As they divide they are displaced toward the extremities where they become postmitotic, differentiate, and eventually are sloughed from the tips of the tentacles and the foot. All of the epithelial cells in the body column are mitotic and maintain the lineage ( 9). The ectodermal (green) and endodermal (blue) epithelial cell lineages form the inside and outside of the body column. ( A) A schematic of Hydra showing that it is composed of three cell lineages. PIWI proteins are expressed in the interstitial stem cells and are enriched in perinuclear granules. In this study we provide a comprehensive analysis of both PIWI protein expression and piRNA sequences in Hydra, which demonstrates that the PIWI–piRNA pathway has ancient and broadly conserved stem cell functions, including somatic functions.įig. These cells indefinitely self-renew and retain the capability of differentiating into the nonmitotic cells that function in the tentacles and foot ( Fig. The epithelial lineages do not have a true stem cell population, but they are mitotic along the entire length of the body column and these progenitor/stem cells are responsible for maintaining the lineage ( 9). The multipotent interstitial stem cells that support the interstitial lineage give rise to three somatic cell types (nerves, nematocytes, and gland cells) and to germ cells ( Fig. The adult Hydra polyp is composed of three distinct cell lineages: the two epithelial lineages (ectoderm and endoderm) and the interstitial lineage ( Fig. Hydra is a morphologically simple multicellular organism belonging to the phylum Cnidaria, which is the sister group to bilaterians ( SI Appendix, Fig. The potential significance of the pathway in stem cells outside the gonad and on nontransposon sequences is largely unexplored. However, detailed analyses have been largely confined to the function of the PIWI–piRNA pathway in the germline and the gonadal somatic cells in a few model bilaterians, with a focus on transposon silencing ( 7). Piwi expression is also found in hematopoietic stem cells in humans, mesenchymal stem cells in mice, and somatic stem cells in cnidarians and ctenophores ( 3– 6). For example, piwi genes are expressed in the pluripotent stem cells of planarians, sponges, and tunicates, and are required for regeneration ( 2). Piwi is also expressed outside the germline, largely in various kinds of stem and progenitor cells. Loss of PIWI proteins in Drosophila, mice, and zebrafish leads to a loss of fertility, due to a disruption in germline stem cell (GSC) formation or maintenance, arrest in meiosis, and other gametogenic defects ( 1). P-element–induced wimpy testis (PIWI) proteins and their bound small PIWI-interacting RNAs (piRNAs) are central players in a regulatory pathway that is essential for germline establishment and maintenance. This comprehensive analysis of the PIWI–piRNA pathway in the somatic stem/progenitor cells of a nonbilaterian animal suggests that this pathway originated with broader stem cell functionality. Finally we demonstrate that hywi function is essential in the somatic epithelial lineages. We also identified putative nontransposon targets of the pathway unique to each lineage. By sequencing the small RNAs specific to the interstitial, ectodermal, and endodermal lineages, we found that the targeting of transposons appears to be largely restricted to the interstitial lineage. piRNAs map to transposons with a 25- to 35-fold enrichment compared with the abundance of transposon transcripts. To explore this function, we generated a Hydra transcriptome for piRNA mapping. Hydra PIWI proteins are strictly cytoplasmic and thus likely act as posttranscriptional regulators. We identified ∼15 million piRNAs associated with Hywi and/or Hyli and found that they exhibit the ping-pong signature of piRNA biogenesis. Here we report that Hydra has two PIWI proteins, Hydra PIWI (Hywi) and Hydra PIWI-like (Hyli), both of which are expressed in all Hydra stem/progenitor cells, but not in terminally differentiated cells. The cnidarian Hydra is a simple metazoan with well-characterized stem/progenitor cells that provides a unique model for analysis of PIWI function. PIWI proteins and their bound PIWI-interacting RNAs (piRNAs) are found in animal germlines and are essential for fertility, but their functions outside of the gonad are not well understood.
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