Cyclin D1 regulates entry into and progression through the cell cycle by controlling the activity of the cyclin-dependent kinases (CDKs). To identify binding partners for cyclin D1, Bienvenu et al. created knock-in mice expressing cyclin D1 with tandem Flag and hemagglutinin (HA) tags and performed a proteomic analysis of cyclin D1 immunoprecipitates from knock-in mouse tissues. Their screen (which they termed “genetic-proteomic”) identified previously known binding partners with roles in cell cycle control (such as CDK4 and CDK6), as well as new binding partners that function in transcriptional regulation. Chromatin immunoprecipitation followed by DNA microarray analysis (ChIP-chip) indicated that cyclin D1 bound to promoter regions near transcription start sites, specifically at DNA recognition sequences characteristic of six transcription factors, including cyclic adenosine monophosphate response element–binding 2 (CREB2) and nuclear factor Y (NF-Y). Compared with retinas from wild-type mice, cyclin D1-null (Ccnd1–/–) mice showed decreased expression of Notch1, which encodes a transmembrane protein that is cleaved to generate an active intracellular domain (NICD) and which promotes proliferation of retinal progenitor cells. Similar to Notch1-deficient mice, Ccnd1–/– mice exhibit retinal hypoplasia, prompting the authors to investigate whether cyclin D1 regulates Notch1 expression. Notch1 increases the abundance of Hes5 mRNA; Hes5, which is a transcription factor, then represses the expression of NeuroD1 and Math5, both of which are transcription factors that promote neuronal differentiation. In Ccnd1–/– retinas, Notch1 abundance was lower than in wild-type retinas; furthermore, the mRNA abundance of Hes5 was decreased, whereas that of NeuroD1 was increased. Injection of retrovirus expressing the NICD into the subretinal space of Ccnd1–/– pups rescued proliferation of retinal progenitor cells, thus confirming that cyclin D1 affects Notch signaling during retinal development. Cyclin D1 increased Notch1 gene transcription by recruiting the histone acetylase CBP (CREB-binding protein) to the Notch1 promoter. Retinas from Ccnd1–/– mice showed lower amounts of CBP occupation of the Notch1 regulatory region and acetylation of histone H4K5 on the Notch1 promoter than those from wild-type mice. Thus, cyclin D1 regulates Notch1 gene transcription during retinal development by binding to CBP and altering histone acetylation.
F. Bienvenu, S. Jirawatnotai, J. E. Elias, C. A. Meyer, K. Mizeracka, A. Marson, G. M. Frampton, M. F. Cole, D. T. Odom, J. Odajima, Y. Geng, A. Zagozdzon, M. Jecrois, R. A. Young, X. S. Liu, C. L. Cepko, S. P. Gygi, P. Sicinski, Transcriptional role of cyclin D1 in development revealed by a genetic-proteomic screen. Nature 463, 374–378 (2010). [PubMed]