The protein kinase Ire1 has a Hac1-independent essential role in iron uptake and virulence of Candida albicans

Abstract

Protein kinases play central roles in virtually all signaling pathways that enable organisms to adapt to their environment. Microbial pathogens must cope with severely restricted iron availability in mammalian hosts to invade and establish themselves within infected tissues. To uncover protein kinase signaling pathways that are involved in the adaptation of the pathogenic yeast Candida albicans to iron limitation, we generated a comprehensive protein kinase deletion mutant library of a wild-type strain. Screening of this library revealed that the protein kinase Ire1, which has a conserved role in the response of eukaryotic cells to endoplasmic reticulum stress, is essential for growth of C. albicans under iron-limiting conditions. Ire1 was not necessary for the activity of the transcription factor Sef1, which regulates the response of the fungus to iron limitation, and Sef1 target genes that are induced by iron depletion were normally upregulated in ire1Δ mutants. Instead, Ire1 was required for proper localization of the high-affinity iron permease Ftr1 to the cell membrane. Intriguingly, iron limitation did not cause increased endoplasmic reticulum stress, and the transcription factor Hac1, which is activated by Ire1-mediated removal of the non-canonical intron in the HAC1 mRNA, was dispensable for Ftr1 localization to the cell membrane and growth under iron-limiting conditions. Nevertheless, expression of a pre-spliced HAC1 copy in ire1Δ mutants restored Ftr1 localization and rescued the growth defects of the mutants. Both ire1Δ and hac1Δ mutants were avirulent in a mouse model of systemic candidiasis, indicating that an appropriate response to endoplasmic reticulum stress is important for the virulence of C. albicans. However, the specific requirement of Ire1 for the functionality of the high-affinity iron permease Ftr1, a well-established virulence factor, even in the absence of endoplasmic reticulum stress uncovers a novel Hac1-independent essential role of Ire1 in iron acquisition and virulence of C. albicans.

Candida albicans SR-Like Protein Kinases Regulate Different Cellular Processes: Sky1 Is Involved in Control of Ion Homeostasis, While Sky2 Is Important for Dipeptide Utilization

“…C. albicans Casky1D andCasky2D (orf19.35) deletion mutants were taken from the existent mutant library of non-essential uncharacterized predicted protein kinases (Ramıŕez-Zavala et al, 2022). All other C. albicans deletion mutants and complemented strains were constructed as described previously (Ramıŕez-Zavala et al, 2017).…”

“…To test whether CgSky1, CaSky1, and CaSky2 are involved in similar cellular functions we generated C. glabrata mutant strains lacking SKY1 (Cgsky1D) and C. albicans strains lacking both SKY1 and SKY2 (Casky1D Casky2D). In addition, we used available C. albicans Casky1D and Casky2D (orf19.35) mutants (Ramıŕez-Zavala et al, 2022). As a control we used the S. cerevisiae BY4741 wild-type strain and a sky1D mutant (Scsky1D).…”

Functional genomic analysis of Candida albicans protein kinases reveals modulators of morphogenesis in diverse environments

“…Additionally, this work contributes to the growing appreciation for Hac1-independent mechanisms of Ire1 function in pathogenic fungi. 46 , 47 , 48 , 49 While previous reports have observed that both the overexpression and depletion of the stress response regulator Hsf1 induces filamentation in the liquid medium, 50 to our knowledge this is the first example of a protein for which overexpression and depletion lead to comparable phenotypes in distinct media states.…”

“… 13 , 54 , 55 , 56 , 57 , 58 With Ire1, there has been growing interest in its potential as a therapeutic target as it is critical for growth under host-relevant stress conditions, including elevated temperature and iron-limitation. 28 , 49 Furthermore, this kinase is reported to have a key role in biofilm formation in vivo , as well as for pathogenicity in a mouse model of systemic infection. 28 , 49 In the pharmaceutical sector, Ire1 has become an attractive therapeutic target for the treatment of several human diseases, including cancer, as illustrated by ORIN1001, a selective inhibitor of Ire1 that entered clinical trials for patients with advanced solid tumors.…”

“… 28 , 49 Furthermore, this kinase is reported to have a key role in biofilm formation in vivo , as well as for pathogenicity in a mouse model of systemic infection. 28 , 49 In the pharmaceutical sector, Ire1 has become an attractive therapeutic target for the treatment of several human diseases, including cancer, as illustrated by ORIN1001, a selective inhibitor of Ire1 that entered clinical trials for patients with advanced solid tumors. 59 , 60 Although the conservation between human and yeast IRE1 raises concerns for potential host toxicity in antifungal drug development, recent studies have demonstrated that fungal selectivity can be improved using structure-guided approaches.…”