Piwi expression in archeocytes and choanocytes in demosponges: insights into the stem cell system in demosponges

Abstract

Little is known about the stem cells of organisms early in metazoan evolution. To characterize the stem cell system in demosponges, we identified Piwi homologs of a freshwater sponge, Ephydatia fluviatilis, as candidate stem cell (archeocyte) markers. EfPiwiA mRNA was expressed in cells with archeocyte cell morphological features. We demonstrated that these EfPiwiA-expressing cells were indeed stem cells by showing their ability to proliferate, as indicated by BrdU-incorporation, and to differentiate, as indicated by the coexpression of EfPiwiA with cell-lineage-specific genes in presumptive committed archeocytes. EfPiwiA mRNA expression was maintained in mature choanocytes forming chambers, in contrast to the transition of gene expression from EfPiwiA to cell-lineage-specific markers during archeocyte differentiation into other cell types. Choanocytes are food-entrapping cells with morphological features similar to those of choanoflagellates (microvillus collar and a flagellum). Their known abilities to transform into archeocytes under specific circumstances and to give rise to gametes (mostly sperm) indicate that even when they are fully differentiated, choanocytes maintain pluripotent stem cell-like potential. Based on the specific expression of EfPiwiA in archeocytes and choanocytes, combined with previous studies, we propose that both archeocytes and choanocytes are components of the demosponge stem cell system. We discuss the possibility that choanocytes might represent the ancestral stem cells, whereas archeocytes might represent stem cells that further evolved in ancestral multicellular organisms.

Pluripotent Gametogenic Stem Cells of Asexually Reproducing Invertebrates

“…Stem cells of invertebrates reproducing asexually are traditionally often but not always referred to as totipotent, if their ability to differentiate into gametes and all somatic cells of the organism is shown (reviews: Isaeva et al, 2008bIsaeva et al, , 2009Rinkevich et al, 2009;Sköld et al, 2009). For instance, archaeocytes of sponges are regarded as totipotent (Simpson, 1984;Müller, 2006) or pluripotent (Funayama, 2008;Funayama et al, 2010). Funayama believes that both archaeocytes and choanocytes of sponges are pluripotent.…”

“…To avoid confusion and misunderstanding, the term "pluripotent" is using here according to the recent reviews (Seydoux & Braun, 2006;Funayama et al, 2010;Strouji & Extavour, 2011). The problem of cell line having unlimited morphogenetic potential stems from A. Weismann' "germ plasm" theory.…”

“…Particularly, pluripotent stem, primary germ, and gonial cells have a common propertyself-renewal through mitotic reproduction over long periods or throughout life span of the organism (Houston & King, 2000;Hogan, 2000;Sköld et al, 2009). For instance, sponge archaeocytes are self-renewing, mitotically active, telomerase-positive and bromodeoxyuridine incorporating cells (Müller, 2006;Funayama et al, 2010). Interstitial cells in hydra and other cnidarians can produce both germline cells and some but not all somatic cell types, since epidermal and gastrodermal cells are also capable of mitotic reproduction; so the stem cell system in hydroids includes interstitial, epidermal and gastrodermal stem cells continuously undergoing the mitotic cycle (Campbell, 1974;Thomas & Edwards, 1991;Bode, 1996).…”

“…Stem cells are an essential and defining feature developed during evolution of all multicellular organisms (Lohman, 2008;Batygina, 2010;Funayama et al, 2010). The study of mammalian embryonic stem cells has become a hot, intensely developing field in biology, biotechnology, and biomedicine.…”

“…However, comparative studies of stem cells in various multicellular organisms are required to understand molecular mechanisms of maintaining pluri/totipotency, "stemness", which still remain far from clear, as well as mechanisms that regulate gametogenesis, reproduction, development and regeneration. According to the generally accepted view, stem cells are cells of embryos or adult organisms capable of self-renewing by mitotic reproduction and differentiation into specialized cell types (Weissman et al, 2001;Cogle et al 2003;Müller, 2006;Lohman 2008;Rinkevich et al, 2009;Sköld et al, 2009;Funayama et al, 2010). Two main types of stem cells should be considered: the cells of the germline and the somatic stem cell lineages (Hogan, 2001;Rinkevich, 2009;Srouji & Extavour, 2011).…”