FRAXA Research Foundation and the Fragile X Research Foundation of Canada have awarded a grant of $100,000 over two years to Dr. Raymond Turner at the University of Calgary in Alberta, Canada. Dr. Turner and postdoctoral fellow Xiaoqin Zhan, PhD are attempting to reactivate a segment of FMRP to reverse symptoms of fragile X in a mouse model of the disorder.
Fragile X syndrome arises through a loss of a single protein, FMRP, and is the greatest inherited cause of autism spectrum disorder worldwide. Yet it has no cure. FMRP affects the expression of thousands of other proteins during development of brain circuits, and these proteins are key to normal function at synapses (junctions between nerve cells). One consequence of losing FMRP is that synapses show less plasticity, or ability to fine-tune their responses based on prior experience.
This team is working to find an effective and safe way to introduce FMRP back into brain neurons. Their clever approach is to attach a “tat” peptide to a segment of the FMRP molecule, to facilitate its journey from the blood stream into brain neurons. In fact, they have been able to inject the tat-FMRP segment into the tail vein of mice and see it function in brain cells less than one hour later.
Tat and Truncated FMRP
TAT, which stands for “Trans-Activator of Transcription” is a tiny string of amino acids which can be attached to a protein, like FMRP. It is useful for protein replacement because it can serve as a taxicab, transporting a protein into the brain, past the infamous blood-brain-barrier. In this case, it can be used to deliver a segment of FMRP to brain cells.
In this project, Drs. Turner and Zhan will attach TAT to a segment of FMRP rather than the full protein. Why just a segment of FMRP? The fragile X protein is very large – too large to easily transport, even with tat. In addition, starting with a simplified version of FMRP avoids some dosage issues that people have seen in previous studies of protein replacement.
A Deeper Dive into the Project
“We use the mouse cerebellar mossy fiber-granule cell pathway to explore the basis for synaptic abnormalities in fragile X,” notes Dr. Turner. “Our preliminary data reveal that FMRP binds to a complex between voltage-gated calcium and potassium channels (Cav3-Kv4) that potentiates the input by increasing granule cell excitability. We find that FMRP knockout mice completely lack mossy fiber evoked potentiation of granule cell excitability. Importantly, intracellular infusion of a short active fragment FMRP (amino acids 1-298) into granule cells of FMRP knockout mice in vitro restores Cav3-Kv4 function and potentiation. In addition, a tat-FMRP(298) construct injected into the bloodstream of mice rapidly crosses the blood-brain barrier to enter neurons throughout the brain. Preliminary behavioral tests conducted with collaborator Dr. Ning Cheng further indicate a reduction in hyperactivity of FMRP knockout mice within hours of injecting tat-FMRP(298).
“These results are exciting in providing a glimpse into the potential to reintroduce FMRP as a potential therapeutic strategy to reduce abnormal behaviors in fragile X syndrome. This project will explore the mechanism by which FMRP(298) restores synaptic plasticity, and the parameters that define the best set of conditions to promote positive behavioral effects through tat-FMRP(298) injection.“