TEMPORAL PATTERNS OF GENE-EXPRESSION OF G1-S CYCLINS AND CDKS DURING THE FIRST AND 2ND MITOTIC CELL-CYCLES IN MOUSE EMBRYOS

Cell-cycle progression in somatic cells is regulated by a family of cy clins and cyclin-dependent kinases (cdks) that form specific complexes as a function of cell-cycle progression. However, the transcript abun dance of G1-S cyclins and cdks during the meiotic and mitotic cell cyc les of mammalian embryos has not been previously reported. Using a rev erse transcription-polymerase chain reaction (PCR) assay that detects changes in either mRNA abundance or polyadenylation state, we examined the relative levels of gene expression for the G1-S cyclins and cdks, as well as for p21, p27, and the retinoblastoma (Rb) gene in mouse oo cytes, metaphase II-arrested eggs, and 1-2-cell embryos. The PCR produ cts for cyclins D1, D3, and A, as well as cdk4, p21, and Rb, displayed similar levels in meiotically incompetent and competent oocytes, as w ell as in metaphase II-arrested eggs. The levels of PCR products for c yclin D2, p27, and two forms of cdk2 were similar in meiotically incom petent and competent oocytes but decreased during oocyte maturation. F inally, the level of PCR products for cyclin E and cdk2 gradually decr eased during the progression from meiotically incompetent oocytes to m etaphase Ii-arrested eggs. When the levels of PCR products for the G1- S regulatory genes were evaluated during the first and second mitotic cell cycles, four main patterns were found: 1) steady levels for cycli n A; 2) steady levels followed by a 2-3-fold increase during the G2 ph ase of the second mitotic cell cycle for cyclins D1, E, cdk2, and p21; 3) a transient increase during the S and/or G2 phases of the first mi totic cell cycle for p27, cyclin D3, and the two forms of cdk2; and 4) higher levels during the first cell cycle and then a decrease with lo wer levels during the second mitotic cell cycle for cyclin D2 and Rb. cdk4 expression displayed a combination of patterns 2 and 3. The incre ase in the amount of PCR product for the cdk4 gene during the first mi totic cell cycle was due to polyadenylation, whereas the increase in t he amount of PCR product for cdk4, cdk2, and cyclins D1 and E in the s econd mitotic cell cycle was a product of activation of the embryonic genome. (C) 1996 Wiley-Liss, Inc.