Mechanical membrane injury induces axonal beading through localized activation of calpain

Diffuse axonal injury (DAI), a major component of traumatic brain injury, is characterized by a sequence of neurochemical reactions initiated at the time of trauma and resulting in axonal degeneration and cell death. Calcium influx through mechanically induced axolemmal pores and subsequent activation of calpains are thought to be responsible for the cytoskeletal damage leading to impaired axonal transport. Focal disruption of cytoskeleton accompanied by the accumulation of transported membranous cargo leads to axonal beading which is the characteristic morphology of DAI. By applying fluid shear stress injury on cultured primary neurons, acute calcium (Ca2+) and calpain responses of axons to mechanical trauma were investigated. Intracellular Ca2+ concentration ([Ca2+]i) shows a steady increase following injury that can be blocked by sealing membrane pores with Poloxamer 188 and by chelating intra- or extracellular Ca2+. Calpain activity increases in response to mechanical injury and this increase depends on Ca2+ availability and on axolemmal permeability. Both the [Ca2+]i increase and calpain activity exhibit focal peaks along the axons which co-localize with mitochondria and predict future axonal bead locations. These findings suggest that mechanoporation may be the initiating mechanism resulting in ensuing calcium fluxes and subsequent calpain activity and that post-injury membrane repair may be a valid therapeutic approach for acute intervention in DAI.