BBA - Molecular Basis of Disease (v.1862, #3)

Microglia are unique cells in the central nervous system (CNS) and of particular importance for the development and homeostasis thereof. Recently, genetic manipulation of microglia in vivo has led to valuable insights about the origin of microglia and their behavior under steady-state conditions. Nevertheless, in pathological settings, their resting and surveillant nature can rapidly turn into either a beneficial or detrimental state significantly shaping disease courses. Therefore, it is tempting to manipulate these cells under pathological conditions in vivo and thereby decipher their contribution to the outcome of frequent neurological diseases such as Alzheimer's disease (AD), Parkinson's disease (PD), multiple sclerosis (MS) or amyotrophic lateral sclerosis (ALS). In this review, we will discuss which transgenic mouse models are currently available and can thus be used to genetically label microglia, to modulate their gene expression or to deplete them during development and under healthy conditions. Furthermore, the hallmarks of neurological disease models and how genetic manipulation of microglia will expand our knowledge about the underlying disease mechanisms will be discussed.This article is part of a Special Issue entitled: Neuro Inflammation edited by Helga E. de Vries and Markus Schwaninger.
Keywords: Microglia; Genetic manipulation; Targeting; Labeling; Yolk sac; CX3CR1;

The central nervous system (CNS) is a very unique system with multiple features that differentiate it from systemic tissues. One of the most captivating aspects of its distinctive nature is the presence of the blood brain barrier (BBB), which seals it from the periphery. Therefore, to preserve tissue homeostasis, the CNS has to rely heavily on resident cells such as microglia. These pivotal cells of the mononuclear lineage have important and dichotomous roles according to various neurological disorders. However, certain insults can overwhelm microglia as well as compromising the integrity of the BBB, thus allowing the infiltration of bone marrow-derived macrophages (BMDMs). The use of myeloablation and bone marrow transplantation allowed the generation of chimeric mice to study resident microglia and infiltrated BMDM separately. This breakthrough completely revolutionized the way we captured these 2 types of mononuclear phagocytic cells. We now realize that microglia and BMDM exhibit distinct features and appear to perform different tasks. Since these cells are central in several pathologies, it is crucial to use chimeric mice to analyze their functions and mechanisms to possibly harness them for therapeutic purpose. This review will shed light on the advent of this methodology and how it allowed deciphering the ontology of microglia and its maintenance during adulthood. We will also compare the different strategies used to perform myeloablation. Finally, we will discuss the landmark studies that used chimeric mice to characterize the roles of microglia and BMDM in several neurological disorders.This article is part of a Special Issue entitled: Neuro Inflammation edited by Helga E. de Vries and Markus Schwaninger.
Keywords: Bone marrow; Myeloid cells; Chimeric mice; Alzheimer's disease; Stroke; Multiple sclerosis;

The pathogenesis of neurological disorders such as multiple sclerosis (MS), amyotrophic lateral sclerosis (ALS) and Alzheimer's disease (AD) is multifactorial and incompletely understood. The development of therapies for these disorders of the central nervous system (CNS) is thus far very challenging. Neuroinflammation is one of the processes that contribute to the pathogenesis of CNS diseases, and therefore represents an important therapeutic target. Myeloid cells derived from the bone marrow are ideal candidates for cell therapy in the CNS as they are capable of targeting the brain and providing neuroprotective and anti-inflammatory effects. In this review, experimental and clinical evidence for the therapeutic potential of myeloid cells in neurological disorders will be discussed. This article is part of a Special Issue entitled: Neuro Inflammation edited by Helga E. de Vries and Markus Schwaninger.
Keywords: Neuroinflammation; Neurodegeneration; Stem cells; Bone marrow; Multiple sclerosis; Amyotrophic lateral sclerosis; Alzheimer’s disease;

Protective features of peripheral monocytes/macrophages in stroke by Michael Gliem; Markus Schwaninger; Sebastian Jander (329-338).
Hematogenous recruitment of monocytes and macrophages has traditionally been viewed as a harmful process causing exacerbation of brain injury after stroke. However, emerging findings suggest equally important protective features. Inflammatory monocytes are rapidly recruited to ischemic brain via a CCR2-dependent pathway and undergo secondary differentiation in the target tissue towards non-inflammatory macrophages, mediating neuroprotection and repair of the ischemic neurovascular unit. In contrast, independent recruitment of non-inflammatory monocytes via CX3CR1 does not occur. Thus, protective features of hematogenous macrophages mainly depend on initial CCR2-dependent cell recruitment. Under therapeutic considerations, specific modulation of monocyte-derived macrophages will therefore be more appropriate than non-selectively blocking their hematogenous recruitment. This article is part of a Special Issue entitled: Neuro Inflammation edited by Helga E. de Vries and Markus Schwaninger.
Keywords: Stroke; Inflammation; Macrophage; Monocyte; Neuroprotection; Repair;

Activation of microglia is a common denominator and a pathophysiological hallmark of the central nervous system (CNS) disorders. Damage or CNS disorders can trigger inflammatory responses in resident microglia and initiate a systemic immune system response. Although a repertoire of inflammatory responses differs in those diseases, there is a spectrum of transcriptionally activated genes that encode various mediators such as growth factors, inflammatory cytokines, chemokines, matrix metalloproteinases, enzymes producing lipid mediators, toxic molocules, all of which contribute to neuroinflammation. The initiation, progression and termination of inflammation requires global activation of gene expression, postranscriptional regulation, epigenetic modifications, changes in chromatin structure and these processes are tightly regulated by specific signaling pathways. This review focuses on the function of “master regulators” and epigenetic mechanisms in microglia activation during neuroinflammation. We review studies showing impact of epigenetic enzyme inhibitors on microglia activation in vitro and in vivo, and critically discuss potential of such molecules to prevent/moderate pathological events mediated by microglia under brain pathologies. This article is part of a Special Issue entitled: Neuro Inflammation edited by Helga E. de Vries and Markus Schwaninger.
Keywords: Neuroinflammation; Microglia; Signaling pathways; Transcription regulation; Epigenetics; HDAC inhibitors;

Dendritic cells in brain diseases by Peter Ludewig; Mattia Gallizioli; Xabier Urra; Sarah Behr; Vanessa H. Brait; Mathias Gelderblom; Tim Magnus; Anna M. Planas (352-367).
Dendritic cells (DCs) are professional antigen presenting cells that constantly survey the environment acting as sentinels of the immune system, including in the CNS. DCs are strategically located near the cerebrospinal fluid, but they can potentially migrate to draining cervical lymph nodes either triggering immunogenic T cell responses or displaying tolerogenic functions. Under physiological conditions, the presence of DCs in the brain parenchyma is minimal but their numbers increase in neuroinflammation. Although DCs belong to a distinct immune cell lineage, they show various phenotypes and share certain common markers with monocytes, macrophages, and microglia. All these cells can express major histocompatibility complex class II, and acquire similar morphologies hampering their precise identification. Neuroinflammation is increasingly recognized in many brain disorders; here we review the literature reporting DCs in the inflamed brain in disease conditions and corresponding animal models of multiple sclerosis, stroke, brain tumors, Alzheimer's disease, Parkinson's disease, and epilepsy. This article is part of a Special Issue entitled: Neuro Inflammation edited by Helga E. de Vries and Markus Schwaninger.
Keywords: Neuroinflammation; Stroke; Multiple sclerosis; Neurodegenerative diseases; Epilepsy; Brain tumor;

Myeloid derived suppressor cells in inflammatory conditions of the central nervous system by Carolina Melero-Jerez; María Cristina Ortega; Verónica Moliné-Velázquez; Diego Clemente (368-380).
The knowledge of the immune system elements and their relationship with other tissues, organs and systems are key approximations for the resolution of many immune-related disorders. The control of the immune response and/or its modulation from the pro-inflammatory to the anti-inflammatory response is being deeply studied in the field. In the last years, the study of myeloid-derived suppressor cells (MDSCs), a group of immature myeloid cells with a high suppressive activity on T cells has been extensively addressed in cancer. In contrast, their role in neuroimmune diseases is far from being totally understood. In this review, we will summarize data about MDSCs coming from the study of neuroinflammatory diseases in general and their potential role in multiple sclerosis, in order to introduce the putative use of this extraordinary promising cell type for future cell-based therapies.This article is part of a Special Issue entitled: Neuro Inflammation edited by Helga E. de Vries and Markus Schwaninger.
Keywords: Multiple sclerosis; Spinal cord injury; Autoimmunity; Neuroregeneration; Cell therapy; T cell suppression;

Genetic manipulation of brain endothelial cells in vivo by Julian C. Assmann; Jakob Körbelin; Markus Schwaninger (381-394).
Brain endothelial cells take center stage of the blood–brain barrier. They maintain homeostasis in the central nervous system (CNS) and are involved in the pathophysiology of many neurological diseases. So far investigations of their function have largely depended on in vitro models that lack the important impact of other cells and compartments in the mammalian CNS. A full evaluation of their role in a systemic context requires in vivo experiments. Here, we review recent innovative tools by which brain endothelial cells can be genetically manipulated in living organisms. We focus on conditional techniques for cell-specific deletion or overexpression of genes in mice. In a translational perspective, we summarize previous attempts to transduce brain endothelial cells in vivo using viral vectors or to transfect them with diverse methods. Available techniques provide the experimental basis for achieving a more refined picture of brain endothelial function in health and disease and to target this cell population for gene therapy. This article is part of a Special Issue entitled: Neuro Inflammation edited by Helga E. de Vries and Markus Schwaninger.
Keywords: Cre; Tet-on; Tet-off; Adeno-associated virus; Slco1c1; Liposomal transfection;

The plasminogen activation system in neuroinflammation by Anupriya Mehra; Carine Ali; Jérôme Parcq; Denis Vivien; Fabian Docagne (395-402).
The plasminogen activation (PA) system consists in a group of proteases and protease inhibitors regulating the activation of the zymogen plasminogen into its proteolytically active form, plasmin. Here, we give an update of the current knowledge about the role of the PA system on different aspects of neuroinflammation. These include modification in blood–brain barrier integrity, leukocyte diapedesis, removal of fibrin deposits in nervous tissues, microglial activation and neutrophil functions. Furthermore, we focus on the molecular mechanisms (some of them independent of plasmin generation and even of proteolysis) and target receptors responsible for these effects. The description of these mechanisms of action may help designing new therapeutic strategies targeting the expression, activity and molecular mediators of the PA system in neurological disorders involving neuroinflammatory processes. This article is part of a Special Issue entitled: Neuro Inflammation edited by Helga E. de Vries and Markus Schwaninger.
Keywords: Plasminogen activator; Blood-brain barrier; Leukocytes; Microglia;

Exosomes as new diagnostic tools in CNS diseases by Katja M. Kanninen; Nea Bister; Jari Koistinaho; Tarja Malm (403-410).
Exosomes are small extracellular vesicles that modulate important functions in physiology and under pathological conditions of the central nervous system (CNS). Exosomal contents, proteins, lipids and various RNA species, are altered during disease. The fact that exosomes are released into the blood stream from blood cells and endothelial cells responding to CNS diseases as well as from the brain and spinal cord, and that they express markers which allow their tracking to the cell of origin, makes the use of exosomes for diagnostic purposes and biomarker discovery particularly appealing. While the utilization of exosomes for diagnostics in diseases affecting the CNS are still in the early stages of discovery and development, it is expected that through further research and fervent development of protocols relating to isolation and purification the true potential of exosomes derived from the CNS will be harnessed for more effective clinical disease diagnosis. In this review we begin with a short introduction to the origin, composition and function of exosomes in the CNS. Next we discuss the current status of methodologies related to isolation and detection of CNS exosomes. We end with an account of exosomes in diagnostics and biomarker discovery, which focuses on three diseases of the CNS: Alzheimer's disease, multiple sclerosis, and stroke. This article is part of a Special Issue entitled: Neuro Inflammation edited by Helga E. de Vries and Markus Schwaninger.
Keywords: Stroke; Multiple sclerosis; Alzheimer's disease; MicroRNA; Exosomes;

Neuroinflammatory biomarkers: From stroke diagnosis and prognosis to therapy by Alba Simats; Teresa García-Berrocoso; Joan Montaner (411-424).
Stroke is the third leading cause of death in industrialized countries and one of the largest causes of permanent disability worldwide. Therapeutic options to fight stroke are still limited and the only approved drug is tissue-plasminogen activator (tPA) and/or mechanical thrombectomy. Post-stroke inflammation is well known to contribute to the expansion of the ischemic lesion, whereas its resolution stimulates tissue repair and neuroregeneration processes. As inflammation highly influences susceptibility of stroke patients to overcome the disease, there is an increasing need to develop new diagnostic, prognostic and therapeutic strategies for post-stroke inflammation. This review provides a brief overview of the contribution of the inflammatory mechanisms to the pathophysiology of stroke. It specially focuses on the role of inflammatory biomarkers to help predicting stroke patients' outcome since some of those biomarkers might turn out to be targets to be therapeutically altered overcoming the urgent need for the identification of potent drugs to modulate stroke-associated inflammation. This article is part of a Special Issue entitled: Neuro Inflammation edited by Helga E. de Vries and Markus Schwaninger.
Keywords: Stroke; Cerebral ischemia; Inflammation; Biomarkers; Therapeutic targets;

Imaging of neuroinflammation in Alzheimer's disease, multiple sclerosis and stroke: Recent developments in positron emission tomography by Bieneke Janssen; Danielle J. Vugts; Uta Funke; Ger T. Molenaar; Perry S. Kruijer; Bart N.M. van Berckel; Adriaan A. Lammertsma; Albert D. Windhorst (425-441).
Neuroinflammation is thought to play a pivotal role in many diseases affecting the brain, including Alzheimer's disease, multiple sclerosis and stroke. Neuroinflammation is characterised predominantly by microglial activation, which can be visualised using positron emission tomography (PET). Traditionally, translocator protein 18 kDa (TSPO) is the target for imaging of neuroinflammation using PET. In this review, recent preclinical and clinical research using PET in Alzheimer's disease, multiple sclerosis and stroke is summarised. In addition, new molecular targets for imaging of neuroinflammation, such as monoamine oxidases, adenosine receptors and cannabinoid receptor type 2, are discussed. This article is part of a Special Issue entitled: Neuro Inflammation edited by Helga E. de Vries and Markus Schwaninger.
Keywords: Positron emission tomography; Neuroinflammation; Microglia; Alzheimer's disease; Multiple sclerosis; Stroke;

Cerebrospinal fluid (CSF) circulation and turnover provides a sink for the elimination of solutes from the brain interstitium, serving an important homeostatic role for the function of the central nervous system. Disruption of normal CSF circulation and turnover is believed to contribute to the development of many diseases, including neurodegenerative conditions such as Alzheimer's disease, ischemic and traumatic brain injury, and neuroinflammatory conditions such as multiple sclerosis. Recent insights into CSF biology suggesting that CSF and interstitial fluid exchange along a brain-wide network of perivascular spaces termed the ‘glymphatic’ system suggest that CSF circulation may interact intimately with glial and vascular function to regulate basic aspects of brain function. Dysfunction within this glial vascular network, which is a feature of the aging and injured brain, is a potentially critical link between brain injury, neuroinflammation and the development of chronic neurodegeneration. Ongoing research within this field may provide a powerful new framework for understanding the common links between neurodegenerative, neurovascular and neuroinflammatory disease, in addition to providing potentially novel therapeutic targets for these conditions. This article is part of a Special Issue entitled: Neuro Inflammation edited by Helga E. de Vries and Markus Schwaninger.
Keywords: CSF; Glymphatic; Alzheimer's disease; Multiple sclerosis; Cerebral ischemia; Diurnal variation;

Molecular alterations of the blood–brain barrier under inflammatory conditions: The role of endothelial to mesenchymal transition by Claudio Derada Troletti; Paul de Goede; Alwin Kamermans; Helga E. de Vries (452-460).
Impairment of the protective properties of the blood–brain barrier (BBB) is a key event during numerous neurological diseases, including multiple sclerosis (MS). Under these pathological conditions, the specialized brain endothelial cells (BECs) lose their protective function leading to neuroinflammation and neurodegeneration. To date, underlying mechanisms for this loss of function remain unclear. Endothelial to mesenchymal transition (EndoMT) is a dynamic process by which endothelial cells (ECs) dedifferentiate into mesenchymal cells and as a result lose their specific phenotype and function. As yet, little is known about the involvement of this process in the impaired function of the BECs under pathological conditions such as MS. Interestingly, several signaling pathways that can induce EndoMT are also involved in different central nervous system (CNS) pathologies associated with BBB dysfunction. In this review, we first discuss the structure and function of the BBB highlighting the changes that occur during MS. Next, we will summarize recent findings on the pathways underlying EndoMT, and finally, we will discuss the potential role of EndoMT during BBB dysfunction in neurological disorders. This article is part of a Special Issue entitled: Neuro Inflammation edited by Helga E. de Vries and Markus Schwaninger.
Keywords: Blood–brain barrier; Multiple sclerosis; Endothelial to mesenchymal transition; Epithelial to mesenchymal transition; Neuroinflammation; Snail;

Immune cell trafficking across the barriers of the central nervous system in multiple sclerosis and stroke by Melissa A. Lopes Pinheiro; Gijs Kooij; Mark R. Mizee; Alwin Kamermans; Gaby Enzmann; Ruth Lyck; Markus Schwaninger; Britta Engelhardt; Helga E. de Vries (461-471).
Each year about 650,000 Europeans die from stroke and a similar number lives with the sequelae of multiple sclerosis (MS). Stroke and MS differ in their etiology. Although cause and likewise clinical presentation set the two diseases apart, they share common downstream mechanisms that lead to damage and recovery. Demyelination and axonal injury are characteristics of MS but are also observed in stroke. Conversely, hallmarks of stroke, such as vascular impairment and neurodegeneration, are found in MS. However, the most conspicuous common feature is the marked neuroinflammatory response, marked by glia cell activation and immune cell influx.In MS and stroke the blood–brain barrier is disrupted allowing bone marrow-derived macrophages to invade the brain in support of the resident microglia. In addition, there is a massive invasion of auto-reactive T-cells into the brain of patients with MS. Though less pronounced a similar phenomenon is also found in ischemic lesions. Not surprisingly, the two diseases also resemble each other at the level of gene expression and the biosynthesis of other proinflammatory mediators.While MS has traditionally been considered to be an autoimmune neuroinflammatory disorder, the role of inflammation for cerebral ischemia has only been recognized later. In the case of MS the long track record as neuroinflammatory disease has paid off with respect to treatment options. There are now about a dozen of approved drugs for the treatment of MS that specifically target neuroinflammation by modulating the immune system. Interestingly, experimental work demonstrated that drugs that are in routine use to mitigate neuroinflammation in MS may also work in stroke models. Examples include Fingolimod, glatiramer acetate, and antibodies blocking the leukocyte integrin VLA-4. Moreover, therapeutic strategies that were discovered in experimental autoimmune encephalomyelitis (EAE), the animal model of MS, turned out to be also effective in experimental stroke models. This suggests that previous achievements in MS research may be relevant for stroke. Interestingly, the converse is equally true. Concepts on the neurovascular unit that were developed in a stroke context turned out to be applicable to neuroinflammatory research in MS. Examples include work on the important role of the vascular basement membrane and the BBB for the invasion of immune cells into the brain. Furthermore, tissue plasminogen activator (tPA), the only established drug treatment in acute stroke, modulates the pathogenesis of MS. Endogenous tPA is released from endothelium and astroglia and acts on the BBB, microglia and other neuroinflammatory cells. Thus, the vascular perspective of stroke research provides important input into the mechanisms on how endothelial cells and the BBB regulate inflammation in MS, particularly the invasion of immune cells into the CNS. In the current review we will first discuss pathogenesis of both diseases and current treatment regimens and will provide a detailed overview on pathways of immune cell migration across the barriers of the CNS and the role of activated astrocytes in this process. This article is part of a Special Issue entitled: Neuro Inflammation edited by Helga E. de Vries and Markus Schwaninger.
Keywords: Blood–brain barrier; Immune cell trafficking; Multiple sclerosis; Stroke; Astrocyte;

Glial influences on BBB functions and molecular players in immune cell trafficking by Marc-André Lécuyer; Hania Kebir; Alexandre Prat (472-482).
The blood–brain barrier (BBB) constitutes an elaborate structure formed by specialized capillary endothelial cells, which together with pericytes and perivascular glial cells regulates the exchanges between the central nervous system (CNS) and the periphery. Intricate interactions between the different cellular constituents of the BBB are crucial in establishing a functional BBB and maintaining the delicate homeostasis of the CNS microenvironment. In this review, we discuss the role of astrocytes and microglia in inducing and maintaining barrier properties under physiological conditions as well as their involvement during neuroinflammatory pathologies. This article is part of a Special Issue entitled: Neuro Inflammation edited by Helga E. de Vries and Markus Schwaninger.
Keywords: Blood–brain barrier (BBB); Neurovascular unit (NVU); Neuroinflammation; Glial cells; Astrocytes; Endothelial cells; Microglia; Multiple sclerosis (MS); Experimental autoimmune encephalomyelitis (EAE); Tight junction molecules; IL-17;

Reactive gliosis in the pathogenesis of CNS diseases by Milos Pekny; Marcela Pekna (483-491).
Astrocytes maintain the homeostasis of the central nervous system (CNS) by e.g. recycling of neurotransmitters and providing nutrients to neurons. Astrocytes function also as key regulators of synaptic plasticity and adult neurogenesis. Any insult to the CNS tissue triggers a range of molecular, morphological and functional changes of astrocytes jointly called reactive (astro)gliosis. Reactive (astro)gliosis is highly heterogeneous and also context-dependent process that aims at the restoration of homeostasis and limits tissue damage. However, under some circumstances, dysfunctional (astro)gliosis can become detrimental and inhibit adaptive neural plasticity mechanisms needed for functional recovery. Understanding the multifaceted and context-specific functions of astrocytes will contribute to the development of novel therapeutic strategies that, when applied at the right time-point, will improve the outcome of diverse neurological disorders. This article is part of a Special Issue entitled: Neuro Inflammation edited by Helga E. de Vries and Markus Schwaninger.

Glial cell response after aneurysmal subarachnoid hemorrhage — Functional consequences and clinical implications by Bart J. van Dijk; Mervyn D.I. Vergouwen; Myrna M. Kelfkens; Gabriel J.E. Rinkel; Elly M. Hol (492-505).
Glial cells, both astrocytes and microglia, respond to neurodegenerative processes and to brain damage by a process called reactive gliosis. This response is highly context dependent, varies from mild to severe, and can be protective or detrimental for neural functioning. In patients with a subarachnoid hemorrhage from a ruptured aneurysm, the acute glial response is important to restrict the initial damage. Patients who survive the hemorrhage and early brain injury, often suffer from delayed cerebral ischemia or persisting cognitive impairment. Glia emerge as versatile cells that can modulate synapses and can control the microcirculatory blood flow in the brain. Therefore, a sustained activation of glial cells can affect normal brain functioning. Here we review the current literature on the glial response induced by aneurysmal subarachnoid hemorrhage in humans and in animal models. We discuss how reactive gliosis can affect brain functioning and how it may contribute to early brain injury, delayed cerebral ischemia and cognitive impairment after aneurysmal subarachnoid hemorrhage. This article is part of a Special Issue entitled: Neuro Inflammation edited by Helga E. de Vries and Markus Schwaninger.
Keywords: Stroke; Reactive gliosis; Astrocytes; Microglia; Cognitive impairment; Subarachnoid hemorrhage;

Oxidative injury plays a major role in brain damage in many age-related human brain diseases and is particularly pronounced in the progressive stage of multiple sclerosis. In the latter it is related to the chronic inflammatory process and is amplified by brain changes due to aging and accumulation of disease burden. It induces demyelination and neurodegeneration by direct oxidation of lipids, proteins and DNA as well as by the induction of mitochondrial injury, which results in energy deficiency and further amplification of oxygen radical production. It affects neurons and all types of glia cells, but neurons and oligodendrocytes are most vulnerable. Difference in the susceptibility for oxidative injury between different cellular components of the central nervous system appears to be due to cell type specific differences in anti-oxidant defense mechanisms, iron loading, cellular susceptibility to apoptosis induction and energy demand. This article is part of a Special Issue entitled: Neuro Inflammation edited by Helga E. de Vries and Markus Schwaninger.
Keywords: Oxidative stress; Multiple sclerosis; Neurons; Oligodendrocytes; Astrocytes; Microglia;