Activated microglia proliferate at neurites of mutant huntingtin-expressing neurons

Microglial response to aging and neuroinflammation in the development of neurodegenerative diseases

ABSTRACT

Cellular senescence and chronic inflammation in response to aging are considered to be indicators of brain aging; they have a great impact on the aging process and are the main risk factors for neurodegeneration. Reviewing the microglial response to aging and neuroinflammation in neurodegenerative diseases will help understand the importance of microglia in neurodegenerative diseases. This review describes the origin and function of microglia and focuses on the role of different states of the microglial response to aging and chronic inflammation on the occurrence and development of neurodegenerative diseases, including Alzheimer's disease, Huntington's chorea, and Parkinson's disease. This review also describes the potential benefits of treating neurodegenerative diseases by modulating changes in microglial states. Therefore, inducing a shift from the neurotoxic to neuroprotective microglial state in neurodegenerative diseases induced by aging and chronic inflammation holds promise for the treatment of neurodegenerative diseases in the future.

Circulating levels of inflammatory biomarkers in Huntington's disease: A systematic review and meta-analysis

BACKGROUND

Huntington's disease (HD) is an autosomal dominant disease caused by an abnormally high number of CAG repeats at the huntingtin-encoding gene, HTT. This genetic alteration results in the expression of a mutant form of the protein (mHTT) and the formation of intracellular aggregates, inducing an inflammatory state within the affected areas. This dysfunction of inflammatory response leads to elevated levels of related inflammatory markers in both CNS tissue samples and body fluids. This study aims to investigate peripheral/blood concentrations of inflammatory molecules in HD.

METHODS

A search was conducted in MEDLINE, Scopus, Web of Science, and Embase databases until March 30th, 2023. Random-effect meta-analysis was used for exploring concentrations of inflammatory molecules in HD. Subgroup and sensitivity analyses were used to assess heterogeneity among the included studies. The study protocol has been registered in PROSPERO with the ID number CRD42022296078.

RESULTS

Ten studies were included in the meta-analysis. Plasma levels of Interleukin 6 (IL-6) and IL-10 were higher in HD compared to controls. Other biomarkers, namely, complement component C-reactive protein (CRP), C3, interferon-γ (IFN-γ), IL-1, IL-2, IL-8, and tumor necrosis factor-α (TNF-α), did not show any significant differences between the two groups. In addition, the subgroup analysis results established no significant differences in levels of these biomarkers in body fluids among premanifest and manifest HD patients.

CONCLUSION

The results of this study provide evidence for the presence of higher plasma levels of IL-6 and IL-10 in HD patients in comparison with healthy controls.

Microglial and Astrocytic Responses in the Human Midcingulate Cortex in Huntington's Disease

OBJECTIVE

Patients with Huntington's disease can present with variable difficulties of motor functioning, mood, and cognition. Neurodegeneration occurs in the anterior cingulate cortex of some patients with Huntington's disease and is linked to the presentation of mood symptomatology. Neuroinflammation, perpetrated by activated microglia and astrocytes, has been reported in Huntington's disease and may contribute to disease progression and presentation. This study sought to quantify the density of mutant huntingtin protein and neuroinflammatory glial changes in the midcingulate cortex of postmortem patients with Huntington's disease and determine if either correlates with the presentation of mood, motor, or mixed symptomatology.

METHODS

Free-floating immunohistochemistry quantified 1C2 immunolabeling density as an indicative marker of mutant huntingtin protein, and protein and morphological markers of astrocyte (EAAT2, Cx43, and GFAP), and microglial (Iba1 and HLA-DP/DQ/DR) activation. Relationships among the level of microglial activation, mutant huntingtin burden, and case characteristics were explored using correlative analysis.

RESULTS

We report alterations in activated microglia number and morphology in the midcingulate cortex of Huntington's disease cases with predominant mood symptomatology. An increased proportion of activated microglia was observed in the midcingulate of all Huntington's disease cases and positively correlated with 1C2 burden. Alterations in the astrocytic glutamate transporter EAAT2 were observed in the midcingulate cortex of patients associated with mood symptoms.

INTERPRETATION

This study presents pathological changes in microglia and astrocytes in the midcingulate cortex in Huntington's disease, which coincide with mood symptom presentation. These findings further the understanding of neuroinflammation in Huntington's disease, a necessary step for developing inflammation-targeted therapeutics. ANN NEUROL 2023;94:895-910.

Microglia in neurodegenerative diseases: mechanism and potential therapeutic targets

Microglia activation is observed in various neurodegenerative diseases. Recent advances in single-cell technologies have revealed that these reactive microglia were with high spatial and temporal heterogeneity. Some identified microglia in specific states correlate with pathological hallmarks and are associated with specific functions. Microglia both exert protective function by phagocytosing and clearing pathological protein aggregates and play detrimental roles due to excessive uptake of protein aggregates, which would lead to microglial phagocytic ability impairment, neuroinflammation, and eventually neurodegeneration. In addition, peripheral immune cells infiltration shapes microglia into a pro-inflammatory phenotype and accelerates disease progression. Microglia also act as a mobile vehicle to propagate protein aggregates. Extracellular vesicles released from microglia and autophagy impairment in microglia all contribute to pathological progression and neurodegeneration. Thus, enhancing microglial phagocytosis, reducing microglial-mediated neuroinflammation, inhibiting microglial exosome synthesis and secretion, and promoting microglial conversion into a protective phenotype are considered to be promising strategies for the therapy of neurodegenerative diseases. Here we comprehensively review the biology of microglia and the roles of microglia in neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, multiple system atrophy, amyotrophic lateral sclerosis, frontotemporal dementia, progressive supranuclear palsy, corticobasal degeneration, dementia with Lewy bodies and Huntington's disease. We also summarize the possible microglia-targeted interventions and treatments against neurodegenerative diseases with preclinical and clinical evidence in cell experiments, animal studies, and clinical trials.

From Pathogenesis to Therapeutics: A Review of 150 Years of Huntington's Disease Research

Huntington's disease (HD) is a debilitating neurodegenerative genetic disorder caused by an expanded polyglutamine-coding (CAG) trinucleotide repeat in the huntingtin (HTT) gene. HD behaves as a highly penetrant dominant disorder likely acting through a toxic gain of function by the mutant huntingtin protein. Widespread cellular degeneration of the medium spiny neurons of the caudate nucleus and putamen are responsible for the onset of symptomology that encompasses motor, cognitive, and behavioural abnormalities. Over the past 150 years of HD research since George Huntington published his description, a plethora of pathogenic mechanisms have been proposed with key themes including excitotoxicity, dopaminergic imbalance, mitochondrial dysfunction, metabolic defects, disruption of proteostasis, transcriptional dysregulation, and neuroinflammation. Despite the identification and characterisation of the causative gene and mutation and significant advances in our understanding of the cellular pathology in recent years, a disease-modifying intervention has not yet been clinically approved. This review includes an overview of Huntington's disease, from its genetic aetiology to clinical presentation and its pathogenic manifestation. An updated view of molecular mechanisms and the latest therapeutic developments will also be discussed.

Mechanisms of Microglia Proliferation in a Rat Model of Facial Nerve Anatomy

Although microglia exist as a minor glial cell type in the normal state of the brain, they increase in number in response to various disorders and insults. However, it remains unclear whether microglia proliferate in the affected area, and the mechanism of the proliferation has long attracted the attention of researchers. We analyzed microglial mitosis using a facial nerve transection model in which the blood-brain barrier is left unimpaired when the nerves are axotomized. Our results showed that the levels of macrophage colony-stimulating factor (M-CSF), cFms (the receptor for M-CSF), cyclin A/D, and proliferating cell nuclear antigen (PCNA) were increased in microglia in the axotomized facial nucleus (axotFN). In vitro experiments revealed that M-CSF induced cFms, cyclin A/D, and PCNA in microglia, suggesting that microglia proliferate in response to M-CSF in vivo. In addition, M-CSF caused the activation of c-Jun N-terminal kinase (JNK) and p38, and the specific inhibitors of JNK and p38 arrested the microglial mitosis. JNK and p38 were shown to play roles in the induction of cyclins/PCNA and cFms, respectively. cFms was suggested to be induced through a signaling cascade of p38-mitogen- and stress-activated kinase-1 (MSK1)-cAMP-responsive element binding protein (CREB) and/or p38-activating transcription factor 2 (ATF2). Microglia proliferating in the axotFN are anticipated to serve as neuroprotective cells by supplying neurotrophic factors and/or scavenging excite toxins and reactive oxygen radicals.

Interleukin-6 as a marker of Huntington's disease progression: Systematic review and meta-analysis

Huntington's disease (HD) is a rare, inherited disorder with a broad spectrum of manifestations that vary with disease severity and progression. Although genetic testing can readily confirm the initial diagnosis of HD, markers sensitive to HD progression are needed to aid the development of individual treatment plans. The current analysis aims to identify plasma Interleukin-6 (IL-6) as a marker of disease progression in HD patients. A systematic search of PubMed and Medline from conception through October 2021 was conducted. Studies reporting plasma IL-6 levels of mutation-positive HD patients and healthy controls that met inclusion criteria were selected. The search strategy collected 303 studies, 9 of which met analysis inclusion criteria. From included studies, plasma IL-6 levels of 469 individuals with the HD mutation and 206 healthy controls were collected. Plasma IL-6 levels were meta-analytically compared between healthy controls and individuals with the confirmed HD mutation at all stages of disease and correlated to performance on standardized measures of total cognitive and motor function. Plasma IL-6 was significantly increased in HD groups compared to controls (g = 0.73, 95% CI = 0.31,1.16, P < 0.01) and increased significantly throughout most stages of disease progression, notably between pre-manifest and manifest (g = 0.31, 95% CI = 0.04,0.59, P < 0.05) and early and moderate HD stages (g = 0.52, 95% CI = 0.18,0.86, P < 0.01). Significant correlations between plasma IL-6 levels and HD symptomatic progression were identified, with increased cytokine levels associated with more severe motor impairments (r = 0.179, 95% CI = 0.0479,0.304, P = 0.008) and more extreme disabilities in activities of daily living and/or work tasks (r = -0.229, 95% CI = -0.334, -0.119, P < 0.001). Conclusively, plasma IL-6 levels correlate with disease and motor symptom progression and may act as a viable marker for clinical use. Analysis is limited by small study numbers and highlights the need for future work to identify definitive ranges or rates of change of plasma IL-6 levels that correlate to progressive HD disease states.

Inflammasome activation and assembly in Huntington's disease

Huntington's disease (HD) is a rare neurodegenerative disease characterized by motor, cognitive, and psychiatric symptoms. Inflammasomes are multiprotein complexes capable of sensing pathogen-associated and damage-associated molecular patterns, triggering innate immune pathways. Activation of inflammasomes results in a pro-inflammatory cascade involving, among other molecules, caspases and interleukins. NLRP3 (nucleotide-binding domain, leucine-rich-repeat containing family, pyrin domain-containing 3) is the most studied inflammasome complex, and its activation results in caspase-1 mediated cleavage of the pro-interleukins IL-1β and IL-18 into their mature forms, also inducing a gasdermin D mediated form of pro-inflammatory cell death, i.e. pyroptosis. Accumulating evidence has implicated NLRP3 inflammasome complex in neurodegenerative diseases. The evidence in HD is still scant and mostly derived from pre-clinical studies. This review aims to present the available evidence on NLRP3 inflammasome activation in HD and to discuss whether targeting this innate immune system complex might be a promising therapeutic strategy to alleviate its symptoms.

Hunting for the cause: Evidence for prion-like mechanisms in Huntington’s disease

The hypothesis that pathogenic protein aggregates associated with neurodegenerative diseases spread from cell-to-cell in the brain in a manner akin to infectious prions has gained substantial momentum due to an explosion of research in the past 10–15 years. Here, we review current evidence supporting the existence of prion-like mechanisms in Huntington’s disease (HD), an autosomal dominant neurodegenerative disease caused by expansion of a CAG repeat tract in exon 1 of the huntingtin (HTT) gene. We summarize information gained from human studies and in vivo and in vitro models of HD that strongly support prion-like features of the mutant HTT (mHTT) protein, including potential involvement of molecular features of mHTT seeds, synaptic structures and connectivity, endocytic and exocytic mechanisms, tunneling nanotubes, and nonneuronal cells in mHTT propagation in the brain. We discuss mechanisms by which mHTT aggregate spreading and neurotoxicity could be causally linked and the potential benefits of targeting prion-like mechanisms in the search for new disease-modifying therapies for HD and other fatal neurodegenerative diseases.

Neuroinflammation in Huntington's Disease: A Starring Role for Astrocyte and Microglia

Huntington's disease (HD) is a neurodegenerative genetic disorder caused by a CAG repeat expansion in the huntingtin gene. HD causes motor, cognitive, and behavioral dysfunction. Since no existing treatment affects the course of this disease, new treatments are needed. Inflammation is frequently observed in HD patients before symptom onset. Neuroinflammation, characterized by the presence of reactive microglia, astrocytes and inflammatory factors within the brain, is also detected early. However, in comparison to other neurodegenerative diseases, the role of neuroinflammation in HD is much less known. Work has been dedicated to altered microglial and astrocytic functions in the context of HD, but less attention has been given to glial participation in neuroinflammation. This review describes evidence of inflammation in HD patients and animal models. It also discusses recent knowledge on neuroinflammation in HD, highlighting astrocyte and microglia involvement in the disease and considering anti-inflammatory therapeutic approaches.

Distinct phases of adult microglia proliferation: a Myc-mediated early phase and a Tnfaip3-mediated late phase

Microgliosis is a hallmark of many neurological diseases, including Alzheimer’s disease, stroke, seizure, traumatic brain and spinal cord injuries, and peripheral and optic nerve injuries. Recent studies have shown that the newly self-renewed microglia have specific neurological functions. However, the mechanism of adult microglia proliferation remains largely unclear. Here, with single-cell RNA sequencing, flow cytometry, and immunohistochemistry, we demonstrate that the sciatic nerve injury induced two distinct phases of microglia proliferation in mouse spinal cord, each with different gene expression profiles. We demonstrate that the transcription factor Myc was transiently upregulated in spinal cord microglia after nerve injury to mediate an early phase microglia proliferation. On the other hand, we reveal that the tumor-necrosis factor alpha-induced protein 3 (Tnfaip3) was downregulated to mediate the Myc-independent late-phase microglia proliferation. We show that cyclin dependent kinase 1, a kinase with important function in the M phase of the cell cycle, was involved only in the early phase. We reveal that although the early phase was neither necessary nor sufficient for the late phase proliferation, the late-phase suppressed the early phase microglia proliferation in the spinal cord. Finally, we demonstrate that the termination of spinal cord microglia proliferation required both Myc and Tnfaip3 to resume their baseline expression. Thus, we have delineated an interactive signaling network in the proliferation of differentiated microglia.

Nicotinamide Adenine Dinucleotide Phosphate Oxidases Are Everywhere in Brain Disease, but Not in Huntington's Disease?

Huntington’s disease (HD) is an inherited neurodegenerative disorder characterized by neuronal loss and tissue atrophy mainly in the striatum and cortex. In the early stages of the disease, impairment of neuronal function, synaptic dysfunction and white matter loss precedes neuronal death itself. Relative to other neurodegenerative diseases such as Alzheimer’s and Parkinson’s disease and Amyotrophic Lateral Sclerosis, where the effects of either microglia or NADPH oxidases (NOXs) are recognized as important contributors to disease pathogenesis and progression, there is a pronounced lack of information in HD. This information void contrasts with evidence from human HD patients where blood monocytes and microglia are activated well before HD clinical symptoms (PET scans), and the clear signs of oxidative stress and inflammation in post mortem HD brain. Habitually, NOX activity and oxidative stress in the central nervous system (CNS) are equated with microglia, but research of the last two decades has carved out important roles for NOX enzyme function in neurons. Here, we will convey recent information about the function of NOX enzymes in neurons, and contemplate on putative roles of neuronal NOX in HD. We will focus on NOX-produced reactive oxygen species (ROS) as redox signaling molecules in/among neurons, and the specific roles of NOXs in important processes such as neurogenesis and lineage specification, neurite outgrowth and growth cone dynamics, and synaptic plasticity where NMDAR-dependent signaling, and long-term depression/potentiation are redox-regulated phenomena. HD animal models and induced pluripotent stem cell (iPSC) studies have made it clear that the very same physiological processes are also affected in HD, and we will speculate on possible roles for NOX in the pathogenesis and development of disease. Finally, we also take into account the limited information on microglia in HD and relate this to any contribution of NOX enzymes.

Iron activates microglia and directly stimulates indoleamine-2,3-dioxygenase activity in the N171-82Q mouse model of Huntington's disease

Huntington's disease (HD) is a neurodegenerative disorder caused by a dominant CAG-repeat expansion in the huntingtin gene. Microglial activation is a key feature of HD pathology, and is present before clinical disease onset. The kynurenine pathway (KP) of tryptophan degradation is activated in HD, and is thought to contribute to disease progression. Indoleamine-2,3-dioxygenase (IDO) catalyzes the first step in this pathway; this and other pathway enzymes reside with microglia. While HD brain microglia accumulate iron, the role of iron in promoting microglial activation and KP activity is unclear. Here we utilized the neonatal iron supplementation model to investigate the relationship between iron, microglial activation and neurodegeneration in adult HD mice. We show in the N171-82Q mouse model of HD microglial morphologic changes consistent with immune activation. Neonatal iron supplementation in these mice promoted neurodegeneration and resulted in additional microglial activation in adults as determined by increased soma volume and decreased process length. We further demonstrate that iron activates IDO, both in brain lysates and purified recombinant protein (EC50 = 1.24 nM). Brain IDO activity is increased by HD. Neonatal iron supplementation further promoted IDO activity in cerebral cortex, altered KP metabolite profiles, and promoted HD neurodegeneration as measured by brain weights and striatal volumes. Our results demonstrate that dietary iron is an important activator of microglia and the KP pathway in this HD model, and that this occurs in part through a direct effect on IDO. The findings are relevant to understanding how iron promotes neurodegeneration in HD.

Human Huntington's disease pluripotent stem cell-derived microglia develop normally but are abnormally hyper-reactive and release elevated levels of reactive oxygen species

Background

Neuroinflammation may contribute to the pathogenesis of Huntington’s disease, given evidence of activated microglia and elevated levels of inflammatory molecules in disease gene carriers, even those many years from symptom onset. We have shown previously that monocytes from Huntington’s disease patients are hyper-reactive to stimulation in a manner dependent on their autonomous expression of the disease-causing mutant HTT protein. To date, however, whether human microglia are similarly hyper-responsive in a cell-autonomous manner has not been determined.

Methods

Microglial-like cells were derived from human pluripotent stem cells (PSCs) expressing mutant HTT containing varying polyglutamine lengths. These included lines that are otherwise isogenic, such that any observed differences can be attributed with certainty to the disease mutation itself. Analyses by quantitative PCR and immunofluorescence microscopy respectively of key genes and protein markers were undertaken to determine whether Huntington’s disease PSCs differentiated normally to a microglial fate. The resultant cultures and their supernatants were then assessed by various biochemical assays and multiplex ELISAs for viability and responses to stimulation, including the release of pro-inflammatory cytokines and reactive oxygen species. Conditioned media were applied to PSC-derived striatal neurons, and vice versa, to determine the effects that the secretomes of each cell type might have on the other.

Results

Human PSCs generated microglia successfully irrespective of the expression of mutant HTT. These cells, however, were hyper-reactive to stimulation in the production of pro-inflammatory cytokines such as IL-6 and TNFα. They also released elevated levels of reactive oxygen species that have neurotoxic potential. Accompanying such phenotypes, human Huntington’s disease PSC-derived microglia showed increased levels of apoptosis and were more susceptible to exogenous stress. Such stress appeared to be induced by supernatants from human PSC-derived striatal neurons expressing mutant HTT with a long polyglutamine tract.

Conclusions

These studies show, for the first time, that human Huntington’s disease PSC-derived microglia are hyper-reactive due to their autonomous expression of mutant HTT. This provides a cellular basis for the contribution that neuroinflammation might make to Huntington’s disease pathogenesis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02147-6.

Microglia regulate synaptic development and plasticity

Synapses are fundamental structures of neural circuits that transmit information between neurons. Thus, the process of neural circuit formation via proper synaptic connections shapes the basis of brain functions and animal behavior. Synapses continuously undergo repeated formation and elimination throughout the lifetime of an organism, reflecting the dynamics of neural circuit function. The structural transformation of synapses has been described mainly in relation to neural activity-dependent strengthening and weakening of synaptic functions, that is, functional plasticity of synapses. An increasing number of studies have unveiled the roles of microglia, brain-resident immune cells that survey the brain parenchyma with highly motile processes, in synapse formation and elimination as well as in regulating synaptic function. Over the past 15 years, the molecular mechanisms underlying microglia-dependent regulation of synaptic plasticity have been thoroughly studied, and researchers have reported that the disruption of microglia-dependent regulation causes synaptic dysfunction that leads to brain diseases. In this review, we will broadly introduce studies that report the roles of microglia in synaptic plasticity and the possible underlying molecular mechanisms.

Assessing Microglial Dynamics by Live Imaging

Microglia are highly dynamic in the brain in terms of their ability to migrate, proliferate, and phagocytose over the course of an individual's life. Real-time imaging is a useful tool to examine how microglial behavior is regulated and how it affects the surrounding environment. However, microglia are sensitive to environmental stimuli, so they possibly change their state during live imaging in vivo, mainly due to surgical damage, and in vitro due to various effects associated with culture conditions. Therefore, it is difficult to perform live imaging without compromising the properties of the microglia under physiological conditions. To overcome this barrier, various experimental conditions have been developed; recently, it has become possible to perform live imaging of so-called surveillant microglia in vivo, ex vivo, and in vitro, although there are various limitations. Now, we can choose in vivo, ex vivo, or in vitro live imaging systems according to the research objective. In this review, we discuss the advantages and disadvantages of each experimental system and outline the physiological significance and molecular mechanisms of microglial behavior that have been elucidated by live imaging.

Microglial innate memory and epigenetic reprogramming in neurological disorders

Microglia are myeloid-derived cells recognized as brain-resident macrophages. They act as the first and main line of immune defense in the central nervous system (CNS). Microglia have high phenotypic plasticity and are essential for regulating healthy brain homeostasis, and their dysregulation underlies the onset and progression of several CNS pathologies through impaired inflammatory responses. Aberrant microglial activation, following an inflammatory insult, is associated with epigenetic dysregulation in various CNS pathologies. Emerging data suggest that certain stimuli to myeloid cells determine enhanced or attenuated responses to subsequent stimuli. These phenomena, generally termed innate immune memory (IIM), are highly dependent on epigenetic reprogramming. Microglial priming has been reported in several neurological diseases and corresponds to a state of increased permissiveness or exacerbated response, promoted by continuous exposure to a chronic pro-inflammatory environment. In this article, we provide extensive evidence of these epigenetic-mediated phenomena under neurological conditions and discuss their contribution to pathogenesis and their clinical implications, including those concerning potential novel therapeutic approaches.

The microbiota-microglia axis in central nervous system disorders

The innate immune system in the central nervous system (CNS) is mainly represented by specialized tissue-resident macrophages, called microglia. In the past years, various species-, host- and tissue-specific as well as environmental factors were recognized that essentially affect microglial properties and functions in the healthy and diseased brain. Host microbiota are mostly residing in the gut and contribute to microglial activation states, for example, via short-chain fatty acids (SCFAs) or aryl hydrocarbon receptor (AhR) ligands. Thereby, the gut microorganisms are deemed to influence numerous CNS diseases mediated by microglia. In this review, we summarize recent findings of the interaction between the host microbiota and the CNS in health and disease, where we specifically highlight the resident gut microbiota as a crucial environmental factor for microglial function as what we coin "the microbiota-microglia axis."

Microglial depletion prevents extracellular matrix changes and striatal volume reduction in a model of Huntington's disease

Huntington's disease is associated with a reactive microglial response and consequent inflammation. To address the role of these cells in disease pathogenesis, we depleted microglia from R6/2 mice, a rapidly progressing model of Huntington's disease marked by behavioural impairment, mutant huntingtin (mHTT) accumulation, and early death, through colony-stimulating factor 1 receptor inhibition (CSF1Ri) with pexidartinib (PLX3397) for the duration of disease. Although we observed an interferon gene signature in addition to downregulated neuritogenic and synaptic gene pathways with disease, overt inflammation was not evident by microglial morphology or cytokine transcript levels in R6/2 mice. Nonetheless, CSF1Ri-induced microglial elimination reduced or prevented disease-related grip strength and object recognition deficits, mHTT accumulation, astrogliosis, and striatal volume loss, the latter of which was not associated with reductions in cell number but with the extracellular accumulation of chondroitin sulphate proteoglycans (CSPGs)-a primary component of glial scars. A concurrent loss of proteoglycan-containing perineuronal nets was also evident in R6/2 mice, and microglial elimination not only prevented this but also strikingly increased perineuronal nets in the brains of naïve littermates, suggesting a new role for microglia as homeostatic regulators of perineuronal net formation and integrity.

An interplay between immune response and neurodegenerative disease progression: An assessment using Drosophila as a model

Neurodegeneration, the slow and progressive loss of neurons in the central nervous system has become a major challenge to public health worldwide particularly with elderly people. Until recently, the brain and immune system were studied exclusively, independent of each other representing two distinct systems. Recent studies ensue crosstalk between these two systems to maintain homeostasis. Though the progressive loss of specific neuronal subsets is a hallmark of neurodegenerative disease, emerging evidences indicate that immune response also plays a critical role in disease progression. Due to conservation of mechanisms that govern neural development and innate immune activation in flies and humans, and availability of powerful genetic tools, the fruit fly Drosophila melanogaster is one of the best model organisms to investigate the immune response in neurodegenerative disease. Owing to significant homology between human and Drosophila immune system and recent reports on interplay between immune system and neurodegenerative disease progression, the main focus of the review is to develop a comprehensive understanding of how neuro-immune interactions contribute to neurodegeneration using Drosophila as a model system.

Cyclic GMP-AMP synthase promotes the inflammatory and autophagy responses in Huntington disease

Huntington disease (HD) is a genetic disorder caused by glutamine-expansion in the huntingtin (mHTT) protein, which affects motor, psychiatric, and cognitive function, but the mechanisms remain unclear. mHTT is known to induce DNA damage and affect autophagy, both associated with inflammatory responses, but what mediates all these were unknown. Here we report that cGAS, a DNA damage sensor, is highly upregulated in the striatum of a mouse model and HD human patient’s tissue. We found ribosomes, which make proteins, are robustly accumulated on the cGAS mRNA in HD cells. cGAS depletion decreases—and cGAS expression increases—both inflammatory and autophagy responses in HD striatal cells. Thus, cGAS is a therapeutic target for HD. Blocking cGAS will prevent/slow down HD symptoms.

Huntington disease (HD) is caused by an expansion mutation of the N-terminal polyglutamine of huntingtin (mHTT). mHTT is ubiquitously present, but it induces noticeable damage to the brain’s striatum, thereby affecting motor, psychiatric, and cognitive functions. The striatal damage and progression of HD are associated with the inflammatory response; however, the underlying molecular mechanisms remain unclear. Here, we report that cGMP-AMP synthase (cGAS), a DNA sensor, is a critical regulator of inflammatory and autophagy responses in HD. Ribosome profiling revealed that the cGAS mRNA has high ribosome occupancy at exon 1 and codon-specific pauses at positions 171 (CCG) and 172 (CGT) in HD striatal cells. Moreover, the protein levels and activity of cGAS (based on the phosphorylated STING and phosphorylated TBK1 levels), and the expression and ribosome occupancy of cGAS-dependent inflammatory genes (Ccl5 and Cxcl10) are increased in HD striatum. Depletion of cGAS diminishes cGAS activity and decreases the expression of inflammatory genes while suppressing the up-regulation of autophagy in HD cells. In contrast, reinstating cGAS in cGAS-depleted HD cells activates cGAS activity and promotes inflammatory and autophagy responses. Ribosome profiling also revealed that LC3A and LC3B, the two major autophagy initiators, show altered ribosome occupancy in HD cells. We also detected the presence of numerous micronuclei, which are known to induce cGAS, in the cytoplasm of neurons derived from human HD embryonic stem cells. Collectively, our results indicate that cGAS is up-regulated in HD and mediates inflammatory and autophagy responses. Thus, targeting the cGAS pathway may offer therapeutic benefits in HD.

Microglial physiological properties and interactions with synapses are altered at presymptomatic stages in a mouse model of Huntington's disease pathology

BACKGROUND

Huntington's disease (HD) is a dominantly inherited neurodegenerative disorder that affects cognitive and motor abilities by primarily targeting the striatum and cerebral cortex. HD is caused by a mutation elongating the CAG repeats within the Huntingtin gene, resulting in HTT protein misfolding. Although the genetic cause of HD has been established, the specific susceptibility of neurons within various brain structures has remained elusive. Microglia, which are the brain's resident macrophages, have emerged as important players in neurodegeneration. Nevertheless, few studies have examined their implication in HD.

METHODS

To provide novel insights, we investigated the maturation and dysfunction of striatal microglia using the R6/2 mouse model of HD. This transgenic model, which presents with 120+/-5 CAG repeats, displays progressive motor deficits beginning at 6 weeks of age, with full incapacitation by 13 weeks. We studied microglial morphology, phagocytic capacity, and synaptic contacts in the striatum of R6/2 versus wild-type (WT) littermates at 3, 10, and 13 weeks of age, using a combination of light and transmission electron microscopy. We also reconstructed dendrites and determined synaptic density within the striatum of R6/2 and WT littermates, at nanoscale resolution using focused ion beam scanning electron microscopy.

RESULTS

At 3 weeks of age, prior to any known motor deficits, microglia in R6/2 animals displayed a more mature morphological phenotype than WT animals. Microglia from R6/2 mice across all ages also demonstrated increased phagocytosis, as revealed by light microscopy and transmission electron microscopy. Furthermore, microglial processes from 10-week-old R6/2 mice made fewer contacts with synaptic structures than microglial processes in 3-week-old R6/2 mice and age-matched WT littermates. Synaptic density was not affected by genotype at 3 weeks of age but increased with maturation in WT mice. The location of synapses was lastly modified in R6/2 mice compared with WT controls, from targeting dendritic spines to dendritic trunks at both 3 and 10 weeks of age.

CONCLUSIONS

These findings suggest that microglia may play an intimate role in synaptic alteration and loss during HD pathogenesis.

The Role of Microglia and Astrocytes in Huntington's Disease

Huntington's disease (HD) is an autosomal dominant neurodegenerative disease. HD patients present with movement disorders, behavioral and psychiatric symptoms and cognitive decline. This review summarizes the contribution of microglia and astrocytes to HD pathophysiology. Neuroinflammation in the HD brain is characterized by a reactive morphology in these glial cells. Microglia and astrocytes are critical in regulating neuronal activity and maintaining an optimal milieu for neuronal function. Previous studies provide evidence that activated microglia and reactive astrocytes contribute to HD pathology through transcriptional activation of pro-inflammatory genes to perpetuate a chronic inflammatory state. Reactive astrocytes also display functional changes in glutamate and ion homeostasis and energy metabolism. Astrocytic and microglial changes may further contribute to the neuronal death observed with the progression of HD. Importantly, the degree to which these neuroinflammatory changes are detrimental to neurons and contribute to the progression of HD pathology is not well understood. Furthermore, recent observations provide compelling evidence that activated microglia and astrocytes exert a variety of beneficial functions that are essential for limiting tissue damage and preserving neuronal function in the HD brain. Therefore, a better understanding of the neuroinflammatory environment in the brain in HD may lead to the development of targeted and innovative therapeutic opportunities.

Cell-Autonomous and Non-cell-Autonomous Pathogenic Mechanisms in Huntington's Disease: Insights from In Vitro and In Vivo Models

Huntington's disease (HD) is an autosomal dominant disorder caused by an expansion in the trinucleotide CAG repeat in exon-1 in the huntingtin gene, located on chromosome 4. When the number of trinucleotide CAG exceeds 40 repeats, disease invariably is manifested, characterized by motor, cognitive, and psychiatric symptoms. The huntingtin (Htt) protein and its mutant form (mutant huntingtin, mHtt) are ubiquitously expressed but although multiple brain regions are affected, the most vulnerable brain region is the striatum. Striatal medium-sized spiny neurons (MSNs) preferentially degenerate, followed by the cortical pyramidal neurons located in layers V and VI. Proposed HD pathogenic mechanisms include, but are not restricted to, excitotoxicity, neurotrophic support deficits, collapse of the protein degradation mechanisms, mitochondrial dysfunction, transcriptional alterations, and disorders of myelin. Studies performed in cell type-specific and regionally selective HD mouse models implicate both MSN cell-autonomous properties and cell-cell interactions, particularly corticostriatal but also with non-neuronal cell types. Here, we review the intrinsic properties of MSNs that contribute to their selective vulnerability and in addition, we discuss how astrocytes, microglia, and oligodendrocytes, together with aberrant corticostriatal connectivity, contribute to HD pathophysiology. In addition, mHtt causes cell-autonomous dysfunction in cell types other than MSNs. These findings have implications in terms of therapeutic strategies aimed at preventing neuronal dysfunction and degeneration.

Heat-Shock Proteins in Neuroinflammation

The heat-shock response, one of the main pro-survival mechanisms of a living organism, has evolved as the biochemical response of cells to cope with heat stress. The most well-characterized aspect of the heat-shock response is the accumulation of a conserved set of proteins termed heat-shock proteins (HSPs). HSPs are key players in protein homeostasis acting as chaperones by aiding the folding and assembly of nascent proteins and protecting against protein aggregation. HSPs have been associated with neurological diseases in the context of their chaperone activity, as they were found to suppress the aggregation of misfolded toxic proteins. In recent times, HSPs have proven to have functions apart from the classical molecular chaperoning in that they play a role in a wider scale of neurological disorders by modulating neuronal survival, inflammation, and disease-specific signaling processes. HSPs are gaining importance based on their ability to fine-tune inflammation and act as immune modulators in various bodily fluids. However, their effect on neuroinflammation processes is not yet fully understood. In this review, we summarize the role of neuroinflammation in acute and chronic pathological conditions affecting the brain. Moreover, we seek to explore the existing literature on HSP-mediated inflammatory function within the central nervous system and compare the function of these proteins when they are localized intracellularly compared to being present in the extracellular milieu.

Intranasal Administration of Mesenchymal Stem Cells Ameliorates the Abnormal Dopamine Transmission System and Inflammatory Reaction in the R6/2 Mouse Model of Huntington Disease

Intrastriatal administration of mesenchymal stem cells (MSCs) has shown beneficial effects in rodent models of Huntington disease (HD). However, the invasive nature of surgical procedure and its potential to trigger the host immune response may limit its clinical use. Hence, we sought to evaluate the non-invasive intranasal administration (INA) of MSC delivery as an effective alternative route in HD. GFP-expressing MSCs derived from bone marrow were intranasally administered to 4-week-old R6/2 HD transgenic mice. MSCs were detected in the olfactory bulb, midbrain and striatum five days post-delivery. Compared to phosphate-buffered saline (PBS)-treated littermates, MSC-treated R6/2 mice showed an increased survival rate and attenuated circadian activity disruption assessed by locomotor activity. MSCs increased the protein expression of DARPP-32 and tyrosine hydroxylase (TH) and downregulated gene expression of inflammatory modulators in the brain 7.5 weeks after INA. While vehicle treated R6/2 mice displayed decreased Iba1 expression and altered microglial morphology in comparison to the wild type littermates, MSCs restored both, Iba1 level and the thickness of microglial processes in the striatum of R6/2 mice. Our results demonstrate significantly ameliorated phenotypes of R6/2 mice after MSCs administration via INA, suggesting this method as an effective delivering route of cells to the brain for HD therapy.

IKKβ slows Huntington's disease progression in R6/1 mice

Neuroinflammation is an important contributor to neuronal pathology and death in neurodegenerative diseases and neuronal injury. Therapeutic interventions blocking the activity of the inflammatory kinase IKKβ, a key regulator of neuroinflammatory pathways, is protective in several animal models of neurodegenerative disease and neuronal injury. In Huntington's disease (HD), however, significant questions exist as to the impact of blocking or diminishing the activity of IKKβ on HD pathology given its potential role in Huntingtin (HTT) degradation. In cell culture, IKKβ phosphorylates HTT serine (S) 13 and activates HTT degradation, a process that becomes impaired with polyQ expansion. To investigate the in vivo relationship of IKKβ to HTT S13 phosphorylation and HD progression, we crossed conditional tamoxifen-inducible IKKβ knockout mice with R6/1 HD mice. Behavioral assays in these mice showed a significant worsening of HD pathological phenotypes. The increased behavioral pathology correlated with reduced levels of endogenous mouse full-length phospho-S13 HTT, supporting the importance of IKKβ in the phosphorylation of HTT S13 in vivo. Notably, many striatal autophagy genes were up-regulated in HD vs. control mice; however, IKKβ knockout partially reduced this up-regulation in HD, increased striatal neurodegeneration, and enhanced an activated microglial response. We propose that IKKβ is protective in striatal neurons early in HD progression via phosphorylation of HTT S13. As IKKβ is also required for up-regulation of some autophagy genes and HTT is a scaffold for selective autophagy, IKKβ may influence autophagy through multiple mechanisms to maintain healthy striatal function, thereby reducing neuronal degeneration to slow HD onset.

Microglia-mediated neuroinflammation in neurodegenerative diseases

Microglia, being the resident immune cells of the central nervous system, play an important role in maintaining tissue homeostasis and contributes towards brain development under normal conditions. However, when there is a neuronal injury or other insult, depending on the type and magnitude of stimuli, microglia will be activated to secrete either proinflammatory factors that enhance cytotoxicity or anti-inflammatory neuroprotective factors that assist in wound healing and tissue repair. Excessive microglial activation damages the surrounding healthy neural tissue, and the factors secreted by the dead or dying neurons in turn exacerbate the chronic activation of microglia, causing progressive loss of neurons. It is the case observed in many neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, Huntington's disease and amyotrophic lateral sclerosis. This review gives a detailed account of the microglia-mediated neuroinflammation in various neurodegenerative diseases. Hence, resolving chronic inflammation mediated by microglia bears great promise as a novel treatment strategy to reduce neuronal damage and to foster a permissive environment for further regeneration effort.

Mortal engines: Mitochondrial bioenergetics and dysfunction in neurodegenerative diseases

Mitochondria are best known for their role in ATP generation. However, studies over the past two decades have shown that mitochondria do much more than that. Mitochondria regulate both necrotic and apoptotic cell death pathways, they store and therefore coordinate cellular Ca²⁺ signaling, they generate and metabolize important building blocks, by-products and signaling molecules, and they also generate and are targets of free radical species that modulate many aspects of cell physiology and pathology. Most estimates suggest that although the brain makes up only 2 percent of body weight, utilizes about 20 percent of the body's total ATP. Thus, mitochondrial dysfunction greatly impacts brain functions and is indeed associated with numerous neurodegenerative diseases. Furthermore, a number of abnormal disease-associated proteins have been shown to interact directly with mitochondria, leading to mitochondrial dysfunction and subsequent neuronal cell death. Here, we discuss the role of mitochondrial dynamics impairment in the pathological processes associated with neurodegeneration and suggest that a therapy targeting mitochondrialdysfunction holds a great promise.

IKKβ and mutant huntingtin interactions regulate the expression of IL-34: implications for microglial-mediated neurodegeneration in HD

Neuronal interleukin-34 (IL-34) promotes the expansion of microglia in the central nervous system-microglial activation and expansion are in turn implicated in the pathogenesis of Huntington's disease (HD). We thus examined whether the accumulation of an amyloidogenic exon-1 fragment of mutant huntingtin (mHTTx1) modulates the expression of IL-34 in dopaminergic neurons derived from a human embryonic stem cell line. We found that mHTTx1 aggregates induce IL-34 production selectively in post-mitotic neurons. Exposure of neurons to DNA damaging agents or the excitotoxin NMDA elicited similar results suggesting that IL-34 induction may be a general response to neuronal stress including the accumulation of misfolded mHTTx1. We further determined that knockdown or blocking the activity of IκB kinase beta (IKKβ) prevented the aggregation of mHTTx1 and subsequent IL-34 production. While elevated IL-34 itself had no effect on the aggregation or the toxicity of mHTTx1 in neuronal culture, IL-34 expression in a rodent brain slice model with intact neuron-microglial networks exacerbated mHTTx1-induced degeneration of striatal medium-sized spiny neurons. Conversely, an inhibitor of the IL-34 receptor reduced microglial numbers and ameliorated mHTTx1-mediated neurodegeneration. Together, these findings uncover a novel function for IKKβ/mHTTx1 interactions in regulating IL-34 production, and implicate a role for IL-34 in non-cell-autonomous, microglial-dependent neurodegeneration in HD.

Selective proliferative response of microglia to alternative polarization signals

Background

Microglia are resident myeloid cells of the central nervous system (CNS) that are maintained by self-renewal and actively participate in tissue homeostasis and immune defense. Under the influence of endogenous or pathological signals, microglia undertake biochemical transformations that are schematically classified as the pro-inflammatory M1 phenotype and the alternatively activated M2 state. Dysregulated proliferation of M1-activated microglia has detrimental effects, while an increased number of microglia with the alternative, pro-resolving phenotype might be beneficial in brain pathologies; however, the proliferative response of microglia to M2 signals is not yet known. We thus evaluated the ability of interleukin-4 (IL-4), a typical M2 and proliferative signal for peripheral macrophages, to induce microglia proliferation and compared it with other proliferative and M2 polarizing stimuli for macrophages, namely colony-stimulating factor-1 (CSF-1) and the estrogen hormone, 17β-estradiol (E₂).

Methods

Recombinant IL-4 was delivered to the brain of adult mice by intracerebroventricular (i.c.v.) injection; whole brain areas or ex vivo-sorted microglia were analyzed by real-time PCR for assessing the mRNA levels of genes related with cell proliferation (Ki67, CDK-1, and CcnB2) and M2 polarization (Arg1, Fizz1, Ym-1) or by FACS analyses of in vivo BrdU incorporation in microglia. Primary cultures of microglia and astrocytes were also tested for proliferative effects.

Results

Our results show that IL-4 only slightly modified the expression of cell cycle-related genes in some brain areas but not in microglia, where it strongly enhanced M2 gene expression; on the contrary, brain delivery of CSF-1 triggered proliferation as well as M2 polarization of microglia both in vivo and in vitro. Similar to IL-4, the systemic E₂ administration failed to induce microglia proliferation while it increased M2 gene expression.

Conclusions

Our data show that, in contrast to the wider responsiveness of peripheral macrophages, microglia proliferation is stimulated by selected M2 polarizing stimuli suggesting a role for the local microenvironment and developmental origin of tissue macrophages in regulating self-renewal following alternative activating stimuli.

Electronic supplementary material

The online version of this article (10.1186/s12974-017-1011-6) contains supplementary material, which is available to authorized users.

Microglial Activation in the Pathogenesis of Huntington's Disease

Huntington’s disease (HD) is an autosomal dominantly inherited neurodegenerative disorder caused by expanded CAG trinucleotide repeats (>36) in exon 1 of HTT gene that encodes huntingtin protein. Although HD is characterized by a predominant loss of neurons in the striatum and cortex, previous studies point to a critical role of aberrant accumulation of mutant huntingtin in microglia that contributes to the progressive neurodegeneration in HD, through both cell-autonomous and non-cell-autonomous mechanisms. Microglia are resident immune cells in the central nervous system (CNS), which function to surveil the microenvironment at a quiescent state. In response to various pro-inflammatory stimuli, microglia become activated and undergo two separate phases (M1 and M2 phenotype), which release pro-inflammatory cytokines (IL-1β, IL-6, and TNF-α), anti-inflammatory cytokines, and growth factors (TGF-β, CD206, and Arg1), respectively. Immunoregulation by microglial activation could be either neurotoxic or neuroprotective. In this review, we summarized current understanding about microglial activation in the pathogenesis and progression of HD, with a primary focus of M1 and M2 phenotype of activated microglia and their corresponding signaling pathways.

Frequency of nuclear mutant huntingtin inclusion formation in neurons and glia is cell-type-specific

Huntington's disease (HD) is an autosomal dominant inherited neurodegenerative disorder that is caused by a CAG expansion in the Huntingtin (HTT) gene, leading to HTT inclusion formation in the brain. The mutant huntingtin protein (mHTT) is ubiquitously expressed and therefore nuclear inclusions could be present in all brain cells. The effects of nuclear inclusion formation have been mainly studied in neurons, while the effect on glia has been comparatively disregarded. Astrocytes, microglia, and oligodendrocytes are glial cells that are essential for normal brain function and are implicated in several neurological diseases. Here we examined the number of nuclear mHTT inclusions in both neurons and various types of glia in the two brain areas that are the most affected in HD, frontal cortex, and striatum. We compared nuclear mHTT inclusion body formation in three HD mouse models that express either full‐length HTT or an N‐terminal exon1 fragment of mHTT, and we observed nuclear inclusions in neurons, astrocytes, oligodendrocytes, and microglia. When studying the frequency of cells with nuclear inclusions in mice, we found that half of the population of neurons contained nuclear inclusions at the disease end stage, whereas the proportion of GFAP‐positive astrocytes and oligodendrocytes having a nuclear inclusion was much lower, while microglia hardly showed any nuclear inclusions. Nuclear inclusions were also present in neurons and all studied glial cell types in human patient material. This is the first report to compare nuclear mHTT inclusions in glia and neurons in different HD mouse models and HD patient brains. GLIA 2016;65:50–61

Macrophage populations and self-renewal: Changing the paradigm

The origin of macrophages has been considered since several decades to be a continuum from bone marrow (BM) to tissue via monocytes as precursors. The development of new tools such as genetic lineage tracing, parabiosis and BM chimeras changed the paradigm of macrophage origin. In steady state, most resident macrophages are of embryonic origin, whereas a monocyte origin remains prominent in pathological conditions. The findings of a proliferation of mature macrophages will oblige us to reappraise the relationship between proliferation and differentiation in macrophages. This review is based on the recent explosion of high impact articles on macrophage biology. It summarizes new data on the origin of macrophages and their self-renewal potential in steady states. While monocytes are required for intestinal macrophage development, the microglia is independent of monocyte influx and skin macrophages provide an excellent model of the balance between monocyte input and self-renewal. In addition, macrophage proliferation requires intrinsic and extrinsic factors including growth factors and cytokines. It also analyzes the impact of this new paradigm in human diseases such as athrosclerosis, cancer, infectious diseases and neurodegenerative diseases. In atherosclerosis, the finding of macrophage proliferation within the lesions will change our understanding of disease pathophysiology, this new paradigm may have therapeutical impact in the future.

Disruption of immune cell function by mutant huntingtin in Huntington's disease pathogenesis

Innate immune dysfunction is increasingly recognised as a key characteristic of neurodegenerative disease. In the fatal inherited neurological disorder, Huntington's disease, altered innate immune cell function and increased inflammation are observed in the brain and the periphery of disease gene carriers many years before symptom onset, suggesting a potentially early and important role in disease pathogenesis. This is due, at least in part, to the intrinsic effects of the disease-causing protein, mutant huntingtin, expressed in innate immune cells themselves. Understanding whether such innate immune dysfunction in Huntington's disease can be targeted to slow the onset and/or the progression of the disease has significant therapeutic implications and is the subject of much current research.

Is membrane homeostasis the missing link between inflammation and neurodegenerative diseases?

Systemic inflammation and infections are associated with neurodegenerative diseases. Unfortunately, the molecular bases of this link are still largely undiscovered. We, therefore, review how inflammatory processes can imbalance membrane homeostasis and theorize how this may have an effect on the aggregation behavior of the proteins implicated in such diseases. Specifically, we describe the processes that generate such imbalances at the molecular level, and try to understand how they affect protein folding and localization. Overall, current knowledge suggests that microglia pro-inflammatory mediators can generate membrane damage, which may have an impact in terms of triggering or accelerating disease manifestation.

Innate immune activation in neurodegenerative disease

The triggering of innate immune mechanisms is emerging as a crucial component of major neurodegenerative diseases. Microglia and other cell types in the brain can be activated in response to misfolded proteins or aberrantly localized nucleic acids. This diverts microglia from their physiological and beneficial functions, and leads to their sustained release of pro-inflammatory mediators. In this Review, we discuss how the activation of innate immune signalling pathways - in particular, the NOD-, LRR- and pyrin domain-containing 3 (NLRP3) inflammasome - by aberrant host proteins may be a common step in the development of diverse neurodegenerative disorders. During chronic activation of microglia, the sustained exposure of neurons to pro-inflammatory mediators can cause neuronal dysfunction and contribute to cell death. As chronic neuroinflammation is observed at relatively early stages of neurodegenerative disease, targeting the mechanisms that drive this process may be useful for diagnostic and therapeutic purposes.

Role of microglia adenosine A(2A) receptors in retinal and brain neurodegenerative diseases

Neuroinflammation mediated by microglial cells in the brain has been commonly associated with neurodegenerative diseases. Whether this microglia-mediated neuroinflammation is cause or consequence of neurodegeneration is still a matter of controversy. However, it is unequivocal that chronic neuroinflammation plays a role in disease progression and halting that process represents a potential therapeutic strategy. The neuromodulator adenosine emerges as a promising targeting candidate based on its ability to regulate microglial proliferation, chemotaxis, and reactivity through the activation of its G protein coupled A2A receptor (A2AR). This is in striking agreement with the ability of A2AR blockade to control several brain diseases. Retinal degenerative diseases have been also associated with microglia-mediated neuroinflammation, but the role of A2AR has been scarcely explored. This review aims to compare inflammatory features of Parkinson's and Alzheimer's diseases with glaucoma and diabetic retinopathy, discussing the therapeutic potential of A2AR in these degenerative conditions.

Microglial dynamics and role in the healthy and diseased brain: a paradigm of functional plasticity

The study of the dynamics and functions of microglia in the healthy and diseased brain is a matter of intense scientific activity. The application of new techniques and new experimental approaches has allowed the identification of novel microglial functions and the redefinition of classic ones. In this review, we propose the study of microglial functions, rather than their molecular profiles, to better understand and define the roles of these cells in the brain. We review current knowledge on the role of surveillant microglia, proliferating microglia, pruning/neuromodulatory microglia, phagocytic microglia, and inflammatory microglia and the molecular profiles that are associated with these functions. In the remodeling scenario of microglial biology, the analysis of microglial functional states will inform about the roles in health and disease and will guide us to a more precise understanding of the multifaceted roles of this never-resting cells.

Mutant Huntingtin promotes autonomous microglia activation via myeloid lineage-determining factors

Huntington's disease (HD) is a fatal neurodegenerative disorder caused by an extended polyglutamine repeat in the N terminus of the Huntingtin protein (HTT). Reactive microglia and elevated cytokine levels are observed in the brains of HD patients, but the extent to which neuroinflammation results from extrinsic or cell-autonomous mechanisms in microglia is unknown. Using genome-wide approaches, we found that expression of mutant Huntingtin (mHTT) in microglia promoted cell-autonomous pro-inflammatory transcriptional activation by increasing the expression and transcriptional activities of the myeloid lineage-determining factors PU.1 and C/EBPs. We observed elevated levels of PU.1 and its target genes in the brains of mouse models and individuals with HD. Moreover, mHTT-expressing microglia exhibited an increased capacity to induce neuronal death ex vivo and in vivo in the presence of sterile inflammation. These findings suggest a cell-autonomous basis for enhanced microglia reactivity that may influence non-cell-autonomous HD pathogenesis.

An in vitro perspective on the molecular mechanisms underlying mutant huntingtin protein toxicity

Huntington's disease (HD) is a devastating neurodegenerative disorder whose main hallmark is brain atrophy. However, several peripheral organs are considerably affected and their symptoms may, in fact, manifest before those resulting from brain pathology. HD is of genetic origin and caused by a mutation in the huntingtin gene. The mutated protein has detrimental effects on cell survival, but whether the mutation leads to a gain of toxic function or a loss of function of the altered protein is still highly controversial. Most currently used in vitro models have been designed, to a large extent, to investigate the effects of the aggregation process in neuronal-like cells. However, as the pathology involves several other organs, new in vitro models are critically needed to take into account the deleterious effects of mutant huntingtin in peripheral tissues, and thus to identify new targets that could lead to more effective clinical interventions in the early course of the disease. This review aims to present current in vitro models of HD pathology and to discuss the knowledge that has been gained from these studies as well as the new in vitro tools that have been developed, which should reflect the more global view that we now have of the disease.

Huntington's disease and the striatal medium spiny neuron: cell-autonomous and non-cell-autonomous mechanisms of disease

Huntington's disease is an autosomal dominant disorder caused by a mutation in the gene encoding the protein huntingtin on chromosome 4. The mutation is an expanded CAG repeat in the first exon, encoding a polyglutamine tract. If the polyglutamine tract is > 40, penetrance is 100% and death is inevitable. Despite the widespread expression of huntingtin, HD has long been considered primarily as a disease of the striatum. It is characterized by selective vulnerability with dysfunction followed by death of the medium size spiny neuron. Considerable effort is being expended to determine whether striatal damage is cell-autonomous, non-cell-autonomous, requiring cell-cell and region to region communication, or both. We review data supporting both mechanisms. We also attempt to organize the data into common mechanisms that may arise outside the medium, spiny neuron, but ultimately have their greatest impact in the striatum.

Immunofluorescent spectral analysis reveals the intrathecal cannabinoid agonist, AM1241, produces spinal anti-inflammatory cytokine responses in neuropathic rats exhibiting relief from allodynia

During pathological pain, the actions of the endocannabinoid system, including the cannabinoid 2 receptor (CB(2)R), leads to effective anti-allodynia and modifies a variety of spinal microglial and astrocyte responses. Here, following spinal administration of the CB(2)R compound, AM1241, we examined immunoreactive alterations in markers for activated p38 mitogen-activated protein kinase, interleukin-1β (IL-1β), the anti-inflammatory cytokine, interleukin-10 (IL-10) as well as degradative endocannabinoid enzymes, and markers for altered glial responses in neuropathic rats. In these studies, the dorsal horn of the spinal cord and dorsal root ganglia were examined. AM1241 produced profound anti-allodynia with corresponding immunoreactive levels of p38 mitogen-activated kinase, IL-1β, IL-10, the endocannabinoid enzyme monoacylglycerol lipase, and astrocyte activation markers that were similar to nonneuropathic controls. In contrast, spinal AM1241 did not suppress the increased microglial responses observed in neuropathic rats. The differences in fluorescent markers were determined within discrete anatomical regions by applying spectral analysis methods, which virtually eliminated nonspecific signal during the quantification of specific immunofluorescent intensity. These data reveal expression profiles that support the actions of intrathecal AM1241 control pathological pain through anti-inflammatory mechanisms by modulating critical glial factors, and additionally decrease expression levels of endocannabinoid degradative enzymes.

Microglia in the developing brain: a potential target with lifetime effects

Microglia are a heterogenous group of monocyte-derived cells serving multiple roles within the brain, many of which are associated with immune and macrophage like properties. These cells are known to serve a critical role during brain injury and to maintain homeostasis; yet, their defined roles during development have yet to be elucidated. Microglial actions appear to influence events associated with neuronal proliferation and differentiation during development, as well as, contribute to processes associated with the removal of dying neurons or cellular debris and management of synaptic connections. These long-lived cells display changes during injury and with aging that are critical to the maintenance of the neuronal environment over the lifespan of the organism. These processes may be altered by changes in the colonization of the brain or by inflammatory events during development. This review addresses the role of microglia during brain development, both structurally and functionally, as well as the inherent vulnerability of the developing nervous system. A framework is presented considering microglia as a critical nervous system-specific cell that can influence multiple aspects of brain development (e.g., vascularization, synaptogenesis, and myelination) and have a long term impact on the functional vulnerability of the nervous system to a subsequent insult, whether environmental, physical, age-related, or disease-related.

Features of microglia and neuroinflammation relevant to environmental exposure and neurotoxicity

Microglia are resident cells of the brain involved in regulatory processes critical for development, maintenance of the neural environment, injury and repair. They belong to the monocytic-macrophage lineage and serve as brain immune cells to orchestrate innate immune responses; however, they are distinct from other tissue macrophages due to their relatively quiescent phenotype and tight regulation by the CNS microenvironment. Microglia actively survey the surrounding parenchyma and respond rapidly to changes such that any disruption to neural architecture or function can contribute to the loss in regulation of the microglia phenotype. In many models of neurodegeneration and neurotoxicity, early events of synaptic degeneration and neuronal loss are accompanied by an inflammatory response including activation of microglia, perivascular monocytes, and recruitment of leukocytes. In culture, microglia have been shown to be capable of releasing several potentially cytotoxic substances, such as reactive oxygen intermediates, nitric oxide, proteases, arachidonic acid derivatives, excitatory amino acids, and cytokines; however, they also produce various neurotrophic factors and quench damage from free radicals and excitotoxins. As the primary source for pro-inflammatory cytokines, microglia are implicated as pivotal mediators of neuroinflammation and can induce or modulate a broad spectrum of cellular responses. Neuroinflammation should be considered as a balanced network of processes whereby subtle modifications can shift the cells toward disparate outcomes. For any evaluation of neuroinflammation and microglial responses, within the framework of neurotoxicity or degeneration, one key question in determining the consequence of neuroinflammation is whether the response is an initiating event or the consequence of tissue damage. As examples of environmental exposure-related neuroinflammation in the literature, we provide an evaluation of data on manganese and diesel exhaust particles.