Akt (PKB) is a serine threonine protein kinase downstream of the phosphoinositide 3-kinase (PI3K) pathway. In mammals, Akt is ubiquitously expressed and is associated with regulation of cellular proliferation, metabolism, cell growth and cell death. Akt has been widely studied for its central role in physiology and disease, in particular cancer where it has become an attractive pharmacological target. However, the mechanisms by which Akt signaling regulates stem cell behavior in the complexity of the whole body are poorly understood. Planarians are flatworms with large populations of stem cells capable of dividing to support adult tissue renewal and regeneration. The planarian ortholog Smed-Akt is molecularly conserved providing unique opportunities to analyze the function of Akt during cellular turnover and repair of adult tissues. Our findings abrogating Smed-Akt with RNA-interference in the planarian Schmidtea mediterranea led to a gradual decrease in stem cell (neoblasts) numbers. The reduced neoblast numbers largely affected the maintenance of adult tissues including the nervous and excretory systems and ciliated structures in the ventral epithelia, which impaired planarian locomotion. Downregulation of Smed-Akt function also resulted in an increase of cell death throughout the animal. However, in response to amputation, levels of cell death were decreased and failed to localize near the injury site. Interestingly, the neoblast mitotic response was increased around the amputation area but the regenerative blastema failed to form. We demonstrate Akt signaling is essential for organismal physiology and in late stages of the Akt phenotype the reduction in neoblast numbers may impair regeneration in planarians. Functional disruption of Smed-Akt alters the balance between cell proliferation and cell death leading to systemic impairment of adult tissue renewal. Our results also reveal novel roles for Akt signaling during regeneration, specifically for the timely localization of cell death near the injury site. Thus, Akt signaling regulates neoblast biology and mediates in the distribution of injury-mediated cell death during tissue repair in planarians.

Target of Rapamycin (TOR) controls an evolutionarily conserved signaling pathway that modulates cellular growth and division by sensing levels of nutrients, energy and stress. As such, TOR signaling is a crucial component of tissues and organs that translates systemic signals into cellular behavior. The ubiquitous nature of TOR signaling, together with the difficulty of analyzing tissue during cellular turnover and repair, have limited our understanding of how this kinase operates throughout the body. Here, we use the planarian model system to address TOR regulation at the organismal level. The planarian TOR homolog (Smed-TOR) is ubiquitously expressed, including stem cells (neoblasts) and differentiated tissues. Inhibition of TOR with RNA interference severely restricts cell proliferation, allowing the study of neoblasts with restricted proliferative capacity during regeneration and systemic cell turnover. Strikingly, TOR signaling is required for neoblast response to amputation and localized growth (blastema). However, in the absence of TOR signaling, regeneration takes place only within differentiated tissues. In addition, TOR is essential for maintaining the balance between cell division and cell death, and its dysfunction leads to tissue degeneration and lack of organismal growth in the presence of nutrients. Finally, TOR function is likely to be mediated through TOR Complex 1 as its disruption recapitulates signs of the TOR phenotype. Our data reveal novel roles for TOR signaling in controlling adult stem cells at a systemic level and suggest a new paradigm for studying TOR function during physiological turnover and regeneration.

Cellular fate decisions are influenced by their topographical location in the adult body. For instance, tissue repair and neoplastic growth are greater in anterior than in posterior regions of adult animals. However, the molecular underpinnings of these regional differences are unknown. We identified a regional switch in the adult planarian body upon systemic disruption of homologous recombination with RNA-interference of Rad51. Rad51 knock-down increases DNA double strand breaks (DSB) throughout the body, but stem cells react differently depending on their location along the anteroposterior axis. In the presence of extensive DSB, cells in the anterior part of the body resist death, whereas cells in the posterior region undergo apoptosis. Furthermore, we found that presence of brain tissue is capable of inducing proliferation of cells with DNA damage and that the retinoblastoma pathway enables overproliferation of cells with DSB while attending to the demands of tissue growth and repair. Our results implicate both autonomous and non-autonomous mechanisms as key mediators of regional cell behavior and cellular transformation in the adult body.