Astroglial FMRP-dependent translational down-regulation of mGluR5 underlies glutamate transporter GLT1 dysregulation in the fragile X mouse

Abstract

Fragile X syndrome (FXS) is a neurodevelopmental disorder caused by the loss-of-function of fragile X mental retardation protein (FMRP). The loss of FMRP function in neurons abolishes its suppression on mGluR1/5-dependent dendritic protein translation, enhancing mGluR1/5-dependent synaptic plasticity and other disease phenotypes in FXS. In this study, we describe a new activation function of FMRP in regulating protein expression in astroglial cells. We found that astroglial glutamate transporter subtype glutamate transporter 1 (GLT1) and glutamate uptake is significantly reduced in the cortex of fmr1(-/-) mice. Correspondingly, neuronal excitability is also enhanced in acute fmr1(-/-) (but not in fmr1(+/+) control) cortical slices treated with low doses (10 μm) of the GLT1-specific inhibitor dihydrokainate (DHK). Using mismatched astrocyte and neuron co-cultures, we demonstrate that the loss of astroglial (but not neuronal) FMRP particularly reduces neuron-dependent GLT1 expression and glutamate uptake in co-cultures. Interestingly, protein (but not mRNA) expression and the (S)-3,5-dihydroxyphenylglycine-dependent Ca(2+) responses of astroglial mGluR5 receptor are also selectively reduced in fmr1(-/-) astrocytes and brain slices, attenuating neuron-dependent GLT1 expression. Subsequent FMRP immunoprecipitation and QRT-PCR analysis showed that astroglial mGluR5 (but not GLT1) mRNA is associated with FMRP. In summary, our results provide evidence that FMRP positively regulates translational expression of mGluR5 in astroglial cells, and FMRP-dependent down-regulation of mGluR5 underlies GLT1 dysregulation in fmr1(-/-) astrocytes. The dysregulation of GLT1 and reduced glutamate uptake may potentially contribute to enhanced neuronal excitability observed in the mouse model of FXS.

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

“…Altered Glu transport has been recently reported in Fmr1 KO astrocytes co-cultured with either WT or Fmr1 KO neurons (53). However, unlike our present finding in the preCGG KI model, Fmr1 KO astrocyte cultures showed similar levels of expression of both GLT-1 and GLAST when compared with WT controls.…”

Glutamate Clearance Is Locally Modulated by Presynaptic Neuronal Activity in the Cerebral Cortex

“…Genetic and pharmacological manipulations of transporter expression (Rothstein et al, 2005;Omrani et al, 2009;Goodrich et al, 2013;Higashimori et function. For example, genetic ablation of the neuronal glutamate transporter EAAC1, which constitutes a small fraction of the total glutamate clearance capacity, alters glutamatergic signaling and synaptic plasticity (Scimemi et al, 2009;Holmseth et al, 2012).…”

Reduced excitatory amino acid transporter 1 and metabotropic glutamate receptor 5 expression in the cerebellum of fragile X mental retardation gene 1 premutation carriers with fragile X-associated tremor/ataxia syndrome

“…Notably, it was recently reported that Glu uptake is reduced in cortical extracts of the FMR1 KO mice, although this is thought to be mediated by GLT1; Downregulation of mGluR5 and GLAST were also observed in these fractions at early but not at later developmental stages (Higashimori et al 2013). The preCGG KI mice show rotarod deficits (Van Dam et al 2005), and subtle but significant increased foot slips on the ladder rung test that could be analogous to a mild ataxia (Hunsaker et al 2011), and poor skilled forelimb motor learning (Diep et al 2012).…”