The role of D-type cyclins in regulation of early lymphocyte development

An important checkpoint during early B and T lymphopoiesis is the expression of the respective pre-B cell and pre-T cell receptor. Signaling initiated by the expression of either receptor promotes several events, including a burst of proliferation; however, how cells coordinate this clonal expansion remains unclear. The D-type cyclins have been established as critical regulators of proliferation of hematopoietic cells, and cyclin D3 specifically is required for pre-T cell development. We have defined a role for cyclin D3 in B cell development. Our results demonstrate that D3 is upregulated upon transition to the large pre-B cell stage. In the absence of cyclin D3, large pre-B cell development is impaired due to a reduction in early B cell proliferation. Additionally, the development of cyclin D3-deficient B cells depends on cyclin D2 as the combined loss of cyclin D2 and D3 results in an abrogation of B cell development. These findings define a critical role for cyclin D3 in early B cell development.;To extend these results we have investigated the cyclin D3-dependent functions that are required for lymphocyte development. We have evaluated the known functions of D-type cyclins in regulating proliferation, specifically the titration of cell cycle inhibitors and phosphorylation of the retinoblastoma tumor suppressor, Rb. To this end we have generated cyclin D3/p27 and cyclin D3/Rb deficient animals. Our findings indicate that p27 or Rb loss partially restored the defect in proliferation and development of D3-deficient B and T cells. Although these results genetically demonstrate the importance of both titration of p27 and phosphorylation of Rb as D3-dependent functions, neither p27 nor Rb loss was sufficient to completely rescue cyclin D3-deficient pre-B and pre-T cell development. This incomplete rescue suggests that both functions of cyclin D3 must be inactivated during early B and T lymphopoiesis.;Finally, we have aimed to determine the functional redundancy between cyclin D2 and D3 in early lymphocyte development. We have demonstrated that cyclin D2 is expressed in early B and T cells and that this expression is increased upon cyclin D3 loss. This observation suggests that cyclin D2 cannot function as D3 does to promote lymphocyte development. To further investigate this possibility, we are generating a cyclin D2 → D3 "knockin" mouse, and analysis of B and T lymphocyte development from this animal will help resolve whether cyclin D2 and D3 have different functions.