ERK signaling controls blastema cell differentiation during planarian regeneration

ERK-activated CK-2 triggers blastema formation during appendage regeneration

Appendage regeneration relies on the formation of blastema, a heterogeneous cellular structure formed at the injury site. However, little is known about the early injury-activated signaling pathways that trigger blastema formation during appendage regeneration. Here, we provide compelling evidence that the extracellular signal-regulated kinase (ERK)-activated casein kinase 2 (CK-2), which has not been previously implicated in appendage regeneration, triggers blastema formation during leg regeneration in the American cockroach, Periplaneta americana. After amputation, CK-2 undergoes rapid activation through ERK-induced phosphorylation within blastema cells. RNAi knockdown of CK-2 severely impairs blastema formation by repressing cell proliferation through down-regulating mitosis-related genes. Evolutionarily, the regenerative role of CK-2 is conserved in zebrafish caudal fin regeneration via promoting blastema cell proliferation. Together, we find and demonstrate that the ERK-activated CK-2 triggers blastema formation in both cockroach and zebrafish, helping explore initiation factors during appendage regeneration.

Establishment of a new method to isolate viable x-ray-sensitive cells from planarian by fluorescence-activated cell sorting

Planarians show outstanding regenerative ability due to the proliferation of neoblasts. Hence the method to isolate planarian neoblasts is important to understand the regeneration process. In our previous study, we reported a method to isolate planarian neoblasts of Dugesia japonica using fluorescence-activated cell sorting (FACS). However, we have not yet succeeded in cultivating these cells even under in vivo conditions after transplantation into x-ray-irradiated planarians. This suggests that dissociated cells might enter apoptotic or necrotic states in the process of fluorescent dye staining and sorting. Here, we developed a new method to isolate viable neoblasts, which can proliferate in the x-ray-irradiated planarians. First, the toxicity of various fluorescence dyes was investigated. All nuclear fluorescent dyes such as Hoechst 33342, DRAQ5, and DyeCycle, showed, more or less, toxicity to mammalian culture cells. In contrast, cytoplasmic fluorescent dye for live cells, calcein AM, was less toxic on these cells. Next, we stained the dissociated planarian cells with only calcein AM, and then collected the x-ray-sensitive fraction. Although the purity of neoblasts was slightly lower than that of the original staining method (ca. 97% → ca. 89%), the sorted cells could actively proliferate when they were injected into x-ray-irradiated planarians. This simple staining and sorting method will provide new opportunities to isolate viable neoblasts and understand regenerating processes.

Enduring questions in regenerative biology and the search for answers

The potential for basic research to uncover the inner workings of regenerative processes and produce meaningful medical therapies has inspired scientists, clinicians, and patients for hundreds of years. Decades of studies using a handful of highly regenerative model organisms have significantly advanced our knowledge of key cell types and molecular pathways involved in regeneration. However, many questions remain about how regenerative processes unfold in regeneration-competent species, how they are curtailed in non-regenerative organisms, and how they might be induced (or restored) in humans. Recent technological advances in genomics, molecular biology, computer science, bioengineering, and stem cell research hold promise to collectively provide new experimental evidence for how different organisms accomplish the process of regeneration. In theory, this new evidence should inform the design of new clinical approaches for regenerative medicine. A deeper understanding of how tissues and organs regenerate will also undoubtedly impact many adjacent scientific fields. To best apply and adapt these new technologies in ways that break long-standing barriers and answer critical questions about regeneration, we must combine the deep knowledge of developmental and evolutionary biologists with the hard-earned expertise of scientists in mechanistic and technical fields. To this end, this perspective is based on conversations from a workshop we organized at the Banbury Center, during which a diverse cross-section of the regeneration research community and experts in various technologies discussed enduring questions in regenerative biology. Here, we share the questions this group identified as significant and unanswered, i.e., known unknowns. We also describe the obstacles limiting our progress in answering these questions and how expanding the number and diversity of organisms used in regeneration research is essential for deepening our understanding of regenerative capacity. Finally, we propose that investigating these problems collaboratively across a diverse network of researchers has the potential to advance our field and produce unexpected insights into important questions in related areas of biology and medicine.

Schistosoma mansoni excretory-secretory products induce protein kinase signalling, hyperkinesia, and stem cell proliferation in the opposite sex

Adult male and female schistosomes in copula dwell within human blood vessels and lay eggs that cause the major Neglected Tropical Disease human schistosomiasis. How males and females communicate to each other is poorly understood; however, male-female physical interaction is known to be important. Here, we investigate whether excretory-secretory products (ESPs), released into the external milieu by mature Schistosoma mansoni, might induce responses in the opposite sex. We demonstrate that ESPs adhere to the surface of opposite sex worms inducing the activation of extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein kinase (p38 MAPK) pathways, particularly in the parasite tegument. Furthermore, we show that mature worms stimulated signalling in juvenile worms. Strikingly, we demonstrate that ESPs from the opposite sex promote stem cell proliferation, in an ERK- and p38 MAPK-dependent manner, in the tegument and within the testes of males, and the ovaries and vitellaria of females. Hyperkinesia also occurs following opposite sex ESP exposure. Our findings support the hypothesis that male and female schistosomes may communicate over distance to modulate key processes underlying worm development and disease progression, opening unique avenues for schistosomiasis control.

Ultrafast distant wound response is essential for whole-body regeneration

Injury induces systemic responses, but their functions remain elusive. Mechanisms that can rapidly synchronize wound responses through long distances are also mostly unknown. Using planarian flatworms capable of whole-body regeneration, we report that injury induces extracellular signal-regulated kinase (Erk) activity waves to travel at a speed 10-100 times faster than those in other multicellular tissues. This ultrafast propagation requires longitudinal body-wall muscles, elongated cells forming dense parallel tracks running the length of the organism. The morphological properties of muscles allow them to act as superhighways for propagating and disseminating wound signals. Inhibiting Erk propagation prevents tissues distant to the wound from responding and blocks regeneration, which can be rescued by a second injury to distal tissues shortly after the first injury. Our findings provide a mechanism for long-range signal propagation in large, complex tissues to coordinate responses across cell types and highlight the function of feedback between spatially separated tissues during whole-body regeneration.

The scaling of goals from cellular to anatomical homeostasis: an evolutionary simulation, experiment and analysis

Complex living agents consist of cells, which are themselves competent sub-agents navigating physiological and metabolic spaces. Behaviour science, evolutionary developmental biology and the field of machine intelligence all seek to understand the scaling of biological cognition: what enables individual cells to integrate their activities to result in the emergence of a novel, higher-level intelligence with large-scale goals and competencies that belong to it and not to its parts? Here, we report the results of simulations based on the TAME framework, which proposes that evolution pivoted the collective intelligence of cells during morphogenesis of the body into traditional behavioural intelligence by scaling up homeostatic competencies of cells in metabolic space. In this article, we created a minimal in silico system (two-dimensional neural cellular automata) and tested the hypothesis that evolutionary dynamics are sufficient for low-level setpoints of metabolic homeostasis in individual cells to scale up to tissue-level emergent behaviour. Our system showed the evolution of the much more complex setpoints of cell collectives (tissues) that solve a problem in morphospace: the organization of a body-wide positional information axis (the classic French flag problem in developmental biology). We found that these emergent morphogenetic agents exhibit a number of predicted features, including the use of stress propagation dynamics to achieve the target morphology as well as the ability to recover from perturbation (robustness) and long-term stability (even though neither of these was directly selected for). Moreover, we observed an unexpected behaviour of sudden remodelling long after the system stabilizes. We tested this prediction in a biological system-regenerating planaria-and observed a very similar phenomenon. We propose that this system is a first step towards a quantitative understanding of how evolution scales minimal goal-directed behaviour (homeostatic loops) into higher-level problem-solving agents in morphogenetic and other spaces.

Cell Proliferation Indices in Regenerating Alitta virens (Annelida, Errantia)

In recent years, interest in the possible molecular regulators of cell proliferation and differentiation in a wide range of regeneration models has grown significantly, but the cell kinetics of this process remain largely a mystery. Here we try to elucidate the cellular aspects of regeneration by EdU incorporation in intact and posteriorly amputated annelid Alitta virens using quantitative analysis. We found that the main mechanism of blastema formation in A. virens is local dedifferentiation; mitotically active cells of intact segments do not significantly contribute to the blastemal cellular sources. Amputation-induced proliferation occurred predominantly within the epidermal and intestinal epithelium, as well as wound-adjacent muscle fibers, where clusters of cells at the same stage of the cell cycle were found. The resulting regenerative bud had zones of high proliferative activity and consisted of a heterogeneous population of cells that differed in their anterior-posterior positions and in their cell cycle parameters. The data presented allowed for the quantification of cell proliferation in the context of annelid regeneration for the first time. Regenerative cells showed an unprecedentedly high cycle rate and an exceptionally large growth fraction, making this regeneration model especially valuable for studying coordinated cell cycle entry in vivo in response to injury.

Ultrafast and long-range coordination of wound responses is essential for whole-body regeneration

Injury induces systemic, global responses whose functions remain elusive. In addition, mechanisms that rapidly synchronize wound responses through long distances across the organismal scale are mostly unknown. Using planarians, which have extreme regenerative ability, we report that injury induces Erk activity to travel in a wave-like manner at an unexpected speed (∼1 mm/h), 10-100 times faster than those measured in other multicellular tissues. This ultrafast signal propagation requires longitudinal body-wall muscles, elongated cells forming dense parallel tracks running the length of the organism. Combining experiments and computational models, we show that the morphological properties of muscles allow them to minimize the number of slow intercellular signaling steps and act as bidirectional superhighways for propagating wound signals and instructing responses in other cell types. Inhibiting Erk propagation prevents cells distant to the wound from responding and blocks regeneration, which can be rescued by a second injury to distal tissues within a narrow time window after the first injury. These results suggest that rapid responses in uninjured tissues far from wounds are essential for regeneration. Our findings provide a mechanism for long-range signal propagation in large and complex tissues to coordinate cellular responses across diverse cell types, and highlights the function of feedback between spatially separated tissues during whole-body regeneration.

Physiological and molecular mechanisms of insect appendage regeneration

Regeneration, as a fascinating scientific field, refers to the ability of animals replacing lost tissue or body parts. Many metazoan organisms have been reported with the regeneration phenomena, but showing evolutionarily variable abilities. As the most diverse metazoan taxon, hundreds of insects show strong appendage regeneration ability. The regeneration process and ability are dependent on many factors, including macroscopic physiological conditions and microscopic molecular mechanisms. This article reviews research progress on the physiological conditions and internal underlying mechanisms controlling appendage regeneration in insects.

Djptpn11 is indispensable for planarian regeneration by affecting early wound response genes expression and the Wnt pathway

Planarian is an ideal model system of studying regeneration. Stem cell system and positional control genes (PCGs) are two important factors for perfect regeneration of planarians and they combine to promote their regeneration. Even so, how wounds regulate proliferation and neoblast fate is still important areas to address. Ptpn11 (Protein tyrosine phosphatase non-receptor type 11), one of PTP (Protein tyrosine phosphatase) family members, plays an important role in cellular processes including cell survival, proliferation, differentiation and apoptosis. Nevertheless, the role of ptpn11 in the planarian regeneration has not been fully studied. In this study, we identify the Djptpn11 gene to observe its function in planarian regeneration. The results reveal that the regeneration is severely inhibited and cause the disorder homeostasis in planarians. Furthermore, the stem cells proliferation and differentiation decreases while the apoptosis increases following Djptpn11 RNAi. At the same time, Djptpn11 affects the expression levels of early wound response genes (Djegr2, Dj1-jun, Djrunt1, Djwnt1 and Djnotum). Djwnt1 and Djnotum are two key Wnt signaling pathway genes and Djptpn11 affects the expression levels of Djwnt1 and Djnotum in the early and late stages of planarian regeneration. In general, Djptpn11 is indispensable for the homeostasis and regeneration of planarian by affecting the stem cells, early wound response genes and the Wnt pathway.

An insight into planarian regeneration

Background

Planarian has attracted increasing attentions in the regeneration field for its usefulness as an important biological model organism attributing to its strong regeneration ability. Both the complexity of multiple regulatory networks and their coordinate functions contribute to the maintenance of normal cellular homeostasis and the process of regeneration in planarian. The polarity, size, location and number of regeneration tissues are regulated by diverse mechanisms. In this review we summarize the recent advances about the importance genetic and molecular mechanisms for regeneration control on various tissues in planarian.

Methods

A comprehensive literature search of original articles published in recent years was performed in regards to the molecular mechanism of each cell types during the planarian regeneration, including neoblast, nerve system, eye spot, excretory system and epidermal.

Results

Available molecular mechanisms gave us an overview of regeneration process in every tissue. The sense of injuries and initiation of regeneration is regulated by diverse genes like follistatin and ERK signaling. The Neoblasts differentiate into tissue progenitors under the regulation of genes such as egfr‐3. The regeneration polarity is controlled by Wnt pathway, BMP pathway and bioelectric signals. The neoblast within the blastema differentiate into desired cell types and regenerate the missing tissues. Those tissue specific genes regulate the tissue progenitor cells to differentiate into desired cell types to complete the regeneration process.

Conclusion

All tissue types in planarian participate in the regeneration process regulated by distinct molecular factors and cellular signaling pathways. The neoblasts play vital roles in tissue regeneration and morphology maintenance. These studies provide new insights into the molecular mechanisms for regulating planarian regeneration.

The planarian wound epidermis gene equinox is required for blastema formation in regeneration

Regeneration often involves the formation of a blastema, an outgrowth or regenerative bud formed at the plane of injury where missing tissues are produced. The mechanisms that trigger blastema formation are therefore fundamental for regeneration. Here, we identify a gene, which we named equinox, that is expressed within hours of injury in the planarian wound epidermis. equinox encodes a predicted secreted protein that is conserved in many animal phyla. Following equinox inhibition, amputated planarians fail to maintain wound-induced gene expression and to subsequently undergo blastema outgrowth. Associated with these defects is an inability to reestablish lost positional information needed for missing tissue specification. Our findings link the planarian wound epidermis, through equinox, to regeneration of positional information and blastema formation, indicating a broad regulatory role of the wound epidermis in diverse regenerative contexts.

Developmental biology is poised to discover altogether new principles in biology in the 21st century

In the 20th century, developmental biology spearheaded a revolution in our understanding of complex biological problems. Its success rests in great part on a truly unique approach that has recruited a diversity of systems and research organisms rather than focusing on isolated cells or molecules, while also employing a wide variety of technological and intellectual approaches. But what will developmental biology contribute to this century? Advances in technology and instrumentation are presently moving at neck-breaking speed and herald the advent of an age of technological wonders in which previously inaccessible biology is now tangibly within our grasps. For instance, single-cell RNAseq has revealed novel, transient cell states in both stem and differentiated cells that are specified by defined changes in gene expression frequency during regeneration. Additionally, genome-wide epigenetic analyses combined with gene editing and transgenic methodologies have identified the existence of regeneration responsive enhancers in adult vertebrate tissues. These circumstances combined with our discipline’s diversity of experimental and intellectual approaches offer unimaginable opportunities for developmental biologists not only to discover new biology but also to reveal entirely new principles of biology.

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.

Migratory regulation by MTA homologous genes is essential for the uniform distribution of planarian adult pluripotent stem cells

The migration of adult stem cells in vivo is an important issue, but the complex tissue structures involved, and limited accessibility of the cells hinder a detailed investigation. To overcome these problems, the freshwater planarian Dugesia japonica was used because it has a simple body plan and abundant adult pluripotent stem cells (neoblasts) distributed uniformly throughout its body. To investigate the migratory mechanisms of neoblasts, two planarian homologous genes of metastatic tumor antigen (MTA-A and MTA-B), a protein involved in cancer metastasis that functions through histone deacetylation, were identified, and their function was analyzed using RNA interference (RNAi). MTA-A or MTA-B knockdown disrupted homeostatic tissue turnover and regeneration in planarians. Whereas neoblasts in MTA-A (RNAi) and MTA-B (RNAi) animals were maintained, neoblast differentiation was inhibited. Furthermore, the normal uniform neoblast distribution pattern changed to a branch-like pattern in MTA-A (RNAi) and MTA-B (RNAi) animals. To examine the neoblast migratory ability, a partial X-ray irradiation assay was performed in D. japonica. Using this assay system, the MTA-A knockdown neoblasts migrated collectively in a branch-like pattern, and the MTA-B knockdown neoblasts were not able to migrate. These results indicated that MTA-A was required for the exit of neoblasts from the branch-like region, and that MTA-B was required for neoblast migration. Thus, the migration mediated by MTA-A and MTA-B enabled uniform neoblast distribution and was required for neoblast differentiation to achieve tissue homeostasis and regeneration.

Loss of plac8 expression rapidly leads pluripotent stem cells to enter active state during planarian regeneration

The regenerative ability of planarians relies on their adult pluripotent stem cell population. Although all stem cells express a piwi homolog, recently it has become possible to classify the piwi+ stem cell population into specialized subpopulations according to the expression of genes related to differentiation. However, piwi+ stem cells behave practically as a homogeneous population after amputation, during which stem cells show accelerated proliferation, named ‘induced hyperproliferation’. Here, we show that plac8-A was expressed in almost all of the stem cells, and that a decrease of the plac8-A expression level led to induced hyperproliferation uniformly in a broad stem cell subpopulation after amputation. This reduction of plac8-A expression was caused by activated JNK signaling after amputation. Pharmacological inhibition of JNK signaling caused failure to induce hyperproliferation and resulted in regenerative defects. Such defects were abrogated by simultaneous knockdown of plac8-A expression. Thus, JNK-dependent suppression of plac8-A expression is indispensable for stem cell dynamics involved in regeneration. These findings suggest that plac8-A acts as a molecular switch of piwi+ stem cells for entry into the regenerative state after amputation.

Proliferation maintains the undifferentiated status of stem cells: The role of the planarian cell cycle regulator Cdh1

The coincidence of cell cycle exit and differentiation has been described in a wide variety of stem cells and organisms for decades, but the causal relationship is still unclear due to the complicated regulation of the cell cycle. Here, we used the planarian Dugesia japonica since they may possess a simple cell cycle regulation in which Cdh1 is one of the factors responsible for exiting the cell cycle. When cdh1 was functionally inhibited, the planarians could not maintain their tissue homeostasis and could not regenerate their missing body parts. While the knockdown of cdh1 caused pronounced accumulation of the stem cells, the progenitor and differentiated cells were decreased. Further analyses indicated that the stem cells with cdh1 knockdown did not undergo differentiation even though they received ERK signaling activation as an induction signal. These results suggested that stem cells could not acquire differentiation competence without cell cycle exit. Thus, we propose that cell cycle regulation determines the differentiation competence and that cell cycle exit to G0 enables stem cells to undergo differentiation.

Regeneration of Planarian Auricles and Reestablishment of Chemotactic Ability

Detection of chemical stimuli is crucial for living systems and also contributes to quality of life in humans. Since loss of olfaction becomes more prevalent with aging, longer life expectancies have fueled interest in understanding the molecular mechanisms behind the development and maintenance of chemical sensing. Planarian flatworms possess an unsurpassed ability for stem cell-driven regeneration that allows them to restore any damaged or removed part of their bodies. This includes anteriorly-positioned lateral flaps known as auricles, which have long been thought to play a central role in chemotaxis. The contribution of auricles to the detection of positive chemical stimuli was tested in this study using Girardia dorotocephala, a North American planarian species known for its morphologically prominent auricles. Behavioral experiments staged under laboratory conditions revealed that removal of auricles by amputation leads to a significant decrease in the ability of planarians to find food. However, full chemotactic capacity is observed as early as 2 days post-amputation, which is days prior from restoration of auricle morphology, but correlative with accumulation of ciliated cells in the position of auricle regeneration. Planarians subjected to x-ray irradiation prior to auricle amputation were unable to restore auricle morphology, but were still able to restore chemotactic capacity. These results indicate that although regeneration of auricle morphology requires stem cells, some restoration of chemotactic ability can still be achieved in the absence of normal auricle morphology, corroborating with the initial observation that chemotactic success is reestablished 2-days post-amputation in our assays. Transcriptome profiles of excised auricles were obtained to facilitate molecular characterization of these structures, as well as the identification of genes that contribute to chemotaxis and auricle development. A significant overlap was found between genes with preferential expression in auricles of G. dorotocephala and genes with reduced expression upon SoxB1 knockdown in Schmidtea mediterranea, suggesting that SoxB1 has a conserved role in regulating auricle development and function. Models that distinguish between possible contributions to chemotactic behavior obtained from cellular composition, as compared to anatomical morphology of the auricles, are discussed.

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.

Signaling gradients in surface dynamics as basis for planarian regeneration

Based on experimental data, we introduce and analyze a system of reaction-diffusion equations for the regeneration of planarian flatworms. We model dynamics of head and tail cells expressing positional control genes that translate into localized signals which in turn guide stem cell differentiation. Tissue orientation and positional information are encoded in a long range wnt-related signaling gradient. Our system correctly reproduces typical cut and graft experiments, and improves on previous models by preserving polarity in regeneration over orders of magnitude in body size during growth phases. Key to polarity preservation in our model flatworm is the sensitivity of cell differentiation to gradients of wnt-related signals relative to the tissue surface. This process is particularly relevant in small tissue layers close to cuts during their healing, and modeled in a robust fashion through dynamic boundary conditions.

Planarian stem cells sense the identity of the missing pharynx to launch its targeted regeneration

In order to regenerate tissues successfully, stem cells must detect injuries and restore missing cell types through largely unknown mechanisms. Planarian flatworms have an extensive stem cell population responsible for regenerating any organ after amputation. Here, we compare planarian stem cell responses to different injuries by either amputation of a single organ, the pharynx, or removal of tissues from other organs by decapitation. We find that planarian stem cells adopt distinct behaviors depending on what tissue is missing to target progenitor and tissue production towards missing tissues. Loss of non-pharyngeal tissues only increases non-pharyngeal progenitors, while pharynx removal selectively triggers division and expansion of pharynx progenitors. By pharmacologically inhibiting either mitosis or activation of the MAP kinase ERK, we identify a narrow window of time during which stem cell division and ERK signaling produces pharynx progenitors necessary for regeneration. These results indicate that planarian stem cells can tailor their output to match the regenerative needs of the animal.

2-D08 treatment regulates C2C12 myoblast proliferation and differentiation via the Erk1/2 and proteasome signaling pathways

SUMOylation is one of the post-translational modifications that involves the covalent attachment of the small ubiquitin-like modifier (SUMO) to the substrate. SUMOylation regulates multiple biological processes, including myoblast proliferation, differentiation, and apoptosis. 2-D08 is a synthetically available flavone, which acts as a potent cell-permeable SUMOylation inhibitor. Its mechanism of action involves preventing the transfer of SUMO from the E2 thioester to the substrate without influencing SUMO-activating enzyme E1 (SAE-1/2) or E2 Ubc9-SUMO thioester formation. However, both the effects and mechanisms of 2-D08 on C2C12 myoblast cells remain unclear. In the present study, we found that treatment with 2-D08 inhibits C2C12 cell proliferation and differentiation. We confirmed that 2-D08 significantly hampers the viability of C2C12 cells. Additionally, it inhibited myogenic differentiation, decreasing myosin heavy chain (MHC), MyoD, and myogenin expression. Furthermore, we confirmed that 2-D08-mediated anti-myogenic effects impair myoblast differentiation and myotube formation, reducing the number of MHC-positive C2C12 cells. In addition, we found that 2-D08 induces the activation of ErK1/2 and the degradation of MyoD and myogenin in C2C12 cells. Taken together, these results indicated that 2-D08 treatment results in the deregulated proliferation and differentiation of myoblasts. However, further research is needed to investigate the long-term effects of 2-D08 on skeletal muscles.

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.

The Act of Controlling Adult Stem Cell Dynamics: Insights from Animal Models

Adult stem cells (ASCs) are the undifferentiated cells that possess self-renewal and differentiation abilities. They are present in all major organ systems of the body and are uniquely reserved there during development for tissue maintenance during homeostasis, injury, and infection. They do so by promptly modulating the dynamics of proliferation, differentiation, survival, and migration. Any imbalance in these processes may result in regeneration failure or developing cancer. Hence, the dynamics of these various behaviors of ASCs need to always be precisely controlled. Several genetic and epigenetic factors have been demonstrated to be involved in tightly regulating the proliferation, differentiation, and self-renewal of ASCs. Understanding these mechanisms is of great importance, given the role of stem cells in regenerative medicine. Investigations on various animal models have played a significant part in enriching our knowledge and giving In Vivo in-sight into such ASCs regulatory mechanisms. In this review, we have discussed the recent In Vivo studies demonstrating the role of various genetic factors in regulating dynamics of different ASCs viz. intestinal stem cells (ISCs), neural stem cells (NSCs), hematopoietic stem cells (HSCs), and epidermal stem cells (Ep-SCs).

The Ras/MAPK pathway is required for regenerative growth of wing discs in the black cutworm Agrotis ypsilon

Regeneration is a common phenomenon in various organisms by which tissues restore the damaged or naturally detached parts. In insects, appendage regeneration takes place during the embryonic, larval and pupal stages for individual survival. The wing disc of black cutworm Agrotis ypsilon has the capacity of regeneration after ablation, but understanding of molecular mechanisms in wing disc regeneration is still limited. After ablation of partial or whole wing discs before the fifth instar larval stage, the adult wings appeared to be normal. In the last two larval stages, ablation of the left wing disc led to smaller corresponding adult wing. Cell proliferation was reduced in the ablated wing disc but was gradually recovered two days post ablation. Transcriptome analysis found that genes in the mitogen-activated protein kinase (MAPK) pathway were upregulated. Repression of gene expression in this pathway, including Ras oncogene at 64B (Ras64B), Downstream of raf1 (Dsor1), and cAMP-dependent protein kinase catalytic subunit 3 (Pka-C3) by RNA interference after ablation, led to diminishment of both adult wings, suggesting that the MAPK signaling is essential for wing growth. Additionally, cell proliferation was still decelerated by injecting Ras64B, Dsor, or Pka-C3 dsRNA two days after ablation, indicating that the MAPK signaling-regulated cell proliferation is essential for growth. These results provide molecular clues to the regulation of cell proliferation during regeneration in lepidopteran insects.

Tubgcp3 is a mitotic regulator of planarian epidermal differentiation

Tubgcp3/GCP3 (The centrosomal protein γ-tubulin complex protein 3) is a component of the γ-tubulin small complexes (γ-TuSCs) and γ-tubulin ring complexes (γ-TuRCs), which play critical roles in mitotic spindle formation during mitosis. However, its function in stem cell development has 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 identified a homolog of Tubgcp3 in planarian Dugesia japonica, and found that Tubgcp3 is required for the maintenance of epidermal lineage. RNAi targeting Tubgcp3 resulted in tissue homeostasis and regeneration defect. Knockdown of Tubgcp3 reduced cell divisions and led to a loss of the mature epidermal cells. Our findings indicate that Tubgcp3 is a mitotic regulator and plays a crucial role in planarian epidermal differentiation.

MEK/ERK Signaling Regulates Reconstitution of the Dopaminergic Nerve Circuit in the Planarian Dugesia japonica

Planarian Dugesia japonica is a flatworm that can autonomously regenerate its own body after an artificial amputation. A recent report showed the role of the mitogen-activated protein kinase/extracellular signal-regulated kinase (MEK/ERK) pathway in the head morphogenesis during the planarian regeneration process after amputation; however, neuron-specific regeneration mechanisms have not yet been reported. Here, whether MEK/ERK pathway was involved in the dopaminergic neuronal regeneration in planarians was investigated. Planarians regenerated their body within 14 days after amputation; however, the head region morphogenesis was inhibited by MEK inhibitor U0126 (3 or 10 μM). Furthermore, the number of planarian tyrosine hydroxylase (DjTH)-positive dopaminergic neurons in the regenerated head region was also decreased by U0126. The 6-hydroxydopamine (6-OHDA), a dopaminergic neurotoxin, can decrease the number of dopaminergic neurons; however, planarians can regenerate dopaminergic neurons after injecting 6-OHDA into the intestinal tract. MEK inhibitor PD98059 (30 μM) or U0126 (10 μM) significantly decreased dopaminergic neurons 5 days after the 6-OHDA injection. During the regeneration process of dopaminergic neurons, phosphorylated histone H3 (H3P)-positive stem cells known as "neoblasts" were increased in the head region; however, MEK inhibitors significantly decreased the number of H3P-positive neoblasts. These results suggested that dopaminergic neuronal regeneration in planarian was regulated by the MEK/ERK pathway.

Reactive oxygen species rescue regeneration after silencing the MAPK-ERK signaling pathway in Schmidtea mediterranea

Despite extensive research on molecular pathways controlling the process of regeneration in model organisms, little is known about the actual initiation signals necessary to induce regeneration. Recently, the activation of ERK signaling has been shown to be required to initiate regeneration in planarians. However, how ERK signaling is activated remains unknown. Reactive Oxygen Species (ROS) are well-known early signals necessary for regeneration in several models, including planarians. Still, the probable interplay between ROS and MAPK/ERK has not yet been described. Here, by interfering with major mediators (ROS, EGFR and MAPK/ERK), we were able to identify wound-induced ROS, and specifically H₂O₂, as upstream cues in the activation of regeneration. Our data demonstrate new relationships between regeneration-related ROS production and MAPK/ERK activation at the earliest regeneration stages, as well as the involvement of the EGFR-signaling pathway. Our results suggest that (1) ROS and/or H₂O₂ have the potential to rescue regeneration after MEK-inhibition, either by H₂O₂-treatment or light therapy, (2) ROS and/or H₂O₂ are required for the activation of MAPK/ERK signaling pathway, (3) the EGFR pathway can mediate ROS production and the activation of MAPK/ERK during planarian regeneration.

The transcriptome of anterior regeneration in earthworm Eudrilus eugeniae

The oligochaete earthworm, Eudrilus eugeniae is capable of regenerating both anterior and posterior segments. The present study focuses on the transcriptome analysis of earthworm E. eugeniae to identify and functionally annotate the key genes supporting the anterior blastema formation and regulating the anterior regeneration of the worm. The Illumina sequencing generated a total of 91,593,182 raw reads which were assembled into 105,193 contigs using CLC genomics workbench. In total, 40,946 contigs were annotated against the NCBI nr and SwissProt database and among them, 15,702 contigs were assigned to 14,575 GO terms. Besides a total of 9389 contigs were mapped to 416 KEGG biological pathways. The RNA-Seq comparison study identified 10,868 differentially expressed genes (DEGs) and of them, 3986 genes were significantly upregulated in the anterior regenerated blastema tissue samples of the worm. The GO enrichment analysis showed angiogenesis and unfolded protein binding as the top enriched functions and the pathway enrichment analysis denoted TCA cycle as the most significantly enriched pathway associated with the upregulated gene dataset of the worm. The identified DEGs and their function and pathway information can be effectively utilized further to interpret the key cellular, genetic and molecular events associated with the regeneration of the worm.

Identification and characterization of a fibroblast growth factor gene in the planarian Dugesia japonica

Planarians belong to the phylum Platyhelminthes and can regenerate their missing body parts after injury via activation of somatic pluripotent stem cells called neoblasts. Previous studies suggested that fibroblast growth factor (FGF) signaling plays a crucial role in the regulation of head tissue differentiation during planarian regeneration. To date, however, no FGF homologues in the Platyhelminthes have been reported. Here, we used a planarian Dugesia japonica model and identified an fgf gene termed Djfgf, which encodes a putative secreted protein with a core FGF domain characteristic of the FGF8/17/18 subfamily in bilaterians. Using Xenopus embryos, we found that DjFGF has FGF activity as assayed by Xbra induction. We next examined Djfgf expression in non-regenerating intact and regenerating planarians. In intact planarians, Djfgf was expressed in the auricles in the head and the pharynx. In the early process of regeneration, Djfgf was transiently expressed in a subset of differentiated cells around wounds. Notably, Djfgf expression was highly induced in the process of head regeneration when compared to that in the tail regeneration. Furthermore, assays of head regeneration from tail fragments revealed that combinatorial actions of the anterior extracellular signal-regulated kinase (ERK) and posterior Wnt/ß-catenin signaling restricted Djfgf expression to a certain anterior body part. This is the region where neoblasts undergo active proliferation to give rise to their differentiating progeny in response to wounding. The data suggest the possibility that DjFGF may act as an anterior counterpart of posteriorly localized Wnt molecules and trigger neoblast responses involved in planarian head regeneration.

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.

Djmek is involved in planarian regeneration by regulation of cell proliferation and apoptosis

Dugesia japonica, belonging to Platyhelminthes, plays an important role in the animal evolution and is well known for its extraordinary regenerative ability. Mitogen activated protein kinase (MAPK) pathway is an important cell signaling pathway that converts extracellular stimuli into a wide range of cellular responses. The MAP-extracellular signal-regulated kinase (MEK) is a main component of MAPK/ERK signaling, but there are few studies on mek gene in planarians. In this study, we observe the expression patterns of Djmek1 and Djmek2 in planarians, and find that both of the two genes are required for the planarian regeneration. At the same time, we also find that both Djmek1 and Djmek2 are involved in the planarian regeneration by regulation of cell proliferation and apoptosis. Together, our findings show that the functions of the two genes are similar and complementary, and they play an important role in the regeneration of planarians.

What is the role of PIWI family proteins in adult pluripotent stem cells? Insights from asexually reproducing animals, planarians

Planarians have a remarkable regenerative ability owing to their adult pluripotent stem cells (aPSCs), which are called "neoblasts." Planarians maintain a considerable number of neoblasts throughout their adulthood to supply differentiated cells for the maintenance of tissue homeostasis and asexual reproduction (fission followed by regeneration). Thus, planarians serve as a good model to study the regulatory mechanisms of in vivo aPSCs. In asexually reproducing invertebrates, such as sponge, Hydra, and planaria, piwi family genes are the markers most commonly expressed in aPSCs. While piwi family genes are known as guardians against transposable elements in the germline cells of animals that only sexually propagate, their functions in the aPSC system have remained elusive. In this review, we introduce recent knowledge on the PIWI family proteins in the aPSC system in planarians and other organisms and discuss how PIWI family proteins contribute to the regulation of the aPSC system.

Sirtuin-1 regulates organismal growth by altering feeding behavior and intestinal morphology in planarians

Nutrient availability upon feeding leads to an increase in body size in the planarian Schmidtea mediterranea However, it remains unclear how food consumption integrates with cell division at the organismal level. Here, we show that the NAD-dependent protein deacetylases sirtuins are evolutionarily conserved in planarians, and specifically demonstrate that the homolog of human sirtuin-1 (SIRT1) (encoded by Smed-Sirt-1), regulates organismal growth by impairing both feeding behavior and intestinal morphology. Disruption of Smed-Sirt-1 with RNAi or pharmacological inhibition of Sirtuin-1 leads to reduced animal growth. Conversely, enhancement of Sirtuin-1 activity with resveratrol accelerates growth. Differences in growth rates were associated with changes in the amount of time taken to locate food and overall food consumption. Furthermore, Smed-Sirt-1(RNAi) animals displayed reduced cell death and increased stem cell proliferation accompanied by impaired expression of intestinal lineage progenitors and reduced branching of the gut. Taken together, our findings indicate that Sirtuin-1 is a crucial metabolic hub capable of controlling animal behavior, tissue renewal and morphogenesis of the adult intestine.

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.

Positional information and tissue scaling during development and regeneration

In order to contribute to the appropriate tissues during development, cells need to know their position within the embryo. This positional information is conveyed by gradients of signaling molecules, termed morphogens, that are produced in specific regions of the embryo and induce concentration-dependent responses in target tissues. Positional information is remarkably robust, and embryos often develop with the correct proportions even if large parts of the embryo are removed. In this Review, we discuss classical embryological experiments and modern quantitative analyses that have led to mechanistic insights into how morphogen gradients adapt, scale and properly pattern differently sized domains. We analyze these experimental findings in the context of mathematical models and synthesize general principles that apply to multiple systems across species and developmental stages.

Evolutionary view of pluripotency seen from early development of non-mammalian amniotes

Early embryonic cells are capable of acquiring numerous developmental fates until they become irreversibly committed to specific lineages depending on intrinsic determinants and/or regional interactions. From fertilization to gastrulation, such pluripotent cells first increase in number and then turn to undergoing differentiation. Mechanisms regulating pluripotency in each species attract great interest in developmental biology. Also, outlining the evolutionary background of pluripotency can enhance our understanding of mammalian pluripotency and provide a broader view of early development of vertebrates. Here, we introduce integrative models of pluripotent states in amniotes (mammals, birds and reptiles) to offer a comprehensive overview of widely accepted knowledge about mammalian pluripotency and our recent findings in non-mammalian amniotes, such as chicken and gecko. In particular, we describe 1) the IL6/Stat3 signaling pathway as a positive regulator of naive pluripotency, 2) Fgf/Erk signaling as a process that prepares cells for differentiation, 3) the role of the interactions between these two signaling pathways during the transition from pluripotency to differentiation, and 4) functional diversification of two transcription factors, Class V POUs and Nanog. In the last section, we also briefly discuss possible relationships of unique cell cycle properties of early embryonic cells with signaling pathways and developmental potentials in the pluripotent cell states.

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.

DjERas plays an important role in planarian regeneration and homeostasis

The mechanisms of cell turnover including cell proliferation and cell differentiation were complex. Planarians possess amazing regeneration ability and undergo cell turnover throughout life. We identified a homologous gene of ERas by RNAi in Dugesia japonica. Knocking-down DjERas resulted in regeneration and homeostasis defects. Furthermore, we found that the expression of neoblasts and late progeny marker gene decreased in DjERas RNAi planarians. Our studies indicated that down-regulation of DjERas inhibited the proliferation and differentiation of stem cells through the conserved signaling pathway, resulted in the inability of the planarian to regenerate and maintain homeostasis. Our results suggest that DjERas plays a crucial role in the process of cell turnover.

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.

Neural control of body-plan axis in regenerating planaria

Control of axial polarity during regeneration is a crucial open question. We developed a quantitative model of regenerating planaria, which elucidates self-assembly mechanisms of morphogen gradients required for robust body-plan control. The computational model has been developed to predict the fraction of heteromorphoses expected in a population of regenerating planaria fragments subjected to different treatments, and for fragments originating from different regions along the anterior-posterior and medio-lateral axis. This allows for a direct comparison between computational and experimental regeneration outcomes. Vector transport of morphogens was identified as a fundamental requirement to account for virtually scale-free self-assembly of the morphogen gradients observed in planarian homeostasis and regeneration. The model correctly describes altered body-plans following many known experimental manipulations, and accurately predicts outcomes of novel cutting scenarios, which we tested. We show that the vector transport field coincides with the alignment of nerve axons distributed throughout the planarian tissue, and demonstrate that the head-tail axis is controlled by the net polarity of neurons in a regenerating fragment. This model provides a comprehensive framework for mechanistically understanding fundamental aspects of body-plan regulation, and sheds new light on the role of the nervous system in directing growth and form.

Planarian regeneration as a model of anatomical homeostasis: Recent progress in biophysical and computational approaches

Planarian behavior, physiology, and pattern control offer profound lessons for regenerative medicine, evolutionary biology, morphogenetic engineering, robotics, and unconventional computation. Despite recent advances in the molecular genetics of stem cell differentiation, this model organism's remarkable anatomical homeostasis provokes us with truly fundamental puzzles about the origin of large-scale shape and its relationship to the genome. In this review article, we first highlight several deep mysteries about planarian regeneration in the context of the current paradigm in this field. We then review recent progress in understanding of the physiological control of an endogenous, bioelectric pattern memory that guides regeneration, and how modulating this memory can permanently alter the flatworm's target morphology. Finally, we focus on computational approaches that complement reductive pathway analysis with synthetic, systems-level understanding of morphological decision-making. We analyze existing models of planarian pattern control and highlight recent successes and remaining knowledge gaps in this interdisciplinary frontier field.

The role of fibroblast growth factor signalling in Echinococcus multilocularis development and host-parasite interaction

BACKGROUND

Alveolar echinococcosis (AE) is a lethal zoonosis caused by the metacestode larva of the tapeworm Echinococcus multilocularis. The infection is characterized by tumour-like growth of the metacestode within the host liver, leading to extensive fibrosis and organ-failure. The molecular mechanisms of parasite organ tropism towards the liver and influences of liver cytokines and hormones on parasite development are little studied to date.

METHODOLOGY/PRINCIPAL FINDINGS

We show that the E. multilocularis larval stage expresses three members of the fibroblast growth factor (FGF) receptor family with homology to human FGF receptors. Using the Xenopus expression system we demonstrate that all three Echinococcus FGF receptors are activated in response to human acidic and basic FGF, which are present in the liver. In all three cases, activation could be prevented by addition of the tyrosine kinase (TK) inhibitor BIBF 1120, which is used to treat human cancer. At physiological concentrations, acidic and basic FGF significantly stimulated the formation of metacestode vesicles from parasite stem cells in vitro and supported metacestode growth. Furthermore, the parasite's mitogen activated protein kinase signalling system was stimulated upon addition of human FGF. The survival of metacestode vesicles and parasite stem cells were drastically affected in vitro in the presence of BIBF 1120.

CONCLUSIONS/SIGNIFICANCE

Our data indicate that mammalian FGF, which is present in the liver and upregulated during fibrosis, supports the establishment of the Echinococcus metacestode during AE by acting on an evolutionarily conserved parasite FGF signalling system. These data are valuable for understanding molecular mechanisms of organ tropism and host-parasite interaction in AE. Furthermore, our data indicate that the parasite's FGF signalling systems are promising targets for the development of novel drugs against AE.

The planarian Schmidtea mediterranea as a model system for the discovery and characterization of cell-penetrating peptides and bioportides

The general utility of the planarian Schmidtea mediterranea, an organism with remarkable regenerative capacity, was investigated as a convenient three-dimensional model to analyse the import of cell-penetrating peptides (CPPs) and bioportides (bioactive CPPs) into complex tissues. The unpigmented planarian blastema, 3 days post head amputation, is a robust platform to assess the penetration of red-fluorescent CPPs into epithelial cells and deeper tissues. Three planarian proteins, Ovo, ZicA and Djeya, which collectively control head remodelling and eye regeneration following decapitation, are a convenient source of novel cationic CPP vectors. One example, Djeya1 (RKLAFRYRRIKELYNSYR), is a particularly efficient and seemingly inert CPP vector that could be further developed to assist the delivery of bioactive payloads across the plasma membrane of eukaryotic cells. Eye regeneration, following head amputation, was utilized in an effort to identify bioportides capable of influencing stem cell-dependent morphogenesis. These investigations identified the tetradecapeptide mastoparan (INLKALAALAKKIL) as a bioportide able to influence the gross morphology of head development. We conclude that, compared with cellular monolayers, the S. mediterranea system provides many advantages and will support the identification of bioportides able to selectively modify the biology of totipotent neoblasts and, presumably, other mammalian stem cell types.

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.

JmjC Domain-Encoding Genes Are Conserved in Highly Regenerative Metazoans and Are Associated with Planarian Whole-Body Regeneration

The capacity for regeneration varies greatly among metazoans, yet little is known about the evolutionary processes leading to such different regeneration abilities. In particular, highly regenerative species such as planarians and cnidarians can regenerate the whole body from an amputated fragment; however, a common molecular basis, if any, among these species remains unclear. Here, we show that genes encoding Jumonji C (JmjC) domain-containing proteins are associated with high regeneration ability. We classified 132 fully sequenced metazoans into two groups with high or low regeneration abilities and identified 118 genes conserved in the high regenerative group that were lost in species in the low regeneration group during evolution. Ninety-six percent of them were JmjC domain-encoding genes. We denoted the candidate genes as high regenerative species-specific JmjC domain-encoding genes (HRJDs). We observed losses of HRJDs in Helobdella robusta, which lost its high regeneration ability during evolution based on phylogenetic analysis. By RNA sequencing analyses, we observed that HRJD orthologs were differentially expressed during regeneration in two Cnidarians, as well as Platyhelminthes and Urochordata, which are highly regenerative species. Furthermore, >50% of the head and tail parts of amputated planarians (Dugesia japonica) died during regeneration after RNA interference of HRJD orthologs. These results indicate that HRJD are strongly associated with a high regeneration ability in metazoans. HRJD paralogs regulate gene expression by histone demethylation; thus, HRJD may be related to epigenetic regulation controlling stem cell renewal and stem cell differentiation during regeneration. We propose that HRJD play a central role in epigenetic regulation during regeneration.

Molecular cloning and characterization of a novel Y-box gene from planarian Dugesia japonica

Y-box binding protein (YB protein) is an ancient conserved multifunctional DNA/RNA-binding protein. A novel YB protein DjY2 gene from planarian Dugesia japonica was cloned by RACE method and characterized. This cDNA contains 689 bp with a putative open reading frame of 197 amino acids. It has a predicted molecular mass of 22.14 kDa and an isoelectric point of 9.67. Whole-mount in situ hybridization and relative quantitative real-time PCR were used to study the spatial and temporal expression pattern of DjY2 in the process of planarian regeneration. Results showed that DjY2 was expressed in many parts of the body in intact planarian, but the expression level was low in head and pharynx. The transcripts of DjY2 was significantly increased both at the head parts and the tail parts after amputation, especially at the site of cutting. The spatial expression gradually recovered to the state of intact planarian with the time of regeneration. Our results indicated that DjY2 might participate in the process of regeneration in planarian.

Danger signals activate a putative innate immune system during regeneration in a filamentous fungus

The ability to respond to injury is a biological process shared by organisms of different kingdoms that can even result in complete regeneration of a part or structure that was lost. Due to their immobility, multicellular fungi are prey to various predators and are therefore constantly exposed to mechanical damage. Nevertheless, our current knowledge of how fungi respond to injury is scarce. Here we show that activation of injury responses and hyphal regeneration in the filamentous fungus Trichoderma atroviride relies on the detection of two danger or alarm signals. As an early response to injury, we detected a transient increase in cytosolic free calcium ([Ca2+]c) that was promoted by extracellular ATP, and which is likely regulated by a mechanism of calcium-induced calcium-release. In addition, we demonstrate that the mitogen activated protein kinase Tmk1 plays a key role in hyphal regeneration. Calcium- and Tmk1-mediated signaling cascades activated major transcriptional changes early following injury, including induction of a set of regeneration associated genes related to cell signaling, stress responses, transcription regulation, ribosome biogenesis/translation, replication and DNA repair. Interestingly, we uncovered the activation of a putative fungal innate immune response, including the involvement of HET domain genes, known to participate in programmed cell death. Our work shows that fungi and animals share danger-signals, signaling cascades, and the activation of the expression of genes related to immunity after injury, which are likely the result of convergent evolution.

Role of MEKK1 in the anterior-posterior patterning during planarian regeneration

Planarians have established a unique body pattern along the anterior-posterior (AP) axis, which consists of at least four distinct body regions arranged in an anterior to posterior sequence: head, prepharyngeal, pharyngeal (containing a pharynx), and tail regions, and possess high regenerative ability. How they reconstruct the regional continuity in a head-to-tail sequence after amputation still remains unknown. We use as a model planarian Dugesia japonica head regeneration from tail fragments, which involves dynamic rearrangement of the body regionality of preexisting tail tissues along the AP axis, and show here that RNA interference of the gene D. japonica mek kinase 1 (Djmekk1) caused a significant anterior shift in the position of pharynx regeneration at the expense of the prepharyngeal region, while keeping the head region relatively constant in size, and accordingly led to development of a relatively longer tail region. Our data suggest that DjMEKK1 regulates anterior extracellular signal-regulated kinase (ERK) and posterior β-catenin signaling pathways in a positive and negative manner, respectively, to establish a proper balance resulting in the regeneration of planarian's scale-invariant trunk-to-tail patterns across individuals. Furthermore, we demonstrated that DjMEKK1 negatively modulates planarian β-catenin activity via its serine/threonine kinase domain, but not its PHD/RING finger domain, by testing secondary axis formation in Xenopus embryos. The data suggest that Djmekk1 plays an instructive role in the coordination between the establishment of the prepharyngeal region and posteriorizing of pharynx formation by balancing the two opposing morphogenetic signals along the AP axis during planarian regeneration.