R-Baclofen Reverses a Social Behavior Deficit and Elevated Protein Synthesis in a Mouse Model of Fragile X Syndrome

BACKGROUND

Fragile X syndrome (FXS) is the most common known inherited form of intellectual disability and the single genomic cause of autism spectrum disorders. It is caused by the absence of a fragile X mental retardation gene (Fmr1) product, FMRP, an RNA-binding translation suppressor. Elevated rates of protein synthesis in the brain and an imbalance between synaptic signaling via glutamate and γ-aminobutyric acid (GABA) are both considered important in the pathogenesis of FXS. In a mouse model of FXS (Fmr1 knockout [KO]), treatment with R-baclofen reversed some behavioral and biochemical phenotypes. A remaining crucial question is whether R-baclofen is also able to reverse increased brain protein synthesis rates.

METHODS

To answer this question, we measured regional rates of cerebral protein synthesis in vivo with the L-[1-(14)C]leucine method in vehicle- and R-baclofen-treated wildtype and Fmr1 KO mice. We further probed signaling pathways involved in the regulation of protein synthesis.

RESULTS

Acute R-baclofen administration corrected elevated protein synthesis and reduced deficits on a test of social behavior in adult Fmr1 KO mice. It also suppressed activity of the mammalian target of rapamycin pathway, particularly in synaptosome-enriched fractions, but it had no effect on extracellular-regulated kinase 1/2 activity. Ninety min after R-baclofen treatment, we observed an increase in metabotropic glutamate receptor 5 expression in the frontal cortex, a finding that may shed light on the tolerance observed in human studies with this drug.

CONCLUSIONS

Our results suggest that treatment via activation of the GABA (GABA receptor subtype B) system warrants further study in patients with FXS.

Cerebral protein synthesis in a knockin mouse model of the fragile X premutation

The (CGG)n-repeat in the 5′-untranslated region of the fragile X mental retardation gene (FMR1) gene is polymorphic and may become unstable on transmission to the next generation. In fragile X syndrome, CGG repeat lengths exceed 200, resulting in silencing of FMR1 and absence of its protein product, fragile X mental retardation protein (FMRP). CGG repeat lengths between 55 and 200 occur in fragile X premutation (FXPM) carriers and have a high risk of expansion to a full mutation on maternal transmission. FXPM carriers have an increased risk for developing progressive neurodegenerative syndromes and neuropsychological symptoms. FMR1 mRNA levels are elevated in FXPM, and it is thought that clinical symptoms might be caused by a toxic gain of function due to elevated FMR1 mRNA. Paradoxically, FMRP levels decrease moderately with increasing CGG repeat length in FXPM. Lowered FMRP levels may also contribute to the appearance of clinical problems. We previously reported increases in regional rates of cerebral protein synthesis (rCPS) in the absence of FMRP in an Fmr1 knockout mouse model and in a FXPM knockin (KI) mouse model with 120 to 140 CGG repeats in which FMRP levels are profoundly reduced (80%–90%). To explore whether the concentration of FMRP contributes to the rCPS changes, we measured rCPS in another FXPM KI model with a similar CGG repeat length and a 50% reduction in FMRP. In all 24 brain regions examined, rCPS were unaffected. These results suggest that even with 50% reductions in FMRP, normal protein synthesis rates are maintained.

Voluntary exercise regionally augments rates of cerebral protein synthesis

Exercise is a natural form of neurophysiologic stimulation that has known benefits for mental health, maintenance of cerebral function, and stress reduction. Exercise is known to induce an upregulation of brain-derived neurotrophic factor and this is thought to be involved in associated increases in neural plasticity. Protein synthesis is also an essential component of adaptive plasticity. We hypothesized that exercise may stimulate changes in brain protein synthesis as part of its effects on plasticity. Here, we applied the quantitative autoradiographic L-[1-(14)C]leucine method to the in vivo determination of regional rates of cerebral protein synthesis (rCPS) in adult rats following a seven day period of voluntary wheel-running and their sedentary counterparts. In four of 21 brain regions examined, the mean values of rCPS in the exercised rats were statistically significantly higher than in sedentary controls; regions affected were paraventricular hypothalamic nucleus, ventral hippocampus as a whole, CA1 pyramidal cell layer in ventral hippocampus, and frontal cortex. Increases in rCPS approached statistical significance in dentate gyrus of the ventral hippocampus. Our results affirm the value of exercise in encouraging hippocampal and possibly cortical neuroplasticity, and also suggest that exercise may modulate stimulation of stress-response pathways. Ultimately, our study indicates that measurement of rCPS with PET might be used as a marker of brain response to exercise in human subjects.

Regional rates of cerebral protein synthesis measured with L-[1-11C]leucine and PET in conscious, young adult men: normal values, variability, and reproducibility

We report regional rates of cerebral protein synthesis (rCPS) measured with the fully quantitative L-[1-(11)C]leucine positron emission tomography (PET) method. The method accounts for the fraction (lambda) of unlabeled amino acids in the precursor pool for protein synthesis derived from arterial plasma; the remainder (1-lambda) comes from tissue proteolysis. We determined rCPS and lambda in 18 regions and whole brain in 10 healthy men (21 to 24 years). Subjects underwent two 90-min dynamic PET studies with arterial blood sampling at least 2 weeks apart. Rates of cerebral protein synthesis varied regionally and ranged from 0.97+/-0.70 to 2.25+/-0.20 nmol/g per min. Values of rCPS were in good agreement between the two PET studies. Mean differences in rCPS between studies ranged from 9% in cortical regions to 15% in white matter. The lambda value was comparatively more uniform across regions, ranging from 0.63+/-0.03 to 0.79+/-0.02. Mean differences in lambda between studies were 2% to 8%. Intersubject variability in rCPS was on average 6% in cortical areas, 9% in subcortical regions, and 12% in white matter; intersubject variability in lambda was 2% to 8%. Our data indicate that in human subjects low variance and highly reproducible measures of rCPS can be made with the L-[1-(11)C]leucine PET method.