PRECLINICAL DEVELOPMENT OF A NOVEL SEROTONIN RECEPTOR MODULATOR FOR FRAGILE X SYNDROME

Abstract

Fragile X syndrome (FXS), caused by FMR1 gene silencing and subsequent loss of the RNA-binding protein FMRP, is the leading monogenic cause of intellectual disability and autism spectrum disorder, characterized by social deficits and repetitive behaviors. Currently, there are no approved medications for FXS and only two approved for autism. MMP-9 is an extracellular matrix regulating enzyme and a significant target of FMRP—the loss of FMRP in FXS leads to overexpression of MMP-9. MMP-9 degrades perineuronal nets (PNNs) that stabilize glutamatergic and GABAergic neurotransmission, and imbalance in these neurotransmitter systems is a hallmark of FXS, suggesting correcting MMP-9 overactivity may restore neuronal homeostasis in FXS by normalizing PNNs. For example, we show significantly decreased PNN density in hippocampal CA2 in juvenile Fmr1 knockout mice, a neural system critical for social memory, compared to wild-type mice. In parallel with PNN deficits, we show a trend that juvenile Fmr1 KO mice have increased MMP-9.

We showed that acute administration of FPT, a novel medication candidate with potent 5-HT1AR, 5-HT1BR, 5-HT1DR agonist activity, moderate 5-HT2CR activity, and weak agonist/competitive antagonist 5-HT7R activity, increases social interactions, decreases anxiety, and decreases repetitive behaviors in adult Fmr1 KO and WT mice. However, we did not investigate FPT’s effects on social behavior in juvenile mice—focusing on critical neurodevelopmental periods—nor have we explored mechanisms underlying FPT’s therapeutic-like effects. In susceptible juvenile Fmr1 KO mice, FPT eliminated the audiogenic seizure phenotype. We investigated whether FPT would be efficacious after chronic treatment or if tolerance would develop. After four-day repeated treatment followed by a one-day washout period, FPT increased social interactions in juvenile Fmr1 KO mice and WT mice. FPT restored the CA2 PNN deficit in juvenile Fmr1 KO mice to WT levels such that there were significant genotype and interaction effects and a near-significant treatment effect. Additionally, there was a near-significant interaction effect of genotype and treatment for the MMP-9 results. MMP-9 trended towards an increase in vehicle-treated Fmr1 KO mice compared to WT mice (vehicle- or FPT-treated), and FPT restored MMP-9 to WT levels. Modulating distinct 5-HTRs can regulate MMP-9 expression, neuroanatomical plasticity, and social behaviors. We conclude that FPT increases social behavior in juvenile Fmr1 KO mice and works, in part, by decreasing MMP-9 levels and restoring PNN levels in hippocampal CA2 via a 5-HT1AR, 5-HT1BR, 5-HT1DR, 5-HT2CR, and/or 5-HT7R mechanism.