Types or States? Cellular Dynamics and Regenerative Potential

Acoel single-cell atlas reveals expression dynamics and heterogeneity of adult pluripotent stem cells

Adult pluripotent stem cell (aPSC) populations underlie whole-body regeneration in many distantly-related animal lineages, but how the underlying cellular and molecular mechanisms compare across species is unknown. Here, we apply single-cell RNA sequencing to profile transcriptional cell states of the acoel worm Hofstenia miamia during postembryonic development and regeneration. We identify cell types shared across stages and their associated gene expression dynamics during regeneration. Functional studies confirm that the aPSCs, also known as neoblasts, are the source of differentiated cells and reveal transcription factors needed for differentiation. Subclustering of neoblasts recovers transcriptionally distinct subpopulations, the majority of which are likely specialized to differentiated lineages. One neoblast subset, showing enriched expression of the histone variant H3.3, appears to lack specialization. Altogether, the cell states identified in this study facilitate comparisons to other species and enable future studies of stem cell fate potentials.

Planarians to schistosomes: an overview of flatworm cell-types and regulators

Schistosomiasis remains a major neglected tropical disease that afflicts over 200 million people globally. Schistosomes, the aetiological agent of schistosomiasis, are parasitic flatworms that propagate between molluscan and mammalian hosts. Inside the mammalian host, schistosomes rapidly grow over 100-fold in size and develop into a sexually mature male or female that thrives in the bloodstream for several decades. Recent work has identified schistosome stem cells as the source that drives parasite transmission, reproduction and longevity. Moreover, studies have begun to uncover molecular programmes deployed by stem cells that are essential for tissue development and maintenance, parasite survival and immune evasion. Such programmes are reminiscent of neoblast-driven development and regeneration of planarians, the free-living flatworm relative of schistosomes. Over the last few decades, research in planarians has employed modern functional genomic tools that significantly enhanced our understanding of stem cell-driven animal development and regeneration. In this review, we take a broad stroke overview of major flatworm organ systems at the cellular and molecular levels. We summarize recent advances on genetic regulators that play critical roles in differentiation and maintenance of flatworm cell types. Finally, we provide perspectives on how investigation of basic parasite biology is critical to discovering new approaches to battle schistosomiasis.

Wnt signaling in whole-body regeneration

Regeneration abilities are widespread among animals and select species can restore any body parts removed by wounds that sever the major body axes. This capability of whole-body regeneration as exemplified in flatworm planarians, Acoels, and Cnidarians involves initial responses to injury, the assessment of wound site polarization, determination of missing tissue and programming of blastema fate, and patterned outgrowth to restore axis content and proportionality. Wnt signaling drives many shared and conserved aspects of the biology of whole-body regeneration in the planarian species Schmidtea mediterranea and Dugesia japonica, in the Acoel Hofstenia miamia, and in Cnidarians Hydra and Nematostella. These overlapping mechanisms suggest whole-body regeneration might be an ancestral property across diverse animal taxa.

Identification of putative enhancer-like elements predicts regulatory networks active in planarian adult stem cells

Planarians have become an established model system to study regeneration and stem cells, but the regulatory elements in the genome remain almost entirely undescribed. Here, by integrating epigenetic and expression data we use multiple sources of evidence to predict enhancer elements active in the adult stem cell populations that drive regeneration. We have used ChIP-seq data to identify genomic regions with histone modifications consistent with enhancer activity, and ATAC-seq data to identify accessible chromatin. Overlapping these signals allowed for the identification of a set of high-confidence candidate enhancers predicted to be active in planarian adult stem cells. These enhancers are enriched for predicted transcription factor (TF) binding sites for TFs and TF families expressed in planarian adult stem cells. Footprinting analyses provided further evidence that these potential TF binding sites are likely to be occupied in adult stem cells. We integrated these analyses to build testable hypotheses for the regulatory function of TFs in stem cells, both with respect to how pluripotency might be regulated, and to how lineage differentiation programs are controlled. We found that our predicted GRNs were independently supported by existing TF RNAi/RNA-seq datasets, providing further evidence that our work predicts active enhancers that regulate adult stem cells and regenerative mechanisms.

m6A is required for resolving progenitor identity during planarian stem cell differentiation

Regeneration and tissue homeostasis require accurate production of missing cell lineages. Cell production is driven by changes to gene expression, which is shaped by multiple layers of regulation. Here, we find that the ubiquitous mRNA base-modification, m6A, is required for proper cell fate choice and cellular maturation in planarian stem cells (neoblasts). We mapped m6A-enriched regions in 7,600 planarian genes and found that perturbation of the m6A pathway resulted in progressive deterioration of tissues and death. Using single-cell RNA sequencing of >20,000 cells following perturbation of the m6A pathway, we identified an increase in expression of noncanonical histone variants, and that inhibition of the pathway resulted in accumulation of undifferentiated cells throughout the animal in an abnormal transcriptional state. Analysis of >1,000 planarian gene expression datasets revealed that the inhibition of the chromatin modifying complex NuRD had almost indistinguishable consequences, unraveling an unappreciated link between m6A and chromatin modifications. Our findings reveal that m6A is critical for planarian stem cell homeostasis and gene regulation in tissue maintenance and regeneration.

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.

Monitoring Chromatin Regulation in Planarians Using Chromatin Immunoprecipitation Followed by Sequencing (ChIP-seq)

Planarians are an accessible model system to study animal regeneration and stem cells. Over the last two decades, new molecular techniques have provided us with powerful tools to understand whole-body regeneration and pluripotent adult stem cells specifically. We describe a method for performing Chromatin Immunoprecipitation followed by sequencing (ChIP-seq) on planarian cells that relies on FACS to isolate different cell populations followed by immunoprecipitation and library preparation for next-generation sequencing. Whole-genome profiling of histone modifications enables a greater understanding of epigenetic mechanisms in development, pluripotency, and differentiation. This protocol adds to the growing list of functional genomic approaches to study whole-body regeneration in animals.

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.

Planarian stem cells specify fate yet retain potency during the cell cycle

Planarian whole-body regeneration is enabled by stem cells called neoblasts. At least some neoblasts are individually pluripotent. Neoblasts are also heterogeneous, with subpopulations of specialized neoblasts having different specified fates. Fate specification in neoblasts is regulated by fate-specific transcription factor (FSTF) expression. Here, we find that FSTF expression is common in neoblast S/G2/M cell-cycle phases but less common in G1. We find that specialized neoblasts can divide to produce progeny with asymmetric cell fates, suggesting that they could retain pluripotency. Furthermore, no known neoblast class was present in all neoblast colonies, suggesting that pluripotency is not the exclusive property of any known class. We tested this possibility with single-cell transplantations, which indicate that at least some specialized neoblasts are likely clonogenic. On the basis of these findings, we propose a model for neoblast pluripotency in which neoblasts can undergo specialization during the cell cycle without loss of potency.

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.

The stem cell marker Prom1 promotes axon regeneration by down-regulating cholesterol synthesis via Smad signaling

Axon regeneration is regulated by a neuron-intrinsic transcriptional program that is suppressed during development but that can be reactivated following peripheral nerve injury. Here we identify Prom1, which encodes the stem cell marker prominin-1, as a regulator of the axon regeneration program. Prom1 expression is developmentally down-regulated, and the genetic deletion of Prom1 in mice inhibits axon regeneration in dorsal root ganglion (DRG) cultures and in the sciatic nerve, revealing the neuronal role of Prom1 in injury-induced regeneration. Elevating prominin-1 levels in cultured DRG neurons or in mice via adeno-associated virus-mediated gene delivery enhances axon regeneration in vitro and in vivo, allowing outgrowth on an inhibitory substrate. Prom1 overexpression induces the consistent down-regulation of cholesterol metabolism-associated genes and a reduction in cellular cholesterol levels in a Smad pathway-dependent manner, which promotes axonal regrowth. We find that prominin-1 interacts with the type I TGF-β receptor ALK4, and that they synergistically induce phosphorylation of Smad2. These results suggest that Prom1 and cholesterol metabolism pathways are possible therapeutic targets for the promotion of neural recovery after injury.

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.

The Cellular and Molecular Basis for Planarian Regeneration

Regeneration is one of the great mysteries of biology. Planarians are flatworms capable of dramatic feats of regeneration, which have been studied for over 2 centuries. Recent findings identify key cellular and molecular principles underlying these feats. A stem cell population (neoblasts) generates new cells and is comprised of pluripotent stem cells (cNeoblasts) and fate-specified cells (specialized neoblasts). Positional information is constitutively active and harbored primarily in muscle, where it acts to guide stem cell-mediated tissue turnover and regeneration. I describe here a model in which positional information and stem cells combine to enable regeneration.

A planarian RPS3 homolog is critical to the modulation of planarian regeneration

Planarians are an important model for stem cell and regeneration biology, because they can regenerate any missing body structure in a short time. As an important component of ribosomes, ribosomal proteins can synthesize proteins and play a central role in cell cycle checkpoint, cell survival/senescence/apoptosis, and organismal growth and development. In this study, we identified and amplified the homologous gene of RPS3 in Dugesia japonica. Double-stranded RNA mediated RNAi revealed that when the Dj-RPS3 function was lost by planarians; they did not form blastemas and died 100%. Further investigation, confirmed that Dj-RPS3 was involved in regulating the proliferative and early differentiation of neoblasts.

JAK/STAT signaling in stem cells and regeneration: from Drosophila to vertebrates

The JAK/STAT pathway is a conserved metazoan signaling system that transduces cues from extracellular cytokines into transcriptional changes in the nucleus. JAK/STAT signaling is best known for its roles in immunity. However, recent work has demonstrated that it also regulates critical homeostatic processes in germline and somatic stem cells, as well as regenerative processes in several tissues, including the gonad, intestine and appendages. Here, we provide an overview of JAK/STAT signaling in stem cells and regeneration, focusing on Drosophila and highlighting JAK/STAT pathway functions in proliferation, survival and cell competition that are conserved between Drosophila and vertebrates.

SoxB1 Activity Regulates Sensory Neuron Regeneration, Maintenance, and Function in Planarians

SoxB1 genes play fundamental roles in neurodevelopmental processes and maintaining stem cell multipotency, but little is known about their function in regeneration. We addressed this question by analyzing the activity of the SoxB1 homolog soxB1-2 in the planarian Schmidtea mediterranea. Expression and functional analysis revealed that soxB1-2 marks ectodermal-lineage progenitors, and its activity is required for differentiation of subsets of ciliated epidermal and neuronal cells. Moreover, we show that inhibiting soxB1-2 or its candidate target genes leads to abnormal sensory neuron regeneration that causes planarians to display seizure-like movements or phenotypes associated with the loss of sensory modalities. Our analyses highlight soxB1-2-regulated genes that are expressed in sensory neurons and are homologous to factors implicated in epileptic disorders in humans and animal models of epilepsy, indicating that planarians can serve as a complementary model to investigate genetic causes of epilepsy.

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

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

Physiological evaluation of the behavior and epidermis of freshwater planarians (Girardia tigrina and Girardia sp.) exposed to stressors

Planarians are metazoan freshwater flatworms which are free-living organisms. Their body has pluripotent stem cell promoters of tissue regeneration capacity. The water temperature and the potential of hydrogen (pH) of lentic ecosystems are important factors involved in the distribution and abundance of these animals. Although the pH factor is directly related to the physiology and behavior of planarians, their adaptive and regenerating capacities still remain unknown. The Critical Thermal Maximum (CTM) is a very widespread method used in the evaluation of thermal tolerance. In this study, Girardia tigrina (Girard, 1850) and Girardia sp., a species found in Brazil, which is under study as a new species, had their epidermis assessed by scanning electron microscopy (SEM) to analyze their physiological structures before and after exposure to different stressors. SEM was used as a method to evaluate the planarians' epidermis as a result of the increasing temperature (CTM) and pH alterations, the latter with the use of a new methodology defined as Critical Hydrogen ion concentration Maximum (CHM). In increasing temperatures from 20°C to 37°C, both Girardia tigrina and Girardia sp. proved to be adaptable to thermal stress. Girardia sp. was shown to be more resistant to higher temperatures. However, Girardia tigrina was more resistant to extreme pH conditions (4.0 to 10.0). SEM analysis showed morphological differences among planarian species, such as the arrangement of the structures and cell types of the dorsal epidermis. Moreover, planarians demonstrated the ability to change the surrounding pH of their external environment in order to maintain the function of their physiological mechanisms, suggesting that these animals have a complex survival system, possibly related to protonephridia, flame cells and excretory pores.This article has an associated First Person interview with the first author of the paper.

Planarian flatworms as a new model system for understanding the epigenetic regulation of stem cell pluripotency and differentiation

Planarian flatworms possess pluripotent stem cells (neoblasts) that are able to differentiate into all cell types that constitute the adult body plan. Consequently, planarians possess remarkable regenerative capabilities. Transcriptomic studies have revealed that gene expression is coordinated to maintain neoblast pluripotency, and ensure correct lineage specification during differentiation. But as yet they have not revealed how this regulation of expression is controlled. In this review, we propose that planarians represent a unique and effective system to study the epigenetic regulation of these processes in an in vivo context. We consolidate evidence suggesting that although DNA methylation is likely present in some flatworm lineages, it does not regulate neoblast function in Schmidtea mediterranea. A number of phenotypic studies have documented the role of histone modification and chromatin remodelling complexes in regulating distinct neoblast processes, and we focus on four important examples of planarian epigenetic regulators: Nucleosome Remodeling Deacetylase (NuRD) complex, Polycomb Repressive Complex (PRC), the SET1/MLL methyltransferases, and the nuclear PIWI/piRNA complex. Given the recent advent of ChIP-seq in planarians, we propose future avenues of research that will identify the genomic targets of these complexes allowing for a clearer picture of how neoblast processes are coordinated at the epigenetic level. These insights into neoblast biology may be directly relevant to mammalian stem cells and disease. The unique biology of planarians will also allow us to investigate how extracellular signals feed into epigenetic regulatory networks to govern concerted neoblast responses during regenerative polarity, tissue patterning, and remodelling.

A Conceptual Framework for Cell Identity Transitions in Plants

Multicellular organisms develop from a single cell that proliferates to form different cell types with specialized functions. Sixty years ago, Waddington suggested the 'epigenetic landscape' as a useful metaphor for the process. According to this view, cells move through a rugged identity space along genetically encoded trajectories, until arriving at one of the possible final fates. In plants in particular, these trajectories have strong spatial correlates, as cell identity is intimately linked to its relative position within the plant. During regeneration, however, positional signals are severely disrupted and differentiated cells are able to undergo rapid non-canonical identity changes. Moreover, while pluripotent properties have long been ascribed to plant cells, the introduction of induced pluripotent stem cells in animal studies suggests such plasticity may not be unique to plants. As a result, current concepts of differentiation as a gradual and hierarchical process are being reformulated across biological fields. Traditional studies of plant regeneration have placed strong emphasis on the emergence of patterns and tissue organization, and information regarding the events occurring at the level of individual cells is only now beginning to emerge. Here, I review the historical and current concepts of cell identity and identity transitions, and discuss how new views and tools may instruct the future understanding of differentiation and plant regeneration.

Planarians and the History of Animal Regeneration: Paradigm Shifts and Key Concepts in Biology

Regeneration has captured human imagination for much of recorded history. Its sociological influence is evident in ancient and modern folklore, art, politics, and even language. In many ways, the study of regeneration helped establish the field of biology as a legitimate scientific discipline. Furthermore, regeneration research yielded critical insights that challenged flawed scientific models and uncovered fundamental principles underpinning the workings of life on this planet. This chapter details some ways in which the study of animal regeneration-with special emphasis on planarian regeneration-influenced the evolution of thought in biology. This includes contributions to the discovery of stem cells, the nature of heredity, and key concepts in pattern formation.

[The identity of normal and cancer stem cells]

What is a stem cell? Is stemness an intrinsic or extrinsic property? What role does the microenvironment play in the stemness identity? We distinguish four identities for normal and cancerous stem cells and explore their consequences for therapeutic strategy choice in the oncology setting. Acquisition of genetic and epigenetic alterations during cell transformation and disease progression questions the stability of the stemness property's identity in cancers.

Organ specific gene expression in the regenerating tail of Macrostomum lignano

Temporal and spatial characterization of gene expression is a prerequisite for the understanding of cell-, tissue-, and organ-differentiation. In a multifaceted approach to investigate gene expression in the tail plate of the free-living marine flatworm Macrostomum lignano, we performed a posterior-region-specific in situ hybridization screen, RNA sequencing (RNA-seq) of regenerating animals, and functional analyses of selected tail-specific genes. The in situ screen revealed transcripts expressed in the antrum, cement glands, adhesive organs, prostate glands, rhabdite glands, and other tissues. Next we used RNA-seq to characterize temporal expression in the regenerating tail plate revealing a time restricted onset of both adhesive organs and copulatory apparatus regeneration. In addition, we identified three novel previously unannotated genes solely expressed in the regenerating stylet. RNA interference showed that these genes are required for the formation of not only the stylet but the whole male copulatory apparatus. RNAi treated animals lacked the stylet, vesicula granulorum, seminal vesicle, false seminal vesicle, and prostate glands, while the other tissues of the tail plate, such as adhesive organs regenerated normally. In summary, our findings provide a large resource of expression data during homeostasis and regeneration of the morphologically complex tail regeneration and pave the way for a better understanding of organogenesis in M. lignano.

Planarian Epidermal Stem Cells Respond to Positional Cues to Promote Cell-Type Diversity

Successful regeneration requires that progenitors of different lineages form the appropriate missing cell types. However, simply generating lineages is not enough. Cells produced by a particular lineage often have distinct functions depending on their position within the organism. How this occurs in regeneration is largely unexplored. In planarian regeneration, new cells arise from a proliferative cell population (neoblasts). We used the planarian epidermal lineage to study how the location of adult progenitor cells results in their acquisition of distinct functional identities. Single-cell RNA sequencing of epidermal progenitors revealed the emergence of distinct spatial identities as early in the lineage as the epidermal neoblasts, with further pre-patterning occurring in their post-mitotic migratory progeny. Establishment of dorsal-ventral epidermal identities and functions, in response to BMP signaling, required neoblasts. Our work identified positional signals that activate regionalized transcriptional programs in the stem cell population and subsequently promote cell-type diversity in the epidermis.

Eye Absence Does Not Regulate Planarian Stem Cells during Eye Regeneration

Dividing cells called neoblasts contain pluripotent stem cells and drive planarian flatworm regeneration from diverse injuries. A long-standing question is whether neoblasts directly sense and respond to the identity of missing tissues during regeneration. We used the eye to investigate this question. Surprisingly, eye removal was neither sufficient nor necessary for neoblasts to increase eye progenitor production. Neoblasts normally increase eye progenitor production following decapitation, facilitating regeneration. Eye removal alone, however, did not induce this response. Eye regeneration following eye-specific resection resulted from homeostatic rates of eye progenitor production and less cell death in the regenerating eye. Conversely, large head injuries that left eyes intact increased eye progenitor production. Large injuries also non-specifically increased progenitor production for multiple uninjured tissues. We propose a model for eye regeneration in which eye tissue production by planarian stem cells is not directly regulated by the absence of the eye itself.

The NuRD complex component p66 suppresses photoreceptor neuron regeneration in planarians

Regeneration involves precise control of cell fate to produce an appropriate complement of tissues formed within a blastema. Several chromatin-modifying complexes have been identified as required for regeneration in planarians, but it is unclear whether this class of molecules uniformly promotes the production of differentiated cells. We identify a function for p66, encoding a DNA-binding protein component of the NuRD (nucleosome remodeling and deacetylase) complex, as well as the chromodomain helicase chd4, in suppressing production of photoreceptor neurons (PRNs) in planarians. This suppressive effect appeared restricted to PRNs because p66 inhibition did not influence numbers of eye pigment cup cells (PCCs) and decreased numbers of brain neurons and epidermal progenitors. PRNs from p66(RNAi) animals differentiated with some abnormalities but nonetheless produced arrestin+ projections to the brain. p66 inhibition produced excess ovo+otxA+ PRN progenitors without affecting numbers of ovo+otxA- PCC progenitors, and ovo and otxA were each required for the p66(RNAi) excess PRN phenotype. Together these results suggest that p66 acts through the NuRD complex to suppress PRN production by limiting expression of lineage-specific transcription factors.

Light-induced depigmentation in planarians models the pathophysiology of acute porphyrias

Porphyrias are disorders of heme metabolism frequently characterized by extreme photosensitivity. This symptom results from accumulation of porphyrins, tetrapyrrole intermediates in heme biosynthesis that generate reactive oxygen species when exposed to light, in the skin of affected individuals. Here we report that in addition to producing an ommochrome body pigment, the planarian flatworm Schmidtea mediterranea generates porphyrins in its subepithelial pigment cells under physiological conditions, and that this leads to pigment cell loss when animals are exposed to intense visible light. Remarkably, porphyrin biosynthesis and light-induced depigmentation are enhanced by starvation, recapitulating a common feature of some porphyrias – decreased nutrient intake precipitates an acute manifestation of the disease. Our results establish planarians as an experimentally tractable animal model for research into the pathophysiology of acute porphyrias, and potentially for the identification of novel pharmacological interventions capable of alleviating porphyrin-mediated photosensitivity or decoupling dieting and fasting from disease pathogenesis.

DOI:http://dx.doi.org/10.7554/eLife.14175.001

Porphyrias are rare diseases that involve ring-shaped molecules called porphyrins accumulating in various parts of the body. Porphyrins are produced as part of the normal process that makes an important molecule called heme, which is required to transport oxygen. However, high levels of porphyrins can be toxic. For example, porphyrins deposited in the skin can cause swelling and blistering when the skin is exposed to bright light. Other disease symptoms include neurological issues ranging from anxiety and confusion to seizures or paralysis. It has been speculated that porphyrias may have affected several historical figures, including the artist Vincent van Gogh.

In addition to their role in heme production, porphyrins also have other roles. For example, they are used as pigments in the wing feathers of some owls. Researchers are trying to understand more about how organisms regulate porphyrin production so that it might be possible to develop more effective treatments for porphyria in humans.

Here, Stubenhaus et al. studied how a flatworm called Schmidtea mediterranea makes porphyrins. A group of undergraduate students noticed that these animals – which are normally brown in color – turned white when they were exposed to sunlight for several days. Stubenhaus et al. found that S. mediterranea makes porphyrins in the pigment cells of its skin using the same genes that make porphyrins in humans. Together with other molecules called ommochromes, the porphyrins give rise to the normal color of this flatworm. However, when the animals are exposed to intense light for extended periods of time, which is unlikely to occur in the wild, porphyrin production leads to loss of the pigment cells.

The experiments also show that starvation increases the rate of pigment cell loss in light-exposed flatworms, which mirrors the worsening of disease symptoms some porphyria patients experience when they diet or fast. Stubenhaus et al. propose that flatworms are useful models in which to study the molecular processes that are responsible for porphyrias in humans. Further research is required to determine the exact chemical structure of the porphyrin and ommochrome molecules produced in different flatworm species. Stubenhaus et al. also plan to use flatworms to screen for drugs that could potentially be developed into new treatments for porphyria.

DOI:http://dx.doi.org/10.7554/eLife.14175.002

The EGFR signaling pathway controls gut progenitor differentiation during planarian regeneration and homeostasis

The planarian Schmidtea mediterranea maintains and regenerates all its adult tissues through the proliferation and differentiation of a single population of pluripotent adult stem cells (ASCs) called neoblasts. Despite recent advances, the mechanisms regulating ASC differentiation into mature cell types are poorly understood. Here, we show that silencing of the planarian EGF receptor egfr-1 by RNA interference (RNAi) impairs gut progenitor differentiation into mature cells, compromising gut regeneration and maintenance. We identify a new putative EGF ligand, nrg-1, the silencing of which phenocopies the defects observed in egfr-1(RNAi) animals. These findings indicate that egfr-1 and nrg-1 promote gut progenitor differentiation, and are thus essential for normal cell turnover and regeneration in the planarian gut. Our study demonstrates that the EGFR signaling pathway is an important regulator of ASC differentiation in planarians.

Wnt, Ptk7, and FGFRL expression gradients control trunk positional identity in planarian regeneration

Mechanisms enabling positional identity re-establishment are likely critical for tissue regeneration. Planarians use Wnt/beta-catenin signaling to polarize the termini of their anteroposterior axis, but little is known about how regeneration signaling restores regionalization along body or organ axes. We identify three genes expressed constitutively in overlapping body-wide transcriptional gradients that control trunk-tail positional identity in regeneration. ptk7 encodes a trunk-expressed kinase-dead Wnt co-receptor, wntP-2 encodes a posterior-expressed Wnt ligand, and ndl-3 encodes an anterior-expressed homolog of conserved FGFRL/nou-darake decoy receptors. ptk7 and wntP-2 maintain and allow appropriate regeneration of trunk tissue position independently of canonical Wnt signaling and with suppression of ndl-3 expression in the posterior. These results suggest that restoration of regional identity in regeneration involves the interpretation and re-establishment of axis-wide transcriptional gradients of signaling molecules.

Feedback control in planarian stem cell systems

Background

In planarian flatworms, the mechanisms underlying the activity of collectively pluripotent adult stem cells (neoblasts) and their descendants can now be studied from the level of the individual gene to the entire animal. Flatworms maintain startling developmental plasticity and regenerative capacity in response to variable nutrient conditions or injury. We develop a model for cell dynamics in such animals, assuming that fully differentiated cells exert feedback control on neoblast activity.

Results

Our model predicts a number of whole organism level and general cell biological and behaviours, some of which have been empirically observed or inferred in planarians and others that have not. As previously observed empirically we find: 1) a curvilinear relationship between external food and planarian steady state size; 2) the fraction of neoblasts in the steady state is constant regardless of planarian size; 3) a burst of controlled apoptosis during regeneration after amputation as the number of differentiated cells are adjusted towards their homeostatic/steady state level. In addition our model describes the following properties that can inform and be tested by future experiments: 4) the strength of feedback control from differentiated cells to neoblasts (i.e. the activity of the signalling system) and from neoblasts on themselves in relation to absolute number depends upon the level of food in the environment; 5) planarians adjust size when food level reduces initially through increased apoptosis and then through a reduction in neoblast self-renewal activity; 6) following wounding or excision of differentiated cells, different time scales characterize both recovery of size and the two feedback functions; 7) the temporal pattern of feedback controls differs noticeably during recovery from a removal or neoblasts or a removal of differentiated cells; 8) the signaling strength for apoptosis of differentiated cells depends upon both the absolute and relative deviations of the number of differentiated cells from their homeostatic level; and 9) planaria prioritize resource use for cell divisions.

Conclusions

We offer the first analytical framework for organizing experiments on planarian flatworm stem cell dynamics in a form that allows models to be compared with quantitative cell data based on underlying molecular mechanisms and thus facilitate the interplay between empirical studies and modeling. This framework is the foundation for studying cell migration during wound repair, the determination of homeostatic levels of differentiated cells by natural selection, and stochastic effects.

Electronic supplementary material

The online version of this article (doi:10.1186/s12918-016-0261-8) contains supplementary material, which is available to authorized users.

Planarian Body-Wall Muscle: Regeneration and Function beyond a Simple Skeletal Support

The body-wall musculature of adult planarians consists of intricately organized muscle fibers, which after amputation are regenerated rapidly and with great precision through the proliferation and differentiation of pluripotent stem cells. These traits make the planarian body-wall musculature a potentially useful model for the study of cell proliferation, differentiation, and pattern formation. Planarian body-wall muscle shows some ambiguous features common to both skeletal and smooth muscle cells. However, its skeletal nature is implied by the expression of skeletal myosin heavy-chain genes and the myogenic transcription factor myoD. Where and when planarian stem cells become committed to the myogenic lineage during regeneration, how the new muscle cells are integrated into the pre-existing muscle net, and the identity of the molecular pathway controlling the myogenic gene program are key aspects of planarian muscle regeneration that need to be addressed. Expression of the conserved transcription factor myoD has been recently demonstrated in putative myogenic progenitors. Moreover, recent studies suggest that differentiated muscle cells may provide positional information to planarian stem cells during regeneration. Here, I review the limited available knowledge on planarian muscle regeneration.