酸敏感离子通道研究进展

组织酸化是生理和病理下常见的现象.神经元可以通过酸敏感的离子通道（ASICs）来感受细胞周围的pH值的降低.ASICs属于NaC/DEG家族的一个成员.目前,已发现了6个ASICs亚基,它们在外周和中枢神经系统中广泛表达,其同聚体和异聚体通道有着各种不同的电生理学特性.ASICs在机体感觉尤其是痛觉中起着至关重要的作用.     

Acid-sensing ion channels (ASICs): therapeutic targets for neurological diseases and their regulation

Extracellular acidification occurs not only in pathological conditions such as inflammation and brain ischemia, but also in normal physiological conditions such as synaptic transmission. Acid-sensing ion channels (ASICs) can detect a broad range of physiological pH changes during pathological and synaptic cellular activities. ASICs are voltage-independent, proton-gated cation channels widely expressed throughout the central and peripheral nervous system. Activation of ASICs is involved in pain perception, synaptic plasticity, learning and memory, fear, ischemic neuronal injury, seizure termination, neuronal degeneration, and mechanosensation. Therefore, ASICs emerge as potential therapeutic targets for manipulating pain and neurological diseases. The activity of these channels can be regulated by many factors such as lactate, Zn²⁺, and Phe-Met-Arg-Phe amide (FMRFamide)-like neuropeptides by interacting with the channel’s large extracellular loop. ASICs are also modulated by G protein-coupled receptors such as CB₁ cannabinoid receptors and 5-HT₂. This review focuses on the physiological roles of ASICs and the molecular mechanisms by which these channels are regulated. [BMB Reports 2013; 46(6): 295-304]     

FMRFamide-gated sodium channel and ASIC channels: a new class of ionotropic receptors for FMRFamide and related peptides

FMRFamide and related peptides typically exert their action through G-protein coupled receptors. However, two ionotropic receptors for these peptides have recently been identified. They are both members of the epithelial amiloride-sensitive Na+ channel and degenerin (ENaC/DEG) family of ion channels. The invertebrate FMRFamide-gated Na+ channel (FaNaC) is a neuronal Na+-selective channel which is directly gated by micromolar concentrations of FMRFamide and related tetrapeptides. Its response is fast and partially desensitizing, and FaNaC has been proposed to participate in peptidergic neurotransmission. On the other hand, mammalian acid-sensing ion channels (ASICs) are not gated but are directly modulated by FMRFamide and related mammalian peptides like NPFF and NPSF. ASICs are activated by external protons and are therefore extracellular pH sensors. They are expressed both in the central and peripheral nervous system and appear to be involved in many physiological and pathophysiological processes such as hippocampal long-term potentiation and defects in learning and memory, acquired fear-related behavior, retinal function, brain ischemia, pain sensation in ischemia and inflammation, taste perception, hearing functions, and mechanoperception. The potentiation of ASIC activity by endogenous RFamide neuropeptides probably participates in the response to noxious acidosis in sensory and central neurons. Available data also raises the possibility of the existence of still unknown FMRFamide related endogenous peptides acting as direct agonists for ASICs.     

酸敏感离子通道的功能及其相关调控

酸敏感离子通道（ASICs）是一类由胞外酸化所激活的阳离子通道.目前,已发现了6个ASICs亚基,它们在外周和中枢神经系统中广泛表达.利用基因敲除等技术,已证明它们在触觉、痛觉、酸味觉以及学习记忆中具有重要作用.同时,它们也参与某些病理反应.ASICs可以被神经肽、温度、金属离子和缺血相关物质等调控,从而整合细胞周围的多种信号以行使其功能.     

Acid-sensing ion channels contribute to the effect of acidosis on the function of dendritic cells

As an H(+)-gated subgroup of the degenerin/epithelial Na(+) channel family, acid-sensing ion channels (ASICs) were reported to be involved in various physiological and pathological processes in neurons. However, little is known about the role of ASICs in the function of dendritic cells (DCs). In this study, we investigated the expression of ASICs in mouse bone marrow-derived DCs and their possible role in the function of DCs. We found that ASIC1, ASIC2, and ASIC3 are expressed in DCs at the mRNA and protein levels, and extracellular acid can evoke ASIC-like currents in DCs. We also demonstrated that acidosis upregulated the expression of CD11c, MHC class II, CD80, and CD86 and enhanced the Ag-presenting ability of DCs via ASICs. Moreover, the effect of acidosis on DCs can be abolished by the nonsteroidal anti-inflammatory drugs ibuprofen and diclofenac. These results suggest that ASICs are involved in the acidosis-mediated effect on DC function.     

Trypsin cleaves acid-sensing ion channel 1a in a domain that is critical for channel gating

Acid-sensing ion channels (ASICs) are neuronal Na(+) channels that are members of the epithelial Na(+) channel/degenerin family and are transiently activated by extracellular acidification. ASICs in the central nervous system have a modulatory role in synaptic transmission and are involved in cell injury induced by acidosis. We have recently demonstrated that ASIC function is regulated by serine proteases. We provide here evidence that this regulation of ASIC function is tightly linked to channel cleavage. Trypsin cleaves ASIC1a with a similar time course as it changes ASIC1a function, whereas ASIC1b, whose function is not modified by trypsin, is not cleaved. Trypsin cleaves ASIC1a at Arg-145, in the N-terminal part of the extracellular loop, between a highly conserved sequence and a sequence that is critical for ASIC1a inhibition by the venom of the tarantula Psalmopoeus cambridgei. This channel domain controls the inactivation kinetics and co-determines the pH dependence of ASIC gating. It undergoes a conformational change during inactivation, which renders the cleavage site inaccessible to trypsin in inactivated channels.     

Differential effects of N-glycans on surface expression suggest structural differences between the acid-sensing ion channel (ASIC) 1a and ASIC1b

ASICs (acid-sensing ion channels) are H(+)-gated Na(+) channels with a widespread expression pattern in the central and the peripheral nervous system. ASICs have a simple topology with two transmembrane domains, cytoplasmic termini and a large ectodomain between the transmembrane domains; this topology has been confirmed by the crystal structure of chicken ASIC1. ASIC1a and ASIC1b are two variants encoded by the asic1 gene. The variable part of the protein includes the cytoplasmic N-terminus, the first transmembrane domain and approximately the first third of the ectodomain. Both variants contain two consensus sequences for N-linked glycosylation in the common, distal part of the ectodomain. In contrast with ASIC1a, ASIC1b contains two additional consensus sequences in the variable, proximal part of the ectodomain. Here we show that all the extracellular asparagine residues within the putative consensus sequences for N-glycosylation carry glycans. The two common distal glycans increase surface expression of the channels, but are no absolute requirement for channel activity. In sharp contrast, the presence of at least one of the two proximal glycans, which are specific to ASIC1b, is an absolute requirement for surface expression of ASIC1b. This result suggests substantial differences in the structure of the proximal ectodomain between the two ASIC1 variants.     

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.     

Molecular cloning, functional expression and chromosomal localization of an amiloride-sensitive Na(+) channel from human small intestine

Amiloride-sensitive Na(+) channels belonging to the recently discovered NaC/DEG family of genes have been found in several human tissues including epithelia and central and peripheral neurons. We describe here the molecular cloning of a cDNA encoding a novel human amiloride-sensitive Na(+) channel subunit that is principally expressed in the small intestine and has been called hINaC (human intestine Na(+) channel). This protein is similar to the recently identified rodent channel BLINaC and is relatively close to the acid sensing ion channels (ASICs) (79 and 29% amino acid identity, respectively). ASICs are activated by extracellular protons and probably participate in sensory neurons to nociception linked to tissue acidosis. hINaC is not activated by lowering the external pH but gain-of-function mutations can be introduced and reveal when expressed in Xenopus oocytes, an important Na(+) channel activity which is blocked by amiloride (IC(50)=0.5 microM). These results suggest the existence of a still unknown physiological activator for hINaC (e.g. an extracellular ligand). The presence of this new amiloride-sensitive Na(+) channel in human small intestine probably has interesting physiological as well as physiopathological implications that remain to be clarified. The large activation of this channel by point mutations may be associated with a degenerin-like behavior as previously observed for channels expressed in nematode mechanosensitive neurons. The hINaC gene has been mapped on the 4q31.3-q32 region of the human genome.     

Polymorphisms and genetic linkage of histamine receptors

Histamine is a biogenic amine that plays an essential role in controlling many physiological functions, both in the central nervous system (CNS) and the peripheral nervous system (PNS). Most of these physiological effects are mediated through interactions with four histamine receptor subtypes, all of which are members of the larger family of rhodopsin-like class A G-protein coupled receptors (GPCRs) (Leurs et al., 2011; Lim et al., 2009). Here, we focus on the genetic variations and polymorphisms localized on the genes encoding for human histamine receptors where it provides an up to date collection of all polymorphisms found on genes encoding the histamine receptor subtypes and their association to diseases.     

Acid-sensing ion channels: trafficking and pathophysiology

Acid-sensing ion channels (ASICs) are proton-gated cation channels that are widely expressed in both the peripheral and central nervous systems. ASICs contribute to a variety of pathophysiological conditions that involve tissue acidosis, such as ischemic stroke, epileptic seizures and multiple sclerosis. Although much progress has been made in researching the structure-function relationship and pharmacology of ASICs, little is known about the trafficking of ASICs and its contribution to ASIC function. The recent identification of the mechanism of membrane insertion and endocytosis of ASIC1a highlights the emerging role of ASIC trafficking in regulating its pathophysiological functions. In this review, we summarize the recent advances and discuss future directions on this topic.     

Acid-sensing ion channels: trafficking and pathophysiology

Acid-sensing ion channels (ASICs) are proton-gated cation channels that are widely expressed in both the peripheral and central nervous systems. ASICs contribute to a variety of pathophysiological conditions that involve tissue acidosis, such as ischemic stroke, epileptic seizures and multiple sclerosis. Although much progress has been made in researching the structure-function relationship and pharmacology of ASICs, little is known about the trafficking of ASICs and its contribution to ASIC function. The recent identification of the mechanism of membrane insertion and endocytosis of ASIC1a highlights the emerging role of ASIC trafficking in regulating its pathophysiological functions. In this review, we summarize the recent advances and discuss future directions on this topic.     

Extracellular matrix: functions in the nervous system

An astonishing number of extracellular matrix glycoproteins are expressed in dynamic patterns in the developing and adult nervous system. Neural stem cells, neurons, and glia express receptors that mediate interactions with specific extracellular matrix molecules. Functional studies in vitro and genetic studies in mice have provided evidence that the extracellular matrix affects virtually all aspects of nervous system development and function. Here we will summarize recent findings that have shed light on the specific functions of defined extracellular matrix molecules on such diverse processes as neural stem cell differentiation, neuronal migration, the formation of axonal tracts, and the maturation and function of synapses in the peripheral and central nervous system.     

细胞外调控分子对酸敏感离子通道的功能调控及其机制

酸敏感离子通道(acid-Sensing ion channels,ASlCs)是一类由细胞外质子(H )激活的配体门控阳离子通道.迄今为止,人们在哺乳动物体内已经发现了6种ASICs亚基蛋白,它们分布在多种组织器官中.越来越多的研究表明:ASICs参与了机体的生理、病理过程,如:学习、记忆、痛觉、脑中风和肿瘤.在过去的10年中,人们发现多种内源性或外源性分子可以调控ASICs通道活性.由于这些细胞外调控分子与多种生理和病理功能有关,因此研究细胞外调控分子对ASICs的调控及其分子机制,可以帮助我们更多地了解ASICs功能以及结构信息,也为人们设计ASICs靶点特异性药物提供了理论依据.文章将系统地介绍细胞外调控分子对ASICs的功能调控及其作用机制,特别是该研究领域的最新进展.     

Central effects of neuromedin U in the regulation of energy homeostasis

Neuromedin U (NMU) is a brain-gut peptide whose peripheral activities are well-understood but whose central actions have yet to be clarified. The recent identification of two NMU receptors in rat brain has provided a springboard for further investigation into its role in the central nervous system. Intracerebroventricular administration of NMU to free-feeding rats decreased food intake and body weight. Conversely, NMU increased gross locomotor activity, body temperature, and heat production. NMU, a potent endogenous anorectic peptide, serves as a catabolic signaling molecule in the brain. Further investigation of the biochemical and physiological functions of NMU will help our better understanding of the mechanisms of energy homeostasis.     

Gaseous messengers in the nervous system

Nitric oxide is the first gaseous messenger whose functions were comprehensively studied in different systems of organism. Recently, new data on the physiological role of other endogenous gases: carbon monoxide and hydrogen sulfide, appeared. The role of gases in gastrointestinal tract and cardiovascular system have been established; however, data on their function and mechanisms of action in nervous system are insufficient. This article highlights the current information on the role of gaseous messengers in central and peripheral nervous system.     

Differential Regulation of Proton-Sensitive Ion Channels by Phospholipids: A Comparative Study between ASICs and TRPV1

Protons are released in pain-generating pathological conditions such as inflammation, ischemic stroke, infection, and cancer. During normal synaptic activities, protons are thought to play a role in neurotransmission processes. Acid-sensing ion channels (ASICs) are typical proton sensors in the central nervous system (CNS) and the peripheral nervous system (PNS). In addition to ASICs, capsaicin- and heat-activated transient receptor potential vanilloid 1 (TRPV1) channels can also mediate proton-mediated pain signaling. In spite of their importance in perception of pH fluctuations, the regulatory mechanisms of these proton-sensitive ion channels still need to be further investigated. Here, we compared regulation of ASICs and TRPV1 by membrane phosphoinositides, which are general cofactors of many receptors and ion channels. We observed that ASICs do not require membrane phosphatidylinositol 4-phosphate (PI(4)P) or phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) for their function. However, TRPV1 currents were inhibited by simultaneous breakdown of PI(4)P and PI(4,5)P₂. By using a novel chimeric protein, CF-PTEN, that can specifically dephosphorylate at the D3 position of phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P₃), we also observed that neither ASICs nor TRPV1 activities were altered by depletion of PI(3,4,5)P₃ in intact cells. Finally, we compared the effects of arachidonic acid (AA) on two proton-sensitive ion channels. We observed that AA potentiates the currents of both ASICs and TRPV1, but that they have different recovery aspects. In conclusion, ASICs and TRPV1 have different sensitivities toward membrane phospholipids, such as PI(4)P, PI(4,5)P₂, and AA, although they have common roles as proton sensors. Further investigation about the complementary roles and respective contributions of ASICs and TRPV1 in proton-mediated signaling is necessary.     

中枢神经系统中的胃泌素释放肽：在厌恶性情绪记忆中的作用及机制研究

胃泌素释放肽(gastrin-releasing peptide,GRP)是蛙皮素(bombesin,BB/BN)在哺乳动物中的同系物,在中枢神经系统中广泛分布,是一种重要的脑内神经调质,参与动物的多种生理功能和本能行为,在大脑的高级功能方面也发挥一定的作用.在神经系统中,随着GRP水平的改变,动物的记忆特别是与恐惧、焦虑相关记忆的形成、巩固和消退以及突触可塑性均发生不同程度的变化.GRP及其受体还被认为与神经系统性疾病有关,是潜在的神经系统性疾病的治疗靶点,但其相关的机制尚未明确.很多研究者基于不同实验方法提出了相关假设.本文从传统药理学、遗传学和电生理学方面对GRP系统在厌恶性情绪驱动的记忆、突触可塑性变化以及在中枢神经系统中的作用机制进行综述,希望为进一步明确GRP系统在中枢神经系统中的作用研究提供新的思路.     

The tarantula toxin psalmotoxin 1 inhibits acid-sensing ion channel (ASIC) 1a by increasing its apparent H+ affinity

Acid-sensing ion channels (ASICs) are ion channels activated by extracellular protons. They are involved in higher brain functions and perception of pain, taste, and mechanical stimuli. Homomeric ASIC1a is potently inhibited by the tarantula toxin psalmotoxin 1. The mechanism of this inhibition is unknown. Here we show that psalmotoxin 1 inhibits ASIC1a by a unique mechanism: the toxin increases the apparent affinity for H(+) of ASIC1a. Since ASIC1a is activated by H(+) concentrations that are only slightly larger than the resting H(+) concentration, this increase in H(+) affinity is sufficient to shift ASIC1a channels into the desensitized state. As activation of ASIC1a has recently been linked to neurodegeneration associated with stroke, our results suggest chronic desensitization of ASIC1a by a slight increase of its H(+) affinity as a possible way of therapeutic intervention in stroke.     

Up-regulation of acid-gated Na(+) channels (ASICs) by cystic fibrosis transmembrane conductance regulator co-expression in Xenopus oocytes.

Cystic fibrosis transmembrane conductance regulator (CFTR) functions as both a chloride channel and an epithelial transport regulator, interacting with Na(+) (epithelial sodium channel), Cl(-), renal outer medullary potassium channel(+), and H(2)O channels and some exchangers (i.e. Na(+)/H(+)) and co-transporters (Na(+)-HCO(3)(minus sign), Na(+)-K(+)-2Cl(-)). Acid-sensitive ion channels (ASICs), members of the epithelial sodium channel/degenerin superfamily, were originally cloned from neuronal tissue, and recently localized in epithelia. Because CFTR has been immunocytochemically and functionally identified in rat, murine, and human brain, the regulation of ASICs by CFTR was tested in oocytes. Our observations show that the proton-gated Na(+) current formed by the heteromultimeric ASIC1a/2a channel was up-regulated by wild type but not by Delta F508-CFTR. In contrast, the acid-gated Na(+) current associated with either the homomultimeric ASIC1a or ASIC2a channel was not influenced by wild type CFTR. The apparent equilibrium dissociation constant for extracellular Na(+) for ASIC1a/2a was increased by CFTR, but CFTR had no effect on the gating behavior or acid sensitivity of ASIC1a/2a. CFTR had no effect on the pH activation of ASIC1a/2a. We conclude that wild type CFTR elevates the acid-gated Na(+) current of ASIC1a/2a in part by altering the kinetics of extracellular Na(+) interaction.