Focal increases of axoplasmic Ca2+, aggregation of sodium-calcium exchanger, N-type Ca2+ channel, and actin define the sites of spheroids in axons undergoing oxidative stress.

Axonal spheroids occur as part of the pathology of a variety of neurologic diseases. Reactive oxygen species (ROS) trigger formation of spheroids, axonal severing, and Ca(2+) overload. The mechanisms by which ROS lead to the spheroid formation at specific axonal sites remain elusive. Here, using adult mouse primary neurons, we investigate the role of Ca(2+), its regulating systems, and cytoskeletal changes in formation of axonal spheroids triggered by ROS. The results reveal that dramatically higher axoplasmic Ca(2+) levels occur at the sites of axonal spheroids than in the rest of the axon. High focal axoplasmic Ca(2+) levels correlate with focal aggregation of the reverse Na(+)/Ca(2+) exchanger 1, voltage-gated N-type Ca(2+) channel α1B subunit, and actin at the sites of spheroids in individual axons. This study provides new insights into the mechanism of a spheroid formation at specific sites along axons undergoing oxidative stress and a basis for new neuroprotective strategies.