Characterization of a piRNA binding protein Miwi in mouse oocytes

Argonaute proteins and Piwi proteins bind with microRNA (mRNA) and Piwi-interacting RNA (piRNA), respectively, to form functional complexes. Piwi proteins are mostly restricted to germ cells and stem cells, and the Piwi-piRNA pathway is required for normal spermatogenesis. Although piRNAs were also recently identified in mammalian oocytes, expression of Piwi proteins in the ovary has not been well characterized. Previous studies did not detect mRNA of Miwi, a murine homologue of Piwi proteins, in total RNA of mouse ovary tissue. We demonstrated herein the presence of Miwi in murine oocytes. Reverse transcription polymerase chain reaction (RT-PCR), Western blot, and immunofluorescence based on quantum dots immune labeling technique were used to investigate the expression profile of Miwi in oocytes of adult and neonatal females at 0, 1, 2, 3, and 4 weeks postpartum. Although RT-PCR was negative in total RNA of the adult ovary, both RT-PCR and Western blot detected Miwi in oocytes of adult mice, and ovaries of neonatal females. Miwi transcript and protein peaked at 1 and 2 weeks postpartum, respectively. Miwi mRNA was detectable in newborn mouse ovaries, implying its transcription was initiated at least in the primordial follicle. Its protein was strong in late primary and secondary follicles, but appeared to decrease as maturation proceeded. The exclusion of anti-Miwi immunofluorescence from some cytoplasmic granules was observed. Given that diverse biologic and molecular functions have been revealed for the Piwi-piRNA pathway in germline cells of many species, Miwi might be an important functional protein in murine folliculogenesis.