A Two-Compartment Model of VEGF Distribution in the Mouse

Citation: Yen P, Finley SD, Engel-Stefanini MO, Popel AS ( A Two-Compartment Model of VEGF Distribution in the Mouse Phillip Yen. 0 Stacey D. Finley 0 Marianne O. Engel-Stefanini 0 Aleksander S. Popel 0 Christos Chatziantoniou, Institut National de la Sante et de la Recherche Medicale, France 0 Department of Biomedical Engineering, Johns Hopkins University School of Medicine , Baltimore, Maryland , United States of America Vascular endothelial growth factor (VEGF) is a key regulator of angiogenesis - the growth of new microvessels from existing microvasculature. Angiogenesis is a complex process involving numerous molecular species, and to better understand it, a systems biology approach is necessary. In vivo preclinical experiments in the area of angiogenesis are typically performed in mouse models; this includes drug development targeting VEGF. Thus, to quantitatively interpret such experimental results, a computational model of VEGF distribution in the mouse can be beneficial. In this paper, we present an in silico model of VEGF distribution in mice, determine model parameters from existing experimental data, conduct sensitivity analysis, and test the validity of the model. The multiscale model is comprised of two compartments: blood and tissue. The model accounts for interactions between two major VEGF isoforms (VEGF120 and VEGF164) and their endothelial cell receptors VEGFR-1, VEGFR-2, and co-receptor neuropilin-1. Neuropilin-1 is also expressed on the surface of parenchymal cells. The model includes transcapillary macromolecular permeability, lymphatic transport, and macromolecular plasma clearance. Simulations predict that the concentration of unbound VEGF in the tissue is approximately 50-fold greater than in the blood. These concentrations are highly dependent on the VEGF secretion rate. Parameter estimation was performed to fit the simulation results to available experimental data, and permitted the estimation of VEGF secretion rate in healthy tissue, which is difficult to measure experimentally. The model can provide quantitative interpretation of preclinical animal data and may be used in conjunction with experimental studies in the development of pro- and anti-angiogenic agents. The model approximates the normal tissue as skeletal muscle and includes endothelial cells to represent the vasculature. As the VEGF system becomes better characterized in other tissues and cell types, the model can be expanded to include additional compartments and vascular elements. - Funding: This work was supported by the National Institutes of Health (NIH) grants R01 HL101200 and R01 CA138264. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. . These authors contributed equally to this work. Vascular endothelial growth factor (VEGF) belongs to a family of cytokines that play an important role in angiogenesis the formation of new capillaries from pre-existing vessels. The VEGF family in mammals is composed of VEGF-A, VEGF-B, VEGF-C, VEGF-D and placental growth factor (PlGF). The most well-studied member is VEGF-A (generally referred to as VEGF) that consists of several splice isoforms including VEGF121, VEGF145, VEGF165, VEGF189 and VEGF206 in humans, where the subscripted number indicates the number of amino acids [1,2]. The amino acid number for all splice isoforms is one less than in humans for rodent VEGF orthologs. The roles of VEGF189 and VEGF206 in vivo are currently still unclear [3]. Therefore, in our model, we consider the two most abundant isoforms of VEGF-A in the mouse: VEGF120 and VEGF164. The tyrosine-kinase receptors of VEGF include VEGFR-1 (Flt-1), VEGFR-2 (Flk-1 or KDR in humans), and VEGFR-3 (Flt-4). VEGFR-1 and VEGFR-2 are the primary receptors for VEGF-A and play a major role in angiogenesis, while VEGFR-3 binds VEGFC and VEGF-D and plays a major role in lymphangiogenesis. VEGFR-1 and VEGFR-2 are predominantly expressed on endothelial cells; however, these receptors have also been shown to be present on bone marrow-derived cells [4] and other cell types such as neurons and cancer cells. The binding of VEGF-A to VEGFR-2 is believed to be the main signaling pathway for angiogenesis [5]. In addition to the tyrosine-kinase receptors, VEGF-A binds to co-receptor neuropilin-1 (NRP-1). NRP-1 was first found to be expressed on certain tumor and endothelial cell surfaces [5], and has been shown to enhance the binding of VEGF165 to VEGFR-2. VEGF can also bind to heparan sulfate proteoglycans in the extracellular matrix (ECM), endothelial cell basement membrane (EBM) and parenchymal cell basement membrane (PBM). Computational models of VEGF-mediated angiogenesis have been developed to study various aspects of the angiogenic process [6]. A single-compartment model of the human tissue was initially developed to study the kinetic ligand-receptor interactions of multiple VEGF isoforms with endothelial cell surface receptors (VEGFR-1, VEGFR-2, NRP-1) and extracellular matrix binding sites [7]. This model was later expanded to include three compartments, including a tumor compartment, to study tumor angiogenesis [8] and peripheral arterial disease [9]. These compartment models describe spatially averaged VEGF distributions and receptor bindings in the tissue, blood and tumor. However, these models were based on human data and are not immediately applicable to animal data. Mouse animal models have been extensively used to study cardiovascular diseases such as peripheral arterial disease and coronary artery disease [10]. Mouse tumor xenograft models are also commonly used to study different cancers and to develop anti-tumor therapies. Mice are convenient animal models to study human diseases because the overall biology of the mouse is in many respects similar to that of humans, and the two species share many similar characteristics of pathological conditions [11]. Anatomically based three-dimensional models of VEGF-mediated angiogenesis have also been developed to study processes such as endothelial cell migration and proliferation, and capillary sprout formation [12,13]; however, 3D models are typically limited to smaller scales: microscopic and mesoscopic. Under physiological conditions, VEGF level in the mouse blood is low (,1.5 pM) [14], possibly as a result of VEGF having a short halflife in this species (3 minutes) [14,15]. One example of an important mouse study is the work performed by Rudge et al., who describe a high-affinity VEGF antagonist called Aflibercept, engineered to sequester VEGF by forming a complex [16]. The protein is a fully human soluble decoy receptor made by fusing the second Ig domain of human VEGFR-1 to the third Ig domain of human VEGFR-2 with the constant region (Fc) of human IgG1 [17]. Aflibercept, known commercially as VEGF Trap, forms an inert complex with VEGF, and the (...truncated)