Scientists in Professor Mark Hankins’ lab were curious to find out whether there could be a range of proteins that detect light, given the diversity of systems that are known to be regulated by light in vertebrates (such as circadian rhythms, DNA repair and even metabolism and gene expression). They discovered that zebrafish have more than 40 light-sensing proteins (opsins). This is a much larger repertoire than previously found in any other vertebrate.
The research, published in Genome Research and funded by the Biotechnology and Biological Sciences Research Council and the Australian Research Council, found opsins in the eye and brain, but also in organs not commonly thought to be directly affected by light, such as the heart, fins, gills, gonads, liver and gut. The team also found four completely new opsin families in zebrafish, some of which are also present amphibians, reptiles, birds and some mammals such as the platypus.
Wayne Davies, a former post-doc in Professor Hankins’ lab and now Associate Professor at the University of Western Australia, said that these surprising findings in the zebrafish could now be used to improve understanding of light-detection systems in other vertebrates.
“Light detection that aids vision in vertebrates is well understood, but the impact of light detection on other bodily functions has remained more of a mystery,” he said.
We knew that the zebrafish had an extensive compliment of visual opsin proteins; the new data reveal a remarkable array of 32 non-visual opsins that we have mapped to the zebrafish anatomy and that underpin an array of light-sensitive processes that extend throughout the body.
- Professor Mark Hankins from the Nuffield Laboratory of Opthalmology
Specialised receptors in vertebrates’ eyes detect light to assist with colour vision and the formation of images. But non-visual systems use light detection in other ways, such as to synchronise biological rhythms for sleep and seasonal breeding. Mammals use a single type of receptor in the eye which detects light and relays the information to a central ‘master’ clock in the brain, which then regulates bodily functions. But in bony fish, cellular clocks are controlled by light directly, bypassing the cranial master control centre. From the evolutionary perspective it appears there has been a reduction and centralisation of light receptive mechanisms.
In humans, disruption of photosensory systems during activities such as shift-work and long-distance travel can lead to sleep dysfunction and jet-lag, as well as more serious physical and mental health issues.
The discoveries in zebrafish mean that researchers can now begin to work out how light can control normal physiology at the cell level, and use this knowledge to assist with future research into diseases found in humans and other animals.