Heterozygosity for neurodevelopmental disorder-associatedTRIOvariants yields distinct deficits in behavior, neuronal development, and synaptic transmission in mice

Abstract

Heterozygosity for rare genetic variants inTRIOis associated with neurodevelopmental disorders (NDDs) including schizophrenia (SCZ), autism spectrum disorder (ASD) and intellectual disability. TRIO uses its two guanine nucleotide exchange factor (GEF) domains to activate GTPases (GEF1: Rac1 and RhoG; GEF2: RhoA) that control neuronal migration, synapse development and function. It remains unclear whether and how discreteTRIOvariants differentially impact these neurodevelopmental events. Here, we elucidate how heterozygosity for NDD-associatedTriovariants –+/K1431M(ASD),+/K1918X(SCZ), and+/M2145T(bipolar disorder, BPD) – impact mouse behavior, brain development, and synapse structure and function. Heterozygosity for differentTriovariants impacts motor, social, and cognitive behaviors in distinct ways that align with clinical phenotypes in humans. ASD- and SCZ-linkedTriovariants differentially impact head and brain size with corresponding changes in dendritic arbors of motor cortex layer 5 pyramidal neurons (M1 L5 PNs). Although dendritic spine density and synaptic ultrastructure were only modestly altered in theTriovariant heterozygotes, we observe significant changes in synaptic function and plasticity including excitatory/inhibitory imbalance and long-term potentiation defects. We also identify distinct changes in glutamate synaptic release in+/K1431Mand+/M2145Tcortico-cortical synapses, associated with deficiencies in crucial presynaptic release regulators. WhileTRIO K1431Mhas impaired ability to promote GTP exchange on Rac1,+/K1431Mmice exhibit increased Rac1 activity, suggesting possible compensation by other GEFs. Our work reveals that discrete disease-associatedTriovariants yield overlapping but distinct NDD-associated phenotypes in mice and demonstrates, for the first time, an essential role for Trio in presynaptic glutamate release, underscoring the importance of studying the impact of variant heterozygosity in vivo.