Neurosteroid hydroxylase CYP7B - Vivid reporter activity in dentate gyrus of gene-targeted mice and abolition of a widespread pathway of steroid and oxysterol hydroxylation

The major adrenal steroid dehydroepiandrosterone (DHEA) enhances memory and immune function but has no known dedicated receptor; local metabolism may govern its activity. We described a cytochrome P450 expressed in brain and other tissues, CYP7B, that catalyzes the 7 alpha -hydroxylation of oxystero ls and 3 beta -hydroxysteroids including DHEA, We report here that CYP7B mR NA and 7 alpha -hydroxylation activity are widespread in rat tissues. Howev er, steroids related to DHEA are reported to be modified at positions other than 7 alpha, exemplified by prominent 6 alpha -hydroxylation of 5 alpha - androstane-3 beta ,17 beta -diol (A/anediol) in some rodent tissues includi ng brain. To determine whether CYP7B is responsible for these and other act ivities we disrupted the mouse Cyp7b gene by targeted insertion of an IRES- lacZ reporter cassette, placing reporter enzyme activity (beta -galactosida se) under Cyp7b promoter control. In heterozygous mouse brain, chromogenic detection of reporter activity was strikingly restricted to the dentate gyr us. Staining did not exactly reproduce the in situ hybridization expression pattern; post-transcriptional control is inferred. Lower level staining wa s detected in cerebellum, liver, and kidney, and which largely paralleled m RNA distribution. Liver and kidney expression was sexually dimorphic. Mice homozygous for the insertion are viable and superficially normal, but ex vi vo metabolism of DHEA to 7 alpha -hydroxy-DHEA was abolished in brain, sple en, thymus, heart, lung, prostate, uterus, and mammary gland; lower abundan ce metabolites were also eliminated. 7 alpha -Hydroxylation of 25-hydroxych olesterol and related substrates was also abolished, as was presumed 6 alph a -hydroxylation of A/anediol. These different enzyme activities therefore derive from the Cyp7b gene. CYP7B is thus a major extrahepatic steroid and oxysterol hydroxylase and provides the predominant route for local metaboli sm of DHEA and related molecules in brain and other tissues.