The Cellular Neuroscience Research Team

Multiple Sclerosis

Multiple Sclerosis is one of the most common forms of neurodegenerative disorder in the world. The exact cause of MS is not still known, however, research suggests that a combination of several factors may play a role. MS is not considered a hereditary disease. However, a number of genetic variations have been shown to increase the risk of developing the disease [1]. Some studies have suggested a viral component to MS [2]. People in certain geographical regions are also known to be at higher risk for MS [1]. At present, the autoimmune theory represents the most widely accepted explanation of MS pathology. There is an alternative theory which is becoming more widely accepted, that of a biphasic disease consisting of an initial inflammatory phase, followed by oligodendrocyte (OL) death, demyelination, and loss of axons [3-5]. Ultimately this interferes with nerve impulse transmission to effecter targets and manifests as a variety of disease-induced symptoms such as: weakness, fatigue, cognitive dysfunction and sensory abnormalities including neuropathic pain (NPP) [6,7].

Approximately 75% of MS patients suffer from NPP, placing it as the second worst disease induced symptom [8-12]. Several studies involving the use of cytokine inhibitors, knockout mice, or direct application of cytokines with subsequent investigation of electrical activity and behavioral changes, support the involvement of TNFalpha in the development of chronic NPP [13-15].TNFalpha has also been linked to the intracellular signaling pathways that play a role in the pathogenic activation of DRG cells following inflammation or injury [14,16]. Further, TNFalpha has also been shown to directly induce neuronal production of neuropeptides and inflammatory agents such as substance P and calcitonin gene related peptide (CGRP) within the DRG and spinal cord [17,18]. The documented effects of CGRP and Substance P on neuropathic pain are well known [17]. The dorsal root ganglia (DRG) regulate pain [19]. DRG are the sensory ganglia associated with the spinal nerves. The main function of DRG is to regulate and maintain sensory homeostasis. During period(s) of immune activation, rapid and sustainable bursts of inflammatory cytokine activity in the DRG (e.g. TNFalpha, IL-12, IFNgamma) may serve as the abnormal stimulus that eventually disrupts this sensory equilibrium [16,20,21]. The vasculature surrounding DRG is highly permeable and may facilitate the bi-directional transport of cytokines between the DRG and peripheral blood [22]. As a result, DRG could function as a pivotal reservoir for MS-induced inflammatory cytokines accounting for direct effects on sensory neurons [16,23,24].

Model describing the role of the DRG in the inflammatory induction of MS and NPP We have developed a model to describe the role of the DRG in the inflammatory induction of MS and NPP [16]. Antigenic induction of inflammatory cytokines in the DRG and/or spinal cord in the early stages of MS identifies a novel mechanism for MS induced neuropathic pain and may trigger the induction of the neurotrophins brain derived neurotrophic factor (BDNF) and nerve growth factor (NGF) that regulate downstream effects on myelin [14,16,24,25]. We are studying the role of cytokines and neurotrophins in the development of MS in rodent models of NPP and MS (the sciatic nerve axotomy and experimental autoimmune encephalomyelitis models). We are studying the expression patterns of these proteins in the DRG and the spinal cord at different stages of the disease processes.

Myelination

Myelin is vital for normal mammalian development. Myelin facilitates efficient conductance of signals along the nerve. The importance of myelination to normal brain function is evident from the pathology of numerous disorders, e.g. leukodystrophies [26], Pelizaeus-Merzbacher Disease [27,28], cerebral palsy [29,30]. Further, Schizophrenia [31-33], age related cognitive decline [34], major depression and bipolar disorder[33], Down's Syndrome [35], and autism [36-38] are all characterized by myelin defects. Oligodendrocytes (OL) produce and maintain CNS myelin. The OL originates as a pre- progenitor cell in the telencephalon of the developing brain [39]. From there the pre- progenitor cells migrate across the sub-pallial layer to the sub-ventricular zone (SVZ), where they proliferate and differentiate into OL progenitors (OPs) [40]. The OPs then undergo further migration away from the SVZ, to populate the developing white matter tracts of the brain [41]. Once the OPs have migrated to their final destination, they undergo further proliferation, prior to maturing into the myelinating OL. There are five basic phases of the OL lineage: generation, migration, proliferation, differentiation and myelination. Four of these five phases are dependent on the successful migration of the precursor cells away from their site of generation, to their site of proliferation and differentiation prior to successful myelination. Our research is focused on the regulation of OL progenitor cell (OP) migration, and differentiation, during normal and abnormal cortical development. Our overall goal is to elucidate the regulatory mechanisms surrounding OL distribution, and myelination in the developing brain, and compare normal behaviour with that seen in disease models.

There are currently two myelin projects running in the lab.

  1. The role of receptor tyrosine kinases in the regulation of oligodendrocyte behavior [42-44]

  2. The role of MeCP2 in the regulation of oligodendrocyte behavior [45,46]

We use assays of migration [47], proliferation and differentiation [48], alongside molecular biology analysis (Western Blot, immunoreactivity, RT-PCR) and imaging (light and fluorescent microscopy, time-lapse videomicroscopy) to analyse cell behaviour. In addition, calcium flux measurements are performed in collaboration with Dr. Ratna Bose (Department of Pharmacology) [43].

Experimental Model

We have identified several different factors that regulate the migration of OP through the developing CNS [29,42-44,46,48-51]. Based on these findings, we have developed a model to describe OP migration away from the germinal matrix.

Model to explain OP migration in the developing CNS

Research model

In this model OP are stimulated to migrate away from the germinal matrix by a combination of chemorepellents, such as netrin [52], and motogenic factors, such as PDGF [44]. Once the cells have begun to migrate, they will move around randomly until they find a permissive substrate upon which to migrate [41]. Such permissive substrates are provided by the axons of the neuronal cytoarchitecture, and possibly also by radial glial cells. Once the OPs have found a permissive substrate upon which to migrate, they will move along that path until they are given the signal to stop migrating. This signal is most likely a localised concentration of the chemokine, CXCL1 (formerly Gro-alpha) [49], at which point the OP are exposed to a combination of mitogenic factors which enhances their proliferation [53]. Once sufficient OP have been generated to myelinate the axons of the specific location, OP differentiation is triggered. OP that are in excess of the number of OL required to myelinate that area either undergo apoptosis [54], or remain as OP to effect minimal repair of the adult CNS [48,55].

Where the paths cross
Myelinating spinal cord culture, myelin in green (MBP immunostain) and neurons in red (neurofilament immunostain)

We are using in vitro assays to further our understanding of the role of inflammatory mediators and neurotrophins in the onst of MS. We have developed an assay of myelination so that we can assess the effect of inflammatory mediators on myelination and remyelination in vitro. In addition, we can assess specific signalling events that are involved in demyelination resulting from inflammation.




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Updated 9:58 PM 21/12/2018
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Dr. Emma E. Frost