Planarians are a model system for studying adult stem cells, as they possess the neoblasts, a population of pluripotent adult stem cells able to give rise to both somatic and germ cells. Although over the last years several efforts have been made to shed light on neoblast biology, only recent evidence indicate that this population of cells is heterogeneous. In this study we irradiated planarians with different non-lethal X-ray doses (1-5 Gy) and we identified subpopulations of neoblasts with diverse levels of tolerance to X-rays. We demonstrated that a dramatic reduction of neoblasts occurred soon after non-lethal irradiations and that de-novo proliferation of some radioresistant cells re-established the primary neoblast number. In particular, a strong proliferation activity occurred at the ventral side of irradiated animals close to the nervous system. The produced cells migrated towards the dorsal parenchyma and, together with some dorsal radioresistant cells, reconstituted the entire neoblast population demonstrating the extreme plasticity of this adult stem cell system.

SummaryRetinoblastoma-associated proteins 46 and 48 (RbAp46 and RbAp48) are factors that are components of different chromatin-modelling complexes, such as polycomb repressive complex 2, the activity of which is related to epigenetic gene regulation in stem cells. To date, no direct findings are available on the in vivo role of RbAp48 in stem-cell biology. We recently identified DjRbAp48 -a planarian (Dugesia japonica) homologue of human RBAP48 -expression of which is restricted to the neoblasts, the adult stem cells of planarians. In vivo silencing of DjRbAp48 induces lethality and inability to regenerate, even though neoblasts proliferate and accumulate after wounding. Despite a partial reduction in neoblast number, we were always able to detect a significant number of these cells in DjRbAp48 RNAi animals. Parallel to the decrease in neoblasts, a reduction in the number of differentiated cells and the presence of apoptoticlike neoblasts were detectable in RNAi animals. These findings suggest that DjRbAp48 is not involved in neoblast maintenance, but rather in the regulation of differentiation of stem-cell progeny. We discuss our data, taking into account the possibility that DjRbAp48 might control the expression of genes necessary for cell differentiation by influencing chromatin architecture.

Prohibitins are pleiotropic proteins, whose multiple roles are emerging as key elements in the regulation of cell survival and proliferation. Indeed, prohibitins interact with several intracellular proteins strategically involved in the regulation of cell cycle progression in response to extracellular growth signals. Prohibitins also have regulatory functions in mitochondrial fusion and cristae morphogenesis, phenomena related to the ability of self-renewing embryonic stem cells to undergo differentiation, during which mitochondria develop numerous cristae, increase in number, and generate an extensive reticular network. We recently identified a Prohibitin 2 homolog (DjPhb2) that is expressed in adult stem cells (neoblasts) of planarians, a well-known model system for in vivo studies on stem cells and tissue regeneration. Here, we show that in DjPhb2 silenced planarians, most proliferating cells disappear, with the exception of a subpopulation of neoblasts localized along the dorsal body midline. Neoblast depletion impairs regeneration and, finally, leads animals to death. Our in vivo findings demonstrate that prohibitin 2 plays an important role in regulating stem cell biology, being involved in both the control of cell cycle progression and mitochondrial cristae morphogenesis.

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