[Frontiers in Bioscience 7, 12-23, January 1, 2002]


Lori L. Isom

Department of Pharmacology, The University of Michigan, Ann Arbor, MI


1. Abstract
3. The role of ion channels in cell adhesion
3.1. Sodium channel b subunits form a gene family
3.2. Structure-function experiments define the Ig loop region of b1 and sites of a-b1 interaction
3.3. b subunits as cell adhesion molecules
3.3.1. Sodium channel b subunits interact with extracellular matrix proteins and influence cell migration
3.3.2. Sodium channel b subunits interact homophilically to cause cellular aggregation and ankyrin recruitment
3.4. Sodium channel b subunits interact with RPTPb, Neurofascin, and contactin
3.5. Sodium channel b subunits are involved in human disease
3.6.Other suspects
4. Perspective
5. Acknowledgements
6. References


Voltage-gated sodium channels are unique in that they combine action potential conduction with cell adhesion. Mammalian sodium channels are heterotrimers, composed of a central, pore-forming a subunit and two auxiliary b subunits. The a subunits are members of a large gene family containing the voltage-gated sodium, potassium, and calcium channels. Sodium channel a subunits form a gene subfamily with at least eleven members. Mutations in sodium channel a subunit genes have been linked to paroxysmal disorders such as epilepsy, long QT syndrome (LQT), and hyperkalemic periodic paralysis in humans, and motor endplate disease and cerebellar ataxia in mice. Three genes encode the sodium channel b subunits with at least one alternative splice product. Unlike the pore-forming a subunits, the sodium channel b subunits are not structurally related to b subunits of calcium and potassium channels. Sodium channel b subunits are multifunctional. They modulate channel gating and regulate the level of channel expression at the plasma membrane. We have shown that b subunits also function as cell adhesion molecules (CAMs) in terms of interaction with extracellular matrix molecules, regulation of cell migration, cellular aggregation, and interaction with the cytoskeleton. A mutation in SCN1B has been shown to cause GEFS+1 epilepsy in human families. We propose that the sodium channel signaling complex at nodes of Ranvier involves b subunits as channel modulators as well as CAMs, other CAMs such as neurofascin and contactin, RPTPb, and extracellular matrix molecules such as tenascin. Finally, we explore other subunits of voltage-gated ion channels as potential CAM candidates.