Microglia activation in central nervous system disorders: A review of recent mechanistic investigations and development efforts

Intercellular Adhesion Molecule 1 (ICAM-1): An Inflammatory Regulator with Potential Implications in Ferroptosis and Parkinson's Disease

Intercellular adhesion molecule 1 (ICAM-1/CD54), a transmembrane glycoprotein, has been considered as one of the most important adhesion molecules during leukocyte recruitment. It is encoded by the ICAM1 gene and plays a central role in inflammation. Its crucial role in many inflammatory diseases such as ulcerative colitis and rheumatoid arthritis are well established. Given that neuroinflammation, underscored by microglial activation, is a key element in neurodegenerative diseases such as Parkinson's disease (PD), we investigated whether ICAM-1 has a role in this progressive neurological condition and, if so, to elucidate the underpinning mechanisms. Specifically, we were interested in the potential interaction between ICAM-1, glial cells, and ferroptosis, an iron-dependent form of cell death that has recently been implicated in PD. We conclude that there exist direct and indirect (via glial cells and T cells) influences of ICAM-1 on ferroptosis and that further elucidation of these interactions can suggest novel intervention for this devastating disease.

Apilarnil exerts neuroprotective effects and alleviates motor dysfunction by rebalancing M1/M2 microglia polarization, regulating miR-155 and miR-124 expression in a rotenone-induced Parkinson's disease rat model

Microglial activation contributes to the neuropathology of Parkinson's disease (PD). Inhibiting M1 while simultaneously boosting M2 microglia activation may therefore be a potential treatment for PD. Apilarnil (API) is a bee product produced from drone larvae. Recent research has demonstrated the protective effects of API on multiple body systems. Nevertheless, its impact on PD or the microglial M1/M2 pathway has not yet been investigated. Thus, we intended to evaluate the dose-dependent effects of API in rotenone (ROT)-induced PD rat model and explore the role of M1/M2 in mediating its effect. Seventy-two Wistar rats were equally grouped as; control, API, ROT, and groups in which API (200, 400, and 800 mg/kg, p.o.) was given simultaneously with ROT (2 mg/kg, s.c.) for 28 days. The high dose of API (800 mg/kg) showed enhanced motor function, higher expression of tyrosine hydroxylase and dopamine levels, less dopamine turnover and α-synuclein expression, and a better histopathological picture when compared to the ROT group and the lower two doses. API's high dose exerted its neuroprotective effects through abridging the M1 microglial activity, illustrated in the reduced expression of miR-155, Iba-1, CD36, CXCL10, and other pro-inflammatory markers' levels. Inversely, API high dose enhanced M2 microglial activity, witnessed in the elevated expression of miR-124, CD206, Ym1, Fizz1, arginase-1, and other anti-inflammatory indices, in comparison to the diseased group. To conclude, our study revealed a novel neuroprotective impact for API against experimentally induced PD, where the high dose showed the highest protection via rebalancing M1/M2 polarization.

β2-Integrins Regulate Microglial Responses and the Functional Outcome of Hemorrhagic Stroke In Vivo

Stroke is one of the leading causes of death and long-term disabilities worldwide. In addition to interruption of blood flow, inflammation is widely recognized as an important factor mediating tissue destruction in stroke. Depending on their phenotype, microglia, the main leukocytes in the CNS, are capable of either causing further tissue damage or promoting brain restoration after stroke. β2-integrins are cell adhesion molecules that are constitutively expressed on microglia. The function of β2-integrins has been investigated extensively in animal models of ischemic stroke, but their role in hemorrhagic stroke is currently poorly understood. We show in this study that dysfunction of β2-integrins is associated with improved functional outcome and decreased inflammatory cytokine expression in the brain in a mouse model of hemorrhagic stroke. Furthermore, β2-integrins affect microglial phenotype and cytokine responses in vivo. Therefore, our findings suggest that targeting β2-integrins in hemorrhagic stroke may be beneficial.

Harnessing nanomedicine for modulating microglial states in the central nervous system disorders: Challenges and opportunities

Microglia are essential for maintaining homeostasis and responding to pathological events in the central nervous system (CNS). Their dynamic and multidimensional states in different environments are pivotal factors in various CNS disorders. However, therapeutic modulation of microglial states is challenging due to the intricate balance these cells maintain in the CNS environment and the blood-brain barrier's restriction of drug delivery. Nanomedicine presents a promising avenue for addressing these challenges, offering a method for the targeted and efficient modulation of microglial states. This review covers the challenges faced in microglial therapeutic modulation and potential use of nanoparticle-based drug delivery systems. We provide an in-depth examination of nanoparticle applications for modulating microglial states in a range of CNS disorders, encompassing neurodegenerative and autoimmune diseases, infections, traumatic injuries, stroke, tumors, chronic pain, and psychiatric conditions. This review highlights the recent advancements and future prospects in nanomedicine for microglial modulation, paving the way for future research and clinical applications of therapeutic interventions in CNS disorders.

Unlocking the therapeutic potential of P2X7 receptor: a comprehensive review of its role in neurodegenerative disorders

The P2X7 receptor (P2X7R), an ATP-gated ion channel, has emerged as a crucial player in neuroinflammation and a promising therapeutic target for neurodegenerative disorders. This review explores the current understanding of P2X7R's structure, activation, and physiological roles, focusing on its expression and function in microglial cells. The article examines the receptor's involvement in calcium signaling, microglial activation, and polarization, as well as its role in the pathogenesis of Alzheimer's disease, Parkinson's disease, multiple sclerosis, and amyotrophic lateral sclerosis. The review highlights the complex nature of P2X7R signaling, discussing its potential neuroprotective and neurotoxic effects depending on the disease stage and context. It also addresses the development of P2X7R antagonists and their progress in clinical trials, identifying key research gaps and future perspectives for P2X7R-targeted therapy development. By providing a comprehensive overview of the current state of knowledge and future directions, this review serves as a valuable resource for researchers and clinicians interested in exploring the therapeutic potential of targeting P2X7R for the treatment of neurodegenerative disorders.

The complex role of the chemokine CX3CL1/Fractalkine in major depressive disorder: A narrative review of preclinical and clinical studies

Evidence suggests that neuroinflammation exhibits a dual role in the pathogenesis of major depressive disorder (MDD), both potentiating the onset of depressive symptoms and developing as a consequence of them. Our narrative review focuses on the role of the chemokine fractalkine (FKN) (also known as CX3CL1), which has gained increasing interest for its ability to induce changes to microglial phenotypes through interaction with its corresponding receptor (CX3CR1) that may impact neurophysiological processes relevant to MDD. Despite this, there is a lack of a clear understanding of the role of FKN in MDD. Overall, our review of the literature shows the involvement of FKN in MDD, both in preclinical models of depression, and in clinical studies of depressed patients. Preclinical studies (N = 8) seem to point towards two alternative hypotheses for FKN's role in MDD: a) FKN may drive pro-inflammatory changes to microglia that contribute towards MDD pathogenesis; or b) FKN may inhibit pro-inflammatory changes to microglia, thereby exerting a protective effect against MDD pathogenesis. Evidence for a) primarily derives from preclinical chronic stress models of depression in mice, whereas for b) from preclinical inflammation models of depression. Whereas, in humans, clinical studies (N = 4) consistently showed a positive association between FKN and presence of MDD, however it is not clear whether FKN is driving or moderating MDD pathogenesis. Future studies should aim for larger and more controlled clinical cohorts, in order to advance our understanding of FKN role both in the context of stress and/or inflammation.

SARS-CoV-2 neurotropism-induced anxiety/depression-like behaviors require Microglia activation

The coronavirus disease 2019 (COVID-19) pandemic, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has been associated with a wide range of "long COVID" neurological symptoms. However, the mechanisms governing SARS-CoV-2 neurotropism and its effects on long-term behavioral changes remain poorly understood. Using a highly virulent mouse-adapted SARS-CoV-2 strain, denoted as SARS2-N501Y MA30 , we demonstrated that intranasal inoculation of SARS2-N501Y MA30 results in viral dissemination to multiple brain regions, including the amygdala and hippocampus. Behavioral assays indicated a marked elevation in anxiety- and depression-like behaviors post infection. A comparative analysis of RNA expression profiles disclosed alterations in the post-infected brains. Additionally, we observed dendritic spine remodeling on neurons within the amygdala after infection. Infection with SARS2-N501Y MA30 was associated with microglial activation and a subsequent increase in microglia-dependent neuronal activity in the amygdala. Pharmacological inhibition of microglial activity subsequent to viral spike inoculation mitigates microglia-dependent neuronal hyperactivity. Transcriptomic analysis of infected brains revealed the upregulation of inflammatory and cytokine-related pathways, implicating microglia-driven neuroinflammation in the pathogenesis of neuronal hyperactivity and behavioral abnormality. Overall, these data provide critical insights into the neurological consequences of SARS-CoV-2 infection and underscore microglia as a potential therapeutic target for ameliorating virus-induced neurobehavioral abnormalities.

Enhanced levels of fractalkine and HSP60 in cerebrospinal fluid of sporadic amyotrophic lateral sclerosis patients

Amyotrophic Lateral Sclerosis (ALS) is a multifactorial neurodegenerative disorder with a significant contribution of non-cell autonomous mechanisms to motor neuronal degeneration. Amongst a plethora of molecules, fractalkine (C-X3-C motif chemokine ligand 1), and Heat Shock Protein 60 (HSP60), are key modulators of microglial activation. The contribution of these molecules in Sporadic ALS (SALS) remains unexplored. To investigate this, fractalkine levels were estimated in Cerebrospinal fluid (CSF) of SALS patients (ALS-CSF; n = 44) by Enzyme-linked Immunosorbent Assay (ELISA) and correlated with clinical parameters including disease severity and duration. CSF HSP60 levels were estimated by Western blotting (ALS-CSF; n = 19). Also, CSF levels of Chitotriosidase-1 (CHIT-1), a microglia-specific neuroinflammatory molecule, were measured and its association, if any, with fractalkine and HSP60 was investigated. Both fractalkine and HSP60 levels were significantly elevated in ALS-CSF. Similar to our earlier observation, CHIT-1 levels were also upregulated. Fractalkine showed a moderate negative correlation with the ALS-Functional Rating Scale (ALSFRS) score indicating its significant rise in mild cases which plateaued in cases with high disease severity. However, no obvious correlation was found between fractalkine, HSP60, and CHIT-1. Our study hints that high fractalkine levels in mild cases might be conferring neuroprotection by combating microglial activation and highlights its importance as a novel therapeutic target for SALS. On the other hand, significantly enhanced levels of HSP60, a pro-inflammatory molecule, hint towards its role in accentuating microgliosis, although, it doesn't act synergistically with CHIT-1. Our study suggests that fractalkine and HSP60 act independently of CHIT-1 to suppress and accentuate neuroinflammation, respectively.

The role of neuroinflammation in neurodegenerative diseases: current understanding and future therapeutic targets

Neuroinflammation refers to a highly complicated reaction of the central nervous system (CNS) to certain stimuli such as trauma, infection, and neurodegenerative diseases. This is a cellular immune response whereby glial cells are activated, inflammatory mediators are liberated and reactive oxygen and nitrogen species are synthesized. Neuroinflammation is a key process that helps protect the brain from pathogens, but inappropriate, or protracted inflammation yields pathological states such as Parkinson's disease, Alzheimer's, Multiple Sclerosis, and other neurodegenerative disorders that showcase various pathways of neurodegeneration distributed in various parts of the CNS. This review reveals the major neuroinflammatory signaling pathways associated with neurodegeneration. Additionally, it explores promising therapeutic avenues, such as stem cell therapy, genetic intervention, and nanoparticles, aiming to regulate neuroinflammation and potentially impede or decelerate the advancement of these conditions. A comprehensive understanding of the intricate connection between neuroinflammation and these diseases is pivotal for the development of future treatment strategies that can alleviate the burden imposed by these devastating disorders.

Nicotinic Acetylcholine Receptors in Glial Cells as Molecular Target for Parkinson's Disease

Parkinson's disease (PD) is a progressive neurodegenerative disease characterized by resting tremor, bradykinesia, rigidity, and postural instability that also includes non-motor symptoms such as mood dysregulation. Dopamine (DA) is the primary neurotransmitter involved in this disease, but cholinergic imbalance has also been implicated. Current intervention in PD is focused on replenishing central DA, which provides remarkable temporary symptomatic relief but does not address neuronal loss and the progression of the disease. It has been well established that neuronal nicotinic cholinergic receptors (nAChRs) can regulate DA release and that nicotine itself may have neuroprotective effects. Recent studies identified nAChRs in nonneuronal cell types, including glial cells, where they may regulate inflammatory responses. Given the crucial role of neuroinflammation in dopaminergic degeneration and the involvement of microglia and astrocytes in this response, glial nAChRs may provide a novel therapeutic target in the prevention and/or treatment of PD. In this review, following a brief discussion of PD, we focus on the role of glial cells and, specifically, their nAChRs in PD pathology and/or treatment.

Molecular Mechanisms in Pathophysiology of Mucopolysaccharidosis and Prospects for Innovative Therapy

Mucopolysaccharidoses (MPSs) are a group of inborn errors of the metabolism caused by a deficiency in the lysosomal enzymes required to break down molecules called glycosaminoglycans (GAGs). These GAGs accumulate over time in various tissues and disrupt multiple biological systems, including catabolism of other substances, autophagy, and mitochondrial function. These pathological changes ultimately increase oxidative stress and activate innate immunity and inflammation. We have described the pathophysiology of MPS and activated inflammation in this paper, starting with accumulating the primary storage materials, GAGs. At the initial stage of GAG accumulation, affected tissues/cells are reversibly affected but progress irreversibly to: (1) disruption of substrate degradation with pathogenic changes in lysosomal function, (2) cellular dysfunction, secondary/tertiary accumulation (toxins such as GM2 or GM3 ganglioside, etc.), and inflammatory process, and (3) progressive tissue/organ damage and cell death (e.g., skeletal dysplasia, CNS impairment, etc.). For current and future treatment, several potential treatments for MPS that can penetrate the blood-brain barrier and bone have been proposed and/or are in clinical trials, including targeting peptides and molecular Trojan horses such as monoclonal antibodies attached to enzymes via receptor-mediated transport. Gene therapy trials with AAV, ex vivo LV, and Sleeping Beauty transposon system for MPS are proposed and/or underway as innovative therapeutic options. In addition, possible immunomodulatory reagents that can suppress MPS symptoms have been summarized in this review.

Exercise, Spinal Microglia and Neuropathic Pain: Potential Molecular Mechanisms

As one of the most common neuropathic disorders, neuropathic pain often has a negative impact on patients with persistent pain, mood disorders and sleep disturbances. Currently, neuropathic pain is not treated with any specific drug, instead, drugs for other diseases are used as replacements in clinics, but most have adverse effects. In recent years, the role of spinal cord microglia in the pathogenesis of neuropathic pain has been widely recognized, and they are being explored as potential therapeutic targets. Spinal microglia are known to be involved in the pathogenic mechanisms of neuropathic pain through purine signaling, fractalkine signaling, and p38 MAPK signaling. Exercise is a safe and effective treatment, and numerous studies have demonstrated its effectiveness in improving neurological symptoms. Nevertheless, it remains unclear what the exact molecular mechanism is. This review summarized the specific molecular mechanisms of exercise in alleviating neuropathic pain by mediating the activity of spinal microglia and maintaining the phenotypic homeostasis of spinal microglia through purine signaling, fractalkine signaling and p38 MAPK signaling. In addition, it has been proposed that different intensities and types of exercise affect the regulation of the above-mentioned signaling pathways differently, providing a theoretical basis for the improvement of neuropathic pain through exercise.

Contextualizing the Role of Osteopontin in the Inflammatory Responses of Alzheimer's Disease

Alzheimer's disease (AD) is characterized by progressive accumulations of extracellular amyloid-beta (Aβ) aggregates from soluble oligomers to insoluble plaques and hyperphosphorylated intraneuronal tau, also from soluble oligomers to insoluble neurofibrillary tangles (NFTs). Tau and Aβ complexes spread from the entorhinal cortex of the brain to interconnected regions, where they bind pattern recognition receptors on microglia and astroglia to trigger inflammation and neurotoxicity that ultimately lead to neurodegeneration and clinical AD. Systemic inflammation is initiated by Aβ's egress into the circulation, which may be secondary to microglial activation and can confer both destructive and reparative actions. Microglial activation pathways and downstream drivers of Aβ/NFT neurotoxicity, including inflammatory regulators, are primary targets for AD therapy. Osteopontin (OPN), an inflammatory cytokine and biomarker of AD, is implicated in Aβ clearance and toxicity, microglial activation, and inflammation, and is considered to be a potential therapeutic target. Here, using the most relevant works from the literature, we review and contextualize the evidence for a central role of OPN and associated inflammation in AD.

The dual face of microglia (M1/M2) as a potential target in the protective effect of nutraceuticals against neurodegenerative diseases

The pathophysiology of different neurodegenerative illnesses is significantly influenced by the polarization regulation of microglia and macrophages. Traditional classifications of macrophage phenotypes include the pro-inflammatory M1 and the anti-inflammatory M2 phenotypes. Numerous studies demonstrated dynamic non-coding RNA modifications, which are catalyzed by microglia-induced neuroinflammation. Different nutraceuticals focus on the polarization of M1/M2 phenotypes of microglia and macrophages, offering a potent defense against neurodegeneration. Caeminaxin A, curcumin, aromatic-turmerone, myricetin, aurantiamide, 3,6'-disinapoylsucrose, and resveratrol reduced M1 microglial inflammatory markers while increased M2 indicators in Alzheimer's disease. Amyloid beta-induced microglial M1 activation was suppressed by andrographolide, sulforaphane, triptolide, xanthoceraside, piperlongumine, and novel plant extracts which also prevented microglia-mediated necroptosis and apoptosis. Asarone, galangin, baicalein, and a-mangostin reduced oxidative stress and pro-inflammatory cytokines, such as interleukin (IL)-1, IL-6, and tumor necrosis factor-alpha in M1-activated microglia in Parkinson's disease. Additionally, myrcene, icariin, and tenuigenin prevented the nod-like receptor family pyrin domain-containing 3 inflammasome and microglial neurotoxicity, while a-cyperone, citronellol, nobiletin, and taurine prevented NADPH oxidase 2 and nuclear factor kappa B activation. Furthermore, other nutraceuticals like plantamajoside, swertiamarin, urolithin A, kurarinone, Daphne genkwa flower, and Boswellia serrata extracts showed promising neuroprotection in treating Parkinson's disease. In Huntington's disease, elderberry, curcumin, iresine celosia, Schisandra chinensis, gintonin, and pomiferin showed promising results against microglial activation and improved patient symptoms. Meanwhile, linolenic acid, resveratrol, Huperzia serrata, icariin, and baicalein protected against activated macrophages and microglia in experimental autoimmune encephalomyelitis and multiple sclerosis. Additionally, emodin, esters of gallic and rosmarinic acids, Agathisflavone, and sinomenine offered promising multiple sclerosis treatments. This review highlights the therapeutic potential of using nutraceuticals to treat neurodegenerative diseases involving microglial-related pathways.

GM1 oligosaccharide efficacy against α-synuclein aggregation and toxicity in vitro

Fibrillary aggregated α-synuclein represents the neurologic hallmark of Parkinson's disease and is considered to play a causative role in the disease. Although the causes leading to α-synuclein aggregation are not clear, the GM1 ganglioside interaction is recognized to prevent this process. How GM1 exerts these functions is not completely clear, although a primary role of its soluble oligosaccharide (GM1-OS) is emerging. Indeed, we recently identified GM1-OS as the bioactive moiety responsible for GM1 neurotrophic and neuroprotective properties, specifically reverting the parkinsonian phenotype both in in vitro and in vivo models. Here, we report on GM1-OS efficacy against the α-synuclein aggregation and toxicity in vitro. By amyloid seeding aggregation assay and NMR spectroscopy, we demonstrated that GM1-OS was able to prevent both the spontaneous and the prion-like α-synuclein aggregation. Additionally, circular dichroism spectroscopy of recombinant monomeric α-synuclein showed that GM1-OS did not induce any change in α-synuclein secondary structure. Importantly, GM1-OS significantly increased neuronal survival and preserved neurite networks of dopaminergic neurons affected by α-synuclein oligomers, together with a reduction of microglia activation. These data further demonstrate that the ganglioside GM1 acts through its oligosaccharide also in preventing the α-synuclein pathogenic aggregation in Parkinson's disease, opening a perspective window for GM1-OS as drug candidate.

AIF1: Function and Connection with Inflammatory Diseases

Macrophages are a type of immune cell distributed throughout all tissues of an organism. Allograft inflammatory factor 1 (AIF1) is a calcium-binding protein linked to the activation of macrophages. AIF1 is a key intracellular signaling molecule that participates in phagocytosis, membrane ruffling and F-actin polymerization. Moreover, it has several cell type-specific functions. AIF1 plays important roles in the development of several diseases: kidney disease, rheumatoid arthritis, cancer, cardiovascular diseases, metabolic diseases and neurological disorders, and in transplants. In this review, we present a comprehensive review of the known structure, functions and role of AIF1 in inflammatory diseases.