[Molecular pathogenesis of motor neuron diseases]

Spinal and bulbar muscular atrophy (SBMA) and amyotrophic lateral sclerosis (ALS) are representative motor neuron diseases in which selective neuronal degeneration occurs. In this paper, some molecular aspects are discussed related to the pathogenesis of the neuronal degeneration. SBMA is a an X-linked neurodegenerative disease caused by the expansion of a CAG repeat in the first exon of the androgen receptor (AR) gene. To date, eight CAG repeat diseases have been identified, including spinal and bulbar muscular atrophy (SBMA), Huntington's disease (HD), dentatorubralpallidoluysian atrophy (DRPLA), and five spinocerebellar ataxias (SCAs 1, 2, 3, 6, 7). These disorders very likely share a common pathogenesis caused by the gain of a toxic function associated with the expanded polyglutamine tract. Several mechanisms have been postulated as a pathogenic process for neurodegeneration caused by the expanded polyglutamine tract. In SBMA, nuclear inclusions (NIs) containing mutant AR protein have been observed in regions of SBMA central nervous system susceptible to degenerations. Transcriptional factors or their cofactors, such as CREB or creb-binding protein (CBP) sequestrated in NIs, may alter the major intracellular transcriptional signal transduction and ultimately may result in neuronal degeneration. The components in the ubiquitin-proteasome pathway also colocalized in NIs and contribute to the path-ogenesis of SBMA. We generated two types of transgenic mice expressing 239Q under the control of human AR promoter and full-size AR containing 97Q. Marked neurological symptoms and extensive nuclear inclusions were observed in both transgenic lines, but there was no neuronal cell death, suggesting that major neurological phenotype was due to neuronal dysfunction instead of neuronal cell death. As for the therapeutic strategies, the overexpression of Hsp70 and Hsp40 chaperones acted together to protect a cultured neuronal cell model of SBMA from inclusion formation and cell death by mutant AR with expanded polyglutamine tract. In regard to ALS, we are screening the gene expression profiles of the motor neurons from the human ALS and SOD transgenic mouse spinal cord. Motor neurons were microdissected from the spinal cord samples by a lazer-captured microdissection system. Gene expression profiles were screened by cDNA microarray and molecular indexing. Several new molecules were cloned and characterized for their function and relation to neuronal cell dysfunction. Some molecules characterized in this procedure were briefly described.