Screening for potential prophylactics targeting sporozoite motility through the skin

Plasmodium sporozoite shows distinct motility patterns in responses to three-dimensional environments

During malaria infection, Plasmodium sporozoites, the fast-moving stage of the parasite, are injected by a mosquito into the skin of the mammalian host. In the skin, sporozoites need to migrate through the dermal tissue to enter the blood vessel. Sporozoite motility is critical for infection but not well understood. Here, we used collagen hydrogels with tunable fiber structures, as an in vitro model for the skin. After injecting sporozoites into the hydrogel, we analyzed their motility in three-dimension (3D). We found that sporozoites demonstrated chiral motility, in that they mostly follow right-handed helical trajectories. In high-concentration collagen gel, sporozoites have lower instantaneous speed, but exhibit straighter tracks compared to low-concentration collagen gel, which leads to longer net displacement and faster dissemination. Taken together, our study indicates an inner mechanism for sporozoites to adapt to the environment, which could help with their successful exit from the skin tissue.

Imaging as a (pre)clinical tool in parasitology

Imaging of parasites is central to diagnosis of many parasitic diseases and has thus far played an important role in the development of antiparasitic strategies. The development of novel imaging technologies has revolutionized medicine in fields other than parasitology and has also opened up new avenues for the visualization of parasites. Here we review the role imaging technology has played so far in parasitology and how it may spur further advancement. We point out possibilities to improve current microscopy-based diagnostic methods and how to extend them with radiological imaging modalities. We also highlight in vivo tracking of parasites as a readout for efficacy of new antiparasitic strategies and as a source of fundamental insights for rational design.

Assessment of the drugability of initial malaria infection through miniaturized sporozoite assays and high-throughput screening

The sporozoite stages of malaria parasites are the primary cause of infection of the vertebrate host and are targeted by (experimental) vaccines. Yet, little is known about their susceptibility to chemical intervention. Phenotypic high-throughput screens have not been feasible due to a lack of in vitro systems. Here we tested 78 marketed and experimental antimalarial compounds in miniaturized assays addressing sporozoite viability, gliding motility, hepatocyte traversal, and intrahepatocytic schizogony. None potently interfered with sporozoite viability or motility but ten compounds acted at the level of schizogony with IC50s < 100 nM. To identify compounds directly targeting sporozoites, we screened 81,000 compounds from the Global Health Diversity and reFRAME libraries in a sporozoite viability assay using a parasite expressing a luciferase reporter driven by the circumsporozoite promoter. The ionophore gramicidin emerged as the single hit from this screening campaign. Its effect on sporozoite viability translated into reduced gliding motility and an inability of sporozoites to invade human primary hepatocytes and develop into hepatic schizonts. While providing proof of concept for a small molecule sporontocidal mode of action, our combined data indicate that liver schizogony is more accessible to chemical intervention by (candidate) antimalarials.

Rapid and Specific Action of Methylene Blue against Plasmodium Transmission Stages

Methylene blue (MB) is the oldest synthetic anti-infective. Its high potency against asexual and sexual stages of malaria parasites is well documented. This study aimed to investigate possible additional activities of MB in interfering with parasite transmission and determine target stages in Anopheles vectors and humans. MB's transmission-blocking activity was first evaluated by an ex vivo direct membrane feeding assay (DMFA) using Plasmodium falciparum field isolates. To investigate anti-mosquito stage activity, Plasmodium berghei-infected Anopheles stephensi mosquitoes were fed a second blood meal on mice that had been treated with methylene blue, 3, 6- and 15-days after the initial infectious blood meal. Anti-sporozoite and liver stage activities were evaluated in vitro and in vivo via sporozoite invasion and liver stage development assays, respectively. MB exhibited a robust inhibition of P. falciparum transmission in An. gambiae, even when added shortly before the DMFA but only a moderate effect against P. berghei oocyst development. Exposure of mature P. berghei and P. falciparum sporozoites to MB blocked hepatocyte invasion, yet P. berghei liver stage development was unaffected by MB. Our results indicate previously underappreciated rapid specific activities of methylene blue against Plasmodium transmission stages, preventing the establishment of both mosquito midgut and liver infections as the first essential steps in both hosts.

Targeting malaria parasites inside mosquitoes: ecoevolutionary consequences

Proof-of-concept studies demonstrate that antimalarial drugs designed for human treatment can also be applied to mosquitoes to interrupt malaria transmission. Deploying a new control tool is ideally undertaken within a stewardship programme that maximises a drug's lifespan by minimising the risk of resistance evolution and slowing its spread once emerged. We ask: what are the epidemiological and evolutionary consequences of targeting parasites within mosquitoes? Our synthesis argues that targeting parasites inside mosquitoes (i) can be modelled by readily expanding existing epidemiological frameworks; (ii) provides a functionally novel control method that has potential to be more robust to resistance evolution than targeting parasites in humans; and (iii) could extend the lifespan and clinical benefit of antimalarials used exclusively to treat humans.

Screening the Pathogen Box for Inhibition of Plasmodium falciparum Sporozoite Motility Reveals a Critical Role for Kinases in Transmission Stages

As the malaria parasite becomes resistant to every drug that we develop, the identification and development of novel drug candidates are essential. Many studies have screened compounds designed to target the clinically important blood stages. However, if we are to shrink the malaria map, new drugs that block the transmission of the parasite are needed. Sporozoites are the infective stage of the malaria parasite, transmitted to the mammalian host as mosquitoes probe for blood. Sporozoite motility is critical to their ability to exit the inoculation site and establish infection, and drug-like compounds targeting motility are effective at blocking infection in the rodent malaria model. In this study, we established a moderate-throughput motility assay for sporozoites of the human malaria parasite Plasmodium falciparum, enabling us to screen the 400 drug-like compounds from the pathogen box provided by the Medicines for Malaria Venture for their activity. Compounds exhibiting inhibitory effects on P. falciparum sporozoite motility were further assessed for transmission-blocking activity and asexual-stage growth. Five compounds had a significant inhibitory effect on P. falciparum sporozoite motility in the nanomolar range. Using membrane feeding assays, we demonstrate that four of these compounds had inhibitory activity against the transmission of P. falciparum to the mosquito. Interestingly, of the four compounds with inhibitory activity against both transmission stages, three are known kinase inhibitors. Together with a previous study that found that several of these compounds could inhibit asexual blood-stage parasite growth, our findings provide new antimalarial drug candidates that have multistage activity.

Contemporary exploitation of natural products for arthropod-borne pathogen transmission-blocking interventions

An integrated approach to innovatively counter the transmission of various arthropod-borne diseases to humans would benefit from strategies that sustainably limit onward passage of infective life cycle stages of pathogens and parasites to the insect vectors and vice versa. Aiming to accelerate the impetus towards a disease-free world amid the challenges posed by climate change, discovery, mindful exploitation and integration of active natural products in design of pathogen transmission-blocking interventions is of high priority. Herein, we provide a review of natural compounds endowed with blockade potential against transmissible forms of human pathogens reported in the last 2 decades from 2000 to 2021. Finally, we propose various translational strategies that can exploit these pathogen transmission-blocking natural products into design of novel and sustainable disease control interventions. In summary, tapping these compounds will potentially aid in integrated combat mission to reduce disease transmission trends.

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.

Malaria parasites differentially sense environmental elasticity during transmission

Transmission of malaria-causing parasites to and by the mosquito relies on active parasite migration and constitutes bottlenecks in the Plasmodium life cycle. Parasite adaption to the biochemically and physically different environments must hence be a key evolutionary driver for transmission efficiency. To probe how subtle but physiologically relevant changes in environmental elasticity impact parasite migration, we introduce 2D and 3D polyacrylamide gels to study ookinetes, the parasite forms emigrating from the mosquito blood meal and sporozoites, the forms transmitted to the vertebrate host. We show that ookinetes adapt their migratory path but not their speed to environmental elasticity and are motile for over 24 h on soft substrates. In contrast, sporozoites evolved more short-lived rapid gliding motility for rapidly crossing the skin. Strikingly, sporozoites are highly sensitive to substrate elasticity possibly to avoid adhesion to soft endothelial cells on their long way to the liver. Hence, the two migratory stages of Plasmodium evolved different strategies to overcome the physical challenges posed by the respective environments and barriers they encounter.

Comparative intravital imaging of human and rodent malaria sporozoites reveals the skin is not a species-specific barrier

Malaria infection starts with the injection of Plasmodium sporozoites into the host’s skin. Sporozoites are motile and move in the skin to find and enter blood vessels to be carried to the liver. Here, we present the first characterization of P. falciparum sporozoites in vivo, analyzing their motility in mouse skin and human skin xenografts and comparing their motility to two rodent malaria species. These data suggest that in contrast to the liver and blood stages, the skin is not a species‐specific barrier for Plasmodium. Indeed, P. falciparum sporozoites enter blood vessels in mouse skin at similar rates to the rodent malaria parasites. Furthermore, we demonstrate that antibodies targeting sporozoites significantly impact the motility of P. falciparum sporozoites in mouse skin. Though the sporozoite stage is a validated vaccine target, vaccine trials have been hampered by the lack of good animal models for human malaria parasites. Pre‐clinical screening of next‐generation vaccines would be significantly aided by the in vivo platform we describe here, expediting down‐selection of candidates prior to human vaccine trials.

Quantification of wild-type and radiation attenuated Plasmodium falciparum sporozoite motility in human skin

Given the number of global malaria cases and deaths, the need for a vaccine against Plasmodium falciparum (Pf) remains pressing. Administration of live, radiation-attenuated Pf sporozoites can fully protect malaria-naïve individuals. Despite the fact that motility of these attenuated parasites is key to their infectivity and ultimately protective efficacy, sporozoite motility in human tissue (e.g. skin) remains wholly uncharacterized to date. We show that the ability to quantitatively address the complexity of sporozoite motility in human tissue provides an additional tool in the development of attenuated sporozoite vaccines. We imaged Pf movement in the skin of its natural host and compared wild-type and radiation-attenuated GFP-expressing Pf sporozoites. Using custom image analysis software and human skin explants we were able to quantitatively study their key motility features. This head-to-head comparison revealed that radiation attenuation impaired the capacity of sporozoites to vary their movement angle, velocity and direction, promoting less refined movement patterns. Understanding and overcoming these changes in motility will contribute to the development of an efficacious attenuated parasite malaria vaccine.

Targeting Malaria in Flight

In recent work, Paton et al. (2019) report that mosquitoes exposed to atovaquone, a known antimalarial, are resistant to infection by the malaria parasite Plasmodium. Their results may be the foundation for the discovery of a new arsenal of antimalarials.

Regulation of Plasmodium sporozoite motility by formulation components

BACKGROUND

The protective efficacy of the most promising malaria whole-parasite based vaccine candidates critically depends on the parasite's potential to migrate in the human host. Key components of the parasite motility machinery (e.g. adhesive proteins, actin/myosin-based motor, geometrical properties) have been identified, however the regulation of this machinery is an unknown process.

METHODS

In vitro microscopic live imaging of parasites in different formulations was performed and analysed, with the quantitative analysis software SMOOTIn vitro, their motility; their adherence capacity, movement pattern and velocity during forward locomotion.

RESULTS

SMOOTIn vitro enabled the detailed analysis of the regulation of the motility machinery of Plasmodium berghei in response to specific (macro)molecules in the formulation. Albumin acted as an essential supplement to induce parasite attachment and movement. Glucose, salts and other whole serum components further increased the attachment rate and regulated the velocity of the movement.

CONCLUSIONS

Based on the findings can be concluded that a complex interplay of albumin, glucose and certain salts and amino acids regulates parasite motility. Insights in parasite motility regulation by supplements in solution potentially provide a way to optimize the whole-parasite malaria vaccine formulation.

Cyanobacterial peptides as a tour de force in the chemical space of antiparasitic agents

Parasites are scarcely addressed target for antimicrobial peptides despite their big impact in health and global economy. The notion of antimicrobial peptides is frequently associated to the innate immune defense of vertebrates and invertebrate vectors, as the ultimate recipients of the parasite infection. These antiparasite peptides are produced by ribosomal synthesis, with few post-translational modifications, and their diversity come mostly from their amino acid sequence. For many of them permeabilization of the cell membrane of the targeted pathogen is crucial for their microbicidal mechanism. In contrast, cyanobacterial peptides are produced either by ribosomal or non-ribosomal biosynthesis. Quite often, they undergo heavy modifications, such as the inclusion of non-proteinogenic amino acids, lipid acylation, cyclation, N^α-methylation, or heterocyclic rings. Furthermore, the few targets identified for cyanobacterial peptides in parasites are intracellular. Some cyanobacterial antiparasite peptides are active at picomolar concentrations, whereas those from higher eukaryotes usually work in the micromolar range. In all, cyanobacterial peptides are an appealing target to develop new antiparasite therapies and a challenge in the invention of new synthetic methods for peptides. This review aims to provide an updated appraisal of antiparasite cyanobacterial peptides and to establish a side-by -side comparison with those antiparasite peptides from higher eukaryotes.