Acid-sensing ion channel 2a assembles with epithelial Na⁺ channel β and γ subunits to form mechanosensitive ion...

ASIC2 and ENaC subunits are required for stretch-induced mechanoreceptor currents in renal vascular smooth muscle cells and pressure-induced constriction responses in small renal arteries and arterioles. To examine whether ENaC subunits and ASIC2 can form mechanosensitive ion channels, we co-expressed ASIC2a with ENaC β and γ subunits in Xenopus oocytes to test the channel's response to flow-induced shear stress by changing the perfusion rates with Na⁺ as the conducting ion. Under baseline conditions (0.5 ml/min), oocytes co-expressing ASIC2a and β/γENaC (ASIC2a+β/γENaC) displayed larger inward Na⁺ currents than ASIC2a alone. In oocytes expressing ASIC2a+β/γENaC, but not ASIC2a alone, baseline currents increased in response to a fast perfusion (5 ml/min). The inward Na⁺ currents carried by ASIC2a+β/γENaC were insensitive to 10 μM amiloride but were inhibited when extracellular Na⁺ was replaced with NMDG⁺. A gain-of-function ASIC2a mutant, ASIC2aG430V, had readily detectable amiloride-sensitive baseline currents. Interestingly, flow-mediated channel activation was dependent on subunit composition. A high perfusion rate (5 ml/min) elicited a two-fold increase in Na⁺ currents in oocytes expressing ASIC2aG430V or ASIC2aG430V+βENaC, which was further increased to approximately four-fold when γENaC or both β and γ subunits were co-expressed with ASIC2aG430V (ASIC2aG430V+γENaC or ASIC2aG430V+β/γENaC). Exposure to acidic pH evoked greater channel activity in oocytes expressing ASIC2aG430V+β/γENaC than ASIC2aG430V alone. In contrast, ENaC β and γ subunits did not alter the channel's permeability to monovalent cations of different sizes. Collectively, our experimental findings and structural modeling suggest that ASIC2a assembles with ENaC β and γ subunits to form mechanosensitive channels.