Improving Axon Regeneration in the Chronically Injured Spinal Cord Through Delayed Macrophage Manipulation

Scientific abstract: Macrophages inhibit axon regeneration and persist chronically within spinal cord lesions. The goals of this project are to elucidate the role of chronically dwelling macrophages on regenerative failure, as well as improve regenerative treatments by depleting macrophages in chronic spinal cord injury (SCI). Investigating chronic SCI is a top priority for individuals living with paralysis and therapeutic investigations applicable to chronic SCI settings remain understudied. Axon regeneration has been the focus of treatment efforts to restore lost function in severe cases of chronic SCI, however regeneration in chronic SCI is limited compared to acute settings. Advances in regenerative therapies for chronic SCI have demonstrated proof-of-principal that limited regeneration is possible and can be enhanced through combinatorial approaches. Axons which have been coerced to regenerate in chronic SCI avoid growing through the macrophage-rich lesion core that persists chronically after injury. Instead, axons regrow through regions of spared tissue or locations with astrocyte bridges spanning the lesion. Due to limited regions capable of supporting axon regeneration, most fibers still do not successfully regenerate. SCI lesion cores contain permanently elevated densities of macrophages and other inhibitory fibrotic scar components. Acutely after SCI, macrophages collapse growth cones, lead to axon dieback away from the lesion, and orchestrate cascades that result in the deposition of extracellular matrix compounds that are inhibitory to axon growth. Indeed, infiltrating macrophages have been determined to be a primary contributor to astrocyte and fibroblast activation. However, the role of these macrophages on preventing regeneration in chronic SCI is poorly understood. This project will determine the role of chronically dwelling macrophages within the SCI lesion on inhibiting axons that are stimulated to regenerate chronically after injury. We have optimized methods to deplete macrophages residing within the lesion core at chronic SCI time points, as well as developed a novel gene delivery approach capable of targeting all spinal-projecting neurons than have been damaged after injury. Aim 1 of this work will utilize our macrophage depletion technique to investigate the role of chronically dwelling macrophages after SCI on the maintenance of a growth-inhibitory environment. Aim 2 will determine the contribution of macrophages to regenerative failure in chronic SCI settings by depleting macrophages from the SCI lesion using concurrent regenerative strategies initiated chronically after injury. Data collected from this project will determine the role of chronic neuroinflammation on impedance of axon growth and repair and will develop an approach for future experiments aimed at translating chronic regenerative treatments into real improvements for individuals with spinal cord injury.