Intra-axonal calcium changes after axotomy in wild-type and slow Wallerian degeneration axons

Abstract

Calcium accumulation induces the breakdown of cytoskeleton and axonal fragmentation in the late stages of Wallerian degeneration. In the early stages there is no evidence for any long-lasting, extensive increase in intra-axonal calcium but there does appear to be some redistribution. We hypothesized that changes in calcium distribution could have an early regulatory role in axonal degeneration in addition to the late executionary role of calcium. Schmidt–Lanterman clefts (SLCs), which allow exchange of metabolites and ions between the periaxonal and extracellular space, are likely to have an increased role when axon segments are separated from the cell body, so we used the oxalate-pyroantimonate method to study calcium at SLCs in distal stumps of transected wild-type and slow Wallerian degeneration (WldS) mutant sciatic nerves, in which Wallerian degeneration is greatly delayed. In wild-type nerves most SLCs show a step gradient of calcium distribution, which is lost at around 20% of SLCs within 3mm of the lesion site by 4–24h after nerve transection. To investigate further the association with Wallerian degeneration, we studied nerves from WldS rats. The step gradient of calcium distribution in WldS is absent in around 20% of the intact nerves beneath SLCs but 4–24h following injury, calcium distribution in transected axons remained similar to that in uninjured nerves. We then used calcium indicators to study influx and buffering of calcium in injured neurites in primary culture. Calcium penetration and the early calcium increase in this system were indistinguishable between WldS and wild-type axons. However, a significant difference was observed during the following hours, when calcium increased in wild-type neurites but not in WldS neurites. We conclude that there is little relationship between calcium distribution and the early stages of Wallerian degeneration at the time points studied in vivo or in vitro but that WldS neurites fail to show a later calcium rise that could be a cause or consequence of the later stages of Wallerian degeneration.