Skip to main page content
Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
. 2018 Aug 14;115(33):8430-8435.
doi: 10.1073/pnas.1806614115. Epub 2018 Jul 30.

Acid-sensing Ion Channels Emerged Over 600 Mya and Are Conserved Throughout the Deuterostomes

Affiliations
Free PMC article

Acid-sensing Ion Channels Emerged Over 600 Mya and Are Conserved Throughout the Deuterostomes

Timothy Lynagh et al. Proc Natl Acad Sci U S A. .
Free PMC article

Abstract

Acid-sensing ion channels (ASICs) are proton-gated ion channels broadly expressed in the vertebrate nervous system, converting decreased extracellular pH into excitatory sodium current. ASICs were previously thought to be a vertebrate-specific branch of the DEG/ENaC family, a broadly conserved but functionally diverse family of channels. Here, we provide phylogenetic and experimental evidence that ASICs are conserved throughout deuterostome animals, showing that ASICs evolved over 600 million years ago. We also provide evidence of ASIC expression in the central nervous system of the tunicate, Oikopleura dioica Furthermore, by comparing broadly related ASICs, we identify key molecular determinants of proton sensitivity and establish that proton sensitivity of the ASIC4 isoform was lost in the mammalian lineage. Taken together, these results suggest that contributions of ASICs to neuronal function may also be conserved broadly in numerous animal phyla.

Keywords: ASIC evolution; invertebrate; ligand recognition; nervous system; proton sensing.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
Deuterostome ASICs identified through phylogenetic and functional analyses. (A) Maximum-likelihood tree inferred from alignment of 17 ASIC-like subunits from one vertebrate, one tunicate, two cephalochordates, one hemichordate, and two echinoderms, rooted to mouse BASIC. (B and C) pH-gated currents in oocytes expressing indicated cRNAs. Resting pH 7.5 (B) or 9.0 (C). [Scale bars: x 20 s; y 0.1 μA (B. belcheri A and P. flava B) or 3 μA (all others).] B. belcheri BChi is a chimera (SI Appendix, Supplementary Text). (D) In situ hybridization targeting O. dioica group A ASIC (black arrows). Illustrated CNS landmarks: CaG, caudal ganglion; ccn, cerebrocaudal nerve; CeG, cerebral ganglion; fbn, first brain nerve. (Inset) Control experiment using complementary sense probe showing nonspecific staining in the pharynx (yellow arrows in both sense and antisense experiments).
Fig. 2.
Fig. 2.
Determinants of proton sensitivity in broadly related ASICs. (A) Chicken ASIC1 structure (PDB ID code 2QTS), different subunits colored orange, blue, and black. Selected side chains are shown in red (acidic pocket), blue (β5/β6 loop from adjacent subunit), and orange (β1 strand). All numbers in this figure refer to equivalent residues in mouse ASIC1a. (B and C) Normalized peak current responses to decreasing pH for mouse ASIC1a or B. belcheri group BChi ASIC mutants carrying indicated deletions (mean ± SEM, n = 5–7). (D) Magnified view of A showing H73 and D78 from adjacent subunits. (E) Example current responses to acidic pH (gray/black bars) from resting pH of 7.5, alone or with 2 mM DTT. pH for H73C/D87C 6.0/5.5/5.0/4.5/4.0; WT 7.0/6.8/6.5/6.0. [Scale bars: x 20 s; y 100 nA (H73C/D78C) and 3 µA (WT).] (F) Normalized responses to acidic pH with/without 2 mM DTT for WT and indicated mouse ASIC1a mutants (mean ± SEM, n = 4). (G) Magnified view of A showing K211 and selected vicinal side-chain/main-chain atoms. Dashed lines: distance = 2.9 Å. (H) Valine (V) and 2-hydroxy-3-methylbutanoic acid (ϖ), red ellipse highlighting decreased electron density. (I) Normalized responses to acidic pH for regular substitutions and noncanonical substitutions (mean ± SEM, n = 5–7).
Fig. 3.
Fig. 3.
Proton sensitivity in vertebrate ASIC4. (A) Maximum-likelihood tree of ASIC4 amino acid sequences, rooted to zebrafish ASIC1.1. Bootstrap support (per 100) shown for selected branches. Example pH 4.0-gated currents for indicated ASICs shown on right. (Scale bars: x 10 s; y 0.1 µA.) “Δ24” constructs lack 24 N-terminal amino acids following the starting methionine. (B) Average (column) and individual (dots) pH 4.0-gated peak current amplitudes (color as in A). n = 10, except for uninjected, n = 25.
Fig. 4.
Fig. 4.
ASIC1a β1 segment confers proton sensitivity on ASIC4. (A) Cartoon illustrating ASIC1a segments that were substituted into ASIC4 (segment 2 circled for clarity). (B) Example currents (Left) and mean peak current amplitude (black columns, Right) in response to pH 4.0 at indicated constructs. (C) Example currents (Left) and mean (±SEM, n = 5–6), normalized responses (Right) to decreasing pH. (Scale bars in B and C: x 5 s, y 0.1 μA.)

Similar articles

See all similar articles

Cited by 12 articles

See all "Cited by" articles

Publication types

LinkOut - more resources

Feedback