Microglia in CNS development: Shaping the brain for the future

Proteomic analysis of P. gingivalis-Lipopolysaccharide induced neuroinflammation in SH-SY5Y and HMC3 cells

Chronic periodontitis and its keystone pathogen, Porphyromonas gingivalis, have increasingly been linked with Alzheimer's disease (AD). However, P.gingivalis-lipopolysaccharide (LPS) mediated release of neuroinflammatory proteins contributes to AD remains underexplored. In this study, we utilized data-independent acquisition mass spectrometry to characterize P.gingivalis-LPS induced profile of differentially expressed proteins associated with the neuroinflammatory response in human neuroblastoma (SH-SY5Y) and human microglial (HMC3) cells. We reported a set of 124 proteins in SH-SY5Y cells and 96 proteins in HMC3 cells whose levels were significantly upregulated or downregulated by exposure to P. gingivalis-LPS. Our findings demonstrate that P. gingivalis-LPS contributed to the elevated expressions of dementia biomarkers and pro-inflammatory cytokines that include APP, Aβ1-42, Aβ1-40, T-Tau, p-Tau, VEGF, TGF-β, IL-1β, IL-6 and TNF-α through 2 distinct pathways of extracellular sensing by cell surface receptors and intracellular cytosolic receptors. Interestingly, intracellular signaling proteins activated with P. gingivalis-LPS transfection using Lipofectamine™ 2000 had significantly higher fold change protein expression compared to the extracellular signaling with P. gingivalis-LPS treatment. Additionally, we also explored P. gingivalis-LPS mediated activation of caspase-4 dependent non canonical inflammasome pathway in both SH-SY5Y and HMC3 cells. In summary, P. gingivalis-LPS induced neuroinflammatory protein expression in SH-SY5Y and HMC3 cells, provided insights into the specific inflammatory pathways underlying the potential link between P. gingivalis-LPS infection and the pathogenesis of Alzheimer's disease and related dementias.

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.

Maternal social environment shapes yolk testosterone allocation and embryonic neural gene expression in tree swallows

Offspring from females breeding in competitive social environments are often exposed to more testosterone (T) during embryonic development, which can affect traits from growth to behavior in potentially adaptive ways. Despite the important role of maternally derived steroids in shaping offspring development, the molecular mechanisms driving these processes are currently unclear. Here, we use tree swallows (Tachycineta bicolor) to explore the effects of the maternal social environment on yolk T concentrations and genome-wide patterns of neural gene expression in embryos. We measured aggressive interactions among females breeding at variable densities and collected their eggs at two timepoints, including the day laid to measure yolk T concentrations and on embryonic day 11 to measure gene expression in whole brain samples. We found that females breeding in high-density sites experienced elevated rates of physical aggression and their eggs had higher yolk T concentrations. A differential gene expression and weighted gene co-expression network analysis indicated that embryos from high-density sites experienced an upregulation of genes involved in hormone, circulatory, and immune processes, and these gene expression patterns were correlated with yolk T levels and aggression. Genes implicated in neural development were additionally downregulated in embryos from high-density sites. These data highlight how early neurogenomic processes may be affected by the maternal social environment, giving rise to phenotypic plasticity in offspring.

Maternal SARS-CoV-2 impacts fetal placental macrophage programs and placenta-derived microglial models of neurodevelopment

BACKGROUND

The SARS-CoV-2 virus activates maternal and placental immune responses. Such activation in the setting of other infections during pregnancy is known to impact fetal brain development. The effects of maternal immune activation on neurodevelopment are mediated at least in part by fetal brain microglia. However, microglia are inaccessible for direct analysis, and there are no validated non-invasive surrogate models to evaluate in utero microglial priming and function. We have previously demonstrated shared transcriptional programs between microglia and Hofbauer cells (HBCs, or fetal placental macrophages) in mouse models.

METHODS AND RESULTS

We assessed the impact of maternal SARS-CoV-2 on HBCs isolated from 24 term placentas (N = 10 SARS-CoV-2 positive cases, 14 negative controls). Using single-cell RNA-sequencing, we demonstrated that HBC subpopulations exhibit distinct cellular programs, with specific subpopulations differentially impacted by SARS-CoV-2. Assessment of differentially expressed genes implied impaired phagocytosis, a key function of both HBCs and microglia, in some subclusters. Leveraging previously validated models of microglial synaptic pruning, we showed that HBCs isolated from placentas of SARS-CoV-2 positive pregnancies can be transdifferentiated into microglia-like cells (HBC-iMGs), with impaired synaptic pruning behavior compared to HBC models from negative controls.

CONCLUSION

These findings suggest that HBCs isolated at birth can be used to create personalized cellular models of offspring microglial programming.

Prenatal Immune Challenge Differentiates the Effect of Aripiprazole and Risperidone on CD200-CD200R and CX3CL1-CX3CR1 Dyads and Microglial Polarization: A Study in Organotypic Cortical Cultures

Microglia are the primary innate immune cells of the central nervous system and extensively contribute to brain homeostasis. Dysfunctional or excessive activity of microglia may be associated with several neuropsychiatric disorders, including schizophrenia. Therefore, we examined whether aripiprazole and risperidone could influence the expression of the Cd200-Cd200r and Cx3cl1-Cx3cr1 axes, which are crucial for the regulation of microglial activity and interactions of these cells with neurons. Additionally, we evaluated the impact of these drugs on microglial pro- and anti-inflammatory markers (Cd40, Il-1β, Il-6, Cebpb, Cd206, Arg1, Il-10 and Tgf-β) and cytokine release (IL-6, IL-10). The research was executed in organotypic cortical cultures (OCCs) prepared from the offspring of control rats (control OCCs) or those exposed to maternal immune activation (MIA OCCs), which allows for the exploration of schizophrenia-like disturbances in animals. All experiments were performed under basal conditions and after additional stimulation with lipopolysaccharide (LPS), following the "two-hit" hypothesis of schizophrenia. We found that MIA diminished the mRNA level of Cd200r and affected the OCCs' response to additional LPS exposure in terms of this parameter. LPS downregulated the Cx3cr1 expression and profoundly changed the mRNA levels of pro- and anti-inflammatory microglial markers in both types of OCCs. Risperidone increased Cd200 expression in MIA OCCs, while aripiprazole treatment elevated the gene levels of the Cx3cl1-Cx3cr1 dyad in control OCCs. The antipsychotics limited the LPS-generated increase in the expression of proinflammatory factors (Il-1β and Il-6) and enhanced the mRNA levels of anti-inflammatory components (Cd206 and Tgf-β) of microglial polarization, mostly in the absence of the MIA procedure. Finally, we observed a more pronounced modulating impact of aripiprazole on the expression of pro- and anti-inflammatory cytokines when compared to risperidone in MIA OCCs. In conclusion, our data suggest that MIA might influence microglial activation and crosstalk of microglial cells with neurons, whereas aripiprazole and risperidone could beneficially affect these changes in OCCs.

Multilineage-differentiating stress-enduring cells: a powerful tool for tissue damage repair

Multilineage-differentiating stress-enduring (Muse) cells are a type of pluripotent cell with unique characteristics such as non-tumorigenic and pluripotent differentiation ability. After homing, Muse cells spontaneously differentiate into tissue component cells and supplement damaged/lost cells to participate in tissue repair. Importantly, Muse cells can survive in injured tissue for an extended period, stabilizing and promoting tissue repair. In addition, it has been confirmed that injection of exogenous Muse cells exerts anti-inflammatory, anti-apoptosis, anti-fibrosis, immunomodulatory, and paracrine protective effects in vivo. The discovery of Muse cells is an important breakthrough in the field of regenerative medicine. The article provides a comprehensive review of the characteristics, sources, and potential mechanisms of Muse cells for tissue repair and regeneration. This review serves as a foundation for the further utilization of Muse cells as a key clinical tool in regenerative medicine.

Loss of Function in the Neurodevelopmental Disease and Schizophrenia-Associated Gene CYFIP1 in Human Microglia-like Cells Supports a Functional Role in Synaptic Engulfment

BACKGROUND

The CYFIP1 gene, located in the neurodevelopmental risk locus 15q11.2, is highly expressed in microglia, but its role in human microglial function as it relates to neurodevelopment is not well understood.

METHODS

We generated multiple CRISPR (clustered regularly interspaced short palindromic repeat) knockouts of CYFIP1 in patient-derived models of microglia to characterize function and phenotype. Using microglia-like cells reprogrammed from peripheral blood mononuclear cells, we quantified phagocytosis of synaptosomes (isolated and purified synaptic vesicles) from human induced pluripotent stem cell (iPSC)-derived neuronal cultures as an in vitro model of synaptic pruning. We repeated these analyses in human iPSC-derived microglia-like cells derived from 3 isogenic wild-type/knockout line pairs derived from 2 donors and further characterized microglial development and function through morphology and motility.

RESULTS

CYFIP1 knockout using orthogonal CRISPR constructs in multiple patient-derived cell lines was associated with a statistically significant decrease in synaptic vesicle phagocytosis in microglia-like cell models derived from both peripheral blood mononuclear cells and iPSCs. Morphology was also shifted toward a more ramified profile, and motility was significantly reduced. However, iPSC-CYFIP1 knockout lines retained the ability to differentiate to functional microglia.

CONCLUSIONS

The changes in microglial phenotype and function due to the loss of function of CYFIP1 observed in this study implicate a potential impact on processes such as synaptic pruning that may contribute to CYFIP1-related neurodevelopmental disorders. Investigating risk genes in a range of central nervous system cell types, not solely neurons, may be required to fully understand the way in which common and rare variants intersect to yield neuropsychiatric disorders.

Continuous low-level dietary exposure to glyphosate elicits dose and sex-dependent synaptic and microglial adaptations in the rodent brain

Prolonged exposure to low levels of dietary contaminants is a context in modern life that could alter organ physiology gradually. Here, we aimed to investigate the impact of continuous exposure to acceptable daily intake (ADI) and non-observable adverse effect level (NOAEL) of glyphosate from gestation to adulthood using C57BL/6J mice and incorporating these levels into their food pellets. From adulthood, we analyzed neurophysiological and neuro-glia cellular adaptations in male and female animals. Using ex-vivo hippocampal slice electrophysiology, we found a reduced efficacy of Schaffer collateral-to-CA1 excitatory synapses in glyphosate-exposed dietary conditions, with ADI and NOAEL dose-dependent effects. Short-term facilitation of excitatory synaptic transmission was specifically increased in NOAEL conditions, with a predominant influence in males, suggesting a reduced probability of neurotransmitter release. Long-term synaptic potentiation (LTP) was decreased in NOAEL-exposed mice. Next, we explore whether these neurophysiological modifications are associated with neuro-glia changes in the somatosensory cortex and hippocampus. High-resolution confocal microscopy analyses unveil a dose-dependent increased density of excitatory Vglut1+ Homer1+ synapses. Microglial Iba1+ cells displayed a shortening of their ramifications, a sign of cellular reactivity that was more pronounced in males at NOAEL levels. The morphology of GFAP+ astrocytes was generally not modified. Finally, we asked whether mouse-specific cross-correlations exist among all data sets generated. This examination included the novel object recognition (NOR) test performed before ex vivo functional and immunohistochemical examinations. We report a negative linear regression between the number of synapses and NOR or LTP maintenance when plotting ADI and NOAEL datasets. These results outline synaptic and microglial cell adaptations resulting from prenatal and continuous dietary low levels of glyphosate, discernible in, but not limited to, adult males exposed to the NOAEL. We discuss the potential significance of these findings to real-world consumer situations and long-term brain resilience.

Neuroprotective effects of gemfibrozil in neurological disorders: Focus on inflammation and molecular mechanisms

BACKGROUND

Gemfibrozil (Gem) is a drug that has been shown to activate PPAR-α, a nuclear receptor that plays a key role in regulating lipid metabolism. Gem is used to lower the levels of triglycerides and reduce the risk of coronary heart disease in patients. Experimental studies in vitro and in vivo have shown that Gem can prevent or slow the progression of neurological disorders (NDs), including cerebral ischemia (CI), Alzheimer's disease (AD), Parkinson's disease (PD), and multiple sclerosis (MS). Neuroinflammation is known to play a significant role in these disorders.

METHOD

The literature review for this study was conducted by searching Scopus, Science Direct, PubMed, and Google Scholar databases.

RESULT

The results of this study show that Gem has neuroprotective effects through several cellular and molecular mechanisms such as: (1) Gem has the ability to upregulate pro-survival factors (PGC-1α and TFAM), promoting the survival and function of mitochondria in the brain, (2) Gem strongly inhibits the activation of NF-κB, AP-1, and C/EBPβ in cytokine-stimulated astroglial cells, which are known to increase the expression of iNOS and the production of NO in response to proinflammatory cytokines, (3) Gem protects dopamine neurons in the MPTP mouse model of PD by increasing the expression of PPARα, which in turn stimulates the production of GDNF in astrocytes, (4) Gem reduces amyloid plaque pathology, reduces the activity of glial cells, and improves memory, (5) Gem increases myelin genes expression (MBP and CNPase) via PPAR-β, and (6) Gem increases hippocampal BDNF to counteract depression.

CONCLUSION

According to the study, Gem was investigated for its potential therapeutic effect in NDs. Further research is needed to fully understand the therapeutic potential of Gem in NDs.

Neuron-derived exosomes mediate sevoflurane-induced neurotoxicity in neonatal mice via transferring lncRNA Gas5 and promoting M1 polarization of microglia

Sevoflurane exposure during rapid brain development induces neuronal apoptosis and causes memory and cognitive deficits in neonatal mice. Exosomes that transfer genetic materials including long non-coding RNAs (lncRNAs) between cells play a critical role in intercellular communication. However, the lncRNAs found in exosomes derived from neurons treated with sevoflurane and their potential role in promoting neurotoxicity remain unknown. In this study, we investigated the role of cross-talk of newborn mouse neurons with microglial cells in sevoflurane-induced neurotoxicity. Mouse hippocampal neuronal HT22 cells were exposed to sevoflurane, and then co-cultured with BV2 microglial cells. We showed that sevoflurane treatment markedly increased the expression of the lncRNA growth arrest-specific 5 (Gas5) in neuron-derived extracellular vesicles, which inhibited neuronal proliferation and induced neuronal apoptosis by promoting M1 polarization of microglia and the release of inflammatory cytokines. We further revealed that the exosomal lncRNA Gas5 significantly upregulated Foxo3 as a competitive endogenous RNA of miR-212-3p in BV2 cells, and activated the NF-κB pathway to promote M1 microglial polarization and the secretion of inflammatory cytokines, thereby exacerbating neuronal damage. In neonatal mice, intracranial injection of the exosomes derived from sevoflurane-treated neurons into the bilateral hippocampi significantly increased the proportion of M1 microglia, inhibited neuronal proliferation and promoted apoptosis, ultimately leading to neurotoxicity. Similar results were observed in vitro in BV2 cells treated with the CM from HT22 cells after sevoflurane exposure. We conclude that sevoflurane induces the transfer of lncRNA Gas5-containing exosomes from neurons, which in turn regulates the M1 polarization of microglia and contributes to neurotoxicity. Thus, modulating the expression of lncRNA Gas5 or the secretion of exosomes could be a strategy for addressing sevoflurane-induced neurotoxicity.

Maternal SARS-CoV-2 impacts fetal placental macrophage programs and placenta-derived microglial models of neurodevelopment

The SARS-CoV-2 virus activates maternal and placental immune responses, which in the setting of other infections occurring during pregnancy are known to impact fetal brain development. The effects of maternal immune activation on neurodevelopment are mediated at least in part by fetal brain microglia. However, microglia are inaccessible for direct analysis, and there are no validated non-invasive surrogate models to evaluate in utero microglial priming and function. We have previously demonstrated shared transcriptional programs between microglia and Hofbauer cells (HBCs, or fetal placental macrophages) in mouse models. Here, we assessed the impact of maternal SARS-CoV-2 on HBCs isolated from term placentas using single-cell RNA-sequencing. We demonstrated that HBC subpopulations exhibit distinct cellular programs, with specific subpopulations differentially impacted by SARS-CoV-2. Assessment of differentially expressed genes implied impaired phagocytosis, a key function of both HBCs and microglia, in some subclusters. Leveraging previously validated models of microglial synaptic pruning, we showed that HBCs isolated from placentas of SARS-CoV-2 positive pregnancies can be transdifferentiated into microglia-like cells, with altered morphology and impaired synaptic pruning behavior compared to HBC models from negative controls. These findings suggest that HBCs isolated at birth can be used to create personalized cellular models of offspring microglial programming.

Central respiratory command and microglia: An early-life partnership

Microglia, brain-resident macrophages, are key players in brain development, regulating synapse density, shaping neural circuits, contributing to plasticity, and maintaining nervous tissue homeostasis. These functions are ensured from early prenatal development until maturity, in normal and pathological states of the central nervous system. Microglia dysfunction can be involved in several neurodevelopmental disorders, some of which are associated with respiratory deficits. Breathing is a rhythmic motor behavior generated and controlled by hindbrain neuronal networks. The operation of the central respiratory command relies on the proper development of these rhythmogenic networks, formation of their appropriate interactions, and their lifelong constant adaptation to physiological needs. This review, focusing exclusively on the perinatal period, outlines recent advances obtained in rodents in determining the roles of microglia in the establishment and functioning of the respiratory networks and their involvement in certain pathologies.

Microglial transglutaminase 2 deficiency causes impaired synaptic remodelling and cognitive deficits in mice

Microglia are the primary source of transglutaminase 2 (TGM2) in the brain; however, the roles of microglial TGM2 in neural development and disease are still not well known. The aim of this study is to elucidate the role and mechanisms of microglial TGM2 in the brain. A mouse line with a specific knockout of Tgm2 in microglia was generated. Immunohistochemistry, Western blot and qRT-PCR assays were performed to evaluate the expression levels of TGM2, PSD-95 and CD68. Confocal imaging, immunofluorescence staining and behavioural analyses were conducted to identify phenotypes of microglial TGM2 deficiency. Finally, RNA sequencing, qRT-PCR and co-culture of neurons and microglia were used to explore the potential mechanisms. Deletion of microglial Tgm2 causes impaired synaptic pruning, reduced anxiety and increased cognitive deficits in mice. At the molecular level, the phagocytic genes, such as Cq1a, C1qb and Tim4, are significantly down-regulated in TGM2-deficient microglia. This study elucidates a novel role of microglial TGM2 in regulating synaptic remodelling and cognitive function, indicating that microglia Tgm2 is essential for proper neural development.

Maternal immune activation and role of placenta in the prenatal programming of neurodevelopmental disorders

Maternal infection during pregnancy, leading to maternal immune activation (mIA) and cytokine release, increases the offspring risk of developing a variety of neurodevelopmental disorders (NDDs), including schizophrenia. Animal models have provided evidence to support these mechanistic links, with placental inflammatory responses and dysregulation of placental function implicated. This leads to changes in fetal brain cytokine balance and altered epigenetic regulation of key neurodevelopmental pathways. The prenatal timing of such mIA-evoked changes, and the accompanying fetal developmental responses to an altered in utero environment, will determine the scope of the impacts on neurodevelopmental processes. Such dysregulation can impart enduring neuropathological changes, which manifest subsequently in the postnatal period as altered neurodevelopmental behaviours in the offspring. Hence, elucidation of the functional changes that occur at the molecular level in the placenta is vital in improving our understanding of the mechanisms that underlie the pathogenesis of NDDs. This has notable relevance to the recent COVID-19 pandemic, where inflammatory responses in the placenta to SARS-CoV-2 infection during pregnancy and NDDs in early childhood have been reported. This review presents an integrated overview of these collective topics and describes the possible contribution of prenatal programming through placental effects as an underlying mechanism that links to NDD risk, underpinned by altered epigenetic regulation of neurodevelopmental pathways.

Pediatric Brain Tumours: Lessons from the Immune Microenvironment

In spite of recent advances in tumour molecular subtyping, pediatric brain tumours (PBTs) remain the leading cause of cancer-related deaths in children. While some PBTs are treatable with favourable outcomes, recurrent and metastatic disease for certain types of PBTs remains challenging and is often fatal. Tumour immunotherapy has emerged as a hopeful avenue for the treatment of childhood tumours, and recent immunotherapy efforts have been directed towards PBTs. This strategy has the potential to combat otherwise incurable PBTs, while minimizing off-target effects and long-term sequelae. As the infiltration and activation states of immune cells, including tumour-infiltrating lymphocytes and tumour-associated macrophages, are key to shaping responses towards immunotherapy, this review explores the immune landscape of the developing brain and discusses the tumour immune microenvironments of common PBTs, with hopes of conferring insights that may inform future treatment design.

Identification of a novel de novo mutation of SETBP1 and new findings of SETBP1 in tumorgenesis

BACKGROUND

In the past decade, SETBP1 has attracted a lot of interest on that the same gene with different type or level (germline or somatic) of variants could provoke different pathologic consequences such as Schinzel-Giedon syndrome, SETBP1 Haploinsufficiency Disorder (SETBP1-HD) and myeloid malignancies. Whole exome sequencing was conducted to detect the etiology of a pregnant woman with mental retardation. As a new oncogene and potential marker of myeloid malignancies, somatic SETBP1 variants in other cancers were rarely studied. We performed a pan-cancer analysis of SETBP1 gene in different cancers for the first time.

RESULTS

A novel heterozygous mutation of the SETBP1 gene (c.1724_1727del, p.D575Vfs*4) was found in the patient and the fetus and the mutation was predicted to result in a truncated protein. Reduced SETBP1 expression was associated with SETBP1-HD. The pan-cancer analysis of SETBP1 showed that SETBP1 overexpression should be given special attention in Bladder Urothelial Carcinoma (BLCA) and Stomach adenocarcinoma (STAD).

CONCLUSIONS

The de novo SETBP1 mutation was the genetic cause of SETBP1-HD in the family. BLCA and STAD might be related to SETBP1 overexpression.

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.

LPS-stimulated microglial cells promote ganglion cell death in organotypic cultures of quail embryo retina

During development microglia colonize the central nervous system (CNS) and play an important role in programmed cell death, not only because of their ability to remove dead cells by phagocytosis, but also because they can promote the death of neuronal and glial cells. To study this process, we used as experimental systems the developing in situ quail embryo retina and organotypic cultures of quail embryo retina explants (QEREs). In both systems, immature microglia show an upregulation of certain inflammatory markers, e.g., inducible NO synthase (iNOS), and nitric oxide (NO) under basal conditions, which can be further enhanced with LPS-treatment. Hence, we investigated in the present study the role of microglia in promoting ganglion cell death during retinal development in QEREs. Results showed that LPS-stimulation of microglia in QEREs increases (i) the percentage of retinal cells with externalized phosphatidylserine, (ii) the frequency of phagocytic contacts between microglial and caspase-3-positive ganglion cells, (iii) cell death in the ganglion cell layer, and (iv) microglial production of reactive oxygen/nitrogen species, such as NO. Furthermore, iNOS inhibition by L-NMMA decreases cell death of ganglion cells and increases the number of ganglion cells in LPS-treated QEREs. These data demonstrate that LPS-stimulated microglia induce ganglion cell death in cultured QEREs by a NO-dependent mechanism. The fact that phagocytic contacts between microglial and caspase-3-positive ganglion cells increase suggests that this cell death might be mediated by microglial engulfment, although a phagocytosis-independent mechanism cannot be excluded.

The Physio-Pathological Role of Group I Metabotropic Glutamate Receptors Expressed by Microglia in Health and Disease with a Focus on Amyotrophic Lateral Sclerosis

Microglia cells are the resident immune cells of the central nervous system. They act as the first-line immune guardians of nervous tissue and central drivers of neuroinflammation. Any homeostatic alteration that can compromise neuron and tissue integrity could activate microglia. Once activated, microglia exhibit highly diverse phenotypes and functions related to either beneficial or harmful consequences. Microglia activation is associated with the release of protective or deleterious cytokines, chemokines, and growth factors that can in turn determine defensive or pathological outcomes. This scenario is complicated by the pathology-related specific phenotypes that microglia can assume, thus leading to the so-called disease-associated microglia phenotypes. Microglia express several receptors that regulate the balance between pro- and anti-inflammatory features, sometimes exerting opposite actions on microglial functions according to specific conditions. In this context, group I metabotropic glutamate receptors (mGluRs) are molecular structures that may contribute to the modulation of the reactive phenotype of microglia cells, and this is worthy of exploration. Here, we summarize the role of group I mGluRs in shaping microglia cells' phenotype in specific physio-pathological conditions, including some neurodegenerative disorders. A significant section of the review is specifically focused on amyotrophic lateral sclerosis (ALS) since it represents an entirely unexplored topic of research in the field.

Role of α-synuclein in microglia: autophagy and phagocytosis balance neuroinflammation in Parkinson's disease

BACKGROUND

Parkinson's disease (PD) is the second most common neurodegenerative disease, and is characterized by accumulation of α-synuclein (α-syn). Neuroinflammation driven by microglia is an important pathological manifestation of PD. α-Syn is a crucial marker of PD, and its accumulation leads to microglia M1-like phenotype polarization, activation of NLRP3 inflammasomes, and impaired autophagy and phagocytosis in microglia. Autophagy of microglia is related to degradation of α-syn and NLRP3 inflammasome blockage to relieve neuroinflammation. Microglial autophagy and phagocytosis of released α-syn or fragments from apoptotic neurons maintain homeostasis in the brain. A variety of PD-related genes such as LRRK2, GBA and DJ-1 also contribute to this stability process.

OBJECTIVES

Further studies are needed to determine how α-syn works in microglia.

METHODS

A keyword-based search was performed using the PubMed database for published articles.

CONCLUSION

In this review, we discuss the interaction between microglia and α-syn in PD pathogenesis and the possible mechanism of microglial autophagy and phagocytosis in α-syn clearance and inhibition of neuroinflammation. This may provide a novel insight into treatment of PD.

Botulinum neurotoxin A ameliorates depressive-like behavior in a reserpine-induced Parkinson's disease mouse model via suppressing hippocampal microglial engulfment and neuroinflammation

Depression is one of the common non-motor symptoms of Parkinson's disease (PD). In the clinic, botulinum neurotoxin A (BoNT/A) has been used to treat depression. In this study, we investigated the mechanisms underlying the anti-depressive effect of BoNT/A in a PD mouse model. Mice were administered reserpine (3 μg/mL in the drinking water) for 10 weeks. From the 10^th week, BoNT/A (10 U·kg^-1·d^-1) was injected into the cheek for 3 consecutive days. We showed that chronic administration of reserpine produced the behavioral phenotypes of depression and neurochemical changes in the substantia nigra pars compacta (SNpc) and striatum. BoNT/A treatment significantly ameliorated the depressive-like behaviors, but did not improve TH activity in SNpc of reserpine-treated mice. We demonstrated that BoNT/A treatment reversed reserpine-induced complement and microglia activation in the hippocampal CA1 region. Furthermore, BoNT/A treatment significantly attenuated the microglial engulfment of presynaptic synapses, thus ameliorating the apparent synapse and spine loss in the hippocampus in the reserpine-treated mice. Moreover, BoNT/A treatment suppressed microglia-mediated expression of pro-inflammatory cytokines TNF-α and IL-1β in reserpine-treated mice. In addition, we showed that BoNT/A (0.1 U/mL) ameliorated reserpine-induced complement and microglia activation in mouse BV2 microglial cells in vitro. We conclude that BoNT/A ameliorates depressive-like behavior in a reserpine-induced PD mouse model through reversing the synapse loss mediated by classical complement induced-microglial engulfment as well as alleviating microglia-mediated proinflammatory responses. BoNT/A ameliorates depressive-like behavior, and reverses synapse loss mediated by classical complement pathway-initiated microglia engulfment as well as alleviates microglia-mediated proinflammatory response in the reserpine-induced Parkinson's disease mouse model.

HIF-1α participates in secondary brain injury through regulating neuroinflammation

A deeper understanding of the underlying biological mechanisms of secondary brain injury induced by traumatic brain injury (TBI) will greatly advance the development of effective treatments for patients with TBI. Hypoxia-inducible factor-1 alpha (HIF-1α) is a central regulator of cellular response to hypoxia. In addition, growing evidence shows that HIF-1α plays the important role in TBI-induced changes in biological processes; however, detailed functional mechanisms are not completely known. The aim of the present work was to further explore HIF-1α-mediated events after TBI. To this end, next-generation sequencing, coupled with cellular and molecular analysis, was adopted to interrogate vulnerable events in a rat controlled cortical impact model of TBI. The results demonstrated that TBI induced accumulation of HIF-1α at the peri-injury site at 24 h post-injury, which was associated with neuronal loss. Moreover, gene set enrichment analysis unveiled that neuroinflammation, especially an innate inflammatory response, was significantly evoked by TBI, which could be attenuated by the inhibition of HIF-1α. Furthermore, the inhibition of HIF-1α could mitigate the activation of microglia and astrocytes. Taken together, all these data implied that HIF-1α might contribute to secondary brain injury through regulating neuroinflammation.

Recent Advances in Brain Organoid Technology for Human Brain Research

Brain organoids are self-assembled three-dimensional aggregates with brain-like cell types and structures and have emerged as new model systems that can be used to investigate human neurodevelopment and neurological disorders. However, brain organoids are not as mature and functional as real human brains due to limitations of the culture system with insufficient developmental patterning signals and a lack of components that are important for brain development and function, such as the non-neural population and vasculature. In addition, establishing the desired brain-like environment and monitoring the complex neural networks and physiological functions of the brain organoids remain challenging. The current protocols to generate brain organoids also have problems with heterogeneity and batch variation due to spontaneous self-organization of brain organoids into complex architectures of the brain. To address these limitations of current brain organoid technologies, various engineering platforms, such as extracellular matrices, fluidic devices, three-dimensional bioprinting, bioreactors, polymeric scaffolds, microelectrodes, and biochemical sensors, have been employed to improve neuronal development and maturation, reduce structural heterogeneity, and facilitate functional analysis and monitoring. In this review, we provide an overview of the latest engineering techniques that overcome these limitations in the production and application of brain organoids.

Exosomes derived from mesenchymal stem cells overexpressing miR-210 inhibits neuronal inflammation and contribute to neurite outgrowth through modulating microglia polarization

Inflammatory responses play a critical role in the progress of neurodegenerative disorders. MSC-Exos is considered to have an anti-inflammatory effect on the treatment strategy for brain injury. However, the therapeutic effect and possible mechanism of Exosomal miR-210 on microglia polarization-induced neuroinflammation and neurite outgrowth have not been reported. MSC-Exos were isolated by ultracentrifugation, identified by Nanosight NS300, transmission electron microscopy, and western bolt. In vitro, to explore the protective mechanism of MSC-Exos against neuroinflammation, the microglial BV2 cell was exposed to lipopolysaccharide to assess inflammatory changes. The intake of 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (Dil)-MSC-Exos into microglia was observed by fluorescence microscopy. The results showed that Exosomal miR-210 treatment significantly inhibited the production of nitric oxide and pro-inflammatory cytokines. Exosomal miR-210 treatment also increased the number of M2 microglia cells and inhibited M1 microglia polarization. In addition, western blot demonstrated that Exosomal miR-210 reduced neuronal apoptosis. Thus, Exosomal miR-210 attenuated neuronal inflammation and promoted neurite outgrowth. Exosomal miR-210 from MSCs attenuated neuronal inflammation and contributed to neurogenesis possibly by inhibiting microglial M1 polarization.

Microglial heterogeneity and complement component 3 elimination within emerging multisensory midbrain compartments during an early critical period

The lateral cortex of the inferior colliculus (LCIC) is a midbrain shell region that receives multimodal inputs that target discrete zones of its compartmental (modular-matrix) framework. This arrangement emerges perinatally in mice (postnatal day, P0-P12) as somatosensory and auditory inputs segregate into their respective modular and matrix terminal patterns. Microglial cells (MGCs) perform a variety of critical functions in the developing brain, among them identifying areas of active circuit assembly and selectively pruning exuberant or underutilized connections. Recent evidence in other brain structures suggest considerable MGC heterogeneity across the lifespan, particularly during established developmental critical periods. The present study examines the potential involvement of classical complement cascade signaling (C3-CR3/CD11b) in refining early multisensory networks, and identifies several microglial subsets exhibiting distinct molecular signatures within the nascent LCIC. Immunostaining was performed in GAD67-green fluorescent protein (GFP) and CX3CR1-GFP mice throughout and after the defined LCIC critical period. GAD labeling highlights the emerging LCIC modularity, while CX3CR1 labeling depicts MGCs expressing the fractalkine receptor. C3 expression is widespread throughout the LCIC neuropil early on, prior to its conspicuous absence from modular zones at P8, and more global disappearance following critical period closure. CD11b-expressing microglia while homogeneously distributed at birth, are biased to modular fields at P8 and then the surrounding matrix by P12. Temporal and spatial matching of the disappearance of C3 by LCIC compartment (i.e., modules then matrix) with CD11b-positive MGC occupancy implicates complement signaling in the selective refinement of early LCIC connectivity. Multiple-labeling studies for a variety of established MGC markers (CD11b, CX3CR1, Iba1, TMEM119) indicate significant MGC heterogeneity in the LCIC as its compartments and segregated multisensory maps emerge. Marker colocalization was the exception rather than the rule, suggesting that unique MGC subpopulations exist in the LCIC and perhaps serve distinct developmental roles. Potential mechanisms whereby microglia sculpt early multisensory LCIC maps and how such activity/inactivity may underlie certain neurodevelopmental conditions, including autism spectrum disorder and schizophrenia, are discussed.

Autism Spectrum Disorders: A Recent Update on Targeting Inflammatory Pathways with Natural Anti-Inflammatory Agents

Autism spectrum disorder (ASD) is a heterogeneous category of developmental psychiatric disorders which is characterized by inadequate social interaction, less communication, and repetitive phenotype behavior. ASD is comorbid with various types of disorders. The reported prevalence is 1% in the United Kingdom, 1.5% in the United States, and ~0.2% in India at present. The natural anti-inflammatory agents on brain development are linked to interaction with many types of inflammatory pathways affected by genetic, epigenetic, and environmental variables. Inflammatory targeting pathways have already been linked to ASD. However, these routes are diluted, and new strategies are being developed in natural anti-inflammatory medicines to treat ASD. This review summarizes the numerous preclinical and clinical studies having potential protective effects and natural anti-inflammatory agents on the developing brain during pregnancy. Inflammation during pregnancy activates the maternal infection that likely leads to the development of neuropsychiatric disorders in the offspring. The inflammatory pathways have been an effective target for the subject of translational research studies on ASD.

Neurotoxicity evoked by organophosphates and available countermeasures

Organophosphorus compounds (OP) are a constant problem, both in the military and in the civilian field, not only in the form of acute poisoning but also for their long-lasting consequences. No antidote has been found that satisfactorily protects against the toxic effects of organophosphates. Likewise, there is no universal cure to avert damage after poisoning. The key mechanism of organophosphate toxicity is the inhibition of acetylcholinesterase. The overstimulation of nicotinic or muscarinic receptors by accumulated acetylcholine on a synaptic cleft leads to activation of the glutamatergic system and the development of seizures. Further consequences include generation of reactive oxygen species (ROS), neuroinflammation, and the formation of various other neuropathologists. In this review, we present neuroprotection strategies which can slow down the secondary nerve cell damage and alleviate neurological and neuropsychiatric disturbance. In our opinion, there is no unequivocal approach to ensure neuroprotection, however, sooner the neurotoxicity pathway is targeted, the better the results which can be expected. It seems crucial to target the key propagation pathways, i.e., to block cholinergic and, foremostly, glutamatergic cascades. Currently, the privileged approach oriented to stimulating GABA_AR by benzodiazepines is of limited efficacy, so that antagonizing the hyperactivity of the glutamatergic system could provide an even more efficacious approach for terminating OP-induced seizures and protecting the brain from permanent damage. Encouraging results have been reported for tezampanel, an antagonist of GluK1 kainate and AMPA receptors, especially in combination with caramiphen, an anticholinergic and anti-glutamatergic agent. On the other hand, targeting ROS by antioxidants cannot or already developed neuroinflammation does not seem to be very productive as other processes are also involved.

Lactoferrin for Mental Health: Neuro-Redox Regulation and Neuroprotective Effects across the Blood-Brain Barrier with Special Reference to Neuro-COVID-19

Overall mental health depends in part on the blood-brain barrier, which regulates nutrient transfer in-and-out of the brain and its central nervous system. Lactoferrin, an innate metal-transport protein, synthesized in the substantia nigra, particularly in dopaminergic neurons and activated microglia is vital for brain physiology. Lactoferrin rapidly crosses the blood-brain barrier via receptor-mediated transcytosis and accumulates in the brain capillary endothelial cells. Lactoferrin receptors are additionally present on glioma cells, brain micro-vessels, and neurons. As a regulator of neuro-redox, microglial lactoferrin is critical for protection/repair of neurons and healthy brain function. Iron imbalance and oxidative stress are common among patients with neurodegenerative disorders such as Parkinson's disease, Alzheimer's disease, dementia, depression, and multiple sclerosis. As an endogenous iron-chelator, lactoferrin prevents iron accumulation and dopamine depletion in Parkinson's disease patients. Oral lactoferrin supplementation could modulate the p-Akt/PTEN pathway, reduce Aβ deposition, and ameliorate cognitive decline in Alzheimer's disease. Novel lactoferrin-based nano-therapeutics have emerged as effective drug-delivery systems for clinical management of neurodegenerative disorders. Recent emergence of the Coronavirus disease-2019 (COVID-19) pandemic, initially considered a respiratory illness, demonstrated a broader virulence spectrum with the ability to cross the blood-brain barrier and inflict a plethora of neuropathological manifestations in the brain - the Neuro-COVID-19. Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections are widely reported in Parkinson's disease, Alzheimer's disease, dementia, and multiple sclerosis patients with aggravated clinical outcomes. Lactoferrin, credited with several neuroprotective benefits in the brain could serve as a potential adjuvant in the clinical management of Neuro-COVID-19.

New insight in the cross-talk between microglia and schizophrenia: From the perspective of neurodevelopment

Characterized by psychotic symptoms, negative symptoms and cognitive deficits, schizophrenia had a catastrophic effect on patients and their families. Multifaceted reliable evidence indicated that schizophrenia is a neurodevelopmental disorder. Microglia, the immune cells in central nervous system, related to many neurodevelopmental diseases. Microglia could affect neuronal survival, neuronal death and synaptic plasticity during neurodevelopment. Anomalous microglia during neurodevelopment may be associated with schizophrenia. Therefore, a hypothesis proposes that the abnormal function of microglia leads to the occurrence of schizophrenia. Nowadays, accumulating experiments between microglia and schizophrenia could afford unparalleled probability to assess this hypothesis. Herein, this review summarizes the latest supporting evidence in order to shed light on the mystery of microglia in schizophrenia.

Multifaceted microglia during brain development: Models and tools

Microglia, the brain resident macrophages, are multifaceted glial cells that belong to the central nervous and immune systems. As part of the immune system, they mediate innate immune responses, regulate brain homeostasis and protect the brain in response to inflammation or injury. At the same time, they can perform a wide array of cellular functions that relate to the normal functioning of the brain. Importantly, microglia are key actors of brain development. Indeed, these early brain invaders originate outside of the central nervous system from yolk sac myeloid progenitors, and migrate into the neural folds during early embryogenesis. Before the generation of oligodendrocytes and astrocytes, microglia thus occupy a unique position, constituting the main glial population during early development and participating in a wide array of embryonic and postnatal processes. During this developmental time window, microglia display remarkable features, being highly heterogeneous in time, space, morphology and transcriptional states. Although tremendous progress has been made in our understanding of their ontogeny and roles, there are several limitations for the investigation of specific microglial functions as well as their heterogeneity during development. This review summarizes the current murine tools and models used in the field to study the development of these peculiar cells. In particular, we focus on the methodologies used to label and deplete microglia, monitor their behavior through live-imaging and also discuss the progress currently being made by the community to unravel microglial functions in brain development and disorders.

Microglial motility is modulated by neuronal activity and correlates with dendritic spine plasticity in the hippocampus of awake mice

Microglia, the resident immune cells of the brain, play a complex role in health and disease. They actively survey the brain parenchyma by physically interacting with other cells and structurally shaping the brain. Yet, the mechanisms underlying microglial motility and significance for synapse stability, especially in the hippocampus during adulthood, remain widely unresolved. Here, we investigated the effect of neuronal activity on microglial motility and the implications for the formation and survival of dendritic spines on hippocampal CA1 neurons in vivo. We used repetitive two-photon in vivo imaging in the hippocampus of awake and anesthetized mice to simultaneously study the motility of microglia and their interaction with dendritic spines. We found that CA3 to CA1 input is sufficient to modulate microglial process motility. Simultaneously, more dendritic spines emerged in mice after awake compared to anesthetized imaging. Interestingly, the rate of microglial contacts with individual dendritic spines and dendrites was associated with the stability, removal, and emergence of dendritic spines. These results suggest that microglia might sense neuronal activity via neurotransmitter release and actively participate in synaptic rewiring of the hippocampal neural network during adulthood. Further, this study has profound relevance for hippocampal learning and memory processes.

Muse cells decrease the neuroinflammatory response by modulating the proportion of M1 and M2 microglia in vitro

Neuroinflammation hinders repair of the central nervous system (CNS). Stem cell transplantation is a very promising approach for treatment of CNS injuries. However, it is difficult to select seed cells that can both facilitate nerve regeneration and improve the microenvironment in the CNS. In this study, we isolated multilineage-differentiating stress-enduring (Muse) cells from bone marrow mesenchymal stem cells. We explored the anti-inflammatory effect and mechanism of Muse cells in vitro by coculture of Muse cells with lipopolysaccharide-stimulated microglia. Our results showed that Muse cells effectively reduced the transcription and secretion of tumor necrosis factor α and interleukin-1β and increased the expression of transforming growth factor-β and interleukin-10 in microglia. In addition, Muse cells decreased the number of M1 microglia and increased the proportion of M2 microglia in an inflammatory environment more effectively than bone marrow mesenchymal stem cells. We also show that Muse cells inhibited the protein expression of toll-like receptor 4 (TLR4) and myeloid differentiation primary response protein (MyD88) and inhibited the expression of the phosphorylated forms of transcription factor p65, nuclear factor (NF)-κB inhibitor alpha, and p38 mitogen-activated protein kinase (MAPK) in microglia. Therefore, we suggest Muse cells cause antineuroinflammatory effects by inhibition of the TLR4/MyD88/NF-κB and p38 MAPK signaling pathways in microglia. Our results shed light on the function of Muse cells in relation to CNS diseases and provide insight into the selection of seed cells.

Microglial pattern recognition via IL-33 promotes synaptic refinement in developing corticothalamic circuits in mice

Microglia are critical regulators of brain development that engulf synaptic proteins during postnatal synapse remodeling. However, the mechanisms through which microglia sense the brain environment are not well defined. Here, we characterized the regulatory program downstream of interleukin-33 (IL-33), a cytokine that promotes microglial synapse remodeling. Exposing the developing brain to a supraphysiological dose of IL-33 altered the microglial enhancer landscape and increased binding of stimulus-dependent transcription factors including AP-1/FOS. This induced a gene expression program enriched for the expression of pattern recognition receptors, including the scavenger receptor MARCO. CNS-specific deletion of IL-33 led to increased excitatory/inhibitory synaptic balance, spontaneous absence-like epileptiform activity in juvenile mice, and increased seizure susceptibility in response to chemoconvulsants. We found that MARCO promoted synapse engulfment, and Marco-deficient animals had excess thalamic excitatory synapses and increased seizure susceptibility. Taken together, these data define coordinated epigenetic and functional changes in microglia and uncover pattern recognition receptors as potential regulators of postnatal synaptic refinement.

Longitudinal Positron Emission Tomography Imaging with P2X7 Receptor-Specific Radioligand 18F-FTTM in a Kainic Acid Rat Model of Temporal Lobe Epilepsy

P2X7 receptors (P2X7R), as a brain inflammation biomarker, play important roles in the epileptogenic progress. Mounting evidence supports their activation in the brain during epilepsy, and inhibition of the P2X7 receptor reduces the seizure frequency and severity. In this study, we investigate P2X7R-targeted (¹⁸F-FTTM) position emission tomography (PET) imaging in a rat model of temporal lobe epilepsy to obtain further insights into the role of P2X7R during epileptogenesis. ¹⁸F-FTTM (5-10% radiochemical yield, over 99% radiochemical purity, and a specific activity of 270-300 MBq/nmol, n = 6, EOS) was first synthesized. Then, the rat models induced by intrahippocampal injection of saline (1.2 μL, n = 15) or kainic acid (1.2 μL, 0.5 μg/μL, n = 35) were examined using ¹⁸F-FTTM Micro-PET/CT longitudinal imaging, respectively. The imaging results showed that increases in the ¹⁸F-FTTM uptake was evident after status epilepticus (SE) in the epileptogenesis-associated brain regions, such as the hippocampus, amygdala, or temporal cortex, and this peaked during the latent period. The histopathological analysis revealed that the P2X7R PET uptake reached a peak at 7 days after SE and was mostly related to microglial activation. Thus, P2X7R-targeted PET imaging agent ¹⁸F-FTTM may act as a useful tool for identifying brain inflammation during epilepsy. P2X7R PET is a highly potent longitudinal biomarker of epilepsy and could be of interest to determine the therapeutic windows in epilepsy and to monitor treatment response, and it warrants further clinical studies.

Systemic maternal inflammation promotes ASD via IL-6 and IFN-γ

Autism spectrum disorder (ASD) is a neurological disorder that manifests during early development, impacting individuals through their ways of communicating, social behaviors, and their ability to perform day-to-day activities. There have been different proposed mechanisms on how ASD precipitates within a patient, one of which being the impact cytokines have on fetal development once a mother's immune system has been activated (referred to as maternal immune activation, MIA). The occurrence of ASD has long been associated with elevated levels of several cytokines, including interleukin-6 (IL-6) and interferon gamma (IFN-γ). These proinflammatory cytokines can achieve high systemic levels in response to immune activating pathogens from various extrinsic sources. Transfer of cytokines such as IL-6 across the placental barrier allows accumulation in the fetus, potentially inducing neuroinflammation and consequently altering neurodevelopmental processes. Individuals who have been later diagnosed with ASD have been observed to have elevated levels of IL-6 and other proinflammatory cytokines during gestation. Moreover, the outcome of MIA has been associated with neurological effects such as impaired social interaction and an increase in repetitive behavior in animal models, supporting a mechanistic link between gestational inflammation and development of ASD-like characteristics. The present review attempts to provide a concise overview of the available preclinical and clinical data that suggest cross-talk between IL-6 and IFN-γ through both extrinsic and intrinsic factors as a central mechanism of MIA that may promote the development of ASD.

Microglia shape the embryonic development of mammalian respiratory networks

Microglia, brain-resident macrophages, play key roles during prenatal development in defining neural circuitry function, including ensuring proper synaptic wiring and maintaining homeostasis. Mammalian breathing rhythmogenesis arises from interacting brainstem neural networks that are assembled during embryonic development, but the specific role of microglia in this process remains unknown. Here, we investigated the anatomical and functional consequences of respiratory circuit formation in the absence of microglia. We first established the normal distribution of microglia within the wild-type (WT, Spi1^+/+ (Pu.1 WT)) mouse (Mus musculus) brainstem at embryonic ages when the respiratory networks are known to emerge (embryonic day (E) 14.5 for the parafacial respiratory group (epF) and E16.5 for the preBötzinger complex (preBötC)). In transgenic mice depleted of microglia (Spi1^−/− (Pu.1 KO) mutant), we performed anatomical staining, calcium imaging, and electrophysiological recordings of neuronal activities in vitro to assess the status of these circuits at their respective times of functional emergence. Spontaneous respiratory-related activity recorded from reduced in vitro preparations showed an abnormally slow rhythm frequency expressed by the epF at E14.5, the preBötC at E16.5, and in the phrenic motor nerves from E16.5 onwards. These deficits were associated with a reduced number of active epF neurons, defects in commissural projections that couple the bilateral preBötC half-centers, and an accompanying decrease in their functional coordination. These abnormalities probably contribute to eventual neonatal death, since plethysmography revealed that E18.5 Spi1^−/− embryos are unable to sustain breathing activity ex utero. Our results thus point to a crucial contribution of microglia in the proper establishment of the central respiratory command during embryonic development.

Microglial control of neuronal development via somatic purinergic junctions

Microglia, the resident immune cells of the brain, play important roles during development. Although bi-directional communication between microglia and neuronal progenitors or immature neurons has been demonstrated, the main sites of interaction and the underlying mechanisms remain elusive. By using advanced methods, here we provide evidence that microglial processes form specialized contacts with the cell bodies of developing neurons throughout embryonic, early postnatal, and adult neurogenesis. These early developmental contacts are highly reminiscent of somatic purinergic junctions that are instrumental for microglia-neuron communication in the adult brain. The formation and maintenance of these junctions is regulated by functional microglial P2Y12 receptors, and deletion of P2Y12Rs disturbs proliferation of neuronal precursors and leads to aberrant cortical cytoarchitecture during development and in adulthood. We propose that early developmental formation of somatic purinergic junctions represents an important interface for microglia to monitor the status of immature neurons and control neurodevelopment.

p27kip1 Modulates the Morphology and Phagocytic Activity of Microglia

p27^kip1 is a multifunctional protein that promotes cell cycle exit by blocking the activity of cyclin/cyclin-dependent kinase complexes as well as migration and motility via signaling pathways that converge on the actin and microtubule cytoskeleton. Despite the broad characterization of p27^kip1 function in neural cells, little is known about its relevance in microglia. Here, we studied the role of p27^kip1 in microglia using a combination of in vitro and in situ approaches. While the loss of p27^kip1 did not affect microglial density in the cerebral cortex, it altered their morphological complexity in situ. However, despite the presence of p27^kip1 in microglial processes, as shown by immunofluorescence in cultured cells, loss of p27^kip1 did not change microglial process motility and extension after applying laser-induced brain damage in cortical brain slices. Primary microglia lacking p27^kip1 showed increased phagocytic uptake of synaptosomes, while a cell cycle dead variant negatively affected phagocytosis. These findings indicate that p27^kip1 plays specific roles in microglia.

The role of microglia in neuropsychiatric disorders and suicide

This narrative review examines the possible role of microglial cells, first, in neuroinflammation and, second, in schizophrenia, depression, and suicide. Recent research on the interactions between microglia, astrocytes and neurons and their involvement in pathophysiological processes of neuropsychiatric disorders is presented. This review focuses on results from postmortem, positron emission tomography (PET) imaging studies, and animal models of schizophrenia and depression. Third, the effects of antipsychotic and antidepressant drug therapy, and of electroconvulsive therapy on microglial cells are explored and the upcoming development of therapeutic drugs targeting microglia is described. Finally, there is a discussion on the role of microglia in the evolutionary progression of human lineage. This view may contribute to a new understanding of neuropsychiatric disorders.

Glial-neuron crosstalk in health and disease: A focus on metabolism, obesity, and cognitive impairment

Dementia is a complex set of disorders affecting normal cognitive function. Recently, several clinical studies have shown that diabetes, obesity, and components of the metabolic syndrome (MetS) are associated with cognitive impairment, including dementias such as Alzheimer's disease. Maintaining normal cognitive function is an intricate process involving coordination of neuron function with multiple brain glia. Well-orchestrated bioenergetics is a central requirement of neurons, which need large amounts of energy but lack significant energy storage capacity. Thus, one of the most important glial functions is to provide metabolic support and ensure an adequate energy supply for neurons. Obesity and metabolic disease dysregulate glial function, leading to a failure to respond to neuron energy demands, which results in neuronal damage. In this review, we outline evidence for links between diabetes, obesity, and MetS components to cognitive impairment. Next, we focus on the metabolic crosstalk between the three major glial cell types, oligodendrocytes, astrocytes, and microglia, with neurons under physiological conditions. Finally, we outline how diabetes, obesity, and MetS components can disrupt glial function, and how this disruption might impair glia-neuron metabolic crosstalk and ultimately promote cognitive impairment.

The Antiepileptic Drug and Toxic Teratogen Valproic Acid Alters Microglia in an Environmental Mouse Model of Autism

Autism spectrum disorder (ASD), a neurodevelopmental condition affecting approximately 1 in 44 children in North America, is thought to be a connectivity disorder. Valproic acid (VPA) is a multi-target drug widely used to treat epilepsy. It is also a toxic teratogen as well as a histone deacetylase inhibitor, and fetal exposure to VPA increases the risk of ASD. While the VPA model has been well-characterized for behavioral and neuronal deficits including hyperconnectivity, microglia, the principal immune cells of CNS that regulate dendrite and synapse formation during early brain development, have not been well-characterized and may provide potential hints regarding the etiology of this disorder. Therefore, in this study, we determined the effect of prenatal exposure to VPA on microglial numbers during early postnatal brain development. We found that prenatal exposure to VPA causes a significant reduction in the number of microglia in the primary motor cortex (PMC) during early postnatal brain development, particularly at postnatal day 6 (P6) and postnatal day 10 (P10) in male mice. The early microglial reduction in the VPA model coincides with active cortical synaptogenesis and is significant because it may potentially play a role in mediating impaired connectivity in ASD.

Human Down syndrome microglia are up for a synaptic feast

In this issue of Cell Stem Cell, Jin et al. report that human Down syndrome microglia exhibit enhanced synaptic engulfment and accelerated tau-induced cellular senescence in human-mouse chimeric brains. They show that inhibiting interferon signaling rescues both developmental and tau-associated phenotypes, rendering it a potential therapeutic target for Down syndrome.

Hypothesis: By-products of vascular disruption carried in the CSF affect prenatal brain development

Prenatal CNS disruptions can be associated with physically separate findings. Examples include cognitive issues in septo-optic dysplasia and sporadic and WNT1-related unilateral cerebellar hypoplasia, and physical findings such as thinning of the corpus callosum, ventriculomegaly, hippocampal abnormalities, olfactory tract and bulb hypoplasia, and distant cortical dysplasias with schizencephaly. Similar effects to toxicities with intraventricular hemorrhage in prematurity could occur earlier in development. CSF transportation of disruption by-products would provide access to vulnerable areas through inflammatory effects on blood-brain barrier permeability. Outcomes are influenced by location and volume of byproducts in the CSF, timing, transport, and inflammatory responses. A particular association of vermis disruption with cognitive issues may be related to CSF flow distortions that avoid toxin dilutions in the third ventricle. Symmetrical contralateral cortical dysplasia with schizencephaly may reflect immunovascular field-related vulnerabilities seen in situations such as vitiligo.

Neonatal Rat Glia Cultured in Physiological Normoxia for Modeling Neuropathological Conditions In Vitro

Cell culture conditions were proven to highly affect crucial biological processes like proliferation, differentiation, intercellular crosstalk, and senescence. Oxygen tension is one of the major factors influencing cell metabolism and thus, modulating cellular response to pathophysiological conditions. In this context, the presented study aimed at the development of a protocol for efficient culture of rat neonatal glial cells (microglia, astrocytes, and oligodendrocytes) in oxygen concentrations relevant to the nervous tissue. The protocol allows for obtaining three major cell populations, which play crucial roles in sustaining tissue homeostasis and are known to be activated in response to a wide spectrum of external stimuli. The cells are cultured in media without supplement addition to avoid potential modulation of cell processes. The application of active biomolecules for coating culturing surfaces might be useful for mirroring physiological cell interactions with extracellular matrix components. The cell fractions can be assembled as cocultures to further evaluate investigated mechanisms, intercellular crosstalk, or cell response to tested pharmacological compounds. Applying additional procedures, like transient oxygen and glucose deprivation, allows to mimic in vitro the selected pathophysiological conditions. The presented culture system for neonatal rat glial cells is a highly useful tool for in vitro modeling selected neuropathological conditions.

Generation of vascularized brain organoids to study neurovascular interactions

Brain organoids have been used to recapitulate the processes of brain development and related diseases. However, the lack of vasculatures, which regulate neurogenesis and brain disorders, limits the utility of brain organoids. In this study, we induced vessel and brain organoids, respectively, and then fused two types of organoids together to obtain vascularized brain organoids. The fused brain organoids were engrafted with robust vascular network-like structures and exhibited increased number of neural progenitors, in line with the possibility that vessels regulate neural development. Fusion organoids also contained functional blood–brain barrier-like structures, as well as microglial cells, a specific population of immune cells in the brain. The incorporated microglia responded actively to immune stimuli to the fused brain organoids and showed ability of engulfing synapses. Thus, the fusion organoids established in this study allow modeling interactions between the neuronal and non-neuronal components in vitro, particularly the vasculature and microglia niche.

Understanding how the organs form and how their cells behave is essential to finding the causes and treatment for developmental disorders, as well as understanding certain diseases. However, studying most organs in live animals or humans is technically difficult, expensive and invasive. To address this issue, scientists have developed models called ‘organoids’ that recapitulate the development of organs using stem cells in the lab. These models are easier to study and manipulate than the live organs.

Brain organoids have been used to recapitulate brain formation as well as developmental, degenerative and psychiatric brain conditions such as microcephaly, autism and Alzheimer’s disease. However, these brain organoids lack the vasculature (the network of blood vessels) that supplies a live brain with nutrients and regulates its development, and which has important roles in brain disorders. Partly due to this lack of blood vessels, brain organoids also do not develop a blood brain barrier, the structure that prevents certain contents of the blood, including pathogens, toxins and even certain drugs from entering the brain. These characteristics limit the utility of existing brain organoids.

To overcome these limitations, Sun, Ju et al. developed brain organoids and blood vessel organoids independently, and then fused them together to obtain vascularized brain organoids. These fusion organoids developed a robust network of blood vessels that was well integrated with the brain cells, and produced more neural cell precursors than brain organoids that had not been fused. This result is consistent with the idea that blood vessels can regulate brain development.

Analyzing the fusion organoids revealed that they contain structures similar to the blood-brain barrier, as well as microglial cells (immune cells specific to the brain). When exposed to lipopolysaccharide – a component of the cell wall of certain bacteria – these cells responded by initiating an immune response in the fusion organoids. Notably, the microglial cells were also able to engulf connections between brain cells, a process necessary for the brain to develop the correct structures and work normally.

Sun, Ju et al. have developed a new organoid system that will be of broad interest to researchers studying interactions between the brain and the circulatory system. The development of brain-blood-barrier-like structures in the fusion organoids could also facilitate the development of drugs that can cross this barrier, making it easier to treat certain conditions that affect the brain. Refining this model to allow the fusion organoids to grow for longer times in the lab, and adding blood flow to the system will be the next steps to establish this system.

The Anti-Inflammatory Agent Bindarit Attenuates the Impairment of Neural Development through Suppression of Microglial Activation in a Neonatal Hydrocephalus Mouse Model

Neonatal hydrocephalus presents with various degrees of neuroinflammation and long-term neurologic deficits in surgically treated patients, provoking a need for additional medical treatment. We previously reported elevated neuroinflammation and severe periventricular white matter damage in the progressive hydrocephalus (prh) mutant which contains a point mutation in the Ccdc39 gene, causing loss of cilia-mediated unidirectional CSF flow. In this study, we identified cortical neuropil maturation defects such as impaired excitatory synapse maturation and loss of homeostatic microglia, and swimming locomotor defects in early postnatal prh mutant mice. Strikingly, systemic application of the anti-inflammatory small molecule bindarit significantly supports healthy postnatal cerebral cortical development in the prh mutant. While bindarit only mildly reduced the ventricular volume, it significantly improved the edematous appearance and myelination of the corpus callosum. Moreover, the treatment attenuated thinning in cortical Layers II-IV, excitatory synapse formation, and interneuron morphogenesis, by supporting the ramified-shaped homeostatic microglia from excessive cell death. Also, the therapeutic effect led to the alleviation of a spastic locomotor phenotype of the mutant. We found that microglia, but not peripheral monocytes, contribute to amoeboid-shaped activated myeloid cells in prh mutants' corpus callosum and the proinflammatory cytokines expression. Bindarit blocks nuclear factor (NF)-kB activation and its downstream proinflammatory cytokines, including monocyte chemoattractant protein-1, in the prh mutant. Collectively, we revealed that amelioration of neuroinflammation is crucial for white matter and neuronal maturation in neonatal hydrocephalus. Future studies of bindarit treatment combined with CSF diversion surgery may provide long-term benefits supporting neuronal development in neonatal hydrocephalus.SIGNIFICANCE STATEMENT In neonatal hydrocephalus, little is known about the signaling cascades of neuroinflammation or the impact of such inflammatory insults on neural cell development within the perinatal cerebral cortex. Here, we report that proinflammatory activation of myeloid cells, the majority of which are derived from microglia, impairs periventricular myelination and cortical neuronal maturation using the mouse prh genetic model of neonatal hydrocephalus. Administration of bindarit, an anti-inflammatory small molecule that blocks nuclear factor (NF)-kB activation, restored the cortical thinning and synaptic maturation defects in the prh mutant brain through suppression of microglial activation. These data indicate the potential therapeutic use of anti-inflammatory reagents targeting neuroinflammation in the treatment of neonatal hydrocephalus.

Developmental exposure to environmental levels of cadmium induces neurotoxicity and activates microglia in zebrafish larvae: From the perspectives of neurobehavior and neuroimaging

Cadmium (Cd) is a worldwide environmental pollutant that postures serious threats to humans and ecosystems. Over the years, its adverse effects on the central nervous system (CNS) have been concerned, whereas the underlying cellular/molecular mechanisms remain unclear. In this study, taking advantages of zebrafish model in high-throughput imaging and behavioral tests, we have explored the potential developmental neurotoxicity of Cd at environmentally relevant levels, from the perspectives of neurobehavior and neuroimaging. Briefly, Cd²⁺ exposure resulted in a general impairment of zebrafish early development. Zebrafish neurobehavioral patterns including locomotion and reactivity to environmental signals were significantly perturbed upon Cd²⁺ exposure. Importantly, a combination of in vivo two-photon neuroimaging, flow cytometry and gene expression analyses revealed notable neurodevelopmental disorders as well as neuroimmune responses induced by Cd²⁺ exposure. Both cell-cycle arrest and apoptosis contributed jointly to a significant decrease of neuronal density in zebrafish larvae exposed to Cd²⁺. The dramatic morphological alterations of microglia from multi-branched to amoeboid, the microgliosis, as well as the modulation of gene expression profiles demonstrated a strong activation of microglia and neuroinflammation triggered by environmental levels of Cd²⁺. Together, our study points to the developmental toxicity of Cd in inducing CNS impairment and neuroinflammation thereby providing visualized etiological evidence of this heavy metal induced neurodevelopmental disorders. It's tempting to speculate that this research model might represent a promising tool not only for understanding the molecular mechanisms of Cd-induced neurotoxicity, but also for developing pharmacotherapies to mitigate the neurological damage resulting from exposure to Cd, and other neurotoxicants.

Beneficial Effects of Spirulina Consumption on Brain Health

Spirulina is a microscopic, filamentous cyanobacterium that grows in alkaline water bodies. It is extensively utilized as a nutraceutical food supplement all over the world due to its high levels of functional compounds, such as phycocyanins, phenols and polysaccharides, with anti-inflammatory, antioxidant, immunomodulating properties both in vivo and in vitro. Several scientific publications have suggested its positive effects in various pathologies such as cardiovascular diseases, hypercholesterolemia, hyperglycemia, obesity, hypertension, tumors and inflammatory diseases. Lately, different studies have demonstrated the neuroprotective role of Spirulina on the development of the neural system, senility and a number of pathological conditions, including neurological and neurodegenerative diseases. This review focuses on the role of Spirulina in the brain, highlighting how it exerts its beneficial anti-inflammatory and antioxidant effects, acting on glial cell activation, and in the prevention and/or progression of neurodegenerative diseases, in particular Parkinson’s disease, Alzheimer’s disease and Multiple Sclerosis; due to these properties, Spirulina could be considered a potential natural drug.

Perinatal Penicillin Exposure Affects Cortical Development and Sensory Processing

The prevalent use of antibiotics in pregnant women and neonates raises concerns about long-term risks for children’s health, but their effects on the central nervous system is not well understood. We studied the effects of perinatal penicillin exposure (PPE) on brain structure and function in mice with a therapeutically relevant regimen. We used a battery of behavioral tests to evaluate anxiety, working memory, and sensory processing, and immunohistochemistry to quantify changes in parvalbumin-expressing inhibitory interneurons (PV+ INs), perineuronal nets (PNNs), as well as microglia density and morphology. In addition, we performed mesoscale calcium imaging to study neural activity and functional connectivity across cortical regions, and two-photon imaging to monitor dendritic spine and microglial dynamics. We found that adolescent PPE mice have abnormal sensory processing, including impaired texture discrimination and altered prepulse inhibition. Such behavioral changes are associated with increased spontaneous neural activities in various cortical regions, and delayed maturation of PV+ INs in the somatosensory cortex. Furthermore, adolescent PPE mice have elevated elimination of dendritic spines on the apical dendrites of layer 5 pyramidal neurons, as well as increased ramifications and spatial coverage of cortical microglia. Finally, while synaptic defects are transient during adolescence, behavioral abnormalities persist into adulthood. Our study demonstrates that early-life exposure to antibiotics affects cortical development, leaving a lasting effect on brain functions.