JNK controls the onset of mitosis in planarian stem cells and triggers apoptotic cell death required for regeneration and remodeling

A PAK family kinase and the Hippo/Yorkie pathway modulate WNT signaling to functionally integrate body axes during regeneration

Successful regeneration of missing tissues requires seamless integration of positional information along the body axes. Planarians, which regenerate from almost any injury, use conserved, developmentally important signaling pathways to pattern the body axes. However, the molecular mechanisms which facilitate cross talk between these signaling pathways to integrate positional information remain poorly understood. Here, we report a p21-activated kinase (smed-pak1) which functionally integrates the anterior-posterior (AP) and the medio-lateral (ML) axes. pak1 inhibits WNT/β-catenin signaling along the AP axis and, functions synergistically with the β-catenin-independent WNT signaling of the ML axis. Furthermore, this functional integration is dependent on warts and merlin-the components of the Hippo/Yorkie (YKI) pathway. Hippo/YKI pathway is a critical regulator of body size in flies and mice, but our data suggest the pathway regulates body axes patterning in planarians. Our study provides a signaling network integrating positional information which can mediate coordinated growth and patterning during planarian regeneration.

Mechanistic regulation of planarian shape during growth and degrowth

Adult planarians can grow when fed and degrow (shrink) when starved while maintaining their whole-body shape. It is unknown how the morphogens patterning the planarian axes are coordinated during feeding and starvation or how they modulate the necessary differential tissue growth or degrowth. Here, we investigate the dynamics of planarian shape together with a theoretical study of the mechanisms regulating whole-body proportions and shape. We found that the planarian body proportions scale isometrically following similar linear rates during growth and degrowth, but that fed worms are significantly wider than starved worms. By combining a descriptive model of planarian shape and size with a mechanistic model of anterior-posterior and medio-lateral signaling calibrated with a novel parameter optimization methodology, we theoretically demonstrate that the feedback loop between these positional information signals and the shape they control can regulate the planarian whole-body shape during growth. Furthermore, the computational model produced the correct shape and size dynamics during degrowth as a result of a predicted increase in apoptosis rate and pole signal during starvation. These results offer mechanistic insights into the dynamic regulation of whole-body morphologies.

Biomarkers of chemotherapy-induced cardiotoxicity: toward precision prevention using extracellular vesicles

Detrimental side effects of drugs like doxorubicin, which can cause cardiotoxicity, pose barriers for preventing cancer progression, or treating cancer early through molecular interception. Extracellular vesicles (EVs) are valued for their potential as biomarkers of human health, chemical and molecular carcinogenesis, and therapeutics to treat disease at the cellular level. EVs are released both during normal growth and in response to toxicity and cellular death, playing key roles in cellular communication. Consequently, EVs may hold promise as precision biomarkers and therapeutics to prevent or offset damaging off-target effects of chemotherapeutics. EVs have promise as biomarkers of impending cardiotoxicity induced by chemotherapies and as cardioprotective therapeutic agents. However, EVs can also mediate cardiotoxic cues, depending on the identity and past events of their parent cells. Understanding how EVs mediate signaling is critical toward implementing EVs as therapeutic agents to mitigate cardiotoxic effects of chemotherapies. For example, it remains unclear how mixtures of EV populations from cells exposed to toxins or undergoing different stages of cell death contribute to signaling across cardiac tissues. Here, we present our perspective on the outlook of EVs as future clinical tools to mitigate chemotherapy-induced cardiotoxicity, both as biomarkers of impending cardiotoxicity and as cardioprotective agents. Also, we discuss how heterogeneous mixtures of EVs and transient exposures to toxicants may add complexity to predicting outcomes of exogenously applied EVs. Elucidating how EV cargo and signaling properties change during dynamic cellular events may aid precision prevention of cardiotoxicity in anticancer treatments and development of safer chemotherapeutics.

Understanding the molecular mechanism of regeneration through apoptosis-induced compensatory proliferation studies - updates and future aspects

AICP is a crucial process that maintaining tissue homeostasis and regeneration. In the past, cell death was perceived merely as a means to discard cells without functional consequences. However, during regeneration, effector caspases orchestrate apoptosis, releasing signals that activate stem cells, thereby compensating for tissue loss across various animal models. Despite significant progress, the activation of Wnt3a by caspase-3 remains a focal point of research gaps in AICP mechanisms, spanning from lower to higher regenerative animals. This inquiry into the molecular intricacies of caspase-3-induced Wnt3a activation contributes to a deeper understanding of the links between regeneration and cancer mechanisms. Our report provides current updates on AICP pathways, delineating research gaps and highlighting the potential for future investigations aimed at enhancing our comprehension of this intricate process.

Apoptotic signaling: Beyond cell death

Apoptosis is the best described form of regulated cell death, and was, until relatively recently, considered irreversible once particular biochemical points-of-no-return were activated. In this manuscript, we examine the mechanisms cells use to escape from a self-amplifying death signaling module. We discuss the role of feedback, dynamics, propagation, and noise in apoptotic signaling. We conclude with a revised model for the role of apoptosis in animal development, homeostasis, and disease.

MRLC controls apoptotic cell death and functions to regulate epidermal development during planarian regeneration and homeostasis

Adult stem cells (ASCs) are pluripotent cells with the capacity to self-renew and constantly replace lost cells due to physiological turnover or injury. Understanding the molecular mechanisms of the precise coordination of stem cell proliferation and proper cell fate decision is important to regeneration and organismal homeostasis. The planarian epidermis provides a highly tractable model to study ASC complex dynamic due to the distinct spatiotemporal differentiation stages during lineage development. Here, we identified the myosin regulatory light chain (MRLC) homologue in the Dugesia japonica transcriptome. We found high expression levels of MRLC in wound region during regeneration and also expressed in late epidermal progenitors as an essential regulator of the lineage from neoblasts to mature epidermal cells. We investigated the function of MRLC using in situ hybridization, real-time polymerase chain reaction and double fluorescent and uncovered the potential mechanism. Knockdown of MRLC leads to a remarkable increase in cell death, causes severe abnormalities during regeneration and homeostasis and eventually leads to animal death. The global decrease in epidermal cell in MRLC RNAi animals induces accelerated epidermal proliferation and differentiation. Additionally, we find that MRLC is co-expressed with cdc42 and acts cooperatively to control the epidermal lineage development by affecting cell death. Our results uncover an important role of MRLC, as an inhibitor of apoptosis, involves in epidermal development.

Djsnon, a downstream gene of Djfoxk1, is required for the regeneration of the planarian central nervous system

Regulators of adult neurogenesis are crucial targets for neuronal repair. Freshwater planarians are ideal model systems for studying neuronal regeneration as they can regenerate their entire central nervous system (CNS) using pluripotent adult stem cells. Here, we identified Djfoxk1 in planarian Dugesia japonica to be required for planarian CNS regeneration. Knockdown of Djfoxk1 inhibits the regeneration of the cephalic ganglia, resulting in the failure of eye regeneration. By RNAi screening of Djfoxk1 downstream genes, we identified Djsnon as another regulator of planarian neuronal regeneration. Inhibition of Djsnon with RNA interference (RNAi) results in similar phenotypes caused by Djfoxk1 RNAi without affecting cell proliferation and wound healing. Our findings show that Djsnon as a downstream gene of Djfoxk1 regulates the regeneration of the planarian CNS.

Wnt/β-catenin signalling is required for pole-specific chromatin remodeling during planarian regeneration

For successful regeneration, the identity of the missing tissue must be specified according to the pre-existing tissue. Planarians are ideal for the study of the mechanisms underlying this process; the same field of cells can regrow a head or a tail according to the missing body part. After amputation, the differential activation of the Wnt/β-catenin signal specifies anterior versus posterior identity. Initially, both wnt1 and notum (Wnt inhibitor) are expressed in all wounds, but 48 hours later they are restricted to posterior or anterior facing wounds, respectively, by an unknown mechanism. Here we show that 12 hours after amputation, the chromatin accessibility of cells in the wound region changes according to the polarity of the pre-existing tissue in a Wnt/β-catenin-dependent manner. Genomic analyses suggest that homeobox transcription factors and chromatin-remodeling proteins are direct Wnt/β-catenin targets, which trigger the expression of posterior effectors. Finally, we identify FoxG as a wnt1 up-stream regulator, probably via binding to its first intron enhancer region.

From injury to patterning—MAPKs and Wnt signaling in Hydra

Hydra has a regenerative capacity that is not limited to individual organs but encompasses the entire body. Various global and integrative genome, transcriptome and proteome approaches have shown that many of the signaling pathways and transcription factors present in vertebrates are already present in Cnidaria, the sister group of Bilateria, and are also activated in regeneration. It is now possible to investigate one of the central questions of regeneration biology, i.e., how does the patterning system become activated by the injury signals that initiate regeneration. This review will present the current data obtained in Hydra and draw parallels with regeneration in Bilateria. Important findings of this global analysis are that the Wnt signaling pathway has a dual function in the regeneration process. In the early phase Wnt is activated generically and in a second phase of pattern formation it is activated in a position specific manner. Thus, Wnt signaling is part of the generic injury response, in which mitogen-activated protein kinases (MAPKs) are initially activated via calcium and reactive oxygen species (ROS). The MAPKs, p38, c-Jun N-terminal kinases (JNKs) and extracellular signal-regulated kinases (ERK) are essential for Wnt activation in Hydra head and foot regenerates. Furthermore, the antagonism between the ERK signaling pathway and stress-induced MAPKs results in a balanced induction of apoptosis and mitosis. However, the early Wnt genes are activated by MAPK signaling rather than apoptosis. Early Wnt gene activity is differentially integrated with a stable, β-Catenin-based gradient along the primary body axis maintaining axial polarity and activating further Wnts in the regenerating head. Because MAPKs and Wnts are highly evolutionarily conserved, we hypothesize that this mechanism is also present in vertebrates but may be activated to different degrees at the level of early Wnt gene integration.

The role of the immune microenvironment in bone, cartilage, and soft tissue regeneration: from mechanism to therapeutic opportunity

Bone, cartilage, and soft tissue regeneration is a complex spatiotemporal process recruiting a variety of cell types, whose activity and interplay must be precisely mediated for effective healing post-injury. Although extensive strides have been made in the understanding of the immune microenvironment processes governing bone, cartilage, and soft tissue regeneration, effective clinical translation of these mechanisms remains a challenge. Regulation of the immune microenvironment is increasingly becoming a favorable target for bone, cartilage, and soft tissue regeneration; therefore, an in-depth understanding of the communication between immune cells and functional tissue cells would be valuable. Herein, we review the regulatory role of the immune microenvironment in the promotion and maintenance of stem cell states in the context of bone, cartilage, and soft tissue repair and regeneration. We discuss the roles of various immune cell subsets in bone, cartilage, and soft tissue repair and regeneration processes and introduce novel strategies, for example, biomaterial-targeting of immune cell activity, aimed at regulating healing. Understanding the mechanisms of the crosstalk between the immune microenvironment and regeneration pathways may shed light on new therapeutic opportunities for enhancing bone, cartilage, and soft tissue regeneration through regulation of the immune microenvironment.

Injury-induced MAPK activation triggers body axis formation in Hydra by default Wnt signaling

Hydra's almost unlimited regenerative potential is based on Wnt signaling, but so far it is unknown how the injury stimulus is transmitted to discrete patterning fates in head and foot regenerates. We previously identified mitogen-activated protein kinases (MAPKs) among the earliest injury response molecules in Hydra head regeneration. Here, we show that three MAPKs-p38, c-Jun N-terminal kinases (JNKs), and extracellular signal-regulated kinases (ERKs)-are essential to initiate regeneration in Hydra, independent of the wound position. Their activation occurs in response to any injury and requires calcium and reactive oxygen species (ROS) signaling. Phosphorylated MAPKs hereby exhibit cross talk with mutual antagonism between the ERK pathway and stress-induced MAPKs, orchestrating a balance between cell survival and apoptosis. Importantly, Wnt3 and Wnt9/10c, which are induced by MAPK signaling, can partially rescue regeneration in tissues treated with MAPK inhibitors. Also, foot regenerates can be reverted to form head tissue by a pharmacological increase of β-catenin signaling or the application of recombinant Wnts. We propose a model in which a β-catenin-based stable gradient of head-forming capacity along the primary body axis, by differentially integrating an indiscriminate injury response, determines the fate of the regenerating tissue. Hereby, Wnt signaling acquires sustained activation in the head regenerate, while it is transient in the presumptive foot tissue. Given the high level of evolutionary conservation of MAPKs and Wnts, we assume that this mechanism is deeply embedded in our genome.

Comparative analysis of the survival and regeneration potential of juvenile and matured earthworm, Eudrilus eugeniae, upon in vivo and in vitro maintenance

Eudrilus eugeniae is a clitellum-dependent earthworm that requires intact clitellum segments for its survival and regeneration. The present study aims to interconnect the survival and regeneration ability that varies between in vivo and in vitro maintenance upon different sites of amputation. The amputated portion of the worm that possesses intact clitellum (13th-18th segments) survived and had the potential to regenerate, whereas worms with partial or without clitellum segments only survived and were unable to regenerate. Besides segment length and clitellum segments, clitellum factors also determined the survival, blastemal initiation and differentiation potential. The survivability and regeneration potential of worms were augmented upon in vitro maintenance. Notably, the amputated segments (1st-10th segments) and posterior segments of similar length, which usually die within the 4th day in vivo, survived for more than 60 days in vitro but lacked the regeneration ability. On the other hand, the amputated posterior segments (30th to 37th segments) from juvenile worms, maintained in in vitro condition, survived and initiated blastema with multiple buds but lacked the ability to regenerate. Interestingly, the equal half of adult worm blastema that is maintained in in vitro conditions were able to form the blastema-like structure with the help of a unique stick. The anterior blastema failed to retain the regenerative structure but the posterior portion of the amputated blastema, which is also associated with a small portion of the body segment, showed the ability to retain the regenerative structure. Our results conclude that the survivability is enhanced upon in vitro maintenance and this condition favours the adult dedifferentiated blastemal and stem cell-enriched juvenile posterior segments to form a regenerative blastema.

Interactive toxicity of copper and cadmium in regenerating and adult planarians

In a polluted environment, metals are present as complex mixtures. As a result, organisms are exposed to different metals at the same time, which affects both metal-specific as well as overall toxicity. Detailed information about the molecular mechanisms underlying the adverse effects of combined exposures remains limited in terms of different life stages. In this study, the freshwater planarian Schmidtea mediterranea was used to investigate developmental and physiological responses associated with a combined exposure to Cu and Cd. In addition, the cellular and molecular mechanisms underlying the provoked adverse effects were studied in different exposure scenarios. Mixed exposure resulted in a decline in survival, diverse non-lethal morphological changes, neuroregenerative impairments, altered behaviour and a limited repair capacity. Underlying to these effects, the cellular redox state was altered in all exposure conditions. In adult animals, this led to DNA damage and corresponding transcriptional changes in cell cycle and DNA repair genes. In regenerating animals, changes in hydrogen peroxide and glutathione contents led to regenerative defects. Overall, our results demonstrate that (1) developing organisms are more susceptible to metal exposures, and (2) the toxicity of an individual metal increases significantly in a mixed exposure scenario. These aspects have to be included in current risk assessment strategies.

An Emerging Frontier in Intercellular Communication: Extracellular Vesicles in Regeneration

Regeneration requires cellular proliferation, differentiation, and other processes that are regulated by secreted cues originating from cells in the local environment. Recent studies suggest that signaling by extracellular vesicles (EVs), another mode of paracrine communication, may also play a significant role in coordinating cellular behaviors during regeneration. EVs are nanoparticles composed of a lipid bilayer enclosing proteins, nucleic acids, lipids, and other metabolites, and are secreted by most cell types. Upon EV uptake by target cells, EV cargo can influence diverse cellular behaviors during regeneration, including cell survival, immune responses, extracellular matrix remodeling, proliferation, migration, and differentiation. In this review, we briefly introduce the history of EV research and EV biogenesis. Then, we review current understanding of how EVs regulate cellular behaviors during regeneration derived from numerous studies of stem cell-derived EVs in mammalian injury models. Finally, we discuss the potential of other established and emerging research organisms to expand our mechanistic knowledge of basic EV biology, how injury modulates EV biogenesis, cellular sources of EVs in vivo, and the roles of EVs in organisms with greater regenerative capacity.

DjPtpn11 is an essential modulator of planarian (Dugesia japonica) regeneration

Freshwater planarian Dugesia japonica is an excellent model organism for investigating stem cell behavior during regeneration. Despite studies showing that numerous genetic factors are involved in regeneration, much more research is required to fully understand the molecular mechanisms that orchestrate regeneration. In this study, we identified an evolutionarily conserved gene DjPtpn11(DjShp2). DjPtpn11 transcripts are expressed in neoblasts and some differentiated cells, with a high expression at the newly formed blastema. Its silencing by RNA interference (RNAi) affected anterior regeneration and inhibited the regeneration of posterior regions, including cholinergic and serotonergic neuron regeneration. In adult planarians, DjPtpn11 knockdown did not affect neoblast survival and proliferation but might prevent the stem cell migration and differentiation through ERK signaling. DjPtpn11 was demonstrated to be necessary for the anterior blastema cell differentiation partially via regulating ERK-DjMkpA activity. DjPtpn11 also influenced posterior specification via DjIslet, suggesting that DjPtpn11 may be involved in regulating the Wnt signaling pathway during the development of posterior blastema. Together, these data identified that DjPtpn11 is an essential modulator for the regeneration of planarians, and it may influence the appropriate differentiation of blastema cells.

Defined diets for freshwater planarians

BACKGROUND

Planarian flatworms are popular invertebrate models for basic research on stem cell biology and regeneration. These animals are commonly maintained on a diet of homogenized calf liver or boiled egg yolk in the laboratory, introducing a source of uncontrolled experimental variability.

RESULTS

Here, we report the development of defined diets, prepared entirely from standardized, commercially sourced ingredients, for the freshwater species Schmidtea mediterranea, Dugesia japonica, and Girardia dorotocephala. These food sources provide an opportunity to test the effects of specific nutritional variables on biological phenomena of interest. Defined diet consumption was not sufficient for growth and only partially induced the increase in stem cell division that normally accompanies feeding, suggesting these responses are not solely determined by caloric intake. Our defined diet formulations enable delivery of double-stranded RNA for gene knockdown in a manner that provides unique advantages in some experimental contexts. We also present a new approach for preserving tissue integrity during hydrogen peroxide bleaching of liver-fed animals.

CONCLUSIONS

These tools will empower research on the connections between diet, metabolism, and stem cell biology in the experimentally tractable planarian system.

Strategies for Improving Photodynamic Therapy Through Pharmacological Modulation of the Immediate Early Stress Response

Photodynamic therapy (PDT) is a minimally to noninvasive treatment modality that has emerged as a promising alternative to conventional cancer treatments. PDT induces hyperoxidative stress and disrupts cellular homeostasis in photosensitized cancer cells, resulting in cell death and ultimately removal of the tumor. However, various survival pathways can be activated in sublethally afflicted cancer cells following PDT. The acute stress response is one of the known survival pathways in PDT, which is activated by reactive oxygen species and signals via ASK-1 (directly) or via TNFR (indirectly). The acute stress response can activate various other survival pathways that may entail antioxidant, pro-inflammatory, angiogenic, and proteotoxic stress responses that culminate in the cancer cell's ability to cope with redox stress and oxidative damage. This review provides an overview of the immediate early stress response in the context of PDT, mechanisms of activation by PDT, and molecular intervention strategies aimed at inhibiting survival signaling and improving PDT outcome.

A MEKK1 - JNK mitogen activated kinase (MAPK) cascade module is active in Echinococcus multilocularis stem cells

Background

The metacestode larval stage of the fox-tapeworm Echinococcus multilocularis causes alveolar echinococcosis by tumour-like growth within the liver of the intermediate host. Metacestode growth and development is stimulated by host-derived cytokines such as insulin, fibroblast growth factor, and epidermal growth factor via activation of cognate receptor tyrosine kinases expressed by the parasite. Little is known, however, concerning signal transmission to the parasite nucleus and cross-reaction with other parasite signalling systems.

Methodology/Principal findings

Using bioinformatic approaches, cloning, and yeast two-hybrid analyses we identified a novel mitogen-activated kinase (MAPK) cascade module that consists of E. multilocularis orthologs of the tyrosine kinase receptor interactor Growth factor receptor-bound 2, EmGrb2, the MAPK kinase kinase EmMEKK1, a novel MAPK kinase, EmMKK3, and a close homolog to c-Jun N-terminal kinase (JNK), EmMPK3. Whole mount in situ hybridization analyses indicated that EmMEKK1 and EmMPK3 are both expressed in E. multilocularis germinative (stem) cells but also in differentiated or differentiating cells. Treatment with the known JNK inhibitor SP600125 led to a significantly reduced formation of metacestode vesicles from stem cells and to a specific reduction of proliferating stem cells in mature metacestode vesicles.

Conclusions/Significance

We provide evidence for the expression of a MEKK1-JNK MAPK cascade module which, in mammals, is crucially involved in stress responses, cytoskeletal rearrangements, and apoptosis, in E. multilocularis stem cells. Inhibitor studies indicate an important role of JNK signalling in E. multilocularis stem cell survival and/or maintenance. Our data are relevant for molecular and cellular studies into crosstalk signalling mechanisms that govern Echinococcus stem cell function and introduce the JNK signalling cascade as a possible target of chemotherapeutics against echinococcosis.

The metacestode larva of the tapeworm E. multilocularis grows infiltrative, like a malignant tumour, within the liver of the host thus causing the lethal disease alveolar echinococcosis. Previous work established that the metacestode senses signals of host hormones and cytokines by expressing surface receptors that share high homology with respective host receptors. However, little is known how these signals are transmitted from the parasite cell surface to the nucleus to alter gene expression. In this work, the authors present a module of several protein kinases that typically transmit cytokine signals from surface receptors to central regulators called mitogen-activated protein kinases (MAPK). The authors demonstrate that this module is active in parasite stem cells, which drive the development of metacestode larva. They also show that inhibitors directed against one component of the module, EmMPK3, affect maintenance and/or survival of stem cells in the metacestode and prevent the formation of metacestode larva from parasite cell cultures. This information facilitates molecular and cellular studies to unravel the complex signalling network that regulate Echinococcus stem cell proliferation in response to host signals. Furthermore, these data could open new ways of anti-parasitic chemotherapy by introducing EmMPK3 as a possible drug target.

JNK Mediates Differentiation, Cell Polarity and Apoptosis During Amphioxus Development by Regulating Actin Cytoskeleton Dynamics and ERK Signalling

c-Jun N-terminal kinase (JNK) is a multi-functional protein involved in a diverse array of context-dependent processes, including apoptosis, cell cycle regulation, adhesion, and differentiation. It is integral to several signalling cascades, notably downstream of non-canonical Wnt and mitogen activated protein kinase (MAPK) signalling pathways. As such, it is a key regulator of cellular behaviour and patterning during embryonic development across the animal kingdom. The cephalochordate amphioxus is an invertebrate chordate model system straddling the invertebrate to vertebrate transition and is thus ideally suited for comparative studies of morphogenesis. However, next to nothing is known about JNK signalling or cellular processes in this lineage. Pharmacological inhibition of JNK signalling using SP600125 during embryonic development arrests gastrula invagination and causes convergence extension-like defects in axial elongation, particularly of the notochord. Pharynx formation and anterior oral mesoderm derivatives like the preoral pit are also affected. This is accompanied by tissue-specific transcriptional changes, including reduced expression of six3/6 and wnt2 in the notochord, and ectopic wnt11 in neurulating embryos treated at late gastrula stages. Cellular delamination results in accumulation of cells in the gut cavity and a dorsal fin-like protrusion, followed by secondary Caspase-3-mediated apoptosis of polarity-deficient cells, a phenotype only partly rescued by co-culture with the pan-Caspase inhibitor Z-VAD-fmk. Ectopic activation of extracellular signal regulated kinase (ERK) signalling in the neighbours of extruded notochord and neural cells, possibly due to altered adhesive and tensile properties, as well as defects in cellular migration, may explain some phenotypes caused by JNK inhibition. Overall, this study supports conserved functions of JNK signalling in mediating the complex balance between cell survival, apoptosis, differentiation, and cell fate specification during cephalochordate morphogenesis.

Decoding Stem Cells: An Overview on Planarian Stem Cell Heterogeneity and Lineage Progression

Planarians are flatworms capable of whole-body regeneration, able to regrow any missing body part after injury or amputation. The extraordinary regenerative capacity of planarians is based upon the presence in the adult of a large population of somatic pluripotent stem cells. These cells, called neoblasts, offer a unique system to study the process of stem cell specification and differentiation in vivo. In recent years, FACS-based isolation of neoblasts, RNAi functional analyses as well as high-throughput approaches such as single-cell sequencing have allowed a rapid progress in our understanding of many different aspects of neoblast biology. Here, we summarize our current knowledge on the molecular signatures that define planarian neoblasts heterogeneity, which includes a percentage of truly pluripotent stem cells, and guide the commitment of pluripotent neoblasts into lineage-specific progenitor cells, as well as their differentiation into specific planarian cell types.

Apoptosis Quantification in Tissue: Development of a Semi-Automatic Protocol and Assessment of Critical Steps of Image Processing

Apoptosis is associated with numerous phenotypical characteristics, and is thus studied with many tools. In this study, we compared two broadly used apoptotic assays: TUNEL and staining with an antibody targeting the activated form of an effector caspase. To compare them, we developed a protocol based on commonly used tools such as image filtering, z-projection, and thresholding. Even though it is commonly used in image-processing protocols, thresholding remains a recurring problem. Here, we analyzed the impact of processing parameters and readout choice on the accuracy of apoptotic signal quantification. Our results show that TUNEL is quite robust, even if image processing parameters may not always allow to detect subtle differences of the apoptotic rate. On the contrary, images from anti-cleaved caspase staining are more sensitive to handle and necessitate being processed more carefully. We then developed an open-source Fiji macro automatizing most steps of the image processing and quantification protocol. It is noteworthy that the field of application of this macro is wider than apoptosis and it can be used to treat and quantify other kind of images.

Beyond Casual Resemblances: Rigorous Frameworks for Comparing Regeneration Across Species

The majority of animal phyla have species that can regenerate. Comparing regeneration across animals can reconstruct the molecular and cellular evolutionary history of this process. Recent studies have revealed some similarity in regeneration mechanisms, but rigorous comparative methods are needed to assess whether these resemblances are ancestral pathways (homology) or are the result of convergent evolution (homoplasy). This review aims to provide a framework for comparing regeneration across animals, focusing on gene regulatory networks (GRNs), which are substrates for assessing process homology. The homology of the wound-induced activation of Wnt signaling and of adult stem cells are discussed as examples of ongoing studies of regeneration that enable comparisons in a GRN framework. Expanding the study of regeneration GRNs in currently studied species and broadening taxonomic sampling for these approaches will identify processes that are unifying principles of regeneration biology across animals. These insights are important both for evolutionary studies of regeneration and for human regenerative medicine. Expected final online publication date for the Annual Review of Cell and Developmental Biology, Volume 37 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Whole-Body Regeneration in the Lobate Ctenophore Mnemiopsis leidyi

Ctenophores (a.k.a. comb jellies) are one of the earliest branching extant metazoan phyla. Adult regenerative ability varies greatly within the group, with platyctenes undergoing both sexual and asexual reproduction by fission while others in the genus Beroe having completely lost the ability to replace missing body parts. We focus on the unique regenerative aspects of the lobate ctenophore, Mnemiopsis leidyi, which has become a popular model for its rapid wound healing and tissue replacement, optical clarity, and sequenced genome. M. leidyi’s highly mosaic, stereotyped development has been leveraged to reveal the polar coordinate system that directs whole-body regeneration as well as lineage restriction of replacement cells in various regenerating organs. Several cell signaling pathways known to function in regeneration in other animals are absent from the ctenophore’s genome. Further research will either reveal ancient principles of the regenerative process common to all animals or reveal novel solutions to the stability of cell fates and whole-body regeneration.

From Cell Death to Regeneration: Rebuilding After Injury

The ability to regrow lost or damaged tissues is widespread, but highly variable among animals. Understanding this variation remains a challenge in regeneration biology. Numerous studies from Hydra to mouse have shown that apoptosis acts as a potent and necessary mechanism in regeneration. Much is known about the involvement of apoptosis during normal development in regulating the number and type of cells in the body. In the context of regeneration, apoptosis also regulates cell number and proliferation in tissue remodeling. Apoptosis acts both early in the process to stimulate regeneration and later to regulate regenerative patterning. Multiple studies indicate that apoptosis acts as a signal to stimulate proliferation within the regenerative tissues, producing the cells needed for full regeneration. The conservation of apoptosis as a regenerative mechanism demonstrated across species highlights its importance and motivates the continued investigation of this important facet of programmed cell death. This review summarizes what is known about the roles of apoptosis during regeneration, and compares regenerative apoptosis with the mechanisms and function of apoptosis in development. Defining the complexity of regenerative apoptosis will contribute to new knowledge and perspectives for understanding mechanisms of apoptosis induction and regulation.

Analysis of Fox genes in Schmidtea mediterranea reveals new families and a conserved role of Smed-foxO in controlling cell death

The forkhead box (Fox) genes encode transcription factors that control several key aspects of development. Present in the ancestor of all eukaryotes, Fox genes underwent several duplications followed by loss and diversification events that gave rise to the current 25 families. However, few Fox members have been identified from the Lophotrochozoa clade, and specifically from planarians, which are a unique model for understanding development, due to the striking plasticity of the adult. The aim of this study was to identify and perform evolutionary and functional studies of the Fox genes of lophotrochozoan species and, specifically, of the planarian Schmidtea mediterranea. Generating a pipeline for identifying Forkhead domains and using phylogenetics allowed us the phylogenetic reconstruction of Fox genes. We corrected the annotation for misannotated genes and uncovered a new family, the QD, present in all metazoans. According to the new phylogeny, the 27 Fox genes found in Schmidtea mediterranea were classified into 12 families. In Platyhelminthes, family losses were accompanied by extensive gene diversification and the appearance of specific families, the A(P) and N(P). Among the newly identified planarian Fox genes, we found a single copy of foxO, which shows an evolutionary conserved role in controlling cell death.

Anticancer and Immunomodulatory Effects of Polysaccharides

Cancer is one of the leading causes of death and one of the most important public health problems in the world. And every year, millions of new cancers and hundreds of thousands of cancer-related deaths are reported worldwide. In recent decades, a number of biologically active polysaccharides and polysaccharide-protein complexes have been isolated from plants, lichen, algae, yeast, fungi and mushroom, and due to their antitumor and immunomodulatory properties, these compounds have received considerable attention. Overall, the two key mechanisms by which polysaccharides act on tumor cells are direct action (inhibition of cancer cell growth and induction of programmed cell death/apoptosis) and indirect action (stimulation of immunity). Immunosuppressive effects are recognizable in both cancer patients and tumor bearing animals, suggesting that the immune system plays an important role in the immune surveillance of cancer cells. Thus, enhancement of the host immune response has been evaluated as a possible way of inhibiting tumor growth without damaging the host. In addition to their therapeutic and prophylactic properties, the polysaccharides are effective and less toxic than chemotherapy. The anticancer activity and immunomodulatory effects of most polysaccharides have shown the promising and real potential for the benefits of human health.

Cellular and molecular responses-mediated by DjMEK1/2 are necessary for planarian regeneration

The planarian flatworm is an ideal model to study the regeneration due to its robust regenerative ability. A variety of cellular response activities have been reported to be involved in the regeneration process, including the mitogen-activated protein kinase (MAPK) signaling. However, the mechanism of MAPK pathway in regenerative responses is still unclear. In this study, we employed the planarian, Dugesia japonica, as the model to investigate the function of MAP-extracellular signal-regulated kinase (MEK), an important component of MAPK signaling pathway, in the regeneration process. We found that MEK was required for the missing tissue response after several amputation and subsequent regeneration. MEK not only affected the size of blastema in the early stage of regeneration by regulating stem cell proliferation, but also determined the planarian's regeneration through balancing cell proliferation and apoptosis. In addition, the activation of Wnt pathway partially rescued regenerative defects induced by inhibition of MEK. Taken together, our results highlight a crucial role of MEK signaling in the planarian regeneration.

Molecular impact of launch related dynamic vibrations and static hypergravity in planarians

Although many examples of simulated and real microgravity demonstrating their profound effect on biological systems are described in literature, few reports deal with hypergravity and vibration effects, the levels of which are severely increased during the launch preceding the desired microgravity period. Here, we used planarians, flatworms that can regenerate any body part in a few days. Planarians are an ideal model to study the impact of launch-related hypergravity and vibration during a regenerative process in a "whole animal" context. Therefore, planarians were subjected to 8.5 minutes of 4 g hypergravity (i.e. a human-rated launch level) in the Large Diameter Centrifuge (LDC) and/or to vibrations (20-2000 Hz, 11.3 G _rms) simulating the conditions of a standard rocket launch. The transcriptional levels of genes (erg-1, runt-1, fos, jnk, and yki) related with the early stress response were quantified through qPCR. The results show that early response genes are severely deregulated after static and dynamic loads but more so after a combined exposure of dynamic (vibration) and static (hypergravity) loads, more closely simulating real launch exposure profiles. Importantly, at least four days after the exposure, the transcriptional levels of those genes are still deregulated. Our results highlight the deep impact that short exposures to hypergravity and vibration have in organisms, and thus the implications that space flight launch could have. These phenomena should be taken into account when planning for well-controlled microgravity studies.

Caspase dependent apoptosis is required for anterior regeneration in Aeolosoma viride and its related gene expressions are regulated by the Wnt signaling pathway

Although apoptosis has been widely observed during the regenerative process, the mechanisms by which it is regulated and its roles in regeneration remained unclear. In this study, we introduced Aeolosoma viride, a fresh water annelid with an extraordinary regenerative ability as our model organism to study the functions and regulations of apoptotic caspases. Here we showed that major events of apoptosis were detected near the wounded area and showed spatial correlation with the expression patterns of caspase gene namely Avi-caspase X and two apoptosis regulators namely Avi-Bax and Avi-Bcl-xL. Next, we investigated how Avi-caspase X gene expression and apoptosis influence regeneration following head amputation. RNA interference of Avi-caspase X reduced the amounts of apoptotic cells, as well as the percentage of successful regeneration, suggesting a critical role for apoptosis in anterior regeneration of A. viride. In addition, we also discovered that the expression of apoptotic caspases was regulated by the canonical Wnt signaling pathway. Together, our study showed that caspase dependent apoptosis was critical to the anterior regeneration of A. viride, and could be regulated by the canonical Wnt signaling pathway.

Injury Delays Stem Cell Apoptosis after Radiation in Planarians

Stem cells are continuously exposed to multiple stresses, including radiation and tissue injury. As central drivers of tissue repair and regeneration, it is necessary to understand how their behavior is influenced by these stressors. Planarians have an abundant population of stem cells that are rapidly eliminated after radiation exposure via apoptosis. Low doses of radiation eliminate the majority of these stem cells, allowing a few to remain [1]. Here, we combine radiation with injury to define how stem cells respond to tissue damage. We find that a variety of injuries induced within a defined window of time surrounding radiation cause stem cells to outlast those in uninjured animals. Injury stimulates localized cell death adjacent to wounds [2], in the same regions where stem cells persist. This persistence occurs in the absence of proliferation. Instead, stem cells are retained near the wound due to delayed apoptosis, which we quantify by combining fluorescence-activated cell sorting (FACS) with annexin V staining. Pharmacological inhibition of the mitogen-activated protein (MAP) kinase extracellular signal-regulated kinase (ERK) prevents stem cell persistence after injury, implicating wound-induced ERK activity in this response. By combining radiation with injury, our work reveals a novel connection between dying cells and stem cells that remain. Furthermore, the ability to induce stem cell persistence after radiation provides a paradigm to study mechanisms that may contribute to unanticipated consequences of injury, such as tumorigenesis.

Planarian cell number depends on blitzschnell, a novel gene family that balances cell proliferation and cell death

Control of cell number is crucial to define body size during animal development and to restrict tumoral transformation. The cell number is determined by the balance between cell proliferation and cell death. Although many genes are known to regulate those processes, the molecular mechanisms underlying the relationship between cell number and body size remain poorly understood. This relationship can be better understood by studying planarians, flatworms that continuously change their body size according to nutrient availability. We identified a novel gene family, blitzschnell (bls), that consists of de novo and taxonomically restricted genes that control cell proliferation:cell death ratio. Their silencing promotes faster regeneration and increases cell number during homeostasis. Importantly, this increase in cell number leads to an increase in body size only in a nutrient-rich environment; in starved planarians, silencing results in a decrease in cell size and cell accumulation that ultimately produces overgrowths. bls expression is downregulated after feeding and is related to activity of the insulin/Akt/mTOR network, suggesting that the bls family evolved in planarians as an additional mechanism for restricting cell number in nutrient-fluctuating environments.

Pro-apoptotic and pro-proliferation functions of the JNK pathway of Drosophila: roles in cell competition, tumorigenesis and regeneration

The Jun N-terminal kinase (JNK) is a member of the mitogen-activated protein kinase family. It appears to be conserved in all animal species where it regulates important physiological functions involved in apoptosis, cell migration, cell proliferation and regeneration. In this review, we focus on the functions of JNK in Drosophila imaginal discs, where it has been reported that it can induce both cell death and cell proliferation. We discuss this apparent paradox in the light of recent findings and propose that the pro-apoptotic and the pro-proliferative functions are intrinsic properties of JNK activity. Whether one function or another is predominant depends on the cellular context.

tec-1 kinase negatively regulates regenerative neurogenesis in planarians

Negative regulators of adult neurogenesis are of particular interest as targets to enhance neuronal repair, but few have yet been identified. Planarians can regenerate their entire CNS using pluripotent adult stem cells, and this process is robustly regulated to ensure that new neurons are produced in proper abundance. Using a high-throughput pipeline to quantify brain chemosensory neurons, we identify the conserved tyrosine kinase tec-1 as a negative regulator of planarian neuronal regeneration. tec-1RNAi increased the abundance of several CNS and PNS neuron subtypes regenerated or maintained through homeostasis, without affecting body patterning or non-neural cells. Experiments using TUNEL, BrdU, progenitor labeling, and stem cell elimination during regeneration indicate tec-1 limits the survival of newly differentiated neurons. In vertebrates, the Tec kinase family has been studied extensively for roles in immune function, and our results identify a novel role for tec-1 as negative regulator of planarian adult neurogenesis.

JNK-mediated Slit-Robo signaling facilitates epithelial wound repair by extruding dying cells

Multicellular organisms repair injured epithelium by evolutionarily conserved biological processes including activation of c-Jun N-terminal kinase (JNK) signaling. Here, we show in Drosophila imaginal epithelium that physical injury leads to the emergence of dying cells, which are extruded from the wounded tissue by JNK-induced Slit-Roundabout2 (Robo2) repulsive signaling. Reducing Slit-Robo2 signaling in the wounded tissue suppresses extrusion of dying cells and generates aberrant cells with highly upregulated growth factors Wingless (Wg) and Decapentaplegic (Dpp). The inappropriately elevated Wg and Dpp impairs wound repair, as halving one of these growth factor genes cancelled wound healing defects caused by Slit-Robo2 downregulation. Our data suggest that JNK-mediated Slit-Robo2 signaling contributes to epithelial wound repair by promoting extrusion of dying cells from the wounded tissue, which facilitates transient and appropriate induction of growth factors for proper wound healing.

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.

Comparative transcriptomics in Syllidae (Annelida) indicates that posterior regeneration and regular growth are comparable, while anterior regeneration is a distinct process

Background

Annelids exhibit remarkable postembryonic developmental abilities. Most annelids grow during their whole life by adding segments through the action of a segment addition zone (SAZ) located in front of the pygidium. In addition, they show an outstanding ability to regenerate their bodies. Experimental evidence and field observations show that many annelids are able to regenerate their posterior bodies, while anterior regeneration is often limited or absent. Syllidae, for instance, usually show high abilities of posterior regeneration, although anterior regeneration varies across species. Some syllids are able to partially restore the anterior end, while others regenerate all lost anterior body after bisection. Here, we used comparative transcriptomics to detect changes in the gene expression profiles during anterior regeneration, posterior regeneration and regular growth of two syllid species: Sphaerosyllis hystrix and Syllis gracilis; which exhibit limited and complete anterior regeneration, respectively.

Results

We detected a high number of genes with differential expression: 4771 genes in S. hystrix (limited anterior regeneration) and 1997 genes in S. gracilis (complete anterior regeneration). For both species, the comparative transcriptomic analysis showed that gene expression during posterior regeneration and regular growth was very similar, whereas anterior regeneration was characterized by up-regulation of several genes. Among the up-regulated genes, we identified putative homologs of regeneration-related genes associated to cellular proliferation, nervous system development, establishment of body axis, and stem-cellness; such as rup and JNK (in S. hystrix); and glutamine synthetase, elav, slit, Hox genes, β-catenin and PL10 (in S. gracilis).

Conclusions

Posterior regeneration and regular growth show no significant differences in gene expression in the herein investigated syllids. However, anterior regeneration is associated with a clear change in terms of gene expression in both species. Our comparative transcriptomic analysis was able to detect differential expression of some regeneration-related genes, suggesting that syllids share some features of the regenerative mechanisms already known for other annelids and invertebrates.

TINP1 homolog is required for planarian regeneration

The planarian flatworm is an ideal system for the study of regeneration in vivo. In this study, we focus on TINP1, which is one of the most conserved proteins in eukaryotic organisms. We found that TINP1 was expressed in parenchymal region through whole body as well as central nervous system (CNS) during the course of regeneration. RNA interference targeting DjTINP1 caused lysis defects in regenerating tissues and a decreased in cell division and expression levels of DjpiwiA and Djpcna. Furthermore, the expression levels of DjTINP1 were decreased when we inhibited the TGF-β signal by knockdown of smad4, which is the sole co-smad and has been proved to control the blastema patterning and central nervous system (CNS) regeneration in planarians. These findings suggest that DjTINP1 participate in the maintenance of neoblasts and be required for proper cell proliferation in planarians as a downstream gene of the TGF-β signal pathway.

The first inorganic mitogens: Cerium oxide and cerium fluoride nanoparticles stimulate planarian regeneration via neoblastic activation

We report the first experimental evidence for the mitogenic action of cerium(IV) oxide and cerium(III) fluoride nanoparticles (CONs and CFNs) on the regeneration of a whole organism - freshwater flatworms Schmidtea mediterranea (planarian). Both types of cerium-containing nanoparticles are shown to be a highly potent mitogen for planaria. Both CONs and CFNs, in micro- and nanomolar concentrations, markedly accelerate planarian blastema growth, due to the enhancement of cellular proliferation, causing an increase in the mitotic index and in the quantity of blastema cells in regenerating planaria. CONs provided maximum activity at concentrations which were two orders of magnitude lower than those for CeF₃. The valence state of cerium in cerium-containing nanoparticles plays a significant role in the planarian regeneration mechanism: CeO₂ nanoparticles containing predominantly Ce⁴⁺ species presumably scavenge wound induced reactive oxygen species and moderately activate gene expression processes, while the regenerative action of CeF₃ nanoparticles containing only Ce³⁺ species is manifested in the pronounced expression of the genes involved in cell division, differentiation and migration. This is the first report on the effect of cerium-containing nanoparticles on tissue regeneration in vivo, further revealing the mechanisms of their biological action, which enhances the possibility of their use in cellular technologies.

Cdc42 regulate the apoptotic cell death required for planarian epidermal regeneration and homeostasis

Rho GTPases have been shown previously to play important roles in several cellular processes by regulating the organization of the actin and microtubule cytoskeletons. However, the mechanisms of Rho GTPases that integrate the cellular responses during regeneration have not been thoroughly elucidated. The planarian flatworm, which contains a large number of adult somatic stem cells (neoblasts), is a unique model to study stem cell lineage development in vivo. Here, we focus on cdc42, which is an extensively characterized member among Rho GTPases. We found that cdc42 is required for the maintenance of epidermal lineage. Cdc42 RNAi induced a sustained increased of cell death and led to a loss of the mature epidermal cells but without affected cell division. Our results indicate that cdc42 function as an inhibitor to block the excessive apoptotic cell death in planarian epidermal regeneration and homeostasis.

Analysis of sea star larval regeneration reveals conserved processes of whole-body regeneration across the metazoa

Background

Metazoan lineages exhibit a wide range of regenerative capabilities that vary among developmental stage and tissue type. The most robust regenerative abilities are apparent in the phyla Cnidaria, Platyhelminthes, and Echinodermata, whose members are capable of whole-body regeneration (WBR). This phenomenon has been well characterized in planarian and hydra models, but the molecular mechanisms of WBR are less established within echinoderms, or any other deuterostome system. Thus, it is not clear to what degree aspects of this regenerative ability are shared among metazoa.

Results

We characterize regeneration in the larval stage of the Bat Star (Patiria miniata). Following bisection along the anterior-posterior axis, larvae progress through phases of wound healing and re-proportioning of larval tissues. The overall number of proliferating cells is reduced following bisection, and we find evidence for a re-deployment of genes with known roles in embryonic axial patterning. Following axial respecification, we observe a significant localization of proliferating cells to the wound region. Analyses of transcriptome data highlight the molecular signatures of functions that are common to regeneration, including specific signaling pathways and cell cycle controls. Notably, we find evidence for temporal similarities among orthologous genes involved in regeneration from published Platyhelminth and Cnidarian regeneration datasets.

Conclusions

These analyses show that sea star larval regeneration includes phases of wound response, axis respecification, and wound-proximal proliferation. Commonalities of the overall process of regeneration, as well as gene usage between this deuterostome and other species with divergent evolutionary origins reveal a deep similarity of whole-body regeneration among the metazoa.

Electronic supplementary material

The online version of this article (10.1186/s12915-019-0633-9) contains supplementary material, which is available to authorized users.

Planarians Customize Their Stem Cell Responses Following Genotoxic Stress as a Function of Exposure Time and Regenerative State

Aiming to in vivo characterize the responses of pluripotent stem cells and regenerative tissues to carcinogenic stress, we employed the highly regenerative organism Schmidtea mediterranea. Its broad regenerative capacities are attributable to a large pool of pluripotent stem cells, which are considered key players in the lower vulnerability toward chemically induced carcinogenesis observed in regenerative organisms. Schmidtea mediterranea is, therefore, an ideal model to study pluripotent stem cell responses with stem cells residing in their natural environment. Including microenvironmental alterations is important, as the surrounding niche influences the onset of oncogenic events. Both short- (3 days) and long-term (17 days) exposures to the genotoxic carcinogen methyl methanesulfonate (50 µM) were evaluated during homeostasis and animal regeneration, two situations that render altered cellular niches. In both cases, MMS-induced DNA damage was observed, which provoked a decrease in proliferation on the short term. The outcome of DNA damage responses following long-term exposure differed between homeostatic and regenerating animals. During regeneration, DNA repair systems were more easily activated than in animals in homeostasis, where apoptosis was an important outcome. Knockdown experiments confirmed the importance of DNA repair systems during carcinogenic exposure in regenerating animals as knockdown of rad51 induced a stem cell-depleted phenotype, after regeneration was completed.

Detection of Cell Death in Planarians

Planarians are freshwater flatworms, well known for their ability to regenerate a complete organism from any piece of their body. Furthermore, planarians are constantly growing and degrowing throughout their lives, maintaining a functional and proportioned body. These properties rely on the presence of a population of adult stem cells and on the tight control of their cell renewal, which is based on the balance between the proliferation of new cells and their differentiation, and the death of unnecessary cells. Due to the importance of these two processes in planarian biology, over the years, researchers have optimized molecular techniques to detect both cell proliferation and cell death in planarians. Here, we present the two main protocols currently used for cell death detection and quantification in the planarian field: Caspase-3 activity quantification and TUNEL assay.

A carcinogenic trigger to study the function of tumor suppressor genes in Schmidtea mediterranea

Planarians have been long known for their regenerative ability, which hinges on pluripotency. Recently, however, the planarian model has been successfully established for routine toxicological screens aimed to assess overproliferation, mutagenicity and tumorigenesis. In this study, we focused on planarian tumor suppressor genes (TSGs) and their role during chemically induced carcinogenic stress in Schmidtea mediterranea Combining in silico and proteomic screens with exposure to human carcinogen type 1A agent cadmium (Cd), we showed that many TSGs have a function in stem cells and that, in general, exposure to Cd accelerated the onset and increased the severity of the observed phenotype. This suggested that the interaction between environmental and genetic factors plays an important role in tumor development in S. mediterranea Therefore, we further focused on the synergistic effects of Cd exposure and p53 knockdown (KD) at the cellular and molecular levels. Cd also produced a specific proteomic landscape in homeostatic animals, with 172 proteins differentially expressed, 43 of which were downregulated. Several of these proteins have tumor suppressor function in human and other animals, namely Wilms Tumor 1 Associated Protein (WT1), Heat Shock Protein 90 (HSP90), Glioma Pathogenesis-Related Protein 1 (GLIPR1) and Matrix Metalloproteinase B (Smed-MMPB). Both Glipr1 and MmpB KD produced large outgrowths, epidermal lesions and epidermal blisters. The epidermal blisters that formed as a consequence of Smed-MmpB KD were populated by smedwi1+ cells, many of which were actively proliferating, while large outgrowths contained ectopically differentiated structures, such as photoreceptors, nervous tissue and a small pharynx. In conclusion, Smed-MmpB is a planarian TSG that prevents stem cell proliferation and differentiation outside the proper milieu.

Staying in shape: Planarians as a model for understanding regenerative morphology

A key requirement of tissue/organ regeneration is the ability to induce appropriate shape in situ. Regenerated structures need to be integrated with pre-existing ones, through the combined regulation of new tissue growth and the scaling of surrounding tissues. This requires a tightly coordinated control of individual cell functions such as proliferation and stem cell differentiation. While great strides have been made in elucidating cell growth and differentiation mechanisms, how overall shape is generated during regeneration remains unknown. This is because a significant gap remains in our understanding of how cell behaviors are coordinated at the level of tissues and organs. The highly regenerative planarian flatworm has emerged as an important model for defining and understanding regenerative shape mechanisms. This review provides an overview of the main processes known to regulate tissue and animal shape during planarian regeneration: adult stem cell regulation, the reestablishment of body axes, tissue remodeling in pre-existing structures, organ scaling and the maintenance of body proportion, and the bioelectrical regulation of animal morphology. In order for the field to move forward, it will be necessary to identify shape mutants as a means to uncover the molecular mechanisms that synchronize all these separate processes to produce the worm's final regenerative shape. This knowledge will also aid efforts to define the mechanisms that control the termination of regenerative processes.

A lineage CLOUD for neoblasts

In planarians, pluripotency can be studied in vivo in the adult animal, making these animals a unique model system where pluripotency-based regeneration (PBR)-and its therapeutic potential-can be investigated. This review focuses on recent findings to build a cloud model of fate restriction likelihood for planarian stem and progenitor cells. Recently, a computational approach based on functional and molecular profiling at the single cell level was proposed for human hematopoietic stem cells. Based on data generated both in vivo and ex vivo, we hypothesized that planarian stem cells could acquire multiple direction lineage biases, following a "badlands" landscape. Instead of a discrete tree-like hierarchy, where the potency of stem/progenitor cells reduces stepwise, we propose a Continuum of LOw-primed UnDifferentiated Planarian Stem/Progenitor Cells (CLOUD-PSPCs). Every subclass of neoblast/progenitor cells is a cloud of likelihood, as the single cell transcriptomics data indicate. The CLOUD-HSPCs concept was substantiated by in vitro data from cell culture; therefore, to confirm the CLOUD-PSPCs model, the planarian community needs to develop new tools, like live cell tracking. Future studies will allow a deeper understanding of PBR in planarian, and the possible implications for regenerative therapies in human.

It is not all about regeneration: Planarians striking power to stand starvation

All living forms, prokaryotes as eukaryotes, have some means of adaptation to food scarcity, which extends the survival chances under extreme environmental conditions. Nowadays we know that dietary interventions, including fasting, extends lifespan of many organisms and can also protect against age-related diseases including in humans. Therefore, the capacity of adapting to periods of food scarcity may have evolved billions of years ago not only to allow immediate organismal survival but also to be able to extend organismal lifespan or at least to lead to a healthier remaining lifespan. Planarians have been the center of attention since more than two centuries because of their astonishing power of full body regeneration that relies on a large amount of adult stem cells or neoblasts. However, they also present an often-overlooked characteristic. They are able to stand long time starvation. Planarians have adapted to periods of fasting by shrinking or degrowing. Here we will review the published data about starvation in planarians and conclude with the possibility of starvation being one of the processes that rejuvenate the planarian, thus explaining the historical notion of non-ageing planarians.

Wound-induced cell proliferation during animal regeneration

Many animal species are capable of replacing missing tissues that are lost upon injury or amputation through the process of regeneration. Although the extent of regeneration is variable across animals, that is, some animals can regenerate any missing cell type whereas some can only regenerate certain organs or tissues, regulated cell proliferation underlies the formation of new tissues in most systems. Notably, many species display an increase in proliferation within hours or days upon wounding. While different cell types proliferate in response to wounding in various animal taxa, comparative molecular data are beginning to point to shared wound-induced mechanisms that regulate cell division during regeneration. Here, we synthesize current insights about early molecular pathways of regeneration from diverse model and emerging systems by considering these species in their evolutionary contexts. Despite the great diversity of mechanisms underlying injury-induced cell proliferation across animals, and sometimes even in the same species, similar pathways for proliferation have been implicated in distantly related species (e.g., small diffusible molecules, signaling from apoptotic cells, growth factor signaling, mTOR and Hippo signaling, and Wnt and Bmp pathways). Studies that explicitly interrogate molecular and cellular regenerative mechanisms in understudied animal phyla will reveal the extent to which early pathways in the process of regeneration are conserved or independently evolved. This article is categorized under: Comparative Development and Evolution > Body Plan Evolution Adult Stem Cells, Tissue Renewal, and Regeneration > Regeneration Comparative Development and Evolution > Model Systems.

Post-translational regulation of planarian regeneration

Most mammals cannot easily overcome degenerative disease or traumatic injuries. In contrast, an innate ability to regenerate is observed across animal phyla. Freshwater planarians are amongst the organisms that are capable of stem cell-mediated whole-body regeneration and have served as an exemplary model to study how pluripotency is maintained and regulated in vivo. Here, we review findings on the role of post-translational modifications and the genes regulating phosphorylation, ubiquitylation, and chromatin remodeling in planarian regeneration. Furthermore, we discuss how technological advances for identifying cellular targets of these processes will fill gaps in our knowledge of the signaling mechanisms that underlie regeneration in planarians, which should inform how tissue repair can be stimulated in non-regenerative model organisms and in humans.