Adult somatic stem cells in the human parasite Schistosoma mansoni

Abstract

Schistosomiasis is among the most prevalent human parasitic diseases, affecting more than 200 million people worldwide. The aetiological agents of this disease are trematode flatworms (Schistosoma) that live and lay eggs within the vasculature of the host. These eggs lodge in host tissues, causing inflammatory responses that are the primary cause of morbidity. Because these parasites can live and reproduce within human hosts for decades, elucidating the mechanisms that promote their longevity is of fundamental importance. Although adult pluripotent stem cells, called neoblasts, drive long-term homeostatic tissue maintenance in long-lived free-living flatworms (for example, planarians), and neoblast-like cells have been described in some parasitic tapeworms, little is known about whether similar cell types exist in any trematode species. Here we describe a population of neoblast-like cells in the trematode Schistosoma mansoni. These cells resemble planarian neoblasts morphologically and share their ability to proliferate and differentiate into derivatives of multiple germ layers. Capitalizing on available genomic resources and RNA-seq-based gene expression profiling, we find that these schistosome neoblast-like cells express a fibroblast growth factor receptor orthologue. Using RNA interference we demonstrate that this gene is required for the maintenance of these neoblast-like cells. Our observations indicate that adaptation of developmental strategies shared by free-living ancestors to modern-day schistosomes probably contributed to the success of these animals as long-lived obligate parasites. We expect that future studies deciphering the function of these neoblast-like cells will have important implications for understanding the biology of these devastating parasites.

Figure 1. Proliferation of somatic cells in adult schistosomes

a–b, EdU labeling in (a) male and (b) female parasites. c–d, Distribution of mesenchymal PSCs in (c) male and (d) female parasites. Phalloidin staining for actin shows male enteric and dorso-ventral muscles and female enteric and uterine muscles. e, Strategy to characterize PSC morphology. f, The morphology of EdU⁻ and EdU⁺ cells. Arrowhead indicates a nucleolus. g, FISH for histone h2b with EdU labeling. Arrowhead indicates a cytoplasmic projection. (a–d, g) are confocal projections; (a–b) are derived from tiled stacks. Scale bars: (a–b) 500 μm, (c–d) 20 μm, (f–g) 5 μm.

Figure 2. Transcriptional profiling identifies genes expressed in proliferative cells

a, EdU incorporation is abrogated at D3 following irradiation. b, Strategy to identify PSC-expressed genes. c, Volcano plot showing expression differences in control versus irradiated parasites. n = 3 for each group. d, WISH for various transcripts in unirradiated and D5 post-irradiation parasites. n > 3 parasites. e, EdU labeling and FISH for Sm-fgfrA. 1988/2000 EdU⁺ PSCs were Sm-fgfrA⁺ following a 20–22 hour pulse (n = 20 male parasites). (a, e) are confocal projections. Scale bars: (a, e) 20 μm, (d) 100 μm.

Figure 3. PSCs self-renew and differentiate

a, EdU-BrdU double labeling. Arrowheads, EdU⁺BrdU⁺ nuclei. Arrows, EdU⁺ “doublets”. b, Percentage (±s.d.) EdU⁺ doublets (green) that are BrdU⁻-BrdU⁻ (top), BrdU⁺-BrdU⁺ (middle, BrdU is magenta), or BrdU⁺-BrdU⁻ (Bottom). n = 21 parasites. c, Strategy to monitor cellular differentiation. d–g, EdU and sWGA labeling showing EdU⁺ cells in (d,e) male intestine or (f,g) dorsal musculature at (d,f) D1 and (e,g) D7 following a pulse. (d,e) Insets, intestinal basal surface (dashed lines) and lumen (green). Arrowheads, EdU⁺ (e) intestinal cells or (g) muscle cells. Images are confocal projections. Scale bars: (a,d–g) 20 μm (b) 5 μm.

Figure 4. Sm-fgfrA is required for the maintenance of somatic stem cells

a, EdU labeling and DIC images in control and Sm-fgfrA(RNAi) at RNAi D17. b, Percentage EdU⁺ nuclei/total nuclei in dissociated tissues from control(RNAi) (n = 3002 nuclei) and Sm-fgfrA(RNAi) (n = 3642 nuclei) parasites. Error bars, 95% confidence intervals, p < 0.0001 χ². c, WISH for histone h2b (top row) and nanos2 (bottom row) transcripts in control (left column) versus Sm-fgfrA(RNAi) (right column) parasites at RNAi D20-21. n > 5 parasites/experiment. Scale bars: (a) 100 μm (c) 200 μm.