Mitochondrial dysfunction plays a key role in progressive axonal loss in Multiple Sclerosis

Multiple Sclerosis is the most common inflammatory demyelinating disease of the central nervous system and is the leading cause of non traumatic neurological disability in young adults. In recent years it has become increasingly evident that axonal degeneration is a key player in the pathogenesis of disability in MS but the mechanisms that lead to axonal damage are not fully understood. It seems likely that the causes of axonal damage vary at different stages of the disease and several theories have evolved that address the mechanisms leading to axonal loss in the acute stages of demyelination. There has been relatively little attention given to investigation of the mechanisms involved in chronic axonal loss in the progressive stages of MS. We propose a hypothesis that mitochondria play a key role in this chronic axonal loss. Following demyelination there is redistribution of sodium channels along the axon and mitochondria are recruited to the demyelinated regions to meet the increased energy requirements necessary to maintain conduction. The mitochondria present within the chronically demyelinated axons will be functioning at full capacity. The axon may well be able to function for many years due to these adaptive mechanisms but we propose that eventually, despite antioxidant defences, free radical damage will accumulate and mitochondrial function will become compromised. ATP concentration within the axon will decrease and the effect on axonal function will be profound. The actual cause of cell death could be due to a number of mechanisms related to mitochondrial dysfunction including failure of ionic homeostasis, calcium influx, mitochondrial mediated cell death or impaired axonal transport. Whatever the cause of axonal loss our hypothesis is that mitochondria are central to this process. We explore steps to test this hypothesis and discuss the possible therapeutic approaches which target the mitochondrial mechanisms that may contribute to chronic axonal loss.