GABAergic/glycinergic and glutamatergic neurons mediate distinct neurodevelopmental phenotypes of STXBP1 encephalopathy

An increasing number of pathogenic variants in presynaptic proteins involved in the synaptic vesicle cycle are being discovered in neurodevelopmental disorders. The clinical features of these synaptic vesicle cycle disorders are diverse, but the most prevalent phenotypes include intellectual disability, epilepsy, movement disorders, cerebral visual impairment, and psychiatric symptoms (Verhage and Sørensen, 2020; Bonnycastle et al., 2021; John et al., 2021; Melland et al., 2021). Among this growing list of synaptic vesicle cycle disorders, the most frequent is encephalopathy caused by heterozygous pathogenic variants in syntaxin-binding protein 1 (STXBP1, also known as MUNC18-1) (Verhage and Sørensen, 2020; John et al., 2021). STXBP1 is an essential protein for presynaptic neurotransmitter release. Its haploinsufficiency is the main disease mechanism and impairs both excitatory and inhibitory neurotransmitter release. However, the disease pathogenesis and cellular origins of the broad spectrum of neurological phenotypes are poorly understood. Here we generate cell type-specific haploinsufficient male and female mice and show that haploinsufficiency in GABAergic/glycinergic neurons causes developmental delay, epilepsy, motor, cognitive, and psychiatric deficits, recapitulating majority of the phenotypes observed in the constitutive haploinsufficient mice and encephalopathy. In contrast, haploinsufficiency in glutamatergic neurons results in a small subset of cognitive and seizure phenotypes distinct from those caused by haploinsufficiency in GABAergic/glycinergic neurons. Thus, the contrasting roles of excitatory and inhibitory signaling reveal GABAergic/glycinergic dysfunction as a key disease mechanism of encephalopathy and suggest the possibility to selectively modulate disease phenotypes by targeting specific neurotransmitter systems. Heterozygous pathogenic variants in are in the top five causes of pediatric epilepsies and one of the most frequent causes of neurodevelopmental disorders. They affect presynaptic neurotransmitter release and a broad spectrum of neurological features common among neurodevelopmental disorders, but the disease pathogenesis and cell types subserving these phenotypes remain unclear. Here we report the distinct roles of GABAergic/glycinergic and glutamatergic neurons in the pathogenesis of encephalopathy. These results will aid the development of therapeutic interventions by suggesting the potential outcomes of therapeutic stra