External mechanical loading overrules cell-cell mechanical communication in sprouting angiogenesis during early bone regeneration

Sprouting angiogenesis plays a key role during bone regeneration. For example, insufficient early revascularization of the injured site can lead to delayed or non-healing. During sprouting, endothelial cells are known to be mechano-sensitive and respond to local mechanical stimuli. Endothelial cells interact and communicate mechanically with their surroundings, such as outer-vascular stromal cells, through cell-induced traction forces. In addition, external physiological loads act at the healing site, resulting in tissue deformations and impacting cellular arrangements. How these two distinct mechanical cues (cell-induced and external) impact angiogenesis and sprout patterning in early bone healing remains however largely unknown. Therefore, the aim of this study was to investigate the relative role of externally applied and cell-induced mechanical signals in driving sprout patterning at the onset of bone healing. To investigate cellular self-organisation in early bone healing, an in silico model accounting for the mechano-regulation of sprouting angiogenesis and stromal cell organization was developed. Computer model predictions were compared to in vivo experiments of a mouse osteotomy model stabilized with a rigid or a semirigid fixation system. We found that the magnitude and orientation of principal strains within the healing region can explain experimentally observed sprout patterning, under both fixation conditions. Furthermore, upon simulating the selective inhibition of either cell-induced or externally applied mechanical cues, external mechanical signals appear to overrule the mechanical communication acting on a cell-cell interaction level. Such findings illustrate the relevance of external mechanical signals over the local cell-mediated mechanical cues and could be used in the design of fracture treatment strategies for bone regeneration.Author summary: After bone injury, the timely growth of new blood vessels from existing ones—a process known as sprouting angiogenesis—is essential for a proper bone healing process. Although we know that sprouting angiogenesis is influenced by mechanical cues, how cell-induced forces or external physiological loads interact to drive sprouting angiogenesis during the early stages of bone healing remains largely unknown. To investigate this, we developed a computational framework that simulates sprouting angiogenesis during the early healing phase, since the different mechanical cues are difficult to isolate and investigate experimentally. Through the comparison of computer model predictions with the experimental data, we identified the magnitude and direction of the principal strains within the healing zone as key drivers of vessel invasion and patterning. After confirming that our computer model correctly captured vessel and cell organization during early bone healing, we performed in silico experiments to better understand the relative contribution of the various mechanical cues to the sprouting process. We found that externally applied mechanical loads overrule the cell-cell mechanical communication. Our findings suggest that external mechanical loads could be taken into consideration to design more effective treatment strategies for the regeneration of bone. This study demonstrates the importance of combining in silico and experimental techniques to gain new insights behind complex mechano-biological processes such as the mechanical regulation of sprouting angiogenesis.