Introduction of ASIC
Acid-sensitive ion channels (ASICs) are a class of cation channels activated by extracellular acidification. Currently, six ASICs have been discovered, which are widely expressed in the peripheral and central nervous systems. Techniques such as gene knockout, have proven that they play an important role in tactile, pain, sour taste, learning and memory. At the same time, they are also involved in certain pathological reactions. ASICs can be regulated by neuropeptides, temperature, metal ions and ischemic related substances, thereby integrating the cells around them. There have been a variety of signals to regulate its function.
The family member of ASICs and their structure, distribution and functional characteristics
ASICs are a branch of the NaC (Na+ channel)/DEG (degenerin) superfamily. Like other members of the family, ASICs may consist of four subunits, each consisting of more than 500 amino acids, whose structure contains two hydrophobic transmembrane regions, a large cysteine-rich extracellular loop and intracellular N-terminus and C-terminus. Four conserved regions in this structure play an important role in the function of ASICs. The second transmembrane segment (TM2) forms a pore lining; nine conserved amino acid sequences near the medial segment of TM1 affect channel openness probability, ion permeability, and Na+ selectivity. The extracellular proximity to TM2 (Gly430) mutation leads to the continuous opening of the channel, indicating that this region is related to the gating of the channel; a cysteine-rich conserved fragment in the extracellular domain is involved in maintaining the basic function of the channel. ASICs are widely distributed in vivo, but different subunits are distributed in specificity. For example, ASIC1a is expressed in brain, spinal cord and dorsal root ganglia (DRG), but its splice variant ASIC1b is specifically present in DRG; ASIC2a was originally shown to be expressed only in the central nervous system. However, recent studies have shown that ASIC2a is also expressed in DRG; ASIC2b is widely distributed in the body and expressed in both brain and DRG; ASIC3 is mainly expressed in DRG neurons. However, there is also a small amount of expression in the central nervous system; ASIC4 is a new member of the recently discovered ASICs, which is also widely distributed and expressed in the brain, spinal cord, inner ear and some DRG neurons. The understanding is mainly from the study of expression channels. The ASIC1a homopolymer channel is highly sensitive to H+ (pH50 = 6.0) and the current only shows fast inactivation. The channel mainly transports Na+, and also transports Ca2+. The ASIC1b homopolymer channel has similar H+ sensitivity and kinetic properties to the ASIC1a channel, except that it is only permeable Na+. The sensitive of ASIC2a homopolymer channel is very low (pH50 = 4.35), suggesting that H+ may not be an endogenous ligand for this subunit. The homopolymer expressed by ASIC2b in Xenopus oocytes is non-functional. It is usually co-expressed as a helper subunit and other ASICs subunits to form heteromers and change the channel selectivity of ion. The current mediated by ASIC3 consists of two components: a fast deactivation component and a steady state component. The two current components have different sensitivities to H+, and their pH50 is 6.5 and 3. 5. ASIC4 can neither form a functional homopolymer channel nor form a functional heteromer channel. In natural neurons, there are far more types of acid-induced currents (some acids can cause electrical currents in the nervous system, such as sulfonic acid, and the process requires the assistance of the ASCI family) than the above, therefore, there must be some heteropolymer channels that mediate the diversity of currents. Now, five kinds of heteromeric ASICs channels have been discovered, ASIC1a +2a, ASIC2a +3, ASIC2a +2b, ASIC3 +2b, and ASIC1a+3. They typically exhibit different current characteristics, ion selectivity, and pH sensitivity than homopolymers. Among the above five heteromeric channels, ASIC1a+2a was shown to be expressed in the brain, while ASIC2a+3 was shown to be expressed in both brain and DRG.
The physiological and pathological functions of ASICs
The wide distribution of ASICsin vivo suggests important physiopathological significance. However, due to the lack of effective drugs that selectively target ASICs and their subunits, studies on the function of ASICs are still difficult. Fortunately, genes manipulation technology has greatly promoted the functional study of receptors and ion channels. Using gene knockout mice combined with electrophysiology and behavioral techniques, we now have some preliminary understanding of the physiological functions of ASICs. For example, ASICs are involved in the sense of touch, pain, sour taste formation and learning and memory. At the same time, there is evidence that ASICs are involved in certain pathological reactions such as ischemia, hypoxia, and epilepsy. The role of ASICs in the sense of touch and pain stimulates the skin to produce various sensations such as touch and pain, however, we know very little about the molecular mechanisms of sensation. Since MEC-4 and MEC-10 in the DEG/NaC family have been shown to be associated with mechanical stimuli, it is suggested that ASICs with the same origin may have similar function. In addition, experiments have shown that ASICs are expressed in specific epidermal mechanoreceptors. Therefore, ASICs may be one of the targets of mechanical stimulation. The study of ASIC2 and ASIC3 knockout mice provides strong evidence for the role of ASICs in the sense of touch. Priceet al found the hair of ASIC2 knockout mice or the sensitivity of the skin to light touch was significantly reduced. They subsequently found that the responsiveness of ASIC3 knockout mice to light touch was enhanced. These results suggest that ASIC2 and ASIC3 are involved in the formation of mechanical irritations in the hair or skin. The different effects of light touch suggest that they may form the central component of the tactile sensation complex in the form of heteropolymers. Due to the close association between tissue acidification and pain sensation, ASICs are initially thought to be associated with pain sensation as acid receptors. The ASIC3-mediated current has a steady-state composition (most ASICs-mediated reactions are fully desensitized in seconds to tens of seconds), suggesting that it is most likely involved in the formation of pain sensation. This view is supported by further research on ASIC3 knockout mice. Behavioral analysis of knockout mice indicates that ASIC3 not only plays an important role in acid-induced pain, but also mediates other forms such as thermal stimulation, noxious mechanical stimulation, etc. But the asic3gene knockout does not completely remove any form of pain, suggesting that ASIC3 is only part of the pain receptor. In addition, studies of myocardial sensory afferent nerves suggest that homopolymer ASIC3 may mediate the H+-induced current which suggests that ASIC3 plays an important role in angina pectoris and chest pain. ASICs can sense acid changes around cells, so they may play an important role in sour taste. To prove this hypothesis, Ugawa et al. established a cDNA library of rat circumvallate papilla and line gene census. Another exciting thing is that they really found a subunit gene of ASICs - asic2a gene. The asic2a gene in situ hybridization experiment revealed that the mRNA of asic2a was concentrated in the taste bud cells of the contoured nipple, but not in the surrounding tissues of the taste bud cells. Further immunohistochemical method was used to identify the top of the ASIC2a distributed in the taste bud cells. Expressing taste bud ASIC2a in oocytes showed that it was not only sensitive to acid, but also reacted differently to carbonic acid and hydrochloric acid at the same pH, which was consistent with taste buds' perception of acid. These results strongly suggested that ASIC2a was involved in taste bud cells' perception of acid. ASICs play a role in learning and memory, and synaptic vesicles are acidic (pH 5.6), so during synaptic transmission, especially when a large number of vesicles are released, they lead to local acidification of the synaptic cleft. This localized acidification with synaptic transmission is measured with an acid-sensitive electrode. The actual acidification may be greater due to the sensitivity of the electrode and the like. On the other hand, some subunits of ASICs such as ASIC1a are extremely sensitive to acid. Therefore, ASICs may be involved in synaptic transmission. Recently, Wemmie et al. used gene knockout mice to discover that ASIC1a is involved in synaptic activity and affects long-term potentiation (LTP). Although normal synaptic transmission has not changed in ASIC1a knockout mice, high frequency stimulation (HFS)-induced LTP is severely impaired—enhanced excitatory postsynaptic potential (EPSP) after HFS, it gradually decreases to baseline levels within a few minutes. This LTP damage can be reversed after the NMDA receptor is up-regulated, as in the absence of Mg2+ or PKC agonists. Therefore, Wemmie et al. proposed the possible mechanisms of ASIC1a involved in synaptic plasticity: the release of many vesicles generated by HSS stimulation leads to acidification of the synaptic cleft, thereby activating ASIC1a, causing a stronger depolarization of the postsynaptic membrane, thereby removing Mg2+ from the NMDA receptor, blocking effect thus promoting the formation of LTP. Behavioral experiments show that ASIC1a knockout mice do exhibit spatial learning and memory impairment, further confirming the role of ASIC1a in learning and memory. Knockout mice also showed damage from blink reflexes, suggesting that cerebellar long-term depression (LTD) may also be affected. This suggests that the mechanism by which ASIC1a is involved in synaptic plasticity can be quite complex.