Developmental regulation and molecular nature of an activity in murine oocytes that transfers histones onto sperm DNA

At fertilization, the remodelling of the sperm nucleus into the male pronucleus is critical for normal development. Morphological and functional changes to the nucleus are underpinned by biochemical changes in the chromatin composition, most notably the removal of sperm specific protamines and assembly of histones onto the paternal DNA. This exchange is controlled by oocyte factors, as exemplified in Xenopus by nucleoplasmin. Though mammalian factors remain unidentified, a functional assay based on antibodies recognizing core histones has been developed to test the activity in oocytes that transfers histones onto sperm DNA, named histone transfer activity (HTA). The assay was applied to growing and maturing murine oocytes to determine when during oogenesis HTA develops, and to probe potential regulatory mechanisms. Fully-grown oocytes develop HTA upon maturation, in a protein-synthesis dependent manner. Large, growing oocytes also develop HTA upon entry into M-phase. Small growing meiotically incompetent oocytes, ones that do not spontaneously enter M-phase, do not develop HTA, though this can be overcome by culture of oocytes to meiotic competence, or by treatment with strontium to induce intracellular calcium oscillations. Taken together these findings form a model of how HTA develops throughout oogenesis. Finally, an attempt is made to identify a potential mammalian HTA factor. Transcripts for two remodelling factors, mNAP and Npm3, are identified in the murine oocyte, and injection of anti-sense oligonucleotides reveals that Npm3 plays a significant role in the deposition of histories and the remodelling of sperm chromatin at fertilization. Combined with the findings of the HTA assay, the data forms a testable model of how Npm3 may be regulated throughout oogenesis.