Planarians Maintain a Constant Ratio of Different Cell Types During Changes in Body Size by Using the Stem Cell System

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

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

Mechanistic regulation of planarian shape during growth and degrowth

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

LIM-HD transcription factors control axial patterning and specify distinct neuronal and intestinal cell identities in planarians

Adult planarians can regenerate the gut, eyes and even a functional brain. Proper identity and patterning of the newly formed structures require signals that guide and commit their adult stem cells. During embryogenesis, LIM-homeodomain (LIM-HD) transcription factors act in a combinatorial 'LIM code' to control cell fate determination and differentiation. However, our understanding about the role these genes play during regeneration and homeostasis is limited. Here, we report the full repertoire of LIM-HD genes in Schmidtea mediterranea. We found that lim homeobox (lhx) genes appear expressed in complementary patterns along the cephalic ganglia and digestive system of the planarian, with some of them being co-expressed in the same cell types. We have identified that Smed-islet1, -lhx1/5-1, -lhx2/9-3, -lhx6/8, -lmx1a/b-2 and -lmx1a/b-3 are essential to pattern and size the planarian brain as well as for correct regeneration of specific subpopulations of dopaminergic, serotonergic, GABAergic and cholinergic neurons, while Smed-lhx1/5.2 and -lhx2/9.2 are required for the proper expression of intestinal cell type markers, specifically the goblet subtype. LIM-HD are also involved in controlling axonal pathfinding (lhx6/8), axial patterning (islet1, lhx1/5-1, lmx1a/b-3), head/body proportions (islet2) and stem cell proliferation (lhx3/4, lhx2/9-3, lmx1a/b-2, lmx1a/b-3). Altogether, our results suggest that planarians might present a combinatorial LIM code that controls axial patterning and axonal growing and specifies distinct neuronal and intestinal cell identities.

Convergent evolution of the sensory pits in and within flatworms

BACKGROUND

Unlike most free-living platyhelminths, catenulids, the sister group to all remaining flatworms, do not have eyes. Instead, the most prominent sensory structures in their heads are statocysts or sensory pits. The latter, found in the family Stenostomidae, are concave depressions located laterally on the head that represent one of the taxonomically important traits of the family. In the past, the sensory pits of flatworms have been homologized with the cephalic organs of nemerteans, a clade that occupies a sister position to platyhelminths in some recent phylogenies. To test for this homology, we studied morphology and gene expression in the sensory pits of the catenulid Stenostomum brevipharyngium.

RESULTS

We used confocal and electron microscopy to investigate the detailed morphology of the sensory pits, as well as their formation during regeneration and asexual reproduction. The most prevalent cell type within the organ is epidermally-derived neuron-like cells that have cell bodies embedded deeply in the brain lobes and long neurite-like processes extending to the bottom of the pit. Those elongated processes are adorned with extensive microvillar projections that fill up the cavity of the pit, but cilia are not associated with the sensory pit. We also studied the expression patterns of some of the transcription factors expressed in the nemertean cephalic organs during the development of the pits. Only a single gene, pax4/6, is expressed in both the cerebral organs of nemerteans and sensory pits of S. brevipharyngium, challenging the idea of their deep homology.

CONCLUSIONS

Since there is no morphological or molecular correspondence between the sensory pits of Stenostomum and the cerebral organs of nemerteans, we reject their homology. Interestingly, the major cell type contributing to the sensory pits of stenostomids shows ultrastructural similarities to the rhabdomeric photoreceptors of other flatworms and expresses ortholog of the gene pax4/6, the pan-bilaterian master regulator of eye development. We suggest that the sensory pits of stenostomids might have evolved from the ancestral rhabdomeric photoreceptors that lost their photosensitivity and evolved secondary function. The mapping of head sensory structures on plathelminth phylogeny indicates that sensory pit-like organs evolved many times independently in flatworms.

A Comprehensive conceptual and computational dynamics framework for autonomous regeneration of form and function in biological organisms

In biology, regeneration is a mysterious phenomenon that has inspired self-repairing systems, robots, and biobots. It is a collective computational process whereby cells communicate to achieve an anatomical set point and restore original function in regenerated tissue or the whole organism. Despite decades of research, the mechanisms involved in this process are still poorly understood. Likewise, the current algorithms are insufficient to overcome this knowledge barrier and enable advances in regenerative medicine, synthetic biology, and living machines/biobots. We propose a comprehensive conceptual framework for the engine of regeneration with hypotheses for the mechanisms and algorithms of stem cell-mediated regeneration that enables a system like the planarian flatworm to fully restore anatomical (form) and bioelectric (function) homeostasis from any small- or large-scale damage. The framework extends the available regeneration knowledge with novel hypotheses to propose collective intelligent self-repair machines with multi-level feedback neural control systems driven by somatic and stem cells. We computationally implemented the framework to demonstrate the robust recovery of both form and function (anatomical and bioelectric homeostasis) in an in silico worm that, in a simple way, resembles the planarian. In the absence of complete regeneration knowledge, the framework contributes to understanding and generating hypotheses for stem cell mediated form and function regeneration, which may help advance regenerative medicine and synthetic biology. Further, as our framework is a bio-inspired and bio-computing self-repair machine, it may be useful for building self-repair robots/biobots and artificial self-repair systems.

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.

Defined diets for freshwater planarians

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

Quantification of planarian behaviors

Research on individual behaviors can help to reveal the processes and mechanisms that mediate an animal's habits and interactions with the environment. Importantly, individual behaviors arise as outcomes of genetic programs, morphogenesis, physiological processes, and neural functions; thus, behavioral analyses can be used to detect disorders in these processes. Planarians belong to an early branching bilateral group of organisms that possess a simple central nervous system. Furthermore, planarians display various behavioral responses to the environment via their nervous system. Planarians also have remarkable regenerative abilities, including whole-brain regeneration. Therefore, the combination of planarians' phylogenetic position, behavioral properties, regenerative ability, and genetic accessibility provides a unique opportunity to understand the basic mechanisms underlying the anatomical properties of neural morphogenesis and the dynamic physiological processes and neural function. Here, we describe a step-by-step protocol for conducting simple behavioral analyses in planarians with the aim of helping to introduce researchers to the utility of performing behavioral analyses in planarians. Since the conditions of planarians impact experimental results and reproducibility, this protocol begins with a method for maintaining planarians. Next, we introduce the behavioral tests as well as the methods for quantifying them using minimal and cost-effective equipment and materials. Finally, we present a unique RNAi technique that enables conditional silencing of neural activity in the brain of planarians.

Characterization of the dynamics and variability of neuronal subtype responses during growth, degrowth, and regeneration of Nematostella vectensis

BACKGROUND

The ability to regenerate body parts is a feature of metazoan organisms and the focus of intense research aiming to understand its basis. A number of mechanisms involved in regeneration, such as proliferation and tissue remodeling, affect whole tissues; however, little is known on how distinctively different constituent cell types respond to the dynamics of regenerating tissues. Preliminary studies suggest that a number of organisms alter neuronal numbers to scale with changes in body size. In some species with the ability of whole-body axis regeneration, it has additionally been observed that regenerates are smaller than their pre-amputated parent, but maintain the correct morphological proportionality, suggesting that scaling of tissue and neuronal numbers also occurs. However, the cell dynamics and responses of neuronal subtypes during nervous system regeneration, scaling, and whole-body axis regeneration are not well understood in any system. The cnidarian sea anemone Nematostella vectensis is capable of whole-body axis regeneration, with a number of observations suggesting the ability to alter its size in response to changes in feeding. We took advantage of Nematostella's transparent and "simple" body plan and the NvLWamide-like mCherry fluorescent reporter transgenic line to probe the response of neuron populations to variations in body size in vivo in adult animals during body scaling and regeneration.

RESULTS

We utilized the previously characterized NvLWamide-like::mCherry transgenic reporter line to determine the in vivo response of neuronal subtypes during growth, degrowth, and regeneration. Nematostella alters its size in response to caloric intake, and the nervous system responds by altering neuronal number to scale as the animal changes in size. Neuronal numbers in both the endodermal and ectodermal nerve nets decreased as animals shrunk, increased as they grew, and these changes were reversible. Whole-body axis regeneration resulted in regenerates that were smaller than their pre-amputated size, and the regenerated nerve nets were reduced in neuronal number. Different neuronal subtypes had distinct responses during regeneration, including consistent, not consistent, and conditional increases in number. Conditional responses were regulated, in part, by the size of the remnant fragment and the position of the amputation site. Regenerates and adults with reduced nerve nets displayed normal behaviors, indicating that the nerve net retains functionality as it scales.

CONCLUSION

These data suggest that the Nematostella nerve net is dynamic, capable of scaling with changes in body size, and that neuronal subtypes display differential regenerative responses, which we propose may be linked to the scale state of the regenerating animals.

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.

STRIPAK Limits Stem Cell Differentiation of a WNT Signaling Center to Control Planarian Axis Scaling

Regeneration involves regulating tissue proportionality across considerable size ranges through unknown mechanisms. In planarians, which scale reversibly over 40× through regeneration, we identify the Striatin-interacting phosphatase and kinase (STRIPAK) complex as a potent negative regulator of axis length. Inhibition of two proteins in the STRIPAK complex, mob4 and striatin, dramatically increased posterior length, through expansion of a posterior wnt1+ signaling center within midline muscle cells. wnt1 was required for tail expansion after mob4 inhibition and dynamically reestablishes proportionality after amputation in normal animals, indicating STRIPAK represses Wnt signaling for scaling. Regulation of wnt1 expansion was stem cell dependent, demonstrating that control of signaling-center production through stem cell differentiation underlies proportional growth in adult regenerative tissue.

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.

Role of neoblasts in the patterned postembryonic growth of the platyhelminth Macrostomum lignano

Neoblasts are motile pluripotent stem cells unique to the flatworm phyla Platyhelminthes and Acoela. The role of neoblasts in tissue regeneration has received much attention in recent studies. Here we review data pertinent to the structure and embryonic origin of these stem cells, and their participation in normal cell turnover. Next, we present data proving that neoblasts also account for the addition of cells during postembryonic growth. Bromodeoxyuridine (BrdU) pulse chase experiments demonstrate that the incorporation of neoblast-derived cells into the different tissues of the juvenile worm follows a stereotyped pattern, whereby cells within the parenchymal layer (muscle, gland) incorporate new cells most rapidly, followed by the epidermal domain surrounding the mouth, dorsal epidermis, and, lastly, the nervous system.

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.

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

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

Regenerative tissue remodeling in planarians - The mysteries of morphallaxis

Biologists have long marveled at the ability of planarian flatworms to regenerate any parts of their bodies in just a little over a week. While great progress has been made in deciphering the mechanisms by which new tissue is formed at sites of amputation, we know relatively little about the complementary remodeling response that occurs in uninjured tissues to restore anatomical scale and proportion. This review explores the mysterious biology of this process, first described in hydra by the father of experimental zoology, Abraham Trembley, and later termed 'morphallaxis' by the father of experimental genetics, Thomas Hunt Morgan. The perceptive work of these early pioneers, together with recent studies using modern tools, has revealed some of the key features of regenerative tissue remodeling, including repatterning of the body axes, reproportioning of organs like the brain and gut, and a major increase in the rate of cell death. Yet a mechanistic solution to this longstanding problem in the field will require further study by the next generation of planarian researchers.

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

The establishment of size and shape remains a fundamental challenge in biological research that planarian flatworms uniquely epitomize. Planarians can regenerate complete and perfectly proportioned animals from tiny and arbitrarily shaped tissue pieces; they continuously renew all organismal cell types from abundant pluripotent stem cells, yet maintain shape and anatomy in the face of constant turnover; they grow when feeding and literally degrow when starving, while scaling form and function over as much as a 40-fold range in body length or an 800-fold change in total cell numbers. This review provides a broad overview of the current understanding of the planarian stem cell system, the mechanisms that pattern the planarian body plan and how the interplay between patterning signals and cell fate choices orchestrates regeneration. What emerges is a conceptual framework for the maintenance and regeneration of the planarian body plan on basis of the interplay between pluripotent stem cells and self-organizing patterns and further, the general utility of planarians as model system for the mechanistic basis of size and shape.

Transcriptional signatures of somatic neoblasts and germline cells in Macrostomum lignano

The regeneration-capable flatworm Macrostomum lignano is a powerful model organism to study the biology of stem cells in vivo. As a flatworm amenable to transgenesis, it complements the historically used planarian flatworm models, such as Schmidtea mediterranea. However, information on the transcriptome and markers of stem cells in M. lignano is limited. We generated a de novo transcriptome assembly and performed the first comprehensive characterization of gene expression in the proliferating cells of M. lignano, represented by somatic stem cells, called neoblasts, and germline cells. Knockdown of a selected set of neoblast genes, including Mlig-ddx39, Mlig-rrm1, Mlig-rpa3, Mlig-cdk1, and Mlig-h2a, confirmed their crucial role for the functionality of somatic neoblasts during homeostasis and regeneration. The generated M. lignano transcriptome assembly and gene expression signatures of somatic neoblasts and germline cells will be a valuable resource for future molecular studies in M. lignano.

DOI:http://dx.doi.org/10.7554/eLife.20607.001

Muscle-derived matrix metalloproteinase regulates stem cell proliferation in planarians

BACKGROUND

Matrix metalloproteinases (MMPs) are a large family of regulatory enzymes that function in extracellular matrix degradation and facilitate a diverse range of cellular processes. Despite the significant focus on the activities of MMPs in human disease, there is a lack of substantial knowledge regarding their normal physiological roles and their role in regulating aspects of stem cell biology. The freshwater planarian Schmidtea mediterranea (S. mediterranea) is an excellent system in which to study robust and nearly unlimited regeneration, guided by a population of mitotically active stem cells, termed neoblasts.

RESULTS

We characterized MMPs in the context of planarian stem cells, specifically exploring the role of S. mediterranea MT-MMPB. Using in situ hybridization and available functional genomic tools, we observed that mt-mmpB is expressed in the dorsoventral muscle cells, and its loss results in a reduction in animal size accompanied by a decrease in mitotic cells, suggesting that it plays a unique role in regulating stem cell proliferation.

CONCLUSIONS

The novel findings of this study bring to light the unique and critical roles that muscles play in regulating neoblast function, and more broadly, highlight the importance of MMPs in stem cell biology. Developmental Dynamics 245:963-970, 2016. © 2016 Wiley Periodicals, Inc.

Go ahead, grow a head! A planarian's guide to anterior regeneration

The unique ability of some planarian species to regenerate a head de novo, including a functional brain, provides an experimentally accessible system in which to study the mechanisms underlying regeneration. Here, we summarize the current knowledge on the key steps of planarian head regeneration (head‐versus‐tail decision, anterior pole formation and head patterning) and their molecular and cellular basis. Moreover, instructive properties of the anterior pole as a putative organizer and in coordinating anterior midline formation are discussed. Finally, we highlight that regeneration initiation occurs in a two‐step manner and hypothesize that wound‐induced and existing positional cues interact to detect tissue loss and together determine the appropriate regenerative outcomes.

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.

Wnt/Notum spatial feedback inhibition controls neoblast differentiation to regulate reversible growth of the planarian brain

Mechanisms determining final organ size are poorly understood. Animals undergoing regeneration or ongoing adult growth are likely to require sustained and robust mechanisms to achieve and maintain appropriate sizes. Planarians, well known for their ability to undergo whole-body regeneration using pluripotent adult stem cells of the neoblast population, can reversibly scale body size over an order of magnitude by controlling cell number. Using quantitative analysis, we showed that after injury planarians perfectly restored brain:body proportion by increasing brain cell number through epimorphosis or decreasing brain cell number through tissue remodeling (morphallaxis), as appropriate. We identified a pathway controlling a brain size set-point that involves feedback inhibition between wnt11-6/wntA/wnt4a and notum, encoding conserved antagonistic signaling factors expressed at opposite brain poles. wnt11-6/wntA/wnt4a undergoes feedback inhibition through canonical Wnt signaling but is likely to regulate brain size in a non-canonical pathway independently of beta-catenin-1 and APC. Wnt/Notum signaling tunes numbers of differentiated brain cells in regenerative growth and tissue remodeling by influencing the abundance of brain progenitors descended from pluripotent stem cells, as opposed to regulating cell death. These results suggest that the attainment of final organ size might be accomplished by achieving a balance of positional signaling inputs that regulate the rates of tissue production.

Types or States? Cellular Dynamics and Regenerative Potential

Many of our organs can maintain and repair themselves during homeostasis and injury, as a result of the action of tissue-specific, multipotent stem cells. However, recent evidence from mammalian systems suggests that injury stimulates dramatic plasticity, or transient changes in cell potential, in both stem cells and more differentiated cells. Planarian flatworms possess abundant stem cells, making them an exceptional model for understanding the cellular behavior underlying homeostasis and regeneration. Recent discoveries of cell lineages and regeneration-specific events provide an initial framework for unraveling the complex cellular contributions to regeneration. In this review, we discuss the concept of cellular plasticity in the context of planarian regeneration, and consider the possibility that pluripotency may be a transient, probabilistic state exhibited by stem cells.

On the organ trail: insights into organ regeneration in the planarian

Advances in stem cell biology have led to the derivation of diverse cell types, yet challenges remain in creating complex tissues and functional organs. Unlike humans, some animals regenerate all missing tissues and organs successfully after dramatic injuries. Studies of organisms with exceptional regenerative capacity, like planarians, could complement in vitro studies and reveal mechanistic themes underlying regeneration on the scale of whole organs and tissues. In this review, we outline progress in understanding planarian organ regeneration, with focus on recent studies of the nervous, digestive, and excretory systems. We further examine molecular mechanisms underlying establishment of diverse cell fates from the planarian stem cell pool. Finally, we explore conceptual directions for future studies of organ regeneration in planarians.

Planarian shows decision-making behavior in response to multiple stimuli by integrative brain function

INTRODUCTION

Planarians belong to an evolutionarily early group of organisms that possess a central nervous system including a well-organized brain with a simple architecture but many types of neurons. Planarians display a number of behaviors, such as phototaxis and thermotaxis, in response to external stimuli, and it has been shown that various molecules and neural pathways in the brain are involved in controlling these behaviors. However, due to the lack of combinatorial assay methods, it remains obscure whether planarians possess higher brain functions, including integration in the brain, in which multiple signals coming from outside are coordinated and used in determining behavioral strategies.

RESULTS

In the present study, we designed chemotaxis and thigmotaxis/kinesis tracking assays to measure several planarian behaviors in addition to those measured by phototaxis and thermotaxis assays previously established by our group, and used these tests to analyze planarian chemotactic and thigmotactic/kinetic behaviors. We found that headless planarian body fragments and planarians that had specifically lost neural activity following regeneration-dependent conditional gene knockdown (Readyknock) of synaptotagmin in the brain lost both chemotactic and thigmotactic behaviors, suggesting that neural activity in the brain is required for the planarian's chemotactic and thigmotactic behaviors. Furthermore, we compared the strength of phototaxis, chemotaxis, thigmotaxis/kinesis, and thermotaxis by presenting simultaneous binary stimuli to planarians. We found that planarians showed a clear order of predominance of these behaviors. For example, when planarians were simultaneously exposed to 400 lux of light and a chemoattractant, they showed chemoattractive behavior irrespective of the direction of the light source, although exposure to light of this intensity alone induces evasive behavior away from the light source. In contrast, when the light intensity was increased to 800 or 1600 lux and the same dose of chemoattractant was presented, planarian behaviors were gradually shifted to negative phototaxis rather than chemoattraction. These results suggest that planarians may be capable of selecting behavioral strategies via the integration of discrete brain functions when exposed to multiple stimuli.

CONCLUSIONS

The planarian brain processes external signals received through the respective sensory neurons, thereby resulting in the production of appropriate behaviors. In addition, planarians can adjust behavioral features in response to stimulus conditions by integrating multiple external signals in the brain.

An automated training paradigm reveals long-term memory in planarians and its persistence through head regeneration

Planarian flatworms are a popular system for research into the molecular mechanisms that enable these complex organisms to regenerate their entire body, including the brain. Classical data suggest that they may also be capable of long-term memory. Thus, the planarian system may offer the unique opportunity to study brain regeneration and memory in the same animal. To establish a system for the investigation of the dynamics of memory in a regenerating brain, we developed a computerized training and testing paradigm that avoided the many issues that confounded previous, manual attempts to train planarians. We then used this new system to train flatworms in an environmental familiarization protocol. We show that worms exhibit environmental familiarization, and that this memory persists for at least 14 days - long enough for the brain to regenerate. We further show that trained, decapitated planarians exhibit evidence of memory retrieval in a savings paradigm after regenerating a new head. Our work establishes a foundation for objective, high-throughput assays in this molecularly tractable model system that will shed light on the fundamental interface between body patterning and stored memories. We propose planarians as key emerging model species for mechanistic investigations of the encoding of specific memories in biological tissues. Moreover, this system is lik ely to have important implications for the biomedicine of stem-cell-derived treatments of degenerative brain disorders in human adults.

Stem cell systems and regeneration in planaria

Planarians are members of the Platyhelminthes (flatworms). These animals have evolved a remarkable stem cell system. A single pluripotent adult stem cell type ("neoblast") gives rise to the entire range of cell types and organs in the planarian body plan, including a brain, digestive-, excretory-, sensory- and reproductive systems. Neoblasts are abundantly present throughout the mesenchyme and divide continuously. The resulting stream of progenitors and turnover of differentiated cells drive the rapid self-renewal of the entire animal within a matter of weeks. Planarians grow and literally de-grow ("shrink") by the food supply-dependent adjustment of organismal turnover rates, scaling body plan proportions over as much as a 50-fold size range. Their dynamic body architecture further allows astonishing regenerative abilities, including the regeneration of complete and perfectly proportioned animals even from tiny tissue remnants. Planarians as an experimental system, therefore, provide unique opportunities for addressing a spectrum of current problems in stem cell research, including the evolutionary conservation of pluripotency, the dynamic organization of differentiation lineages and the mechanisms underlying organismal stem cell homeostasis. The first part of this review focuses on the molecular biology of neoblasts as pluripotent stem cells. The second part examines the fascinating mechanistic and conceptual challenges posed by a stem cell system that epitomizes a universal design principle of biological systems: the dynamic steady state.

Modeling planarian regeneration: a primer for reverse-engineering the worm

A mechanistic understanding of robust self-assembly and repair capabilities of complex systems would have enormous implications for basic evolutionary developmental biology as well as for transformative applications in regenerative biomedicine and the engineering of highly fault-tolerant cybernetic systems. Molecular biologists are working to identify the pathways underlying the remarkable regenerative abilities of model species that perfectly regenerate limbs, brains, and other complex body parts. However, a profound disconnect remains between the deluge of high-resolution genetic and protein data on pathways required for regeneration, and the desired spatial, algorithmic models that show how self-monitoring and growth control arise from the synthesis of cellular activities. This barrier to progress in the understanding of morphogenetic controls may be breached by powerful techniques from the computational sciences-using non-traditional modeling approaches to reverse-engineer systems such as planaria: flatworms with a complex bodyplan and nervous system that are able to regenerate any body part after traumatic injury. Currently, the involvement of experts from outside of molecular genetics is hampered by the specialist literature of molecular developmental biology: impactful collaborations across such different fields require that review literature be available that presents the key functional capabilities of important biological model systems while abstracting away from the often irrelevant and confusing details of specific genes and proteins. To facilitate modeling efforts by computer scientists, physicists, engineers, and mathematicians, we present a different kind of review of planarian regeneration. Focusing on the main patterning properties of this system, we review what is known about the signal exchanges that occur during regenerative repair in planaria and the cellular mechanisms that are thought to underlie them. By establishing an engineering-like style for reviews of the molecular developmental biology of biomedically important model systems, significant fresh insights and quantitative computational models will be developed by new collaborations between biology and the information sciences.

TORC1 is required to balance cell proliferation and cell death in planarians

Multicellular organisms are equipped with cellular mechanisms that enable them to replace differentiated cells lost to normal physiological turnover, injury, and for some such as planarians, even amputation. This process of tissue homeostasis is generally mediated by adult stem cells (ASCs), tissue-specific stem cells responsible for maintaining anatomical form and function. To do so, ASCs must modulate the balance between cell proliferation, i.e. in response to nutrients, and that of cell death, i.e. in response to starvation or injury. But how these two antagonistic processes are coordinated remains unclear. Here, we explore the role of the core components of the TOR pathway during planarian tissue homeostasis and regeneration and identified an essential function for TORC1 in these two processes. RNAi-mediated silencing of TOR in intact animals resulted in a significant increase in cell death, whereas stem cell proliferation and stem cell maintenance were unaffected. Amputated animals failed to increase stem cell proliferation after wounding and displayed defects in tissue remodeling. Together, our findings suggest two distinct roles for TORC1 in planarians. TORC1 is required to modulate the balance between cell proliferation and cell death during normal cell turnover and in response to nutrients. In addition, it is required to initiate appropriate stem cell proliferation during regeneration and for proper tissue remodeling to occur to maintain scale and proportion.

Chance and necessity in eye evolution

Charles Darwin has proposed the theory that evolution of live organisms is based on random variation and natural selection. Jacques Monod in his classic book Chance and Necessity, published 40 years ago, presented his thesis “that the biosphere does not contain a predictable class of objects or events, but constitutes a particular occurrence, compatible indeed with the first principles, but not deducible from those principals and therefore, essentially unpredictable.” Recent discoveries in eye evolution are in agreement with both of these theses. They confirm Darwin's assumption of a simple eye prototype and lend strong support for the notion of a monophyletic origin of the various eye types. Considering the complexity of the underlying gene regulatory networks the unpredictability is obvious. The evolution of the Hox gene cluster and the specification of the body plan starting from an evolutionary prototype segment is discussed. In the course of evolution, a series of similar prototypic segments gradually undergoes cephalization anteriorly and caudalization posteriorly through diversification of the Hox genes.

ERK signaling controls blastema cell differentiation during planarian regeneration

The robust regenerative ability of planarians depends on a population of somatic stem cells called neoblasts, which are the only mitotic cells in adults and are responsible for blastema formation after amputation. The molecular mechanism underlying neoblast differentiation associated with blastema formation remains unknown. Here, using the planarian Dugesia japonica we found that DjmkpA, a planarian mitogen-activated protein kinase (MAPK) phosphatase-related gene, was specifically expressed in blastema cells in response to increased extracellular signal-related kinase (ERK) activity. Pharmacological and genetic [RNA interference (RNAi)] approaches provided evidence that ERK activity was required for blastema cells to exit the proliferative state and undergo differentiation. By contrast, DjmkpA RNAi induced an increased level of ERK activity and rescued the differentiation defect of blastema cells caused by pharmacological reduction of ERK activity. These observations suggest that ERK signaling plays an instructive role in the cell fate decisions of blastema cells regarding whether to differentiate or not, by inducing DjmkpA as a negative regulator of ERK signaling during planarian regeneration.

Stem cell-based growth, regeneration, and remodeling of the planarian intestine

Although some animals are capable of regenerating organs, the mechanisms by which this is achieved are poorly understood. In planarians, pluripotent somatic stem cells called neoblasts supply new cells for growth, replenish tissues in response to cellular turnover, and regenerate tissues after injury. For most tissues and organs, however, the spatiotemporal dynamics of stem cell differentiation and the fate of tissue that existed prior to injury have not been characterized systematically. Utilizing in vivo imaging and bromodeoxyuridine pulse-chase experiments, we have analyzed growth and regeneration of the planarian intestine, the organ responsible for digestion and nutrient distribution. During growth, we observe that new gut branches are added along the entire anteroposterior axis. We find that new enterocytes differentiate throughout the intestine rather than in specific growth zones, suggesting that branching morphogenesis is achieved primarily by remodeling of differentiated intestinal tissues. During regeneration, we also demonstrate a previously unappreciated degree of intestinal remodeling, in which pre-existing posterior gut tissue contributes extensively to the newly formed anterior gut, and vice versa. By contrast to growing animals, differentiation of new intestinal cells occurs at preferential locations, including within newly generated tissue (the blastema), and along pre-existing intestinal branches undergoing remodeling. Our results indicate that growth and regeneration of the planarian intestine are achieved by co-ordinated differentiation of stem cells and the remodeling of pre-existing tissues. Elucidation of the mechanisms by which these processes are integrated will be critical for understanding organogenesis in a post-embryonic context.

dlx and sp6-9 Control optic cup regeneration in a prototypic eye

Optic cups are a structural feature of diverse eyes, from simple pit eyes to camera eyes of vertebrates and cephalopods. We used the planarian prototypic eye as a model to study the genetic control of optic cup formation and regeneration. We identified two genes encoding transcription factors, sp6-9 and dlx, that were expressed in the eye specifically in the optic cup and not the photoreceptor neurons. RNAi of these genes prevented formation of visible optic cups during regeneration. Planarian regeneration requires an adult proliferative cell population with stem cell-like properties called the neoblasts. We found that optic cup formation occurred only after migration of progressively differentiating progenitor cells from the neoblast population. The eye regeneration defect caused by dlx and sp6-9 RNAi can be explained by a failure to generate these early optic cup progenitors. Dlx and Sp6-9 genes function as a module during the development of diverse animal appendages, including vertebrate and insect limbs. Our work reveals a novel function for this gene pair in the development of a fundamental eye component, and it utilizes these genes to demonstrate a mechanism for total organ regeneration in which extensive cell movement separates new cell specification from organ morphogenesis.

Some invertebrates, such as planarians and Hydra, can regenerate fully after amputations that remove large parts of the body. We investigated how cells in the body of planarians provide new cells for eye regeneration after complete head removal. Planarians possess highly potent regenerative cells (neoblasts) in a compartment inside the worm, and these cells must be present in a body fragment for it to regenerate. We identify a pair of transcription factors, sp6-9 and dlx, that are expressed in the optic cup, and use expression of these genes as markers to demonstrate that lineage restriction of eye cells during regeneration begins within the neoblast compartment. dlx and sp6-9 are essential for formation of optic cup progenitors, and inhibition of these genes with RNA interference results in eyes that lack optic cups after regeneration. During eye development in both flies and vertebrates, progenitors form within a patterned epithelium. Interestingly, planarian eye precursors only aggregate once they have stopped cycling and undergone extensive migration. At this stage they already express markers of the terminally differentiated state. Therefore, we identify a mechanism for eye formation during regeneration and a novel function for a conserved gene pair in eye regeneration.

EGFR signaling regulates cell proliferation, differentiation and morphogenesis during planarian regeneration and homeostasis

Similarly to development, the process of regeneration requires that cells accurately sense and respond to their external environment. Thus, intrinsic cues must be integrated with signals from the surrounding environment to ensure appropriate temporal and spatial regulation of tissue regeneration. Identifying the signaling pathways that control these events will not only provide insights into a fascinating biological phenomenon but may also yield new molecular targets for use in regenerative medicine. Among classical models to study regeneration, freshwater planarians represent an attractive system in which to investigate the signals that regulate cell proliferation and differentiation, as well as the proper patterning of the structures being regenerated. Recent studies in planarians have begun to define the role of conserved signaling pathways during regeneration. Here, we extend these analyses to the epidermal growth factor (EGF) receptor pathway. We report the characterization of three epidermal growth factor (EGF) receptors in the planarian Schmidtea mediterranea. Silencing of these genes by RNA interference (RNAi) yielded multiple defects in intact and regenerating planarians. Smed-egfr-1(RNAi) resulted in decreased differentiation of eye pigment cells, abnormal pharynx regeneration and maintenance, and the development of dorsal outgrowths. In contrast, Smed-egfr-3(RNAi) animals produced smaller blastemas associated with abnormal differentiation of certain cell types. Our results suggest important roles for the EGFR signaling in controlling cell proliferation, differentiation and morphogenesis during planarian regeneration and homeostasis.

Analysis of motor function modulated by cholinergic neurons in planarian Dugesia japonica

Recent studies of the freshwater planarian Dugesia japonica have revealed fundamental mechanisms and unique aspects of neuroscience and neuroregeneration. Here, we identified the gene for planarian choline acetyltransferase (Djchat), which is essential for acetylcholine (ACh) biosynthesis. Immunofluorescence studies using anti-Dugesia japonica ChAT (DjChAT) antibody revealed that cholinergic neurons are widely distributed in the planarian nervous system, including the brain, ventral nerve cords, optic nerves, and pharyngeal nerve plexus. In order to investigate the function of cholinergic neurons in planarians, we used both pharmacological and RNA interference (RNAi) approaches. Administration of physostigmine (an acetylcholinesterase inhibitor) clearly elevated the amount of ACh, and then induced sudden muscle contraction behavior in a concentration-dependent manner. In addition, we found that pretreatment with tubocurarine (a muscle nicotinic ACh receptor antagonist) or atropine (a non-selective muscarinic ACh receptor antagonist), but not pretreatment with mecamylamine (a neural nicotinic ACh receptor antagonist), significantly extended the latency time for physostigmine-induced contraction behavior, suggesting that muscle nicotinic ACh receptors and muscarinic ACh receptors contribute to physostigmine-induced contraction behavior. We also confirmed that ACh biosynthesis ability and DjChAT-immunoreactivity were eliminated in Djchat(RNAi) planarians. Moreover, the decrease of the level of ACh induced by Djchat(RNAi) caused extension of the latency time for contraction behavior. Our findings support the possibility that the cholinergic functions of planarians are similar to those of vertebrates, suggesting that planarians are simple but useful model organisms for getting insight into the cholinergic nervous system in higher animals.