Schistosome biology illuminated: Schistosomiasis is caused by a parasitic flatworm about which little is known. Therefore, options to combat human disease caused by schistosome infection are limited. To aid in our quest to develop treatments, two studies undertook molecular investigations of the parasite Schistosoma mansoni . By generating a single-cell atlas, Wendt et al. identified the developmental trajectory of the flatworm, including the blood-feeding gut required for its survival in the host. From these data, they found a gene required for gut development that, when knocked out through RNA interference, confers reduced pathology in infected mice. Wang et al. performed a large-scale RNA interference survey of S. mansoni and identified an essential pair of protein kinases that can be targeted by approved pharmacological intervention (see the Perspective by Anderson and Duraisingh). These molecular investigations add to our understanding of the schistosome parasite and provide biological information that may help to combat this neglected tropical disease. Science , this issue p. 1644 , p. 1649 ; see also p. 1562

Epigenetic mechanisms and chromatin structure play an important role in development. Their impact is therefore expected to be strong in parasites with complex life cycles and multiple, strikingly different, developmental stages, i.e. developmental plasticity. Some studies have already described how the chromatin structure, through histone modifications, varies from a developmental stage to another in a few unicellular parasites. While H3K4me3 profiles remain relatively constant, H3K27 trimethylation and bivalent methylation show strong variation. Inhibitors (A366 and GSK343) of H3K27 histone methyltransferase activity in S. mansoni efficiently blocked miracidium to sporocyst transition indicating that H3K27 trimethylation is required for life cycle progression. As S. mansoni is a multicellular parasite that significantly affects both the health and economy of endemic areas, a better understanding of fluke developmental processes within the definitive host will likely highlight novel disease control strategies. Towards this goal, we also studied H4K20me1 in female cercariae and adults. In particular, we found that bivalent trimethylation of H3K4 and H3K27 at the transcription start site of genes is a landmark of the cercarial stage. In cercariae, H3K27me3 presence and strong enrichment in H4K20me1 over long regions (10–100 kb) is associated with development related genes. Here, we provide a broad overview of the chromatin structure of a metazoan parasite throughout its most important lifecycle stages. The five developmental stages studied here present distinct chromatin structures, indicating that histone methylation plays an important role during development. Hence, components of the histone methylation (and demethylation) machinery may provide suitable Schistosomiasis control targets. Schistosoma mansoni is a parasitic flatworm and causative agent of intestinal schistosomiasis, a neglected tropical disease affecting 67 million people worldwide. The parasite has a complex life cycle involving two consecutive obligate hosts (a poikilotherm snail and a homeotherm mammal) and two transitions between these hosts as free-swimming larvae. Here, we show that the chromatin structure of five different developmental stages is characterized by specific changes in chemical modifications of histones, basic proteins that are closely associated with DNA (trimethylation of lysines 4 and 27 and histone H3, and monomethylation of lysine 20 on histone H4). These modifications occur around protein coding genes as well as within repetitive genomic elements. A functional role for histone methylation during schistosome development was elucidated by the use of epi-drugs targeting G9a/GLP and EZH2 histone methyltransferase orthologs in S. mansoni. Our results indicate that histone methylation plays an important role during schistosome development and suggest that the enzymes responsible for maintaining these chromatin modifications are suitable targets for anti-schistosomal drugs.

In September 2022, the Drug Discovery Unit at the University of Dundee, UK, organised an international meeting at the Wellcome Collection in London to explore the current clinical situation and challenges associated with treating schistosomiasis. The aim of this meeting was to discuss the need for new treatments in view of the clinical situation and to ascertain what the key requirements would be for any potential new anti-schistosomals. This information will be essential to inform ongoing drug discovery efforts for schistosomiasis. We also discussed the potential drug discovery pathway and associated criteria for progressing compounds to the clinic. To date, praziquantel (PZQ) is the only drug available to treat all species causing schistosomiasis, but it is often unable to completely clear parasites from an infected patient, partially due to its inactivity against juvenile worms. PZQ-mediated mass drug administration campaigns conducted in endemic areas (e.g., sub-Saharan Africa, where schistosomiasis is primarily prevalent) have contributed to reducing the burden of disease but will not eliminate the disease as a public health problem. The potential for Schistosoma to develop resistance towards PZQ, as the sole treatment available, could become a concern. Consequently, new anthelmintic medications are urgently needed, and this Perspective aims to capture some of the learnings from our discussions on the key criteria for new treatments.

The G×E concept, in which genotype × environment interactions bring about the phenotype, is widely used to describe biological phenomena. We propose to extend the initial notion of the concept, replacing G by 'inheritance system'. This system, comprised of both genome and epigenome components, collectively interacts with the environment to shape the development of a phenotype. In the case of the human blood fluke Schistosoma mansoni, responsible for intestinal bilharzia, the phenotypic trait that is most relevant to global health is infection success. Taking a systems biology view we show how genetic and epigenetic interactions result in ephemeral, but also heritable, phenotypic variations that are important for infection success.

Schistosomiasis endangers the lives of greater than 200 million people every year and is predominantly controlled by a single class chemotherapy, praziquantel (PZQ). Development of PZQ replacement (to combat the threat of PZQ insensitivity/resistance arising) or combinatorial (to facilitate the killing of PZQ-insensitive juvenile schistosomes) chemotherapies would help sustain this control strategy into the future. Here, we re-categorise two families of druggable epigenetic targets in Schistosoma mansoni, the histone methyltransferases (HMTs) and the histone demethylases (HDMs). Amongst these, a S. mansoni Lysine Specific Demethylase 1 (SmLSD1, Smp_150560) homolog was selected for further analyses. Homology modelling of SmLSD1 and in silico docking of greater than four thousand putative inhibitors identified seven (L1 - L7) showing more favourable binding to the target pocket of SmLSD1 vs Homo sapiens HsLSD1; six of these seven (L1 - L6) plus three structural analogues of L7 (L8 - L10) were subsequently screened against schistosomula using the Roboworm anthelmintic discovery platform. The most active compounds (L10 - pirarubicin > L8 - danunorubicin hydrochloride) were subsequently tested against juvenile (3 wk old) and mature (7 wk old) schistosome stages and found to impede motility, suppress egg production and affect tegumental surfaces. When compared to a surrogate human cell line (HepG2), a moderate window of selectivity was observed for the most active compound L10 (selectivity indices - 11 for schistosomula, 9 for juveniles, 1.5 for adults). Finally, RNA interference of SmLSD1 recapitulated the egg-laying defect of schistosomes co-cultivated in the presence of L10 and L8. These preliminary results suggest that SmLSD1 represents an attractive new target for schistosomiasis; identification of more potent and selective SmLSD1 compounds, however, is essential. Nevertheless, the approaches described herein highlight an interdisciplinary strategy for selecting and screening novel/repositioned anti-schistosomals, which can be applied to any druggable (epigenetic) target encoded by the parasite's genome.

Praziquantel represents the frontline chemotherapy used to treat schistosomiasis, a neglected tropical disease (NTD) caused by infection with macro-parasitic blood fluke schistosomes. While this drug is safe, its inability to kill all schistosome lifecycle stages within the human host often requires repeat treatments. This limitation, amongst others, has led to the search for novel anti-schistosome replacement or combinatorial chemotherapies. Here, we describe a repositioning strategy to assess the anthelmintic activity of epigenetic probes/inhibitors obtained from the Structural Genomics Consortium. Thirty-seven epigenetic probes/inhibitors targeting histone readers, writers and erasers were initially screened against Schistosoma mansoni schistosomula using the high-throughput Roboworm platform. At 10 μM, 14 of these 37 compounds (38%) negatively affected schistosomula motility and phenotype after 72 hours of continuous co-incubation. Subsequent dose-response titrations against schistosomula and adult worms revealed epigenetic probes targeting one reader (NVS-CECR2-1), one writer (LLY-507 and BAY-598) and one eraser (GSK-J4) to be particularly active. As LLY-507/BAY-598 (SMYD2 histone methyltransferase inhibitors) and GSK-J4 (a JMJD3 histone demethylase inhibitor) regulate an epigenetic process (protein methylation) known to be critical for schistosome development, further characterisation of these compounds/putative targets was performed. RNA interference (RNAi) of one putative LLY-507/BAY-598 S. mansoni target (Smp_000700) in adult worms replicated the compound-mediated motility and egg production defects. Furthermore, H3K36me2, a known product catalysed by SMYD2 activity, was also reduced by LLY-507 (25%), BAY-598 (23%) and siSmp_000700 (15%) treatment of adult worms. Oviposition and packaging of vitelline cells into in vitro laid eggs was also significantly affected by GSK-J4 (putative cell permeable prodrug inhibitor of Smp_034000), but not by the related structural analogue GSK-J1 (cell impermeable inhibitor). Collectively, these results provide further support for the development of next-generation drugs targeting schistosome epigenetic pathway components. In particular, the progression of histone methylation/demethylation modulators presents a tractable strategy for anti-schistosomal control. Human schistosomiasis is caused by infection with parasitic blood fluke worms. Global control of this NTD is currently facilitated by administration of a single drug, praziquantel (PZQ). This mono-chemotherapeutic strategy of schistosomiasis control presents challenges as PZQ is not active against all human-dwelling schistosome lifecycle stages and the evolution of PZQ resistant parasites remains a threat. Therefore, new drugs to be used in combination with or in replacement of PZQ are urgently needed. Here, continuing our studies on Schistosoma mansoni epigenetic processes, we performed anthelmintic screening of 37 epigenetic probes/epigenetic inhibitors obtained from the Structural Genomics Consortium (SGC). The results of these studies highlighted that schistosome protein methylation/demethylation processes are acutely vulnerable. In particular, compounds affecting schistosome SMYD (LLY-507, BAY-598) or JMJD (GSK-J4) homologues are especially active on schistosomula and adult worms during in vitro phenotypic drug screens. The active epigenetic probes identified here as well as their corresponding S. mansoni protein targets offers new starting points for the development of next-generation anti-schistosomals.