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Departments of Molecular Genetics and Microbiology,1 Medicine,2 Pharmacology and Cancer Biology,3 Howard Hughes Medical Institute, Duke University Medical Center, Durham, North Carolina4
Received 16 July 2004/ Accepted 21 August 2004
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ABSTRACT |
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subunit Gpa1 to control the adenylyl cyclase (Cac1). Aca1 interacted with the C terminus of Cac1 in the yeast two-hybrid system. By molecular and genetic approaches, Aca1 was shown to play a critical role in mating by regulating cell fusion and filamentous growth in a cAMP-dependent manner. Aca1 also regulates melanin and capsule production via the Cac1-cAMP-protein kinase A pathway. Genetic epistasis studies support models in which Aca1 and Gpa1 are necessary and sufficient components that cooperate to activate adenylyl cyclase. Taken together, these studies further define the cAMP signaling cascade controlling virulence of this ubiquitous human fungal pathogen. |
INTRODUCTION |
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The MAPK cascade regulates mating processes involving morphological differentiation, such as the dikaryotic mycelia, basidia, and basidiospores, which are produced in response to peptide pheromones secreted by opposite mating-type cells (24, 32, 43). The MAPK pathway is composed of mating-type-specific (Ste3/a, Ste20/a, Ste11/a, and Ste12/a) and nonspecific (Gpb1, Ste7, and Cpk1) elements (12, 49). Gene disruption experiments revealed that the MAPK pathway is required for mating and cell type-specific differentiation but not for virulence (12). However, mating type has been associated with the virulence of serotype D (variety neoformans) strains by Kwon-Chung et al., who showed that the -mating type is more virulent than the a-mating type (25). Furthermore, Del Poeta et al. demonstrated that the MF1 pheromone gene is induced during the late stages of central nervous system infection (13). In contrast, mating type has not yet been associated with virulence in congenic serotype A (variety grubii) strains (35). In response to nitrogen limitation, desiccation, and MFa pheromone, haploid strains can also filament and sporulate by a process known as haploid fruiting (50, 54).
The ability to grow at high temperature is mediated by at least two signaling pathways: a Ras (Ras1/Ras2) signaling pathway and a calcineurin-dependent pathway (1, 37, 52). The Ras pathway also promotes mating via the MAPK pathway (1). Although Ras plays a prominent role in regulating adenylyl cyclase and cAMP signaling in Saccharomyces cerevisiae (22, 33, 39), in both C. neoformans and Ustilago maydis Ras lacks the domain involved in cyclase binding and no longer functions in cAMP signaling (1, 28, 29).
The cAMP-protein kinase A (PKA)-dependent signaling pathway is of particular interest based on its ability to control both virulence factors (melanin and capsule production) and morphological differentiation (mating and filamentous growth) of C. neoformans. Elements of this pathway include the G protein Gpa1, adenylyl cyclase Cac1, and PKA consisting of the catalytic subunits Pka1/Pka2 and the regulatory subunit Pkr1 (2, 3, 14, 21). Disruption of the GPA1 or CAC1 genes confers defects in melanin and capsule formation and mating that are suppressed by exogenous cAMP (2, 3). As a result, gpa1
and cac1 mutant strains are attenuated for virulence or avirulent in rabbit and murine models of cryptococcal meningitis (2, 3). The functions of Pka1 and Pka2 have diverged between serotype A variety C. grubii and D variety C. neoformans strains. Whereas Pka1 plays an essential role in capsule and melanin production in serotype A, Pka2 has assumed this role in serotype D (21). Pka1 and Pka2 are regulated by the upstream regulatory subunit Pkr1 (14). Deletion of the PKR1 gene results in overproduction of capsule and hypervirulence (14).
In the model yeast S. cerevisiae, there is a dual input to adenylyl cyclase activation and cAMP production. One input is via Ras1/2 and the other via Gpa2 (for a review, see reference 29). The elements downstream of adenylyl cyclase are also divergent between the model yeast and pathogenic fungus. Three PKA catalytic subunit isoforms, Tpk1, Tpk2, and Tpk3, are functionally redundant for yeast vegetative growth but specialized for pseudohyphal development. Tpk2 promotes pseudohyphal growth, whereas Tpk1 and Tpk3 inhibit filamentation (38, 42, 45). In another ascomycetous, pathogenic fungus, Candida albicans, only two isoforms, Tpk1 and Tpk2, have been identified and both promote hyphal differentiation, although each responds to different inducing signals (5, 44). In contrast, in C. neoformans the cAMP pathway appears to function as a linear Gpa1-Cac1-PKA cascade (Pka1 for serotype A or Pka2 for serotype D) (2, 3, 14, 21).
Here, we identify and characterize an upstream regulatory element of adenylyl cyclase, Aca1 (for adenylyl cyclase-associated protein 1). Yeast two-hybrid studies demonstrate that Aca1 binds to the C terminus of Cac1 and also forms homodimers similar to S. cerevisiae CAP/Srv2. Aca1 constitutes an upstream element of the Cac1-cAMP-signaling pathway independent of Gpa1 and Ras1, and regulates mating, capsule and melanin production, and virulence of C. neoformans. aca1 gpa1 double mutants exhibited phenotypes equivalent to cac1 mutants, indicating that Aca1 and Gpa1 are together necessary and sufficient for activation of the Cac1-cAMP pathway. Finally, by analyzing pka1 pka2 mutants we discovered that Pka2 can play a limited redundant role with Pka1 during mating and melanin production, demonstrating that under some physiological conditions the Cac1-cAMP signaling pathway is also bifurcated to Pka1 and Pka2 in C. neoformans.
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MATERIALS AND METHODS |
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Yeast two-hybrid assay. To construct plasmids expressing the Gal4 DNA binding domain (BD) or activation domain (AD) fused to the full-length ACA1 open reading frame (ORF), the ACA1 gene was amplified by reverse transcription-PCR (RT-PCR) with the primers 13086 and 13087 (13086/13087; see Table S1 in the supplemental material for each primer sequence) and cloned into two-hybrid vectors pGBT9 and pGAD424, generating pGBT-ACA1 and pGAD-ACA1. Similarly, the C-terminal region (2126 to 2260 amino acids [aa]) of CAC1 was amplified by using the primers 12873/12874 and cloned into pGBT9 and pGAD424, generating pGBT-CAC12126-2260 and pGAD-CAC12126-2260, respectively. The full-length GPA1 and RAS1 ORFs were also generated by RT-PCR by using primers 10961/10962 for GPA1 and primers 12875/12876 for RAS1 and cloned into pGBT9, generating pGBT-GPA1 and pGBT-RAS1, respectively. The reporter yeast strain PJ69-4A was cotransformed, and at least three independent Leu+ Trp+ transformants grown on SD medium lacking leucine and tryptophan (SD–Leu–Trp) were further analyzed for growth ability on SD–Leu–Trp–His and SD–Leu–Trp–His–Ade media.
Disruption of the ACA1 gene.
The aca1-null mutant (aca1) was generated in the congenic C. neoformans serotype A MAT (H99) and MATa (KN99) strain background (35) by PCR overlap as previously described (10). In the first round of PCR, the 5' and 3' regions of the ACA1 gene were generated by PCR (ExTaq; Takara Co.) by using H99 or KN99 genomic DNA and primers (Fig. 2): 10341/10342 for the 5' end and 10343/10344 for the 3' end. Natr or Neor dominant selectable markers were also generated by PCR with M13 forward (M13F) and reverse (M13R) primers by using plasmid pNATSTM#43 with a unique signature tag (kindly provided by Jennifer K. Lodge, St. Louis University School of Medicine; tag sequences available upon request) or pJAF1 (17) as templates, respectively. ACA1 disruption fragments were generated by PCR overlap with the primers 10341/10344. A 4.3-kb ACA1 disruption construct was gel extracted and precipitated onto 600-µg gold microcarrier beads (0.8 µm; Bioworld, Inc.) and biolistically transformed into strain H99 or KN99 as described previously (11). Stable transformants were selected on YPD medium containing nourseothricin (100 mg/liter) or G418 (200 mg/liter).
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mutant strains, diagnostic PCR was performed by analyzing the 5' junction of disrupted aca1 alleles with primers 10345 and 8994. Screened transformants were further confirmed by Southern blot analysis with a PstI-digested genomic DNA and an ACA1-specific probe made by PCR with primers 10409 and 10410. Uracil auxotrophic aca1 mutants (YSB108 and YSB109) (Table 1) were generated by inducing spontaneous ura5 mutations in strains YSB6 and YSB58, respectively, on SD medium containing 5-fluoroorotic acid (5-FOA).
To construct the aca1+ACA1 reconstituted strains, H99 genomic DNA containing the entire ACA1 gene was isolated from a C. neoformans H99 bacterial artificial chromosome (BAC) library using the ACA1 specific probe described above. The 3-kb SalI-HindIII fragment containing the ACA1 gene (Fig. 1) was cloned into plasmid pJAF12 containing the Neor marker, generating pNEOSRV2. Plasmids pNATSRV2 and pURASRV2 were further constructed by inserting the 3-kb XhoI-HindIII fragment of pNEOSRV2 into pJAF13 or pJAF7 containing a Natr or URA5 selectable marker, respectively. SbfI-linearized plasmids pNEOSRV2, pNATSRV2, or pURASRV2 were biolistically transformed into strains YSB6, YSB58, or ura5 YSB108/YSB109 (Table 1), respectively.
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Disruption of the GPA1, CAC1, RAS1, GPB1, PKA1, and PKA2 genes in the H99 and KN99 strain backgrounds. Although the GPA1, CAC1, RAS1, GPB1, PKA1, and PKA2 genes have been previously disrupted and characterized in serotype A C. neoformans (1-3, 50), these mutants were isolated in mutagenized auxotrophic H99 derivatives that may contain other unintended mutations. Therefore, these genes were disrupted in the congenic prototrophic H99 and KN99 genetic background by using methods equivalent to those described above. The 5' regions of each gene were generated with the following primers: 10913/11736 for GPA1, 11608/11609 for CAC1, 11600/11601 for RAS1, 11592/11593 for GPB1, 12911/12912 for PKA1, and 12916/12917 for PKA2. The 3' regions for each gene were generated with the following primer pairs: 11737/10957 for GPA1, 11610/11611 for CAC1, 11602/11603 for RAS1, 11594/11595 for GPB1, 12913/12936 for PKA1, and 12918/12919 for PKA2. The Neor marker was generated as described above. Natr markers were similarly generated with the following plasmids as templates: pNATSTM#146 for GPB1, pNATSTM#150 for RAS1, pNATSTM#5 for GPA1, pNATSTM#159 for CAC1, pNATSTM#191 for PKA1, and pNATSTM#205 for PKA2. Overlap PCR and biolistic transformation were performed as described above and the genotype of each mutant was confirmed by diagnostic PCR and Southern blot (not shown).
Construction of serotype A aca1 gpa1, aca1 cac1, aca1 ras1, ras1 cac1, pka1 pka2, and crg1 aca1 double-mutant strains.
For epistasis analysis, the GPA1, CAC1, and RAS1 genes were further disrupted in MAT aca1 (YSB6) and MATa aca1 (YSB58) mutant strains by using the same disruption cassette described above, generating MAT and MATa aca1 gpa1, aca1 cac1, and aca1 ras1 double-mutant strains (Table 1). The RAS1 gene was also disrupted in MAT cac1 (YSB42) and MATa cac1 (YSB79) mutant strains, creating ras1 cac1 double mutants (Table 1). The PKA2 gene was further disrupted in the MAT pka1 mutant (YSB188), generating pka1 pka2 double mutants (Table 1). Stable double-mutant strains (Natr Neor) were selected on YPD medium containing both nourseothricin and G418. Positive transformants were further screened by diagnostic PCR and confirmed by Southern blot analysis as described. The MATa crg1 aca1 mutant (YSB96) is an F1 progeny from a meiotic cross between strains JKH43 (MATa pde2::NAT crg1::URA5, unpublished) and YSB58. Strain JKH43 is an F1 progeny from a meiotic cross between strains JKH33 (MAT pde2::NAT [unpublished data]) and PPW196.
Mating, fusion, and confrontation assays. Mating experiments were performed by coincubating cells of opposite mating type on V8 mating medium with or without 1 or 10 mM cAMP in the dark at room temperature for 1 to 4 weeks.
Cell fusion efficiency was measured as previously described (21) with minor modifications. A total of 107 cells of each MAT and MATa strain bearing Natr or Neor markers, respectively, per ml was mixed in an equal volume, and 5 µl of this cell mixture was spotted onto V8 medium, followed by incubation for 24 h at room temperature in the dark. The cells were scraped and resuspended in 1 ml of distilled H2O, and 20 µl of cell suspension was plated onto YPD medium containing nourseothricin and G418. The number of colonies on each plate was determined after 4 days of incubation at room temperature.
For the confrontation assay to monitor pheromone production and response of cells, cells were streaked in confrontation with a cells on filament agar, followed by incubation for 5 days at room temperature in the dark. Images of mating and confrontation assays were captured with a Nikon Eclipse E400 microscope equipped with a Nikon CoolPix digital camera.
Assay for capsule production. Each strain (a single colony from solid YPD medium) was incubated for 16 h at 30°C in liquid YPD medium, spotted (3 x 105 cells) onto agar-based DME medium, and further incubated for 24 h at 37°C. After incubation, the capsule was stained by India ink and observed microscopically. Differential interference microscopy for cell and capsule images was performed with a Zeiss Axioskop 2 equipped with an AxioCam MRM digital camera. Quantitative measurement of capsule size was performed as previously described (57) by microscopically measuring the diameters of the capsule and the cell by using AxioVision 3.1 software (Zeiss). The relative capsule diameter, calculated as (Dw – Dc x 100)/Dw, where Dw and Dc indicate the diameters of the whole-cell body (cell plus capsule) and cell body only, respectively, was statistically compared between each mutant and wild-type strains by using the Bonferroni multiple comparison test performed with Prism 4.0 (GraphPad Software) (57).
Assay for melanin production. Each strain was incubated for 16 h at 30°C in YPD medium, spotted (3 x 105 cells) onto Niger seed medium containing 0.1% glucose, and incubated for 3 days at 30°C or 37°C. Melanin production was monitored daily and photographed. Quantitative measurements of laccase activity were performed as previously described (21). Briefly, 108 cells of each strain were grown in 25 ml of L-DOPA medium at 30°C for 16 h. The cultures were shifted to 25°C and further incubated for 6 and 24 h; then, 1 ml of culture was centrifuged, and the optical density at 475 nm (OD475) of the supernatant was measured with a spectrophotometer.
cAMP assays. Cells from a single colony grown on solid YPD medium were incubated at 30°C for 20 h in YPD medium. Cells were collected by centrifugation and washed twice with water and once with MES buffer (10 mM morpholineethanesulfonic acid [pH 6.0], 0.1 mM EDTA). Cells were resuspended in MES buffer at an OD600 of 2.0 and further incubated at 30°C with shaking for glucose starvation. After 2 h of incubation, glucose was added to a final concentration of 2%. At various time points (0, 0.5, 1, and 3 min), 1 ml of cell suspension was sampled, filtered through 0.45-µm-pore-size membrane filters (Millipore), immediately extracted with butanol-saturated formic acid (4.1% [vol/vol]), and lyophilized. The intracellular cAMP concentration was determined by using a cAMP [125I] Direct Biotrak Scintillation Proximity Assay System (Amersham Biosciences) and normalized to the wet weight of cells.
Virulence assays.
Yeast strains were grown at 30°C in YPD medium overnight with shaking (250 rpm), collected by centrifugation, and washed twice with sterile phosphate-buffered saline (PBS), and the final concentration was adjusted to 2 x 106 CFU/ml with sterile PBS. Female A/Jcr mice (NCI/Charles River Laboratories) mice (20 to 24 g each) were infected with wild-type (H99, six mice), aca1 (YSB6, nine mice), aca1+ACA1 reconstituted (YSB117, ten mice), and cac1 (YSB42, 10 mice) mutant strains by intranasal inhalation. Each mouse was inoculated with 105 CFU, in a volume of 50 µl, via nasal inhalation as previously described (9). Mice that appeared moribund (i.e., lethargic, rapid weight loss, or in pain) were sacrificed by using CO2 inhalation. Survival data from the murine experiments were statistically analyzed between paired groups by using the log-rank test with the Prism program 4.0. The animal protocol used for these experiments was approved by the Duke University Animal Use Committee.
Nucleotide sequence accession number. The sequence for the ACA1 gene from strain H99 has been assigned GenBank accession number AY629559.
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RESULTS |
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subunit Gpa1. The mating defect of cac1 mutants is more severe than that of gpa1 mutants and cac1 mutants are avirulent, whereas gpa1 mutants are attenuated but can result in lethal infection (3, 14). Ras, which is one of the upstream activators for adenylyl cyclase in S. cerevisiae, is not an upstream signaling component in the Cac1-cAMP-PKA pathway modulating capsule and melanin production in C. neoformans (1, 52). We focused our search for another potential upstream signaling component in the cAMP-signaling pathway on CAP/Srv2, which is known to be an effector protein between Ras and adenylyl cyclase in S. cerevisiae (15, 16). Furthermore, in C. albicans, the CAP/Srv2 homolog Cap1 mediates cAMP-dependent signaling to govern morphogenic transitions and virulence (4). Therefore, we hypothesized that a CAP/Srv2 homolog may mediate Ras or cAMP signaling in C. neoformans. By BLAST searches, a single CAP/Srv2 homolog was identified in both the serotype A and D C. neoformans genomes. This gene was designated ACA1 for adenylyl cyclase-associated protein 1. The serotype A Aca1 protein shares 73% identity with serotype D Aca1 and 32 to 35% identity with homologs in S. cerevisiae, C. albicans, and humans. Based on cDNA sequence analysis of 5' and 3' RACE products, the serotype A C. neoformans ACA1 spans a 1,908-bp ORF (from start to stop codon) and is interrupted by eight introns (GenBank accession number AY629599). The ACA1 gene encodes a 507-amino-acid protein. The C. neoformans Aca1 protein shows a domain structure typical of CAP/Srv2 homologs in other organisms. First, Aca1 contains the RLExAT/VxRLE motif (16RLEAVTSRLE25) that mediates Ras/cAMP signaling and proper CAP/Srv2 localization (55). Second, Aca1 contains a central proline rich region (269PPPPPP274), which may be involved in protein folding to enable interactions between the N- and C-terminal regions of CAP/Srv2 (55). Third, Aca1 has a single SH3 binding motif (PxxP; x, any amino acid, 335PLKP338), which plays an important role in proper cellular localization (19). This motif is also implicated in binding to Abp1 (actin-binding protein 1), which modulates actin cytoskeleton regulation in S. cerevisiae (19). Finally, Aca1 has a highly conserved C-terminal CAP-signature sequence involved in actin-monomer binding in the model yeast (18).
To further confirm that Aca1 is an adenylyl cyclase-associated protein, we demonstrated that Aca1 and Cac1 physically interact. In S. cerevisiae, the N-terminal domain (1 to 36 aa) of CAP/Srv2 binds to the C-terminal domain (1,822 to 2,026 aa) of the adenylyl cyclase Cyr1 protein through coiled-coil interactions mediated by tandem repeats of a heptad motif (xxxxx or xxxxx, where is a hydrophobic amino acid and x is any amino acid) present on both proteins (36). We found that the heptad repeats are also highly conserved in the C. neoformans Aca1 and Cac1 proteins (Fig. 1A). Notably, two leucine residues in both ScCAP (L20 and L27) and ScCyr1 (L1916 and L1923) that are known to be essential for their interaction are highly conserved in Aca1 and Cac1 (Fig. 1A), suggesting the potential for coiled-coil interactions between the two proteins. This hypothesis was confirmed by a yeast two-hybrid assay showing an interaction between Aca1 and the C-terminal region (2,126 to 2,260 aa) of Cac1 (Fig. 1B).
Furthermore, we also found that Aca1 can associate with itself (Fig. 1B), indicative of homodimerization or multimerization of Aca1, although the Aca1-Aca1 interaction appears to be weaker than the Aca1-Cac12126-2260 interaction (Fig. 1B). In S. cerevisiae, dimer or multimer formation by CAP/Srv2 is important for localization but is not essential for cAMP signaling (55, 58). In contrast to the association of Aca1 with Cac1, no interaction was observed between Aca1 and Gpa1 or Ras1 (Fig. 1B), implying that Aca1 might activate Cac1 independent of Gpa1 and Ras1. Taken together, the high degree of structural conservation between Aca1 and other CAP/Srv2 homologs and the physical interaction observed between Aca1 and Cac1 demonstrate that C. neoformans Aca1 is a bona fide member of the CAP/Srv2 protein family.
Disruption of the ACA1 gene and recapitulation of gpb1, ras1, gpa1, cac1, pka1, and pka2 mutations in the congenic serotype A MAT and MATa strains.
To characterize the function of Aca1 in known signaling pathways of C. neoformans, the ACA1 gene was disrupted. aca1::NAT and aca1::NEO disruption alleles were introduced into strains H99 and KN99, respectively, by biolistic transformation, deleting an internal 1,880-bp (bp 1 to 1880 from the start) of the 1,908-bp ORF (Fig. 2A). The genotypes of two independent MAT aca1 mutants (YSB6 and YSB7) and three independent MATa aca1 mutants (YSB58, YSB59, and YSB60) were verified by diagnostic PCR (not shown) and Southern blot analysis (Fig. 2B). The wild-type ACA1 gene was reintroduced into the aca1 mutants by targeting to the ACA1 locus with linearized ACA1-NEO or ACA1-NAT constructs (Fig. 2B). Successful disruption and reintegration of ACA1 were further confirmed by RT-PCR analysis, demonstrating the loss and recovery of ACA1 expression in the aca1 mutant and aca1+ACA1 reconstituted strains, respectively (Fig. 2C). All of the independent MAT or MATa aca1 mutant strains showed identical in vitro phenotypes, and representative results from strains YSB6 (MAT aca1) and YSB58 (MATa aca1) (Table 1) are shown here.
For comparative phenotypic analysis of aca1 mutants with other cAMP-dependent or -independent mutants, we then redisrupted the GPB1, RAS1, GPA1, CAC1, PKA1, and PKA2 genes in the congenic serotype A MAT H99 and MATa KN99 strains, which were recently generated by Nielsen et al. (35). We regenerated these mutant strains for the following reasons. First, the previous mutants had been isolated in different mutagenized auxotrophic parental strain backgrounds (ade2 or ura5), which complicates comparative phenotypic analysis between those mutants and mutants generated directly in H99 or KN99 by using dominant selectable markers. Second, utilization of auxotrophic strains as parental strains for gene disruption might cause misinterpretation of phenotypes through direct or indirect effects of auxotrophic mutations or unexpected background mutations introduced during their generation by UV or gamma irradiation. Finally, the GPB1, RAS1, GPA1, CAC1, PKA1, and PKA2 genes had not been disrupted previously in the KN99 MATa genetic background. Disruption of these genes in the congenic MATa background makes it possible to investigate serotype A versus a bilateral mating efficiency, instead of by using heterologous serotype D mating-type tester strains.
We regenerated all mutants by using the Natr and Neor dominant selectable markers in the congenic H99 and KN99 backgrounds as described in Materials and Methods. In general, newly constructed MAT and MATa gpb1 (YSB49 and YSB76), ras1 (YSB51 and YSB73), gpa1 (YSB83 and YSB85), cac1 (YSB42 and YSB79), pka1 (YSB188 and YSB191), and pka2 (YSB194 and YSB198) mutants (Table 1) displayed phenotypes comparable to mutants generated previously (1-3, 50), in terms of defective mating (gpb1, ras1, gpa1, cac1, and pka1 mutants), reduced capsule and melanin production (gpa1, cac1, and pka1 mutants), or inability to grow at high temperature (ras1 mutants) (data not shown). The genotypes for each mutant strain were confirmed by Southern blot analyses and at least two independent mutants for each gene exhibited identical phenotypes.
Aca1 is not essential for growth at high temperature.
C. neoformans ras1 mutants exhibit a temperature-sensitive growth defect (1). Based on the functional connection between Ras1 and CAP/Srv2 in S. cerevisiae, we tested growth of aca1 mutants at high temperature to address the question of whether Aca1 is involved in Ras1 signaling. ras1 mutants newly constructed in the congenic H99 and KN99 background (YSB51 and YSB73) showed severe growth defects at 37 to 39°C (not shown), in accord with previous studies (1, 52). In contrast, aca1 mutants had no growth defect at 30, 37, or 39°C (not shown). MAT and MATa wild-type (H99 and KN99), aca1 mutants (YSB6 and YSB58), and aca1+ACA1 reconstituted strains (YSB117 and YSB118) showed similar growth rates in YPD medium at either 30 or 37°C. When the RAS1 gene was disrupted in the aca1 (aca1 ras1 mutants YSB174 and YSB175) or cac1 (ras1 cac1 mutants YSB182 and YSB185) mutant background, the temperature-sensitive growth defect conferred by the ras1 mutant was readily apparent at 37° and 39°C. We conclude that Aca1 is not involved in the Ras1-specific signaling pathway regulating growth at high temperature.
Aca1 promotes mating via the cAMP-signaling pathway in parallel with Gpa1.
In previous studies, Cac1 and Gpa1 were found to be involved in mating (2, 3). We hypothesized that Aca1 should play a similar role in mating if it is also a component of the cAMP signaling pathway. In unilateral mating crosses with the wild-type strains (H99 or KN99) of opposite mating type, both MAT and MATa aca1 mutants mated less efficiently on V8 medium than did the wild type (Fig. 3A). The defect in unilateral mating was more pronounced at earlier time points, and basidiospores were observed at later time points (Fig. 3A). aca1 mutants displayed more dramatic defects in bilateral mating crosses (aca1 x aca1) in which only very short filaments were produced within the mating patches but no basidiospores were observed, even after 4 weeks of incubation (Fig. 3A).
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mutants result from defects in cell fusion or mating filamentation, cell fusion efficiency was determined with Natr or Neor marked aca1 mutants (YSB6 and YSB58) and normalized to the fusion of control strains (YSB119 and YSB121, aca1+ACA1 reconstituted strains). aca1 mutants were less efficient in cell fusion compared than the wild-type strain (Fig. 3B) but still yielded fusion products (11.9% ± 1.4%), indicating that the aca1 disruption results in both defective mating filamentation and reduced cell fusion frequency. In support of this conclusion, all aca1/aca1 diploid strains isolated from cell fusion assays exhibited less prolific filamentous growth compared to wild-type diploid control strains (Fig. 3C).
Defects in filamentous growth conferred by the aca1 mutation were further assayed by using confrontation assays. For the present study, we used pheromone hypersensitive crg1 mutants, which lack an RGS protein that normally desensitizes cells to pheromone exposure (35, 47). When MAT crg1 mutants were confronted with MATa crg1 mutants, both cell types formed filaments toward the mating partner in response to pheromone (Fig. 3D). Disruption of ACA1 in the MATa crg1 mutant (aca1 crg1 double mutants; Table 1) still resulted in normal conjugation tube formation from confronting crg1 cells but defective filamentous growth from the a aca1 crg1 mutant (Fig. 3D), indicating that Aca1 is not required for pheromone production but is involved in filamentation during response to pheromone.
To further determine whether Aca1 regulates mating in a cAMP-dependent manner, the mating defect of aca1 mutants was compared to those of gpb1 and ras1 (cAMP-independent) versus gpa1 and cac1 (cAMP-dependent) mutants. We found that gpb1, ras1, gpa1, and cac1 mutants all showed more severe unilateral mating defects than aca1 mutants (Fig. 4A). The addition of 1 or 10 mM exogenous cAMP, however, greatly enhanced unilateral mating and bilateral mating, respectively, of gpa1, cac1, and aca1 mutant cells to levels comparable to wild type (Fig. 4A). In contrast, unilateral or bilateral mating defects of ras1 and gpb1 mutants were not rescued by 1 or 10 mM cAMP (Fig. 4B), further confirming that Ras1 and Gpb1 transmit mating signals through cAMP-independent signaling pathways.
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mutants (Fig. 3B). cac1 mutants showed even more severe cell fusion defects than either gpa1 or aca1 mutants (Fig. 3B). In contrast to the less severe cell fusion defect observed in gpa1 mutants compared to aca1 mutants, gpa1/gpa1 diploid strains were completely blocked in filamentous growth, as were cac1/cac1 diploid strains (Fig. 3C). These results suggest that Gpa1 and Aca1 independently contribute to activate the Cac1-cAMP pathway during the mating process. Compared to these cAMP pathway mutants, disruption of the RAS1 or GPB1 genes conferred a complete lack of cell fusion even in unilateral mating, which contrasts with the reduced but apparent cell fusion events in cells lacking components of the cAMP pathway (Fig. 3B). Therefore, the data indicate that the Cac1-cAMP pathway controls mating, is independently activated by either (or both) Aca1 or Gpa1, and functions in a pathway distinct from Ras1 and Gpb1.
Previous studies identified the two PKA catalytic subunits Pka1 and Pka2, and Pka1 plays the major role in mating and filamentous growth in serotype A (14, 21). In accord with these results, pka1 mutant strains (YSB188 and YSB191) displayed a severe mating defect (particularly in bilateral matings), whereas pka2 mutant strains (YSB194 and YSB198) had no mating defect (Fig. 4B). However, the finding that the pka1 mutant showed less severe unilateral mating defects than the cac1 mutant suggested that Pka2 might play a limited role in mating in pka1 mutant cells (Fig. 4). In fact, pka1 pka2 double-mutant strains showed a more severe unilateral mating defect than pka1 mutant cells, similar to the cac1 mutant, suggesting that the mating signal from Cac1 is bifurcated into Pka1 and Pka2, with Pka1 serving as the principal signaling element of the cAMP pathway.
Aca1 is required for capsule production via the Cac1-cAMP-Pka1-dependent signaling pathway.
Previous studies showed that the Gpa1-Cac1-cAMP-Pka1 signaling pathway is the major cascade for capsule regulation (2, 3), whereas Ras1 is not required (1). Consistent with our assignment of Aca1 as a component of the cAMP-signaling pathway, Aca1 was required for capsule production (Fig. 5). aca1 mutants were highly defective in capsule production in response to a variety of capsule-inducing signals, including agar-based DME (Fig. 5), low iron, and 10% serum (not shown). These mutants were hypocapsular and not acapsular, and a residual level of capsule was still observed, similar to other cAMP pathway mutants (i.e., gpa1, cac1, or pka1) (Fig. 5). Based on quantitative measurements of relative capsule sizes, the defect in capsule production of aca1 mutants was comparable to those of gpa1, cac1 and pka1 (Fig. 5B). Addition of 10 mM cAMP to agar-based DME media completely restored capsule production in aca1, gpa1, and cac1 mutants but not of pka1 mutants, demonstrating that Aca1 functions upstream of Pka1 in the cAMP pathway.
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cac1 double-mutant strains (YSB42 and YSB79) were generated (Table 1). aca1 cac1 double mutants did not display any additive defects in capsule production relative to aca1 or cac1 single-mutant strains and still produced minimal capsules compared to a complete lack of capsule in the acapsular strain cap59 (Fig. 5). The data suggest then that Aca1 signals in a linear fashion with Cac1 and Pka1. As reported (1), Ras1 is not involved in capsule production, and aca1 ras1 double mutants exhibited the same level of capsule defect observed with aca1 single mutants (Fig. 5). Also in accord with a previous study (21), Pka2 was found to be completely dispensable for capsule production. pka2 mutants produced wild-type levels of capsule, and pka1 pka2 double mutants (YSB200) did not exhibit any additive defects in capsule production compared to pka1 single-mutant strains (data not shown). In conclusion, Aca1 transmits capsule inducing signals through the Cac1-cAMP-PKA-dependent signaling pathway.
Aca1 and Gpa1 individually contribute to melanin production via the Cac1-cAMP-PKA-dependent pathway.
Melanin plays a role as an antioxidant during host infection and serves as a virulence factor regulated by the Gpa1-Cac1-cAMP-PKA signaling pathway. Therefore, we investigated the role of Aca1 in melanin production, both by visual inspection of melanin accumulation in cells on Niger seed medium and by quantitative measurements of laccase activity. Disruption of the ACA1 gene attenuated melanin production, and this defect was more apparent at 37°C than at 30°C (Fig. 6A). The defect of the aca1 mutant was comparable to gpa1 and cac1 mutants at 37°C. Melanin synthesis in the aca1, gpa1, and cac1 mutants was restored by exogenous cAMP (10 mM), further confirming that Aca1, Gpa1, and Cac1 function in a cAMP-dependent manner. Interestingly, we found that gpa1 and aca1 mutants exhibited only minor defects in melanin biosynthesis at 30°C compared to a more pronounced defect of cac1 mutants (Fig. 6A). Quantitative measurement of laccase activity in L-DOPA medium further confirmed that aca1 and gpa1 mutants are deficient in melanin production compared to wild-type, but neither is as defective as cac1 mutants (Fig. 6B). These data suggest that Cac1 requires multiple inputs to activate the synthesis of melanin.
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aca1 double-mutant strains showed a more severe melanin defect than either gpa1 or aca1 single mutant, and this synergistic defect was comparable to cac1 mutants on both Niger seed and L-DOPA media (Fig. 6). The melanin defect of gpa1 aca1 mutants was completely rescued by addition of exogenous cAMP (10 mM) (Fig. 6A). In contrast, introducing an aca1 mutation into the cac1 mutant background did not exacerbate the melanin defect of cac1 mutant cells (Fig. 6). These findings support a model in which Gpa1 and Aca1 are two parallel upstream components activating the Cac1-cAMP-PKA signaling pathway for production of melanin.
As previously reported (21), pka1 mutants displayed defects in melanin synthesis similar to other cAMP cascade mutants at 37°C on Niger seed medium, which could not be rescued by exogenous cAMP (Fig. 6A). We note however that pka1 mutants showed a less severe defect in melanin accumulation at 30°C on Niger seed medium than cac1 mutants, implicating the potential role of the other catalytic subunit Pka2 in this pathway (Fig. 6A). Supporting this hypothesis, pka1 pka2 double mutants were found to be more melanin defective in both 30 and 37°C compared to pka1 mutants, similar to cac1 or aca1 gpa1 double mutants. Similarly, pka1 pka2 double mutants largely failed to produce melanin in L-DOPA medium similar to cac1 mutants (Fig. 6B). The pka2 single-mutant strain exhibited wild-type melanin production (Fig. 6), as reported previously (21). These data demonstrate that Pka1 plays the predominant role in cAMP signaling to control melanin synthesis but, in its absence, Pka2 can fulfill a limited signaling capacity.
Aca1 is required for glucose-induced but not basal cAMP levels.
The role of Aca1 in controlling cAMP signaling in conjunction with Gpa1 and Cac1 was confirmed by measuring cAMP levels during glucose sensing. In accord with a previous report (3), cAMP levels in the wild type rapidly increased after glucose readdition to glucose-starved cells (Fig. 7). In contrast, the gpa1 and cac1 mutant exhibited significantly lower cAMP concentrations at all time points and lacked any detectable cAMP pulse (Fig. 7), and thus both Gpa1 and Cac1 are required for both glucose-induced cAMP signaling and maintenance of basal cAMP levels. The aca1 mutant was defective in induced cAMP production but maintained basal cAMP levels equivalent to the wild-type at the zero time point (Fig. 7). cAMP levels in the aca1 mutant were significantly higher than those in the gpa1 and cac1 mutants, indicating that Aca1 plays a role in glucose-induced but not basal cAMP levels. Reintroduction of the ACA1 gene restored a wild-type cAMP signaling pattern in the aca1 mutant (Fig. 7). The basal cAMP levels in the aca1 mutant were further reduced to the levels of the cac1 mutant after disruption of the GPA1 gene (aca1 gpa1) (Fig. 7), a finding consistent with the identical in vitro phenotypes observed in aca1 gpa1 and cac1 mutants. Taken together, these findings confirm that Aca1 controls cAMP signaling through Cac1 in a manner distinct from Gpa1.
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(YSB6), aca1+ACA1 reconstituted (YSB117), or cac1 (YSB42) mutant strains by intranasal inhalation, which mimics the natural route of human infection by C. neoformans. Inhaled cells first infect the lung and then disseminate to the brain, causing meningoencephalitis. Survival of infected mice was monitored for 40 days. All mice infected with the wild-type strain became moribund between days 19 and 24 postinfection (median survival, 23 days) (Fig. 8). In contrast, all mice infected with the aca1 mutant strains survived throughout the course of the experiment without showing any signs of illness (P < 0.0001 compared to the wild-type or reconstituted strains) (Fig. 8). The aca1 mutants were as avirulent as cac1 mutants and reintroduction of the ACA1 gene into the aca1 mutants completely restored virulence (Fig. 8). Animals infected with the aca1+ACA1 reconstituted strain became moribund between days 23 and 25 postinfection (median survival of 23 days; P = 0.2027 compared to wild-type strains). These findings provide evidence that Aca1 regulates virulence of C. neoformans via the Cac1-cAMP pathway.
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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mutant were restored to wild-type by exogenous cAMP. Finally, Aca1 controls intracellular cAMP levels in response to glucose readdition. The aca1 mutant maintained a basal level of cAMP but failed to induce cAMP production after the readdition of glucose. Similar to the aca1 mutant, the C. albicans cap1 mutant was found to be defective in induced cAMP levels, but maintained a normal basal level during the bud-hypha transition (4). Therefore, Aca1 encodes a member of CAP/Srv2 protein family regulating cAMP signaling in C. neoformans.
Aca1 was found to be an important virulence regulator for C. neoformans. The complete absence of virulence of aca1 mutant strains is likely attributable to lack of capsule and melanin production rather than to defects in mating and filamentation because other mutants showing only defects in mating and filamentous growth (e.g., ste11, ste7, and cpk1 mutants) are virulent (12). Previously, the adenylyl cyclase-associated protein (Cap1) was found to be a virulence regulator for the ascomycetous pathogenic fungus C. albicans (4). Interestingly, avirulence of the C. albicans cap1 mutant appears to result from its inability to switch between yeast and hyphal growth (4), an essential virulence factor for C. albicans (30). Therefore, the adenylyl cyclase-associated proteins Aca1 and Cap1 serve as important virulence regulators for C. neoformans and C. albicans via conserved cAMP-signaling pathways that drive unique developmental cascades required for the acquisition of virulence.
Construction of ras1, gpb1, and cAMP cascade mutants by using the congenic serotype A C. neoformans with dominant selectable markers.
Another major contribution of the present study is the recreation of cAMP-independent (gpb1 and ras1) and cAMP-dependent (gpa1, cac1, pka1, and pka2) mutants by using dominant selectable markers and the congenic serotype A C. neoformans MAT and MATa strains, H99 and KN99, as parental strains. In previous studies, the functions of Gpb1, Ras1, Gpa1, Cac1, and Pka1/2 were analyzed by using mutants created in uracil or adenine auxotrophic parental strain backgrounds (1-3, 50). Here, however, we suggest that use of auxotrophic strains as parents for mutant construction should be avoided, if possible, because earlier auxotrophic strains generated by UV or gamma irradiation may contain undesired background mutations and ura5 mutants have recently been found to be temperature sensitive (23). We found few, if any, examples of problems based on these concerns, but it seems prudent to advocate advancing the state of the art in the field at this time.
New insights into cAMP-dependent and -independent signaling pathways regulating virulence and differentiation of C. neoformans.
Our studies further expand existing knowledge about signal transduction systems regulating virulence factors (melanin and capsule) and morphological differentiation (mating and filamentation), which are summarized in Fig. 9. In this model, Aca1 functions as an upstream component of the Cac1-cAMP signaling pathway in parallel with Gpa1. First, aca1 mutant strains showed subtle phenotypic differences compared to gpa1 mutant strains in cell fusion efficiency and filamentation during mating and in melanin synthesis. Second, disruption of both GPA1 and ACA1 conferred additive defects in melanin production, resulting in defects similar to those observed in cac1 mutants, providing evidence that Aca1 and Gpa1 are two necessary and sufficient upstream regulators for Cac1. aca1 cac1 double-mutant strains were comparable to cac1 single mutants, showing that Aca1 signals in a linear fashion with Cac1. We speculate that Aca1 and Gpa1 constitute two upstream components of adenylyl cyclase in C. neoformans and serve roles similar to Ras1/2 and Gpa2 in S. cerevisiae.
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and pka2 mutants newly constructed in the present study showed phenotypes consistent with those of the previous study; however, we did uncover a minor role for Pka2 in mating and melanin production through generation of pka1 pka2 double-mutant strains. pka1 pka2 double-mutant strains displayed a more severe defect in unilateral mating crosses and melanin biosynthesis than pka1 mutants, similar to the cac1 mutant or aca1 gpa1 mutant strains. Thus, Pka1 plays the major and Pka2 a minor role, and the two are necessary and sufficient downstream elements of the Cac1-cAMP signaling pathway for complete mating and melanin production.
We found that Ras1 also has a minor role in melanin production, independent of cAMP signaling. Although ras1 single-mutant strains did not show significant defects in melanin production in accord with a previous report (1), aca1 ras1 and cac1 ras1 double-mutant strains exhibited a more severe melanin defect than aca1
, H99), aca1
, YSB6), aca1
, YSB117), gpa1
, YSB42) were glucose starved for 2 h. After the readdition of glucose, a portion of cell suspension was extracted, and its intracellular cAMP concentration was measured at the indicated times as described in Materials and Methods. Each datum point and error bar indicate the mean and standard deviation, respectively, for duplicate samples of two independent experiments. At all time points except zero, the cAMP levels were significantly higher in the wild-type or aca1
