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Summary HtrA serine proteases are highly conserved and essential ATP-independent proteases with chaperone activity. Bacteria express a variable number of HtrA homologues that contribute to the virulence and pathogenicity of bacterial pathogens. Lyme disease spirochetes possess a single HtrA protease homologue, Borrelia burgdorferi HtrA (BbHtrA). Previous studies established that, like the human orthologue HtrA1, BbHtrA is proteolytically active against numerous extracellular proteins in vitro. In this study, we utilized size exclusion chromatography and blue native polyacrylamide gel electrophoresis (BN-PAGE) to demonstrate BbHtrA oligomeric structures that were substrate independent and salt sensitive. Examination of the influence of transition metals on the activity of BbHtrA revealed that this protease is inhibited by Zn2+ > Cu2+ > Mn2+. Extending this analysis to two other HtrA proteases, E. coli DegP and HtrA1, revealed that all three HtrA proteases were reversibly inhibited by ZnCl2 at all micro molar concentrations examined. Commercial inhibitors for HtrA proteases are not available and physiologic HtrA inhibitors are unknown. Our observation of conserved zinc inhibition of HtrA proteases will facilitate structural and functional studies of additional members of this important class of proteases.

Summary The Lyme disease spirochaete, Borrelia burgdorferi, causes damage to diverse host tissues and induces inflammation but the mechanisms of injury are poorly understood. We recently reported that a surface-exposed B. burgdorferi protease, which is expressed during human disease and is conserved within the major Lyme disease spirochaete species, degrades the extracellular matrix proteoglycan, aggrecan. Here we demonstrate that BbHtrA also degrades fibronectin and numerous proteoglycans found in skin, joints and neural tissues. BbHtrA degradation of fibronectin released known pro-inflammatory fibronectin fragments FnIII13–14 and Fnf-29, which may amplify the inflammatory processes triggered by the presence of the bacteria. When this hypothesis was tested directly by exposing chondrocytes to BbHtrA in vitro, inflammatory cytokines (sICAM-1 and IL-6) and chemokines (CXCL1, CCL1, CCL2 and CCL5) that are hallmarks of Lyme disease were induced. These results provide the first evidence that, by utilizing BbHtrA, B. burgdorferi may actively participate in its dissemination and in the tissue damage and inflammation observed in Lyme disease.