Approximately 10% of the photoreceptor outer segment (OS) is turned over each day, requiring large amounts of lipid and protein to be moved from the inner segment to the OS. Defects in intraphotoreceptor transport can lead to retinal degeneration and blindness. The transport mechanisms are unknown, but because the OS is a modified cilium, intraflagellar transport (IFT) is a candidate mechanism. IFT involves movement of large protein complexes along ciliary microtubules and is required for assembly and maintenance of cilia. We show that IFT particle proteins are localized to photoreceptor connecting cilia. We further find that mice with a mutation in the IFT particle protein gene, Tg737/IFT88, have abnormal OS development and retinal degeneration. Thus, IFT is important for assembly and maintenance of the vertebrate OS.

The mammalian circadian system consists of a central oscillator in the suprachiasmatic nucleus of the hypothalamus, which coordinates peripheral clocks in organs throughout the body. Although circadian clocks control the rhythmic expression of a large number of genes involved in metabolism and other aspects of circadian physiology, the consequences of genetic disruption of circadiancontrolled pathways remain poorly defined. Here we report that the targeted disruption of Nocturnin (Ccrn4l) in mice, a gene that encodes a circadian deadenylase, confers resistance to dietinduced obesity. Mice lacking Nocturnin remain lean on high-fat diets, with lower body weight and reduced visceral fat. However, unlike lean lipodystrophic mouse models, these mice do not have fatty livers and do not exhibit increased activity or reduced food intake. Gene expression data suggest that Nocturnin knockout mice have deficits in lipid metabolism or uptake, in addition to changes in glucose and insulin sensitivity. Our data support a pivotal role for Nocturnin downstream of the circadian clockwork in the posttranscriptional regulation of genes necessary for nutrient uptake, metabolism, and storage.C ircadian clocks are present in most tissues of the body, where they control the expression of 5-10% of the tissue-specific mRNAs through both transcriptional and posttranscriptional regulation (1, 2). The widespread importance of circadian clock regulation is evident in that generalized disruption of normal clock function results in tumor formation, sleep disorders, and metabolic problems (reviewed in refs. 3 and 4). For example, mutations in the central clock genes Clock or Bmal1 result in metabolic changes found in obesity and the metabolic syndrome (5-8), and numerous genes involved in fatty acid, cholesterol, and glucose metabolism in liver are regulated in circadian or diurnal patterns (9-15), indicating that the clock plays a broad role in regulating metabolism. Nonetheless, the large number of genes, metabolic pathways, and cell/tissue types that are under general circadian control impose a major challenge in understanding the molecular details. Further advances in this area require refined understanding of the specific circadian output pathways by which the clocks regulate physiology.For cycling mRNAs to closely reflect daily rhythmic transcriptional drive, their half-lives must be relatively short. There are several examples of rhythmic posttranscriptional regulation in which the mRNA half-life or adenylation state changes over the course of the day (16-19), but very little is known about the mechanisms responsible. A likely contributor is Nocturnin (Ccrn4l, Noc), which has been implicated in the posttranscriptional regulation of mRNA stability and/or translatability by the circadian clock (20). Noc is expressed rhythmically in many tissues, with particularly high-amplitude rhythms in liver where mRNA levels are increased 100-fold in early night (21). Noc is at a pivotal position to play a role in shaping the rhythmic pattern of gene ...

The optic vesicle comprises a pool of bi-potential progenitor cells from which the retinal pigment epithelium (RPE) and neural retina fates segregate during ocular morphogenesis. Several transcription factors and signaling pathways have been shown to be important for RPE maintenance and differentiation, but an understanding of the initial fate specification and determination of this ocular cell type is lacking. We show that Yap/Taz-Tead activity is necessary and sufficient for optic vesicle progenitors to adopt RPE identity in zebrafish. A Tead-responsive transgene is expressed within the domain of the optic cup from which RPE arises, and Yap immunoreactivity localizes to the nuclei of prospective RPE cells. yap (yap1) mutants lack a subset of RPE cells and/or exhibit coloboma. Loss of RPE in yap mutants is exacerbated in combination with taz (wwtr1) mutant alleles such that, when Yap and Taz are both absent, optic vesicle progenitor cells completely lose their ability to form RPE. The mechanism of Yap-dependent RPE cell type determination is reliant on both nuclear localization of Yap and interaction with a Tead co-factor. In contrast to loss of Yap and Taz, overexpression of either protein within optic vesicle progenitors leads to ectopic pigmentation in a dosage-dependent manner. Overall, this study identifies Yap and Taz as key early regulators of RPE genesis and provides a mechanistic framework for understanding the congenital ocular defects of Sveinsson's chorioretinal atrophy and congenital retinal coloboma.

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In lower vertebrates, cone retinomotor movements occur in response to changes in lighting conditions and to an endogenous circadian clock. In the light, cone myoids contract, while in the dark, they elongate . In order to test the hypothesis that melatonin and dopamine may be involved in the regulation of cone movement, we have used an in vitro eyecup preparation from Xenopus laevis that sustains light-and dark-adaptive cone retinomotor movement . Melatonin mimics darkness by causing cone elongation . Dark-and melatonin-induced cone elongation are blocked by dopamine . Dopamine also stimulates cone contraction in dark-adapted eyecups. The effect of dopamine appears to be mediated specifically by a dopamine receptor, possibly of the D2 type . The dopamine agonist apomorphine and the putative D2 agonist LY171555 induced cone contraction . In contrast, the putative D, agonist SKF38393-A and specific a,-, a2-, and /3-adrenergic receptor agonists were without effect . Furthermore, the dopamine antagonist spiroperidol not only blocked light-induced cone contraction, but also stimulated cone elongation in the light. These results suggest that dopamine is part of the light signal for cone contraction, and that its suppression is part of the dark signal for cone elongation . Melatonin may affect cone movement indirectly through its influence on the dopaminergic system .