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Abstract

X-chromosome inactivation (XCI) is an epigenetic process unique to female mammals, and creates a “mosaic” of cells expressing either the maternal or paternal X-chromosome. Females heterozygous for genes leading to X-linked neurodevelopmental disorders (NDD) exhibit a variety of phenotypes. This thesis investigated the hypotheses that (1) variability in the ratio and location of cells expressing the mutant vs wild type (WT) X-chromosome is an underlying cause of phenotypic variability and that (2) baseline sex differences in the brain play a role in the differential manifestation of the X-linked NDDs based on sex. I studied oligodendrocyte (OL) development and myelination in male and female mice to establish foundation differences in OL biology, as well as in Fragile X Syndrome (FXS), a NDD caused by the loss of function of an X-linked gene Fmr1. Moreover, these studies were performed to characterize cellular behavior during adolescence, which is a critical period for behavioral development in FXS, as well as for developmental myelination by OLs. To probe OLs in the mosaic brain, I utilized a novel Cre-loxP based dual-color X-linked cell marking system (NG2;XGFP/FxtdT), which enables distinguishing WT and mutant OLs in Fmr1+/- females using fluorescent reporters. Using single-cell transcriptomics, in vivo fate-mapping analysis, and extensive confocal and 2-photon imaging in the mice, I found that in general oligodendrocyte precursor cells (OPCs) in the male cortex differentiated more rapidly into OLs than in the female, and such faster maturation was also observed when these female OPCs were mutant in a mosaic environment – showing that there exists both developmental and pathological faster maturation of OLs. Detailed transcriptomic and histological analysis also revealed that these differential rates of maturation, as well as simply existing in a mosaic environment, leads to changes in morphological and myelination patterns of these OLs. Additionally, these analyses also reveal that OLs that mature faster upregulate genes and proteins involved in the cellular response to environmental glutamate. Altogether, the results lead to the conclusion that not only do baseline sex differences in the biology of OLs exist, but these differences also form an important background for the unique character of OLs when present in the environment of X-linked mosaicism in females, being affected by environmental factors such as glutamate which is a potential master regulator of OL development.