A novel role of EphA4/Stat6/Mertk signaling in cleaning up the brain after trauma

Abstract

Abstract Traumatic brain injury (TBI) is a major cause of disabilities and deaths from acute injuries in the US and worldwide. Neuroinflammation and neuronal apoptosis are hallmarks of TBI; however, the mechanism of clearance of apoptotic debris by efferocytosis remains ill-defined. Our previous studies demonstrated that EphA4 receptor in peripheral-derived macrophage (PDM) mediates the pro-inflammatory response after injury. Here, we aimed to evaluate whether PDM-specific EphA4 restricts efferocytosis following TBI. We generated wild-type (WT) and EphA4-knockout (KO), GFP +bone marrow (BM) chimeric mice and performed Controlled cortical impact injury. We observed that KO mice displayed improved motor function, cerebral blood flow, reduced cortical cell death as measured by TUNEL, and enhanced efferocytosis compared to WT mice. The percentage of GFP +/Iba1 −expressing PDMs that engulf NeuN +or TUNEL +cells was significantly higher in KO mice (p<0.05, n=5). We further performed an in vitro efferocytosis assay using pHrodo-labeled apoptotic Jurkat cells co-cultured with GFP +WT and KO BM-derived macrophages. We found a significant increase in efferocytosis efficiency in KO compared to WT cells (p<0.05, n=4). In addition, mRNA isolated from KO cells show increased expression of the efferocytosis receptor Mertk and its ligands Gas6 and Pros1 and phospho-array analysis shows increased p-Stat6, a key regulator of Mertk/Gas6. Interestingly, the use of Stat6 and Mertk inhibitors attenuated efferocytosis effects in KO macrophages in-vitro. Our findings implicate the EphA4/Stat6/Mertk axis as a novel regulator of apoptotic debris clearance that may be targeted to augment resolution of inflammation following TBI. National Institute of Neurological Disorders and Stroke of the National Institutes of Health, NS096281, NS119540, and NS121103 (MHT)