The protein phosphatase isoform PP1γ1 substitutes for PP1γ2 to support spermatogenesis but not normal sperm function and fertility.

Four isoforms of serine/threonine phosphatase type I, PP1α, PP1β, PP1γ1 and PP1γ2, are derived from three genes. The PP1γ1 and PP1γ2 isoforms are alternately spliced transcripts of the Ppp1cc gene. While PP1γ1 is ubiquitous in somatic cells, PP1γ2 expressed exclusively in testicular germ cells and sperm. Ppp1cc knockout male mice (-/-), lacking both PP1γ1 and PP1γ2 are sterile due to impaired sperm morphogenesis. Fertility and normal sperm function can be restored by transgenic expression of PP1γ2 alone in testis of Ppp1cc (-/-) mice. The purpose of this study was to determine whether the PP1γ1 isoform is functionally equivalent to PP1γ2 in supporting spermatogenesis and male fertility. Significant levels of transgenic PP1γ1 expression occurred only when the transgene lacked a 1kb 3'UTR region immediately following the stop codon of the PP1γ1 transcript. PP1γ1 was also incorporated into sperm at levels comparable to PP1γ2 in sperm from wild type mice. Spermatogenesis was restored in mice expressing PP1γ1 in the absence of PP1γ2. However, males from the transgenic rescue lines were sub-fertile. Sperm from the PP1γ1 rescue mice were unable to fertilize eggs in vitro. Intra-sperm localization of PP1γ1 and the association of the protein regulators of the phosphatase were altered in epididymal sperm in transgenic PP1γ1 compared to PP1γ2. Thus the ubiquitous isoform PP1γ1, not normally expressed in differentiating germ cells, could replace PP1γ2 to support spermatogenesis and spermiation. However, PP1γ2, which is the PP1 isoform in mammalian sperm, has an isoform-specific role in supporting normal sperm function and fertility.