Understanding Down Syndrome Brain Development Using Human iPSC-Based Mouse Chimeras

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Down syndrome (DS), caused by triplication of human chromosome 21 (HSA21), is the most common genetic origin of intellectual disability. Studying DS disease mechanism is challenging because functional human DS brain tissues are scarcely available and transgenic mouse models of DS demonstrate incomplete/inaccurate expression of HSA21 genes. The advent of human induced pluripotent stem cell (hiPSC) technology has led to the generation of DS patient-derived hiPSCs, which presents an unprecedented opportunity for studying the pathogenesis of DS with unlimited human brain cells in vitro. While using the hiPSC-based in vitro models, basic aspects of the disease phenotypes can be examined, the disruption of neural circuits in the developing brain under disease conditions remains to be studied with hiPSCs. Ultimately, specific developmental and disease mechanisms can only be modeled in live animals to identify links between cellular phenotypes and behavioral performance. Therefore, we propose to employ hiPSC-based chimeric mouse brain models to study the neuropathophysiology of DS in vivo. Microglia play critical roles in brain development and are also an active player in learning and memory processes. Surprisingly, very little information is available on how trisomy of HSA21 alters the development and functions of microglia and what roles microglia play in the abnormal brain development and cognitive deficits in DS. In addition, mounting evidence indicates that rodent microglia are not able to fully mirror the properties of human microglia in normal and disease conditions. In this study, we will use our recently created hiPSC microglial chimeric mouse model to unravel the role of microglia in DS pathogenesis in an in vivo system with intact neural networks. We hypothesize that unlike engrafted normal human microglia, engrafted diseased DS human microglia will show abnormal biological properties and functions, such as synaptic pruning function in vivo. These abnormal properties of DS microglia will result in their negative regulation of the synaptic activity and plasticity of the hippocampal neural network, critically contributing to the cognitive deficits seen in DS. This hypothesis will be tested in three specific aims. Aim 1: we will determine the differences between DS and control hiPSC-derived microglia in vivo in human microglial chimeric mouse brains. Aim 2: Using the microglial chimeric mouse model, we will further examine the impact of integration of DS microglia on synaptic plasticity of the hippocampus and learning and memory behavior of the animals. Aim 3: We will normalize the expression of the HSA21 genes by CRISPR/Cas9 to examine how this will alter the properties of DS microglia. Moreover, single-cell RNA-sequencing analysis of hiPSC microglial chimeric mouse brains will be performed to compare gene expression profiles of control and DS microglia. Findings from our study using a powerful, new hiPSC microglial chimeric mouse model will provide novel insights into the pathological roles of human microglia in DS. Identifying the potential molecules that can be targeted to improve microglial function may provide a new therapeutic avenue for the treatment of DS.