Circadian Clocks: Translation Lost

Antibody class switching mediated by yeast endonuclease-generated DNA breaks

Antibody class switching in activated B cells uses class switch recombination (CSR), which joins activation-induced cytidine deaminase (AID)-dependent double-strand breaks (DSBs) within two large immunoglobulin heavy chain (IgH) locus switch (S) regions that lie up to 200 kilobases apart. To test postulated roles of S regions and AID in CSR, we generated mutant B cells in which donor Smu and accepter Sgamma1 regions were replaced with yeast I-SceI endonuclease sites. We found that site-specific I-SceI DSBs mediate recombinational IgH locus class switching from IgM to IgG1 without S regions or AID. We propose that CSR evolved to exploit a general DNA repair process that promotes joining of widely separated DSBs within a chromosome.

Retinoid-related Orphan Receptors (RORs): Roles in Cellular Differentiation and Development

Retinoid-related orphan receptors RORalpha, -beta, and -gamma are transcription factors belonging to the steroid hormone receptor superfamily. During embryonic development RORs are expressed in a spatial and temporal manner and are critical in the regulation of cellular differentiation and the development of several tissues. RORalpha plays a key role in the development of the cerebellum particularly in the regulation of the maturation and survival of Purkinje cells. In RORalpha-deficient mice, the reduced production of sonic hedgehog by these cells appears to be the major cause of the decreased proliferation of granule cell precursors and the observed cerebellar atrophy. RORalpha has been implicated in the regulation of a number of other physiological processes, including bone formation. RORbeta expression is largely restricted to several regions of the brain, the retina, and pineal gland. Mice deficient in RORbeta develop retinal degeneration that results in blindness. RORgamma is essential for lymph node organogenesis. In the intestine RORgamma is required for the formation of several other lymphoid tissues: Peyer's patches, cryptopatches, and isolated lymphoid follicles. RORgamma plays a key role in the generation of lymphoid tissue inducer (LTi) cells that are essential for the development of these lymphoid tissues. In addition, RORgamma is a critical regulator of thymopoiesis. It controls the differentiation of immature single-positive thymocytes into double-positive thymocytes and promotes the survival of double-positive thymocytes by inducing the expression of the anti-apoptotic gene Bcl-X(L). Interestingly, all three ROR receptors appear to play a role in the control of circadian rhythms. RORalpha positively regulates the expression of Bmal1, a transcription factor that is critical in the control of the circadian clock. This review intends to provide an overview of the current status of the functions RORs have in these biological processes.

Circadian clocks — the fall and rise of physiology

Circadian clocks control the daily life of most light-sensitive organisms - from cyanobacteria to humans. Molecular processes generate cellular rhythmicity, and cellular clocks in animals coordinate rhythms through interaction (known as coupling). This hierarchy of clocks generates a complex, approximately 24-hour temporal programme that is synchronized with the rotation of the Earth. The circadian system ensures anticipation and adaptation to daily environmental changes, and functions on different levels - from gene expression to behaviour. Circadian research is a remarkable example of interdisciplinarity, unravelling the complex mechanisms that underlie a ubiquitous biological programme. Insights from this research will help to optimize medical diagnostics and therapy, as well as adjust social and biological timing on the individual level.