Two distinct types of dying back axonal degeneration in vitro

In many neurodegenerative diseases and following traumas, dying back degeneration is a common phenomenon that aggravates the pathology and may eventually lead to death of the affected neurone. We aimed to investigate the mechanism of dying back degeneration with an in vitro axonal injury model.

We cultured adult mouse dorsal root ganglion neurones and with a precise laser beam, cut the axons they extended. Preparations were imaged continuously and images were analysed to describe and quantify ensuing events. Potential contributions of calpains and caspases to the degeneration were explored using specific inhibitors and immunohistochemistry. In vivo implications of the results were sought in nerve sections after sciatic nerve cut.

The proximal part of the transected axons went under basically two types of dying back degeneration, fragmentation and retraction. In fragmentation the cytoplasm became condensed and with concomitant axial collapse the axon disintegrated into small pieces. In retraction, the severed axon was pulled back to the soma in an organized manner. We demonstrated that fragmentation was associated with a high risk of cell death, while survival rate with retraction was as high as those of uninjured neurones. Regeneration of transected axon was more likely after retraction than following fragmentation. Activities of caspase-3 and calpains but not of caspase-6 were found linked with retraction and regeneration but not with the fragmentation.

This study describes two quite distinct types of dying back degeneration that lead an injured neurone to quite different fates.