| Analysis of the impact of cAMP misregulation and muscarinic acetylcholine receptor signaling in a Drosophila model for Fragile X
by Anita Pepper and Tom Jongens , 5/1/2008
In
the last several years, we have developed a Drosophila Fragile X
model, based on loss of function mutations in the Drosophila homologue
of FMR1, called dfmr1. Using this
model, we and others have identified several phenotypes that
bear similarity to symptoms displayed by fragile X patients.
These include neuro-anatomical defects, circadian defects,
reduced social interaction, and cognitive deficits. The
overall goal of our laboratory is to use this model to
understand how the loss of dfmr1 gene function
leads to the phenotypes observed in the fly model and to
identify approaches to ameliorate them. This, we hope, will
provide a step forward in finding therapeutic approaches to
treat fragile X patients.
Recently there has
been a significant breakthrough in identifying the cause of at least
some of the phenotypes displayed by our fly model as well as
by a mouse fragile X model. Collectively,
several observations from investigators in the fragile X field indicate
that loss of FMR1
activity leads to enhanced metabotropic glutamate
receptor (mGluR) signaling. This finding has lead to the proposal by Mark
Bear, Kim Huber and Steve Warren that many of the symptoms of
this disease are due to enhanced signaling from this
receptor ("the mGluR Theory"). We, in collaboration with
Sean McBride, were able to provide significant support for
this model by showing that treatment of our fly model,
with drugs that reduce mGluR signaling, could rescue
some neuronal-anatomical defects and the social interaction and memory deficits. A
significant amount of additional support for this model has
come from several other groups that have found that additional
phenotypes displayed by the fly model, as well as phenotypes
displayed by the mouse model can be rescued by similar drug
treatment, or by genetic reduction of this receptor. These
results are very exciting in that they point to a potential
treatment for Fragile X syndrome.
One caveat with this treatment
approach is the lack of currently available FDA approved drugs
to use for treatment. Therefore it is important to explore
additional routes to decrease the signaling pathways activated
by the metabotropic glutamate receptor. Furthermore it
is important to remember that not all phenotypes displayed
by Fragile X models are rescued by reducing mGluR signaling,
thus other pathways for therapeutic intervention should be explored.
With funds provided by
FRAXA, we will explore two new approaches to treat
phenotypes displayed by the fly fragile X model. One pathway we
are focusing on is cAMP regulation, as the pathway that
produces this important small signaling molecule has been shown to
be down-regulated in fragile X patients and the fly and mouse
models of fragile X. We are therefore exploring the effects of
increasing cAMP production on fly model phenotypes. Also,
following on the finding of Kim Huber's lab that have shown
that signaling by another neuronal receptor, the muscarinic
acetylcholine receptor, is upregulated in absence of FMR1 activity, we will explore the
effects of down-regulating the activity of this receptor on
the phenotypes displayed by our fly model. This work is being
done in collaboration with Sean McBride.
Pharmacological Rescue of the
Drosophila Fragile X Model
3/2/2005: Potential Treatment for Fragile X
Demonstrated in Fruit Flies
(Press Release from University of Pennsylvania School of Medicine)
Fragile X
Syndrome is one of the most commonly inherited forms of mental
retardation, with an incidence of 1 in 4,000 males and 1 in
8,000 females. Not many medications exist to help Fragile X
patients. Now, in a fruit fly model of the disease,
researchers from the University of Pennsylvania School of
Medicine and their colleagues have shown that it is possible
to reverse some of the symptoms of the disorder using drugs
that dampen specific neuronal overactivity. Their findings
appear in the March 3, 2005 issue of Neuron.
Senior author Thomas A.
Jongens, PhD, Associate Professor of Genetics at Penn, and
colleagues from Albert Einstein College of Medicine and Drexel
University College of Medicine, as well as other labs, have
developed and characterized a Drosophila fly model for Fragile
X. This model is based on mutants that lack the dfmr1 gene,
which encodes a protein similar to human FMR1 protein.
"Interestingly, work by my lab and others have found that the
dfmr1 mutants display many physical and behavioral
characteristics similar to symptoms displayed by Fragile X
patients," says Jongens. These include structural defects in
certain neurons, enlarged testes, failure to maintain proper
day/night activity patterns; attention deficits and
hyperactivity, and defects in behavior-dependent learning and
memory.
"Our thinking was that
since so many of the behavioral and physical phenotypes in the
fly model were similar to symptoms observed in fragile X
patients and a mouse fragile X model, FMR1 and dfmr1 must be
regulating similar biological processes in human, mice, and
flies," states Jongens.
A mouse model of Fragile
X also shows symptoms similar to those of Fragile X patients.
Studies outside of Penn using the mouse model have
demonstrated that Fragile X patients have a tendency to break
down synaptic connections (sites used for neuron to neuron
communication) more readily than the general population. This
breakdown is due to an increased activity in the metabotropic
glutamate receptor (mGluR), which is located on the surface of
neurons, including in the hippocampus -- the memory and
learning center in the brain. In turn, this increased activity
compromises neurotransmission for memory-associated functions.
Jongens and colleagues
surmised that mGluR overactivity may be at the root of many of
the Fragile X symptoms. Using such drugs as lithium chloride
and others, known as antagonists, that block mGluR's activity,
the team tested to see if the drugs could rescue any of the
observed behavioral and memory defects observed in the fly
model.
"What we found was very
striking," says Jongens. They found that the drug treatments
restored memory-dependent courtship behavior in mutant flies
and reversed some of the neuronal structural defects. The
group used lithium because it is known to have activities
analogous to blocking mGluR-receptor activity, and it is
already an FDA-approved drug used to treat other ailments in
humans such as bipolar disorder.
"The big take-home
message from our work is that maintaining proper regulation of
mGluR signaling is a conserved function of the dFMR1 and FMRP
proteins and that loss of dfmr1 function in flies leads to at
least a subset of the cognitive and behavioral defects
observed in the fly model," says Jongens. "These results
provide a potential route by which symptoms of Fragile X
patients may be ameliorated."
First authors on the
paper are Sean M.J. McBride, Albert Einstein College of
Medicine, and Catherine H. Choi, Drexel University College of
Medicine. This work was funded by the National Institutes of
Health and the FRAXA Research Foundation.
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