Grant reports

2008

The role of FMRP in dopaminergic modulation of synaptic plasticity in the prefrontal cortex

Fragile X syndrome is one of the most common inherited causes of mental impairment. It originates from the loss of FMRP due to the mutation of the FMR1 gene. The neurotransmitter Dopamine, in the prefrontal cortex (PFC) of the brain plays an important role in its cognitive functions, including working memory, reward response, and attention. Previous studies suggest that the dysfunction of dopamine might be involved in cognitive impairments in Fragile X syndrome. We hypothesized that FMRP might be involved in dopamine regulation of synaptic plasticity in the PFC.

In this project, we will systematically study the role of FMRP in dopamine regulation of excitatory transmission and synaptic plasticity (including LTP and LTD) in the PFC of wild-type and Fmr1 KO(Fragile X) mice. We will use electrophysiological and biochemical methods to determine the effect of dopamine receptor activation on synaptic glutamate receptors and the role of FMRP in dopamine regulation of synaptic plasticity in the PFC. We will further carry out all the necessary molecular biological experiments to investigate how FMRP is involved in the dopamine receptor signaling pathway. To demonstrate the interaction between FMRP and the dopamine neurotransmitter system, we will investigate the role of dopamine-FMRP in different forms of fear memory, including trace fear memory, which is related to the function of the PFC.

This project will provide the direct evidence that FMRP might be involved in the dopamine regulation of synaptic plasticity in the PFC. The proposed studies will help us to further elucidate the cellular mechanisms for impaired learning and memory in Fragile X syndrome. We hope that new insights into potential therapeutic targets for Fragile X syndrome will be obtained in the project.

Testing Metabotropic Glutamate Receptor and GABA Ligands in Fragile X Syndrome Mice

Fragile X syndrome (FXS) is a genetic disorder caused, in most cases, by an inherited mutation of the FMR1 gene, which in turn prevents the production of its correspon protein. The FMR protein encoded by this gene is normally operative in neuronal branches (dendrites) where it controls local protein synthesis. The FMRP is involved in bin and transporting genetic messages to the synapse where they are held in an inactive state until needed. The loss of this protein in FXS causes many defects including seizur about 20% of FXS patients and abnormal dendritic spine formation and the inhibition of synaptic growth, and interference with learning and memory. It is believed that the de in neuronal shape and structure may underlie many of the symptoms seen in the animal models and in patients with Fragile X.

This lab will test the hypothesis that drugs that stimulate the mGluR receptors of neurons (called group I mGluR agonists) will worsen the pathological features and se susceptibility of the disorder while drugs that suppress mGluR receptors of neurons (called Group I mGluRs antagonists) will make them better. It will also test compound bind to another type of neuronal receptor believed to be affected in Fragile X Syndrome, known as the GABA receptor. The primary emphasis will be on testing various drug compounds for use as potential therapeutic treatments for Fragile X.

Role of PTEN in Fragile X syndrome

Fragile X syndrome (FXS), a leading cause of mental impairment, results from a CGG expansion in the 5′ untranslated region of the FMR 1 gene and the resulting decreased production of its associated protein (fragile X mental retardation protein, FMRP). This causes the structural abnormalities and immature shapes in the dendritic spines of the neurons of people with Fragile X. It is now believed that these changes underly the cognitive, behavioural and autistic features seen in this disorder.

Normal brain cells contain phosphatase (an enzyme) called PTEN. Interestingly, PTEN mutations have been reported in individuals with autism spectrum disorders. Recent evidence also indicates that PTEN inactivation causes increased growth of the main branch (the axon) of each neuron, and an increase in the number of dendritic spines on each neuron. Given that the deficiency of both PTEN and FMRP is associated with autism and leads to similar abnormalities of dendritic spines, we have examined the effect of FMRP on the production of protein PTEN in cultured neurons of mouse brain (cortex and hippocampus). We found that the production of PTEN was significantly increased in neurons transfected (injected) with wild-type FMRP. This result provides novel evidence suggesting that FMRP may regulate PTEN production in neuronal cells and that dysfunction of this regulation may contribute to the molecular abnormailities of FXS. Accordingly, in this project we propose to test our working hypothesis that the phosphatase PTEN and its interacting signals may be involved in FMRP deficiency-induced abnormality of dendritic spines. The evidence obtained from this study would provide an important basis for the development of a therapeutic strategy to treat Fragile X patients.

The role of interactive protein relations and synaptic balance in Fragile X Syndrome

The Fragile X Syndrome (FXS) is the most common inherited form of mental impairment in all races and ethnic groups. Affected individuals display a variety of intellectual deficits from learning problems to autism. FXS is caused by a loss of the FX protein (FMRP), that functions in local protein synthesis. A key advance in FXS research was the generation of a mouse model and an exciting discovery is that synaptic contacts of adult FMRP knockout (KO) mice display characteristics of an early development, indicating a deficit in synaptic maturation. Accordingly, the activity induced increase in PSD95 –a locally synthesized scaffolding protein important for synapse maturation-is found to require FMRP, suggesting that deficits in local synthesis of PSD95 may lead to abnormal synapse development.

Our work indicates the relationship of scaffolding and adhesion proteins (e.g.NLGs) to regulate synapse development and specificity. Hence we hypothesize that altered PSD95 production leads to an altered balance of excitatory and inhibitory synapses (E/I ratio) and that re-establishment of this relationship will be important to rescue synaptic balance. Thus, a comprehensive analysis will be performed on KO mice to determine alterations in the production or distribution of synaptic proteins. We will also quantify the E/I ratio and test if production of PSD95 or other synaptic proteins in cultured neurons will rescue the E/I balance.

The proposed studies will test a novel mechanism by which appropriate amounts of molecules are critical for synapse development and control of the E/I ratio. Important insights into synapse development and the relevance of the E/I ratio in FXS will be obtained. Furthermore, fundamental models of synapse development will be tested in Fragile X mice to find novel strategies in the therapeutic treatment of FXS patients.

Targeting glutamate receptors to stop the onset of Fragile-X syndrome

Fragile-X syndrome (FXS) is the most common form of inherited mental retardation with impairments ranging from learning problems to severe intellectual disabilities. Although it is established that the Fmr1 gene is silenced in FXS, it is still unclear how this accounts for the onset of symptoms. It has been proposed that FXS in children and adults occurs because their brains exhibit an exaggerated form of signaling between nerve cells. This signaling involves the removal of ionotropic glutamate receptors (iGluRs) from the surface of nerve cells in a process called long-term depression (LTD). LTD is thought to be important for many brain functions including our ability to learn. The effects of an exaggerated LTD on the structure of the neurons could therefore explain many of the clinical features of FXS. While this finding is helping the development of new types of therapy, it is possible that other iGluR signaling pathways are also disrupted.

The Bowie lab has recently identified a novel type of iGluR signaling that is distinct from LTD. Importantly, it is found in the newborn brain exactly at a time point when FXS symptoms first appear. The Bowie lab will test to see if sufferers of FXS lack this important signaling pathway. Their ongoing studies have already worked out some of the key molecules that contribute to this pathway. Consequently, the aim of the Bowie lab is to use this information to develop novel medications to counteract the onset of FXS in young children.