The Role of the Fibrotic Scar in Repair Following Neuroinflammation

Abstract

Multiple sclerosis (MS) is a neuroinflammatory disease of the central nervous system (CNS) in which the body’s immune system attacks the myelin sheath that surrounds and insulates axons. In many cases this myelin is not repaired by myelinating oligodendrocytes, which decreases the efficiency of action potential conduction and leads to neural dysfunction. We hypothesized that a barrier preventing oligodendrocyte lineage cells from repairing damaged myelin is a fibrotic scar. Following CNS injury, a scar consisting of an outer glial scar made up of reactive astrocytes and an inner fibrotic scar made of proteins such as collagen I forms around the site of trauma. In MS the glial scar has also been characterized, but the presence of a fibrotic scar has not been investigated. I have shown that following induction of experimental autoimmune encephalomyelitis (EAE) in mice, which is used as a model of MS, an extensive fibrotic scar forms in the lesioned tissue. Scar‐forming cells were visualized in this tissue using a Col1a1GFP mouse model. The number of these cells increased rapidly in the lesion site following symptom onset and remained high throughout the course of the disease. Lineage tracing and single cell RNA sequencing determined that these cells arise from the proliferation of fibroblasts and not other cells such as pericytes turning on the production of collagen. The objective of this project is to determine the role of the fibrotic scar in tissue repair and to determine mechanisms of scar formation in the CNS. To determine the role of the fibrotic scar following EAE I used the herpes thymidine kinase system to ablate proliferating fibroblasts. Using this paradigm I was able to reduce scar formation by 60% in EAE and found that this reduction resulted in a decrease in motor symptoms in the later stages of disease concurrent with an increase in oligodendrocyte lineage cells in the inflammatory lesions. To understand the signaling pathways that play a role in CNS scar formation, I used FACS to purify Col1a1GFP+ fibroblasts from spinal cords of healthy mice and mice with EAE and analyzed their transcriptome by RNA sequencing. I found that these cells upregulate inflammatory signaling pathways in disease such as the interferon gamma pathway. Deleting the interferon gamma receptor in CNS fibroblasts resulted in a decrease in scar formation following EAE and may be a potential therapeutic target for CNS disorders with fibrotic scarring. In conclusion I identified a fibrotic scar forms following neuroinflammation that arises from the proliferation of CNS fibroblasts, plays a role in disease recovery and forms in part through interferon gamma signaling.