Abstract
One of the key defining features of an endogenous circadian clock is that it can be entrained or set to local time. Though a number of cues can perform this role, light is the predominant environmental signal that acts to entrain circadian pacemakers in most species. For the past 20 years, a great deal of work has been performed on the light input pathway in mammals and the role of intrinsically photosensitive retinal ganglion cells (ipRGCs)/melanopsin in detecting and sending light information to the suprachiasmatic nucleus (SCN). In teleost fishes, reptiles and birds, the biology of light sensitivity is more complicated as cells and tissues can be directly light responsive. Non-visual light signalling was described many years ago in the context of seasonal, photoperiodic responses in birds and lizards. In the case of teleosts, in particular the zebrafish model system, not only do peripheral tissues have a circadian pacemaker, but possess clear, direct light sensitivity. A surprisingly wide number of opsin photopigments have been described within these tissues, which may underpin this fundamental ability to respond to light, though no specific functional link for any given opsin yet exists. In this study, we show that zebrafish cells show wide spectral sensitivities, as well as express a number of opsin photopigments – several of which are under direct clock control. Furthermore, we also show that light outside the visual range, both ultraviolet and infrared light, can induce clock genes in zebrafish cells. These same wavelengths can phase shift the clock, except infrared light, which generates no shift even though genes such as per2 and cry1a are induced.
Highlights
The most ancient and predictable environmental cue for life on Earth is the onset of sunrise and sunset
We show that zebrafish cell lines, rather like the adult tissues (Davies et al, 2015), display a diversity of expressed opsins, a number of which are under clock-control and as such show robust daily rhythms in expression
To examine which opsins are present in the cells, both PAC2 cells, and transformed cells expressing a clock-dominant negative construct were kept on a 12:12 light dark cycle at constant temperature, and cells were harvested at ZT3 and ZT15
Summary
The most ancient and predictable environmental cue for life on Earth is the onset of sunrise and sunset. It is hard to imagine any other environmental stimulus that an animal or plant experiences that lacks any biologically significant noise. Most life under the Sun, from bacteria to plants to humans have evolved a circadian clock which internally represents this highly predictable change in day and night. Zebrafish Clock Entrainment pacemaker is that it needs to be entrained or set each day to the environmental light-dark cycle. Natural selection acts on correct phase relationships rather than clock period, such that it is essential both internal and external oscillations are appropriately phase aligned. Entrainment is an essential and defining feature of the circadian clock and the topic addressed in this study
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