Voltage-gated Na+ channels: potential for beta subunits as therapeutic targets

Abstract

Background: Voltage gated Na(+) channels (VGSCs) contain a pore-forming alpha subunit and one or more beta subunits. VGSCs are involved in a wide variety of pathophysiologies, including epilepsy, cardiac arrhythmia, multiple sclerosis, periodic paralysis, migraine, neuropathic and inflammatory pain, Huntington's disease and cancer. Increasing evidence implicates the beta subunits as key players in these disorders.

Objective: To review the recent literature describing the multifunctional roles of VGSC beta subunits in the context of their role(s) in disease.

Methods: An extensive review of the literature on beta subunits.

Results/conclusion: beta subunits are multifunctional. As components of VGSC complexes, beta subunits mediate signaling processes regulating electrical excitability, adhesion, migration, pathfinding and transcription. beta subunits may prove useful in disease diagnosis and therapy.

Figure 1. Basic functional architecture of voltage-gated Na⁺ channels (VGSCs)

VGSCs contain one pore-forming α subunit in association with two β subunits (Catterall, 1992a). The α subunits consist of four homolgous domains, each containing six transmembrane segments (1–6). Segment 4 contains positively charged residues making the voltage sensor (Catterall, 2000). The β subunits contain one extracellular immunoglobulin (Ig) loop, one transmembrane segment, and an intracellular C-terminal domain (Isom et al., 1994). The tyrosine phosphorylation site in β1 is shown (Malhotra et al., 2004). ψ, glycosylation sites. β2 and β4 are covalently linked to the α subunit through disulfide bonds, although the residues(s) responsible for this interaction have not yet been identified.

Figure 2. Adhesion partners of β1

β1 interacts homophilically and heterophilically with tenascin-R, NrCAM, neurofascin (NF)155, NF186, contactin, N-cadherin and VGSC β2 (Xiao et al., 1999; Malhotra et al., 2000; Malhotra et al., 2002; McEwen and Isom, 2004; McEwen et al., 2004). Interactions with cell adhesion molecules can be trans or cis, although are shown here in trans. The intracellular domain of β1 recruits ankyrin_G or ankyrin_B (Ank) in response to homophilic adhesion and this interaction is dependent on tyrosine phosphorylation of Y181 (Malhotra et al., 2002; Malhotra et al., 2004). EGF-L, epidermal growth factor-like repeat; FN III, fibronectin III-like repeat; Ig, immunoglobulin domain; GPI, glycophosphatidylinositol anchor; Cadh, cadherin repeat.

Figure 3. Model of β1-mediated neurite extension at the growth cone and axonal fasciculation

Trans-adhesive interactions between β1 on an opposing glial cell and a lipid raft β1-contactin complex at the growth cone of a migrating cerebellar granule neuron initiate a signaling cascade via fyn kinase, resulting in neurite outgrowth and migration (Brackenbury et al., 2008a). In addition, β1-β1 trans-adhesion between neurons is proposed to result in axonal fasciculation (Brackenbury et al., 2008a). The possible functional association of β1 with α subunits in these processes is unclear.

Figure 4. Proteolytic processing of β subunits

Sequential cleavage of β subunits by (1) α-secretase and/or β-site amyloid precursor protein-cleaving enzyme (BACE1), followed by (2) γ-secretase yields soluble extracellular N-terminal domains and small intracellular C-terminal domains (Kim et al., 2005; Wong et al., 2005). The soluble immunoglobulin domains are responsible for regulation of adhesion and migration (Davis et al., 2004; Kim et al., 2005). β subunit intracellular domains regulate neurite outgrowth and are putative regulators of VGSC gene transcription (Kim et al., 2007; Miyazaki et al., 2007).

Figure 5. The VGSC macromolecular signaling complex

Pore-forming VGSC α subunits associate with two β subunits (in this case, β1 and β2) (Catterall, 2000). Through the immunoglobulin domains of the β subunits, the complex can associate heterophilically with cell adhesion molecules, e.g. contactin (McEwen and Isom, 2004; McEwen et al., 2004; Brackenbury et al., 2008a). Secretases, e.g. the β-site amyloid precursor protein-cleaving enzyme (BACE1) and γ-secretase may exist in the complex in order to permit regulated proteolytic cleavage of the VGSC β subunits to release intracellular domains (ICDs) (Kim et al., 2005; Wong et al., 2005; Kim et al., 2007). β1 interacts with receptor protein tyrosine phosphatase β (RPTPβ) (Ratcliffe et al., 2000), and depending on the phosphorylation state of Y181 (P), can recruit the cytoskeleton via ankyrin (Ank) (Malhotra et al., 2000; Malhotra et al., 2002). β1 likely interacts with fyn kinase to permit neurite outgrowth/migration (Brackenbury et al., 2008a).