Cellular and Molecular Responses to Traumatic Brain Injury

Traumatic brain injury (TBI) is a relatively unknown disease considering the tens of millions of people affected around the world each year. Many TBI patients die from their injuries and survivors often suffer from life-long disabilities. The primary injury initiates a variety of cellular and molecular processes that are both beneficial and detrimental for the brain, but that are not fully understood. The focus of this thesis has been to study the role of astrocytes in clearance of dead cells after TBI and to identify injury specific proteins that may function as biomarkers, by using cell cultures, animal models and in cerebrospinal fluid (CSF) from TBI patients. The result demonstrates a new function in that astrocytes, the most numerous cell type in the brain, engulf dead cells after injury both in cell cultures and in adult mice and thereby save neurons from contact-induced apoptosis. Astrocytes are effective phagocytes, but degrade the ingested dead cells very slowly. Moreover, astrocytes express the lysosome-alkalizing proteins Rab27a and Nox2 as well as major histocompatibility complex class II, the receptors on which antigens are being presented. By lowering the pH of the lysosomes with acidic nanoparticles, the degradation increases, but the astrocytes still remained less effective than macrophages. Taken together, the data indicates that the low acidification in astrocytes can preserve antigens and that astrocytes may be able to activate T cells. The expression and secretion of injury-specific proteins was studied in a cell culture model of TBI by separate mass spectrometry analysis of cells and medium. Interestingly, close to 30 % of the injury-specific proteins in medium are linked to actin, for example ezrin of the ezrin/radixin/moesin (ERM) protein family. Ezrin, but none of the other ERM proteins or actin, is actively secreted after injury. Extracellular ezrin also increases in CSF in response to experimental TBI in rats and is present in CSF from TBI patients, indicating that ezrin is a potential biomarker for TBI.