Discriminating between commensal and pathogenic states of opportunistic pathogens is critical for host mucosal defense and homeostasis. The opportunistic human fungal pathogen Candida albicans is also a constituent of the normal oral flora and grows either as yeasts or hyphae. We demonstrate that oral epithelial cells orchestrate an innate response to C. albicans via NF-κB and a biphasic MAPK response. Activation of NF-κB and the first MAPK phase, constituting c-Jun activation, is independent of morphology and due to fungal cell wall recognition. Activation of the second MAPK phase, constituting MKP1 and c-Fos activation, is dependent upon hypha formation and fungal burdens and correlates with proinflammatory responses. Such biphasic response may allow epithelial tissues to remain quiescent under low fungal burdens while responding specifically and strongly to damage-inducing hyphae when burdens increase. MAPK/MKP1/c-Fos activation may represent a “danger response” pathway that is critical for identifying and responding to the pathogenic switch of commensal microbes.
► NF-κB and MAPK control epithelial effector responses against Candida albicans ► c-Jun activation is independent of morphology and due to fungal cell wall recognition ► MAPK/MKP-1/c-Fos pathway activation is dependent on fungal hyphae and burdens ► MAPK discriminatory response may dictate C. albicans mucosal colonization in vivo
We previously reported that a bi-phasic innate immune MAPK response, constituting activation of the mitogen-activated protein kinase (MAPK) phosphatase MKP1 and c-Fos transcription factor, discriminates between the yeast and hyphal forms of Candida albicans in oral epithelial cells (ECs). Since the vast majority of mucosal Candida infections are vaginal, we sought to determine whether a similar bi-phasic MAPK-based immune response was activated by C. albicans in vaginal ECs. Here, we demonstrate that vaginal ECs orchestrate an innate response to C. albicans via NF-κB and MAPK signaling pathways. However, unlike in oral ECs, the first MAPK response, defined by c-Jun transcription factor activation, is delayed until 2 h in vaginal ECs but is still independent of hypha formation. The ‘second’ or ‘late’ MAPK response, constituting MKP1 and c-Fos transcription factor activation, is identical to oral ECs and is dependent upon both hypha formation and fungal burdens. NF-κB activation is immediate but independent of morphology. Furthermore, the proinflammatory response in vaginal ECs is different to oral ECs, with an absence of G-CSF and CCL20 and low level IL-6 production. Therefore, differences exist in how C. albicans activates signaling mechanisms in oral and vaginal ECs; however, the activation of MAPK-based pathways that discriminate between yeast and hyphal forms is retained between these mucosal sites. We conclude that this MAPK-based signaling pathway is a common mechanism enabling different human epithelial tissues to orchestrate innate immune responses specifically against C. albicans hyphae.
Oral epithelial cells discriminate between the yeast and hyphal forms of Candida albicans via the mitogen-activated protein kinase (MAPK) signaling pathway. This occurs through phosphorylation of the MAPK phosphatase MKP1 and activation of the c-Fos transcription factor by the hyphal form. Given that fungal cell wall polysaccharides are critical in host recognition and immune activation in myeloid cells, we sought to determine whether β-glucan and N- or O-glycosylation was important in activating the MAPK/MKP1/c-Fos hypha-mediated response mechanism and proinflammatory cytokines in oral epithelial cells. Using a series of β-glucan and N- and O-mannan mutants, we found that N-mannosylation (via Δoch1 and Δpmr1 mutants) and O-mannosylation (via Δpmt1 and Δmnt1 Δmnt2 mutants), but not phosphomannan (via a Δmnn4 mutant) or β-1,2 mannosylation (via Δbmt1 to Δbmt6 mutants), were required for MKP1/c-Fos activation, proinflammatory cytokine production, and cell damage induction. However, the N- and O-mannan mutants showed reduced adhesion or lack of initial hypha formation at 2 h, resulting in little MKP1/c-Fos activation, or restricted hypha formation/pseudohyphal formation at 24 h, resulting in minimal proinflammatory cytokine production and cell damage. Further, the α-1,6-mannose backbone of the N-linked outer chain (corresponding to a Δmnn9 mutant) may be required for epithelial adhesion, while the α-1,2-mannose component of phospholipomannan (corresponding to a Δmit1 mutant) may contribute to epithelial cell damage. β-Glucan appeared to play no role in adhesion, epithelial activation, or cell damage. In summary, N- and O-mannosylation defects affect the ability of C. albicans to induce proinflammatory cytokines and damage in oral epithelial cells, but this may be due to indirect effects on fungal pathogenicity rather than mannose residues being direct activators of the MAPK/MKP1/c-Fos hypha-mediated immune response.
Oral epithelial cells detect the human pathogenic fungus Candida albicans via NF-κB and a bi-phasic mitogen-activated protein kinase (MAPK) signaling response. However, discrimination between C. albicans yeast and hyphal forms is mediated only by the MAPK pathway, which constitutes activation of the MAPK phosphatase MKP1 and the c-Fos transcription factor and is targeted against the hyphal form. Given that C. albicans is not the only Candida species capable of filamentation or causing mucosal infections, we sought to determine whether this MAPK/MKP1/c-Fos mediated response mechanism was activated by other pathogenic Candida species, including C. dubliniensis, C. tropicalis, C. parapsilosis, C. glabrata and C. krusei. Although all Candida species activated the NF-κB signaling pathway, only C. albicans and C. dubliniensis were capable of inducing MKP1 and c-Fos activation, which directly correlated with hypha formation. However, only C. albicans strongly induced cytokine production (G-CSF, GM-CSF, IL-6 and IL-1α) and cell damage. Candida dubliniensis, C. tropicalis and C. parapsilosis were also capable of inducing IL-1α and this correlated with mild cell damage and was dependent upon fungal burdens. Our data demonstrate that activation of the MAPK/MKP1/c-Fos pathway in oral epithelial cells is specific to C. dubliniensis and C. albicans hyphae.
Candida albicans; Candida dubliniensis; Hypha formation; MAPK; MKP1; c-Fos; NF-κB; Oral epithelium; Innate immunity
The fungus C. albicans uses adhesins to interact with human epithelial surfaces in the processes of colonization and pathogenesis. The C. albicans ALS (agglutinin-like sequence) gene family encodes eight large cell-surface glycoproteins (Als1-Als7 and Als9) that have adhesive function. This study utilized C. albicans Δals mutant strains to investigate the role of the Als family in oral epithelial cell adhesion and damage, cytokine induction and activation of a MAPK-based (MKP1/c-Fos) signaling pathway that discriminates between yeast and hyphae. Of the eight Δals mutants tested, only the Δals3 strain showed significant reductions in oral epithelial cell adhesion and damage, and cytokine production. High fungal:epithelial cell multiplicities of infection were able to rescue the cell damage and cytokine production phenotypes, demonstrating the importance of fungal burden in mucosal infections. Despite its adhesion, damage and cytokine induction phenotypes, the Δals3 strain induced MKP1 phosphorylation and c-Fos production to a similar extent as control cells. Our data demonstrate that Als3 is involved directly in epithelial adhesion but indirectly in cell damage and cytokine induction, and is not the factor targeted by oral epithelial cells to discriminate between the yeast and hyphal form of C. albicans.
The opportunistic fungus Candida albicans is one of the leading causes of infections in immunocompromised patients, and innate immunity provides a principal mechanism for protection from the pathogen. In the present work, the role of integrin αXβ2 in the pathogenesis of fungal infection was assessed. Both purified αXβ2 and αXβ2-expressing human epithelial kidney 293 cells recognized and bound to the fungal hyphae of SC5314 strain of C. albicans but not to the yeast form or to hyphae of a strain deficient in the fungal mannoprotein, Pra1. The binding of the integrin to the fungus was inhibited by β-glucans but not by mannans, implicating a lectin-like activity in recognition but distinct in specificity from that of αMβ2. Mice deficient in αXβ2 were more prone to systemic infection with the LD50 fungal inoculum decreasing 3-fold in αXβ2-deficient mice compared with wild-type mice. After challenging i.v. with 1.5 × 104 cell/g, 60% of control C57BL/6 mice died within 14 d compared with 100% mortality of αXβ2-deficient mice within 9 d. Organs taken from αXβ2-deficient mice 16 h postinfection revealed a 10-fold increase in fungal invasion into the brain and a 2-fold increase into the liver. These data indicate that αXβ2 is important for protection against systemic C. albicans infections and macrophage subsets in the liver, Kupffer cells, and in the brain, microglial cells use αXβ2 to control fungal invasion.
Innate immune responses mediated by Toll-like receptors (TLRs), a class of pattern-recognition receptors, play a critical role in the defense against microbial pathogens. However, excessive TLR-mediated responses result in sepsis, autoimmunity, and chronic inflammation. To prevent deleterious activation of TLRs, cells have evolved multiple mechanisms that inhibit innate immune reactions. Stimulation of TLRs induces the expression of the gene encoding the mitogen-activated protein kinase (MAPK) phosphatase-1 (MKP-1), a nuclear-localized dual-specificity phosphatase that preferentially dephosphorylates p38 MAPK and c-Jun N-terminal kinase (JNK), resulting in the attenuation of TLR-triggered production of proinflammatory cytokines. MKP-1 is posttranslationally modified by multiple mechanisms, including phosphorylation. A study now demonstrates that MKP-1 is also acetylated on a key lysine residue following stimulation of TLRs. Acetylation of MKP-1 promotes the interaction of MKP-1 with its substrate p38 MAPK, which results in dephosphorylation of p38 MAPK and the inhibition of innate immunity.
The opportunistic fungus Candida albicans is one of the leading causes of infections in immunocompromised patients, and innate immunity provides a principal mechanism for protection from the pathogen. In the present work, the role of integrin αXβ2 in the pathogenesis of fungal infection was assessed. Both purified αXβ2 and αXβ2-expressing HEK293 cells recognized and bound to the fungal hyphae of SC5314 strain of C. albicans but not to the yeast form or to hyphae of a strain deficient in the fungal mannoprotein, Pra1. The binding of the integrin to the fungus was inhibited by β-glucans but not by mannans, implicating a lectin-like activity in recognition but distinct in specificity from that of αMβ2. Mice deficient in αXβ2 were more prone to systemic infection with the LD50 fungal inoculum decreasing three-fold in αXβ2 deficient mice compared to wild-type mice. After challenging intravenously with 1.5 × 104 cell/g, 60% of control C57BL/6 mice died within 14 days compared to 100% mortality of αXβ2-deficient mice within 9 days. Organs taken from αXβ2-deficient mice 16 hrs after infection revealed a 10-fold increase in fungal invasion into brain and 2-fold increase into liver. These data indicate that αXβ2 is important for protection against systemic C. albicans infections and macrophage subsets in liver, Kupffer cells, while in brain, microglial cells employ αXβ2 to control fungal invasion.
integrin αXβ2; Pra1; C. albicans; systemic fungal infections; protection; murine model; adhesion molecules; macrophages; microglia; Kupffer cells
The mechanism regulating radiation-induced anti-apoptotic response, a
limiting factor in improving cell radiosensitivity, remains elusive.
Mitogen-activated protein kinase (MAPK) phosphatase (MKP)-1 is the major
member of MKPs that dephosphorylates and inactivates MAPK. Here we provide the
evidence that MKP-1 was negatively bridging between NF-κB-mediated
prosurvival pathway and c-Jun N-terminal kinase (JNK)-mediated proapoptotic
response. MKP-1 was induced by γ-radiation and repressed
radiation-induced pro-apoptotic status. NF-κB RelA/p50 heterodimer was
recruited to MKP-1 gene promoter to induce MKP-1 transcription. Deletion of
the NF-κB-binding site or inactivation of NF-κB by its small
interfering RNA significantly decreased the radiation-induced MKP-1 promoter
activity. In addition, MKP-1-deficient mouse embryonic fibroblasts exhibited a
prolonged activation of JNK but not p38 or extracellular signal-regulated
kinase subfamilies of MAPKs. The prolonged activation of JNK was not induced
by treatment with tumor necrosis factor α or interleukin-6, and
inactivation of JNK but not p38 or ERK abolished radiation-induced
proapoptotic status, indicating that JNK is specifically inhibited by
radiation-induced MKP-1. Three MKP-1 wild type human tumor cell lines treated
with MKP-1 small interfering RNA showed an increased proapoptotic response
that can be rescued by overexpression of wild type mouse MKP-1. Together,
these results suggest that MKP-1 is a NF-κB-mediated prosurvival
effector in attenuating JNK-mediated pro-apoptotic response;
NF-κB/MKP-1-mediated negative JNK regulation represents a potential
therapeutic target for adjusting cell radiosensitivity.
Invading C. albicans hyphae are recognized by macrophages, activate the caspase-1/IL-1β pathway, and lead to the activation of IL-17 pathway to control the C. albicans infection.
In the mucosa, the immune pathways discriminating between colonizing and invasive Candida, thus inducing tolerance or inflammation, are poorly understood. Th17 responses induced by Candida albicans hyphae are central for the activation of mucosal antifungal immunity. An essential step for the discrimination between yeasts and hyphae and induction of Th17 responses is the activation of the inflammasome by C. albicans hyphae and the subsequent release of active IL-1β in macrophages. Inflammasome activation in macrophages results from differences in cell-wall architecture between yeasts and hyphae and is partly mediated by the dectin-1/Syk pathway. These results define the dectin-1/inflammasome pathway as the mechanism that enables the host immune system to mount a protective Th17 response and distinguish between colonization and tissue invasion by C. albicans.
Candida; colonization; invasion; IL-1β; IL-17
Dual-specificity MAP kinase phosphatases (MKPs) provide a complex negative regulatory network that acts to shape the duration, magnitude and spatiotemporal profile of MAP kinase activities in response to both physiological and pathological stimuli. Individual MKPs may exhibit either exquisite specificity towards a single mitogen-activated protein kinase (MAPK) isoform or be able to regulate multiple MAPK pathways in a single cell or tissue. They can act as negative feedback regulators of MAPK activity, but can also provide mechanisms of crosstalk between distinct MAPK pathways and between MAPK signalling and other intracellular signalling modules. In this review, we explore the current state of knowledge with respect to the regulation of MKP expression levels and activities, the mechanisms by which individual MKPs recognize and interact with different MAPK isoforms and their role in the spatiotemporal regulation of MAPK signalling.
dual specificity protein phosphatase; DUSP; ERK; JNK mitogen activated protein kinase; MAPK; MAPK localisation; MAPK phosphatase; MKP; p38
The opportunistic fungal pathogen Candida albicans is both a benign gut commensal and a frequently fatal systemic pathogen. The interaction of C. albicans with the host's innate immune system is the primary factor in this balance; defects in innate immunity predispose the patient to disseminated candidiasis. Because of the central importance of phagocytic cells in defense against fungal infections, we have investigated the response of C. albicans to phagocytosis by mammalian macrophages using genomic transcript profiling. This analysis reveals a dramatic reprogramming of transcription in C. albicans that occurs in two successive steps. In the early phase cells shift to a starvation mode, including gluconeogenic growth, activation of fatty acid degradation, and downregulation of translation. In a later phase, as hyphal growth enables C. albicans to escape from the macrophage, cells quickly resume glycolytic growth. In addition, there is a substantial nonmetabolic response imbedded in the early phase, including machinery for DNA damage repair, oxidative stress responses, peptide uptake systems, and arginine biosynthesis. Further, a surprising percentage of the genes that respond specifically to macrophage contact have no known homologs, suggesting that the organism has undergone substantial evolutionary adaptations to the commensal or pathogen lifestyle. This transcriptional reprogramming is almost wholly absent in the related, but nonpathogenic, yeast Saccharomyces cerevisiae, suggesting that these large-scale and coordinated changes contribute significantly to the ability of this organism to survive and cause disease in vivo.
Candida albicans is an opportunistic fungal pathogen that normally exists as a harmless commensal in humans. In instances where host debilitation occurs, Candida can cause a range of clinical infections, and whilst these are primarily superficial, effecting mucosal membranes, systemic infections can develop in severely immunocompromised individuals. The mechanism of host immunity during commensal carriage of C. albicans has been intensively studied. In this paper, we present the most recent information concerning host recognition of C. albicans leading to cytokine production and the subsequent T-cell responses generated in response to C. albicans. Particular focus is given to the role of the IL-12 cytokine family including IL-12, IL-23, IL-27, and IL-35, in host immunity to Candida. CD4+ T-cells are considered crucial in the regulation of immunity and inflammation. In this regard, the role of Th1/2, helper cells, together with the recently identified Th17 and Treg cells in candidosis will be discussed. Understanding the detailed mechanisms that underlie host immunity to Candida not only will be of benefit in terms of the infections caused by this organism but could also be exploited in the development of therapeutic interventions for other diseases.
Mitogen-activated protein kinases (MAPKs) are key regulators of cellular physiology and immune responses and abnormality in MAPKs is implicated in many diseases. MAPKs are activated by MAPK kinases through phosphorylation of the threonine and tyrosine residues in the conserved Thr-Xaa-Tyr domain, where Xaa represents amino acid residues characteristic of distinct MAPK subfamilies. Since MAPKs play a crucial role in a variety of cellular processes, a delicate regulatory network has evolved to control their activities. Over the past two decades, a group of dual specificity MAPK phosphatases (MKPs) have been identified that deactivate MAPKs. Since MAPKs can enhance MKP activities, MKPs are considered as an important feedback control mechanism that limits the MAPK cascades. This review outlines the role of MKP-1, a prototypical MKP family member, in physiology and disease. We will first discuss the basic biochemistry and regulation of MKP-1. Next, we will present the current consensus on the immunological and physiological functions of MKP-1 in infectious, inflammatory, metabolic, and nervous system diseases as revealed by studies using animal models. We will also discuss the emerging evidence implicating MKP-1 in human disorders. Finally, we will conclude with a discussion of the potential for pharmacomodulation of MKP-1 expression.
MKP-1; MAPK; immunology; mouse; human; inflammation
Oropharyngeal and vaginal candidiases are the most common forms of mucosal fungal infections and are primarily caused by Candida albicans, a dimorphic fungal commensal organism of the gastrointestinal and lower female reproductive tracts. Clinical and experimental observations suggest that local immunity is important in host defense against candidiasis. Accordingly, cytokines and chemokines are present at the oral and vaginal mucosa during C. albicans infections. Since mucosal epithelial cells produce a variety of cytokines and chemokines in response to microorganisms and since C. albicans is closely associated with mucosal epithelial cells as a commensal, we sought to identify cytokines and/or chemokines produced by primary oral and vaginal epithelial cells and cell lines in response to C. albicans. The results showed that proinflammatory cytokines were produced by oral and/or vaginal epithelial cells at various levels constitutively with considerable interleukin-1α (IL-1α) and tumor necrosis factor alpha, but not IL-6, produced in response to C. albicans. In contrast, Th1-type (IL-12 and gamma interferon) and Th2-type-immunoregulatory (IL-10 and transforming growth factor β) cytokines and the chemokines monocyte chemoattractant protein 1 and IL-8 were produced in low to undetectable concentrations with little additional production in response to C. albicans. Taken together, these results indicate that cytokines and chemokines are variably produced by oral and vaginal epithelial cells constitutively, as well as in response to C. albicans, and are predominated by proinflammatory cytokines.
The scattering of Madin-Darby canine kidney (MDCK) epithelial cells
by scatter factor/hepatocyte growth factor (SF/HGF) is associated with
transcriptional induction of the urokinase gene, which occurs
essentially through activation of an EBS/AP1 response element. We have
investigated the signal transduction pathways leading to this
transcriptional response. We found that SF/HGF induces rapid and
sustained phosphorylation of the extracellular signal-regulated kinase
(ERK) MAPK while stimulating weakly and then repressing phosphorylation
of the JUN N-terminal kinase (JNK) MAPK for several hours. This delayed
repression of JNK was preceded by phosphorylation of the MKP2
phosphatase, and both MKP2 induction and JNK dephosphorylation were
under the control of MEK, the upstream kinase of ERK. ERK and MKP2
stimulate the EBS/AP1-dependent transcriptional response to SF/HGF, but
not JNK, which inhibits this response. We further demonstrated that
depending on cell density, the RAS-ERK-MKP2 pathway controls this
transrepressing effect of JNK. Together, these data demonstrate that in
a sequential manner SF/HGF activates ERK and MKP2, which in turn
dephosphorylates JNK. This sequence of events provides a model for
efficient cell scattering by SF/HGF at low cell density.
Candida albicans is an opportunistic fungal pathogen of humans that resides commensally on epithelial surfaces, but can cause inflammation when pathogenic. Resolvins are a class of anti-inflammatory lipids derived from omega-3 polyunsaturated fatty acids (PUFA) that attenuate neutrophil migration during the resolution phase of inflammation. In this report we demonstrate that C. albicans biosynthesizes resolvins that are chemically identical to those produced by human cells. In contrast to the trans-cellular biosynthesis of human Resolvin E1 (RvE1), RvE1 biosynthesis in C. albicans occurs in the absence of other cellular partners. C. albicans biosynthesis of RvE1 is sensitive to lipoxygenase and cytochrome P450 monoxygenase inhibitors. We show that 10nM RvE1 reduces neutrophil chemotaxis in response to IL-8; 1nM RvE1 enhanced phagocytosis of Candida by human neutrophils, as well as intracellular ROS generation and killing, while having no direct affect on neutrophil motility. In a mouse model of systemic candidiasis, RvE1 stimulated clearance of the fungus from circulating blood. These results reveal an inter-species chemical signaling system that modulates host immune functions and may play a role in balancing host carriage of commensal and pathogenic C. albicans.
Perception of external stimuli and generation of an appropriate response are crucial for host colonization by pathogens. In pathogenic fungi, mitogen activated protein kinase (MAPK) pathways regulate dimorphism, biofilm/mat formation, and virulence. Signaling mucins, characterized by a heavily glycosylated extracellular domain, a transmembrane domain, and a small cytoplasmic domain, are known to regulate various signaling pathways. In Candida albicans, the mucin Msb2 regulates the Cek1 MAPK pathway. We show here that Msb2 is localized to the yeast cell wall and is further enriched on hyphal surfaces. A msb2Δ/Δ strain formed normal hyphae but had biofilm defects. Cek1 (but not Mkc1) phosphorylation was absent in the msb2Δ/Δ mutant. The extracellular domain of Msb2 was shed in cells exposed to elevated temperature and carbon source limitation, concomitant with germination and Cek1 phosphorylation. Msb2 shedding occurred differentially in cells grown planktonically or on solid surfaces in the presence of cell wall and osmotic stressors. We further show that Msb2 shedding and Cek1 phosphorylation were inhibited by addition of Pepstatin A (PA), a selective inhibitor of aspartic proteases (Saps). Analysis of combinations of Sap protease mutants identified a sap8Δ/Δ mutant with reduced MAPK signaling along with defects in biofilm formation, thereby suggesting that Sap8 potentially serves as a major regulator of Msb2 processing. We further show that loss of either Msb2 (msb2Δ/Δ) or Sap8 (sap8Δ/Δ) resulted in higher C. albicans surface β-glucan exposure and msb2Δ/Δ showed attenuated virulence in a murine model of oral candidiasis. Thus, Sap-mediated proteolytic cleavage of Msb2 is required for activation of the Cek1 MAPK pathway in response to environmental cues including those that induce germination. Inhibition of Msb2 processing at the level of Saps may provide a means of attenuating MAPK signaling and reducing C. albicans virulence.
► First demonstration of post-mitotic fusion of macrophages infected with Candida albicans. ► Post-mitotic fusion was attributed to C. albicans hyphae but not yeast form cells. ► Post-mitotic fusion may inhibit macrophage proliferation and the formation of new uninfected phagocytes.
The pathogenicity of the opportunistic human fungal pathogen Candida albicans depends on its ability to inhibit effective destruction by host phagocytes. Using live cell video microscopy, we show here for the first time that C. albicans inhibits cell division in macrophages undergoing mitosis. Inhibition of macrophage cell division is dependent on the ability of C. albicans to form hyphae, as it is rarely observed following phagocytosis of UV-killed or morphogenesis-defective mutant Candida. Interestingly, failed cell division following phagocytosis of hyphal C. albicans is surprisingly common, and leads to the formation of large multinuclear macrophages. This raises question as to whether inhibition of macrophage cell division is another virulence attribute of C. albicans or enables host macrophages to contain the pathogen.
Candida albicans; Macrophage; Phagocytosis; Mitosis; Innate immunology
In response to a wide variety of environmental stimuli, the opportunistic fungal pathogen Candida albicans exits the budding cycle, producing germ tubes and hyphae concomitant with expression of virulence genes, such as that encoding hyphal wall protein 1 (HWP1). Biochemical studies implicate cyclic AMP (cAMP) increases in promoting bud-hypha transitions, but genetic evidence relating genes that control cAMP levels to bud-hypha transitions has not been reported. Adenylate cyclase-associated proteins (CAPs) of nonpathogenic fungi interact with Ras and adenylate cyclase to increase cAMP levels under specific environmental conditions. To initiate studies on the relationship between cAMP signaling and bud-hypha transitions in C. albicans, we identified, cloned, characterized, and disrupted the C. albicans CAP1 gene. C. albicans strains with inactivated CAP1 budded in conditions that led to germ tube formation in isogenic strains with CAP1. The addition of 10 mM cAMP and dibutyryl cAMP promoted bud-hypha transitions and filamentous growth in the cap1/cap1 mutant in liquid and solid media, respectively, showing clearly that cAMP promotes hypha formation in C. albicans. Increases in cytoplasmic cAMP preceding germ tube emergence in strains having CAP1 were markedly diminished in the budding cap1/cap1 mutant. C. albicans strains with deletions of both alleles of CAP1 were avirulent in a mouse model of systemic candidiasis. The avirulence of a germ tube-deficient cap1/cap1 mutant coupled with the role of Cap1 in regulating cAMP levels shows that the Cap1-mediated cAMP signaling pathway is required for bud-hypha transitions, filamentous growth, and the pathogenesis of candidiasis.
Candida albicans is an opportunistic fungal pathogen whose virulence is related to its ability to switch between yeast, pseudohyphal, and true-hyphal morphologies. To ask how long-distance nuclear migration occurs in C. albicans hyphae, we identified the fundamental properties of nuclear movements and microtubule dynamics using time-lapse microscopy. In hyphae, nuclei migrate to, and divide across, the presumptive site of septation, which forms 10 to 15 μm distal to the basal cell. The mother nucleus returns to the basal cell, while the daughter nucleus reiterates the process. We used time-lapse microscopy to identify the mechanisms by which C. albicans nuclei move over long distances and are coordinated with hyphal morphology. We followed nuclear migration and spindle dynamics, as well as the time and position of septum specification, defined it as the presumptum, and established a chronology of nuclear, spindle, and morphological events. Analysis of microtubule dynamics revealed that premitotic forward nuclear migration is due to the repetitive sliding of astral microtubules along the cell cortex but that postmitotic forward and reverse nuclear migrations are due primarily to spindle elongation. Free microtubules exhibit cell cycle regulation; they are present during interphase and disappear at the time of spindle assembly. Finally, a growth defect in strains expressing Tub2-green fluorescent protein revealed a connection between hyphal elongation and the nuclear cell cycle that is coordinated by hyphal length and/or volume.
The RNA-binding protein (RBP) HuR plays a vital role in the mammalian stress response, effecting changes in the proliferation and survival of damaged cells. HuR prominently influences the stress response by regulating the stability and translation of mRNAs encoding stress-response proteins. Recently, HuR was found to affect mitogen-activated protein kinase (MAPK) signaling, at least in part by post-transcriptionally promoting the expression of MAPK phosphatase-1 (MKP-1). As anticipated for a pivotal regulator of the MAPKs c-jun N-terminal kinase (JNK) and p38, MKP-1 expression is tightly regulated transcriptionally, post-transcriptionally and post-translationally. HuR’s influence on MKP-1 expression helps to ensure the appropriate abundance of MKP-1 and consequently the appropriate cellular response to stress stimuli.
mRNA-binding protein; ribonucleoprotein complex; mRNA turnover; translational control; ELAV; MAP kinase; ERK; JNK; p38
Mitogen-activated protein kinase (MAPK) pathways play crucial roles in developmental and adaptive responses. Depending on the stimulus, MAPK activation regulates a wide variety of plant cell responses, such as proliferation, differentiation and cell death, which normally require precise spatial and temporal control. In this context, protein phosphatases play important roles by regulating the duration and magnitude of MAPK activities. During infection by non-host and incompatible host microorganisms, MAPK activity can promote a local cell death mechanism called hypersensitive response (HR), which is part of the plant defence response. HR-like responses require sustained MAPK activity and correlate with oxidative burst. We recently showed that MAPK phosphatase MKP2 positively controls biotic and abiotic stress responses in Arabidopsis. MKP2 interacts with MPK6 in HR-like responses triggered by fungal elicitors, suggesting that MKP2 protein is part of the mechanism involved in MAPK regulation during HR. Here we discuss the interplay of MAPK and MKP2 phosphatase signaling during cell death responses elicited by host-pathogen interactions.
Arabidopsis; hypersensitive response (HR); MAPK; MPK6; MKP2; ROS
Ywp1 is a prominent glycosylphosphatidylinositol (GPI)-anchored glycoprotein of the cell wall of Candida albicans; it is present in the yeast form of this opportunistic fungal pathogen but absent from filamentous forms and chlamydospores. Yeast cells that lack Ywp1 are more adhesive and form thicker biofilms, implying an antiadhesive activity for Ywp1, with a possible role in yeast dispersal. The antiadhesive effect of Ywp1 is transplantable from yeast to hyphae, as hyphae that are forced to express YWP1 lose adhesion in an in vitro assay. Deletion of the GPI anchor results in loss of Ywp1 to the surrounding medium and reduction of the antiadhesive effect, implying an importance of time-dependent residency in the cell wall. Anchor-negative versions of Ywp1 possessing or lacking a C-terminal green fluorescent protein (GFP) tag were created in C. albicans and harvested from culture supernatants; in addition to serving as quantifiable markers for Ywp1 secretion, they revealed that the cleaved 11-kDa propeptide of Ywp1 remains strongly but noncovalently associated with the Ywp1 core. This association is resistant to highly acidic and basic solutions, 8 M urea, and 1% SDS (below 45°C). Above 50°C, SDS dissociates the isolated complex, but even higher temperatures are required to dissociate the propeptide from native Ywp1 that is anchored in a cell wall. This property has permitted detection, for the first time, of orthologs of Ywp1 in other members of the Candida clade. The cleaved propeptide, which carries the sole N-glycan of Ywp1, must participate in the antiadhesive effect of Ywp1.
The common fungal pathogen Candida albicans has the ability to grow as a yeast or as a hypha and can alternate between these morphotypes. The overall biomass of both morphotypes increases with growth. However, only yeasts, but not hyphae, exist as discrete cellular entities. Multiplicity of infection (MOI) is a useful parameter to determine the initial inoculum of yeasts for in vitro infection assays. Since the amount of hyphae is difficult to quantify, comparable starting conditions in such assays cannot be determined accurately for yeasts and hyphae using MOI. To circumvent this problem, we have established a set of correlation coefficients to convert fungal metabolic activity and optical density to dry mass. Using these correlations, we were able to accurately compare ROS production and IL-8 release by polymorphonuclear neutrophils upon infection with equal dry mass amounts of yeast and hyphal morphotypes. Neutrophil responses depended on the initial form of infection, irrespective of C. albicans wild-type yeasts transforming to hyphal growth during the assay. Infection with a high mass of live C. albicans yeasts resulted in lower neutrophil ROS and this decrease stems from efficient ROS detoxification by C. albicans without directly affecting the phagocyte ROS machinery. Moreover, we show that dead C. albicans induces significantly less ROS and IL-8 release than live fungi, but thimerosal-killed C. albicans were still able to detoxify neutrophil ROS. Thus, the dry mass approach presented in this study reveals neutrophil responses to different amounts and morphotypes of C. albicans and serves as a template for studies that aim to identify morphotype-specific responses in a variety of immune cells.