Key Points
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Invasive fungal infections are one of the most devastating consequence of the rapidly increasing number of immunocompromised patients. Despite the rising incidence and mortality of infections with Cryptococcus neoformans, Candida albicans and Aspergillus fumigatus, the antifungal armamentarium remains limited, and novel targets focused on fundamental molecular pathogenesis are needed.
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The calcineurin pathway is a conserved stress-response signalling pathway that has revolutionized today's immunosuppression. The calcineurin inhibitors FK506 and cyclosporine A (CsA) bind to human calcineurin and block signal transduction and T-cell activation, leading to efficient immunosuppression that can prevent organ rejection. Additionally, calcineurin inhibitors have been used clinically to treat a myriad of other conditions.
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In this article, the authors propose that inhibiting fungal calcineurin pathways could be an effective method of halting the growth of invasive fungal pathogens, thereby preventing or treating disease. The two currently available inhibitors FK506 and CsA delivered in their present intravenous formulations will not solve this problem, as they possess inherent and well-utilized cross-reactive human immunosuppressive activity. However, molecular dissection of the fungal calcineurin pathway is yielding promising results and concepts for new drug development.
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Current calcineurin inhibitors have been shown to possess antifungal activity against the major fungal pathogens, but under different conditions. Several calcineurin pathway genes in each of the major human fungal pathogens have also been disrupted and show effects on fungal growth and virulence. However, the effects are not uniform, highlighting that while the pathway is genetically conserved, the divergent disruption phenotypes require each individual pathogen to be studied in detail.
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In C. neoformans, calcineurin inhibition leads to temperature-sensitive growth and fungal clearing in animal models and at the relevant temperatures in patients. Additionally, calcineurin inhibition leads to defects in hyphal elongation and therefore the inability to mate. In C. albicans, calcineurin A is required for growth under stress, growth in serum and virulence in systemic animal models. In A. fumigatus calcineurin A is not essential, in contrast to its essential role in the less pathogenic A. nidulans. Disruption of calcineurin A in A. fumigatus yielded extremely blunted hyphae that were unable to invade animal tissue and resulted in near avirulence in multiple animal models.
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Molecular findings over the past decade have pointed to the fungal calcineurin pathway as a novel antifungal target with impressive effects at halting growth and inhibiting virulence in the three major fungal pathogens that affect immunocompromised patients. The future will hold further detailed molecular dissection of the pathway to optimize fungal killing and effectively harness the calcineurin pathway to both prevent and treat invasive fungal infections.
Abstract
The number of immunocompromised patients with invasive fungal infections continues to increase and new antifungal therapies are not keeping pace with the growing incidence of these infections and their associated mortality. Calcineurin inhibition is currently used to exert effective immunosuppression following organ transplantation and in treating various other conditions. However, the calcineurin pathway is also intricately involved in the growth and pathogenesis of the three major fungal pathogens of humans, Cryptococcus neoformans, Candida albicans and Aspergillus fumigatus, and the exploitation of fungal calcineurin pathways holds great promise for the future development of novel antifungal agents. This Review summarizes our current understanding of calcineurin biology in these fungal species, and its exciting potential role in treating invasive fungal infections.
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Acknowledgements
The authors thank the many postdoctoral fellows, students and technicians who have assisted over the years in deciphering this important pathway in pathogenic fungi.
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Supplementary information S1 (figure)
Clustal W analysis of calcineurin A proteins in major human fungal pathogens and humans. (PDF 60 kb)
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Glossary
- Dopamine and cAMP-regulated phosphoprotein
-
Involved in regulating the state of phosphorylation and the activity of a large number of phosphoproteins, ion channels and neurotransmitter receptors.
- N-methyl-D-aspartatic acid receptor
-
(NMDA receptor). NMDA receptors mediate excitatory effects in the brain when they are stimulated by endogenous ligands such as glutamic acid.
- Immunophilin
-
A family of cis–trans peptidylprolyl isomerases that includes cyclophilins and FK506-binding proteins (FKBPs). These proteins were originally discovered as cellular receptors for immunosuppressive drugs, including cyclosporine A and FK506. The complexes that form between immunophilins and their cognate ligands are the functional modules for immunosuppression. Immunophilins are now known to function at the crossroads of protein folding and trafficking, and signal transduction.
- Graft-versus-host disease
-
Tissue damage in a recipient of allogeneic tissue (usually a bone-marrow or solid-organ transplant) that results from the activity of donor cytotoxic T lymphocytes recognizing the tissues of the recipient as foreign. Graft-versus-host disease varies markedly in extent, but it can be life-threatening in severe cases.
- Segmental glomerulosclerosis
-
Renal disease with an unknown etiology or mechanism leading to injury of renal cells and collapse of renal glomerular capillaries, sclerosis and ultimately clinical renal failure.
- Calmodulin
-
A small Ca2+-binding protein, the most important transducer of intracellular Ca2+ signals. It interacts with, and regulates the activity of, a range of proteins that control many cellular processes, including protein phosphorylation and dephosphorylation, cyclic-nucleotide formation and breakdown, cytoskeletal rearrangement, gene transcription and membrane potential.
- Echinocandins
-
A class of intravenous antifungal agents that interfere with fungal cell-wall biosynthesis by non-competitive inhibition of β-1,3-D-glucan synthase, an enzyme present in fungi but absent in mammalian cells. β-1,3-glucan, an essential cell-wall polysaccharide, forms a fibril of three helically entwined linear polysaccharides and provides structural integrity for the fungal cell wall.
- Azole
-
Class of intravenous and oral antifungal agents that inhibit the fungal cytochrome P45014DM (also known as lanosterol 14α-demethylase), which catalyzes a late step in ergosterol biosynthesis in the fungal cell membrane. The drugs bind to the heme group in the target protein and block demethylation of the C-14 of lanosterol, leading to substitution of methylated sterols in the membrane and depletion of ergosterol.
- Ergosterol
-
The main sterol in the fungal cell membrane. Ergosterol is responsible, and essential, for structural and regulatory membrane features such as fluidity and permeability (equivalent to cholesterol in mammalian cells).
- Aflatoxin
-
A naturally occuring mycotoxin, most notably produced by Aspergillus flavus and Aspergillus parasiticus, which can destroy agricultural crops as well as function as a carcinogen in humans.
- Hydrophobin
-
A small hydrophobic protein found in the surface rodlet layer of Aspergillus spp. conidia that has been associated with virulence.
- Non-homologous end-joining
-
A pathway that rejoins DNA strand breaks without relying on significant homology.
- Bronchiolitis obliterans
-
A disease of the lungs with a fixed airway obstruction and inflammation in which the bronchioles are plugged with granulation tissue, leading to shortness of breath and cough.
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Steinbach, W., Reedy, J., Cramer, R. et al. Harnessing calcineurin as a novel anti-infective agent against invasive fungal infections. Nat Rev Microbiol 5, 418–430 (2007). https://doi.org/10.1038/nrmicro1680
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DOI: https://doi.org/10.1038/nrmicro1680
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