Significant Expression Levels of Transgenic PPP1CC2 in Testis and Sperm Are Required to Overcome the Male Infertility Phenotype of Ppp1cc Null Mice

PPP1CC2, one of four isoforms of the ser/thr protein phosphatase PP1, is a mammalian-specific splice variant of the Ppp1cc gene, and the only isoform whose expression is confined almost completely to spermatogenic cells. Additionally, PPP1CC2 is the sole isoform found in mammalian spermatozoa. Although PPP1CC1, the other Ppp1cc product, is expressed in many tissues including testis, the only phenotype resulting from deletion of Ppp1cc gene is male infertility. To determine which of the products of Ppp1cc is essential for male fertility, we created two PPP1CC2 transgenes, eTg-G2 and pTg-G2, where Ppp1cc2 expression was driven by the putative endogenous promoter of Ppp1cc or by the testis specific human Pgk2 promoter, respectively. Our results demonstrate that the 2.6-kb genomic region directly upstream of the Ppp1cc structural gene can drive expression of Ppp1cc2, and recapitulate the wild-type tissue specificity of PPP1CC2 in transgenic mice. More importantly, we show that expression of PPP1CC2 alone, via either promoter, is able not only to restore normal spermatogenesis, but the fertility of Ppp1cc null mice as well, provided that transgenic PPP1CC2 expression in testis reaches at least a lower threshold level equivalent to approximately 50% of its expression by a Ppp1cc +/− male. We conclude that the endogenous Ppp1cc promoter normally functions in the testis to maintain a sufficient level of PPP1CC2 expression for normal spermatogenesis to occur, and that production of spermatozoa capable of fertilization in vivo can take place in the complete absence of PPP1CC1 expression.