Functional reconstitution of a purified proline permease from Candida albicans: interaction with the antifungal cispentacin

Molecular Umbrella as a Nanocarrier for Antifungals

A molecular umbrella composed of two O-sulfated cholic acid residues was applied for the construction of conjugates with cispentacin, containing a “trimethyl lock” (TML) or o-dithiobenzylcarbamoyl moiety as a cleavable linker. Three out of five conjugates demonstrated antifungal in vitro activity against C. albicans and C. glabrata but not against C. krusei, with MIC₉₀ values in the 0.22–0.99 mM range and were not hemolytic. Antifungal activity of the most active conjugate 24c, containing the TML–pimelate linker, was comparable to that of intact cispentacin. A structural analogue of 24c, containing the Nap-NH₂ fluorescent probe, was accumulated in Candida cells, and TML-containing conjugates were cleaved in cell-free extract of C. albicans cells. These results suggest that a molecular umbrella can be successfully applied as a nanocarrier for the construction of cleavable antifungal conjugates.

Amino Acid Metabolism and Transport Mechanisms as Potential Antifungal Targets

Discovering new drugs for treatment of invasive fungal infections is an enduring challenge. There are only three major classes of antifungal agents, and no new class has been introduced into clinical practice in more than a decade. However, recent advances in our understanding of the fungal life cycle, functional genomics, proteomics, and gene mapping have enabled the identification of new drug targets to treat these potentially deadly infections. In this paper, we examine amino acid transport mechanisms and metabolism as potential drug targets to treat invasive fungal infections, including pathogenic yeasts, such as species of Candida and Cryptococcus, as well as molds, such as Aspergillus fumigatus. We also explore the mechanisms by which amino acids may be exploited to identify novel drug targets and review potential hurdles to bringing this approach into clinical practice.

Caloric restriction restores the chronological life span of the Goa1 null mutant of Candida albicans in spite of high cell levels of ROS

The Candida albicans Goa1p is required for mitochondrial functions. In a strain lacking GOA1 (GOA31), respiration, mitochondrial membrane potential, complex I (CI) activity of the electron transport chain, and ATP synthesis are significantly decreased. A shortened chronological life span (CLS) of GOA31 occurs in 2% glucose that is associated with an increase in cell reactive oxidant species (ROS) and apoptosis. We now show that caloric restriction (CR) in media containing 0.5% glucose instead of 2% glucose-SC extends the CLS to the level of parental and gene-reconstituted strains. Paradoxically, ROS levels in GOA31 far exceed those of control strains in 0.5% glucose and, as a consequence, increased lipid peroxidation occurs even though CLS is restored. Microarray analysis was used to characterize transcriptional changes during CR in GOA31. We found that CR shifts cells of all strains to a non-glucose carbon metabolism (β-oxidation). Our model of ROS formation in GOA31 follows the paradigm that the generation of oxygen radicals from β-oxidation of cell lipids via FADH(2) (CII) and NADH (CI) creates an unfavorable cellular FADH(2)/NADH ratio that causes a transient overload in CII activity resulting in excess free cell radicals. In GOA31 the CI and peroxisomal dysfunctions increase the levels of ROS compared to control strains. Recovery from high levels of ROS may be associated with an increase in iron and sugar transporters, as well as an anti-stress response that includes the SOD1 and GPX1. Thus, CR creates a favorable growth environment, but cells of GOA31 must overcome a high but transient ROS production.

Asymmetric distribution of phosphatidylethanolamine in C. albicans: possible mediation by CDR1, a multidrug transporter belonging to ATP binding cassette (ABC) superfamily

By using two molecular probes, we demonstrate that only 4% of total phosphatidylethanolamine (PtdEtn) in the plasma membrane (PM) of a human pathogenic yeast, Candida albicans, is present in its external half. Evidence is presented to show that the availability of PtdEtn could be related to the expression of a multidrug transporter CDR1 of C. albicans, and the process is energy-dependent. A homozygous CDR1 disruptant strain of C. albicans shows almost 23% reduction in the external labelling of PtdEtn. This report shows that, similar to human MDRs, yeast multidrug transporter could also be involved in aminophospholipid translocation.

Molecular mode of action of the antifungal beta-amino acid BAY 10-8888

BAY 10-8888 is a cyclic beta-amino acid that is related to cispentacin and that has antifungal activity. Candida albicans cells accumulated BAY 10-8888 intracellularly to a concentration about 200 that in the medium when grown in media with a variety of nitrogen sources. In complex growth medium, BAY 10-8888 transport activity was markedly reduced and was paralleled by a decrease in its antifungal activity. Uptake of BAY 10-8888 was mediated by an H+-coupled amino acid transporter with specificity for branched-chain amino acids (isoleucine, leucine, and valine) and showed a KT (Michaelis constant of the transport reaction) of 0.95 mM and a Vmax of 18.9 nmol x min-1 x 10(7) cells-1. Similar to the transport of natural amino acids in Saccharomyces cerevisiae, the transport of BAY 10-8888 into the cell was unidirectional. Efflux occurred by diffusion and was not carrier mediated. Inside the cell BAY 10-8888 inhibited specifically isoleucyl-tRNA synthetase, resulting in inhibition of protein synthesis and cell growth. Intracellular isoleucine reversed BAY 10-8888-induced growth inhibition. BAY 10-8888 was not incorporated into proteins. BAY 10-8888 inhibited isoleucyl-tRNA synthetase with the same concentration dependency as protein biosynthesis in intact cells assuming 200-fold accumulation.

Decreased accumulation or increased isoleucyl-tRNA synthetase activity confers resistance to the cyclic beta-amino acid BAY 10-8888 in Candida albicans and Candida tropicalis

BAY 10-8888, a cyclic beta-amino acid, exerts its antifungal activity by inhibition of isoleucyl-tRNA synthetase activity after accumulation to a millimolar concentration inside the cell. We have selected and characterized BAY 10-8888-resistant Candida albicans mutants. Reduced BAY 10-8888 accumulation as well as increased isoleucyl-tRNA synthetase activity was observed in these mutants. Some of the mutants were cross-resistant to cispentacin, a structurally related beta-amino acid, while sensitivities to 5-fluorocytosine and fluconazole remained unchanged in all mutants. All except two in vitro-resistant mutants were pathogenic in a murine candidiasis model, and BAY 10-8888 failed to cure the infection. Furthermore, we have characterized BAY 10-8888 transport and isoleucyl-tRNA synthetase activity in several Candida tropicalis strains which showed MICs higher than those of other Candida strains. An analysis of the C. tropicalis strains revealed that intracellular concentrations of BAY 10-8888 were in the millimolar range, comparable to those for C. albicans. However, these isolates expressed isoleucyl-tRNA synthetase activities about fourfold higher than those for C. albicans. To test the possibility of resistance modeling, we determined the correlations between the intracellular concentration of BAY 10-8888, the specific activity of isoleucyl-tRNA synthetase, the number of free, i.e., noninhibited, isoleucyl-tRNA synthetase molecules/cell, and growth, assuming a linear relation. We found significant correlations between growth and the intracellular concentration of BAY 10-8888 and between growth and the number of free isoleucyl-tRNA synthetase molecules/cell, but not between growth and the specific activity of isoleucyl-tRNA synthetase.

Deletion of transmembrane domain 12 of CDR1, a multidrug transporter from Candida albicans, leads to altered drug specificity: expression of a yeast multidrug transporter in baculovirus expression system

Cdr1p, an ATP-binding cassette transporter from the pathogenic yeast Candida albicans, confers resistance to several unrelated drugs including anti-Candida drugs (Prasad et al., 1995b). We demonstrate that the deletion of 237 bp (79 aa) from the 3' end of CDR1 (which encompasses the transmembrane domain (TM) 12 of the putative transporter) did not result in the total loss of its ability to efflux cytotoxic agents. While the expression of deltaCDR1 in yeast resulted in impaired sensitivity to drugs like cycloheximide, anisomycin, sulfomethuron methyl and antifungal nystatin, its ability to confer resistance remained unaltered to drugs such as o-phenanthroline, 4-nitroquinoline-N-oxide, cerulenin, azoles, oligomycin, erythromycin, and benomyl. Similar to human MDR1p. Cdr1p might also have localized drug binding sites in TM 12, but that might not be the case for all the drugs. The TM 12 deletion also did not lead to any significant impairment in NTPase activities. Both ATPase and UTPase activities of complete Cdr1p and deltaCdr1p were not significantly altered, as was the case with respect to their ability to efflux Rh123 and steroid hormone like [3H]-beta-estradiol. To further dissect the functionality of Cdr1p, its truncated version was overexpressed in a baculovirus-insect cell expression system. The synthesis of deltaCdr1p in Sf9 cells was temporally regulated as a function of the baculovirus polyhedrin gene promoter. The Sf9 derived deltaCdr1p was approximately 130 kDa, which was lower than the expected size, probably due to the differences in glycosylation. This, however, did not affect the functionality of deltaCdr1p. The deletion of TM 12 did not affect the targeting of the protein and deltaCdr1p was exclusively localized in plasma membrane of Sf9 cells as detected by immunofluorescence. The expression of deltaCdr1p in the baculovirus-insect expression system generated a high drug-stimulated plasma membrane-bound ATPase activity which was not demonstrable when deltaCdr1p was expressed in yeast.

Purified arginine permease of Candida albicans is functionally active in a reconstituted system

We have for the first time purified arginine permease from a pathogenic yeast, Candida albicans, to homogeneity by affinity chromatography using L-arginine-linked agarose matrix as affinity column. The purified protein (PP) was of 66 kDa with no subunit structure. Two kinetically distinct binding affinities of PP were evident where high affinity binding (S1) revealed a dependence on acidic pH while pH did not have dramatic effect on low affinity (S2) binding. The specificity of L-arginine binding to PP with regard to other amino acids, structural analogues and inhibitors, was essentially similar to arginine transport observed in the intact cells of C. albicans (Rao et al., 1986). The purified arginine permease was reconstituted into proteoliposomes and its functionality was tested by imposing a valinomycin-induced membrane potential. All the characteristic features of L-arginine transport displayed by the reconstituted system were similar to those observed in intact cells. Thus homogeneous purified arginine permease was also functionally active.