Injection of antisense oligonucleotides was used to investigate the function of c-mos in murine oocytes. Oocytes injected with antisense c-mos oligonucleotides completed the first meiotic division but failed to initiate meiosis H. Instead, loss of c-mos function led to chromosome decondensation, reformation of a nucleus after meiosis I, and cleavage to two cells. Therefore, c-mos is required for meiosis II during murine oocyte maturation.The ability of activated oncogenes to induce neoplastic transformation suggests that their normal cellular progenitors, the protooncogenes, may also play important roles in regulating cell growth and differentiation. While some protooncogenes are expressed in many cell types, others display more restricted patterns of expression, suggesting that they play a role in particular developmental schemes. c-mos is unique among the protooncogenes in that its major sites of expression appear limited to male and female germ cells of the mouse and of several other species (1-8), suggesting a specific function for c-mos in meiotic cell types.The murine oocyte, which is arrested at the diplotene stage of meiotic prophase, accumulates large amounts of c-mos RNA during its growth (2, 3). The c-mos transcripts in these oocytes lack detectable poly(A) tails but become polyadenylylated after resumption of meiosis (9). Such posttranscriptional polyadenylylation is indicative of recruitment of maternal mRNAs for translation in both the mouse and lower organisms (10-13). Consistent with the fate of other maternal mRNAs in the mouse (14), c-mos RNA is degraded by the two-cell stage (2, 15, 16). These results imply that c-mos is a maternal mRNA that is translated and may function during meiosis.In the present study, we have used injection of antisense oligonucleotides to investigate the function of c-mos in the murine oocyte. Meiosis in the murine oocyte is easily followed in vitro. After the reinitiation of meiosis, germinal vesicle (nuclear) breakdown (GVBD) occurs within 4 hr. The first polar body is extruded 6-8 hr later, indicating the completion of meiosis I. Upon reaching metaphase II, the now mature oocyte (egg) will once again arrest awaiting fertilization. After sperm penetration, the egg completes meiosis II, extrudes the second polar body, and cleaves to two cells within 24 Typically, 20-35 cumulus-enclosed oocytes were washed twice through 2.5 ml of DPBS containing 5% fetal calf serum to remove IBMX and initiate maturation. Oocytes were transferred to 350 1LI of DPBS containing 5% fetal calf serum under silicone oil, visualized by using Hoffman diffractioninterference contrast optics, and injected in the cytoplasm with 10 pl of oligonucleotide (1 ptgld) in 0.6x DPBS containing 0.15 mM EDTA; a beveled injection pipette with an outer diameter of 2.5 pm= was used. Injection volume was controlled with a Picoinjector 100 (Medical Systems, Greenvale, NY
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