643396

research-article2016

MSJ0010.1177/1352458516643396Multiple Sclerosis JournalSK Ludwin, VTS Rao

MULTIPLE SCLEROSIS MSJ JOURNAL

Topical Review

Astrocytes in multiple sclerosis Samuel K Ludwin, Vijayaraghava TS Rao, Craig S Moore and Jack P Antel

Abstract:  Recent experimental and clinical studies on astrocytes are unraveling the capabilities of these multi-functional cells in normal homeostasis, and in central nervous system (CNS) disease. This review focuses on understanding their behavior in all aspects of the initiation, evolution, and resolution of the multiple sclerosis (MS) lesion. Astrocytes display remarkable flexibility and variability of their physical structure and biochemical output, each aspect finely tuned to the specific stage and location of the disease, participating in both pathogenic and beneficial changes seen in acute and progressive forms. As examples, chemo-attractive or repulsive molecules may facilitate the entry of destructive immune cells but may also aid in the recruitment of oligodendrocyte precursors, essential for repair. Pro-inflammatory cytokines may attack pathogenic cells and also destroy normal oligodendrocytes, myelin, and axons. Protective trophic factors may also open the blood–brain barrier and modulate the extracellular matrix to favor recruitment and persistence of CNS-specific immune cells. A chronic glial scar may confer structural support following tissue loss and inhibit ingress of further noxious insults and also inhibit migration of reparative cells and molecules into the damaged tissue. Continual study into these processes offers the therapeutic opportunities to enhance the beneficial capabilities of these cells while limiting their destructive effects.

Multiple Sclerosis Journal 2016, Vol. 22(9) 1114­–1124 DOI: 10.1177/ 1352458516643396 © The Author(s), 2016. Reprints and permissions: http://www.sagepub.co.uk/ journalsPermissions.nav

Keywords:  Astrocytes, multiple sclerosis, reactivity, immunopathology, neuroprotection Date received: 25 February 2016; revised: 9 March 2016; accepted: 9 March 2016

Introduction Although pathological astrocytes were described in the very earliest of the reports on multiple sclerosis (MS) in the 19th century, and were even considered to be the primary cellular target or agent of the disease by the great Charcot,1 for most of the next 120 years or so, astrocytes were considered to be passive reactors to the immune storm being wreaked on the brain. However, there has been a surge of interest in the study of astrocytes in health and disease. In contrast to earlier suggestions that these are relatively stable and fixed cells, providing mainly structural and metabolic support and contribution to the blood–brain barrier (BBB), it has now become abundantly clear that they are dynamic, carry out a large number of diverse functions, and in fact are vital for the maintenance of the homeostasis of the normal brain. These cells play a critical role in diverse disease processes and are responsible for both protective and deleterious events occurring during the course of disease (see below). Although many features are common to all astrocytes, varying topographical locations, or differing pathological insults, may lead to corresponding specific functional reactions.2,3

The focus of this review will be the description of the astrocytic changes occurring in MS; the interpretation of these changes draw on observations derived from both clinical and experimental studies. Much has been written on immune cells and oligodendrocytes in MS, but the astrocyte has generally been regarded as a secondary player in the disease and has consequently received less attention. MS, being a complex process with confounding phenomena and epiphenomena, displays correspondingly confusing responses in the astrocytes. To a large degree, many aspects of astrocytic reactivity have developed through evolution to protectively respond to injury, whether by pro- or anti-inflammatory responses, chemo-attractant or repellant production, or glial scar formation. Some of these are exquisitely and appropriately timed according to circumstances of location, etiology, and severity of the disease process and therefore, therapeutic intervention to modify these responses must take into consideration the risk of interfering with the body’s natural defense systems. Basic issues raised in this review include the following: Is the astrocyte itself a disease target, or merely a reactor to the effects of the disease? What role does

Correspondence to: SK Ludwin Department of Pathology and Molecular Medicine, Queen’s University, Kingston, ON K7L 4V1, Canada. [email protected]. ca Samuel K Ludwin Neuroimmunology, Montreal Neurological Institute, McGill University, Montreal, QC, Canada/ Department of Pathology and Molecular Medicine, Queen’s University, Kingston, ON, Canada Vijayaraghava TS Rao Jack P Antel Neuroimmunology, Montreal Neurological Institute, McGill University, Montreal, QC, Canada Craig S Moore Division of BioMedical Sciences, Faculty of Medicine, Memorial University of Newfoundland, St. John’s, NL, Canada

1114 http://msj.sagepub.com

SK Ludwin, VTS Rao et al.

Figure 1.  Normal astrocytes. Cortical astrocytes are depicted with (a) a Golgi silver stain and (b) an anti-GFAP immunestain. The cell bodies are rounded, the cytoplasm scanty, and the cell processes spidery and lacy. The relationship of astrocyte processes to the blood vessels (BV) is well seen in (a).

the astrocyte play in causing tissue damage, or in elimination of noxious effectors? What is its role in neuroprotection, and in limitation of damage? What is its role in enhancing axonal regeneration or remyelination? Does the formation of a glial scar imply that the tissue is unsalvageable, or is the scar itself hindering repair? Astrocytes in the normal brain Astrocytes3,4 are the most common cells in the brain, outnumbering neurons by a 4:1 margin. Interestingly, astrocytes have a topographic distribution marked out to define a territory of neurons and axons, which they support. This is most readily observed in the cortex, which when stained with antibodies to glial fibrillary acidic protein (GFAP) shows cytoplasmic fields consisting of delicate lacy processes (Figure 1). Not all cortical astrocytes will stain with GFAP or the other markers commonly used, such as Aldh-1 and glutathione synthetase. In the white matter, the morphology, as well as the territorial fields, is more difficult to see, as their processes are aligned along the axons in an isomorphic manner. The cortical cells have been termed protoplasmic, while those in the white matter are designated as fibrous. In both the cortex and the white matter, cytoplasmic processes

can be seen lining the perivascular spaces; they are critical for the formation and maintenance of the BBB.5–8 Similarly, processes from superficial astrocytes extend to, and line the superficial sub-pial cortex, forming the glial limitans. Variations are present within these categories. In the cortex, the “inter-laminar” astrocytes, between the second and third layers, are the cells subtending the glial limitans. Their processes are thicker and more oriented than those in the deeper cortex, and they stain more intensely and more uniformly than the latter. This no doubt reflects a greater concentration of intermediate GFAP filaments, important for structural rigidity. Astrocytes contribute to multiple physiologic functions in the central nervous system (CNS).3,4,9,10 They assist in local blood flow regulation through prostaglandin E and to water clearance from the interstitial “glymphatic” channels to paravenous channels; the latter process is dependent on the aquaporin 4 (Aq4) water channel expressed on astrocyte end-feet.11 The chemokines and cytokines produced by astrocytes participate in maintenance of tissue homeostasis and immune regulation. Astrocytes contribute to the formation of the extracellular matrix (ECM), mainly glycosamino-glycans and tenascin C. They maintain homeostasis of the neurons and axons within

http://msj.sagepub.com 1115

Multiple Sclerosis Journal 22(9) their territories, regulating the transport and passage of fluids, ions, toxins, and neurotransmitters, in and out of these cells. Significantly they also provide trophic factor support, essential for the well-being of the axons and neurons, and through the Jagged/Notch and Wnt pathways facilitate inter-cellular signaling with oligodendrocytes12 and between neurons and oligodendrocytes. They have a role in regulating synaptogenesis and maintaining synaptic function through gammaaminobutyric acid (GABA), metabotropic glutamate, and purine receptors. These modulate neuronal activity and transmission, leading to the concept of the tripartite synapse.13 During the development, they form the radial glial fibers along which primitive neurons and glia migrate out from the germinal matrix to populate the gray and white matter, while within the germinal matrix, they proliferate and may even contribute to neurogenesis. Currently, there is great interest in the role they play in myelination and remyelination,14,15 of critical importance in the repair of MS lesions. Although many of these functions are common to most astrocytes, specific populations of astrocytes may be particularly specialized for certain functions. We have recently investigated this feature by measuring the expression of a panel of microRNAs from specifically labeled, individually captured human astrocytes, taken from the topographical populations in situ described above.2 Expression of selective microRNAs varied between astrocytes from the interlaminar, the deep cortex, and the white matter, supporting different functionalities in these populations. Reactive astrocytes In a sense, the distinction between normal astrocytes and astrocytes reacting to disease or injury is a tenuous one, as the former are always reacting to alterations in the normal environment in which they are sited; it is certain that with increasingly sophisticated technology, we will be able to detect these subtle reactive changes in the “normal” situation. The morphological changes in reactive astrocytes have been known for over a century and a half; increasingly, these are now being linked to specific chemical, molecular, and genetic properties of the cells. Although this review focuses on astrocytes in MS, insights into the molecular properties of reactive astrocytes have been greatly enhanced by studying other diseases as well as experimental models. Whether astrocyte reactions are specific to MS is at present unclear, but disease-specific genomic changes have been reported in astrocytes from stroke and inflammatory lesions.16 The intensity and type of the changes vary concomitantly with the degree of tissue damage and the distance from the lesion or insult

Figure 2.  The graded reaction of astrocytes to injury. A necrotic cavity (TC) following trauma is seen in the deep gray matter of this mouse brain, stained with antibodies to GFAP. The hippocampus (H) is seen top left. There is intense staining of the astrocytes around the cavity, which tapers off with increasing distance from the lesion. Increased staining is also seen in the hippocampal cortex. Less reactive normal appearing tissue is seen in the lower right corner.

epicenter (Figure 2),17,18 emphasizing the exquisitely finely tuned nature of the response. An ongoing debate has been on whether by decreasing inflammation and enhancing repair gliosis is beneficial or, when pro-inflammatory, detrimental. The consensus is obviously that astrocytic reactions can be both, depending on the stage and severity of the damage, among other factors.3,10,19–22 The future challenges on this point will be to recognize when the beneficial response to tissue damage ends, and a functional switch to permanent non-permissive gliosis begins. The terms plastic and non-plastic have been suggested to describe these differing states.23 However, to be noted is that a solid glial scar, while being non- permissive to axonal and cellular migration,24 also limits further ingress of noxious factors and cells and provides structural stability to areas where tissue has been lost.18,25 A summary of the astrocytic reaction and regression in MS is illustrated Figure 3. Morphology of the astrocytic response in MS This varies in accordance with the stage and type of lesion. MS lesions are usually designated as (a) active

1116 http://msj.sagepub.com

SK Ludwin, VTS Rao et al.

Figure 3.  A stylized representation of the functions, reactions, and regression of astrocytes during the course of multiple sclerosis. More details of the factors at play can be found in the text and Table 1. (a) An astrocyte in a physiologic state illustrates some of the main features and functions. (b) The astrocyte has become acutely reactive secondary to tissue insult and has elaborated both structural and biochemical changes. The outcome of these reaction results in three different scenarios, depending on the severity and the timing of the termination of the insult; complete restitution results in a return to the physiologic state, while if the insult is moderate, (c) a mixed picture of regeneration and scarring may be found. (d) A rigid glial scar is seen, secondary to severe tissue loss.

(acute), (b) inactive (chronic), (c) chronic active, an inactive core, with evidence of ongoing or recent activity, usually at the peripheral rim, and (d) early or preactive lesions, where no plaque is present, but breakdown of the BBB, mild perivascular inflammatory reaction, and reactive astrocytes is present.26–30 In relapsing/remitting disease, the lesions are often either active or chronic active. Inactive lesions are typical of progressive disease, whether primary or secondary. In addition, astrocyte reactivity may also be seen in the so-called normal appearing white matter (NAWM),20 the diffusely abnormal white matter (DAWM),31 and experimentally has been seen to accompany Wallerian degeneration.23

Active plaques The astrocytes are very hypertrophic and prominent, with swollen gemistocytic cytoplasm containing glial

intermediate filaments and swollen abnormal processes (Figure 4). The nuclei are large, but usually not hyperchromatic, although the reaction may be florid enough to suggest neoplasia on biopsy. Large multinucleate astrocytes (Creutzfeldt cells) may be found. They are intimately associated with oligodendrocytes, which they often appear to ingest, as well as T cells, suggestive of either a trophic or a cleaning-up function. Although not always shown convincingly in MS tissue, in both experimental demyelination and trauma, astrocytes proliferate in the appropriate situations.17,32–34 Ultrastructurally, the astrocyte processes filled with glycogen extend around demyelinating fibers and show increased numbers and packing of intermediate filaments associated with GFAP staining. Astrocytes also show phagocytosis, with myelin and cellular debris in their cytoplasm. Changes in cytoplasmic appearance (“glassy”) may be present on electron microscopy (EM). In experimental studies,

http://msj.sagepub.com 1117

Multiple Sclerosis Journal 22(9)

Figure 4.  Morphological features of the active plaque. (a) The active plaque (APl) at the left of the figure is marked by intense macrophage activity, as seen with immuno-staining for CD68, a macrophage marker. The activity is less at the plaque rim (R) and decreases even more in the more normal white matter on the right (WM). An adjacent section stained with anti-GFAP antibodies (b) shows a matching astrocytic reaction, decreasing from the plaque to the more normal surrounding white matter. Higher magnification shows the features of hypertrophic reactive astrocytes, stained with (c) hematoxylin–eosin and (d) anti-GFAP antibodies. (c) The cytoplasm is swollen, and many of the nuclei of these cells are enlarged.

through their role in microglia attraction, they help to clear debris.35 Angiogenesis is also a feature of the acute lesion,36 with astrocytes expressing vascular endothelial growth factor (VEGF) in response to hypoxia-inducible factor (HIF) up-regulation in macrophages. Early damage to the astrocytic foot processes on blood vessels, with resulting bare patches of basal lamina, has been shown, suggesting that this cell too may be a primary target of the injury.37 Inactive plaques Inactive plaques show astrocytes with small somata and nuclei and long thin processes; in the white matter, they may be arrayed in an isomorphic pattern suggesting prior tract architecture (Figure 5). The

processes may be packed with filaments on electron microscopy, but often do not stain intensely for GFAP, due to the polymerization of the protein, with masking of the epitopes. There is usually axonal dropout of varying severity, and those that remain are often surrounded by filament-rich enlarged astrocyte processes forming the glial scar. Axo-glial membrane specializations resembling gap junctions can be found;38 this could be a compensatory effort by the astrocyte to take over some of the trophic functions of oligodendrocytes, but it may also represent a more permanent additional barrier to oligodendrocytes wrapping. Varying numbers of surviving axons and myelinated fibers may be found within the inactive core, and occasional inflammatory cells and mildly reactive astrocytes may be found.

1118 http://msj.sagepub.com

SK Ludwin, VTS Rao et al.

Figure 5.  Morphological features of the inactive plaque. (a) Sharply delineated inactive (chronic) plaques (Pl) are seen as clear spaces lying within the myelinated white matter, stained blue with a Loyez stain for myelin. The plaque on the left has a faint bluish stain, suggestive of some remyelination. (b) The astrocytes in the inactive plaque, stained for GFAP, have scanty cytoplasm (arrows), and their thickened processes are elongated in an isomorphic parallel orientation, forming a glial scar.

Chronic active plaques These same features may be found in the central core, but the peripheral rim shows active demyelination, with macrophage phagocytosis and varying degrees of inflammatory infiltration. However, some evidence of activity may also be seen in the central core as well. The activity represents ongoing damage; this may be an evolving lesion, or a new insult. In addition, especially at the rim, thinly myelinated sheaths and oligodendrocytes expressing myelin proteins indicate remyelination alongside the demyelination and phagocytosis. In these areas, the astrocytes show varying degrees of reactivity as has been described above, which often extends beyond the active rim into the surrounding intact tissue. In view of their known functions, these reactive cells may be contributing to the breakdown and clearance of debris, to the trophic protection of damaged tissue, or to remyelination. Early (“Pre-lesions”) These are identified by focal mild perivascular lymphocytic infiltration and breakdown of the BBB, with

protein leakage and endothelial junctional protein alteration.39 Focal BBB leakage may also be seen on magnetic resonance imaging (MRI) and experimental imaging/pathological studies have shown the concordance of these findings.40 Accompanying astrocytosis may be the first indication to the investigator of the presence of a disease process.

NAWM Reactive astrocytosis is also common in the NAWM, often accompanied by small foci of lymphocytic infiltration, demyelination and microglial/macrophage activation, and axonal damage. This region has been shown to be highly active with marked upregulation of genes and secreted factors, many similar to those seen within active lesions.20 Similar changes may also be seen in the lesions of DAWM, which consists of regions of myelin changes in which lipids are preferentially decreased in the absence of change in myelin protein change.31 These changes may at times be difficult to distinguish from Wallerian degeneration, which is an important feature of the progressive disease.41

http://msj.sagepub.com 1119

Multiple Sclerosis Journal 22(9) Cortical lesions In the demyelinative sub-pial, intra-cortical and cortical component of leukocortical lesions, there is also reactive astrocytosis, although mirroring the inflammatory infiltrate, the reaction is far more muted than in the corresponding white matter. Reactivity is more marked in the very rarely seen instances of acute inflammation in biopsied cortical lesions.42 In MS, the cortical inter-astrocytic gap junction proteins CX30 and CX43, partners of oligodendrocyte connexins, are up-regulated in normal appearing gray matter (NAGM) and cortical lesions. This indicates ongoing astrocytosis, and a breakdown of signaling to the oligodendrocytes, perhaps leading to remyelination failure.12 Further evidence of gray matter involvement is seen in the alterations in the metabolic status in the NAGM; this is evidenced by reduced transcription of the astrocytic genes involved in the astrocyteneuron lactate shuttle and the glutamate/glutamine cycle.43,44 Molecular and biochemical features of reactive astrocytosis in MS

pro-inflammatory microRNA. Preliminary results from another study62 showed a down-regulation of miR-146a, a putative anti-inflammatory microRNA, in active plaques. In view of the importance of microRNAs in determining functional switches, and the potential therapeutic application of miR silencing, further investigations on these molecules are required. Chronic lesions Besides the non-permissive nature of the more chronic glial scar, astrocytes in chronic MS and EAE may cause further inflammatory damage by producing lactosyl ceramide, (normally used for axonal and myelin formation and maintenance) through the enzyme B4galactosyl transferase 5.63 This may contribute to disease progression and is in keeping with previous work in chronic EAE,64 where the astrocytes secreted CCL2, thereby continuing the inflammatory process; astrocytes with knock-out of CCL2 had diminished chronic inflammation. It has been suggested that astrocytes may also contribute to progression by the continued elaboration of pro-inflammatory factors and by contributing to axonal mitochondrial dysfunction.65

Active MS lesions The most striking feature in MS lesions, as well as in other clinical and experimental situations, is a major up-regulation of GFAP, seen immunohistochemically and with in situ hybridization for GFAP mRNA and the structural protein vimentin (see Table 1). Also upregulated are other astrocyte markers, glutamine synthetase 1 and aldehyde dehydrogenase 1 family, member L1 (ALDH1L1),4,57 nestin, Aq, and S100. Reactive astrocytes in active lesions express a plethora of factors including pro- and anti-inflammatory cytokines, chemo-attractant and repellant (e.g. semaphorin) molecules, and trophic factors including VEGF, in response to HIF secretion by macrophages,23 as well as brain-derived neurotrophic factor (BDNF) and tropomyosin-related kinase (TrkB),45 ciliary neurotrophic factor (CNTF), insulin-like growth factor-1 (IGF-1), and neurotrophin-3 (NT3).46 In addition, the sodium channel Nav1.5 is up-regulated in astrocytes47 as is the potassium channel KIR4.1.58 Reducing astrocyte response to the pro-inflammatory effects of sphingosine 1 phosphate (S1P) by ablating the SIP-1 receptor on astrocytes is shown to reduce severity of experimental autoimmune encephalomyelitis (EAE), providing a further potential basis for the effect of this class of drug on the activity of MS.59

Astrocytes and remyelination Early studies using ethidium bromide demyelination confirmed the need for these cells,66 as elimination of astrocytes resulted in remyelination only by Schwann cells, unless transplanted astrocytes were reintroduced.67 Many proteins have been shown to either enhance or inhibit remyelination (see Table 1).14,15 In MS, up-regulation of the ECM molecules, chondroitin sulfate proteoglycan (CSPG), hyaluronic acid, and tenascinC48 inhibit remyelination directly and also contribute to the glial scar, which may act as a barrier to oligodendrocyte migration and thus remyelination.68 The role of platelet-derived growth factor a (PDGFa) and fibroblast growth factor 2 (FGF2), found in astrocytes, is more complex; they promote oligodendrocyte precursor cell (OPC) renewal but block differentiation. On the other hand, astrocytes also produce tissue inhibitor of metalloproteinase 1 (TIMP-1), which inhibits matrix metalloproteinases in the ECM, thus enhancing remyelination.15,49 Tumor necrosis factor a (TNFa) and FGF-b, expressed on astrocytes in MS and EAE lesions, can mediate myelin injury or promote repair, depending on the tumor necrosis factor receptor (TNFR1 vs 2) engaged.69

In addition, a differential expression of 10 astrocyteassociated microRNAs from active versus inactive plaques has been reported,60,61 notably miR-155, a

Neuromyelitis optica (Devic’s disease) Long considered to be a variant of MS, neuromyelitis optica (NMO) has been clearly delineated as a separate

1120 http://msj.sagepub.com

SK Ludwin, VTS Rao et al. Table 1.  Molecules produced or up-regulated by reactive astrocytes and demonstrated in MS.14,15,19,36,37,43–56 S. no.

Function

Protein/molecule

1 2

Proliferation Trophic/growth/ differentiation

3

Inflammation producing

4 5 6

Inflammation resolving Chemo-attraction (can be destructive or beneficial) Remyelination promotion

7

Remyelination inhibition

8 9 10 11

Migration Cell adhesion Angiogenesis BBB breakdown

CXCL12 (pathogenic T and B cells) BAFF, PDGF, VEGF, IL-6, IL-11 Retinaldehyde dehydrogenase through retinoic acid Nav 1.5 Channel on axons CNTF (neuron protectant) TNFα, IFNγ, IL17, LTα, NO synthetase, CD200, CXCL12, IL-1β, IL-6 & CCL20 IL-6, IL-27, and IL-10 CXCL1, CXCL8, CXCL10, IL-1β, semaphorins 3A and 3F, and MCP-1 CXCL1, CXCL8, IL-6, IL-11, semaphorin 3F, FGF1 (through CXCL8 and LIF), retinoic acid, excitatory amino acid transporter (EAAT), γ-Secretase, CNTF, BDNF, TrkB, and NGF CXCL10, CXCL1, MMPs, and TIMP ECM molecules: hyaluronan, tenascin C, chondroitin sulfate (suppress myelination through CXCL10) Semaphorin 3A, Jagged-1, Lingo, and fibronectin TG2 (tissue transglutaminase) V-CAM-1, I-CAM, and TG2 VEGF and PDGF VEGF, PDGF, V-CAM, MMPs, and IL-1β

MS: multiple sclerosis; CXCL: c-x-c motif ligand chemokine; BAFF: B cell activating factor; VEGF: vascular endothelial growth factor; PDGF: platelet-derived growth factor; IL: interleukin; CNTF: ciliary neurotrophic factor; TNFα: tumor necrosis factor α; IFNγ: interferon γ;  LTα: lymphotoxin α; NO: nitric oxide; CCL: c-c motif ligand chemokine; MCP: monocyte chemo-attractant protein; FGF1: fibroblast growth factor 1; BDNF: brain-derived neurotrophic factor; TrkB: tropomyosin-related kinase; NGF: nerve growth factor; MMP: matrix metalloproteinase; TIMP: tissue inhibitor of metalloproteinase; ECM: extracellular matrix; V-CAM-1: vascular cell adhesion molecule 1; I-CAM: inter-cellular adhesion molecule; TG2: transglutaminase. (1) Molecules may fit into more than one category; in each instance, the result may be destructive or beneficial, depending on location or timing, for example, a chemo-attractant molecule may attract pathogenic monocytes or microglia and also remyelinating oligodendrocyte precursors. (2) Other molecules, not listed in the table, have been described in reactive astrocytes in other human pathological conditions, as well as experimentally.

condition, following the identification of antibodies to the astrocytic water channel protein, Aq4, in the serum of most of the patients.70,71 Over the years the clinical, radiological, pathological, and immunological features have resulted in the concept of the NMO spectrum disorders.72,73 Numerous systems have attempted to classify the variants, which include cases with no detectable antibodies,74 and puzzling cases of what have been described as “optico-spinal MS.” Indeed, it has been suggested that a classification should be based on molecular markers.75 The pathology shows a severe, at times necrotizing, demyelinating disease, with antibody and complement deposition, T-cell, macrophage and granulocyte infiltration, and axonal destruction. The striking feature is the loss of Aq4 staining in the lesions, accompanied by lesser degrees of GFAP loss, disintegration of the astrocytic foot processes on the blood vessels and the external glia limitans, leading to vascular permeability, leukocyte chemotaxis, and loss of glutamate homeostasis;71 necrosis and drop out of

the astrocytes often precedes the demyelination.72 GFAP may also be present in the cerebrospinal fluid (CSF). At times, around the lesions, reactive astrocytosis may be seen. The demonstration of this demyelinating astropathy has renewed interest in exploring the possibility of a similar situation occurring in MS. Although astrocytic damage has been described in acute MS37 (see earlier), as yet no similar antibodies have been described. However, astropathy without demyelination has been described in acute hemorrhagic leukoencephalitis, thought to be either a metabolic or a hyperacute immune disease, perhaps offering further avenues for investigation.76 The therapeutic implications for the elimination of such antibodies in both diseases are important and obvious. Summary This review has documented the critical roles of the astrocyte in MS, both beneficial and pathogenic.

http://msj.sagepub.com 1121

Multiple Sclerosis Journal 22(9) It should also be remembered that many of these pathogenic or protective responses derive from the normal reaction of surveillance cells to metabolic, infective, toxic, or ischemic insults; unfortunately, these may also have the by-product of further tissue damage or inhibition of repair. Understanding the origin, the timing, and the modulation of these responses will be essential in future therapeutic planning. Declaration of Conflicting Interests The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

11. Iliff JJ, Wang M, Liao Y, et al. A paravascular pathway facilitates CSF flow through the brain parenchyma and the clearance of interstitial solutes, including amyloid beta. Sci Transl Med 2012; 4: 147ra111. 12. Markoullis K, Sargiannidou I, Schiza N, et al. Oligodendrocyte gap junction loss and disconnection from reactive astrocytes in multiple sclerosis gray matter. J Neuropathol Exp Neurol 2014; 73: 865–879. 13. Sun W, McConnell E, Pare JF, et al. Glutamatedependent neuroglial calcium signaling differs between young and adult brain. Science 2013; 339: 197–200.

Funding The author(s) received no financial support for the research, authorship, and/or publication of this article.

14. Barnett SC and Linington C. Myelination: Do astrocytes play a role? Neuroscientist 2013; 19: 442–450.

References

16. Zamanian JL, Xu L, Foo LC, et al. Genomic analysis of reactive astrogliosis. J Neurosci 2012; 32: 6391–6410.

1. Charcot JM. Histologie de la sclerose en plaque. Paris: Gaz Hop, 1868; 41: 554–566. 2. Rao VTS, Ludwin SK, Fuh SC, et al. MicroRNA expression patterns in human astrocytes in relation to anatomical location and age. J Neuropathol Exp Neurol 2016, http://www.ncbi.nlm.nih.gov/ pubmed/26802178

15. Moore CS, Abdullah SL, Brown A, et al. How factors secreted from astrocytes impact myelin repair. J Neurosci Res 2011; 89: 13–21.

17. Ludwin SK. Reaction of oligodendrocytes and astrocytes to trauma and implantation. A combined autoradiographic and immunohistochemical study. Lab Invest 1985; 52: 20–30.

3. Sofroniew MV and Vinters HV. Astrocytes: Biology and pathology. Acta Neuropathol 2010; 119: 7–35.

18. Anderson MA, Ao Y and Sofroniew MV. Heterogeneity of reactive astrocytes. Neurosci Lett 2014; 565: 23–29.

4. Khakh BS and Sofroniew MV. Diversity of astrocyte functions and phenotypes in neural circuits. Nat Neurosci 2015; 18: 942–952.

19. Williams A, Piaton G and Lubetzki C. Astrocytes– Friends or foes in multiple sclerosis? Glia 2007; 55: 1300–1312.

5. Alvarez JI, Katayama T and Prat A. Glial influence on the blood brain barrier. Glia 2013; 61: 1939–1958.

20. Zeis T, Graumann U, Reynolds R, et al. Normalappearing white matter in multiple sclerosis is in a subtle balance between inflammation and neuroprotection. Brain 2008; 131: 288–303.

6. Alvarez JI, Dodelet-Devillers A, Kebir H, et al. The Hedgehog pathway promotes blood-brain barrier integrity and CNS immune quiescence. Science 2011; 334: 1727–1731. 7. Cheslow L and Alvarez JI. Glial-endothelial crosstalk regulates blood–Brain barrier function. Curr Opin Pharmacol 2016; 26: 39–46. 8. Lecuyer MA, Kebir H and Prat A. Glial influences on BBB functions and molecular players in immune cell trafficking. Biochim Biophys Acta 2016; 1862: 472–482. 9. Barres BA. The mystery and magic of glia: A perspective on their roles in health and disease. Neuron 2008; 60: 430–440. 10. Lundgaard I, Osorio MJ, Kress BT, et al. White matter astrocytes in health and disease. Neuroscience 2014; 276: 161–173.

21. Pekny M, Wilhelmsson U and Pekna M. The dual role of astrocyte activation and reactive gliosis. Neurosci Lett 2014; 565: 30–38. 22. Roskoski R Jr, Wilgus H and Vrana KE. Inactivation of tyrosine hydroxylase by pterin substrates following phosphorylation by cyclic AMPdependent protein kinase. Mol Pharmacol 1990; 38: 541–546. 23. Ludwin SK. The pathogenesis of multiple sclerosis: Relating human pathology to experimental studies. J Neuropathol Exp Neurol 2006; 65: 305–318. 24. Ludwin SK. Oligodendrocytes from optic nerves subjected to long term Wallerian degeneration retain the capacity to myelinate. Acta Neuropathol 1992; 84: 530–537.

1122 http://msj.sagepub.com

SK Ludwin, VTS Rao et al. 25. Voskuhl RR, Peterson RS, Song B, et al. Reactive astrocytes form scar-like perivascular barriers to leukocytes during adaptive immune inflammation of the CNS. J Neurosci 2009; 29: 11511–11522.

39. Alvarez JI, Saint-Laurent O, Godschalk A, et al. Focal disturbances in the blood–Brain barrier are associated with formation of neuroinflammatory lesions. Neurobiol Dis 2015; 74: 14–24.

26. Lassmann H and Wekerle H. The pathology of multiple sclerosis. In: Compston A, Confavreux C, Lassmann H, et al. (eds) McAlpine’s multiple sclerosis. 4th ed. Edinburgh: Churchill Livingstone, Elsevier, 2006, pp. 557–600.

40. Absinta M, Nair G, Sati P, et al. Direct MRI detection of impending plaque development in multiple sclerosis. Neurol Neuroimmunol Neuroinflamm 2015; 2: e145.

27. Ludwin SK and Raine CS. The neuropathology of multiple sclerosis. In: Raine CS, McFarland H and Hohlfeld R (eds) Multiple sclerosis: a comprehensive text. 2nd ed. Philadelphia, PA: Saunders Elsevier, 2008, pp. 151–177. 28. Kutzelnigg A and Lassmann H. Pathology of multiple sclerosis and related inflammatory demyelinating diseases. Handb Clin Neurol 2014; 122: 15–58. 29. Lucchinetti C, Bruck W, Parisi J, et al. Heterogeneity of multiple sclerosis lesions: Implications for the pathogenesis of demyelination. Ann Neurol 2000; 47: 707–717. 30. Metz I, Weigand SD, Popescu BF, et al. Pathologic heterogeneity persists in early active multiple sclerosis lesions. Ann Neurol 2014; 75: 728–738. 31. Laule C, Pavlova V, Leung E, et al. Diffusely abnormal white matter in multiple sclerosis: Further histologic studies provide evidence for a primary lipid abnormality with neurodegeneration. J Neuropathol Exp Neurol 2013; 72: 42–52. 32. Popiela A, Zdrojewicz Z and Kasiak J. Clinical problems related to postmenopausal osteoporosis. Pol Tyg Lek 1991; 46: 568–570. 33. Ludwin SK. An autoradiographic study of cellular proliferation in remyelination of the central nervous system. Am J Pathol 1979; 95: 683–696. 34. Victor J, Turcant A, Tadei A, et al. Pharmacokinetics and efficacy of delayed-action hydroquinidine in patients with ventricular extrasystole. Ann Cardiol Angeiol 1987; 36: 163–166. 35. Skripuletz T, Hackstette D, Bauer K, et al. Astrocytes regulate myelin clearance through recruitment of microglia during cuprizone-induced demyelination. Brain 2013; 136: 147–167. 36. Ludwin SK. The neuropathology of multiple sclerosis. Neuroimaging Clin N Am 2000; 10: 625–648, vii.

41. Stadelmann C, Wegner C and Bruck W. Inflammation, demyelination, and degeneration— Recent insights from MS pathology. Biochim Biophys Acta 2011; 1812: 275–282. 42. Lucchinetti CF, Popescu BF, Bunyan RF, et al. Inflammatory cortical demyelination in early multiple sclerosis. N Engl J Med 2011; 365: 2188–2197. 43. Zeis T, Allaman I, Gentner M, et al. Metabolic gene expression changes in astrocytes in Multiple Sclerosis cerebral cortex are indicative of immune-mediated signaling. Brain Behav Immun 2015; 48: 313–325. 44. Finsterwald C, Magistretti PJ and Lengacher S. Astrocytes: New targets for the treatment of neurodegenerative diseases. Curr Pharm Des 2015; 21: 3570–3581. 45. Stadelmann C, Kerschensteiner M, Misgeld T, et al. BDNF and gp145trkB in multiple sclerosis brain lesions: Neuroprotective interactions between immune and neuronal cells? Brain 2002; 125: 75–85. 46. Nair A, Frederick TJ and Miller SD. Astrocytes in multiple sclerosis: A product of their environment. Cell Mol Life Sci 2008; 65: 2702–2720. 47. Black JA, Newcombe J and Waxman SG. Astrocytes within multiple sclerosis lesions upregulate sodium channel Nav1.5. Brain 2010; 133: 835–846. 48. Nash B, Ioannidou K and Barnett SC. Astrocyte phenotypes and their relationship to myelination. J Anat 2011; 219: 44–52. 49. Moore CS, Milner R, Nishiyama A, et al. Astrocytic tissue inhibitor of metalloproteinase-1 (TIMP1) promotes oligodendrocyte differentiation and enhances CNS myelination. J Neurosci 2011; 31: 6247–6254. 50. Rothhammer V and Quintana FJ. Control of autoimmune CNS inflammation by astrocytes. Semin Immunopathol 2015; 37: 625–638.

37. Brosnan CF and Raine CS. The astrocyte in multiple sclerosis revisited. Glia 2013; 61: 453–465.

51. Nijland PG, Molenaar RJ, van der Pol SM, et al. Differential expression of glucose-metabolizing enzymes in multiple sclerosis lesions. Acta Neuropathol Commun 2015; 3: 79.

38. Raine CS. Membrane specialisations between demyelinated axons and astroglia in chronic EAE lesions and multiple sclerosis plaques. Nature 1978; 275: 326–327.

52. Senecal V, Deblois G, Beauseigle D, et al. Production of IL-27 in multiple sclerosis lesions by astrocytes and myeloid cells: Modulation of local immune responses. Glia 2016; 64: 553–569.

http://msj.sagepub.com 1123

Multiple Sclerosis Journal 22(9) 53. Mohan H, Friese A, Albrecht S, et al. Transcript profiling of different types of multiple sclerosis lesions yields FGF1 as a promoter of remyelination. Acta Neuropathol Commun 2014; 2: 168. 54. Mizee MR, Nijland PG, van der Pol SM, et al. Astrocyte-derived retinoic acid: A novel regulator of blood-brain barrier function in multiple sclerosis. Acta Neuropathol 2014; 128: 691–703. 55. Van Strien ME, Drukarch B, Bol JG, et al. Appearance of tissue transglutaminase in astrocytes in multiple sclerosis lesions: A role in cell adhesion and migration? Brain Pathol 2011; 21: 44–54. 56. Stoffels JM, de Jonge JC, Stancic M, et al. Fibronectin aggregation in multiple sclerosis lesions impairs remyelination. Brain 2013; 136: 116–131. 57. Yang Y, Vidensky S, Jin L, et al. Molecular comparison of GLT1+ and ALDH1L1+ astrocytes in vivo in astroglial reporter mice. Glia 2011; 59: 200–207. 58. Satoh J-I, Tabunoki H, Ishida T, et al. Reactive astrocytes express the potassium channel Kir4.1 in active multiple sclerosis lesions. Clin Exp Neuroimmunol 2013; 4: 19–28. 59. Choi JW, Gardell SE, Herr DR, et al. FTY720 (fingolimod) efficacy in an animal model of multiple sclerosis requires astrocyte sphingosine 1-phosphate receptor 1 (S1P1) modulation. Proc Natl Acad Sci U S A 2011; 108: 751–756.

Visit SAGE journals online http://msj.sagepub.com

 SAGE journals

experimental autoimmune encephalomyelitis. J Neuroimmunol 2014; 274: 53–61. 65. Correale J and Farez MF. The role of astrocytes in multiple sclerosis progression. Front Neurol 2015; 6: 180. 66. Blakemore WF and Crang AJ. The relationship between type-1 astrocytes, Schwann cells and oligodendrocytes following transplantation of glial cell cultures into demyelinating lesions in the adult rat spinal cord. J Neurocytol 1989; 18: 519–528. 67. Franklin RJ, Crang AJ and Blakemore WF. Transplanted type-1 astrocytes facilitate repair of demyelinating lesions by host oligodendrocytes in adult rat spinal cord. J Neurocytol 1991; 20: 420–430. 68. Sofroniew MV. Molecular dissection of reactive astrogliosis and glial scar formation. Trends Neurosci 2009; 32: 638–647. 69. Finsen B, Antel J and Owens T. TNFalpha: Kill or cure for demyelinating disease? Mol Psychiatry 2002; 7: 820–821. 70. Lennon VA, Wingerchuk DM, Kryzer TJ, et al. A serum autoantibody marker of neuromyelitis optica: Distinction from multiple sclerosis. Lancet 2004; 364: 2106–2112. 71. Zekeridou A and Lennon VA. Aquaporin-4 autoimmunity. Neurol Neuroimmunol Neuroinflamm 2015; 2: e110.

60. Junker A, Krumbholz M, Eisele S, et al. MicroRNA profiling of multiple sclerosis lesions identifies modulators of the regulatory protein CD47. Brain 2009; 132: 3342–3352.

72. Lucchinetti CF, Guo Y, Popescu BF, et al. The pathology of an autoimmune astrocytopathy: Lessons learned from neuromyelitis optica. Brain Pathol 2014; 24: 83–97.

61. Junker A, Hohlfeld R and Meinl E. The emerging role of microRNAs in multiple sclerosis. Nat Rev Neurol 2011; 7: 56–59.

73. Wingerchuk DM and Weinshenker BG. Neuromyelitis optica (Devic’s syndrome). Handb Clin Neurol 2014; 122: 581–599.

62. Rao VTS, Fuh SC, Moore CS, et al. Expression profiles of inflammation associated microRNAs in astrocytes from multiple sclerosis lesions. In: Proceedings of the ECTRIMS-ACTRIMS 2014 (Abstract), Boston, MA, 10–13 September 2014.

74. Wingerchuk DM, Banwell B, Bennett JL, et al. International consensus diagnostic criteria for neuromyelitis optica spectrum disorders. Neurology 2015; 85: 177–189.

63. Mayo L, Trauger SA, Blain M, et al. Regulation of astrocyte activation by glycolipids drives chronic CNS inflammation. Nat Med 2014; 20: 1147–1156. 64. Kim RY, Hoffman AS, Itoh N, et al. Astrocyte CCL2 sustains immune cell infiltration in chronic

75. Pittock SJ. Demyelinating disease: NMO spectrum disorders: Clinical or molecular classification? Nat Rev Neurol 2016; 12: 129–130. 76. Robinson CA, Adiele RC, Tham M, et al. Early and widespread injury of astrocytes in the absence of demyelination in acute haemorrhagic leukoencephalitis. Acta Neuropathol Commun 2014; 2: 52.

1124 http://msj.sagepub.com