Involvement of Mouse Nucleoplasmin 2 in the Decondensation of Sperm Chromatin after Fertilization1

The tightly condensed chromatin of spermatozoa is rapidly decondensed after the spermatozoa enter oocytes. Although no factor involved in sperm chromatin decondensation (SCD) has been identified in mammals, it has been suggested that a factor related to SCD activity is present in the germinal vesicle (GV) of oocytes. Here, we found that the nucleolus-like body (NLB), which is a component of the GV, is involved in SCD in murine oocytes. When NLBs were microsurgically removed from GV-stage oocytes, SCD was significantly retarded in the paternal genome after fertilization following meiotic maturation. We found that the retardation of SCD in the NLB-removed oocytes was restored by the microinjection of mRNA encoding nucleoplasmin 2 (NPM2), a component of NLBs. Furthermore, SCD was retarded in the fertilized oocytes from Npm2-knockout females, and recombinant NPM2 alone could induce the SCD in vitro. These data provide evidence that NPM2 is involved in sperm chromatin remodeling in mammals.