Entry of OpaA+ gonococci into HEp-2 cells requires concerted action of glycosaminoglycans, fibronectin and integrin receptors

Heparan sulphate proteoglycans are increasingly implicated as eukaryotic cell surface receptors for bacterial pathogens. Here, we report that Neisseria gonorrhoeae adheres to proteoglycan receptors on HEp-2 epithelial cells but that internalization of the bacterium by this cell type requires the serum glycoprotein fibronectin. Fibronectin was shown to bind specifically to gonococci producing the OpaA adhesin, Binding assays with fibronectin fragments located the bacterial binding site near the N-terminal end of the molecule. However, none of the tested fibronectin fragments supported gonococcal entry into the eukaryotic cells; a 120 kDa fragment carrying the cell adhesion domain with the amino acid sequence RGD even inhibited the fibronectin-mediated uptake of MS11-OpaA. This inhibition could be mimicked by an RGD-containing hexapeptide and by alpha 5 beta 1 integrin-specific antibodies, suggesting that interaction of the central region of fibronectin with integrin receptors facilitated bacterial uptake. Fibronectin was unable to promote gonococcal entry into HEp-2 cells that had been treated with the enzyme heparinase ill, which degrades the glycosaminoglycan side-chains of proteoglycan receptors. On the basis of these results, we propose a novel cellular uptake pathway for bacteria, which involves the binding of the pathogen to glycosaminoglycans that, in turn, act as co-receptors facilitating fibronectin-mediated bacterial uptake through integrin receptors. in this scenario, fibronectin would act as a molecular bridge linking the Opa-proteoglycan complex with host cell integrin receptors.