Circadian Clocks in Fish-What Have We Learned so far?

Abstract

Zebrafish represent the one alternative vertebrate, genetic model system to mice that can be easily manipulated in a laboratory setting. With the teleost Medaka (Oryzias latipes), which now has a significant following, and over 30,000 other fish species worldwide, there is great potential to study the biology of environmental adaptation using teleosts. Zebrafish are primarily used for research on developmental biology, for obvious reasons. However, fish in general have also contributed to our understanding of circadian clock biology in the broadest sense. In this review, we will discuss selected areas where this contribution seems most unique. This will include a discussion of the issue of central versus peripheral clocks, in which zebrafish played an early role; the global nature of light sensitivity; and the critical role played by light in regulating cell biology. In addition, we also discuss the importance of the clock in controlling the timing of fundamental aspects of cell biology, such as the temporal control of the cell cycle. Many of these findings are applicable to the majority of vertebrate species. However, some reflect the unique manner in which "fish" can solve biological problems, in an evolutionary context. Genome duplication events simply mean that many fish species have more gene copies to "throw at a problem", and evolution seems to have taken advantage of this "gene abundance". How this relates to their poor cousins, the mammals, remains to be seen.

Keywords: DNA repair; cell cycle; circadian clock; development; non-visual light detection; zebrafish.

Figure 1

Zebrafish tissues are rhythmic and directly light-responsive. All zebrafish cell types and tissue/organs examined to date are directly light-responsive and do not require a centralised photosensitive structure to turn on light-induced transcription. Cells and organs can be entrained directly by light stimuli through the use of visual and non-visual peripheral opsins. The light signal starts transcription of light-sensitive genes, such as stress responses and DNA repair, as well as the clock genes per2 and cry1a, which sets the circadian clock. The peripherally entrained clock in turn regulates a plethora of downstream cellular processes [14].

Figure 2

A selection of rhythmic clock-target genes regulated in the early stages of zebrafish larval development. A Nanostring-based gene expression analysis of a wide selection of genes examined between 72–168 h post-fertilization. The selected data shown in panels a, b and c reveals a wide range of genes that show robust oscillations during embryo development, when larvae are raised on a light-dark cycle. Constant light (in red) stops the circadian pacemaker in the embryo, as well as the rhythmic expression in downstream, clock-regulated genes. (a) shows that numerous cell cycle regulators have robust transcriptional daily rhythms. (b) shows changes in three genes involved in neuro-development and differentiation, with neuroD showing very high amplitude rhtyhms. (c) shows rhythms in three genes involved in cell fate decisions in the intestine. (d) shows how neuroD only begins to show robust oscillations from day 4–5 of development onwards. (Taken from Laranjeiro and Whitmore, 2014) [26].

Figure 3

This figure shows specific circadian data collected from a zebrafish melanoma model. (a) Transgenic fish were generated, which develop a melanoma in which cells are both GFP-tagged and contain a per3-luciferase reporter gene. The lower panel shows a cross-section through a fish revealing the extend of the GFP-positive tumour. (b) Luminenescent gene expression recordings from both a developing melanoma and neighbouring skin reveal that circadian gene expression rhythms are significantly lower amplitude in the tumour and damp faster than in healthy skin. (c) Two strongly light responsive genes, Per2 and 6-4 photolyase, so no transcriptional light induction or rhythmicity in the developing tumour compared to adjacent, healthy skin. (Taken from Hamilton, Diaz-de-Cerio and Whitmore, 2015) [77].