Voltage-gated sodium channel gene repertoire of lampreys: gene duplications, tissue-specific expression and discovery of a long-lost gene

Abstract

Studies of the voltage-gated sodium (Nav) channels of extant gnathostomes have made it possible to deduce that ancestral gnathostomes possessed four voltage-gated sodium channel genes derived from a single ancestral chordate gene following two rounds of genome duplication early in vertebrates. We investigated the Nav gene family in two species of lampreys (the Japanese lamprey Lethenteron japonicum and sea lamprey Petromyzon marinus) (jawless vertebrates-agnatha) and compared them with those of basal vertebrates to better understand the origin of Nav genes in vertebrates. We noted six Nav genes in both lamprey species, but orthology with gnathostome (jawed vertebrate) channels was inconclusive. Surprisingly, the Nav2 gene, ubiquitously found in invertebrates and believed to have been lost in vertebrates, is present in lampreys, elephant shark (Callorhinchus milii) and coelacanth (Latimeria chalumnae). Despite repeated duplication of the Nav1 family in vertebrates, Nav2 is only in single copy in those vertebrates in which it is retained, and was independently lost in ray-finned fishes and tetrapods. Of the other five Nav channel genes, most were expressed in brain, one in brain and heart, and one exclusively in skeletal muscle. Invertebrates do not express Nav channel genes in muscle. Thus, early in the vertebrate lineage Nav channels began to diversify and different genes began to express in heart and muscle.

Keywords: agnathans; evolution; lamprey; voltage-gated sodium channel.

Figure 1.

Maximum-likelihood tree (100 bootstrap replicates) for chordate voltage-gated sodium (Nav) channel genes. Nav2, an invertebrate Nav channel gene believed to be absent in chordates, was observed in two lamprey species, the elephant shark and coelacanth. Nav1 channel genes duplicated in chordates.

Figure 2.

The selectivity filter of Nav2 and Nav1 voltage-gated sodium channels. Each domain of the four-domain channel contributes to the selectivity filter. Key amino acids for sodium selectivity are highlighted (red). Note difference in domain III (K in Nav1, E in Nav2). (Online version in colour.)

Figure 3.

Expression analysis (RNA-seq. TPM) of Nav2 and Nav1 voltage-gated sodium channel genes in brain, heart, kidney, muscle, notochord, ovary and testis of the Japanese lamprey.

Figure 4.

Expression analysis (RNA-seq, RPM) of Nav2 and Nav1 voltage-gated sodium channel genes in brain, gill, rope (male-specific adipose tissue), lips, muscle and larval kidney of the sea lamprey.

Figure 5.

Schematic of possible histories of gene gain and loss for lamprey Nav1 channel genes.