Multiscale analysis of arterial bypass remodeling: Mechanical characterization and influence of endothelial injury on vascular smooth muscle cell proliferation

Up to 50% of arterial bypass grafts re-occlude due to vascular smooth muscle cell (VSMC) over-proliferation thought to result from wall stress and endothelial (EC) injury. A deeper understanding of graft remodeling by measuring macroscale and microscale mechanics and effects of EC injury on VSMC proliferation could lead to therapies that specifically prevent occlusive over-proliferation while allowing EC healing and beneficial remodeling.; In an ultrasound study of lower extremity peripheral bypass grafts, we observed a significant increase in wall stiffness and thickness over 6 months, resulting in lower wall tension near common arterial levels. Changes in wall thickness and stiffness did not correlate with lumenal narrowing or occlusion, suggesting that wall thickening and stiffening lowers wall tension while occlusion may be a more local phenomenon.; A novel microindentation method was developed to measure mechanical properties of soft, hydrated biomaterials and tissues. We found that the microscale modulus of polyacrylamide, used to study VSMC behavior, correlates with the nanoscale modulus measured by atomic force microscopy as well as macroscopic tensile tests. Using microindentation, we characterized the microscale modulus of human greater saphenous vein and found that microscale modulus is much lower than the macroscale elastic modulus resisting arterial pressure. These results allow the manufacture of substrates that accurately mimic vascular mechanics for the investigation of mechanical effects on VSMCs.; Experiments with direct transmembrane co-culture found that VSMCs directly opposite the injured/recovered portion of an EC injury front had significantly increased proliferation. This increase was not apparent in other regions of the same cultures or in uninjured co-cultures. The inhibition of the PDGF receptor significantly reduced VSMC proliferation in the injured/recovered region. Connexin43 aggregation in co-cultured cells suggests gap junction formation and the disruption of gap junctions also resulted in lower proliferation rates in the injured/recovered region. These results demonstrate that an EC injury front stimulates VSMC proliferation only in a localized region and this stimulation may depend on both PDGF and gap junction signaling. Further research into these mechanisms could lead to therapies that specifically target VSMC over-proliferation due to EC injury without hindering other remodeling processes that reduces wall tension.