OVEREXPRESSION OF MOUSE FOLLISTATIN CAUSES REPRODUCTIVE DEFECTS IN TRANSGENIC MICE

Follistatin is an activin-binding protein that can act as an activin a ntagonist in vitro. Follistatin also binds heparin sulfate proteoglyca ns and may function as a reservoir for activins in vivo. In the mouse, follistatin mRNA is first detected in the deciduum on embryonic day 5 .5 and later in the developing hindbrain, somites, vibrissae, teeth, e pidermis, and muscle. We have previously shown that follistatin-defici ent mice have numerous embryonic defects including shiny, taut skin, g rowth retardation, and cleft palate leading to death within hours of b irth. To further define the roles of follistatin during mammalian repr oduction and development, we created gain-of-function mutant mice in w hich mouse follistatin is overexpressed. The mouse metallothionein (MT )-I promoter was placed upstream of the six-exon mouse follistatin (FS ) gene. To distinguish wild-type and transgenic follistatin mRNA, the 3'-untranslated region of the mouse follistatin gene was replaced with the SV40 untranslated and polyA sequences. Three male and two female founder transgenic mice were produced, were fertile, and transmitted t he transgene to offspring. Northern blot analysis demonstrated that th e transgene mRNA was expressed at varying levels in the livers of offs pring from four of five of the transgenic lines and was expressed in t he testes in all five lines. In MT-FS line 4, which had the highest ex pression of the transgene mRNA in the liver, the transgene transcripts were also present in multiple other tissues. Phenotypically, the MT-F S transgenic lines had defects in the testis, ovary, and hair. Mice fr om MT-FS lines 7 and 10 had slightly decreased testis size, whereas mi ce from lines 4, 5, and 9 had much smaller testes and shiny, somewhat irregular, fur. Histological analysis of the adult testes from line 5 and 9 males showed variable degrees of Leydig cell hyperplasia, an arr est of spermatogenesis, and seminiferous tubular degeneration leading to infertility. Female transgenic mice from lines 4 and 9 had thin ute ri and small ovaries due to a block in folliculogenesis at various sta ges. Many of the line 9 female mice eventually became infertile, and a ll of the line 4 female mice were infertile. Suppressed serum FSH leve ls were seen in only the line 4 transgenic male and female mice, the l ine with widespread expression of the transgene. Serum FSH levels were not significantly different in gonadectomized wild-type and line 5 tr ansgenic male mice despite high levels of the follistatin transgene mR NA in the liver of these transgenic mice. These results suggest that f ollistatin exerts its effects at the levels of the gonads and pituitar y as a local regulator of activin and possibly other transforming grow th factor-beta family members.