Research led by UCL (University College London), Yale and
University of California, San Francisco has shown that the hormone
estrogen alleviates the sleep disruption experienced by zebrafish
genetically designed to help understand the biology of autism
spectrum disorder (ASD).
The scientists set out to investigate the function of genes
linked to autism and seizures in humans by using zebrafish as a
model system. They unexpectedly found that estrogens have a
selective effect in calming hyperactive fish during the night which
will help scientists to understand the brain pathways affected in
ASD. The finding is also intriguing given ASD is four times more
common in men than women.
Dr Jason Rihel of UCL said: "We're surprised to see that
estrogens in particular have such a selective effect in correcting
hyperactive behaviour in our ASD fish model, and we're
investigating this further to understand the mechanisms of this
response. We don't know if the hormone is targeting single or
multiple pathways, as estrogen is involved in lots of processes,
but we're keen to find out."
The team discovered that a plant-derived estrogen, called
biochanin A, and the human sex hormone, ?-estradiol, were most
effective at selectively stopping night time hyperactivity without
affecting the daytime activity of the fish. The FDA approved drug
risperidone, which was the first approved to treat irritability and
aggressive behaviour in ASD patients, stopped hyperactivity but
made fish less active during the day, suggesting estrogens are
better at selectively targeting pathways that are relevant to
ASD.
Dr Ellen Hoffman of Yale University said: "This research helps
scientists to understand the function of an autism risk gene in the
developing brain, which is important for understanding the biology
of autism. The mechanism of action of the estrogens on zebrafish
behaviour remains unknown and there is considerably more work to do
before these findings can be applied to humans."
The study used a simple animal model with genetic mutations to
understand ASD at different levels, from cellular processes and
brain circuitry, through to behaviour. Zebrafish were chosen as
they have complex behaviours like sleep at a young age and are
transparent, allowing brain development and activity to be
visualised with markers. They are also amenable to drug screening,
as compounds added to the water will enter the larval zebrafish
brain.
Normal zebrafish were studied alongside those predisposed to
seizures and sleep disruption due a mutation in the CNTNAP2 (often
pronounced 'catnap') gene, which has been associated with human
ASD. To identify specific processes involved in ASD-linked
hyperactivity, the behavioural patterns of the mutant fish were
compared to normal zebrafish exposed to 550 psychoactive compounds.
The compounds predicted to trigger or suppress the abnormal
behaviour became candidates for follow-up experiments.
Dr Rihel added: "We used the drug screen as a tool to help
identify the core mechanisms involved in ASD rather than to find
new therapies. For example, drugs predicted to cause sleeplessness
in the normal fish population might indicate the same pathways are
affected in the mutant population. Conversely, those drugs that
selectively promote sleep may hint at ways to rescue the mutant
population from hyperactivity."
Four of the top 10 compounds the screen predicted to supress
sleeplessness were known to have estrogenic activity, suggesting
estrogens as a whole class of molecules may have a positive effect
on the mutant fish population. This was tested on both normal and
mutant fish siblings using different doses of biochanin A and
?-estradiol, which showed that both compounds selectively suppress
the mutant night time hyperactivity.
The team plan to use this information to test the effect of
estrogens in fish with other genetic mutations linked to ASD before
progressing to mammalian models. These further studies will help
map the pathways involved in ASD and understand the mechanism of
action by which estrogens can specifically alter behaviour.
The paper, Estrogens Suppress a Behavioral Phenotype in Zebrafish Mutants of
the Autism Risk Gene, CNTNAP2, is published inNeuron.