Singular features of trypanosomatids' phosphotransferases involved in cell energy management

In silico prediction of the metabolism of Blastocrithidia nonstop, a trypanosomatid with non-canonical genetic code

BACKGROUND

Almost all extant organisms use the same, so-called canonical, genetic code with departures from it being very rare. Even more exceptional are the instances when a eukaryote with non-canonical code can be easily cultivated and has its whole genome and transcriptome sequenced. This is the case of Blastocrithidia nonstop, a trypanosomatid flagellate that reassigned all three stop codons to encode amino acids.

RESULTS

We in silico predicted the metabolism of B. nonstop and compared it with that of the well-studied human parasites Trypanosoma brucei and Leishmania major. The mapped mitochondrial, glycosomal and cytosolic metabolism contains all typical features of these diverse and important parasites. We also provided experimental validation for some of the predicted observations, concerning, specifically presence of glycosomes, cellular respiration, and assembly of the respiratory complexes.

CONCLUSIONS

In an unusual comparison of metabolism between a parasitic protist with a massively altered genetic code and its close relatives that rely on a canonical code we showed that the dramatic differences on the level of nucleic acids do not seem to be reflected in the metabolisms. Moreover, although the genome of B. nonstop is extremely AT-rich, we could not find any alterations of its pyrimidine synthesis pathway when compared to other trypanosomatids. Hence, we conclude that the dramatic alteration of the genetic code of B. nonstop has no significant repercussions on the metabolism of this flagellate.

Released Parasite-Derived Kinases as Novel Targets for Antiparasitic Therapies

The efficient manipulation of their host cell is an essential feature of intracellular parasites. Most molecular mechanisms governing the subversion of host cell by protozoan parasites involve the release of parasite-derived molecules into the host cell cytoplasm and direct interaction with host proteins. Among these released proteins, kinases are particularly important as they govern the subversion of important host pathways, such as signalling or metabolic pathways. These enzymes, which catalyse the transfer of a phosphate group from ATP onto serine, threonine, tyrosine or histidine residues to covalently modify proteins, are involved in numerous essential biological processes such as cell cycle or transport. Although little is known about the role of most of the released parasite-derived kinases in the host cell, they are examples of kinases hijacking host cellular pathways such as signal transduction or apoptosis, which are essential for immune response evasion as well as parasite survival and development. Here we present the current knowledge on released protozoan kinases and their involvement in host-pathogen interactions. We also highlight the knowledge gaps remaining before considering those kinases - involved in host signalling subversion - as antiparasitic drug targets.

Apoferritin and Apoferritin-Capped Metal Nanoparticles Inhibit Arginine Kinase of Trypanosoma brucei

The aim of this study was to explore the inhibitory potential of apoferritin or apoferritin-capped metal nanoparticles (silver, gold and platinum) against Trypanosomabrucei arginine kinase. The arginine kinase activity was determined in the presence and absence of apoferritin or apoferritin-capped metal nanoparticles. In addition, kinetic parameters and relative inhibition of enzyme activity were estimated. Apoferritin or apoferritin-capped metal nanoparticles’ interaction with arginine kinase of T. brucei led to a >70% reduction in the enzyme activity. Further analysis to determine kinetic parameters suggests a mixed inhibition by apoferritin or apoferritin-nanoparticles, with a decrease in V_max. Furthermore, the K_m of the enzyme increased for both ATP and L-arginine substrates. Meantime, the inhibition constant (K_i) values for the apoferritin and apoferritin-nanoparticle interaction were in the submicromolar concentration ranging between 0.062 to 0.168 nM and 0.001 to 0.057 nM, respectively, for both substrates (i.e., L-arginine and ATP). Further kinetic analyses are warranted to aid the development of these nanoparticles as selective therapeutics. Also, more studies are required to elucidate the binding properties of these nanoparticles to arginine kinase of T. brucei.

Leishmania donovani adenylate kinase 2a prevents ATP-mediated cell cytolysis in macrophages

In Leishmania spp. ATP utilizing enzymes serves as a key role in preserving integrity of host cells for survival of parasite. Earlier reports suggested that Adenylate kinase (AK) a phosphotransferase enzyme released by Leishmania donovani secretome, involved in modulating levels of NTPs. In the present study, we cloned, expressed and characterized recombinant putative AK. Based on a sequence and phylogeny analysis, we identified the prominent features of the seven AK isoforms of Leishmania donovani and assigned our putative AK as LdAK2a. The K_m value of LdAK2a for ATP and AMP substrate were 204 μM and 184 μM, respectively and V_max was calculated as 1.6 μmol min^-1 mg^-1 protein. Ap5A, a known inhibitor of AK inhibited LdAK2a with estimated K_i values of 280 nM and 230 nM for ATP and AMP respectively. CD spectral studies were carried out to estimate its structural stability. Recombinant LdAK2a was found to prevent ATP mediated cell cytolysis of Raw 264.7 macrophages in vitro, which was determined by LDH assay and MMP assay. This is the first report which validates that Leishmanial AK2a can prevent ATP mediated cytolysis of macrophage cells and thereby probably play a role in preserving integrity of host cells for survival of parasite.

Structure and function of Per-ARNT-Sim domains and their possible role in the life-cycle biology of Trypanosoma cruzi

Per-ARNT-Sim (PAS) domains of proteins play important roles as modules for signalling and cellular regulation processes in widely diverse organisms such as Archaea, Bacteria, protists, plants, yeasts, insects and vertebrates. These domains are present in many proteins where they are used as sensors of stimuli and modules for protein interactions. Characteristically, they can bind a broad spectrum of molecules. Such binding causes the domain to trigger a specific cellular response or to make the protein containing the domain susceptible to responding to additional physical or chemical signals. Different PAS proteins have the ability to sense redox potential, light, oxygen, energy levels, carboxylic acids, fatty acids and several other stimuli. Such proteins have been found to be involved in cellular processes such as development, virulence, sporulation, adaptation to hypoxia, circadian cycle, metabolism and gene regulation and expression. Our analysis of the genome of different kinetoplastid species revealed the presence of PAS domains also in different predicted kinases from these protists. Open-reading frames coding for these PAS-kinases are unusually large. In addition, the products of these genes appear to contain in their structure combinations of domains uncommon in other eukaryotes. The physiological significance of PAS domains in these parasites, specifically in Trypanosoma cruzi, is discussed.

Crystal structure and biophysical characterization of the nucleoside diphosphate kinase from Leishmania braziliensis

BACKGROUND

Nucleoside diphosphate kinase (NDK) is a housekeeping enzyme that plays key roles in nucleotide recycling and homeostasis in trypanosomatids. It is also secreted by the intracellular parasite Leishmania to modulate the host response. These functions make NDK an attractive target for drug design and for studies aiming at a better understanding of the mechanisms mediating host-pathogen interactions.

RESULTS

We report the crystal structure and biophysical characterization of the NDK from Leishmania braziliensis (LbNDK). The subunit consists of six α-helices along with a core of four β-strands arranged in a β2β3β1β4 antiparallel topology order. In contrast to the NDK from L. major, the LbNDK C-terminal extension is partially unfolded. SAXS data showed that LbNDK forms hexamers in solution in the pH range from 7.0 to 4.0, a hydrodynamic behavior conserved in most eukaryotic NDKs. However, DSF assays show that acidification and alkalization decrease the hexamer stability.

CONCLUSIONS

Our results support that LbNDK remains hexameric in pH conditions akin to that faced by this enzyme when secreted by Leishmania amastigotes in the parasitophorous vacuoles (pH 4.7 to 5.3). The unusual unfolded conformation of LbNDK C-terminus decreases the surface buried in the trimer interface exposing new regions that might be explored for the development of compounds designed to disturb enzyme oligomerization, which may impair the important nucleotide salvage pathway in these parasites.

Interaction of metal nanoparticles with recombinant arginine kinase from Trypanosoma brucei: thermodynamic and spectrofluorimetric evaluation

BACKGROUND

Trypanosoma brucei, responsible for African sleeping sickness, is a lethal parasite against which there is need for new drug protocols. It is therefore relevant to attack possible biomedical targets with specific preparations and since arginine kinase does not occur in humans but is present in the parasite it becomes a suitable target.

METHODS

Fluorescence quenching, thermodynamic analysis and FRET have shown that arginine kinase from T. brucei interacted with silver or gold nanoparticles.

RESULTS

The enzyme only had one binding site. At 25°C the dissociation (Kd) and Stern-Volmer constants (KSV) were 15.2nM, 0.058nM(-1) [Ag]; and 43.5nM, 0.052nM(-1) [Au] and these decreased to 11.2nM, 0.041nM(-1) [Ag]; and 24.2nM, 0.039nM(-1) [Au] at 30°C illustrating static quenching and the formation of a non-fluorescent fluorophore-nanoparticle complex. Silver nanoparticles bound to arginine kinase with greater affinity, enhanced fluorescence quenching and easier access to tryptophan molecules than gold. Negative ΔH and ΔG values implied that the interaction of both Ag and Au nanoparticles with arginine kinase was spontaneous with electrostatic forces. FRET confirmed that the nanoparticles were bound 2.11nm [Ag] and 2.26nm [Au] from a single surface tryptophan residue.

CONCLUSIONS

The nanoparticles bind close to the arginine substrate through a cysteine residue that controls the electrophilic and nucleophilic characters of the substrate arginine-guanidinium group crucial for enzymatic phosphoryl transfer between ADP and ATP.

GENERAL SIGNIFICANCE

The nanoparticles of silver and gold interact with arginine kinase from T. brucei and may prove to have far reaching consequences in clinical trials.

The phosphoarginine energy-buffering system of trypanosoma brucei involves multiple arginine kinase isoforms with different subcellular locations

Phosphagen energy-buffering systems play an essential role in regulating the cellular energy homeostasis in periods of high-energy demand or energy supply fluctuations. Here we describe the phosphoarginine/arginine kinase system of the kinetoplastid parasite Trypanosoma brucei, consisting of three highly similar arginine kinase isoforms (TbAK1-3). Immunofluorescence microscopy using myc-tagged protein versions revealed that each isoform is located in a specific subcellular compartment: TbAK1 is exclusively found in the flagellum, TbAK2 in the glycosome, and TbAK3 in the cytosol of T. brucei. The flagellar location of TbAK1 is dependent on a 22 amino acid long N-terminal sequence, which is sufficient for targeting a GFP-fusion protein to the trypanosome flagellum. The glycosomal location of TbAK2 is in agreement with the presence of a conserved peroxisomal targeting signal, the C-terminal tripeptide ‘SNL’. TbAK3 lacks any apparent targeting sequences and is accordingly located in the cytosol of the parasite. Northern blot analysis indicated that each TbAK isoform is differentially expressed in bloodstream and procyclic forms of T. brucei, while the total cellular arginine kinase activity was 3-fold higher in bloodstream form trypanosomes. These results suggest a substantial change in the temporal and spatial energy requirements during parasite differentiation. Increased arginine kinase activity improved growth of procyclic form T. brucei during oxidative challenges with hydrogen peroxide. Elimination of the total cellular arginine kinase activity by RNA interference significantly decreased growth (>90%) of procyclic form T. brucei under standard culture conditions and was lethal for this life cycle stage in the presence of hydrogen peroxide. The putative physiological roles of the different TbAK isoforms in T. brucei are further discussed.