Stem Cells, Patterning and Regeneration in Planarians: Self-Organization at the Organismal Scale

The extracellular matrix niche of muscle stem cells

Preserving the potency of stem cells in adult tissues is very demanding and relies on the concerted action of various cellular and non-cellular elements in a precise stoichiometry. This balanced microenvironment is found in specific anatomical "pockets" within the tissue, known as the stem cell niche. In this review, we explore the interplay between stem cells and their niches, with a primary focus on skeletal muscle stem cells and the extracellular matrix (ECM). Quiescent muscle stem cells, known as satellite cells are active producers of a diverse array of ECM molecules, encompassing major constituents like collagens, laminins, and integrins, some of which are explored in this review. The conventional perception of ECM as merely a structural scaffold is evolving. Collagens can directly interact as ligands with receptors on satellite cells, while other ECM proteins have the capacity to sequester growth factors and regulate their release, especially relevant during satellite cell turnover in homeostasis or activation upon injury. Additionally, we explore an evolutionary perspective on the ECM across a range of multicellular organisms and discuss a model wherein satellite cells are self-sustained by generating their own niche. Considering the prevalence of ECM proteins in the connective tissue of various organs it is not surprising that mutations in ECM genes have pathological implications, including in muscle, where they can lead to myopathies. However, the particular role of certain disease-related ECM proteins in stem cell maintenance highlights the potential contribution of stem cell deregulation to the progression of these disorders.

Evolutionary dynamics of whole-body regeneration across planarian flatworms

Regenerative abilities vary dramatically across animals. Even amongst planarian flatworms, well-known for complete regeneration from tiny body fragments, some species have restricted regeneration abilities while others are almost entirely regeneration incompetent. Here, we assemble a diverse live collection of 40 planarian species to probe the evolution of head regeneration in the group. Combining quantification of species-specific head-regeneration abilities with a comprehensive transcriptome-based phylogeny reconstruction, we show multiple independent transitions between robust whole-body regeneration and restricted regeneration in freshwater species. RNA-mediated genetic interference inhibition of canonical Wnt signalling in RNA-mediated genetic interference-sensitive species bypassed all head-regeneration defects, suggesting that the Wnt pathway is linked to the emergence of planarian regeneration defects. Our finding that Wnt signalling has multiple roles in the reproductive system of the model species Schmidtea mediterranea raises the possibility that a trade-off between egg-laying, asexual reproduction by fission/regeneration and Wnt signalling drives regenerative trait evolution. Although quantitative comparisons of Wnt signalling levels, yolk content and reproductive strategy across our species collection remained inconclusive, they revealed divergent Wnt signalling roles in the reproductive system of planarians. Altogether, our study establishes planarians as a model taxon for comparative regeneration research and presents a framework for the mechanistic evolution of regenerative abilities.

WTAP regulates stem cells via TRAF6 to maintain planarian homeostasis and regeneration

WTAP, a highly conserved Wilms' tumor 1 interacting protein, is involved in a variety of biological processes. However, functional studies of WTAP in planarians have not been reported. In this study, we examined the spatiotemporal expression pattern of planarian DjWTAP and investigated its functions in planarians regeneration and homeostasis. Knocking-down DjWTAP resulted in severe morphological defects leading to lethality within 20 days. Silencing DjWTAP promoted the proliferation of PiwiA⁺ cells but impaired the lineage differentiation of epidermal, neural, digestive, and excretory cell types, suggesting a critical role for DjWTAP in stem cell self-renewal and differentiation in planarian. To further investigate the mechanisms underlying the defective differentiation, RNA-seq was employed to determine the transcriptomic alterations upon DjWTAP RNA interference. Histone 4 (H4), Histone-lysine N-methyltransferase-SETMAR like, and TNF receptor-associated factor 6 (TRAF6), were significantly upregulated in response to DjWTAP RNAi. Knocking-down TRAF6 largely rescued the defective tissue homeostasis and regeneration resulted from DjWTAP knockdown in planarians, suggesting that DjWTAP maintains planarian regeneration and homeostasis via TRAF6.

Do Not Lose Your Head Over the Unequal Regeneration Capacity in Prolecithophoran Flatworms

One of the central questions in studying the evolution of regeneration in flatworms remains whether the ancestral flatworm was able to regenerate all body parts, including the head. If so, this ability was subsequently lost in most existent flatworms. The alternative hypothesis is that head regeneration has evolved within flatworms, possibly several times independently. In the well-studied flatworm taxon Tricladida (planarians), most species are able to regenerate a head. Little is known about the regeneration capacity of the closest relatives of Tricladida: Fecampiida and Prolecithophora. Here, we analysed the regeneration capacity of three prolecithophoran families: Pseudostomidae, Plagiostomidae, and Protomonotresidae. The regeneration capacity of prolecithophorans varies considerably between families, which is likely related to the remaining body size of the regenerates. While all studied prolecithophoran species were able to regenerate a tail-shaped posterior end, only some Pseudostomidae could regenerate a part of the pharynx and pharynx pouch. Some Plagiostomidae could regenerate a head including the brain and eyes, provided the roots of the brain were present. The broad spectrum of regeneration capacity in Prolecithophora suggests that head regeneration capacity is not an apomorphy of Adiaphanida.

Heterologous reporter expression in the planarian Schmidtea mediterranea through somatic mRNA transfection

Planarians have long been studied for their regenerative abilities. Moving forward, tools for ectopic expression of non-native proteins will be of substantial value. Using a luminescent reporter to overcome the strong autofluorescence of planarian tissues, we demonstrate heterologous protein expression in planarian cells and live animals. Our approach is based on the introduction of mRNA through several nanotechnological and chemical transfection methods. We improve reporter expression by altering untranslated region (UTR) sequences and codon bias, facilitating the measurement of expression kinetics in both isolated cells and whole planarians using luminescence imaging. We also examine protein expression as a function of variations in the UTRs of delivered mRNA, demonstrating a framework to investigate gene regulation at the post-transcriptional level. Together, these advances expand the toolbox for the mechanistic analysis of planarian biology and establish a foundation for the development and expansion of transgenic techniques in this unique model system.

Head removal enhances planarian electrotaxis

Certain animal species utilize electric fields for communication, hunting and spatial orientation. Freshwater planarians move toward the cathode in a static electric field (cathodic electrotaxis). This planarian behavior was first described by Raymond Pearl more than a century ago. However, planarian electrotaxis has received little attention since, and the underlying mechanisms and evolutionary significance remain unknown. To close this knowledge gap, we developed an apparatus and scoring metrics for automated quantitative and mechanistic studies of planarian behavior upon exposure to a static electric field. Using this automated setup, we characterized electrotaxis in the planarian Dugesia japonica and found that this species responds to voltage instead of current, in contrast to results from previous studies using other planarian species. Surprisingly, we found differences in electrotaxis ability between small (shorter) and large (longer) planarians. To determine the cause of these differences, we took advantage of the regenerative abilities of planarians and compared electrotaxis in head, tail and trunk fragments of various lengths. We found that tail and trunk fragments electrotaxed, whereas head fragments did not, regardless of size. Based on these data, we hypothesized that signals from the head may interfere with electrotaxis when the head area/body area reached a critical threshold. In support of this hypothesis, we found that (1) smaller intact planarians that cannot electrotax have a relatively larger head-to-body-ratio than large planarians that can electrotax, and (2) the electrotaxis behavior of cut head fragments was negatively correlated with the head-to-body ratio of the fragments. Moreover, we could restore cathodic electrotaxis in head fragments via decapitation, directly demonstrating inhibition of electrotaxis by the head.

Summary: A new method for quantitative studies of planarian electrotaxis shows that Dugesia japonica move toward the cathode. This behavior is enhanced by removal of the head.

Cellular diversity and developmental hierarchy in the planarian nervous system

Our ability to dissect cell type diversity, development, and plasticity in the nervous system has been transformed by the recent surge of massive sequencing studies at the single-cell level. A large body of this work has focused primarily on organisms with nervous systems established early in development. Using planarian flatworms in which neurons are constantly respecified, replenished, and regenerated, we analyze several existing single-cell transcriptomic datasets and observe features in neuron identity, differentiation, maturation, and function that may provide the planarian nervous system with high levels of adaptability required to respond to various cues including injury. This analysis allows us to place many prior observations made by functional characterizations in a general framework and provide additional hypothesis and predictions to test in future investigations.

DDX24 is required for muscle fiber organization and the suppression of wound-induced Wnt activity necessary for pole re-establishment during planarian regeneration

Planarians have a remarkable ability to undergo whole-body regeneration. Successful regeneration outcome is determined by processes like polarity establishment at the wound site, which is followed by pole (organizer) specification. Interestingly, these determinants are almost exclusively expressed by muscles in these animals. However, the molecular toolkit that enables the functional versatility of planarian muscles remains poorly understood. Here we report that SMED_DDX24, a D-E-A-D Box RNA helicase, is necessary for planarian survival and regeneration. We found that DDX24 is enriched in muscles and its knockdown disrupts muscle fiber organization. This leads to defective pole specification, which in turn results in misregulation of many positional control genes specifically during regeneration. ddx24 RNAi also upregulates wound-induced Wnt signalling. Suppressing this ectopic Wnt activity rescues the knockdown phenotype by enabling better anterior pole regeneration. To summarize, our work highlights the role of an RNA helicase in muscle fiber organization, and modulating amputation-induced wnt levels, both of which seem critical for pole re-organization, thereby regulating whole-body regeneration.

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.

Src acts with WNT/FGFRL signaling to pattern the planarian anteroposterior axis

Tissue identity determination is crucial for regeneration, and the planarian anteroposterior (AP) axis uses positional control genes expressed from body wall muscle to determine body regionalization. Canonical Wnt signaling establishes anterior versus posterior pole identities through notum and wnt1 signaling, and two Wnt/FGFRL signaling pathways control head and trunk domains, but their downstream signaling mechanisms are not fully understood. Here, we identify a planarian Src homolog that restricts head and trunk identities to anterior positions. src-1(RNAi) animals formed enlarged brains and ectopic eyes and also duplicated trunk tissue, similar to a combination of Wnt/FGFRL RNAi phenotypes. src-1 was required for establishing territories of positional control gene expression in Schmidtea mediterranea, indicating that it acts at an upstream step in patterning the AP axis. Double RNAi experiments and eye regeneration assays suggest src-1 can act in parallel to at least some Wnt and FGFRL factors. Co-inhibition of src-1 with other posterior-promoting factors led to dramatic patterning changes and a reprogramming of Wnt/FGFRLs into controlling new positional outputs. These results identify src-1 as a factor that promotes robustness of the AP positional system that instructs appropriate regeneration.

Restoration of DNA integrity and cell cycle by electric stimulation in planarian tissues damaged by ionizing radiation

Exposure to high levels of ionizing γ-radiation leads to irreversible DNA damage and cell death. Here, we establish that exogenous application of electric stimulation enables cellular plasticity to reestablish stem cell activity in tissues damaged by ionizing radiation. We show that sub-threshold direct current stimulation (DCS) rapidly restores pluripotent stem cell populations previously eliminated by lethally γ-irradiated tissues of the planarian flatworm Schmidtea mediterranea. Our findings reveal that DCS enhances DNA repair, transcriptional activity, and cell cycle entry in post-mitotic cells. These responses involve rapid increases in cytosolic [Ca2+] through the activation of L-type Cav channels and intracellular Ca2+ stores leading to the activation of immediate early genes and ectopic expression of stem cell markers in postmitotic cells. Overall, we show the potential of electric current stimulation to reverse the damaging effects of high dose γ-radiation in adult tissues. Furthermore, our results provide mechanistic insights describing how electric stimulation effectively translates into molecular responses capable of regulating fundamental cellular functions without the need for genetic or pharmacological intervention.

Articulating the "stem cell niche" paradigm through the lens of non-model aquatic invertebrates

Stem cells (SCs) in vertebrates typically reside in "stem cell niches" (SCNs), morphologically restricted tissue microenvironments that are important for SC survival and proliferation. SCNs are broadly defined by properties including physical location, but in contrast to vertebrates and other "model" organisms, aquatic invertebrate SCs do not have clearly documented niche outlines or properties. Life strategies such as regeneration or asexual reproduction may have conditioned the niche architectural variability in aquatic or marine animal groups. By both establishing the invertebrates SCNs as independent types, yet allowing inclusiveness among them, the comparative analysis will allow the future functional characterization of SCNs.

Principles of regeneration revealed by the planarian eye

One approach to elucidating the principles of regeneration is to investigate mechanisms that regenerate a target organ. Planarian eyes are discrete, visible structures that are dispensable for viability, making them powerful for studying the logic of regeneration. Fate specification in eye regeneration occurs in stem cells (neoblasts), generating eye progenitors. Eye progenitor production is not responsive to the presence or absence of the eye, with regeneration explained by constant progenitor production in the appropriate positional environment. Eye progenitors display coarse spatial specification. A combination of eye-extrinsic cues and self-organization with differentiated eye cells dictate where migratory eye progenitors target. Finally, guidepost-like cells influence regenerating axons to facilitate the restoration of eye circuitry. These findings from the eye as a case study present a model that explains how regeneration can occur.

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.

A Spatiotemporal Characterisation of Redox Molecules in Planarians, with a Focus on the Role of Glutathione during Regeneration

A strict coordination between pro- and antioxidative molecules is needed for normal animal physiology, although their exact function and dynamics during regeneration and development remains largely unknown. Via in vivo imaging, we were able to locate and discriminate between reactive oxygen species (ROS) in real-time during different physiological stages of the highly regenerative planarian Schmidtea mediterranea. All ROS signals were strong enough to overcome the detected autofluorescence. Combined with an in situ characterisation and quantification of the transcription of several antioxidant genes, our data showed that the planarian gut and epidermis have a well-equipped redox system. Pharmacological inhibition or RNA interference of either side of the redox balance resulted in alterations in the regeneration process, characterised by decreased blastema sizes and delayed neurodevelopment, thereby affecting tails more than heads. Focusing on glutathione, a central component in the redox balance, we found that it is highly present in planarians and that a significant reduction in glutathione content led to regenerative failure with tissue lesions, characterised by underlying stem cell alterations. This exploratory study indicates that ROS and antioxidants are tightly intertwined and should be studied as a whole to fully comprehend the function of the redox balance in animal physiology.

Direct Current Electric Stimulation Alters the Frequency and the Distribution of Mitotic Cells in Planarians

Background: The use of direct current electric stimulation (DCS) is an effective strategy to treat disease and enhance body functionality. Thus, treatment with DCS is an attractive biomedical alternative, but the molecular underpinnings remain mostly unknown. The lack of experimental models to dissect the effects of DCS from molecular to organismal levels is an important caveat. Here, we introduce the planarian flatworm Schmidtea mediterranea as a tractable organism for in vivo studies of DCS. We developed an experimental method that facilitates the application of direct current electrical stimulation to the whole planarian body (pDCS). Materials and Methods: Planarian immobilization was achieved by combining treatment with anesthesia, agar embedding, and low temperature via a dedicated thermoelectric cooling unit. Electric currents for pDCS were delivered using pulled glass microelectrodes. The electric potential was supplied through a constant voltage power supply. pDCS was administered up to six hours, and behavioral and molecular effects were measured by using video recordings, immunohistochemistry, and gene expression analysis. Results: The behavioral immobilization effects are reversible, and pDCS resulted in a redistribution of mitotic cells along the mediolateral axis of the planarian body. The pDCS effects were dependent on the polarity of the electric field, which led to either increase in reductions in mitotic densities associated with the time of pDCS. The changes in mitotic cells were consistent with apparent redistribution in gene expression of the stem cell marker smedwi-1. Conclusion: The immobilization technique presented in this work facilitates studies aimed at dissecting the effects of exogenous electric stimulation in the adult body. Treatment with DCS can be administered for varying times, and the consequences evaluated at different levels, including animal behavior, cellular and transcriptional changes. Indeed, treatment with pDCS can alter cellular and transcriptional parameters depending on the polarity of the electric field and duration of the exposure.

Why isn't sex optional? Stem-cell competition, loss of regenerative capacity, and cancer in metazoan evolution

Animals that can reproduce vegetatively by fission or budding and also sexually via specialized gametes are found in all five primary animal lineages (Bilateria, Cnidaria, Ctenophora, Placozoa, Porifera). Many bilaterian lineages, including roundworms, insects, and most chordates, have lost the capability of vegetative reproduction and are obligately gametic. We suggest a developmental explanation for this evolutionary phenomenon: obligate gametic reproduction is the result of germline stem cells winning a winner-take-all competition with non-germline stem cells for control of reproduction and hence lineage survival. We develop this suggestion by extending Hamilton's rule, which factors the relatedness between parties into the cost/benefit analysis that underpins cooperative behaviors, to include similarity of cellular state. We show how coercive or deceptive cell-cell signaling can be used to make costly cooperative behaviors appear less costly to the cooperating party. We then show how competition between stem-cell lineages can render an ancestral combination of vegetative reproduction with facultative sex unstable, with one or the other process driven to extinction. The increased susceptibility to cancer observed in obligately-sexual lineages is, we suggest, a side-effect of deceptive signaling that is exacerbated by the loss of whole-body regenerative abilities. We suggest a variety of experimental approaches for testing our predictions.

Dynamic Polarization of the Multiciliated Planarian Epidermis between Body Plan Landmarks

Polarity is a universal design principle of biological systems that manifests at all organizational scales, yet its coordination across scales remains poorly understood. Here, we make use of the extreme anatomical plasticity of planarian flatworms to probe the interplay between global body plan polarity and local cell polarity. Our quantitative analysis of ciliary rootlet orientation in the epidermis reveals a dynamic polarity field with head and tail as independent determinants of anteroposterior (A/P) polarization and the body margin as determinant of mediolateral (M/L) polarization. Mathematical modeling rationalizes the global polarity field and its response to experimental manipulations as superposition of separate A/P and M/L fields, and we identify the core PCP and Ft/Ds pathways as their molecular mediators. Overall, our study establishes a framework for the alignment of cellular polarity vectors relative to planarian body plan landmarks and establishes the core PCP and Ft/Ds pathways as evolutionarily conserved 2D-polarization module.

PlanMine 3.0-improvements to a mineable resource of flatworm biology and biodiversity

Flatworms (Platyhelminthes) are a basally branching phylum that harbours a wealth of fascinating biology, including planarians with their astonishing regenerative abilities and the parasitic tape worms and blood flukes that exert a massive impact on human health. PlanMine (http://planmine.mpi-cbg.de/) has the mission objective of providing both a mineable sequence repository for planarians and also a resource for the comparative analysis of flatworm biology. While the original PlanMine release was entirely based on transcriptomes, the current release transitions to a more genomic perspective. Building on the recent availability of a high quality genome assembly of the planarian model species Schmidtea mediterranea, we provide a gene prediction set that now assign existing transcripts to defined genomic coordinates. The addition of recent single cell and bulk RNA-seq datasets greatly expands the available gene expression information. Further, we add transcriptomes from a broad range of other flatworms and provide a phylogeny-aware interface that makes evolutionary species comparisons accessible to non-experts. At its core, PlanMine continues to utilize the powerful InterMine framework and consistent data annotations to enable meaningful inter-species comparisons. Overall, PlanMine 3.0 thus provides a host of new features that makes the fascinating biology of flatworms accessible to the wider research community.

Cell-type diversity and regionalized gene expression in the planarian intestine

Proper function and repair of the digestive system are vital to most animals. Deciphering the mechanisms involved in these processes requires an atlas of gene expression and cell types. Here, we applied laser-capture microdissection (LCM) and RNA-seq to characterize the intestinal transcriptome of Schmidtea mediterranea, a planarian flatworm that can regenerate all organs, including the gut. We identified hundreds of genes with intestinal expression undetected by previous approaches. Systematic analyses revealed extensive conservation of digestive physiology and cell types with other animals, including humans. Furthermore, spatial LCM enabled us to uncover previously unappreciated regionalization of gene expression in the planarian intestine along the medio-lateral axis, especially among intestinal goblet cells. Finally, we identified two intestine-enriched transcription factors that specifically regulate regeneration (hedgehog signaling effector gli-1) or maintenance (RREB2) of goblet cells. Altogether, this work provides resources for further investigation of mechanisms involved in gastrointestinal function, repair and regeneration.

Does regeneration recapitulate phylogeny? Planaria as a model of body-axis specification in ancestral eumetazoa

Metazoan body plans combine well-defined primary, secondary, and in many bilaterians, tertiary body axes with structural asymmetries at multiple scales. Despite decades of study, how axis-defining symmetries and system-defining asymmetries co-emerge during both evolution and development remain open questions. Regeneration studies in asexual planaria have demonstrated an array of viable forms with symmetrized and, in some cases, duplicated body axes. We suggest that such forms may point toward an ancestral eumetazoan form with characteristics of both cnidarians and placazoa.

Poly(ADP-Ribose) Polymerase-3 Regulates Regeneration in Planarians

Protein ADP-ribosylation is a reversible post-translational modification (PTM) process that plays fundamental roles in cell signaling. The covalent attachment of ADP ribose polymers is executed by PAR polymerases (PARP) and it is essential for chromatin organization, DNA repair, cell cycle, transcription, and replication, among other critical cellular events. The process of PARylation or polyADP-ribosylation is dynamic and takes place across many tissues undergoing renewal and repair, but the molecular mechanisms regulating this PTM remain mostly unknown. Here, we introduce the use of the planarian Schmidtea mediterranea as a tractable model to study PARylation in the complexity of the adult body that is under constant renewal and is capable of regenerating damaged tissues. We identified the evolutionary conservation of PARP signaling that is expressed in planarian stem cells and differentiated tissues. We also demonstrate that Smed-PARP-3 homolog is required for proper regeneration of tissues in the anterior region of the animal. Furthermore, our results demonstrate, Smed-PARP-3(RNAi) disrupts the timely location of injury-induced cell death near the anterior facing wounds and also affects the regeneration of the central nervous system. Our work reveals novel roles for PARylation in large-scale regeneration and provides a simplified platform to investigate PARP signaling in the complexity of the adult body.

The evolutionary terrestrialization of planarian flatworms (Platyhelminthes, Tricladida, Geoplanidae): a review and research programme

The terrestrialization of animal life from aquatic ancestors is a key transition during the history of life. Planarian flatworms form an ideal group of model organisms to study this colonization of the land because they have freshwater, marine, and terrestrial representatives. The widespread occurrence of terrestrial flatworms is a testament to their remarkable success occupying a new niche on land. This lineage of terrestrial worms provides a unique glimpse of an evolutionary pathway by which a group of early divergent aquatic, invertebrate metazoans has moved onto land. Land flatworms are among the first groups of animals to have evolved terrestrial adaptations and to have extensively radiated. Study of this terrestrialization process and the anatomical key innovations facilitating their colonization of the land will contribute greatly to our understanding of this important step in Metazoan history. The context and scientific background are reviewed regarding the evolutionary terrestrialization of land flatworms. Furthermore, a framework of a research programme is sketched, which has as its main objective to test hypotheses on the evolution of land planarians, specifically whether particular anatomical and physiological key innovations have contributed to their evolutionary successful terrestrial colonization and radiation. In this context special attention is paid to the respiration in aquatic and terrestrial planarians. The research programme depends on a comprehensive phylogenetic analysis of all major taxa of the land flatworms on the basis of both molecular and anatomical data. The data sets should be analyzed phylogenetically with a suite of phylogenetic inference methods. Building on such robust reconstructions, it will be possible to study associations between key innovations and the evolutionary terrestrialization process.

Adjustments in the control of mitochondrial respiratory capacity to tolerate temperature fluctuations

As the world's climate changes, life faces an evolving thermal environment. Mitochondrial oxidative phosphorylation (OXPHOS) is critical to ensure sufficient cellular energy production, and it is strongly influenced by temperature. The thermally induced changes to the regulation of specific steps within the OXPHOS process are poorly understood. In our study, we used the eurythermal species of planarian Dugesia tigrina to study the thermal sensitivity of the OXPHOS process at 10, 15, 20, 25 and 30°C. We conducted cold acclimation experiments where we measured the adjustment of specific steps in OXPHOS at two assay temperatures (10 and 20°C) following 4 weeks of acclimation under normal (22°C) or low (5°C) temperature conditions. At the low temperature, the contribution of the NADH pathway to the maximal OXPHOS capacity, in a combined pathway (NADH and succinate), was reduced. There was partial compensation by an increased contribution of the succinate pathway. As the temperature decreased, OXPHOS became more limited by the capacity of the phosphorylation system. Acclimation to the low temperature resulted in positive adjustments of the NADH pathway capacity due, at least in part, to an increase in complex I activity. The acclimation also resulted in a better match between OXPHOS and phosphorylation system capacities. Both of these adjustments following acclimation were specific to the low assay temperature. We conclude that there is substantial plasticity in the mitochondrial OXPHOS process following thermal acclimation in D. tigrina, and this probably contributes to the wide thermal range of the species.

Model systems for regeneration: planarians

Planarians are a group of flatworms. Some planarian species have remarkable regenerative abilities, which involve abundant pluripotent adult stem cells. This makes these worms a powerful model system for understanding the molecular and evolutionary underpinnings of regeneration. By providing a succinct overview of planarian taxonomy, anatomy, available tools and the molecular orchestration of regeneration, this Primer aims to showcase both the unique assets and the questions that can be addressed with this model system.

A modular laboratory course using planarians to study genes involved in tissue regeneration

Undergraduate research experiences are excellent opportunities to engage students in science alongside experienced scientists, but at large institutions, it is challenging to accommodate all students. To address and engage a larger number of students, we developed a modular laboratory course based on the course-based undergraduate research experiences model. This new course was integrated with the scientific aims of a research laboratory studying the cellular and molecular mechanisms underlying tissue regeneration in planarians. In this course, students were asked to identify genes with roles in planarian biology. Students analyzed and cloned an assigned gene, determined its expression pattern in situ and examined its function in regeneration. Additionally, we developed critical thinking and scientific communication skills by incorporating activities focused on critical concepts. Students obtained high quality primary data and were successful in completing and mastering the course learning outcomes. They benefitted by developing basic research skills, learning to perform, trouble-shooting experiments, reading and critically analyzing primary literature, and using the information to defend and explain their experimental results. Through this course, students also increased their confidence and ability to perform independent scientific research. The course was designed to make it accessible to the community to implement and adapt as appropriate in diverse institutions. © 2019 International Union of Biochemistry and Molecular Biology, 47(5):547-559, 2019.

Muscle functions as a connective tissue and source of extracellular matrix in planarians

Regeneration and tissue turnover require new cell production and positional information. Planarians are flatworms capable of regenerating all body parts using a population of stem cells called neoblasts. The positional information required for tissue patterning is primarily harbored by muscle cells, which also control body contraction. Here we produce an in silico planarian matrisome and use recent whole-animal single-cell-transcriptome data to determine that muscle is a major source of extracellular matrix (ECM). No other ECM-secreting, fibroblast-like cell type was detected. Instead, muscle cells express core ECM components, including all 19 collagen-encoding genes. Inhibition of muscle-expressed hemicentin-1 (hmcn-1), which encodes a highly conserved ECM glycoprotein, results in ectopic peripheral localization of cells, including neoblasts, outside of the muscle layer. ECM secretion and hmcn-1-dependent maintenance of tissue separation indicate that muscle functions as a planarian connective tissue, raising the possibility of broad roles for connective tissue in adult positional information.

How the cellular source of positional information compares across regenerative animals is unclear. Here, the authors find that planarian muscle, which harbours positional information, acts as a connective tissue by being a major site of matrisome gene expression and by maintaining tissue architecture.

Body size-dependent energy storage causes Kleiber's law scaling of the metabolic rate in planarians

Kleiber's law, or the 3/4 -power law scaling of the metabolic rate with body mass, is considered one of the few quantitative laws in biology, yet its physiological basis remains unknown. Here, we report Kleiber's law scaling in the planarian Schmidtea mediterranea. Its reversible and life history-independent changes in adult body mass over 3 orders of magnitude reveal that Kleiber's law does not emerge from the size-dependent decrease in cellular metabolic rate, but from a size-dependent increase in mass per cell. Through a combination of experiment and theoretical analysis of the organismal energy balance, we further show that the mass allometry is caused by body size dependent energy storage. Our results reveal the physiological origins of Kleiber's law in planarians and have general implications for understanding a fundamental scaling law in biology.