Professor and Chair of Biomedical Engineering University of Virginia, United States
Introduction:: In idiopathic pulmonary fibrosis (IPF), progressive extracellular matrix stiffening and dysregulated levels of growth factors, such as vascular endothelial growth factor (VEGF), disrupt endothelial cell (EC) to pericyte cell communication. Physical coupling of ECs with pericytes in the microcirculation is necessary for maintaining normal microvessel density and tissue perfusion in the wound healing response. Multi-scale computational models allow us to investigate how these different overlapping cues are integrated to lead to cell and tissue level outcomes, as well as isolate how perturbations in one variable affect the overall system. We have developed a multi-scale computational model that simulates intracellular signaling in ECs and pericytes, heterotypic cell-cell communication, and the dynamic lung microenvironment to study how changes in microenvironmental VEGF and stiffness affect microvessel stability. We hypothesize that endothelial cell-pericyte coupling will decrease with decreased VEGF and increased microenvironmental stiffness.
Materials and Methods:: The multi-scale model is comprised of logic-based ordinary differential equations representing intracellular signaling networks in ECs and pericytes that are integrated into an agent-based model representing the lung environment. Simulated cells interact with one another, sensing and dynamically altering their microenvironment (Figure 1A). The logic-based network signaling models were developed using the Netflux toolkit in MATLAB. The agent-based model of the spatiotemporal 2D lung environment was constructed in NetLogo.
Results, Conclusions, and Discussions:: At a stiffness of healthy lung (2 kPa) and fibrotic foci (10kPa) low VEGF levels potentiated angiogenesis leading to an increase in vessel area. At stiffnesses closer to that of mature lung fibrosis, 15 kPa and 20 kPa, the presence of VEGF had no significant effect on vessel area (Figure 1B). Interestingly, when we quantify endothelial cell apoptosis we observe that at a stiffness of 15 kPa, between fibrotic foci and mature fibrosis, there is a significant increase in apoptosis that is not associated with significant differences in vessel area (Figure 1C). This potentially indicates rapid and disorganized angiogenesis and regression in response to competing microenvironmental cues. These results may in part help explain conflicting reports of vascularity in fibrotic foci and suggest that targeting VEGF-R may be a viable therapeutic option for IPF.
By simulating intracellular biochemical signaling networks, heterotypic cell interactions, and the dynamic multi-cell microenvironment, the model predicted that low levels of VEGF and increased microenvironmental stiffness causes hyper-remodeling of capillaries during IPF progression as signified by rapid rates of EC proliferation and apoptosis. Our multi-scale model demonstrates feasibility of using computational methods to simulate complex IPF microenvironments, and demonstrates the importance of considering EC-pericyte communication in predicting cellular responses to pro-fibrotic environments. Future work will explore the use of different schemes of progressive stiffening such as incorporating the generation of fibrotic scar on a patch-by-patch basis using a random walk method, similar to those proposed in computational mechanical models of IPF scar progression.
