Cloning, Localization, and Functional Expression of Sodium Channel β1A Subunits

Abstract

Auxiliary ␤1 subunits of voltage-gated sodium channels have been shown to be cell adhesion molecules of the Ig superfamily. Co-expression of ␣ and ␤1 subunits modulates channel gating as well as plasma membrane expression levels. We have cloned, sequenced, and expressed a splice variant of ␤1, termed ␤1A, that results from an apparent intron retention event. ␤1 and ␤1A are structurally homologous proteins with type I membrane topology; however, they contain little to no amino acid homology beyond the shared Ig loop region. ␤1A mRNA expression is developmentally regulated in rat brain such that it is complementary to ␤1. ␤1A mRNA is expressed during embryonic development, and then its expression becomes undetectable after birth, concomitant with the onset of ␤1 expression. In contrast, ␤1A mRNA is expressed in adult adrenal gland and heart. Western blot analysis revealed ␤1A protein expression in heart, skeletal muscle, and adrenal gland but not in adult brain or spinal cord. Immunocytochemical analysis of ␤1A expression revealed selective expression in brain and spinal cord neurons, with high expression in heart and all dorsal root ganglia neurons. Co-expression of ␣IIA and ␤1A subunits in Chinese hamster lung 1610 cells results in a 2.5-fold increase in sodium current density compared with cells expressing ␣IIA alone. This increase in current density reflected two effects of ␤1A: 1) an increase in the proportion of cells expressing detectable sodium currents and 2) an increase in the level of functional sodium channels in expressing cells. Sodium channels isolated from brain are composed of a central pore-forming ␣ subunit and two auxiliary subunits, ␤1 and ␤2, which do not form the pore yet play critical roles in channel modulation and expression. A mutation in the ␤1 gene (SCN1B) has been implicated to play a role in febrile seizures and generalized epilepsy, GEFSϩ (1). The primary structure of the ␤1 subunit predicts an integral membrane glycoprotein with type I transmembrane topology as well as an extracellular Ig fold (2, 3). ␤ 1 subunits can be classified as members of the V-set of the Ig superfamily, which includes many cell adhesion molecules. ␤1 and ␣ subunit co-expression has been well characterized in Xenopus oocytes and in mammalian cells. In oocytes, co-expression of type IIA (SCN2A) or I (SCN4A) ␣ subunits with ␤1 increases the proportion of sodium channels that function in a fast gating mode, accelerates the macroscopic rates of activation and inactivation, shifts the voltage dependence of inactivation in the hyperpolarizing direction, and increases the peak current amplitude consistent with increases in channel expression (4 -9). In Chinese hamster lung (CHL) 1 cells, stable coexpression of ␤1 with ␣IIA results in increased channel expression levels at the plasma membrane as well as moderate hyperpolarizing shifts in the voltage dependence of channel activation and inactivation (10).␤1 mRNA is expressed only after birth in the developing brain (5, 11). However, previous studies showing the devel...

Characterization of Sodium Channel α- and β-Subunits in Rat and Mouse Cardiac Myocytes

“…We have also observed ␤ 1 A staining of surface membranes of cardiac myocytes when viewed in cross section. 3 It has been proposed that cardiac sodium channels may be targeted and clustered to specific locations in a manner similar to that observed for sodium channels in brain. 25 The presence of ␤-subunits in cardiac myocytes may facilitate sodium channel localization and clustering to discrete functional domains via cell-adhesive interactions.…”

“…2 Recently, 2 additional members of the ␤-subunit gene family have been identified: ␤ 1 A and ␤ 3 . 3,4 The subunit structure of cardiac sodium channels has not been well defined. At least 2 ␣-subunit mRNAs are expressed in heart: Nav1.5 and Nav1.1.…”

Intron Retention Generates a Novel Id3 Isoform That Inhibits Vascular Lesion Formation

“…Recombinant herstatin protein is capable of antagonizing Her-2-mediated growth in breast cancer cells, and herstatin message is down-regulated in carcinoma cells, providing a potential link between herstatin production and growth inhibition in breast cancer, yet it remains unclear whether herstatin protein is produced in vivo (15,22,23). Protein expression from intron-containing message has been described in the literature (24,25) for several protein isoforms including periaxin and sodium channel subunits. However, the in vivo consequences of these events are not clear.…”

β-Subunits of voltage-gated sodium channels in human prostate cancer: quantitative in vitro and in vivo analyses of mRNA expression

“…[7][8][9][10] At least one of these (b1) can be expressed in different isoforms, generated by alternative splicing and intron-retention mechanisms. [11][12][13] VGSCbs can affect VGSC functioning via multiple mechanisms, including modification of voltage-dependent gating, activation and inactivation. 7,[14][15][16][17] These subunits can also increase the amplitude of VGSC currents by (i) modulating the intracellular trafficking of VGSCa protein (thereby incorporating more VGSCs into the cell's plasma membrane) and (ii) increasing their cell surface stability via association with cytoskeletal molecules such as ankyrin.…”