A protein-farnesyl transferase inhibitor interferes with the serum-induced conversion of Candida albicans from a cellular yeast form to a filamentous form

Physiological adventures in Candida albicans: farnesol and ubiquinones

SUMMARYFarnesol was first identified as a quorum-sensing molecule, which blocked the yeast to hyphal transition in Candida albicans, 22 years ago. However, its interactions with Candida biology are surprisingly complex. Exogenous (secreted or supplied) farnesol can also act as a virulence factor during pathogenesis and as a fungicidal agent triggering apoptosis in other competing fungi. Farnesol synthesis is turned off both during anaerobic growth and in opaque cells. Distinctly different cellular responses are observed as exogenous farnesol levels are increased from 0.1 to 100 µM. Reported changes include altered morphology, stress response, pathogenicity, antibiotic sensitivity/resistance, and even cell lysis. Throughout, there has been a dearth of mechanisms associated with these observations, in part due to the absence of accurate measurement of intracellular farnesol levels (Fi). This obstacle has recently been overcome, and the above phenomena can now be viewed in terms of changing Fi levels and the percentage of farnesol secreted. Critically, two aspects of isoprenoid metabolism present in higher organisms are absent in C. albicans and likely in other yeasts. These are pathways for farnesol salvage (converting farnesol to farnesyl pyrophosphate) and farnesylcysteine cleavage, a necessary step in the turnover of farnesylated proteins. Together, these developments suggest a unifying model, whereby high, threshold levels of Fi regulate which target proteins are farnesylated or the extent to which they are farnesylated. Thus, we suggest that the diversity of cellular responses to farnesol reflects the diversity of the proteins that are or are not farnesylated.

Old and new strategies in therapy and diagnosis against fungal infections

Fungal infections represent a serious global health threat. The new emerging pathogens and the spread of different forms of resistance are now hardly challenging the tools available in therapy and diagnostics. With the commonly used diagnoses, fungal identification is often slow and inaccurate, and, on the other hand, some drugs currently used as treatments are significantly affected by the decrease in susceptibility. Herein, the antifungal arsenal is critically summarized. Besides describing the old approaches and their mechanisms, advantages, and limitations, the focus is dedicated to innovative strategies which are designed, identified, and developed to take advantage of the discrepancies between fungal and host cells. Relevant pathways and their role in survival and virulence are discussed as their suitability as sources of antifungal targets. In a similar way, molecules with antifungal activity are reported as potential agents/precursors of the next generation of antimycotics. Particular attention was devoted to biotechnological entities, to their novelty and reliability, to drug repurposing and restoration, and to combinatorial applications yielding significant improvements in efficacy. KEY POINTS: • New antifungal agents and targets are needed to limit fungal morbidity and mortality. • Therapeutics and diagnostics suffer of delays in innovation and lack of targets. • Biologics, drug repurposing and combinations are the future of antifungal treatments.

Anti-Yeasts, Antioxidant and Healing Properties of Henna Pre-Treated by Moist Heat and Molecular Docking of Its Major Constituents, Chlorogenic and Ellagic Acids, with Candida albicans and Geotrichum candidum Proteins

Lawsonia inermis, known as henna, has traditionally been utilized in cosmetics and folk medicine because of their valuable health effects. A lack of information about the processes that increase or decrease release, as well as the biological activities of constituents of natural origin, is an important pharmacological problem. This investigation evaluates the influence of moist heat on the flavonoid and phenolic contents of henna powder and their biological activities. HPLC analysis reflected the existence of 20 and 19 compounds of flavonoids and phenolics in the extract of unpre-treated henna by moist heat (UPMH) and pre-treated henna by moist heat (PMH). Several compounds such as chlorogenic acid, ellagic acid, rutin, rosmarinic acid, kaempferol, and pyrocatechol occurred with high concentrations of 57,017.33, 25,821.09, 15,059.88, 6345.08, 1248.42, and 819.19 µg/mL UPMH while occurred with low concentrations of 44,286.51, 17,914.26, 3809.85, 5760.05, 49.01, and 0.0 µg/mL, respectively in PMH. C. albicans, C. tropicalis, and G. candidum were more affected by UPMH with inhibition zones of 30.17 ± 0.29, 27 ± 0.5, and 29 ± 1.5 mm than PMH with inhibition zones of 29 ± 0.5, 25.33 ± 0.58, and 24.17 ± 0.29 mm, respectively. UPMH henna exhibited less MIC and MFC against the tested yeasts than PMH. Moreover, UPMH henna showed good wound healing, where the rat of migration, wound closure %, and area difference % were 14.806 um, 74.938 um2, and 710.667% compared with PMH henna 11.360 um, 59.083 um2, 545.333%, respectively. Antioxidant activity of UPMH and PMH henna. Promising antioxidant activity was recorded for both UPMH or PMH henna with IC50 5.46 µg/mL and 7.46 µg/mL, respectively. The docking interaction of chlorogenic acid and ellagic acid with the crystal structures of G. candidum (4ZZT) and C. albicans (4YDE) was examined. The biological screening demonstrated that the compounds had favorable docking results with particular proteins. Chlorogenic acid had robust behavior in the G. candidum (4ZZT) active pocket and displayed a docking score of -7.84379 Kcal/mol, higher than ellagic acid's -6.18615 Kcal/mol.

Differential requirements of protein geranylgeranylation for the virulence of human pathogenic fungi

Protein prenylation is a crucial post-translational modification largely mediated by two heterodimeric enzyme complexes, farnesyltransferase and geranylgeranyltransferase type-I (GGTase-I), each composed of a shared α-subunit and a unique β-subunit. GGTase-I enzymes are validated drug targets that contribute to virulence in Cryptococcus neoformans and to the yeast-to-hyphal transition in Candida albicans. Therefore, we sought to investigate the importance of the α-subunit, RamB, and the β-subunit, Cdc43, of the A. fumigatus GGTase-I complex to hyphal growth and virulence. Deletion of cdc43 resulted in impaired hyphal morphogenesis and thermo-sensitivity, which was exacerbated during growth in rich media. The Δcdc43 mutant also displayed hypersensitivity to cell wall stress agents and to cell wall synthesis inhibitors, suggesting alterations of cell wall biosynthesis or stress signaling. In support of this, analyses of cell wall content revealed decreased amounts of β-glucan in the Δcdc43 strain. Despite strong in vitro phenotypes, the Δcdc43 mutant was fully virulent in two models of murine invasive aspergillosis, similar to the control strain. We further found that a strain expressing the α-subunit gene, ramB, from a tetracycline-inducible promoter was inviable under non-inducing in vitro growth conditions and was virtually avirulent in both mouse models. Lastly, virulence studies using C. albicans strains with tetracycline-repressible RAM2 or CDC43 expression revealed reduced pathogenicity associated with downregulation of either gene in a murine model of disseminated infection. Together, these findings indicate a differential requirement for protein geranylgeranylation for fungal virulence, and further inform the selection of specific prenyltransferases as promising antifungal drug targets for each pathogen.

Lonafarnib synergizes with azoles against Aspergillus spp. and Exophiala spp

Farnesylation, which is catalyzed by farnesyltransferase, promotes membrane association of the modified protein and protein-protein interactions, and plays an important role in a number of physiological processes of pathogenic fungi, including stress response, environmental adaption and virulence. Lonafarnib is an orally bioavailable nonpeptide tricyclic farnesyltransferase inhibitor with excellent pharmacokinetic and safety profile. In the present study, we investigated the in vitro activities of lonafarnib alone or combined with azoles, including itraconazole, voriconazole, and posaconazole, against 22 strains of Aspergillus spp. and 18 strains of Exophiala dermatitidis via broth microdilution checkerboard technique. Lonafarnib alone was inactive against all isolates tested. However, synergistic effects between lonafarnib and itraconazole were observed in 86% Aspergillus strains and 94% E. dermatitidis strains. In addition, lonafarnib/posaconazole combination also exhibited synergism against 59% of Aspergillus strains and 100% E. dermatitidis strains. However, synergistic effects of lonafarnib/voriconazole were only observed in 32% Aspergillus strains and 28% E. dermatitidis strains. The effective working ranges of lonafarnib were 2-4 μg/ml and 1-4 μg/ml against Aspergillus isolates and E. dermatitidis isolates, respectively. No antagonism was observed in all combinations. This study demonstrated that lonafarnib could enhance the in vitro antifungal activity of itraconazole, posaconazole and voriconazole against Aspergillus spp. and E. dermatitidis, suggesting that azoles, especially itraconazole and posaconazole, combined with farnesyltransferase inhibitor might provide a potential strategy to the management of Aspergillus and Exophiala infections. However, further studies are warranted to elucidate the underlying mechanism and to investigate the potential of reliable and safe application in clinical practice.

Serum responsive proteome reveals correlation between oxidative phosphorylation and morphogenesis in Candida albicans ATCC10231

To understand the impact of fetal bovine serum (FBS) on metabolism and cellular architecture in addition to morphogenesis, we have identified FBS responsive proteome of Candida albicans. FBS induced 34% hyphae and 60% pseudohyphae in C. albicans at 30 °C while 98% hyphae at 37 °C. LC-MS/MS analysis revealed that 285 proteins modulated significantly in response to FBS at 30 °C and 37 °C. Out of which 152 were upregulated and 62 were downregulated at 30 °C while 18 were up and 53 were downregulated at 37 °C. Functional annotation suggests that FBS may inhibit glycolysis and fermentative pathway and enhance oxidative phosphorylation (OxPhos), TCA cycle, amino acid and fatty acid metabolism indicating a use of alternative energy source by C. albicans. OxPhos inhibition assay using sodium azide corroborated the correlation between inhibition of glycolysis and enhanced OxPhos with pseudohyphae formation. C. albicans induced hyphae in response to FBS irrespective of down regulation of Ras1,Asr1/Asr2, indicates the possible involvement of MAPK and cAMP-PKA independent pathway. The Cell wall of cells grown in presence of FBS at 30 °C was rich in mannan, Beta 1,3-glucan and chitin while membranes were rich in ergosterol compared to those grown at 37 °C.

SIGNIFICANCE OF THE STUDY

This is the first study suggesting a correlation between OxPhos and morphogenesis especially pseudohyphae formation in C. albicans. Our data also indicate that fetal bovine serum (FBS) induced morphogenesis is multifactorial and may involve MAPK and cAMP-PKA independent pathway. In addition to morphogenesis, our study provides an insight in to the modulation of metabolism and cellular architecture of C. albicans in response to FBS.

Antifungal Drugs: The Current Armamentarium and Development of New Agents

Invasive fungal infections are becoming an increasingly important cause of human mortality and morbidity, particularly for immunocompromised populations. The fungal pathogens Candida albicans , Cryptococcus neoformans , and Aspergillus fumigatus collectively contribute to over 1 million human deaths annually. Hence, the importance of safe and effective antifungal therapeutics for the practice of modern medicine has never been greater. Given that fungi are eukaryotes like their human host, the number of unique molecular targets that can be exploited for drug development remains limited. Only three classes of molecules are currently approved for the treatment of invasive mycoses. The efficacy of these agents is compromised by host toxicity, fungistatic activity, or the emergence of drug resistance in pathogen populations. Here we describe our current arsenal of antifungals and highlight current strategies that are being employed to improve the therapeutic safety and efficacy of these drugs. We discuss state-of-the-art approaches to discover novel chemical matter with antifungal activity and highlight some of the most promising new targets for antifungal drug development. We feature the benefits of combination therapy as a strategy to expand our current repertoire of antifungals and discuss the antifungal combinations that have shown the greatest potential for clinical development. Despite the paucity of new classes of antifungals that have come to market in recent years, it is clear that by leveraging innovative approaches to drug discovery and cultivating collaborations between academia and industry, there is great potential to bolster the antifungal armamentarium.

Ancient pathogens in museal dry bone specimens: analysis of paleocytology and aDNA

Bone samples investigated in this study derive from the pathologic-anatomical collection of the Natural History Museum of Vienna. In order to explore the survival of treponemes and treponemal ancient DNA in museal dry bone specimens, we analyzed three individuals known to have been infected with Treponema pallidum pallidum. No reproducible evidence of surviving pathogen's ancient DNA (aDNA) was obtained, despite the highly sensitive extraction and amplification techniques (TPP15 and arp). Additionally, decalcification fluid of bone sections was smear stained with May-Gruenwald-Giemsa. The slides were examined using direct light microscope and dark field illumination. Remnants of spirochetal structures were detectable in every smear. Our results demonstrate that aDNA is unlikely to survive, but spirochetal remains are stainable and thus detectable.

Exploration of Aspergillus fumigatus Ras pathways for novel antifungal drug targets

Ras pathway signaling is a critical virulence determinant for pathogenic fungi. Localization of Ras to the plasma membrane (PM) is required for Ras network interactions supporting fungal growth and virulence. For example, loss of Aspergillus fumigatus RasA signaling at the PM via inhibition of palmitoylation leads to decreased growth, altered hyphal morphogenesis, decreased cell wall integrity and loss of virulence. In order to be properly localized and activated, Ras proteins must transit a series of post-translational modification (PTM) steps. These steps include farnesylation, proteolytic cleavage of terminal amino acids, carboxymethylation, and palmitoylation. Because Ras activation drives tumor development, Ras pathways have been extensively studied in mammalian cells as a potential target for anti-cancer therapy. Inhibitors of mammalian Ras interactions and PTM components have been, or are actively being, developed. This review will focus on the potential for building upon existing scaffolds to exploit fungal Ras proteins for therapy, synthesizing data from studies employing both mammalian and fungal systems.

The quorum-sensing molecules farnesol/homoserine lactone and dodecanol operate via distinct modes of action in Candida albicans

Living as a commensal, Candida albicans must adapt and respond to environmental cues generated by the mammalian host and by microbes comprising the natural flora. These signals have opposing effects on C. albicans, with host cues promoting the yeast-to-hyphal transition and bacteria-derived quorum-sensing molecules inhibiting hyphal development. Hyphal development is regulated through modulation of the cyclic AMP (cAMP)/protein kinase A (PKA) signaling pathway, and it has been postulated that quorum-sensing molecules can affect filamentation by inhibiting the cAMP pathway. Here, we show that both farnesol and 3-oxo-C(12)-homoserine lactone, a quorum-sensing molecule secreted by Pseudomonas aeruginosa, block hyphal development by affecting cAMP signaling; they both directly inhibited the activity of the Candida adenylyl cyclase, Cyr1p. In contrast, the 12-carbon alcohol dodecanol appeared to modulate hyphal development and the cAMP signaling pathway without directly affecting the activity of Cyr1p. Instead, we show that dodecanol exerted its effects through a mechanism involving the C. albicans hyphal repressor, Sfl1p. Deletion of SFL1 did not affect the response to farnesol but did interfere with the response to dodecanol. Therefore, quorum sensing in C. albicans is mediated via multiple mechanisms of action. Interestingly, our experiments raise the possibility that the Burkholderia cenocepacia diffusible signal factor, BDSF, also mediates its effects via Sfl1p, suggesting that dodecanol's mode of action, but not farnesol or 3-oxo-C(12)-homoserine lactone, may be used by other quorum-sensing molecules.

Modulation of morphogenesis in Candida albicans by various small molecules

The pathogenic yeast Candida albicans, a member of the mucosal microbiota, is responsible for a large spectrum of infections, ranging from benign thrush and vulvovaginitis in both healthy and immunocompromised individuals to severe, life-threatening infections in immunocompromised patients. A striking feature of C. albicans is its ability to grow as budding yeast and as filamentous forms, including hyphae and pseudohyphae. The yeast-to-hypha transition contributes to the overall virulence of C. albicans and may even constitute a target for the development of antifungal drugs. Indeed, impairing morphogenesis in C. albicans has been shown to be a means to treat candidiasis. Additionally, a large number of small molecules such as farnesol, fatty acids, rapamycin, geldanamycin, histone deacetylase inhibitors, and cell cycle inhibitors have been reported to modulate the yeast-to-hypha transition in C. albicans. In this minireview, we take a look at molecules that modulate morphogenesis in this pathogenic yeast. When possible, we address experimental findings regarding their mechanisms of action and their therapeutic potential. We discuss whether or not modulating morphogenesis constitutes a strategy to treat Candida infections.

Conjugated linoleic acid inhibits hyphal growth in Candida albicans by modulating Ras1p cellular levels and downregulating TEC1 expression

The polymorphic yeast Candida albicans exists in yeast and filamentous forms. Given that the morphogenetic switch coincides with the expression of many virulence factors, the yeast-to-hypha transition constitutes an attractive target for the development of new antifungal agents. Since an untapped therapeutic potential resides in small molecules that hinder C. albicans filamentation, we characterized the inhibitory effect of conjugated linoleic acid (CLA) on hyphal growth and addressed its mechanism of action. CLA inhibited hyphal growth in a dose-dependent fashion in both liquid and solid hypha-inducing media. The fatty acid blocked germ tube formation without affecting cellular growth rates. Global transcriptional profiling revealed that CLA downregulated the expression of hypha-specific genes and abrogated the induction of several regulators of hyphal growth, including TEC1, UME6, RFG1, and RAS1. However, neither UME6 nor RFG1 was necessary for CLA-mediated hyphal growth inhibition. Expression analysis showed that the downregulation of TEC1 expression levels by CLA depended on RAS1. In addition, while RAS1 transcript levels remained constant in CLA-treated cells, its protein levels declined with time. With the use of a strain expressing GFP-Ras1p, CLA treatment was also shown to affect Ras1p localization to the plasma membrane. These findings suggest that CLA inhibits hyphal growth by affecting the cellular localization of Ras1p and blocking the increase in RAS1 mRNA and protein levels. Combined, these effects should prevent the induction of the Ras1p signaling pathway. This study provides the biological and molecular explanations that underlie CLA's ability to inhibit hyphal growth in C. albicans.

Farnesol and dodecanol effects on the Candida albicans Ras1-cAMP signalling pathway and the regulation of morphogenesis

Candida albicans hypha formation which has been stimulated via the Ras1-cAMP-Efg1 signalling cascade is inhibited by farnesol, a C. albicans autoregulatory factor, and small molecules such as dodecanol. In cultures containing farnesol or dodecanol, hypha formation was restored upon addition of dibutyryl-cAMP. The CAI4-Ras1(G13V) strain, which carries a dominant-active variant of Ras1 and forms hyphae in the absence of inducing stimuli, grew as yeast in medium with farnesol or dodecanol; the heat shock sensitivity of the CAI4-Ras1(G13V) strain was also suppressed by these compounds. Neither Pde1 nor Pde2 was necessary for the repression of hyphal growth by farnesol or dodecanol. Two transcripts, CTA1 and HSP12, which are at higher levels upon mutation of Ras1 or Cdc35, were increased in abundance in cells grown with farnesol or dodecanol. Microscopic analysis of strains carrying CTA1 and HWP1 promoter fusions grown with intermediate concentrations of farnesol or dodecanol indicated a link between cells with the increased expression of cAMP-repressed genes and cells repressed for hypha formation. Because several cAMP-controlled outputs are affected by farnesol and dodecanol, our findings suggest that these compounds impact activity of the Ras1-Cdc35 pathway, thus leading to an alteration of C. albicans morphology.

The RAM1 gene encoding a protein-farnesyltransferase beta-subunit homologue is essential in Cryptococcus neoformans

Many small G proteins require post-translational modification to allow functional association to the cell membrane. This process often involves the enzymic addition of hydrophobic prenyl groups to a conserved cysteine residue near the C-terminus of the protein. The enzymes that catalyse these reactions include protein farnesyltransferase and protein geranylgeranyltransferases. The human fungal pathogen Cryptococcus neoformans requires functional Ras and Rho proteins in order to undergo normal growth and differentiation. Since farnesylation and geranylgeranylation are likely required for the proper function of these small G proteins, we hypothesized that inhibition of these prenylation events would alter the growth and cellular morphogenesis of this fungus. We cloned the RAM1 gene encoding the single protein-farnesyltransferase beta-chain homologue in C. neoformans. Using a gene-disruption strategy in a diploid C. neoformans strain, we demonstrated that this gene encodes an essential function, in contrast to the case in Saccharomyces cerevisiae, in which the homologous RAM1 gene is not essential for growth. Pharmacological inhibition of farnesyltransferase activity resulted in dose-dependent cytostasis of C. neoformans, as well as prevention of hyphal differentiation. Simultaneous inhibition of farnesylation and calcineurin signalling results in a synthetic effect on growth. Protein farnesylation is required for the growth and cellular differentiation of C. neoformans and may provide novel targets for antifungal therapy.

Protein prenyltransferases: anchor size, pseudogenes and parasites

Lipid modification of eukaryotic proteins by protein prenyltransferases is required for critical signaling pathways, cell cycle progression, cytoskeleton remodeling, induction of apoptosis and vesicular trafficking. This review analyzes the influence of distinct states of sequential posttranslational processing that can be obtained after single or double prenylation, reversible palmitoylation, proteolytic cleavage of the C-terminus and possible reversible carboxymethylation. This series of modifications, as well as the exact length of the prenyl anchor, are determinants in protein-membrane and specific protein-protein interactions of protein prenyltransferase substrates. Furthermore, the occurrence and distribution of pseudogenes of protein prenyltransferase subunits are discussed. Besides being developed as anti-cancer agents, prenyltransferase inhibitors are effective against an increasing number of parasitic diseases. Extensive screens for protein prenyltransferases in genomic data of fungal and protozoan pathogens unveil a series of new pharmacologic targets for prenyltransferase inhibition, including the parasites Brugia malayi, Onchocerca volvulus, Aspergillus nidulans, Pneumocystis carinii, Entamoeba histolytica, Strongyloides stercoralis, Trichinella spiralis and Cryptosporidium parvum.

A Tudor protein with multiple SNc domains from pea seedlings: cellular localization, partial characterization, sequence analysis, and phylogenetic relationships

A major high molecular weight protein (HMP) in the cytoskeletal fraction from pea has been purified. A combination of chromatographic techniques and protease fragment analysis also facilitated the isolation of the encoding cDNA, disclosing the sequence of the complete open reading frame. The protein possesses four complete N-terminal Staphylococcal nuclease (SNc) domains, a central Tudor domain and a partial SNc domain at the C-terminus, which may act as a coiled-coil cytoskeleton interaction motif. Cell fractionation studies showed that the protein was abundant in the cytoskeleton fraction in dark-grown pea seedlings, but essentially was absent from the nucleus. Gel filtration column chromatography indicated that the native protein exists as a dimer, while isoelectric focusing suggested that there were at least four HMP isotypes. The protein co-eluted with ribosomes from a heparin affinity column in vitro, consistent with ribosome/polysome interactions in vivo. Significantly, sequence analysis of the C-terminal SNc motif may accurately predict nuclear versus cytoplasmic localization resulting in potentially very different functional roles for this protein family in different organisms. An antibody to HMP from peas was also raised and an HMP with a similar molecular mass was detected in the cytoskeleton fractions and to a lesser extent in the nuclear fraction (250 g pellet) from rice and wheat seedlings.