Microglia remodel synapses by presynaptic trogocytosis and spine head filopodia induction

Rapid phagosome isolation enables unbiased multiomic analysis of human microglial phagosomes

Microglia are the resident macrophages of the central nervous system (CNS). Their phagocytic activity is central during brain development and homeostasis-and in a plethora of brain pathologies. However, little is known about the composition, dynamics, and function of human microglial phagosomes under homeostatic and pathological conditions. Here, we developed a method for rapid isolation of pure and intact phagosomes from human pluripotent stem cell-derived microglia under various in vitro conditions, and from human brain biopsies, for unbiased multiomic analysis. Phagosome profiling revealed that microglial phagosomes were equipped to sense minute changes in their environment and were highly dynamic. We detected proteins involved in synapse homeostasis, or implicated in brain pathologies, and identified the phagosome as the site where quinolinic acid was stored and metabolized for de novo nicotinamide adenine dinucleotide (NAD+) generation in the cytoplasm. Our findings highlight the central role of phagosomes in microglial functioning in the healthy and diseased brain.

Emerging evidence of context-dependent synapse elimination by phagocytes in the CNS

Precise synapse elimination is essential for the establishment of a fully developed neural circuit during brain development and higher function in adult brain. Beyond immune and nutrition support, recent groundbreaking studies have revealed that phagocytic microglia and astrocytes can actively and selectively eliminate synapses in normal and diseased brains, thereby mediating synapse loss and maintaining circuit homeostasis. Multiple lines of evidence indicate that the mechanisms of synapse elimination by phagocytic glia are not universal but rather depend on specific contexts and detailed neuron-glia interactions. The mechanism of synapse elimination by phagocytic glia is dependent on neuron-intrinsic factors and many innate immune and local apoptosis-related molecules. During development, microglial synapse engulfment in the visual thalamus is primarily influenced by the classic complement pathway, whereas in the barrel cortex, the fractalkine pathway is dominant. In Alzheimer's disease, microglia employ complement-dependent mechanisms for synapse engulfment in tauopathy and early β-amyloid pathology, but microglia are not involved in synapse loss at late β-amyloid stages. Phagocytic microglia also engulf synapses in a complement-dependent way in schizophrenia, anxiety, and stress. In addition, phagocytic astrocytes engulf synapses in a MEGF10-dependent way during visual development, memory, and stroke. Furthermore, the mechanism of a phenomenon that phagocytes selectively eliminate excitatory and inhibitory synapses is also emphasized in this review. We hypothesize that elucidating context-dependent synapse elimination by phagocytic microglia and astrocytes may reveal the molecular basis of synapse loss in neural disorders and provide a rationale for developing novel candidate therapies that target synapse loss and circuit homeostasis.

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

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

The role of microglial TREM2 in development: A path toward neurodegeneration?

The nervous and the immune systems undergo a continuous cross talk, starting from early development and continuing throughout adulthood and aging. Defects in this cross talk contribute to neurodevelopmental and neurodegenerative diseases. Microglia are the resident immune cells in the brain that are primarily involved in this bidirectional communication. Among the microglial genes, trem2 is a key player, controlling the functional state of microglia and being at the forefront of many processes that require interaction between microglia and other brain components, such as neurons and oligodendrocytes. The present review focuses on the early developmental window, describing the early brain processes in which TREM2 is primarily involved, including the modulation of synapse formation and elimination, the control of neuronal bioenergetic states as well as the contribution to myelination processes and neuronal circuit formation. By causing imbalances during these early maturation phases, dysfunctional TREM2 may have a striking impact on the adult brain, making it a more sensitive target for insults occurring during adulthood and aging.

A hominoid-specific signaling axis regulating the tempo of synaptic maturation

Human cortical neurons (hCNs) exhibit high dendritic complexity and synaptic density, and the maturation process is greatly protracted. However, the molecular mechanism governing these specific features remains unclear. Here, we report that the hominoid-specific gene TBC1D3 promotes dendritic arborization and protracts the pace of synaptogenesis. Ablation of TBC1D3 in induced hCNs causes reduction of dendritic growth and precocious synaptic maturation. Forced expression of TBC1D3 in the mouse cortex protracts synaptic maturation while increasing dendritic growth. Mechanistically, TBC1D3 functions via interaction with MICAL1, a monooxygenase that mediates oxidation of actin filament. At the early stage of differentiation, the TBC1D3/MICAL1 interaction in the cytosol promotes dendritic growth via F-actin oxidation and enhanced actin dynamics. At late stages, TBC1D3 escorts MICAL1 into the nucleus and downregulates the expression of genes related with synaptic maturation through interaction with the chromatin remodeling factor ATRX. Thus, this study delineates the molecular mechanisms underlying human neuron development.

From Blur to Brilliance: The Ascendance of Advanced Microscopy in Neuronal Cell Biology

The intricate network of the brain's neurons and synapses poses unparalleled challenges for research, distinct from other biological studies. This is particularly true when dissecting how neurons and their functional units work at a cell biological level. While traditional microscopy has been foundational, it was unable to reveal the deeper complexities of neural interactions. However, an imaging renaissance has transformed our capabilities. Advancements in light and electron microscopy, combined with correlative imaging, now achieve unprecedented resolutions, uncovering the most nuanced neural structures. Maximizing these tools requires more than just technical proficiency. It is crucial to align research aims, allocate resources wisely, and analyze data effectively. At the heart of this evolution is interdisciplinary collaboration, where various experts come together to translate detailed imagery into significant biological insights. This review navigates the latest developments in microscopy, underscoring both the promise of and prerequisites for bending this powerful tool set to understanding neuronal cell biology.

Microglia and Astrocytes in Alzheimer's Disease: Significance and Summary of Recent Advances

Alzheimer's disease, one of the most common forms of dementia, is characterized by a slow progression of cognitive impairment and neuronal loss. Currently, approved treatments for AD are hindered by various side effects and limited efficacy. Despite considerable research, practical treatments for AD have not been developed. Increasing evidence shows that glial cells, especially microglia and astrocytes, are essential in the initiation and progression of AD. During AD progression, activated resident microglia increases the ability of resting astrocytes to transform into reactive astrocytes, promoting neurodegeneration. Extensive clinical and molecular studies show the involvement of microglia and astrocyte-mediated neuroinflammation in AD pathology, indicating that microglia and astrocytes may be potential therapeutic targets for AD. This review will summarize the significant and recent advances of microglia and astrocytes in the pathogenesis of AD in three parts. First, we will review the typical pathological changes of AD and discuss microglia and astrocytes in terms of function and phenotypic changes. Second, we will describe microglia and astrocytes' physiological and pathological role in AD. These roles include the inflammatory response, "eat me" and "don't eat me" signals, Aβ seeding, propagation, clearance, synapse loss, synaptic pruning, remyelination, and demyelination. Last, we will review the pharmacological and non-pharmacological therapies targeting microglia and astrocytes in AD. We conclude that microglia and astrocytes are essential in the initiation and development of AD. Therefore, understanding the new role of microglia and astrocytes in AD progression is critical for future AD studies and clinical trials. Moreover, pharmacological, and non-pharmacological therapies targeting microglia and astrocytes, with specific studies investigating microglia and astrocyte-mediated neuronal damage and repair, may be a promising research direction for future studies regarding AD treatment and prevention.

Bone marrow-derived myeloid cells transiently colonize the brain during postnatal development and interact with glutamatergic synapses

Although the roles of embryonic yolk sac-derived, resident microglia in neurodevelopment were extensively studied, the possible involvement of bone marrow-derived cells remains elusive. In this work, we used a fate-mapping strategy to selectively label bone marrow-derived cells and their progeny in the brain (FLT3+IBA1+). FLT3+IBA1+ cells were confirmed to be transiently present in the healthy brain during early postnatal development. FLT3+IBA1+ cells have a distinct morphology index at postnatal day(P)0, P7, and P14 compared with neighboring microglia. FLT3+IBA1+ cells also express the microglial markers P2RY12 and TMEM119 and interact with VGLUT1 synapses at P14. Scanning electron microscopy indeed showed that FLT3+ cells contact and engulf pre-synaptic elements. Our findings suggest FLT3+IBA1+ cells might assist microglia in their physiological functions in the developing brain including synaptic pruning which is performed using their purinergic sensors. Our findings stimulate further investigation on the involvement of peripheral macrophages during homeostatic and pathological development.

The role of microglia in early neurodevelopment and the effects of maternal immune activation

Activation of the maternal immune system during gestation has been associated with an increased risk for neurodevelopmental disorders in the offspring, particularly schizophrenia and autism spectrum disorder. Microglia, the tissue-resident macrophages of the central nervous system, are implicated as potential mediators of this increased risk. Early in development, microglia start populating the embryonic central nervous system and in addition to their traditional role as immune responders under homeostatic conditions, microglia are also intricately involved in various early neurodevelopmental processes. The timing of immune activation may interfere with microglia functioning during early neurodevelopment, potentially leading to long-term consequences in postnatal life. In this review we will discuss the involvement of microglia in brain development during the prenatal and early postnatal stages of life, while also examining the effects of maternal immune activation on microglia and neurodevelopmental processes. Additionally, we discuss recent single cell RNA-sequencing studies focusing on microglia during prenatal development, and hypothesize how early life microglial priming, potentially through epigenetic reprogramming, may be related to neurodevelopmental disorders.

Microglia mediate the early-life programming of adult glucose control

Mammalian glucose homeostasis is, in part, nutritionally programmed during early neonatal life, a critical window for the formation of synapses between hypothalamic glucoregulatory centers. Although microglia are known to prune synapses throughout the brain, their specific role in refining hypothalamic glucoregulatory circuits remains unknown. Here, we show that microglia in the mediobasal hypothalamus (MBH) of mice actively engage in synaptic pruning during early life. Microglial phagocytic activity is induced following birth, regresses upon weaning from maternal milk, and is exacerbated by feeding dams a high-fat diet while lactating. In particular, we show that microglia refine perineuronal nets (PNNs) within the neonatal MBH. Indeed, transiently depleting microglia before weaning (P6-16), but not afterward (P21-31), remarkably increased PNN abundance in the MBH. Furthermore, mice lacking microglia only from P6-16 had glucose intolerance due to impaired glucose-responsive pancreatic insulin secretion in adulthood, a phenotype not seen if microglial depletion occurred after weaning. Viral retrograde tracing revealed that this impairment is linked to a reduction in the number of neurons in specific hypothalamic glucoregulatory centers that synaptically connect to the pancreatic β-cell compartment. These findings show that microglia facilitate synaptic plasticity in the MBH during early life through a process that includes PNN refinement, to establish hypothalamic circuits that regulate adult glucose homeostasis.

Neuroglia in cognitive reserve

The concept of cognitive reserve was born to account for the disjunction between the objective extent of brain damage in pathology and its clinical and intellectual outcome. The cognitive reserve comprises structural (brain reserve) and functional (brain maintenance, resilience, compensation) aspects of the nervous tissue reflecting exposome-driven life-long plasticity, which defines the ability of the brain to withstand aging and pathology. The mechanistic background of this concept was primarily focused on adaptive changes in neurones and neuronal networks. We present arguments favoring the more inclusive view, positing that neuroglia are fundamental for defining the cognitive reserve through homeostatic, neuroprotective, and neurodegenerative mechanisms. Neuroglia are critical for the life-long shaping of synaptically connected neuronal circuits as well as the brain connectome thus defining cognitive reserve. Neuroglial homeostatic and protective physiological responses define brain maintenance and resilience, while neuroglia regenerative capabilities are critical for brain compensation in pathology. Targeting neuroglia may represent an untrodden path for prolonging cognitive longevity.

Microglial TNFα controls daily changes in synaptic GABAARs and sleep slow waves

Microglia sense the changes in their environment. How microglia actively translate these changes into suitable cues to adapt brain physiology is unknown. We reveal an activity-dependent regulation of cortical inhibitory synapses by microglia, driven by purinergic signaling acting on P2RX7 and mediated by microglia-derived TNFα. We demonstrate that sleep induces microglia-dependent synaptic enrichment of GABAARs in a manner dependent on microglial TNFα and P2RX7. We further show that microglia-specific depletion of TNFα alters slow waves during NREM sleep and blunt memory consolidation in sleep-dependent learning tasks. Together, our results reveal that microglia orchestrate sleep-intrinsic plasticity of synaptic GABAARs, sculpt sleep slow waves, and support memory consolidation.

Microglial sex differences in innate high anxiety and modulatory effects of minocycline

Microglia modulate synaptic refinement in the central nervous system (CNS). We have previously shown that a mouse model with innate high anxiety-related behavior (HAB) displays higher CD68+ microglia density in the key regions of anxiety circuits compared to mice with normal anxiety-related behavior (NAB) in males, and that minocycline treatment attenuated the enhanced anxiety of HAB male. Given that a higher prevalence of anxiety is widely reported in females compared to males, little is known concerning sex differences at the cellular level. Herein, we address this by analyzing microglia heterogeneity and function in the HAB and NAB brains of both sexes. Single-cell RNA sequencing revealed ten distinct microglia clusters varied by their frequency and gene expression profile. We report striking sex differences, especially in the major microglia clusters of HABs, indicating a higher expression of genes associated with phagocytosis and synaptic engulfment in the female compared to the male. On a functional level, we show that female HAB microglia engulfed a greater amount of hippocampal vGLUT1+ excitatory synapses compared to the male. We moreover show that female HAB microglia engulfed more synaptosomes compared to the male HAB in vitro. Due to previously reported effects of minocycline on microglia, we finally administered oral minocycline to HABs of both sexes and showed a significant reduction in the engulfment of synapses by female HAB microglia. In parallel to our microglia-specific findings, we further showed an anxiolytic effect of minocycline on female HABs, which is complementary to our previous findings in the male HABs. Our study, therefore, identifies the altered function of synaptic engulfment by microglia as a potential avenue to target and resolve microglia heterogeneity in mice with innate high anxiety.

Clusterin is a Potential Therapeutic Target in Alzheimer's Disease

In recent years, Clusterin, a glycosylated protein with multiple biological functions, has attracted extensive research attention. It is closely associated with the physiological and pathological states within the organism. Particularly in Alzheimer's disease (AD) research, Clusterin plays a significant role in the disease's occurrence and progression. Numerous studies have demonstrated a close association between Clusterin and AD. Firstly, the expression level of Clusterin in the brain tissue of AD patients is closely related to pathological progression. Secondly, Clusterin is involved in the deposition and formation of β-amyloid, which is a crucial process in AD development. Furthermore, Clusterin may affect the pathogenesis of AD through mechanisms such as regulating inflammation, controlling cell apoptosis, and clearing pathological proteins. Therefore, further research on the relationship between Clusterin and AD will contribute to a deeper understanding of the etiology of this neurodegenerative disease and provide a theoretical basis for developing early diagnostic and therapeutic strategies for AD. This also makes Clusterin one of the research focuses as a potential biomarker for AD diagnosis and treatment monitoring.

Microglia contribute to neuronal synchrony despite endogenous ATP-related phenotypic transformation in acute mouse brain slices

Acute brain slices represent a workhorse model for studying the central nervous system (CNS) from nanoscale events to complex circuits. While slice preparation inherently involves tissue damage, it is unclear how microglia, the main immune cells and damage sensors of the CNS react to this injury and shape neuronal activity ex vivo. To this end, we investigated microglial phenotypes and contribution to network organization and functioning in acute brain slices. We reveal time-dependent microglial phenotype changes influenced by complex extracellular ATP dynamics through P2Y12R and CX3CR1 signalling, which is sustained for hours in ex vivo mouse brain slices. Downregulation of P2Y12R and changes of microglia-neuron interactions occur in line with alterations in the number of excitatory and inhibitory synapses over time. Importantly, functional microglia modulate synapse sprouting, while microglial dysfunction results in markedly impaired ripple activity both ex vivo and in vivo. Collectively, our data suggest that microglia are modulators of complex neuronal networks with important roles to maintain neuronal network integrity and activity. We suggest that slice preparation can be used to model time-dependent changes of microglia-neuron interactions to reveal how microglia shape neuronal circuits in physiological and pathological conditions.

Impacts of age and environment on postnatal microglial activity: Consequences for cognitive function following early life adversity

Early life adversity (ELA) increases the likelihood of later-life neuropsychiatric disorders and cognitive dysfunction. Importantly, ELA, neuropsychiatric disorders, and cognitive deficits all involve aberrant immune signaling. Microglia are the primary neuroimmune cells and regulate brain development. Microglia are particularly sensitive to early life insults, which can program their responses to future challenges. ELA in the form of maternal separation (MS) in rats alters later-life microglial morphology and the inflammatory profile of the prefrontal cortex, a region important for cognition. However, the role of microglial responses during MS in the development of later cognition is not known. Therefore, here we aimed to determine whether the presence of microglia during MS mediates long-term impacts on adult working memory. Clodronate liposomes were used to transiently deplete microglia from the brain, while empty liposomes were used as a control. We hypothesized that if microglia mediate the long-term impacts of ELA on working memory in adulthood, then depleting microglia during MS would prevent these deficits. Importantly, microglial function shifts throughout the neonatal period, so an exploratory investigation assessed whether depletion during the early versus late neonatal period had different effects on adult working memory. Surprisingly, empty liposome treatment during the early, but not late, postnatal period induced microglial activity changes that compounded with MS to impair working memory in females. In contrast, microglial depletion later in infancy impaired later life working memory in females, suggesting that microglial function during late infancy plays an important role in the development of cognitive function. Together, these findings suggest that microglia shift their sensitivity to early life insults across development. Our findings also highlight the potential for MS to impact some developmental processes only when compounded with additional neuroimmune challenges in a sex-dependent manner.

Microglia target synaptic sites early during excitatory circuit disassembly in neurodegeneration

During development, microglia prune excess synapses to refine neuronal circuits. In neurodegeneration, the role of microglia-mediated synaptic pruning in circuit remodeling and dysfunction is important for developing therapies aimed at modulating microglial function. Here we analyzed the role of microglia in the synapse disassembly of degenerating postsynaptic neurons in the inner retina. After inducing transient intraocular pressure elevation to injure retinal ganglion cells, microglia increase in number, shift to ameboid morphology, and exhibit greater process movement. Furthermore, due to the greater number of microglia, there is increased colocalization of microglia with synaptic components throughout the inner plexiform layer and with excitatory synaptic sites along individual ganglion cell dendrites. Microglia depletion partially restores ganglion cell function, suggesting that microglia activation may be neurotoxic in early neurodegeneration. Our results demonstrate the important role of microglia in synapse disassembly in degenerating circuits, highlighting their recruitment to synaptic sites early after neuronal injury.

Viral-mediated inflammation by Poly I:C induces the chemokine CCL5 in NK cells and its receptors CCR1 and CCR5 in microglia in the neonatal rat cerebellum

Objectives

To study the effect of viral inflammation induced by Polyinosinic:polycytidylic acid (PIC) on the cerebellum during a critical period of development in rats.

Methods

Neonatal rat pups were treated with PIC on postnatal days (PN) 8 and 10 after which we quantified RNA using Nanostring, qRT-PCR and RNAscope and analyzed immune cells through flow cytometry and immunohistochemistry on PN11. Using the same paradigm, we also analyzed play juvenile behavior, anxiety-like behavior, motor balance using the balance beam and the rotarod assays as well as fine motor behavior using the sunflower seed opening test.

Results

We determined that male and female pups treated with PIC reacted with a significant increase in CCL5, a chemotactic cytokine that attracts T-cells, eosinophils and basophils to the site of inflammation, at PN11. PIC treatment also increased the expression of two receptors for CCL5, CCR1 and CCR5 in the cerebellar vermis in both males and females at PN11. In-situ hybridization (RNAscope®) for specific transcripts revealed that microglia express both CCL5 receptors under inflammatory and non-inflammatory conditions in both males and females. PIC treatment also increased the total number of CCL5+ cells in the developing cerebellum which were determined to be both natural killer cells and T-cells. There were modest but significant impacts of PIC treatment on large and fine motor skills and juvenile play behavior.

Conclusions

Our findings suggest an important role for CCL5 and other immune cells in mediating inflammation in the developing cerebellum that potentially impact the maturation of cerebellar neurons during a critical period of development.

Microglia phagocytic mechanisms: Development informing disease

Microglia are tissue-resident macrophages and professional phagocytes of the central nervous system (CNS). In development, microglia-mediated phagocytosis is important for sculpting the cellular architecture. This includes the engulfment of dead/dying cells, pruning extranumerary synapses and axons, and phagocytosing fragments of myelin sheaths. Intriguingly, these developmental phagocytic mechanisms by which microglia sculpt the CNS are now appreciated as important for eliminating synapses, myelin, and proteins during neurodegeneration. Here, we discuss parallels between neurodevelopment and neurodegeneration, which highlights how development is informing disease. We further discuss recent advances and challenges towards therapeutically targeting these phagocytic pathways and how we can leverage development to overcome these challenges.

Microglia-neuron-vascular interactions in ischemia

Cerebral ischemia is a devastating condition that results in impaired blood flow in the brain leading to acute brain injury. As the most common form of stroke, occlusion of cerebral arteries leads to a characteristic sequence of pathophysiological changes in the brain tissue. The mechanisms involved, and comorbidities that determine outcome after an ischemic event appear to be highly heterogeneous. On their own, the processes leading to neuronal injury in the absence of sufficient blood supply to meet the metabolic demand of the cells are complex and manifest at different temporal and spatial scales. While the contribution of non-neuronal cells to stroke pathophysiology is increasingly recognized, recent data show that microglia, the main immune cells of the central nervous system parenchyma, play previously unrecognized roles in basic physiological processes beyond their inflammatory functions, which markedly change during ischemic conditions. In this review, we aim to discuss some of the known microglia-neuron-vascular interactions assumed to contribute to the acute and delayed pathologies after cerebral ischemia. Because the mechanisms of neuronal injury have been extensively discussed in several excellent previous reviews, here we focus on some recently explored pathways that may directly or indirectly shape neuronal injury through microglia-related actions. These discoveries suggest that modulating gliovascular processes in different forms of stroke and other neurological disorders might have presently unexplored therapeutic potential in combination with neuroprotective and flow restoration strategies.

Lifelong absence of microglia alters hippocampal glutamatergic networks but not synapse and spine density

Microglia sculpt developing neural circuits by eliminating excess synapses in a process called synaptic pruning, by removing apoptotic neurons, and by promoting neuronal survival. To elucidate the role of microglia during embryonic and postnatal brain development, we used a mouse model deficient in microglia throughout life by deletion of the fms-intronic regulatory element (FIRE) in the Csf1r locus. Surprisingly, young adult Csf1rΔFIRE/ΔFIRE mice display no changes in excitatory and inhibitory synapse number and spine density of CA1 hippocampal neurons compared with Csf1r+/+ littermates. However, CA1 neurons are less excitable, receive less CA3 excitatory input and show altered synaptic properties, but this does not affect novel object recognition. Cytokine profiling indicates an anti-inflammatory state along with increases in ApoE levels and reactive astrocytes containing synaptic markers in Csf1rΔFIRE/ΔFIRE mice. Notably, these changes in Csf1rΔFIRE/ΔFIRE mice closely resemble the effects of acute microglial depletion in adult mice after normal development. Our findings suggest that microglia are not mandatory for synaptic pruning, and that in their absence pruning can be achieved by other mechanisms.

The paradoxical role of zinc on microglia

Zinc is an essential trace element for humans, and its homeostasis is essential for the health of the central nervous system. Microglia, the resident immune cells in the central nervous system, play the roles of sustaining, nourishing, and immune surveillance. Microglia are sensitive to microenvironment changes and are easily activated to M1 phenotype to enhance disease progression or the M2 phenotype to improve peripheral nerves injury repair. Zinc is requisite for microglial activation, However, the cytotoxicity outcome of zinc against microglia, the activated microglia phenotype, and activated microglia function are ambiguous. Herein, we have reviewed the neurological function of zinc and microglia, particularly the ambiguous role of zinc on microglia. We also pay attention to the role of zinc homeostasis on microglial function within the central nervous system disease. Finally, we observe the relationship between zinc and microglia, attempting to design new therapeutic measures against major nervous system disorders.

Microglia pack a toolbox for life

After decades of being overlooked, a recent wave of studies have explored the roles of microglia in brain health and disease. Microglia perform important physiological functions to set up and maintain proper neural network functions, as well as orchestrate responses to toxic stimuli to limit harm. Many microglial transcriptional programs, extracellular sensing molecules, and functional outputs are seen throughout life. A stark example is the similarity of microglial responses to stressors during neurodevelopment and neurodegeneration. The same themes often match that of other tissue-resident macrophages, presenting an opportunity to apply known concepts as therapeutics develop. We argue that microglial signaling during development and neurologic disease overlap with one another and with other tissue-resident macrophage pathways, in part due to similar sensed stimuli and a conserved sensome of receptors and signaling molecules, akin to a toolkit.

Transcranial Magneto-Acoustic Stimulation Protects Synaptic Rehabilitation from Amyloid-Beta Plaques via Regulation of Microglial Functions

Transcranial magneto-acoustic stimulation (TMAS), which is characterized by high spatiotemporal resolution and high penetrability, is a non-invasive neuromodulation technology based on the magnetic-acoustic coupling effect. To reveal the effects of TMAS treatment on amyloid-beta (Aβ) plaque and synaptic plasticity in Alzheimer's disease, we conducted a comparative analysis of TMAS and transcranial ultrasound stimulation (TUS) based on acoustic effects in 5xFAD mice and BV2 microglia cells. We found that the TMAS-TUS treatment effectively reduced amyloid plaque loads and plaque-associated neurotoxicity. Additionally, TMAS-TUS treatment ameliorated impairments in long-term memory formation and long-term potentiation. Moreover, TMAS-TUS treatment stimulated microglial proliferation and migration while enhancing the phagocytosis and clearance of Aβ. In 5xFAD mice with induced microglial exhaustion, TMAS-TUS treatment-mediated Aβ plaque reduction, synaptic rehabilitation improvement, and the increase in phospho-AKT levels were diminished. Overall, our study highlights that stimulation of hippocampal microglia by TMAS treatment can induce anti-cognitive impairment effects via PI3K-AKT signaling, providing hope for the development of new strategies for an adjuvant therapy for Alzheimer's disease.

The ins and outs of microglial cells in brain health and disease

Microglia are the brain's resident macrophages that play pivotal roles in immune surveillance and maintaining homeostasis of the Central Nervous System (CNS). Microglia are functionally implicated in various cerebrovascular diseases, including stroke, aneurysm, and tumorigenesis as they regulate neuroinflammatory responses and tissue repair processes. Here, we review the manifold functions of microglia in the brain under physiological and pathological conditions, primarily focusing on the implication of microglia in glioma propagation and progression. We further review the current status of therapies targeting microglial cells, including their re-education, depletion, and re-population approaches as therapeutic options to improve patient outcomes for various neurological and neuroinflammatory disorders, including cancer.

Immune proteins C1q and CD47 may contribute to aberrant microglia-mediated synapse loss in the aging monkey brain that is associated with cognitive impairment

Cognitive impairment in learning, memory, and executive function occurs in normal aging even in the absence of Alzheimer's disease (AD). While neurons do not degenerate in humans or monkeys free of AD, there are structural changes including synapse loss and dendritic atrophy, especially in the dorsolateral prefrontal cortex (dlPFC), and these correlate with cognitive age-related impairment. Developmental studies revealed activity-dependent neuronal properties that lead to synapse remodeling by microglia. Microglia-mediated phagocytosis that may eliminate synapses is regulated by immune "eat me" and "don't eat me" signaling proteins in an activity-dependent manner, so that less active synapses are eliminated. Whether this process contributes to age-related synapse loss remains unknown. The present study used a rhesus monkey model of normal aging to investigate the balance between the "eat me" signal, complement component C1q, and the "don't eat me" signal, transmembrane glycoprotein CD47, relative to age-related synapse loss in dlPFC Area 46. Results showed an age-related elevation of C1q and reduction of CD47 at PSD95+ synapses that is associated with cognitive impairment. Additionally, reduced neuronal CD47 RNA expression was found, indicating that aged neurons were less able to produce the protective signal CD47. Interestingly, microglia do not show the hypertrophic morphology indicative of phagocytic activity. These findings suggest that in the aging brain, changes in the balance of immunologic proteins give microglia instructions favoring synapse elimination of less active synapses, but this may occur by a process other than classic phagocytosis such as trogocytosis.

The Influence of Microglia on Neuroplasticity and Long-Term Cognitive Sequelae in Long COVID: Impacts on Brain Development and Beyond

Microglial cells, the immune cells of the central nervous system, are key elements regulating brain development and brain health. These cells are fully responsive to stressors, microenvironmental alterations and are actively involved in the construction of neural circuits in children and the ability to undergo full experience-dependent plasticity in adults. Since neuroinflammation is a known key element in the pathogenesis of COVID-19, one might expect the dysregulation of microglial function to severely impact both functional and structural plasticity, leading to the cognitive sequelae that appear in the pathogenesis of Long COVID. Therefore, understanding this complex scenario is mandatory for establishing the possible molecular mechanisms related to these symptoms. In the present review, we will discuss Long COVID and its association with reduced levels of BDNF, altered crosstalk between circulating immune cells and microglia, increased levels of inflammasomes, cytokines and chemokines, as well as the alterations in signaling pathways that impact neural synaptic remodeling and plasticity, such as fractalkines, the complement system, the expression of SIRPα and CD47 molecules and altered matrix remodeling. Together, these complex mechanisms may help us understand consequences of Long COVID for brain development and its association with altered brain plasticity, impacting learning disabilities, neurodevelopmental disorders, as well as cognitive decline in adults.

The emerging neuroimmune hypothesis of bipolar disorder: An updated overview of neuroimmune and microglial findings

Bipolar disorder (BD) is a severe and multifactorial disease, with onset usually in young adulthood, which follows a progressive course throughout life. Replicated epidemiological studies have suggested inflammatory mechanisms and neuroimmune risk factors as primary contributors to the onset and development of BD. While not all patients display overt markers of inflammation, significant evidence suggests that aberrant immune signaling contributes to all stages of the disease and seems to be mood phase dependent, likely explaining the heterogeneity of findings observed in this population. As the brain's immune cells, microglia orchestrate the brain's immune response and play a critical role in maintaining the brain's health across the lifespan. Microglia are also highly sensitive to environmental changes and respond to physiological and pathological events by adapting their functions, structure, and molecular expression. Recently, it has been highlighted that instead of a single population of cells, microglia comprise a heterogeneous community with specialized states adjusted according to the local molecular cues and intercellular interactions. Early evidence has highlighted the contribution of microglia to BD neuropathology, notably for severe outcomes, such as suicidality. However, the roles and diversity of microglial states in this disease are still largely undermined. This review brings an updated overview of current literature on the contribution of neuroimmune risk factors for the onset and progression of BD, the most prominent neuroimmune abnormalities (including biomarker, neuroimaging, ex vivo studies) and the most recent findings of microglial involvement in BD neuropathology. Combining these different shreds of evidence, we aim to propose a unifying hypothesis for BD pathophysiology centered on neuroimmune abnormalities and microglia. Also, we highlight the urgent need to apply novel multi-system biology approaches to characterize the diversity of microglial states and functions involved in this enigmatic disorder, which can open bright perspectives for novel biomarkers and therapeutic discoveries.

Microglial over-pruning of synapses during development in autism-associated SCN2A-deficient mice and human cerebral organoids

Autism spectrum disorder (ASD) is a major neurodevelopmental disorder affecting 1 in 36 children in the United States. While neurons have been the focus of understanding ASD, an altered neuro-immune response in the brain may be closely associated with ASD, and a neuro-immune interaction could play a role in the disease progression. As the resident immune cells of the brain, microglia regulate brain development and homeostasis via core functions including phagocytosis of synapses. While ASD has been traditionally considered a polygenic disorder, recent large-scale human genetic studies have identified SCN2A deficiency as a leading monogenic cause of ASD and intellectual disability. We generated a Scn2a-deficient mouse model, which displays major behavioral and neuronal phenotypes. However, the role of microglia in this disease model is unknown. Here, we reported that Scn2a-deficient mice have impaired learning and memory, accompanied by reduced synaptic transmission and lower spine density in neurons of the hippocampus. Microglia in Scn2a-deficient mice are partially activated, exerting excessive phagocytic pruning of post-synapses related to the complement C3 cascades during selective developmental stages. The ablation of microglia using PLX3397 partially restores synaptic transmission and spine density. To extend our findings from rodents to human cells, we established a microglia-incorporated human cerebral organoid model carrying an SCN2A protein-truncating mutation identified in children with ASD. We found that human microglia display increased elimination of post-synapse in cerebral organoids carrying the SCN2A mutation. Our study establishes a key role of microglia in multi-species autism-associated models of SCN2A deficiency from mouse to human cells.

Astrocytic phagocytosis in the medial prefrontal cortex jeopardises postoperative memory consolidation in mice

Memory impairment is one of the main characteristics of postoperative cognitive dysfunction. It remains elusive how postoperative pathological changes of the brain link to the memory impairment. The clinical setting of perioperation was mimicked via partial hepatectomy under sevoflurane anaesthesia together with preoperative restraint stress (Hep-Sev-stress) in mice. Memory changes were assessed with fear conditioning. The medial prefrontal cortex (mPFC)-dorsal hippocampus connectivity was evaluated with injecting neurotracer 28 days before surgery. Astrocytic activation was limited via injecting AAV-GFAP-hM4Di-eGFP into the mPFC. Astrocytic and microglial phagocytosis of synapses were visualised with co-labelling hippocampal neuronal axon terminals with PSD-95 and S100β or Iba1. Neuroinflammation and oxidative stress status were also detected. Hep-Sev-stress impaired the memory consolidation (mean [standard error], 49.91 [2.55]% vs. 35.40 [3.97]% in the contextual memory, p = 0.007; 40.72 [2.78]% vs. 27.77 [2.22]% in cued memory, p = 0.002) and the cued memory retrieval (39.00 [3.08]% vs. 24.11 [2.06]%, p = 0.001) in mice when compared with these in the naïve controls. Hep-Sev-stress damaged the connectivity from the dorsal hippocampus to mPFC but not from the mPFC to the dorsal hippocampus and increased the astrocytic but not microglial phagocytosis of hippocampal neuronal axon terminals in the mPFC. The intervention also induced neuroinflammation and oxidative stress in the dorsal hippocampus and the mPFC in a regional-dependent manner. Limiting astrocyte activation in the mPFC alleviated memory consolidation impairment induced by Hep-Sev-stress. Postoperative memory consolidation was impaired due to astrocytic phagocytosis-induced connectivity injury from the dorsal hippocampus to the medial prefrontal cortex.

Clearance of β-amyloid and synapses by the optogenetic depolarization of microglia is complement selective

Microglia actively monitor the neighboring brain microenvironments and constantly contact synapses with their unique ramified processes. In neurodegenerative diseases, including Alzheimer's disease (AD), microglia undergo morphological and functional alterations. Whether the direct manipulation of microglia can selectively or concurrently modulate synaptic function and the response to disease-associated factors remains elusive. Here, we employ optogenetic methods to stimulate microglia in vitro and in vivo. Membrane depolarization rapidly changes microglia morphology and leads to enhanced phagocytosis. We found that the optogenetic stimulation of microglia can efficiently promote β-amyloid (Aβ) clearance in the brain parenchyma, but it can also enhance synapse elimination. Importantly, the inhibition of C1q selectively prevents synapse loss induced by microglia depolarization but does not affect Aβ clearance. Our data reveal independent microglia-mediated phagocytosis pathways toward Aβ and synapses. Our results also shed light on a synergistic strategy of depolarizing microglia and inhibiting complement functions for the clearance of Aβ while sparing synapses.

C5aR1 signaling promotes region- and age-dependent synaptic pruning in models of Alzheimer's disease

INTRODUCTION

Synaptic loss is a hallmark of Alzheimer's disease (AD) that correlates with cognitive decline in AD patients. Complement-mediated synaptic pruning has been associated with this excessive loss of synapses in AD. Here, we investigated the effect of C5aR1 inhibition on microglial and astroglial synaptic pruning in two mouse models of AD.

METHODS

A combination of super-resolution and confocal and tridimensional image reconstruction was used to assess the effect of genetic ablation or pharmacological inhibition of C5aR1 on the Arctic48 and Tg2576 models of AD.

RESULTS

Genetic ablation or pharmacological inhibition of C5aR1 partially rescues excessive pre-synaptic pruning and synaptic loss in an age and region-dependent fashion in two mouse models of AD, which correlates with improved long-term potentiation (LTP).

DISCUSSION

Reduction of excessive synaptic pruning is an additional beneficial outcome of the suppression of C5a-C5aR1 signaling, further supporting its potential as an effective targeted therapy to treat AD.

HIGHLIGHTS

C5aR1 ablation restores long-term potentiation in the Arctic model of AD. C5aR1 ablation rescues region specific excessive pre-synaptic loss. C5aR1 antagonist, PMX205, rescues VGlut1 loss in the Tg2576 model of AD. C1q tagging is not sufficient to induce VGlut1 microglial ingestion. Astrocytes contribute to excessive pre-synaptic loss at late stages of the disease.

Microglia: Activity-dependent regulators of neural circuits

It has been more than a century since Pío del Río-Hortega first characterized microglia in histological stains of brain tissue. Since then, significant advances have been made in understanding the role of these resident central nervous system (CNS) macrophages. In particular, it is now known that microglia can sense neural activity and modulate neuronal circuits accordingly. We review the mechanisms by which microglia detect changes in neural activity to then modulate synapse numbers in the developing and mature CNS. This includes responses to both spontaneous and experience-driven neural activity. We further discuss activity-dependent mechanisms by which microglia regulate synaptic function and neural circuit excitability. Together, our discussion provides a comprehensive review of the activity-dependent functions of microglia within neural circuits in the healthy CNS, and highlights exciting new open questions related to understanding more fully microglia as key components and regulators of neural circuits.

Microglial inflammation in genome instability: A neurodegenerative perspective

The maintenance of genome stability is crucial for cell homeostasis and tissue integrity. Numerous human neuropathologies display chronic inflammation in the central nervous system, set against a backdrop of genome instability, implying a close interplay between the DNA damage and immune responses in the context of neurological disease. Dissecting the molecular mechanisms of this crosstalk is essential for holistic understanding of neuroinflammatory pathways in genome instability disorders. Non-neuronal cell types, specifically microglia, are major drivers of neuroinflammation in the central nervous system with neuro-protective and -toxic capabilities. Here, we discuss how persistent DNA damage affects microglial homeostasis, zooming in on the cytosolic DNA sensing cGAS-STING pathway and the downstream inflammatory response, which can drive neurotoxic outcomes in the context of genome instability.

Microglia in brain aging: An overview of recent basic science and clinical research developments

Aging is characterized by progressive degeneration of tissues and organs, and it is positively associated with an increased mortality rate. The brain, as one of the most significantly affected organs, experiences age-related changes, including abnormal neuronal activity, dysfunctional calcium homeostasis, dysregulated mitochondrial function, and increased levels of reactive oxygen species. These changes collectively contribute to cognitive deterioration. Aging is also a key risk factor for neurodegenerative diseases, such as Alzheimer's disease and Parkinson's disease. For many years, neurodegenerative disease investigations have primarily focused on neurons, with less attention given to microglial cells. However, recently, microglial homeostasis has emerged as an important mediator in neurological disease pathogenesis. Here, we provide an overview of brain aging from the perspective of the microglia. In doing so, we present the current knowledge on the correlation between brain aging and the microglia, summarize recent progress of investigations about the microglia in normal aging, Alzheimer's disease, Parkinson's disease, Huntington's disease, and amyotrophic lateral sclerosis, and then discuss the correlation between the senescent microglia and the brain, which will culminate with a presentation of the molecular complexity involved in the microglia in brain aging with suggestions for healthy aging.

Complement tunes glutamate release and supports synaptic impairments in an animal model of multiple sclerosis

BACKGROUND AND PURPOSE

To deepen our knowledge of the role of complement in synaptic impairment in experimental autoimmune encephalomyelitis (EAE) mice, we investigated the distribution of C1q and C3 proteins and the role of complement as a promoter of glutamate release in purified nerve endings (synaptosomes) and astrocytic processes (gliosomes) isolated from the cortex of EAE mice at the acute stage of the disease (21 ± 1 day post-immunization).

EXPERIMENTAL APPROACH

EAE cortical synaptosomes and gliosomes were analysed for glutamate release efficiency (measured as release of preloaded [3 H]D-aspartate ([3 H]D-ASP)), C1q and C3 protein density, and for viability and ongoing apoptosis.

KEY RESULTS

In healthy mice, complement releases [3 H]D-ASP from gliosomes more efficiently than from synaptosomes. The releasing activity occurs in a dilution-dependent manner and involves the reversal of the excitatory amino acid transporters (EAATs). In EAE mice, the complement-induced releasing activity is significantly reduced in cortical synaptosomes but amplified in cortical gliosomes. These adaptations are paralleled by decreased density of the EAAT2 protein in synaptosomes and increased EAAT1 staining in gliosomes. Concomitantly, PSD95, GFAP, and CD11b, but not SNAP25, proteins are overexpressed in the cortex of the EAE mice. Similarly, C1q and C3 protein immunostaining is increased in EAE cortical synaptosomes and gliosomes, although signs of ongoing apoptosis or altered viability are not detectable.

CONCLUSION AND IMPLICATIONS

Our results unveil a new noncanonical role of complement in the CNS of EAE mice relevant to disease progression and central synaptopathy that suggests new therapeutic targets for the management of MS.

Microglia maintain structural integrity during fetal brain morphogenesis

Microglia (MG), the brain-resident macrophages, play major roles in health and disease via a diversity of cellular states. While embryonic MG display a large heterogeneity of cellular distribution and transcriptomic states, their functions remain poorly characterized. Here, we uncovered a role for MG in the maintenance of structural integrity at two fetal cortical boundaries. At these boundaries between structures that grow in distinct directions, embryonic MG accumulate, display a state resembling post-natal axon-tract-associated microglia (ATM) and prevent the progression of microcavities into large cavitary lesions, in part via a mechanism involving the ATM-factor Spp1. MG and Spp1 furthermore contribute to the rapid repair of lesions, collectively highlighting protective functions that preserve the fetal brain from physiological morphogenetic stress and injury. Our study thus highlights key major roles for embryonic MG and Spp1 in maintaining structural integrity during morphogenesis, with major implications for our understanding of MG functions and brain development.

The Maternal Microbiome as a Map to Understanding the Impact of Prenatal Stress on Offspring Psychiatric Health

Stress and psychiatric disorders have been independently associated with disruption of the maternal and offspring microbiome and with increased risk of the offspring developing psychiatric disorders, both in clinical studies and in preclinical studies. However, the role of the microbiome in mediating the effect of prenatal stress on offspring behavior is unclear. While preclinical studies have identified several key mechanisms, clinical studies focusing on mechanisms are limited. In this review, we discuss 3 specific mechanisms by which the microbiome could mediate the effects of prenatal stress: 1) altered production of short-chain fatty acids; 2) disruptions in TH17 (T helper 17) cell differentiation, leading to maternal and fetal immune activation; and 3) perturbation of intestinal and microbial tryptophan metabolism and serotonergic signaling. Finally, we review the existing clinical literature focusing on these mechanisms and highlight the need for additional mechanistic clinical research to better understand the role of the microbiome in the context of prenatal stress.

Reorganization of adolescent prefrontal cortex circuitry is required for mouse cognitive maturation

Most cognitive functions involving the prefrontal cortex emerge during late development. Increasing evidence links this delayed maturation to the protracted timeline of prefrontal development, which likely does not reach full maturity before the end of adolescence. However, the underlying mechanisms that drive the emergence and fine-tuning of cognitive abilities during adolescence, caused by circuit wiring, are still unknown. Here, we continuously monitored prefrontal activity throughout the postnatal development of mice and showed that an initial activity increase was interrupted by an extensive microglia-mediated breakdown of activity, followed by the rewiring of circuit elements to achieve adult-like patterns and synchrony. Interfering with these processes during adolescence, but not adulthood, led to a long-lasting microglia-induced disruption of prefrontal activity and neuronal morphology and decreased cognitive abilities. These results identified a nonlinear reorganization of prefrontal circuits during adolescence and revealed its importance for adult network function and cognitive processing.

The transplant rejection response involves neutrophil and macrophage adhesion-mediated trogocytosis and is regulated by NFATc3

The anti-foreign tissue (transplant rejection) response, mediated by the immune system, has been the biggest obstacle to successful organ transplantation. There are still many enigmas regarding this process and some aspects of the underlying mechanisms driving the immune response against foreign tissues remain poorly understood. Here, we found that a large number of neutrophils and macrophages were attached to the graft during skin transplantation. Furthermore, both types of cells could autonomously adhere to and damage neonatal rat cardiomyocyte mass (NRCM) in vitro. We have demonstrated that Complement C3 and the receptor CR3 participated in neutrophils/macrophages-mediated adhesion and damage this foreign tissue (NRCM or skin grafts). We have provided direct evidence that the damage to these tissues occurs by a process referred to as trogocytosis, a damage mode that has never previously been reported to directly destroy grafts. We further demonstrated that this process can be regulated by NFAT, in particular, NFATc3. This study not only enriches an understanding of host-donor interaction in transplant rejection, but also provides new avenues for exploring the development of novel immunosuppressive drugs which prevent rejection during transplant therapy.

Glial cells as a promising therapeutic target of glaucoma: beyond the IOP

Glial cells, a type of non-neuronal cell found in the central nervous system (CNS), play a critical role in maintaining homeostasis and regulating CNS functions. Recent advancements in technology have paved the way for new therapeutic strategies in the fight against glaucoma. While intraocular pressure (IOP) is the most well-known modifiable risk factor, a significant number of glaucoma patients have normal IOP levels. Because glaucoma is a complex, multifactorial disease influenced by various factors that contribute to its onset and progression, it is imperative that we consider factors beyond IOP to effectively prevent or slow down the disease's advancement. In the realm of CNS neurodegenerative diseases, glial cells have emerged as key players due to their pivotal roles in initiating and hastening disease progression. The inhibition of dysregulated glial function holds the potential to protect neurons and restore brain function. Consequently, glial cells represent an enticing therapeutic candidate for glaucoma, even though the majority of glaucoma research has historically concentrated solely on retinal ganglion cells (RGCs). In addition to the neuroprotection of RGCs, the proper regulation of glial cell function can also facilitate structural and functional recovery in the retina. In this review, we offer an overview of recent advancements in understanding the non-cell-autonomous mechanisms underlying the pathogenesis of glaucoma. Furthermore, state-of-the-art technologies have opened up possibilities for regenerating the optic nerve, which was previously believed to be incapable of regeneration. We will also delve into the potential roles of glial cells in the regeneration of the optic nerve and the restoration of visual function.

Current perspectives on microglia-neuron communication in the central nervous system: Direct and indirect modes of interaction

BACKGROUND

The incessant communication that takes place between microglia and neurons is essential the development, maintenance, and pathogenesis of the central nervous system (CNS). As mobile phagocytic cells, microglia serve a critical role in surveilling and scavenging the neuronal milieu to uphold homeostasis.

AIM OF REVIEW

This review aims to discuss the various mechanisms that govern the interaction between microglia and neurons, from the molecular to the organ system level, and to highlight the importance of these interactions in the development, maintenance, and pathogenesis of the CNS.

KEY SCIENTIFIC CONCEPTS OF REVIEW

Recent research has revealed that microglia-neuron interaction is vital for regulating fundamental neuronal functions, such as synaptic pruning, axonal remodeling, and neurogenesis. The review will elucidate the intricate signaling pathways involved in these interactions, both direct and indirect, to provide a better understanding of the fundamental mechanisms of brain function. Furthermore, gaining insights into these signals could lead to the development of innovative therapies for neural disorders.

The Biological Significance of Trogocytosis

Trogocytosis is the intercellular transfer of membrane and membrane-associated proteins between cells. Trogocytosis is an underappreciated phenomenon that has historically routinely been dismissed as an artefact. With a greater understanding of the process and the implications it has on biological systems, trogocytosis has the potential to become a paradigm changer. The presence on a cell of molecules they don't endogenously express can alter the biological activity of the cell and could also lead to the acquisition of new functions. To better appreciate this phenomenon, it is important to understand how these intercellular membrane exchanges influence the function and activity of the donor and the recipient cells. In this chapter, we will examine how the molecules acquired by trogocytosis influence the biology of a variety of systems including mammalian fertilization, treatment of hemolytic disease of the newborn, viral and parasitic infections, cancer immunotherapy, and immune modulation.

General Pathophysiology of Microglia

Microglia, which are the resident innate immune cells of the central nervous system (CNS), have emerged as critical for maintaining health by not only ensuring proper development, activity, and plasticity of neurones and glial cells but also maintaining and restoring homeostasis when faced with various challenges across the lifespan. This chapter is dedicated to the current understanding of microglia, including their beneficial versus detrimental roles, which are highly complex, rely on various microglial states, and intimately depend on their spatiotemporal context. Microglia are first contextualized within the perspective of finding therapeutic strategies to cure diseases in the twenty-first century-the overall functions of neuroglia with relation one to another and to neurones, and their shared CNS environment. A historical framework is provided, and the main principles of glial neuropathology are enunciated. The current view of microglial nomenclature is then covered, notably by discussing the rejected concepts of microglial activation, their polarisation into M1 and M2 phenotypes, and neuroinflammation. The transformation of the microglial population through the addition, migration, and elimination of individual members, as well as their dynamic metamorphosis between a wide variety of structural and functional states, based on the experienced physiological and pathological stimuli, is subsequently discussed. Lastly, the perspective of microglia as a cell type endowed with a health status determining their outcomes on adaptive CNS plasticity as well as disease pathology is proposed for twenty-first-century approaches to disease prevention and treatment.

Progress in Structural and Functional In Vivo Imaging of Microglia and Their Application in Health and Disease

The first line of defense for the central nervous system (CNS) against injury or disease is provided by microglia. Microglia were long believed to stay in a dormant/resting state, reacting only to injury or disease. This view changed dramatically with the development of modern imaging techniques that allowed the study of microglial behavior in the intact brain over time, to reveal the dynamic nature of their responses. Over the past two decades, in vivo imaging using multiphoton microscopy has revealed numerous new functions of microglia in the developing, adult, aged, injured, and diseased CNS. As the most dynamic cells in the brain, microglia continuously contact all structures and cell types, such as glial and vascular cells, neuronal cell bodies, axons, dendrites, and dendritic spines, and are believed to play a central role in sculpting neuronal networks throughout life. Following trauma, or in neurodegenerative or neuroinflammatory diseases, microglial responses range from protective to harmful, underscoring the need to better understand their diverse roles and states in different pathological conditions. In this chapter, we introduce multiphoton microscopy and discuss recent advances in structural and functional imaging technologies that have expanded our toolbox to study microglial states and behaviors in new ways and depths. We also discuss relevant mouse models available for in vivo imaging studies of microglia and review how such studies are constantly refining our understanding of the multifaceted role of microglia in the healthy and diseased CNS.

Microglia-mediated calcium-permeable AMPAR accumulation in the nucleus accumbens drives hyperlocomotion during cocaine withdrawal

During withdrawal from cocaine, calcium permeable-AMPA receptors (CP-AMPAR) progressively accumulate in nucleus accumbens (NAc) synapses, a phenomenon linked to behavioral sensitization and drug-seeking. Recently, it has been suggested that neuroimmune alterations might promote aberrant changes in synaptic plasticity, thus contributing to substance abuse-related behaviors. Here, we investigated the role of microglia in NAc neuroadaptations after withdrawal from cocaine-induced conditioned place preference (CPP). We depleted microglia using PLX5622-supplemented diet during cocaine withdrawal, and after the place preference test, we measured dendritic spine density and the presence of CP-AMPAR in the NAc shell. Microglia depletion prevented cocaine-induced changes in dendritic spines and CP-AMPAR accumulation. Furthermore, microglia depletion prevented conditioned hyperlocomotion without affecting drug-context associative memory. Microglia displayed fewer number of branches, resulting in a reduced arborization area and microglia control domain at late withdrawal. Our results suggest that microglia are necessary for the synaptic adaptations in NAc synapses during cocaine withdrawal and therefore represent a promising therapeutic target for relapse prevention.

Synapse Regulation

Microglia are the resident immune cells of the brain. As such, they rapidly detect changes in normal brain homeostasis and accurately respond by fine-tuning in a tightly regulated manner their morphology, gene expression, and functional behavior. Depending on the nature of these changes, microglia can thicken and retract their processes, proliferate and migrate, release numerous signaling factors and compounds influencing neuronal physiology (e.g., cytokines and trophic factors), in addition to secreting proteases able to transform the extracellular matrix, and phagocytosing various types of cellular debris, etc. Because microglia also transform rapidly (on a time scale of minutes) during experimental procedures, studying these very special cells requires methods that are specifically non-invasive. The development of such methods has provided unprecedented insights into the roles of microglia during normal physiological conditions. In particular, transcranial two-photon in vivo imaging revealed that presumably "resting" microglia continuously survey the brain parenchyma with their highly motile processes, in addition to modulating their structural and functional interactions with neuronal circuits along the changes in neuronal activity and behavioral experience occurring throughout the lifespan. In this chapter, we will describe how surveillant microglia interact with synaptic elements and modulate the number, maturation, function, and plasticity of synapses in the healthy developing, mature, and aging brain, with consequences on neuronal activity, learning and memory, and the behavioral outcome.