Modeling links softening of myelin and spectrin scaffolds of axons after a concussion to increased vulnerability to repeated injuries

Abstract

Damage to the microtubule lattice, which serves as a rigid cytoskeletal backbone for the axon, is a hallmark mechanical initiator of pathophysiology after concussion. Understanding the mechanical stress transfer from the brain tissue to the axonal cytoskeleton is essential to determine the microtubule lattice’s vulnerability to mechanical injury. Here, we develop an ultrastructural model of the axon’s cytoskeletal architecture to identify the components involved in the dynamic load transfer during injury. Corroborative in vivo studies were performed using a gyrencephalic swine model of concussion via single and repetitive head rotational acceleration. Computational analysis of the load transfer mechanism demonstrates that the myelin sheath and the actin/spectrin cortex play a significant role in effectively shielding the microtubules from tissue stress. We derive failure maps in the space spanned by tissue stress and stress rate to identify physiological conditions in which the microtubule lattice can rupture. We establish that a softer axonal cortex leads to a higher susceptibility of the microtubules to failure. Immunohistochemical examination of tissue from the swine model of single and repetitive concussion confirms the presence of postinjury spectrin degradation, with more extensive pathology observed following repetitive injury. Because the degradation of myelin and spectrin occurs over weeks following the first injury, we show that softening of the myelin layer and axonal cortex exposes the microtubules to higher stress during repeated incidences of traumatic brain injuries. Our predictions explain how mechanical injury predisposes axons to exacerbated responses to repeated injuries, as observed in vitro and in vivo.

Dynamics of the axon plasma membrane skeleton

“…6,15 Since then, it has been acknowledged that the APMS provides lateral stiffness to the axon via the actin rings 32 and that it shields microtubules from axial stress. 12 It has also been shown that the APMS can act as a tensile shock absorber. 21…”

“…It was recently shown that the APMS can significantly shield microtubules during applied tissue stress. 12 Additionally, it has been demonstrated that the loss of beta-spectrin, one of the main components of APMS, in C. elegans leads to spontaneous breaking of axons, which is caused by mechanical strains generated by mere animal movement, and that such axon breaking phenotype can be prevented by paralyzing the animal to reduce movement induced mechanical strains. 13,14 It was also recently shown that actin rings are required to maintain microtubule organization.…”

Mechanical role of the submembrane spectrin scaffold in red blood cells and neurons

“…AFM data based on local indentation of the axonal membrane followed by coarse-grained molecular dynamics simulations also suggest that the spectrin scaffold significantly influences membrane mechanics and that damage to this skeleton can compromise axonal integrity under injury-causing conditions (Zhang et al, 2017). A recent computational analysis of the composite axon further highlights the role of the spectrin scaffold, along with other elements, under concussion or traumatic brain injury-causing conditions (Kant et al, 2021). As long axons are subjected to large deformations during body movements, especially at the joints, or during head impacts, spectrin-based tension buffering mechanisms might have evolved to offer some protection against stretch-induced damage to axons.…”

Axons-on-a-chip for mimicking non-disruptive diffuse axonal injury underlying traumatic brain injury

“…The mechanical impacts on brain tissue are transient, lasting only milliseconds to microseconds, 3,24 leading to primary mechanical injuries to the axon, including ruptured PM, distorted cytoskeleton and elevated [Ca 2+ ] axon . [25][26][27][28] Most of the current microfluidic-based AoC devices are based on the disruptive axonal severing (axotomy), 5,18,19 or uni-axial overstretching, in which tensile forces are exerted on axons or the substrates to which they tightly adhere longitudinally. 14,20 Although these models significantly advanced our understanding of axonal injury, their flaws persist, including (1) inability to mimic the mechanical stress in transverse directions; (2) incompatibility with live-imaging microscopy to capture the instant subcellular responses during impacts; and (3) inability to restrict the force loading to the axon.…”