Enhanced Asynchronous Ca2+ Oscillations Associated with Impaired Glutamate Transport in Cortical Astrocytes Expressing Fmr1 Gene Premutation Expansion

Premutation CGG repeat expansions (55-200 CGG repeats; preCGG) within the fragile X mental retardation 1 (FMR1) gene can cause fragile X-associated tremor/ataxia syndrome. Defects in early neuronal migration and morphology, electrophysiological activity, and mitochondria trafficking have been described in a premutation mouse model, but whether preCGG mutations also affect astrocyte function remains unknown. PreCGG cortical astrocytes (similar to 170 CGG repeats) displayed 3-fold higher Fmr1 mRNA and 30% lower FMR1 protein (FMRP) when compared with WT. PreCGG astrocytes showed modest reductions in expression of glutamate (Glu) transporters GLT-1 and GLAST and attenuated Glu uptake (p<0.01). Consistent with astrocyte cultures in vitro, aged preCGG mice cerebral cortex also displayed reduced GLAST and GLT-1 expression. Approximately 65% of the WT and preCGG cortical astrocytes displayed spontaneous asynchronous Ca2+ oscillations. PreCGG astrocytes exhibited nearly 50% higher frequency of asynchronous Ca2+ oscillations (p < 0.01) than WT, a difference mimicked by chronic exposure of WT astrocytes to L-trans-pyrrolidine-2,4-dicarboxylic acid (L-trans-PDC) or by partial suppression of GLAST using siRNA interference. Acute challenge with Glu augmented the frequency of Ca2+ oscillations in both genotypes. Additionally, 10 mu M Glu elicited a sustained intracellular Ca2+ rise in a higher portion of preCGG astrocytes when compared with WT. Pharmacological studies showed that mGluR5, but not NMDA receptor, contributed to Glu hypersensitivity in preCGG astrocytes. These functional defects in preCGG astrocytes, especially in Glu signaling, may contribute to fragile X-associated tremor/ataxia syndrome neuropathology.