FMRFamide-gated sodium channel diversity relationship to other DEG/ENaC channels, and the molecular basis for neuropeptide activity.

Abstract

The trimeric degenerin/epithelial sodium channel (DEG/ENaC) superfamily is diverse, including constitutively active, mechanosensitive, acid-gated, and neuropeptide-gated channels. The latter are typified by gastropod FMRFamide-gated sodium channels (FaNaCs). In this work, we explored FaNaC diversity and the molecular basis for neuropeptide activity, using phylogenetics and electrophysiology. First, we establish a broader FaNaC family than previously realized, including FaNaCs in numerous invertebrates and Wamide-gated channels (WaNaCs) in worms that evolved out of FaNaCs to bind these longer neuropeptides. Secondly, our comparative and experimental analysis of the FaNaC family identifies an extracellular pocket lined by residues whose mutation can drastically reduce or increase neuropeptide potency. Remarkably, we also observe that analogous mutations in mammalian acid-sensing ion channels (ASICs) alter the potency with which the synthetic peptide FRRFa modulates acid-gated currents in these channels. Together, these results uncover a pocket that determines neuropeptide activity in DEG/ENaC channels and establishes evolutionary relationships between neuropeptide-gated members of the superfamily.