A photoperiodic time measurement served by the biphasic expression of Cryptochrome1ab in the zebrafish eye

Abstract

The zebrafish (Danio rerio) is a model species that is used to study the circadian clock. it possesses light-entrainable circadian clocks in both central and peripheral tissues, and its core circadian factor cryptochromes (CRYs) have diverged significantly during evolution. In order to elucidate the functional diversity and involvement of cRYs in photoperiodic mechanisms, we investigated the daily expression profiles of six Cry transcripts in central (brain and eye) and peripheral (fin, skin and muscle) tissues. the zCry genes exhibited gene-specific diurnal conserved variations, and were divided into morning and evening groups. notably, zCry1ab exhibited biphasic expression profiles in the eye, with peaks in the morning and evening. comparing ocular zCry1ab expression in different photoperiods (18L:6D, 14L:10D, 10L:14D and 6L:18D) revealed that zCry1ab expression duration changed depending on the photoperiod: it increased at midnight and peaked before lights off. zCry1ab expression in constant light or dark after entrainment under long-or short-day conditions suggested that the evening clock and photic input pathway are involved in photoperiod-dependent zCry1ab expression. Laser microdissection followed by qRt-pcR analysis showed that the evening peak of zCry1ab was likely ascribed to visual photoreceptors. These results suggest the presence of an eye-specific photoperiodic time measurement served by zCry1ab.The circadian clock consists of three parts: the input pathway(s), the oscillation system and the output pathway(s). Light is the most important signal that activates the input pathway to synchronize the oscillator with daily external cycles. The oscillator generates time signals to control physiological circadian functions through the output pathway. Internal time signals are integrated with the environmental light signal to measure day length, which triggers seasonal photoperiodic responses in many organisms living in temperate zones 1 .The zebrafish (Danio rerio) circadian clock is one of the most studied among fish species, and is reported to exist in not only the central tissues but also the embryo, larva, peripheral tissues and cultured cells (refs. 2-11 in Table 1 , refs. 12-15 ). These zebrafish circadian clocks are directly light-entrainable and this feature is also seen in the other fish cells 16 in contrast to the mammalian circadian clocks that are not light-entrainable except for those in the eye.Among vertebrate core clock components such as period (PER), CLOCK, BMAL and cryptochrome (CRY) proteins, CRYs have both a wide evolutionary background and divergent molecular functions 6 . CRYs are structurally classified into several groups, and together form a large protein family with the photo repair enzymes, photolyases (PHRs). Among CRY/PHR family proteins, animal-type CRYs (CRY1 and CRY2) play a central role in the circadian clock oscillator. Animal-type CRY1 and CRY2 function as transcriptional repressors in the core loop of the circadian clock oscillation system 17,18 , while fruit fly CRY (...

Midnight/midday-synchronized expression of cryptochrome genes in the eyes of three teleost species, zebrafish, goldfish, and medaka

“…zCry1ab , as previously reported [ 22 ], showed two peaks near the beginning of the light period (ZT23.7 in LD; ZT21.5 in SD) and just before the end (ZT13.0 in LD; ZT9.0 in SD), under both LD and SD conditions (Fig. 2 c and d; Table 1 ).…”

“…We previously compared the expression profiles of the Cry genes in the zebrafish eye and reported that their peak times could be located on midnight-morning and evening periods, and that Cry1ab has a double peak [ 22 ]. In this study, expression profiles of 16 Cry genes were obtained by evaluating the gene expression every 2 h, under different photoperiods (14L10D and 10L14D) and in the constant dark just after the different photoperiodic entrainments.…”

“…Genes showing sunrise-synchronized peaks in zebrafish and goldfish seem to be influenced by light. One of such genes, zCry1aa , is strongly induced by light in both zebrafish tissues [ 22 ] and cultured cell lines [ 32 , 33 ]. This seemed to be the case also in the zebrafish eye under both the SD and LD conditions, because a peak around noon weakened or disappeared under both LD-DD and SD-DD conditions (Fig.…”

“…To understand the functional differentiation of the teleost Cry paralogs, we analyzed the expression of the Cry genes in zebrafish [ 22 ]. Among the six zCry s examined, zCry1ab showed dual peaks, one in the morning and the other at the end of the light period, in an eye-specific manner.…”

“…Each of these peaks is synchronized at midnight and at the end of light as the day length changes, and therefore the zCry1ab expression profile fluctuates in response to the day length. These results led us to propose a mechanism for the discrimination of day length in which the states of multiple circadian oscillators vary depending on the day length, resulting in the photoperiodic expression of zCry1ab [ 22 ]. Such a mechanism may control the day-length-dependent changes in the expression of the melanopsin gene in the zebrafish eye [ 23 ].…”