Convergent evolution of a genotoxic stress response in a parasite-specific p53 homolog

A single-cell atlas of the miracidium larva of Schistosoma mansoni reveals cell types, developmental pathways, and tissue architecture

Schistosoma mansoni is a parasitic flatworm that causes the major neglected tropical disease schistosomiasis. The miracidium is the first larval stage of the life cycle. It swims and infects a freshwater snail, transforms into a mother sporocyst, where its stem cells generate daughter sporocysts that give rise to human-infective cercariae larvae. To understand the miracidium at cellular and molecular levels, we created a whole-body atlas of its ~365 cells. Single-cell RNA sequencing identified 19 transcriptionally distinct cell clusters. In situ hybridisation of tissue-specific genes revealed that 93% of the cells in the larva are somatic (57% neural, 19% muscle, 13% epidermal or tegument, 2% parenchyma, and 2% protonephridia) and 7% are stem. Whereas neurons represent the most diverse somatic cell types, trajectory analysis of the two main stem cell populations indicates that one of them is the origin of the tegument lineage and the other likely contains pluripotent cells. Furthermore, unlike the somatic cells, each of these stem populations shows sex-biased transcriptional signatures suggesting a cell-type-specific gene dosage compensation for sex chromosome-linked loci. The miracidium represents a simple developmental stage with which to gain a fundamental understanding of the molecular biology and spatial architecture of schistosome cells.

Horizontal gene transfer of a functional cki homolog in the human pathogen Schistosoma mansoni

Schistosomes are parasitic flatworms responsible for the neglected tropical disease schistosomiasis, causing devastating morbidity and mortality in the developing world. The parasites are protected by a skin-like tegument, and maintenance of this tegument is controlled by a schistosome ortholog of the tumor suppressor TP53. To understand mechanistically how p53-1 controls tegument production, we identified a cyclin dependent kinase inhibitor homolog (cki) that was co-expressed with p53-1. RNA interference of cki resulted in a hyperproliferation phenotype, that, in combination with p53-1 RNA interference yielded abundant tumor-like growths, indicating that cki and p53-1 are bona fide tumor suppressors in Schistosoma mansoni. Interestingly, cki homologs are widely present throughout parasitic flatworms but evidently absent from their free-living ancestors, suggesting this cki homolog came from an ancient horizontal gene transfer event. This in turn implies that the evolution of parasitism in flatworms may have been aided by a highly unusual means of metazoan genetic inheritance.

Starvation resistance in planarians: multiple strategies to get a thrifty phenotype

Starvation resistance is a life-saving mechanism for many organisms facing food availability fluctuation in the natural environment. Different strategies have been episodically identified for some model organisms, the first of which was the ability to suppress metabolic rate. Among the identified strategies, the ability of planarians to shrink their body under fasting conditions and revert the process after feeding (the growth-degrowth process) represents a fascinating mechanism to face long periods of fasting. The growth-degrowth process is strictly related to the capability of planarians to continuously maintain tissue homeostasis and body proportions even in challenging conditions, thanks to the presence of a population of pluripotent stem cells. Here, we take advantage of several previous studies describing the growth-degrowth process and of recent progress in the understanding of planarian homeostasis mechanisms, to identify tissue-selective transcriptional downregulation as a driving strategy for the development of a thrifty phenotype, and the p53 transcription factor as a player in adjusting tissue homeostasis in accordance with food availability.

Genome-wide transcriptome analysis of Echinococcus multilocularis larvae and germinative cell cultures reveals genes involved in parasite stem cell function

The lethal zoonosis alveolar echinococcosis is caused by tumour-like growth of the metacestode stage of the tapeworm Echinococcus multilocularis within host organs. We previously demonstrated that metacestode proliferation is exclusively driven by somatic stem cells (germinative cells), which are the only mitotically active parasite cells that give rise to all differentiated cell types. The Echinococcus gene repertoire required for germinative cell maintenance and differentiation has not been characterised so far. We herein carried out Illumina sequencing on cDNA from Echinococcus metacestode vesicles, from metacestode tissue depleted of germinative cells, and from Echinococcus primary cell cultures. We identified a set of ~1,180 genes associated with germinative cells, which contained numerous known stem cell markers alongside genes involved in replication, cell cycle regulation, mitosis, meiosis, epigenetic modification, and nucleotide metabolism. Interestingly, we also identified 44 stem cell associated transcription factors that are likely involved in regulating germinative cell differentiation and/or pluripotency. By in situ hybridization and pulse-chase experiments, we also found a new general Echinococcus stem cell marker, EmCIP2Ah, and we provide evidence implying the presence of a slow cycling stem cell sub-population expressing the extracellular matrix factor Emkal1. RNA-Seq analyses on primary cell cultures revealed that metacestode-derived Echinococcus stem cells display an expanded differentiation capability and do not only form differentiated cell types of the metacestode, but also cells expressing genes specific for protoscoleces, adult worms, and oncospheres, including an ortholog of the schistosome praziquantel target, EmTRPMPZQ. Finally, we show that primary cell cultures contain a cell population expressing an ortholog of the tumour necrosis factor α receptor family and that mammalian TNFα accelerates the development of metacestode vesicles from germinative cells. Taken together, our analyses provide a robust and comprehensive characterization of the Echinococcus germinative cell transcriptome, demonstrate expanded differentiation capability of metacestode derived stem cells, and underscore the potential of primary germinative cell cultures to investigate developmental processes of the parasite. These data are relevant for studies into the role of Echinococcus stem cells in parasite development and will facilitate the design of anti-parasitic drugs that specifically act on the parasite germinative cell compartment.

Inhibition of ATM kinase rescues planarian regeneration after lethal radiation

As stem cells divide, they acquire mutations that can be passed on to daughter cells. To mitigate potentially deleterious outcomes, cells activate the DNA damage response (DDR) network, which governs several cellular outcomes following DNA damage, including repairing DNA or undergoing apoptosis. At the helm of the DDR are three PI3-like kinases including Ataxia-Telangiectasia Mutated (ATM). We report here that knockdown of ATM in planarian flatworms enables stem cells to withstand lethal doses of radiation which would otherwise induce cell death. In this context, stem cells circumvent apoptosis, replicate their DNA, and recover function using homologous recombination-mediated DNA repair. Despite radiation exposure, atm knockdown animals survive long-term and regenerate new tissues. These effects occur independently of ATM's canonical downstream effector p53. Together, our results demonstrate that in planarians, ATM promotes radiation-induced apoptosis. This acute, ATM-dependent apoptosis is a key determinant of long-term animal survival. Our results suggest that inhibition of ATM in these organisms could, therefore, potentially favor cell survival after radiation without obvious effects on stem cell behavior.

Actin restricts cell proliferation and promotes differentiation during planarian regeneration

Actin is an integral component of the cytoskeleton, which plays an important role in various fundamental cellular processes, such as affecting the polarity of embryonic cells during embryonic development in various model organisms. Meanwhile, previous studies have demonstrated that the polymerization of the actin cytoskeleton can affect cell migration, proliferation, and differentiation. Actin polymerization state regulated osteogenic differentiation and affected cell proliferation. However, the function of actin in regenerative biology has not been thoroughly elucidated. The planarian flatworm, which contains a large number of adult somatic stem cells (neoblasts), is an ideal model organism to study regenerative biology. Here, we identified a homolog of actin in planarian Dugesia japonica and found that RNAi targeting actin during planarian regeneration results in the formation of protrusions on the dorsal side, where the division of phospho-H3 mitotic cells is increased. In addition, a decrease in differentiation is observed in regenerating tissues after Djactin RNAi. These results indicate that Djactin functions in proliferation and differentiation control in planarian regeneration.