Structural basis for species-selective targeting of Hsp90 in a pathogenic fungus

New strategies are needed to counter the escalating threat posed by drug-resistant fungi. The molecular chaperone Hsp90 affords a promising target because it supports survival, virulence and drug-resistance across diverse pathogens. Inhibitors of human Hsp90 under development as anticancer therapeutics, however, exert host toxicities that preclude their use as antifungals. Seeking a route to species-selectivity, we investigate the nucleotide-binding domain (NBD) of Hsp90 from the most common human fungal pathogen, Candida albicans. Here we report structures for this NBD alone, in complex with ADP or in complex with known Hsp90 inhibitors. Encouraged by the conformational flexibility revealed by these structures, we synthesize an inhibitor with >25-fold binding-selectivity for fungal Hsp90 NBD. Comparing co-crystals occupied by this probe vs. anticancer Hsp90 inhibitors revealed major, previously unreported conformational rearrangements. These insights and our probe's species-selectivity in culture support the feasibility of targeting Hsp90 as a promising antifungal strategy.

Chemical Genomics-Based Antifungal Drug Discovery: Targeting Glycosylphosphatidylinositol (GPI) Precursor Biosynthesis

Steadily increasing antifungal drug resistance and persistent high rates of fungal-associated mortality highlight the dire need for the development of novel antifungals. Characterization of inhibitors of one enzyme in the GPI anchor pathway, Gwt1, has generated interest in the exploration of targets in this pathway for further study. Utilizing a chemical genomics-based screening platform referred to as the Candida albicans fitness test (CaFT), we have identified novel inhibitors of Gwt1 and a second enzyme in the glycosylphosphatidylinositol (GPI) cell wall anchor pathway, Mcd4. We further validate these targets using the model fungal organism Saccharomyces cerevisiae and demonstrate the utility of using the facile toolbox that has been compiled in this species to further explore target specific biology. Using these compounds as probes, we demonstrate that inhibition of Mcd4 as well as Gwt1 blocks the growth of a broad spectrum of fungal pathogens and exposes key elicitors of pathogen recognition. Interestingly, a strong chemical synergy is also observed by combining Gwt1 and Mcd4 inhibitors, mirroring the demonstrated synthetic lethality of combining conditional mutants of GWT1 and MCD4. We further demonstrate that the Mcd4 inhibitor M720 is efficacious in a murine infection model of systemic candidiasis. Our results establish Mcd4 as a promising antifungal target and confirm the GPI cell wall anchor synthesis pathway as a promising antifungal target area by demonstrating that effects of inhibiting it are more general than previously recognized.

Inhibiting GPI anchor biosynthesis in fungi stresses the endoplasmic reticulum and enhances immunogenicity

In fungi, the anchoring of proteins to the plasma membrane via their covalent attachment to glycosylphosphatidylinositol (GPI) is essential and thus provides a valuable point of attack for the development of antifungal therapeutics. Unfortunately, studying the underlying biology of GPI-anchor synthesis is difficult, especially in medically relevant fungal pathogens because they are not genetically tractable. Compounding difficulties, many of the genes in this pathway are essential in Saccharomyces cerevisiae. Here, we report the discovery of a new small molecule christened gepinacin (for GPI acylation inhibitor) which selectively inhibits Gwt1, a critical acyltransferase required for the biosynthesis of fungal GPI anchors. After delineating the target specificity of gepinacin using genetic and biochemical techniques, we used it to probe key, therapeutically relevant consequences of disrupting GPI anchor metabolism in fungi. We found that, unlike all three major classes of antifungals in current use, the direct antimicrobial activity of this compound results predominantly from its ability to induce overwhelming stress to the endoplasmic reticulum. Gepinacin did not affect the viability of mammalian cells nor did it inhibit their orthologous acyltransferase. This enabled its use in co-culture experiments to examine Gwt1's effects on host-pathogen interactions. In isolates of Candida albicans, the most common fungal pathogen in humans, exposure to gepinacin at sublethal concentrations impaired filamentation and unmasked cell wall β-glucan to stimulate a pro-inflammatory cytokine response in macrophages. Gwt1 is a promising antifungal drug target, and gepanacin is a useful probe for studying how disrupting GPI-anchor synthesis impairs viability and alters host-pathogen interactions in genetically intractable fungi.

A rhizosphere fungus enhances Arabidopsis thermotolerance through production of an HSP90 inhibitor

The molecular chaperone HEAT SHOCK PROTEIN90 (HSP90) is essential for the maturation of key regulatory proteins in eukaryotes and for the response to temperature stress. Earlier, we have reported that fungi living in association with plants of the Sonoran desert produce small molecule inhibitors of mammalian HSP90. Here, we address whether elaboration of the HSP90 inhibitor monocillin I (MON) by the rhizosphere fungus Paraphaeosphaeria quadriseptata affects plant HSP90 and plant environmental responsiveness. We demonstrate that MON binds Arabidopsis (Arabidopsis thaliana) HSP90 and can inhibit the function of HSP90 in lysates of wheat (Triticum aestivum) germ. MON treatment of Arabidopsis seedlings induced HSP101 and HSP70, conserved components of the stress response. Application of MON, or growth in the presence of MON, allowed Arabidopsis wild type but not AtHSP101 knockout mutant seedlings to survive otherwise lethal temperature stress. Finally, cocultivation of P. quadriseptata with Arabidopsis enhanced plant heat stress tolerance. These data demonstrate that HSP90-inhibitory compounds produced by fungi can influence plant growth and responses to the environment.

HspBP1, an Hsp70 cochaperone, has two structural domains and is capable of altering the conformation of the Hsp70 ATPase domain

We present here the first structural information for HspBP1, an Hsp70 cochaperone. Using circular dichroism, HspBP1 was determined to be 35% helical. Although HspBP1 is encoded by seven exons, limited proteolysis shows that it has only two structural domains. Domain I, amino acids 1-83, is largely unstructured. Domain II, amino acids 84-359, is predicted to be 43% helical using circular dichroism. Using limited proteolysis we have also shown that HspBP1 association changes the conformation of the ATPase domain of Hsp70. Only domain II of HspBP1 is required to bring about this conformational change. Truncation mutants of HspBP1 were tested for their ability to inhibit the renaturation of luciferase and bind to Hsp70 in reticulocyte lysate. A carboxyl terminal truncation mutant that was slightly longer than domain I was inactive in these assays, but domain II was sufficient to perform both functions. Domain II was less active than full-length HspBP1 in these assays, and addition of amino acids from domain I improved both functions. These studies show that HspBP1 domain II can bind Hsp70, change the conformation of the ATPase domain, and inhibit Hsp70-associated protein folding.

Specific labeling of isoprenylated proteins: application to study inhibitors of the post-translational farnesylation and geranylgeranylation

Specific labeling of either farnesylated or geranylgeranylated proteins in human PC-3 prostate cancer cell line was obtained by suppression of mevalonic acid biosynthesis with lovastatin, 50 microM, followed by supplementation of cell culture medium with either [3H]farnesyl- or [3H]geranylgeranyl-pyrophosphate. The immunoprecipitation of either a farnesylated (p21 ras) or geranylgeranylated (p21 rap 1) protein demonstrated that labeling was specific since proteins were detected only if the appropriate isoprenoid was added to the culture medium. TLC analysis indicated that no conversion of one isoprenoid to the other occurred in these conditions. The selective labeling of either farnesylated or geranylgeranylated proteins may be a valuable tool for the development of inhibitors of isoprenoid transferases as a potential new class of antitumor agents.

6-[18F]fluoro-L-dopa and cerebral blood flow in unilaterally MPTP-treated monkeys

Intravenous administration of 15O-labeled water and 6-[18F]-L-fluorodopa were used to assess abnormal striatal activity in monkeys after long-term recovery of unilateral lesions of the dopaminergic nigro-striatal system induced by the neurotoxin MPTP. PET data were examined in relation to behavioral and biological parameters. Cerebral blood flow and 6-[18F]-L-DOPA uptake were found to be significantly reduced in the lesioned striatum, compared to the unaffected side and to normal controls. There was no correlation between cerebral blood flow and any of the behavioral parameters. The uptake rate constant of 18F-DOPA from blood to striatum and the ratios of striatum to occipital areas were highly correlated to the concentrations of homovanillic acid in the cerebrospinal fluid of the same animals but not to the rotational behavior. This MPTP-induced model of striatal dopamine deficiency in primates presents similarities with idiopathic Parkinson's disease and may be used to evaluate the effects of dopaminergic lesions and transplants on brain function.

Suramin rapidly alters cellular tyrosine phosphorylation in prostate cancer cell lines

Suramin, a synthetic polysulfonated anionic compound, is known to abrogate the activity of a variety of growth factors that serve as ligands for receptor-class protein-tyrosine kinases. Based on this information, we initially hypothesized that suramin treatment would be associated with decreased tyrosine phosphorylation. Upon testing this hypothesis in prostate cancer cell lines, we found that the most conspicuous effect of suramin was to increase the tyrosine phosphorylation of several distinct proteins. Further analyses indicate that suramin-induced increases in tyrosine phosphorylation represent a generalized, but not universal, phenomenon found in cell lines derived from a variety of human tissues. These rapid and specific suramin-induced alterations represent a novel finding for a non-polypeptide pharmaceutical agent and question the hypothesis that suramin exerts its antitumor action simply by abrogation of growth factor action.

Positron emission tomography with 18F-dopa: interpretation and biological correlates in nonhuman primates

Positron emission tomography (PET) was carried out, with 18F-DOPA as a ligand, in normal control monkeys and "parkinsonian" monkeys who had been treated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. The following approaches were used in data analysis: ratio of 18F accumulation in specific to nonspecific brain areas and 18F-DOPA influx constant obtained using either the actual plasma 18F-DOPA or the 18F activity in a nonspecific brain area as the input function. The results from these analyses were compared to one another and to biological parameters relevant to dopaminergic function. The striatum/cortex ratio and the rate constant calculated from plasma 18F-DOPA appeared to be the most sensitive analytic techniques.

Comparison of src-family cDNAs reveals distinct mechanisms underlying focus formation in transfected fibroblasts

Despite the intensive study of both cellular transformation and src-family protein-tyrosine kinases, there have been no direct comparisons of transforming potency for normal members of this gene-family. In this study, the focus-forming activity of normal c-src, fyn, and lck cDNAs were compared in NIH 3T3 cell transfection assays. Focus formation was studied quantitatively, and individual foci were analyzed for phosphotyrosine content and expression of appropriate translational products. Each foci arising from c-src transfectants had a marked increase in phosphotyrosine content, and the majority of these foci expressed a c-src protein with an aberrant carboxyl terminus. Foci derived from lck transfectants also had a marked increase in phosphotyrosine content, and some foci expressed a lck protein with an aberrant carboxyl terminus. In contrast, foci from fyn-transfected cells were not distinguished from G418-selected mass cultures in terms of total phosphotyrosine content or expression of p59fyn. These studies support the previously published concept that overexpression of the normal fyn protein contributes to focus formation in transfected NIH 3T3 cells but suggest that the focus-forming activity observed after c-src or lck transfections is frequently attributable to mutational events. Because lck mutations have not been previously described in transformed foci, we characterized the lck transcript expressed in two foci and identified a novel point mutation that encodes a lck protein with increased in vivo kinase and focus-forming activity.

Delayed L-phenylalanine infusion allows for simultaneous kinetic analysis and improved evaluation of specific-to-nonspecific fluorine-18-dopa uptake in brain

The accumulation of 3-O-methyl-6-[18F]fluoro-L-DOPA (18F-30M-DOPA) in the brain from the circulation is responsible for most of the nonspecific background during 18F-DOPA positron emission tomography scanning. To increase the sensitivity of 18F-DOPA for imaging presynaptic dopamine systems, we took advantage of 18F-30M-DOPA's rapid clearance from the brain (T1/2 approximately 15-20 min). The infusion of the unlabeled amino acid L-phenylalanine, starting 75 min after 18F-DOPA administration, prevents 18F-30M-DOPA entrance into the brain through competition at the large amino acid transport system of the blood brain barrier. This method produces high specific-to-nonspecific contrast images of 18F accumulation beginning 15-30 min after onset of amino acid infusion and better sensitivity to small changes in 18F-DOPA uptake while still allowing for kinetic analysis of the data in the early time points. Kinetic and anatomical data were found to be strongly correlated.

Distribution and kinetics of 3-O-methyl-6-[18F]fluoro-L-dopa in the rhesus monkey brain

Most attempts to model accurately [18F]-DOPA imaging of the dopamine system are based on the assumptions that its main peripheral metabolite, 3-O-methyl-6-[18F]fluoro-L-DOPA ([18F]3-OM-DOPA), crosses the blood-brain barrier but is present as a homogenous distribution throughout the brain, in part because it is not converted into [18F]DOPA in significant quantities. These assumptions were based mainly on data in rodents. Little information is available in the primate. To verify the accuracy of the above assumptions, we administered 18F-labeled 3-OM-DOPA to normal rhesus monkeys and animals with lesions of the DA nigrostriatal system. No selective 18F regional accumulation in brain was apparent in normal or lesioned animals. The plasma metabolite analysis revealed that only the negatively charged metabolites (e.g., sulfated conjugates) that do not cross the blood-brain barrier were found in significant quantities in the plasma. A one-compartment, three-parameter model was adequate to describe the kinetics of [18F]3-OM-DOPA. In conclusion, assumptions concerning [18F]3-OM-DOPA's behavior in brain appear acceptable for [18F]DOPA modeling purposes.

Postinjection L-phenylalanine increases basal ganglia contrast in PET scans of 6-18F-DOPA

The sensitivity of 18F-DOPA positron emission tomography for imaging presynaptic dopamine systems is limited by the amount of specific-to-nonspecific accumulation of radioactivity in brain. In rhesus monkeys, we have been able to increase this ratio by taking advantage of the lag time between 18F-DOPA injection and the formation of its main metabolite, the amino acid 18F-fluoromethoxydopa, the entrance of which into brain is responsible for most of the brain's nonspecific radioactivity. By infusing an unlabeled amino acid, L-phenylalanine, starting 15 min after 18F-DOPA administration, we preferentially blocked the accumulation of 18F-fluoromethoxydopa by preventing its entrance into brain through competition at the large neutral amino acid transport system of the blood-brain barrier. This method appears as reliable as the original and more sensitive, as demonstrated by the comparison of normal and MPTP-treated animals under both conditions.

New rapid analysis method demonstrates differences in 6-[18F] fluoro-L-dopa plasma input curves with and without carbidopa and in hemi-MPTP lesioned monkeys

Kinetic modeling of the PET tracer 6-[18F]fluoro-L-dopa ([18F]Dopa), used to measure presynaptic dopamine function, requires the accurate determination of the plasma input curve. We have developed a new method that uses alumina extraction preceded by cation and anion exchange resins to determine the parent compound, [18F]Dopa and its critical metabolite 3-O-methyl-6-[18F]fluoro-L-dopa. Using this method we found that carbidopa increases the plasma input of [18F]Dopa while decreasing the rate of metabolite formation, and that previous drug treatment can significantly effect [18F]Dopa metabolism.

6-18F-L-dopa imaging of the dopamine neostriatal system in normal and clinically normal MPTP-treated rhesus monkeys

Positron emission tomography following intravenous administration of 6-[18F]-L-fluorodopa was used to investigate the usefulness of PET for the assessment of normal and abnormal dopaminergic function. For this purpose, the incracerebral distribution of 6-[18F]-L-fluorodopa and its metabolites was evaluated in normal control and asymptomatic MPTP-treated rhesus monkeys. MPTP is a neurotoxic compound which destroys selectively the dapaminergic neurons of the nigrostriatal pathways in primates. The 18F accumulation was found to be significantly reduced in the striatum, putamen more than caudate, of the MPTP-treated animals compared to the normal controls. The 18F accumulation in dopamine-poor areas did not differ between the two groups. The ratios of striatum to dopamine-poor brain area were highly correlated to the concentrations of the dopamine metabolite, homovanillic acid, in the cerebrospinal fluid of the same animals. The findings are consistent with the hypothesis that "silent damage" to the dopaminergic nigral neurons may precede the onset of parkinsonism by many years and that PET scanner examination using 6-[18F]-L-fluorodopa may be useful in the detection of subtle dopaminergic dysfunctions as may exist in DA-related motor syndromes and neuropsychiatric disorders.

Role of serotonergic input in the down-regulation of beta-adrenoceptors following long-term clorgyline treatment

Administration of the selective monoamine oxidase (MAO) type A-inhibiting antidepressant clorgyline (1 mg/kg per day) to rats for 21 days caused a significant decrease in cortical [3H]dihydroalprenolol binding. Selective lesioning of central serotonergic axons by 5,7-dihydroxytryptamine (5,7-DHT; confirmed by the presence of the serotonin syndrome in response to a 40 mg/kg dose of 5-hydroxytryptophan (5-HTP) or inhibition of 5-HT synthesis by parachlorophenylalanine (PCPA) caused significant 5-HT and 5-HIAA depletions in the cortex without much effect on NE and DA concentrations, but did not have any significant effect on beta-adrenoceptor density, and furthermore failed to attenuate clorgyline-induced decreases in beta-adrenoceptor density. Clorgyline treatment partially antagonized 5-HT depletion by the 5,7-DHT lesion or PCPA treatment. These findings suggest that due to their ability to raise 5-HT concentrations, MAO-inhibiting antidepressants may be a better alternative than the tricyclics in treating depressed patients with reduced 5-HT if down-regulation of beta-adrenoceptors is critical for antidepressant efficacy.

Blockade of in vivo binding of 125I-labeled 3-iodobenzamide (IBZM) to dopamine receptors by D2 antagonist and agonist

The in vivo binding of [125I]3-iodobenzamide (IBZM), a substituted benzamide, to DA receptor binding sites in the caudate nucleus, nucleus accumbens, and olfactory tubercle was investigated by using ex vivo autoradiography. The in vivo binding of IBZM seems to be selective to D2 dopamine receptors, since the binding was blocked by pretreatment of animals with D2 agonist LY-171555 or antagonist YM-09151-2. Furthermore, in vitro binding assays in striatal membranes confirmed that IBZM binding was highly selective to D2 sites. Thus, IBZM, when labeled with 123I (T1/2: 13h; 159 kev), could be a potential ligand for imaging D2 dopamine receptors by single photon emission computerized tomography procedures.

Foot shock induces time and region specific adrenergic receptor changes in rat brain

Rats were subjected to 1 hr or 2 hr of electric foot shock for 1 day or 7 days and adrenergic receptor binding was evaluated in the hypothalamus, brainstem and cortex. beta-Adrenergic receptor density in the hypothalamus was dramatically reduced following 1 hr of shock. Following repeated shock, alpha 2-adrenergic receptors in the cortex and brainstem were observed to increase. Cortical alpha 2-adrenergic receptors were more sensitive to stress than the alpha 2-adrenergic receptors of the brainstem, alterations in the latter only reaching statistical significance following 7 days of shock and 24 hr of recovery. alpha 1- and beta-adrenergic receptors in the brainstem and cortex were relatively resistant to stress induced changes. The significance of type of stress, duration of stress, and strain of rat for understanding the current data are discussed in the context of prior reports of stress induced receptor changes.