Roles of NG2 glial cells in diseases of the central nervous system

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

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

Role of Glial Cells in Neuronal Function, Mood Disorders, and Drug Addiction

Mood disorders and substance use disorder (SUD) are of immense medical and social concern. Although significant progress on neuronal involvement in mood and reward circuitries has been achieved, it is only relatively recently that the role of glia in these disorders has attracted attention. Detailed understanding of the glial functions in these devastating diseases could offer novel interventions. Here, following a brief review of circuitries involved in mood regulation and reward perception, the specific contributions of neurotrophic factors, neuroinflammation, and gut microbiota to these diseases are highlighted. In this context, the role of specific glial cells (e.g., microglia, astroglia, oligodendrocytes, and synantocytes) on phenotypic manifestation of mood disorders or SUD are emphasized. In addition, use of this knowledge in the potential development of novel therapeutics is touched upon.

Roles of Epigenetics and Glial Cells in Drug-Induced Autism Spectrum Disorder

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by severe deficits in social communication and interaction, repetitive movements, abnormal focusing on objects, or activity that can significantly affect the quality of life of the afflicted. Neuronal and glial cells have been implicated. It has a genetic component but can also be triggered by environmental factors or drugs. For example, prenatal exposure to valproic acid or acetaminophen, or ingestion of propionic acid, can increase the risk of ASD. Recently, epigenetic influences on ASD have come to the forefront of investigations on the etiology, prevention, and treatment of this disorder. Epigenetics refers to DNA modifications that alter gene expression without making any changes to the DNA sequence. Although an increasing number of pharmaceuticals and environmental chemicals are being implicated in the etiology of ASD, here, we specifically focus on the molecular influences of the abovementioned chemicals on epigenetic alterations in neuronal and glial cells and their potential connection to ASD. We conclude that a better understanding of these phenomena can lead to more effective interventions in ASD.

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

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

The emerging role of the chondroitin sulfate proteoglycan family in neurodegenerative diseases

Chondroitin sulfate (CS) is a kind of linear polysaccharide that is covalently linked to proteins to form proteoglycans. Chondroitin sulfate proteoglycans (CSPGs) consist of a core protein, with one or more CS chains covalently attached. CSPGs are precisely regulated and they exert a variety of physiological functions by binding to adhesion molecules and growth factors. Widely distributed in the nervous system in human body, CSPGs contribute to the major component of extracellular matrix (ECM), where they play an important role in the development and maturation of the nervous system, as well as in the pathophysiological response to damage to the central nervous system (CNS). While there are more than 30 types of CSPGs, this review covers the roles of the most important ones, including versican, aggrecan, neurocan and NG2 in the pathogenesis of neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis and multiple sclerosis. The updated reports of the treatment of neurodegenerative diseases are involving CSPGs.

Spatiotemporal Profile and Morphological Changes of NG2 Glia in the CA1 Region of the Rat Hippocampus after Transient Forebrain Ischemia

Neuron-glial antigen-2 (NG2) glia undergo proliferation and morphological changes following brain insults. Here, we show that NG2 glia is activated in a characteristic time- and layer-specific manner in the ischemia-vulnerable CA1 region of the rat hippocampus. Resting NG2 glia of the pyramidal cell layer (somatic region) shared morphological features with those of the neighboring dendritic stratum radiatum. During the postischemic period, reactive NG2 glia of the pyramidal cell layer exhibited shortened, scarcely branched processes, while those of the stratum radiatum had multiple branching processes with their arborization being almost indiscernible 7~14 days after reperfusion. Immunoelectron microscopy demonstrated that NG2 immunoreactivity was specifically associated with the plasma membrane and the adjacent extracellular matrix of NG2 glia in the stratum radiatum at 14 days. NG2 glia also exhibited differences in their numbers and proliferation profiles in the two examined hippocampal strata after ischemia. In addition, induced NG2 expression in activated microglia/macrophages exhibited a characteristic strata-dependent pattern in the ischemic CA1 hippocampus. NG2 induction was prominent in macrophage-like phenotypes which were predominantly localized in the pyramidal cell layer, compared with activated stellate microglial cells in the stratum radiatum. Thus, our data demonstrate that activation of NG2 glia and the induction of NG2 expression in activated microglia/macrophages occur in a distinct time- and layer-specific manner in the ischemic CA1 hippocampus. These characteristic profiles of reactive NG2 glia could be secondary to the degeneration processes occurring in the cell bodies or dendritic domains of hippocampal CA1 pyramidal neurons after ischemic insults.

Neurodegeneration and Neuro-Regeneration-Alzheimer's Disease and Stem Cell Therapy

Aging causes many changes in the human body, and is a high risk for various diseases. Dementia, a common age-related disease, is a clinical disorder triggered by neurodegeneration. Brain damage caused by neuronal death leads to cognitive decline, memory loss, learning inabilities and mood changes. Numerous disease conditions may cause dementia; however, the most common one is Alzheimer’s disease (AD), a futile and yet untreatable illness. Adult neurogenesis carries the potential of brain self-repair by an endogenous formation of newly-born neurons in the adult brain; however it also declines with age. Strategies to improve the symptoms of aging and age-related diseases have included different means to stimulate neurogenesis, both pharmacologically and naturally. Finally, the regulatory mechanisms of stem cells neurogenesis or a functional integration of newborn neurons have been explored to provide the basis for grafted stem cell therapy. This review aims to provide an overview of AD pathology of different neural and glial cell types and summarizes current strategies of experimental stem cell treatments and their putative future use in clinical settings.

The Delivery Challenge in Neurodegenerative Disorders: The Nanoparticles Role in Alzheimer's Disease Therapeutics and Diagnostics

Alzheimer’s disease (AD) is one of the main causes of disability and dependency among elderly people. AD is a neurodegenerative disorder characterized by a progressive and irreversible cognitive impairment, whose etiology is unclear because of the complex molecular mechanisms involved in its pathophysiology. A global view of the AD pathophysiology is described in order to understand the need for an effective treatment and why nanoparticles (NPs) could be an important weapon against neurodegenerative diseases by solving the general problem of poor delivery into the central nervous system (CNS) for many drugs. Drug delivery into the CNS is one of the most challenging objectives in pharmaceutical design, due to the limited access to the CNS imposed by the blood-brain barrier (BBB). The purpose of this review is to present a comprehensive overview of the use of NPs as delivery systems for therapeutic and diagnostic purposes in models of AD.

The Biology of Glial Cells and Their Complex Roles in Alzheimer's Disease: New Opportunities in Therapy

Even though Alzheimer's disease (AD) is of significant interest to the scientific community, its pathogenesis is very complicated and not well-understood. A great deal of progress has been made in AD research recently and with the advent of these new insights more therapeutic benefits may be identified that could help patients around the world. Much of the research in AD thus far has been very neuron-oriented; however, recent studies suggest that glial cells, i.e., microglia, astrocytes, oligodendrocytes, and oligodendrocyte progenitor cells (NG2 glia), are linked to the pathogenesis of AD and may offer several potential therapeutic targets against AD. In addition to a number of other functions, glial cells are responsible for maintaining homeostasis (i.e., concentration of ions, neurotransmitters, etc.) within the central nervous system (CNS) and are crucial to the structural integrity of neurons. This review explores the: (i) role of glial cells in AD pathogenesis; (ii) complex functionalities of the components involved; and (iii) potential therapeutic targets that could eventually lead to a better quality of life for AD patients.

In vivo spatiotemporal dynamics of NG2 glia activity caused by neural electrode implantation

Neural interface technology provides direct sampling and analysis of electrical and chemical events in the brain in order to better understand neuronal function and treat neurodegenerative disease. However, intracortical electrodes experience inflammatory reactions that reduce long-term stability and functionality and are understood to be facilitated by activated microglia and astrocytes. Emerging studies have identified another cell type that participates in the formation of a high-impedance glial scar following brain injury; the oligodendrocyte precursor cell (OPC). These cells maintain functional synapses with neurons and are a crucial source of neurotrophic support. Following injury, OPCs migrate toward areas of tissue injury over the course of days, similar to activated microglia. The delayed time course implicates these OPCs as key components in the formation of the outer layers of the glial scar around the implant. In vivo two-photon laser scanning microscopy (TPLSM) was employed to observe fluorescently-labeled OPC and microglia reactivity up to 72 h following probe insertion. OPCs initiated extension of cellular processes (2.5 ± 0.4 μm h-1) and cell body migration (1.6 ± 0.3 μm h-1) toward the probe beginning 12 h after insertion. By 72 h, OPCs became activated at a radius of about 190.3 μm away from the probe surface. This study characterized the early spatiotemporal dynamics of OPCs involved in the inflammatory response induced by microelectrode insertion. OPCs are key mediators of tissue health and are understood to have multiple fate potentials. Detailed spatiotemporal characterization of glial behavior under pathological conditions may allow identification of alternative intervention targets for mitigating the formation of a glial scar and subsequent neurodegeneration that debilitates chronic neural interfaces.

Morphological characterization of NG2 glia and their association with neuroglial cells in the 3-nitropropionic acid-lesioned striatum of rat

Our aim was to examine the spatiotemporal profiles and phenotypic characteristics of neuron-glia antigen 2 (NG2) glia and their associations with neuroglial cells in striatal lesions due to the mitochondrial toxin 3-nitropropionic acid (3-NP). In control striatum, weak NG2 immunoreactivity was restricted to resting NG2 glia with thin processes, but prominent NG2 expression was noted on activated microglia/macrophages, and reactive NG2 glia in the lesion core after 3-NP injection. Activation of NG2 glia, including enhanced proliferation and morphological changes, had a close spatiotemporal relationship with infiltration of activated microglia into the lesion core. Thick and highly branched processes of reactive NG2 glia formed a cellular network in the astrocyte-free lesion core and primarily surrounded developing cavities 2–4 weeks post-lesion. NG2 glia became associated with astrocytes in the lesion core and the border of cavities over the chronic interval of 4–8 weeks. Immunoelectron microscopy indicated that reactive NG2 glia had large euchromatic nuclei with prominent nucleoli and thick and branched processes that ramified distally. Thus, our data provide detailed information regarding the morphologies of NG2 glia in the lesion core, and support the link between transformation of NG2 glia to the reactive form and microglial activation/recruitment in response to brain insults.

Glia-neuron interactions in neurological diseases: Testing non-cell autonomy in a dish

For the past century, research on neurological disorders has largely focused on the most prominently affected cell types - the neurons. However, with increasing knowledge of the diverse physiological functions of glial cells, their impact on these diseases has become more evident. Thus, many conditions appear to have more complex origins than initially thought. Since neurological pathologies are often sporadic with unknown etiology, animal models are difficult to create and might only reflect a small portion of patients in which a mutation in a gene has been identified. Therefore, reliable in vitro systems to studying these disorders are urgently needed. They might be a pre-requisite for improving our understanding of the disease mechanisms as well as for the development of potential new therapies. In this review, we will briefly summarize the function of different glial cell types in the healthy central nervous system (CNS) and outline their implication in the development or progression of neurological conditions. We will then describe different types of culture systems to model non-cell autonomous interactions in vitro and evaluate advantages and disadvantages. This article is part of a Special Issue entitled SI: Exploiting human neurons.

CellMapper: rapid and accurate inference of gene expression in difficult-to-isolate cell types

We present a sensitive approach to predict genes expressed selectively in specific cell types, by searching publicly available expression data for genes with a similar expression profile to known cell-specific markers. Our method, CellMapper, strongly outperforms previous computational algorithms to predict cell type-specific expression, especially for rare and difficult-to-isolate cell types. Furthermore, CellMapper makes accurate predictions for human brain cell types that have never been isolated, and can be rapidly applied to diverse cell types from many tissues. We demonstrate a clinically relevant application to prioritize candidate genes in disease susceptibility loci identified by GWAS.

Juvenile striatal white matter is resistant to ischemia-induced damage

White matter injury following ischemic stroke is a major cause of functional disability. Injury to both myelinated axons and oligodendrocytes, the myelin producing cells in the central nervous system, occurs in experimental models of ischemic stroke. Age-related changes in white matter vulnerability to ischemia have been extensively studied and suggest that both the perinatal and the aged periods are times of increased white matter vulnerability. However, sensitivity of white matter following stroke in the juvenile brain has not been evaluated. Interestingly, the late pediatric period is an important developmental stage, as it is the time of maximal myelination. The current study demonstrates that neurons in late pediatric/juvenile striatum are vulnerable to ischemic damage, with neuronal injury being comparable in juvenile and adult mice following ischemia. By contrast, actively myelinating striatal oligodendrocytes in the juvenile brain are resistant to ischemia, whereas adult oligodendrocytes are quite sensitive. As a result, myelin sheaths are remarkably intact and axons survive well in the injured striatum of juvenile mice. In addition to relative resistance of juvenile white matter, other glial responses were very different in juvenile and adult mice following cerebral ischemia, including differences in astrogliosis, fibrosis, NG2-cell reactivity, and vascular integrity. Together, these responses lead to long-term preservation of brain parenchyma in juvenile mice, compared to severe tissue loss and scarring in adult mice. Overall, the current study suggests that equivalent ischemic insults may result in less functional deficit in children compared to adults and an environment more conducive to long-term recovery. GLIA 2016;64:1972-1986.

Evaluation of autologous tissue sources for the isolation of endothelial cells and adipose tissue-derived mesenchymal stem cells to pre-vascularize tissue-engineered vascular grafts

Currently used synthetic vascular grafts bear a high infection risk due to insufficient microvascularization of the graft wall disabling the infiltration of immune cells. Tissue-engineered grafts with a functional pre-vascularization thus would be desirable. However, autologous tissue sources for capillary forming cells need to be evaluated. Here, peripheral blood outgrowth endothelial cells (PB-OEC) from 17 healthy donors and pericyte-like mesenchymal stem cells derived from adipose tissue (ASC) of 17 patients scheduled for visceral surgery were characterized and investigated regarding their ability to form capillary-like networks in plasma-derived fibrin gels. To obtain proliferating PB-OEC with endothelial cell-specific properties (CD31-, VE-cadherin-expression, ac-LDL uptake and three-dimensional (3D)-tube formation in fibrin gels) both enrichment of CD34

Protective Properties of Neural Extracellular Matrix

The extracellular matrix (ECM) of the central nervous system (CNS) occupies a large part of the neural tissue. It serves a variety of functions ranging from support of cell migration and regulating synaptic transmission and plasticity to the active modulation of the neural tissue after injury. In addition, evidence for neuroprotective properties of ECM components has accumulated more recently. In contrast to other connective tissues, the central nervous ECM is mainly composed of glycosaminoglycans, which can be present unbound in the form of hyaluronan or bound to proteins, thus forming proteoglycans. A subtype of this molecular family are the chondroitin sulphate proteoglycans (CSPGs), which are composed of a core protein that carries at least one covalently bound glycosaminoglycan side chain with a certain degree of sulphation. Several studies could show neuroprotective features of CSPGs against excitotoxicity, amyloid-ß toxicity, or oxidative stress. Recently, we could provide evidence for a neuroprotective function of a specialized form of ECM, the so-called perineuronal net ensheathing a subtype of neurons. Here, we will give an overview on recently emerging aspects of neuroprotective properties of CSPGs and perineuronal nets that might be relevant for our understanding on the distribution and progression of brain pathology and future perspectives toward modifying neurodegenerative diseases.

Oligodendrocyte Lineage Cells in Chronic Demyelination of Multiple Sclerosis Optic Nerve

Reports that chronically demyelinated multiple sclerosis brain and spinal cord lesions contained immature oligodendrocyte lineage cells have generated major interest aimed at the potential for promotion of endogenous repair. Despite the prominence of the optic nerve as a lesion site and its importance in clinical disease assessment, no detailed studies of multiple sclerosis‐affected optic nerve exist. This study aims to provide insight into the cellular pathology of chronic demyelination in multiple sclerosis through direct morphological and immunohistochemical analysis of optic nerve in conjunction with observations from an experimental cat optic nerve model of successful remyelination. Myelin staining was followed by immunohistochemistry to differentially label neuroglia. Digitally immortalized sections were then analyzed to generate quantification data and antigenic phenotypes including maturational stages within the oligodendrocyte lineage. It was found that some chronically demyelinated multiple sclerosis optic nerve lesions contained oligodendroglial cells and that heterogeneity existed in the presence of myelin sheaths, oligodendrocyte maturational stages and extent of axonal investment. The findings advance our understanding of oligodendrocyte activity in chronically demyelinated human optic nerve and may have implications for studies aimed at enhancement of endogenous repair in multiple sclerosis.

Diosgenin promotes oligodendrocyte progenitor cell differentiation through estrogen receptor-mediated ERK1/2 activation to accelerate remyelination

Differentiation of oligodendrocyte progenitor cells (OPCs) into mature oligodendrocytes is a prerequisite for remyelination after demyelination, and impairment of this process is suggested to be a major reason for remyelination failure. Diosgenin, a plant-derived steroid, has been implicated for therapeutic use in many diseases, but little is known about its effect on the central nervous system. In this study, using a purified rat OPC culture model, we show that diosgenin significantly and specifically promotes OPC differentiation without affecting the viability, proliferation, or migration of OPC. Interestingly, the effect of diosgenin can be blocked by estrogen receptor (ER) antagonist ICI 182780 but not by glucocorticoid and progesterone receptor antagonist RU38486, nor by mineralocorticoid receptor antagonist spirolactone. Moreover, it is revealed that both ER-alpha and ER-beta are expressed in OPC, and diosgenin can activate the extracellular signal-regulated kinase 1/2 (ERK1/2) in OPC via ER. The pro-differentiation effect of diosgenin can also be obstructed by the ERK inhibitor PD98059. Furthermore, in the cuprizone-induced demyelination model, it is demonstrated that diosgenin administration significantly accelerates/enhances remyelination as detected by Luxol fast blue stain, MBP immunohistochemistry and real time RT-PCR. Diosgenin also increases the number of mature oligodendrocytes in the corpus callosum while it does not affect the number of OPCs. Taking together, our results suggest that diosgenin promotes the differentiation of OPC into mature oligodendrocyte through an ER-mediated ERK1/2 activation pathway to accelerate remyelination, which implicates a novel therapeutic usage of this steroidal natural product in demyelinating diseases such as multiple sclerosis (MS).