Planarian Stem Cell Heterogeneity

Starvation resistance in planarians: multiple strategies to get a thrifty phenotype

Starvation resistance is a life-saving mechanism for many organisms facing food availability fluctuation in the natural environment. Different strategies have been episodically identified for some model organisms, the first of which was the ability to suppress metabolic rate. Among the identified strategies, the ability of planarians to shrink their body under fasting conditions and revert the process after feeding (the growth-degrowth process) represents a fascinating mechanism to face long periods of fasting. The growth-degrowth process is strictly related to the capability of planarians to continuously maintain tissue homeostasis and body proportions even in challenging conditions, thanks to the presence of a population of pluripotent stem cells. Here, we take advantage of several previous studies describing the growth-degrowth process and of recent progress in the understanding of planarian homeostasis mechanisms, to identify tissue-selective transcriptional downregulation as a driving strategy for the development of a thrifty phenotype, and the p53 transcription factor as a player in adjusting tissue homeostasis in accordance with food availability.

Microtubule-associated protein 1B is implicated in stem cell commitment and nervous system regeneration in planarians

Microtubule-associated 1B (MAP1B) proteins are expressed at the nervous system level where they control cytoskeleton activity and regulate neurotransmitter release. Here, we report about the identification of a planarian MAP1B factor (DjMap1B) that is enriched in cephalic ganglia and longitudinal nerve cords but not in neoblasts, the plentiful population of adult stem cells present in planarians, thanks to which these animals can continuously cell turnover and regenerate any lost body parts. DjMap1B knockdown induces morphological anomalies in the nervous system and affects neoblast commitment. Our data put forward a correlation between a MAP1B factor and stem cells and suggest a function of the nervous system in non-cell autonomous control of planarian stem cells.

Cell quiescence in planarian stem cells, interplay between p53 and nutritional stimuli

Cell quiescence appeared early in evolution as an adaptive response to adverse conditions (i.e. nutrient depletion). In metazoans, quiescence has been involved in additional processes like tissue homeostasis, which is made possible by the presence of adult stem cells (ASCs). Cell cycle control machinery is a common hub for quiescence entrance, and evidence indicates a role for p53 in establishing the quiescent state of undamaged cells. Mechanisms responsible for waking up quiescent cells remain elusive, and nutritional stimulus, as a legacy of its original role, still appears to be a player in quiescence exit. Planarians, rich in ASCs, represent a suitable system in which we characterized a quiescent population of ASCs, the dorsal midline cord (DMC) cells, exhibiting unique transcriptional features and maintained quiescent by p53 and awakened upon feeding. The function of DMC cells is puzzling and we speculate that DMC cells, despite retaining ancient properties, might represent a functional drift in which quiescence has been recruited to provide evolutionary advantages.

Djhsp70s, especially Djhsp70c, play a key role in planarian regeneration and tissue homeostasis by regulating cell proliferation and apoptosis

Heat shock protein 70 family (HSP70s) is one of the most conserved and important group of HSPs as molecular chaperones, which plays an important role in cytoprotection, anti-apoptosis and so on. However, the molecular mechanism of HSP70s in animal regeneration remains to be delineated. In this study, we investigate the roles of HSP70s in regeneration of planarian. The four genes, Djhsp70a, Djhsp70b, Djhsp70c, and Djhsp70d of the HSP70s, are selected from the transcriptome database, because of their high expression levels in planarians. We then study the biological roles of each gene by conducting various experimental techniques, including RNAi, RT-PCR, WISH, Whole-mount immunostaining and TUNEL. The results show: (1) External stressors, such as temperature, tissue damage and ionic liquid upregulate the expression of Djhsp70s significantly. (2) The gene expression of Djhsp70s in planarians exhibits dynamic patterns. According to the result of WISH, the Djhsp70s are mainly expressed in parenchymal tissues on both sides of the body as well as blastema. It is consistent with the data of qRT-PCR. (3) After RNA interference of Djhsp70s, the worms experience cephalic regression and lysis, body curling, stagnant regeneration and death. (4) Knockdown of Djhsp70s affect the cell proliferation and apoptosis. These results suggest that Djhsp70s are not only conserved in cytoprotection, but involved in homeostasis maintenance and regeneration process by regulating coordination of cell proliferation and apoptosis in planarians.

Artificially altered gravity elicits cell homeostasis imbalance in planarian worms, and cerium oxide nanoparticles counteract this effect

Gravity alterations elicit complex and mostly detrimental effects on biological systems. Among these, a prominent role is occupied by oxidative stress, with consequences for tissue homeostasis and development. Studies in altered gravity are relevant for both Earth and space biomedicine, but their implementation using whole organisms is often troublesome. Here we utilize planarians, simple worm model for stem cell and regeneration biology, to characterize the pathogenic mechanisms brought by artificial gravity alterations. In particular, we provide a comprehensive evaluation of molecular responses in intact and regenerating specimens, and demonstrate a protective action from the space-apt for nanotechnological antioxidant cerium oxide nanoparticles.

Stem Cells and Innate Immunity in Aquatic Invertebrates: Bridging Two Seemingly Disparate Disciplines for New Discoveries in Biology

The scopes related to the interplay between stem cells and the immune system are broad and range from the basic understanding of organism's physiology and ecology to translational studies, further contributing to (eco)toxicology, biotechnology, and medicine as well as regulatory and ethical aspects. Stem cells originate immune cells through hematopoiesis, and the interplay between the two cell types is required in processes like regeneration. In addition, stem and immune cell anomalies directly affect the organism's functions, its ability to cope with environmental changes and, indirectly, its role in ecosystem services. However, stem cells and immune cells continue to be considered parts of two branches of biological research with few interconnections between them. This review aims to bridge these two seemingly disparate disciplines towards much more integrative and transformative approaches with examples deriving mainly from aquatic invertebrates. We discuss the current understanding of cross-disciplinary collaborative and emerging issues, raising novel hypotheses and comments. We also discuss the problems and perspectives of the two disciplines and how to integrate their conceptual frameworks to address basic equations in biology in a new, innovative way.

Sub-Lethal 5-Fluorouracil Dose Challenges Planarian Stem Cells Promoting Transcriptional Profile Changes in the Pluripotent Sigma-Class Neoblasts

Under physiological conditions, the complex planarian neoblast system is a composite of hierarchically organized stem cell sub-populations with sigma-class neoblasts, including clonogenic neoblasts, endowed with larger self-renewal and differentiation capabilities, thus generating all the other sub-populations and dominating the regenerative process. This complex system responds to differentiated tissue demands, ensuring a continuous cell turnover in a way to replace aged specialized cells and maintain tissue functionality. Potency of the neoblast system can be appreciated under challenging conditions in which these stem cells are massively depleted and the few remaining repopulate the entire body, ensuring animal resilience. These challenging conditions offer the possibility to deepen the relationships among different neoblast sub-populations, allowing to expose uncanonical properties that are negligible under physiological conditions. In this paper, we employ short, sub-lethal 5-fluorouracil treatment to specifically affect proliferating cells passing through the S phase and demonstrate that S-phase slowdown triggers a shift in the transcriptional profile of sigma neoblasts, which reduces the expression of their hallmark sox-P1. Later, some cells reactivate sox-P1 expression, suggesting that some neoblasts in the earlier steps of commitment could modulate their expression profile, reacquiring a wider differentiative potential.

Long-term Outcome of Autologous Hematopoietic Stem Cell Infusion in Cirrhosis: Waning Effect over Time

Background and Aims: Long-term data on cell-based therapies, including hematopoietic stem cell infusion in cirrhosis, are sparse and lacking. Methods: Patients with cirrhosis of non-viral etiology received either standard-of-care (n = 23) or autologous CD34+ cell infusion through the hepatic artery (n = 22). Study patients received granulocyte colony-stimulating factor (commonly known as G-CSF) injections at 520 µgm per day for 3 days, followed by leukapheresis and CD34+ cell infusion into the hepatic artery. The Control group received standard-of-care treatment. Results: Mean CD34+ cell count on the third day of G-CSF injection was 27.00 ± 20.43 cells/µL 81.84 ± 11.99 viability and purity of 80-90%. Significant improvement in the model of end-stage liver disease (commonly known as MELD) score (15.75 ± 5.13 vs. 19.94 ± 6.68, p = 0.04) was noted at end of 3 months and 1 year (15.5 ± 5.3 vs. 19.8 ± 6.4, p = 0.04) but was not statistically different at end of the second (17.2 ± 5.5 vs. 20.3 ± 6.8, p = 0.17) and third-year (18.4 ± 6.1 vs. 21.3 ± 6.4, p = 0.25). No difference in mortality (6/23 vs. 5/23) was noted. Conclusions: Autologous CD34+ cell infusion effectively improved liver function and MELD score up to 1 year but the sustained benefit was not maintained at the end of 3 years, possibly due to ongoing progression of the underlying disease.

Dynamics of interaction and effects of microplastics on planarian tissue regeneration and cellular homeostasis

Increasing microplastics pollution of marine and terrestrial water is a concerning issue for ecosystems and human health. Nevertheless, the interaction of microplastics with freshwater biota is still a poorly explored field. In order to achieve information concerning the uptake, distribution and effect of microplastics in planarians, Dugesia japonica specimens have been fed with mixtures of food and differently shaped and sized plastic particles. Feeding activity and food intake were non-altered by the presence of high concentrations of different types of plastic particles. However, the persistence of microplastic within the planarian body was a function of size/shape, being small spheres (<10 μm in diameter) and short fibers (14 μm large and 5/6 μm length) more persisting than larger spheres and longer fibers which were eliminated almost entirely by ejection in a few hours. Transmission electron microscopy analysis demonstrated that at least part of microplastics was phagocytized by the enterocytes. Chronic exposure to small plastic did not alter the regenerative ability but caused a significant reduction of the gut epithelium thickness and lipid content of enterocytes, together with the induction of apoptotic cell death, modulation of Djgata 4/5/6 expression and reduced growth rate. The ability of microplastic to perturb planarian homeostasis is concerning being them extremely resilient against mechanical and chemical insults and suggests possible harmful effects upon other more susceptible species in freshwater ecosystems.

Opposite effects of low intensity light of different wavelengths on the planarian regeneration rate

Planarian freshwater flatworms have the unique ability to regenerate due to stem cell activity. The process of regeneration is extremely sensitive to various factors, including light radiation. Here, the effect of low-intensity LED light of different wavelengths on regeneration, stem cell proliferation and gene expression associated with these processes was studied. LED matrices with different wavelengths (red (λ_max = 635 nm), green (λ_max = 520 nm) and blue (λ_max = 463 nm), as well as LED laser diodes (red (λ_max = 638.5 nm), green (λ_max = 533 nm) and blue (λ_max = 420 nm), were used in the experiments. Computer-assisted morphometry, whole-mount immunocytochemical study and RT-PCR were used to analyze the biological effects of LED light exposure on the planarian regeneration in vivo. It was found that a one-time exposure of regenerating planarians with low-intensity red light diodes stimulated head blastema growth in a dose-dependent manner (up to 40%). The green light exposure of planarians resulted in the opposite effect, showing a reduced head blastema growth rate by up to 21%. The blue light exposure did not lead to any changes in the rate of head blastema growth. The maximum effects of light exposure were observed at a dose of 175.2 mJ/cm². No significant differences were revealed in the dynamics of neoblasts' (planarian stem cells) proliferation under red and green light exposure. However, the RT-PCR gene expression analysis of 46 wound-induced genes revealed their up-regulation upon red LED light exposure, and down-regulation upon green light exposure. Thus, we have demonstrated that the planarian regeneration process is rather sensitive to the effects of low-intensity light radiation of certain wavelengths, the biological activity of red and green light being dictated by the different expression of the genes regulating transcriptional activity.