Microglia drive synaptic and functional connectivity deficits in the Ts65Dn mouse model of Down syndrome by affecting inhibition.

Microglia, the resident immune cells of the brain, play a crucial role in sculpting neuronal circuits during development, and their dysfunction is increasingly implicated in neurodevelopmental disorders such as Down syndrome (DS). Here, we reveal a previously unrecognized pathological mechanism whereby microglia contribute to synaptic and neuronal activity deficits in DS: a selective disruption of microglia–interneuron interactions. Using primary neuron–microglia co-cultures from Ts65Dn mice, we show that while trisomy in neurons drives excitatory synaptic deficits and major microglial morphological changes, microglial trisomy disrupts the regulation of inhibitory synapses in a cell-autonomous manner. To investigate these pathological interactions in vivo, we developed a novel spatial distribution analysis tool that, combined with chemogenetic approaches targeting parvalbumin (PV) interneurons in Ts65Dn mice, allowed us to reveal a disrupted microglia–PV interneuron crosstalk characterized by reduced physical association and impaired microglial responsiveness to PV activity. Finally, by targeting microglia via P2Y12 receptor inhibition, we restored cortical connectivity, rescued PV interneuron function, and improved cognitive performance in Ts65Dn mice. Overall, these findings establish microglia–interneuron dysregulation as a key driver of neuronal activity and synaptic dysfunction in the Ts65Dn mouse model of DS and identify the microglia as a promising therapeutic target to counter circuit dysfunction and cognitive deficits.