Dynamics of spinal microglia repopulation following an acute depletion

Temporal changes of spinal microglia in murine models of neuropathic pain: a scoping review

Neuropathic pain (NP) is an ineffectively treated, debilitating chronic pain disorder that is associated with maladaptive changes in the central nervous system, particularly in the spinal cord. Murine models of NP looking at the mechanisms underlying these changes suggest an important role of microglia, the resident immune cells of the central nervous system, in various stages of disease progression. However, given the number of different NP models and the resource limitations that come with tracking longitudinal changes in NP animals, many studies fail to truly recapitulate the patterns that exist between pain conditions and temporal microglial changes. This review integrates how NP studies are being carried out in murine models and how microglia changes over time can affect pain behavior in order to inform better study design and highlight knowledge gaps in the field. 258 peer-reviewed, primary source articles looking at spinal microglia in murine models of NP were selected using Covidence. Trends in the type of mice, statistical tests, pain models, interventions, microglial markers and temporal pain behavior and microglia changes were recorded and analyzed. Studies were primarily conducted in inbred, young adult, male mice having peripheral nerve injury which highlights the lack of generalizability in the data currently being collected. Changes in microglia and pain behavior, which were both increased, were tested most commonly up to 2 weeks after pain initiation despite aberrant microglia activity also being recorded at later time points in NP conditions. Studies using treatments that decrease microglia show decreased pain behavior primarily at the 1- and 2-week time point with many studies not recording pain behavior despite the involvement of spinal microglia dysfunction in their development. These results show the need for not only studying spinal microglia dynamics in a variety of NP conditions at longer time points but also for better clinically relevant study design considerations.

Activation of TGR5 in the injured nerve site according to a prevention protocol mitigates partial sciatic nerve ligation-induced neuropathic pain by alleviating neuroinflammation

ABSTRACT

Neuropathic pain is a pervasive medical challenge currently lacking effective treatment options. Molecular changes at the site of peripheral nerve injury contribute to both peripheral and central sensitization, critical components of neuropathic pain. This study explores the role of the G-protein-coupled bile acid receptor (GPBAR1 or TGR5) in the peripheral mechanisms underlying neuropathic pain induced by partial sciatic nerve ligation in male mice. TGR5 was upregulated in the injured nerve site and predominantly colocalized with macrophages. Perisciatic nerve administration of the TGR5 agonist, INT-777 according to a prevention protocol (50 μg/μL daily from postoperative day [POD] 0 to POD6) provided sustained relief from mechanical allodynia and spontaneous pain, whereas the TGR5 antagonist, SBI-115 worsened neuropathic pain. Transcriptome sequencing linked the pain relief induced by TGR5 activation to reduced neuroinflammation, which was further evidenced by a decrease in myeloid cells and pro-inflammatory mediators (eg, CCL3, CXCL9, interleukin [IL]-6, and tumor necrosis factor [TNF] α) and an increase in CD86-CD206+ anti-inflammatory macrophages at POD7. Besides, myeloid-cell-specific TGR5 knockdown in the injured nerve site exacerbated both neuropathic pain and neuroinflammation, which was substantiated by bulk RNA-sequencing and upregulated expression levels of inflammatory mediators (including CCL3, CCL2, IL-6, TNF α, and IL-1β) and the increased number of monocytes/macrophages at POD7. Furthermore, the activation of microglia in the spinal cord on POD7 and POD14 was altered when TGR5 in the sciatic nerve was manipulated. Collectively, TGR5 activation in the injured nerve site mitigates neuropathic pain by reducing neuroinflammation, while TGR5 knockdown in myeloid cells worsens pain by enhancing neuroinflammation.

Deplete and repeat: microglial CSF1R inhibition and traumatic brain injury

Traumatic brain injury (TBI) is a public health burden affecting millions of people. Sustained neuroinflammation after TBI is often associated with poor outcome. As a result, increased attention has been placed on the role of immune cells in post-injury recovery. Microglia are highly dynamic after TBI and play a key role in the post-injury neuroinflammatory response. Therefore, microglia represent a malleable post-injury target that could substantially influence long-term outcome after TBI. This review highlights the cell specific role of microglia in TBI pathophysiology. Microglia have been manipulated via genetic deletion, drug inhibition, and pharmacological depletion in various pre-clinical TBI models. Notably, colony stimulating factor 1 (CSF1) and its receptor (CSF1R) have gained much traction in recent years as a pharmacological target on microglia. CSF1R is a transmembrane tyrosine kinase receptor that is essential for microglia proliferation, differentiation, and survival. Small molecule inhibitors targeting CSF1R result in a swift and effective depletion of microglia in rodents. Moreover, discontinuation of the inhibitors is sufficient for microglia repopulation. Attention is placed on summarizing studies that incorporate CSF1R inhibition of microglia. Indeed, microglia depletion affects multiple aspects of TBI pathophysiology, including neuroinflammation, oxidative stress, and functional recovery with measurable influence on astrocytes, peripheral immune cells, and neurons. Taken together, the data highlight an important role for microglia in sustaining neuroinflammation and increasing risk of oxidative stress, which lends to neuronal damage and behavioral deficits chronically after TBI. Ultimately, the insights gained from CSF1R depletion of microglia are critical for understanding the temporospatial role that microglia develop in mediating TBI pathophysiology and recovery.

Established and emerging techniques for the study of microglia: visualization, depletion, and fate mapping

The central nervous system (CNS) is an essential hub for neuronal communication. As a major component of the CNS, glial cells are vital in the maintenance and regulation of neuronal network dynamics. Research on microglia, the resident innate immune cells of the CNS, has advanced considerably in recent years, and our understanding of their diverse functions continues to grow. Microglia play critical roles in the formation and regulation of neuronal synapses, myelination, responses to injury, neurogenesis, inflammation, and many other physiological processes. In parallel with advances in microglial biology, cutting-edge techniques for the characterization of microglial properties have emerged with increasing depth and precision. Labeling tools and reporter models are important for the study of microglial morphology, ultrastructure, and dynamics, but also for microglial isolation, which is required to glean key phenotypic information through single-cell transcriptomics and other emerging approaches. Strategies for selective microglial depletion and modulation can provide novel insights into microglia-targeted treatment strategies in models of neuropsychiatric and neurodegenerative conditions, cancer, and autoimmunity. Finally, fate mapping has emerged as an important tool to answer fundamental questions about microglial biology, including their origin, migration, and proliferation throughout the lifetime of an organism. This review aims to provide a comprehensive discussion of these established and emerging techniques, with applications to the study of microglia in development, homeostasis, and CNS pathologies.

Repurposing of pexidartinib for microglia depletion and renewal

Pexidartinib (PLX3397) is a small molecule receptor tyrosine kinase inhibitor of colony stimulating factor 1 receptor (CSF1R) with moderate selectivity over other members of the platelet derived growth factor receptor family. It is approved for treatment of tenosynovial giant cell tumors (TGCT). CSF1R is highly expressed by microglia, which are macrophages of the central nervous system (CNS) that defend the CNS against injury and pathogens and contribute to synapse development and plasticity. Challenged by pathogens, apoptotic cells, debris, or inflammatory molecules they adopt a responsive state to propagate the inflammation and eventually return to a homeostatic state. The phenotypic switch may fail, and disease-associated microglia contribute to the pathophysiology in neurodegenerative or neuropsychiatric diseases or long-lasting detrimental brain inflammation after brain, spinal cord or nerve injury or ischemia/hemorrhage. Microglia also contribute to the growth permissive tumor microenvironment of glioblastoma (GBM). In rodents, continuous treatment for 1-2 weeks via pexidartinib food pellets leads to a depletion of microglia and subsequent repopulation from the remaining fraction, which is aided by peripheral monocytes that search empty niches for engraftment. The putative therapeutic benefit of such microglia depletion or forced renewal has been assessed in almost any rodent model of CNS disease or injury or GBM with heterogeneous outcomes, but a tendency of partial beneficial effects. So far, microglia monitoring e.g. via positron emission imaging is not standard of care for patients receiving Pexidartinib (e.g. for TGCT), so that the depletion and repopulation efficiency in humans is still largely unknown. Considering the virtuous functions of microglia, continuous depletion is likely no therapeutic option but short-lasting transient partial depletion to stimulate microglia renewal or replace microglia in genetic disease in combination with e.g. stem cell transplantation or as part of a multimodal concept in treatment of glioblastoma appears feasible. The present review provides an overview of the preclinical evidence pro and contra microglia depletion as a therapeutic approach.

New insight on microglia activation in neurodegenerative diseases and therapeutics

Microglia are immune cells within the central nervous system (CNS) closely linked to brain health and neurodegenerative diseases such as Alzheimer's disease and Parkinson's disease. In response to changes in the surrounding environment, microglia activate and change their state and function. Several factors, example for circadian rhythm disruption and the development of neurodegenerative diseases, influence microglia activation. In this review, we explore microglia's function and the associated neural mechanisms. We elucidate that circadian rhythms are essential factors influencing microglia activation and function. Circadian rhythm disruption affects microglia activation and, consequently, neurodegenerative diseases. In addition, we found that abnormal microglia activation is a common feature of neurodegenerative diseases and an essential factor of disease development. Here we highlight the importance of microglia activation in neurodegenerative diseases. Targeting microglia for neurodegenerative disease treatment is a promising direction. We introduce the progress of methods targeting microglia for the treatment of neurodegenerative diseases and summarize the progress of drugs developed with microglia as targets, hoping to provide new ideas for treating neurodegenerative diseases.

Macrophages and microglia in inflammation and neuroinflammation underlying different pain states

Pain is a main symptom in inflammation, and inflammation induces pain via inflammatory mediators acting on nociceptive neurons. Macrophages and microglia are distinct cell types, representing immune cells and glial cells, respectively, but they share similar roles in pain regulation. Macrophages are key regulators of inflammation and pain. Macrophage polarization plays different roles in inducing and resolving pain. Notably, macrophage polarization and phagocytosis can be induced by specialized pro-resolution mediators (SPMs). SPMs also potently inhibit inflammatory and neuropathic pain via immunomodulation and neuromodulation. In this review, we discuss macrophage signaling involved in pain induction and resolution, as well as in maintaining physiological pain. Microglia are macrophage-like cells in the central nervous system (CNS) and drive neuroinflammation and pathological pain in various inflammatory and neurological disorders. Microglia-produced inflammatory cytokines can potently regulate excitatory and inhibitory synaptic transmission as neuromodulators. We also highlight sex differences in macrophage and microglial signaling in inflammatory and neuropathic pain. Thus, targeting macrophage and microglial signaling in distinct locations via pharmacological approaches, including immunotherapies, and non-pharmacological approaches will help to control chronic inflammation and chronic pain.

Time-dependent and selective microglia-mediated removal of spinal synapses in neuropathic pain

Neuropathic pain is a debilitating condition resulting from damage to the nervous system. Imbalance of spinal excitation and inhibition has been proposed to contribute to neuropathic pain. However, the structural basis of this imbalance remains unknown. Using a preclinical model of neuropathic pain, we show that microglia selectively engulf spinal synapses that are formed by central neurons and spare those of peripheral sensory neurons. Furthermore, we reveal that removal of inhibitory and excitatory synapses exhibits distinct temporal patterns, in which microglia-mediated inhibitory synapse removal precedes excitatory synapse removal. We also find selective and gradual increase in complement depositions on dorsal horn synapses that corresponds to the temporal pattern of microglial synapse pruning activity and type-specific synapse loss. Together, these results define a specific role for microglia in the progression of neuropathic pain pathogenesis and implicate these immune cells in structural remodeling of dorsal horn circuitry.

What microglia depletion approaches tell us about the role of microglia on synaptic function and behavior

Microglia are dynamic cells, constantly surveying their surroundings and interacting with neurons and synapses. Indeed, a wealth of knowledge has revealed a critical role of microglia in modulating synaptic transmission and plasticity in the developing brain. In the past decade, novel pharmacological and genetic strategies have allowed the acute removal of microglia, opening the possibility to explore and understand the role of microglia also in the adult brain. In this review, we summarized and discussed the contribution of microglia depletion strategies to the current understanding of the role of microglia on synaptic function, learning and memory, and behavior both in physiological and pathological conditions. We first described the available microglia depletion methods highlighting their main strengths and weaknesses. We then reviewed the impact of microglia depletion on structural and functional synaptic plasticity. Next, we focused our analysis on the effects of microglia depletion on behavior, including general locomotor activity, sensory perception, motor function, sociability, learning and memory both in healthy animals and animal models of disease. Finally, we integrated the findings from the reviewed studies and discussed the emerging roles of microglia on the maintenance of synaptic function, learning, memory strength and forgetfulness, and the implications of microglia depletion in models of brain disease.

Restorative therapy using microglial depletion and repopulation for central nervous system injuries and diseases

Microglia are important resident immune cells in the central nervous system (CNS) and play an important role in its development, homeostasis, and disease treatments. Activated microglia perform diverse functions in mouse models of CNS neurodegenerative diseases or deficits. In humans, microglia have been linked to various neurodegenerative diseases. Following brain or spinal cord injury, microglia express pro- and anti-inflammatory phenotypes at different stages of recovery. With the development of pharmacological and genetic tools for microglial depletion, studies have demonstrated that microglial depletion exerts both positive and negative effects in the treatment of CNS diseases. Notably, microglial depletion provides an empty niche that stimulates production of new microglia. Microglial depletion and repopulation can not only treat diseases by eliminating dysfunctional microglia but can also provide an indication of the molecular mechanisms of diseases. Although this approach has shown impressive results, its use is still in its infancy. In this review, we summarize the current pharmacological and genetic tools for microglial depletion and highlight recent advances in microglial repopulation therapy for the treatment and functional recovery of neurological diseases and deficits. Finally, we briefly discuss the therapeutic challenges and prospective uses of microglial repopulation therapy.

Should We Open Fire on Microglia? Depletion Models as Tools to Elucidate Microglial Role in Health and Alzheimer's Disease

Microglia play a critical role in both homeostasis and disease, displaying a wide variety in terms of density, functional markers and transcriptomic profiles along the different brain regions as well as under injury or pathological conditions, such as Alzheimer's disease (AD). The generation of reliable models to study into a dysfunctional microglia context could provide new knowledge towards the contribution of these cells in AD. In this work, we included an overview of different microglial depletion approaches. We also reported unpublished data from our genetic microglial depletion model, Cx3cr1CreER/Csf1rflx/flx, in which we temporally controlled microglia depletion by either intraperitoneal (acute model) or oral (chronic model) tamoxifen administration. Our results reported a clear microglial repopulation, then pointing out that our model would mimic a context of microglial replacement instead of microglial dysfunction. Next, we evaluated the origin and pattern of microglial repopulation. Additionally, we also reviewed previous works assessing the effects of microglial depletion in the progression of Aβ and Tau pathologies, where controversial data are found, probably due to the heterogeneous and time-varying microglial phenotypes observed in AD. Despite that, microglial depletion represents a promising tool to assess microglial role in AD and design therapeutic strategies.

Don't know what you got till it's gone: microglial depletion and neurodegeneration

In the central nervous system, immunologic surveillance and response are carried out, in large part, by microglia. These resident macrophages derive from myeloid precursors in the embryonic yolk sac, migrating to the brain and eventually populating local tissue prior to blood-brain barrier formation. Preserved for the duration of lifespan, microglia serve the host as more than just a central arm of innate immunity, also contributing significantly to the development and maintenance of neurons and neural networks, as well as neuroregeneration. The critical nature of these varied functions makes the characterization of key roles played by microglia in neurodegenerative disorders, especially Alzheimer's disease, of paramount importance. While genetic models and rudimentary pharmacologic approaches for microglial manipulation have greatly improved our understanding of central nervous system health and disease, significant advances in the selective and near complete in vitro and in vivo depletion of microglia for neuroscience application continue to push the boundaries of research. Here we discuss the research efficacy and utility of various microglial depletion strategies, including the highly effective CSF1R inhibitor models, noteworthy insights into the relationship between microglia and neurodegeneration, and the potential for therapeutic repurposing of microglial depletion and repopulation.

Investigating Microglia in Health and Disease: Challenges and Opportunities

Microglia are tissue-resident macrophages implicated in central nervous system (CNS) development, homeostasis, and response to injury. Recent advances in transcriptomics, multiplex protein expression analysis, and experimental depletion of microglia have cemented their importance. However, it is still unclear which models are best suited to investigate microglia and explore their function in human disease. Here, we discuss issues regarding off-targeting during experimental manipulation, and differences and similarities between human and rodent microglia. With new developments in transgenic lines and human-rodent chimeras, we anticipate that in coming years, a clearer picture of microglia function in health and disease will emerge.

Microglia and Their Promising Role in Ischemic Brain Injuries: An Update

Ischemic brain injuries are common diseases with high morbidity, disability, and mortality rates, which have significant impacts on human health and life. Microglia are resident cells of the central nervous system (CNS). The inflammatory responses mediated by microglia play an important role in the occurrence and development of ischemic brain injuries. This article summarizes the activation, polarization, depletion, and repopulation of microglia after ischemic brain injuries, proposing new treatment strategies for such injuries through the modulation of microglial function.

miRNA 146a-5p-loaded poly(d,l-lactic-co-glycolic acid) nanoparticles impair pain behaviors by inhibiting multiple inflammatory pathways in microglia

Aims: We investigated whether miRNA (miR) 146a-5p-loaded nanoparticles (NPs) can attenuate neuropathic pain behaviors in the rat spinal nerve ligation-induced neuropathic pain model by inhibiting activation of the NF-κB and p38 MAPK pathways in spinal microglia. Materials & methods: After NP preparation, miR NPs were assessed for their physical characteristics and then injected intrathecally into the spinal cords of rat spinal nerve ligation rats to test their analgesic effects. Results: miR NPs reduced pain behaviors for 11 days by negatively regulating the inflammatory response in spinal microglia. Conclusion: The anti-inflammatory effects of miR 146a-5p along with nanoparticle-based materials make miR NPs promising tools for treating neuropathic pain.

Microglia in the developing retina

Microglia are increasingly shown to be key players in neuron development and synapse connectivity. However, the underlying mechanisms by which microglia regulate neuron function remain poorly understood in part because such analysis is challenging in the brain where neurons and synapses are intermingled and connectivity is only beginning to be mapped. Here, we discuss the features and function of microglia in the ordered mammalian retina where the laminar organization of neurons and synapses facilitates such molecular studies. We discuss microglia origins and consider the evidence for molecularly distinct microglia subpopulations and their potential for differential roles with a particular focus on the early stages of retina development. We then review the models and methods used for the study of these cells and discuss emerging data that link retina microglia to the genesis and survival of particular retina cell subtypes. We also highlight potential roles for microglia in shaping the development and organization of the vasculature and discuss cellular and molecular mechanisms involved in this process. Such insights may help resolve the mechanisms by which retinal microglia impact visual function and help guide studies of related features in brain development and disease.

Microglia in Pain: Detrimental and Protective Roles in Pathogenesis and Resolution of Pain

The previous decade has seen a rapid increase in microglial studies on pain, with a unique focus on microgliosis in the spinal cord after nerve injury and neuropathic pain. Numerous signaling molecules are altered in microglia and contribute to the pathogenesis of pain. Here, we discuss how microglial signaling regulates spinal cord synaptic plasticity in acute and chronic pain conditions with different degrees and variations of microgliosis. We highlight that microglial mediators such as pro- and anti-inflammatory cytokines are powerful neuromodulators that regulate synaptic transmission and pain via neuron-glial interactions. We also reveal an emerging role of microglia in the resolution of pain, in part via specialized pro-resolving mediators including resolvins, protectins, and maresins. We also discuss a possible role of microglia in chronic itch.

Contribution of microglial reaction to increased nociceptive responses in high-fat-diet (HFD)-induced obesity in male mice

The progressive increase in the prevalence of obesity in the population can result in increased healthcare costs and demands. Recent studies have revealed a positive correlation between pain and obesity, although the underlying mechanisms still remain unknown. Here, we aimed to clarify the role of microglia in altered pain behaviors induced by high-fat diet (HFD) in male mice. We found that C57BL/6CR mice on HFD exhibited enhanced spinal microglial reaction (increased cell number and up-regulated expression of p-p38 and CD16/32), increased tumor necrosis factor-α (TNF-α) mRNA and brain-derived neurotrophic factor (BDNF) protein expression as well as a polarization of spinal microglial toward a pro-inflammatory phenotype. Moreover, we found that using PLX3397 (a selective colony-stimulating factor-1 receptor (CSF1R) kinase inhibitor) to eliminate microglia in HFD-induced obesity mice, inflammation in the spinal cord was rescued, as was abnormal pain hypersensitivity. Intrathecal injection of Mac-1-saporin (a saporin-conjugated anti-mac1 antibody) resulted in a decreased number of microglia and attenuated both mechanical allodynia and thermal hyperalgesia in HFD-fed mice. These results indicate that the pro-inflammatory functions of spinal microglia have a special relevance to abnormal pain hypersensitivity in HFD-induced obesity mice. In conclusion, our data suggest that HFD induces a classical reaction of microglia, characterized by an enhanced phosphorylation of p-38 and increased CD16/32 expression, which may in part contribute to increased nociceptive responses in HFD-induced obesity mice.

Proliferation and Differentiation in the Adult Subventricular Zone Are Not Affected by CSF1R Inhibition

Microglia are reported to have significant roles in regulating normal mammalian adult neurogenesis. There are two neurogenic niches in the adult mammal brain: the subgranular zone (SGZ) in the hippocampus, and the subventricular zone (SVZ), which makes up the lining of the lateral ventricles. While the microglia interactions on adult neurogenesis in the hippocampus have been characterized, the SVZ niche is not as well investigated. The SVZ niche is unique in that the newborn neurons migrate a much longer distance through multiple brain structures compared to newborn neurons in the hippocampus, making it more difficult to fully characterize how microglia influence this process. To examine the SVZ niche and migration pathway, we used the colony stimulating factor 1 receptor (CSF1R) antagonist PLX5622, which promotes brain wide microglia ablation. Microglia ablation resulted in no changes in the numbers of neural stem cells (NSCs), transient amplifying cells, and neuroblasts. Microglia ablation in the olfactory bulb (OB) was decreased compared to the SVZ. CSF1R inhibition had no effect on the ability of microglia to proliferate. Thus, our data suggest that microglia are not required for normal functioning SVZ adult neurogenesis.

Involvement of lysophosphatidic acid-induced astrocyte activation underlying the maintenance of partial sciatic nerve injury-induced neuropathic pain

We have previously demonstrated that lysophosphatidic acid (LPA) plays key roles in the initial mechanisms for neuropathic pain (NeuP) development. Here, we examined whether LPA receptor mechanisms and LPA production are related to the glial activation at a late stage after partial sciatic nerve ligation (pSNL) by use of microglial inhibitor, Mac1-saporin or astrocyte inhibitor, and L-α-aminoadipate (L-AA). Although single intrathecal injection of LPA1/3 antagonist, Ki-16425 did not affect the pain threshold at day 7 after the spinal cord injury, repeated treatments of each compound gradually reversed the basal pain threshold to the control level. The intrathecal administration of a microglia inhibitor, Mac-1-saporin reversed the late hyperalgesia and LPA production at day 14 after the pSNL, whereas L-AA inhibited the hyperalgesia, but had no effect on LPA production. The involvement of LPA receptors in astrocyte activation in vivo was evidenced by the findings that Ki-16425 treatments abolished the upregulation of CXCL1 in activated astrocytes in the spinal dorsal horn of mice at day 14 after the pSNL, and that Ki-16425 reversed the LPA-induced upregulation of several chemokine gene expressions in primary cultured astrocytes. Finally, we found that significant hyperalgesia was observed with intrathecal administration of primary cultured astrocytes, which had been stimulated by LPA in a Ki-16425-reversible manner. All these findings suggest that LPA production and LPA1/3 receptor activation through differential glial mechanisms play key roles in the maintenance as well as initiation mechanisms in NeuP.

Neurons and Microglia; A Sickly-Sweet Duo in Diabetic Pain Neuropathy

Diabetes is a common condition characterized by persistent hyperglycemia. High blood sugar primarily affects cells that have a limited capacity to regulate their glucose intake. These cells include capillary endothelial cells in the retina, mesangial cells in the renal glomerulus, Schwann cells, and neurons of the peripheral and central nervous systems. As a result, hyperglycemia leads to largely intractable complications such as retinopathy, nephropathy, hypertension, and neuropathy. Diabetic pain neuropathy is a complex and multifactorial disease that has been associated with poor glycemic control, longer diabetes duration, hypertension, advanced age, smoking status, hypoinsulinemia, and dyslipidemia. While many of the driving factors involved in diabetic pain are still being investigated, they can be broadly classified as either neuron -intrinsic or -extrinsic. In neurons, hyperglycemia impairs the polyol pathway, leading to an overproduction of reactive oxygen species and reactive nitrogen species, an enhanced formation of advanced glycation end products, and a disruption in Na⁺/K⁺ ATPase pump function. In terms of the extrinsic pathway, hyperglycemia leads to the generation of both overactive microglia and microangiopathy. The former incites a feed-forward inflammatory loop that hypersensitizes nociceptor neurons, as observed at the onset of diabetic pain neuropathy. The latter reduces neurons' access to oxygen, glucose and nutrients, prompting reductions in nociceptor terminal expression and losses in sensation, as observed in the later stages of diabetic pain neuropathy. Overall, microglia can be seen as potent and long-lasting amplifiers of nociceptor neuron activity, and may therefore constitute a potential therapeutic target in the treatment of diabetic pain neuropathy.

VEGFR2 promotes central endothelial activation and the spread of pain in inflammatory arthritis

Chronic pain can develop in response to conditions such as inflammatory arthritis. The central mechanisms underlying the development and maintenance of chronic pain in humans are not well elucidated although there is evidence for a role of microglia and astrocytes. However in pre-clinical models of pain, including models of inflammatory arthritis, there is a wealth of evidence indicating roles for pathological glial reactivity within the CNS. In the spinal dorsal horn of rats with painful inflammatory arthritis we found both a significant increase in CD11b⁺ microglia-like cells and GFAP⁺ astrocytes associated with blood vessels, and the number of activated blood vessels expressing the adhesion molecule ICAM-1, indicating potential glio-vascular activation. Using pharmacological interventions targeting VEGFR2 in arthritic rats, to inhibit endothelial cell activation, the number of dorsal horn ICAM-1⁺ blood vessels, CD11b⁺ microglia and the development of secondary mechanical allodynia, an indicator of central sensitization, were all prevented. Targeting endothelial VEGFR2 by inducible Tie2-specific VEGFR2 knock-out also prevented secondary allodynia in mice and glio-vascular activation in the dorsal horn in response to inflammatory arthritis. Inhibition of VEGFR2 in vitro significantly blocked ICAM-1-dependent monocyte adhesion to brain microvascular endothelial cells, when stimulated with inflammatory mediators TNF-α and VEGF-A₁₆₅a. Taken together our findings suggest that a novel VEGFR2-mediated spinal cord glio-vascular mechanism may promote peripheral CD11b⁺ circulating cell transmigration into the CNS parenchyma and contribute to the development of chronic pain in inflammatory arthritis. We hypothesise that preventing this glio-vascular activation and circulating cell translocation into the spinal cord could be a new therapeutic strategy for pain caused by rheumatoid arthritis.

Region-Specific Phenotypes of Microglia: The Role of Local Regulatory Cues

Microglia are ubiquitous, macrophage like cells within the central nervous system (CNS) that play critical roles in supporting neuronal health and viability. They can also influence neuronal membrane properties and synaptic connectivity, positioning microglia as key cellular players in both physiological and pathological contexts. Microglia have generally been assumed to be equivalent throughout the CNS, but accumulating evidence indicates that their properties vary substantially across distinct CNS regions. In comparison to our understanding of neuronal diversity and its functional importance, our knowledge about causes and consequences of microglial regional heterogeneity is extremely limited. To fully understand how microglia influence the function of specific neuronal populations and shape heightened susceptibility of some neurons to damage and disease, greater focus on microglial heterogeneity is needed.

Impaired Autophagy of GABAergic Interneurons in Neuropathic Pain

Neuropathic pain (NP) is caused by lesions of the peripheral fibers and central neurons in the somatosensory nervous system and affects 7-10% of the general population. Although the distinct cause of neuropathic pain has been investigated in primary afferent neurons over the years, pain modulation by central sensitization remains controversial. NP is believed to be driven by cell type-specific spinal synaptic plasticity in the dorsal horn. Upon intense afferent stimulation, spinothalamic tract neurons are potentiated, whereas GABAergic interneurons are inhibited leading to long-term depression. Growing evidences suggest that the inhibition of GABAergic neurons plays pivotal roles in the manifestation of neuropathic and inflammatory pain states. Downregulation of GABA transmission and impairment of GABAergic interneurons in the dorsal horn are critical consequences after spinal cord and peripheral nerve injuries. These impairments in GABAergic interneurons may be associated with dysfunctional autophagy, resulting in neuropathic pain. Here, we review an emerging number of investigations that suggest a pivotal role of impaired autophagy of GABAergic interneurons in NP. We discuss relevant research spurring the development of new targets and therapeutic agents of NP and emphasize the need for a multidisciplinary approach to manage NP in the future.

Spinal microglia are required for long-term maintenance of neuropathic pain

While spinal microglia play a role in early stages of neuropathic pain etiology, whether they are useful targets to reverse chronic pain at late stages remains unknown. Here, we show that microglia activation in the spinal cord persists for >3 months following nerve injury in rodents, beyond involvement of proinflammatory cytokine and chemokine signalling. In this chronic phase, selective depletion of spinal microglia in male rats with the targeted immunotoxin Mac1-saporin and blockade of brain-derived neurotrophic factor-TrkB signalling with intrathecal TrkB Fc chimera, but not cytokine inhibition, almost completely reversed pain hypersensitivity. By contrast, local spinal administration of Mac1-saporin did not affect nociceptive withdrawal threshold in control animals nor did it affect the strength of afferent-evoked synaptic activity in the spinal dorsal horn in normal conditions. These findings show that the long-term, chronic phase of nerve injury-induced pain hypersensitivity is maintained by microglia-neuron interactions. The findings also effectively separate the central signalling pathways underlying the maintenance phase of the pathology from the early and peripheral inflammatory reactions to injury, pointing to different targets for the treatment of acute vs chronic injury-induced pain.

Repopulating retinal microglia restore endogenous organization and function under CX3CL1-CX3CR1 regulation

Microglia have been discovered to undergo repopulation following ablation. However, the functionality of repopulated microglia and the mechanisms regulating microglia repopulation are unknown. We examined microglial homeostasis in the adult mouse retina, a specialized neural compartment containing regular arrays of microglia in discrete synaptic laminae that can be directly visualized. Using in vivo imaging and cell-fate mapping techniques, we discovered that repopulation originated from residual microglia proliferating in the central inner retina that subsequently spread by centrifugal migration to fully recapitulate pre-existing microglial distributions and morphologies. Repopulating cells fully restored microglial functions including constitutive "surveying" process movements, behavioral and physiological responses to retinal injury, and maintenance of synaptic structure and function. Microglial repopulation was regulated by CX3CL1-CX3CR1 signaling, slowing in CX3CR1 deficiency and accelerating with exogenous CX3CL1 administration. Microglial homeostasis following perturbation can fully recover microglial organization and function under the regulation of chemokine signaling between neurons and microglia.

Microglia in health and pain: impact of noxious early life events

NEW FINDINGS

What is the topic of this review? This review discusses the origins and development of microglia, and how stress, pain or inflammation in early life disturbs microglial function during critical developmental periods, leading to altered pain sensitivity and/or increased risk of chronic pain in later life. What advances does it highlight? We highlight recent advances in understanding how disrupted microglial function impacts the developing nervous system and the consequences for pain processing and susceptibility for development of chronic pain in later life. The discovery of microglia is accredited to Pío del Río-Hortega, who recognized this 'third element' of CNS cells as being morphologically distinct from neurons and astrocytes. For decades after this finding, microglia were altogether ignored or relegated as simply being support cells. Emerging from virtual obscurity, microglia have now gained notoriety as immune cells that assume a leading role in the development, maintenance and protection of a healthy CNS. Pioneering studies have recently shed light on the origins of microglia, their role in the developing nervous system and the complex roles they play beyond the immune response. These studies reveal that altered microglial function can have a profoundly negative impact on the developing brain and may be a determinant in a range of neurodevelopmental disorders and neurodegenerative diseases. The realization that aberrant microglial function also critically underlies chronic pain, a debilitating disorder that afflicts over 1.5 billion people worldwide, was a major conceptual leap forward in the pain field. Adding to this advance is emerging evidence that early life noxious experiences can have a long-lasting impact on central pain processing and adult pain sensitivity. With microglia now coming of age, in this review we examine the association between adverse early life events, such as stress, injury or inflammation, and the influence of sex differences, on the role of microglia in pain physiology in adulthood.

Lessons Learned about Neurodegeneration from Microglia and Monocyte Depletion Studies

While bone marrow-derived Ly6C^hi monocytes can infiltrate the central nervous system (CNS) they are developmentally and functionally distinct from resident microglia. Our understanding of the relative importance of these two populations in the distinct processes of pathogenesis and resolution of inflammation during neurodegenerative disorders was limited by a lack of tools to specifically manipulate each cell type. During recent years, the development of experimental cell-specific depletion models has enabled this issue to be addressed. Herein we compare and contrast the different depletion approaches that have been used, focusing on the respective functionalities of microglia and monocyte-derived macrophages in a range of neurodegenerative disease states, and discuss their prospects for immunotherapy.

An updated assessment of microglia depletion: current concepts and future directions

Microglia are the principal resident immune cells in the central nervous system and are believed to be versatile players in both inflammatory and physiological contexts. On the one hand, in order to safeguard the microenvironment microglia can be rapidly activated by contact with microbial products or cell debris, thereby exerting the functions of innate immunity via phagocytosis and secretion of cytokines and chemokines. Conversely, microglia can also assist in brain development, synaptic plasticity and neural repair through the production of neurotrophic factors and clearance of myelin debris. It is now well accepted that the dysfunction of microglia and microglia-induced neuroinflammation are implicated in the occurrence and progression of many neurological diseases. Although the past decade has witnessed major progress in understanding of multi-tasking microglia, what remains largely enigmatic is the relative importance of microglia at different disease stages and how microglia should be targeted for optimal therapeutic efficacy. Notably, microglia depletion through genetic targeting or pharmacological therapies can be viewed as effective tools to stimulate new microglia to repopulate the central nervous system. Microglia depletion and subsequent repopulation at defined stages in various experimental animal model disorders allow us to extend our knowledge of molecular mechanisms, thus holding promise for designing strategies to resolve neuroinflammation and promote recovery. Herein we highlight the highly plastic and diverse phenotypes of microglia and outline the lessons learned from microglia depletion approaches.

Infiltrating cells from host brain restore the microglial population in grafted cortical tissue

Transplantation of embryonic cortical tissue is considered as a promising therapy for brain injury. Grafted neurons can reestablish neuronal network and improve cortical function of the host brain. Microglia is a key player in regulating neuronal survival and plasticity, but its activation and dynamics in grafted cortical tissue remain unknown. Using two-photon intravital imaging and parabiotic model, here we investigated the proliferation and source of microglia in the donor region by transplanting embryonic cortical tissue into adult cortex. Live imaging showed that the endogenous microglia of the grafted tissue were rapidly lost after transplantation. Instead, host-derived microglia infiltrated and colonized the graft. Parabiotic model suggested that the main source of infiltrating cells is the parenchyma of the host brain. Colonized microglia proliferated and experienced an extensive morphological transition and eventually differentiated into resting ramified morphology. Collectively, these results demonstrated that donor tissue has little contribution to the activated microglia and host brain controls the microglial population in the graft.