UC San Diego

Three Prime Repair Exonuclease 1 (TREX1) gene mutations have been associated with Aicardi-Goutières Syndrome (AGS) – a rare, severe pediatric autoimmune disorder that primarily affects the brain and has a poorly understood etiology. Microglia are brain-resident macrophages indispensable for brain development and implicated in multiple neuroinflammatory diseases. However, the role of TREX1 – a DNase that cleaves cytosolic nucleic acids, preventing viral- and autoimmune-related inflammatory responses – in microglia biology remains to be elucidated. Chapter 1 serves as an introduction and a review of the literature providing key background on the following: human cortical brain development, glial contributions to brain development, organoid models to study human brain development, and using brain organoids for modeling neurodevelopmental disorders. Chapter 2 is an original study investigating TREX1 functions in human microglia. We leverage a model of human embryonic stem cell (hESC)-derived engineered microglia-like cells, bulk and single-cell transcriptomics, optical and transmission electron microscopy, and three-month-old assembloids composed of microglia and regionalized neural organoids to interrogate TREX1 functions in human microglia. Our analyses suggest that TREX1 influences cholesterol metabolism, leading to an active microglial morphology with increased phagocytosis in the absence of TREX1. Notably, regulating cholesterol metabolism with an HMG-CoA reductase inhibitor, FDA-approved atorvastatin, rescues these microglial phenotypes. Chapter 3 details our original work elucidating the functions of TREX1-KO microglia on human brain development. We develop the first in vitro human brain assembloid model where microglia are present during neuro- and gliogenesis, including oligodendrocyte development. Using single-cell transcriptomics, we discover that TREX1-KO microglia selectively affect oligodendrocyte maturation and myelination. Together, these results suggest routes for therapeutic intervention in pathologies such as AGS based on microglia-specific molecular and cellular mechanisms.