ERK-dependent hyperexcitability of BLA neurons projecting to dCA3 underlies social dysfunction in a male mouse model of fragile X syndrome

BACKGROUND: Social dysfunction is a core symptom of autism spectrum disorder (ASD), including fragile X syndrome (FXS), but its underlying neural circuits and molecular mechanisms remain poorly understood. Previous studies have implicated the amygdala and hippocampus in social behaviour, yet the specific pathways and signalling events linking genetic deficits to behavioural dysfunction have not been fully delineated. METHODS: Using activity-dependent c-Fos mapping, fibre photometry, closed-loop optogenetics, pharmacological inhibition, and shRNA-mediated knockdown, we investigated the role of the BLA-dCA3 projection and ERK signalling in male Fmr1 KO mice, complemented by re-analysis of human ASD snRNA-seq data and whole-cell patch-clamp recordings. FINDINGS: We found that BLA-dCA3 projecting neurons are aberrantly hyperactivated in Fmr1 KO mice during interactions with both novel and familiar mice, and that closed-loop activation of this pathway in WT mice during familiar interaction impairs social discrimination, whereas its inhibition in KO mice rescues the deficit. Additionally, ERK signalling is upregulated in the BLA of both patients with ASD and Fmr1 KO mice; knocking down FMR1 in the adult BLA recapitulates both the social deficit and ERK hyperactivation, while pharmacological ERK inhibition rescues social behaviour and normalises neuronal hyperexcitability. INTERPRETATION: These findings pinpoint the BLA-dCA3 circuit and BLA-specific ERK signalling as critical mediators of social discrimination deficits in FXS. Our study establishes a causal link from FMRP loss to circuit dysfunction and ERK pathway dysregulation, and suggests that targeting this pathway may offer a promising strategy for treating social dysfunction in ASD and FXS. FUNDING: This work was supported by grants from the National Science and Technology Major Project (2025ZD0214701), the National Natural Science Foundation of China (32171014, 31970940, and 32500889), Nanhu Brain-Computer Interface Institute (010904018), and the Zhejiang Provincial Natural Science Foundation of China (LMS25C090004).