Abstract

Damage to axons determines the clinical deficit in many neurological conditions including multiple sclerosis and traumatic spinal cord injury. In this presentation I want to introduce new approaches which allow us to follow the degeneration and regeneration of single axons in the spinal cord of living animals. To investigate the behaviour of single axons after transaction we have developed an in vivo imaging approach that reveals the sequence of events that unfold after spinal cord injury with high temporal and spatial resolution. Using a combination of modern imaging and transgenic labeling of neuronal subsets we monitor individual fluorescent axons over several days after transection in the spinal cord of living mice. Our results indicate that axonal die-back after transection is mediated by an acute form of axonal degeneration which is similar in mechanisms to delayed Wallerian degeneration. In vivo imaging further reveals that many axons attempt regeneration within 24 hours after lesion. This growth response appears to fail due to the lack of directional information. Time-lapse imaging of single axons can thus provide a powerful analytical tool for assessing the pathogenesis and therapy of spinal cord injuries. More recently we have adapted this in vivo imaging approach to investigate the pathogenesis of immune-mediated axon damage in an animal model of multiple sclerosis. By time-lapse imaging of fluorescently labeled axons we could follow the slow and spatially restricted degeneration of axons in inflammatory CNS lesions. This “focal axonal degeneration” appears to be a novel type of axonal degeneration that can be differentiated from post-traumatic forms of axonal degeneration like Wallerian degeneration e.g. by its limited extension and slow speed of progression. We could further identify intermediate stages of “focal axonal degeneration” that can persist for several days and progress either to the degeneration or full recovery of the affected axons. The early stages of “focal axonal degeneration” are often associated with persistent macrophage contacts suggesting that macrophage-derived mediators play a crucial role for the induction of this process. Using these models of spinal cord injury and multiple sclerosis I hope to illustrate how in vivo imaging can help us to understand the pathogenesis of axon injury and pave the way towards the development of targeted neuroprotective therapies.