Microglia play a pivotal role in the coordination of brain development and have emerged as a critical determinant in the progression of neurodegenerative diseases; however, the role of microglia in the onset and progression of neurodevelopmental disorders is less clear. Here we show that conditional deletion of αVβ8 from the central nervous system (Itgb8ΔCNS mice) blocks microglia in their normal stepwise development from immature precursors to mature microglia. These “dysmature” microglia appear to result from reduced TGFβ signaling during a critical perinatal window, are distinct from microglia with induced reduction in TGFβ signaling during adulthood, and directly cause a unique neurodevelopmental syndrome characterized by oligodendrocyte maturational arrest, interneuron loss, and spastic neuromotor dysfunction. Consistent with this, early (but not late) microglia depletion completely reverses this phenotype. Together, these data identify novel roles for αVβ8 and TGFβ signaling in coordinating microgliogenesis with brain development and implicate abnormally programmed microglia or their products in human neurodevelopmental disorders that share this neuropathology.

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The srbi gene encodes a lipoprotein receptor with high affinity for high density lipoprotein that is mainly expressed in the liver and in steroidogenic tissues. Disruption of this gene in mice and mutations in humans lead to alterations in lipoprotein metabolism and/or fertility. During murine development, scavenger receptor class B member I (SR-BI) is present in the yolk sac and the placenta and is only expressed in the embryo itself late in gestation. In humans, it has been detected in trophoblast cells and placenta. Although the proportion of mice carrying a null mutation in SR-BI obtained from heterozygous intercrosses is lower than the expected by the Mendelian ratio, suggesting the involvement of this receptor in intrauterine development, the cause of this demise has remained unknown. In this work, we show that embryos lacking SR-BI exhibit a high prevalence of exencephaly with a sex bias toward females. Immunolocalization studies confirmed that SR-BI is not expressed in the embryo at early stages of development and allowed a more detailed description of its localization in the cells that mediate maternal-fetal transport of nutrients. SR-BI-null embryos contain less cholesterol than their wild-type littermates, suggesting the involvement of SR-BI in materno-fetal cholesterol transport. Newborn SR-BI-deficient pups exhibit intrauterine growth restriction, suggesting that this receptor is also important for fetal growth. Altogether, the results of our work suggest that the presence of SR-BI in extraembryonic tissues is involved in the maternal-fetal transport of cholesterol and/or other lipids with a role during neural tube closure and fetal growth.

SR-BI is the main receptor for high density lipoproteins (HDL) and mediates the bidirectional transport of lipids, such as cholesterol and vitamin E, between these particles and cells. During early development, SR-BI is expressed in extraembryonic tissueScavenger Receptor Class B type I (SR-BI) is the main receptor for high density lipoproteins (HDL), and numerous studies have described its role in mediating the bidirectional transport of lipids between these lipoproteins and cells 1 . In the liver, SR-BI is involved in the uptake of cholesterol from HDL and its excretion in bile, the final step in reverse cholesterol transport. SR-BI also participates in the uptake of cholesterol in steroidogenic tissues, such as the adrenal glands and ovaries, to be used as a substrate for steroid hormone synthesis 2 . Important information on the roles of SR-BI other than in cholesterol homeostasis and cholesterol provision for steroidogenesis, such as platelet aggregation, erythrocyte maturation and oocyte meiosis, has been generated from the SR-BI knock out (SR-BI −/− ) mouse since it was generated almost two decades ago 3 .In generating SR-BI −/− mice via heterozygous intercrosses, researchers noted that the proportion of weaned homozygous null mice was half that expected by the Mendelian ratio 3 . This evidence, together with the fact that SR-BI is present in murine trophoblasts involved in maternal-foetal nutrient exchange at different stages of gestation 4 , led researchers to postulate that this HDL receptor might be involved in embryonic development. We recently showed that nearly 50% of SR-BI −/− embryos fail to close the anterior neural tube and develop cranial NTD and exencephaly 5 , leading to perinatal death, which explains the deviation from the Mendelian ratio previously reported in weaned SR-BI null mice 3 . Among the spectrum of defective neurulation conditions conferred by abnormal closure at different portions of the neural tube, only cranial NTD is observed in SR-BI −/− embryos.During murine early development, SR-BI is not detected in the embryo itself but rather in trophoblast giant cells (TGC) from the parietal yolk sac 4,5 . TGC play a critical role in embryonic uptake of various nutrients from the maternal blood supply before the establishment of a mature placenta 6 . Despite the prominent role of SR-BI

High density lipoproteins (HDL) take up cholesterol from peripheral tissues via ABC transporters and deliver it to the liver via scavenger receptor class B type I (SR-B1). HDL are the main lipoproteins present in follicular fluid (FF). They are thought to derive from plasma, but their origin is still controversial. SR-B1 knock-out (KO) mice have provided important evidence linking HDL metabolism and female fertility. These mice have cholesterol-rich circulating HDL and female infertility that can be restored by treating mice with the cholesterol-lowering drug probucol. Ovulated oocytes from SR-B1 KO females are dysfunctional and show excess cholesterol. The mechanisms explaining the contribution of FF HDL to oocyte cholesterol homeostasis are unknown. Here, using quantitation of filipin fluorescence we show that in SR-B1 KO ovaries, cholesterol excess is first observed in immature oocytes in antral follicles. By performing cross-transplant experiments between WT and apolipoprotein A-I deficient (ApoA-I KO) mice, which lack the main protein component of HDL, we provide evidence supporting the plasmatic origin of FF HDL. Also, we demonstrate that probucol treatment in SR-B1 KO females results in lowering of cholesterol content in their oocytes. Incubation of oocytes from SR-B1 KO mice with purified WT HDL reduces their cholesterol content, suggesting that HDL promote efflux of excess cholesterol from oocytes. In agreement with this hypothesis, we identified ABC transporters in oocytes and observed that ABCA1 KO oocytes have excess cholesterol and lower viability than WT oocytes.