title: Quantifying the proteolytic release of extracellular matrix-sequestered VEGF with a computational model.
abstract: BACKGROUND
  VEGF proteolysis by plasmin or matrix metalloproteinases (MMPs) is believed to play an important role in regulating vascular patterning in vivo by releasing VEGF from the extracellular matrix (ECM). However, a quantitative understanding of the kinetics of VEGF cleavage and the efficiency of cell-mediated VEGF release is currently lacking. To address these uncertainties, we develop a molecular-detailed quantitative model of VEGF proteolysis, used here in the context of an endothelial sprout.
  
  
  RESULTS
  To study a cell's ability to cleave VEGF, the model captures MMP secretion, VEGF-ECM binding, VEGF proteolysis from VEGF165 to VEGF114 (the expected MMP cleavage product of VEGF165) and VEGF receptor-mediated recapture. Using experimental data, we estimated the effective bimolecular rate constant of VEGF165 cleavage by plasmin to be 328 M(-1) s(-1) at 25 degrees C, which is relatively slow compared to typical MMP-ECM proteolysis reactions. While previous studies have implicated cellular proteolysis in growth factor processing, we show that single cells do not individually have the capacity to cleave VEGF to any appreciable extent (less than 0.1% conversion). In addition, we find that a tip cell's receptor system will not efficiently recapture the cleaved VEGF due to an inability of cleaved VEGF to associate with Neuropilin-1.
  
  
  CONCLUSIONS
  Overall, VEGF165 cleavage in vivo is likely to be mediated by the combined effect of numerous cells, instead of behaving in a single-cell-directed, autocrine manner. We show that heparan sulfate proteoglycans (HSPGs) potentiate VEGF cleavage by increasing the VEGF clearance time in tissues. In addition, we find that the VEGF-HSPG complex is more sensitive to proteases than is soluble VEGF, which may imply its potential relevance in receptor signaling. Finally, according to our calculations, experimentally measured soluble protease levels are approximately two orders of magnitude lower than that needed to reconcile levels of VEGF cleavage seen in pathological situations.
authors.affiliation: Department of Biomedical Engineering, Johns Hopkins University School of Medicine, Baltimore, Maryland, United States of America. pvempat1@jhu.edu
authors.forename: Prakash
authors.initials: P
authors.lastname: Vempati
authors.forename: Feilim
authors.initials: F
authors.lastname: Mac Gabhann
authors.forename: Aleksander S
authors.initials: AS
authors.lastname: Popel
mesh_terms: D000818:Animals; D001485:Basement Membrane / Q000378:metabolism; D003198:Computer Simulation; D005109:Extracellular Matrix / Q000378:metabolism*; D006801:Humans; D008962:Models, Theoretical; D042442:Vascular Endothelial Growth Factors / Q000378:metabolism*
chemical_list: D042442:Vascular Endothelial Growth Factors