Down-regulation of hypusine biosynthesis in Plasmodium by inhibition of S-adenosyl-methionine-decarboxylase

Aminoquinolines afford resistance to cerebral malaria in susceptible mice

OBJECTIVES

Malaria treatment is impeded by increasing resistance to conventional antimalarial drugs. Here we explored the activity of ten novel benzothiophene, thiophene and benzene aminoquinolines.

METHODS

In vitro testing was performed by the lactate dehydrogenase assay in chloroquine (CQ)-sensitive Plasmodium falciparum strain 3D7 and CQ-resistant (CQ^R) P. falciparum strain Dd2. In vivo activity was evaluated by a modified Thompson test using C57BL/6 mice infected with Plasmodium berghei ANKA strain.

RESULTS

Nine of the ten compounds had a lower 50% inhibitory concentration (IC₅₀) than CQ against the CQ^R strain Dd2. Five of these compounds that were available for in vivo evaluation were shown to be non-toxic. All five compounds administered at a dose of 160mg/kg/day for 3 days prolonged the survival of treated compared with untreated mice. Untreated control mice died by Day 7 with a mean parasitaemia of 15%. Among treated mice, a dichotomous outcome was observed, with a two-third majority of treated mice dying by Day 17 with a low mean parasitaemia of 5%, whilst one-third survived longer with a mean hyperparasitaemia of 70%; specifically, five of these mice survived a mean of 25 days, whilst two even survived past Day 31.

CONCLUSIONS

The significant antimalarial potential of this aminoquinoline series is illustrated by its excellent in vitro activity against the CQ^RP. falciparum strain and significant in vivo activity. Interestingly, compounds ClAQ7, ClAQ9 and ClAQ11 were able to confer resistance to cerebral malaria and afford a switch to hyperparasitaemia to mice prone to the neurological syndrome.

Vitamin B6-dependent enzymes in the human malaria parasite Plasmodium falciparum: a druggable target?

Malaria is a deadly infectious disease which affects millions of people each year in tropical regions. There is no effective vaccine available and the treatment is based on drugs which are currently facing an emergence of drug resistance and in this sense the search for new drug targets is indispensable. It is well established that vitamin biosynthetic pathways, such as the vitamin B6 de novo synthesis present in Plasmodium, are excellent drug targets. The active form of vitamin B6, pyridoxal 5-phosphate, is, besides its antioxidative properties, a cofactor for a variety of essential enzymes present in the malaria parasite which includes the ornithine decarboxylase (ODC, synthesis of polyamines), the aspartate aminotransferase (AspAT, involved in the protein biosynthesis), and the serine hydroxymethyltransferase (SHMT, a key enzyme within the folate metabolism).

Pharmacological potential of biogenic amine-polyamine interactions beyond neurotransmission

Histamine, serotonin and dopamine are biogenic amines involved in intercellular communication with multiple effects on human pathophysiology. They are products of two highly homologous enzymes, histidine decarboxylase and l-aromatic amino acid decarboxylase, and transmit their signals through different receptors and signal transduction mechanisms. Polyamines derived from ornithine (putrescine, spermidine and spermine) are mainly involved in intracellular effects related to cell proliferation and death mechanisms. This review summarizes structural and functional evidence for interactions between components of all these amine metabolic and signalling networks (decarboxylases, transporters, oxidases, receptors etc.) at cellular and tissue levels, distinct from nervous and neuroendocrine systems, where the crosstalk among these amine-related components can also have important pathophysiological consequences. The discussion highlights aspects that could help to predict and discuss the effects of intervention strategies.

Unique posttranslational modifications in eukaryotic translation factors and their roles in protozoan parasite viability and pathogenesis

Protozoan parasites are one of the major causes of diseases worldwide. The vector transmitted parasites exhibit complex life cycles involving interactions between humans, protozoa, and arthropods. In order to adapt themselves to the changing microenvironments, they have to undergo complex morphological and metabolic changes. These changes can be brought about by expressing a new pool of proteins in the cell or by modifying the existing repertoire of proteins via posttranslational modifications (PTMs). PTMs involve covalent modification and processing of proteins thereby modulating their functions. Some of these changes may involve PTMs of parasite proteins to help the parasite survive within the host and the vector. Out of many PTMs known, three are unique since they occur only on single proteins: ethanolamine phosphoglycerol (EPG) glutamate, hypusine and diphthamide. These modifications occur on eukaryotic elongation factor 1A (eEF1A), eukaryotic initiation factor 5A (eIF5A) and eukaryotic elongation factor 2 (eEF2), respectively. Interestingly, the proteins carrying these unique modifications are all involved in the elongation steps of translation. Here we review these unique PTMs, which are well conserved in protozoan parasites, and discuss their roles in viability and pathogenesis of parasites. Characterization of these modifications and studying their roles in physiology as well as pathogenesis will provide new insights in parasite biology, which may also help in developing new therapeutic interventions.

Translation initiation factor eIF-5A, the hypusine-containing protein, is phosphorylated on serine and tyrosine and O-glycosylated in Trichomonas vaginalis

The eukaryotic translation factor eIF-5A is highly conserved throughout eukaryotes and undergoes an unusual polyamine-dependent post-translational modification called hypusination. Trichomonas vaginalis has two tveif-5a genes (tveif-5a1 and tveif-5a2), each encoding a 19-kDa protein. In this report, we describe the detection of two forms with different isoelectric points (5.2 and 5.5) that correspond to the precursor and mature TveIF-5A, respectively. In addition, we demonstrated that only the mature form of TveIF-5A is phosphorylated and glycosylated via two-dimensional gel electrophoresis-western blot (2DE-WB) assays using anti-phosphoserine and anti-phosphotyrosine antibodies and the SNA, ConA and MAA lectins. Interestingly, when the protozoa were grown in 1,4-diamino-2-butanone (DAB), an inhibitor of putrescine biosynthesis, and transferred to medium containing exogenous putrescine, a new spot with an isoelectric point of 5.3 was observed, presumably corresponding to a phosphorylated intermediate or deoxyhypusine form. Our data indicate that, in T. vaginalis, phosphorylations and glycosylations are necessary to obtain the mature TveIF-5A, and we confirm the identity of the precursor, intermediate and mature forms of TveIF-5A by mass spectrometry analysis.

Translational control of eIF5A in various diseases

Translational control is a crucial component in the development and progression of different diseases. Translational control may involve selective translation of specific mRNAs, which promote cell proliferation or lead to alterations in translation factor levels and activities. Eukaryotic initiation factor 5A (eIF5A) is the only known protein to contain the unusual amino acid hypusine [N (ε)-(4-amino-2-hydroxybutyl)-lysine], which is formed from the polyamine spermidine by two catalytic steps. eIF5A is involved in translation, elongation and stimulating peptide bond formation. Hypusination of eIF5A is essential for its activity in promoting cell proliferation. Meanwhile, there is evidence that eIF5A is a key protein in the pathogenicity of different diseases, such as diabetes, several human cancers, malaria and HIV-1 infections. Hitherto, the available data suggest that eIF5A has a role of a cell context-dependent function being more proliferative in the case of several human cancers and being involved under stress conditions in diabetes. Secondly, in HIV-1 infections and in diabetes, eIF5A also has a nuclear function by its sequence-specific binding of mRNAs as an mRNA-shuttle in conjunction with nuclear membrane export proteins. This binding may also influence the half-lives of mRNAs or their sequestration. Based on these data, there is a considerable therapeutic interest in eIF5A as a selective target for drug development through inhibition of hypusination.

Putrescine is required for the expression of eif-5a in Trichomonas vaginalis

Recently, we found that Trichomonas vaginalis contains a eukaryotic translation initiation factor 5A (TveIF-5A) with unknown function in this parasite. eIF-5A is the only cellular protein dependent of polyamines to form a hypusine residue, an unusual basic amino acid that is post-translationally formed by modification of a single specific lysine residue in an eIF-5A precursor protein. The purpose of this study was to determine the effect of a putrescine analogue, 1,4-diamino-2-butanone (DAB), on tveif-5a mRNA and TveIF-5A protein expression. TveIF-5A protein expression was reduced by inhibition of putrescine biosynthesis, and tveif-5a mRNA levels were reduced ∼90%, as shown by western blot and immunofluorescence assays. Cycloheximide treatment reduced the amount of mature TveIF-5A protein at 4h and decreased the tveif-5a transcript level at 2h, according to western blot, RT-PCR and qRT-PCR analyses. Actinomycin D treatment showed that the tveif-5a mRNA had half-life of ∼2.5h in DAB-treated parasites. The half-life of tveif-5a mRNA was ∼4.5h under exogenous putrescine conditions. These results suggest that putrescine is required for tveif-5a mRNA stability, and it is necessary for the expression, stability and maturation of TveIF-5A protein.

Binding and inhibition of human spermidine synthase by decarboxylated S-adenosylhomocysteine

Aminopropyltransferases are essential enzymes that form polyamines in eukaryotic and most prokaryotic cells. Spermidine synthase (SpdS) is one of the most well-studied enzymes in this biosynthetic pathway. The enzyme uses decarboxylated S-adenosylmethionine and a short-chain polyamine (putrescine) to make a medium-chain polyamine (spermidine) and 5'-deoxy-5'-methylthioadenosine as a byproduct. Here, we report a new spermidine synthase inhibitor, decarboxylated S-adenosylhomocysteine (dcSAH). The inhibitor was synthesized, and dose-dependent inhibition of human, Thermatoga maritima, and Plasmodium falciparum spermidine synthases, as well as functionally homologous human spermine synthase, was determined. The human SpdS/dcSAH complex structure was determined by X-ray crystallography at 2.0 Å resolution and showed consistent active site positioning and coordination with previously known structures. Isothermal calorimetry binding assays confirmed inhibitor binding to human SpdS with K(d) of 1.1 ± 0.3 μM in the absence of putrescine and 3.2 ± 0.1 μM in the presence of putrescine. These results indicate a potential for further inhibitor development based on the dcSAH scaffold.