CELLULAR EXPRESSION OF AN HMGCR PROMOTER CAT FUSION GENE IN TRANSGENIC MOUSE-BRAIN - EVIDENCE FOR A DEVELOPMENTAL REGULATION IN OLIGODENDROCYTES

The HMGCR gene encodes the 3-hydroxy-3-methylglutaryl coenzyme A reduc tase, which is the key enzyme for cholesterol synthesis. Mice transgen ic for the prokaryotic chloramphenicol acetyl transferase (CAT) report er gene fused with a 5' Bam H1 fragment including the promoter sequenc e for murine HMGCR gene have been obtained. Homozygote transgenic mice were derived from a particular line selected for similar regulation o f endogenous HMGCR and the transgene expression by nutritional conditi ons in different tissue. In addition, high expression of the transgene was evidenced in the brain. Cellular expression of the CAT gene in th e central nervous system (CNS) was investigated by immunohistochemistr y (IHC). This study was performed on frozen sections of the developing and adult brain, using a rabbit anti-CAT antiserum especially raised for that purpose. CAT expression was observed in some rare individuals in different neural cell types including Purkinje cells and astrocyte s. But the most outstanding observation was the high level of CAT expr ession correlated with differentiated pattern of oligodendrocyte (Ol) distribution observed in white-matter tracts. Double and triple labeli ng for CAT and stage-specific antigens were performed on transgenic Ol -enriched preparations and cultures. This study showed a normal sequen ce of differentiation in the transgenic oligodendroglial cell lineage and demonstrated a strict correlation between late differentiation and activation of the CAT gene in these cells: CAT expression started in transgenic Ols between galactocerebroside (GC)-positive and myelin bas ic protein (MBP)-positive stages and was detected in MBP-positive cell s during the myelination period. After myelination, the number of CAT- positive Ols decreased in the adult brain. These observations demonstr ate a developmental regulation of the CAT transgene in Ols during myel ination in CNS and reinforce the hypothesis of endogenous synthesis as major source of cholesterol during myelination. (C) 1994 Wiley-Liss, Inc.