Loss of FMRP Impaired Hippocampal Long-Term Plasticity and Spatial Learning in Rats

Abstract

Fragile X syndrome (FXS) is a neurodevelopmental disorder caused by mutations in the <i>FMR1</i> gene that inactivate expression of the gene product, the fragile X mental retardation 1 protein (FMRP). In this study, we used clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) technology to generate <i>Fmr1</i> knockout (KO) rats by disruption of the fourth exon of the <i>Fmr1</i> gene. Western blotting analysis confirmed that the FMRP was absent from the brains of the <i>Fmr1</i> KO rats (<i>Fmr1^exon4-KO</i> ). Electrophysiological analysis revealed that the theta-burst stimulation (TBS)-induced long-term potentiation (LTP) and the low-frequency stimulus (LFS)-induced long-term depression (LTD) were decreased in the hippocampal Schaffer collateral pathway of the <i>Fmr1^exon4-KO</i> rats. Short-term plasticity, measured as the paired-pulse ratio, remained normal in the KO rats. The synaptic strength mediated by the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) was also impaired. Consistent with previous reports, the <i>Fmr1^exon4-KO</i> rats demonstrated an enhanced 3,5-dihydroxyphenylglycine (DHPG)-induced LTD in the present study, and this enhancement is insensitive to protein translation. In addition, the <i>Fmr1^exon4-KO</i> rats showed deficits in the probe trial in the Morris water maze test. These results demonstrate that deletion of the <i>Fmr1</i> gene in rats specifically impairs long-term synaptic plasticity and hippocampus-dependent learning in a manner resembling the key symptoms of FXS. Furthermore, the <i>Fmr1^exon4-KO</i> rats displayed impaired social interaction and macroorchidism, the results consistent with those observed in patients with FXS. Thus, <i>Fmr1^exon4-KO</i> rats constitute a novel rat model of FXS that complements existing mouse models.