Ion channels are proteins that form miniscule pores in nerve cell membranes, and when open they generate currents across the membrane to propagate nerve signals. Andrew Spencer from the University of Alberta,Canada, explains that one class of ion channels, the potassium channels, play a major role in shaping the action potentials in many tissues, including heart, and as a consequence have been subject to strong natural selection resulting in a wide variety of potassium channels, each finely tuned to a particular function. One family of potassium ion channels found in vertebrate hearts, the Kv4 voltage-gated potassium channels, comprises three individual members (Kv4.1, Kv4.2 and Kv4.3) that evolved from one of our ancestor's early Kv4 channels. However, the hearts of simpler invertebrates only carry a single Kv4 channel, like our ancestors. Knowing that sea squirts (tunicates) are our most ancient living relatives, Vicenta Salvador-Recatalà and Andrew Spencer wondered whether our tunicate ancestors are more like invertebrates,with only one Kv4 channel, or whether the Kv4 gene had been duplicated early enough in evolutionary history for tunicates to carry multiple Kv4 ion channels. Salvador-Recatalà decided to investigate how many copies of the gene the tunicate Ciona interstinalis possesses,and whether the ion channel plays a role in the tunicate's simple heart(p. 731).
Searching through the C. intestinalis genome,Salvador-Recatalà, Warren Gallin and Spencer identified a single Kv4 gene; the vertebrate channel must have become duplicated after tunicates diverged from our family tree. Salvador-Recatalà also knew that an accessory protein, KChIP, modulates the function of modern Kv4 channels, and wondered whether our tunicate predecessors still carry this gene? Trawling through the genome again, the team was delighted to discover and clone the first non-vertebrate KChIP subunit gene.
Curious to know how the tunicate's potassium channel functions,Salvador-Recatalà, Peter Ruben and Jennifer Abbruzzese began investigating the ion channel's electrical properties by injecting Xenopus egg cells with different combinations of Kv4 and KChIP RNA. Knowing that the cells would use the RNA to produce the ion channel and modulating protein, the team monitored the electrical properties of the modified cells and found that when they introduced the channel alone, the egg cells generated a significant potassium current. However, when the team generated both Kv4 and KChIP proteins in the cells, they saw a dramatic shift in the Kv4 channel's electrical properties; KChIP increased the strength of the egg cell's potassium current as well as prolonging the current's duration,suggesting that the ancient channel contributes to tunicate heart function. Spencer suspects that KChIP probably aids insertion of the ion channel in the cell's membrane, increasing the number of ion channels in the membrane, and the potassium current in turn.
Probing the ion channel's function further, the team removed the first 32 amino acids of the Kv4 protein, which interact with KChIP in vertebrates, to see whether this affected the tunicate channel's function; KChIP no longer modulated the truncated ion channel's function, suggesting that KChIP interacts with tunicate Kv4 through the channel's N terminus, just like modern KChIP. So tunicates invented the blueprint for Kv4 ion channel modulation while vertebrates have refined ion channel function further by expanding their repertoire of Kv4 genes.