Brain glucose induces tolerance of Cryptococcus neoformans to amphotericin B during meningitis

Abstract

In bacteria, antibiotic tolerance, the ability of a susceptible population to survive high doses of cidal drugs, has been shown to compromise therapeutic outcomes.In comparison, whether fungicide tolerance can be induced by host-derived factors during fungal diseases remains unproven. Here, through a systematic evaluation of metabolite-drug-fungal interactions in the leading fungal meningitis pathogen, Cryptococcus neoformans, we found that glucose, on which the brain depends for fuel, induces fungal tolerance to amphotericin B (AmB) in mouse brain tissue and patient cerebrospinal fluid via the fungal glucose repression activator Mig1. Mig1-mediated tolerance limits treatment efficacy for cryptococcal meningitis in mice and occurs through inhibiting the synthesis of ergosterol, the target of AmB, and promoting the production of inositolphosphorylceramide, which competes with AmB for ergosterol. Furthermore, AmB combined with an inhibitor of fungal-specific inositolphosphorylceramide synthase, aureobasidin A (AbA), shows a better efficacy against cryptococcal meningitis in mice than do clinically recommended therapies. Our discoveries provide evidence of host metabolite-induced fungicidal tolerance, which limits treatment efficacy for fungal meningitis.Main: Cryptococcus neoformans is the leading cause of fungal meningitis, causing approximately 180,000 deaths annually 1,2 , and the only fungicidal drug available to treat it is amphotericin B (AmB) 3,4 . Genetic resistance to AmB, defined as an elevated minimum inhibitory concentration (MIC) is extremely rare among C. neoformans and other meningitis-causing fungi 5-7 , and the clinical outcomes of cryptococcal meningitis are thought to be largely independent of AmB resistance 7-9 .Recently, treatment outcomes of candidiasis in a large proportion of patients infected by genetically susceptible Candida isolates have been shown to be influenced by fungal tolerance to the fungistatic drug fluconazole, highlighting the previously neglected importance of survival modes other than antifungal resistance 10 . In these cases, fungistatic tolerance is defined as growth of a subpopulation of clonal cells at drug concentrations above the MIC 10,11 . Unlike microbiostatic drugs that inhibit the growth of microbes without destroying them, tolerance to microbicidal drugs, whose activities are irreversible, reflects the abilities of susceptible microbial cells to survive a transient exposure to a microbiocide at concentrations that would otherwise be lethal 12,13 . This tolerance can be quantified using time-kill curve-based approaches (e.g. MDK99 assessment) as documented for several bacterial and fungal pathogens 5,6,12,[14][15][16] . Notably, the time-kill curve assays performed in independent retrospective laboratory-based studies on otherwise AmB-susceptible C. neoformans strains isolated from patients with cryptococcal meningitis have shown correlations with clinical outcomes 5,6 . Despite the identification of such correlations, it remains unproven whether AmB tolerance ca...

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“…Drug-related factors impacting drug delivery into human nail tissue include drug molecular size, surface charge, lipophilicity, and polarity [ 1 , 10 ]. Recent findings suggest that factors present in the natural environment or in situ behavior of the pathogens may influence the effectiveness of a drug differently compared with controlled laboratory conditions [ 11 , 12 ]. During onychomycosis treatment, the keratin matrix that composes the dorsal plate of the nail is considered an obstacle to nail plate penetration.…”

Population heterogeneity in Cryptococcus neoformans: Impact on pathogenesis

“…By screening a library of 340 metabolites, the authors identified that C . neoformans tolerance to AmB increases in high-glucose media, as more cells survived AmB treatment in the presence of high glucose in vitro [ 18 ]. Similarly, AmB was more lethal against C .…”

“…The mutant strain lacking the MIG1 gene showed significantly reduced fungal load after AmB treatment, in the brains of mice with cryptococcal meningitis, confirming that brain glucose induces tolerance against AmB in vivo as well. Interestingly, a combination of AmB + aureobasidin A (sphingolipid biosynthesis inhibitor) showed better outcomes than the currently recommended combination treatment AmB+ flucytosine, in infected mice [ 18 ]. Although more research is needed, these 2 studies are great examples of the promises that adjuvant therapy holds.…”