RESUMO
In Leishmania mexicana parasites, a unique glucose transporter, LmxGT1, is selectively targeted to the flagellar membrane, suggesting a possible sensory role that is often associated with ciliary membrane proteins. Expression of LmxGT1 is down-regulated â¼20-fold by increasing cell density but is up-regulated â¼50-fold by depleting glucose from the medium, and the permease is strongly down-regulated when flagellated insect-stage promastigotes invade mammalian macrophages and transform into intracellular amastigotes. Regulation of LmxGT1 expression by glucose and during the lifecycle operates at the level of protein stability. Significantly, a ∆lmxgt1 null mutant, grown in abundant glucose, undergoes catastrophic loss of viability when parasites deplete glucose from the medium, a property not exhibited by wild-type or add-back lines. These results suggest that LmxGT1 may function as a glucose sensor that allows parasites to enter the stationary phase when they deplete glucose and that in the absence of this sensor, parasites do not maintain viability when they run out of glucose. However, alternate roles for LmxGT1 in monitoring glucose availability are considered. The absence of known sensory receptors with defined ligands and biologic functions in Leishmania and related kinetoplastid parasites underscores the potential significance of these observations.
Assuntos
Proteínas Facilitadoras de Transporte de Glucose/metabolismo , Leishmania mexicana/metabolismo , Proteínas de Protozoários/metabolismo , Animais , Linhagem Celular , Feminino , Flagelos/metabolismo , Regulação da Expressão Gênica , Genes de Protozoários , Glucose/metabolismo , Proteínas Facilitadoras de Transporte de Glucose/genética , Humanos , Leishmania mexicana/genética , Leishmania mexicana/patogenicidade , Leishmaniose Cutânea/metabolismo , Leishmaniose Cutânea/parasitologia , Camundongos , Camundongos Endogâmicos BALB C , Mutação , Proteínas de Protozoários/genética , Psychodidae/parasitologia , Proteínas Recombinantes de Fusão/genética , Proteínas Recombinantes de Fusão/metabolismoRESUMO
Gluconeogenesis is an active pathway in Leishmania amastigotes and is essential for their survival within the mammalian cells. However, our knowledge about this pathway in trypanosomatids is very limited. We investigated the role of glycerol kinase (GK), phosphoenolpyruvate carboxykinase (PEPCK), and pyruvate phosphate dikinase (PPDK) in gluconeogenesis by generating the respective Leishmania mexicana Δgk, Δpepck, and Δppdk null mutants. Our results demonstrated that indeed GK, PEPCK, and PPDK are key players in the gluconeogenesis pathway in Leishmania, although stage-specific differences in their contribution to this pathway were found. GK participates in the entry of glycerol in promastigotes and amastigotes; PEPCK participates in the entry of aspartate in promastigotes, and PPDK is involved in the entry of alanine in amastigotes. Furthermore, the majority of alanine enters into the pathway via decarboxylation of pyruvate in promastigotes, whereas pathway redundancy is suggested for the entry of aspartate in amastigotes. Interestingly, we also found that l-lactate, an abundant glucogenic precursor in mammals, was used by Leishmania amastigotes to synthesize mannogen, entering the pathway through PPDK. On the basis of these new results, we propose a revision in the current model of gluconeogenesis in Leishmania, emphasizing the differences between amastigotes and promastigotes. This work underlines the importance of studying the trypanosomatid intracellular life cycle stages to gain a better understanding of the pathologies caused in humans.
Assuntos
Gluconeogênese , Glicerol Quinase/metabolismo , Leishmania mexicana/metabolismo , Fosfoenolpiruvato Carboxilase/metabolismo , Proteínas de Protozoários/metabolismo , Piruvato Ortofosfato Diquinase/metabolismo , Vias Biossintéticas/efeitos dos fármacos , Southern Blotting , Western Blotting , DNA de Protozoário/genética , Glucose/metabolismo , Glucose/farmacologia , Glicerol Quinase/genética , Humanos , Leishmania mexicana/genética , Leishmania mexicana/crescimento & desenvolvimento , Estágios do Ciclo de Vida , Mutação , Fosfoenolpiruvato Carboxilase/genética , Proteínas de Protozoários/genética , Piruvato Ortofosfato Diquinase/genéticaRESUMO
The LmxGT1 glucose transporter is selectively targeted to the flagellum of the kinetoplastid parasite Leishmania mexicana, but the mechanism for targeting this and other flagella-specific membrane proteins among the Kinetoplastida is unknown. To address the mechanism of flagellar targeting, we employed in vivo cross-linking, tandem affinity purification, and mass spectrometry to identify a novel protein, KHARON1 (KH1), which is important for the flagellar trafficking of LmxGT1. Kh1 null mutant parasites are strongly impaired in flagellar targeting of LmxGT1, and trafficking of the permease was arrested in the flagellar pocket. Immunolocalization revealed that KH1 is located at the base of the flagellum, within the flagellar pocket, where it associates with the proximal segment of the flagellar axoneme. We propose that KH1 mediates transit of LmxGT1 from the flagellar pocket into the flagellar membrane via interaction with the proximal portion of the flagellar axoneme. KH1 represents the first component involved in flagellar trafficking of integral membrane proteins among parasitic protozoa. Of considerable interest, Kh1 null mutants are strongly compromised for growth as amastigotes within host macrophages. Thus, KH1 is also important for the disease causing stage of the parasite life cycle.
Assuntos
Flagelos/metabolismo , Glucose/metabolismo , Leishmania mexicana/metabolismo , Proteínas de Transporte de Monossacarídeos/metabolismo , Proteínas de Protozoários/metabolismo , Sequência de Aminoácidos , Animais , Animais Geneticamente Modificados , Sequência de Bases , Cromatografia de Afinidade , Primers do DNA , Dados de Sequência Molecular , Transporte Proteico , Proteínas de Protozoários/química , Homologia de Sequência de AminoácidosRESUMO
The genome of Leishmania mexicana encompasses a cluster of three glucose transporter genes designated LmxGT1, LmxGT2 and LmxGT3. Functional and genetic studies of a cluster null mutant (Δlmxgt1-3) have dissected the roles of these proteins in Leishmania metabolism and virulence. However, null mutants were recovered at very low frequency, and comparative genome hybridizations revealed that Δlmxgt1-3 mutants contained a linear extrachromosomal 40 kb amplification of a region on chromosome 29 not amplified in wild type parasites. These data suggested a model where this 29-40k amplicon encoded a second site suppressor contributing to parasite survival in the absence of GT1-3 function. To test this, we quantified the frequency of recovery of knockouts in the presence of individual overexpressed open reading frames covering the 29-40k amplicon. The data mapped the suppressor activity to PIFTC3, encoding a component of the intraflagellar transport pathway. We discuss possible models by which PIFTC3 might act to facilitate loss of GTs specifically. Surprisingly, by plasmid segregation we showed that continued PIFTC3 overexpression was not required for Δlmxgt1-3 viability. These studies provide the first evidence that genetic suppression can occur by providing critical biological functions transiently. This novel form of genetic suppression may extend to other genes, pathways and organisms.
Assuntos
Técnicas de Inativação de Genes , Leishmania mexicana/genética , Proteínas de Transporte de Monossacarídeos/genética , Supressão Genética , Leishmania mexicana/metabolismo , Viabilidade Microbiana , Modelos BiológicosRESUMO
Glucose is a major nutrient in the insect vector stage of Leishmania parasites. Glucose transporter null mutants of Leishmania mexicana exhibit profound phenotypic changes in both insect stage promastigotes and mammalian host stage amastigotes that reside within phagolysosomes of host macrophages. Some of these phenotypic changes could be either mediated or attenuated by changes in gene expression that accompany deletion of the glucose transporter genes. To search for changes in protein expression, the profile of proteins detected on two-dimensional gels was compared for wild type and glucose transporter null mutant promastigotes. A total of 50 spots whose intensities changed significantly and consistently in multiple experiments were detected, suggesting that a cohort of proteins is altered in expression levels in the null mutant parasites. Following identification of proteins by mass spectrometry, 3 such regulated proteins were chosen for more detailed analysis: mitochondrial aldehyde dehydrogenase, ribokinase, and hexokinase. Immunoblots employing antisera against these enzymes confirmed that their levels were upregulated, both in glucose transporter null mutants and in wild type parasites starved for glucose. Quantitative reverse transcriptase PCR (qRT-PCR) revealed that the levels of mRNAs encoding these enzymes were also enhanced. Global expression profiling using microarrays revealed a limited number of additional changes, although the sensitivity of the microarrays to detect modest changes in amplitude was less than that of two-dimensional gels. Hence, there is likely to be a network of proteins whose expression levels are altered by genetic ablation of glucose transporters, and much of this regulation may be reflected by changes in the levels of the cognate mRNAs. Some of these changes in protein expression may reflect an adaptive response of the parasites to limitation of glucose.
Assuntos
Deleção de Genes , Perfilação da Expressão Gênica , Leishmania mexicana/genética , Leishmania mexicana/metabolismo , Proteínas de Transporte de Monossacarídeos/deficiência , Proteoma/análise , Proteínas de Protozoários/análise , Eletroforese em Gel Bidimensional , Immunoblotting , Espectrometria de Massas , Análise em Microsséries , RNA Mensageiro/biossíntese , RNA de Protozoário/biossíntese , Reação em Cadeia da Polimerase Via Transcriptase ReversaRESUMO
A glucose transporter null mutant of the parasitic protozoan Leishmania mexicana, in which three linked glucose transporter genes have been deleted by targeted gene replacement, is unable to replicate as amastigote forms within phagolysomes of mammalian host macrophages and is avirulent. Spontaneous suppressors of the null mutant have been isolated that partially restore replication of parasites within macrophages. These suppressor mutants have amplified the gene for an alternative hexose transporter, the LmGT4 permease (previously called the D2 permease), on a circular extrachromosomal element, and they overexpress LmGT4 mRNA and protein. The suppressors have also regained the ability to transport hexoses, and they have reverted other phenotypes of the null mutant exhibiting enhanced resistance to oxidative killing, heat shock and starvation for nutrients, as well as augmented levels of the storage carbohydrate beta-mannan, increased cell size and increased growth as insect stage promastigotes compared with the unsuppressed mutant. Complementation of the null mutant with the LmGT4 gene on a multicopy episomal expression vector also reverted these phenotypes, confirming that suppression results from amplification of the LmGT4 gene. These results underscore the importance of hexose transporters for the infectious stage of the parasite life cycle.
Assuntos
Amplificação de Genes , Leishmania mexicana/genética , Proteínas de Transporte de Monossacarídeos/metabolismo , Proteínas de Protozoários/metabolismo , Animais , Hibridização Genômica Comparativa , Genes de Protozoários , Teste de Complementação Genética , Hexoses/metabolismo , Leishmania mexicana/metabolismo , Macrófagos/parasitologia , Camundongos , Camundongos Endogâmicos BALB C , Proteínas de Transporte de Monossacarídeos/genética , Mutação , Análise de Sequência com Séries de Oligonucleotídeos , Fenótipo , Proteínas de Protozoários/genética , RNA de Protozoário/genéticaRESUMO
Glucose is a major source of energy and carbon in promastigotes of Leishmania mexicana, and its uptake is mediated by three glucose transporters whose genes are encoded within a single cluster. A null mutant in which the glucose transporter gene cluster was deleted by homologous gene replacement was generated previously and shown to grow more slowly than wild type promastigotes but not to be viable as amastigotes in primary tissue culture macrophages or in axenic culture. Further phenotypic characterization demonstrates that the null mutant is unable to import glucose, mannose, fructose, or galactose and that each of the three glucose transporter isoforms, LmGT1, LmGT2, and LmGT3, is capable of transporting each of these hexoses. Complementation of the null mutant with each isoform is able to restore growth in each of the four hexoses to wild type levels. Null mutant promastigotes are reduced in size to about 2/3 the volume of wild type parasites. In addition, the null mutants are significantly more sensitive to oxidative stress than their wild type counterparts. These results underscore the importance of glucose transporters in the parasite life cycle and suggest reasons for their non-viability in the disease-causing amastigote stage.
Assuntos
Leishmania mexicana/genética , Leishmania mexicana/metabolismo , Proteínas de Transporte de Monossacarídeos/genética , Proteínas de Transporte de Monossacarídeos/metabolismo , Proteínas de Protozoários/genética , Proteínas de Protozoários/metabolismo , Animais , Genes de Protozoários , Teste de Complementação Genética , Hexoses/metabolismo , Concentração de Íons de Hidrogênio , Técnicas In Vitro , Leishmania mexicana/crescimento & desenvolvimento , Macrófagos/parasitologia , Camundongos , Camundongos Endogâmicos BALB C , Mutação , Estresse Oxidativo , Fenótipo , Isoformas de Proteínas/genética , Isoformas de Proteínas/metabolismoRESUMO
Leishmania mexicana are parasitic protozoa that express a variety of glycoconjugates that play important roles in their biology as well as the storage carbohydrate beta-mannan, which is an essential virulence factor for survival of intracellular amastigote forms in the mammalian host. Glucose transporter null mutants, which are viable as insect form promastigotes but not as amastigotes, do not take up glucose and other hexoses but are still able to synthesize these glycoconjugates and beta-mannan, although at reduced levels. Synthesis of these carbohydrate-containing macromolecules could be accounted for by incorporation of non-carbohydrate precursors into carbohydrates by gluconeogenesis. However, the significantly reduced level of the virulence factor beta-mannan in the glucose transporter null mutants compared with wild-type parasites may contribute to the non-viability of these null mutants in the disease-causing amastigote stage of the life cycle.
Assuntos
Proteínas Facilitadoras de Transporte de Glucose/genética , Proteínas Facilitadoras de Transporte de Glucose/metabolismo , Leishmania mexicana/metabolismo , Leishmania mexicana/patogenicidade , Acetatos/metabolismo , Alanina/metabolismo , Animais , Ácido Aspártico/metabolismo , Linhagem Celular , Gluconeogênese , Proteínas Facilitadoras de Transporte de Glucose/deficiência , Glicerol/metabolismo , Leishmania mexicana/genética , Mananas/metabolismo , Proteínas de Protozoários/biossíntese , Proteínas de Protozoários/genética , Proteínas de Protozoários/metabolismo , VirulênciaRESUMO
Both insect and mammalian life cycle stages of Leishmania mexicana take up glucose and express all three isoforms encoded by the LmGT glucose transporter gene family. To evaluate glucose transporter function in intact parasites, a null mutant line has been created by targeted disruption of the LmGT locus that encompasses the LmGT1, LmGT2, and LmGT3 genes. This deltalmgt null mutant exhibited no detectable glucose transport activity. The growth rate of the deltalmgt knockout in the promastigote stage was reduced to a rate comparable with that of WT cells grown in the absence of glucose. deltalmgt cells also exhibited dramatically reduced infectivity to macrophages, demonstrating that expression of LmGT isoforms is essential for viability of amastigotes. Furthermore, WT L. mexicana were not able to grow as axenic culture form amastigotes if glucose was withdrawn from the medium, implying that glucose is an essential nutrient in this life cycle stage. Expression of either LmGT2 or LmGT3, but not of LmGT1, in deltalmgt null mutants significantly restored growth as promastigotes, but only LmGT3 expression substantially rescued amastigote growth in macrophages. Subcellular localization of the three isoforms was investigated in deltalmgt cells expressing individual LmGT isoforms. Using anti-LmGT antiserum and GFP-tagged LmGT fusion proteins, LmGT2 and LmGT3 were localized to the cell body, whereas LmGT1 was localized specifically to the flagellum. These results establish that each glucose transporter isoform has distinct biological functions in the parasite.