Light Induction of a Vertebrate Clock Gene Involves Signaling through Blue-Light Receptors and MAP Kinases

Abstract

The signaling pathways that couple light photoreception to entrainment of the circadian clock have yet to be deciphered. Two prominent groups of candidates for the circadian photoreceptors are opsins (e.g., melanopsin) and blue-light photoreceptors (e.g., cryptochromes). We have previously showed that the zebrafish is an ideal model organism in which to study circadian regulation and light response in peripheral tissues. Here, we used the light-responsive zebrafish cell line Z3 to dissect the response of the clock gene zPer2 to light. We show that the MAPK (mitogen-activated protein kinase) pathway is essential for this response, although other signaling pathways may also play a role. Moreover, action spectrum analyses of zPer2 transcriptional response to monochromatic light demonstrate the involvement of a blue-light photoreceptor. The Cry1b and Cry3 cryptochromes constitute attractive candidates as photoreceptors in this setting. Our results establish a link between blue-light photoreceptors, probably cryptochromes, and the MAPK pathway to elicit light-induced transcriptional activation of clock genes.

Circadian time-keeping during early stages of development

“…Light treatment has been shown to increase zper2 mRNA levels in a zebrafish cell line (15) and, recently, in the pineal gland of 24-h embryos (25). To determine the effect of light on the temporal expression pattern of zper2 during the first day of development, embryos were exposed to different photoperiodic regimes and inspected for zper2 mRNA expression at short intervals by whole-mount ISH.…”

“…Consequently, circadian rhythmicity of melatonin production and photic entrainment of the oscillator can be observed when zebrafish pineal glands are placed in culture (12). Interestingly, peripheral clock-containing tissues in zebrafish were shown to be directly entrained by light without the involvement of the retina or the pineal gland; the mechanism for this photoreception and entrainment is currently unknown (13)(14)(15). In a zebrafish cell line, a light pulse synchronizes and stabilizes the period of otherwise unsynchronized and stochastic cellular oscillators, thus inducing overt rhythms of gene expression (16).…”

E-box function in a period gene repressed by light

“…Expression of the zfper2 gene is induced within the first 2 h after light exposure (30,36). Given the role of Per proteins in the circadian timing mechanism, it is tempting to speculate that light-induced zfper2 may down-regulate zfper4 expression by means of the E-box elements.…”

“…Three zebrafish per genes have been described to date, homologs of mper1, 2, and 3 (30,(33)(34)(35). Whereas the clock regulates expression of zfper1 and 3, light activates zfper2 (30,36). A blue light photoreceptor coupled to the mitogenactivated protein kinase pathway has been implicated in mediating light-induced expression of zfper2 (36).…”

Novel functions for Period 3 and Exo-rhodopsin in rhythmic transcription and melatonin biosynthesis within the zebrafish pineal organ

“…The light-dependent onset of aanat2 transcription in the pineal requires Per2 [63,64]. Transcription of the per2 gene itself is induced by light [12,63,64]. Thus, Per2 has the potential to act as an intermediate between Exorh on the cell membrane and clock components such as Clock/BMAL on the aanat2 promoter ( Figure 6).…”