Gary Bassell, PhD, Principal Investigator
Ravi Muddashetty, PhD, FRAXA Fellow
Emory University

FRAXA Awards:
  $40,000 in 2008 
  $40,000 in 2007 
  

Previous awards at Albert Einstein College of Medicine:
  $15,000 in 2004
  $35,000 in 2003
  $65,000 in 2002
  $30,000 each in 2001 & 2000

Nerve cells have two types of long processes called axons and dendrites. During brain development, axons and dendrites extend outward to form connections (synapses) with other nerve cells. Dr. Bassell's team has developed powerful molecular genetic techniques to visualize the movements of mRNAs and proteins. They can track FMR1 mRNAs and FMRP particles as they move through these processes in tissue from Fragile X knockout mice and normal controls. Their work indicates an important role for FMRP in development of neurons.

by Gary Bassell and Ravi Muddashetty, 5/2007

Fragile X syndrome is caused by the inherited loss of the Fragile X Mental Retardation protein (FMRP), an mRNA binding protein that has been hypothesized to regulate the local synthesis of specific proteins at synapses in response to activation of glutamate receptors. In the absence of FMRP, there is excess protein synthesis.

A major goal has been to identify which proteins are defective in their local production at synapses in FXS. The inability to regulate local protein synthesis at synapses in FXS is believed to contribute to the impairments in cognitive function in FXS.

In a recent study from Gary Bassell's laboratory, Dr. Muddashetty and colleagues at Emory University have discovered FMRP targets at synapses: postsynaptic density-95 (PSD-95) mRNA, AMPA receptor subunits, GluR1 and GluR1 mRNAs, and -CaMKII. The synaptic protein synthesis for each of these targets was dysregulated in response to activation of group 1 metabotropic glutamate receptors (mGluR) in a mouse model of Fragile X syndrome (Muddashetty et al., J. Neuroscience 2007). Excitingly, each of these targets are known to have critical roles in synaptic plasticity, learning and memory. This study has advances our understanding on how brain synapses in FXS are altered and opens up potential new targets for clinical intervention.

With the support of a FRAXA fellowship, Dr. Muddashetty will investigate the molecular mechanism whereby FMRP may directly regulate the translation of these mRNAs at synapses in response to activation of mGluR and/or other receptors. The experimental approach will involve use of both biochemical and microscopic imaging methods applied to cortical and hippocampal neurons and synaptoneurosomes from wild type and Fmr1 KO mice. Local translation of these mRNAs in dendrites of live neurons will be studied using FRAP following expression of EGFP reporters fused their untranslated regions (UTR). Experiments will also be done to assess the ability of the mGluR5 antagonist, MPEP, to correct alterations in dendritic mRNA translation in cultured neurons from the Fmr1 KO.

An understanding of how the local synthesis of specific proteins in dendrites may be dysregulated in the Fmr1 knockout mouse model for FXS is essential to understand the function of FMRP at synapses. We believe this research will provide new insight into how FMRP is involved in the glutamatergic regulation of dendritic and synaptic protein synthesis. These studies will also provide quantifiable cell-based assays for screening of potential therapeutic compounds and their derivatives to intervene at the underlying core defects in Fragile X Syndrome.

Gary Bassell, PhD, Principal Investigator
Laura Antar, FRAXA Fellow
Jason Dictenberg, PhD, FRAXA Fellow
Albert Einstein College of Medicine

by Laura Antar, 4/2004

A major challenge for Fragile X research is trying to understand the normal function of FMRP in the brain. With this knowledge, one can design pharmacologic treatments that might compensate for the loss of FMRP.

We know that FMRP is an mRNA binding protein, which means that it can transport mRNAs to specific sites in the cell and then regulate the translation of mRNAs at those sites into proteins. We show that FMRP is found in the majority of synapses in the brain. Synapses are the sites where two neurons communicate via chemical and electrical signals. At excitatory synapses, one neuron in the brain releases a chemical message called glutamate, onto a second neuron. The second neuron receives this message through its receptors. Excitatory synapses are critically involved in brain development, learning and memory. It is known that these synapses have both structural and functional defects in Fragile X.

The Bassell laboratory has developed microscopic imaging tools to visualize FMRP in live neurons, and we have been able to track the movements of FMRP along dendrites to synapses. This was done first by using recombinant DNA technology to fuse FMRP to a protein called Green Fluorescent Protein (GFP). The fluorescent FMRP protein was then introduced into cultured neurons that taken from embryonic rat brain. Cultured neurons provide a powerful tool to enable visualization of FMRP behavior in response to synapse activation.

Activation of a specific receptor type, metabotropic glutamate receptor (mGluR5), can stimulate the movement of both FMRP and Fmr1 mRNA (an mRNA that FMRP is known to bind) to dendrites. We showed that at synapses, FMRP movement is regulated, but the movement of Fmr1 mRNA is not. This suggests that FMRP may stop regulating the expression of specific mRNAs, such as Fmr1 mRNA, in response to excitatory synaptic activity. It may do this by releasing the mRNA from its grasp. This indicates that different proteins may be expressed in synapses in Fragile X patients (who lack FMRP) as compared with unaffected people, who have the protein. Such a difference in synaptic protein expression could affect cell structure and learning and memory.

This study was published in the Journal of Neuroscience (Antar et al., 2004; 24:2648). Our work has important implications toward understanding how regulation of mRNA localization and translation of mRNAs into proteins may be altered in Fragile X syndrome. Previous research has shown that mGluRs are important for a form of synaptic memory (called mGluR-dependent long term depression (LTD)) which is known to require protein synthesis, yet is abnormally enhanced in mice that do not produce FMRP. Our study showed that a pharmacological agent, MPEP, that reduces the ability of the receptor to receive its chemical signal, is able to change both the dendritic and synaptic movement of FMRP. Our lab, and others, believes that there are important clinical implications in the design of drugs, such as MPEP, that may modulate specific signaling pathways that are imbalanced in Fragile X patients.

more FRAXA research reports