Drosophila Cdk4 is required for normal growth and is dispensable for cell cycle progression.

Complexes of D-type cyclins and cdk4 or 6 are thought to govern progression through the G(1) phase of the cell cycle. In Drosophila, single genes for Cyclin D and Cdk4 have been identified, simplifying genetic analysis. Here, we show that Drosophila Cdk4 interacts with Cyclin D and the Rb homolog RBF as expected, but is not absolutely essential. Flies homozygous for null mutations develop to the adult stage and are fertile, although only to a very limited degree. Overexpression of inactive mutant Cdk4, which is able to bind Cyclin D, does not enhance the Cdk4 mutant phenotype, confirming the absence of additional Cyclin D-dependent cdks. Our results indicate, therefore, that progression into and through the cell cycle can occur in the absence of Cdk4. However, the growth of cells and of the organism is reduced in Cdk4 mutants, indicating a role of D-type cyclin-dependent protein kinases in the modulation of growth rates.

The Drosophila cyclin D-Cdk4 complex promotes cellular growth.

Mammalian cyclin D-Cdk4 complexes have been characterized as growth factor-responsive cell cycle regulators. Their levels rise upon growth factor stimulation, and they can phosphorylate and thus neutralize Retinoblastoma (Rb) family proteins to promote an E2F-dependent transcriptional program and S-phase entry. Here we characterize the in vivo function of Drosophila Cyclin D (CycD). We find that Drosophila CycD-Cdk4 does not act as a direct G(1)/S-phase regulator, but instead promotes cellular growth (accumulation of mass). The cellular response to CycD-Cdk4-driven growth varied according to cell type. In undifferentiated proliferating wing imaginal cells, CycD-Cdk4 caused accelerated cell division (hyperplasia) without affecting cell cycle phasing or cell size. In endoreplicating salivary gland cells, CycD-Cdk4 caused excessive DNA replication and cell enlargement (hypertrophy). In differentiating eyes, CycD-Cdk4 caused cell enlargement (hypertrophy) in post-mitotic cells. Interaction tests with a Drosophila Rb homolog, RBF, indicate that CycD-Cdk4 can counteract the cell cycle suppressive effects of RBF, but that its growth promoting activity is mediated at least in part via other targets.

Zygotic degradation of two maternal Cdc25 mRNAs terminates Drosophila's early cell cycle program.

In Drosophila embryos the maternal/zygotic transition (MZT) in cell cycle control normally follows mitosis 13. Here we show that this transition requires degradation of two maternal mRNAs, string and twine, which encode Cdc25 phosphatases. Although twine is essential for meiosis and string is essential for most mitotic cycles, the two genes have mutually complementing, overlapping functions in the female germ line and the early embryo. Deletion of both gene products from the female germ line arrests germ-line development. Reducing the maternal dose of both products can lower the number of early embryonic mitoses to 12, whereas increasing maternal Cdc25(twine) can increase the number of early mitoses to 14. Blocking the activation of zygotic transcription stabilizes maternal string and twine mRNAs and also allows an extra maternal mitosis, which is Cdc25 dependent. We propose that Drosophila's MZT comprises a chain reaction in which (1) proliferating nuclei deplete factors (probably mitotic cyclins) required for cell cycle progression; (2) this depletion causes the elongation of interphases and allows zygotic transcription; (3) new gene products accumulate that promote degradation of maternal mRNAs, including string and twine; and (4) consequent loss of Cdc25 phosphatase activity allows inhibitory phosphorylation of Cdc2 by Dwee1 kinase, effecting G2 arrest. Unlike timing or counting mechanisms, this mechanism can compensate for losses or additions of nuclei by altering the timing and number of the maternal cycles and thus will always generate the correct cell density at the MZT.