Identification and functional analysis of human oocyte-specific linker histone H1

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We investigated osH1 protein expression in the context of biologic significance in human fertilization. A polyclonal rabbit antibody raised against synthetic peptides representing parts of the osH1 sequence was used for Western blotting of extracts from oocytes and ovarian tissues, which demonstrated a band at 36kDa, the calculated molecular mass of osH1. Western analysis detected the protein in chromatin of germinal-vesicle-stage oocytes through two-cell embryos. Thirty four oocytes remaining unfertilized after intracytoplasmic sperm injection were examined by fluorescence immunohistochemistry. Chromatin of injected sperm remained condensed without osH1 in 45% ; remained condensed with osH1 incorporated in 10% ; and had decondensed with osH1 incorporated in 45%. No oocyte showed decondensed sperm chromatin without osH1. Thus, osH1 incorporation into sperm chromatin precedes sperm chromatin decondensation, although whether incorporation of osH1 is needed for development after its disappearance is unclear. Immunohistochemical analysis in ovarian tissues with polyclonal anti-osH1 antibody revealed that the osH1 protein was localized in oocytes of all stages from primordial follicles through Graafian follicles. No reactivity of anti-osH1 antibody was noted in granulosa or theca cells. While the signal was more intense in the nucleus, the cytoplasm also was stained in oocytes examined at all stages. Full length cDNA of the os H1 was cloned, and the recombinant osH1 protein was expressed using wheat germ cell-free protein synthesis technology.