Expression of Plasmodium falciparum G6PD-6PGL in laboratory parasites and in patient isolates in G6PD-deficient and normal Nigerian children

Giardia lamblia G6PD::6PGL Fused Protein Inhibitors Decrease Trophozoite Viability: A New Alternative against Giardiasis

Treatments to combat giardiasis have been reported to have several drawbacks, partly due to the drug resistance and toxicity of current antiparasitic agents. These constraints have prompted many researchers to investigate new drugs that act against protozoan parasites. Enzyme inhibition is an important means of regulating pathogen metabolism and has recently been identified as a significant alternative target in the search for new treatments. Glucose-6-phosphate dehydrogenase and 6-phosphogluconolactonase (G6PD::6PGL) is a bifunctional enzyme involved in the pentose phosphate pathway (PPP) in Giardia lamblia (G. lamblia). The G. lamblia enzyme is unusual since, unlike the human enzyme, it is a fused enzyme. Here, we show, through inhibition assays, that an in-house chemical library of 120 compounds and four target compounds, named CNZ-7, CNZ-8, CMC-1, and FLP-2, are potent inhibitors of the G. lamblia G6PD::6PGL fused enzyme. With a constant (k2) of 2.3, 3.2, and 2.8 M−1 s−1, respectively, they provoke alterations in the secondary and tertiary protein structure and global stability. As a novel approach, target compounds show antigiardial activity, with IC50 values of 8.7, 15.2, 15.3, and 24.1 µM in trophozoites from G. lamblia. Moreover, these compounds show selectivity against G. lamblia, since, through counter-screening in Caco-2 and HT29 human cells, they were found to have low toxicity. This finding positions these compounds as a potential and attractive starting point for new antigiardial drugs.

Glucose-6-phosphate dehydrogenase deficiency and reduced haemoglobin levels in African children with severe malaria

BACKGROUND

Extensive studies investigating the role of host genetic factors during malaria associate glucose-6-phosphate dehydrogenase deficiency with relative protection. G6PD deficiency had been reported to associate with anti-malarial drug induced with haemolytic anaemia.

METHODS

A total of 301 Gabonese, Ghanaian, and Kenyan children aged 6-120 months with severe malaria recruited in a multicentre trial on artesunate were included in this sub-study. G6PD normal (type B), heterozygous (type A(+)) and deficient (type A(-)) genotypes were determined by direct sequencing of the common African mutations G202A and A376G. Furthermore, multivariate analyses were executed to associate possible contributions of G6PD deficiency with baseline haemoglobin levels, parasitaemia and with severe malarial anaemia.

RESULTS

Two hundred and seventy-eight children (132 females and 146 males) were successfully genotyped for G6PD variants. The overall prevalence of G6PD deficiency was 13 % [36/278; 3 % (4/132) female homozygous and 22 % (32/146) male hemizygous], 14 % (40/278) children were female heterozygous while 73 % (202/278) were G6PD normal [67 % (88/132) females and 78 % (114/146) males] individuals. Multivariate regression revealed a significant association of moderately and severely deficient G6PD genotypes with haemoglobin levels according to the baseline data (p < 0.0001; G6PD heterozygous: p < 0.0001; G6PD deficient: p = 0.009), but not with severe malarial anaemia (p = 0.66). No association of G6PD genotypes with baseline parasitaemia.

CONCLUSIONS

In this study, moderately (type A(+)) and severely (type A(-)) G6PD deficiency showed significant association with lower haemoglobin concentrations at baseline in African children with severe malaria without leading to severe malarial anaemia. In addition, there was no association of G6PD variant types with parasite densities on admission.

African glucose-6-phosphate dehydrogenase alleles associated with protection from severe malaria in heterozygous females in Tanzania

X-linked Glucose-6-phosphate dehydrogenase (G6PD) A- deficiency is prevalent in sub-Saharan Africa populations, and has been associated with protection from severe malaria. Whether females and/or males are protected by G6PD deficiency is uncertain, due in part to G6PD and malaria phenotypic complexity and misclassification. Almost all large association studies have genotyped a limited number of G6PD SNPs (e.g. G6PD202 / G6PD376), and this approach has been too blunt to capture the complete epidemiological picture. Here we have identified 68 G6PD polymorphisms and analysed 29 of these (i.e. those with a minor allele frequency greater than 1%) in 983 severe malaria cases and controls in Tanzania. We establish, across a number of SNPs including G6PD376, that only female heterozygotes are protected from severe malaria. Haplotype analysis reveals the G6PD locus to be under balancing selection, suggesting a mechanism of protection relying on alleles at modest frequency and avoiding fixation, where protection provided by G6PD deficiency against severe malaria is offset by increased risk of life-threatening complications. Our study also demonstrates that the much-needed large-scale studies of severe malaria and G6PD enzymatic function across African populations require the identification and analysis of the full repertoire of G6PD genetic markers.

Glucose-6-phosphate dehydrogenase (G6PD) is an essential enzyme that protects red blood cells from oxidative damage. Numerous genetic variants of G6PD, residing in the X chromosome, are found among African populations: mutations causing A- deficiency can lead to serious clinical outcomes (including hemolytic anemia) but also confer protection against severe malaria. Epidemiological studies have used some of the genetic markers that cause A- deficiency to establish who is protected from severe malaria, with differing results. Whether females, with one or two copies of mutant genes, males with one copy, or both genders are protected is uncertain. This uncertainty is due to G6PD and malaria phenotypic complexity and misclassification, and to genetic differences between populations and the limited numbers of genetic markers (usually 2) considered. In this study we analysed more than 30 G6PD genetic markers in 506 Tanzanian children with severe malaria and 477 without malaria. We found that only females with one normal and one mutant copy of the gene (heterozygotes) were protected from severe malaria. Further, we established that the G6PD gene is under evolutionary pressure with the likely mechanism being selection by malaria. Our work demonstrates that studies of severe malaria and G6PD enzymatic function across African populations require, in addition to complete and accurate G6PD phenotypic classification, the identification and analysis of the full repertoire of G6PD genetic markers.

Mechanisms of genetically-based resistance to malaria

Malaria remains one of the most prevalent parasitoses worldwide. About 350 to 500 million febrile episodes are observed yearly in African children alone and more than 1 million people die because of malaria each year. Multiple factors have hampered the effective control of this disease, some of which include the complex biology of the Plasmodium parasites, their high polymorphism and their increasingly high resistance to antimalarial drugs, mainly in endemic regions. The ancient interaction between malarial parasites and humans has led to the fixation in the population of several inherited alterations conferring protection against malaria. Some of the mechanisms underlying protection against this disease are described in this review for hemoglobin-inherited disorders (thalassemia, sickle-cell trait, HbC and HbE), erythrocyte polymorphisms (ovalocytosis and Duffy blood group), enzymopathies (G6PD deficiency and PK deficiency) and immunogenetic variants (HLA alleles, complement receptor 1, NOS2, tumor necrosis factor-α promoter and chromosome 5q31-q33 polymorphisms).

Transient silencing of Plasmodium falciparum bifunctional glucose-6-phosphate dehydrogenase- 6-phosphogluconolactonase

The bifunctional enzyme glucose-6-phosphate dehydrogenase-6-phosphogluconolactonase (G6PD-6PGL) found in Plasmodium falciparum has unique structural and functional characteristics restricted to this genus. This study was designed to examine the effects of RNA-mediated PfG6PD-6PGL gene silencing in cultures of P. falciparum on the expression of parasite antioxidant defense genes at the transcription level. The highest degree of G6PD-6PGL silencing achieved was 86% at the mRNA level, with a recovery to almost normal levels within 24 h, indicating only transient diminished expression of the PfG6PD-6PGL gene. PfG6PD-6PGL silencing caused arrest of the trophozoite stage and enhanced gametocyte formation. In addition, an immediate transcriptional response was shown by thioredoxin reductase suggesting that P. falciparum G6PD-6PGL plays a physiological role in the specific response of the parasite to intracellullar oxidative stress. P. falciparum transfection with an empty DNA vector also promoted intracellular stress, as determined by mRNA up-regulation of antioxidant genes. Collectively, our findings point to an important role for this enzyme in the parasite's infection cycle. The different characteristics of G6PD-6PGL with respect to its homologue in the host make it an ideal target for therapeutic strategies.

Impact of red blood cell polymorphisms on the antibody response to Plasmodium falciparum in Senegal

The evidence of protection afforded by red blood cell polymorphisms against either clinical malaria or Plasmodium falciparum blood levels varies with the study site and the type of malaria transmission. Nevertheless, no clear implication of an antibody-related effect has yet been established in the protection related to red blood cell polymorphisms. We performed a prospective study, where plasma IgG and IgG subclasses directed to recombinant proteins from the merozoite surface protein 2 (MSP2/3D7 and MSP2/FC27) and the ring-infected erythrocyte surface antigen (RESA) were determined in a cohort of 413 Senegalese children before the annual malaria transmission season. The antibody response was dependent on age, and to a lesser extent, on the village of residence. IgG3 responders to all proteins, IgG responders to RESA and MSP2/3D7, as well as IgG2 to RESA and IgG1 responders to MSP2/3D7, presented enhanced mean values of parasite density, as evaluated during an 18-month follow-up. The levels of IgG and IgG3 to MSP2/3D7 were negatively associated with the risk of occurrence of a malaria attack during the following transmission season. Compared to normal children, sickle cell trait carriers presented lower levels of IgG to MSP2/3D7. Similarly, G6PD A- girls had lower levels of IgG and IgG3 to MSP2/FC27 than did G6PD normal girls. The impact of these particular genetic polymorphisms on the modulation of the antibody response is discussed.

Data mining of the transcriptome of Plasmodium falciparum: the pentose phosphate pathway and ancillary processes

The general paradigm that emerges from the analysis of the transcriptome of the malaria parasite Plasmodium falciparum is that the expression clusters of genes that code for enzymes engaged in the same cellular function is coordinated. Here the consistency of this perception is examined by analysing specific pathways that metabolically-linked. The pentose phosphate pathway (PPP) is a fundamental element of cell biochemistry since it is the major pathway for the recycling of NADP+ to NADPH and for the production of ribose-5-phosphate that is needed for the synthesis of nucleotides. The function of PPP depends on the synthesis of NADP+ and thiamine pyrophosphate, a co-enzyme of the PPP enzyme transketolase. In this essay, the transcription of gene coding for enzymes involved in the PPP, thiamine and NAD(P)+ syntheses are analysed. The genes coding for two essential enzymes in these pathways, transaldolase and NAD+ kinase could not be found in the genome of P. falciparum. It is found that the transcription of the genes of each pathway is not always coordinated and there is usually a gene whose transcription sets the latest time for the full deployment of the pathway's activity. The activity of PPP seems to involve only the oxidative arm of PPP that is geared for maximal NADP+ reduction and ribose-5-phosphate production during the early stages of parasite development. The synthesis of thiamine diphosphate is predicted to occur much later than the expression of transketolase. Later in the parasite cycle, the non-oxidative arm of PPP that can use fructose-6-phosphate and glyceraldehyde-3-phosphate supplied by glycolysis, becomes fully deployed allowing to maximize the production of ribose-5-phosphate. These discrepancies require direct biochemical investigations to test the activities of the various enzymes in the developing parasite. Notably, several transcripts of PPP enzyme-coding genes display biphasic pattern of transcription unlike most transcripts that peak only once during the parasite cycle. The physiological meaning of this pattern requires further investigation.