Microglia Regulate Cell Genesis in a Sex-dependent Manner in the Neonatal Hippocampus.

Microglia, the innate immune cells of the brain, regulate brain development through many processes such as synaptic pruning, supporting cell genesis and phagocytosing living and dying cells. There are sex differences in these same developmental processes throughout the brain, thus microglia may contribute to brain sex differences. We examined whether microglia support a known sex difference in neonatal hippocampal neurogenesis and whether juvenile hippocampal neurogenesis was impacted by the loss of neonatal microglia. We used central infusion of liposomal clodronate to selectively deplete microglia and found decreased cell genesis in the male, but not female, dentate gyrus and hippocampus. We found that loss of microglia decreased cell genesis in the cortex and amygdala of both males and females. We assessed the expression of several cytokines and growth factors that have previously been shown to support cell genesis. We found that expression of Il1b and Tnf were decreased in the hippocampus due to microglia depletion however, there were no sex differences in the expression of any immune genes. In adolescence, there was an increase in the number of mitotic cells in the subgranular zone of the dentate gyrus of previously microglia depleted rats however, the number of newly-born neurons was unchanged in the adolescent animals. We also sought to determine whether there was a sex difference in the number of progenitor cells in the dentate gyrus in the neonatal period. We found no sex differences in the number of progenitor cells. Overall, these studies show that microglia are important for regulating region-specific sex differences in cell genesis in the developing brain.

Small cells with big implications: Microglia and sex differences in brain development, plasticity and behavioral health.

Brain sex differences are programmed largely by sex hormone secretions and direct sex chromosome effects in early life, and are subsequently modulated by early life experiences. The brain's resident immune cells, called microglia, actively contribute to brain development. Recent research has shown that microglia are sexually dimorphic, especially during early life, and may participate in sex-specific organization of the brain and behavior. Likewise, sex differences in immune cells and their signaling in the adult brain have been found, although in most cases their function remains unclear. Additionally, immune cells and their signaling have been implicated in many disorders in which brain development or plasticity is altered, including autism, schizophrenia, pain disorders, major depression, and postpartum depression. This review summarizes what is currently known about sex differences in neuroimmune function in development and during other major phases of brain plasticity, as well as the current state of knowledge regarding sex-specific neuroimmune function in psychiatric disorders.

Microglia and Beyond: Innate Immune Cells As Regulators of Brain Development and Behavioral Function.

Innate immune cells play a well-documented role in the etiology and disease course of many brain-based conditions, including multiple sclerosis, Alzheimer's disease, traumatic brain and spinal cord injury, and brain cancers. In contrast, it is only recently becoming clear that innate immune cells, primarily brain resident macrophages called microglia, are also key regulators of brain development. This review summarizes the current state of knowledge regarding microglia in brain development, with particular emphasis on how microglia during development are distinct from microglia later in life. We also summarize the effects of early life perturbations on microglia function in the developing brain, the role that biological sex plays in microglia function, and the potential role that microglia may play in developmental brain disorders. Finally, given how new the field of developmental neuroimmunology is, we highlight what has yet to be learned about how innate immune cells shape the development of brain and behavior.

Sex differences in microglial phagocytosis in the neonatal hippocampus.

Microglia regulate brain development through many processes, such as promoting neurogenesis, supporting cell survival, and phagocytizing progenitor, newly-born, and dying cells. Many of these same developmental processes show robust sex differences, yet very few studies have assessed sex differences in microglia function during development. Hormonally-induced sexual differentiation of the brain occurs during the perinatal period, thus we examined sex differences in microglial morphology, phagocytosis, and proliferation in the hippocampus during the early postnatal period. We found that the neonatal female hippocampus had significantly more microglia with phagocytic cups than the male hippocampus. We subsequently found that female microglia phagocytized more neural progenitor cells and healthy cells compared to males, but there were no sex differences in the number of newly-born or dying cells targeted by microglial phagocytosis. We found that the number of phagocytic microglia in females was reduced to male-typical levels by treatment with estradiol, the hormone responsible for masculinizing the rodent brain. Females also had higher expression of several phagocytic pathway genes in the hippocampus compared to males. In contrast to robust sex differences in phagocytic microglia, we found no sex differences in the number of microglia with amoeboid, transitioning, or ramified morphologies or differences in three-dimensional reconstructions of microglial morphology. While we did not find a baseline sex difference in microglial proliferation during or following the prenatal gonadal hormone surge in males, we found that estradiol treatment increased microglia proliferation in females. Overall, these data show that there are important sex differences in microglia function in the hippocampus during the early neonatal period.

Microglia depletion in early life programs persistent changes in social, mood-related, and locomotor behavior in male and female rats.

Microglia, the innate immune cells of the central nervous system, regulate brain development by promoting cell genesis, pruning synapses, and removing dying, newly-born or progenitor cells. However, the role of microglia in the early life programming of behavior under normal conditions is not well characterized. We used central infusion of liposomal clodronate to selectively deplete microglia from the neonatal rat brain and subsequently assessed the impact of microglial depletion on programming of juvenile and adult motivated behaviors. Liposomal clodronate treatment on postnatal days one and four led to greater than 70% loss of forebrain microglia by postnatal day 6 that lasted for approximately ten days. Neonatal microglia depletion led to reduced juvenile and adult anxiety behavior on the elevated plus maze and open field test, and increased locomotor activity. On a test of juvenile social play, microglial depletion led to decreased chase behaviors relative to control animals. There was no change in active social behavior in adults on a reciprocal social interaction test, but there was decreased passive interaction time and an increased number of social avoidance behaviors in clodronate treated rats relative to controls. There was an overall decrease in behavioral despair on the forced swim test in adult rats treated neonatally with clodronate. Females, but not males, treated neonatally with clodronate showed a blunted corticosterone response after acute stress in adulthood. These results show that microglia are important for the early life programming of juvenile and adult motivated behavior.

The immune system as a novel regulator of sex differences in brain and behavioral development.

Sexual differentiation of the brain occurs early in life as a result of sex-typical hormone action and sex chromosome effects. Immunocompetent cells are being recognized as underappreciated regulators of sex differences in brain and behavioral development, including microglia, astrocytes, and possibly other less well studied cell types, including T cells and mast cells. Immunocompetent cells in the brain are responsive to steroid hormones, but their role in sex-specific brain development is an emerging field of interest. This Review presents a summary of what is currently known about sex differences in the number, morphology, and signaling profile of immune cells in the developing brain and their role in the early-life programming of sex differences in brain and behavior. We review what is currently known about sex differences in the response to early-life perturbations, including stress, inflammation, diet, and environmental pollutants. We also discuss how and why understanding sex differences in the developing neuroimmune environment may provide insight into understanding the etiology of several neurodevelopmental disorders. This Review also highlights what remains to be discovered in this emerging field of developmental neuroimmunology and underscores the importance of filling in these knowledge gaps. © 2016 Wiley Periodicals, Inc.

Centrally Synthesized Estradiol Is a Potent Anti-Inflammatory in the Injured Zebra Finch Brain.

In homeotherms, injury to the brain, such as a penetrating wound, increases microglial cytokine expression and astroglial aromatase (estrogen synthase). In songbirds, injury-induced synthesis of estrogens is neuroprotective as aromatase inhibition and replacement with estradiol (E2) exacerbates and mitigates the extent of damage, respectively. The influence of induced aromatization on inflammation, however, remains unstudied. We hypothesized that injury-induced aromatization, via E2 synthesis, may affect neuroinflammation after a penetrating brain injury. Using adult zebra finches, we first documented an increase in the transcription of cytokines but not aromatase, 2 hours after the injury. Twenty-four hours after the injury, however, aromatase was dramatically elevated and cytokine expression had returned to baseline, suggesting that aromatization may be involved in the decrease of cytokines and neuroinflammation. In two subsequent experiments, we tested the influence of the inhibition of induced aromatization and aromatase inhibition with concomitant central E2 replacement on the transcription of the cytokines TNF-α, IL-1ß, and IL-6, the enzyme cyclooxygenase-2 (cox-2), and its product prostaglandin E2 (PGE2). Administration of fadrozole, an aromatase inhibitor, caused a sustained elevation of IL-1ß in females and TNF-α, cox-2, and PGE2 in both sexes. This prolonged neuroinflammation appears to be due to a failure to synthesize E2 locally because intracranial E2 replacement lowered IL-1ß in females, TNF-α in males, and cox-2 and PGE2 in both sexes. IL-6 was not affected by injury, aromatase inhibition, or E2 replacement in either sex. These data suggest that E2 synthesis after a penetrating brain injury is a potent and inducible anti-inflammatory signal, with specific modulation of discrete cytokine signaling.