Molecules affecting myelin stability: a novel hypothesis regarding the pathogenesis of multiple sclerosis

In this Mini-Review we present a new hypothesis in support of the neurodegenerative theory as a mechanism for the pathogenesis of multiple sclerosis (MS). The pathogenesis of MS results from changes in two distinct CNS compartments. These are the "myelin" and "nonmyelin" compartments. The myelin compartment is where primary demyelination, amidst attempts at remyelination, is superseded in the CNS by ongoing disease. Recent evidence obtained via magnetic resonance imaging and spectroscopy techniques supports the view that the normal-appearing white matter (NAWM) in the MS brain is altered. Several biochemical changes in NAWM have been determined. These include the cationicity of myelin basic protein (MBP) as a result of the action of peptidyl argininedeiminase (PAD) activity converting arginyl residues to citrulline. The accompanying loss of positive charge makes myelin susceptible to vesiculation and MBP more susceptible to proteolytic activity. An increase of MBP autocatalysis in the MS brain might also contribute to the generation of immunodominant epitopes. Accompanying the destruction of myelin in the myelin compartment is the activation of astrocytes and microglia. These contribute to the inflammatory response and T-cell activation leading to autoimmunity. The complex environment that exists in the demyelinating brain also affects the "nonmyelin" compartment. The inappropriate up-regulation of molecules, including those of the Jagged-1-Notch-1 signal transduction pathway, affects oligodendrocyte precursor cell (OPC) differentiation. Other effectors of oligodendrocyte maturation include stathmin, a microtubule-destabilizing protein, which prevents healing in the demyelinating brain. The hypothesis we present suggests a therapeutic strategy that should 1) target the effectors within the myelin compartment and 2) enable resident OPC maturation in the nonmyelin compartment, allowing for effective repair of myelin loss. The net effect of this new therapeutic strategy is the modification of the disease environment and the stimulation of healing and repair.