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Cryptococcus neoformans is an environmental pathogen that remodels its cellular physiology to survive within mammals and, in susceptible hosts, cause life-threatening meningoencephalitis. Of the many distinctions between the external environment and mammalian tissues, CO 2 concentration in the host is two orders of magnitude higher than in the environment and represents a critical stress for C. neoformans . C. neoformans strains that do not replicate at host CO 2 concentrations are less virulent in mouse models of infection, further supporting CO 2 tolerance as a virulence trait. To further understand the genetic determinants of C. neoformans CO 2 tolerance, we performed a near genome-wide screen for deletion mutants with altered CO 2 fitness using a competitive growth assay. A total of 301 of 4,692 deletion mutants showed altered CO 2 tolerance (245 reduced fitness; 56 increased fitness) demonstrating the global effect of host CO 2 on C. neoformans physiology. Based on this data set as well as a metabolomic analysis of C. neoformans adaptation to host CO 2 , we show that remodeling of central carbon metabolism, oxidative stress buffering, and membrane homeostasis represent an integrated response to CO 2 stress that is mediated in part by the TOR-Ypk1 signaling axis. We propose that CO 2 -induced capsule formation leads to reduced cellular glucose which, in turn, triggers remodeling of central carbon metabolism toward utilization of alternative carbon sources and increased mitochondrial respiration/reactive oxygen generation. Thus, these data provide a near genome-wide profile of the genetic determinants of C. neoformans CO 2 tolerance as well as a model for how this important environmental human fungal pathogen alters its physiology to proliferate in the host.

Cryptococcus spp. are environmental fungi that first must adapt to the host environment before they can cause life-threatening meningitis in immunocompromised patients. Host CO 2 concentrations are 100-fold higher than the external environment and strains unable to grow at host CO 2 concentrations are not pathogenic. Using a genetic screening and transcriptional profiling approach, we report that the TOR pathway is critical for C. neoformans adaptation to host CO 2 partly through Ypk1-dependent remodeling of phosphatidylserine asymmetry at the plasma membrane. We also describe a C. neoformans ABC/PDR transporter ( PDR9 ) that is highly expressed in CO 2 -sensitive environmental strains, suppresses CO 2 -induced phosphatidylserine/phospholipid remodeling, and increases susceptibility to host concentrations of CO 2 . Interestingly, regulation of plasma membrane lipid asymmetry by the TOR-Ypk1 axis is distinct in C. neoformans compared to S. cerevisiae . Finally, host CO 2 concentrations suppress the C. neoformans pathways that respond to host temperature (Mpk1) and pH (Rim101), indicating that host adaptation requires a stringent balance among distinct stress responses. Here, Ristow et al show that the environmental fungus Cryptococcus adapts to the higher carbon dioxide levels present in human tissue through activation of the Target-of-Rapamycin stress response pathway leading to reorganization of its outer membrane lipids and host adaptation.

Efforts to control the global malaria health crisis are undermined by antimalarial resistance. Identifying mechanisms of resistance will uncover the underlying biology of the Plasmodium falciparum malaria parasites that allow evasion of our most promising therapeutics and may reveal new drug targets. We utilized fosmidomycin (FSM) as a chemical inhibitor of plastidial isoprenoid biosynthesis through the methylerythritol phosphate (MEP) pathway. We have thus identified an unusual metabolic regulation scheme in the malaria parasite through the essential glycolytic enzyme, glyceraldehyde 3-phosphate dehydrogenase (GAPDH). Two parallel genetic screens converged on independent but functionally analogous resistance alleles in GAPDH. Metabolic profiling of FSM-resistant gapdh mutant parasites indicates that neither of these mutations disrupt overall glycolytic output. While FSM-resistant GAPDH variant proteins are catalytically active, they have reduced assembly into the homotetrameric state favored by wild-type GAPDH. Disrupted oligomerization of FSM-resistant GAPDH variant proteins is accompanied by altered enzymatic cooperativity and reduced susceptibility to inhibition by free heme. Together, our data identifies a new genetic biomarker of FSM-resistance and reveals the central role of GAPDH in MEP pathway control and antimalarial sensitivity.

Acetyl-CoA synthetases (Acs) have emerged as drug targets for the treatment of cancer, metabolic diseases as well as fungal and parasitic infections. Although a variety of small molecule Acs inhibitors have been discovered, the systematic optimization of these molecules has been slowed by a lack of structural information regarding their mechanism of inhibition. Through a chemical genetic-based, synthetic lethal screen of the human fungal pathogen Cryptococcus neoformans , we identified an isoxazole-based Acs inhibitor with antifungal activity and high selectivity for the C. neoformans Acs1 relative to human ACSS2 as well as to other fungal Acs enzymes. Xray crystallography of the isoxazole- Cn Acs1 complex revealed that the isoxazole occupies both the acetyl- and CoA-binding sites of Cn Acs1. Biochemically, the isoxazoles display uncompetitive inhibition kinetics that are similar to antimalarial Acs inhibitors also proposed to target the CoA binding site. Consequently, these data provide structural and mechanistic insights into the remarkable selectivity of CoA pocket-targeting Acs inhibitors. As such, targeting fungal and parasitic Acs enzymes for the development of novel anti-infectives can be achieved with high selectivity and, thereby, low host toxicity. Acetyl-CoA synthetases have been proposed as targets for development of new antimicrobial drugs. Here, Jezewski et al. identify isoxazole-based compounds with activity against the pathogenic fungus Cryptococcus neoformans , and describe their mechanism of action as inhibitors of fungal acetyl-CoA synthetases.

During its complex life cycle, the malaria parasite survives dramatic environmental stresses, including large temperature shifts. Protein prenylation is required during asexual replication of Plasmodium falciparum , and the canonical heat shock protein 40 protein (HSP40; PF3D7_1437900) is posttranslationally modified with a 15-carbon farnesyl isoprenyl group. During its complex life cycle, the malaria parasite survives dramatic environmental stresses, including large temperature shifts. Protein prenylation is required during asexual replication of Plasmodium falciparum , and the canonical heat shock protein 40 protein (HSP40; PF3D7_1437900) is posttranslationally modified with a 15-carbon farnesyl isoprenyl group. In other organisms, farnesylation of Hsp40 orthologs controls their localization and function in resisting environmental stress. In this work, we find that plastidial isopentenyl pyrophosphate (IPP) synthesis and protein farnesylation are required for malaria parasite survival after cold and heat shock. Furthermore, loss of HSP40 farnesylation alters its membrane attachment and interaction with proteins in essential pathways in the parasite. Together, this work reveals that farnesylation is essential for parasite survival during temperature stress. Farnesylation of HSP40 may promote thermotolerance by guiding distinct chaperone-client protein interactions.

The adaptation of environmental fungal pathogens to the mammalian host is critical to pathogenesis. Of these adaptations, the remodeling of carbon metabolism is particularly important. Cryptococcus neoformans is an important human fungal pathogen for which the external environment is its primary niche. Previous work has shown that two nonessential acetyl-CoA metabolism enzymes, ATP-citrate lyase ( ACL1 ) and acetyl-CoA synthetase ( ACS1 ), play important roles in C. neoformans infection. Here, we took a genetic interaction approach to studying the interplay between these two enzymes along with an enzyme initially called ACS2 but which we have found is an acetoacetyl-CoA synthetase; we have renamed the gene 2- k eto b utyryl C oA synthetase 1 ( KBC1 ) based on its biochemical activity and the systematic name of its substrate. ACL1 and ACS1 represent a synthetic lethal pair of genes based on our genetic interaction studies. Double mutants of KBC1 with either ACS1 or ACL1 do not have significant synthetic phenotypes in vitro , although we find that deletion of any one of these enzymes reduces fitness within macrophages. Importantly, the acs1 Δ kbc1 Δ double mutant has significantly reduced fitness in the CNS relative to either single mutant as well as WT (~2 log 10 CFU reduction in fungal burden), indicating the important role these enzymes play during infection. The expression of both ACS1 and KBC1 is increased in vivo relative to in vitro conditions. The acs1 Δ mutant is hypersusceptible to fluconazole in vivo despite its minimal in vitro phenotypes. These data not only provide insights into the in vivo mechanism of action for a new class of antifungal Acs inhibitors but also into metabolic adaptations of C. neoformans to the host environment. IMPORTANCE The adaptation of environmental fungal pathogens to the mammalian host is critical to pathogenesis. Of these adaptations, the remodeling of carbon metabolism is particularly important. Here, we generated a focused set of double mutants of nonessential genes ( ACL1 , ACS1 , KBC1 ) involved in acetyl-CoA metabolism and examined their fitness in vitro and during CNS infection. From these studies, we found that all three enzymes play important roles during infection but that the role of ACS1 / KBC1 was minimal in vitro . Consistent with these observations, the expression of ACS1 and KBC1 was increased in vivo relative to standard in vitro conditions. Furthermore, strains lacking both ACL1 and ACS1 were not viable. These data clearly show that C. neoformans employs multiple carbon metabolism pathways to adapt to the host environment. They also provide insights into the potential mechanism of action for anti-cryptococcal Acs inhibitors.

Coagulase-positive staphylococci, which frequently colonize the mucosal surfaces of animals, also cause a spectrum of opportunistic infections including skin and soft tissue infections, urinary tract infections, pneumonia, and bacteremia. However, recent advances in bacterial identification have revealed that these common veterinary pathogens are in fact zoonoses that cause serious infections in human patients. The global spread of multidrug-resistant zoonotic staphylococci, in particular the emergence of methicillin-resistant organisms, is now a serious threat to both animal and human welfare. Accordingly, new therapeutic targets that can be exploited to combat staphylococcal infections are urgently needed. Enzymes of the methylerythritol phosphate pathway (MEP) of isoprenoid biosynthesis represent potential targets for treating zoonotic staphylococci. Here we demonstrate that fosmidomycin (FSM) inhibits the first step of the isoprenoid biosynthetic pathway catalyzed by deoxyxylulose phosphate reductoisomerase (DXR) in staphylococci. In addition, we have both enzymatically and structurally determined the mechanism by which FSM elicits its effect. Using a forward genetic screen, the glycerol-3-phosphate transporter GlpT that facilitates FSM uptake was identified in two zoonotic staphylococci, Staphylococcus schleiferi and Staphylococcus pseudintermedius. A series of lipophilic ester prodrugs (termed MEPicides) structurally related to FSM were synthesized, and data indicate that the presence of the prodrug moiety not only substantially increased potency of the inhibitors against staphylococci but also bypassed the need for GlpT-mediated cellular transport. Collectively, our data indicate that the prodrug MEPicides selectively and robustly inhibit DXR in zoonotic staphylococci, and further, that DXR represents a promising, druggable target for future development.

Glycolysis controls cellular energy, redox balance, and biosynthesis. Antiglycolytic therapies are under investigation for treatment of obesity, cancer, aging, autoimmunity, and microbial diseases. Interrupting glycolysis is highly valued as a therapeutic strategy, because glycolytic disruption is generally tolerated in mammals. Unfortunately, anemia is a known dose-limiting side effect of these inhibitors and presents a major caveat to development of antiglycolytic therapies. We developed specific inhibitors of enolase – a critical enzyme in glycolysis – and validated their metabolic and cellular effects on human erythrocytes. Enolase inhibition increases erythrocyte susceptibility to oxidative damage and induces rapid and premature erythrocyte senescence, rather than direct hemolysis. We apply our model of red cell toxicity to address questions regarding erythrocyte glycolytic disruption in the context of Plasmodium falciparum malaria pathogenesis. Our study provides a framework for understanding red blood cell homeostasis under normal and disease states and clarifies the importance of erythrocyte reductive capacity in malaria parasite growth.

Unique and essential aspects of parasite metabolism are excellent targets for development of new antimalarials. An improved understanding of parasite metabolism and drug resistance mechanisms is urgently needed. The antibiotic fosmidomycin targets the synthesis of essential isoprenoid compounds from glucose and is a candidate for antimalarial development. Our report identifies a novel mechanism of drug resistance and further describes a family of metabolic regulators in the parasite. Using a novel forward genetic approach, we also uncovered mutations that suppress drug resistance in the glycolytic enzyme PFK9. Thus, we identify an unexpected genetic mechanism of adaptation to metabolic insult that influences parasite fitness and tolerance of antimalarials. In the malaria parasite Plasmodium falciparum , synthesis of isoprenoids from glycolytic intermediates is essential for survival. The antimalarial fosmidomycin (FSM) inhibits isoprenoid synthesis. In P. falciparum , we identified a loss-of-function mutation in HAD2 ( P. falciparum 3D7_1226300 [PF3D7_1226300]) as necessary for FSM resistance. Enzymatic characterization revealed that HAD2, a member of the haloacid dehalogenase-like hydrolase (HAD) superfamily, is a phosphatase. Harnessing a growth defect in resistant parasites, we selected for suppression of HAD2-mediated FSM resistance and uncovered hypomorphic suppressor mutations in the locus encoding the glycolytic enzyme phosphofructokinase 9 (PFK9). Metabolic profiling demonstrated that FSM resistance is achieved via increased steady-state levels of methylerythritol phosphate (MEP) pathway and glycolytic intermediates and confirmed reduced PFK9 function in the suppressed strains. We identified HAD2 as a novel regulator of malaria parasite metabolism and drug sensitivity and uncovered PFK9 as a novel site of genetic metabolic plasticity in the parasite. Our report informs the biological functions of an evolutionarily conserved family of metabolic regulators and reveals a previously undescribed strategy by which malaria parasites adapt to cellular metabolic dysregulation. IMPORTANCE Unique and essential aspects of parasite metabolism are excellent targets for development of new antimalarials. An improved understanding of parasite metabolism and drug resistance mechanisms is urgently needed. The antibiotic fosmidomycin targets the synthesis of essential isoprenoid compounds from glucose and is a candidate for antimalarial development. Our report identifies a novel mechanism of drug resistance and further describes a family of metabolic regulators in the parasite. Using a novel forward genetic approach, we also uncovered mutations that suppress drug resistance in the glycolytic enzyme PFK9. Thus, we identify an unexpected genetic mechanism of adaptation to metabolic insult that influences parasite fitness and tolerance of antimalarials.

Conditional expression systems are widely used to genetically validate antifungal drug targets in mouse models of infection. Copper-regulated expression using the promoter of the CTR4 gene has been sporadically used for this purpose in C. neoformans . There is an urgent need for new antifungals to treat cryptococcal meningoencephalitis, a leading cause of mortality in people living with HIV/AIDS. An important aspect of antifungal drug development is the validation of targets to determine whether they are required for the survival of the organism in animal models of disease. In Cryptococcus neoformans , a copper-regulated promoter (pCTR4-2) has been used previously to modulate gene expression in vivo . The premise for these experiments is that copper concentrations differ depending on the host niche. Here, we directly test this premise and confirm that the expression of CTR4 , the promoter used to regulate gene expression, is much lower in the mouse lung compared to the brain. To further explore this approach, we applied it to the gene encoding 1,3-β-glucan synthase, FKS1 . In vitro , reduced expression of FKS1 has little effect on growth but does activate the cell wall integrity stress response and increase susceptibility to caspofungin, a direct inhibitor of Fks1. These data suggest that compensatory pathways that reduce C. neoformans resistance do so through posttranscriptional effects. In vivo , however, a less pronounced reduction in FKS1 expression leads to a much more significant reduction in lung fungal burden (~1 log 10 CFU), indicating that the compensatory responses to a reduction in FKS1 expression are not as effective in vivo as they are in vitro . In summary, use of copper-regulated expression of putative drug targets in vitro and in vivo can provide insights into the biological consequences of reduced activity of the target during infection. IMPORTANCE Conditional expression systems are widely used to genetically validate antifungal drug targets in mouse models of infection. Copper-regulated expression using the promoter of the CTR4 gene has been sporadically used for this purpose in C. neoformans . Here, we show that CTR4 expression is low in the lung and high in the brain, establishing the basic premise behind this approach. We applied the approach to the study of FKS1 , the gene encoding the target of the echinocandin class of 1,3-β-glucan synthase inhibitors. Our in vitro and in vivo studies indicate that C. neoformans tolerates extremely low levels of FKS1 expression. This observation provides a potential explanation for the poor activity of 1,3-β-glucan synthase inhibitors toward C. neoformans .