NRSA Fellowship for Timothy Kopper: Contributions of Myelin Derived Arachidonic Acid to Macrophage Polarization and Secondary Damage after Spinal Cord Injury

Spinal cord injury (SCI) produces a chronic inflammatory state primarily mediated by resident microglia and infiltrating monocytes (here, collectively referred to as macrophages). These chronically activated SCI macrophages adopt a pro-inflammatory, pathological state that continues to cause additional damage after the initial injury and inhibits recovery. Myelin debris generated after SCI can directly stimulate macrophages to adopt these maladaptive functions; however, the mechanisms by which this stimulation occur are poorly understood. Myelin contains an inflammatory lipid molecule, arachidonic acid, stored in an inactive form within myelin’s extensive membranes. After injury, macrophages clear and accumulate myelin debris long after injury. Here, we propose that the inflammatory environment within the SCI lesion induces the expression of the enzyme cytosolic phospholipase A2 (cPLA2) within macrophages. cPLA2 releases arachidonic acid from cellular membranes. Free, active arachidonic acid invokes direct pro-inflammatory functions or, through synthesis, forms various eicosanoids with pro-inflammatory properties. This is the basis for the central hypothesis that increased cPLA2 activity in SCI macrophages releases arachidonic acid from myelin debris thereby driving chronic SCI inflammation and tissue damage. . Using in-vitro and in-vivo approaches, Aim 1 of this proposal will identify cPLA2 as the regulator of myelin-mediated inflammation in macrophages. We will thereby demonstrate the importance of myelin-derived arachidonic acid in these critical inflammatory responses. Aim 2 will utilize cPLA2-/- bone marrow chimeric mice to interrogate the importance and influence of macrophage cPLA2 in the pathogenesis of SCI. Chronic proinflammatory macrophage activation is a key mediator of secondary damage and impaired recovery after SCI, yet the mechanisms behind this process are poorly understood. Here I propose a novel mechanism by which myelin debris and subsequent processing by macrophage cPLA2 continuously produces free arachidonic acid thereby driving the chronic inflammation observed after SCI.