Brain-region-specific humanized cortical interneuron mice

Abstract GABAergic cortical interneurons (cINs) play critical roles in balancing, synchronizing, and gating brain activity by inhibiting other neurons. Their malfunction, especially those of medial ganglionic eminence (MGE)-derived cINs, has been associated with various neurodevelopmental brain disorders, such as schizophrenia (SCZ) and autism spectrum disorders (ASD). Considering the fact that the divergence between human brains and rodent brains has resulted in the failure of many central nervous system (CNS) therapeutics validated in rodent models, it is critical to study human neurons to better understand the mechanisms of these cIN-associated brain disorders. Human fetal brain tissues are not accessible for mechanistic studies, but we have developed a method to efficiently generate homogeneous populations of MGE-type human cINs from pluripotent stem cells (PSCs) of healthy or diseased subjects. We have extensively characterized them and demonstrated their authenticity and functionality, making it possible to study the converging functional consequences of complex genetics in real patient neurons, which cannot be studied in mouse neurons due to a lack of conservation of non-coding regions, where most of risk loci are present. However, in vitro cultured neurons lack other critical components of the brain environment, such as astrocytes, oligodendrocytes, microglia and blood vessels, which can significantly impact their function. There have been efforts to optimize in vitro culture systems to better recapitulate in vivo physiological environments by adding other brain cellular components, but there are still limitations as to how closely they can simulate in vivo situations. To resolve this issue, in our previous study, we pioneered human neuron-mouse brain chimeras to study the function of human SCZ neurons in physiological environments. Although we were able to successfully identify SCZ cIN-intrinsic connectivity deficits in mouse brains, we were not able to analyze the impacts of grafted neurons on brain circuits and behaviors due to the presence of healthy mouse neurons in the grafted mice. Thus, in this proposed study, we will perform brain-region-specific cIN-ablation in NodScid gamma (NSG) mice, followed by the replacement of ablated host cINs with human cINs to generate region-specific humanized cIN chimeras. Based on previous studies, including ours, that show successful restoration of compromised mouse inhibition by grafted human cINs, these mice will allow us to analyze the functional impacts of grafted human cINs on the brain circuits and behaviors in physiological in vivo environments. This novel physiological model system will help us tease apart cell-type- and brain-region-specific disease mechanisms for complex brain disorders, and aid in developing novel therapeutics.