Identification of the Active Site in the Heparin II Domain of Fibronectin that Increases Outflow Facility in Cultured Monkey Anterior Segments

PURPOSE—To determine the active site in the Heparin II (HepII) domain of fibronectin that regulates outflow facility in cultured anterior segments and disrupts the actin cytoskeleton in transformed human trabecular meshwork (TM-1) cells. METHODS—Outflow facility was determined by two-level, constant-pressure perfusion in cultured anterior segments of rhesus and cynomolgus monkey eyes. One segment from each pair was exchanged with either the HepII domain or an integrin/syndecan binding peptide (IDAPS or PPRARI) from the HepII domain. To assay changes in the actin cytoskeleton, TM-1 cells were incubated for 24 hours with or without the HepII domain, PPRARI, or IDAPS. Changes were monitored with phase and immunofluorescence microscopy. RESULTS—HepII domain (100 µg/mL) and PPRARI (500 µg/mL) increased outflow facility by 31% ± 13% (n = 9, P < 0.05) and 24% ± 9% (n = 8, P < 0.05), respectively in cultured anterior segments after an overnight infusion. Perfusion with IDAPS (500 µg/mL) had no effect on outflow facility. In TM-1 cultures, 250 µg/mL of the HepII domain or 4 mg/mL of PPRARI disrupted the assembly of actin filaments. A lower concentration of PPRARI (2 mg/mL) disrupted the actin cytoskeleton when used in combination with a nondisrupting concentration of the HepII domain (30– 60 µg/mL). In contrast, IDAPS did not disrupt the actin cytoskeleton under any condition tested. CONCLUSIONS—The active site in the HepII domain that regulates outflow facility in cultured anterior segments and disrupts the actin cytoskeleton in TM-1 cells is the syndecan/integrin binding sequence, PPRARI. Aqueous humor drainage via the conventional outflow pathway accounts for one half to two thirds of the total aqueous outflow in a healthy human eye. 1,2 One feature that contributes to outflow resistance via this pathway is the contractile property of the trabecular meshwork (TM). Thus, agents that disrupt the organization of the actin cytoskeleton, cell–cell junctions, and cell-matrix contacts that maintain tissue integrity tend to increase outflow facility in enucleated human and bovine eye organ perfusion cultures and in live monkey eyes. 3 In contrast, agents that support actin cytoskeleton assembly increase resistance and reduce outflow facility. 4