Circadian clock function

In all types of mammalian cells, the molecular mechanism underlying circadian clock function is based upon interconnected transcription/translation feedback loops [2] (Fig. 2). Two proteins that function as transcriptional... 

Surprisingly, cryptochrome was not found solely in the retina. In addition to low levels found in all tissues, Cryl was also expressed in the SCN, and Cryl mRNA oscillated with circadian rhythmicity, which indicated an additional role for cryptochrome in circadian clock function. Based on the mRNA expression pattern, we postulated a role for Cry2 in circadian photoreception in the retina as well as a hght-independent role for Cryl in the circadian clock mechanism in the SCN. 

Helfrich-Forster C, Winter C, Hofbauer A, Hall JC, Stanewsky R 2001 The circadian clock of fruit flies is blind after elimination of all known photoreceptors. Neuron 30 249-261 Lin FJ, Song W, Meyer-Bernstein E, Naidoo N, Sehgal A 2001 Photic signaling by cryptochrome in the Drosophilacitcidiaa system. Mol Cell Biol 21 7287-7294 Mas P, Devlin PF, Panda S, Kay SA 2000 Functional Interaction of phytochrome B and cryptochrome 2. Nature 408 207-211... 

Bae K, Jin X, Maywood ES, Hastings MH, Reppert SM, Weaver DR 2001 Differential functions of mPerl, mPer2, and mPer3 in the SCN circadian clock. Neuron 30 525—536... 

Circadian clocks are molecular time-keeping mechanisms found in a broad range of cell types from a variety of organisms. The primary roles of these clocks are to maintain their own 24 hour molecular rhythm and to drive the rhythmic expression of genes that control output processes in physiology, metabohsm and behaviour. Core features of the clock are its ability to synchronize to daily environmental speitgehers (e.g. hght—dark or temperature cycles), and then maintain rhythmic function when placed in constant conditions. 

Allada R, Emery P, Takahashi JS, Rosbash M 2001 Stopping time the genetics of fly and mouse circadian clocks. Annu Rev Neurosci 24 1091-1119 Blau J, Young MW 1999 Cycling vrille expression is required for a functional Drosophila clock. Cell 99 661-671... 

Dunlap It would be an enormous amount of work to execute a screen for mutants in the FLO. When you think about it, the situation now with respect to the non-circadian rhythms represented by FLO and similar oscillators is very similar to where we all were with circadian clocks in the early 1980s we had some putative mutants that affected clock expression/function but we had no idea whether they were core circadian clock molecules or simply had pleiotropic effects on the expression of the circadian rhythm. We had no selection, only screens. As we all know it was a lot of work to convincingly show that molecules like FRQ and PER were essential for the circadian clock. I m not sure it s worth making the investment in positional cloning of an allele that might not inform you at all of a mechanism for FLO. And even if you did, since the FLO rhythm is not a circadian rhythm, where is this going to be published In our experience, a gene that affects an oscillator like the FLO that is really not circadian and that no one thinks is involved with the circadian mechanism won t interest a student. 

Antoch MP, Song E-J, Chang A-M et al 1997 Functional identification of the mouse circadian clock gene by transgenic BAC rescue. Cell 89 655-667... 

Camacho F, Cilio M, Guo Y et al 2001 Human casein kinase Idelta phosphorylation of human circadian clock proteins period 1 and 2. FEES Lett 489 159-165 Delaunay F, Thisse C, Marchand O, Laudet V, Thisse B 2000 An inherited functional circadian clock in zebrafish embryos. Science 289 297-300 Dunlap J 1998 Circadian rhythms. An end in the beginning. Science 280 1548-1549 Ebisawa T, Uchiyama M, Kajimura N et al 2001 Association of structural polymorphisms in the human period3 gene with delayed sleep phase syndrome. EMBO Rep 2 342-346 Edery I, Zwiebel LJ, Dembinska ME, Rosbash M 1994 Temporal phosphorylation of the Drosophila period protein. Proc Natl Acad Sci USA 91 2260-2264 Ishida N, Kaneko M, AUada R 1999 Biological clocks. Proc Natl Acad Sci USA 96 8819-8820... 

In addition to CKle, a second casein kinase 1 orthologue, CK1(5, has been implicated in the mammalian circadian clock. CK15 and CKle both bind and phosphorylate mammalian PER proteins in vitro (Keesler et al 2000, Vielhaber et al 2000, Camacho et al 2001), and are physically associated with PER and CRY in vivo (Lee et al 2001). In tau mutants, PER proteins continue to be phosphorylated in spite of the lowered function measured for CKle in vitro (Lee et al 2001). It has been suggested that the residual phosphorylation might be supplied by PER-associated CK15 in the mutants, and that CKle and CK1(5 have overlapping functions in the mammahan circadian system (Lee et al 2001). 

Fig. 10.2 Waves through cell cycle phases in absence (a, b) or presence (c, d) of entrainment by the circadian clock. The variability of durations for all cell cycle phases is equal to 0% (left column) or 15% (right column). The curves, generated by numerical simulations of the cell cycle automaton model, show the proportions of cells in Cl, S, G2 or M phase as a function of time, for days 10-13. The time step used for simulations is equal to 1 min. The duration of the cell cycle before or in the absence of entrainment is 22 h. The successive phases of the cell cycle have the following mean durations G1 (9 h),...

The top panels in Fig. 10.2 show the oscillations in the fraction of cells in the different cell cycle phases, as a function of time, in the absence of entrainment by the circadian clock. In the case considered, the duration of the cell cycle is 22 h, and the variability V is equal to 0% (Fig. 10.2a) or 15% (Fig. 10.2b). When variability is set to zero, no desynchronization occurs and the oscillations in the successive phases of the cell cycle are manifested as square waves that keep a constant amplitude in a given phase. Conversely, when variability increases up to 15% in the absence of entrainment (Fig. 10.2b), the amplitude of the oscillations decreases, reflecting enhanced desynchronization. 

We have already alluded to the effect of synchronization governed by variability V. To further address this point, Fig. 10.6 shows, as a function of V, the cytotoxic effect of the 5-FU profile considered in Fig. 10.2b, with the peak at 4 a.m., in the presence of entrainment of the 22-h cycle by the circadian clock. The results indicate that the cumulated cell kill increases when V rises from 0% to 20%. For this circadian schedule of 5-FU, which is the least toxic to the cells (see above), we see that the better the synchronization, the smaller the number of cells killed. Here, in the presence of entrainment, a larger increase occurs between V 10% and... 

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