Cross-section of the spinal cord of a transgenic mouse expressing green fluorescent protein with a subset of labelled axons. The red box (magnification ×2) delineates the superficial dorsal funiculus where sensory axons can be imaged in vivo. Reproduced, with permission, from Nature Medicine.

A technical report published recently in Nature Medicine shows that it is feasible to image injured axons in the spinal cord in vivo, disclosing new features of axonal degeneration and providing a powerful tool for evaluating therapies that might enhance regeneration.

Using transgenic mice that expressed green fluorescent protein in neurons of the dorsal root ganglia (DRG), Kerschensteiner et al. succeeded in visualizing DRG axons over several spinal segments with wide-field microscopy. They then lesioned a bundle of axons containing a single labelled fibre, and followed the fate of its proximal and distal ends for several days.

They found that the axon ends were stable for the first 10–20 min after the lesion, but then underwent a sudden fragmentation, a process that the authors termed 'acute axonal degeneration' (AAD). This dieback process lasted less than 5 min, but accounted for nearly 90% of the axonal loss that was observed 4 h after injury. Moreover, if AAD of the proximal axon spread beyond a branch point, it also resulted in disconnection of the unlesioned branch from the soma.

By 30 h after injury, both the proximal and distal axons had died back symmetrically; the degeneration extended 300 μm from the lesion site in both directions. By contrast, at later time points, the distal axon experienced the well-known process of Wallerian degeneration, whereas the proximal axon remained stable and, in some cases, even attempted to regrow. However, these attempts to regenerate were remarkably inefficient; the regenerating axons seemed to lack directional information. The authors found no axon that managed to grow back to the lesion site, even though they grew long distances (as long as 1 mm 2 weeks after the lesion). Instead, fibres grew laterally or even in a 'U-turn' trajectory.

It is interesting to compare this erratic behaviour with the ability of peripheral axons to grow in a straight trajectory after a lesion, following a route close to their original path. The inability of central axons to navigate in the proper direction might account, at least in part, for their limited success in extending past the lesion site.

Despite their different time courses, AAD and Wallerian degeneration might share similar molecular mechanisms. The authors found that in 'Wallerian-degeneration slow' mice, which show delayed Wallerian degeneration, AAD was largely absent. Moreover, calpain, which is known to participate in Wallerian degeneration, also seems to be a mediator of AAD, as is shown by the ability of calpain inhibitors to prevent it in vivo. Additional similarities and differences between AAD and Wallerian degeneration might now be established using this technique.

Beyond the description of the degeneration process, this method can also be used to monitor the efficacy of interventions aimed at preventing degeneration or promoting regeneration, as exemplified by the use of calpain inhibitors in this report. Moreover, as the response of axons to damage is relevant not only to spinal cord injury but also to conditions such as multiple sclerosis and amyotrophic lateral sclerosis, this technique might be used to provide new insights into these diseases.