MOUNT SINAI JOURNAL OF MEDICINE 78:231���243, 2011 231 Axonal Damage in Multiple Sclerosis Jeffery D. Haines, PhD,1 Matilde Inglese, MD, PhD,2 and Patrizia Casaccia, MD, PhD1 1Departments of Neuroscience Neurology and Genetics and Genomics, Mount Sinai School of Medicine, New York, NY 2Department of Neurology and Radiology, Mount Sinai School of Medicine, New York, NY OUTLINE HISTOPATHOLOGICAL EVIDENCE OF AXONAL DAMAGE IN MULTIPLE SCLEROSIS Morphological and Histopathological Characteristics of Axonal Damage Evidence for Axonal Damage as Consequence of Primary Demyelination Evidence in Favor of Hypothesis That Axonal Damage Can Occur Independently of Demyelination CLINICAL DATA SUPPORTING NEURONAL DAMAGE IN MS BIOCHEMICAL DATA SUPPORTING NEURONAL DAMAGE IN MULTIPLE SCLEROSIS IMAGING DATA SUPPORTING NEURONAL DAMAGE IN MULTIPLE SCLEROSIS Magnetic Resonance Imaging Studies Spectroscopic Measurement of Metabolites Positron Emission Tomography POTENTIAL MECHANISMS OF DAMAGE Axonal Damage Consequent to Demyelination Axonal Damage Consequent to Direct Cytotoxic Attack Antibody/Complement-Mediated Lesion Damage Caused by Dysfunctional Glial-Neuronal Crosstalk Address Correspondence to: Patrizia Casaccia Departments of Neuroscience and Genetics and Genomics Mount Sinai School of Medicine New York, NY Email: patrizia.casaccia@mssm.edu Damage Caused by Exposure to Glutamate and Cytokines CONCLUSION ABSTRACT Multiple sclerosis is a debilitating disease of the central nervous system that has been characteris- tically classified as an immune-mediated destruc- tion of myelin, the protective coating on nerve fibers. Although the mechanisms responsible for the immune attack to central nervous system myelin have been the subject of intense investigation, more recent studies have focused on the neurodegener- ative component, which is cause of clinical dis- ability in young adults and appears to be only partially controlled by immunomodulatory therapies. Here, we review distinct, but not mutually exclu- sive, mechanisms of pathogenesis of axonal damage in multiple sclerosis patients that are either conse- quent to long-term demyelination or independent from it. We propose that the complexity of axonal degeneration and the heterogeneity of the under- lying pathogenetic mechanisms should be taken into consideration for the design of targeted ther- apeutic intervention. Mt Sinai J Med 78:231���243, 2011. ��� 2011 Mount Sinai School of Medicine Key Words: axon, demyelination, multiple sclerosis, myelin, neurodegeneration, neuroimaging, oligoden- drocyte, pathogenesis. Multiple sclerosis (MS) is a debilitating neurological disorder of young adults with a high prevalence in North America and Europe, afflicting almost 2.5 million individuals worldwide,1,2 and with an incidence rate that continues to rise.3 Approximately 85% of MS patients begin with a relapsing-remitting (RRMS) course of the disease, characterized by clinically debilitating events followed by return to baseline.4 After a variable period of time, the majority of RRMS patients develop a secondary progressive (SPMS) form of the disease, which is characterized by persistent and progressive disease advancement without remissions.5 In a small proportion of MS patients (10%���15%), the disease advances rapidly Published online in Wiley Online Library (wileyonlinelibrary.com). DOI:10.1002/msj.20246 ��� 2011 Mount Sinai School of Medicine

232 J. D. HAINES ET AL.: AXONAL DAMAGE IN MS without periods of remittance and is termed primary progressive MS (PPMS).6 Historically, axonal damage has been recognized as a histopathological hallmark Axonal damage in multiple sclerosis might be concurrent to demyelination, but not necessarily consequent to myelin destruction. of MS since the very early descriptions of the disease by Charcot in the late 1800s.7 More recent clinical, histopathological, and neuroimaging evidence has shed new light on these early findings and supported the concept that the debilitating disease course and long-term disability in MS patients was consequent to axonal loss possibly consequent to demyelination.8,9 The features of axonal damage in MS, however, were similar to those detected in other neurological diseases lacking demyelination, such as amyotrophic lateral sclerosis (ALS), and suggested the possibility that axonal damage in MS might be concurrent to demyelination, but not necessarily consequent to myelin destruction.10 This review discusses the hypothesis that axonal damage underlying the debilitating disease progression observed in MS patients may be caused by mechanisms other than long-term demyelination. HISTOPATHOLOGICAL EVIDENCE OF AXONAL DAMAGE IN MULTIPLE SCLEROSIS Morphological and Histopathological Characteristics of Axonal Damage Axonal damage is a common feature of many neu- rodegenerative diseases. The morphological charac- teristics of early axonal damage include the presence of varicosities and spheroid structures,11 which are associated with impaired transport of proteins and organelles along the axon.12 Disrupted axonal trans- port can be detected by immunohistochemistry using antibodies specific for the amyloid precursor protein (APP) as a marker. In physiological conditions APP is rapidly transported along the axons and cannot be easily detected by immunoreactivity.13,14 In case of axonal dysfunction and altered transport, in con- trast, APP accumulates in localized axonal enlarge- ments that become immunoreactive and that can be detected in active, remyelinating, and inactive MS lesions.15,16 An additional marker of axonal damage is the nonphosphorylated form of neurofilament heavy chain (NFH). In healthy axons, NFH is phosphory- lated and this correlates with fast axonal transport.17 In compromised axons, in contrast, the nonphos- phorylated form of NFH, which can be identified by SMI-32 immunoreactivity, can be used as maker of neurodegeneration,18 ALS,19 and MS.20 Evidence for Axonal Damage as Consequence of Primary Demyelination A correlation between axonal damage and demyeli- nation has been determined from histopathological examination of MS postmortem tissue. Histopatho- logical analysis of early MS lesions showed that most axonal transections occur during the process of active demyelination.21,22 Furthermore, regional axonal loss in the corpus callosum correlated with the cere- bral white matter (WM) lesion volume distribution and was suggested to be a result of degenerated axons transected in demyelinated lesions.23 Axons in MS lesions also stained with antibodies spe- cific for nonphosphorylated NFH and total axonal loss were correlated to the degree of inflammatory demyelination,21 suggesting that neurodegeneration occurs as the consequence of myelin loss. Further evidence supporting this notion was the observation that the extent of remyelination extended lifespan of mice and provided a pro- tective effect on axons.24 Perhaps the most strik- ing evidence to substantiate axonal degeneration as a consequence of demyelination was obtained from animal models of MS, including experimental autoimmune encephalomyelitis (EAE) and cuprizone- induced demyelination. In EAE, animals are immu- nized with antigenic myelin extracts or peptides (eg, myelin-oligodendrocyte glycoprotein [MOG]), which elicit an immune T cell���mediated disease characterized by demyelination.25,26 After long-term demyelination in EAE, axonal loss is observed in rats immunized with MOG22 and in guinea pigs,27 sub- stantiating the concept that axonal damage occurs following the loss of myelin support. Axonal loss can also be detected in the cuprizone-induced model of toxic demyelination in aged mice,28 which have less- efficient remyelination compared with young mice.29 Evidence in Favor of Hypothesis That Axonal Damage Can Occur Independently of Demyelination The hypothesis that axonal damage in MS may occur independently of chronic demyelination has been suggested by several studies that will be reviewed DOI:10.1002/MSJ