Characterisation of DRASIC in the mouse inner ear

Genetic exploration of roles of acid-sensing ion channel subtypes in neurosensory mechanotransduction including proprioception

Although acid-sensing ion channels (ASICs) are proton-gated ion channels responsible for sensing tissue acidosis, accumulating evidence has shown that ASICs are also involved in neurosensory mechanotransduction. However, in contrast to Piezo ion channels, evidence of ASICs as mechanically gated ion channels has not been found using conventional mechanoclamp approaches. Instead, ASICs are involved in the tether model of mechanotransduction, with the channels gated via tethering elements of extracellular matrix and intracellular cytoskeletons. Methods using substrate deformation-driven neurite stretch and micropipette-guided ultrasound were developed to reveal the roles of ASIC3 and ASIC1a, respectively. Here we summarize the evidence supporting the roles of ASICs in neurosensory mechanotransduction in knockout mouse models of ASIC subtypes and provide insight to further probe their roles in proprioception.

Pathology and physiology of acid‑sensitive ion channels in the digestive system (Review)

As a major proton-gated cation channel, acid-sensitive ion channels (ASICs) can perceive large extracellular pH changes. ASICs play an important role in the occurrence and development of diseases of various organs and tissues including in the heart, brain, and gastrointestinal tract, as well as in tumor proliferation, invasion, and metastasis in acidosis and regulation of an acidic microenvironment. The permeability of ASICs to sodium and calcium ions is the basis of their physiological and pathological roles in the body. This review summarizes the physiological and pathological mechanisms of ASICs in digestive system diseases, which plays an important role in the early diagnosis, treatment, and prognosis of digestive system diseases related to ASIC expression.

Acid-sensing ion channel 3: An analgesic target

Acid-sensing ion channel 3 (ASIC3) belongs to the epithelial sodium channel/degenerin (ENaC/DEG) superfamily. There are 7 different ASIC subunits encoded by 5 different genes. Most ASIC subunits form trimeric ion channels that upon activation by extracellular protons mediate a transient inward current inducing cellular excitability. ASIC subunits exhibit differential tissue expression and biophysical properties, and the ability of subunits to form homo- and heteromeric trimers further increases the complexity of currents measured and their pharmacological properties. ASIC3 is of particular interest, not only because it exhibits high expression in sensory neurones, but also because upon activation it does not fully inactivate: a transient current is followed by a sustained current that persists during a period of extracellular acidity, i.e. ASIC3 can encode prolonged acidosis as a nociceptive signal. Furthermore, certain mediators sensitize ASIC3 enabling smaller proton concentrations to activate it and other mediators can directly activate the channel at neutral pH. Moreover, there is a plethora of evidence using transgenic mouse models and pharmacology, which supports ASIC3 as being a potential target for development of analgesics. This review will focus on current understanding of ASIC3 function to provide an overview of how ASIC3 contributes to physiology and pathophysiology, examining the mechanisms by which it can be modulated, and highlighting gaps in current understanding and future research directions.

Diverse identities and sites of action of cochlear neurotransmitters

Accurate encoding of acoustic stimuli requires temporally precise responses to sound integrated with cellular mechanisms that encode the complexity of stimuli over varying timescales and orders of magnitude of intensity. Sound in mammals is initially encoded in the cochlea, the peripheral hearing organ, which contains functionally specialized cells (including hair cells, afferent and efferent neurons, and a multitude of supporting cells) to allow faithful acoustic perception. To accomplish the demanding physiological requirements of hearing, the cochlea has developed synaptic arrangements that operate over different timescales, with varied strengths, and with the ability to adjust function in dynamic hearing conditions. Multiple neurotransmitters interact to support the precision and complexity of hearing. Here, we review the location of release, action, and function of neurotransmitters in the mammalian cochlea with an emphasis on recent work describing the complexity of signaling.

Acid-sensing ion channels (ASICs) influence excitability of stellate neurons in the mouse cochlear nucleus

Acid-sensing ion channels (ASICs) are voltage-independent and proton-gated channels. In this study, we aimed to test the hypothesis whether ASICs might be involved in modifying the excitability of stellate cells in the cochlear nucleus (CN). We determined gene expressions of ASIC1, ASIC2 and ASIC3 in the CN of BALB/mice. ASIC currents in stellate cells were characterized by using whole-cell patch-clamp technique. In the voltage-clamp experiments, inward currents were recorded upon application of 2-[N-Morpholino ethanesulfonic acid]-normal artificial cerebrospinal fluid (MES-aCSF), whose pH 50 was 5.84. Amiloride inhibited the acid-induced currents in a dose-dependent manner. Inhibition of the ASIC currents by extracellular Ca²⁺ and Pb²⁺ (10 μM) was significant evidence for the existence of homomeric ASIC1a subunits. ASIC currents were increased by 20% upon extracellular application of Zn²⁺ (300 μM) (p < 0.05, n = 13). In current-clamp experiments, application of MES-aCSF resulted in the depolarization of stellate cells. The results show that the ASIC currents in stellate cells of the cochlear nucleus are carried largely by the ASIC1a and ASIC2a channels. ASIC channels affect the excitability of the stellate cells and therefore they appear to have a role in the processing of auditory information.

Protons as Messengers of Intercellular Communication in the Nervous System

In this review, evidence demonstrating that protons (H⁺) constitute a complex, regulated intercellular signaling mechanisms are presented. Given that pH is a strictly regulated variable in multicellular organisms, localized extracellular pH changes may constitute significant signals of cellular processes that occur in a cell or a group of cells. Several studies have demonstrated that the low pH of synaptic vesicles implies that neurotransmitter release is always accompanied by the co-release of H⁺ into the synaptic cleft, leading to transient extracellular pH shifts. Also, evidence has accumulated indicating that extracellular H⁺ concentration regulation is complex and implies a source of protons in a network of transporters, ion exchangers, and buffer capacity of the media that may finally establish the extracellular proton concentration. The activation of membrane transporters, increased production of CO₂ and of metabolites, such as lactate, produce significant extracellular pH shifts in nano- and micro-domains in the central nervous system (CNS), constituting a reliable signal for intercellular communication. The acid sensing ion channels (ASIC) function as specific signal sensors of proton signaling mechanism, detecting subtle variations of extracellular H⁺ in a range varying from pH 5 to 8. The main question in relation to this signaling system is whether it is only synaptically restricted, or a volume modulator of neuron excitability. This signaling system may have evolved from a metabolic activity detection mechanism to a highly localized extracellular proton dependent communication mechanism. In this study, evidence showing the mechanisms of regulation of extracellular pH shifts and of the ASICs and its function in modulating the excitability in various systems is reviewed, including data and its role in synaptic neurotransmission, volume transmission and even segregated neurotransmission, leading to a reliable extracellular signaling mechanism.

Acid-sensing ion channels: dual function proteins for chemo-sensing and mechano-sensing

Background

Acid-sensing ion channels (ASICs) are a group of amiloride-sensitive ligand-gated ion channels belonging to the family of degenerin/epithelial sodium channels. ASICs are predominantly expressed in both the peripheral and central nervous system and have been characterized as potent proton sensors to detect extracellular acidification in the periphery and brain.

Main body

Here we review the recent studies focusing on the physiological roles of ASICs in the nervous system. As the major acid-sensing membrane proteins in the nervous system, ASICs detect tissue acidosis occurring at tissue injury, inflammation, ischemia, stroke, and tumors as well as fatiguing muscle to activate pain-sensing nerves in the periphery and transmit pain signals to the brain. Arachidonic acid and lysophosphocholine have been identified as endogenous non-proton ligands activating ASICs in a neutral pH environment. On the other hand, ASICs are found involved in the tether model mechanotransduction, in which the extracellular matrix and cytoplasmic cytoskeletons act like a gating-spring to tether the mechanically activated ion channels and thus transmit the stimulus force to the channels. Accordingly, accumulating evidence has shown ASICs play important roles in mechanotransduction of proprioceptors, mechanoreceptors and nociceptors to monitor the homoeostatic status of muscle contraction, blood volume, and blood pressure as well as pain stimuli.

Conclusion

Together, ASICs are dual-function proteins for both chemosensation and mechanosensation involved in monitoring physiological homoeostasis and pathological signals.

Gene expression profiling of the inner ear

The identification of transcriptional differences has served as an important starting point in understanding the molecular mechanisms behind biological processes and systems. The developmental biology of the inner ear, the biology of hearing and of course the pathology of deafness are all processes that warrant a molecular description if we are to improve human health. To this end, technological innovation has meant that larger scale analysis of gene transcription has been possible for a number of years now, extending our molecular analysis of genes to beyond those that are currently in vogue for a given system. In this review, some of the contributions gene profiling has made to understanding developmental, pathological and physiological processes in the inner ear are highlighted.

Acid-Sensing Ion Channels Expression, Identity and Role in the Excitability of the Cochlear Afferent Neurons

Acid-sensing ion channels (ASICs) are activated by an increase in the extracellular proton concentration. There are four genes (ASIC1-4) that encode six subunits, and they are involved in diverse neuronal functions, such as mechanosensation, learning and memory, nociception, and modulation of retinal function. In this study, we characterize the ASIC currents of spiral ganglion neurons (SGNs). These ASIC currents are primarily carried by Na(+), exhibit fast activation and desensitization, display a pH50 of 6.2 and are blocked by amiloride, indicating that these are ASIC currents. The ASIC currents were further characterized using several pharmacological tools. Gadolinium and acetylsalicylic acid reduced these currents, and FMRFamide, zinc (at high concentrations) and N,N,N',N'-tetrakis-(2-piridilmetil)-ethylenediamine increased them, indicating that functional ASICs are composed of the subunits ASIC1, ASIC2, and ASIC3. Neomycin and streptomycin reduced the desensitization rate of the ASIC current in SGNs, indicating that ASICs may contribute to the ototoxic action of aminoglycosides. RT-PCR of the spiral ganglion revealed significant expression of all ASIC subunits. By immunohistochemistry the expression of the ASIC1a, ASIC2a, ASIC2b, and ASIC3 subunits was detected in SGNs. Although only a few SGNs exhibited action potential firing in response to an acidic stimulus, protons in the extracellular solution modulated SGN activity during sinusoidal stimulation. Our results show that protons modulate the excitability of SGNs via ASICs.

Acid-sensing ion channels and their modulators

According to a modern look acid-sensing ion channels (ASICs) are one of the most important receptors that perceive pH change in the body. ASICs represent proton-gated Na+-selective channels, which are expressed in neurons of the central and peripheral nervous system. These channels are attracting attention of researchers around the world, as they are involved in various physiological processes in the body. Drop of pH may occur in tissues in norm (e.g. the accumulation of lactic acid, the release of protons upon ATP hydrolysis) and pathology (inflammation, ischemic stroke, tissue damage and seizure). These processes are accompanied by unpleasant pain sensations, which may be short-lived or can lead to chronic inflammatory diseases. Modulators of ASIC channels activity are potential candidates for new effective analgesic and neuroprotection drugs. This review summarizes available information about structure, function, and physiological role of ASIC channels. In addition a description of all known ligands of these channels and their practical relevance is provided.

Acid-Sensing Ion Channels and nociception in the peripheral and central nervous systems

Since their molecular cloning in the late 90's, Acid-Sensing Ion Channels (ASICs) have been shown to be involved in many aspects of nociception, both in peripheral and central neurons. In rodents, the combination of specific or non-specific pharmacological modulators of ASICs, together with in vivo knockdown and/or knockout animals has revealed their contribution to the detection, the modulation and the sensitization of the pain message by primary and secondary sensory neurons. Functional ASICs are homo or heterotrimers of different homologous subunits (ASIC1-3). Channels containing ASIC3 or ASIC1 subunits, appear to be important in peripheral nociceptors, where they are subject to intense regulation, while ASIC1a-containing channels also have a prominent role in central neurons, including spinal cord neurons that modulate and transmit the pain signal to the brain. In humans, experiments performed in healthy volunteers using drugs already used in the clinic and acting as poorly-selective inhibitors of ASICs, together with recent in vitro data obtained from stem cell-derived sensory neurons both support a role for these channels in nociception. These data thus suggest a real translational potential in the development of inhibitory strategies of ASICs for the treatment of pain. This article is part of the Special Issue entitled 'Acid-Sensing Ion Channels in the Nervous System'.

Acid-sensing ion channels (ASICs) 2 and 4.2 are expressed in the retina of the adult zebrafish

Acid-sensing ion channels (ASICs) are H(+)-gated, voltage-insensitive cation channels involved in synaptic transmission, mechanosensation and nociception. Different ASICs have been detected in the retina of mammals but it is not known whether they are expressed in adult zebrafish, a commonly used animal model to study the retina in both normal and pathological conditions. We study the expression and distribution of ASIC2 and ASIC4 in the retina of adult zebrafish and its regulation by light using PCR, in situ hybridization, western blot and immunohistochemistry. We detected mRNA encoding zASIC2 and zASIC4.2 but not zASIC4.1. ASIC2, at the mRNA or protein level, was detected in the outer nuclear layer, the outer plexiform layer, the inner plexiform layer, the retinal ganglion cell layer and the optic nerve. ASIC4 was expressed in the photoreceptors layer and to a lesser extent in the retinal ganglion cell layer. Furthermore, the expression of both ASIC2 and ASIC4.2 was down-regulated by light and darkness. These results are the first demonstration that ASIC2 and ASIC4 are expressed in the adult zebrafish retina and suggest that zebrafish could be used as a model organism for studying retinal pathologies involving ASICs.

Genetic exploration of the role of acid-sensing ion channels

Advanced gene targeting technology and related tools in mice have been incorporated into studies of acid-sensing ion channels (ASICs). A single ASIC subtype can be knocked out specifically and screened thoroughly for expression in the nervous system at the cellular level. Mapping studies have further shed light on the initiation and identification of related behavioral phenotypes. Here we review studies involving genetically engineered mouse models used to investigate the physiological function of individual ASIC subtypes: ASIC1 (and ASIC1a), ASIC2, ASIC3 and ASIC4. We discuss the detailed expression studies and significant phenotypes revealed with gene knockout for most known Asic subtypes. Each strategy designed to manipulate mouse genetics has advantages and disadvantages. We discuss the limitations of these Asic-knockout models and propose future directions to solve the genetic issues. This article is part of the Special Issue entitled 'Acid-Sensing Ion Channels in the Nervous System'.

Elevated Expression of Acid-Sensing Ion Channel 3 Inhibits Epilepsy via Activation of Interneurons

Recent studies have indicated that acid-sensing ion channels may play a significant role in the termination of epilepsy. In particular, acid-sensing ion channel 3 (ASIC3) is expressed in the central nervous system and is most sensitive to extracellular pH. However, whether ASIC3 plays a role in epilepsy is unknown. In this study, qRT-PCR, Western blot, immunohistochemistry, double immunofluorescence labeling, and slice recordings were used. We first detected elevated ASIC3 expression patterns in the brains of temporal lobe epilepsy patients and epileptic rats. ASIC3 was expressed in neurons and glia in both humans and in an experimental model of epilepsy, and ASIC3 was colocalized with inhibitory GABAergic interneurons. By blocking ASIC3 with its antagonist APETx2, we observed that injected APETx2 shortened the latency to seizure and increased the incidence of generalized tonic clonic seizure compared to the control group in models of both pilocarpine- and pentylenetetrazole (PTZ)-induced seizures. Additionally, blocking ASIC3 significantly decreased the frequency of action potential (AP) firing in interneurons. Moreover, APETx2 significantly reduced the amplitudes and frequencies of miniature inhibitory postsynaptic currents (mIPSCs) while showed no differences with the APETx2 + bicuculline group and the bicuculline group. These findings suggest that elevated levels of ASIC3 may serve as an anti-epileptic mechanism via postsynaptic mechanisms in interneurons. It could represent a novel therapeutic strategy for epilepsy treatment.

The mouse as a model for age-related hearing loss - a mini-review

The most common form of sensory disability is age-related hearing loss (ARHL), also referred to as presbycusis. ARHL is a complex disorder with a mixture of genetic and environmental components, a combination that leads to a progressive decline in hearing function with increased age. In the last 15 years, there has been a vast increase in our knowledge of the genes that underlie congenital deafness and the critical components of hearing. In contrast, knowledge of the pathological processes involved in ARHL remains very limited. The mouse has proved an essential tool in the identification of early-onset deafness genes and in revealing the basic mechanisms of hearing. As focus is now turning toward elucidating the most common form of hearing loss, ARHL, the mouse will again play a fundamental role in this research. Here, we review the need for an animal model and discuss the suitability of the mouse as an ARHL model. Finally, we outline the ways in which hearing researchers are utilising the mouse in the investigation of ARHL and provide perspectives on the need for these data to be integrated with the results of human genetic studies.

[Acid-Sensing Ion Channels (ASICs) in pain]

The discovery of new drug targets represents a real opportunity for developing fresh strategies against pain. Ion channels are interesting targets because they are directly involved in the detection and the transmission of noxious stimuli by sensory fibres of the peripheral nervous system and by neurons of the spinal cord. Acid-Sensing Ion Channels (ASICs) have emerged as important players in the pain pathway. They are neuronal, voltage-independent depolarizing sodium channels activated by extracellular protons. The ASIC family comprises several subunits that need to associate into homo- or hetero-trimers to form a functional channel. The ASIC1 and ASIC3 isoforms are particularly important in sensory neurons, whereas ASIC1a, alone or in association with ASIC2, is essential in the central nervous system. The potent analgesic effects associated with their inhibition in animals (which can be comparable to those of morphine) and data suggesting a role in human pain illustrate the therapeutic potential of these channels.

Effect of deep tissue incision on pH responses of afferent fibers and dorsal root ganglia innervating muscle

BACKGROUND

Understanding the mechanisms underlying deep tissue pain in the postoperative period is critical to improve therapies. Using the in vitro plantar flexor digitorum brevis muscle-nerve preparation and patch clamp recordings from cultured dorsal root ganglia neurons innervating incised and unincised muscle, the authors investigated responses to various pH changes.

METHODS

Incision including the plantar flexor digitorum brevis muscle or sham operation was made in the rat hind paw. On postoperative day 1, in vitro single-fiber recording was undertaken. On the basis of previous studies, the authors recorded from at least 40 fibers per group. Also DiI-labeled dorsal root ganglia innervating muscle from rats undergoing incision and a sham operation were cultured and tested for acid responses, using whole cell patch clamp recordings.

RESULTS

The prevalence of responsive group IV afferents to lactic acid pH 6.5 in the incision group (15 of 67; 22.3%) was greater than that in the control group (2 of 35; 5.7%; P=0.022). In dorsal root ganglia neurons innervating muscle, incision increased mean current amplitudes of acid-evoked currents; the acid-sensing ion channel blocker, amiloride 300 μM, inhibited more than 75% of the acid-evoked current, whereas, the transient receptor vanilloid receptor 1 blocker (AMG9810 1 μM) did not cause significant inhibition.

CONCLUSION

The authors' experiments demonstrated that incision increases the responses of flexor digitorum brevis muscle afferent fibers to weak acid solutions, and increased acid-evoked currents in dorsal root ganglia innervating muscle. The authors' data suggest that up-regulation of acid-sensing ion channels might underlie this increased chemosensitivity caused by surgery.

Ion channels as drug targets in central nervous system disorders

Ion channel targeted drugs have always been related with either the central nervous system (CNS), the peripheral nervous system, or the cardiovascular system. Within the CNS, basic indications of drugs are: sleep disorders, anxiety, epilepsy, pain, etc. However, traditional channel blockers have multiple adverse events, mainly due to low specificity of mechanism of action. Lately, novel ion channel subtypes have been discovered, which gives premises to drug discovery process led towards specific channel subtypes. An example is Na(+) channels, whose subtypes 1.3 and 1.7-1.9 are responsible for pain, and 1.1 and 1.2 - for epilepsy. Moreover, new drug candidates have been recognized. This review is focusing on ion channels subtypes, which play a significant role in current drug discovery and development process. The knowledge on channel subtypes has developed rapidly, giving new nomenclatures of ion channels. For example, Ca(2+)s channels are not any more divided to T, L, N, P/Q, and R, but they are described as Ca(v)1.1-Ca(v)3.3, with even newer nomenclature α1A-α1I and α1S. Moreover, new channels such as P2X1-P2X7, as well as TRPA1-TRPV1 have been discovered, giving premises for new types of analgesic drugs.

Inner ear insult ablates the arousal response to hypoxia and hypercarbia

INTRODUCTION

Sudden Infant Death Syndrome (SIDS) remains the leading cause of infant mortality in Western societies. A prior study identified an association between hearing suppression on the newborn hearing test and subsequent death from SIDS. This is the first finding of an abnormality in SIDS cases prior to death. A following study identified that inner ear dysfunction precipitates a marked suppression of the hypercapnic ventilatory response (HCVR). Failure of arousal has been proposed to be a key component in SIDS. The objective of the present study was to assess whether inner ear dysfunction not only weakens the hypercapnic response, but also plays a role in suppressing the arousal response to suffocating gas mixtures.

METHODS

Wild-type mice (n=28) received intra-tympanic gentamicin (IT-Gent) injections bilaterally or unilaterally to precipitate inner ear hair cell dysfunction. Three control groups (n=22) received intra-tympanic saline (IT-Saline) bilaterally or unilaterally (right or left), or intra-peritoneal gentamicin (IP-Gent). The body movement arousal responses to severe hypoxia-hypercarbia combined (5% CO2 in nitrogen) were tested under light anesthesia 8 days following the administration of gentamicin or saline.

RESULTS

After injections, the bilateral and unilateral IT-Gent-treated animals behaved similarly to controls, however the HCVR as well as the arousal movements in response to severe hypoxia-hypercarbia were suppressed in IT-Gent-treated animals compared to control animals (P<0.05). Thus the HCVR was significantly decreased in the bilateral (n=9) and unilateral IT-Gent-treated mice (n=19) compared to bilateral (n=7) and unilateral IT-Saline (n=9) control groups (p<0.05). Arousal movements were suppressed in the bilateral IT-Gent group (n=9) compared to bilateral IT-Saline controls (n=7, P<0.0001) and in the unilateral IT-Gent group (n=19) compared to unilateral IT-Saline controls (n=10, P<0.0001).

DISCUSSION

The findings support the theory that inner ear dysfunction could be relevant in the pathophysiology of SIDS. The inner ear appears to play a key role in arousal from suffocating gas mixtures that has not been previously identified.

Neurosensory mechanotransduction through acid-sensing ion channels

Acid-sensing ion channels (ASICs) are voltage-insensitive cation channels responding to extracellular acidification. ASIC proteins have two transmembrane domains and a large extracellular domain. The molecular topology of ASICs is similar to that of the mechanosensory abnormality 4- or 10-proteins expressed in touch receptor neurons and involved in neurosensory mechanotransduction in nematodes. The ASIC proteins are involved in neurosensory mechanotransduction in mammals. The ASIC isoforms are expressed in Merkel cell-neurite complexes, periodontal Ruffini endings and specialized nerve terminals of skin and muscle spindles, so they might participate in mechanosensation. In knockout mouse models, lacking an ASIC isoform produces defects in neurosensory mechanotransduction of tissue such as skin, stomach, colon, aortic arch, venoatrial junction and cochlea. The ASICs are thus implicated in touch, pain, digestive function, baroreception, blood volume control and hearing. However, the role of ASICs in mechanotransduction is still controversial, because we lack evidence that the channels are mechanically sensitive when expressed in heterologous cells. Thus, ASIC channels alone are not sufficient to reconstruct the path of transducing molecules of mechanically activated channels. The mechanotransducers associated with ASICs need further elucidation. In this review, we discuss the expression of ASICs in sensory afferents of mechanoreceptors, findings of knockout studies, technical issues concerning studies of neurosensory mechanotransduction and possible missing links. Also we propose a molecular model and a new approach to disclose the molecular mechanism underlying the neurosensory mechanotransduction.

Acid-sensing ion channels: trafficking and synaptic function

Extracellular acidification occurs in the brain with elevated neural activity, increased metabolism, and neuronal injury. This reduction in pH can have profound effects on brain function because pH regulates essentially every single biochemical reaction. Therefore, it is not surprising to see that Nature evolves a family of proteins, the acid-sensing ion channels (ASICs), to sense extracellular pH reduction. ASICs are proton-gated cation channels that are mainly expressed in the nervous system. In recent years, a growing body of literature has shown that acidosis, through activating ASICs, contributes to multiple diseases, including ischemia, multiple sclerosis, and seizures. In addition, ASICs play a key role in fear and anxiety related psychiatric disorders. Several recent reviews have summarized the importance and therapeutic potential of ASICs in neurological diseases, as well as the structure-function relationship of ASICs. However, there is little focused coverage on either the basic biology of ASICs or their contribution to neural plasticity. This review will center on these topics, with an emphasis on the synaptic role of ASICs and molecular mechanisms regulating the spatial distribution and function of these ion channels.

Acid sensing ion channel 1 in lateral hypothalamus contributes to breathing control

Acid-sensing ion channels (ASICs) are present in neurons and may contribute to chemoreception. Among six subunits of ASICs, ASIC1 is mainly expressed in the central nervous system. Recently, multiple sites in the brain including the lateral hypothalamus (LH) have been found to be sensitive to extracellular acidification. Since LH contains orexin neurons and innervates the medulla respiratory center, we hypothesize that ASIC1 is expressed on the orexin neuron and contributes to acid-induced increase in respiratory drive. To test this hypothesis, we used double immunofluorescence to determine whether ASIC1 is expressed on orexin neurons in the LH, and assessed integrated phrenic nerve discharge (iPND) in intact rats in response to acidification of the LH. We found that ASIC1 was co-localized with orexinA in the LH. Microinjection of acidified artificial cerebrospinal fluid increased the amplitude of iPND by 70% (pH 7.4 v.s. pH 6.5:1.05±0.12 v.s. 1.70±0.10, n = 6, P<0.001) and increased the respiratory drive (peak amplitude of iPND/inspiratory time, PA/Ti) by 40% (1.10±0.23 v.s. 1.50±0.38, P<0.05). This stimulatory effect was abolished by blocking ASIC1 with a nonselective inhibitor (amiloride 10 mM), a selective inhibitor (PcTX1, 10 nM) or by damaging orexin neurons in the LH. Current results support our hypothesis that the orexin neuron in the LH can exert an excitation on respiration via ASIC1 during local acidosis. Since central acidification is involved in breathing dysfunction in a variety of pulmonary diseases, understanding its underlying mechanism may improve patient management.

Acid sensing ion channel 1 in lateral hypothalamus contributes to breathing control

Acid-sensing ion channels (ASICs) are present in neurons and may contribute to chemoreception. Among six subunits of ASICs, ASIC1 is mainly expressed in the central nervous system. Recently, multiple sites in the brain including the lateral hypothalamus (LH) have been found to be sensitive to extracellular acidification. Since LH contains orexin neurons and innervates the medulla respiratory center, we hypothesize that ASIC1 is expressed on the orexin neuron and contributes to acid-induced increase in respiratory drive. To test this hypothesis, we used double immunofluorescence to determine whether ASIC1 is expressed on orexin neurons in the LH, and assessed integrated phrenic nerve discharge (iPND) in intact rats in response to acidification of the LH. We found that ASIC1 was co-localized with orexinA in the LH. Microinjection of acidified artificial cerebrospinal fluid increased the amplitude of iPND by 70% (pH 7.4 v.s. pH 6.5:1.05±0.12 v.s. 1.70±0.10, n = 6, P<0.001) and increased the respiratory drive (peak amplitude of iPND/inspiratory time, PA/Ti) by 40% (1.10±0.23 v.s. 1.50±0.38, P<0.05). This stimulatory effect was abolished by blocking ASIC1 with a nonselective inhibitor (amiloride 10 mM), a selective inhibitor (PcTX1, 10 nM) or by damaging orexin neurons in the LH. Current results support our hypothesis that the orexin neuron in the LH can exert an excitation on respiration via ASIC1 during local acidosis. Since central acidification is involved in breathing dysfunction in a variety of pulmonary diseases, understanding its underlying mechanism may improve patient management.

Potential roles of stem cells in the management of sensorineural hearing loss

BACKGROUND

In the management of sensorineural hearing loss, effective therapy for degenerated hair cells, third order neurons, ganglions, dendrites and synaptic areas of the vestibulo-cochleo-cerebral pathway remains an enigma. Transplantation of stem and progenitor cells appears to be an emerging potential solution, and is the focus of this review.

AIM

To review recent developments in the management of sensorineural hearing loss in the field of stem cell research.

MATERIALS AND METHOD

A systematic review of the English language literature included all experimental and non-experimental studies with a Jadad score of three or more, published between 2000 and 2010 and included in the following databases: Cochrane Library Ear, Nose and Throat Disorders; Medline; Google Scholar; Hinari; and the Online Library of Toronto University.

RESULTS

Of the 455 and 29 600 articles identified from Medline and Google Scholar, respectively, 48 met the inclusion criteria. These were independently reviewed and jointly analysed.

CONCLUSION

Although there is not yet any evidence from successful human studies, stem cell and 'alternative stem cell' technology seems to represent the future of sensorineural hearing loss management.

[The diversity of aging models]

Most of the signalling pathways involved in aging regulation have been recently found well conserved at various levels throughout the evolution. Taking this into account, a diversity of model organisms, including worms, rodents, and lemurs as well, allows to address different questions: how to understand the interactions between genetic and environmental factors while challenging theories of aging, to preserve hearing integrity, to fight against senescence of neural stem cells, or to explore brain fitness from gene expression to cognitive and social behavior? Here are the main issues that can be considered, stressing the complementarities of the models. The differentiation of aging physiological aspects from those induced by age-related pathologies will also be specified. By emphasizing recent ability of technologies to promote new aging insights, we discuss towards a better understanding of mechanisms governing aging.

High throughput gene expression analysis of the inner ear

The mouse auditory and vestibular epithelia consist of a complex array of many different cell types. Over the last decade microarrays were used to characterize gene expression in the inner ear. Studies were performed on wild type mice to identify deafness genes, transcriptional networks activated during development, or identify miRNA with a functional role in the ear. Other studies focused on the molecular response of the inner ear to stimuli ranging from ototoxic medications to hypergravity and caloric restriction. Finally, microarrays were used to identify transcriptional networks activated downstream of deafness genes. As template-free high throughput gene expression profiling methods such as RNA-seq are increasingly popular, we offer a critical review of the data generated over the last decade relating to microarrays for gene expression profiling of the inner ear. Moreover, as most of the published data is available through the gene expression omnibus (GEO), we demonstrate the feasibility of integrating data from independent experiments to reach novel insights.

ENaCs and ASICs as therapeutic targets

The epithelial Na(+) channel (ENaC) and acid-sensitive ion channel (ASIC) branches of the ENaC/degenerin superfamily of cation channels have drawn increasing attention as potential therapeutic targets in a variety of diseases and conditions. Originally thought to be solely expressed in fluid absorptive epithelia and in neurons, it has become apparent that members of this family exhibit nearly ubiquitous expression. Therapeutic opportunities range from hypertension, due to the role of ENaC in maintaining whole body salt and water homeostasis, to anxiety disorders and pain associated with ASIC activity. As a physiologist intrigued by the fundamental mechanics of salt and water transport, it was natural that Dale Benos, to whom this series of reviews is dedicated, should have been at the forefront of research into the amiloride-sensitive sodium channel. The cloning of ENaC and subsequently the ASIC channels has revealed a far wider role for this channel family than was previously imagined. In this review, we will discuss the known and potential roles of ENaC and ASIC subunits in the wide variety of pathologies in which these channels have been implicated. Some of these, such as the role of ENaC in Liddle's syndrome are well established, others less so; however, all are related in that the fundamental defect is due to inappropriate channel activity.

Targeting ASIC3 for pain, anxiety, and insulin resistance

The acid-sensing ion channel 3 (ASIC3) is a pH sensor that responds to mild extracellular acidification and is predominantly expressed in nociceptors. There is much interest in targeting ASIC3 to relieve pain associated with tissue acidosis, and selective drugs targeting ASIC3 have been used to relieve acid-evoked pain in animal models and human studies. There is accumulating evidence that ASIC3 is widely expressed in many neuronal and non-neuronal cells, such as neurons in the brain and adipose cells, albeit to a lesser extent than in nociceptors. Asic3-knockout mice have reduced anxiety levels and enhanced insulin sensitivity, suggesting that antagonizing ASIC3 has additional benefits. This view is tempered by recent studies suggesting that Asic3-knockout mice may experience cardiovascular disturbances. Due to the development of ASIC3 antagonists as analgesics, we review here the additional benefits, safety, risks, and strategy associated with antagonizing ASIC3 function.

ASIC3 channels in multimodal sensory perception

Acid-sensing ion channels (ASICs), which are members of the sodium-selective cation channels belonging to the epithelial sodium channel/degenerin (ENaC/DEG) family, act as membrane-bound receptors for extracellular protons as well as nonproton ligands. At least five ASIC subunits have been identified in mammalian neurons, which form both homotrimeric and heterotrimeric channels. The highly proton sensitive ASIC3 channels are predominantly distributed in peripheral sensory neurons, correlating with their roles in multimodal sensory perception, including nociception, mechanosensation, and chemosensation. Different from other ASIC subunit composing ion channels, ASIC3 channels can mediate a sustained window current in response to mild extracellular acidosis (pH 7.3-6.7), which often occurs accompanied by many sensory stimuli. Furthermore, recent evidence indicates that the sustained component of ASIC3 currents can be enhanced by nonproton ligands including the endogenous metabolite agmatine. In this review, we first summarize the growing body of evidence for the involvement of ASIC3 channels in multimodal sensory perception and then discuss the potential mechanisms underlying ASIC3 activation and mediation of sensory perception, with a special emphasis on its role in nociception. We conclude that ASIC3 activation and modulation by diverse sensory stimuli represent a new avenue for understanding the role of ASIC3 channels in sensory perception. Furthermore, the emerging implications of ASIC3 channels in multiple sensory dysfunctions including nociception allow the development of new pharmacotherapy.

Acid-sensing ion channels (ASICs): pharmacology and implication in pain

Tissue acidosis is a common feature of many painful conditions. Protons are indeed among the first factors released by injured tissues, inducing a local pH fall that depolarizes peripheral free terminals of nociceptors and leads to pain. ASICs are excitatory cation channels directly gated by extracellular protons that are expressed in the nervous system. In sensory neurons, they act as "chemo-electrical" transducers and are involved in somatic and visceral nociception. Two highly specific inhibitory peptides isolated from animal venoms have considerably helped in the understanding of the physiological roles of these channels in pain. At the peripheral level, ASIC3 is important for inflammatory pain. Its expression and its activity are potentiated by several pain mediators present in the "inflammatory soup" that sensitize nociceptors. ASICs have also been involved in some aspects of mechanosensation and mechanonociception, notably in the gastrointestinal tract, but the underlying mechanisms remain to be determined. At the central level, ASIC1a is largely expressed in spinal cord neurons where it has been proposed to participate in the processing of noxious stimuli and in central sensitization. Blocking ASIC1a in the spinal cord also produces a potent analgesia in a broad range of pain conditions through activation of the opiate system. Targeting ASIC channels at different levels of the nervous system could therefore be an interesting strategy for the relief of pain.

Mice lacking Asic3 show reduced anxiety-like behavior on the elevated plus maze and reduced aggression

Sensing external stimulation is crucial for central processing in the brain and subsequent behavioral expression. Although sensory alteration or deprivation may result in behavioral changes, most studies related to the control of behavior have focused on central mechanisms. Here we created a sensory deficit model of mice lacking acid-sensing ion channel 3 (Asic3(-/-)) to probe behavioral alterations. ASIC3 is predominately distributed in the peripheral nervous system. RT-PCR and immunohistochemistry used to examine the expression of Asic3 in the mouse brain showed near-background mRNA and protein levels of ASIC3 throughout the whole brain, except for the sensory mesencephalic trigeminal nucleus. Consistent with the expression results, Asic3 knockout had no effect on synaptic plasticity of the hippocampus and the behavioral tasks of motor function, learning and memory. In anxiety behavior tasks, Asic3(-/-) mice spent more time in the open arms of an elevated plus maze than did their wild-type littermates. Asic3(-/-) mice also displayed less aggressiveness toward intruders but more stereotypic repetitive behaviors during resident-intruder testing than did wild-type littermates. Therefore, loss of ASIC3 produced behavioral changes in anxiety and aggression in mice, which suggests that ASIC3-dependent sensory activities might relate to the central process of emotion modulation.

Asic3(-/-) female mice with hearing deficit affects social development of pups

Background

Infant crying is an important cue for mothers to respond adequately. Inappropriate response to infant crying can hinder social development in infants. In rodents, the pup-mother interaction largely depends on pup's calls. Mouse pups emit high frequency to ultrasonic vocalization (2–90 kHz) to communicate with their dam for maternal care. However, little is known about how the maternal response to infant crying or pup calls affects social development over the long term.

Methodology/Principal Findings

Here we used mice lacking acid-sensing ion channel 3 (Asic3^−/−) to create a hearing deficit to probe the effect of caregiver hearing on maternal care and adolescent social development. Female Asic3^−/− mice showed elevated hearing thresholds for low to ultrasonic frequency (4–32 kHz) on auditory brain stem response, which thus hindered their response to their pups' wriggling calls and ultrasonic vocalization, as well as their retrieval of pups. In adolescence, pups reared by Asic3^−/− mice showed a social deficit in juvenile social behaviors as compared with those reared by wild-type or heterozygous dams. The social-deficit phenotype in juvenile mice reared by Asic3^−/− mice was associated with the reduced serotonin transmission of the brain. However, Asic3^−/− pups cross-fostered to wild-type dams showed rescued social deficit.

Conclusions/Significance

Inadequate response to pups' calls as a result of ASIC3-dependent hearing loss confers maternal deficits in caregivers and social development deficits in their young.

Moderate exercise increases expression for sensory, adrenergic, and immune genes in chronic fatigue syndrome patients but not in normal subjects

Chronic fatigue syndrome (CFS) is characterized by debilitating fatigue, often accompanied by widespread muscle pain that meets criteria for fibromyalgia syndrome (FMS). Symptoms become markedly worse after exercise. Previous studies implicated dysregulation of the sympathetic nervous system (SNS), and immune system (IS) in CFS and FMS. We recently demonstrated that acid sensing ion channel (probably ASIC3), purinergic type 2X receptors (probably P2X4 and P2X5) and the transient receptor potential vanilloid type 1 (TRPV1) are molecular receptors in mouse sensory neurons detecting metabolites that cause acute muscle pain and possibly muscle fatigue. These molecular receptors are found on human leukocytes along with SNS and IS genes. Real-time, quantitative PCR showed that 19 CFS patients had lower expression of beta-2 adrenergic receptors but otherwise did not differ from 16 control subjects before exercise. After a sustained moderate exercise test, CFS patients showed greater increases than control subjects in gene expression for metabolite detecting receptors ASIC3, P2X4, and P2X5, for SNS receptors alpha-2A, beta-1, beta-2, and COMT and IS genes for IL10 and TLR4 lasting from 0.5 to 48 hours (P < .05). These increases were also seen in the CFS subgroup with comorbid FMS and were highly correlated with symptoms of physical fatigue, mental fatigue, and pain. These new findings suggest dysregulation of metabolite detecting receptors as well as SNS and IS in CFS and CFS-FMS.

PERSPECTIVE

Muscle fatigue and pain are major symptoms of CFS. After moderate exercise, CFS and CFS-FMS patients show enhanced gene expression for receptors detecting muscle metabolites and for SNS and IS, which correlate with these symptoms. These findings suggest possible new causes, points for intervention, and objective biomarkers for these disorders.

Expression of ASIC2 in ciliated cells and stereociliated cells

Acid-sensing ion channel 2 (ASIC2) plays a role as a mechanorecptor and acid receptor in the peripheral and central nervous systems. However, several recent studies have suggested that ASIC2 is expressed in several organs, in addition to the nervous system. We have examined the expression and distribution of ASIC2 in rat ciliated cells (trachea and oviduct) and stereociliated cells (epididymis, Corti organ, and ampullary crest) by immunohistochemistry and transmission electron microscopy (TEM). Immunohistochemistry revealed that ASIC2 was expressed in both ciliated cells and stereociliated cells, but the localization differed between these cell types. In ciliated cells, ASIC2 was coexpressed with a cilial marker (acetylated tubulin). In stereociliated cells stained with a stereocilial marker (phalloidin), ASIC2 was observed in the cell body. Observation by TEM suggested that ASIC2 expression was present at the apical side of the cilial membrane in ciliated cells and at the apical side of the cell body in stereociliated cells. This study thus indicates that the proton receptor ASIC2 is expressed in both ciliated and stereociliated cells.

Hypotonic stimuli enhance proton-gated currents of acid-sensing ion channel-1b

Acid-sensing ion channels (ASICs) are strong candidates for mammalian mechanoreceptors. We investigated whether mouse acid-sensing ion channel-1b (ASIC1b) is sensitive to mechanical stimuli using oocyte electrophysiology, because ASIC1b is located in the mechanosensory stereocilia of cochlear hair cells. Hypotonic stimuli that induced membrane stretch of oocytes evoked no significant current in ASIC1b-expressing oocytes at pH 7.5. However, acid (pH 4.0 or 5.0)-evoked currents in the oocytes were substantially enhanced by the hypotonicity, showing mechanosensitivity of ASIC1b and possible mechanogating of the channel in the presence of other components. Interestingly, the ASIC1b channel was permeable to K(+) (a principal charge carrier for cochlear sensory transduction) and the affinity of the channel for amiloride (IC(50) (inhibition constant)=approximately 48.3 microM) was quite similar to that described for the mouse hair cell mechanotransducer current. Taken together, these data raise the possibility that ASIC1b participates in cochlear mechanoelectrical transduction.

Identification and characterization of a subset of mouse sensory neurons that express acid-sensing ion channel 3

Acid-sensing ion channel 3 (ASIC3) is the most sensitive acid sensor in sensory neurons that innervate into skin, muscle, heart, and visceral tissues. ASIC3 is involved in ischemia sensing, nociception, mechanosensation, and hearing, but how ASIC3-expressing neurons differ in their firing properties is still unknown. We hypothesized that ASIC3-expressing neurons have specialized firing properties, which, coupled with the heterogeneity of acid-sensing properties, accounts for various physiological roles. Here, we successfully identified ASIC3-expressing lumbar dorsal root ganglion (DRG) neurons whose transient proton-gated currents were blocked by salicylic acid (SA). The salicylic acid-sensitive (SAS) neurons did not exist in DRG neurons of mice lacking ASIC3. SAS neurons expressed distinct electrophysiological properties as compared with other DRG neurons. Especially, SAS neurons fired action potentials (APs) with large overshoot and long afterhyperpolarization duration, which suggests that they belong to nociceptors. SAS neurons also exhibited multiple nociceptor markers such as capsaicin response (38%), action potential (AP) with inflection (35%), or tetrodotoxin resistance (31%). Only in SAS neurons but not other DRG neurons was afterhyperpolarization duration correlated with resting membrane potential and AP duration. Our studies reveal a unique feature of ASIC3-expressing DRG neurons and a basis for their heterogeneous functions.

Ion channel gene expression in the inner ear

The ion channel genome is still being defined despite numerous publications on the subject. The ion channel transcriptome is even more difficult to assess. Using high-throughput computational tools, we surveyed all available inner ear cDNA libraries to identify genes coding for ion channels. We mapped over 100,000 expressed sequence tags (ESTs) derived from human cochlea, mouse organ of Corti, mouse and zebrafish inner ear, and rat vestibular end organs to Homo sapiens, Mus musculus, Danio rerio, and Rattus norvegicus genomes. A survey of EST data alone reveals that at least a third of the ion channel genome is expressed in the inner ear, with highest expression occurring in hair cell-enriched mouse organ of Corti and rat vestibule. Our data and comparisons with other experimental techniques that measure gene expression show that every method has its limitations and does not per se provide a complete coverage of the inner ear ion channelome. In addition, the data show that most genes produce alternative transcripts with the same spectrum across multiple organisms, no ion channel gene variants are unique to the inner ear, and many splice variants have yet to be annotated. Our high-throughput approach offers a qualitative computational and experimental analysis of ion channel genes in inner ear cDNA collections. A lack of data and incomplete gene annotations prevent both rigorous statistical analyses and comparisons of entire ion channelomes derived from different tissues and organisms.

Gene expression profiling analysis of the inner ear

Recent developments in molecular genetics, including progress in the human genome project, have allowed identification of genes at an unprecedented rate. To date gene expression profiling studies have focused on identifying transcripts that are specifically or preferentially enriched within the inner ear on the assumption that they are more likely to be important for auditory and vestibular function. It is now apparent that some genes preferentially expressed in the cochleo-vestibular system are not crucial for hearing or balance or their functions are compensated for by other genes. In addition, transcripts expressed at low abundance in the inner ear are generally under-represented in gene profiling studies. In this review, we highlight the limitations of current gene expression profiling strategies as a discovery tool for genes involved in cochleo-vestibular development and function. We argue that expression profiling based on hierarchical clustering of transcripts by gene ontology, combined with tissue enrichment data, is more effective for inner ear gene discovery. This approach also provides a framework to assist and direct the functional characterization of gene products.

Gene expression changes during step-wise differentiation of embryonic stem cells along the inner ear hair cell pathway

CONCLUSION

Our study outlines an alternative approach for the selection and investigation of genes involved in inner ear function.

OBJECTIVE

To gain understanding of the gene pathways involved in the development of the normal cochlea.

MATERIALS AND METHODS

Microarray technology currently offers the most efficient approach to investigate gene expression and identify pathways involved in cell differentiation. Epidermal growth factor (EGF) induces cultures derived from the organ of Corti to proliferate and produce new hair cells. Since pluripotent embryonic stem (ES) cells have the capacity to generate all tissues, we induced murine ES cells to differentiate towards ectodermal and neuroectodermal cell types and from there investigated their commitment towards the hair cell lineage in the presence of EGF. Cells were collected at three points along the differentiation pathway and their expression profiles were determined using the Soares NMIE mouse inner ear cDNA library printed in microarray format.

RESULTS

Three genes up-regulated after addition of EGF (serine (or cysteine) proteinase inhibitor, clade H, member 1 (Serpinh1), solute carrier family 2 (facilitated glucose transporter), member 10 (Slc2a10) and secreted acidic cysteine-rich glycoprotein (Sparc)) were selected for further analysis and characterization. Of the three genes, Serpinh1 and Slc2a10 have never been implicated in the hearing process.

Peptides inhibitors of acid-sensing ion channels

Acid-sensing ion channels (ASICs) channels are proton-gated cationic channels mainly expressed in central and peripheric nervous system and related to the epithelial amiloride-sensitive Na(+) channels and to the degenerin family of ion channels. ASICs comprise four proteins forming functional channel subunits (ASIC1a, ASIC1b, ASIC2a, and ASIC3) and two proteins (ASIC2b and ASIC4) without yet known activators. Functional channels are activated by external pH variations ranging from pH(0.5) 6.8 to 4.0 and currents are characterized by either rapid kinetics of inactivation (ASIC1a, ASIC1b, ASIC3) or slow kinetics of inactivation (ASIC2a) and sometimes the presence of a plateau phase (ASIC3). ASIC1a and ASIC3, which are expressed in nociceptive neurons, have been implicated in inflammation and knockout mice studies support the role of ASIC3 in various pain processes. ASIC1a seems more related to synaptic plasticity, memory, learning and fear conditioning in the CNS. ASIC2a contributes to hearing in the cochlea, sour taste sensation, and visual transduction in the retina. The pharmacology of ASICs is limited to rather nonselective drugs such as amiloride, nonsteroid anti-inflammatory drugs, and neuropeptides. Recently, two peptides, PcTx1 and APETx2, isolated from a spider and a sea anemone, have been characterized as selective and high-affinity inhibitors for ASIC1a and ASIC3 channels, respectively. PcTx1 inhibits ASIC1a homomers with an affinity of 0.7 nM (IC(50)) without any effect on ASIC1a containing heteromers and thus helped to characterize ASIC1a homomeric channels in peripheric and central neurons. PcTx1 acts as a gating modifier since it shifts the channel from the resting to an inactivated state by increasing its affinity for H(+). APETx2 is less selective since it inhibits several ASIC3-containing channels (IC(50) from 63 nM to 2 microM) and to date its mode of action is unknown. Nevertheless, APETx2 structure is related to other sea anemone peptides, which act as gating modifiers on Nav and Kv channels.

Acid-sensing ion channel-1b in the stereocilia of mammalian cochlear hair cells

We investigated whether amiloride-blockable proton-gated cation channels ASIC1a (acid-sensing ion channel-1a) and ASIC1b are expressed in the stereocilia of mouse cochlear hair cells. In-situ hybridization studies showed that ASIC1b transcripts, but not ASIC1a transcripts, were expressed in the inner and outer hair cells. Fluorescent immunohistochemical and immunogold electron microscopic analyses revealed that the ASIC1b channels were located at the insertions of the stereocilia into the hair cells. Our findings provide a novel molecular key to the understanding of cochlear physiology and pathophysiology.

Acid-sensing ion channels: advances, questions and therapeutic opportunities

Extracellular acid can have important effects on neuron function. In central and peripheral neurons, acid-sensing ion channels (ASICs) have emerged as key receptors for extracellular protons, and recent studies suggest diverse roles for these channels in the pathophysiology of pain, ischemic stroke and psychiatric disease. ASICs have also been implicated in mechanosensation in the peripheral nervous system and in neurotransmission in the central nervous system. Here, we briefly review advances in our understanding of ASICs, their potential contributions to disease, and the possibility for their therapeutic modification.

Mechano-electrical transduction: new insights into old ideas

The gating-spring theory of hair cell mechanotransduction channel activation was first postulated over twenty years ago. The basic tenets of this hypothesis have been reaffirmed in hair cells from both auditory and vestibular systems and across species. In fact, the basic findings have been reproduced in every hair cell type tested. A great deal of information regarding the structural, mechanical, molecular and biophysical properties of the sensory hair bundle and the mechanotransducer channel has accumulated over the past twenty years. The goal of this review is to investigate new data, using the gating spring hypothesis as the framework for discussion. Mechanisms of channel gating are presented in reference to the need for a molecular gating spring or for tethering to the intra- or extracellular compartments. Dynamics of the sensory hair bundle and the presence of motor proteins are discussed in reference to passive contributions of the hair bundle to gating compliance. And finally, the molecular identity of the channel is discussed in reference to known intrinsic properties of the native transducer channel.