Blocking an enzyme called GSK3 alpha seems to ease many symptoms of fragile X syndrome — including abnormal protein production, seizures induced by loud tones, learning and memory deficits, and excess nerve cell activity —without the known problems of similar approaches, research in mice suggests.
Fragile X syndrome results from mutations in the FMR1 gene, which provides instructions for making a protein known as FMRP. Patients experience symptoms such as intellectual disability, attention deficits, anxiety, and hyperactive behavior, as well as seizures caused by loud noises or bright lights.
Evidence has shown that the loss of FMRP in these patients causes a rise in protein synthesis at neuronal synapses — specialized junctions where nerve cells communicate — and that this increase is mediated by a receptor called metabotropic glutamate receptor 5 (mGluR5).
Compounds blocking mGluR5 have shown promise in animal models, easing most symptoms of fragile X. Yet none of the molecules that reached clinical trials have succeeded in patients. There was also evidence that these compounds caused hallucinations in some patients, and that they lose efficacy with long-term dosing.
An alternative therapeutic target is a protein called glycogen synthase kinase 3 (GSK3), as lithium (which reduces GSK3 activation) and specific inhibitors of this protein also lessened most symptoms of fragile X in animal models.
These are also not without complications: Lithium requires dosages associated with adverse events in children, and blockers cause a rise in a protein called beta-catenin that can result in cancer. However, studies have suggested that these adverse events could be avoided if only one form of GSK3 was targeted.
“GSK3 inhibitors had been tested in Fragile X models before, but it’s never gone anywhere because of the toxicity issue,” Florence Wagner, director of medicinal chemistry at the Broad Institute’s Stanley Center for Psychiatric Research and one of the senior authors of the study, said in a press release.
“Studies had been published showing that if you selectively knock out either alpha or beta [the two GSK3 forms], it wouldn’t trigger beta-catenin accumulation,” she added.
Researchers at the Massachusetts Institute of Technology recently developed selective blockers of either GSK3 alpha (BRD0705), GSK3 beta (BRD3731), or both forms of this protein (BRD0320) and have now studied these inhibitors in a mouse model of fragile X.
After demonstrating that the selective inhibitors against either GSK3 alpha or beta did not raise beta-catenin levels and reached the brain at high enough levels for a therapeutic effect, the researchers examined how they affected a number of symptoms and cellular features of fragile X in animals.
Results showed that the GSK3 alpha inhibitor significantly lowered protein synthesis at the synapse back to normal levels, prevented seizures in the presence of loud auditory stimulus, and decreased excess firing (activation) of nerve cells in the cerebral cortex (the outermost part of the brain). These benefits, however, were not seen with the GSK3 beta inhibitor, although it was binding to the target protein as expected.
The GSK3 alpha inhibitor also corrected memory to levels similar to healthy animals, but notably, it did not cause the excess locomotion seen in mice given the mGluR5 modulators. This excess locomotion was likely induced by the hallucinations also experienced by patients receiving these modulators, the team said.
These findings indicate that “inhibition of GSK3α could achieve the same therapeutic benefit as optimal inhibition of mGluR5 without dose limitations,” the researchers wrote. In addition, the GSK3 alpha inhibitor did not lose efficacy with repeated dosing, again contrasting with mGluR5 blockers.
The team also found that while inhibitors of GSK3 alpha and mGluR5 had similar benefits on protein production, the GSK3 alpha inhibitor exerted its effects through a distinct mechanism, likely explaining why this compound has fewer adverse effects.
“The results of our experiments indicated that selective inhibition of GSK3 alpha was sufficient to ameliorate several phenotypes in the [fragile X] mouse model,” the researchers wrote. “The extent of this phenotypic rescue was comparable to what has been observed by inhibiting mGluR5,” but inhibition of GSK3 alpha neither increased hallucinations and locomotion nor lost its effect with repeat dosing, they added.
While the findings are promising for people with fragile X, “the safety of chronic inhibition of GSK3 alpha will need to be fully evaluated in extensive preclinical studies before contemplating prolonged treatment in human patients,” the team concluded.
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