The challenges of axon survival: introduction to the special issue on axonal degeneration.

Early axon loss is a common feature of many neurodegenerative disorders. It renders neurons functionally inactive, or less active if axon branches are lost, in a manner that is often irreversible. In the CNS, there is no long-range axon regeneration and even peripheral nerve axons are unlikely to reinnervate their targets while the cause of the problem persists. In most disorders, axon degeneration precedes cell death so it is not simply a consequence of it, and it is now clear that axons have at least one degeneration mechanism that differs from that of the soma. It is important to understand these degeneration mechanisms and their contribution to axon loss in neurodegenerative disorders. In this way, it should become possible to prevent axon loss as well as cell death. This special edition considers the roles and mechanisms of axon degeneration in amyotrophic lateral sclerosis, Charcot-Marie-Tooth disease, hereditary spastic paraplegia, ischemic injury, traumatic brain injury, Alzheimer's disease, glaucoma, Huntington's disease and Parkinson's disease. Using examples from these and other disorders, this introduction considers some of the reasons for axon vulnerability. It also illustrates how molecular genetics and studies of Wallerian degeneration have contributed to our understanding of axon degeneration mechanisms.