Physiology of the volume-sensitive/regulatory anion channel VSOR/VRAC. Part 1: from its discovery and phenotype characterization to the molecular entity identification

Abstract

The volume-sensitive outwardly rectifying or volume-regulated anion channel, VSOR/VRAC, which was discovered in 1988, is expressed in most vertebrate cell types and is essentially involved in cell volume regulation after swelling and in the induction of cell death. This series of review articles describes what is already known and what remains to be uncovered about the functional and molecular properties as well as the physiological and pathophysiological roles of VSOR/VRAC. This Part 1 review article describes, from the physiological standpoint, first its discovery and significance in cell volume regulation, second its phenotypical properties, and third its molecular identification. Although the pore-forming core molecules and the volume-sensing subcomponent of VSOR/VRAC were identified as LRRC8 members and TRPM7 in 2014 and 2021, respectively, it is stressed that the identification of the molecular entity of VSOR/VRAC is still not complete enough to explain the full set of phenotypical properties.

Physiology of the volume-sensitive/regulatory anion channel VSOR/VRAC: part 2: its activation mechanisms and essential roles in organic signal release

“…Electrophysiological and pharmacological evidence was provided for the VSOR/VRAC role in glutamate release from mouse astrocytes induced by hypoosmotic and ischemic stress [36]. Accordingly, DCPIB, which is a relatively most VSOR/VRAC-specific blocker among available anion channel blockers (see Reviews [1,12]), was shown to inhibit osmotic swelling-induced glutamate release from rat primary astrocytes [37] and human retinal glial Müller MIO-M1 cells [38] as well as the hypotonicity-induced release of aspartate, which is a non-metabolized analog of glutamate, from rat astrocytes [37,39]. However, DCPIB was unexpectedly found to inhibit not only glutamate release mediated by VSOR/ VRAC but also that mediated by connexin hemichannels as well as glutamate uptake via glutamate transporter GLT1 in rat glial cells [40].…”

“…The homomeric LRRC8A channels reconstituted in liposomes were also found to be activated only in low Γ in solutions [31]. The channel activity was observed even in the absence of ATP and in the presence of a high concentration of free Mg 2+ on the intracellular side, in contrast to the phenotypical properties of native VSOR/VRAC existing in living cells (see Table 1 in Part 1 article [12]). Since the reduction in Γ in should increase the surface potential on the peripheral surface of highly charged domains of channel-forming proteins, physicochemical/electrostatic repulsion or attraction would take place between any pairs of closely adjacent charged domains or proteins.…”

“…Deneka et al [28] suggested that the hydrophilic leucine-rich repeats (LRR) domains (LRRDs: see Fig. 2 in the Part 1 article [12]) are involved in the low Γ in -induced activation of VSOR/ VRAC, because many basic (negatively charged) and acidic (positively charged) residues exist on the molecular surface of cytoplasmic LRRD. In association with the activation of LRRC8A channels, such physicochemical conformational changes in LRRDs were recently observed [112] by using five synthetic nanobodies called sybodies (sbs): namely, LRRC8A channels expressed in Lrrc8-knockout (LRRC8 −/− ) HEK293 cells were found to be activated by sb4 and sb5 but inhibited by sb1, sb2, and sb3, the former two sbs and latter three sbs of which were shown to bind to the concave inside and convex outside, respectively, of the horseshoe-shaped LRRDs by cryo-EM.…”

“…As summarized in the Part 1 article [12], TRPM7, which was shown to exert as a mechano-sensitive swelling-activated cation channel by Numata et al [150,151], serves as the swelling-sensing subcomponent of VSOR/VRAC not only by enhancing LRRC8A mRNA expression via steady-state Ca 2+ influx but also by exhibiting real-time functional coupling to VSOR/VRAC activity with showing a physical interaction to LRRC8A protein [11]. It is likely that the TRPM7-mediated Ca 2+ influx is somehow implicated in the VSOR/VRAC activation caused by cell swelling or membrane expansion.…”

“…Soon thereafter, its phenotypical properties were wellcharacterized [6][7][8], but the molecular nature remained unknown for over a quarter of a century. In recent years, the pore-forming core component and the swelling-sensing subcomponents of VSOR/VRAC were identified as LRRC8 members in 2014 [9,10] and TRPM7 in 2021 [11], respectively, as summarized in the Part 1 article [12].…”