A Notch feeling of somite segmentation and beyond.

Vertebrate segmentation is manifested during embryonic development as serially repeated units termed somites that give rise to vertebrae, ribs, skeletal muscle and dermis. Many theoretical models including the "clock and wavefront" model have been proposed. There is compelling genetic evidence showing that Notch-Delta signaling is indispensable for somitogenesis. Notch receptor and its target genes, Hairy/E(spl) homologues, are known to be crucial for the ticking of the segmentation clock. Through the work done in mouse, chick, Xenopus and zebrafish, an oscillator operated by cyclical transcriptional activation and delayed negative feedback regulation is emerging as the fundamental mechanism underlying the segmentation clock. Ubiquitin-dependent protein degradation and probably other posttranslational regulations are also required. Fgf8 and Wnt3a gradients are important in positioning somite boundaries and, probably, in coordinating tail growth and segmentation. The circadian clock is another biochemical oscillator, which, similar to the segmentation clock, is operated with a negative transcription-regulated feedback mechanism. While the circadian clock uses a more complicated network of pathways to achieve homeostasis, it appears that the segmentation clock exploits the Notch pathway to achieve both signal generation and synchronization. We also discuss mathematical modeling and future directions in the end.

Orphan nuclear receptors, molecular clockwork, and the entrainment of peripheral oscillators.

Here we summarize our work on two aspects of circadian timing: the roles of orphan nuclear receptors in the molecular clockwork, and phase entrainment of peripheral oscillators. With reference to the former, studies on cis-acting regulatory elements within the Bmal1 promoter revealed that REV-ERBalpha, an orphan nuclear receptor provides a link between the positive and negative limbs of the molecular oscillator. Specifically, REV-ERBalpha controls the cyclic transcription of Bmal1 and Clock, the positive limb components. In turn, the circadian expression of Rev-Erbalpha itself is driven directly by the molecular oscillator: it is activated by BMAL1 and CLOCK, and repressed by PERIOD1/2 and CRYPTOCHROME1/2 proteins (the negative limb members). With regard to phase entrainment, it was initially believed that only the suprachiasmatic nucleus (SCN) was capable of generating circadian rhythms. However, circadian oscillators have recently been discovered in many peripheral tissues. In the absence of a functional SCN pacemaker, these peripheral clocks dampen after a few days. Hence, the SCN must periodically synchronize these subsidiary timekeepers. It may accomplish this task mostly through an indirect route: namely, by setting the time of feeding. In addition to feeding cycles, body temperature rhythms and cyclically secreted hormones might also serve as zeitgebers for peripheral clocks.

Phosphorylation of CREB Ser142 regulates light-induced phase shifts of the circadian clock.

Biological rhythms are driven in mammals by a central circadian clock located in the suprachiasmatic nucleus (SCN). Light-induced phase shifting of this clock is correlated with phosphorylation of CREB at Ser133 in the SCN. Here, we characterize phosphorylation of CREB at Ser142 and describe its contribution to the entrainment of the clock. In the SCN, light and glutamate strongly induce CREB Ser142 phosphorylation. To determine the physiological relevance of phosphorylation at Ser142, we generated a mouse mutant, CREB(S142A), lacking this phosphorylation site. Light-induced phase shifts of locomotion and expression of c-Fos and mPer1 in the SCN are significantly attenuated in CREB(S142A) mutants. Our findings provide genetic evidence that CREB Ser142 phosphorylation is involved in the entrainment of the mammalian clock and reveal a novel phosphorylation-dependent regulation of CREB activity.