A Single-Cell Atlas of the Schistosoma mansoni Miracidium Larva Reveals Cellular Diversity and Distinct Stem Cell Populations

Core Concepts

The miracidium larva of the parasitic flatworm Schistosoma mansoni is a critical transmission stage that carries stem cells to infect a snail host. This single-cell atlas provides a comprehensive molecular characterization of the miracidium's 19 distinct cell types, including two transcriptionally distinct stem cell populations, and reveals insights into the larva's development and function.

Abstract

This study presents a single-cell RNA sequencing (scRNA-seq) atlas of the Schistosoma mansoni miracidium larva, which is the first free-living stage in the parasite's complex life cycle. The miracidium is composed of approximately 365 cells, with 93% being somatic cells (57% neural, 19% muscle, 13% tegument, 2% parenchyma, 2% protonephridia) and the remaining 7% being stem cells.
Key findings:

The miracidium has 19 transcriptionally distinct cell types, including five neural clusters, two muscle clusters, one ciliary plate cluster, one tegument cluster, one protonephridial cluster, and two parenchymal clusters.
The neural cells show a high degree of diversity, with the brain containing at least 15 subclusters of neurons expressing different neuropeptide precursor genes.
The two muscle clusters exhibit distinct transcriptomic signatures, corresponding to the orthogonal grid of circular and longitudinal body wall muscles.
The tegument cells are located in the posterior two-thirds of the larva, with their nuclei situated below the body wall muscle and cytoplasmic protrusions reaching between the muscle filaments.
The miracidium contains two transcriptionally distinct stem cell populations - one resembling the Kappa-like stem cells and the other the Delta/Phi-like stem cells observed in the mother sporocyst stage. These two stem cell populations show sex-specific gene expression patterns due to incomplete dosage compensation of sex-linked genes.
RNA velocity analysis suggests that the Delta/Phi-like stem cell population may be the origin of the tegument lineage, while the Kappa-like stem cells likely contain the pluripotent cells that will develop into the cercariae larvae.

This comprehensive single-cell atlas provides unprecedented insights into the cellular organization and developmental potential of the miracidium larva, a critical stage in the Schistosoma life cycle.

Stats

The miracidium larva is composed of approximately 365 cells.
93% of the miracidium cells are somatic, while the remaining 7% are stem cells.
The miracidium contains 19 transcriptionally distinct cell types.
There are two transcriptionally distinct stem cell populations in the miracidium.

Quotes

"The miracidium is composed of ∼365 cells; i) DIC image of miracidium, ii) 3D projection of confocal z-stack of DAPI-stained miracidium with nuclei segmented to enable counting (larval anterior pole at the top in all images)."
"Complexin was expressed in and around >209 nuclei, indicating that 57% of cells in the miracidium were neurons; out of which 129 cpx+ nuclei formed the nuclear rind of the brain (or neural mass/ring), and the remaining 80 were situated peripherally, either anterior or posterior to the brain."
"Meg6 (Smp_163710) was expressed in 66% of the cells in the tegument cluster; ISH showed 46 Meg6+ cells, and the nuclei were in the posterior two-thirds of the larva."

Key Insights Distilled From

A single-cell atlas of the miracidium larva of the human blood fluke Schistosoma mansoni: cell types, developmental pathways and tissue architecture

by Rawlinson,K.... at www.biorxiv.org 03-27-2023

https://www.biorxiv.org/content/10.1101/2023.03.27.533868v4

Deeper Inquiries

How do the transcriptional differences between the two stem cell populations in the miracidium relate to their developmental potential and fate during the transition to the mother sporocyst stage

The transcriptional differences between the two stem cell populations in the miracidium, namely the Delta/Phi-like and Kappa-like clusters, are indicative of their distinct developmental potentials and fates during the transition to the mother sporocyst stage. The Delta/Phi-like stem cells express a set of transcription factors associated with the flatworm tegument/epidermal lineage, such as p53-1, zfp-1, and six3-1. This suggests that these stem cells are primed to give rise to tegument cells, which are essential for the parasite's survival and transmission. On the other hand, the Kappa-like stem cells show enrichment in genes related to transcription and metabolism, indicating a quiescent state rather than an activated state. This suggests that the Kappa-like stem cells may serve as a reservoir for future stem cell populations or contribute to other somatic tissues in the miracidium and beyond. The RNA velocity analysis further supports this by showing the progression of Delta/Phi stem cells towards tegument cells, indicating a lineage-specific differentiation process.

What are the functional implications of the incomplete dosage compensation of sex-linked genes in the miracidium, and how does this impact the biology and transmission of the parasite

The incomplete dosage compensation of sex-linked genes in the miracidium has significant functional implications for the biology and transmission of the parasite. The clustering of stem cells by sex, with distinct transcriptional profiles based on the expression of ZSR and WSR genes, suggests that there may be tissue-specific variations in gene expression between male and female miracidia. This could impact the developmental potential and fate of cells, leading to sex-specific differences in cell differentiation and function. In the context of Schistosoma mansoni, where males and females have different reproductive strategies and roles in the life cycle, these sex-linked gene expression patterns could influence the development of reproductive organs, mating behavior, and ultimately the success of transmission. The differential expression of sex-linked genes in specific tissues, such as the protonephridia, may also play a role in regulating physiological processes and adaptation to the host environment. Overall, the incomplete dosage compensation of sex-linked genes adds another layer of complexity to the developmental program of the parasite and may contribute to its ability to adapt and thrive in diverse environments.

Given the simplicity of the miracidium larva, how can this system be leveraged to gain broader insights into the developmental programs and cell fate decisions that underlie the complex life cycle of Schistosoma mansoni

The simplicity of the miracidium larva provides a unique opportunity to gain broader insights into the developmental programs and cell fate decisions that underlie the complex life cycle of Schistosoma mansoni. By creating a single-cell atlas of the miracidium, researchers can identify and characterize the diverse cell types present in this stage, including somatic cells, stem cells, neurons, tegumental cells, and more. This comprehensive understanding of the cellular composition and transcriptional profiles of the miracidium can serve as a foundation for studying the molecular mechanisms driving development, differentiation, and tissue-specific functions. Additionally, the identification of distinct stem cell populations with different transcriptional profiles and developmental potentials offers a valuable model system to investigate the regulation of stem cell fate and lineage commitment in the context of parasite development. Leveraging the miracidium system can also provide insights into the evolutionary adaptations and strategies employed by Schistosoma mansoni to complete its life cycle and ensure successful transmission. By studying the miracidium at the single-cell level, researchers can uncover novel targets for intervention and control strategies against schistosomiasis.