A recoverable state of axon injury persists for hours after spinal cord contusion in vivo

Abstract

Therapeutic strategies for spinal cord injury (SCI) commonly focus on regenerating disconnected axons. An alternative approach would be to maintain continuity of damaged axons, especially after contusion. The viability of such neuropreservative strategies depends on the degree to which initially injured axons can recover. Here we use morphological and molecular in vivo imaging after contusion SCI in mice to show that injured axons persist in a metastable state for hours. Intra-axonal calcium dynamics influence fate, but the outcome is not invariably destructive in that many axons with calcium elevations recover homeostasis without intervention. Calcium enters axons primarily through mechanopores. Spontaneous pore resealing allows calcium levels to normalize and axons to survive long term. Axon loss can be halted by blocking calcium influx or calpain, even with delayed initiation. Our data identify an inherent self-preservation process in contused axons and a window of opportunity for rescuing connectivity after nontransecting SCI.

In vivo imaging reveals rapid astrocyte depletion and axon damage in a model of neuromyelitis optica‐related pathology

“…Similar to experimental autoimmune encephalomyelitis (EAE), a widely used rodent model of MS, axons exposed to AQP4‐IgG and complement developed early focal swellings. In EAE, however, swollen axons are often severed,21 an irreversible form of damage commonly observed after blunt trauma 22. Such axonal transections were not evident in our NMO‐related model, at least during the first 8 hours, suggesting that there could be potential for reversing axonal damage early in an NMO lesion, as has been observed previously in other models of neuronal damage 21, 22, 37, 38.…”

“…Our in vivo model provides a unique opportunity to investigate whether NMO‐related astrocyte toxicity affects surrounding axons, given that axon damage can be followed by two‐photon imaging in great detail 21, 22. We used combinatorial transgenic labeling of astrocytes and axons ( Aldh1l1 :GFP ×  Thy1 :OFP double transgenic mice) and followed changes in axons for up to 6 hours after application of NMO samples (Figs 2A and 4A, Supplementary Video 3).…”

“…In EAE, however, swollen axons are often severed,21 an irreversible form of damage commonly observed after blunt trauma 22. Such axonal transections were not evident in our NMO‐related model, at least during the first 8 hours, suggesting that there could be potential for reversing axonal damage early in an NMO lesion, as has been observed previously in other models of neuronal damage 21, 22, 37, 38. In addition to revealing new aspects of axon injury, our in vivo model also allows investigating the propagation of injury across the glial‐neuronal network starting from a well‐defined initial lesion to astrocytes.…”

“…Percentage of cell loss was defined as percentage of cells that lost >50% of their fluorescence, which was accompanied by clear changes in morphology (rounding, process blebbing). Axon morphology in a neuroinflammatory setting was staged as described previously 21, 22. Only axons that were observable across all time points during the experiment over at least 50 µm were analyzed.…”

Autophagy induction stabilizes microtubules and promotes axon regeneration after spinal cord injury

“…Interestingly, induced autophagy stabilizes MTs by down-regulating SCG10. Notably, local and temporal Tat-Bec administration attenuates axonal retraction within the critical window that has been shown to be crucial for the regeneration potential of injured axons (60). In line with this notion, Tat-Bec administration also promotes axon regeneration and consequently improves locomotor ability after SCI.…”