Sodium channel b4 subunit: down-regulation and possible involvement in neuritic degeneration in Huntington’s disease transgenic mice Fumitaka Oyama,* ,1 Haruko Miyazaki,* ,   ,1 Naoaki Sakamoto,* Celine Becquet,* Yoko Machida,* Kumi Kaneko,* Chiharu Uchikawa,* Taishi Suzuki,* Masaru Kurosawa,* Tetsurou Ikeda,* Akira Tamaoka, Takashi Sakurai* and Nobuyuki Nukina* *Laboratory for Structural Neuropathology, RIKEN Brain Science Institute, Saitama, Japan  Department of Neurology, Institute of Clinical Medicine, University of Tsukuba, Ibaraki, Japan Abstract Sodium channel b4 is a very recently identified auxiliary subunit of the voltage-gated sodium channels. To find the primarily affected gene in Huntington’s disease (HD) patho- genesis, we profiled HD transgenic mice using a high-density oligonucleotide array and identified b4 as an expressed sequence tag (EST) that was significantly down-regulated in the striatum of HD model mice and patients. Reduction in b4 started at a presymptomatic stage in HD mice, whereas other voltage-gated ion channel subunits were decreased later. In contrast, spinal cord neurons, which generate only negligible levels of expanded polyglutamine aggregates, maintained normal levels of b4 expression even at the symptomatic stage. Overexpression of b4 induced neurite outgrowth in Neuro2a cells, and caused a thickening of dendrites and increased density of dendritic spines in hip- pocampal primary neurons, indicating that b4 modulates neurite outgrowth activities. These results suggest that down-regulation of b4 may lead to abnormalities of sodium channel and neurite degeneration in the striatum of HD transgenic mice and patients with HD. Keywords: Huntington’s disease, neurite outgrowth activity, polyglutamine, sodium channel b4 subunit. J. Neurochem. (2006) 98, 518–529. The voltage-gated sodium channel in the CNS comprises a complex of pore-forming a subunits with both covalently and non-covalently linked b subunits (Catterall 2000). The non-covalently linked b subunits include b1 (Isom et al. 1992) and b3 (Morgan et al. 2000; Qu et al. 2001), whereas the disulfide-linked moieties consist of b2 (Isom et al. 1995). The b subunits are auxiliary components modulating sodium channel electrophysiological function (Catterall 2000; Mor- gan et al. 2000; Qu et al. 2001). In addition, b1 and b2 function as cell adhesion molecules through their extracel- lular Ig-like domain. They interact with ankyrin, tenascin and neurofascin, which may be responsible for targeting sodium channel complexes to the nodes of Ranvier (Srinivasan et al. 1998; Malhotra et al. 2000; Ratcliffe et al. 2000). Recently, a fourth b subunit has been reported (Yu et al. 2003). The b4 subunit is structurally related to b2 and binds the a subunit by a disulfide bond. Co-expression of b4 with brain Na v 1.2a or skeletal muscle Na v 1.4 subunits in the human embryonic cell line tsA-201 resulted in a negative Received November 21, 2005; revised manuscript received March 7, 2006; accepted March 8, 2006. Address correspondence and reprint requests to Nobuyuki Nukina, Laboratory for Structural Neuropathology, RIKEN Brain Science Insti- tute, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan. E-mail: nukina@brain.riken.jp 1 These authors contributed equally to this article. Abbreviations used: BACE1, b site amyloid precursor protein-clea- ving enzyme; dbcAMP, N6,2¢-O-dibutyryladenosine-3¢,5¢-cyclic mono- phosphate; DH, dorsal horn; (E)GFP, (enhanced) green fluorescent protein; EST, expressed sequence tag; GAPDH, glyceraldehyde-3- phosphate dehydrogenase; HD, Huntington’s disease; HD190QG, transgenic mouse expressing 190 repeat polyglutamine as a fusion with EGFP; HRP, horseradish peroxidase; LGP, lateral globus pallidus; NeuN, neuron-specific nuclear protein; NF, neurofilament; NII, neuronal intra- nuclear inclusion; ORF, open reading frame; PBS, phosphate-buffered saline; polyQ, polyglutamine; Put, putamen; RACE, rapid amplification of cDNA ends; SDS, sodium dodecyl sulfate; Str, striatum; Ot, olfactory tubercle; TG, transgenic; Ubi, ubiquitin; VH, ventral horn; WT, wild type. Journal of Neurochemistry , 2006, 98, 518–529 doi:10.1111/j.1471-4159.2006.03893.x 518 Journal Compilation Ó 2006 International Society for Neurochemistry, J. Neurochem. (2006) 98, 518–529 Ó 2006 The Authors