Acquisition of Plasmin Activity byFusobacterium nucleatumsubsp. nucleatumand Potential Contribution to Tissue Destruction during Periodontitis

ABSTRACTFusobacterium nucleatumsubsp. nucleatumhas been associated with a variety of oral and nonoral infections such as periodontitis, pericarditis, bone infections, and brain abscesses. Several studies have shown the role of plasmin, a plasma serine protease, in increasing the invasive capacity of microorganisms. In this study, we investigated the binding of human plasminogen toF. nucleatumsubsp. nucleatum, and its subsequent activation into plasmin. Plasminogen-binding activity of bacterial cells was demonstrated by a solid-phase dot blot assay using an anti-plasminogen antibody. The binding activity was heat resistant and involved cell-surface lysine residues since it was abolished in the presence of the lysine analog ɛ-aminocaproic acid. Activation of plasminogen-coated bacteria occurred following incubation with either streptokinase, urokinase-type plasminogen activator (u-PA), or aPorphyromonas gingivalisculture supernatant. In the case of the P. gingivalisculture supernatant, a cysteine protease was likely involved in the activation. The plasmin activity generated on the cell surface of F. nucleatumsubsp.nucleatumcould be inhibited by aprotinin. Activation of plasminogen by u-PA was greatly enhanced when plasminogen was bound to bacteria rather than in a free soluble form. u-PA-activated plasminogen-coated F. nucleatumsubsp.nucleatumwas found to degrade fibronectin, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Tissue inhibitor of metalloproteinase-1 was also degraded by the plasmin activity generated on the bacterial cells. This study suggests a possible role for plasminogen, which is present in affected periodontal sites, in promoting tissue destruction and invasion by nonproteolytic bacteria such as F. nucleatumsubsp. nucleatum.