Sex differences in microglial appetites during development: inferences and implications.

Most if not all neurological disorders exhibit some degree of sex bias in their incidence, presentation, or pathologic progression. For instance, autism spectrum disorder (ASD), learning disabilities, attention deficit disorder, and early-onset schizophrenia are more common in males, whereas depression, anxiety, anorexia, and Alzheimer’s disease are more common in females. At first glance the clustering of disorders within each sex appear to have little in common; however, upon closer inspection it is clear they segregate by timing – that is, disorders that emerge early in life are more common in males, whereas disorders that emerge later in life (at adolescence or beyond) are more common in females. This pattern is a compelling clue into the origin and pathophysiology of neurological disorders that has been relatively ignored in scientific research and in clinical practice. We ignore this clue at our peril, given that in heterogeneous disorders such as ASD, the sex of the individual is perhaps the most predictive variable of all, which strongly suggests the sex of an individual is critical in the neurobiology underlying the disorder.

Microglia are the resident immune cells of the brain that are implicated in the neuropathology of multiple neurological disorders. These dynamic cells are also well characterized for their critical roles in normal brain development and function, beyond host defense or wound repair, including the phagocytosis of extraneous synapses and apoptotic cells, and roles in learning dependent synapse formation, cortical wiring, and neuronal survival (see (Prinz and Priller, 2014; Schafer and Stevens, 2015) for review). Impairment of the developmental functions of microglia – demonstrated primarily via genetic manipulations of select genes - can adversely impact brain connectivity and behavior (Zhan et al., 2014), and thus disease risk.

Despite the weight of evidence that neurological disorders that segregate by time of onset within each sex also involve a significant neuroimmune or neuroinflammatory component (Hanamsagar and Bilbo, 2016; Nelson and Lenz, 2017), sex differences in the developmental functions of microglia have scarcely been investigated. In this issue of Brain, Behavior, and Immunity, Nelson and colleagues (Nelson et al., 2017) explored the hypothesis that microglia exhibit a sex difference in phagocytic capacity within the neonatal hippocampus of male and female rats. Testosterone spikes in late gestation in male rodents (~embryonic day 18–20), and following conversion to estradiol within the central nervous system (CNS) by aromatase, leads to sexual differentiation of the hippocampus (among other brain regions) during the early postnatal period, via impacts on cell proliferation, dendritic spine number, and cell death. Males have higher levels of cell proliferation in the hippocampus postnatally than females, and females treated with estradiol exhibit male-typical levels of proliferation. Microglia are expert neural custodians during this same period, via the engulfment and lysosomal degradation/phagocytosis of cells (dying and otherwise), synapses, and cellular debris, and are known to regulate the size of the neural stem cell niche of the developing cortex via limits on the number of neural progenitors (Cunningham et al., 2013). Here, Nelson et al. demonstrate for the first time that males exhibit fewer microglia with phagocytic cups compared to females, which may account for the increased level of cell proliferation. Moreover, treatment of newborn females with estradiol reduces the number of phagocytic microglia and phagocytic genes to male levels. There is no change in the total number of microglia between sexes, and female microglia specifically phagocytize more Sox2+ neural progenitor cells, but not newly dividing or pyknotic (dying) cells, compared to male microglia. Finally, and somewhat paradoxically, treatment of females with estradiol increases microglial proliferation despite the lack of a sex difference in proliferation under normal developmental conditions (i.e., in response to the prenatal surge in testosterone in males).

These data are largely consistent with independent findings of a sex difference in microglial morphology in the developing hippocampus of rats (Schwarz et al., 2011), and of transcriptomic differences in developing male vs. female microglia of mice, in which female microglia develop with a distinct transcriptional maturation trajectory to that of males (Hanamsagar et al., 2017). Taken together, these data suggest functionally important sex differences in microglial phagocytic function in the postnatal hippocampus, arguably one of the most important functions of this dynamic cell type during brain development, at least early in life. The authors examined only a limited window of time, but given the striking segregation of neurological disorders by time of onset in males vs. females, it will be interesting and important in future studies to examine additional time points or critical windows.

Implications of such a sex difference could be profound. Due to their macrophage/antigen presenting cell lineage, microglia are exquisitely sensitive to disruptions of homeostasis and thus environmental influences, perhaps more so than any other CNS cell type. The data presented here suggest a specific mechanism in which the impact of disrupted homeostasis by multiple factors on brain development and long-term function (and therefore disease vulnerability) is likely to be sex-specific. Moreover, though this report explored the impact of sex steroid hormones on one aspect of microglial function, it is interesting to speculate how genetic sex might also interact with endocrine factors to shape their eventual phenotype. Microglia are unique among brain cells because they are derived from the fetal yolk sac during gestation (Ginhoux et al., 2010) and reflect the genetic sex of their fetal origin. Thus, XX vs. XY fetal microglia experience the same intrauterine environment prior to their migration into the CNS. This brings up the fascinating possibility that primitive microglia “carry the signal” of maternal environmental influences into the developing brain, and that the signal conveyed may be sexually dimorphic dependent on the genetic sex of the cell (and thus its response to a given intrinsic or extrinsic environmental cue). There is a precedent for this possibility within the placental immune response, which is sexually dimorphic-dependent on sex chromosomes of the fetal placental compartment (Clifton, 2010). Once arrived into the parenchyma, rapidly maturing fetal microglia are further impacted by the hormonal environment, specifically by increased testosterone around the day of birth in males, as demonstrated in this report.

Again, these scenarios remain speculative, but it seems clear that the data presented here, along with a handful of other reports, likely represent only the tip of the iceberg in sexually dimorphic microglial function within the developing brain. Given our increasing understanding of the unique ontogeny and critical roles of these fascinating cells in normal and abnormal brain development – along with the knowledge that virtually all neurological disorders demonstrate some degree of sexual dimorphism – sex of the cell is no longer something the field can continue to ignore.