Heparan sulfate mediates bFGF transport through basement membrane by diffusion with rapid reversible binding

Basic fibroblast growth factor (bFGF) is a pluripotent cytokine with a wide range of target cells. Heparan sulfate binds bFGF, and this interaction has been demonstrated to protect bFGF against physical denaturation and protease degradation. The high concentrations of heparan sulfate in basement membranes have implicated these matrices as storage sites for bFGF in vivo. However, the mechanisms by which basement membranes modulate bFGF storage and release is unknown. To gain insight into these mechanisms, we have developed experimental and mathematical models of extracellular growth factor transport through basement membrane. Intact Descemet's membranes isolated from bovine corneas were mounted within customized diffusion cells and growth factor transport was measured under a variety of conditions that decoupled the diffusion process from the heparan sulfate binding phenomenon. Transport experiments were conducted with bFGF and interleukin 1beta. In addition, bFGF-heparan sulfate binding was disrupted in diffusion studies with high ionic strength buffer and buffers containing protamine sulfate. Transport of bFGF was enhanced dramatically when heparan sulfate binding was inhibited. This process was modeled as a problem of diffusion with fast reversible binding. Experimental parameters were incorporated into a mathematical model and independent simulations were run that showed that the experimental data were accurately predicted by the mathematical model. Thus, this study indicated that basement membranes function as dynamic regulators of growth factor transport, allowing for rapid response to changing environmental conditions. The fundamental principles controlling bFGF transport through basement membrane that have been identified here might have applications in understanding how growth factor distribution is regulated throughout an organism during development and in the adult state.