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The Formation and Properties of the Fibrotic Scar After Spinal Cord Injury

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Spinal cord injury (SCI) is a devastating injury, which can be caused by motor vehicle accidents, violence, and non-traumatic causes. These injuries can leave patients with lifelong paralysis, as well as incontinence and life threatening autonomic dysreflexia. There is currently no FDA approved treatment for SCI. Spinal injury disrupts the blood-brain/spinal barrier and causes a neuroinflammatory response at the lesion site. The influx of inflammatory cells results in demyelination and secondary destruction of axons that managed to survive the initial impact. After SCI, a scar forms at the lesion site. This spinal scar is composed of both glial and fibrotic components and typically endures for the remainder of a patient’s life. The chronic spinal scar presents both molecular and mechanical barriers to neuronal regeneration. Here, we have used a variety of approaches to explore the formation and functional role of fibrotic scarring after SCI in mice. We explored the use of Immune Modifying Nanoparticles (IMPs) for the acute treatment of SCI in mice. IMP treatment selectively depletes circulating monocytes and reduces the inflammatory influx of hematogenous monocytes into spinal lesion. Acute IMP administration after SCI reduced M1 macrophage polarization and decreased the expression of inflammatory cytokines at the lesion site. IMP treatment markedly reduced the presence of the fibrotic scar at chronic time points (12 weeks post injury) without altering glial GFAP expression. The lesion sites of IMP treated mice contained increased numbers of both serotonergic axons and total axons. Most importantly, IMP treated mice demonstrated improved behavioral recovery after both moderate and severe contusion injuries. These findings indicate that early-infiltrating hematogenous monocytes are important contributors to fibrosis after SCI. In light of these promising results, we went on to explore the effect of IMPs on the mechanical properties of injured spinal tissue. Scar tissue throughout the body is typically stiffer than healthy, uninjured, tissue. However, the mechanical properties of the chronically injured spinal cord are largely unknown, and there is no consensus about the rheological environment within spinal lesions. We demonstrated that the chronically injured spinal tissue is mechanically stiffer than uninjured spinal cord. We then hypothesized that this stiffening is due to the presence of the chronic fibrotic scar. Using IMPs as a tool to reduce fibrosis, we demonstrated that IMP treatment rescues the increased tissue stiffness observed after SCI. We also discovered that that IMP treatment after SCI changes the fibrillar alignment of the glial scar network along the glial/fibrotic interface. Glial scars in IMP treated mice more closely resemble the astrocytic network present in the uninjured spinal cord. These results suggest that changes in 3D tissue stiffness after SCI may participate in altering astrocytic phenotype and in maintenance of the chronic glial scar. This highly aligned network of astrocytes within the glial scar may also contribute to stiffening of the mechanical environment. As a whole, our results suggest a mechanism in which IMP-induced decreases in chronic scarring might render the lesion core more physically accommodating to axonal growth and regeneration. These results suggest, but do not directly prove, that reduced fibrosis may be responsible for the improved behavioral outcomes observed in mice after IMP treatment. We sought to generate an orthogonal line of evidence to directly link changes in fibrotic scarring to improved behavioral recovery. Towards this goal, we explored the effects of eliminating an isoform of cellular fibronectin containing Extra Domain A domain (FnEDA) on both fibrosis and functional recovery after SCI. We demonstrated that FnEDA is expressed acutely following SCI and persists as a structural component of the insoluble fibrotic scar at chronic time points. The lack of FnEDA in a FnEDA-null mouse did not decrease the acute accumulation and assembly of fibronectin but markedly diminished chronic fibrotic scarring after SCI. Glial scarring was unchanged after SCI in FnEDA-null mice, as assessed by GFAP protein expression. We found that FnEDA was important for the long-term stability of the insoluble fibronectin matrix at both subacute and chronic time points after SCI. Functional recovery was significantly improved in FnEDA-null mice, and transgenic animals displayed an increased number of axons penetrating the lesion site as compared to wildtype siblings. The results of these studies provide insight into the role of fibrosis after SCI and suggest that the chronic fibrotic scar is indeed detrimental to motor recovery. Our work with FnEDA provides an independent line of evidence associating increased fibrotic scarring with poor behavioral recovery after SCI. Considered together with IMP treatment studies, the work in this thesis suggests that reductions in chronic fibrosis result in improved long-term behavioral outcomes after SCI. I conclude that fibrosis has primarily deleterious effects in the injured spinal cord and represents a barrier to scar resolution and recovery. Therapies targeted at mitigating the formation and maintenance of the chronic fibrotic scar, such as IMPs, may prove to be a promising and novel approach to addressing the unmet clinical need of SCI in the future.

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