Ceruloplasmin as a source of Cu for a fungal pathogen

Copper is an essential metal for virtually all organisms, yet little is known about the extracellular sources of this micronutrient. In serum, the most abundant extracellular Cu-binding molecule is the multi‑copper oxidase ceruloplasmin (Cp). Cp levels increase during infection and inflammation, and pathogens can be exposed to high Cp at sites of infection. It is not known whether Cp might serve as a Cu source for microbial pathogens and we tested this using the opportunistic fungal pathogen Candida albicans. We find that C. albicans can use whole serum as a Cu source and that this Cu is sensed by the transcription factor protein Mac1. Mac1 activates expression of Mn-SOD3 superoxide dismutase and represses Cu/Zn-SOD1 during Cu starvation and both responses are regulated by serum Cu. We also show that purified human Cp can act as a sole source of Cu for the fungus and likewise modulates the Mac1 Cu stress response. To investigate whether Cp is a Cu source in serum, we compared the ability of C. albicans to use serum from wild type versus Cp^−/− mutant mice. We find that serum lacking Cp is deficient in its ability to trigger the Mac1 Cu response. C. albicans did accumulate Cu from Cp^−/− serum, but this Cu was not efficiently sensed by Mac1. We conclude that Cp and non-Cp Cu sources are not equivalent and are handled differently by the fungal cell. Overall, these studies are the first to show that Cp is a preferred source of Cu for a pathogen.