Expression of 3 beta-hydroxysteroid dehydrogenase type I and type VI isoforms in the mouse testis during development

Six isoforms of the enzyme 3 beta-hydroxysteroid dehydrogenase (3 beta HSD) have been identified in the mouse, each the product of a distinct gene. Tw o of these isoforms (type I and type VI) are detectable in the adult testis but changes in their expression during development are unknown. In this st udy we have examined changes in testicular expression and localization of m RNA encoding the type I and type VI isoforms of 3 beta HSD. Total 3 beta HS D (type I plus type VI) mRNA was measured by reverse transcription-polymera se chain reaction and showed a peak of expression at day 5 after birth foll owed by a decline and then a further rise after day 10 that continued up to adulthood. When each isoform was measured individually it was clear that t he type I isoform was expressed at all ages from embryonic day 13 to adulth ood. In contrast, the type VI isoform was only expressed at significant lev els during fetal life on embryonic day 13 and then not again until after da y 10 postnatally. Expression of the type VI isoform mRNA increased markedly after day 10 so that by adulthood it was the predominant 3 beta HSD isofor m present in the testis. Closer examination of the timing of type VI expres sion showed that the isoform mRNA was first detectable at a significant lev el on day 11. In-situ hybridization confirmed that the type I isoform is th e only one expressed in the fetal/neonatal animal and showed that expressio n was limited to the interstitial tissue. In the adult, both type I and typ e VI expression was within the interstitial tissue. The timing of 3 beta HS D type VI mRNA expression suggests, strongly, that this isoform is expresse d only by adult-type Leydig cells in the mouse testis and that this develop ment starts shortly before day 11. The limited expression of the type VI is oform means that it will be a useful marker in studies of adult Leydig cell development.