|Trafficking of FMRP and Associated mRNAs in Response to Activation of Metabotropic Glutamate Receptors
by Laura Antar, 4/30/2004
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 have shown 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.