D-cyclins (cyclins D1, D2, and D3) are components of the core cell cycle machinery. To directly test the ability of each D-cyclin to drive development of various lineages, we generated mice expressing only cyclin D1, or only cyclin D2, or only cyclin D3. We found that these "single-cyclin" embryos develop normally until late gestation. Our analyses revealed that in single-cyclin embryos, the tissue-specific expression pattern of D-cyclins was lost. Instead, mutant embryos ubiquitously expressed the remaining D-cyclin. These findings suggest that the functions of the three D-cyclins are largely exchangeable at this stage. Later in life, single-cyclin mice displayed focused abnormalities, resulting in premature mortality. "Cyclin D1-only" mice developed severe megaloblastic anemia, "cyclin D2-only" mice presented neurological abnormalities, and "cyclin D3-only" mice lacked normal cerebella. Analyses of the affected tissues revealed that these compartments failed to sufficiently up-regulate the remaining, intact D-cyclin. In particular, we found that in cerebellar granule neuron precursors, the N-myc transcription factor communicates with the cell cycle machinery via cyclins D1 and D2, but not D3, explaining the inability of D3-only mice to up-regulate cyclin D3 in this compartment. Hence, the requirement for a particular cyclin in a given tissue is likely caused by specific transcription factors, rather than by unique properties of cyclins. The key components of the core cell cycle machinery are proteins termed D-type cyclins . Three D-cyclins-cyclin D1, cyclin D2, and cyclin D3-operate in mammalian cells. The three proteins are encoded by separate genes, but show significant amino acid similarity (50%-60% identity throughout the coding region; Inaba et al. 1992;Xiong et al. 1992). The levels of D-cyclins are controlled largely by the extracellular environment. Thus, D-cyclins are induced by mitogens, and their levels decline when mitogens are removed or when antimitogens are added (Matsushime et al. 1991). For these reasons, D-cyclins are regarded as sensors of the extracellular environment that link the mitogenic pathways to the core cell cycle machinery. Once induced, D-cyclins associate with partner cyclin-dependent kinases CDK4 and CDK6 and drive phosphorylation and subsequent inactivation of the retinoblastoma tumor suppressor gene product, pRB, and pRB-related proteins p107 and p130 (Matsushime et al. , 1994Bates et al. 1994;Meyerson and Harlow 1994). This, in turn, causes release or derepression of the E2F transcription factors and allows entry of cells into the S phase (Adams 2001). Ectopic expression of D-cyclins' inhibitor, p16 INK4a , was shown to block the proliferation of several cell types, underscoring a critical, essential function for D-cyclins in cell cycle progression (Lukas et al. 1995a;Ortega et al. 2002).During mouse development, the three D-cyclins are expressed in a dynamic and highly orchestrated fashion, often in mutually exclusive cell types. For example, a rapid switch from cyclin D...

SUMMARYThe endocycle is a variant cell cycle consisting of successive DNA synthesis and Gap phases that yield highly polyploid cells. Although essential for metazoan development, relatively little is known about its control or physiologic role in mammals. Using novel lineage-specific cre mice we identified two opposing arms of the E2F program, one driven by canonical transcription activation (E2F1, E2F2 and E2F3) and the other by atypical repression (E2F7 and E2F8), that converge on the regulation of endocycles in vivo. Ablation of canonical activators in the two endocycling tissues of mammals, trophoblast giant cells in the placenta and hepatocytes in the liver, augmented genome ploidy, whereas ablation of atypical repressors diminished ploidy. These two antagonistic arms coordinate the expression of a unique G2/M transcriptional program that is critical for mitosis, karyokinesis and cytokinesis. These results provide in vivo evidence for a direct role of E2F family members in regulating non-traditional cell cycles in mammals.

SUMMARY Cyclin A, the first cyclin ever cloned, is thought to be an essential component of the cell cycle engine. Mammalian cells encode two A-type cyclins, testis-specific cyclin A1 and ubiquitously expressed cyclin A2. Here we tested the requirement for cyclin A function using conditional knockout mice lacking both A-type cyclins. We found that acute ablation of cyclin A in fibroblasts did not affect cell proliferation, but led to prolonged expression of cyclin E across the cell cycle. However, combined ablation of all A- and E-type cyclins extinguished cell division. Hence, in fibroblasts cyclins A and E play redundant roles in cell proliferation. In contrast, ablation of cyclin A in bone marrow obliterated hematopoiesis. We found that cyclin A function was essential for proliferation of hematopoietic and embryonal stem cells. In these compartments cyclin A-Cdk complexes are expressed at particularly high levels, which may render stem cells dependent on cyclin A.