Jee-Yeon Hwang,^1,6,11,13,* Hannah R. Monday,^1,10,11 Jingqi Yan,^1,5,11 Andrea Gompers,^1,7,11 Adina R. Buxbaum,^1,2,3,8 Kirsty J. Sawicka,^1,9 Robert H. Singer,^1,2,3,12 Pablo E. Castillo,^1,4,12 and R. Suzanne Zukin^1,12,*

¹Dominick P. Purpura Department of Neuroscience, Albert Einstein College of Medicine, New York, NY 10461, USA
²Department of Structural & Cell Biology, Albert Einstein College of Medicine, New York, NY 10461, USA
³Janelia Research Campus, Howard Hughes Medical Institute, Ashburn, VA 20147, USA
⁴Department of Psychiatry and Behavioral Sciences, Albert Einstein College of Medicine, New York, NY 10461, USA
⁵Center for Gene Regulation in Health and Disease, Department of Biological, Geological, and Environmental Sciences, Cleveland State University, Cleveland, OH 44115, USA
⁶Department of Pharmacology and Neuroscience, Creighton University School of Medicine, Omaha, NE 68178, USA
⁷Center for Immunology and Infectious Diseases, University of California, Davis, Davis, CA 95616, USA
⁸Present address: Department of Neurosciences, University of California, San Diego, La Jolla, CA 92093, USA
⁹Present address: Cancer Research UK Cambridge Institute, Li Ka Shing Centre, University of Cambridge, Cambridge, UK
¹⁰Present address: Department of Molecular and Cellular Biology and Helen Wills Neuroscience Institute, University of California, Berkeley, Berkeley, CA 94720, USA
¹¹These authors contributed equally
¹²These authors contributed equally
¹³Lead contact
^*Correspondence

Fragile X syndrome (FXS) is a leading cause of inherited intellectual disability and autism. Whereas dysregulated RNA translation in Fmr1 knockout (KO) mice, a model of FXS, is well studied, little is known about aberrant transcription. Using single-molecule mRNA detection, we show that mRNA encoding the AMPAR subunit GluA2 (but not GluA1) is elevated in dendrites and at transcription sites of hippocampal neurons of Fmr1 KO mice, indicating elevated GluA2 transcription. We identify CPEB3, a protein implicated in memory consolidation, as an upstream effector critical to GluA2 mRNA expression in FXS. Increased GluA2 mRNA is translated into an increase in GluA2 subunits, a switch in synaptic AMPAR phenotype from GluA2-lacking, Ca²⁺-permeable to GluA2-containing, Ca²⁺-impermeable, reduced inhibitory synaptic transmission, and loss of NMDAR-independent LTP at glutamatergic synapses onto CA1 inhibitory interneurons. These factors could contribute to an excitatory/inhibitory imbalance—a common theme in FXS and other autism spectrum disorders.