Mechanism of lysosomal cathepsin B degradation of Cryptococcus neoformans

Abstract

Cryptococcus neoformans is an opportunistic fungal pathogen that is spread through inhalation and predominantly causes disease in immunocompromised patients. The infection begins in the lungs as pneumonia and can disseminate into the central nervous system causing a fatal brain infection called cryptococcal meningitis. Dendritic cells (DCs) in the respiratory tract can phagocytose and kill inhaled C. neoformans in the lysosomal compartment. In addition, DC lysosomal extract has anti-cryptococcal activity. A purified lysosomal protease, cathepsin B, exhibits significant growth inhibition of Cryptococcus by an unknown mechanism resulting in osmotic lysis of the organism. Due to the morphology observed after cathepsin B treatment, we hypothesize that the cathepsin B is targeting the fungal cell wall integrity pathway. For these studies, we have treated C. neoformans with cathepsin B and have shown that cathepsin B has anti-cryptococcal activity. We are currently examining the cell wall integrity pathway with/without treatment by Western blot, and are examining the phosphorylation of PKA (the first protein in the signaling cascade) to determine if cathepsin B affects this pathway. The minimum inhibitory concentration assay of cathepsin B is also being examined in comparison with amphotericin B, a common antifungal drug known to inhibit C. neoformans growth. Future studies will examine phosphorylation of PKA and additional proteins in the cell wall integrity pathway and the effects of cathepsin B on cryptococcal virulence factors to identify additional mechanisms of cathepsin B anti-cryptococcal activity.