension. We previously reported that systemic heterozygous ATP2B1-null (ATP2B1+/-) mice exhibited hypertension due to impaired endothelial nitric oxide synthase (eNOS) activity and decreased nitric oxide (NO) production. The ATP2B1 gene encodes plasma membrane calcium ATPase 1 (PMCA1), which has been thought to regulate only intracellular Ca2+ concentration. However, recently, it has been suggested that ATP2B1 works not only at cellular levels, but also throughout the entire body, including in the calcium metabolism, using small intestine-specific ATP2B1 knockout mice. To clarify the roles of ATP2B1 in the entire body and the effects of ATP2B1 on blood pressure, we examined the alterations of calcium related factors in ATP2B1+/- mice. ATP2B1+/- mice exhibited hypocalcemia. The expression of ATP2B1 in the kidney and small intestine decreased, and hypercalciuria was confirmed in ATP2B1+/- mice. The intact-PTH levels were lower, and bone mineral density was increased in these mice. These results suggest that hypocalcemia is mainly a result of inhibited bone resorption without compensation by PTH secretion in the case of ATP2B1 knockout. Moreover, NO production may be affected by reduced PTH secretion, which may cause the increase in vascular contractility in these mice. The ATP2B1 gene is important for not only intra-cellular calcium regulation but also for calcium homeostasis and blood pressure control.

The role of Plasma Membrane Calcium ATPases (PMCAs) in neurodegenerative disorders.

Selective degeneration of differentiated neurons in the brain is the unifying feature of neurodegenerative disorders such as Parkinson's disease (PD) or Alzheimer's disease (AD). A broad spectrum of evidence indicates that initially subtle, but temporally early calcium dysregulation may be central to the selective neuronal vulnerability observed in these slowly progressing, chronic disorders. Moreover, it has long been evident that excitotoxicity and its major toxic effector mechanism, neuronal calcium overload, play a decisive role in the propagation of secondary neuronal death after acute brain injury from trauma or ischemia. Under physiological conditions, neuronal calcium homeostasis is maintained by a fine-tuned interplay between calcium influx and releasing mechanisms (Ca2+-channels), and calcium efflux mechanisms (Ca2+-pumps and -exchangers). Central functional components of the calcium efflux machinery are the Plasma Membrane Calcium ATPases (PMCAs), which represent high-affinity calcium pumps responsible for the ATP-dependent removal of calcium out of the cytosol. Beyond a growing body of experimental evidence, it is their high expression level, their independence of secondary ions or membrane potential, their profound redox regulation and autoregulation, their postsynaptic localization in close proximity to the primary mediators of pathological calcium influx, i.e. NMDA receptors, as well as evolutionary considerations which all suggest a pivotal role of the PMCAs in the etiology of neurodegeneration and make them equally challenging and alluring candidates for drug development. This review aims to summarize the recent literature on the role of PMCAs in the pathogenesis of neurodegenerative disorders.

A complex of Neuroplastin and Plasma Membrane Ca2+ ATPase controls T cell activation.

The outcome of T cell activation is determined by mechanisms that balance Ca2+ influx and clearance. Here we report that murine CD4 T cells lacking Neuroplastin (Nptn -/-), an immunoglobulin superfamily protein, display elevated cytosolic Ca2+ and impaired post-stimulation Ca2+ clearance, along with increased nuclear levels of NFAT transcription factor and enhanced T cell receptor-induced cytokine production. On the molecular level, we identified plasma membrane Ca2+ ATPases (PMCAs) as the main interaction partners of Neuroplastin. PMCA levels were reduced by over 70% in Nptn -/- T cells, suggesting an explanation for altered Ca2+ handling. Supporting this, Ca2+ extrusion was impaired while Ca2+ levels in internal stores were increased. T cells heterozygous for PMCA1 mimicked the phenotype of Nptn -/- T cells. Consistent with sustained Ca2+ levels, differentiation of Nptn -/- T helper cells was biased towards the Th1 versus Th2 subset. Our study thus establishes Neuroplastin-PMCA modules as important regulators of T cell activation.

Role of exosomes in the reproductive tract Oviductosomes mediate interactions of oviductal secretion with gametes/early embryo.

The oviductal epithelial membrane releases into the luminal environment extracellular vesicles (EVs) which are pleomorphic in nature and fall into two categories: exosomes and microvesicles. Both of these membrane vesicles are referred to as Oviductosomes (OVS), and to date have been identified in the murine and bovine species. Bovine EVs derived in vivo and from in vitro culture show differences in their protein cargo which includes CD9 and HSC70 biochemical markers and fertility-modulating proteins such as oviduct-specific glycoprotein (OVGP) and Plasma Membrane Ca(2+) ATPase 4 (PMCA4). PMCA4, an essential multifunctional sperm protein, is hormonally-regulated with elevated levels seen in proestrus/estrus. OVS deliver PMCA4 to sperm via a fusogenic mechanism involving the interaction between CD9 and integrins which are present on their surfaces. Studies of OVS are needed to determine the components of their cargoes and their interaction with oocytes and the very early embryo. Based on our present knowledge of their interaction with sperm, they are expected to play pivotal roles in regulating fertility and promise to inform the current IVF practice.

Plasma membrane calcium ATPases (PMCAs) as potential targets for the treatment of essential hypertension.

The incidence of hypertension, the major modifiable risk factor for cardiovascular disease, is increasing. Thus, there is a pressing need for the development of new and more effective strategies to prevent and treat hypertension. Development of these relies on a continued evolution of our understanding of the mechanisms which control blood pressure (BP). Resistance arteries are important in the regulation of total peripheral resistance and BP; changes in their structure and function are strongly associated with hypertension. Anti-hypertensives which both reduce BP and reverse changes in resistance arterial structure reduce cardiovascular risk more than therapies which reduce BP alone. Hence, identification of novel potential vascular targets which modify BP is important. Hypertension is a multifactorial disorder which may include a genetic component. Genome wide association studies have identified ATP2B1, encoding the calcium pump plasma membrane calcium ATPase 1 (PMCA1), as having a strong association with BP and hypertension. Knockdown or reduced PMCA1 expression in mice has confirmed a physiological role for PMCA1 in BP and resistance arterial regulation. Altered expression or inhibition of PMCA4 has also been shown to modulate these parameters. The mechanisms whereby PMCA1 and 4 can modulate vascular function remain to be fully elucidated but may involve regulation of intracellular calcium homeostasis and/or comprise a structural role. However, clear physiological links between PMCA and BP, coupled with experimental studies directly linking PMCA1 and 4 to changes in BP and arterial function, suggest that they may be important targets for the development of new pharmacological modulators of BP.

Impaired nitric oxide production and increased blood pressure in systemic heterozygous ATP2B1 null mice.

BACKGROUND: In the 'Millennium Genome Project', we identified ATP2B1 as a gene responsible for hypertension through single-nucleotide polymorphism analysis. The ATP2B1 gene encodes the plasma membrane calcium ATPase isoform 1, which contributes to the maintenance of intracellular calcium homeostasis by removing calcium ions. METHOD: Since ATP2B1 knockout mice are reported to be embryo-lethal, we generated systemic heterozygous ATP2B1 null (ATP2B1(+/-)) mice, and evaluated the implication of ATP2B1 in blood pressure. RESULTS: ATP2B1(+/-) mice revealed significantly higher SBP as measured by a radiotelemetric method. Phenylephrine-induced vasoconstriction was significantly increased in vascular rings from ATP2B1(+/-) mice, and the difference in this contraction disappeared in the presence of a nitric oxide synthase (NOS) inhibitor. Vasorelaxation to acetylcholine was significantly attenuated in vascular rings from ATP2B1(+/-) mice. In addition, cultured endothelial cells of ATP2B1(+/-) mice showed that the phosphorylation (Ser-1177) level of endothelial NOS protein was significantly lower, and nitric oxide production in endothelial cells and aorta was lower compared with those in control mice. In contrast, neural NOS expression in vascular smooth muscle cells from ATP2B1(+/-) mice and control mice were not significantly different. CONCLUSION: These results suggest that decreased ATP2B1 gene expression is associated with impaired endothelial NOS activity and nitric oxide production, and the ATP2B1 gene plays a crucial role in the regulation of blood pressure.

The plasma membrane Ca2+-ATPase2 (PMCA2) is involved in the regulation of Purkinje cell dendritic growth in cerebellar organotypic slice cultures.

Purkinje cells are the principal neurons of the cerebellar cortex and have an extensive and elaborate dendritic tree. Chronic activation of type I metabotropic glutamate receptors inhibits Purkinje cell dendritic growth in organotypic cerebellar slice cultures. This effect is mediated by calcium influx through P/Q-type and T-type Ca(2+) channels. We have now studied the role of the plasma membrane Ca(2+)-ATPase2 (PMCA2), a major calcium extrusion pump, for Purkinje cell dendritic development. We found that PMCA2 is strongly expressed in the plasma membrane and dendritic spines of Purkinje cells in organotypic slice cultures compatible with a role for controlling the local dendritic calcium equilibrium. Inhibition of PMCA2 activity by carboxyeosin resulted in a moderate reduction of Purkinje cell dendritic tree size indicating that the extrusion of calcium by PMCA2 is important for maintaining the dendritic calcium concentration and controlling dendritic growth. When inhibition of PMCA2 was combined with stimulation of type I metabotropic glutamate receptors, it partially rescued dendritic morphology. This protection can be explained by a compensatory inactivation of voltage-gated calcium channels in Purkinje cells after PMCA2 inhibition. Our results demonstrate that PMCA2 activity is an important regulator of the dendritic calcium equilibrium controlling Purkinje cell dendritic growth.

A new Atp2b2 deafwaddler allele, dfw(i5), interacts strongly with Cdh23 and other auditory modifiers.

Tight regulation of calcium (Ca2+) concentrations in the stereocilia bundles of auditory hair cells of the inner ear is critical to normal auditory transduction. The plasma membrane Ca2+ ATPase 2 (PMCA2), encoded by the Atp2b2 gene, is the primary mechanism for clearance of Ca2+ from auditory stereocilia, keeping intracellular levels low, and also contributes to maintaining adequate levels of extracellular Ca2+ in the endolymph. This study characterizes a novel null Atp2b2 allele, dfw(i5), by examining cochlear anatomy, vestibular function and auditory physiology in mutant mice. Loss of auditory function in PMCA2 mutants can be attributed to dysregulation of intracellular Ca2+ inside the stereocilia bundles. However, extracellular Ca2+ ions surrounding the stereocilia are also required for rigidity of cadherin 23, a component of the stereocilia tip-link encoded by the Cdh23 gene. This study further resolves the interaction between Atp2b2 and Cdh23 in a gene dosage and frequency-dependent manner, and finds that low frequencies are significantly affected by the interaction. In +/dfw(i5) mice, one mutant copy of Cdh23 is sufficient to cause broad frequency hearing impairment. Additionally, we report another modifying interaction with Atp2b2 on auditory sensitivity, possibly caused by an unidentified hearing loss gene in mice.

Mitochondria and plasma membrane Ca2+-ATPase control presynaptic Ca2+ clearance in capsaicin-sensitive rat sensory neurons.

The central processes of primary nociceptors form synaptic connections with the second-order nociceptive neurons located in the dorsal horn of the spinal cord. These synapses gate the flow of nociceptive information from the periphery to the CNS, and plasticity at these synapses contributes to centrally mediated hyperalgesia and allodynia. Although exocytosis and synaptic plasticity are controlled by Ca(2+) at the release sites, the mechanisms underlying presynaptic Ca(2+) signalling at the nociceptive synapses are not well characterized. We examined the presynaptic mechanisms regulating Ca(2+) clearance following electrical stimulation in capsaicin-sensitive nociceptors using a dorsal root ganglion (DRG)/spinal cord neuron co-culture system. Cytosolic Ca(2+) concentration ([Ca(2+)]i) recovery following electrical stimulation was well approximated by a monoexponential function with a ∼2 s. Inhibition of sarco-endoplasmic reticulum Ca(2+)-ATPase did not affect presynaptic [Ca(2+)]i recovery, and blocking plasmalemmal Na(+)/Ca(2+) exchange produced only a small reduction in the rate of [Ca(2+)]i recovery (∼12%) that was independent of intracellular K(+). However, [Ca(2+)]i recovery in presynaptic boutons strongly depended on the plasma membrane Ca(2+)-ATPase (PMCA) and mitochondria that accounted for ∼47 and 40%, respectively, of presynaptic Ca(2+) clearance. Measurements using a mitochondria-targeted Ca(2+) indicator, mtPericam, demonstrated that presynaptic mitochondria accumulated Ca(2+) in response to electrical stimulation. Quantitative analysis revealed that the mitochondrial Ca(2+) uptake is highly sensitive to presynaptic [Ca(2+)]i elevations, and occurs at [Ca(2+)]i levels as low as ∼200-300 nm. Using RT-PCR, we detected expression of several putative mitochondrial Ca(2+) transporters in DRG, such as MCU, Letm1 and NCLX. Collectively, this work identifies PMCA and mitochondria as the major regulators of presynaptic Ca(2+) signalling at the first sensory synapse, and underlines the high sensitivity of the mitochondrial Ca(2+) uniporter in neurons to cytosolic Ca(2+).

Plasma membrane calcium ATPase regulates bone mass by fine-tuning osteoclast differentiation and survival.

The precise regulation of Ca(2+) dynamics is crucial for proper differentiation and function of osteoclasts. Here we show the involvement of plasma membrane Ca(2+) ATPase (PMCA) isoforms 1 and 4 in osteoclastogenesis. In immature/undifferentiated cells, PMCAs inhibited receptor activator of NF-κB ligand-induced Ca(2+) oscillations and osteoclast differentiation in vitro. Interestingly, nuclear factor of activated T cell c1 (NFATc1) directly stimulated PMCA transcription, whereas the PMCA-mediated Ca(2+) efflux prevented NFATc1 activation, forming a negative regulatory loop. PMCA4 also had an anti-osteoclastogenic effect by reducing NO, which facilitates preosteoclast fusion. In addition to their role in immature cells, increased expression of PMCAs in mature osteoclasts prevented osteoclast apoptosis both in vitro and in vivo. Mice heterozygous for PMCA1 or null for PMCA4 showed an osteopenic phenotype with more osteoclasts on bone surface. Furthermore, PMCA4 expression levels correlated with peak bone mass in premenopausal women. Thus, our results suggest that PMCAs play important roles for the regulation of bone homeostasis in both mice and humans by modulating Ca(2+) signaling in osteoclasts.

Disruption of the interaction between PMCA2 and calcineurin triggers apoptosis and enhances paclitaxel-induced cytotoxicity in breast cancer cells.

Cancer is caused by defects in the signalling mechanisms that govern cell proliferation and apoptosis. It is well known that calcium-dependent signalling pathways play a critical role in cell regulation. A tight control of calcium homeostasis by transporters and channel proteins is required to assure a proper functioning of the calcium-sensitive signal transduction pathways that regulate cell growth and apoptosis. The plasma membrane calcium ATPase 2 (PMCA2) has been recently identified as a negative regulator of apoptosis that can play a significant role in cancer progression by conferring cells resistance to apoptosis. We have previously reported an inhibitory interaction between PMCA2 and the calcium-activated signalling molecule calcineurin in breast cancer cells. Here, we demonstrate that disruption of the PMCA2/calcineurin interaction in a variety of human breast cancer cells results in activation of the calcineurin/NFAT pathway, upregulation in the expression of the pro-apoptotic protein Fas Ligand and in a concomitant loss of cell viability. Reduction in cell viability is the consequence of an increase in cell apoptosis. Impairment of the PMCA2/calcineurin interaction enhances paclitaxel-mediated cytotoxicity of breast tumoral cells. Our results suggest that therapeutic modulation of the PMCA2/calcineurin interaction might have important clinical applications to improve current treatments for breast cancer patients.

PMCA2 via PSD-95 controls calcium signaling by α7-containing nicotinic acetylcholine receptors on aspiny interneurons.

Local control of calcium concentration within neurons is critical for signaling and regulation of synaptic communication in neural circuits. How local control can be achieved in the absence of physical compartmentalization is poorly understood. Challenging examples are provided by nicotinic acetylcholine receptors that contain α7 nicotinic receptor subunits (α7-nAChRs). These receptors are highly permeable to calcium and are concentrated on aspiny dendrites of interneurons, which lack obvious physical compartments for constraining calcium diffusion. Using functional proteomics on rat brain, we show that α7-nAChRs are associated with plasma membrane calcium-ATPase pump isoform 2 (PMCA2). Analysis of α7-nAChR function in hippocampal interneurons in culture shows that PMCA2 activity limits the duration of calcium elevations produced by the receptors. Unexpectedly, PMCA2 inhibition triggers rapid calcium-dependent loss of α7-nAChR clusters. This extreme regulatory response is mediated by CaMKII, involves proteasome activity, depends on the second intracellular loop of α7-nAChR subunits, and is specific in that it does not alter two other classes of calcium-permeable ionotropic receptors on the same neurons. A critical link is provided by the scaffold protein PSD-95 (postsynaptic density-95), which is associated with α7-nAChRs and constrains their mobility as revealed by single-particle tracking on neurons. The PSD-95 link is required for PMCA2-mediated removal of α7-nAChR clusters. This three-component combination of PMCA2, PSD-95, and α7-nAChR offers a novel mechanism for tight control of calcium dynamics in neurons.

Calcium signaling in closely related protozoan groups (Alveolata): non-parasitic ciliates (Paramecium, Tetrahymena) vs. parasitic Apicomplexa (Plasmodium, Toxoplasma).

The importance of Ca2+-signaling for many subcellular processes is well established in higher eukaryotes, whereas information about protozoa is restricted. Recent genome analyses have stimulated such work also with Alveolates, such as ciliates (Paramecium, Tetrahymena) and their pathogenic close relatives, the Apicomplexa (Plasmodium, Toxoplasma). Here we compare Ca2+ signaling in the two closely related groups. Acidic Ca2+ stores have been characterized in detail in Apicomplexa, but hardly in ciliates. Two-pore channels engaged in Ca2+-release from acidic stores in higher eukaryotes have not been stingently characterized in either group. Both groups are endowed with plasma membrane- and endoplasmic reticulum-type Ca2+-ATPases (PMCA, SERCA), respectively. Only recently was it possible to identify in Paramecium a number of homologs of ryanodine and inositol 1,3,4-trisphosphate receptors (RyR, IP3R) and to localize them to widely different organelles participating in vesicle trafficking. For Apicomplexa, physiological experiments suggest the presence of related channels although their identity remains elusive. In Paramecium, IP3Rs are constitutively active in the contractile vacuole complex; RyR-related channels in alveolar sacs are activated during exocytosis stimulation, whereas in the parasites the homologous structure (inner membrane complex) may no longer function as a Ca2+ store. Scrutinized comparison of the two closely related protozoan phyla may stimulate further work and elucidate adaptation to parasitic life. See also "Conclusions" section.

Involvement of the transepithelial calcium transport disruption and the formation of epididymal stones in roosters.

Epididymal lithiasis is a dysfunction of unknown origin characterized by the formation of calcium stones into the lumen of efferent ductules of roosters. Affected animals present an imbalance in the hormonal responsive systems that regulate the expression of proteins involved in the transepithelial calcium transport, as TRPV6, CaBP-D28K, NCX1, and PMCA. Because the efferent ductules are the major site of fluid and calcium reabsorption in excurrent ducts, it was hypothesized that impairment in local calcium homeostasis would lead to lithiasis. To test this hypothesis, we addressed the expression of these proteins in the epididymal region of affected animals. The present study focused on the investigation of the occurrence, tissue distribution, and physiological impact of the transepithelial calcium transport in roosters under normal and pathological conditions. The results showed that affected roosters presented a significant increase in TRPV6 and CaBP-D28k levels, whereas NCX1 and PMCA were not changed. Such alterations were more conspicuous in the proximal efferent ductules, in which was also observed accumulation of calcium within the epithelial cells. These findings provided the first evidences for the involvement of alteration in the expression of proteins essential for calcium reabsorption as a plausible mechanism for the formation of calcium stones within efferent ductules.

Calcium pumps: why so many?

Ca(2+)-ATPases (pumps) are key to the regulation of Ca(2+) in eukaryotic cells: nine are known today, belonging to three multigene families. The three endo(sarco)plasmic reticulum (SERCA) and the four plasma membrane (PMCA) pumps have been known for decades, the two Secretory Pathway Ca(2+) ATPase (SPCA) pumps have only become known recently. The number of pump isoforms is further increased by alternative splicing processes. The three pump types share the basic features of the catalytic mechanism, but differ in a number of properties related to tissue distribution, regulation, and role in the cellular homeostasis of Ca(2+). The molecular understanding of the function of all pumps has received great impetus from the solution of the three-dimensional (3D) structure of one of them, the SERCA pump. This landmark structural advance has been accompanied by the emergence and rapid expansion of the area of pump malfunction. Most of the pump defects described so far are genetic and produce subtler, often tissue and isoform specific, disturbances that affect individual components of the Ca(2+)-controlling and/or processing machinery, compellingly indicating a specialized role for each Ca(2+) pump type and/or isoform.

Characterization of postsynaptic Ca2+ signals at the Drosophila larval NMJ.

Postsynaptic intracellular Ca(2+) concentration ([Ca(2+)](i)) has been proposed to play an important role in both synaptic plasticity and synaptic homeostasis. In particular, postsynaptic Ca(2+) signals can alter synaptic efficacy by influencing transmitter release, receptor sensitivity, and protein synthesis. We examined the postsynaptic Ca(2+) transients at the Drosophila larval neuromuscular junction (NMJ) by injecting the muscle fibers with Ca(2+) indicators rhod-2 and Oregon Green BAPTA-1 (OGB-1) and then monitoring their increased fluorescence during synaptic activity. We observed discrete postsynaptic Ca(2+) transients along the NMJ during single action potentials (APs) and quantal Ca(2+) transients produced by spontaneous transmitter release. Most of the evoked Ca(2+) transients resulted from the release of one or two quanta of transmitter and occurred largely at synaptic boutons. The magnitude of the Ca(2+) signals was correlated with synaptic efficacy; the Is terminals, which produce larger excitatory postsynaptic potentials (EPSPs) and have a greater quantal size than Ib terminals, produced a larger Ca(2+) signal per terminal length and larger quantal Ca(2+) signals than the Ib terminals. During a train of APs, the postsynaptic Ca(2+) signal increased but remained localized to the postsynaptic membrane. In addition, we showed that the plasma membrane Ca(2+)-ATPase (PMCA) played a role in extruding Ca(2+) from the postsynaptic region of the muscle. Drosophila melanogaster has a single PMCA gene, predicted to give rise to various isoforms by alternative splicing. Using RT-PCR, we detected the expression of multiple transcripts in muscle and nervous tissues; the physiological significance of the same is yet to be determined.

Ca2+-signaling, alternative splicing and endoplasmic reticulum stress responses.

Ca(2+)-signaling, alternative splicing, and stress responses by the endoplasmic reticulum are three important cellular activities which can be strongly interconnected to alter the expression of protein isoforms in a tissue dependent manner or during development depending on the environmental conditions. This integrated network of signaling pathways permits a high degree of versatility and adaptation to metabolic, developmental and stress processes. Defects in its regulation may lead to cellular malfunction.

Identification of a novel point mutation of mouse Atp2b2 induced by N-ethyl-N-nitrosourea mutagenesis.

N-ethyl-N-nitrosourea (ENU)-induced mutagenesis is an important approach in the study of gene function and the establishment of human disease models. Here we report an ENU-induced mutation, Elfin, as a mouse model with hearing loss. Homozygous mutants were deaf and displayed severe ataxia, while heterozygous mice had a significant hearing loss. Histological analysis of the inner ear revealed that Elfin had progressive degeneration of the organ of Corti, spiral ganglion cells and an absence of otoconia in the vestibular system. The new mutation was mapped to chromosome 6 between microsatellite markers D6Mit39 and D6Mit254, where the Ca(2+)-ATPase type 2 (Atp2b2) gene resides. Sequence analysis revealed a unique T-to-A transition mutation at amino acid 655 resulting in Ile-to-Asn substitution. These results for the Elfin mutant confirm the role of ATP2B2 in balance, hearing and formation of otoconia and suggest it may serve as a new model of human hereditary hearing loss.

atp2b1a regulates Ca(2+) export during differentiation and regeneration of mechanosensory hair cells in zebrafish.

The molecular mechanisms of development of mechanosensory hair cells have been tackled successfully due to in vivo studies in the zebrafish lateral line. The enhancer trap (ET) transgenic line, SqET4 was instrumental in these studies even despite a lack of a link of its GFP expression pattern to a particular gene(s). We mapped the Tol2 transposon insertion of the SqET4 transgenics onto Chr. 4 next to a gene encoding Atp2b1a (Pmca1) - one of the four PMCAs acting to export Ca(2+) from a cell. atp2b1a expression recapitulates that of GFP during the development of mechanoreceptors of the inner ear and lateral line. atp2b1a expression correlates with the regeneration of these cells. Thus, SqET4 represents the Tg:atp2b1a-GFP line, which links Ca(2+) metabolism and the differentiation of mechanoreceptors. The morpholino-mediated knockdown of atp2b1a blocks Ca(2+) export and affects the division of hair cell progenitors, resulting in their accumulation. Under the control of a master gene of hair cells, Atoh1a, Atp2b1a functions during progenitor cell proliferation and hair cell differentiation. Given the similarity between the phenotypes of atp2b1a morphants and embryos treated with the pan-PMCA inhibitor 5(6)-carboxyeosin, Atp2b1a emerges as member of the Atp2b family responsible for Ca(2+) export during the development of hair cells.