A highly sensitive, PCR-based method for the detection of Plasmodium falciparum clones in microtiter plates

Genetic factors regulating Plasmodium falciparum gametocytogenesis identified by phenotypic screens

Successful transmission of Plasmodium falciparum from one person to another relies on the complete intraerythrocytic development of non-pathogenic sexual gametocytes infectious for anopheline mosquitoes. Understanding the genetic factors that regulate gametocyte development is vital for identifying transmission-blocking targets in the malaria parasite life cycle. Toward this end, we conducted a forward genetic study to characterize the development of gametocytes from sexual commitment to mature stage V. We described a new analysis pipeline for the piggyBac transposon-based mutagenesis phenotypic screen to identify genes that influence both early and late gametocyte stages. We classified individual mutants that increased or decreased parasite abundance as the hypoproducer and hyperproducer phenotypes, respectively, revealing distinctive temporal genetic factors early and late in the sexual development cycle. The study identifies that disruption in factors involved in transcription, protein trafficking and DNA repair are associated with decreasing gametocyte production, while modifications in phosphatase activity are linked to hyperproduction of gametocytes. Our study provides an optimized approach on genotype-phenotype evaluation, offering a new resource for understanding potential targets for therapeutic intervention strategies to disrupt transmission.

Cultivation of Asexual Intraerythrocytic Stages of Plasmodium falciparum

Successfully developed in 1976, the continuous in vitro culture of Plasmodium falciparum has many applications in the field of malaria research. It has become an important experimental model that directly uses a human pathogen responsible for a high prevalence of morbidity and mortality in many parts of the world and is a major source of biological material for immunological, biochemical, molecular, and pharmacological studies. Until present, the basic techniques described by Trager and Jensen and Haynes et al. remain unchanged in many malaria research laboratories. Nonetheless, different factors, including culture media, buffers, serum substitutes and supplements, sources of erythrocytes, and conditions of incubation (especially oxygen concentration), have been modified by different investigators to adapt the original technique in their laboratories or enhance the in vitro growth of the parasites. The possible effects and benefits of these modifications for the continuous cultivation of asexual intraerythrocytic stages of P. falciparum, as well as future challenges in developing a serum-free cultivation system and axenic cultures, are discussed.

High-throughput screening platform identifies small molecules that prevent sequestration of Plasmodium falciparum-infected erythrocytes

BACKGROUND

We developed a 2-step approach to screen molecules that prevent and/or reverse Plasmodium falciparum-infected erythrocyte (IE) binding to host receptors. IE adhesion and sequestration in vasculature causes severe malaria, and therefore antiadhesion therapy might be useful as adjunctive treatment. IE adhesion is mediated by the polymorphic family (approximately 60 members) of P. falciparum EMP1 (PfEMP1) multidomain proteins.

METHODS

We constructed sets of PfEMP1 domains that bind ICAM-1, CSA, or CD36, receptors that commonly support IE binding. Combinations of domain-coated beads were assayed by Bio-Plex technology as a high-throughput molecular platform to screen antiadhesion molecules (antibodies and small molecules). Molecules identified as so-called hits in the screen (first step) then could be assayed individually for inhibition of binding of live IE to receptors (second step).

RESULTS

In proof-of-principle studies, the antiadhesion activity of several antibodies was concordant in Bio-Plex and live IE assays. Using this 2-step approach, we identified several molecules in a small molecule library of 10 000 compounds that could inhibit and reverse binding of IEs to ICAM-1 and CSA receptors.

CONCLUSION

This 2-step screening approach should be efficient for identification of antiadhesion drug candidates for falciparum malaria.

Molecular detection of malaria in South punjab with higher proportion of mixed infections

BACKGROUND

Malaria is well known for its fatalities worldwide, Plasmodium vivax and the Plasmodium falciparum are the two important species of malaria reported from Pakistan and creating lots of morbidities across the country.

METHOD

Study was conducted to determine the Surveillance of malaria in South Punjab by microscopy and Polymerase chain reaction (PCR).

RESULT

samples out of 100 patients were found positive for malarial parasites. One patient was found with mixed infection, whereas P. falciparum and P. vivax infections were detected in 17 and 22 patients, respectively. In nested PCR, genus-specific primers for Plasmodium species. in round 1 and species-specific primers for P. falciparum and P. vivax in round 2 were used. By the application of PCR 41% were found to be infected by Plasmodium spp. Among Plasmodium positive patients: mixed, P. falciparum and P. vivax infection were detected in 10, 15 and 16 patients, respectively. Thirty nine microscopically positive patients confirmed to have Plasmodium spp. One negative by PCR, 2 microscopically negative patients had shown Plasmodium spp. infection (P. falciparum and P. vivax) by PCR. In total samples, P. falciparum, P. vivax and mixed infection accounted for 36.6%, 39.0% and 24.3%, respectively.

CONCLUSION

Microscopy was found deficient for interpretation of mixed infections, low parasitaemia, and species specific diagnosis. The sensitivity, specificity and efficacy of nested PCR was calculated 95%, 98% and 97%, respectively, showing PCR as a more effective and efficient diagnostic tool for malaria.

Expression profile of the Plasmodium falciparum intra-erythrocytic stage protein, PF3D7_1363700

Background

Efforts to control malaria are demanding due to drug-resistant parasites, insecticide-resistant mosquitoes and poor health infrastructure in malaria-endemic countries. Therefore, the research and development of additional malaria control methods are crucial. For host-parasite interactions, surface antigens and secreted proteins are likely to be involved in infectivity and invasion of host tissues and therefore can be effective targets for control by vaccines, drug therapy, or novel mosquito control methods. In an effort to identify and characterize genes that may have a role in host-parasite interaction, this study describes the expression profile of Plasmodium falciparum PF3D7_1363700.

Methods

A P. falciparum gene, PF3D7_1363700, was identified by a search of the annotated Plasmodium genome database. Protein alignments of PF3D7_1363700 orthologues from various Plasmodium species were performed to demonstrate protein similarity. Transcript expression profiles of PF3D7_1363700 were determined via reverse-transcriptase PCR and protein expression was investigated by immunofluorescence assays, western blot analysis and green fluorescent trafficking studies.

Results

The PF3D7_1363700 protein demonstrates significant similarity with orthologues in other Plasmodium species and appears to be unique to Apicomplexans. The PF3D7_1363700 transcription profile demonstrated expression during the intra-erythrocytic, oocyst sporozoite, and salivary gland sporozoite stages while the PF3D7_1363700 protein was only detected during the intra-erythrocytic stages.

Conclusions

This research utilized an in silico approach to identify a well-conserved protein known as PF3D7_1363700. By molecular, biochemical and cellular analyses, PF3D7_1363700 was discovered to be an intra-erythrocytic-specific stage protein that is unique to Apicomplexans.

Functional analysis of Plasmodium vivax dihydrofolate reductase-thymidylate synthase genes through stable transformation of Plasmodium falciparum

Mechanisms of drug resistance in Plasmodium vivax have been difficult to study partially because of the difficulties in culturing the parasite in vitro. This hampers monitoring drug resistance and research to develop or evaluate new drugs. There is an urgent need for a novel method to study mechanisms of P. vivax drug resistance. In this paper we report the development and application of the first Plasmodium falciparum expression system to stably express P. vivax dhfr-ts alleles. We used the piggyBac transposition system for the rapid integration of wild-type, single mutant (117N) and quadruple mutant (57L/58R/61M/117T) pvdhfr-ts alleles into the P. falciparum genome. The majority (81%) of the integrations occurred in non-coding regions of the genome; however, the levels of pvdhfr transcription driven by the P. falciparum dhfr promoter were not different between integrants of non-coding and coding regions. The integrated quadruple pvdhfr mutant allele was much less susceptible to antifolates than the wild-type and single mutant pvdhfr alleles. The resistance phenotype was stable without drug pressure. All the integrated clones were susceptible to the novel antifolate JPC-2067. Therefore, the piggyBac expression system provides a novel and important tool to investigate drug resistance mechanisms and gene functions in P. vivax.

A high-throughput method to detect Plasmodium falciparum clones in limiting dilution microplates

Background

Molecular and cellular studies of Plasmodium falciparum require cloning of parasites by limiting dilution cultivation, typically performed in microplates. The parasite's slow replication rate combined with laborious methods for identification of positive wells has limited these studies. A new high-throughput method for detecting growth without compromising parasite viability is reported.

Methods

In vitro parasite cultivation is associated with extracellular acidification. A survey of fluorescent pH indicators identified 5-(and-6)-carboxy SNARF-1 as a membrane-impermeant dye with a suitable pK_a value. Conditions for facile detection of viable parasites in 96-well microplates were optimized and used for limiting dilution cloning of genetic cross progeny and transfected parasites.

Results

5-(and-6)-carboxy SNARF-1 is a two-emission wavelength dye that accurately reported extracellular pH in parasite cultures. It readily detected parasite growth in microplate wells and yielded results comparable to labour-intensive examination of Giemsa-stained smears. The dye is non-toxic, allowing parasite detection without transfer of culture material to additional plates for separate assays. This dye was used with high-throughput limiting dilution culture to generate additional progeny clones from the HB3 × Dd2 genetic cross.

Conclusions

This fluorescence-based assay represents a low-cost, efficient method for detection of viable parasites in microplate wells; it can be easily expanded by automation.

An improved method for undertaking limiting dilution assays for in vitro cloning of Plasmodium falciparum parasites

Background

Obtaining single parasite clones is required for many techniques in malaria research. Cloning by limiting dilution using microscopy-based assessment for parasite growth is an arduous and labor-intensive process. An alternative method for the detection of parasite growth in limiting dilution assays is using a commercial ELISA histidine-rich protein II (HRP2) detection kit.

Methods

Detection of parasite growth was undertaken using HRP2 ELISA and compared to thick film microscopy. An HRP2 protein standard was used to determine the detection threshold of the HRP2 ELISA assay, and a HRP2 release model was used to extrapolate the amount of parasite growth required for a positive result.

Results

The HRP2 ELISA was more sensitive than microscopy for detecting parasite growth. The minimum level of HRP2 protein detection of the ELISA was 0.11ng/ml. Modeling of HRP2 release determined that 2,116 parasites are required to complete a full erythrocytic cycle to produce sufficient HRP2 to be detected by the ELISA. Under standard culture conditions this number of parasites is likely to be reached between 8 to 14 days of culture.

Conclusions

This method provides an accurate and simple way for the detection of parasite growth in limiting dilution assays, reducing time and resources required in traditional methods. Furthermore the method uses spent culture media instead of the parasite-infected red blood cells, enabling culture to continue.