| In contrast to the traditional view of multiple sclerosis (MS) as an autoimmune disease, several studies suggest that disease begins prior to the activation of the immune system and may involve an inherent defect in myelin. Proteolysis is thought to play a role in myelin breakdown and the matrix metalloproteinases (MMPs) have been implicated. To examine processes involved in initiation of demyelinating disease, the expression of MMPs was characterized in a spontaneously demyelinating mouse model of MS (ND4 line). These mice, which overexpress the myelin protein DM20, allowed investigation of MMP expression prior to and during demyelination. Stromelysin-1 (MMP-3) was found to be highly upregulated prior to onset of clinical signs in ND4 mice, suggesting a possible role for this enzyme in disease initiation. Attempts at modulating levels of MMP-3 were made by crossing the ND4 1ine with a TIMP-1-overexpressing line of transgenic mice. Levels of MMP-3 were reduced in the double transgenics from this cross (TIMP+, DM20+), however, clinical signs were only reduced in one population of double transgenics. The antimitotic drug paclitaxel was also used to suppress clinical signs in ND4 mice as well as MMP-3 protein levels. Since myelin basic protein (MBP) exists as a family of charge isomers whose relative levels differ in MS, the susceptibility of individual charge isomers to MMP-3 digestion was examined. MMP-3 showed greater activity on MBP-component 8 (C8) than on component-1 (C1). Because C8 levels are increased in MS myelin, this may have significance in the susceptibility of MS myelin to proteolytic breakdown. C1 isolated from MS tissue was also more susceptible to MMP-3 digestion than C1 isolated from normal individuals. MBP peptides resulting from MMP-3 digestion were identified and several peptides fell within known immunodominant regions of the protein. While examining proteolytic digestion of MBP, a novel cleavage mechanism was observed. The findings reported in this thesis suggest that upregulation of MMPs prior to onset of demyelinating disease may have roles in disease initiation since these enzymes are capable of generating immunodominant fragments and myelin proteins from MS patients are more susceptible to their degradation. |
