The lipids of Pneumocystis carinii

Non-Cellular Layers of the Respiratory Tract: Protection against Pathogens and Target for Drug Delivery

Epithelial barriers separate the human body from the environment to maintain homeostasis. Compared to the skin and gastrointestinal tract, the respiratory barrier is the thinnest and least protective. The properties of the epithelial cells (height, number of layers, intercellular junctions) and non-cellular layers, mucus in the conducting airways and surfactant in the respiratory parts determine the permeability of the barrier. The review focuses on the non-cellular layers and describes the architecture of the mucus and surfactant followed by interaction with gases and pathogens. While the penetration of gases into the respiratory tract is mainly determined by their hydrophobicity, pathogens use different mechanisms to invade the respiratory tract. Often, the combination of mucus adhesion and subsequent permeation of the mucus mesh is used. Similar mechanisms are also employed to improve drug delivery across the respiratory barrier. Depending on the payload and target region, various mucus-targeting delivery systems have been developed. It appears that the mucus-targeting strategy has to be selected according to the planned application.

Pneumocystis: from a doubtful unique entity to a group of highly diversified fungal species

At the end of the 20th century the unique taxonomically enigmatic entity called Pneumocystis carinii was identified as a heterogeneous group of microscopic Fungi, constituted of multiple stenoxenic biological entities largely spread across ecosystems, closely adapted to, and coevolving in parallel with, mammal species. The discoveries and reasoning that led to the current conceptions about the taxonomy of Pneumocystis at the species level are examined here. The present review also focuses on the biological, morphological and phylogenetical features of Pneumocystis jirovecii, Pneumocystis oryctolagi, Pneumocystis murina, P. carinii and Pneumocystis wakefieldiae, the five Pneumocystis species described until now, mainly on the basis of the phylogenetic species concept. Interestingly, Pneumocystis organisms exhibit a successful adaptation enabling them to dwell and replicate in the lungs of both immunocompromised and healthy mammals, which can act as infection reservoirs. The role of healthy carriers in aerial disease transmission is nowadays recognized as a major contribution to Pneumocystis circulation, and Pneumocystis infection of nonimmunosuppressed hosts has emerged as a public health issue. More studies need to be undertaken both on the clinical consequences of the presence of Pneumocystis in healthy carriers and on the intricate Pneumocystis life cycle to better define its epidemiology, to adapt existing therapies to each clinical context and to discover new drug targets.

Pulmonary alveolar proteinosis

Three adult patients, two of whom were HIV-seropositive, presented with a dyspnea of two-to-three-day duration associated with dry cough. High-resolution CT scans of the chest revealed a widespread air-space consolidation with "crazy-paving" pattern in all cases, suggesting a pulmonary alveolar proteinosis (PAP). Bronchoalveolar lavage (BAL) retrieved varying amounts of turbid fluid containing abundant, coarsely granular material that stained positively using periodic acid-Schiff (PAS) and PAS with prior diastase digestion. Pneumocystis carinii (PC) cysts were identified in Gomori methenamine silver-stained BAL sediments obtained from the two HIV-positive patients. By electron microscopy, numerous myelin figures were found in the BAL sediments in all cases, confirming a PAP. Histologic examination of lung tissues obtained by open biopsy confirmed a PAP in two cases, with one case showing, in addition, PC cysts.

Characterization of a lanosterol 14 alpha-demethylase from Pneumocystis carinii

Pneumocystis carinii (PC) causes severe pneumonia in immunocompromised patients. PC is intrinsically resistant to treatment with azole antifungal medications. The enzyme lanosterol 14 alpha-demethylase (Erg11) is the target for azole antifungals. We cloned PCERG11 and compared its sequence to Erg11 proteins present in azole-resistant organisms, and performed chromosomal and Northern blot analysis for PCERG11. Of 13 potential sites which could confer resistance to azoles, two were identical to azole-resistant Candida. By site-directed mutagenesis we changed these two sites in PCERG11 to those present in azole-sensitive Candida to generate PCERG11-SDM (E113D, T125K). We tested the susceptibility of ERG11 deletion strains of Saccharomyces cerevisiae (SC) expressing PCERG11, PCERG11-SDM, and wild-type SCERG11 to three azole antifungals: fluconazole, itraconazole, and voriconazole. PCERG11 required a 2.2-fold higher dose of voriconazole and 3.5-fold higher dose of fluconazole than SCERG11 for a 50% reduction in growth. No difference was observed in the sensitivity to itraconazole. PCERG11-SDM has increased sensitivity to fluconazole and voriconazole, but not itraconazole. We believe that the molecular structure of the lanosterol 14 alpha-demethylase encoded by PCERG11 confers inherent resistance to azole antifungals and plays an integral part in the overall resistance of this PC to azole therapy.

Comprehensive and definitive structural identities of Pneumocystis carinii sterols

Pneumocystis causes a type of pneumonia in immunodeficient mammals, such as AIDS patients. Mammals cannot alkylate the C-24 position of the sterol side chain, nor can they desaturate C-22. Thus, the reactions leading to these sterol modifications are particularly attractive targets for the development of drugs against fungal and protozoan pathogens that make them. In the present study, the definitive structures of 43 sterol molecular species in rat-derived Pneumocystis carinii were elucidated by nuclear magnetic resonance spectroscopy. Ergosterol, Delta(5,7) sterols, trienes, and tetraenes were not among them. Most (32 of the 43) were 24-alkylsterols, products of S-adenosyl-L-methionine:C-24 sterol methyl transferase (SAM:SMT) enzyme activity. Their abundance is consistent with the suggestion that SAM:SMT is highly active in this organism and that the enzyme is an excellent anti-Pneumocystis drug target. In contrast, the comprehensive analysis strongly suggest that P. carinii does not form Delta(22) sterols, thus C-22 desaturation does not appear to be a drug target in this pathogen. The lanosterol derivatives, 24-methylenelanost-8-en-3 beta-ol and (Z)-24-ethylidenelanost-8-en-3 beta-ol (pneumocysterol), previously identified in human-derived Pneumocystis jiroveci, were also detected among the sterols of the rat-derived P. carinii organisms.

Phospholipid biosynthesis in the oyster protozoan parasite, Perkinsus marinus

Perkinsus marinus is a protozoan parasite that causes high mortality in its commercially and ecologically important host, the Eastern oyster Crassostrea virginica. In order to understand the host-parasite relationship in lipid metabolism, the ability of P. marinus to synthesize phospholipids from polar headgroup precursors was investigated. Pulse/chase experiments were conducted using radiolabled serine, choline, ethanolamine and inositol. Timecourse incubations revealed that in vitro cultured P. marinus meronts can utilize the cytidine diphosphate-diacylglycerol (CDP-DAG) pathway to synthesize phosphatidylinositol (PI) from inositol and phosphatidylserine (PS) from serine. Serine label was also incorporated into phosphatidylethanolamine (PE), phosphatidylcholine (PC) and lysophosphatidylcholine (LPC). Incubations of P. marinus cells with increasing concentrations of radiolabeled serine resulted in more radioactivity recovered in neutral lipids than in polar lipids at the highest substrate concentration tested (344 microM). This suggests that excess serine label was being utilized for fatty acid synthesis and stored as triacylglycerols. Additional incubations were conducted with radiolabeled choline and ethanolamine at concentrations equimolar to the highest serine concentration tested. Ethanolamine label was also incorporated into PE, PS, PC and LPC. Choline label was incorporated into PC. These results suggest the presence of three pathways for de novo synthesis of phospholipids in P. marinus: CDP-choline, CDP-ethanolamine and CDP-DAG. At equivalent substrate concentrations (344 microM) the highest incorporation of labeled substrate into total phospholipids was with serine followed by ethanolamine and choline, respectively. P. marinus phospholipid biosynthetic capabilities appear to be similar to those of Plasmodium and Trypanosoma species.

The Pneumocystis carinii drug target S-adenosyl-L-methionine:sterol C-24 methyl transferase has a unique substrate preference

Pneumocystis is an opportunistic pathogen that can cause pneumonitis in immunodeficient people such as AIDS patients. Pneumocystis remains difficult to study in the absence of culture methods for luxuriant growth. Recombinant protein technology now makes it possible to avoid some major obstacles. The P. carinii expressed sequence tag (EST) database contains 11 entries of a sequence encoding a protein homologous to S-adenosyl-L-methionine (SAM):C-24 sterol methyl transferase (SMT), suggesting high activity of this enzyme in the organism. We sequenced the erg6 cDNA, identified the putative peptide motifs for the sterol and SAM binding sites in the deduced amino acid sequence and expressed the protein in Escherichia coli. Unlike SAM:SMT from other organisms, the P. carinii enzyme had higher affinities for lanosterol and 24-methylenelanosterol than for zymosterol, the preferred substrate in other fungi. Cycloartenol was not a productive substrate. With lanosterol and 24-methylenelanosterol as substrates, the major reaction products were 24-methylenelanosterol and pneumocysterol respectively. Thus, the P. carinii SAM:SMT catalysed the transfer of both the first and the second methyl groups to the sterol C-24 position, and the substrate preference was found to be a unique property of the P. carinii SAM:SMT. These observations, together with the absence of SAM:SMT among mammals, further support the identification of sterol C-24 alkylation reactions as excellent targets for the development of drugs specifically directed against this pathogen.

Pneumocystis carinii: genetic diversity and cell biology

As an important opportunistic pulmonary pathogen, Pneumocystis carinii has been the focus of extensive research over the decades. The use of laboratory animal models has permitted a detailed understanding of the host-parasite interaction but an understanding of the basic biology of P. carinii has lagged due in large part to the inability of the organism to grow well in culture and to the lack of a tractable genetic system. Molecular techniques have demonstrated extensive heterogeneity among P. carinii organisms isolated from different host species. Characterization of the genes and genomes of the Pneumocystis family has supported the notion that the family comprises different species rather than strains within the genus Pneumocystis and contributed to the understanding of the pathophysiology of infection. Many of the technical obstacles in the study of the organisms have been overcome in the past decade and the pace of research into the basic biology of the organism has accelerated. Biochemical pathways have been inferred from the presence of key enzyme activities or gene sequences, and attempts to dissect cellular pathways have been initiated. The Pneumocystis genome project promises to be a rich source of information with regard to the functional activity of the organism and the presence of specific biochemical pathways. These advances in our understanding of the biology of this organism should provide for future studies leading to the control of this opportunistic pathogen.

Uptake of the neutral amino acids glutamine, leucine, and serine by Pneumocystis carinii

Experiments to elucidate the mechanism by which Pneumocystis carinii transports glutamine, leucine, and serine were performed in this study. Uptake of all three radiolabeled amino acids exhibited first-order, saturation kinetics as extracellular substrate concentrations were increased, thus ruling out simple diffusion and indicating carrier-mediated transport. Kinetic analyses of amino acid uptake and the results of competitive inhibition experiments suggested that leucine, serine, and glutamine were taken up via a common transporter system. The uptake of serine was examined in greater detail to characterize the nature of the carrier. Serine uptake was not affected by N, N'-dicyclohexylcarbodiimide, carbonyl cyanide m-chlorophenyl hydrazone, ouabain, gramicidin, valinomycin, sodium azide, salicylhydroxamine acid (SHAM), iodoacetate, iodoacetate plus SHAM, KCN, and azide. Thus serine uptake did not require sodium or energy from ATP, an electrochemical proton gradient or a membrane potential across the cell surface (i.e., proton-motive force). Serine uptake was dependent on glucose in the extracellular compartment. In the presence of glucose, serine uptake was inhibited by chloramphenicol but not cycloheximide. The results from these experiments are most consistent with facilitated diffusion as the mechanism. After 30 min of incubation, most of the radioactivity was in the cellular soluble fraction. In most cases, incorporation into the extractable total lipids and the remaining particulate cellular components were detectable after this incubation period.

Transport of aspartic acid, arginine, and tyrosine by the opportunistic protist Pneumocystis carinii

In order to improve culture media and to discover potential drug targets, uptake of an acidic, a basic, and an aromatic amino acid were investigated. Current culture systems, axenic or co-cultivation with mammalian cells, do not provide either the quantity or quality of cells needed for biochemical studies of this organism. Insight into nutrient acquisition can be expected to lead to improved culture media and improved culture growth. Aspartic acid uptake was directly related to substrate concentration, Q(10) was 1.10 at pH 7.4. Hence the organism acquired this acidic amino acid by simple diffusion. Uptake of the basic amino acid arginine and the aromatic amino acid tyrosine exhibited saturation kinetics consistent with carrier-mediated mechanisms. Kinetic parameters indicated two carriers (K(m)=22.8+/-2.5 microM and K(m)=3.6+/-0.3 mM) for arginine and a single carrier for tyrosine (K(m)=284+/-23 microM). The effects of other L-amino acids showed that the tyrosine carrier was distinct from the arginine carriers. Tyrosine and arginine transport were independent of sodium and potassium ions, and did not appear to require energy from ATP or a proton motive force. Thus facilitated diffusion was identified as the mechanism of uptake. After 30 min of incubation, these amino acids were incorporated into total lipids and the sedimentable material following lipid extraction; more than 90% was in the cellular soluble fraction.

Heterogeneity of Pneumocystis sterol profiles of samples from different sites in the same pair of lungs suggests coinfection by distinct organism populations

Sterol profiles of samples taken from different sites of a Pneumocystis-infected human lung showed large variations in pneumocysterol similar to those that occur among samples from different patients. Thus, the influence of diet or drugs on pneumocysterol accumulation was ruled out, suggesting distinct phenotypic populations as the basis for the heterogeneity.

Inhibitors of sterol biosynthesis and amphotericin B reduce the viability of pneumocystis carinii f. sp. carinii

Pneumocystis carinii synthesizes sterols with a double bond at C-7 of the sterol nucleus and an alkyl group with one or two carbons at C-24 of the side chain. Also, some human-derived Pneumocystis carinii f. sp. hominis strains contain lanosterol derivatives with an alkyl group at C-24. These unique sterols have not been found in other pathogens of mammalian lungs. Thus, P. carinii may have important differences in its susceptibility to drugs known to block reactions in ergosterol biosynthesis in other fungi. In the present study, inhibitors of 3-hydroxy-3-methyglutaryl coenzyme A reductase, squalene synthase, squalene epoxidase, squalene epoxide-lanosterol cyclase, lanosterol demethylase, Delta(8) to Delta(7) isomerase, and S-adenosylmethionine:sterol methyltransferase were tested for their effects on P. carinii viability as determined by quantitation of cellular ATP levels in a population of organisms. Compounds within each category varied in inhibitory effect; the most effective included drugs targeted at squalene synthase, squalene epoxide-lanosterol cyclase, and Delta(8) to Delta(7) isomerase. Some drugs that are potent against ergosterol-synthesizing fungi had little effect against P. carinii, suggesting that substrates and/or enzymes in P. carinii sterol biosynthetic reactions are distinct. Amphotericin B is ineffective in clearing P. carinii infections at clinical doses; however, this drug apparently binds to sterols and causes permeability changes in P. carinii membranes, since it reduced cellular ATP levels in a dose-dependent fashion.

C27 to C32 sterols found in Pneumocystis, an opportunistic pathogen of immunocompromised mammals

Pneumocystis carinii is the paradigm of opportunistic infections in immunocompromised mammals. Prior to the acquired immunodeficiency syndrome (AIDS) pandemic and the use of immunosuppressive therapy in organ transplant and cancer patients, P. carinii was regarded as a curiosity, rarely observed clinically. Interest in this organism exploded when it was identified as the agent of P. carinii pneumonia (PcP), the direct cause of death among many AIDS patients. Aggressive prophylaxis has decreased the number of acute PcP cases, but it remains among the most prevalent opportunistic infections found within this patient population. The taxonomic assignment of P. carinii has long been argued; molecular genetics data now demonstrate that it is a fungus. Several antimycotic drugs are targeted against ergosterol or its biosynthesis, but these are not as effective against PcP as they are against other fungal infections. This can now be explained in part by the identification of the sterols of P. carinii. The organism lacks ergosterol but contains distinct C28 and C29 delta7 24-alkylsterols. Also, 24-methylenelanost-8-en-3beta-ol (C31) and pneumocysterol, (24Z)-ethylidenelanost-8-en-3beta-ol (C32) were recently identified in organisms infecting humans. Together, the delta7 24-alkylsterols and pneumocysterol are regarded as signature lipids of the pathogen that can be useful for the diagnosis of PcP, since no other lung pathogen is known to contain them. Cholesterol (C27), the dominant sterol component in P. carinii, is probably totally scavenged from the host. De novo synthesis of sterols has been demonstrated by the presence of lovastatin-sensitive 3-hydroxy-3-methylglutaryl-CoA reductase activity, the incorporation of radiolabeled mevalonate and squalene into P. carinii sterols, and the reduction in cellular ATP in cells treated with inhibitors of enzymes in sterol biosynthesis.

Sterols of Pneumocystis carinii hominis organisms isolated from human lungs

The opportunistic pathogen Pneumocystis carinii causes pneumonia (P. carinii pneumonia, or PCP) in immunocompromised individuals such as AIDS patients. Rat-derived P. carinii carinii organisms have distinct sterols which are not synthesized by mammals and not found in other microbes infecting mammalian lungs. The dominant sterol present in the organism is cholesterol (which is believed to be scavenged from the host), but other sterols in P. carinii carinii have an alkyl group at C-24 of the sterol side chain (C(28) and C(29) 24-alkylsterols) and a double bond at C-7 of the nucleus. Recently, pneumocysterol (C(32)), which is essentially lanosterol with a C-24 ethylidene group, was detected in lipids extracted from a formalin-fixed human P. carinii-infected lung, and its structures were elucidated by gas-liquid chromatography, mass spectrometry, and nuclear magnetic resonance spectrometry in conjunction with analyses of chemically synthesized authentic standards. The sterol composition of isolated P. carinii hominis organisms has yet to be reported. If P. carinii from animal models is to be used for identifying potential drug targets and for developing chemotherapeutic approaches to clear human infections, it is important to determine whether the 24-alkylsterols of organisms found in rats are also present in organisms in humans. In the present study, sterol analyses of P. carinii hominis organisms isolated from cryopreserved human P. carinii-infected lungs and from bronchoalveolar lavage fluid were performed. Several of the same distinct sterols (e.g., fungisterol and methylcholest-7-ene-3beta-ol) previously identified in P. carinii carinii were also present in organisms isolated from human specimens. Pneumocysterol was detected in only some of the samples.