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, hydroids 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.

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

“…is completely closed at neutral pH). This includes vertebrate ASICs Coric et al 2005;Springauf & Grunder, 2010;Waldmann et al 1997b;Zhang & Canessa, 2002), likely Drosophila Ppk1 (Boiko et al 2012) (although this has not been confirmed by heterologous expression), C. elegans ASIC-1 and DEL-9 (Kaulich et al 2022b) and cnidarian, brachiopod, phoronid, annelid, xenacoelomorpha and hemichordate representatives (Aguilar-Camacho et al 2022;Martí-Solans et al 2022). The other subgroup of the acid-activated channels includes the human ENaCs (Collier & Snyder, 2009;Ji & Benos, 2004) and C. elegans ACD-2 (Kaulich et al 2022b), which show some basal activity that can be blocked by amiloride and enhanced by acidic extracellular pH.…”

Neuropeptides and degenerin/epithelial Na+ channels: a relationship from mammals to cnidarians

“…Some HyNaCs are very sensitive to their peptide ligands and start to open at low nanomolar concentrations of RFamides (Assmann et al., 2014). Although also activated by RFamide neuropeptides, HyNaCs have only a distant relationship to FaNaCs (Aguilar‐Camacho et al., 2022; Assmann et al., 2014; Golubovic et al., 2007). It is also becoming clear that W/Famides in cnidarians, including Hydra‐RFamides, are not one‐to‐one orthologues of any of the bilaterian W/F/Yamide families.…”

“…Of note, Hydra‐RFamide III (KPHLRGRF‐NH 2 ) and Hydra‐RFamide IV (HLRGRF‐NH 2 ) do not activate HyNaCs (Golubovic et al., 2007), although they have the same C‐terminal sequence as Hydra‐RFamide I and II, suggesting that the N‐terminal part of the peptides makes important contributions to binding or activation of HyNaCs. Given that HyNaCs have a relatively close relationship to ASICs (Aguilar‐Camacho et al., 2022; Assmann et al., 2014; Golubovic et al., 2007) homology models can reveal whether they possess a cavity corresponding to the acidic pocket of ASICs. Although the acidic residues of this pocket are not conserved in HyNaCs, it would still be a candidate for the ligand binding pocket of HyNaCs.…”

“…Ctenophores, for example, mainly use glutamate and neuropeptides (Sachkova et al., 2021) and also have a large variety of DEG/ENaCs (Moroz et al., 2014). Likewise, the sea anemone Nematostella vectensis also has a large variety of DEG/ENaCs, the NeNaCs, two of which are closely related to HyNaCs (Aguilar‐Camacho et al., 2022), making neuropeptides candidate ligands for these NeNaCs. A first screen with neuropeptides from Nematostella , however, did not identify a neuropeptide directly activating NeNaCs (Aguilar‐Camacho et al., 2022).…”

“…Likewise, the sea anemone Nematostella vectensis also has a large variety of DEG/ENaCs, the NeNaCs, two of which are closely related to HyNaCs (Aguilar‐Camacho et al., 2022), making neuropeptides candidate ligands for these NeNaCs. A first screen with neuropeptides from Nematostella , however, did not identify a neuropeptide directly activating NeNaCs (Aguilar‐Camacho et al., 2022). Future studies will reveal whether DEG/ENaCs of Ctenophores and other Cnidaria are also activated by neuropeptides and whether these are related to Hydra‐RFamides.…”