Expression of a poriferan potassium channel: insights into the evolution of ion channels in metazoans

Abstract

SUMMARYIon channels establish and regulate membrane potentials in excitable and non-excitable cells. How functional diversification of ion channels contributed to the evolution of nervous systems may be understood by studying organisms at key positions in the evolution of animal multicellularity. We have carried out the first analysis of ion channels cloned from a marine sponge, Amphimedon queenslandica. Phylogenetic comparison of sequences encoding for poriferan inward-rectifier K + (Kir) channels suggests that Kir channels from sponges, cnidarians and triploblastic metazoans each arose from a single channel and that duplications arose independently in the different groups. In Xenopus oocytes, AmqKirA and AmqKirB produced K + currents with strong inward rectification, as seen in the mammalian Kir2 channels, which are found in excitable cells. The pore properties of AmqKir channels demonstrated strong K + selectivity and block by Cs + and Ba 2+. We present an original analysis of sponge ion channel physiology and an examination of the phylogenetic relationships of this channel with other cloned Kir channels. Supplementary material available online at

Mutations in Nature Conferred a High Affinity Phosphatidylinositol 4,5-Bisphosphate-binding Site in Vertebrate Inwardly Rectifying Potassium Channels

“…In addition, AqKir channels, which we engineered to possess the two positive charges present in the PIP 2 -binding site of vertebrate Kir2 channels, showed strong activation by diC 8 PIP 2 , with a sensitivity that was even 3-fold greater than that for vertebrate Kir2.1 channels. Our results confirm that certain positively charged residues (corresponding to residues 80 and 189 in cKir2.2) are absolutely necessary to confer high sensitivity to PIP 2 and represent evolutionary changes that are more recent than the evolution of structures that confer K ϩ selectivity and inward rectification, which are conserved from sponge to human (16). Finally, we propose that other structures present in distant relatives of vertebrate Kir channels must facilitate the activation properties conferred by PIP 2 such that restoration of even a single positive charge (N180K in AqKir) recapitulates the high sensitivity to PIP 2 present in vertebrate Kir channels.…”

“…Conservation of Kir channel selectivity and inward rectification mechanisms (16) have been retained over millions of years of evolution, but acquisition of a new function, PIP 2 regulation by high affinity interaction, is more recent, occurring after the prokaryotic-eukaryotic split.…”

“…To explore the impact of evolutionary pressure on structural changes in ion channel proteins, we studied PIP 2 regulation of AqKir, a K ϩ channel with strong inward rectification, which was previously cloned from the marine sponge Amphimedon queenslandica (16). Sponges are an important comparative group for understanding the relationships between extant unicellular protists and the first multicellular organisms from which all animals evolved (14,(17)(18)(19)(20).…”

Elements of a ‘nervous system’ in sponges

“…However other K channels have been studied, including inward rectifying and two pore K channels (K ir and K 2P ). The K ir channel isolated from Amphimedon shows rapid inactivation, which indicates that the channel resets the membrane potential quicklyas though it might respond to depolarization, a hint that electrical signalling may occur in Amphimedon (Tompkins-MacDonald et al, 2009). The K 2P channel shows sensitivity to amino acids and to pH, but not to temperature, rather like other animal K 2P channels (Wells et al, 2012).…”

A unique alkaline pH-regulated and fatty acid-activated tandem pore domain potassium channel (K2P) from a marine sponge

“…Sponges are sessile, aquatic, multicellular animals that lack a nervous system, but display contractions to expel water (Nickel, 2004;Elliott and Leys, 2007). There is limited information available on membrane conductances in sponges (Zocchi et al, 2001;Tompkins-MacDonald et al, 2009), but hexactinellid (glass) sponges are known to coordinate flagellar movement and demonstrate calcium-dependent action potentials (Leys et al, 1999). Studying ion channels in native sponges using fine electrodes is impeded by the complex glycocalyx and high membrane fluidity (Carpaneto et al, 2003) and thus requires an alternative approach such as the cloning and functional expression of ion channels.…”