Graduate Student University of Maryland, Baltimore
Abstract Text: Traumatic Brain Injury (TBI) is a global health concern that can cause neurological deficit, neurodegeneration, and death. If not resolved, TBI leads to secondary inflammatory responses in the nervous system due to neuronal cell death and release of endogenous Damage Associated Molecular Patterns (DAMPs). In order to control this secondary inflammation and maintain cellular homeostasis, two fundamental catabolic processes occur: macroautophagy and LC3-associated phagocytosis (LAP). Canonical autophagy is activated upon cellular stress, nutrient deprivation, and damage or dysregulation of subcellular components. Non-canonical autophagy (or LAP) is induced upon phagocytosis of extracellular cargoes such as dead/dying cells, immune complexes, and protein aggregates. LAP shares several components of autophagy machinery with canonical autophagy including Beclin1 of class III phosphatidylinositol 3-kinase (PI3KC3) complex and utilizes other unique components such as Rubicon and NADPH oxidase-2 (NOX2). Disruption of either pathway can exacerbate the pathophysiology of TBI. Evidence suggests that autophagy is impaired in neuronal cells after TBI due to activation of cytosolic phospholipase A2 (cPLA2) mediated lysosomal membrane permeabilization. Consequently, inhibition of autophagic flux leads to accumulation of autophagosomes that contribute to neuronal cell death. However, the role of LAP in TBI has not been explored extensively. Studies have shown that LAP deficiency also results in the accumulation of phagocytosed cells and myelin debris in myeloid cells, primarily microglia and macrophage, due to impairment in degradation. It is also reported that LAP-deficient macrophages that engulf dying cells induce type I IFN signaling and M1-like proinflammatory responses. Therefore, we hypothesize that LAP inhibits chronic neuroinflammation resulting from TBI and promotes recovery from the injury. Our data suggests that LAP is involved in the clearance of DAMPs and myelin debris released after injury. TBI induces the accumulation of LAPosomes (LC3+NOX2+ phagosomes) in activated macrophages and microglia. We observed that TBI in Beclin1 KO mice results in DAMP accumulation in the injured cortex tissue and reduced phagocytic capacity of microglia 3 days post-injury. Macrophage/microglia-specific Beclin1 KO mice (LysM-Cre/Becn1-fl/fl mice) also displayed impaired neurological functional outcomes post-injury. In addition, altered pro-inflammatory responses were also observed in LAP-deficient (Rubicon KO) mice following TBI. Furthermore, in vitro assays using Rubicon deficient bone marrow-derived macrophages (BMDMs) support that LAP deficiency leads to increased accumulation of myelin debris and lipid droplets within these cells. Together, these findings indicate that LAP is instrumental in processing and degrading debris and lipid droplets in macrophages/microglia, thereby contributing to resolve neuroinflammation and promoting recovery after TBI.
