Transmission distortion by loss of p21 or p27 cyclin-dependent kinase inhibitors following competitive spermatogonial transplantation.

Spermatogonial stem cells (SSCs) undergo self-renewal division to support spermatogenesis. Although several positive regulators of SSC self-renewal have been identified, little is known about the mechanisms that negatively regulate SSCs. Here we developed a novel transplantation assay for SSCs and demonstrate that p21 and p27 cyclin-dependent kinase inhibitors play critical roles in SSC self-renewal and differentiation. Overexpression of p21 or p27 abrogated proliferation of cultured SSCs in vitro, and their expression levels were downregulated by exogenous self-renewal signals. In contrast, no apparent defects were found in p21 or p27-deficient SSCs by spermatogonial transplantation. However, competitive spermatogonial transplantation with WT SSCs revealed that the loss of either gene causes distortion of germline transmission: p21-deficiency facilitated mutant offspring production, whereas germline transmission was limited by p27-deficiency. Serial transplantation also showed that the loss of p27, but not p21, decreases secondary colony formation, suggesting that appropriate amounts of p27 are necessary for sustaining SSC self-renewal. Thus, p21 and p27 cyclin-dependent kinase inhibitors play critical roles in germline transmission by regulating the balance between SSC self-renewal and differentiation, and competitive spermatogonial transplantation technique will be useful for analyzing subtle defects in spermatogenesis that are not evident by traditional spermatogonial transplantation.