Multiple Sclerosis (MS) is a progressive demyelinating disease that is characterised by demyelination, perivascular inflammation, oligodendrocyte depletion, astroglia proliferation and remyelination. Generally, the aetiology of MS is still unclear, although many environmental factors and the interaction of multiple genes have been proposed to play a role in the disease. MS is usually diagnosed in people between the ages of 20 and 40, and is more common in females than in males (almost 3:1). There are several established experimental demyelination models that, to some extent, reflect the heterogeneity of MS and are therefore seen as suitable to study MS pathogenesis . These models include immune-mediated, virus-induced and toxin-induced models. Biomarkers are crucial for evaluating and assessing the normal biological, pathogenic, and pharmacological response to therapeutic interventions. Knowledge of the molecular basis of this disease through various models and biomarkers is essential to ascertaining and pinpointing the pathway and in turn, developing drugs that can help treat or ameliorate this disease, improving the quality of life of patients.
Models of Multiple Sclerosis (MS)
There are several established experimental demyelination models that, to a certain extent, reflect the heterogeneity of MS and are therefore seen as fitting to study MS pathogenesis. These models include immune-mediated, virus-induced, and toxin-induced models. Experimental autoimmune encephalitis (EAE) is, by far, the most explored model for studying various aspects of autoimmunity in MS pathology. Virus-induced demyelination models support the hypothesis that some environmental factors, such as viral infections, are involved in MS and may be a trigger for the disease. Toxin-induced demyelination models are exploited in the evaluation of the demyelination/remyelination process in the relative absence of immune cells, even though these ways of damaging the myelin do not bear a resemblance to features of the demyelination seen in MS .
Toxin-Induced Demyelination Models
There are a number of agents known to generate demyelination foci, using direct injections of gliotoxins in the white matter, such as ethidium bromide (EtBr) and lysolecithin (LPC), or systemically administered toxins, such as cuprizone. These models are vital for studying remyelination processes in animals. Furthermore, these models ensure good reproducibility and a well-defined anatomical location of the demyelination area .
The toxic effect of this agent, lysophosphatidylcholine (lysolecithin), is able to produce demyelination and it was first described by Susan M. Hall. With detergent-like agent activity, lysolecithin is able to solubilize membranes and is considered to be discerning for myelin-producing cells. Therefore, lysolecithin targets the myelin, leaving other cellular components relatively unaffected, thereby allowing for the enrolment of T and B cells, as well as microglia/macrophage activation at the lesion site, which have a role in clearing myelin debris and in promotion of trophic factors. Lysolecithin injection increases phospholipase A2 activity, which is restricted to activated macrophages. Phospholipase A2 further degrades membrane phosphatidylcholines. Usually, 1% lysolecithin solution is injected into the dorsal funiculus of the spinal cord, caudal cerebellar peduncle, or corpus callosum. Following lysolecithin injection, the formed lesion changes over the next few weeks and is capable of remyelinating completely, starting at the end of the first week after the injection. The remyelination process in this model is faster compared with other toxin-induced demyelination models, mainly because oligodendrocyte progenitor cells (OPCs) are not affected. Demyelinating axons are re-myelinated mainly by oligodendrocytes. However, if the lesion is larger in size, Schwann cells also take part in the remyelination process .
Figure 2 depicts a model of demyelination and remyelination that has been utilised to tease apart the specific mechanisms that contribute towards oligodendrocyte death, oligodendrocyte precursor cell (OPC) migration, differentiation, and remyelination. Cuprizone-induced demyelination was first described in the late 1960s using Swiss mice. Protocols now widely use C57BL/6 mice. Nonetheless, methods have also been developed for rats, which are particularly useful for imaging studies due to their larger brain size. Cuprizone causes detailed oligodendrocyte death and successive demyelination; however, the mechanism is not fully known. It is a copper ion (Cu2+) chelator disturbing cellular metabolism. Mega-mitochondria are observed in the liver of mice following cuprizone intoxication. The presence of these mega-mitochondria indicates a disturbance in metabolic tasks that ultimately results in oligodendrocyte death. The dosage of cuprizone needed is dependent on the strain, age and sex of the mouse. Cuprizone is mixed with the feed at a concentration of 0.2%–0.6% (w/w) for 6 weeks to induce extensive demyelination throughout the brain, of which the corpus callosum is the most widely studied. Intoxication in SJL mice results in a different demyelinating pattern than that seen in C57BL/6 mice. This is further influenced by gender. Mice present with a time dependent weight loss and important behavioural and motor deficiencies .
Biomarkers of Multiple Sclerosis
In 1998, the National Institutes of Health (NIH) deﬁned biomarkers as “a characteristic that is objectively measured and evaluated as an indicator of normal biological processes, pathogenic processes, or pharmacologic responses to therapeutic intervention.” The World Health Organization (WHO), the United Nations, and the International Labour Organization jointly deﬁned a biomarker as “any substance, structure, or process that can be measured in the body or its products and inﬂuence or predict the incidence of outcome or disease.” This detailed deﬁnition included effects of treatments, interventions, and environmental exposures, such as chemicals or nutrients .
Classifications of Biomarkers in MS
Biomarkers are characterized for reasons of systematic approach, and according to their pathophysiological implication in MS pathogenesis.
The TOB-1 gene has a role in opposition to T-cell multiplication, keeping autoreactive cells in a latent state. Its decreased expression leads to a more intense immune response (higher percentage of Th1 and Th17 cells and a lower percentage of T-regulatory cells). TOB-1 polymorphisms represent an independent factor influencing the progression from clinically isolated syndrome (CIS) to clinically definite multiple sclerosis (CDMS) .
Biomarker of Neuroprotection
Vitamin D plays a potential pathogenic role in MS and can be seen in previous epidemiological studies that showed correlation between latitude and sun exposure with relative risk for developing the disease. It suppresses Th1 immune response in multiple levels and enables the production of many neurotrophic factors. 25-Hydroxy vitamin D levels in untreated MS patients inversely correlate with radiologic disease activity. Recently, a vitamin response element (VDRE) was recognized close to the HLA-DRB1∗1501 coding area, with the help of genomics. Vitamin D exhibits an inhibitory role in MS, also at a genetic level, by interacting with VDRE .
Myelin Basic Protein (MBP) and its fragments are found in great quantities in the Cerebrospinal fluid (CSF) of most MS patients during a relapse (80%). A significant correlation of decrease in CSF-MBP, contrast-enhancement in MRI, and clinical disability in response to treatment with methylprednisolone suggests a relationship between inflammation and myelin breakdown in MS .
Biomarkers of Glial Activation Dysfunction
Glial Fibrillary Acidic Protein (GFAP) is a structural protein of the astrocytes whose CSF levels increase in association with gliosis-astrocytosis. High CSF values have been found in Secondary Progressive Multiple sclerosis (SPMS) patients, but rarely in Relapsing Remitting Multiple sclerosis (RRMS) patients, and seem to correlate well with disability progression. CSF-GFAP levels are significantly higher during Neuromyelitis Optica (NMO) relapse, in comparison with MS relapse, and show adequate connection with clinical improvement and disability progression in NMO .
Biomarkers of Remyelination Repair
An example is the Brain-Derived Neurotrophic Factor (BDNF), studies have shown that there exists a Lower CSF-BDNF level in SPMS patients in relation to RRMS patients. These Low BDNF levels are considered to contribute in demyelination and axonal damage progress. An increased BDNF production was observed in Glatiramer Acetate responders, correlating well with clinical improvement .
Multiple sclerosis (MS) is a whimsical disease that affects the central nervous system, disrupting the flow of information within the brain and between the brain and the body. Understanding the mechanism underlying this disease is important for researchers and science communicators to develop therapeutic remedies, thereby effectively disseminating their findings to the public.
Shamsu is currently a graduate student with an interest in the defects in the immune system of the brain as seen in Multiple Sclerosis. He currently explores the ameliorative potentials of plants in conferring neuroprotection. He is extremely passionate about neuroscience.