Mechanical membrane injury induces axonal beading through localized activation of calpain

Abstract

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 (Ca 2+ ) and calpain responses of axons to mechanical trauma were investigated. Intracellular Ca 2+ concentration ([Ca 2+ ] i ) shows a steady increase following injury that can be blocked by sealing membrane pores with Poloxamer 188 and by chelating intra-or extracellular Ca 2+ . Calpain activity increases in response to mechanical injury and this increase depends on Ca 2+ availability and on axolemmal permeability. Both the [Ca 2+ ] 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.

Targeting Voltage-Dependent Calcium Channels with Pregabalin Exerts a Direct Neuroprotective Effect in an Animal Model of Multiple Sclerosis

“…Ca 2+ elevation to pathological levels has serious consequences for neuronal survival since several injury mechanisms are activated, e.g. apoptotic signaling pathways through the activation of the Ca 2+ -dependent proteases calpain and caspases [53], disturbed axonal trafficking through the activation of protease activity [54] and energy deficits through oxidative stress in mitochondria as the main Ca 2+ -buffering organelles [55,56]. These triggers (oxidative stress, mitochondrial dysfunction, and Ca 2+ overload) can lead to abnormalities in glutamate signaling and finally to glutamateinduced excitotoxicity and neurodegeneration [57].…”

The Mechanics of Traumatic Brain Injury: A Review of What We Know and What We Need to Know for Reducing Its Societal Burden

“…As a result, we now know that both neural and glial cells respond to mechanical deformation, that synaptically localized receptors are uniquely mechanosensitive, show immediate alterations in their physiological properties, and changes occur across both excitatory and inhibitory neurons [190][191][192][193][194][195]. At higher loading conditions, an additional mechanism of injury appears, which is the nonspecific, transient opening of pores within the membrane [183,186,[196][197][198][199][200][201][202] after cellular deformation. In contrast to our knowledge on the effects of mechanical deformation on neural and glial cells of the central nervous system (CNS), the role of dynamic pressures in affecting cellular function is not well described.…”

Therapy Development for Diffuse Axonal Injury

“…In particular, activated calpain has been well studied with regard to proteolysis of subaxolemmal spectrin, with the resultant breakdown products detected in damaged axons and in the CSF. 38,41,42,133,134,165,168,[170][171][172] Less well known is the role of calpain on ankyrin proteolysis, which may lead to disordered sodium channel arrangement at the nodes of Ranvier, and its altered binding to neurofascin, which may contribute to axolemmal instability. 173 Several in vivo studies have examined various axonal protective strategies targeting calcium-mediated responses by either direct or indirect means.…”