Sexual differentiation of microglia

Brain-body mechanisms contribute to sexual dimorphism in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is the most common form of human motor neuron disease. It is characterized by the progressive degeneration of upper and lower motor neurons, leading to generalized motor weakness and, ultimately, respiratory paralysis and death within 3-5 years. The disease is shaped by genetics, age, sex and environmental stressors, but no cure or routine biomarkers exist for the disease. Male individuals have a higher propensity to develop ALS, and a different manifestation of the disease phenotype, than female individuals. However, the mechanisms underlying these sex differences remain a mystery. In this Review, we summarize the epidemiology of ALS, examine the sexually dimorphic presentation of the disease and highlight the genetic variants and molecular pathways that might contribute to sex differences in humans and animal models of ALS. We advance the idea that sexual dimorphism in ALS arises from the interactions between the CNS and peripheral organs, involving vascular, metabolic, endocrine, musculoskeletal and immune systems, which are strikingly different between male and female individuals. Finally, we review the response to treatments in ALS and discuss the potential to implement future personalized therapeutic strategies for the disease.

Microglia signaling in health and disease - Implications in sex-specific brain development and plasticity

Microglia, the intrinsic neuroimmune cells residing in the central nervous system (CNS), exert a pivotal influence on brain development, homeostasis, and functionality, encompassing critical roles during both aging and pathological states. Recent advancements in comprehending brain plasticity and functions have spotlighted conspicuous variances between male and female brains, notably in neurogenesis, neuronal myelination, axon fasciculation, and synaptogenesis. Nevertheless, the precise impact of microglia on sex-specific brain cell plasticity, sculpting diverse neural network architectures and circuits, remains largely unexplored. This article seeks to unravel the present understanding of microglial involvement in brain development, plasticity, and function, with a specific emphasis on microglial signaling in brain sex polymorphism. Commencing with an overview of microglia in the CNS and their associated signaling cascades, we subsequently probe recent revelations regarding molecular signaling by microglia in sex-dependent brain developmental plasticity, functions, and diseases. Notably, C-X3-C motif chemokine receptor 1 (CX3CR1), triggering receptors expressed on myeloid cells 2 (TREM2), calcium (Ca2+), and apolipoprotein E (APOE) emerge as molecular candidates significantly contributing to sex-dependent brain development and plasticity. In conclusion, we address burgeoning inquiries surrounding microglia's pivotal role in the functional diversity of developing and aging brains, contemplating their potential implications for gender-tailored therapeutic strategies in neurodegenerative diseases.

Oxidation enhances the toxicity of polyethylene microplastics to mouse eye: Perspective from in vitro and in vivo

Microplastics (MPs) are ubiquitously dispersed in the environment, and undergoing the process of oxidation that alters their physical and chemical properties. Eyes, which directly interface with the external milieu, inevitably encounter MPs. Nonetheless, the ophthalmic toxicity of MPs towards organisms remains unclear. In this study, primary mouse corneal epithelial cells (MCECs), C57BL/6 mice, and CX3CrlGFP/+ mice were utilized to evaluate the toxicity and differences between oxidized low-density polyethylene MPs (modified-MPs) and low-density polyethylene MPs (virgin-MPs) on eyes. The results manifested that virgin-MPs and modified-MPs could be endocytosed by primary MCECs, resulting in a range of cellular damage. Furthermore, they could diminish tear secretion, increase intraocular pressure, and could be internalized into cornea and retina in mice, instigating a series of detrimental reactions. Importantly, modified-MPs exhibited heightened toxicity towards mouse eyes, seemingly due to oxidation enhances the interaction between virgin-MPs/modified-MPs and tissues/cells, and leading to the release of toxic substances increased. In conclusion, our discoveries demonstrate that oxidation exacerbates the harm of virgin-MPs to eyes, and are of great significance for evaluating the risk of MPs to ocular health.

Microglial responses to inflammatory challenge in adult rats altered by developmental exposure to polychlorinated biphenyls in a sex-specific manner

Polychlorinated biphenyls are ubiquitous environmental contaminants linkedc with peripheral immune and neural dysfunction. Neuroimmune signaling is critical to brain development and later health; however, effects of PCBs on neuroimmune processes are largely undescribed. This study extends our previous work in neonatal or adolescent rats by investigating longer-term effects of perinatal PCB exposure on later neuroimmune responses to an inflammatory challenge in adulthood. Male and female Sprague-Dawley rats were exposed to a low-dose, environmentally relevant, mixture of PCBs (Aroclors 1242, 1248, and 1254, 1:1:1, 20 μg / kg dam BW per gestational day) or oil control during gestation and via lactation. Upon reaching adulthood, rats were given a mild inflammatory challenge with lipopolysaccharide (LPS, 50 μg / kg BW, ip) or saline control and then euthanized 3 hours later for gene expression analysis or 24 hours later for immunohistochemical labeling of Iba1+ microglia. PCB exposure did not alter gene expression or microglial morphology independently, but instead interacted with the LPS challenge in brain region- and sex-specific ways. In the female hypothalamus, PCB exposure blunted LPS responses of neuroimmune and neuromodulatory genes without changing microglial morphology. In the female prefrontal cortex, PCBs shifted Iba1+ cells from reactive to hyperramified morphology in response to LPS. Conversely, in the male hypothalamus, PCBs shifted cell phenotypes from hyperramified to reactive morphologies in response to LPS. The results highlight the potential for long-lasting effects of environmental contaminants that are differentially revealed over a lifetime, sometimes only after a secondary challenge. These neuroimmune endpoints are possible mechanisms for PCB effects on a range of neural dysfunction in adulthood, including mental health and neurodegenerative disorders. The findings suggest possible interactions with other environmental challenges that also influence neuroimmune systems.

Interleukin-22 receptor 1-mediated stimulation of T-type Ca2+ channels enhances sensory neuronal excitability through the tyrosine-protein kinase Lyn-dependent PKA pathway

BACKGROUND

Interleukin 24 (IL-24) has been implicated in the nociceptive signaling. However, direct evidence and the precise molecular mechanism underlying IL-24's role in peripheral nociception remain unclear.

METHODS

Using patch clamp recording, molecular biological analysis, immunofluorescence labeling, siRNA-mediated knockdown approach and behavior tests, we elucidated the effects of IL-24 on sensory neuronal excitability and peripheral pain sensitivity mediated by T-type Ca2+ channels (T-type channels).

RESULTS

IL-24 enhances T-type channel currents (T-currents) in trigeminal ganglion (TG) neurons in a reversible and dose-dependent manner, primarily by activating the interleukin-22 receptor 1 (IL-22R1). Furthermore, we found that the IL-24-induced T-type channel response is mediated through tyrosine-protein kinase Lyn, but not its common downstream target JAK1. IL-24 application significantly activated protein kinase A; this effect was independent of cAMP and prevented by Lyn antagonism. Inhibition of PKA prevented the IL-24-induced T-current response, whereas inhibition of protein kinase C or MAPK kinases had no effect. Functionally, IL-24 increased TG neuronal excitability and enhanced pain sensitivity to mechanical stimuli in mice, both of which were suppressed by blocking T-type channels. In a trigeminal neuropathic pain model induced by chronic constriction injury of the infraorbital nerve, inhibiting IL-22R1 signaling alleviated mechanical allodynia, which was reversed by blocking T-type channels or knocking down Cav3.2.

CONCLUSION

Our findings reveal that IL-24 enhances T-currents by stimulating IL-22R1 coupled to Lyn-dependent PKA signaling, leading to TG neuronal hyperexcitability and pain hypersensitivity. Understanding the mechanism of IL-24/IL-22R1 signaling in sensory neurons may pave the way for innovative therapeutic strategies in pain management.

Microgliosis: a double-edged sword in the control of food intake

Maintaining energy balance is essential for survival and health. This physiological function is controlled by the brain, which adapts food intake to energy needs. Indeed, the brain constantly receives a multitude of biological signals that are derived from digested foods or that originate from the gastrointestinal tract, energy stores (liver and adipose tissues) and other metabolically active organs (muscles). These signals, which include circulating nutrients, hormones and neuronal inputs from the periphery, collectively provide information on the overall energy status of the body. In the brain, several neuronal populations can specifically detect these signals. Nutrient-sensing neurons are found in discrete brain areas and are highly enriched in the hypothalamus. In turn, specialized brain circuits coordinate homeostatic responses acting mainly on appetite, peripheral metabolism, activity and arousal. Accumulating evidence shows that hypothalamic microglial cells located at the vicinity of these circuits can influence the brain control of energy balance. However, microglial cells could have opposite effects on energy balance, that is homeostatic or detrimental, and the conditions for this shift are not totally understood yet. One hypothesis relies on the extent of microglial activation, and nutritional lipids can considerably change it.

Modulation of Microglial Function by ATP-Gated P2X7 Receptors: Studies in Rat, Mice and Human

P2X receptors are a family of seven ATP-gated ion channels that trigger physiological and pathophysiological responses in a variety of cells. Five of the family members are sensitive to low concentrations of extracellular ATP, while the P2X6 receptor has an unknown affinity. The last subtype, the P2X7 receptor, is unique in requiring millimolar concentrations to fully activate in humans. This low sensitivity imparts the agonist with the ability to act as a damage-associated molecular pattern that triggers the innate immune response in response to the elevated levels of extracellular ATP that accompany inflammation and tissue damage. In this review, we focus on microglia because they are the primary immune cells of the central nervous system, and they activate in response to ATP or its synthetic analog, BzATP. We start by introducing purinergic receptors and then briefly consider the roles that microglia play in neurodevelopment and disease by referencing both original works and relevant reviews. Next, we move to the role of extracellular ATP and P2X receptors in initiating and/or modulating innate immunity in the central nervous system. While most of the data that we review involve work on mice and rats, we highlight human studies of P2X7R whenever possible.

Longitudinal cerebrospinal fluid measurements show glial hypo- and hyperactivation in predementia Alzheimer's disease

BACKGROUND

Brain innate immune activation is associated with Alzheimer's disease (AD), but degrees of activation may vary between disease stages. Thus, brain innate immune activation must be assessed in longitudinal clinical studies that include biomarker negative healthy controls and cases with established AD pathology. Here, we employ longitudinally sampled cerebrospinal fluid (CSF) core AD, immune activation and glial biomarkers to investigate early (predementia stage) innate immune activation levels and biomarker profiles.

METHODS

We included non-demented cases from a longitudinal observational cohort study, with CSF samples available at baseline (n = 535) and follow-up (n = 213), between 1 and 6 years from baseline (mean 2.8 years). We measured Aβ42/40 ratio, p-tau181, and total-tau to determine Ab (A+), tau-tangle pathology (T+), and neurodegeneration (N+), respectively. We classified individuals into these groups: A-/T-/N-, A+/T-/N-, A+/T+ or N+, or A-/T+ or N+. Using linear and mixed linear regression, we compared levels of CSF sTREM2, YKL-40, clusterin, fractalkine, MCP-1, IL-6, IL-1, IL-18, and IFN-γ both cross-sectionally and longitudinally between groups. A post hoc analysis was also performed to assess biomarker differences between cognitively healthy and impaired individuals in the A+/T+ or N+ group.

RESULTS

Cross-sectionally, CSF sTREM2, YKL-40, clusterin and fractalkine were higher only in groups with tau pathology, independent of amyloidosis (p < 0.001, A+/T+ or N+ and A-/T+ or N+, compared to A-/T-/N-). No significant group differences were observed for the cytokines CSF MCP-1, IL-6, IL-10, IL18 or IFN-γ. Longitudinally, CSF YKL-40, fractalkine and IFN-γ were all significantly lower in stable A+/T-/N- cases (all p < 0.05). CSF sTREM2, YKL-40, clusterin, fractalkine (p < 0.001) and MCP-1 (p < 0.05) were all higher in T or N+, with or without amyloidosis at baseline, but remained stable over time. High CSF sTREM2 was associated with preserved cognitive function within the A+/T+ or N+ group, relative to the cognitively impaired with the same A/T/N biomarker profile (p < 0.01).

CONCLUSIONS

Immune hypoactivation and reduced neuron-microglia communication are observed in isolated amyloidosis while activation and increased fractalkine accompanies tau pathology in predementia AD. Glial hypo- and hyperactivation through the predementia AD continuum suggests altered glial interaction with Ab and tau pathology, and may necessitate differential treatments, depending on the stage and patient-specific activation patterns.

Cystatin F (Cst7) drives sex-dependent changes in microglia in an amyloid-driven model of Alzheimer's disease

Microglial endolysosomal (dys)function is strongly implicated in neurodegenerative disease. Transcriptomic studies show that a microglial state characterised by a set of genes involved in endolysosomal function is induced in both mouse Alzheimer's disease (AD) models and human AD brain, and that the emergence of this state is emphasised in females. Cst7 (encoding cystatin F) is among the most highly upregulated genes in these microglia. However, despite such striking and robust upregulation, the function of Cst7 in neurodegenerative disease is not understood. Here, we crossed Cst7-/- mice with the AppNL-G-F mouse to test the role of Cst7 in a model of amyloid-driven AD. Surprisingly, we found that Cst7 plays a sexually dimorphic role regulating microglia in this model. In females, Cst7-/-AppNL-G-F microglia had greater endolysosomal gene expression, lysosomal burden, and amyloid beta (Aβ) burden in vivo and were more phagocytic in vitro. However, in males, Cst7-/-AppNL-G-F microglia were less inflammatory and had a reduction in lysosomal burden but had no change in Aβ burden. Overall, our study reveals functional roles for one of the most commonly upregulated genes in microglia across disease models, and the sex-specific profiles of Cst7-/--altered microglial disease phenotypes. More broadly, the findings raise important implications for AD including crucial questions on sexual dimorphism in neurodegenerative disease and the interplay between endolysosomal and inflammatory pathways in AD pathology.

CDNF overexpression prevents motor-cognitive dysfunction by intrastriatal CPP-based delivery system in a Parkinson's disease animal model

Parkinson's disease (PD) is characterized by the loss of dopaminergic neurons in the substantia nigra pars compact (SNpc), and no effective treatment has yet been established to prevent PD. Neurotrophic factors, such as cerebral dopamine neurotrophic factor (CDNF), have shown a neuroprotective effect on dopaminergic neurons. Previously, we developed a cell-penetrating-peptide-based delivery system that includes Asn194Lys mutation in the rabies virus glycoprotein-9R peptide (mRVG9R), which demonstrated a higher delivery rate than the wild-type. In this study, using a mouse PD-like model, we evaluated the intrastriatal mRVG9R-KP-CDNF gene therapy through motor and cognitive tests and brain cell analysis. The mRVG9R-KP-CDNF complex was injected into the striatum on days 0 and 20. To induce the PD-like model, mice were intraperitoneally administered Paraquat (PQ) twice a week for 6 weeks. Our findings demonstrate that mRVG9R-KP-CDNF gene therapy effectively protects brain cells from PQ toxicity and prevents motor and cognitive dysfunction in mice. We propose that the mRVG9R-KP-CDNF complex inhibits astrogliosis and microglia activation, safeguarding dopaminergic neurons and oligodendrocytes from PQ-induced damage. This study presents an efficient CDNF delivery system, protecting neurons and glia in the nigrostriatal pathway from PQ-induced damage, which is known to lead to motor and cognitive dysfunction in neurodegenerative diseases such as PD.

Epigenetic regulation of beta-endorphin synthesis in hypothalamic arcuate nucleus neurons modulates neuropathic pain in a rodent pain model

Although beta-endorphinergic neurons in the hypothalamic arcuate nucleus (ARC) synthesize beta-endorphin (β-EP) to alleviate nociceptive behaviors, the underlying regulatory mechanisms remain unknown. Here, we elucidated an epigenetic pathway driven by microRNA regulation of β-EP synthesis in ARC neurons to control neuropathic pain. In pain-injured rats miR-203a-3p was the most highly upregulated miRNA in the ARC. A similar increase was identified in the cerebrospinal fluid of trigeminal neuralgia patients. Mechanistically, we found histone deacetylase 9 was downregulated following nerve injury, which decreased deacetylation of histone H3 lysine-18, facilitating the binding of NR4A2 transcription factor to the miR-203a-3p gene promoter, thereby upregulating miR-203a-3p expression. Further, increased miR-203a-3p was found to maintain neuropathic pain by targeting proprotein convertase 1, an endopeptidase necessary for the cleavage of proopiomelanocortin, the precursor of β-EP. The identified mechanism may provide an avenue for the development of new therapeutic targets for neuropathic pain treatment.

Sex-oriented perspectives in immunopharmacology

Several immunopharmacological agents are effective in the treatment of cancer and immune-mediated conditions, with a favorable impact on life expectancy and clinical outcomes for a large number of patients. Nevertheless, response variation and undesirable effects of these drugs represent major issues, and overall efficacy remains unpredictable. Males and females show a distinct difference in immune system responses, with females generally mounting stronger responses to a variety of stimuli. Therefore, exploring sex differences in the efficacy and safety of immunopharmacological agents would strengthen the practice of precision medicine. As a pharmacological target highlight, programmed cell death 1 ligand 1 (PD-L1) is the first functionally characterized ligand of the coinhibitory programmed death receptor 1 (PD-1). The PD-L1/PD-1 crosstalk plays an important role in the immune response and is relevant in cancer, infectious and autoimmune disease. Sex differences in the response to immune checkpoint inhibitors are well documented, with male patients responding better than female patients. Similarly, higher efficacy of and adherence to tumor necrosis factor inhibitors in chronic inflammatory conditions including rheumatoid arthritis and Crohn's disease have been reported in male patients. The pharmacological basis of sex-specific responses to immune system modulating drugs is actively investigated in other settings such as stroke and type 1 diabetes. Advances in therapeutics targeting the endothelium could soon be wielded against autoimmunity and metabolic disorders. Based on the established sexual dimorphism in immune-related pathophysiology and disease presentation, sex-specific immunopharmacological protocols should be integrated into clinical guidelines.

The role of intracellular calcium-store-mediated calcium signals in in vivo sensor and effector functions of microglia

Under physiological conditions microglia, the immune sentinels of the brain, constantly monitor their microenvironment. In the case of danger, damage or cell/tissue dyshomeostasis, they react with changes in process motility, polarization, directed process movement, morphology and gene expression profile; release pro- and anti-inflammatory mediators; proliferate; and clean brain parenchyma by means of phagocytosis. Based on recent transcriptomic and in vivo Ca2+ imaging data, we argue that the local cell/tissue dyshomeostasis is sensed by microglia via intracellular Ca2+ signals, many of which are mediated by Ca2+ release from the intracellular Ca2+ stores. These signals encode the strength, duration and spatiotemporal pattern of the stimulus and, at the same time, relay this information further to trigger the respective Ca2+ -dependent effector pathways. We also point to the fact that microglial Ca2+ signalling is sexually dimorphic and undergoes profound changes across the organism's lifespan. Interestingly, the first changes in microglial Ca2+ signalling are visible already in 9- to 11-month-old mice, roughly corresponding to 40-year-old humans.

Schizophrenia in Women: Clinical Considerations

Men and women, for biologic and sociocultural reasons, differ in the nature of their risks for schizophrenia and also in their care needs. Women with schizophrenia have several reproduction-associated risks and care needs that require special clinical consideration. They also have several specific risks related to antipsychotics and gender-associated needs not necessarily related to biology. These require clinicians' diagnostic acumen, treatment skills, cultural sensitivity, and advocacy know-how. Although this does not pertain to everyone, awareness on the part of clinicians is essential. This article addresses the current evidence for difference.

Adiponectin receptor 1-mediated stimulation of Cav3.2 channels in trigeminal ganglion neurons induces nociceptive behaviors in mice

BACKGROUND

Adipokines, including adiponectin, are implicated in nociceptive pain; however, the underlying cellular and molecular mechanisms remain unknown.

METHODS

Using electrophysiological recording, immunostaining, molecular biological approaches and animal behaviour tests, we elucidated a pivotal role of adiponectin in regulating membrane excitability and pain sensitivity by manipulating Cav3.2 channels in trigeminal ganglion (TG) neurons.

RESULTS

Adiponectin enhanced T-type Ca2+ channel currents (IT) in TG neurons through the activation of adiponectin receptor 1 (adipoR1) but independently of heterotrimeric G protein-mediated signaling. Coimmunoprecipitation revealed a physical association between AdipoR1 and casein kinase II alpha-subunits (CK2α) in the TG, and inhibiting CK2 activity by chemical inhibitor or siRNA targeting CK2α prevented the adiponectin-induced IT response. Adiponectin significantly activated protein kinase C (PKC), and this effect was abrogated by CK2α knockdown. Adiponectin increased the membrane abundance of PKC beta1 (PKCβ1). Blocking PKCβ1 pharmacologically or genetically abrogated the adiponectin-induced IT increase. In heterologous expression systems, activation of adipoR1 induced a selective enhancement of Cav3.2 channel currents, dependent on PKCβ1 signaling. Functionally, adiponectin increased TG neuronal excitability and induced mechanical pain hypersensitivity, both attenuated by T-type channel blockade. In a trigeminal neuralgia model induced by chronic constriction injury of infraorbital nerve, blockade of adipoR1 signaling suppressed mechanical allodynia, which was prevented by silencing Cav3.2.

CONCLUSION

Our study elucidates a novel signaling cascade wherein adiponectin stimulates TG Cav3.2 channels via adipoR1 coupled to a novel CK2α-dependent PKCβ1. This process induces neuronal hyperexcitability and pain hypersensitivity. Insight into adipoR-Cav3.2 signaling in sensory neurons provides attractive targets for pain treatment.

A 14-day pulse of PLX5622 modifies α-synucleinopathy in preformed fibril-infused aged mice of both sexes

Reactive microglia are observed with aging and in Lewy body disorders, including within the olfactory bulb of men with Parkinson's disease. However, the functional impact of microglia in these disorders is still debated. Resetting these reactive cells by a brief dietary pulse of the colony-stimulating factor 1 receptor (CSF1R) inhibitor PLX5622 may hold therapeutic potential against Lewy-related pathologies. To our knowledge, withdrawal of PLX5622 after short-term exposure has not been tested in the preformed α-synuclein fibril (PFF) model, including in aged mice of both sexes. Compared to aged female mice, we report that aged males on the control diet showed higher numbers of phosphorylated α-synuclein+ inclusions in the limbic rhinencephalon after PFFs were injected in the posterior olfactory bulb. However, aged females displayed larger inclusion sizes compared to males. Short-term (14-day) dietary exposure to PLX5622 followed by control chow reduced inclusion numbers and levels of insoluble α-synuclein in aged males-but not females-and unexpectedly raised inclusion sizes in both sexes. Transient delivery of PLX5622 also improved spatial reference memory in PFF-infused aged mice, as evidenced by an increase in novel arm entries in a Y-maze. Superior memory was positively correlated with inclusion sizes but negatively correlated with inclusion numbers. Although we caution that PLX5622 delivery must be tested further in models of α-synucleinopathy, our data suggest that larger-sized-but fewer-α-synucleinopathic structures are associated with better neurological outcomes in PFF-infused aged mice.

Prenatal benzene exposure in mice alters offspring hypothalamic development predisposing to metabolic disease in later life

Maternal exposure to environmental contaminants during pregnancy poses a significant threat to a developing fetus, as these substances can easily cross the placenta and disrupt the neurodevelopment of offspring. Specifically, the hypothalamus is essential in the regulation of metabolism, notably during critical windows of development. An abnormal hormonal and inflammatory milieu during development can trigger persistent changes in the function of hypothalamic circuits, leading to long-lasting effects on the body's energy homeostasis and metabolism. We recently demonstrated that gestational exposure to clinically relevant levels of benzene induces severe metabolic dysregulation in the offspring. Given the central role of the hypothalamus in metabolic control, we hypothesized that prenatal exposure to benzene impacts hypothalamic development, contributing to the adverse metabolic effects in the offspring. C57BL/6JB dams were exposed to benzene at 50 ppm in the inhalation chambers exclusively during pregnancy (from E0.5 to E19). Transcriptomic analysis of the exposed offspring at postnatal day 21 (P21) revealed hypothalamic changes in genes related to metabolic regulation, inflammation, and neurodevelopment exclusively in males. Moreover, the hypothalamus of prenatally benzene-exposed male offspring displayed alterations in orexigenic and anorexigenic projections, impairments in leptin signaling, and increased microgliosis. Additional exposure to benzene during lactation did not promote further microgliosis or astrogliosis in the offspring, while the high-fat diet (HFD) challenge in adulthood exacerbated glucose metabolism and hypothalamic inflammation in benzene-exposed offspring of both sexes. These findings reveal the persistent adverse effects of prenatal benzene exposure on hypothalamic circuits and neuroinflammation, predisposing the offspring to long-lasting metabolic health conditions.

Steroid hormones: risk and resilience in women's Alzheimer disease

More women have Alzheimer disease (AD) than men, but the reasons for this phenomenon are still unknown. Including women in clinical research and studying their biology is key to understand not just their increased risk but also their resilience against the disease. In this sense, women are more affected by AD than men, but their reserve or resilience mechanisms might delay symptom onset. The aim of this review was to explore what is known about mechanisms underlying women's risk and resilience in AD and identify emerging themes in this area that merit further research. We conducted a review of studies analyzing molecular mechanisms that may induce neuroplasticity in women, as well as cognitive and brain reserve. We also analyzed how the loss of steroid hormones in aging may be linked to AD. We included empirical studies with human and animal models, literature reviews as well as meta-analyses. Our search identified the importance of 17-b-estradiol (E2) as a mechanism driving cognitive and brain reserve in women. More broadly, our analysis revealed the following emerging perspectives: (1) the importance of steroid hormones and their effects on both neurons and glia for the study of risk and resilience in AD, (2) E2's crucial role in women's brain reserve, (3) women's verbal memory advantage as a cognitive reserve factor, and (4) E2's potential role in linguistic experiences such as multilingualism and hearing loss. Future directions for research include analyzing the reserve mechanisms of steroid hormones on neuronal and glial plasticity, as well as identifying the links between steroid hormone loss in aging and risk for AD.

Serotonin sensing by microglia conditions the proper development of neuronal circuits and of social and adaptive skills

The proper maturation of emotional and sensory circuits requires fine-tuning of serotonin (5-HT) level during early postnatal development. Consistently, dysfunctions of the serotonergic system have been associated with neurodevelopmental psychiatric diseases, including autism spectrum disorders (ASD). However, the mechanisms underlying the developmental effects of 5-HT remain partially unknown, one obstacle being the action of 5-HT on different cell types. Here, we focused on microglia, which play a role in brain wiring refinement, and we investigated whether the control of these cells by 5-HT is relevant for neurodevelopment and spontaneous behaviors in mice. Since the main 5-HT sensor in microglia is the 5-HT2B receptor subtype, we prevented 5-HT signaling specifically in microglia by conditional invalidation of the Htr2b gene in these cells. We observed that abrogating the serotonergic control of microglia during early postnatal development affects the phagolysosomal compartment of these cells and their proximity to dendritic spines and perturbs neuronal circuits maturation. Furthermore, this early ablation of microglial 5-HT2B receptors leads to adult hyperactivity in a novel environment and behavioral defects in sociability and flexibility. Importantly, we show that these behavioral alterations result from a developmental effect, since they are not observed when microglial Htr2b invalidation is induced later, at P30 onward. Thus, a primary alteration of 5-HT sensing in microglia, during a critical time window between birth and P30, is sufficient to impair social and flexibility skills. This link between 5-HT and microglia may explain the association between serotonergic dysfunctions and behavioral traits like impaired sociability and inadaptability to novelty, which are prominent in psychiatric disorders such as ASD.

Sexually differentiated microglia and CA1 hippocampal synaptic connectivity

Microglia have been shown to sculpt postnatal circuitry from birth up to adulthood due to their role in both synapse formation, synaptic pruning, and the elimination of weak, redundant synapses. Microglia are differentiated in a sex-dependent manner. In this study, we tested whether sexual differentiation of microglia results in sex-dependent postnatal reorganization of CA1 synaptic connectivity in the hippocampus. The stereological counting of synapses in mice using electron microscopy showed a continuous rise in synapse density until the fourth week, followed by a plateau phase and loss of synapses from the eighth week onwards, with no difference between sexes. This course of alteration in synapse numbers did not differ between sexes. However, selectively, on postnatal day (P) 14 the density of synapses was significantly higher in the female than in the male hippocampus. Higher synapse density in females was paralleled by higher activity of microglia, as indicated by morphological changes, CD68 expression, and proximity of microglia to synaptic sites. In Thy1-GFP mice, consistent with increased synapse numbers, bouton density was also clearly increased in females at P14. At this time point, CD47 expression, the "don't eat me" signal of neurons, was similar in males and females. The decrease in bouton density thereafter in conjunction with increased synapse numbers argues for a role of microglia in the formation of multispine boutons (MSB). Our data in females at P14 support the regulatory role of microglia in synapse density. Sexual differentiation of microglia, however, does not substantially affect long-term synaptic reorganization in the hippocampus.

Chromosomal and gonadal factors regulate microglial sex effects in the aging brain

Biological sex contributes to phenotypic sex effects through genetic (sex chromosomal) and hormonal (gonadal) mechanisms. There are profound sex differences in the prevalence and progression of age-related brain diseases, including neurodegenerative diseases. Inflammation of neural tissue is one of the most consistent age-related phenotypes seen with healthy aging and disease. The pro-inflammatory environment of the aging brain has primarily been attributed to microglial reactivity and adoption of heterogeneous reactive states dependent upon intrinsic (i.e., sex) and extrinsic (i.e., age, disease state) factors. Here, we review sex effects in microglia across the lifespan, explore potential genetic and hormonal molecular mechanisms of microglial sex effects, and discuss currently available models and methods to study sex effects in the aging brain. Despite recent attention to this area, significant further research is needed to mechanistically understand the regulation of microglial sex effects across the lifespan, which may open new avenues for sex informed prevention and treatment strategies.

Genetic and Epigenetic Sexual Dimorphism of Brain Cells during Aging

In recent years, much of the attention paid to theoretical and applied biomedicine, as well as neurobiology, has been drawn to various aspects of sexual dimorphism due to the differences that male and female brain cells demonstrate during aging: (a) a dimorphic pattern of response to therapy for neurodegenerative disorders, (b) different age of onset and different degrees of the prevalence of such disorders, and (c) differences in their symptomatic manifestations in men and women. The purpose of this review is to outline the genetic and epigenetic differences in brain cells during aging in males and females. As a result, we hereby show that the presence of brain aging patterns in males and females is due to a complex of factors associated with the effects of sex chromosomes, which subsequently entails a change in signal cascades in somatic cells.

Sex-Specific Microglial Responses to Glucocerebrosidase Inhibition: Relevance to GBA1-Linked Parkinson's Disease

Microglia are heterogenous cells characterized by distinct populations each contributing to specific biological processes in the nervous system, including neuroprotection. To elucidate the impact of sex-specific microglia heterogenicity to the susceptibility of neuronal stress, we video-recorded with time-lapse microscopy the changes in shape and motility occurring in primary cells derived from mice of both sexes in response to pro-inflammatory or neurotoxic stimulations. With this morpho-functional analysis, we documented distinct microglia subpopulations eliciting sex-specific responses to stimulation: male microglia tended to have a more pro-inflammatory phenotype, while female microglia showed increased sensitivity to conduritol-B-epoxide (CBE), a small molecule inhibitor of glucocerebrosidase, the enzyme encoded by the GBA1 gene, mutations of which are the major risk factor for Parkinson's Disease (PD). Interestingly, glucocerebrosidase inhibition particularly impaired the ability of female microglia to enhance the Nrf2-dependent detoxification pathway in neurons, attenuating the sex differences observed in this neuroprotective function. This finding is consistent with the clinical impact of GBA1 mutations, in which the 1.5-2-fold reduced risk of developing idiopathic PD observed in female individuals is lost in the GBA1 carrier population, thus suggesting a sex-specific role for microglia in the etiopathogenesis of PD-GBA1.

Prenatal benzene exposure alters offspring hypothalamic development predisposing to metabolic disease in later life

The hypothalamus is essential in the regulation of metabolism, notably during critical windows of development. An abnormal hormonal and inflammatory milieu during development can trigger persistent changes in the function of hypothalamic circuits, leading to long-lasting effects on the body’s energy homeostasis and metabolism. We recently demonstrated that gestational exposure to benzene at smoking levels induces severe metabolic dysregulation in the offspring. Given the central role of the hypothalamus in metabolic control, we hypothesized that prenatal exposure to benzene impacts hypothalamic development, contributing to the adverse metabolic effects in the offspring. C57BL/6JB dams were exposed to benzene in the inhalation chambers exclusively during pregnancy (from E0.5 to E19). The transcriptome analysis of the offspring hypothalamus at postnatal day 21 (P21) revealed changes in genes related to metabolic regulation, inflammation, and neurodevelopment exclusively in benzene-exposed male offspring. Moreover, the hypothalamus of prenatally benzene-exposed male offspring displayed alterations in orexigenic and anorexigenic projections, impairments in leptin signaling, and increased microgliosis. Additional exposure to benzene during lactation did not promote further microgliosis or astrogliosis in the offspring, while the high-fat diet (HFD) challenge in adulthood exacerbated glucose metabolism and hypothalamic inflammation in benzene-exposed offspring of both sexes. These findings reveal the persistent impact of prenatal benzene exposure on hypothalamic circuits and neuroinflammation, predisposing the offspring to long-lasting metabolic health conditions.

Microglial homeostasis disruption modulates non-rapid eye movement sleep duration and neuronal activity in adult female mice

Sleep is a natural physiological state, tightly regulated through several neuroanatomical and neurochemical systems, which is essential to maintain physical and mental health. Recent studies revealed that the functions of microglia, the resident immune cells of the brain, differ along the sleep-wake cycle. Inflammatory cytokines, such as interleukin-1β and tumor necrosis factor-α, mainly produced by microglia in the brain, are also well-known to promote sleep. However, the contributing role of microglia on sleep regulation remains largely elusive, even more so in females. Given the higher prevalence of various sleep disorders in women, we aimed to determine the role of microglia in regulating the sleep-wake cycle specifically in female mice. Microglia were depleted in adult female mice with inhibitors of the colony-stimulating factor 1 receptor (CSF1R) (PLX3397 or PLX5622), which is required for microglial population maintenance. This led to a 65-73% reduction of the microglial population, as confirmed by immunofluorescence staining against IBA1 (marker of microglia/macrophages) and TMEM119 (microglia-specific marker) in the reticular nucleus of the thalamus and primary motor cortex. The spontaneous sleep-wake cycle was evaluated at steady-state, during microglial homeostasis disruption and after complete microglial repopulation, upon cessation of treatment with the inhibitors of CSF1R, using electroencephalography (EEG) and electromyography (EMG). We found that microglia-depleted female mice spent more time in non-rapid eye movement (NREM) sleep and had an increased number of NREM sleep episodes, which was partially restored after microglial total repopulation. To determine whether microglia could regulate sleep locally by modulating synaptic transmission, we used patch clamp to record spontaneous activity of pyramidal neurons in the primary motor cortex, which showed an increase of excitatory synaptic transmission during the dark phase. These changes in neuronal activity were modulated by microglial depletion in a phase-dependent manner. Altogether, our results indicate that microglia are involved in the sleep regulation of female mice, further strengthening their potential implication in the development and/or progression of sleep disorders. Furthermore, our findings indicate that microglial repopulation can contribute to normalizing sleep alterations caused by their partial depletion.

Structural and functional distinctions of co-resident microglia and monocyte-derived macrophages after retinal degeneration

BACKGROUND

Both resident microglia and invading peripheral immune cells can respond to injury and degeneration in the central nervous system. However, after dead and dying neurons have been cleared and homeostasis is re-established, it is unknown whether resident immune cells fully resume normal functions and to what degree the peripheral immune cells take up residence.

METHODS

Using flow cytometry, in vivo retinal imaging, immunohistochemistry, and single-cell mRNA sequencing, we assess resident microglia and monocyte-derived macrophages in the retina during and after the loss of photoreceptors in the Arr1^-/- mouse model of inducible degeneration.

RESULTS

We find that photoreceptor loss results in a small, sustained increase in mononuclear phagocytes and, after degeneration wanes, these cells re-establish a spatial mosaic reminiscent of healthy retinas. Transcriptomic analysis revealed the population remained unusually heterogeneous, with several subpopulations expressing gene patterns consistent with mildly activated phenotypes. Roughly a third of "new resident" cells expressed markers traditionally associated with both microglial and monocytic lineages, making their etiology ambiguous. Using an inducible Cre-based fluorescent lineage tracing paradigm to confirm the origins of new resident immune cells, we found approximately equal numbers of microglia and monocyte-derived macrophages after degeneration had subsided. In vivo retinal imaging and immunohistochemical analysis showed that both subpopulations remained functionally responsive to sites of local damage, though on average the monocyte-derived cells had less morphological complexity, expressed higher levels of MHCII, and had less migratory activity than the native resident population.

CONCLUSIONS

Monocytic cells that infiltrate the retina during degeneration differentiate into monocyte-derived macrophages that can remain in the retina long-term. These monocyte-derived macrophages adopt ramified morphologies and microglia-like gene expression. However, they remain distinguishable in morphology and gene expression from resident microglia and appear to differ functionally, showing less responsiveness to subsequent retinal injuries. These findings support the idea that persistent changes in the local immune population that occur in response to cell loss in aging and progressive retinal diseases may include the establishment of subpopulations of bone marrow-derived cells whose ability to respond to subsequent insults wanes over time.

Microglial diversity along the hippocampal longitudinal axis impacts synaptic plasticity in adult male mice under homeostatic conditions

The hippocampus is a plastic brain area that shows functional segregation along its longitudinal axis, reflected by a higher level of long-term potentiation (LTP) in the CA1 region of the dorsal hippocampus (DH) compared to the ventral hippocampus (VH), but the mechanisms underlying this difference remain elusive. Numerous studies have highlighted the importance of microglia-neuronal communication in modulating synaptic transmission and hippocampal plasticity, although its role in physiological contexts is still largely unknown. We characterized in depth the features of microglia in the two hippocampal poles and investigated their contribution to CA1 plasticity under physiological conditions. We unveiled the influence of microglia in differentially modulating the amplitude of LTP in the DH and VH, showing that minocycline or PLX5622 treatment reduced LTP amplitude in the DH, while increasing it in the VH. This was recapitulated in Cx3cr1 knockout mice, indicating that microglia have a key role in setting the conditions for plasticity processes in a region-specific manner, and that the CX3CL1-CX3CR1 pathway is a key element in determining the basal level of CA1 LTP in the two regions. The observed LTP differences at the two poles were associated with transcriptional changes in the expression of genes encoding for Il-1, Tnf-α, Il-6, and Bdnf, essential players of neuronal plasticity. Furthermore, microglia in the CA1 SR region showed an increase in soma and a more extensive arborization, an increased prevalence of immature lysosomes accompanied by an elevation in mRNA expression of phagocytic markers Mertk and Cd68 and a surge in the expression of microglial outward K⁺ currents in the VH compared to DH, suggesting a distinct basal phenotypic state of microglia across the two hippocampal poles. Overall, we characterized the molecular, morphological, ultrastructural, and functional profile of microglia at the two poles, suggesting that modifications in hippocampal subregions related to different microglial statuses can contribute to dissect the phenotypical aspects of many diseases in which microglia are known to be involved.

Identification of Sex-Specific Genetic Variants Associated With Tau PET

Background and Objectives

Important sex differences exist in tau pathology along the Alzheimer disease (AD) continuum, with women showing enhanced tau deposition compared with men, especially during the mild cognitive impairment (MCI) phase. This study aims to identify specific genetic variants associated with sex differences in regional tau aggregation, as measured with PET.

Methods

Four hundred ninety-three participants (women, n = 246; men, n = 247) who self-identified as White from the AD Neuroimaging Initiative study, with genotyping data and 18F-Flortaucipir tau PET data, were included irrespective of clinical diagnosis (cognitively normal [CN], MCI, and AD). We focused on the genetic variants within 10 genes previously shown to have sex-dependent effects on AD to reduce the burden of multiple comparisons: BIN1, MS4A6A, DNAJA2, FERMT2, APOC1, APOC1P1, FAM193B, C2orf47, TYW5, and CR1. Multivariate analysis of variance was applied to identify genetic variants associated with tau PET data in 3 regions of interests (composite regions of Braak I, Braak III/IV, and Braak V/VI stages) in women and men separately. We controlled for age, scanner manufacture, amyloid status, APOE ε4 carriership, diagnosis (CN vs MCI vs AD), and the first 10 genetic principal components to adjust for population stratification.

Results

We identified 3 genetic loci within 3 different genes associated with tau deposits specifically in women: rs79711283 within DNAJA2, rs113357081 within FERMT2, and rs74614106 within TYW5. In men, we also identified 3 loci within CR1 associated with tau deposits: rs115096248, rs113698814, and rs78150633.

Discussion

Our findings revealed sex-specific genetic variants associated with tau deposition independent of APOE ε4, amyloid status, and clinical diagnosis. These results provide potential molecular targets for understanding the mechanism of sex-specific tau aggregation and developing sex-specific gene-guided precision prevention or therapeutic interventions for AD.

Modeling CSF-1 receptor deficiency diseases - how close are we?

The role of colony-stimulating factor-1 receptor (CSF-1R) in macrophage and organismal development has been extensively studied in mouse. Within the last decade, mutations in the CSF1R have been shown to cause rare diseases of both pediatric (Brain Abnormalities, Neurodegeneration, and Dysosteosclerosis, OMIM #618476) and adult (CSF1R-related leukoencephalopathy, OMIM #221820) onset. Here we review the genetics, penetrance, and histopathological features of these diseases and discuss to what extent the animal models of Csf1r deficiency currently available provide systems in which to study the underlying mechanisms involved.

Brain region- and sex-specific transcriptional profiles of microglia

Microglia are resident macrophages of the brain, performing roles related to brain homeostasis, including modulation of synapses, trophic support, phagocytosis of apoptotic cells and debris, as well as brain protection and repair. Studies assessing morphological and transcriptional features of microglia found regional differences as well as sex differences in some investigated brain regions. However, markers used to isolate microglia in many previous studies are not expressed exclusively by microglia or cannot be used to identify and isolate microglia in all contexts. Here, fluorescent activated cell sorting was used to isolate cells expressing the microglia-specific marker TMEM119 from prefrontal cortex (PFC), striatum, and midbrain in mice. RNA-sequencing was used to assess the transcriptional profile of microglia, focusing on brain region and sex differences. We found striking brain region differences in microglia-specific transcript expression. Most notable was the distinct transcriptional profile of midbrain microglia, with enrichment for pathways related to immune function; these midbrain microglia exhibited a profile similar to disease-associated or immune-surveillant microglia. Transcripts more highly expressed in PFC isolated microglia were enriched for synapse-related pathways while microglia isolated from the striatum were enriched for pathways related to microtubule polymerization. We also found evidence for a gradient of expression of microglia-specific transcripts across the rostral-to-caudal axes of the brain, with microglia extracted from the striatum exhibiting a transcriptional profile intermediate between that of the PFC and midbrain. We also found sex differences in expression of microglia-specific transcripts in all 3 brain regions, with many selenium-related transcripts more highly expressed in females across brain regions. These results suggest that the transcriptional profile of microglia varies between brain regions under homeostatic conditions, suggesting that microglia perform diverse roles in different brain regions and even based on sex.

Microglial Priming in Infections and Its Risk to Neurodegenerative Diseases

Infectious diseases of different etiologies have been associated with acute and long-term neurological consequences. The primary cause of these consequences appears to be an inflammatory process characterized primarily by a pro-inflammatory microglial state. Microglial cells, the local effectors' cells of innate immunity, once faced by a stimulus, alter their morphology, and become a primary source of inflammatory cytokines that increase the inflammatory process of the brain. This inflammatory scenario exerts a critical role in the pathogenesis of neurodegenerative diseases. In recent years, several studies have shown the involvement of the microglial inflammatory response caused by infections in the development of neurodegenerative diseases. This has been associated with a transitory microglial state subsequent to an inflammatory response, known as microglial priming, in which these cells are more responsive to stimuli. Thus, systemic inflammation and infections induce a transitory state in microglia that may lead to changes in their state and function, making priming them for subsequent immune challenges. However, considering that microglia are long-lived cells and are repeatedly exposed to infections during a lifetime, microglial priming may not be beneficial. In this review, we discuss the relationship between infections and neurodegenerative diseases and how this may rely on microglial priming.

Significant Sex Differences in the Efficacy of the CSF1R Inhibitor-PLX5622 on Rat Brain Microglia Elimination

Microglia play pivotal roles in central nervous system development, homeostasis, responses to trauma, and neurodegenerative and neuropsychiatric disorders with significant sex-bias in their symptoms and prevalence. Survival of the microglia in adult brains depends on the expression of the colony-stimulating factor 1 receptor (CSF1R). The inhibition of CSF1R by brain-permeant PLX5622 in the chow eliminates, within 5-10 days, ~90% of the microglia in female and male mice, thereby enabling the investigation of the roles of the microglia in health and pathological mice models. Because of a prevailing "impression" that PLX5622 is ineffective in rats, it has hardly been used in studies of adult rats. Here, we report that effective microglia elimination by PLX5622-chow in rats is highly sex-dependent. Our observations provide missing information for the limited use and interpretation of PLX5622 in biomedical studies of the microglia in rat models. The sex differences that are too often overlooked must be carefully considered and clearly emphasized.

Critical Role of Maternal Selenium Nutrition in Neurodevelopment: Effects on Offspring Behavior and Neuroinflammatory Profile

Research in both animals and humans shows that some nutrients are important in pregnancy and during the first years of life to support brain and cognitive development. Our aim was to evaluate the role of selenium (Se) in supporting brain and behavioral plasticity and maturation. Pregnant and lactating female rats and their offspring up to postnatal day 40 were fed isocaloric diets differing in Se content—i.e., optimal, sub-optimal, and deficient—and neurodevelopmental, neuroinflammatory, and anti-oxidant markers were analyzed. We observed early adverse behavioral changes in juvenile rats only in sub-optimal offspring. In addition, sub-optimal, more than deficient supply, reduced basal glial reactivity in sex dimorphic and brain-area specific fashion. In female offspring, deficient and sub-optimal diets reduced the antioxidant Glutathione peroxidase (GPx) activity in the cortex and in the liver, the latter being the key organ regulating Se metabolism and homeostasis. The finding that the Se sub-optimal was more detrimental than Se deficient diet may suggest that maternal Se deficient diet, leading to a lower Se supply at earlier stages of fetal development, stimulated homeostatic mechanisms in the offspring that were not initiated by sub-optimal Se. Our observations demonstrate that even moderate Se deficiency during early life negatively may affect, in a sex-specific manner, optimal brain development.

Sex Differences in Poststroke Inflammation: a Focus on Microglia Across the Lifespan

Stroke is one of the leading causes of death worldwide and currently only few therapeutic options are available. Stroke is a sexually dimorphic disease contributing to the difficulty in finding efficient treatments. Poststroke neuroinflammation is geared largely by brain microglia and infiltrating peripheral immune cells and largely contributes to sex differences in the outcome of stroke. Microglia, since very early in the development, are sexually divergent, imprinting specific sex-related features. The diversity in terms of microglial density, morphology, and transcriptomic and proteomic profiles between sexes remains in the adulthood and is likely to contribute to the observed sex-differences on the postischemic inflammation. The impact of sexual hormones is fundamental: changes in terms of risk and severity have been observed for females before and after menopause underlining the importance of altered circulating sexual hormones. Moreover, aging is a driving force for changes that interact with sex, shifting the inflammatory response in a sex-dependent manner. This review summarizes the present literature on sex differences in stroke-induced inflammatory responses, with the focus on different microglial responses along lifespan.

Microglia in brain development and regeneration

It has recently emerged that microglia, the tissue-resident macrophages of the central nervous system, play significant non-innate immune roles to support the development, maintenance, homeostasis and repair of the brain. Apart from being highly specialized brain phagocytes, microglia modulate the development and functions of neurons and glial cells through both direct and indirect interactions. Thus, recognizing the elements that influence the homeostasis and heterogeneity of microglia in normal brain development is crucial to understanding the mechanisms that lead to early disease pathogenesis of neurodevelopmental disorders. In this Review, we discuss recent studies that have elucidated the physiological development of microglia and summarize our knowledge of their non-innate immune functions in brain development and tissue repair.

Exploring Sex-Related Differences in Microglia May Be a Game-Changer in Precision Medicine

One area of microglial biology that has been relatively neglected until recently is sex differences and this is in spite of the fact that sex is a risk factor in several diseases that are characterized by neuroinflammation and, by extension, microglial activation. Why these sex differences exist is not known but the panoply of differences extend to microglial number, genotype and phenotype. Significantly, several of these sex-related differences are also evident in health and change during life emphasizing the dynamic and plastic nature of microglia. This review will consider how age impacts on sex-related differences in microglia and ask whether the advancement of personalized medicine demands that a greater focus is placed on studying sex-related differences in microglia in Alzheimer's disease, Parkinson's disease and models of inflammatory stress and trauma in order to make true progress in dealing with these conditions.

Protein Expression of the Microglial Marker Tmem119 Decreases in Association With Morphological Changes and Location in a Mouse Model of Traumatic Brain Injury

The activation of microglia and the infiltration of macrophages are hallmarks of neuroinflammation after acute brain injuries, including traumatic brain injury (TBI). The two myeloid populations share many features in the post-injury inflammatory response, thus, being antigenically indistinguishable. Recently Tmem119, a type I transmembrane protein specifically expressed by microglia under physiological conditions, was proposed as a tool to differentiate resident microglia from blood-borne macrophages, not expressing it. However, the validity of Tmem119 as a specific marker of resident microglia in the context of acute brain injury, where microglia are activated and macrophages are recruited, needs validation. Our purpose was to investigate Tmem119 expression and distribution in relation to the morphology of brain myeloid cells present in the injured area after TBI. Mice underwent sham surgery or TBI by controlled cortical impact (CCI). Brains from sham-operated, or TBI mice, were analyzed by in situ hybridization to identify the cells expressing Tmem119, and by Western blot and quantitative immunofluorescence to measure Tmem119 protein levels in the entire brain regions and single cells. The morphology of Iba1+ myeloid cells was analyzed at different times (4 and 7 days after TBI) and several distances from the contused edge in order to associate Tmem119 expression with morphological evolution of active microglia. In situ hybridization indicated an increased Tmem119 RNA along with increased microglial complement C1q activation in the contused area and surrounding regions. On the contrary, the biochemical evaluation showed a drop in Tmem119 protein levels in the same areas. The Tmem119 immunoreactivity decreased in Iba1+ myeloid cells found in the contused cortex at both time points, with the cells showing the hypertrophic ameboid morphology having no Tmem119 expression. The Tmem119 was present on ramifications of resident microglia and its presence was decreased as a consequence of microglial activation in cortical areas close to contusion. Based on the data, we conclude that the decrease of Tmem119 in reactive microglia may depend on the process of microglial activation, which involves the retracting of their branchings to acquire an ameboid shape. The Tmem119 immunoreactivity decreases in reactive microglia to similar levels than the blood-borne macrophages, thus, failing to discriminate the two myeloid populations after TBI.

Running the Female Power Grid Across Lifespan Through Brain Estrogen Signaling

The role of central estrogen in cognitive, metabolic, and reproductive health has long fascinated the lay public and scientists alike. In the last two decades, insight into estrogen signaling in the brain and its impact on female physiology is beginning to catch up with the vast information already established for its actions on peripheral tissues. Using newer methods to manipulate estrogen signaling in hormone-sensitive brain regions, neuroscientists are now identifying the molecular pathways and neuronal subtypes required for controlling sex-dependent energy allocation. However, the immense cellular complexity of these hormone-sensitive brain regions makes it clear that more research is needed to fully appreciate how estrogen modulates neural circuits to regulate physiological and behavioral end points. Such insight is essential for understanding how natural or drug-induced hormone fluctuations across lifespan affect women's health.

Brain transcriptomic profiling reveals common alterations across neurodegenerative and psychiatric disorders

Neurodegenerative and neuropsychiatric disorders (ND-NPs) are multifactorial, polygenic and complex behavioral phenotypes caused by brain abnormalities. Large-scale collaborative efforts have tried to identify the genetic architecture of these conditions. However, the specific and shared underlying molecular pathobiology of brain illnesses is not clear. Here, we examine transcriptome-wide characterization of eight conditions, using a total of 2,633 post-mortem brain samples from patients with Alzheimer’s disease (AD), Parkinson’s disease (PD), Progressive Supranuclear Palsy (PSP), Pathological Aging (PA), Autism Spectrum Disorder (ASD), Schizophrenia (Scz), Major Depressive Disorder (MDD), and Bipolar Disorder (BP)–in comparison with 2,078 brain samples from matched control subjects.

Similar transcriptome alterations were observed between NDs and NPs with the top correlations obtained between Scz-BP, ASD-PD, AD-PD, and Scz-ASD. Region-specific comparisons also revealed shared transcriptome alterations in frontal and temporal lobes across NPs and NDs. Co-expression network analysis identified coordinated dysregulations of cell-type-specific modules across NDs and NPs. This study provides a transcriptomic framework to understand the molecular alterations of NPs and NDs through their shared- and specific gene expression in the brain.

Brain Transcriptome Responses to Dexamethasone Depending on Dose and Sex Reveal Factors Contributing to Sex-Specific Vulnerability to Stress-Induced Disorders

BACKGROUND

Glucocorticoid (GC) receptor (GR) signaling in the hypothalamus (Hyp) and in the superordinate limbic structures, such as the hippocampus (Hip), conveys feedback regulation of the neuroendocrine stress response and acts upon other neurobiological functions that ultimately influence mental health. These responses are strongly influenced by sex, but the molecular causes are still largely unexplored.

METHODS

To investigate GR targets and their GC sensitivity in the Hyp and Hip, we treated juvenile male and female piglets with 10 (D10) or 60 (D60) µg/kg dexamethasone (DEX), a selective GR agonist, and analyzed transcriptome responses compared to a saline control group using RNA sequencing.

RESULTS

Both doses influenced similar biological functions, including cellular response to lipid and immune cell-related functions, but the transcriptional response to D10 was considerably weaker, particularly in the Hip. Weighted Gene Co-expression Network Analysis revealed a network of genes coordinately regulated by DEX in both structures, among which the alpha-arrestin ARRDC2 takes a central position. Distinct functional groups of genes were differentially regulated by DEX between sexes depending on the dose; at D10, these included particularly mitochondrial genes, whereas at D60 interferon signaling and lipid homeostasis genes were enriched. The general and sex-specific transcriptional responses to DEX highlight microglia as the prominent target. Several key marker genes of disease-associated microglia were regulated by DEX depending on sex, such as TREM2 and LPL.

CONCLUSION

The discovered expression signatures suggest that DEX induced a dysfunctional state of microglia in males, while in females microglia were primed, which could entail predisposition for different mental disorders.

Insulin Resistance in the Brain: Evidence Supporting a Role for Inflammation, Reactive Microglia, and the Impact of Biological Sex

Increased intake of highly processed, energy-dense foods combined with a sedentary lifestyle are helping fuel the current overweight and obesity crisis, which is more prevalent in women than in men. Although peripheral organs such as adipose tissue contribute to the physiological development of obesity, emerging work aims to understand the role of the central nervous system to whole-body energy homeostasis and development of weight gain and obesity. The present review discusses the impact of insulin, insulin resistance, free fatty acids, and inflammation on brain function and how these differ between the males and females in the context of obesity. We highlight the potential of microglia, the resident immune cells in the brain, as mediators of neuronal insulin resistance that drive reduced satiety, increased food intake, and thus, obesity.

Estrogen receptors and the aging brain

The female sex hormone estrogen has been ascribed potent neuroprotective properties. It signals by binding and activating estrogen receptors that, depending on receptor subtype and upstream or downstream effectors, can mediate gene transcription and rapid non-genomic actions. In this way, estrogen receptors in the brain participate in modulating neural differentiation, proliferation, neuroinflammation, cholesterol metabolism, synaptic plasticity, and behavior. Circulating sex hormones decrease in the course of aging, more rapidly at menopause in women, and slower in men. This review will discuss what this drop entails in terms of modulating neuroprotection and resilience in the aging brain downstream of spatiotemporal estrogen receptor alpha (ERα) and beta (ERβ) signaling, as well as in terms of the sex differences observed in Alzheimer's disease (AD) and Parkinson's disease (PD). In addition, controversies related to ER expression in the brain will be discussed. Understanding the spatiotemporal signaling of sex hormones in the brain can lead to more personalized prevention strategies or therapies combating neurodegenerative diseases.

ZNF382 controls mouse neuropathic pain via silencer-based epigenetic inhibition of Cxcl13 in DRG neurons

Nerve injury-induced changes of gene expression in dorsal root ganglion (DRG) are critical for neuropathic pain genesis. However, how these changes occur remains elusive. Here we report the down-regulation of zinc finger protein 382 (ZNF382) in injured DRG neurons after nerve injury. Rescuing this down-regulation attenuates nociceptive hypersensitivity. Conversely, mimicking this down-regulation produces neuropathic pain symptoms, which are alleviated by C-X-C motif chemokine 13 (CXCL13) knockdown or its receptor CXCR5 knockout. Mechanistically, an identified cis-acting silencer at distal upstream of the Cxcl13 promoter suppresses Cxcl13 transcription via binding to ZNF382. Blocking this binding or genetically deleting this silencer abolishes the ZNF382 suppression on Cxcl13 transcription and impairs ZNF382-induced antinociception. Moreover, ZNF382 down-regulation disrupts the repressive epigenetic complex containing histone deacetylase 1 and SET domain bifurcated 1 at the silencer-promoter loop, resulting in Cxcl13 transcriptional activation. Thus, ZNF382 down-regulation is required for neuropathic pain likely through silencer-based epigenetic disinhibition of CXCL13, a key neuropathic pain player, in DRG neurons.

Smart biomaterials to enhance the efficiency of immunotherapy in glioblastoma: State of the art and future perspectives

Glioblastoma multiform (GBM) is considered as the most lethal tumor among CNS malignancies. Although immunotherapy has achieved remarkable advances in cancer treatment, it has not shown satisfactory results in GBM patients. Biomaterial science, along with nanobiotechnology, is able to optimize the efficiency of immunotherapy in these patients. They can be employed to provide the specific activation of immune cells in tumor tissue and combinational therapy as well as preventing systemic adverse effects resulting from hyperactivation of immune responses and off-targeting effect. Advance biomaterials in this field are classified into targeting nanocarriers and localized delivery systems. This review will offer an overview of immunotherapy strategies for glioblastoma and advance delivery systems for immunotherapeutics that may have a high potential in glioblastoma treatment.

Prophylactic Knockdown of the miR-183/96/182 Cluster Ameliorates Pseudomonas aeruginosa-Induced Keratitis

Purpose

Previously, we demonstrated that miR-183/96/182 cluster (miR-183C) knockout mice exhibit decreased severity of Pseudomonas aeruginosa (PA)-induced keratitis. This study tests the hypothesis that prophylactic knockdown of miR-183C ameliorates PA keratitis indicative of a therapeutic potential.

Methods

Eight-week-old miR-183C wild-type and C57BL/6J inbred mice were used. Locked nucleic acid-modified anti-miR-183C or negative control oligoribonucleotides with scrambled sequences (NC ORNs) were injected subconjunctivally 1 day before and then topically applied once daily for 5 days post-infection (dpi) (strain 19660). Corneal disease was graded at 1, 3, and 5 dpi. Corneas were harvested for RT-PCR, ELISA, immunofluorescence (IF), myeloperoxidase and plate count assays, and flow cytometry. Corneal nerve density was evaluated in flatmounted corneas by IF staining with anti-β-III tubulin antibody.

Results

Anti-miR-183C downregulated miR-183C in the cornea. It resulted in an increase in IL-1β at 1 dpi, which was decreased at 5 dpi; fewer polymorphonuclear leukocytes (PMNs) at 5 dpi; lower viable bacterial plate count at both 1 and 5 dpi; increased percentages of MHCII+ macrophages (Mϕ) and dendritic cells (DCs), consistent with enhanced activation/maturation; and decreased severity of PA keratitis. Anti-miR-183C treatment in the cornea of naïve mice resulted in a transient reduction of corneal nerve density, which was fully recovered one week after the last anti-miR application. miR-183C targets repulsive axon-guidance receptor molecule Neuropilin 1, which may mediate the effect of anti-miR-183C on corneal nerve regression.

Conclusions

Prophylactic miR-183C knockdown is protective against PA keratitis through its regulation of innate immunity, corneal innervation, and neuroimmune interactions.

Uncovering microglial pathways driving sex-specific neurobiological effects in stress and depression

Women suffer from major depressive disorder (MDD) more often than men and report greater MDD symptom severity. Mounting evidence suggests that sex differences in MDD may be driven, in part, by sex-specific neurobiological mechanisms. Chronic stress is a significant risk factor in MDD, and preclinical rodent models show differential patterns of stress-induced neural remodeling and cognitive-behavioral dysfunction in males and females. For instance, chronic stress leads to synapse loss in the medial prefrontal cortex in male rodents yet has either no effect on- or increases-synapse number in females. Recent reports have implicated microglia, the immune cells of the brain, in MDD, and findings demonstrate sex-specific microglial signatures in both preclinical stress models and MDD patients. Given that microglia can remodel neural architecture, modulate synaptic transmission, and affect subsequent changes in behavior, it is plausible that microglial pathways contribute to differential stress effects on neuroplasticity and function in males and females. As such, this review examines the evidence for sex-specific microglia-neuron interactions in preclinical stress models and in patients with MDD. Discoveries highlighted herein demonstrate divergent microglial contributions in males and females and suggest that future studies investigating stress-linked disorders should be guided by sex-dependent neurobiological and behavioral findings. Examining these pathways represents a clear avenue toward both a richer understanding of brain, behavior, and immunity, and innovative psychoneuroimmunology-based applications in personalized medicine.

Maintaining the Balance of Intestinal Flora through the Diet: Effective Prevention of Illness

The human body is home to a complex community of dynamic equilibrium microbiota, including bacteria, fungi, parasites, and viruses. It is known that the gut microbiome plays a crucial role in regulating innate and adaptive immune responses, intestinal peristalsis, intestinal barrier homeostasis, nutrient uptake, and fat distribution. The complex relationship between the host and microbiome suggests that when this relationship is out of balance, the microbiome may contribute to disease development. The brain-gut-microbial axis is composed of many signal molecules, gastrointestinal mucosal cells, the vagus nerve, and blood-brain barrier, which plays an essential role in developing many diseases. The microbiome can influence the central nervous system function through the brain-gut axis; the central nervous system can also affect the composition and partial functions of the gut microbiome in the same way. Different dietary patterns, specific dietary components, and functional dietary factors can significantly affect intestinal flora's structure, composition, and function, thereby affecting human health. Based on the above, this paper reviewed the relationship between diet, intestinal flora, and human health, and the strategies to prevent mental illness through the dietary modification of intestinal microorganisms.

Inhibition of microglial β-glucocerebrosidase hampers the microglia-mediated antioxidant and protective response in neurons

BACKGROUND

Homozygotic mutations in the GBA gene cause Gaucher's disease; moreover, both patients and heterozygotic carriers have been associated with 20- to 30-fold increased risk of developing Parkinson's disease. In homozygosis, these mutations impair the activity of β-glucocerebrosidase, the enzyme encoded by GBA, and generate a lysosomal disorder in macrophages, which changes morphology towards an engorged phenotype, considered the hallmark of Gaucher's disease. Notwithstanding the key role of macrophages in this disease, most of the effects in the brain have been attributed to the β-glucocerebrosidase deficit in neurons, while a microglial phenotype for these mutations has never been reported.

METHODS

We applied the bioluminescence imaging technology, immunohistochemistry and gene expression analysis to investigate the consequences of microglial β-glucocerebrosidase inhibition in the brain of reporter mice, in primary neuron/microglia cocultures and in cell lines. The use of primary cells from reporter mice allowed for the first time, to discriminate in cocultures neuronal from microglial responses consequent to the β-glucocerebrosidase inhibition; results were finally confirmed by pharmacological depletion of microglia from the brain of mice.

RESULTS

Our data demonstrate the existence of a novel neuroprotective mechanism mediated by a direct microglia-to-neuron contact supported by functional actin structures. This cellular contact stimulates the nuclear factor erythroid 2-related factor 2 activity in neurons, a key signal involved in drug detoxification, redox balance, metabolism, autophagy, lysosomal biogenesis, mitochondrial dysfunctions, and neuroinflammation. The central role played by microglia in this neuronal response in vivo was proven by depletion of the lineage in the brain of reporter mice. Pharmacological inhibition of microglial β-glucocerebrosidase was proven to induce morphological changes, to turn on an anti-inflammatory/repairing pathway, and to hinder the microglia ability to activate the nuclear factor erythroid 2-related factor 2 response, thus increasing the neuronal susceptibility to neurotoxins.

CONCLUSION

This mechanism provides a possible explanation for the increased risk of neurodegeneration observed in carriers of GBA mutations and suggest novel therapeutic strategies designed to revert the microglial phenotype associated with β-glucocerebrosidase inhibition, aimed at resetting the protective microglia-to-neuron communication.

Microglial heterogeneity in chronic pain

In this review, we report existing preclinical evidence on how the CNS compartment as well as sex affect microglia functions in health. We highlight that recent advances in transcriptomics analyses have led to thorough characterization of disease-associated microglial states in mice and humans. We then consider the specific scenario of peripheral nerve or tissue injury which induce expression of a specific subset of genes in microglia in the dorsal horn of the spinal cord. We suggest the intriguing possibility that future studies may disclose the existence of a unique microglia transcriptional profile that is associated with chronic pain conditions. We also collect evidence that microglial activation in pain-related areas of the brain can be observed in models of neuropathic pain in agreement with recent neuroimaging studies in chronic pain patients. Based on the evidence discussed here, we predict that future studies on the neuroimmune interactions in chronic pain should complement our current understanding of microglia functions, but also adventure in using novel approaches such as scRNA-seq, spatial transcriptomics, CYTOF and transmission electron microscopy to provide a more complete characterization of the function, transcriptome and structure of microglia in chronic pain.

CD22 Blockage Restores Age-Related Impairments of Microglia Surveillance Capacity

Microglia, the innate immune cells of the brain, are essential for maintaining homeostasis by their ramified, highly motile processes and for orchestrating the immune response to pathological stimuli. They are implicated in several neurodegenerative diseases like Alzheimer's and Parkinson's disease. One commonality of these diseases is their strong correlation with aging as the highest risk factor and studying age-related alterations in microglia physiology and associated signaling mechanism is indispensable for a better understanding of age-related pathomechanisms. CD22 has been identified as a modifier of microglia phagocytosis in a recent study, but not much is known about the function of CD22 in microglia. Here we show that CD22 surface levels are upregulated in aged versus adult microglia. Furthermore, in the amyloid mouse model PS2APP, Aβ-containing microglia also exhibit increased CD22 signal. To assess the impact of CD22 blockage on microglia morphology and dynamics, we have established a protocol to image microglia process motility in acutely prepared brain slices from CX3CR1-GFP reporter mice. We observed a significant reduction of microglial ramification and surveillance capacity in brain slices from aged versus adult mice. The age-related decrease in surveillance can be restored by antibody-mediated CD22 blockage in aged mice, whereas surveillance in adult mice is not affected by CD22 inhibition. Moreover to complement the results obtained in mice, we show that human iPSC-derived macrophages exhibit an increased phagocytic capacity upon CD22 blockage. Downstream analysis of antibody-mediated CD22 inhibition revealed an influence on BMP and TGFβ associated gene networks. Our results demonstrate CD22 as a broad age-associated modulator of microglia functionality with potential implications for neurodegenerative disorders.