Adaptive evolution of voltage-gated sodium channels: the first 800 million years

Abstract

Voltage-gated Na(+)-permeable (Nav) channels form the basis for electrical excitability in animals. Nav channels evolved from Ca(2+) channels and were present in the common ancestor of choanoflagellates and animals, although this channel was likely permeable to both Na(+) and Ca(2+). Thus, like many other neuronal channels and receptors, Nav channels predated neurons. Invertebrates possess two Nav channels (Nav1 and Nav2), whereas vertebrate Nav channels are of the Nav1 family. Approximately 500 Mya in early chordates Nav channels evolved a motif that allowed them to cluster at axon initial segments, 50 million years later with the evolution of myelin, Nav channels "capitalized" on this property and clustered at nodes of Ranvier. The enhancement of conduction velocity along with the evolution of jaws likely made early gnathostomes fierce predators and the dominant vertebrates in the ocean. Later in vertebrate evolution, the Nav channel gene family expanded in parallel in tetrapods and teleosts (∼9 to 10 genes in amniotes, 8 in teleosts). This expansion occurred during or after the late Devonian extinction, when teleosts and tetrapods each diversified in their respective habitats, and coincided with an increase in the number of telencephalic nuclei in both groups. The expansion of Nav channels may have allowed for more sophisticated neural computation and tailoring of Nav channel kinetics with potassium channel kinetics to enhance energy savings. Nav channels show adaptive sequence evolution for increasing diversity in communication signals (electric fish), in protection against lethal Nav channel toxins (snakes, newts, pufferfish, insects), and in specialized habitats (naked mole rats).

Fig. 1.

Schematic diagram of the evolutionary relationships among some key families in the ion channel superfamily. On the top of the figure is the structure of the channels moving from left to right showing a linear leak K⁺ channel that is composed of two membrane-spanning helices and a pore (blue), a 6TM channel with a single voltage sensor (red), and four domain 4x6TM channels with four voltage sensors. There is uncertainty about the origin of the 4x6TM family, which more likely evolved in eukaryotes than prokaryotes, as indicated in this figure. A more precise and detailed relationship among Cav and Nav channels in basal metazoans and their sister group, the choanoflagellates, is given in Fig. 3. Reprinted from Comparative Biochemistry and Physiology Part B: Biochemistry and Molecular Biology, 129/1, Peter A. V. Anderson, Robert M. Greenberg, Phylogeny of ion channels: Cues to structure and function, 12-17, Copyright (2001), with permission from Elsevier.

Fig. 2.

Hypothetical secondary structure of a Nav channel. Top: The Nav channel is composed of four repeating domains (I–IV), each of which has six membrane-spanning segments (S1–S6), and their connecting loops (in white). Middle: The four domains cluster around a pore. Bottom: The four P loops dip down into the membrane and line the outer mouth of the channel that is evident in an en face view of a single domain. The black dot represents the single amino acid at the deepest position of each of the four P loops that determines Na⁺ ion selectivity. From Liebeskind et al. (16).

Fig. 3.

Maximum likelihood phylogeny of the voltage-gated sodium channel family. The common ancestor of choanoflagellates (represented by Monosiga in green) and animals had a Nav channel that was likely permeable to Ca²⁺ and Na⁺ (pore motif = DEEA). This motif is present in the Nav channels of anthozoan cnidaria (anemones, coral) and the Nav2 channel of invertebrates. The presence of a lysine (K) in the pore improves Na⁺ selectivity (indicated by red star). A lysine is found in the Nav1 channels of bilaterians (DEKA) and Nav channel of medusozoan cnidaria (jellyfish) (DKEA), both of which have more centralized nervous systems than anthozoans and are motile. Additionally, there is strong conservation of a hydrophobic (blue) triplet of amino acids in the “inactivation gate” region. From Liebeskind et al. (16).

Fig. 4.

The Nav channel gene family underwent an expansion in parallel in teleosts and tetrapods. (A) A schematic chromosome with Nav channel genes (SCN, sodium channel) surrounded by other genes. (B) This chromosome, along with all of the other ancestral chordate chromosomes, duplicated twice at the origin of vertebrates (2R). (C) There was an additional round of genome duplication in teleosts (3R) and (D) tandem duplications of Nav channel genes in ancestral tetrapods and amniotes. There is no indication of any loss of Nav channel genes despite losses of surrounding genes in both teleosts and tetrapods. Furthermore, although not shown here, no other ion channel gene family duplicated after the teleost and tetrapod divergence. Thus, there was likely to be strong selection for the preservation of Nav channel gene duplicates. Reprinted from Jenny Widmark, Görel Sundström, Daniel Ocampo Daza, Dan Larhammar, Differential evolution of voltage-gated sodium channels in tetrapods and teleost fishes, Molecular Biology and Evolution, 2011, by permission of Oxford University Press.

Fig. 5.

Nav1.4a is a fast-evolving Nav channel expressed in the electric organs of two independently derived lineages of weakly electric fish. Two paralogous genes, (A) scn4aa, which encodes Nav1.4a, and (B) scn4ab, which encodes Nav1.4ab, are expressed in the muscles of teleost fish. In the two lineages of weakly electric fishes, the mormyroidea and gymnotiformes, the gene for Nav1.4a (scn4aa) lost its expression in muscle and is only expressed in the electric organ. Nav1.4a underwent a burst of accelerated evolution at the origin of each lineage of electric fish. Nav1.4b, which is expressed in muscle and may also be expressed in the electric organ, evolved at a lower rate. The rate of nonsynonymous substitutions/nonsynonymous sites/rate of synonymous substitutions/synonymous site (dN/dS) in each gene is shown by a color scale in which cool colors represent low rates of sequence evolution and hot colors represent high rates. The arrows indicate where Nav1.4a gene expression was lost from muscle in both lineages. The production of either a highly regular wave type or an irregular pulse type of electric organ discharge is indicated in both groups. In both lineages of electric fishes, the electric organ develops from muscle (myogenic), except for one group (Apteronotidae) in which it is derived from the axons of motorneurons. From Arnegard et al. (2010) (5).