Fluorescent tagging of Plasmodium circumsporozoite protein allows imaging of sporozoite formation but blocks egress from oocysts

The Plasmodium circumsporozoite protein

The circumsporozoite protein (CSP) is one of the most studied proteins of the malaria parasite. It is the target of the only licensed malaria vaccines and is essential for sporozoite formation and infectivity. Yet, the mechanisms by which CSP functions and its interactions with other proteins are only beginning to be understood. Here we review the current state of knowledge of CSP structure and function, as sporozoites develop in the mosquito and establish infection in the mammalian host, and outline outstanding questions that need to be addressed.

Unlocking nitrogen compounds' promise against malaria: A comprehensive review

Plasmodium parasites are the primary cause of malaria, leading to high mortality rates, which require clinical attention. Many of the medications used in the treatment have resulted in resistance over time. Artemisinin combination therapy (ACT) has shown significant results for the treatment. However, mutations in the parasite have resulted in resistance, leading to decreased efficiency of the medications that are currently being used. Therefore, there is a critical need to find novel scaffolds that are safe, effective, and of economic advantage. Literature has reported several potent molecules with diverse scaffolds designed, synthesized, and evaluated against different strains of Plasmodium. With this growing list of compounds, it is essential to collect the data in one place to gain a concise overview of the emerging scaffolds in recent years. For this purpose, nitrogen-containing heterocycles such as β-carboline, imidazole, quinazoline, quinoline, thiazole, and thiophene have been highly explored due to their wide biological applications. Besides these, another scaffold, benzodiazepine, which is majorly used as a central nervous system depressant, is emerging as an anti-malarial agent. Hence, this review centers on the latest medication advancements designed to combat malaria, emphasizing special attention to 1,4-benzodiazepines as a novel scaffold for antimalarial drug discovery.

Let it glow: genetically encoded fluorescent reporters in Plasmodium

The use of fluorescent proteins (FPs) in Plasmodium parasites has been key to understand the biology of this obligate intracellular protozoon. FPs like the green fluorescent protein (GFP) enabled to explore protein localization, promoter activity as well as dynamic processes like protein export and endocytosis. Furthermore, FP biosensors have provided detailed information on physiological parameters at the subcellular level, and fluorescent reporter lines greatly extended the malariology toolbox. Still, in order to achieve optimal results, it is crucial to know exactly the properties of the FP of choice and the genetic scenario in which it will be used. This review highlights advantages and disadvantages of available landing sites and promoters that have been successfully applied for the ectopic expression of FPs in Plasmodium berghei and Plasmodium falciparum. Furthermore, the properties of newly developed FPs beyond DsRed and EGFP, in the visualization of cells and cellular structures as well as in the sensing of small molecules are discussed.

Overcoming the egress block of Plasmodium sporozoites expressing fluorescently tagged circumsporozoite protein

Plasmodium sporozoites are the highly motile and invasive forms of the malaria parasite transmitted by mosquitoes. Sporozoites form within oocysts at the midgut wall of the mosquito, egress from oocysts and enter salivary glands prior to transmission. The GPI-anchored major surface protein, the circumsporozoite protein (CSP) is important for Plasmodium sporozoite formation, egress, migration and invasion. To visualize CSP, we previously generated full-length versions of CSP internally tagged with the green fluorescent protein, GFP. However, while these allowed for imaging of sporogony in oocysts, sporozoites failed to egress. Here, we explore different strategies to overcome this block in egress and obtain salivary gland resident sporozoites that express CSP-GFP. Replacing the N-terminal and repeat region with GFP did not allow sporozoite formation. Lowering expression of CSP-GFP at the endogenous locus allowed sporozoite formation but did not overcome egress block. Crossing of CSP-GFP expressing parasites that are blocked in egress with wild-type parasites yielded a small fraction of parasites that entered salivary glands and expressed various levels of CSP-GFP. Expressing CSP-GFP constructs from a silent chromosome region from promoters that are active only post salivary gland invasion yielded normal numbers of fluorescent salivary gland sporozoites, albeit with low levels of fluorescence. We also show that lowering CSP expression by 50% allowed egress from oocysts but not salivary gland entry. In conclusion, Plasmodium berghei parasites with normal CSP expression tolerate a certain level of CSP-GFP without disruption of oocyst egress and salivary gland invasion.

Using qPCR to compare the detection of Plasmodium vivax oocysts and sporozoites in Anopheles farauti mosquitoes between two DNA extraction methods

Background

Currently, the gold standard to assess parasite developmental stages in mosquitoes is light microscopy. Microscopy can miss low-density infections, is time-consuming and not species-specific. Enzyme-linked immunosorbent assay (ELISA) has been the alternative technique to evaluate the infectivity of mosquitoes especially in field studies however it is semi-quantitative. Molecular techniques that have been used to detect the mosquito stages of malaria parasites including P. vivax. Here, we present a quantitative real-time assay (qPCR) that can be used to detect low-density P. vivax oocyst and sporozoite infections while comparing parasites extracted by the conventional DNA extraction and heating methods.

Methods

Colony reared Anopheles farauti mosquitoes were exposed to blood samples collected from infected individuals using a direct membrane feeding assay. The fully fed mosquitoes were kept for 7 and 14 days post-feed before dissection to confirm presence of oocysts and sporozoites. Infected mosquito guts and the salivary glands (with the head and thorax) were stored and DNA was extracted either by heating or by performing conventional column-based DNA extraction. Following DNA extraction the infected samples were subjected to qPCR to detect P. vivax parasites.

Results

DNA extraction of 1 or more oocysts by heating resulted in an overall sensitivity of 78% (57/73) and single oocysts infections were detected with a sensitivity of 82% (15/17) in the heating arm. We observed a 60% (18/30) sensitivity with sporozoites where DNA was extracted using the conventional DNA extraction method. We show that the heating method significantly improved the detection of oocysts over conventional DNA extraction. There was no significant difference in the DNA copy numbers when comparing the detection of oocysts from the conventional DNA extraction versus heating. However, we observed that the DNA copy numbers of the sporozoites detected in the heating arm was significantly higher than in the conventional DNA extraction arm.

Conclusion

We have adapted a qPCR assay which, when coupled with heating to release DNA reduces sample processing time and cost. Direct qPCR after heating will be a useful tool when investigating transmission blocking vaccines or antimalarials or when evaluating field caught mosquitoes for the presence of malaria parasites.

Plasmodium sporozoite disintegration during skin passage limits malaria parasite transmission

During transmission of malaria‐causing parasites from mosquitoes to mammals, Plasmodium sporozoites migrate rapidly in the skin to search for a blood vessel. The high migratory speed and narrow passages taken by the parasites suggest considerable strain on the sporozoites to maintain their shape. Here, we show that the membrane‐associated protein, concavin, is important for the maintenance of the Plasmodium sporozoite shape inside salivary glands of mosquitoes and during migration in the skin. Concavin‐GFP localizes at the cytoplasmic periphery and concavin(−) sporozoites progressively round up upon entry of salivary glands. Rounded concavin(−) sporozoites fail to pass through the narrow salivary ducts and are rarely ejected by mosquitoes, while normally shaped concavin(−) sporozoites are transmitted. Strikingly, motile concavin(−) sporozoites disintegrate while migrating through the skin leading to parasite arrest or death and decreased transmission efficiency. Collectively, we suggest that concavin contributes to cell shape maintenance by riveting the plasma membrane to the subtending inner membrane complex. Interfering with cell shape maintenance pathways might hence provide a new strategy to prevent a malaria infection.

Plasmodium development in Anopheles: a tale of shared resources

Interactions between the Anopheles mosquito vector and Plasmodium parasites shape how malaria is transmitted in endemic regions. The long association of these two organisms has led to evolutionary processes that minimize fitness costs of infection and benefit both players through shared nutrient resources, parasite immune suppression, and mosquito tolerance to infection. In this review we explore recent data describing how Plasmodium falciparum, the deadliest malaria parasite, associates with one of its most important natural mosquito hosts, Anopheles gambiae, and we discuss the implications of these findings for parasite transmission and vector control strategies currently in development.