Binding of platelet factor four (PF 4) to glomerular polyanion

Synthetic polycations have been shown to bind to glomerular polyanion (GPA) and increase glomerular permeability. Here, we show that human platelet factor 4 (PF 4), a platelet secretory protein, binds to GPA. The following methods were used to assess PF 4 binding to GPA: (1) Sections of human and rat renal cortex were incubated with PF 4 or PF 4 was injected intravenously into rats followed by immunofluorescence techniques; (2) ¹²⁵I-PF 4 was added to isolated glomerular basement membrane (GBM) suspensions and binding assessed isotopically; (3) PF 4 was perfused through rat kidneys ex vivo followed by immunoperoxidase methods for electron microscopy (EM). In vitro and in vivo, PF 4 bound to the mesangium and linearly to capillary walls. Isotopic studies showed dose-dependent saturable binding of PF 4 to GBM which was reversed by heparin. By EM, PF 4 binding sites were resolved in the GBM, particularly in the laminae rarae as punctate densities similar in distribution to anionic sites revealed by cationic dyes. Also, endothelial and epithelial cell surfaces stained. An ionic interaction between PF 4 and GPA was indicated by elimination of staining by washing PF 4-treated sections with buffer containing 1.0 and 3.0 M NaCl or with heparin. Pretreatment of rats with polyethyleneimine (a synthetic polycation) blocked PF 4 binding. Heparin administration in vivo removed previously bound PF 4. By virtue of its affinity for GPA and behavior like a polycation, PF 4 may alter glomerular permeability and play a role in glomerular disease.