The most distinctive feature of oocyte-specific linker histones is the specific timing of their expression during embryonic development. In Xenopus nuclear transfer, somatic linker histones in the donor nucleus are replaced with oocyte-specific linker histone B4, leading to the involvement of oocyte-specific linker histones in nuclear reprogramming. We recently have discovered a mouse oocyte-specific linker histone, named H1foo, and demonstrated its expression pattern in normal preimplantation embryos. The present study was undertaken to determine whether the replacement of somatic linker histones with H1foo occurs during the process of mouse nuclear transfer. H1foo was detected in the donor nucleus soon after transplantation. Thereafter, H1foo was restricted to the chromatin in up to two-cell stage embryos. After fusion of an oocyte with a cell expressing GFP (green fluorescent protein)-tagged somatic linker histone H1c, immediate release of H1c in the donor nucleus was observed. In addition, we used fluorescence recovery after photobleaching (FRAP), and found that H1foo is more mobile than H1c in living cells. The greater mobility of H1foo may contribute to its rapid replacement and decreased stability of the embryonic chromatin structure. These results suggest that rapid replacement of H1c with H1foo may play an important role in nuclear remodeling.

Abstract. Oocyte-specific linker histone H1foo is localized in the oocyte nucleus, either diffusely or bound to chromatin, during the processes of meiotic maturation and fertilization. This expression pattern suggests that H1foo plays a key role in the control of gene expression and chromatin modification during oogenesis and early embryogenesis. To reveal the function of H1foo, we microinjected antisense morpholino oligonucleotides (MO) against H1foo into mouse germinalvesicle stage oocytes. The rate of in vitro maturation of the antisense MO group was significantly lower than that of the control group. Eggs that failed to extrude a first polar body following injection of antisense MO arrested at metaphase I. Additionally, co-injection of in vitro synthesized H1foo mRNA along with antisense MO successfully rescued expression of H1foo and improved the in vitro maturation rate. There was no difference in the rate of parthenogenesis between the antisense MO and control groups. These results indicate that H1foo is essential for maturation of germinal vesiclestage oocytes.

Ovary-specific acidic protein (OSAP) is a novel molecule discovered from a genomic project designed to identify ovary-selective genes in mice. Whereas public databases suggest extraovarian expression of OSAP, its tissue distribution has not yet been well documented. Thus, the expression profile of mouse and human OSAP was determined by quantitative real-time RT-PCR using RNAs isolated from various tissues. The results demonstrate that the human and mouse OSAP expression profiles are similar; OSAP is prominently expressed in steroidogenic tissues with the highest level of expression observed in the adrenal gland. Placenta served as an exception and possessed minimal level of OSAP mRNA. Immunohistochemical studies show that mouse OSAP localizes almost exclusively to the steroid-producing cells of the ovary, adrenal gland, and testis. Consistent with predictions made by several subcellular localization algorithms, dual labeling studies in Y-1 mouse adrenocortical cells indicate OSAP resides in the mitochondria. Because of its abundant expression in steroidogenic cells and mitochondrial localization, a role for OSAP in steroidogenesis was determined. OSAP silencing by specific small interfering RNAs significantly inhibits 8-bromoadenosine-cAMP-induced progesterone production in Y-1 cells. Reduction in OSAP levels results in mitochondrial fragmentation and a decrease in the cellular content of mitochondrial DNA, indicative of decreased mitochondrial abundance. Lastly, 8-bromoadenosine-cAMP does not regulate OSAP protein expression in Y-1 cells as is the case for other steroidogenic components known to be induced by cAMP. Collectively these results suggest that OSAP is involved in steroidogenesis, potentially through its ability to maintain mitochondrial abundance and morphology.

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