Protein kinase A catalytic subunit interacts and phosphorylates members of trans-sialidase super-family in Trypanosoma cruzi

Transgenic overexpression of heat shock protein (HSP83) enhances protein kinase A activity, disrupts GP63 surface protease expression and alters promastigote morphology in Leishmania amazonensis

Leishmania parasites undergo morphological changes during their infectious life cycle, including developmental transitions within the sandfly vector, culminating in metacyclic stages that are pre-adapted for infection. Upon entering vertebrate host phagocytes, Leishmania differentiate into intracellular amastigotes, the form that is ultimately transmitted back to the vector to complete the life cycle. Although environmental conditions that induce these cellular transitions are well-established, molecular mechanisms governing Leishmania morphologic differentiation in response to these cues remain largely uncharacterized. Previous studies indicate a key role for HSP83 in both promastigote metacyclogenesis and amastigote differentiation. To further elucidate HSP83 functions in the Leishmania lifecycle, we examined the biological impact of experimentally elevating HSP83 gene expression in Leishmania. Significantly, HSP83 overexpression was associated with altered metacyclic morphology, increased protein kinase A (PKA) activity and decreased expression of the Leishmania major surface protease, GP63. Corroborating these findings, overexpression of the L. amazonensis PKA catalytic subunit resulted in a largely similar phenotype. Our findings demonstrate for the first time in Leishmania, a functional link between HSP83 and PKA in the control of Leishmania gene expression, replication and morphogenesis.

Trypanosoma cruzi DNA Polymerase β Is Phosphorylated In Vivo and In Vitro by Protein Kinase C (PKC) and Casein Kinase 2 (CK2)

DNA polymerase β plays a fundamental role in the life cycle of Trypanosoma cruzi since it participates in the kinetoplast DNA repair and replication. This enzyme can be found in two forms in cell extracts of T. cruzi epimastigotes form. The H form is a phosphorylated form of DNA polymerase β, while the L form is not phosphorylated. The protein kinases which are able to in vivo phosphorylate DNA polymerase β have not been identified yet. In this work, we purified the H form of this DNA polymerase and identified the phosphorylation sites. DNA polymerase β is in vivo phosphorylated at several amino acid residues including Tyr35, Thr123, Thr137 and Ser286. Thr123 is phosphorylated by casein kinase 2 and Thr137 and Ser286 are phosphorylated by protein kinase C-like enzymes. Protein kinase C encoding genes were identified in T. cruzi, and those genes were cloned, expressed in bacteria and the recombinant protein was purified. It was found that T. cruzi possesses three different protein kinase C-like enzymes named TcPKC1, TcPKC2, and TcPKC3. Both TcPKC1 and TcPKC2 were able to in vitro phosphorylate recombinant DNA polymerase β, and in addition, TcPKC1 gets auto phosphorylated. Those proteins contain several regulatory domains at the N-terminus, which are predicted to bind phosphoinositols, and TcPKC1 contains a lipocalin domain at the C-terminus that might be able to bind free fatty acids. Tyr35 is phosphorylated by an unidentified protein kinase and considering that the T. cruzi genome does not contain Tyr kinase encoding genes, it is probable that Tyr35 could be phosphorylated by a dual protein kinase. Wee1 is a eukaryotic dual protein kinase involved in cell cycle regulation. We identified a Wee1 homolog in T. cruzi and the recombinant kinase was assayed using DNA polymerase β as a substrate. T. cruzi Wee1 was able to in vitro phosphorylate recombinant DNA polymerase β, although we were not able to demonstrate specific phosphorylation on Tyr35. Those results indicate that there exists a cell signaling pathway involving PKC-like kinases in T. cruzi.

Signaling pathways involved in environmental sensing in Trypanosoma cruzi

Trypanosoma cruzi is a unicellular parasite and the etiologic agent of Chagas disease. The parasite has a digenetic life cycle alternating between mammalian and insect hosts, where it faces a variety of environmental conditions to which it must adapt in order to survive. The adaptation to these changes is mediated by signaling pathways that coordinate the cellular responses to the new environmental settings. Major environmental changes include temperature, nutrient availability, ionic composition, pH, osmolarity, oxidative stress, contact with host cells and tissues, host immune response, and intracellular life. Some of the signaling pathways and second messengers potentially involved in the response to these changes have been elucidated in recent years and will be the subject of this review.

Methods to Investigate Signal Transduction Pathways in Trypanosoma cruzi: Cyclic Nucleotide Phosphodiesterases Assay Protocols

Intracellular levels of cyclic nucleotide second messengers are regulated predominantly by a large superfamily of phosphodiesterases (PDEs). Most of the different PDE variants play specific physiological functions; in fact, PDEs can associate with other proteins allowing them to be strategically anchored throughout the cell. In this regard, precise cellular expression and compartmentalization of these enzymes produce the specific control of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) gradients in cells and enable their integration with other signaling pathways.In trypanosomatids, some PDEs are essential for their survival and play fundamental roles in the adaptation of these parasites to different environmental stresses, as well as in the differentiation between their different life cycle forms. Given that these enzymes not only are similar to human PDEs but also have differential biochemical properties, and due to the great knowledge of drugs that target human PDEs, trypanosomatid PDEs could be postulated as important therapeutic targets through the repositioning of drugs.In this chapter, we describe a simple and sensitive radioisotope-based method to measure cyclic 3',5'-nucleotide phosphodiesterase using [³H]cAMP.

Large-Scale Phylogenetic Analysis of Trypanosomatid Adenylate Cyclases Reveals Associations with Extracellular Lifestyle and Host-Pathogen Interplay

Receptor adenylate cyclases (RACs) on the surface of trypanosomatids are important players in the host–parasite interface. They detect still unidentified environmental signals that affect the parasites’ responses to host immune challenge, coordination of social motility, and regulation of cell division. A lesser known class of oxygen-sensing adenylate cyclases (OACs) related to RACs has been lost in trypanosomes and expanded mostly in Leishmania species and related insect-dwelling trypanosomatids. In this work, we have undertaken a large-scale phylogenetic analysis of both classes of adenylate cyclases (ACs) in trypanosomatids and the free-living Bodo saltans. We observe that the expanded RAC repertoire in trypanosomatids with a two-host life cycle is not only associated with an extracellular lifestyle within the vertebrate host, but also with a complex path through the insect vector involving several life cycle stages. In Trypanosoma brucei, RACs are split into two major clades, which significantly differ in their expression profiles in the mammalian host and the insect vector. RACs of the closely related Trypanosoma congolense are intermingled within these two clades, supporting early RAC diversification. Subspecies of T. brucei that have lost the capacity to infect insects exhibit high numbers of pseudogenized RACs, suggesting many of these proteins have become redundant upon the acquisition of a single-host life cycle. OACs appear to be an innovation occurring after the expansion of RACs in trypanosomatids. Endosymbiont-harboring trypanosomatids exhibit a diversification of OACs, whereas these proteins are pseudogenized in Leishmania subgenus Viannia. This analysis sheds light on how ACs have evolved to allow diverse trypanosomatids to occupy multifarious niches and assume various lifestyles.

Signal Transduction Pathways as Therapeutic Target for Chagas Disease

Trypanosomatids are a group of flagellated unicellular eukaryotes, causing serious human diseases including Chagas disease (Trypanosoma cruzi), sleeping sickness (Trypanosoma brucei spp.) and Leishmaniasis (Leishmania spp.). The second messenger cAMP is involved in numerous and fundamental processes in these parasites including differentiation between stages, proliferation, osmoregulation, oxidative stress and quorum sensing. Interestingly, its signaling pathway is quite different from that of mammals, including structurally different adenylyl cyclases, the shortage of orthologous effector proteins and the absence of G-protein-coupled-receptors, among others. These characteristics make the proteins involved in these transduction pathways good candidates for therapeutic targets. However, the identification of new unknown druggable targets involves extensive research time and is economically very expensive, making difficult the transition from basic research to the clinical phase. Trypanosomatid PDEs have characteristic binding pockets that allow for a differential inhibition from their human orthologs. Modification in the approved drugs for human to convert them into trypanocidal treatments could lead to more effective therapies, shorter lab time and lower costs. In view of the fact that kinetoplastid PDEs are highly conserved with their mammalian counterparts, and since there are already numerous drugs on the market against human PDEs, the drug repositioning approach is highly promising. The development of new technologies, higher government and industrial involvement and more scientists committed to basic investigation, are the key to ultimately find an effective treatment and cure for the neglected tropical diseases.

Quantitative phosphoproteome and proteome analyses emphasize the influence of phosphorylation events during the nutritional stress of Trypanosoma cruzi: the initial moments of in vitro metacyclogenesis

Phosphorylation is an important event in cell signaling that is modulated by kinases and phosphatases. In Trypanosoma cruzi, the etiological agent of Chagas disease, approximately 2% of the protein-coding genes encode for protein kinases. This parasite has a heteroxenic life cycle with four different development stages. In the midgut of invertebrate vector, epimastigotes differentiate into metacyclic trypomastigotes in a process known as metacyclogenesis. This process can be reproduced in vitro by submitting parasites to nutritional stress (NS). Aiming to contribute to the elucidation of mechanisms that trigger metacyclogenesis, we applied super-SILAC (super-stable isotope labeling by amino acids in cell culture) and LC-MS/MS to analyze different points during NS. This analysis resulted in the identification of 4205 protein groups and 3643 phosphopeptides with the location of 4846 phosphorylation sites. Several phosphosites were considered modulated along NS and are present in proteins associated with various functions, such as fatty acid synthesis and the regulation of protein expression, reinforcing the importance of phosphorylation and signaling events to the parasite. These modulated sites may be triggers of metacyclogenesis.

Reprogramming of Trypanosoma cruzi metabolism triggered by parasite interaction with the host cell extracellular matrix

Trypanosoma cruzi, the etiological agent of Chagas' disease, affects 8 million people predominantly living in socioeconomic underdeveloped areas. T. cruzi trypomastigotes (Ty), the classical infective stage, interact with the extracellular matrix (ECM), an obligatory step before invasion of almost all mammalian cells in different tissues. Here we have characterized the proteome and phosphoproteome of T. cruzi trypomastigotes upon interaction with ECM (MTy) and the data are available via ProteomeXchange with identifier PXD010970. Proteins involved with metabolic processes (such as the glycolytic pathway), kinases, flagellum and microtubule related proteins, transport-associated proteins and RNA/DNA binding elements are highly represented in the pool of proteins modified by phosphorylation. Further, important metabolic switches triggered by this interaction with ECM were indicated by decreases in the phosphorylation of hexokinase, phosphofructokinase, fructose-2,6-bisphosphatase, phosphoglucomutase, phosphoglycerate kinase in MTy. Concomitantly, a decrease in the pyruvate and lactate and an increase of glucose and succinate contents were detected by GC-MS. These observations led us to focus on the changes in the glycolytic pathway upon binding of the parasite to the ECM. Inhibition of hexokinase, pyruvate kinase and lactate dehydrogenase activities in MTy were observed and this correlated with the phosphorylation levels of the respective enzymes. Putative kinases involved in protein phosphorylation altered upon parasite incubation with ECM were suggested by in silico analysis. Taken together, our results show that in addition to cytoskeletal changes and protease activation, a reprogramming of the trypomastigote metabolism is triggered by the interaction of the parasite with the ECM prior to cell invasion and differentiation into amastigotes, the multiplicative intracellular stage of T. cruzi in the vertebrate host.

A Multilayer Network Approach for Guiding Drug Repositioning in Neglected Diseases

Drug development for neglected diseases has been historically hampered due to lack of market incentives. The advent of public domain resources containing chemical information from high throughput screenings is changing the landscape of drug discovery for these diseases. In this work we took advantage of data from extensively studied organisms like human, mouse, E. coli and yeast, among others, to develop a novel integrative network model to prioritize and identify candidate drug targets in neglected pathogen proteomes, and bioactive drug-like molecules. We modeled genomic (proteins) and chemical (bioactive compounds) data as a multilayer weighted network graph that takes advantage of bioactivity data across 221 species, chemical similarities between 1.7 10⁵ compounds and several functional relations among 1.67 10⁵ proteins. These relations comprised orthology, sharing of protein domains, and shared participation in defined biochemical pathways. We showcase the application of this network graph to the problem of prioritization of new candidate targets, based on the information available in the graph for known compound-target associations. We validated this strategy by performing a cross validation procedure for known mouse and Trypanosoma cruzi targets and showed that our approach outperforms classic alignment-based approaches. Moreover, our model provides additional flexibility as two different network definitions could be considered, finding in both cases qualitatively different but sensible candidate targets. We also showcase the application of the network to suggest targets for orphan compounds that are active against Plasmodium falciparum in high-throughput screens. In this case our approach provided a reduced prioritization list of target proteins for the query molecules and showed the ability to propose new testable hypotheses for each compound. Moreover, we found that some predictions highlighted by our network model were supported by independent experimental validations as found post-facto in the literature.

Neglected tropical diseases are human infectious diseases that are often associated with poverty. Historically, lack of interest from the pharmaceutical industry resulted in the lack of good drugs to combat the majority of the pathogens that cause these diseases. Recently, the availability of open chemical information has increased with the advent of public domain chemical resources and the release of data from high throughput screening assays. Our aim in this work was to make use of data from extensively studied organisms like human, mouse, E. coli and yeast, among others, to prioritize and identify candidate drug targets in neglected pathogen proteomes, and drug-like bioactive molecules to foster drug development against neglected diseases. Our approach to the problem relied on applying bioinformatics and computational biology strategies to model large datasets spanning complete proteomes and extensive chemical information from publicly available sources. As a result, we were able to prioritize drug targets and identify potential targets for orphan bioactive drugs.

The ever unfolding story of cAMP signaling in trypanosomatids: vive la difference!

Kinetoplastids are unicellular, eukaryotic, flagellated protozoans containing the eponymous kinetoplast. Within this order, the family of trypanosomatids are responsible for some of the most serious human diseases, including Chagas disease (Trypanosoma cruzi), sleeping sickness (Trypanosoma brucei spp.), and leishmaniasis (Leishmania spp). Although cAMP is produced during the life cycle stages of these parasites, its signaling pathways are very different from those of mammals. The absence of G-protein-coupled receptors, the presence of structurally different adenylyl cyclases, the paucity of known cAMP effector proteins and the stringent need for regulation of cAMP in the small kinetoplastid cells all suggest a significantly different biochemical pathway and likely cell biology. However, each of the main kinetoplastid parasites express four class 1-type cyclic nucleotide-specific phosphodiesterases (PDEA-D), which have highly similar catalytic domains to that of human PDEs. To date, only TbrPDEB, expressed as two slightly different isoforms TbrPDEB1 and B2, has been found to be essential when ablated. Although the genomes contain reasonably well conserved genes for catalytic and regulatory domains of protein kinase A, these have been shown to have varied structural and functional roles in the different species. Recent discovery of a role of cAMP/AMP metabolism in a quorum-sensing signaling pathway in T. brucei, and the identification of downstream cAMP Response Proteins (CARPs) whose expression levels correlate with sensitivity to PDE inhibitors, suggests a complex signaling cascade. The interplay between the roles of these novel CARPs and the quorum-sensing signaling pathway on cell division and differentiation makes for intriguing cell biology and a new paradigm in cAMP signal transduction, as well as potential targets for trypanosomatid-specific cAMP pathway-based therapeutics.

Kinetoplastid Specific RNA-Protein Interactions in Trypanosoma cruzi Ribosome Biogenesis

RNA binding proteins (RBP) play essential roles in the highly conserved and coordinated process of ribosome biogenesis. Our laboratory has previously characterized two essential and abundant RBPs, P34 and P37, in Trypanosoma brucei which are required for several critical steps in ribosome biogenesis. The genes for these proteins have only been identified in kinetoplastid organisms but not in the host genome. We have identified a homolog of the TbP34 and TbP37 in a T. cruzi strain (termed TcP37/NRBD). Although the N-terminal APK-rich domain and RNA recognition motifs are conserved, the C-terminal region which contains putative nuclear and nucleolar localization signals in TbP34 and TbP37 is almost entirely missing from TcP37/NRBD. We have shown that TcP37/NRBD is expressed in T. cruzi epimastigotes at the level of mature mRNA and protein. Despite the loss of the C-terminal domain, TcP37/NRBD is present in the nucleus, including the nucleolus, and the cytoplasm. TcP37/NRBD interacts directly with Tc 5S rRNA, but does not associate with polyadenylated RNA. TcP37/NRBD also associates in vivo and in vitro with large ribosomal protein TcL5 and, unlike the case of T. brucei, this association is strongly enhanced by the presence of 5S rRNA, suggesting that the loss of the C-terminal domain of TcP37/NRBD may alter the interactions within the complex. These results indicate that the unique preribosomal complex comprised of L5, 5S rRNA, and the trypanosome-specific TcP37/NRBD or TbP34 and TbP37 is functionally conserved in trypanosomes despite the differences in the C-termini of the trypanosome-specific protein components.

cAMP signalling in trypanosomatids: role in pathogenesis and as a drug target

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Trypanosoma brucei adenylate cyclases are implicated in modulation of host immune response and social motility.

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First effectors downstream of cAMP signalling were identified in Trypanosoma cruzi and T. brucei.

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Crystal structures reveal a unique pocket in trypanosomatid phosphodiesterases.

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Trypanosomatid phosphodiesterase inhibitors are promising drug candidates.

Despite recent research linking cAMP signalling to virulence in trypanosomatids and detailed studies of trypanosomatid adenylyl cyclases (ACs) and phosphodiesterases (PDEs) since their discoveries 40 years ago, downstream components of the pathway and their biological functions have remained remarkably elusive. However, in recent years, significant discoveries have been made: a role for parasite ACs has been proposed in cytokinesis, evasion of the host immune response, and social motility. cAMP phosphodiesterases PDEB1 and PDEB2 were found to be essential for survival and virulence of Trypanosoma brucei and, in Trypanosoma cruzi, PDEC2 was shown to be required for normal osmoregulation. As we discuss here, these breakthroughs have led to an ongoing surge in the development of PDE inhibitors as lead compounds for trypanocidal drugs.

Down regulation of NO signaling in Trypanosoma cruzi upon parasite-extracellular matrix interaction: changes in protein modification by nitrosylation and nitration

Background

Adhesion of the Trypanosoma cruzi trypomastigotes, the causative agent of Chagas' disease in humans, to components of the extracellular matrix (ECM) is an important step in host cell invasion. The signaling events triggered in the parasite upon binding to ECM are less explored and, to our knowledge, there is no data available regarding •NO signaling.

Methodology/Principal Findings

Trypomastigotes were incubated with ECM for different periods of time. Nitrated and S-nitrosylated proteins were analyzed by Western blotting using anti-nitrotyrosine and S-nitrosyl cysteine antibodies. At 2 h incubation time, a decrease in NO synthase activity, •NO, citrulline, arginine and cGMP concentrations, as well as the protein modifications levels have been observed in the parasite. The modified proteins were enriched by immunoprecipitation with anti-nitrotyrosine antibodies (nitrated proteins) or by the biotin switch method (S-nitrosylated proteins) and identified by MS/MS. The presence of both modifications was confirmed in proteins of interest by immunoblotting or immunoprecipitation.

Conclusions/Significance

For the first time it was shown that T. cruzi proteins are amenable to modifications by S-nitrosylation and nitration. When T. cruzi trypomastigotes are incubated with the extracellular matrix there is a general down regulation of these reactions, including a decrease in both NOS activity and cGMP concentration. Notwithstanding, some specific proteins, such as enolase or histones had, at least, their nitration levels increased. This suggests that post-translational modifications of T. cruzi proteins are not only a reflex of NOS activity, implying other mechanisms that circumvent a relatively low synthesis of •NO. In conclusion, the extracellular matrix, a cell surrounding layer of macromolecules that have to be trespassed by the parasite in order to be internalized into host cells, contributes to the modification of •NO signaling in the parasite, probably an essential move for the ensuing invasion step.

Interaction of Trypanosoma cruzi with the extracellular matrix (ECM) is an essential step in the invasion of mammalian cells. However, the nature of the signaling triggered in the parasite is poorly understood. Herein the key role of nitric oxide in T. cruzi signaling is described, using an ECM preparation, in the absence of host cells. Inhibition of NOS activity, with the expected decrease in •NO production, as well as decrease in cGMP concentration were observed by the incubation of T. cruzi trypomastigotes with ECM. Additionally, lower levels of protein S-nitrosylation and nitration were detected. These post-translational modifications have been analyzed by biotin-switch and protein immunoprecipitation approaches coupled to mass spectrometry. The presence of both modifications was confirmed for specific proteins, as mucin II (S-nitrosylation), histones, enolase and tubulins. To our knowledge, decrease in the •NO signaling pathway upon T. cruzi trypomastigotes adhesion to ECM, affecting both the canonical pathway (•NO-soluble guanylyl cyclase-cGMP) and protein S-nitrosylation and nitration is described for the first time in this parasite.

Identification of novel cyclic nucleotide binding proteins in Trypanosoma cruzi

Cyclic AMP has been implicated as second messenger in a wide range of cellular processes. In the protozoan parasite Trypanosoma cruzi, cAMP is involved in the development of the parasite's life cycle. While cAMP effectors have been widely studied in other eukaryotic cells, little is known about cAMP's mechanism of action in T. cruzi. To date, only a cAMP-dependent protein kinase A (PKA) has been cloned and characterised in this parasite; however experimental evidence indicates the existence of cAMP-dependent, PKA-independent events. In order to identify new cAMP binding proteins as potential cAMP effectors, we carried out in silico studies using the predicted T. cruzi proteome. Using a combination of search methods 27 proteins with putative cNMP binding domains (CBDs) were identified. Phylogenetic analysis of the CBDs presented a homogeneous distribution, with sequences segregated into two main branches: one containing kinases-like proteins and the other gathering hypothetical proteins with different function or no other known. Comparative modelling of the strongest candidates provides support for the hypothesis that these proteins may give rise to structurally viable cyclic nucleotide binding domains. Pull-down and nucleotide displacement assays strongly suggest that TcCLB.508523.80 could bind cAMP and eventually be a new putative PKA-independent cAMP effector in T. cruzi.

Quantitative proteomic and phosphoproteomic analysis of Trypanosoma cruzi amastigogenesis

Chagas disease is a tropical neglected disease endemic in Latin America caused by the protozoan Trypanosoma cruzi. The parasite has four major life stages: epimastigote, metacyclic trypomastigote, bloodstream trypomastigote, and amastigote. The differentiation from infective trypomastigotes into replicative amastigotes, called amastigogenesis, takes place in vivo inside mammalian host cells after a period of incubation in an acidic phagolysosome. This differentiation process can be mimicked in vitro by incubating tissue-culture-derived trypomastigotes in acidic DMEM. Here we used this well-established differentiation protocol to perform a comprehensive quantitative proteomic and phosphoproteomic analysis of T. cruzi amastigogenesis. Samples from fully differentiated forms and two biologically relevant intermediate time points were Lys-C/trypsin digested, iTRAQ-labeled, and multiplexed. Subsequently, phosphopeptides were enriched using a TiO2 matrix. Non-phosphorylated peptides were fractionated via hydrophilic interaction liquid chromatography prior to LC-MS/MS analysis. LC-MS/MS and bioinformatics procedures were used for protein and phosphopeptide quantitation, identification, and phosphorylation site assignment. We were able to identify regulated proteins and pathways involved in coordinating amastigogenesis. We also observed that a significant proportion of the regulated proteins were membrane proteins. Modulated phosphorylation events coordinated by protein kinases and phosphatases that are part of the signaling cascade induced by incubation in acidic medium were also evinced. To our knowledge, this work is the most comprehensive quantitative proteomics study of T. cruzi amastigogenesis, and these data will serve as a trustworthy basis for future studies, and possibly for new potential drug targets.

The Gp85 surface glycoproteins from Trypanosoma cruzi

Trypanosoma cruzi strains show distinctive characteristics as genetic polymorphism and infectivity. Large repertoires of molecules, such as the Gp85 glycoproteins, members of the Gp85/Trans-sialidase superfamily, as well as multiple signaling pathways, are associated with invasion of mammalian cells by the parasite. Due to the large number of expressed members, encoded by more than 700 genes, the research focused on this superfamily conserved sequences is discussed. Binding sites to laminin have been identified at the N-terminus of the Gp85 molecules. Interestingly, the T. cruzi protein phosphorylation profile is changed upon parasite binding to laminin (or fibronectin), particularly the cytoskeletal proteins such as those from the paraflagellar rod and the tubulins, which are both markedly dephosphorylated. Detailed analysis of the signaling cascades triggered upon T. cruzi binding to extracellular matrix (ECM) proteins revealed the involvement of the MAPK/ERK pathway in this event. At the C-terminus, the conserved FLY sequence is a cytokeratin-binding domain and is involved in augmented host cell invasion in vitro and high levels of parasitemia in vivo. FLY, which is associated to tissue tropism and preferentially binds to the heart vasculature may somehow be correlated with the severe cardiac form, an important clinical manifestation of chronic Chagas' disease.

New insights on the sialidase protein family revealed by a phylogenetic analysis in metazoa

Sialidases are glycohydrolytic enzymes present from virus to mammals that remove sialic acid from oligosaccharide chains. Four different sialidase forms are known in vertebrates: the lysosomal NEU1, the cytosolic NEU2 and the membrane-associated NEU3 and NEU4. These enzymes modulate the cell sialic acid content and are involved in several cellular processes and pathological conditions. Molecular defects in NEU1 are responsible for sialidosis, an inherited disease characterized by lysosomal storage disorder and neurodegeneration. The studies on the biology of sialic acids and sialyltransferases, the anabolic counterparts of sialidases, have revealed a complex picture with more than 50 sialic acid variants selectively present in the different branches of the tree of life. The gain/loss of specific sialoconjugates have been proposed as key events in the evolution of deuterostomes and Homo sapiens, as well as in the host-pathogen interactions. To date, less attention has been paid to the evolution of sialidases. Thus we have conducted a survey on the state of the sialidase family in metazoan. Using an in silico approach, we identified and characterized sialidase orthologs from 21 different organisms distributed among the evolutionary tree: Metazoa relative (Monosiga brevicollis), early Deuterostomia, precursor of Chordata and Vertebrata (teleost fishes, amphibians, reptiles, avians and early and recent mammals). We were able to reconstruct the evolution of the sialidase protein family from the ancestral sialidase NEU1 and identify a new form of the enzyme, NEU5, representing an intermediate step in the evolution leading to the modern NEU3, NEU4 and NEU2. Our study provides new insights on the mechanisms that shaped the substrate specificity and other peculiar properties of the modern mammalian sialidases. Moreover, we further confirm findings on the catalytic residues and identified enzyme loop portions that behave as rapidly diverging regions and may be involved in the evolution of specific properties of sialidases.

Identification of Toxoplasma gondii cAMP dependent protein kinase and its role in the tachyzoite growth

Background

cAMP-dependent protein kinase (PKA) has been implicated in the asexual stage of the Toxoplasma gondii life cycle through assaying the effect of a PKA-specific inhibitor on its growth rate. Since inhibition of the host cell PKA cannot be ruled out, a more precise evaluation of the role of PKA, as well as characterization of the kinase itself, is necessary.

Methodology/Principal Finding

The inhibitory effects of two PKA inhibitors, H89, an ATP-competitive chemical inhibitor, and PKI, a substrate-competitive mammalian natural peptide inhibitor, were estimated. In the in vitro kinase assay, the inhibitory effect of PKI on a recombinant T. gondii PKA catalytic subunit (TgPKA-C) was weaker compared to that on mammalian PKA-C. In a tachyzoite growth assay, PKI had little effect on the growth of tachyzoites, whereas H89 strongly inhibited it. Moreover, T. gondii PKA regulatory subunit (TgPKA-R)-overexpressing tachyzoites showed a significant growth defect.

Conclusions/Significance

Our data suggest that PKA plays an important role in the growth of tachyzoites, and the inhibitory effect of substrate-competitive inhibitor PKI on T. gondii PKA was low compared to that of the ATP competitive inhibitor H89.

Cyclic-nucleotide signalling in protozoa

Compared with the impressive progress in understanding signal transduction pathways and mechanisms in mammalian systems, advances in protozoan signalling processes, including cyclic nucleotide metabolism, have been very slow. This is in large part connected to the fact that the components of these pathways are very different in the protozoan parasites, as confirmed by the recently completed genome. For instance, kinetoplastids have no equivalents to the mammalian Class I adenylyl cyclases (ACs) in their genomes nor any of the subunits of the associated G-proteins. The cyclases in kinetoplastid parasites contain a single transmembrane domain, a conserved intracellular catalytic domain and a highly variable extracellular domain - consistent with the expression of multiple receptor-activated cyclases - but no receptor ligands, agonists or antagonists have been identified. Apicomplexan AC and guanylyl cyclase (GC) are even more unusual, potentially being bifunctional, harbouring either a putative ion channel (AC) or a P-type ATPase-like domain (GC) alongside the catalytic region. Phosphodiesterases (PDEs) and cyclic-nucleotide-activated protein kinases are essentially conserved in protozoa, although mostly insensitive to inhibitors of the mammalian proteins. Some of the PDEs have now been validated as promising drug targets. In the following manuscript, we will summarize the existing literature on cAMP and cGMP in protozoa: cyclases, PDEs and cyclic-nucleotide-dependent kinases.