Expanded Functional Diversity of Shaker K+ Channels in Cnidarians Is Driven by Gene Expansion

Abstract

The genome of the cnidarian Nematostella vectensis (starlet sea anemone) provides a molecular genetic view into the first nervous systems, which appeared in a late common ancestor of cnidarians and bilaterians. Nematostella has a surprisingly large and diverse set of neuronal signaling genes including paralogs of most neuronal signaling molecules found in higher metazoans. Several ion channel gene families are highly expanded in the sea anemone, including three subfamilies of the Shaker K+ channel gene family: Shaker (Kv1), Shaw (Kv3) and Shal (Kv4). In order to better understand the physiological significance of these voltage-gated K+ channel expansions, we analyzed the function of 18 members of the 20 gene Shaker subfamily in Nematostella. Six of the Nematostella Shaker genes express functional homotetrameric K+ channels in vitro. These include functional orthologs of bilaterian Shakers and channels with an unusually high threshold for voltage activation. We identified 11 Nematostella Shaker genes with a distinct “silent” or “regulatory” phenotype; these encode subunits that function only in heteromeric channels and serve to further diversify Nematostella Shaker channel gating properties. Subunits with the regulatory phenotype have not previously been found in the Shaker subfamily, but have evolved independently in the Shab (Kv2) family in vertebrates and the Shal family in a cnidarian. Phylogenetic analysis indicates that regulatory subunits were present in ancestral cnidarians, but have continued to diversity at a high rate after the split between anthozoans and hydrozoans. Comparison of Shaker family gene complements from diverse metazoan species reveals frequent, large scale duplication has produced highly unique sets of Shaker channels in the major metazoan lineages.

Major diversification of voltage-gated K + channels occurred in ancestral parahoxozoans

“…Nematostella Shaker channels have been extensively characterized and demonstrate functional conservation between hydrozoans, anthozoans, and bilaterians (15,16,55). To better understand the functional evolution of the Shaker family, we first functionally expressed Nematostella whole Shab, Shaw, and Shal channels in Xenopus oocytes.…”

“…Shaker channels are clustered to the axon initial segment and nodes of Ranvier in vertebrate neurons (11)(12)(13) and underlie the delayed rectifier in squid giant axons (14). The Shaker subfamily is diverse in cnidarians (15,16), and the starlet sea anemone Nematostella vectensis has functional orthologs of most identified Shaker current types observed in bilaterians (16).…”

Functional evolution of Erg potassium channel gating reveals an ancient origin for I Kr

“…No Drosophila voltage-gated K + channel has an I Kr -like phenotype, so the change in DmErg is either uncompensated or dependent on a distinct molecular mechanism such as gating modification by auxiliary subunits. For example, fast inactivation is native to Drosophila and Nematostella Shaker channels (4,42), but provided by a cytoplasmic β-subunit in mammalian Shaker channels (43). The physiological role of Erg channels in Nematostella has not yet been explored, but slow wave contractions of the body (Movie S1) suggest a role for plateau potentials in behavior.…”

“…Vertebrates recently expanded the number of ion channel genes within each of the conserved families because of vertebratespecific gene duplications. Additionally, phylogenetically restricted duplications of ion channel genes appear common throughout the Eumetazoa (1,(3)(4)(5). Thus, there is little 1:1 gene orthology between the eumetazoan phyla (1).…”

Ether-à-go-go family voltage-gated K+ channels evolved in an ancestral metazoan and functionally diversified in a cnidarian–bilaterian ancestor

“…There is also a pattern of high conservation for other voltage-gated K + channels, including the Shaker, Shal and Shaw families, which encode A-currents and delayed rectifiers and probably play important roles in action potential repolarization and patterning (Bouchard et al, 2006;Jegla and Salkoff, 1997;Jegla et al, 2012;Sand et al, 2011). Therefore, the high molecular conservation of channel genes between cnidarians and bilaterians extends to functional conservation, at least at the level of channel biophysics.…”