Use of cilofungin as direct fluorescent probe for monitoring antifungal drug-membrane interaction

The cyclic lipopeptide micafungin induces rupture of isolated mitochondria by reprograming the mitochondrial inner membrane anion channel

The antifungal activity of the drug micafungin, a cyclic lipopeptide that interacts with membrane proteins, may involve inhibition of fungal mitochondria. In humans, mitochondria are spared by the inability of micafungin to cross the cytoplasmic membrane. Using isolated mitochondria, we find that micafungin initiates the uptake of salts, causing rapid swelling and rupture of mitochondria with release of cytochrome c. The inner membrane anion channel (IMAC) is altered by micafungin to transfer both cations and anions. We propose that binding of anionic micafungin to IMAC attracts cations into the ion pore for the rapid transfer of ion pairs.

Membrane-Anchored Cyclic Peptides as Effectors of Mitochondrial Oxidative Phosphorylation

The echinocandins are membrane-anchored, cyclic lipopeptides (CLPs) with antifungal activity due to their ability to inhibit a glucan synthase located in the plasma membrane of fungi such as Candida albicans. A hydrophobic tail of an echinocandin CLP inserts into a membrane, placing a six-amino acid cyclic peptide near the membrane surface. Because processes critical for the function of the electron transfer complexes of mitochondria, such as proton uptake and release, take place near the surface of the membrane, we have tested the ability of two echinocandin CLPs, caspofungin and micafungin, to affect the activity of electron transfer complexes in isolated mammalian mitochondria. Indeed, caspofungin and micafungin both inhibit whole chain electron transfer in isolated mitochondria at low micromolar concentrations. The effects of the CLPs are fully reversible, in some cases simply via the addition of bovine serum albumin to bind the CLPs via their hydrophobic tails. Each CLP affects more than one complex, but they still exhibit specificity of action. Only caspofungin inhibits complex I, and the CLP inhibits liver but not heart complex I. Both CLPs inhibit heart and liver complex III. Caspofungin inhibits complex IV activity, while, remarkably, micafungin stimulates complex IV activity nearly 3-fold. Using a variety of assays, we have developed initial hypotheses for the mechanisms by which caspofungin and micafungin alter the activities of complexes IV and III. The dication caspofungin partially inhibits cytochrome c binding at the low-affinity binding site of complex IV, while it also appears to inhibit the release of protons from the outer surface of the complex, similar to Zn(2+). Anionic micafungin appears to stimulate complex IV activity by enhancing the transfer of protons to the O2 reduction site. For complex III, we hypothesize that each CLP binds to the cytochrome b subunit and the Fe-S subunit to inhibit the required rotational movement of the latter.

The Candida albicans Cdr2p ATP-binding cassette (ABC) transporter confers resistance to caspofungin

Multidrug resistance may pose a serious problem to antifungal therapy. The Candida albicans Cdr2p is one of two ATP-binding cassette (ABC) transporters mediating antifungal resistance in vivo through increased drug efflux. Echinocandins such as caspofungin represent the newest class of antifungals that target cell wall synthesis. We show here by agar plate resistance assays that cross-resistant clinical isolates of C. albicans display high minimal inhibitory concentrations (MICs) to caspofungin when compared with a sensitive ATCC reference strain. Northern analysis and immunoblotting indicate that these isolates also show high levels of CDR1 and CDR2 expression. To determine a possible contribution of Cdr1p or Cdr2p to caspofungin resistance, we have functionally expressed Cdr1p and Cdr2p in appropriate recipient strains of the yeast Saccharomyces cerevisiae. Yeast cells expressing Cdr1p or Cdr2p exhibit cross-resistance to established antifungal drugs such as azoles and terbinafine. However, Cdr2p and, to a much lesser extent, Cdr1p confer caspofungin hyper-resistance when expressed in yeast. Likewise, Cdr2p confers caspofungin resistance when constitutively overexpressed in a drug-sensitive C. albicans strain. We therefore propose that Cdr2p may contribute to clinical candin resistance. Finally, our data suggest that cross-resistance phenotypes of clinical isolates are the consequence of distinct mechanisms that may operate simultaneously.

[Review article. Molecular basis of antimycotic therapy]

A review is presented on the hitherto clinically administered antimycotic drugs, their action mechanisms and limitations as well as on the presently newly developed antifungals and their molecular targets.

Cellular accumulation, localization, and activity of a synthetic cyclopeptamine in fungi

A novel synthetic cyclopeptamine, A172013, rapidly accumulated by passive diffusion into Candida albicans CCH442. Drug influx could not be totally facilitated by the membrane-bound target, beta-(1,3)-glucan synthase, since accumulation was unsaturable at drug concentrations up to 10 microg/ml (about 1.6 x 10(-7) molecules/cell), or 25x MIC. About 55 and 23% of the cell-incorporated drug was associated with the cell wall and protoplasts, respectively. Isolated microsomes contained 95% of the protoplast-associated drug, which was fully active against glucan synthesis in vitro. Drug (0.1 microg/ml) accumulation was rapid and complete after 5 min in several fungi tested, including a lipopeptide/cyclopeptamine-resistant strain of C. albicans (LP3-1). The compound penetrated to comparable levels in both yeast and hyphal forms of C. albicans, and accumulation in Aspergillus niger was 20% that in C. albicans. These data indicated that drug-cell interactions were driven by the amphiphilic nature of the compound and that the cell wall served as a major drug reservoir.

Characterization of a lipopeptide-resistant strain of Candida albicans

Lipopeptides are antifungal agents that inhibit cell wall beta-(1,3)-glucan biosynthesis in fungal organisms. A mutant resistant to lipopeptides was generated by UV mutagenesis and characterized. The Candida albicans mutant (LP3-1) was stable and showed resistance specificity to a broad range of lipopeptides and certain glycolipid inhibitors. Other antifungal agents with diverse modes of action had a normal minimum inhibitory concentration profile for LP3-1 compared with the wild-type strain (CCH 442). In the in vitro beta-(1,3)-glucan synthase assay, both the lipopeptides and papulacandin-related agents had considerably higher 50% inhibitory concentration values in the LP3-1 strain than in the wild-type strain. In reconstitution assays, the resistance factor was associated with the integral membrane pellet rather than the peripheral GTP-binding protein. The LP3-1 strain had a membrane lipid profile similar to that of the parent strain and was virulent in a murine model of systemic candidiasis. Taken together, these results indicate that the resistance factor is associated with the integral membrane component of beta-(1,3)-glucan synthase. Lipopeptides are common antifungal agents encountered during screening of natural products. The LP3-1 strain was resistant to natural product extracts known to contain various lipopeptides. Thus, LP3-1 can be used in a dereplication assay.