Washington | Scientists have identified a gene that when over-activated may cause severe insomnia and could be a potential target for new therapies to treat sleep disorders. Researchers from California Institute of Technology (Caltech) in US performed the first large-scale screening in a vertebrate animal for genes that regulate sleep.
Genetic screens are a powerful method that can help identify the genetic basis of behaviours. They involve mutating the DNA of thousands of animals, raising them, identifying any resulting physical or behavioural differences, and determining which altered gene produced each mutation.
Recently, zebrafish have emerged as a valuable vertebrate model system for studying sleep. Like humans, zebrafish sleep for consolidated periods of time at night. Just after fertilisation, when the zebrafish embryos were still single cells, the researchers injected them with a DNA molecule, called a plasmid, carrying a gene that was inserted into the genome of some of the cells in each fish.
In particular, they wanted to test genes that are predicted to encode for secreted proteins – those, like neuropeptides, that cells make and then release. Many of the genes that have been identified as being involved in sleep encode neuropeptides. Using a genetic switch called heat-shock promoter the researchers were able to control when the fish expressed each inserted gene.
They kept the switch on long enough for the fish to over-express each gene, making many copies of the products. Then they used computerised video trackers to monitor the fish for several days to see which genes affected sleep. Next, the researchers made transgenic zebrafish for each of the genes that had demonstrated strong effects on sleep in the genetic screen.
The most significant change resulted from over-expression of neuromedin U (Nmu), a gene that is also found in mammals and is expressed in a part of the brain called the hypothalamus. After heat shock, the fish that over-express Nmu are much more active both during the day and at night, said David Prober, assistant professor of biology at Caltech. The fish almost don’t sleep at all the night following the heat shock, so they display a very profound form of insomnia, Prober said.
When the researchers mutated the zebrafish so that they did not have Nmu, the larvae were less active during the day. Adult fish without the gene were particularly sluggish first thing in the morning. Like humans, zebrafish normally start to wake up at the end of the night and then become much more active when the lights turn on. The fish without Nmu are defective in this anticipation of dawn, said Prober.
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