Abstract
BackgroundPlanarian stem cells, or neoblasts, drive the almost unlimited regeneration capacities of freshwater planarians. Neoblasts are traditionally described by their morphological features and by the fact that they are the only proliferative cell type in asexual planarians. Therefore, they can be specifically eliminated by irradiation. Irradiation, however, is likely to induce transcriptome-wide changes in gene expression that are not associated with neoblast ablation. This has affected the accurate description of their specific transcriptomic profile.ResultsWe introduce the use of Smed-histone-2B RNA interference (RNAi) for genetic ablation of neoblast cells in Schmidtea mediterranea as an alternative to irradiation. We characterize the rapid, neoblast-specific phenotype induced by Smed-histone-2B RNAi, resulting in neoblast ablation. We compare and triangulate RNA-seq data after using both irradiation and Smed-histone-2B RNAi over a time course as means of neoblast ablation. Our analyses show that Smed-histone-2B RNAi eliminates neoblast gene expression with high specificity and discrimination from gene expression in other cellular compartments. We compile a high confidence list of genes downregulated by both irradiation and Smed-histone-2B RNAi and validate their expression in neoblast cells. Lastly, we analyze the overall expression profile of neoblast cells.ConclusionsOur list of neoblast genes parallels their morphological features and is highly enriched for nuclear components, chromatin remodeling factors, RNA splicing factors, RNA granule components and the machinery of cell division. Our data reveal that the regulation of planarian stem cells relies on posttranscriptional regulatory mechanisms and suggest that planarians are an ideal model for this understudied aspect of stem cell biology.
Highlights
Planarian stem cells, or neoblasts, drive the almost unlimited regeneration capacities of freshwater planarians
Smed-H2B is expressed in NBs but not in the central nervous system (CNS) We first characterized the expression pattern and phenotype of Smed-H2B, a Histone H2B variant known to be expressed in NBs [17]
No expression was detected in brain cells themselves, and the positive cells observed inside the structure of the brain likely correspond to NBs that are infiltrating the brain as a part of the homeostatic tissue turnover of the animal. These results show that SmedH2B is expressed in NBs and is not detected in any other tissue of the planarian body, including the CNS
Summary
Neoblasts, drive the almost unlimited regeneration capacities of freshwater planarians. CBs are electron dense granules located in the perinuclear region, often associated with nuclear pores and mitochondria These structures have both morphological and molecular similarities to germ granules present in the germ line cells of metazoans (collectively known as ‘nuage’). Some are required for NB function and are necessary for both regeneration and the homeostatic cell turnover of the worms These include two Piwi homologues [15,16] and a Bruno [17], a Pumilio [18] and a Tudor homologue gene [19], all involved in post-transcriptional regulation. Some of these markers are expressed in the central nervous system (CNS) of planarians [15,17,18,20,21,22], suggesting a shared requirement for complex posttranscriptional/RNA-based gene regulation in NBs and neurons
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