Dr. Green completed her Ph.D. in Biochemistry and Molecular Biology at the University of Kansas Medical Center in 1991. She then joined the laboratory of Dr. Joseph Besharse in the Department of Anatomy & Cell Biology at the University of Kansas Medical Center, where she worked on the molecular mechanisms of circadian rhythmicity in the retinal photoreceptors of Xenopus laevis. In 1997 she joined the faculty in the Department of Biology at the University of Virginia, continuing her work on circadian rhythms in both Xenopus and mammals. She moved her laboratory to the University of Texas Southwestern Medical Center in the fall of 2009, joining the Department of Neuroscience.
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.
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.
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.
McCarthy, E.v., Baggs, J.E., Geskes, J.M., Hogenesch, J.B. and Green, C.B. (2009) Generation of a novel allelic series of cryptochrome mutants via mutagenesis reveals residues involved in protein:protein interaction and CRY2-specific repression. Mol. Cell Biol. 29: 5465-5476.
Green, C.B., Takahashi, J.S. and Bass, J. (2008) The meter of metabolism. Cell 134: 2-16.
Green, C.B.*, Douris, N.*, Kojima, S., Strayer, C.A., Fogerty, J., Lourim, D., Keller, S. and Besharse, J.C. (2007) Loss of nocturnin, a circadian deadenylase, confers resistance to hepatic steatosis and diet-induced obesity. Proc. Natl. Acad. Sci., U.S.A., 104: 9888-9893.
van der Schalie, E.A., Conte, F.E., Marz, K.E. and Green, C.B. (2007) 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. Mol. Cell Biol. 27: 2120-2129.
Garbarino-Pico, E., Rollag, M.D., Strayer, C.A., Niu, S., Besharse, J.C., and Green, C.B. (2007) Immediate early response of the circadian polyA ribonuclease nocturnin to two extracellular stimuli. RNA 13: 745-755.
Zhu, H., Conte, F. and Green, C.B. (2003) Nuclear localization and transcriptional repression are confined to separable domains in the circadian protein CRYPTOCHROME. Curr. Biol., 13: 1653-1658.
Kojima, S., Matsumoto, K., Hirose, M., Shimada, M., Nagano, M., Shigeyoshi, Y., Hoshino, S., Green, C.B., Sakaki, Y., and Tei, H. (2007) LARK activates post-transcriptional expression of an essential mammalian clock protein, PERIOD1. Proc. Natl. Acad. Sci, U.S.A. 104: 1859-1864.
Baggs, J. and Green, C.B. (2003) Nocturnin, a deadenylase in Xenopus laevis retina: a mechanism for posttranscriptional control of circadian-related mRNA. Curr. Biol. 13: 189-198.