Critical role of DNA checkpoints in mediating genotoxic-stress-induced filamentous growth in Candida albicans

Dynein-dependent nuclear dynamics affect morphogenesis in Candida albicans by means of the Bub2p spindle checkpoint

Candida albicans, the most prevalent fungal pathogen of humans, grows with multiple morphologies. The dynamics of nuclear movement are similar in wild-type yeast and pseudohyphae: nuclei divide across the bud neck. By contrast, in hyphae, nuclei migrate 10-20 microm into the growing germ tube before dividing. We analyzed the role of the dynein-dynactin complex in hyphal and yeast cells using time-lapse fluorescence microscopy. Cells lacking the heavy chain of cytoplasmic dynein or the p150(Glued) subunit of dynactin were defective in the position and orientation of the spindle. Hyphal cells often failed to deliver a nucleus to the daughter cell, resulting in defects in morphogenesis. Under yeast growth conditions, cultures included a mixture of yeast and pseudohyphal-like cells that exhibited distinctive defects in nuclear dynamics: in yeast, nuclei divided within the mother cell, and the spindle position checkpoint protein Bub2p ensured the delivery of the daughter nucleus to the daughter cell before cytokinesis; in pseudohyphal-like cells, pre-mitotic nuclei migrated into the daughter and no checkpoint ensured return of a nucleus to the mother cell before cytokinesis. Analysis of double mutants indicated that Bub2p also mediated the pre-anaphase arrest and polarization of pseudohyphal-like cells. Thus, Bub2p has two distinct roles in C. albicans cells lacking dynein: it mediates pre-anaphase arrest and it coordinates spindle disassembly with mitotic exit.

Transcriptome analysis of the Aspergillus nidulans AtmA (ATM, Ataxia-Telangiectasia mutated) null mutant

ATM is a phosphatidyl-3-kinase-related protein kinase that functions as a central regulator of DNA damage response in eukaryotes. In humans, mutations in ATM cause the devastating neurodegenerative disease Ataxia-Telangiectasia. Previously, we characterized the homologue of ATM (AtmA) in the filamentous fungus Aspergillus nidulans. In addition to its expected role in the DNA damage response, we found that AtmA is also required for polarized hyphal growth. Our results suggested that AtmA probably regulates the function and/or localization of landmark proteins required for the formation of a polarity axis. Here, we extended these studies by investigating which pathways are influenced by AtmA during proliferation and polar growth by comparatively determining the transcriptional profile of A. nidulans wild-type and DeltaatmA mutant strains in different growth conditions. Our results indicate an important role of the pentose phosphate pathway in the fungal proliferation during endogenous DNA damage and polar growth monitored by the AtmA kinase. Furthermore, we identified several genes that have decreased mRNA expression in the DeltaatmA mutant that are involved in the formation of a polarized hyphae and control of polar growth; in the synthesis of phosphatidic acid (e.g. phospholipase D); in the ergosterol biosynthesis (plasma membrane microdomains, lipid rafts); and in intracellular trafficking.

Cyclin-dependent kinases control septin phosphorylation in Candida albicans hyphal development

Cyclin-dependent kinases (Cdks) control cytoskeleton polarization in yeast morphogenesis. However, the target and mechanism remain unclear. Here, we show that the Candida albicans Cdk Cdc28, through temporally controlled association with two cyclins Ccn1 and Hgc1, rapidly establishes and persistently maintains phosphorylation of the septin cytoskeleton protein Cdc11 for hyphal development. Upon hyphal induction, Cdc28-Ccn1 binds to septin complexes and phosphorylates Cdc11 on Ser394, a nonconsensus Cdk target. This phosphorylation requires prior phosphorylation on Ser395 by the septin-associated kinase Gin4. Mutating Ser394 or Ser395 blocked Cdc11 phosphorylation on Ser394 and impaired hyphal morphogenesis. Reconstitution experiments using purified Cdc28-Ccn1, Gin4, and septins reproduced phosphorylations on the same residues. Transient septin-Cdc28 associations were also detected prior to bud and mating-projection emergence in S. cerevisiae. Our study uncovers a direct link between the cell-cycle engine and the septin cytoskeleton that may be part of a conserved mechanism underlying polarized morphogenesis.

Role of DNA mismatch repair and double-strand break repair in genome stability and antifungal drug resistance in Candida albicans

Drug resistance has become a major problem in the treatment of Candida albicans infections. Genome changes, such as aneuploidy, translocations, loss of heterozygosity, or point mutations, are often observed in clinical isolates that have become resistant to antifungal drugs. To determine whether these types of alterations result when DNA repair pathways are eliminated, we constructed yeast strains bearing deletions in six genes involved in mismatch repair (MSH2 and PMS1) or double-strand break repair (MRE11, RAD50, RAD52, and YKU80). We show that the mre11Delta/mre11Delta, rad50Delta/rad50Delta, and rad52Delta/rad52Delta mutants are slow growing and exhibit a wrinkly colony phenotype and that cultures of these mutants contain abundant elongated pseudohypha-like cells. These same mutants are susceptible to hydrogen peroxide, tetrabutyl hydrogen peroxide, UV radiation, camptothecin, ethylmethane sulfonate, and methylmethane sulfonate. The msh2Delta/msh2Delta, pms1Delta/pms1Delta, and yku80Delta/yku80Delta mutants exhibit none of these phenotypes. We observed an increase in genome instability in mre11Delta/mre11Delta and rad50Delta/rad50Delta mutants by using a GAL1/URA3 marker system to monitor the integrity of chromosome 1. We investigated the acquisition of drug resistance in the DNA repair mutants and found that deletion of mre11Delta/mre11Delta, rad50Delta/rad50Delta, or rad52Delta/rad52Delta leads to an increased susceptibility to fluconazole. Interestingly, we also observed an elevated frequency of appearance of drug-resistant colonies for both msh2Delta/msh2Delta and pms1Delta/pms1Delta (MMR mutants) and rad50Delta/rad50Delta (DSBR mutant). Our data demonstrate that defects in double-strand break repair lead to an increase in genome instability, while drug resistance arises more rapidly in C. albicans strains lacking mismatch repair proteins or proteins central to double-strand break repair.