Amphotericin forms an extramembranous and fungicidal sterol sponge

Abstract

Amphotericin has remained the powerful but highly toxic last line of defense in treating life-threatening fungal infections in humans for over 50 years with minimal development of microbial resistance. Understanding how this small molecule kills yeast is thus critical for guiding development of derivatives with an improved therapeutic index and other resistance-refractory antimicrobial agents. In the widely accepted ion channel model for its mechanism of cytocidal action, amphotericin forms aggregates inside lipid bilayers that permeabilize and kill cells. In contrast, we report that amphotericin exists primarily in the form of large, extramembranous aggregates that kill yeast by extracting ergosterol from lipid bilayers. These findings reveal that extraction of a polyfunctional lipid underlies the resistance-refractory antimicrobial action of amphotericin and suggests a roadmap for separating its cytocidal and membrane-permeabilizing activities. This new mechanistic understanding is also guiding development of the first derivatives of amphotericin that kill yeast but not human cells.

Selvamicin, an atypical antifungal polyene from two alternative genomic contexts

“…Such interactions can compromise membrane integrity and inhibit the function of membrane proteins (19,20). Recent studies suggest that ergosterol sequestration into extracellular aggregates may be the dominant mechanism of action (21,22), although several polyenes, including nystatin and amphotericin B, have also long been known to permeabilize membranes by the formation of ergosterol-dependent transmembrane channels (23). The presumed geometry of these channels situates the charged end of the molecule at the lipid-water interface, with the polyene and polyol interacting with ergosterol within the plasma membrane.…”

The fungicidal activity of amphotericin B requires autophagy-dependent targeting to the vacuole under a nutrient-starved condition in Saccharomyces cerevisiae

“…A recent study showed a sterol sponge model in which extramembranous aggregates of AmB extract ergosterol from phospholipid bilayers and thereby kill yeast (Anderson et al, 2014). In fungi, trafficking of AmB to the vacuole may be an attempt to eliminate such a fungistatic compound that increases plasma membrane permeability or causes ergosterol extraction on the cell surface.…”

“…Indeed, the most recently proposed mechanism for AmB lethality is based on its ability to extract ergosterol from the plasma membrane phospholipid layers in the form of large, extramembranous aggregates (Anderson et al, 2014). Notably, pathogenic fungi with a mutation in the ergosterol biosynthetic pathway not only accumulate a synthetic intermediate instead of ergosterol, but are also resistant to AmB (Martel et al, 2010;Young et al, 2003) in addition to azoles (Martel et al, 2010;Pinjon et al, 2003).…”

Polyene macrolide biosynthesis in streptomycetes and related bacteria: recent advances from genome sequencing and experimental studies

“…However, recent studies with non-permeabilising amphotericin analogues suggest that transmembrane channels are not essential for antifungal activity, sequestering of ergosterol disrupts multiple cellular processes and may be sufficient (Szpilman et al, 2008;Gray et al, 2012). The new "sponge model" proposes that aggregates of amphotericin B align on the membrane surface and extract ergosterol from the lipid bilayer (Anderson et al, 2014). This activity is similar to that of methyl--cyclodextrins, which are also known to deplete membranes of cholesterol (Zidovetzki and Levitan, 2007).…”