Shuhui Lim1 and Philipp Kaldis1,2
1- Institute of Molecular and Cell Biology (IMCB), A*STAR (Agency for Science, Technology and Research), Singapore
2- Department of Biochemistry, National University of Singapore (NUS), Singapore
Published in Stem Cells 30, 1509-1520; July 2012.
During neurogenesis, cell cycle regulators play a pivotal role in ensuring proper proliferation, cell cycle exit, and differentiation of neural precursors. However, the precise role of cyclin-dependent kinases (Cdks) in these processes is not well understood. We generated Cdk2 and Cdk4 double knockout (DKO) mice and found a striking ablation of the intermediate zone and cortical plate in mouse embryonic brain. When neural stem cells (NSCs) were isolated and analyzed, DKO NSCs proliferated comparable to wild type as Cdk1 now binds to cyclin D1 and E1 and assumes the role vacated by the loss of Cdk2 and Cdk4 in phosphorylating Rb. Although compensation was sufficient for the maintenance of self-renewal and multilineage potential, DKO NSCs displayed an altered cell cycle profile and were more prone to neuronal differentiation. This was manifested in vivo as a marked reduction in S-phase length and an increased tendency for neurogenic divisions that prevented proper expansion of the basal progenitor pool. Our data thus demonstrate the induction of neurogenic divisions in the absence of critical mediators of G1/S transition—Cdk2 and Cdk4, and highlight their evolutionary importance in the determination of cortical thickness.
Figure Legend: Loss of intermediate zone and cortical plate in Cdk2-/-Cdk4-/- mouse embryonic brain
(A, B): Immunofluorescence staining of E13.5 WT (A) and DKO (B) embryonic brain sectioned sagittally and probed with antibodies against βIII-tubulin (green) and Sox2 (red). (C): Thickness of E13.5 WT (black-white) and DKO (red-white) neocortex. Based on shape of nuclei and Sox2/βIII-tubulin double-staining, the neocortex was subdivided into VZ/SVZ and IZ/CP/MZ, and thickness determined for WT (black) and DKO (red). Data represent mean of four litters, with at least one embryo per genotype per litter. Abbreviations: CP, cortical plate; DKO, double knockout; IZ, intermediate zone; MZ, marginal zone; SVZ, subventricular zone; VZ, ventricular zone; WT, wild type.
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