Brain-resident microglia predominate over infiltrating myeloid cells in activation, phagocytosis and interaction with T-lymphocytes in the MPTP mouse model of Parkinson disease

ICAM-1 may promote the loss of dopaminergic neurons by regulating inflammation in MPTP-induced Parkinson's disease mouse models

Parkinson's disease (PD) is a chronic neurodegenerative disease with unclear pathogenesis that involves neuroinflammation and intestinal microbial dysbiosis. Intercellular adhesion molecule-1 (ICAM-1), an inflammatory marker, participates in neuroinflammation during dopaminergic neuronal damage. However, the explicit mechanisms of action of ICAM-1 in PD have not been elucidated. We established a subacute PD mouse model by the intraperitoneal injection of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and observed motor symptoms and gastrointestinal dysfunction in mice. Immunofluorescence was used to examine the survival of dopaminergic neurons, expression of microglial and astrocyte markers, and intestinal tight junction-associated proteins. Then, we use 16 S rRNA sequencing to identify alterations in the microbiota. Our findings revealed that ICAM-1-specific antibody (Ab) treatment relieved behavioural defects, gastrointestinal dysfunction, and dopaminergic neuronal death in MPTP-induced PD mice. Further mechanistic investigations indicated that ICAM-1Ab might suppress neuroinflammation by inhibiting the activation of astrocytes and microglia in the substantia nigra and relieving colon barrier impairment and intestinal inflammation. Furthermore, 16 S rRNA sequencing revealed that the relative abundances of bacterial Firmicutes, Clostridia, and Lachnospiraceae were elevated in the PD mice. However, ICAM-1Ab treatment ameliorated the MPTP-induced disorders in the intestinal microbiota. Collectively, we concluded that the suppressing ICAM-1 might lead to the a significant decrease of inflammation and restore the gut microbial community, thus ameliorating the damage of DA neurons.

Microalgae-Based Hydrogel for Inflammatory Bowel Disease and Its Associated Anxiety and Depression

Patients diagnosed with inflammatory bowel disease (IBD) exhibit a notable prevalence of psychiatric disorders, such as anxiety and depression. Nevertheless, the etiology of psychiatric disorders associated with IBD remains uncertain, and an efficacious treatment approach has yet to be established. Herein, an oral hydrogel strategy (SP@Rh-gel) is proposed for co-delivery of Spirulina platensis and rhein to treat IBD and IBD-associated anxiety and depression by modulating the microbiota-gut-brain axis. SP@Rh-gel improves the solubility, release characteristics and intestinal retention capacity of the drug, leading to a significant improvement in the oral therapeutic efficacy. Oral administration of SP@Rh-gel can reduce intestinal inflammation and rebalance the disrupted intestinal microbial community. Furthermore, SP@Rh-gel maintains intestinal barrier integrity and reduces the release of pro-inflammatory factors and their entry into the hippocampus through the blood-brain barrier, thereby inhibiting neuroinflammation and maintaining neuroplasticity. SP@Rh-gel significantly alleviates the colitis symptoms, as well as anxiety- and depression-like behaviors, in a chronic colitis mouse model. This study demonstrates the significant involvement of the microbiota-gut-brain axis in the development of IBD with psychiatric disorders and proposes a safe, simple, and highly efficient therapeutic approach for managing IBD and comorbid psychiatric disorders.

Experimental Models to Study Immune Dysfunction in the Pathogenesis of Parkinson's Disease

Parkinson's disease (PD) is a chronic, age-related, progressive multisystem disease associated with neuroinflammation and immune dysfunction. This review discusses the methodological approaches used to study the changes in central and peripheral immunity in PD, the advantages and limitations of the techniques, and their applicability to humans. Although a single animal model cannot replicate all pathological features of the human disease, neuroinflammation is present in most animal models of PD and plays a critical role in understanding the involvement of the immune system (IS) in the pathogenesis of PD. The IS and its interactions with different cell types in the central nervous system (CNS) play an important role in the pathogenesis of PD. Even though culture models do not fully reflect the complexity of disease progression, they are limited in their ability to mimic long-term effects and need validation through in vivo studies. They are an indispensable tool for understanding the interplay between the IS and the pathogenesis of this disease. Understanding the immune-mediated mechanisms may lead to potential therapeutic targets for the treatment of PD. We believe that the development of methodological guidelines for experiments with animal models and PD patients is crucial to ensure the validity and consistency of the results.

The involvement of α-synucleinopathy in the disruption of microglial homeostasis contributes to the pathogenesis of Parkinson's disease

The intracellular deposition and intercellular transmission of α-synuclein (α-syn) are shared pathological characteristics among neurodegenerative disorders collectively known as α-synucleinopathies, including Parkinson's disease (PD). Although the precise triggers of α-synucleinopathies remain unclear, recent findings indicate that disruption of microglial homeostasis contributes to the pathogenesis of PD. Microglia play a crucial role in maintaining optimal neuronal function by ensuring a homeostatic environment, but this function is disrupted during the progression of α-syn pathology. The involvement of microglia in the accumulation, uptake, and clearance of aggregated proteins is critical for managing disease spread and progression caused by α-syn pathology. This review summarizes current knowledge on the interrelationships between microglia and α-synucleinopathies, focusing on the remarkable ability of microglia to recognize and internalize extracellular α-syn through diverse pathways. Microglia process α-syn intracellularly and intercellularly to facilitate the α-syn neuronal aggregation and cell-to-cell propagation. The conformational state of α-synuclein distinctly influences microglial inflammation, which can affect peripheral immune cells such as macrophages and lymphocytes and may regulate the pathogenesis of α-synucleinopathies. We also discuss ongoing research efforts to identify potential therapeutic approaches targeting both α-syn accumulation and inflammation in PD. Video Abstract.

Dopaminergic regulation of inflammation and immunity in Parkinson's disease: friend or foe?

Parkinson's disease (PD) is a neurodegenerative disease affecting 7-10 million people worldwide. Currently, there is no treatment available to prevent or delay PD progression, partially due to the limited understanding of the pathological events which lead to the death of dopaminergic neurons in the substantia nigra in the brain, which is known to be the cause of PD symptoms. The current available treatments aim at compensating dopamine (DA) deficiency in the brain using its precursor levodopa, dopaminergic agonists and some indirect dopaminergic agents. The immune system is emerging as a critical player in PD. Therefore, immune-based approaches have recently been proposed to be used as potential antiparkinsonian agents. It has been well-known that dopaminergic pathways play a significant role in regulating immune responses in the brain. Although dopaminergic agents are the primary antiparkinsonian treatments, their immune regulatory effect has yet to be fully understood. The present review summarises the current available evidence of the immune regulatory effects of DA and its mimics and discusses dopaminergic agents as antiparkinsonian drugs. Based on the current understanding of their involvement in the regulation of neuroinflammation in PD, we propose that targeting immune pathways involved in PD pathology could offer a better treatment outcome for PD patients.

Neuromelanin: Its role in the pathogenesis of idiopathic Parkinson's disease and potential as a therapeutic target

Parkinson's disease is an increasingly prevalent condition that involves the marked loss of dopaminergic neurons in the substantia nigra pars compacta. These neurons pigmented with neuromelanin along with other regions of the brain are almost exclusively victims of neurodegeneration in the disease. The link between neuromelanin and Parkinson's disease has been widely studied for decades. While many studies have outlined the pigment's neuroprotective function as a potent free radical scavenger, antioxidant, and ion-chelator, it has also been observed to play a role in cell death due to mitochondrial dysfunction and oxidative stress, especially in the parkinsonian disease state. This is due to the damaging effects of neuromelanin precursors, neuromelanin-related ion dysregulation and intra- and extraneuronal neuromelanin accumulation. Current and emerging therapeutic endeavours guided by these pathological processes may include antioxidant therapy, proteostasis enhancement, ion chelation and neuromelanin-targeted immunotherapy to prevent the accumulation, formation and effects of neuromelanin and oxidative neuromelanin precursors. Some of these therapeutic strategies are already in nascent stages, while others have produced mixed results in clinical trials. This review aims to provide an update on how neuromelanin and neuromelanin-related substances may be linked to the pathogenesis of Parkinson's disease and how future therapeutic strategies may be able to hamper or prevent neuromelanin-related pathological processes and ultimately modify disease progression in Parkinson's.

Impact of Sex on Neuroimmune contributions to Parkinson's disease

Parkinson's disease (PD) is the second most common neurodegenerative disorder after Alzheimer's disease. Inflammation has been observed in both the idiopathic and familial forms of PD. Importantly, PD is reported more often in men than in women, men having at least 1.5- fold higher risk to develop PD than women. This review summarizes the impact of biological sex and sex hormones on the neuroimmune contributions to PD and its investigation in animal models of PD. Innate and peripheral immune systems participate in the brain neuroinflammation of PD patients and is reproduced in neurotoxin, genetic and alpha-synuclein based models of PD. Microglia and astrocytes are the main cells of the innate immune system in the central nervous system and are the first to react to restore homeostasis in the brain. Analysis of serum immunoprofiles in female and male control and PD patients show that a great proportion of these markers differ between male and female. The relationship between CSF inflammatory markers and PD clinical characteristics or PD biomarkers shows sex differences. Conversely, in animal models of PD, sex differences in inflammation are well documented and the beneficial effects of endogenous and exogenous estrogenic modulation in inflammation have been reported. Targeting neuroinflammation in PD is an emerging therapeutic option but gonadal drugs have not yet been investigated in this respect, thus offering new opportunities for sex specific treatments.

Disease mechanisms as subtypes: Lysosomal dysfunction in the endolysosomal Parkinson's disease subtype

Parkinson's disease (PD) remains one of the most prevalent neurodegenerative disorders. It has become increasingly recognized that PD is not one disease but a constellation of many, with distinct cellular mechanisms driving pathology and neuronal loss in each given subtype. Endolysosomal trafficking and lysosomal degradation are crucial to maintain neuronal homeostasis and vesicular trafficking. It is clear that deficits in endolysosomal signaling data support the existence of an endolysosomal PD subtype. This chapter describes how cellular pathways involved in endolysosomal vesicular trafficking and lysosomal degradation in neurons and immune cells can contribute to PD. Last, as inflammatory processes including phagocytosis and cytokine release are central in glia-neuron interactions, a spotlight on the role of neuroinflammation plays in the pathogenesis of this PD subtype is also explored.

Disease mechanisms as subtypes: Immune dysfunction in Parkinson's disease

In recent years, the contraposition between inflammatory and neurodegenerative processes has been increasingly challenged. Inflammation has been emphasized as a key player in the onset and progression of Parkinson disease (PD) and other neurodegenerative disorders. The strongest indicators of the involvement of the immune system derived from evidence of microglial activation, profound imbalance in phenotype and composition of peripheral immune cells, and impaired humoral immune responses. Moreover, peripheral inflammatory mechanisms (e.g., involving the gut-brain axis) and immunogenetic factors are likely to be implicated. Even though several lines of preclinical and clinical studies are supporting and defining the complex relationship between the immune system and PD, the exact mechanisms are currently unknown. Similarly, the temporal and causal connections between innate and adaptive immune responses and neurodegeneration are unsettled, challenging our ambition to define an integrated and holistic model of the disease. Despite these difficulties, current evidence is providing the unique opportunity to develop immune-targeted approaches for PD, thus enriching our therapeutic armamentarium. This chapter aims to provide an extensive overview of past and present studies that explored the implication of the immune system in neurodegeneration, thus paving the road for the concept of disease modification in PD.

HMGB1 is a Potential and Challenging Therapeutic Target for Parkinson's Disease

Parkinson's disease (PD) is one of the most common degenerative diseases of the human nervous system and has a wide range of serious impacts on human health and quality of life. Recently, research targeting high mobility group box 1 (HMGB1) in PD has emerged, and a variety of laboratory methods for inhibiting HMGB1 have achieved good results to a certain extent. However, given that HMGB1 undergoes a variety of intracellular modifications and three different forms of extracellular redox, the possible roles of these forms in PD are likely to be different. General inhibition of all forms of HMGB1 is obviously not ideal and has become one of the biggest obstacles in the clinical application of targeting HMGB1. In this review, pure mechanistic research of HMGB1 and in vivo research targeting HMGB1 were combined, the effects of HMGB1 on neurons and immune cell responses in PD are discussed in detail, and the problems that need to be focused on in the future are addressed.

Inflammation and immune dysfunction in Parkinson disease

Parkinson disease (PD) is a progressive neurodegenerative disease that affects peripheral organs as well as the central nervous system and involves a fundamental role of neuroinflammation in its pathophysiology. Neurohistological and neuroimaging studies support the presence of ongoing and end-stage neuroinflammatory processes in PD. Moreover, numerous studies of peripheral blood and cerebrospinal fluid from patients with PD suggest alterations in markers of inflammation and immune cell populations that could initiate or exacerbate neuroinflammation and perpetuate the neurodegenerative process. A number of disease genes and risk factors have been identified as modulators of immune function in PD and evidence is mounting for a role of viral or bacterial exposure, pesticides and alterations in gut microbiota in disease pathogenesis. This has led to the hypothesis that complex gene-by-environment interactions combine with an ageing immune system to create the 'perfect storm' that enables the development and progression of PD. We discuss the evidence for this hypothesis and opportunities to harness the emerging immunological knowledge from patients with PD to create better preclinical models with the long-term goal of enabling earlier identification of at-risk individuals to prevent, delay and more effectively treat the disease.

Sequestration of Inflammation in Parkinson's Disease via Stem Cell Therapy

Parkinson’s disease is the second most common neurodegenerative disease. Insidious and progressive, this disorder is secondary to the gradual loss of dopaminergic signaling and worsening neuroinflammation, affecting patients’ motor capabilities. Gold standard treatment includes exogenous dopamine therapy in the form of levodopa–carbidopa, or surgical intervention with a deep brain stimulator to the subcortical basal ganglia. Unfortunately, these therapies may ironically exacerbate the already pro-inflammatory environment. An alternative approach may involve cell-based therapies. Cell-based therapies, whether endogenous or exogenous, often have anti-inflammatory properties. Alternative strategies, such as exercise and diet modifications, also appear to play a significant role in facilitating endogenous and exogenous stem cells to induce an anti-inflammatory response, and thus are of unique interest to neuroinflammatory conditions including Parkinson’s disease. Treating patients with current gold standard therapeutics and adding adjuvant stem cell therapy, alongside the aforementioned lifestyle modifications, may ideally sequester inflammation and thus halt neurodegeneration.

Supramolecular organizing centers at the interface of inflammation and neurodegeneration

The pathogenesis of neurodegenerative diseases involves the accumulation of misfolded protein aggregates. These deposits are both directly toxic to neurons, invoking loss of cell connectivity and cell death, and recognized by innate sensors that upon activation release neurotoxic cytokines, chemokines, and various reactive species. This neuroinflammation is propagated through signaling cascades where activated sensors/receptors, adaptors, and effectors associate into multiprotein complexes known as supramolecular organizing centers (SMOCs). This review provides a comprehensive overview of the SMOCs, involved in neuroinflammation and neurotoxicity, such as myddosomes, inflammasomes, and necrosomes, their assembly, and evidence for their involvement in common neurodegenerative diseases. We discuss the multifaceted role of neuroinflammation in the progression of neurodegeneration. Recent progress in the understanding of particular SMOC participation in common neurodegenerative diseases such as Alzheimer's disease offers novel therapeutic strategies for currently absent disease-modifying treatments.

CIITA expression is regulated by histone deacetylase enzymes and has a role in α-synuclein pre-formed fibril-induced antigen presentation in murine microglial cell line

AIM

Parkinson's disease(PD) is a chronic neurodegenerative disorder related with several genetic and epigenetic factors. In the context of epigenetic factors, histone acetylation is one of the most associated mechanisms with Parkinson's disease progression. This study investigates the effects of the increased histone acetylation on antigen presentation in microglial cells which were induced by pre-formed fibrils of α-synuclein(pFF α-synuclein).

METHODS

Parkinson's disease model was created with pFF α-synuclein administration to the BV-2 microglial cells. BV-2 cells were co-treated with CUDC-907 and TMP-195 to increase histone acetylation in the presence of α-synuclein. Antigen representation was evaluated by determining expression levels of major histocompatibility complex-II(MHC-II) and class-II major histocompatibility complex(CIITA).

RESULTS

Our results showed that pFF α-synuclein significantly increased MHC-II expression, and that effect was most severe at 6 hours of administration of α-synuclein. Increasing histone acetylation via CUDC-907 and TMP-195 enhanced MHC-II levels expression, which was more severe in CUDC-907. Additionally, CIITA expression levels were significantly increased with pFF α-synuclein administration and intensified with the co-treatment of CUDC-907 and TMP-195. Furthermore, pFF α-synuclein caused a time-dependent increase in the IFN-gamma(IFN-ɣ) and interleukin-16(IL-16) levels, and that increase was potentiated with CUDC-907 and TMP-195.

CONCLUSION

Changes in MHC-II and CIITA expression indicate that histone acetylation increases the antigen presentation properties of microglial cells after pFF α-synuclein or histone deacetylase inhibitor(HDACi) administration. Our results show that microglial antigen presentation might have an essential role in the pathology of Parkinson's disease, and α-synuclein likely to play a primary role in this mechanism.

The role of efferocytosis in neuro-degenerative diseases

Efferocytosis has a critical role in maintaining tissues and organs' homeostasis by removing apoptotic cells. It is essential for human health, and disturbances in efferocytosis may result indifferent illnesses. In case of inadequate clearance of the dead cells, the content in the cells would be released. In fact, it induces some damages to the tissue and leads to the prolonged inflammation, so unsuitable phagocytosis of the apoptotic cells is involved in occurrence as well as expansion of numerous human chronic inflammatory diseases. Studies have shown age dependence of the neuro-degenerative diseases, which are largely due to the neuro-inflammation and the loss of neurons and thus cause the brain's functional disorders. Efferocytosis is coupled to anti-inflammatory responses that contribute to the elimination of the dying neurons in neuro-degenerative diseases, so its disruption may make a risk factor in numerous human chronic inflammatory diseases such as multiple sclerosis, Alzheimer's disease, glioblastoma, and Rett syndrome. This study is a review of the efferocytosis molecular pathways and their role in neuro-degenerative diseases in order to discover a new treatment option to cure patients.

T cells, α-synuclein and Parkinson disease

The notion that autoimmune responses to α-synuclein may be involved in the pathogenesis of this disorder stems from reports that mutations in α-synuclein or certain alleles of the major histocompatibility complex (MHC) are associated with the disease and that dopaminergic and norepinephrinergic neurons in the midbrain can present antigenic epitopes. Here, we discuss recent evidence that a defined set of peptides derived from α-synuclein act as antigenic epitopes displayed by specific MHC alleles and drive helper and cytotoxic T cell responses in patients with PD. Moreover, phosphorylated α-synuclein may activate T cell responses in a less restricted manner in PD. While the roles for the acquired immune system in disease pathogenesis remain unknown, preclinical animal models and in vitro studies indicate that T cells may interact with neurons and exert effects related to neuronal death and neuroprotection. These findings suggest that therapeutics that target T cells and ameliorate the incidence or disease severity of inflammatory bowel disorders or CNS autoimmune diseases such as multiple sclerosis may be useful in PD.

Fecal microbiota transplantation protects rotenone-induced Parkinson's disease mice via suppressing inflammation mediated by the lipopolysaccharide-TLR4 signaling pathway through the microbiota-gut-brain axis

BACKGROUND

Parkinson's disease (PD) is a prevalent neurodegenerative disorder, displaying not only well-known motor deficits but also gastrointestinal dysfunctions. Consistently, it has been increasingly evident that gut microbiota affects the communication between the gut and the brain in PD pathogenesis, known as the microbiota-gut-brain axis. As an approach to re-establishing a normal microbiota community, fecal microbiota transplantation (FMT) has exerted beneficial effects on PD in recent studies. Here, in this study, we established a chronic rotenone-induced PD mouse model to evaluate the protective effects of FMT treatment on PD and to explore the underlying mechanisms, which also proves the involvement of gut microbiota dysbiosis in PD pathogenesis via the microbiota-gut-brain axis.

RESULTS

We demonstrated that gut microbiota dysbiosis induced by rotenone administration caused gastrointestinal function impairment and poor behavioral performances in the PD mice. Moreover, 16S RNA sequencing identified the increase of bacterial genera Akkermansia and Desulfovibrio in fecal samples of rotenone-induced mice. By contrast, FMT treatment remarkably restored the gut microbial community, thus ameliorating the gastrointestinal dysfunctions and the motor deficits of the PD mice. Further experiments revealed that FMT administration alleviated intestinal inflammation and barrier destruction, thus reducing the levels of systemic inflammation. Subsequently, FMT treatment attenuated blood-brain barrier (BBB) impairment and suppressed neuroinflammation in the substantia nigra (SN), which further decreased the damage of dopaminergic neurons. Additional mechanistic investigation discovered that FMT treatment reduced lipopolysaccharide (LPS) levels in the colon, the serum, and the SN, thereafter suppressing the TLR4/MyD88/NF-κB signaling pathway and its downstream pro-inflammatory products both in the SN and the colon.

CONCLUSIONS

Our current study demonstrates that FMT treatment can correct the gut microbiota dysbiosis and ameliorate the rotenone-induced PD mouse model, in which suppression of the inflammation mediated by the LPS-TLR4 signaling pathway both in the gut and the brain possibly plays a significant role. Further, we prove that rotenone-induced microbiota dysbiosis is involved in the genesis of PD via the microbiota-gut-brain axis. Video abstract.

Novel compound FLZ alleviates rotenone-induced PD mouse model by suppressing TLR4/MyD88/NF-κB pathway through microbiota-gut-brain axis

Parkinson's disease (PD) is the second most common neurodegenerative disease, but none of the current treatments for PD can halt the progress of the disease due to the limited understanding of the pathogenesis. In PD development, the communication between the brain and the gastrointestinal system influenced by gut microbiota is known as microbiota-gut-brain axis. However, the explicit mechanisms of microbiota dysbiosis in PD development have not been well elucidated yet. FLZ, a novel squamosamide derivative, has been proved to be effective in many PD models and is undergoing the phase I clinical trial to treat PD in China. Moreover, our previous pharmacokinetic study revealed that gut microbiota could regulate the absorption of FLZ in vivo. The aims of our study were to assess the protective effects of FLZ treatment on PD and to further explore the underlying microbiota-related mechanisms of PD by using FLZ as a tool. In the current study, chronic oral administration of rotenone was utilized to induce a mouse model to mimic the pathological process of PD. Here we revealed that FLZ treatment alleviated gastrointestinal dysfunctions, motor symptoms, and dopaminergic neuron death in rotenone-challenged mice. 16S rRNA sequencing found that PD-related microbiota alterations induced by rotenone were reversed by FLZ treatment. Remarkably, FLZ administration attenuated intestinal inflammation and gut barrier destruction, which subsequently inhibited systemic inflammation. Eventually, FLZ treatment restored blood-brain barrier structure and suppressed neuroinflammation by inhibiting the activation of astrocytes and microglia in the substantia nigra (SN). Further mechanistic research demonstrated that FLZ treatment suppressed the TLR4/MyD88/NF-κB pathway both in the SN and colon. Collectively, FLZ treatment ameliorates microbiota dysbiosis to protect the PD model via inhibiting TLR4 pathway, which contributes to one of the underlying mechanisms beneath its neuroprotective effects. Our research also supports the importance of microbiota-gut-brain axis in PD pathogenesis, suggesting its potential role as a novel therapeutic target for PD treatment.

Fecal Microbiota Transplantation Exerts a Protective Role in MPTP-Induced Parkinson's Disease via the TLR4/PI3K/AKT/NF-κB Pathway Stimulated by α-Synuclein

Gut microbiota is closely related to the Parkinson's disease (PD) pathogenesis. Additionally, aggregation of α-synuclein (α-syn) is central to PD pathogenesis. Here we identified the further mechanisms of gut microbiota in PD. A mouse model with PD was established via injection of MPTP. Normal or MPTP-induced PD like animals were treated with FMT from healthy normal mice. Pole test and traction test were performed to examine the effects of FMT on motor function of PD mice. Fecal SCFAs were assessed by gas chromatography-mass spectrometry. The α-syn level in the substantia nigra pars compacta (SN) of mice was measured using western blot. Dopaminergic neurons and microglial activation in the SN were analyzed by immunohistochemistry (IHC) and immunofluorescence (IF) staining. FMT alleviated physical impairment, decreased fecal SCFAs in a mouse model of PD. Additionally, FMT decreased the expression of α-syn, as well as inhibited the activation of microglia in the SN, and blocked the TLR4/PI3K/AKT/NF-κB signaling in the SN and striatum. FMT could protect mice against PD via suppressing α-syn expression and inactivating the TLR4/PI3K/AKT/NF-κB signaling.

Human Monocytes Plasticity in Neurodegeneration

Monocytes play a crucial role in immunity and tissue homeostasis. They constitute the first line of defense during the inflammatory process, playing a role in the pathogenesis and progression of diseases, making them an attractive therapeutic target. They are heterogeneous in morphology and surface marker expression, which suggest different molecular and physiological properties. Recent evidences have demonstrated their ability to enter the brain, and, as a consequence, their hypothetical role in different neurodegenerative diseases. In this review, we will discuss the current knowledge about the correlation between monocyte dysregulation in the brain and/or in the periphery and neurological diseases in humans. Here we will focus on the most common neurodegenerative disorders, such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis and multiple sclerosis.

Dwellers and Trespassers: Mononuclear Phagocytes at the Borders of the Central Nervous System

The central nervous system (CNS) parenchyma is enclosed and protected by a multilayered system of cellular and acellular barriers, functionally separating glia and neurons from peripheral circulation and blood-borne immune cells. Populating these borders as dynamic observers, CNS-resident macrophages contribute to organ homeostasis. Upon autoimmune, traumatic or neurodegenerative inflammation, these phagocytes start playing additional roles as immune regulators contributing to disease evolution. At the same time, pathological CNS conditions drive the migration and recruitment of blood-borne monocyte-derived cells across distinct local gateways. This invasion process drastically increases border complexity and can lead to parenchymal infiltration of blood-borne phagocytes playing a direct role both in damage and in tissue repair. While recent studies and technical advancements have highlighted the extreme heterogeneity of these resident and CNS-invading cells, both the compartment-specific mechanism of invasion and the functional specification of intruding and resident cells remain unclear. This review illustrates the complexity of mononuclear phagocytes at CNS interfaces, indicating how further studies of CNS border dynamics are crucially needed to shed light on local and systemic regulation of CNS functions and dysfunctions.

Neuroinflammation in Parkinson's Disease: Triggers, Mechanisms, and Immunotherapies

Parkinson's disease (PD) is a heterogeneous neurodegenerative disease involving multiple etiologies and pathogenesis, in which neuroinflammation is a common factor. Both preclinical experiments and clinical studies provide evidence for the involvement of neuroinflammation in the pathophysiology of PD, although there are a number of key issues related to neuroinflammatory processes in PD that remain to be addressed. In this review, we highlight the relationship between the common pathological mechanisms of PD and neuroinflammation, including aggregation of α-synuclein, genetic factors, mitochondrial dysfunction, and gut microbiome dysbiosis. We also describe the two positive feedback loops initiated in PD after the immune system is activated, and their role in the pathogenesis of PD. In addition, the interconnections and differences between the central and peripheral immune systems are discussed. Finally, we review the latest progress in immunotherapy research for PD patients, and propose future directions for clinical research.

Acylated Ghrelin as a Multi-Targeted Therapy for Alzheimer's and Parkinson's Disease

Much thought has been given to the impact of Amyloid Beta, Tau and Alpha-Synuclein in the development of Alzheimer's disease (AD) and Parkinson's disease (PD), yet the clinical failures of the recent decades indicate that there are further pathological mechanisms at work. Indeed, besides amyloids, AD and PD are characterized by the culminative interplay of oxidative stress, mitochondrial dysfunction and hyperfission, defective autophagy and mitophagy, systemic inflammation, BBB and vascular damage, demyelination, cerebral insulin resistance, the loss of dopamine production in PD, impaired neurogenesis and, of course, widespread axonal, synaptic and neuronal degeneration that leads to cognitive and motor impediments. Interestingly, the acylated form of the hormone ghrelin has shown the potential to ameliorate the latter pathologic changes, although some studies indicate a few complications that need to be considered in the long-term administration of the hormone. As such, this review will illustrate the wide-ranging neuroprotective properties of acylated ghrelin and critically evaluate the hormone's therapeutic benefits for the treatment of AD and PD.

Microglial autophagy defect causes parkinson disease-like symptoms by accelerating inflammasome activation in mice

Microglial activation-induced neuroinflammation is closely associated with the development of Parkinson disease (PD). Macroautophagy/autophagy regulates many biological processes, but the role of autophagy in microglial activation during PD development remains largely unclear. In this study, we showed that deletion of microglial Atg5 caused PD-like symptoms in mice, characterized by impairment in motor coordination and cognitive learning, loss of tyrosine hydroxylase (TH) neurons, enhancement of neuroinflammation and reduction in dopamine levels in the striatum. Mechanistically, we found that inhibition of autophagy led to NLRP3 (NLR family pyrin domain containing 3) inflammasome activation via PDE10A (phosphodiesterase 10A)-cyclic adenosine monophosphate (cAMP) signaling in microglia, and the sequential upregulation of downstream IL1B/IL-1β in turn increased the expression of MIF (macrophage migration inhibitory factor [glycosylation-inhibiting factor]), a pro-inflammatory cytokine. Inhibition of NLRP3 inflammasome activation by administration of MCC950, a specific inhibitor for NLRP3, decreased MIF expression and neuroinflammatory levels, and rescued the loss of TH neurons in the substantial nigra (SN). Interestingly, we found that serum MIF levels in PD patients were significantly elevated. Taken together, our results reveal an important role of autophagy in microglial activation-driven PD-like symptoms, thus providing potential targets for the clinical treatment of PD. Abbreviations: ATG: autophagy related; cAMP: cyclic adenosine monophosphate; cKO: conditional knockout; NOS2/INOS: nitric oxide synthase 2, inducible; IL1B: interleukin 1 beta; ITGAM/CD-11b: integrin alpha M/cluster of differentiation molecule 11B; MAP1LC3: microtubule-associated protein 1 light chain 3; MIF: macrophage migration inhibitory factor (glycosylation-inhibiting factor); NLRP3: NLR family pyrin domain containing 3; PBS: phosphate-buffered saline; PD: parkinson disease; PDE10A: phosphodiesterase 10A; SN: substantial nigra; TH: tyrosine hydroxylase; TNF: tumor necrosis factor; WT: wild type.

Prefrontal Cortex Transcriptomic Deconvolution Implicates Monocyte Infiltration in Parkinson's Disease

INTRODUCTION

Although not considered a primary cause, neuroinflammation is associated with many neurodegenerative disorders, including Parkinson's disease (PD).

METHODS

To elucidate potential immune involvement in PD, the present study imputed immune cell abundances from bulk RNA-sequencing transcriptomic data of PD postmortem prefrontal cortices. CIBERSORTx, an RNA deconvolution algorithm that implements support vector regression, was used to measure the relative abundances of immune cells from a previously published gene expression dataset. Through this machine-learning approach, relative proportions of 22 immune cell subtypes present in the original brain tissue were estimated.

RESULTS

Prefrontal cortices from PD patients exhibited significantly higher relative abundances of monocytes compared to neuropathologically normal controls (p value = 0.0005). The relative proportions of the other 21 immune subtypes showed no significant differences between control and PD samples.

CONCLUSION AND DISCUSSION

The findings corroborate previous reports and suggest monocytes may be involved in PD pathogenesis.

HMGB1 A box protects neurons by potently inhibiting both microglia and T cell-mediated inflammation in a mouse Parkinson's disease model

In the subacute Parkinson's disease (PD) mice model of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), injection of HMGB1 competitive inhibitor protein HMGB1 A box and the ethyl pyruvate (EP) that inhibit the release of HMGB1 from cells restored the number of dopaminergic neurons and TH+ fibers in the SN and striatum. Our data show that A box up-regulated CD200-CD200R signal of microglia inhibited the activation of microglia mediated by HMGB1, and the production of TNF-α, IL-1β and IL-6 in vivo and in vitro mixed culture system. Microglia overexpressing CD200R produced less inflammatory chemokines and reduced the loss of TH+ neurons. In addition, HMGB1 A box decreased the level of CCL5 and significantly inhibited the infiltration of almost all T cells including Th17 and the proportion of Th17 in CD4+ T cells. In vitro MPP+ induced model and HMGB1-stimulated mesencephalic cell system activated microglia induced the differentiation of naïve T cells to Th17, and A box significantly inhibited this process. To sum up, our results show that HMGB1 A box targeting HMGB1, which effectively reduces the activation of microglia in MPTP PD model by restoring CD200-CD200R signal inhibit microglia mediated neuroinflammation and the differentiation of T cells to Th17.

The role of glia in protein aggregation

Protein aggregation diseases involve intracellular accumulation or extracellular deposition of certain protein species in neuronal or glial cells, leading to neurodegeneration and shortened lifespan. Prime examples include Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), and Huntington's disease (HD), which are affected by overlapping or specific aggregation-prone proteins. Mounting evidence suggests that dysfunctional glial cells may be major drivers for some diseases, and when they are not causal factors, they could still significantly exacerbate or alleviate disease progression by playing a plethora of detrimental or beneficial roles. Here we review the diverse functions performed by glial cells in a variety of protein aggregation diseases, highlighting the complexity of the issue and the interconnected relationships between these multifaceted effects.

Remodeling microglia to a protective phenotype in Parkinson's disease?

Parkinson's disease (PD) is the most widespread movement disorder with a prevalence of 1 in 1000 individuals above 60 years of age. Until now, understanding the pathological mechanisms of PD to translate them into therapy has remained a high research priority. In this review, we highlight evidence describing the involvement of microglial dysfunction in PD. Thereafter, we provide current knowledge suggesting that the substantia nigra pars compacta and putamen, compared to other brain regions, show a reduced microglial density, as well as altered morphological and functional properties in homeostatic conditions, while presenting dystrophic features associated with aging. Further, we describe that this defective microglial programing emerges as early as the second postnatal week, persists until adulthood and impacts negatively on their transcriptional pattern and provision of local trophic support. We emphasize the role of α-synuclein oligomers as a major dysfunctional signal underlining microglial-mediated phenotypic switch and adaptive response contributing to neurodegeneration. Moreover, we explore available avenues should microglia be considered as target for neuroprotective or restorative strategies including preventing the aggregation of α-synuclein protofibrils formation. However, we provide a note of caution regarding the success of microglial-targeted PD strategies, using minocycline as an example. In conclusion, we discuss putative neuroprotective agents that were unsuccessful in previous trials but could be reconsidered by focusing on the stage of microglial-dependent pathogenic events during PD in suitable cohorts of patients.

Sodium Butyrate Exacerbates Parkinson's Disease by Aggravating Neuroinflammation and Colonic Inflammation in MPTP-Induced Mice Model

The abnormal production of short chain fatty acid (SCFAs) caused by gut microbial dysbiosis plays an important role in the pathogenesis and progression of Parkinson's disease (PD). This study sought to evaluate how butyrate, one of SCFAs, affect the pathology in a subacute 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine hydrochloride (MPTP) treated mouse model of PD. Sodium butyrate (NaB; 165 mg/kg/day i.g., 7 days) was administrated from the day after the last MPTP injection. Interestingly, NaB significantly aggravated MPTP-induced motor dysfunction (P < 0.01), decreased dopamine (P < 0.05) and 5-HT (P < 0.05) levels, exacerbated declines of dopaminergic neurons (34%, P < 0.05) and downregulated expression of tyrosine hydroxylase (TH, 47%, P < 0.05), potentiated glia-mediated neuroinflammation by increasing the number of microglia (17%, P < 0.05) and activating astrocytes (28%, P < 0.01). In vitro study also confirmed that NaB could significantly exacerbate pro-inflammatory cytokines expression (IL-1β, 4.11-fold, P < 0.01; IL-18, 3.42-fold, P < 0.01 and iNOS, 2.52-fold, P < 0.05) and NO production (1.55-fold, P < 0.001) in LPS-stimulated BV2 cells. In addition, NaB upregulated the expression of pro-inflammatory cytokines (IL-6, 3.52-fold, P < 0.05; IL-18, 1.72-fold, P < 0.001) and NLRP3 (3.11-fold, P < 0.001) in the colon of PD mice. However, NaB had no effect on NFκB, MyD88 and TNF-α expression in PD mice. Our results indicate that NaB exacerbates MPTP-induced PD by aggravating neuroinflammation and colonic inflammation independently of the NFκB/MyD88/TNF-α signaling pathway.

Harnessing regulatory T cell neuroprotective activities for treatment of neurodegenerative disorders

Emerging evidence demonstrates that adaptive immunity influences the pathobiology of neurodegenerative disorders. Misfolded aggregated self-proteins can break immune tolerance leading to the induction of autoreactive effector T cells (Teffs) with associated decreases in anti-inflammatory neuroprotective regulatory T cells (Tregs). An imbalance between Teffs and Tregs leads to microglial activation, inflammation and neuronal injury. The cascade of such a disordered immunity includes the drainage of the aggregated protein antigens into cervical lymph nodes serving to amplify effector immune responses. Both preclinical and clinical studies demonstrate transformation of this altered immunity for therapeutic gain. We posit that the signs and symptoms of common neurodegenerative disorders such as Alzheimer's and Parkinson's diseases, amyotrophic lateral sclerosis, and stroke can be attenuated by boosting Treg activities.

Inflammasomes: An Emerging Mechanism Translating Environmental Toxicant Exposure Into Neuroinflammation in Parkinson's Disease

Evidence indicates that complex gene-environment interactions underlie the incidence and progression of Parkinson's disease (PD). Neuroinflammation is a well-characterized feature of PD widely believed to exacerbate the neurodegenerative process. Environmental toxicants associated with PD, such as pesticides and heavy metals, can cause cellular damage and stress potentially triggering an inflammatory response. Toxicant exposure can cause stress and damage to cells by impairing mitochondrial function, deregulating lysosomal function, and enhancing the spread of misfolded proteins. These stress-associated mechanisms produce sterile triggers such as reactive oxygen species (ROS) along with a variety of proteinaceous insults that are well documented in PD. These associations provide a compelling rationale for analysis of sterile inflammatory mechanisms that may link environmental exposure to neuroinflammation and PD progression. Intracellular inflammasomes are cytosolic assemblies of proteins that contain pattern recognition receptors, and a growing body of evidence implicates the association between inflammasome activation and neurodegenerative disease. Characterization of how inflammasomes may function in PD is a high priority because the majority of PD cases are sporadic, supporting the widely held belief that environmental exposure is a major factor in disease initiation and progression. Inflammasomes may represent a common mechanism that helps to explain the strong association between exposure and PD by mechanistically linking environmental toxicant-driven cellular stress with neuroinflammation and ultimately cell death.

Glial phagocytic clearance in Parkinson's disease

An emerging picture suggests that glial cells' loss of beneficial roles or gain of toxic functions can contribute to neurodegenerative conditions. Among glial cells, microglia and astrocytes have been shown to play phagocytic roles by engulfing synapses, apoptotic cells, cell debris, and released toxic proteins. As pathogenic protein accumulation is a key feature in Parkinson's disease (PD), compromised phagocytic clearance might participate in PD pathogenesis. In contrast, enhanced, uncontrolled and potentially toxic glial clearance capacity could contribute to synaptic degeneration. Here, we summarize the current knowledge of the molecular mechanisms underlying microglial and astrocytic phagocytosis, focusing on the possible implication of phagocytic dysfunction in neuronal degeneration. Several endo-lysosomal proteins displaying genetic variants in PD are highly expressed by microglia and astrocytes. We also present the evidence that lysosomal defects can affect phagocytic clearance and discuss the therapeutic relevance of restoring or enhancing lysosomal function in PD.

Autoimmunity in Parkinson's Disease: The Role of α-Synuclein-Specific T Cells

Evidence from a variety of studies implicates a role for the adaptive immune system in Parkinson's disease (PD). Similar to multiple sclerosis (MS) patients who display a high number of T cells in the brain attacking oligodendrocytes, PD patients show higher numbers of T cells in the ventral midbrain than healthy, age-matched controls. Mouse models of the disease also show the presence of T cells in the brain. The role of these infiltrating T cells in the propagation of disease is controversial; however, recent studies indicate that they may be autoreactive in nature, recognizing disease-altered self-proteins as foreign antigens. T cells of PD patients can generate an autoimmune response to α-synuclein, a protein that is aggregated in PD. α-Synuclein and other proteins are post-translationally modified in an environment in which protein processing is altered, possibly leading to the generation of neo-epitopes, or self-peptides that have not been identified by the host immune system as non-foreign. Infiltrating T cells may also be responding to such modified proteins. Genome-wide association studies (GWAS) have shown associations of PD with haplotypes of major histocompatibility complex (MHC) class II genes, and a polymorphism in a non-coding region that may increase MHC class II in PD patients. We speculate that the inflammation observed in PD may play both pathogenic and protective roles. Future studies on the adaptive immune system in neurodegenerative disorders may elucidate steps in disease pathogenesis and assist with the development of both biomarkers and treatments.

Aucubin alleviates glial cell activation and preserves dopaminergic neurons in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced parkinsonian mice

Aucubin (AUC) is a major bioactive ingredient in Eucommia ulmoides, Plantain asiatica, and Aucuba japonica, and has been shown to exert anti-inflammatory, antioxidative, and neuroprotective effects. We explore the neuroprotective effects of AUC in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced parkinsonian mice. Mice were administered MPTP (30 mg/kg) daily for 5 days, followed by treatment with AUC for 7 days. Measurement of dopamine levels was performed by high-performance liquid chromatography and tyrosine hydroxylase expression was assessed by western blot. Our results showed that AUC treatment improved mobility in the pole descent test and the traction test, and reduced the loss of dopaminergic neurons in MPTP-induced parkinsonian mice. AUC treatment rescued the decreased dopamine and tyrosine hydroxylase levels in the striatum of parkinsonian mice. Furthermore, AUC treatment reduced both microglia and astrocyte activation in the substantia nigra of parkinsonian mice. These findings suggest that AUC exerts neuroprotective effects, in part by reducing inflammation and preserving dopaminergic neurons. Possible protection mechanisms involved in MPTP-induced parkinsonian mice need to be clarified further.

Astroglia in Alzheimer's Disease

Alzheimer's disease is the most common cause of dementia. Cellular changes in the brains of the patients suffering from Alzheimer's disease occur well in advance of the clinical symptoms. At the cellular level, the most dramatic is a demise of neurones. As astroglial cells carry out homeostatic functions of the brain, it is certain that these cells are at least in part a cause of Alzheimer's disease. Historically, Alois Alzheimer himself has recognised this at the dawn of the disease description. However, the role of astroglia in this disease has been understudied. In this chapter, we summarise the various aspects of glial contribution to this disease and outline the potential of using these cells in prevention (exercise and environmental enrichment) and intervention of this devastating disease.

Counteracting neuroinflammation in experimental Parkinson's disease favors recovery of function: effects of Er-NPCs administration

BACKGROUND

Parkinson's disease (PD) is the second most common neurodegenerative disease, presenting with midbrain dopaminergic neurons degeneration. A number of studies suggest that microglial activation may have a role in PD. It has emerged that inflammation-derived oxidative stress and cytokine-dependent toxicity may contribute to nigrostriatal pathway degeneration and exacerbate the progression of the disease in patients with idiopathic PD. Cell therapies have long been considered a feasible regenerative approach to compensate for the loss of specific cell populations such as the one that occurs in PD. We recently demonstrated that erythropoietin-releasing neural precursors cells (Er-NPCs) administered to MPTP-intoxicated animals survive after transplantation in the recipient's damaged brain, differentiate, and rescue degenerating striatal dopaminergic neurons. Here, we aimed to investigate the potential anti-inflammatory actions of Er-NPCs infused in an MPTP experimental model of PD.

METHODS

The degeneration of dopaminergic neurons was caused by MPTP administration in C57BL/6 male mice. 2.5 × 105 GFP-labeled Er-NPCs were administered by stereotaxic injection unilaterally in the left striatum. Functional recovery was assessed by two independent behavioral tests. Neuroinflammation was investigated measuring the mRNAs levels of pro-inflammatory and anti-inflammatory cytokines, and immunohistochemistry studies were performed to evaluate markers of inflammation and the potential rescue of tyrosine hydroxylase (TH) projections in the striatum of recipient mice.

RESULTS

Er-NPC administration promoted a rapid anti-inflammatory effect that was already evident 24 h after transplant with a decrease of pro-inflammatory and increase of anti-inflammatory cytokines mRNA expression levels. This effect was maintained until the end of the observational period, 2 weeks post-transplant. Here, we show that Er-NPCs transplant reduces macrophage infiltration, directly counteracting the M1-like pro-inflammatory response of murine-activated microglia, which corresponds to the decrease of CD68 and CD86 markers, and induces M2-like pro-regeneration traits, as indicated by the increase of CD206 and IL-10 expression. Moreover, we also show that this activity is mediated by Er-NPCs-derived erythropoietin (EPO) since the co-injection of cells with anti-EPO antibodies neutralizes the anti-inflammatory effect of the Er-NPCs treatment.

CONCLUSION

This study shows the anti-inflammatory actions exerted by Er-NPCs, and we suggest that these cells may represent good candidates for cellular therapy to counteract neuroinflammation in neurodegenerative disorders.

Fractalkine/CX3CR1 is involved in the cross-talk between neuron and glia in neurological diseases

Fractalkine (CX3C chemokine ligand 1, CX3CL1) is an essential chemokine, for regulating adhesion and chemotaxis through binding to CX3CR1, which plays a critical role in the crosstalk between glial cells and neurons by direct or indirect ways in the central nervous system (CNS). Fractalkine/CX3CR1 axis regulates microglial activation and function, neuronal survival and synaptic function by controlling the release of inflammatory cytokines and synaptic plasticity in the course of the neurological disease. The multiple functions of fractalkine/CX3CR1 make it exert neuroprotective or neurotoxic effects, which determines the pathogenesis. However, the role of fractalkine/CX3CR1 in the CNS remains controversial. Whether it can be used as a therapeutic target for neurological diseases needs to be further investigated. In this review, we summarize the studies highlighting fractalkine/CX3CR1-mediated effects and discuss the potential neurotoxic and neuroprotective actions of fractalkine/CX3CR1 in brain injury for providing useful insights into the potential applications of fractalkine/CX3CR1 in neurological diseases.

Boosting phagocytosis and anti-inflammatory phenotype in microglia mediates neuroprotection by PPARγ agonist MDG548 in Parkinson's disease models

BACKGROUND AND PURPOSE

Microglial phenotype and phagocytic activity are deregulated in Parkinson's disease (PD). PPARγ agonists are neuroprotective in experimental PD, but their role in regulating microglial phenotype and phagocytosis has been poorly investigated. We addressed it by using the PPARγ agonist MDG548.

EXPERIMENTAL APPROACH

Murine microglial cell line MMGT12 was stimulated with LPS and/or MDG548, and their effect on phagocytosis of fluorescent microspheres or necrotic neurons was investigated by flow cytometry. Cytokines and markers of microglia phenotype, such as mannose receptor C type 1; MRC1), Ym1 and CD68 were measured by elisa and fluorescent immunohistochemistry. Levels of Beclin-1, which plays a role in microglial phagocytosis, were measured by Western blotting. In the in vivo MPTP-probenecid (MPTPp) model of PD in mice, MDG548 was tested on motor impairment, nigral neurodegeneration, microglial activation and phenotype.

KEY RESULTS

In LPS-stimulated microglia, MDG548 increased phagocytosis of both latex beads and necrotic cells, up-regulated the expression of MRC1, CD68 and to a lesser extent IL-10, while blocking the LPS-induced increase of TNF-α and iNOS. MDG548 also induced Beclin-1. Chronic MPTPp treatment in mice down-regulated MRC1 and TGF-β and up-regulated TNF-α and IL-1β immunoreactivity in activated CD11b-positive microglia, causing the death of nigral dopaminergic neurons. MDG548 arrested MPTPp-induced cell death, enhanced MRC1 and restored cytokine levels.

CONCLUSIONS AND IMPLICATIONS

This study adds a novel mechanism for PPARγ-mediated neuroprotection in PD and suggests that increasing phagocytic activity and anti-inflammatory markers may represent an effective disease-modifying approach.

Partial Depletion of Peripheral M1 Macrophages Reverses Motor Deficits in MPTP-Treated Mouse by Suppressing Neuroinflammation and Dopaminergic Neurodegeneration

Background: Neuroinflammation plays an important role in the pathogenesis of Parkinson's disease (PD). Inflammatory cytokines in the peripheral immune system can induce neuroinflammation in central nervous system (CNS). Whether the peripheral immune system is involved in PD is unclear. The present study investigated the contribution of the peripheral immune system to the neuronal loss in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine(MPTP) model of PD.

Methods: MPTP was intraperitoneally injected into mice to generate a PD model. Mice received clodronate liposomes every 3 days to deplete peripheral macrophages. The percentages of macrophages were measured by flow cytometry at 1, 3, and 7 days after MPTP injection. Neurobehavioral parameters, protein expression, inflammatory cytokines release, and microglia activation were measured by the open field test, western blotting, quantitative polymerase chain reaction (qPCR), and immunofluorescence staining, respectively at 7 days after MPTP injection.

Results: Our study revealed that intraperitoneal injection of MPTP could increase peripheral M1 macrophages levels. It also can induce T cells infiltration and cytokine release. Depletion of M1 macrophages by clodronate liposomes suppressed these inflammatory effects and blunted the loss of TH+ nigral neurons and functional impairments caused by MPTP.

Conclusion: Our results indicated the critical role of M1 macrophages in the pathogenesis of PD and proposed inhibition of M1 macrophages as a promising therapeutic approach for neurodegeneration.

Tolerogenic bone marrow-derived dendritic cells induce neuroprotective regulatory T cells in a model of Parkinson's disease

BACKGROUND

Administration of granulocyte-macrophage colony-stimulating factor (GM-CSF) increases regulatory T cell (Treg) number and function with control of neuroinflammation and neuronal protection in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model of Parkinson's disease (PD). Recently, we demonstrated in an early phase 1 clinical trial that GM-CSF also improves motor skills in PD patients. However, the mechanisms of Treg induction and its effects on neuroprotective responses remain unknown. As GM-CSF induces tolerogenic dendritic cells (DCs) that in turn convert conventional T cells to Tregs, the pathways for DC induction of Tregs were assessed.

METHODS

Following differentiation, bone marrow-derived dendritic cells (BMDCs) were cultured in media or GM-CSF with or without post-culture stimulation with nitrated α-synuclein (N-α-Syn). Expression of cell surface co-stimulatory molecules and proinflammatory cytokines, and induction of Tregs were evaluated. The neuroprotective capacity of tolerogenic BMDCs was assessed by adoptive transfer to MPTP-intoxicated mice. The extent of neuroinflammation and numbers of surviving dopaminergic neurons were assessed in relation to Treg numbers.

RESULTS

Co-culture of differentiated BMDCs with conventional T cells led to Treg induction. Stimulation of BMDCs with N-α-Syn increased expression of co-stimulatory molecules and proinflammatory cytokines, with modest increases in Treg numbers. In contrast, continued culture of BMDCs with GM-CSF modestly altered expression of co-stimulatory molecules and proinflammatory cytokines and chemokines, but decreased Treg induction. Continued culture in GM-CSF and combined stimulation with N-α-Syn reduced Treg induction to the lowest levels. Adoptive transfer of tolerogenic BMDCs to MPTP-intoxicated mice increased splenic Tregs, attenuated neuroinflammatory responses, and protected nigrostriatal dopaminergic neurons.

CONCLUSIONS

GM-CSF acts broadly to differentiate DCs and affect immune transformation from effector to regulatory immune responses. DCs skew such immune responses by increasing Treg numbers and activities that serve to attenuate proinflammatory responses and augment neuroprotection.

Neuroprotective effects of fecal microbiota transplantation on MPTP-induced Parkinson's disease mice: Gut microbiota, glial reaction and TLR4/TNF-α signaling pathway

Parkinson's disease (PD) patients display alterations in gut microbiota composition. However, mechanism between gut microbial dysbiosis and pathogenesis of PD remains unexplored, and no recognized therapies are available to halt or slow progression of PD. Here we identified that gut microbiota from PD mice induced motor impairment and striatal neurotransmitter decrease on normal mice. Sequencing of 16S rRNA revealed that phylum Firmicutes and order Clostridiales decreased, while phylum Proteobacteria, order Turicibacterales and Enterobacteriales increased in fecal samples of PD mice, along with increased fecal short-chain fatty acids (SCFAs). Remarkably, fecal microbiota transplantation (FMT) reduced gut microbial dysbiosis, decreased fecal SCFAs, alleviated physical impairment, and increased striatal DA and 5-HT content of PD mice. Further, FMT reduced the activation of microglia and astrocytes in the substantia nigra, and reduced expression of TLR4/TNF-α signaling pathway components in gut and brain. Our study demonstrates that gut microbial dysbiosis is involved in PD pathogenesis, and FMT can protect PD mice by suppressing neuroinflammation and reducing TLR4/TNF-α signaling.

Microglial Phagocytosis and Its Regulation: A Therapeutic Target in Parkinson's Disease?

The role of phagocytosis in the neuroprotective function of microglia has been appreciated for a long time, but only more recently a dysregulation of this process has been recognized in Parkinson’s disease (PD). Indeed, microglia play several critical roles in central nervous system (CNS), such as clearance of dying neurons and pathogens as well as immunomodulation, and to fulfill these complex tasks they engage distinct phenotypes. Regulation of phenotypic plasticity and phagocytosis in microglia can be impaired by defects in molecular machinery regulating critical homeostatic mechanisms, including autophagy. Here, we briefly summarize current knowledge on molecular mechanisms of microglia phagocytosis, and the neuro-pathological role of microglia in PD. Then we focus more in detail on the possible functional role of microglial phagocytosis in the pathogenesis and progression of PD. Evidence in support of either a beneficial or deleterious role of phagocytosis in dopaminergic degeneration is reported. Altered expression of target-recognizing receptors and lysosomal receptor CD68, as well as the emerging determinant role of α-synuclein (α-SYN) in phagocytic function is discussed. We finally discuss the rationale to consider phagocytic processes as a therapeutic target to prevent or slow down dopaminergic degeneration.

Effects of triptolide on the expression of MHC II in microglia in kainic acid‑induced epilepsy

The purpose of the present study was to determine whether triptolide (T10) had any effect on major histocompatibility complex class II (MHC II) expression in kainic acid (KA)‑activated microglia, and to investigate the underlying molecular mechanism. BV‑2 microglia were pretreated with T10 prior to activation with KA. The expression level of MHC II and class II transactivator (CIITA) mRNA was determined via reverse transcription‑polymerase chain reaction. The expression of MHC II, CIITA and the phosphorylation level of c‑Jun and proto‑oncogene c‑Fos (c‑Fos) was determined by western blotting. The protein expression level of MHC II was determined by immunocytochemistry. It was observed that the mRNA and protein levels of MHC II and CIITA were increased in KA‑activated BV‑2 microglia, and that this increase was almost completely eliminated by T10. AP‑1 is a family of homodimers or heterodimers, composed of Jun family and Fos family proteins. Sequence analysis revealed an AP‑1 DNA binding site in the promoter of CIITA. The phosphorylation of c‑Jun and c‑Fos was increased in KA‑activated microglia, while T10 was able to suppress the phosphorylation of c‑Jun and c‑Fos in KA‑activated microglia. These data suggested that T10 may exert suppressive effects on MHC II expression in KA‑activated microglia, and that the mechanism may involve the regulation of AP‑1 activity.

Microglia Polarization, Gene-Environment Interactions and Wnt/β-Catenin Signaling: Emerging Roles of Glia-Neuron and Glia-Stem/Neuroprogenitor Crosstalk for Dopaminergic Neurorestoration in Aged Parkinsonian Brain

Neuroinflammatory processes are recognized key contributory factors in Parkinson's disease (PD) physiopathology. While the causes responsible for the progressive loss of midbrain dopaminergic (mDA) neuronal cell bodies in the subtantia nigra pars compacta are poorly understood, aging, genetics, environmental toxicity, and particularly inflammation, represent prominent etiological factors in PD development. Especially, reactive astrocytes, microglial cells, and infiltrating monocyte-derived macrophages play dual beneficial/harmful effects, via a panel of pro- or anti-inflammatory cytokines, chemokines, neurotrophic and neurogenic transcription factors. Notably, with age, microglia may adopt a potent neurotoxic, pro-inflammatory “primed” (M1) phenotype when challenged with inflammatory or neurotoxic stimuli that hamper brain's own restorative potential and inhibit endogenous neurorepair mechanisms. In the last decade we have provided evidence for a major role of microglial crosstalk with astrocytes, mDA neurons and neural stem progenitor cells (NSCs) in the MPTP- (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-) mouse model of PD, and identified Wnt/β-catenin signaling, a pivotal morphogen for mDA neurodevelopment, neuroprotection, and neuroinflammatory modulation, as a critical actor in glia-neuron and glia-NSCs crosstalk. With age however, Wnt signaling and glia-NSC-neuron crosstalk become dysfunctional with harmful consequences for mDA neuron plasticity and repair. These findings are of importance given the deregulation of Wnt signaling in PD and the emerging link between most PD related genes, Wnt signaling and inflammation. Especially, in light of the expanding field of microRNAs and inflammatory PD-related genes as modulators of microglial-proinflammatory status, uncovering the complex molecular circuitry linking PD and neuroinflammation will permit the identification of new druggable targets for the cure of the disease. Here we summarize recent findings unveiling major microglial inflammatory and oxidative stress pathways converging in the regulation of Wnt/β-catenin signaling, and reciprocally, the ability of Wnt signaling pathways to modulate microglial activation in PD. Unraveling the key factors and conditons promoting the switch of the proinflammatory M1 microglia status into a neuroprotective and regenerative M2 phenotype will have important consequences for neuroimmune interactions and neuronal outcome under inflammatory and/or neurodegenerative conditions.

Neuroinflammation: Microglia and T Cells Get Ready to Tango

In recent years, many paradigms concerning central nervous system (CNS) immunology have been challenged and shifted, including the discovery of CNS-draining lymphatic vessels, the origin and functional diversity of microglia, the impact of T cells on CNS immunological homeostasis and the role of neuroinflammation in neurodegenerative diseases. In parallel, antigen presentation outside the CNS has revealed the vital role of antigen-presenting cells in maintaining tolerance toward self-proteins, thwarting auto-immunity. Here, we review recent findings that unite these shifted paradigms of microglial functioning, antigen presentation, and CNS-directed T cell activation, focusing on common neurodegenerative diseases. It provides an important update on CNS adaptive immunity, novel targets, and a concept of the microglia T-cell equilibrium.

Depletion of microglia augments the dopaminergic neurotoxicity of MPTP

The activation of microglia and the various substances they produce have been linked to the pathologic development of Parkinson’s disease (PD), but the precise role of microglia in PD remains to be defined. The survival of microglia depends on colony-stimulating factor 1 receptor (CSF1R) signaling, and CSF1R inhibition results in rapid elimination of microglia in the central nervous system. Using a mouse PD model induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) treatment, we showed that the depletion of microglia via the CSF1R inhibitor PLX3397 exacerbated the impairment of locomotor activities and the loss of dopaminergic neurons. Further, depletion of microglia augmented the production of inflammatory mediators and infiltration of leukocytes in the brain after MPTP exposure. Microglia depletion–induced aggravation of MPTP neurotoxicity was also seen in lymphocyte-deficient mice. In addition, the depletion of microglia did not affect the production of brain-derived neurotrophic factor, but it dramatically augmented the production of inflammatory mediators by astrocytes after MPTP treatment. Our findings suggest microglia play a protective role against MPTP-induced neuroinflammation and dopaminergic neurotoxicity.—Yang, X., Ren, H., Wood, K., Li, M., Qiu, S., Shi, F.-D., Ma, C., Liu, Q. Depletion of microglia augments the dopaminergic neurotoxicity of MPTP.

Inhibitory effect of thiacremonone on MPTP-induced dopaminergic neurodegeneration through inhibition of p38 activation

Neuroinflammation is implicated for dopaminergic neurodegeneration. Sulfur compounds extracted from garlic have been shown to have anti-inflammatory properties. Previously, we have investigated that thiacremonone, a sulfur compound isolated from garlic has anti-inflammatory effects on several inflammatory disease models. To investigate the protective effect of thiacremonone against 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced behavioral impairment and dopaminergic neurodegeneration, 8 week old ICR mice were given thiacremonone (10 mg/kg) in drinking water for 1 month and received intraperitoneal injection of MPTP (15 mg/kg, four times with 2 h interval) during the last 7 days of treatment. Our data showed that thiacremonone decreased MPTP-induced behavioral impairments (Rotarod test, Pole test, and Gait test), dopamine depletion and microglia and astrocytes activations as well as neuroinflammation. Higher activation of p38 was found in the substantia nigra and striatum after MPTP injection, but p38 activation was reduced in thiacremonone treated group. In an in vitro study, thiacremonone (1, 2, and 5 μg/ml) effectively decreased MPP+ (0.5 mM)-induced glial activation, inflammatory mediators generation and dopaminergic neurodegeneration in cultured astrocytes and microglial BV-2 cells. Moreover, treatment of p38 MAPK inhibitor SB203580 (10 μM) further inhibited thiacremonone induced reduction of neurodegeneration and neuroinflammation. These results indicated that the anti-inflammatory compound, thiacremonone, inhibited neuroinflammation and dopaminergic neurodegeneration through inhibition of p38 activation.

Differential contribution of microglia and monocytes in neurodegenerative diseases

Neuroinflammation is a hallmark of neurodegenerative diseases including Alzheimer's disease (AD), Parkinson's disease (PD), and amyotrophic lateral sclerosis (ALS). Microglia, the innate immune cells of the CNS, are the first to react to pathological insults. However, multiple studies have also demonstrated an involvement of peripheral monocytes in several neurodegenerative diseases. Due to the different origins of these two cell types, it is important to distinguish their role and function in the development and progression of these diseases. In this review, we will summarize and discuss the current knowledge of the differential contributions of microglia and monocytes in the common neurodegenerative diseases AD, PD, and ALS, as well as multiple sclerosis, which is now regarded as a combination of inflammatory processes and neurodegeneration. Until recently, it has been challenging to differentiate microglia from monocytes, as there were no specific markers. Therefore, the recent identification of specific molecular signatures of both cell types will help to advance our understanding of their differential contribution in neurodegenerative diseases.

Let's make microglia great again in neurodegenerative disorders

All of the common neurodegenerative disorders-Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis and prion diseases-are characterized by accumulation of misfolded proteins that trigger activation of microglia; brain-resident mononuclear phagocytes. This chronic form of neuroinflammation is earmarked by increased release of myriad cytokines and chemokines in patient brains and biofluids. Microglial phagocytosis is compromised early in the disease process, obfuscating clearance of abnormal proteins. This review identifies immune pathologies shared by the major neurodegenerative disorders. The overarching concept is that aberrant innate immune pathways can be targeted for return to homeostasis in hopes of coaxing microglia into clearing neurotoxic misfolded proteins.