Tenascin-R Is a Functional Modulator of Sodium Channel β Subunits

Abstract

Voltage-gated sodium channels isolated from mammalian brain are composed of alpha, beta1, and beta2 subunits. The alpha subunit forms the ion conducting pore of the channel, whereas the beta1 and beta2 subunits modulate channel function, as well as channel plasma membrane expression levels. beta1 and beta2 each contain a single, extracellular Ig-like domain with structural similarity to the neural cell adhesion molecule (CAM), myelin Po. beta2 contains strong amino acid homology to the third Ig domain and to the juxtamembrane region of F3/contactin. Many CAMs of the Ig superfamily have been shown to interact with extracellular matrix molecules. We hypothesized that beta2 may interact with tenascin-R (TN-R), an extracellular matrix molecule that is secreted by oligodendrocytes during myelination and that binds F3-contactin. We show here that cells expressing sodium channel beta1 or beta2 subunits are functionally modulated by TN-R. Transfected cells stably expressing beta1 or beta2 subunits initially recognized and then were repelled from TN-R substrates. The cysteine-rich amino-terminal domain of TN-R expressed as a recombinant peptide, termed EGF-L, appears to be responsible for the repellent effect on beta subunit-expressing cells. The epidermal growth factor-like repeats and fibronectin-like repeats 6-8 are most effective in the initial adhesion of beta subunit-expressing cells. Application of EGF-L to alphaIIAbeta1beta2 channels expressed in Xenopus oocytes potentiated expressed sodium currents without significantly altering current time course or the voltage dependence of current activation or inactivation. Thus, sodium channel beta subunits appear to function as CAMs, and TN-R may be an important regulator of sodium channel localization and function in neurons.

Reduced sodium channel density, altered voltage dependence of inactivation, and increased susceptibility to seizures in mice lacking sodium channel β2-subunits

“…Like the auxiliary subunits of voltage-gated calcium and potassium channels, they modulate gating and voltage dependence as well as regulate expression in the plasma membrane (3). Unlike these other auxiliary subunits, sodium channel ␤-subunits are cell adhesion molecules of the Ig superfamily, which interact with extracellular matrix, transmembrane signaling, and cell adhesion molecules (5)(6)(7)(8)(9)(10)(11)(12). We show here that ␤2-subunits have essential roles in vivo in maintenance of normal electrical excitability of neurons.…”

“…Cloning and functional analysis of the ␤-subunits have implicated them in regulation of channel gating, assembly, and cell surface expression (2)(3)(4). ␤-subunits are also involved in homophilic (5) and heterophilic (6) cell adhesion and in interactions with extracellular matrix and cell adhesion molecules (7)(8)(9), ankyrin (5,(9)(10)(11), and receptor tyrosine phosphatase-␤ (RPTP␤; ref. 12).…”

Sodium Channel β Subunits Mediate Homophilic Cell Adhesion and Recruit Ankyrin to Points of Cell-Cell Contact

“…Multiple antigenic peptides specific to the cytoplasmic domain of ␤2 (KCVRRKKEQKLSTD) or to an extracellular domain of ␤1 (KRRSETTAETFTEWTFR) were synthesized by the Protein and Carbohydrate Structure Core facility at the University of Michigan. These antibodies were described in a previous publication (11). Mouse anti-Drosophila ankyrin antibodies were generated as described previously (17).…”

“…␤1 and ␤2 contain Ig-like extracellular domains and are members of the V-set of the Ig superfamily, which includes many cell adhesion molecules (CAMs) 1 (9). ␤1 and ␤2 have recently been shown to interact with the extracellular matrix proteins tenascin-C and tenascin-R (10,11). Transfected cells expressing ␤1 or ␤2 subunits initially bind to and then are repelled from a tenascin-R substrate (11).…”

“…␤1 and ␤2 have recently been shown to interact with the extracellular matrix proteins tenascin-C and tenascin-R (10,11). Transfected cells expressing ␤1 or ␤2 subunits initially bind to and then are repelled from a tenascin-R substrate (11). Purified sodium channels or the bacterially expressed ␤2 subunit extracellular domain bind tenascin-C and tenascin-R in an enzyme-linked immunosorbent type biochemical assay (10).…”