Disruption of autism-associatedPcdh9gene leads to transcriptional alterations, synapses overgrowth and aberrant excitatory transmission in the CA1

Abstract

ABSTRACTProtocadherins are cell adhesion molecules with crucial role in cell-cell contacts, whose mutations or altered expression have been implicated in multiple brain disorders. In particular, growing evidence links genetic alterations inProtocadherin 9(PCDH9) gene with Autism Spectrum Disorder (ASD) and Major Depression Disorder (MDD). Furthermore,Pcdh9deletion induces neuronal defects in the mouse somatosensory cortex, accompanied by sensorimotor and memory impairment. However, the synaptic and molecular mechanisms underlyingPcdh9physiological function and its involvement in brain pathology remain largely unknown. To this aim, we conducted a comprehensive investigation of PCDH9 role in the mouse hippocampus at the ultrastructural, biochemical, transcriptomic, electrophysiological and network level. We show that PCDH9 mainly localizes at glutamatergic synapses and its expression peaks in the first week after birth, a crucial time window for synaptogenesis. Strikingly,Pcdh9KO neurons exhibit oversized presynaptic terminal and postsynaptic density (PSD) in the CA1. Synapse overgrowth is sustained by the broad up-regulation of synaptic genes and the dysregulation of key drivers of synapse morphogenesis, as revealed by single-nucleus RNAseq. Synaptic and transcriptional defects are accompanied by increased EPSC frequency and disturbances in the hippocampal network activity ofPcdh9KO mice. In conclusion, our work indicates thatPcdh9regulates the morphology and function of excitatory synapses in the CA1, thereby affecting glutamatergic transmission in hippocampal circuitries.