Circadian clocks are endogenous timekeeping mechanisms that control many aspects of physiology and behavior. The Green lab studies the molecular mechanism of circadian timing in mammals, with a particular interest in transcriptional and post-transcriptional regulatory mechanisms. We are currently focused on the following projects: (1) Analyses of the regulation and function of the circadian deadenylase nocturnin, (2) Circadian regulation of metabolism, and (3) Structure/function studies of the core circadian clock components Cryptochromes (CRYs). A summary of the lab?s major projects are listed below.
Nocturnin A major focus of the Green lab is the protein encoded by the Nocturnin gene, named for its high amplitude night-time expression. Nocturnin is a deadenylase, a magnesium-dependent ribonuclease that specifically degrades mRNA polyA tails, suggesting that it plays a role in post-transcriptional regulation of circadian gene expression. Our current hypothesis is that Nocturnin acts on specific circadian-relevant mRNAs and that it recognizes these RNAs via interaction with specific RNA-binding proteins. Our goals are to identify these RNA targets and to determine the mechanism by which Nocturnin regulates their expression, as well as determining more broadly the role that post-transcriptional regulatory mechanisms play in exerting circadian control of behavior and physiology.
Circadian Control of Metabolism Nocturnin knockout mice have a striking metabolic phenotype ? they are resistant to diet-induced obesity and hepatic steatosis, and have altered glucose and lipid metabolism. This is one of several examples of the intimate connection between the circadian system and metabolism. We are continuing to pursue the mechanism behind the lean phenotype of the Nocturnin knockout mice and to further delineate the role of the circadian clock in control of metabolic homeostasis.
Cryptochrome The Cryptochrome proteins are critical transcriptional repressors that are necessary for a functioning circadian clock. These proteins have many structural similarities to the DNA repair enzymes photolyases, but distinct functions. The Green lab has been interested in various structure/function aspects of the cryptochromes and have shown that these proteins have two distinct functional domains ? a core photolyase-like domain that is necessary and sufficient for repression, and a C-terminal tail that is necessary for nuclear localization. We are currently interested in defining the structural aspects of cryptochrome that make it a repressor and are doing this by making chimeric constructs between cryptochrome and the non-repressive but closely related photolyase and via a random mutagenesis screen. We are also pursuing studies that determine how the nuclear entry of cryptochrome is regulated within the circadian clock and how this contributes to the circadian period length.
RESEARCH INTERESTS
Molecular mechanisms of circadian rhythmicity
Circadian control of metabolism
Post-transcriptional regulatory mechanisms
RECENT PUBLICATIONS
Garbarino-Pico, E., Rollag, M.D., Strayer, C.A., Niu, S., Besharse, J.C., and Green, C.B., "Immediate early response of the circadian polyA ribonuclease nocturnin to two extracellular stimuli" RNA, 13:745-755, 2007
van der Schalie, E.A., Conte, F.E., Marz, K.E. and Green, C.B., "Structure/function analysis of Xenopus CRYPTOCHROME 1 and 2 reveals differential nuclear localization mechanisms and functional domains important for interaction with and repression of CLOCK:BMAL1" Molecular Cellular Biology, 27:2120-2129, 2007
Green, C.B.*, Douris, N.*, Kojima, S., Strayer, C.A., Fogerty, J., Lourim, D., Keller, S. and Besharse, J.C., "Loss of nocturnin, a circadian deadenylase, confers resistance to hepatic steatosis and diet-induced obesity" Proceedings of the National Academy of Sciences, U.S.A., 104:9888-9893, 2007
Green, C.B., Takahashi, J.S. and Bass, J, "The meter of metabolism" Cell, 134:2-16, 2008
McCarthy, E.v., Baggs, J.E., Geskes, J.M., Hogenesch, J.B. and Green, C.B., "Generation of a novel allelic series of cryptochrome mutants via mutagenesis reveals residues involved in protein:protein interaction and CRY2-specific repression." Molecular Cellular Biology, 29:5465-5476, 2009
SIGNIFICANT PUBLICATIONS
Green, C.B. and Besharse, J.C., "Identification of a novel vertebrate circadian clock regulated gene encoding the protein nocturnin" Proceedings of the National Academy of Sciences, U.S.A., 93:14884-14888, 1996
Zhu, H. and Green, C.B., "Both Xenopus CRY1 and CRY2 functions require an intact flavin-binding domain, but are differentially sensitive to mutations in a putative flavin electron transport pathway." Current Biology, 11:1945-1949, 2001
Hayasaka, N., LaRue, S., and Green, C.B., ") In vivo disruption of Xenopus CLOCK in the retinal photoreceptor cells abolishes circadian melatonin rhythmicity without affecting its production levels" Journal of Neuroscience, 22:1600-1607, 2002
Baggs, J. and Green, C.B., "Nocturnin, a deadenylase in Xenopus laevis retina: a mechanism for posttranscriptional control of circadian-related mRNA" Current Biology, 13:189-198, 2003
Zhu, H., Conte, F. and Green, C.B., "Nuclear localization and transcriptional repression are confined to separable domains in the circadian protein CRYPTOCHROME" Current Biology, 13:1653-1658, 2003
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