Comparative analysis defines a broader FMRFamide-gated sodium channel family and determinants of neuropeptide sensitivity

Abstract

FMRFamide and similar neuropeptides are important physiological modulators in most invertebrates, but the molecular basis of FMRFamide activity at its receptors is unknown. We therefore sought to identify the molecular determinants of FMRFamide potency in one of its native targets, the excitatory FMRFamide-gated sodium channel (FaNaC) from gastropod mollusks. Using molecular phylogenetics and electrophysiological measurement of function, we identified a broad FaNaC family that includes mollusk and annelid channels gated by FMRFamide, FVRIamides, and/or Wamides (or myoinhibitory peptides). A comparative analysis of this broader FaNaC family and other channels from the overarching DEG/ENaC superfamily, incorporating mutagenesis and experimental dissection of function, identified a pocket of amino acid residues that determines activation of FaNaCs by neuropeptides. Although this pocket has diverged in distantly related DEG/ENaC channels that are activated by other ligands, such as mammalian acid-sensing ion channels, we show that it nonetheless contains residues that determine enhancement of those channels by similar peptides. This study thus identifies amino acid residues that determine FMRFamide activity at FaNaCs and illuminates evolution of ligand recognition in one branch of the DEG/ENaC superfamily of ion channels.

Function and phylogeny support the independent evolution of acid-sensing ion channels in the Placozoa

“…Interestingly, all these described channels form a sister clade with a group of early diverging homologues from cnidarians and the poriferan species Sycon ciliatum. A distinct group of channels within the ENaC superclade contains the FMRFamide and Wamide neuropeptide-gated sodium channels (i.e., FaNaC and WaNaC channels), recently shown to be conserved between the lophotrochozoan lineages of molluscs and annelids (9). These form a sister clade relationship with several channels from the phoronid Phoronis australis and M.lignano.…”

“…Nonetheless, during evolution these channels underwent extensive sequence divergence and genetic expansion/loss in different animal lineages, in several cases obscuring their phylogenetic relationships. Although several recent phylogenetic studies have provided some important insights (8)(9)(10), there are still many unanswered questions about the evolutionary origins ASIC, BASIC, and ENaC channels, and their relationships with the many divergent channels identified in invertebrates.…”

“…Insects like the vinegar fly Drosophila melanogaster also possess a diverse set of Deg/ENaC channels (15), including the proton-activated channel Pickpocket 1 (PPK1) which is expressed in proprioceptive and nociceptive sensory neurons where it also thought to respond to mechanical stimuli (16,17). In molluscs and annelids, Deg/ENaC channels act as neurotransmitter receptors activated by the secreted neuropeptides FMRFamide and Wamide (9,(18)(19)(20). Peptide-gated channels are also found in the cnidarian species Hydra magnipapilatta (a hydrozoan), in the form of HyNaC channels that are activated by RFamide neuropeptides (21,22).…”

The diverse functions of the DEG/ENaC family: linking genetic and physiological insights

“…In 1990, Cottrell and colleagues identified FMRFamide (Phe-Met-Arg-Phe-amide)-gated ion channels in the snail, Helix aspersa (Cottrell et al 1990). Subsequent investigation identified an entire family of FMRF-amide-gated channels in Hydra, the HyNaCs (Assmann et al 2014;Dürrnagel et al 2012;Golubovic et al 2007), as well as additional amidated peptide-gated sodium channels (FaNaCs) from molluscs and annelid channels, gated by FMRFamide, FVRIamides and Wamides (Dandamudi et al 2022;Lingueglia et al 1995;Schmidt et al 2018). Despite extensive screening (Vyvers et al 2018), no native peptide agonist for mammalian ASICs has yet been identified, but several are known to potentiate acid-induced currents.…”