Heterogeneity of microglia and their differential roles in white matter pathology

Microglia are resident immune cells that play multiple roles in central nervous system (CNS) development and disease. Although the classical concept of microglia/macrophage activation is based on a biphasic beneficial‐versus‐deleterious polarization, growing evidence now suggests a much more heterogenous profile of microglial activation that underlie their complex roles in the CNS. To date, the majority of data are focused on microglia in gray matter. However, demyelination is a prominent pathologic finding in a wide range of diseases including multiple sclerosis, Alzheimer's disease, and vascular cognitive impairment and dementia. In this mini‐review, we discuss newly discovered functional subsets of microglia that contribute to white matter response in CNS disease onset and progression. Microglia show different molecular patterns and morphologies depending on disease type and brain region, especially in white matter. Moreover, in later stages of disease, microglia demonstrate unconventional immuno‐regulatory activities such as increased phagocytosis of myelin debris and secretion of trophic factors that stimulate oligodendrocyte lineage cells to facilitate remyelination and disease resolution. Further investigations of these multiple microglia subsets may lead to novel therapeutic approaches to treat white matter pathology in CNS injury and disease.     

Role of Fas--FasL interactions in the pathogenesis and regulation of autoimmune demyelinating disease

Multiple sclerosis (MS) and experimental autoimmune encephalomyelitis (EAE) represent complex processes that lead to destruction of oligodendrocytes (ODCs) and myelin. T cells are integral to the development of these diseases, but whether T cell-mediated cytolytic mechanisms are involved in the destruction of MHC Class II-negative targets, such as oligodendroglia and myelin, in the CNS is unclear. The primary lytic mechanism employed by CD4⁺ T cells is Fas-dependent, but can be MHC-unrestricted. Thus, T cell-mediated Fas–FasL interactions could directly contribute to the pathology of EAE and MS. This review summarizes studies from our laboratory and others that implicate Fas–FasL interactions in both the pathogenesis and regulation of demyelinating diseases.     

Signaling through JAK2-STAT5 pathway is essential for IL-3-induced activation of microglia

Microglia, the resident macrophage of the brain, mediates immune and inflammatory responses in the central nervous system (CNS). Activation of microglia and secretion of inflammatory cytokines associate with the pathogenesis of CNS diseases, including multiple sclerosis (MS), Alzheimer's disease (AD), Parkinson's disease, prion disease, and AIDS dementia. Microbial pathogens, cytokines, chemokines, and costimulatory molecules are potent inducers of microglial activation in the CNS. Signaling through its receptor, IL-3 induces the activation of JAK-STAT and MAP kinase pathways in microglial cells. In this study, we found that in vitro treatment of EOC-20 microglial cells with tyrphostin AG490 blocked IL-3-induced tyrosine phosphorylation of JAK2, STAT5A, and STAT5B signaling proteins. Stable transfection of EOC-20 cells with a dominant negative JAK2 mutant also blocked IL-3-induced tyrosine phosphorylation of JAK2, STAT5A, and STAT5B in microglia. The blockade of JAK2-STAT5 pathway resulted in a decrease in IL-3-induced proliferation and expression of CD40 and major histocompatibility complex class II molecules in microglia. These findings highlight the fact that JAK2-STAT5 signaling pathway plays a critical role in mediating IL-3-induced activation of microglia.     

Role of cytokines and chemokines in prion infections of the central nervous system

Prion infections of the central nervous system (CNS) are characterised by a reactive gliosis and the subsequent degeneration of neuronal tissue. The activation of glial cells, which precedes neuronal death, is likely to be initially caused by the deposition of misfolded, proteinase K-resistant, isoforms (termed PrP(res)) of the prion protein (PrP) in the brain. Cytokines and chemokines released by PrP(res)-activated glia cells may contribute directly or indirectly to the disease development by enhancement and generalisation of the gliosis and via cytotoxicity for neurons. However, the actual role of prion-induced glia activation and subsequent cytokine/chemokine secretion in disease development is still far from clear. In the present work, we review our present knowledge concerning the functional biology of cytokines and chemokines in prion infections of the CNS.     

Multiple sclerosis preceding CNS lymphoma: a case report

Multiple sclerosis (MS) is an autoimmune disease that targets the myelin of the brain, spinal cord, and optic nerves. The inciting trigger that activates this immune response is unknown. Primary central nervous system (CNS) lymphoma is usually a diffuse large B-cell non-Hodgkin's lymphoma that originates in the brain, spinal cord, leptomeninges, or eyes. We report a 33-year-old patient who was diagnosed to have multiple sclerosis initially and a CNS lymphoma was noted 38 months later. Primary CNS lymphoma is a potential complication of chronic immunosuppression. Due to the uses of steroid, immunosuppresants and beta-interferon for multiple sclerosis, our patient developed an immuno-compromised state. These agents might contribute to the occurrence of a primary CNS lymphoma. On the other hand, a demyelinating disease may have preceded the diagnosis of primary CNS lymphoma. A possibility of neoplastic transformation in CNS inflammatory diseases such as multiple sclerosis may occur. The association of coexistent primary CNS lymphoma and multiple sclerosis may be more than coincidental.     

The tyrosine kinase inhibitor imatinib mesylate delays prion neuroinvasion by inhibiting prion propagation in the periphery

Prion diseases are fatal neurodegenerative disorders with no effective therapy. A hallmark of prion disease is the conversion of the normal cellular form of prion protein PrP(C) into a disease-associated isoform PrP(Sc). The authors recently have shown that a tyrosine kinase inhibitor, imatinib mesylate, induces clearance of PrP(Sc) via specific inhibition of c-Abl in prion-infected cell culture models. In this study, the authors assessed the in vivo effects of imatinib mesylate on prion disease using a scrapie-infected mouse model and further investigated prion infectivity of the drug-treated scrapie-infected neuroblastoma (ScN2a) cells. The authors found that imatinib mesylate abolished prion infectivity to almost undetectable level in ScN2a cells and the level of PrP(Sc) was significantly decreased by the drug in scrapie-infected mouse spleens as well as in ScN2a cells. Moreover, the drug treatment at an early phase of peripheral scrapie infection delayed the appearance of PrP(Sc) in the central nervous system (CNS) and onset of clinical disease in mice. However, neither intraperitoneal nor intracerebroventricular delivery of the drug exerted any PrP(Sc) clearance effect in the CNS.     

Prions and peripheral nerves: a deadly rendezvous

The infectious particle causing transmissible spongiform encephalopathy (TSE), a fatal neurodegenerative disease of humans and animals, has been termed prion. Its major component is an aggregated variant of the cellular prion protein, PrP(C). The main target of prion pathology is the central nervous system (CNS), yet most prion diseases are initiated or accompanied by prion replication at extracerebral locations, including secondary lymphoid organs, muscle and, in some instances, blood. How do prions travel from the periphery into the CNS? Is this an active or a passive process and does neuronal prion transport explain the long incubation times in prion diseases? Alternatively, if prion infectivity arises spontaneously in the CNS, as believed from sporadic Creutzfeldt-Jakob patients, how do prions manage to travel from the CNS into the periphery (e.g., spleen, muscle) of the infected host? The mechanisms of neuronal prion transport from the periphery into the CNS or vice versa are heavily investigated and debated but poorly understood. Although research in the past has accumulated knowledge on prion progression from the periphery to the brain, we are far from understanding the molecular mechanisms of neuronal prion transport.     

Death receptor signalling in central nervous system inflammation and demyelination

Death receptors (DRs) are members of the tumor necrosis factor receptor (TNF-R) superfamily that are characterised by the presence of a conserved intracellular death domain and are able to trigger a signalling pathway leading to apoptosis. Strong evidence suggests that DRs contribute to the pathology of tissue destructive diseases, including multiple sclerosis (MS), the most common inflammatory demyelinating disease of the central nervous system (CNS). Here, we review the evidence supporting a role for DRs in MS pathology and its implications for the development of therapeutic strategies for MS and other demyelinating pathologies of the CNS.     

The prion diseases

The prion diseases constitute an unusual group of neurodegenerative disorders. Although they are similar in many ways to other more common diseases, such as Alzheimer disease and amyotrophic lateral sclerosis, they are set apart on the basis of their transmissible nature. In addition to the unique feature of transmissibility, the prion diseases demonstrate that the expression of diverse disease phenotypes is possible from a common etiologic factor. This review provides the reader with a basic understanding of the nature of prions and highlights the clinical and pathologic features of these fascinating diseases.     

Neuroinflamm-aging and neurodegenerative diseases: an overview

Neuroinflammation is considered a chronic activation of the immune response in the central nervous system (CNS) in response to different injuries. This brain immune activation results in various events: circulating immune cells infiltrate the CNS; resident cells are activated; and pro-inflammatory mediators produced and released induce neuroinflammatory brain disease. The effect of immune diffusible mediators on synaptic plasticity might result in CNS dysfunction during neuroinflammatory brain diseases. The CNS dysfunction may induce several human pathological conditions associated with both cognitive impairment and a variable degree of neuroinflammation. Furthermore, age has a powerful effect on enhanced susceptibility to neurodegenerative diseases and age-dependent enhanced neuroinflammatory processes may play an important role in toxin generation that causes death or dysfunction of neurons in neurodegenerative diseases This review will address current understanding of the relationship between ageing, neuroinflammation and neurodegenerative disease by focusing on the principal mechanisms by which the immune system influences the brain plastic phenomena. Also, the present review considers the principal human neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis, multiple sclerosis and psychiatric disorders caused by aging and neuroinflammation.     

Association of endothelial nitric oxide synthase and mitochondrial dysfunction in the hippocampus of scrapie‐infected mice

The elevation of nitric oxide (NO) within the central nervous system (CNS) is known to be associated with the pathogenesis of neurodegenerative diseases such as HIV‐associated dementia (HAD), brain ischemia, Parkinson's disease, and Alzheimer's disease. NO is enzymatically formed by the enzyme nitric oxide synthase (NOS). There are two forms of NOS, the constitutive and the inducible form. The constitutive form is present in endothelial cells (eNOS) and neurons (nNOS). The inducible form (iNOS) is expressed in various cell types including astroglia and microglia of the CNS. Using an animal model, we investigated the involvement of eNOS in the pathology of prion disease. We showed dramatic upregulation of eNOS immunoreactivity in reactive astroglial cells in the hippocampus in the prion disease animal model, scrapie in mice. Expression of eNOS was upregulated in cytosolic and mitochondrial fractions of whole brain. In the hippocampal region, eNOS was widely overexpressed in various components of the cell. We found that eNOS dramatically accumulated in hippocampal mitochondria and was particularly prevalent in structurally dysfunctional mitochondria. In association with the accumulation of eNOS in mitochondria, we showed that mitochondrial superoxide dismutase (Mn‐SOD or SOD2), cytochrome c, and ATP activity were downregulated both in whole brain and in the hippocampal region. These results indicate that eNOS plays a role in the development of dysfunctional mitochondria and this, in turn, could induce some of the histopathological changes seen in prion diseases. © 2010 Wiley‐Liss, Inc.     

Inhibitor of apoptosis protein (IAP) profiling in experimental autoimmune encephalomyelitis (EAE) implicates increased XIAP in T lymphocytes

In multiple sclerosis (MS) and its widely accepted animal model, experimental autoimmune encephalomyelitis (EAE), the failure of autoreactive immune cells to undergo apoptosis is thought to contribute to CNS tissue damage and disease progression. Promoting apoptosis of myelin-reactive immune cells in diseases such as MS, may delay disease progression and decrease the frequency and severity of relapses. X-linked inhibitor of apoptosis (XIAP) is a potent anti-apoptotic protein that inhibits intrinsic, extrinsic, and c-Jun amino-terminal kinase mediated apoptosis and was the only member of the inhibitor of apoptosis (IAP) family upregulated in whole blood from EAE mice. Similar increases in XIAP were also observed in both peripheral and encephalitogenic T lymphocytes. Increased XIAP expression in T cells within areas of demyelination in the CNS suggests that XIAP may be enhancing cell survival and thereby contributing to disease pathology.     

Pathogenesis of prion diseases

Prion diseases are rare neurological disorders that may be of genetic or infectious origin, but most frequently occur sporadically in humans. Their outcome is invariably fatal. As the responsible pathogen, prions have been implicated. Prions are considered to be infectious particles that represent mainly, if not solely, an abnormal, protease-resistant isoform of a cellular protein, the prion protein or PrP^C. As in other neurodegenerative diseases, aggregates of misfolded protein conformers are deposited in the CNS of affected individuals. Pathogenesis of prion diseases comprises mainly two equally important, albeit essentially distinct, topics: first, the mode, spread, and amplification of infectivity in acquired disease, designated as peripheral pathogenesis. In this field, significant advances have implicated an essential role of lymphoid tissues for peripheral prion replication, before a likely neural spread to the CNS. The second is the central pathogenesis, dealing, in addition to spread and replication of prions within the CNS, with the mechanisms of nerve cell damage and death. Although important roles for microglial neurotoxicity, oxidative stress, and complement activation have been identified, we are far from complete understanding, and therapeutic applications in prion diseases still need to be developed.     

Species differences in immune-mediated CNS tissue injury and repair: A (neuro)inflammatory topic

Cells of the adaptive and innate immune systems in the brain parenchyma and in the meningeal spaces contribute to physiologic functions and disease states in the central nervous system (CNS). Animal studies have demonstrated the involvement of immune constituents, along with major histocompatibility complex (MHC) molecules, in neural development and rare genetic disorders (e.g., colony stimulating factor 1 receptor [CSF1R] deficiency). Genome wide association studies suggest a comparable role of the immune system in humans. Although the CNS can be the target of primary autoimmune disorders, no current experimental model captures all of the features of the most common human disorder placed in this category, multiple sclerosis (MS). Such features include spontaneous onset, environmental contributions, and a recurrent/progressive disease course in a genetically predisposed host. Numerous therapeutic interventions related to antigen and cytokine specific therapies have demonstrated effectiveness in experimental autoimmune encephalomyelitis (EAE), the animal model used to define principles underlying immune-mediated mechanisms in MS. Despite the similarities in the two diseases, most treatments used to ameliorate EAE have failed to translate to the human disease. As directly demonstrated in animal models and implicated by correlative studies in humans, adaptive and innate immune constituents within the systemic compartment and resident in the CNS contribute to the disease course of neurodegenerative and neurobehavioral disorders. The expanding knowledge of the molecular properties of glial cells provides increasing insights into species related variables. These variables affect glial bidirectional interactions with the immune system as well as their own production of “immune molecules” that mediate tissue injury and repair.     

Fc receptors and the common gamma-chain in experimental autoimmune encephalomyelitis

Fcgamma receptors (FcgammaRs), composed of a ligand-binding alpha-chain (FcRalpha) sometimes associated with the homodimeric, cell-signaling common gamma-chain (FcRgamma), comprise an important family of effector molecules linking humoral and cell-mediated adaptive immunity and regulating innate immunity. In peripheral autoimmune diseases, FcgammaRs contribute to inflammation and tissue damage through inappropriate activation of macrophages and neutrophils, release of cytokines and oxidants, and destruction of autoantibody-opsonized cells. In the central nervous system (CNS), the role of FcgammaRs in autoimmune disease such as multiple sclerosis (MS) remains largely unexplored despite extensive documentation of CNS-specific antibodies in cerebrospinal fluid and plaques. Several studies have now examined the role of FcgammaRs in experimental autoimmune encephalomyelitis (EAE), the animal model for MS, using mice genetically deficient in one or more FcgammaRs or in FcRgamma. These studies indicate that none of the FcgammaR alpha-chains are critical for EAE development and progression. In contrast, it is unequivocal that FcRgamma contributes to EAE, and surprisingly it seems that this effect is independent of FcgammaRs. Recent studies now indicate that FcRgamma expression in gammadelta T cells, most likely as a component of the TCR/CD3 signaling complex, is a critical requirement for EAE development. These studies support previous evidence implicating a pathogenic role for gammadelta T cells in EAE.     

Therapeutic role of inflammasome inhibitors in neurodegenerative disorders

Neuroinflammation, characterized by the activation of glial cells, is a hallmark in several neurological and neurodegenerative disorders. Inadequate inflammation cannot eliminate the infection of pathogens, while excessive or hyper-reactive inflammation can cause chronic or systemic inflammatory diseases affecting the central nervous system (CNS). In response to a brain injury or pathogen invasion, the pathogen recognition receptors (PRRs) expressed on glial cells are activated via binding to cellular damage-associated molecular patterns (DAMPs) or pathogen-associated molecular patterns (PAMPs). This subsequently leads to the activation of NOD (nucleotide-binding oligomerization domain)-like receptor proteins (NLRs). In neurodegenerative diseases such as HIV-1-associated neurocognitive disorders (HAND), Alzheimer’s disease (AD), Parkinson’s disease (PD), and multiple sclerosis (MS), chronic inflammation is a critical contributing factor for disease manifestation including pathogenesis. Emerging evidence points to the involvement of “inflammasomes”, especially the nucleotide-binding oligomerization domain, leucine-rich repeat, and pyrin domain-containing (NLRP) complex in the development of these diseases. The activated NLRP3 results in the proteolytic activation of caspase-1 that facilitates the cleavage of pro-IL-1β and the secretion of IL-1β and IL-18 proinflammatory cytokines. Accordingly, these and other seminal findings have led to the development of NLRP-targeting small-molecule therapeutics as possible treatment options for neurodegenerative disorders. In this review, we will discuss the new advances and evidence-based literature concerning the role of inflammasomes in neurodegenerative diseases, its role in the neurological repercussions of CNS chronic infection, and the examples of preclinical or clinically tested NLRP inhibitors as potential strategies for the treatment of chronic neurological diseases.     

Effect of copper on extracellular levels of key pro-inflammatory molecules in hypothalamic GN11 and primary neurons

Copper dyshomeostasis is responsible for the neurological symptoms observed in the genetically inherited copper-dependent disorders (e.g., Menkes’ and Wilson's diseases), but it has been also shown to have an important role in neurodegenerative diseases such as Alzheimer disease, prion diseases, Parkinson's disease and amyotrophic lateral sclerosis. It is widely accepted that increased extracellular copper levels contribute to neuronal pathogenic process by increasing the production of dangerous radical oxygen species, but the existence of other molecular mechanisms explaining copper neurotoxicity has not been investigated yet. By using a cellular model based on hypothalamic GN11 cultured neurons exposed to copper supplementation and by analysing the cell conditioned media, we try here to identify new molecular events explaining the association between extracellular copper accumulation and neuronal damages. We show here that increased extracellular copper levels produce a wide complex of alterations in the neuronal extracellular environment. In particular, copper affects the secretion of molecules involved in the protection of neurons against oxidative stress, such as cyclophilin A (CypA), or of molecules capable of shifting neuronal cells towards a pro-inflammatory state, such as IL-1α, IL-12, Rantes, neutrophil gelatinase-associated lipocalin (NGAL) and secreted protein acidic and rich in cysteine (SPARC). Copper pro-inflammatory properties have been confirmed by using primary neurons.     

Mesenchymal stem cells for the treatment of multiple sclerosis and other neurological diseases

The rationale for use of adult stem cells as a treatment for neurological diseases such as multiple sclerosis arose from the hope that they had the capacity to foster repair of the CNS through tissue integration and differentiation into neural cells. Evidence from preclinical studies suggested that mesenchymal stem cells (MSCs), a subset of adult progenitor cells, are an effective therapy in preclinical animal models of neurological diseases such as experimental autoimmune encephalomyelitis, a model for multiple sclerosis, and stroke. In experimental autoimmune encephalomyelitis, intravenous injection of MSCs ameliorates clinical course and decreases demyelination, immune infiltrates, and axonal loss. Surprisingly, these effects do not require full CNS engraftment by MSCs, but rely on the capacity of MSCs to inhibit pathogenic immune responses and release neuroprotective and pro-oligodendrogenic molecules favouring tissue repair. These results led to the conclusion that therapeutic use of MSCs should initially focus on individuals with multiple sclerosis and persistent inflammation. Small clinical studies in different neurological diseases have suggested that MSCs are safe, paving the road for larger phase 2 studies addressing the effect of MSCs on clinical outcomes and markers of disease activity.     

Special issue on microglia and chronic pain

This special issue of Experimental Neurology is devoted to the role of Microglia and Chronic Pain. Chronic pain affects 116 million people per year in the United States, which is more than heart disease, cancer, and diabetes combined. Nervous system trauma and disease are principal contributors to the establishment of chronic pain in people and in animal models. Central nervous system (CNS) injury or tumor development, peripheral nerve injury, multiple sclerosis, diabetes and many other neurological disruptions can serve as the instigating pathophysiolgical conditions that lead to chronic pain. Once considered to function solely as the phagocytotic cells of the CNS, more recent work has demonstrated that persistent activation of the microglial population may contribute to continued dysfunction including chronic pain. In the invited articles for this special issue on Microglia and Chronic Pain, we present evidence for the role of persistent microglial activation in chronic pain after peripheral and central nervous system injury, as well as in diabetic pain, post-herpetic neuralgia pain and related diseases. Collectively, the body of work indicates the importance of understanding the roles of microglial cells in chronic pain which will lead to targeted treatment to attenuate or alleviate chronic neuropathic pain syndromes.