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, Zn(2+), 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.

Kyeong Min Moon, Jimin Kim, Yurim Seong, Byung-Chang Suh, KyeongJin Kang, Han Kyoung Choe & Kyuhyung Kim. BMB Reports 2021;54:393-402. https://doi.org/10.5483/BMBRep.2021.54.8.052

The β-subunits of voltage-gated Ca ²⁺ (Ca _V ) channels regulate the functional expression and several biophysical properties of high-voltage–activated Ca _V channels. We find that Ca _V β-subunits also determine channel regulation by the membrane phospholipid phosphatidylinositol 4,5-bisphosphate (PIP ₂ ). When Ca _V 1.3, -2.1, or -2.2 channels are cotransfected with the β3-subunit, a cytosolic protein, they can be inhibited by activating a voltage-sensitive lipid phosphatase to deplete PIP ₂ . When these channels are coexpressed with a β2a-subunit, a palmitoylated peripheral membrane protein, the inhibition is much smaller. PIP ₂ sensitivity could be increased by disabling the two palmitoylation sites in the β2a-subunit. To further test effects of membrane targeting of Ca _V β-subunits on PIP ₂ regulation, the N terminus of Lyn was ligated onto the cytosolic β3-subunit to confer lipidation. This chimera, like the Ca _V β2a-subunit, displayed plasma membrane localization, slowed the inactivation of Ca _V 2.2 channels, and increased the current density. In addition, the Lyn-β3 subunit significantly decreased Ca _V channel inhibition by PIP ₂ depletion. Evidently lipidation and membrane anchoring of Ca _V β-subunits compete with the PIP ₂ regulation of high-voltage–activated Ca _V channels. Compared with expression with Ca _V β3-subunits alone, inhibition of Ca _V 2.2 channels by PIP ₂ depletion could be significantly attenuated when β2a was coexpressed with β3. Our data suggest that the Ca _V currents in neurons would be regulated by membrane PIP ₂ to a degree that depends on their endogenous β-subunit combinations.

Possible segregation of plasma membrane (PM) phosphoinositide metabolism in membrane lipid domains is not fully understood. We exploited two differently lipidated peptide sequences, L10 and S15, to mark liquid-ordered, cholesterol-rich (L_o) and liquid-disordered, cholesterol-poor (L_d) domains of the PM, often called raft and nonraft domains, respectively. Imaging of the fluorescent labels verified that L10 segregated into cholesterol-rich L_o phases of cooled giant plasma-membrane vesicles (GPMVs), whereas S15 and the dye FAST DiI cosegregated into cholesterol-poor L_d phases. The fluorescent protein markers were used as Förster resonance energy transfer (FRET) pairs in intact cells. An increase of homologous FRET between L10 probes showed that depleting membrane cholesterol shrank L_o domains and enlarged L_d domains, whereas a decrease of L10 FRET showed that adding more cholesterol enlarged L_o and shrank L_d. Heterologous FRET signals between the lipid domain probes and phosphoinositide marker proteins suggested that phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P₂] and phosphatidylinositol 4-phosphate (PtdIns4P) are present in both L_o and L_d domains. In kinetic analysis, muscarinic-receptor-activated phospholipase C (PLC) depleted PtdIns(4,5)P₂ and PtdIns4P more rapidly and produced diacylglycerol (DAG) more rapidly in L_o than in L_d. Further, PtdIns(4,5)P₂ was restored more rapidly in L_o than in L_d. Thus destruction and restoration of PtdIns(4,5)P₂ are faster in L_o than in L_d. This suggests that L_o is enriched with both the receptor G protein/PLC pathway and the PtdIns/PI4-kinase/PtdIns4P pathway. The significant kinetic differences of lipid depletion and restoration also mean that exchange of lipids between these domains is much slower than free diffusion predicts. Significance Lipids of bilayer membranes can segregate laterally into distinct liquid phases of different composition called liquid ordered and liquid disordered, and corresponding in the plasma membrane of living cells to nanodomains called raft and nonraft domains. Using Förster resonance energy transfer and genetically expressible protein probes of lipid domains, we find that several steps of the metabolism of phosphoinositide lipids are concentrated in cholesterol-rich liquid-ordered domains of the plasma membrane. The receptor-mediated breakdown and the restoration of major phosphoinositide pools are faster in the liquid-ordered than in the liquid-disordered domains. Thus, the ordered domains host a key lipid signaling system of the cell.

Significance: Phosphatase and tensin homolog (PTEN) is a phosphatidylinositol 3,4-bisphosphate [PI(3,4)P ₂ ] and phosphatidylinositol 3,4,5-trisphosphate [PI(3,4,5)P ₃ ] 3-phosphatase that plays important roles in cell polarization, division, and development as well as in tumor suppression. Voltage-sensing phosphatases (VSPs) are PTEN family members but have catalytic activity in response to membrane depolarization with a wider range of substrates: They cleave the 5-phosphate of PI(3,4,5)P ₃ and phosphatidylinositol 4,5-bisphosphate as well as the 3-phosphate of PI(3,4)P ₂ and perhaps PI(3,4,5)P ₃ . Using in-cell observations accompanied by quantitative analysis, we demonstrated that VSPs clearly cleave 3-phosphate from PI(3,4,5)P ₃ as PTEN does. Our results suggest that VSPs might act as surrogates for PTEN in several biological events related to membrane depolarization in addition to their inherent role.