Figure 8: A model of the function of the C. neoformans Cu homeostasis machinery during systemic infection.

(a) Model of the roles of Cu transporters during C. neoformans infection. (1) A C. neoformans cell possesses Cu-acquisition (Ctr1 and Ctr4) and Cu-detoxification (Cmt1 and Cmt2) proteins. (2) During pulmonary infection, C. neoformans senses a high-Cu environment. Ctr1 is quickly degraded, but Ctr4 is still expressed. Cmt1 and Cmt2 are expressed to protect fungal cells from antimicrobial Cu in the host lung. (3) During meningoencephalitis, C. neoformans utilizes Ctr1 and Ctr4 for Cu uptake, whereas the expression of Cmt1 and Cmt2 is reduced and therefore these proteins play no function in the brain. We speculate that there are uncharacterized Cu transport machineries in C. neoformans, such as low-affinity or nonspecific transporters. (b) Models of the phenotypes of C. neoformans transporter mutants in vivo. (i) The wild-type strain degrades the Ctr1 Cu⁺ transporter in the lung, while Ctr4 is active and transports the host antimicrobial Cu. (ii) In the brain, Ctr1 and Ctr4 are expressed for Cu acquisition. (iii and iv) In the ctr1Δ strain, the functionally redundant Cu+ transporter Ctr4 can transport Cu in both the lung and brain, enabling growth in both tissues similar to that observed for wild-type C. neoformans. (v) The ctr4Δ strain exhibits increased fungal proliferation in the lung due to the fast protein degradation of Ctr1 and the disruption of CTR4, leading to increased dissemination of fungal cells into the brain. (vi) Because the functionally redundant Cu⁺ transporter Ctr1 is expressed in ctr4Δ cells and the number of ctr4Δ cells in the brain is increased, the ctr4Δ strain exhibits increased fungal virulence. (vii) The cell growth of the ctr1Δctr4Δ strain is increased in the lung, resembling that of the ctr4Δ strain, resulting in increased dissemination of fungal cells in the brain. (viii) Owing to the lack of functional Cu⁺ transporters in the ctr1Δctr4Δ strain, the cell growth is inhibited in the brain. Therefore, the Cu-replete environment in the lung and Cu-deficient conditions in the brain generates a Cu-balance for the ctr1Δctr4Δ strain, resulting in a survival rate equal to that of the wild type.