Inhibition of Aspergillus fumigatus conidia binding to extracellular matrix proteins by sialic acids: a pH effect?
Tiralongo J, Wohlschlager T, Tiralongo E, Kiefel MJ
Date: 23 June 2009
Infection by Aspergillus fumigatus, which causes the life threatening disease invasive aspergillosis, begins with the inhalation of conidia that adhere and germinate in the lung. Previous reports have shown that A. fumigatus conidia express high levels of the negatively charged 9-carbon sugar sialic acid, and that sialic acid appears to mediate the binding of A. fumigatus conidia to basal lamina proteins. However, despite the ability of sialic acid to inhibit A. fumigatus conidia adherence, the exact mechanism by which this binding event occurs remains unresolved. Utilising various free sialic acids and other carbohydrates, sialic acid derivatives, sialoglycoconjugates, glycoproteins, alpha-keto acid related compounds and amino acids we have found that the binding of A. fumigatus conidia to type IV collagen and fibrinogen was inhibited by (i) glycoproteins (in a sialic acid-independent manner), and (ii) free sialic acids, glucuronic acid and alpha-keto acid related compounds. However, inhibition by the latter was found to be the result of a shift in pH from neutral (pH 7.4) to acidic (less than pH 4.6) induced by the relatively high concentrations of free sialic acids, glucuronic acid and alpha-keto acid related compounds used in the binding assays. This suggests that previous reports describing inhibition of A. fumigatus conidia binding by free sialic acid may actually be due to a pH shift similar to that shown here. As previously reported, we found that A. fumigatus conidia only express N-acetylneuraminic acid, the most common sialic acid found in nature. However, A. fumigatus appears to do so by an alternative mechanism to that seen in other organisms. We report here that A. fumigatus (i) does not incorporate sialic acid obtained from the environment, (ii) does not synthesise and incorporate sialic acid from exogenous N-acetylmannosamine, and (iii) lacks homologues of known sialic acid biosynthesising enzymes.
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