A modulatory subunit of acid sensing ion channels in brain and dorsal root ganglion cells

Upregulation of acid-sensing ion channel 1 protein expression by chronic administration of cocaine in the mouse striatum in vivo

Acid-sensing ion channels (ASICs) are ligand-gated cation channels activated by a drop in extracellular pH. They are enriched in the mammalian brain with a high synaptic density. Accumulating evidence suggests that ASIC1 contributes to synaptic activity related to learning/memory and fear conditioning, and also plays critical roles in neurodegenerative diseases. In this study, we explored the effect of the psychostimulant, cocaine, on protein expression of ASICs in the mouse forebrain in vivo. We found that chronic systemic injection of cocaine (20mg/kg, once daily for 5 consecutive days; 14 days of withdrawal) increased ASIC1, but not ASIC2, protein levels in the striatum, including the dorsal (caudate putamen) and the ventral (nucleus accumbens) striatum. No significant changes in ASIC1 or 2 protein levels in the median prefrontal cortex and the hippocampus were observed following the chronic cocaine administration. These data demonstrate that chronic cocaine exposure can upregulate ASIC expression in the striatum in a subunit-selective manner.

Characterization of acid-sensing ion channels in medium spiny neurons of mouse striatum

Acid-sensing ion channels (ASICs) regulate synaptic activities and play important roles in neurodegenerative diseases. They are highly expressed in the striatum, where medium spiny neurons (MSNs) are a major population. Given that the properties of ASICs in MSNs are unknown, in this study, we characterized ASICs in MSNs of the mouse striatum. A rapid drop in extracellular pH induced transient inward currents in all MSNs. The pH value for half-maximal activation was 6.25, close to that obtained in homomeric ASIC1a channels. Based on psalmotoxin 1 and zinc sensitivity, ASIC1a (70.5% of neurons) and heteromeric ASIC1a-2 channels (29.5% of neurons) appeared responsible for the acid-induced currents in MSNs. ASIC currents were diminished in MSNs from ASIC1, but not ASIC2, null mice. Furthermore, a drop in pH induced calcium influx by activating homomeric ASIC1a channels. Activation of ASICs increased the membrane excitability of MSNs and lowering extracellular Ca2+ potentiated ASIC currents. Our data suggest that the homomeric ASIC1a channel represents a majority of the ASIC isoform in MSNs. The potential function of ASICs in the striatum requires further investigation.

Acid-sensing ion channels in neurones of the rat suprachiasmatic nucleus

We used reduced slice reparations to study ASIC-like currents in the rat central clock suprachiasmatic nucleus (SCN). In reduced SCN preparations, a drop of extracellular pH evoked a desensitizing inward current to excite SCN neurones to fire at higher rates. Under voltage-clamped conditions, all SCN neurones responded to a 5 s pH step to 6.4 with an inward current that decayed with an average time constant of 1.2 s to 10% of the peak at the end of step. The current was blocked by amiloride with an IC(50) of 14 microm and was carried mainly by Na(+), suggesting an origin of ASIC-like channels. The SCN neurones were sensitive to neutral pH, with 94% of cells responding to pH 7.0 with an inward current. The study of sensitivity to pH between 7.0 and 4.4 revealed a two-component dose-dependent H(+) activation in most SCN neurones, with the first component (85% in amplitude) having a pH(50) of 6.6, and the second (15%) a pH(50) of 5. The ASIC-like currents were potentiated by lactate and low Ca(2+), but were inhibited by Zn(2+). RT-PCR analysis demonstrated the presence of mRNA for ASIC1a, 2a, 2b, and 3 in SCN. Compared to other central neurones, the unique presence of ASIC3 along with ASIC1a in SCN neurones may contribute to the high pH sensitivity and unusual inhibition by Zn(2+). The high pH sensitivity suggests that the SCN neurones are susceptive to extracellular acidification of physiological origins and that the ASIC current might play a role in regulating SCN excitability.

Distribution of acid-sensing ion channel 3 in the rat hypothalamus

Acid-sensing ion channels (ASICs), the members of the epithelial sodium channel/degenerin (ENaC/DEG) superfamily, are proton-gated voltage-insensitive cation channels. Six ASIC subunits have been identified and characterized in the mammalian nervous system so far. Of these subunits, ASIC3 has been shown to be predominantly expressed in the peripheral nervous system of rodents and implicated in mechnosensation, chemosensation and pain perception. Little is known on ASIC3 in the brain. We thus employed reverse transcription-polymerase chain reaction (RT-PCR) and Western blot to examine the expression of ASIC3 in various rat brain regions, including hippocampus, amygdala, caudate putamen, prefrontal cortex, and hypothalamus. Specific attention was paid to the distribution of ASIC3 in the hypothalamus of rats by using immunohistochemistry. ASIC3 immunoreactivity showed a widespread pattern throughout the hypothalamus, with the highest density in paraventricular nucleus, supraoptic nucleus, suprachiasmatic nucleus, arcuate nucleus, dorsomedial nucleus, median preoptic nucleus, ventromedial preoptic nucleus, and dorsal tuberomammillary nucleus. This study may contribute to the understanding of ASIC3 functions in the CNS.

Acid-sensitive ion channels and receptors

Acidosis is a noxious condition associated with inflammation, ischaemia or defective acid containment. As a consequence, acid sensing has evolved as an important property of afferent neurons with unmyelinated and thinly myelinated nerve fibres. Protons evoke multiple currents in primary afferent neurons, which are carried by several acid-sensitive ion channels. Among these, acid-sensing ion channels (ASICs) and transient receptor potential (TRP) vanilloid-1 (TRPV1) ion channels have been most thoroughly studied. ASICs survey moderate decreases in extracellular pH, whereas TRPV1 is activated only by severe acidosis resulting in pH values below 6. Two-pore-domain K(+) (K(2P)) channels are differentially regulated by small deviations of extra- or intracellular pH from physiological levels. Other acid-sensitive channels include TRPV4, TRPC4, TRPC5, TRPP2 (PKD2L1), ionotropic purinoceptors (P2X), inward rectifier K(+) channels, voltage-activated K(+) channels, L-type Ca(2+) channels, hyperpolarization-activated cyclic nucleotide gated channels, gap junction channels, and Cl(-) channels. In addition, acid-sensitive G protein coupled receptors have also been identified. Most of these molecular acid sensors are expressed by primary sensory neurons, although to different degrees and in various combinations. Emerging evidence indicates that many of the acid-sensitive ion channels and receptors play a role in acid sensing, acid-induced pain and acid-evoked feedback regulation of homeostatic reactions. The existence and apparent redundancy of multiple pH surveillance systems attests to the concept that acid-base regulation is a vital issue for cell and tissue homeostasis. Since upregulation and overactivity of acid sensors appear to contribute to various forms of chronic pain, acid-sensitive ion channels and receptors are considered as targets for novel analgesic drugs. This approach will only be successful if the pathological implications of acid sensors can be differentiated pharmacologically from their physiological function.

The cytoskeletal protein alpha-actinin regulates acid-sensing ion channel 1a through a C-terminal interaction

The acid-sensing ion channel 1a (ASIC1a) is widely expressed in central and peripheral neurons where it generates transient cation currents when extracellular pH falls. ASIC1a confers pH-dependent modulation on postsynaptic dendritic spines and has critical effects in neurological diseases associated with a reduced pH. However, knowledge of the proteins that interact with ASIC1a and influence its function is limited. Here, we show that α-actinin, which links membrane proteins to the actin cytoskeleton, associates with ASIC1a in brain and in cultured cells. The interaction depended on an α-actinin-binding site in the ASIC1a C terminus that was specific for ASIC1a versus other ASICs and for α-actinin-1 and -4. Co-expressing α-actinin-4 altered ASIC1a current density, pH sensitivity, desensitization rate, and recovery from desensitization. Moreover, reducing α-actinin expression altered acid-activated currents in hippocampal neurons. These findings suggest that α-actinins may link ASIC1a to a macromolecular complex in the postsynaptic membrane where it regulates ASIC1a activity.

sigma-1 receptor modulation of acid-sensing ion channel a (ASIC1a) and ASIC1a-induced Ca2+ influx in rat cortical neurons

Acid-sensing ion channels (ASICs) are proton-gated cation channels found in peripheral and central nervous system neurons. The ASIC1a subtype, which has high Ca2+ permeability, is activated by ischemia-induced acidosis and contributes to the neuronal loss that accompanies ischemic stroke. Our laboratory has shown that activation of sigma receptors depresses ion channel activity and [Ca2+](i) dysregulation during ischemia, which enhances neuronal survival. Whole-cell patch-clamp electrophysiology and fluorometric Ca2+ imaging were used to determine whether sigma receptors regulate the function of ASIC in cultured rat cortical neurons. Bath application of the selective ASIC1a blocker, psalmotoxin1, decreased proton-evoked [Ca2+](i) transients and peak membrane currents, suggesting the presence of homomeric ASIC1a channels. The pan-selective sigma-1/sigma-2 receptor agonists, 1,3-di-o-tolyl-guanidine (100 microM) and opipramol (10 microM), reversibly decreased acid-induced elevations in [Ca2+](i) and membrane currents. Pharmacological experiments using sigma receptor-subtype-specific agonists demonstrated that sigma-1, but not sigma-2, receptors inhibit ASIC1a-induced Ca2+ elevations. These results were confirmed using the irreversible sigma receptor antagonist metaphit (50 microM) and the selective sigma-1 antagonist BD1063 (10 nM), which obtunded the inhibitory effects of the sigma-1 agonist, carbetapentane. Activation of ASIC1a was shown to stimulate downstream Ca2+ influx pathways, specifically N-methyl-D-aspartate and (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid/kainate receptors and voltage-gated Ca2+ channels. These subsequent Ca2+ influxes were also inhibited upon activation of sigma-1 receptors. These findings demonstrate that sigma-1 receptor stimulation inhibits ASIC1a-mediated membrane currents and consequent intracellular Ca2+ accumulation. The ability to control ionic imbalances and Ca2+ dysregulation evoked by ASIC1a activation makes sigma receptors an attractive target for ischemic stroke therapy.

Effect of cytosolic pH on epithelial Na+ channel in normal and cystic fibrosis sweat ducts

The activities of cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel and the amiloride-sensitive epithelial Na(+) channel (ENaC) are acutely coordinated in the sweat duct. However, the mechanisms responsible for cross-talk between these ion channels are unknown. Previous studies indicated that luminal pH of sweat ducts varies over 3 pH units and that the cytoplasmic pH affects both CFTR and ENaC. Therefore, using basolaterally alpha-toxin-permeabilized apical membrane preparations of sweat ducts as an experimental system, we tested the hypothesis that the cytosolic pH may mediate the cross-talk between CFTR and ENaC. We showed that while luminal pH had no effect, cytosolic pH acutely affected ENaC activity. That is, acidic pH inhibited, while basic pH activated, ENaC. pH regulation of ENaC appears to be independent of CFTR or endogenous kinase activities because basic pH independently stimulated ENaC (1) in normal ducts even when CFTR was deactivated, (2) in CF ducts that lack CFTR in the plasma membranes and (3) after blocking endogenous kinase activity with staurosporine. Considering the evidence of Na(+)/H(+) exchange (NHE) activity as shown by the expression of mRNA and function of NHE in the basolateral membrane of the sweat duct, we postulate that changes in cytosolic Na(+) ([Na(+)]( i )) may alter cytosolic pH (pH( i )) as salt loads into the cell during electrolyte absorption. These changes may play a role in coordinating the activities of ENaC and CFTR during transepithelial salt transport.

Regulation of ASIC activity by ASIC4--new insights into ASIC channel function revealed by a yeast two-hybrid assay

ASIC4 is a member of the acid-sensing ion channel family that is broadly expressed in the mammalian nervous system, but has no known function. We demonstrate here that transfected ASIC4 is targeted to the plasma membrane in CHO-K1 cells, where it associates with ASIC1a and downregulates exogenous ASIC1a expression. This effect could also be observed on endogenous H+-gated currents in TSA-201 cells and ASIC3 currents in CHO-K1 cells, suggesting a physiological role for ASIC4 in regulating ASIC currents involved in pain mechanisms. Using a yeast two-hybrid assay we found that ASICs interact with proteins involved in diverse functions, including cytoskeletal proteins, enzymes, regulators of endocytosis and G-protein-coupled pathways. ASIC4 is the sole member of this ion channel class to interact strongly with polyubiquitin. The distinct functionally related sets of interacting proteins that bind individual ASICs identified in the yeast two-hybrid screen suggest potential roles for ASICs in a variety of cellular functions.

ASIC3, a sensor of acidic and primary inflammatory pain

Acid-sensing ion channels (ASICs) are cationic channels activated by extracellular acidosis that are expressed in both central and peripheral nervous systems. Although peripheral ASICs seem to be natural sensors of acidic pain (e.g., in inflammation, ischaemia, lesions or tumours), a direct demonstration is still lacking. We show that approximately 60% of rat cutaneous sensory neurons express ASIC3-like currents. Native as well as recombinant ASIC3 respond synergistically to three different inflammatory signals that are slight acidifications (approximately pH 7.0), hypertonicity and arachidonic acid (AA). Moderate pH, alone or in combination with hypertonicity and AA, increases nociceptors excitability and produces pain suppressed by the toxin APETx2, a specific blocker of ASIC3. Both APETx2 and the in vivo knockdown of ASIC3 with a specific siRNA also have potent analgesic effects against primary inflammation-induced hyperalgesia in rat. Peripheral ASIC3 channels are thus essential sensors of acidic pain and integrators of molecular signals produced during inflammation where they contribute to primary hyperalgesia.

SMAD3 functions as a transcriptional repressor of acid-sensing ion channel 3 (ASIC3) in nucleus pulposus cells of the intervertebral disc

The goal of this investigation was to study the regulation of acid-sensing ion channel (ASIC)3 expression by TGFbeta in the nucleus pulposus cells of the intervertebral disc. Analysis of human nucleus pulposus tissue indicated decreased ASIC3 and elevated TGFbeta expression in the degenerate state. In a parallel study, treatment of nucleus pulposus cells with TGFbeta resulted in decreased expression of ASIC3 mRNA and protein. Suppression of ASIC3 promoter activity was evident when the nucleus pulposus cells were treated with TGFbeta or co-transfected with the constitutively active ALK5 or a smad3 construct. On the other hand, co-transfection of dominant negative smad3 or smad7 restored ASIC3 promoter activity. We validated the role of smad3 in controlling ASIC3 expression using cells derived from smad3-null mice. ASIC3 promoter activity in the null cells was 2- to 3-fold higher than the wildtype cells. Moreover, expression of smad3 in null cells decreased ASIC3 promoter activity by almost 50%. Further studies using deletion constructs and trichostatin A treatment showed that the full-length smad3 was necessary, and the suppression involved recruitment of histone deacetylase to the promoter. To determine the mechanism, we evaluated the rat ASIC3 promoter sequence and noted the presence of two smad interacting CAGA box motifs. Gel-shift and supershift analysis indicated that smad3 protein was bound to this motif. Chromatin immunoprecipitation analysis confirmed that smad3 bound both the CAGA elements. Results of these studies clearly show that TGFbeta is highly expressed in the degenerate disc and through smad3 serves as a negative regulator of ASIC3 expression.

Mice lacking acid-sensing ion channels (ASIC) 1 or 2, but not ASIC3, show increased pain behaviour in the formalin test

Extracellular acidification is a component of the inflammatory process and may be a factor driving the pain accompanying it. Acid-sensing ion channels (ASICs) are neuronal proton sensors and evidence suggests they are involved in signalling inflammatory pain. The aims of this study were to (1) clarify the role of ASICs in nociception and (2) confirm their involvement in inflammatory pain and determine whether this was subunit specific. This was achieved by (1) direct comparison of the sensitivity of ASIC1, ASIC2, ASIC3 and TRPV1 knockout mice versus wildtype littermates to acute thermal and mechanical noxious stimuli and (2) studying the behavioural responses of each transgenic strain to hind paw inflammation with either complete Freund's adjuvant (CFA) or formalin. Naïve ASIC1(-/-) and ASIC2(-/-) mice responded normally to acute noxious stimuli, whereas ASIC3(-/-) mice were hypersensitive to high intensity thermal stimuli. CFA injection decreased mechanical and thermal withdrawal thresholds for up to 8 days. ASIC2(-/-) mice had increased mechanical sensitivity on day 1 post-CFA compared to wildtype controls. TRPV1(-/-) mice had significantly reduced thermal, but not mechanical, hyperalgesia on all days after inflammation. Following formalin injection, ASIC1(-/-) and ASIC2(-/-), but not ASIC3(-/-) or TRPV1(-/-), mice showed enhanced pain behaviour, predominantly in the second phase of the test. These data suggest that whilst ASICs may play a role in mediating inflammatory pain, this role is likely to be modulatory and strongly dependent on channel subtype.

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.

Seizure termination by acidosis depends on ASIC1a

Most seizures stop spontaneously; however, the molecular mechanisms that terminate seizures remain unknown. Observations that seizures reduced brain pH and that acidosis inhibited seizures indicate that acidosis halts epileptic activity. Because acid-sensing ion channel 1a (ASIC1a) is exquisitely sensitive to extracellular pH and regulates neuron excitability, we hypothesized that acidosis might activate ASIC1a, which would terminate seizures. Disrupting mouse ASIC1a increased the severity of chemoconvulsant-induced seizures, whereas overexpressing ASIC1a had the opposite effect. ASIC1a did not affect seizure threshold or onset, but shortened seizure duration and prevented seizure progression. CO2 inhalation, long known to lower brain pH and inhibit seizures, required ASIC1a to interrupt tonic-clonic seizures. Acidosis activated inhibitory interneurons through ASIC1a, suggesting that ASIC1a might limit seizures by increasing inhibitory tone. Our results identify ASIC1a as an important element in seizure termination when brain pH falls and suggest both a molecular mechanism for how the brain stops seizures and new therapeutic strategies.

A constitutional translocation t(1;17)(p36.2;q11.2) in a neuroblastoma patient disrupts the human NBPF1 and ACCN1 genes

The human 1p36 region is deleted in many different types of tumors, and so it probably harbors one or more tumor suppressor genes. In a Belgian neuroblastoma patient, a constitutional balanced translocation t(1;17)(p36.2;q11.2) may have led to the development of the tumor by disrupting or activating a gene. Here, we report the cloning of both translocation breakpoints and the identification of a novel gene that is disrupted by this translocation. This gene, named NBPF1 for Neuroblastoma BreakPoint Family member 1, belongs to a recently described gene family encoding highly similar proteins, the functions of which are unknown. The translocation truncates NBPF1 and gives rise to two chimeric transcripts of NBPF1 sequences fused to sequences derived from chromosome 17. On chromosome 17, the translocation disrupts one of the isoforms of ACCN1, a potential glioma tumor suppressor gene. Expression of the NBPF family in neuroblastoma cell lines is highly variable, but it is decreased in cell lines that have a deletion of chromosome 1p. More importantly, expression profiling of the NBPF1 gene showed that its expression is significantly lower in cell lines with heterozygous NBPF1 loss than in cell lines with a normal 1p chromosome. Meta-analysis of the expression of NBPF and ACCN1 in neuroblastoma tumors indicates a role for the NBPF genes and for ACCN1 in tumor aggressiveness. Additionally, DLD1 cells with inducible NBPF1 expression showed a marked decrease of clonal growth in a soft agar assay. The disruption of both NBPF1 and ACCN1 genes in this neuroblastoma patient indicates that these genes might suppress development of neuroblastoma and possibly other tumor types.

Impaired pressure-induced constriction in mouse middle cerebral arteries of ASIC2 knockout mice

Recent studies from our laboratory demonstrated the importance of mechanosensitive epithelial Na+channel (ENaC) proteins in pressure-induced constriction in renal and cerebral arteries. ENaC proteins are closely related to acid-sensing ion channel 2 (ASIC2), a protein known to be required for normal mechanotransduction in certain sensory neurons. However, the role of the ASIC2 protein in pressure-induced constriction has never been addressed. The goal of the current study was to investigate the role of ASIC2 proteins in pressure-induced, or myogenic, constriction in the mouse middle cerebral arteries (MCAs) from ASIC2 wild-type (+/+), heterozygous (+/−), and null (−/−) mice. Constrictor responses to KCl (20–80 mM) and phenylephrine (10−7–10−4M) were not different among groups. However, vasoconstrictor responses to increases in intraluminal pressure (15–90 mmHg) were impaired in MCAs from ASIC2−/−and+/−mice. At 60 and 90 mmHg, MCAs from ASIC2+/+mice generated 13.7 ± 2.1% and 15.8 ± 2.0% tone and ASIC2−/−mice generated 7.4 ± 2.8% and 12.5 ± 2.4% tone, respectively. Surprisingly, MCAs from ASIC2+/−mice generated 1.2 ± 2.2% and 3.9 ± 1.8% tone at 60 and 90 mmHg. The reason underlying the total loss of myogenic tone in the ASIC2+/−is not clear, although the loss of mechanosensitive β- and γ-ENaC proteins may be a contributing factor. These results demonstrate that normal ASIC2 expression is required for normal pressure-induced constriction in the MCA. Furthermore, ASIC2 may be involved in establishing the basal level of myogenic tone.

Current concepts of nociception: nociceptive molecular sensors in sensory neurons

PURPOSE OF REVIEW

A large number of channels that are in some way linked to sensory transduction including nociception have been discovered in recent years. This review summarizes newly discovered channels that are implicated in nociception. Furthermore, details are discussed with emphasis on their possible application to clinical use as analgesics.

RECENT FINDINGS

Studies with null mutant animals deficient in these channel genes reveal that the channels are indeed implicated in physiological as well as pathological nociception.

SUMMARY

Many transient receptor potential channels are thermosensors that detect cold, warm and hot temperatures. These channels are activated not only by natural chemicals such as capsaicin, menthol, and camphor, but by various inflammatory signaling pathways. The acid-sensing ion channel and P2X channel that detect extracellular acidosis and ATP are also implicated in some types of pain. Voltage-gated sodium or calcium channels draw attention because of their involvement in neuropathic pain.

Acid sensing ion channels in dorsal spinal cord neurons

Acid-sensing ion channels (ASICs) are broadly expressed in the CNS, including the spinal cord. However, very little is known about the properties of ASICs in spinal cord neurons compared with brain. We show here that ASIC1a and ASIC2a are the most abundant ASICs in mouse adult spinal cord and are coexpressed by most neurons throughout all the laminas. ASIC currents in cultured embryonic day 14 mouse dorsal spinal neurons mainly flow through homomeric ASIC1a (34% of neurons) and heteromeric ASIC1a plus 2a channels at a ratio of 2:1 (83% of neurons). ASIC2b only has a minor contribution to these currents. The two channel subtypes show different active pH ranges and different inactivation and reactivation kinetics supporting complementary functional properties. One striking property of native dorsal spinal neuron currents and recombinant currents is the pH dependence of the reactivation process. A light sustained acidosis induces a threefold slow-down of the homomeric ASIC1a (from pH 7.4 to pH 7.3) and heteromeric ASIC1a plus 2a (from pH 7.4 to pH 7.2) current reactivation (T(0.5) increasing from 5.77 to 16.84 s and from 0.98 to 3.2 s, respectively), whereas a larger acidosis to pH 6.6 induces a 32-fold slow-down of the ASIC1a plus 2a current reactivation (T(0.5) values increasing to 31.30 s). The pH dependence of ASIC channel reactivation is likely to modulate neuronal excitability associated with repetitive firing in response to extracellular pH oscillations, which can be induced, for example, by intense synaptic activity of central neurons.

Acid-sensing ion channels interact with and inhibit BK K+ channels

Acid-sensing ion channels (ASICs) are neuronal non-voltage-gated cation channels that are activated when extracellular pH falls. They contribute to sensory function and nociception in the peripheral nervous system, and in the brain they contribute to synaptic plasticity and fear responses. Some of the physiologic consequences of disrupting ASIC genes in mice suggested that ASIC channels might modulate neuronal function by mechanisms in addition to their H(+)-evoked opening. Within ASIC channel's large extracellular domain, we identified sequence resembling that in scorpion toxins that inhibit K(+) channels. Therefore, we tested the hypothesis that ASIC channels might inhibit K(+) channel function by coexpressing ASIC1a and the high-conductance Ca(2+)- and voltage-activated K(+) (BK) channel. We found that ASIC1a associated with BK channels and inhibited their current. Reducing extracellular pH disrupted the association and relieved the inhibition. BK channels, in turn, altered the kinetics of ASIC1a current. In addition to BK, ASIC1a inhibited voltage-gated Kv1.3 channels. Other ASIC channels also inhibited BK, although acidosis-dependent relief of inhibition varied. These results reveal a mechanism of ion channel interaction and reciprocal regulation. Finding that a reduced pH activated ASIC1a and relieved BK inhibition suggests that extracellular protons may enhance the activity of channels with opposing effects on membrane voltage. The wide and varied expression patterns of ASICs, BK, and related K(+) channels suggest broad opportunities for this signaling system to alter neuronal function.

Modulation of acid-sensing ion channel activity by nitric oxide

Acid-sensing ion channels (ASICs) are a class of ion channels activated by extracellular protons and are believed to mediate the pain caused by tissue acidosis. Although ASICs have been widely studied, little is known about their regulation by inflammatory mediators. Here, we provide evidence that nitric oxide (NO) potentiates the activity of ASICs. Whole-cell patch-clamp recordings were performed on neonatal rat cultured dorsal root ganglion neurons and on ASIC isoforms expressed in CHO cells. The NO donor S-nitroso-N-acetylpenicillamine (SNAP) potentiates proton-gated currents in DRG neurons and proton-gated currents in CHO cells expressing each of the acid-sensitive ASIC subunits. Modulators of the cGMP/PKG pathway had no effect on the potentiation, but in excised patches from CHO cells expressing ASIC2a, the potentiation could be reversed by externally applied reducing agents. NO therefore has a direct external effect on the ASIC ion channel, probably through oxidization of cysteine residues. Complementary psychophysiological studies were performed using iontophoresis of acidic solutions through the skin of human volunteers. Topical application of the NO donor glyceryl trinitrate significantly increased acid-evoked pain but did not affect heat or mechanical pain thresholds. ASICs may therefore play an important role in the pain associated with metabolic stress and inflammation, where both tissue acidosis and a high level of NO are present.

Presynaptic release probability is increased in hippocampal neurons from ASIC1 knockout mice

Acid-sensing ion channels (ASICs) are H(+)-gated channels that produce transient cation currents in response to extracellular acid. ASICs are expressed in neurons throughout the brain, and ASIC1 knockout mice show behavioral impairments in learning and memory. The role of ASICs in synaptic transmission, however, is not thoroughly understood. We analyzed the involvement of ASICs in synaptic transmission using microisland cultures of hippocampal neurons from wild-type and ASIC knockout mice. There was no significant difference in single action potential (AP)-evoked excitatory postsynaptic currents (EPSCs) between wild-type and ASIC knockout neurons. However, paired-pulse ratios (PPRs) were reduced and spontaneous miniature EPSCs (mEPSCs) occurred at a higher frequency in ASIC1 knockout neurons compared with wild-type neurons. The progressive block of NMDA receptors by an open channel blocker, MK-801, was also faster in ASIC1 knockout neurons. The amplitude and decay time constant of mEPSCs, as well as the size and refilling of the readily releasable pool, were similar in ASIC1 knockout and wild-type neurons. Finally, the release probability, which was estimated directly as the ratio of AP-evoked to hypertonic sucrose-induced charge transfer, was increased in ASIC1 knockout neurons. Transfection of ASIC1a into ASIC1 knockout neurons increased the PPRs, suggesting that alterations in release probability were not the result of developmental compensation within the ASIC1 knockout mice. Together, these findings demonstrate that neurons from ASIC1 knockout mice have an increased probability of neurotransmitter release and indicate that ASIC1a can affect presynaptic mechanisms of synaptic transmission.

Expression of acid-sensing ion channel 3 (ASIC3) in nucleus pulposus cells of the intervertebral disc is regulated by p75NTR and ERK signaling

Although a recent study has shown that skeletal tissues express ASICs, their function is unknown. We show that intervertebral disc cells express ASIC3; moreover, expression is uniquely regulated and needed for survival in a low pH and hypoeromsotic medium. These findings suggest that ASIC3 may adapt disc cells to their hydrodynamically stressed microenvironment.

INTRODUCTION

The nucleus pulposus is an avascular, hydrated tissue that permits the intervertebral disc to resist compressive loads to the spine. Because the tissue is hyperosmotic and avascular, the pH of the nucleus pulposus is low. To determine the mechanisms by which the disc cells accommodate to the low pH and hypertonicity, the expression and regulation of the acid sensing ion channel (ASIC)3 was examined.

MATERIALS AND METHODS

Expression of ASICs in cells of the intervertebral disc was analyzed. To study its regulation, we cloned the 2.8-kb rat ASIC3 promoter and performed luciferase reporter assays. The effect of pharmacological inhibition of ASICs on disc cell survival was studied by measuring MTT and caspase-3 activities.

RESULTS

ASIC3 was expressed in discal tissues and cultured disc cells in vitro. Because studies of neuronal cells have shown that ASIC3 expression and promoter activity is induced by nerve growth factor (NGF), we examined the effect of NGF on nucleus pulposus cells. Surprisingly, ASIC3 promoter activity did not increase after NGF treatment. The absence of induction was linked to nonexpression of tropomyosin-related kinase A (TrkA), a high-affinity NGF receptor, although a modest expression of p75NTR was seen. When treated with p75NTR antibody or transfected with dominant negative-p75NTR plasmid, there was significant suppression of ASIC3 basal promoter activity. To further explore the downstream mechanism of control of ASIC3 basal promoter activity, we blocked p75NTR and measured phospho extracellular matrix regulated kinase (pERK) levels. We found that DN-p75NTR suppressed NGF mediated transient ERK activation. Moreover, inhibition of ERK activity by dominant negative-mitogen activated protein kinase kinase (DN-MEK) resulted in a dose-dependent suppression of ASIC3 basal promoter activity, whereas overexpression of constitutively active MEK1 caused an increase in ASIC3 promoter activity. Finally, to gain insight in the functional importance of ASIC3, we suppressed ASIC activity in nucleus pulposus cells. Noteworthy, under both hyperosmotic and acidic conditions, ASIC3 served to promote cell survival and lower the activity of the pro-apoptosis protein, caspase-3.

CONCLUSIONS

Results of this study indicate that NGF serves to maintain the basal expression of ASIC3 through p75NTR and ERK signaling in discal cells. We suggest that ASIC3 is needed for adaptation of the nucleus pulposus and annulus fibrosus cells to the acidic and hyperosmotic microenvironment of the intervertebral disc.

Candidate amino acids involved in H+ gating of acid-sensing ion channel 1a

Acid-sensing ion channels are ligand-gated cation channels, gated by extracellular H(+). H(+) is the simplest ligand possible, and whereas for larger ligands that gate ion channels complex binding sites in the three-dimensional structure of the proteins have to be assumed, H(+) could in principle gate a channel by titration of a single amino acid. Experimental evidence suggests a more complex situation, however. For example, it has been shown that extracellular Ca(2+) ions compete with H(+); probably Ca(2+) ions bound to the extracellular loop of ASICs stabilize the closed state of the channel and have to be displaced before the channel can open. In such a scheme, amino acids contributing to Ca(2+) binding would also be candidates contributing to H(+) gating. In this study we systematically screened more than 40 conserved, charged amino acids in the extracellular region of ASIC1a for a possible contribution to H(+) gating. We identified four amino acids where substitution strongly affects H(+) gating: Glu(63), His(72)/His(73), and Asp(78). These amino acids are highly conserved among H(+)-sensitive ASICs and are candidates for the "H(+) sensor" of ASICs.

Underlying mechanism of ASIC1a involved in acidosis-induced cytotoxicity in rat C6 glioma cells

AIM

To investigate the underlying mechanism of acid-sensing ion channel (ASIC) 1a involved in the acidosis-induced cytotoxicity of rat C6 glioma cells.

METHODS

The stable ASIC1a-silenced C6 cells built with the RNA interference technology were confirmed by RT-PCR and Western blot analysis. Intracellular calcium ([Ca2+]i) in both the wild-type rat C6 glioma cells and the ASIC1a-silenced C6 cells were analyzed before and after acid application/exposure with the calcium imaging experiment.

RESULTS

The rapid extracellular pH drop induced the increase of [Ca2+]i in the wild-type C6 cells, but not in the ASIC1a-silenced C6 cells. During the prolonged acid exposure, [Ca2+]i was lower in the ASIC1a-silenced C6 cells than that in the control cells.

CONCLUSION

The resultant toxicity of [Ca2+]i might contribute to the acidosis-induced cytotoxicity.

Characterization of human ASIC2a homomeric channels stably expressed in murine Ltk- cells

ASIC2a (BNaC1 or MDEG) is distributed throughout the nervous system and potentially involved in mechanosensation, hearing, vision, and taste functions. However, pharmacological properties of ASIC2 homomers including the mechanism of inhibition by amiloride remain unclear. In this study, we describe the properties of hASIC2a stably expressed in Ltk(-) cells, the first reported stable cell line expressing any ASICs subunit, by standard whole cell voltage clamp method. In response to pH 4.0, at -80 mV, hASIC2a cells exhibited rapidly activating fast transient inward current ( approximately 100 pA/pF) that was followed by a sustained current ( approximately 13 pA/pF). In contrast, untransfected Ltk(-) cells showed only a very small rapidly activating non-inactivating inward current ( approximately 4 pA/pF). The magnitude of hASIC2a transient current was pH dependent with pH(50) values for activation and inactivation of approximately 4.2 and approximately 5.5, respectively. Ion substitution experiments revealed the following rank order of permeability: Na(+)>K(+)>Ca(2+) for the transient current. Amiloride reversibly inhibited the pH 4.0 evoked transient current with IC(50) values of approximately 20 microM at both -30 and -80 mV holding potentials, indicating that the interactions are voltage independent when nearly all amiloride is protonated. Amiloride (100 microM) did not inhibit ASIC2a transient current when pre-applied in pH 7.4 and pH 4.0 currents obtained in absence of amiloride, but it did inhibit currents when co-applied at pH 4.0 suggesting open channel blockade. In summary, ASIC2a stable cell line serves as a useful model system to study the pharmacological properties of ASIC2a currents, potentially contributing to pH-evoked responses in cells of the dorsal root ganglion and the central nervous system.

Regulation of pH in the mammalian central nervous system under normal and pathological conditions: facts and hypotheses

The maintenance of pH homeostasis in the CNS is of key importance for proper execution and regulation of neurotransmission, and deviations from this homeostasis are a crucial factor in the mechanism underlying a spectrum of pathological conditions. The first few sections of the review are devoted to the brain operating under normal conditions. The article commences with an overview of how extrinsic factors modelling the brain at work: neurotransmitters, depolarising stimuli (potassium and voltage changes) and cyclic nucleotides as major signal transducing vehicles affect pH in the CNS. Further, consequences of pH alterations on the major aspects of CNS function and metabolism are outlined. Next, the major cellular events involved in the transport, sequestration, metabolic production and buffering of protons that are common to all the mammalian cells, including the CNS cells. Since CNS function reflects tight interaction between astrocytes and neurons, the pH regulatory events pertinent to either cell type are discussed: overwhelming evidence implicates astrocytes as a key player in pH homeostasis in the brain. The different classes of membrane proteins involved in proton shuttling are listed and their mechanisms of action are given. These include: the Na+/H+ exchanger, different classes of bicarbonate transporters acting in a sodium-dependent- or -independent mode, monocarboxylic acid transporters and the vacuolar-type proton ATPase. A separate section is devoted to carbonic anhydrase, which is represented by multiple isoenzymes capable of pH buffering both in the cell interior and in the extracellular space. Next, impairment of pH regulation and compensatory responses occurring in brain affected by different pathologies: hypoxia/ischemia, epilepsy, hyperammonemic encephalopathies, cerebral tumours and HIV will be described. The review is limited to facts and plausible hypotheses pertaining to phenomena directly involved in pH regulation: changes in pH that accompany metabolic stress but have no distinct implications for the pH regulatory mechanisms are not dealt with. In most cases, the vast body of knowledge derived from in vitro studies remains to be verified in in vivo settings.

Acid-sensing ion channels (ASICs) as pharmacological targets for neurodegenerative diseases

A significant drop of tissue pH or acidosis is a common feature of acute neurological conditions such as ischemic stroke, brain trauma, and epileptic seizures. Acid-sensing ion channels, or ASICs, are proton-gated cation channels widely expressed in peripheral sensory neurons and in the neurons of the central nervous system. Recent studies have demonstrated that activation of these channels by protons plays an important role in a variety of physiological and pathological processes such as nociception, mechanosensation, synaptic plasticity, and acidosis-mediated neuronal injury. This review provides an overview of the recent advance in electrophysiological, pharmacological characterization of ASICs, and their role in neurological diseases. Therapeutic potential of current available ASIC inhibitors is discussed.

Structure of acid-sensing ion channel 1 at 1.9 Å resolution and low pH

Acid-sensing ion channels (ASICs) are voltage-independent, proton-activated receptors that belong to the epithelial sodium channel/degenerin family of ion channels and are implicated in perception of pain, ischaemic stroke, mechanosensation, learning and memory. Here we report the low-pH crystal structure of a chicken ASIC1 deletion mutant at 1.9 A resolution. Each subunit of the chalice-shaped homotrimer is composed of short amino and carboxy termini, two transmembrane helices, a bound chloride ion and a disulphide-rich, multidomain extracellular region enriched in acidic residues and carboxyl-carboxylate pairs within 3 A, suggesting that at least one carboxyl group bears a proton. Electrophysiological studies on aspartate-to-asparagine mutants confirm that these carboxyl-carboxylate pairs participate in proton sensing. Between the acidic residues and the transmembrane pore lies a disulphide-rich 'thumb' domain poised to couple the binding of protons to the opening of the ion channel, thus demonstrating that proton activation involves long-range conformational changes.

ASIC proteins regulate smooth muscle cell migration

The purpose of the present study was to investigate Acid Sensing Ion Channel (ASIC) protein expression and importance in cellular migration. We recently demonstrated that Epithelial Na(+)Channel (ENaC) proteins are required for vascular smooth muscle cell (VSMC) migration; however, the role of the closely related ASIC proteins has not been addressed. We used RT-PCR and immunolabeling to determine expression of ASIC1, ASIC2, ASIC3 and ASIC4 in A10 cells. We used small interference RNA to silence individual ASIC expression and determine the importance of ASIC proteins in wound healing and chemotaxis (PDGF-bb)-initiated migration. We found ASIC1, ASIC2, and ASIC3, but not ASIC4, expression in A10 cells. ASIC1, ASIC2, and ASIC3 siRNA molecules significantly suppressed expression of their respective proteins compared to non-targeting siRNA (RISC) transfected controls by 63%, 44%, and 55%, respectively. Wound healing was inhibited by 10, 20, and 26% compared to RISC controls following suppression of ASIC1, ASIC2, and ASIC3, respectively. Chemotactic migration was inhibited by 30% and 45%, respectively, following suppression of ASIC1 and ASIC3. ASIC2 suppression produced a small, but significant, increase in chemotactic migration (4%). Our data indicate that ASIC expression is required for normal migration and may suggest a novel role for ASIC proteins in cellular migration.

Proton binding sites involved in the activation of acid-sensing ion channel ASIC2a

Most acid-sensing ion channel (ASIC) subunits are activated by protons, but ASIC2b (a splice variant of ASIC2a) is acid-insensitive. Differences in protonatable residues between the extracellular loop regions of ASIC2a and ASIC2b may explain this difference. Site-directed mutagenesis, combined with immunocytochemistry and whole-cell patch clamp, demonstrated that mutating any one of five ASIC2a sites produces channels that traffic normally to the cell surface membrane but are insensitive to protons. One of the mutants forms functional heteromers with ASIC1a and ASIC2a, demonstrating that ion transport is intact in this mutant. These five sites may be involved in the activation of ASIC2a by protons.

Zebrafish acid-sensing ion channel (ASIC) 4, characterization of homo- and heteromeric channels, and identification of regions important for activation by H+

There are four genes for acid-sensing ion channels (ASICs) in the genome of mammalian species. Whereas ASIC1 to ASIC3 form functional H+-gated Na+ channels, ASIC4 is not gated by H+, and its function is unknown. Zebrafish has two ASIC4 paralogs: zASIC4.1 and zASIC4.2. Whereas zASIC4.1 is gated by extracellular H+, zASIC4.2 is not. This differential response to H+ makes zASIC4 paralogs a good model to study the properties of this ion channel. In this study, we found that surface expression of homomeric zASIC4.2 is higher than that of zASIC4.1. Surface expression of zASIC4.1 was much increased by formation of heteromeric channels, suggesting that zASIC4.1 contributes to heteromeric ASICs in zebrafish neurons. Robust surface expression of H+-insensitive zASIC4.2 suggests that zASIC4.2 functions as a homomer and is gated by an as yet unknown stimulus, different from H+. Moreover, we identified a small region just distal to the first transmembrane domain that is crucial for the differential H+ response of the two paralogs. This post-TM1 domain may have a general role in gating of members of this gene family.

A kinase-anchoring protein 150 and calcineurin are involved in regulation of acid-sensing ion channels ASIC1a and ASIC2a

Acid-sensing ion channel (ASIC) 1a and ASIC2a are acid-sensing ion channels in central and peripheral neurons. ASIC1a has been implicated in long-term potentiation of synaptic transmission and ischemic brain injury, whereas ASIC2a is involved in mechanosensation. Although the biological role and distribution of ASIC1a and ASIC2a subunits in brain have been well characterized, little is known about the intracellular regulation of these ion channels that modulates their function. Using pulldown assays and mass spectrometry, we have identified A kinase-anchoring protein (AKAP)150 and the protein phosphatase calcineurin as binding proteins to ASIC2a. Extended pulldown and co-immunoprecipitation assays showed that these regulatory proteins also interact with ASIC1a. Transfection of rat cortical neurons with constructs encoding green fluorescent protein- or hemagglutinin-tagged channels showed expression of ASIC1a and ASIC2a in punctate and clustering patterns in dendrites that co-localized with AKAP150. Inhibition of protein kinase A binding to AKAPs by Ht-31 peptide reduces ASIC currents in cortical neurons and Chinese hamster ovary cells, suggesting a role of AKAP150 in association with protein kinase A in ASIC function. We also demonstrated a regulatory function of calcineurin in ASIC1a and ASIC2a activity. Cyclosporin A, an inhibitor of calcineurin, increased ASIC currents in Chinese hamster ovary cells and in cortical neurons, suggesting that activity of ASICs is inhibited by calcineurin-dependent dephosphorylation. These data imply that ASIC down-regulation by calcineurin could play an important role under pathological conditions accompanying intracellular Ca(2+) overload and tissue acidosis to circumvent harmful activities mediated by these channels.

Association between the ACCN1 gene and multiple sclerosis in Central East Sardinia

Multiple genome screens have been performed to identify regions in linkage or association with Multiple Sclerosis (MS, OMIM 126200), but little overlap has been found among them. This may be, in part, due to a low statistical power to detect small genetic effects and to genetic heterogeneity within and among the studied populations. Motivated by these considerations, we studied a very special population, namely that of Nuoro, Sardinia, Italy. This is an isolated, old, and genetically homogeneous population with high prevalence of MS. Our study sample includes both nuclear families and unrelated cases and controls. A multi-stage study design was adopted. In the first stage, microsatellites were typed in the 17q11.2 region, previously independently found to be in linkage with MS. One significant association was found at microsatellite D17S798. Next, a bioinformatic screening of the region surrounding this marker highlighted an interesting candidate MS susceptibility gene: the Amiloride-sensitive Cation Channel Neuronal 1 (ACCN1) gene. In the second stage of the study, we resequenced the exons and the 3′ untranslated (UTR) region of ACCN1, and investigated the MS association of Single Nucleotide Polymorphisms (SNPs) identified in that region. For this purpose, we developed a method of analysis where complete, phase-solved, posterior-weighted haplotype assignments are imputed for each study individual from incomplete, multi-locus, genotyping data. The imputed assignments provide an input to a number of proposed procedures for testing association at a microsatellite level or of a sequence of SNPs. These include a Mantel-Haenszel type test based on expected frequencies of pseudocase/pseudocontrol haplotypes, as well as permutation based tests, including a combination of permutation and weighted logistic regression analysis. Application of these methods allowed us to find a significant association between MS and the SNP rs28936 located in the 3′ UTR segment of ACCN1 with p = 0.0004 (p = 0.002, after adjusting for multiple testing). This result is in tune with several recent experimental findings which suggest that ACCN1 may play an important role in the pathogenesis of MS.

Fluorescence resonance energy transfer analysis of subunit assembly of the ASIC channel

Acid-sensing ion channels (ASICs) are believed to be homo- or heteromeric complexes, which have been verified by classical methods such as co-immunoprecipitation or electrophysiological assays. However, the exact subunit combinations of ASICs in living cells have not been established yet. Here, we apply assays based on fluorescence resonance energy transfer (FRET) between GFP color mutants CFP and YFP to investigate ASIC assembly directly in living cells. Homomerization as well as heteromerization of different combinations of ASIC subunits were found. In addition, our results suggest the formation of heteromeric 1a/2a channels of stoichiometry consisting of at least two 1a subunits and two 2a subunits. Similar stoichiometry was observed from heteromeric 1a/2b and 2a/2b channels. Our results imply that these heteromeric ASIC channels contain at least four subunits.

Cloning and functional expression of a new epithelial sodium channel delta subunit isoform differentially expressed in neurons of the human and monkey telencephalon

Epithelial sodium channel (ENaC) is a member of the ENaC/degenerin family of amiloride-sensitive, non-voltage gated sodium ion channels. ENaC alpha, beta and gamma subunits are abundantly expressed in epithelial tissues, where they have been well characterized. An ENaC delta subunit has also been described in the human nervous system, although its histological distribution pattern remains unexplored. We have now isolated a novel ENaC delta isoform (delta2) from human brain and studied the expression pattern of both the known (delta1) and the new (delta2) isoforms in the human and monkey telencephalon. ENaC delta2 is produced by a combination of alternative transcription start sites, a frame shift in exon 3 and alternative splicing of exon 4. It forms functional amiloride-sensitive sodium channels when co-expressed with ENaC beta and gamma accessory subunits. Comparison with the classical ENaC channel (alphabetagamma) indicates that the interaction between delta2, beta and gamma is functionally inefficient. Both ENaC delta isoforms are widely expressed in pyramidal cells of the human and monkey cerebral cortex and in different neuronal populations of telencephalic subcortical nuclei, but double-labelling experiments demonstrated a low level of co-localization between isoforms (5-8%), suggesting specific functional roles for each of them.

Localization and comparative analysis of acid-sensing ion channel (ASIC1, 2, and 3) mRNA expression in mouse colonic sensory neurons within thoracolumbar dorsal root ganglia

Reducing colonic mechanosensitivity is an important potential strategy for reducing visceral pain. Mice lacking acid-sensing ion channels (ASIC) 1, 2, and 3 show altered colonic mechanosensory function, implicating ASICs in the mechanotransduction process. Deletion of ASICs affects mechanotransduction in visceral and cutaneous afferents differently, suggesting differential expression. We determined relative expression of ASIC1, 2, and 3 in mouse thoracolumbar dorsal root ganglia (DRG) by quantitative reverse-transcriptase polymerase chain reaction (RT-PCR) analysis (QPCR) and specifically in retrogradely traced colonic neurons isolated via laser capture microdissection. Localization of ASIC expression in DRG was determined with fluorescence in situ hybridization (FISH) and retrograde tracing. QPCR of whole thoracolumbar DRG revealed and abundance of ASIC2 > ASIC1 > ASIC3. Similarly, FISH of all neurons in thoracolumbar DRG demonstrated that ASIC2 was expressed in the most (40 +/- 1%) neurons, followed by ASIC3 (24 +/- 1%), then ASIC1 (18 +/- 1%). Retrograde tracing from the distal colon labeled 4 +/- 1% of neurons in T10-L1 DRG. In contrast to whole DRG, FISH of colonic neurons showed ASIC3 expression in 73 +/- 2%, ASIC2 in 47 +/- 0.5%, and ASIC1 in 30 +/- 2%. QPCR of laser captured colonic neurons revealed that ASIC3 was the most abundant ASIC transcript, followed by ASIC1, then ASIC2. We conclude that ASIC1, 2, and 3 are expressed preferentially in colonic neurons within thoracolumbar DRG. In particular ASIC3, the least abundant in the general population, is the most abundant ASIC transcript in colonic neurons. The prevalence of ASIC3 in neurons innervating the colon supports electrophysiological data showing that it makes a major contribution to colonic mechanotransduction and therefore may be a target for the treatment of visceral pain.

Acidotoxicity in brain ischaemia

Intracellular calcium toxicity remains the central feature in the pathophysiology of ischaemic cell death in brain. Glutamate-gated channels have been thought to be the major sites of ischaemia-induced toxic calcium entry, but the failure of glutamate antagonists in clinical trials has suggested that glutamate-independent mechanisms of calcium entry during ischaemia must exist and may prove central to ischaemic injury. We have shown that ASICs (acid-sensing ion channels) in brain are glutamate-independent vehicles of calcium flux and transport calcium in greater measure in the setting of the two major neurochemical components of ischaemia: acidosis and substrate depletion. Pharmacological blockade of ASICs markedly attenuates stroke injury with a robust therapeutic time window of 5 h following stroke onset. Here, we describe this new mechanism of calcium toxicity in brain ischaemia and offer a potential new therapy for stroke.

Arachidonic acid potentiates acid-sensing ion channels in rat sensory neurons by a direct action

Acid-sensing ion channels (ASICs) are activated by a decrease in extracellular pH. ASICs are expressed in nociceptive sensory neurons, and several lines of evidence suggest that they are responsible for signaling the pain caused by extracellular acidification, but little is understood of the modulation of ASICs by pro-inflammatory factors. Using whole-cell patch clamp we demonstrate that low pH evokes three distinct inward currents in rat dorsal root ganglion neurons: a slowly inactivating transient current, a rapidly inactivating transient current, and a sustained current. All three currents were potentiated by arachidonic acid (AA), to 123%, 171%, and 264% of peak current, respectively. Membrane stretch had no effect on proton-gated currents, implying that AA is unlikely to act via local membrane deformation. The current carried by heterologously expressed ASIC1a and ASIC3 was also potentiated by AA. AA potentiates ASIC activation by a direct mechanism, because inhibition of AA metabolism had no effect on potentiation, and potentiation of single ASIC2a channels could be observed in cell-free patches. Potentiation by lipids with the same chain length as AA increased as the number of double bonds was increased. AA is known to be released in inflammation and the results suggest that AA may be an important pro-inflammatory agent responsible for enhancing acid-mediated pain.

Immunohistochemical analysis of acid-sensing ion channel 2 expression in rat dorsal root ganglion and effects of axotomy

Several studies have suggested that acid-sensing ion channel 2 (ASIC2) plays a role in mechanoperception and acid sensing in the peripheral nervous system. We examined the expression and distribution of ASIC2 in the rat dorsal root ganglion, the co-localization of ASIC2 with tropomyosin-related kinase (trk) receptors, and the effects of axotomy on ASIC2 expression. ASIC2 immunoreactivity was observed in both neurons and satellite cells. ASIC2-positive neurons accounted for 16.5 +/- 2.4% of the total neurons in normal dorsal root ganglion. Most ASIC2-positive neurons were medium-to-large neurons and were labeled with neurofilament 200 kD (NF200). Within these neurons, ASIC2 was not evenly distributed throughout the cytoplasm, but rather was accumulated prominently in the cytoplasm adjacent to the axon hillock and axonal process. We next examined the co-localization of ASIC2 with trk receptors. trkA was expressed in few ASIC2-positive neurons, and trkB and trkC were observed in 85.2% and 53.4% of ASIC2-positive neurons, respectively, while only 6.9% of ASIC2-positive neurons were co-localized with trkC alone. Peripheral axotomy markedly reduced ASIC2 expression in the axotomized dorsal root ganglion neurons. On the other hand, intense ASIC2 staining was observed in satellite cells. These results show that ASIC2 is expressed in the distinct neurochemical population of sensory neurons as well as satellite cells, and that peripheral axotomy induced marked reductions in ASIC2 in neurons.

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.

Distinct ASIC currents are expressed in rat putative nociceptors and are modulated by nerve injury

The H(+)-gated acid-sensing ion channels (ASICs) are expressed in dorsal root ganglion (DRG) neurones. Studies with ASIC knockout mice indicated either a pro-nociceptive or a modulatory role of ASICs in pain sensation. We have investigated in freshly isolated rat DRG neurones whether neurones with different ASIC current properties exist, which may explain distinct cellular roles, and we have investigated ASIC regulation in an experimental model of neuropathic pain. Small-diameter DRG neurones expressed three different ASIC current types which were all preferentially expressed in putative nociceptors. Type 1 currents were mediated by ASIC1a homomultimers and characterized by steep pH dependence of current activation in the pH range 6.8-6.0. Type 3 currents were activated in a similar pH range as type 1, while type 2 currents were activated at pH < 6. When activated by acidification to pH 6.8 or 6.5, the probability of inducing action potentials correlated with the ASIC current density. Nerve injury induced differential regulation of ASIC subunit expression and selective changes in ASIC function in DRG neurones, suggesting a complex reorganization of ASICs during the development of neuropathic pain. In summary, we describe a basis for distinct cellular functions of different ASIC types in small-diameter DRG neurones.

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.

A novel mechanosensitive channel identified in sensory neurons

Mechanosensitive (MS) channels are ion channels gated by different types of mechanical stimuli. MS channels in sensory neurons are thought to be molecular transducers for somatic sensations such as touch, pressure, proprioception and pain. Previously, we reported that two types of MS channels are present in sensory neurons. These channels are termed low threshold (LT) and high threshold (HT) MS channels based on their pressure threshold for activation. Here, we report another type of MS channel present in sensory neurons. The channel is activated by low pressure applied to a patch (threshold approximately 20 mmHg, similar to that in the LT channel). However, because this channel has a smaller single-channel conductance than that of the LT channel, the newly classified MS channel is now called a low threshold small conductance (LTSC) channel. Unlike the LT channel, which has outwardly rectifying currents, the current-voltage relationship of the LTSC is linear. The LTSC was permeable to monovalent cations and Ca2+, and reversibly blocked by gadolinium, a blocker of MS channels. Unlike the LT channel, the LTSC was sensitized by prostaglandin E2, an inflammatory mediator that is known to sensitize nociceptors to mechanical stimuli. LTSC channels were found mostly in small cultured sensory neurons. Thus, these results suggest that the LTSC is a distinct type of MS channel that is different from the LT and HT channels in sensory neurons, and that LTSCs might play a role in mediating somatosensations, including pain.

Sustained currents through ASIC3 ion channels at the modest pH changes that occur during myocardial ischemia

Acid-sensing ion channel 3 (ASIC3) is highly expressed on sensory neurons that innervate heart and skeletal muscle and, therefore, is proposed to detect lactic acidosis and to transduce angina and muscle ischemic pain. A difficulty with this idea is that ASIC3 rapidly desensitizes. How can a desensitizing ion channel mediate a persisting sensation such as angina? Here, we show that rat ASIC3 produces a sustained current within the limited range of extracellular pH (7.3 to 6.7) that occurs during cardiac and skeletal muscle ischemia; experiments use patch clamp on transfected cell lines and on fluorescently tagged sensory neurons that innervate rat heart. No such sustained current occurs with ASIC1a (either as homomers or 1a/3 heteromers), whereas ASIC2a/3 heteromers give much larger currents than ASIC3 homomers. The sustained current persists even over tens of minutes because it is caused by a region of pH where there is overlap between inactivation and activation of the channel. Lactate, an anaerobic metabolite, allows the current to activate at slightly more basic pH. Surprisingly, amiloride, which blocks ASICs when they are activated at lower pH, increases ASIC3 current evoked at pH 7.0. Cardiac sensory neurons exhibit a small, perfectly sustained current when pH changes from 7.4 to 7.0. At least some of this current is carried by ASICs because the current is increased by both Zn(2+), an ASIC modulator, and amiloride. We suggest that this sustained mode is the most relevant form of ASIC3 gating for triggering angina and other ischemic pain.

Acid-sensitive ionic channels in midbrain dopamine neurons are sensitive to ammonium, which may contribute to hyperammonemia damage

Acid-sensitive ion channels (ASICs) are proton-gated and belong to the family of degenerin channels. In the mammalian nervous system, ASICs are most well known in sensory neurons, where they are involved in nociception, occurring when injury or inflammation causes acidification. ASICs also are widely expressed in the CNS, and some synaptic roles have been revealed. Because neuronal activity can produce pH changes, ASICs may respond to local acidic transients and alter the excitability of neuronal circuits more widely than is presently appreciated. Furthermore, ASICs have been found to underlie calcium transients that contribute to neuronal death. Degeneration of midbrain dopamine neurons is characteristic of advanced idiopathic Parkinson's disease. Therefore, we tested for functional ASICs in midbrain dopamine neurons of the ventral tegmental area and substantia nigra compacta. Patch-clamp electrophysiology applied to murine midbrain slices revealed abundant acid-sensitive channels. The ASICs were gated and desensitized by extracellular application of millimolar concentrations of NH(4)Cl. Although the NH(4)Cl solution contains micromolar concentrations of NH(3) at pH 7.4, our evidence indicates that NH(4)(+) gates the ASICs. The proton-gated and the ammonium-gated currents were inhibited by tarantula venom (psalmotoxin), which is specific for the ASIC1a subtype. The results show that acid-sensitive channels are expressed in midbrain dopamine neurons and suggest that ammonium sensitivity is a widely distributed ASIC characteristic in the CNS, including the hippocampus. The ammonium sensitivity suggests a role for ASIC1s in hepatic encephalopathy, cirrhosis, and other neuronal disorders that are associated with hyperammonemia.