Protein modification offers potential target for Friedreich’s ataxia
Mouse study ties flaw in brain receptor process to early FA
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Failure in a protein modification process needed to maintain the function of a brain receptor is a key early mechanism leading to Friedreich’s ataxia, a mouse study showed.
The process, known as palmitoylation, links the loss of frataxin, the protein missing in people with FA, to nerve cell vulnerability before symptoms appear.
Partially restoring frataxin helped increase palmitoylation, suggesting that modulating this protein modification pathway may represent a novel therapeutic target in FA.
The study, “Impaired Glur2 palmitoylation in cerebellar Purkinje cells of a Friedreich ataxia mouse model,” was published in Experimental Neurology.
In FA, a deficiency in frataxin affects the cerebellum, a major structure at the back of the brain that controls balance and coordination. A region within the cerebellum that’s severely damaged in FA is the dentate nucleus, which acts as the main relay station linking the cerebellum to the rest of the brain. The activity of nerve cells in the dentate nucleus is controlled by Purkinje cells, large nerve cells found in the outer layer of the cerebellum.
Experiments yield clues
Studies show that Purkinje cell structure is also disrupted in FA, with cell loss occurring at later stages of disease. The mechanisms linking frataxin deficiency to this cellular dysfunction have not been fully elucidated.
Purkinje cell function is mediated by the AMPA receptor, a protein that’s activated by the chemical messenger glutamate. The receptor is composed of four protein subunits (GluR1 through GluR4), with GluR2 being particularly important for protein function.
Researchers at the University of Pennsylvania and Children’s Hospital of Philadelphia previously discovered that GluR2 protein levels are reduced in a frataxin-deficient mouse model.
To understand why this reduction occurs, the team examined cerebellar tissue from mice with 80% less frataxin than normal mice.
Initial experiments ruled out certain factors that could explain low GluR2 levels, including reduced gene activity for GluR2, fewer Purkinje cells, or reduced GluR2 phosphorylation (a protein modification that regulates receptor trafficking).
Instead, experiments revealed reduced GluR2 palmitoylation in the cerebellums of FA mice. Palmitoylation is another type of modification in which a fatty (lipid) molecule called palmitate is attached to the protein. This helps stabilize AMPA receptors, and without sufficient palmitoylation, mature GluR2 levels drop because unattached receptors are degraded.
While palmitoylated GluR2 was directly observed in Purkinje cell bodies in healthy tissue by a visualization technique, its abundance was markedly reduced in FA Purkinje cells.
To identify a mechanism, the researchers focused on DHHC3, an enzyme that palmitoylates GluR2. Here, DHHC3 protein levels were reduced in the FA cerebellum, specifically within the Purkinje cell layer. In separate cell-based experiments, the team confirmed a direct relationship between GluR2 and DHHC3.
DHHC3’s own palmitoylation, called autopalmitoylation, was also reduced in FA mice compared with controls. Levels of two enzymes that remove palmitate from GluR2 did not differ between groups, indicating that the palmitoylation deficit was more likely due to reduced DHHC3 activity rather than increased palmitate removal.
When the researchers restored frataxin production in FA mice, GluR2 and DHHC3 protein levels increased. And palmitoylated GluR2 in the frataxin-restored group increased by about 50% relative to the original FA mice.
To investigate functional changes in the FA mice, the team recorded AMPA receptor-mediated electrical signals in Purkinje cells in response to stimulation. Those currents dropped after four weeks of reduced frataxin compared with controls, a difference that widened further by eight weeks.
On the rotarod test to assess motor coordination, the total time that mice walked on the rod before falling off did not differ significantly between FA mice and controls at four weeks. While the difference increased by eight weeks, it didn’t reach statistical significance, meaning it could have occurred by chance.
The authors noted that lower GluR2 palmitoylation was detectable at two weeks of frataxin reduction, before Purkinje cell loss and before significant changes in motor behavior or electrophysiology.
“These results implicate disrupted lipid-dependent palmitoylation as a key mechanism linking frataxin loss to [nerve cell] vulnerability and suggest that modulation of palmitoylation pathways may represent a novel avenue for therapeutic intervention in Friedreich ataxia,” the researchers wrote.
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