CD206+ macrophages transventricularly infiltrate the early embryonic cerebral wall to differentiate into microglia

Quercetin ameliorates neuroinflammatory and neurodegenerative biomarkers in the brain and improves neurobehavioral parameters in a repeated intranasal amyloid-beta exposed model of Alzheimer's disease

Objectives: The aim of the present study was to study the potential therapeutic effects of quercetin in protection against repeated intranasal exposure of an amyloid-beta-induced mouse model. Methods: Mice received intranasal Aβ1-42 (5 μg/10 μL) exposure once daily for seven consecutive days. Quercetin was orally administered to them at 30 mg kg-1 and 100 mg kg-1 doses for one week starting from day five following Aβ1-42 peptide administration. Following this, the animals were evaluated for neurobehavioral parameters using a Morris water maze test and a novel object recognition test. Further to this, the biomarkers for neuroinflammation and neurodegeneration were evaluated in the hippocampus and cortex regions of the brain in these animals. Results: Multiple exposures to intranasal Aβ led to a significant decline in the learning and cognitive memory of the animals, whereas oral treatment with quercetin at dosages of 30 and 100 mg kg-1 alleviated Aβ-induced effects. Quercetin treatment significantly reduced Aβ accumulation, oxidative stress and proinflammatory cytokine biomarkers in the brain. In addition, it also alleviated the activation of astrocytic biomarkers, amyloid precursor protein and phosphorylated-tau proteins in the brain. Conclusion: Quercetin was found to be a potent antioxidant, anti-inflammatory compound with protection against neurodegenerative damage and improved learning and cognitive memory in a repeated Aβ-exposure model of AD.

Brain-Engrafted Monocyte-derived Macrophages from Blood and Skull-Bone Marrow Exhibit Distinct Identities from Microglia

Microglia are thought to originate exclusively from primitive macrophage progenitors in the yolk sac (YS) and to persist throughout life without much contribution from definitive hematopoiesis. Here, using lineage tracing, pharmacological manipulation, and RNA-sequencing, we elucidated the presence and characteristics of monocyte-derived macrophages (MDMs) in the brain parenchyma at baseline and during microglia repopulation, and defined the core transcriptional signatures of brain-engrafted MDMs. Lineage tracing mouse models revealed that MDMs transiently express CD206 during brain engraftment as CD206 + microglia precursors in the YS. We found that brain-engrafted MDMs exhibit transcriptional and epigenetic characteristics akin to meningeal macrophages, likely due to environmental imprinting within the meningeal space. Utilizing parabiosis and skull transplantation, we demonstrated that monocytes from both peripheral blood and skull bone marrow can repopulate microglia-depleted brains. Our results reveal the heterogeneous origins and cellular dynamics of brain parenchymal macrophages at baseline and in models of microglia depletion.

Intermittent Fasting Improves Social Interaction and Decreases Inflammatory Markers in Cortex and Hippocampus

Autism spectrum disorder (ASD) is a psychiatric condition characterized by reduced social interaction, anxiety, and stereotypic behaviors related to neuroinflammation and microglia activation. We demonstrated that maternal exposure to Western diet (cafeteria diet or CAF) induced microglia activation, systemic proinflammatory profile, and ASD-like behavior in the offspring. Here, we aimed to identify the effect of alternate day fasting (ADF) as a non-pharmacologic strategy to modulate neuroinflammation and ASD-like behavior in the offspring prenatally exposed to CAF diet. We found that ADF increased plasma beta-hydroxybutyrate (BHB) levels in the offspring exposed to control and CAF diets but not in the cortex (Cx) and hippocampus (Hpp). We observed that ADF increased the CD45 + cells in Cx of both groups; In control individuals, ADF promoted accumulation of CD206 + microglia cells in choroid plexus (CP) and increased in CD45 + macrophages cells and lymphocytes in the Cx. Gestational exposure to CAF diet promoted defective sociability in the offspring; ADF improved social interaction and increased microglia CD206 + in the Hpp and microglia complexity in the dentate gyrus. Additionally, ADF led to attenuation of the ER stress markers (Bip/ATF6/p-JNK) in the Cx and Hpp. Finally, biological modeling showed that fasting promotes higher microglia complexity in Cx, which is related to improvement in social interaction, whereas in dentate gyrus sociability is correlated with less microglia complexity. These data suggest a contribution of intermittent fasting as a physiological stimulus capable of modulating microglia phenotype and complexity in the brain, and social interaction in male mice.

Redefining the ontogeny of hyalocytes as yolk sac-derived tissue-resident macrophages of the vitreous body

BACKGROUND

The eye is a highly specialized sensory organ which encompasses the retina as a part of the central nervous system, but also non-neural compartments such as the transparent vitreous body ensuring stability of the eye globe and a clear optical axis. Hyalocytes are the tissue-resident macrophages of the vitreous body and are considered to play pivotal roles in health and diseases of the vitreoretinal interface, such as proliferative vitreoretinopathy or diabetic retinopathy. However, in contrast to other ocular macrophages, their embryonic origin as well as the extent to which these myeloid cells might be replenished by circulating monocytes remains elusive.

RESULTS

In this study, we combine transgenic reporter mice, embryonic and adult fate mapping approaches as well as parabiosis experiments with multicolor immunofluorescence labeling and confocal laser-scanning microscopy to comprehensively characterize the murine hyalocyte population throughout development and in adulthood. We found that murine hyalocytes express numerous well-known myeloid cell markers, but concomitantly display a distinct immunophenotype that sets them apart from retinal microglia. Embryonic pulse labeling revealed a yolk sac-derived origin of murine hyalocytes, whose precursors seed the developing eye prenatally. Finally, postnatal labeling and parabiosis established the longevity of hyalocytes which rely on Colony Stimulating Factor 1 Receptor (CSF1R) signaling for their maintenance, independent of blood-derived monocytes.

CONCLUSION

Our study identifies hyalocytes as long-living progeny of the yolk sac hematopoiesis and highlights their role as integral members of the innate immune system of the eye. As a consequence of their longevity, immunosenescence processes may culminate in hyalocyte dysfunction, thereby contributing to the development of vitreoretinal diseases. Therefore, myeloid cell-targeted therapies that convey their effects through the modification of hyalocyte properties may represent an interesting approach to alleviate the burden imposed by diseases of the vitreoretinal interface.

A novel preparation for histological analyses of intraventricular macrophages in the embryonic brain

Microglia colonize the brain starting on embryonic day (E) 9.5 in mice, and their population increases with development. We have previously demonstrated that some microglia are derived from intraventricular macrophages, which frequently infiltrate the pallium at E12.5. To address how the infiltration of intraventricular macrophages is spatiotemporally regulated, histological analyses detecting how these cells associate with the surrounding cells at the site of infiltration into the pallial surface are essential. Using two-photon microscopy-based in vivo imaging, we demonstrated that most intraventricular macrophages adhere to the ventricular surface. This is a useful tool for imaging intraventricular macrophages maintaining their original position, but this method cannot be used for observing deeper brain regions. Meanwhile, we found that conventional cryosection-based and naked pallial slice-based observation resulted in unexpected detachment from the ventricular surface of intraventricular macrophages and their mislocation, suggesting that previous histological analyses might have failed to determine their physiological number and location in the ventricular space. To address this, we sought to establish a methodological preparation that enables us to delineate the structure and cellular interactions when intraventricular macrophages infiltrate the pallium. Here, we report that brain slices pretreated with agarose-embedding maintained adequate density and proper positioning of intraventricular macrophages on the ventricular surface. This method also enabled us to perform the immunostaining. We believe that this is helpful for conducting histological analyses to elucidate the mechanisms underlying intraventricular macrophage infiltration into the pallium and their cellular properties, leading to further understanding of the process of microglial colonization into the developing brain.

The molecular determinants of microglial developmental dynamics

Microglia constitute the largest population of parenchymal macrophages in the brain and are considered a unique subset of central nervous system glial cells owing to their extra-embryonic origins in the yolk sac. During development, microglial progenitors readily proliferate and eventually colonize the entire brain. In this Review, we highlight the origins of microglial progenitors and their entry routes into the brain and discuss the various molecular and non-molecular determinants of their fate, which may inform their specific functions. Specifically, we explore recently identified mechanisms that regulate microglial colonization of the brain, including the availability of space, and describe how the expansion of highly proliferative microglial progenitors facilitates the occupation of the microglial niche. Finally, we shed light on the factors involved in establishing microglial identity in the brain.

Spatiotemporal dynamics of the CD11c+ microglial population in the mouse brain and spinal cord from developmental to adult stages

CD11c-positive (CD11c+) microglia have attracted considerable attention because of their potential implications in central nervous system (CNS) development, homeostasis, and disease. However, the spatiotemporal dynamics of the proportion of CD11c+ microglia in individual CNS regions are poorly understood. Here, we investigated the proportion of CD11c+ microglia in six CNS regions (forebrain, olfactory bulb, diencephalon/midbrain, cerebellum, pons/medulla, and spinal cord) from the developmental to adult stages by flow cytometry and immunohistochemical analyses using a CD11c reporter transgenic mouse line, Itgax-Venus. We found that the proportion of CD11c+ microglia in total microglia varied between CNS regions during postnatal development. Specifically, the proportion was high in the olfactory bulb and cerebellum at postnatal day P(4) and P7, respectively, and approximately half of the total microglia were CD11c+. The proportion declined sharply in all regions to P14, and the low percentage persisted over P56. In the spinal cord, the proportion of CD11c+ microglia was also high at P4 and declined to P14, but increased again at P21 and thereafter. Interestingly, the distribution pattern of CD11c+ microglia in the spinal cord markedly changed from gray matter at P4 to white matter at P21. Collectively, our findings reveal the differences in the spatiotemporal dynamics of the proportion of CD11c+ microglia among CNS regions from early development to adult stages in normal mice. These findings improve our understanding of the nature of microglial heterogeneity and its dynamics in the CNS.

Interactions between neural cells and blood vessels in central nervous system development

The sophisticated function of the central nervous system (CNS) is largely supported by proper interactions between neural cells and blood vessels. Accumulating evidence has demonstrated that neurons and glial cells support the formation of blood vessels, which in turn, act as migratory scaffolds for these cell types. Neural progenitors are also involved in the regulation of blood vessel formation. This mutual interaction between neural cells and blood vessels is elegantly controlled by several chemokines, growth factors, extracellular matrix, and adhesion molecules such as integrins. Recent research has revealed that newly migrating cell types along blood vessels repel other preexisting migrating cell types, causing them to detach from the blood vessels. In this review, we discuss vascular formation and cell migration, particularly during development. Moreover, we discuss how the crosstalk between blood vessels and neurons and glial cells could be related to neurodevelopmental disorders.

The niche matters: origin, function and fate of CNS-associated macrophages during health and disease

There are several cellular and acellular structural barriers associated with the brain interfaces, which include the dura, the leptomeninges, the perivascular space and the choroid plexus epithelium. Each structure is enriched by distinct myeloid populations, which mainly originate from erythromyeloid precursors (EMP) in the embryonic yolk sac and seed the CNS during embryogenesis. However, depending on the precise microanatomical environment, resident myeloid cells differ in their marker profile, turnover and the extent to which they can be replenished by blood-derived cells. While some EMP-derived cells seed the parenchyma to become microglia, others engraft the meninges and become CNS-associated macrophages (CAMs), also referred to as border-associated macrophages (BAMs), e.g., leptomeningeal macrophages (MnMΦ). Recent data revealed that MnMΦ migrate into perivascular spaces postnatally where they differentiate into perivascular macrophages (PvMΦ). Under homeostatic conditions in pathogen-free mice, there is virtually no contribution of bone marrow-derived cells to MnMΦ and PvMΦ, but rather to macrophages of the choroid plexus and dura. In neuropathological conditions in which the blood-brain barrier is compromised, however, an influx of bone marrow-derived cells into the CNS can occur, potentially contributing to the pool of CNS myeloid cells. Simultaneously, resident CAMs may also proliferate and undergo transcriptional and proteomic changes, thereby, contributing to the disease outcome. Thus, both resident and infiltrating myeloid cells together act within their microenvironmental niche, but both populations play crucial roles in the overall disease course. Here, we summarize the current understanding of the sources and fates of resident CAMs in health and disease, and the role of the microenvironment in influencing their maintenance and function.

Brain perivascular macrophages: current understanding and future prospects

Brain perivascular macrophages are specialized populations of macrophages that reside in the space around cerebral vessels, such as penetrating arteries and venules. With the help of cutting-edge technologies, such as cell fate mapping and single-cell multi-omics, their multifaceted, pivotal roles in phagocytosis, antigen presentation, vascular integrity maintenance and metabolic regulation have more recently been further revealed under physiological conditions. Accumulating evidence also implies that perivascular macrophages are involved in the pathogenesis of neurodegenerative disease, cerebrovascular dysfunction, autoimmune disease, traumatic brain injury and epilepsy. They can act in either protective or detrimental ways depending on the disease course and stage. However, the underlying mechanisms of perivascular macrophages remain largely unknown. Therefore, we highlight potential future directions in research on perivascular macrophages, including the utilization of genetic mice and novel therapeutic strategies that target these unique immune cells for neuroprotective purposes. In conclusion, this review provides a comprehensive update on the current knowledge of brain perivascular macrophages, shedding light on their pivotal roles in central nervous system health and disease.

Radial glia promote microglial development through integrin αVβ8 -TGFβ1 signaling

Microglia diversity emerges from interactions between intrinsic genetic programs and environment-derived signals, but how these processes unfold and interact in the developing brain remains unclear. Here, we show that radial glia-expressed integrin beta 8 (ITGB8) expressed in radial glia progenitors activates microglia-expressed TGFβ1, permitting microglial development. Domain-restricted deletion of Itgb8 in these progenitors establishes complementary regions with developmentally arrested "dysmature" microglia that persist into adulthood. In the absence of autocrine TGFβ1 signaling, we find that microglia adopt a similar dysmature phenotype, leading to neuromotor symptoms almost identical to Itgb8 mutant mice. In contrast, microglia lacking the TGFβ signal transducers Smad2 and Smad3 have a less polarized dysmature phenotype and correspondingly less severe neuromotor dysfunction. Finally, we show that non-canonical (Smad-independent) signaling partially suppresses disease and development associated gene expression, providing compelling evidence for the adoption of microglial developmental signaling pathways in the context of injury or disease.

Towards an Understanding of Microglia and Border-Associated Macrophages

The central nervous system (CNS) plays a crucial role in regulating bodily functions by sensing and integrating environmental cues and maintaining proper physiological conditions. Recent research has revealed that CNS functions are closely coordinated with the immune system. As even minor disturbances of the immune system in the CNS can lead to various dysfunctions, diseases, or even death, it is highly specialized and segregated from that in peripheral regions. Microglia in the parenchyma and macrophages at the interface between the CNS and peripheral regions are essential immune cells in the CNS that monitor environmental changes. Recent omics analyses have revealed that these cells exhibit highly heterogeneous populations. In this review, we summarize the functions and diversity of microglia in the brain parenchyma and those of macrophages in the border regions, such as the meninges, perivascular spaces, and choroid plexus.

Shaping the cerebral cortex by cellular crosstalk

The cerebral cortex is the brain's outermost layer. It is responsible for processing motor and sensory information that support high-level cognitive abilities and shape personality. Its development and functional organization strongly rely on cell communication that is established via an intricate system of diffusible signals and physical contacts during development. Interfering with this cellular crosstalk can cause neurodevelopmental disorders. Here, we review how crosstalk between migrating cells and their environment influences cerebral cortex development, ranging from neurogenesis to synaptogenesis and assembly of cortical circuits.

The multifaceted roles of embryonic microglia in the developing brain

Microglia are the resident immune cells of the central nervous system (CNS). Microglia originate from erythromyeloid progenitors in the yolk sac at the early embryonic stage, and these progenitors then colonize the CNS through extensive migration and proliferation during development. Microglia account for 10% of all cells in the adult brain, whereas the proportion of these cells in the embryonic brain is only 0.5-1.0%. Nevertheless, microglia in the developing brain widely move their cell body within the structure by extending filopodia; thus, they can interact with surrounding cells, such as neural lineage cells and vascular-structure-composing cells. This active microglial motility suggests that embryonic microglia play a pivotal role in brain development. Indeed, recent increasing evidence has revealed diverse microglial functions at the embryonic stage. For example, microglia control differentiation of neural stem cells, regulate the population size of neural progenitors and modulate the positioning and function of neurons. Moreover, microglia exert functions not only on neural lineage cells but also on blood vessels, such as supporting vascular formation and integrity. This review summarizes recent advances in the understanding of microglial cellular dynamics and multifaceted functions in the developing brain, with particular focus on the embryonic stage, and discusses the fundamental molecular mechanisms underlying their behavior.