Functional analysis in a model sea anemone reveals phylogenetic complexity and a role in cnidocyte discharge of DEG/ENaC ion channels

Abstract

Ion channels of the DEG/ENaC family share a similar structure but serve strikingly diverse biological functions, such as Na+ reabsorption, mechanosensing, proton-sensing, chemosensing and cell-cell communication via neuropeptides. This functional diversity raises the question of the ancient function of DEG/ENaCs. Using an extensive phylogenetic analysis across many different animal groups, we found a surprising diversity of DEG/ENaCs already in Cnidaria (corals, sea anemones, hyrdoids and jellyfish). Using a combination of gene expression analysis, electrophysiological and functional studies combined with pharmacological inhibition as well as genetic knockout in the model cnidarian Nematostella vectensis, we reveal an unanticipated role for a proton-sensitive DEG/ENaC in discharge of N. vectensis cnidocytes, the stinging cells typifying all cnidarians. Our study supports the view that DEG/ENaCs are versatile channels that have been co-opted for diverse functions since their early occurrence in animals and that respond to simple and ancient stimuli, such as omnipresent protons.

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

“…As a result, we were able to produce a tree with strong support for most nodes, including the key node resolving the ASIC and ENaC subfamilies. Of note, a recent comprehensive phylogenetic study similarly delineated two major superclades of Deg/ENaC channels, referred to as clades A and B which somewhat correspond with our ASIC and ENaC superclades, respectively (8). Nevertheless, there are some notable differences.…”

“…2) and is not activated by protons, lacks both the H73 and K211 residues. Notably, these residues are also absent in other Deg/ENaC channels shown to be sensitive to external protons in vitro, including the proton-inhibited T.adhaerens channel TadNaC6 (24), and the proton activated channels TadNaC2, the ENaC-δ channel from human (6), the channels ACD-2, DEL-9, and ASIC-1 from C.elegans (14), Pickpocket1 from D.melanogaster (16), and NeNaC2 from the sea anemone Nematostella vectensis (8). Notably however, TadNaC2, as well as the mouse ASIC 4 and BASIC channels, possess a cationic residue just one amino acid upstream of the K211 position (i.e., R201 in TadNaC2).…”

“…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.…”

“…Peptide-gated channels are also found in the cnidarian species Hydra magnipapilatta (a hydrozoan), in the form of Hy NaC channels that are activated by RFamide neuropeptides (21, 22). In the cnidarian Nematostella vectensis , an anthozoan, Deg/ENaC channels are reported unresponsive to neuropeptides, but rather, proton sensitive with the channel Ne NaC8 being blocked by protons, and the channels Ne NaC2 and Ne NaC14 being proton-activated (8). In this species, the ASIC-like channel Ne NaC2 contributes to cnidocyte discharge, a unique cellular process in cnidarians that leads to expulsion of a venom-laced barb for defense and prey capture (8, 23).…”

“…In the cnidarian Nematostella vectensis , an anthozoan, Deg/ENaC channels are reported unresponsive to neuropeptides, but rather, proton sensitive with the channel Ne NaC8 being blocked by protons, and the channels Ne NaC2 and Ne NaC14 being proton-activated (8). In this species, the ASIC-like channel Ne NaC2 contributes to cnidocyte discharge, a unique cellular process in cnidarians that leads to expulsion of a venom-laced barb for defense and prey capture (8, 23). To date, the most early-diverging Deg/ENaC channel to be functionally characterized in vitro is Tad NaC6 from the placozoan species Trichoplax adhaerens , which forms a Na + leak channels that is blocked by external protons and Ca 2+ ions (24).…”