ABSTRACT
First Person is a series of interviews with the first authors of a selection of papers published in Disease Models & Mechanisms, helping researchers promote themselves alongside their papers. Sabrina Yahiya is first author on ‘ A novel class of sulphonamides potently block malaria transmission by targeting a Plasmodium vacuole membrane protein’, published in DMM. Sabrina conducted the research described in this article while a PhD candidate in Prof. Jake Baum's lab at Imperial College London, UK. She is now a scientist in the Infectious Diseases & Vaccines Group at Kymab, a Sanofi company, in Cambridge, UK, and is interested in the development of therapeutics targeting infectious diseases, with a focus on blocking malaria transmission.
Sabrina Yahiya
How would you explain the main findings of your paper to non-scientific family and friends?
Despite the gains made in malaria control, a staggering 619,000 deaths were attributed to the disease in 2021, as reported by the World Health Organization (World Health Organization, 2022). Plasmodium parasites, the causative agent of malaria, follow a characteristically complex life cycle in which the unicellular parasites morph in adaptation to both mosquito and mammalian hosts. Whilst small-molecule drugs remain the frontline approach to treating malaria cases, existing antimalarials are now threatened by resistance. Frontline antimalarials largely target the symptomatic stages that invade and replicate asexually in mammalian red blood cells, the asexual blood stages. However, of the many stages of the Plasmodium life cycle, a subset of the symptomatic asexual stage parasites commits to sexual differentiation, forming the male and female gametocytes. Whilst gametocytes do not contribute to disease symptoms, they are solely responsible for transmission to the mosquito vector and the subsequent spread of malaria amongst the uninfected population.
Here, our work follows from a drug screen previously performed in our lab at Imperial College London (Delves et al., 2018), which discovered a novel series of potent transmission-blocking N-[(4-hydroxychroman-4-yl)methyl]-sulphonamide (N-4HCS) antimalarials. The N-4HCS compounds specifically prevent formation of male gametes from gametocytes (microgametogenesis) in the mosquito gut following transmission. Following from hit-to-lead optimisation (Rueda-Zubiaurre et al., 2019), our study focuses on the mode by which these molecules act during microgametogenesis in Plasmodium falciparum, the most virulent of the human-infecting Plasmodium species. We first aimed to uncover the gametocyte proteins that interact with the N-4HCS compounds using chemical proteomics, and present evidence that Pfs16, a parasite membrane protein, is the likely target of these compounds. Using a range of techniques, we sought to identify how N-4HCS treatment perturbs gametocyte cellular biology, revealing that the compounds act immediately upon activation of microgametogenesis within a 6-min activity window. Ultimately, we found the N-4HCS compounds to be a powerful starting point for future development of transmission-blocking antimalarials.
“Discovering novel compounds that specifically target microgametogenesis has important implications in both our understanding of Plasmodium biology and malaria disease control.”
What are the potential implications of these results for your field of research?
Discovering novel compounds that specifically target microgametogenesis has important implications in both our understanding of Plasmodium biology and malaria disease control. The function of Pfs16, the P. falciparum gametocyte protein that we identified as the likely target of the N-4HCS compounds, remains elusive. Our discovery that the transmission-blocking N-4HCS compounds inhibit microgametogenesis within a 6-min active window therefore suggests a function of Pfs16 in early microgametogenesis. Structural evidence would bolster our initial finding that N-4HCS compounds target Pfs16, therefore enabling further elucidation of the protein function and its viability as a future antimalarial drug target.
Given the rise of antimalarial resistance to frontline artemisinin combination therapies and the safety concerns associated with the existing transmission-blockers, primaquine and tafenoquine, there is a drive to discover novel compounds. Discovering the potent N-4HCS compounds may therefore lay the foundation for future antimalarial therapies with a stage-specific mode of action.
What are the main advantages and drawbacks of the experimental system you have used as it relates to the disease you are investigating?
Traditionally, antimalarial targets are identified by determining the genetic basis of drug resistance following repeated drug exposure. Given that gametocytes are a non-replicative stage of the Plasmodium life cycle, identification of the targets of molecules targeting this stage is not amenable using this approach. Despite this experimental drawback of studying gametocyte-targeted compounds, we demonstrated that chemical proteomics techniques are a powerful alternative for antimalarial target identification. By modifying N-4HCS compounds, we performed photoaffinity labelling to identify the parasite proteins interacting with the compounds, identifying Pfs16 this way.
Being a population bottleneck within the Plasmodium life cycle, sexual-stage gametocytes and gametes are desirable targets for drug development. Targeting the sexual stages of Plasmodium further presents a huge advantage in preventing human-to-mosquito transmission of malaria, thus protecting the uninfected population. Despite the positive implications in targeting transmission, a gametocyte- or gamete-targeted compound would need to maintain an efficacious concentration at protracted longevity within a host to be effective. Whilst there are chemical and engineering approaches to enhance longevity and enable slow compound release, this is one major drawback of targeting the sexual stages. Alternatively, novel and highly innovative approaches that directly expose the mosquito vector to antimalarials have been reported in recent years (Paton et al., 2019). Given that the N-4HCS compounds target microgametogenesis within the mosquito itself, we envision this to be a promising strategy for N-4HCS administration as ‘antimalarials for mosquitoes’ (discussed below).
Plasmodium falciparum exflagellation. The formation of male haploid microgametes within a mosquito midgut following transmission from human to mosquito. Following three rounds of DNA (blue) replication, egress from the host red blood cell (red) and formation of eight axonemes (green), gametes emerge in the rapid and dynamic process of exflagellation.
Plasmodium falciparum exflagellation. The formation of male haploid microgametes within a mosquito midgut following transmission from human to mosquito. Following three rounds of DNA (blue) replication, egress from the host red blood cell (red) and formation of eight axonemes (green), gametes emerge in the rapid and dynamic process of exflagellation.
What has surprised you the most while conducting your research?
The phenotypic effects of the N-4HCS compounds, particularly the activity window, were amongst the most notable findings in this study. P. falciparum sexual stage biology itself is characteristically complex; upon commitment to sexual differentiation, gametocytes sequester in the host spleen and bone marrow, where they mature over five distinct morphological stages. Upon reaching maturity, stage V gametocytes adopt a falciform morphology and re-enter the host bloodstream, where they are transmissible to Anopheles mosquitoes upon a bloodmeal. Gametocytes can then form male and female gametes (gametogenesis) within the mosquito gut prior to their fusion and fertilisation. Male gamete formation (microgametogenesis) involves rapid and simultaneous host-cell egress, three alternating rounds of DNA replication and endomitosis, and the formation of eight axonemes. Male haploid gametes then emerge in the highly dynamic process of exflagellation in only 15 min, making it one of the most rapid eukaryotic DNA replication events known.
N-4HCS compounds block the process of microgametogenesis with immediate effect, requiring no pre-incubation with gametocytes. Notably, we found that upon activation of microgametogenesis in the absence of compounds and incremental addition of N-4HCS compounds, the inhibition of exflagellation was retained when treated up to 6 min post activation. Surprisingly, we found the cellular phenotype to vary depending on the N-4HCS treatment time following activation when analysed by immunofluorescence microscopy. Both the complexity of microgametogenesis and the powerful activity of N-4HCS compounds is reflected in their ability to act with almost immediate effect, with only a matter of seconds resulting in varied phenotypes.
What do you think is the most significant challenge impacting your research at this time and how will this be addressed over the next 10 years?
Implementation of malaria control strategies targeted at both the Anopheles mosquito and the Plasmodium parasite have been pivotal to the decline in malaria burden. But it is the persistent implementation of the same strategies that has amounted to resistance to frontline antimalarial drugs and insecticides. Whilst there is promise in the recommended rollout of the first malaria vaccine, RTS,S, the vaccine is limited by its efficacy. Altogether, there is an evident need to divert from existing strategies and to focus on innovation and novelty in malaria containment and management. Promisingly, leading industry and academic groups are focusing research efforts on improvements to vaccine efficacy and drug-screening campaigns. There is also a huge amount of innovation within the malaria research community, with exciting new approaches to vector control, parasite killing, disease diagnostics, vaccine approaches and monoclonal antibody therapies.
“In the translation of exciting scientific discoveries to field impact, interdisciplinary collaboration is hugely beneficial and this shared knowledge and research effort would benefit scientists globally.”
What changes do you think could improve the professional lives of scientists?
Collaborative science was at the forefront of this study, drawing on the knowledge of researchers with expertise in parasitology, cellular biology, medicinal chemistry and chemical biology. Drug discovery is highly interdisciplinary in nature and these collaborations were central to enabling our research. It is promising to see the formation of multiple research consortiums spanning industry and academic groups across multiple continents, with a shared agenda to deliver impactful malaria research. In the translation of exciting scientific discoveries to field impact, interdisciplinary collaboration is hugely beneficial and this shared knowledge and research effort would benefit scientists globally.
What's next for you?
Whilst our combined studies had addressed a large proportion of the pre-clinical drug discovery pipeline, from hit discovery to lead optimisation and through to target identification, what remained to be addressed was how our findings could translate into the field. In other words, how do we envision these compounds having a real-world impact in the fight against malaria? Following the completion of my PhD, I was rewarded with a Biotechnology and Biological Sciences Research Council (BBSRC) Flexible Talent Innovation Placement fellowship to test the in vivo efficacy of the N-4HCS compounds at Harvard T.H. Chan School of Public Health, with support from Imperial College London (Prof. Jake Baum) and GlaxoSmithKline (Dr Francisco Javier Gamo). The lab of Prof. Flaminia Catteruccia at Harvard reported that direct exposure of the existing antimalarial atovaquone to Anopheles mosquitoes prevented malaria transmission (Paton et al., 2019). As part of the placement within the Catteruccia lab, we proposed to test whether the in vitro microgametogenesis-targeted activity of N-4HCS compounds would translate to in vivo activity upon direct delivery of the antimalarials to the mosquito itself. We are excited to share these results with the malaria research community soon!
Following on from the fellowship, I moved onto my current role where I am working on vaccines and monoclonal antibody-based therapies for malaria and influenza at Kymab, a Sanofi company.
Sabrina Yahiya's contact details: Kymab, The Bennet Building (B930), Babraham Research Campus, Cambridge CB22 3AT, UK. E-mail: [email protected]