Adaptive changes in the fungal cell wall mediate copper homeostasis
Authors: Probst Corinna , Garcia-Santamarina Sarela , Brooks Jacob T. , Van Der Kloet Inge , Thiele Dennis J. , Alspaugh J. Andrew 
Published: December 2021

Copper homeostasis mechanisms are essential for microbial adaption to changing copper levels within the host during infection. In the opportunistic fungal pathogen Cryptococcus neoformans (Cn), the Cn Cbi1/Bim1 protein is a newly identified copper binding and release protein that is highly induced during copper limitation. Recent studies demonstrated that Cbi1 functions in copper uptake through the Ctr1 copper transporter during copper limitation. However, the mechanism of Cbi1 action is unknown. The fungal cell wall is a dynamic structure primarily composed of carbohydrate polymers, such as chitin and chitosan, polymers known to strongly bind copper ions. We demonstrated that Cbi1 depletion affects cell wall integrity and architecture, connecting copper homeostasis with adaptive changes within the fungal cell wall. The cbi1Δ mutant strain possesses an aberrant cell wall gene transcriptional signature as well as defects in chitin and chitosan deposition. These changes are reflected in altered macrophage activation and changes in the expression of specific virulence-associated phenotypes. Furthermore, using Cn strains defective in chitosan biosynthesis, we demonstrated that cell wall chitosan modulates the ability of the fungal cell to withstand copper stress. In conclusion, our data suggest a dual role for the fungal cell wall, in particular the inner chitin / chitosan layer, in protection against toxic levels of copper and providing a source of metal ion availability during copper starvation. Given the previously described role for Cbi1 in copper uptake, we propose that this copper-binding protein is involved in shuttling copper from the cell wall to the copper transporter Ctr1 for regulated microbial copper uptake.

Author summary

Microorganisms must be equipped to readily acquire essential micro-nutrients like copper from nutritionally poor environments while simultaneously shielding themselves from conditions of metal excess. We explored mechanisms of microbial copper homeostasis in the human opportunistic fungal pathogen Cryptococcus neoformans (Cn) by defining physiological roles of the newly described copper-binding and release protein Cn Cbi1/Bim1. Highly induced during copper limitation, Cbi1 has been shown to interact with the high-affinity copper transporter Ctr1. We defined Cbi1-regulated changes in the fungal cell wall, including controlling levels of the structural carbohydrates chitin and chitosan. These polysaccharides are embedded deeply in the cell wall and are known to avidly bind copper. We also defined the host immunological alterations in response to these cell wall changes. Our data suggest a model in which the fungal cell wall, especially the chito-oligomer layer, serves as a copper-binding structure to shield the cell from states of excess copper, while also serving as a copper storage site during conditions of extracellular copper depletion. Given its ability to bind and release copper, the Cbi1 protein likely shuttles copper from the cell wall to copper transporters for regulated copper acquisition.