Cell Cycle Regulation In Oogenesis

Child Health And Human Development

Investigators

Linked publications, trials & patents

Abstract

The long-term goal of the laboratory is to understand how the cell cycle is coordinated with development during gametogenesis. During the mitotic cycle once per cell cycle DNA replication is achieved because two sequential steps in the process of DNA replication have opposite requirements for Cdk activity. While the formation of prereplication complexes (or licensing) requires the presence of low Cdk activity, the initiation of DNA replication is triggered by high Cdk activity. During the mitotic cycle, low Cdk activity is achieved in G1 after the completion of mitosis and the destruction of the mitotic cyclins. The endocycle is a variant cell cycle in which cells become polyploid by undergoing successive rounds of DNA replication without an intervening mitosis. Although endocycling cells do not undergo mitosis, they must pass through an obligate Gap phase during which Cdk activity is low. How Cdk oscillations are achieved during an endocycle is poorly understood. Over the last year we have shown that the oscillations of the cyclin-dependent kinase inhibitor (CKI) Dacapo (Dap) help introduce the Gap phase during the endocycle. The dap gene encodes a p27KIP1-like CKI that specifically inhibits the activity of CycE/Cdk2 complexes. Our data indicate that Dap inhibits CycE/Cdk2 activity during the Gap phase and promotes the efficient relicensing of DNA replication origins. Intriguingly, a similar role has been proposed for the CKI Sic1 in promoting replication origin licensing in late G1 in S. cerevisiae. In Drosophila a single oocyte develops within a 16-cell germline cyst. While all 16 cells initiate meiosis and undergo premeiotic S phase, only the oocyte retains its meiotic chromosome configuration and remains in the meiotic cycle. The other 15 cells in the cyst enter the endocycle and develop as polyploid nurse cells. A long-standing goal in the field has been to identify factors that are concentrated or activated in the oocyte, that promote meiotic progression and/or the establishment of the oocyte identity. We have isolated a recessive female sterile mutant, missing oocyte (mio), in which the oocyte enters the endocycle and develops as a polyploid nurse cell. Unlike other mutants in which the oocyte becomes polyploid, in mio mutant egg chambers, microtubule-based transport to the pro-oocyte is not grossly aberrant and an oocyte is briefly specified. Ultimately however, mio oocytes abandon the meiotic cycle and enter the endocycle. The mio gene encodes a highly conserved protein that preferentially accumulates in pro-oocyte nuclei in early prophase of meiosis I. Genetic interaction studies indicate that mio influences meiotic progression prior to pachytene and may interact with pathways that control DNA metabolism. Specifically, we find the Mio affects the repair of double-stranded breaks during meiosis, independent of the meiotic checkpoint. Our data strongly suggest that the product of the mio gene acts in the oocyte nucleus to facilitate the execution of the unique cell cycle and developmental programs that produce the mature haploid gamete. Currently, we are working to determine if Mio promotes cell-cycle progression during early embryogenesis. Drosophila ovarian cysts arise through a series of four synchronous incomplete mitotic divisions. After each round of mitosis, a membranous organelle, the fusome, grows along the cleavage furrow and the remnants of the mitotic spindle to connect all cystocytes in a cyst. The fusome is essential for the pattern and synchrony of the mitotic cyst divisions, as well as oocyte differentiation. Using live cell imaging, GFP-tagged proteins, and photobleaching techniques, we have demonstrated that fusomal endomembranes are part of a single continuous endoplasmic reticulum (ER) that is shared by all cystocytes in dividing ovarian cysts. Membrane and lumenal proteins of the common ER freely and rapidly diffuse between cystocytes. The fusomal ER mediates intercellular ER connectivity by linking the cytoplasmic ER membranes of all cystocytes within a cyst. Prior to entry into meiosis and onset of oocyte differentiation, ER continuity between cystocytes is lost. Furthermore, analyses of hts and Dhc64c mutants indicate that intercellular ER continuity within dividing ovarian cysts requires the fusome cytoskeletal component and suggest a possible role for the common ER in synchronizing mitotic cyst divisions.