How the loss of fragile X mental retardation protein (FMRP) in different brain cell types, especially in non-neuron glial cells, induces fragile X syndrome (FXS) phenotypes has just begun to be understood. In the current study, we generated inducible astrocyte-specific Fmr1 conditional knock-out mice (i-astro-Fmr1-cKO) and restoration mice (i-astro-Fmr1-cON) to study the in vivo modulation of FXS synaptic phenotypes by astroglial FMRP. We found that functional expression of glutamate transporter GLT1 is 40% decreased in i-astro-Fmr1-cKO somatosensory cortical astrocytes in vivo, which can be fully rescued by the selective re-expression of FMRP in astrocytes in i-astro-Fmr1-cON mice. Although the selective loss of astroglial FMRP only modestly increases spine density and length in cortical pyramidal neurons, selective re-expression of FMRP in astrocytes significantly attenuates abnormal spine morphology in these neurons of i-astro-Fmr1-cON mice. Moreover, we found that basal protein synthesis levels and immunoreactivity of phosphorylated S6 ribosomal protein (p-s6P) is significantly increased in i-astro-Fmr1-cKO mice, while the enhanced cortical protein synthesis observed in Fmr1 KO mice is mitigated in i-astro-Fmr1-cON mice. Furthermore, ceftriaxone-mediated upregulation of surface GLT1 expression restores functional glutamate uptake and attenuates enhanced neuronal excitability in Fmr1 KO mice. In particular, ceftriaxone significantly decreases the growth rate of abnormally accelerated body weight and completely corrects spine abnormality in Fmr1 KO mice. Together, these results show that the selective loss of astroglial FMRP contributes to cortical synaptic deficits in FXS, presumably through dysregulated astroglial glutamate transporter GLT1 and impaired glutamate uptake. These results suggest the involvement of astrocytemediated mechanisms in the pathogenesis of FXS.

Fragile X syndrome (FXS) is one of the most common inherited intellectual disability (ID) disorders, in which the loss of FMRP protein induces a range of cellular signaling changes primarily through excess protein synthesis. Although neuron-centered molecular and cellular events underlying FXS have been characterized, how different CNS cell types are involved in typical FXS synaptic signaling changes and behavioral phenotypes is largely unknown. Recent evidence suggests that selective loss of astroglial FMRP is able to dysregulate glutamate uptake, increase spine density, and impair motor-skill learning. Here we investigated the effect of astroglial FMRP on synaptic signaling and FXS-related behavioral and learning phenotypes in astroglial Fmr1 cKO and cON mice in which FMRP expression is selectively diminished or restored in astroglia. We found that selective loss of astroglial FMRP contributes to cortical hyperexcitability by enhancing NMDAR-mediated evoked but not spontaneous miniEPSCs and elongating cortical UP state duration. Selective loss of astroglial FMRP is also sufficient to increase locomotor hyperactivity, significantly diminish social novelty preference, and induce memory acquisition and extinction deficits in astroglial Fmr1 cKO mice. Importantly, re-expression of astroglial FMRP is able to significantly rescue the hyperactivity (evoked NMDAR response, UP state duration, and open field test) and social novelty preference in astroglial Fmr1 cON mice. These results demonstrate a profound role of astroglial FMRP in the evoked synaptic signaling, spontaneously occurring cortical UP states, and FXS-related behavioral and learning phenotypes and provide important new insights in the cell type consideration for the FMRP reactivation strategy.

Fragile X syndrome (FXS) is one of the most common inherited intellectual disability (ID) disorders, in which the loss of FMRP protein induces a range of cellular signaling changes primarily through excess protein synthesis. Although neuron‐centered molecular and cellular events underlying FXS have been characterized, how different CNS cell types are involved in typical FXS synaptic signaling changes and behavioral phenotypes is largely unknown. Recent evidence suggests that selective loss of astroglial FMRP is able to dysregulate glutamate uptake, increase spine density, and impair motor‐skill learning. Here we investigated the effect of astroglial FMRP on synaptic signaling and FXS‐related behavioral and learning phenotypes in astroglial Fmr1 cKO and cON mice in which FMRP expression is selectively diminished or restored in astroglia. We found that selective loss of astroglial FMRP contributes to cortical hyperexcitability by enhancing NMDAR‐mediated evoked but not spontaneous miniEPSCs and elongating cortical UP state duration. Selective loss of astroglial FMRP is also sufficient to increase locomotor hyperactivity, significantly diminish social novelty preference, and induce memory acquisition and extinction deficits in astroglial Fmr1 cKO mice. Importantly, re‐expression of astroglial FMRP is able to significantly rescue the hyperactivity (evoked NMDAR response, UP state duration, and open field test) and social novelty preference in astroglial Fmr1 cON mice. These results demonstrate a profound role of astroglial FMRP in the evoked synaptic signaling, spontaneously occurring cortical UP states, and FXS‐related behavioral and learning phenotypes and provide important new insights in the cell type consideration for the FMRP reactivation strategy.