Made for "anchorin": Kv7.2/7.3 (KCNQ2/KCNQ3) channels and the modulation of neuronal excitability in vertebrate axons

Abstract

Kv7.2 and Kv7.3 (encoded by KCNQ2 and KCNQ3) are homologous subunits forming a widely expressed neuronal voltage-gated K(+) (Kv) channel. Hypomorphic mutations in either KCNQ2 or KCNQ3 cause a highly penetrant, though transient, human phenotype-epilepsy during the first months of life. Some KCNQ2 mutations also cause involuntary muscle rippling, or myokymia, which is indicative of motoneuron axon hyperexcitability. Kv7.2 and Kv7.3 are concentrated at axonal initial segments (AISs), and at nodes of Ranvier in the central and peripheral nervous system. Kv7.2 and Kv7.3 share a novel ∼80 residue C-terminal domain bearing an "anchor" motif, which interacts with ankyrin-G and is required for channel AIS (and likely, nodal) localization. This domain includes the sequence IAEGES/TDTD, which is analogous (not homologous) to the ankyrin-G interaction motif of voltage-gated Na(+) (Na(V)) channels. The KCNQ subfamily is evolutionarily ancient, with two genes (KCNQ1 and KCNQ5) persisting as orthologues in extant bilaterian animals from worm to man. However, KCNQ2 and KCNQ3 arose much more recently, in the interval between the divergence of extant jawless and jawed vertebrates. This is precisely the interval during which myelin and saltatory conduction evolved. The natural selection for KCNQ2 and KCNQ3 appears to hinge on these subunits' unique ability to be coordinately localized with Na(V) channels by ankyrin-G, and the resulting enhancement in the reliability of neuronal excitability.

Figure 1. Mammalian Na_V and Kv7 channels possess intracellular anchor motifs with similar sequences, mediating their co-clustering at axon initial segments and nodes of Ranvier

(A) Na_V and Kv7.2/Kv7.3 channel transmembrane topology. Locations of Kv7.2/Kv7.3 peptide sequences required for membrane phospholipid interaction (PIP2, phosphatidylinositol 4,5-bisphosphate) and tetramerization (SID, subunit interaction domain), and the axonal anchor motifs for both channel types are indicated. B) Proposed molecular interactions between jawed vertebrate axonal Na_V and Kv7 channels, ankyrin-G, spectrin, and actin. (C) Cartoon indicating location pattern of ankyrin-G dependent Na_V and Kv7.2/Kv7.3 clustering on myelinated axons, which mediate AP initiation and conduction (AISs, nodes, and branch points, red). After [12].

Figure 2. Phylogenetic analysis reveals that motifs for ankyrin-dependent axonal clustering evolved sequentially, first in chordates (Na_V channels), then in jawed vertebrates (Kv7.2 and Kv7.3)

(A) Phylogram (minimal evolution) of Na_V channels, showing that all vertebrate channels are derived from chordate NaV1. (B) Clustal alignment of Na_V channel DII–DIII loop sequences, showing presence of anchor motifs in chordate NaV1 and all vertebrate channels. The anchor motifs are boxed (red). Shading indicates each residue’s conservation within a set of 28 chordate and non-chordate (not shown) aligned Na_V sequences: bins represent ≤10, 11–20, 21–30, 31–45, 46–60, and 61–100 % conservation. (C) Phylogeny of KCNQ channels, based on analysis of derived amino acid sequences encoded on exons 5–7. Novel genes identified or cloned (see [12]) are shown in red. The branch marking the inferred first appearance of the KCNQ2/3 anchor motif is shown in red. (D) Alignment of KCNQ2 and KCNQ3 C-terminal intracellular sequences near the anchor motifs. Break (vertical black line) indicates location of 5–8 omitted, poorly conserved residues. The derived Kv7.2/Kv7.3 anchor motif sequences (red boxed region) are similar but non-identical to those of chordate Na_V genes. Otherwise no similarity to the Na_V DII–DIII loop sequence shown in B is present. Shading indicates conservation within the 7 KCNQ sequences aligned: shades represent ≤15, 15–30, 31–45, 46–60, 61–75, 76–90, and 91–100% conservation. In A and C, nodes are labeled with bootstrap values (>50%), nodes associated with gene duplications have red dots, and scale bars indicate substitutions per residue. After [12].