Functional Studies of Stem Cells in the Parasitic Flatworm Schistosoma mansoni
Schistosomes are parasitic flatworms that infect over 200 million people, primarily in poverty-stricken developing countries, causing extensive morbidity and mortality. Schistosomes have a great deal in common with other parasitic flatworms such as liver flukes and tapeworms. Namely, they are characterized by a syncytial "skin" known as the tegument that is critical for their survival within their hosts, they possess somatic stem cells referred to as neoblasts, and they are incredibly successful parasites. Until now, there has been little appreciation for the interrelationships between the tegument, neoblasts, and successful parasitism. Our recent work, however, suggested that schistosome neoblasts give rise to cells that are associated with the parasite's tegument. In order to determine whether schistosome neoblasts produced the tegument, we developed novel labeling techniques and found that neoblasts give rise to progenitor cells that ultimately maintain the tegument. We also developed a fluorescence activated cell sorting protocol that we used to isolate neoblasts and tegument progenitors and obtain their transcriptomes. With this transcriptome data, we identified two regulators of tegument maintenance, zfp-1 and zfp-1-1, that are functionally conserved in free-living flatworms, suggesting a common molecular program for "skin" production in all flatworms. Additionally, our work suggested that zfp-1-1 specifically and tegument maintenance generally may be valid therapeutic targets. We next wondered whether schistosome neoblasts were responsible for making or maintaining any non-tegumental tissues in the adult parasite. To study this, we employed single cell RNAseq to create a gene expression atlas of 43,642 cells from adult parasites. This atlas gave us molecular markers for 68 distinct clusters of cells ranging from muscles and neurons to reproductive tissues such as the ovary. It also allowed us to identify a previously unknown sub-population of neoblasts that appears to exist at the top of a schistosome gut lineage. In an effort to study these neoblasts, we found that the schistosome homolog of the nuclear receptor hnf4 is required for normal gut homeostasis and that loss-of-function of hnf4 prevents parasite blood feeding and abrogates disease pathology in vivo. Taken together, this work demonstrates how studying basic developmental processes (i.e. stem cell differentiation) in a pathogenic organism can lead to not only insights into evolutionary biology (the machinery that regulates skin production appears to be conserved across flatworms), but it can also suggest novel therapeutic targets (namely zfp-1-1 and tegument maintenance as well as hnf4 and gut homeostasis).