Clathrin-mediated endocytic signals are required for the regeneration of,as well as homeostasis in, the planarian CNS

Adaptive robustness through incoherent signaling mechanisms in a regenerative brain

Animal behavior emerges from collective dynamics of neurons, making it vulnerable to damage. Paradoxically, many organisms exhibit a remarkable ability to maintain significant behavior even after large-scale neural injury. Molecular underpinnings of this extreme robustness remain largely unknown. Here, we develop a quantitative pipeline to measure long-lasting latent states in planarian flatworm behaviors during whole-brain regeneration. By combining >20,000 animal trials with neural network modeling, we show that long-range volumetric peptidergic signals allow the planarian to rapidly restore coarse behavior output after large perturbations to the nervous system, while slow restoration of small-molecule neuromodulator functions refines precision. This relies on the different time and length scales of neuropeptide and small-molecule transmission to generate incoherent patterns of neural activity that competitively regulate behavior. Controlling behavior through opposing communication mechanisms creates a more robust system than either alone and may serve as a generalizable approach for constructing robust neural networks.

Isolation of planarian viable cells using fluorescence-activated cell sorting for advancing single-cell transcriptome analysis

Preparing viable single cells is critical for conducting single-cell RNA sequencing (scRNA-seq) because the presence of ambient RNA from dead or damaged cells can interfere with data analysis. Here, we developed a method for isolating viable single cells from adult planarian bodies using fluorescence-activated cell sorting (FACS). This method was then applied to both adult pluripotent stem cells (aPSCs) and differentiating/differentiated cells. Initially, we employed a violet instead of ultraviolet (UV) laser to excite Hoechst 33342 to reduce cellular damage. After optimization of cell staining conditions and FACS compensation, we generated FACS profiles similar to those created using a previous method that employed a UV laser. Despite successfully obtaining high-quality RNA sequencing data for aPSCs, non-aPSCs produced low-quality RNA reads (i.e., <60% of cells possessing barcoding mRNAs). Subsequently, we identified an effective FACS gating condition that excluded low-quality cells and tissue debris without staining. This non-staining isolation strategy not only reduced post-dissociation time but also enabled high-quality scRNA-seq results for all cell types (i.e., >80%). Taken together, these findings imply that the non-staining FACS strategy may be beneficial for isolating viable cells not only from planarians but also from other organisms and tissues for scRNA-seq studies.

PBDEs disrupt homeostasis maintenance and regeneration of planarians due to DNA damage, proliferation and apoptosis anomaly

Polybrominated diphenyl ethers (PBDEs) are widely used as brominated flame retardants in the manufacturing industry, belonging to persistent organic pollutants in the environment. Planarians are the freshwater worms, with strong regenerative ability and extreme sensitivity to environmental toxicants. This study aimed to evaluate the potential acute comprehensive effects of PBDE-47/-209 on freshwater planarians. Methods to detect the effects include: detection of oxidative stress, observation of morphology and histology, detection of DNA fragmentation, and detection of cell proliferation and apoptosis. In the PBDE-47 treatment group, planarians showed increased oxidative stress intensity, severe tissue damage, increased DNA fragmentation level, and increased cell proliferation and apoptosis. In the PBDE-209 treatment group, planarians showed decreased oxidative stress intensity, slight tissue damage, almost unchanged DNA fragmentation level and apoptosis, proliferation increased only on the first day after treatment. In conclusion, both PBDE-47 and PBDE-209 are dangerous environmental hazardous material that can disrupt planarians homeostasis, while the toxicity of PBDE-47 is sever than PBDE-209 that PBDE-47 can lead to the death of planarians.

Protein Core Fucosylation Regulates Planarian Head Regeneration via Neoblast Proliferation

Protein glycosylation is an important posttranslational modification that plays a crucial role in cellular function. However, its biological roles in tissue regeneration remain interesting and primarily ambiguous. In this study, we profiled protein glycosylation during head regeneration in planarian Dugesia japonica using a lectin microarray. We found that 6 kinds of lectins showed increased signals and 16 kinds showed decreased signals. Interestingly, we found that protein core fucosylation, manifested by Lens culinaris agglutinin (LCA) staining, was significantly upregulated during planarian head regeneration. Lectin histochemistry indicated that the LCA signal was intensified within the wound and blastemal areas. Furthermore, we found that treatment with a fucosylation inhibitor, 2F-peracetyl-fucose, significantly retarded planarian head regeneration, while supplement with L-fucose could improve DjFut8 expression and stimulate planarian head regeneration. In addition, 53 glycoproteins that bound to LCA were selectively isolated by LCA-magnetic particle conjugates and identified by LC-MS/MS, including the neoblast markers DjpiwiA, DjpiwiB, DjvlgA, and DjvlgB. Overall, our study provides direct evidence for the involvement of protein core fucosylation in planarian regeneration.

tec-1 kinase negatively regulates regenerative neurogenesis in planarians

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

Molecular characterization of clathrin heavy chain (Chc) in Rhipicephalus haemaphysaloides and its effect on vitellogenin (Vg) expression via the clathrin-mediated endocytic pathway

Clathrin plays an important role in arthropods, but its function in ticks has not been explored. Here, we describe the molecular characteristics of the clathrin heavy chain of the tick Rhipicephalus haemaphysaloides and its effects on yolk development. The open reading frame of the clathrin heavy chain (Chc) (Rh-Chc) gene consists of 5103 nucleotides encoding 670 amino acids, which is most closely related to that of Ixodes scapularis and relatively close to Homo sapiens and Drosophila melanogaster. Real-time qPCR revealed that Rh-Chc was expressed at all developmental stages and organs. After Rh-Chc is silenced, ticks did not feed and mortality rate was 100%. Moreover, Rh-Chc co-localized with Vitellogenin receptor (VgR) on oocyte membrane. Immunofluorescence showed that the expression of Vitellogenin (Vg) (Rh-Vg) was also closely related to Rh-Chc. Immunofluorescence showed that the expression of Vg was also closely related to Rh-Chc, Rh-Chc silencing slowed the development of oocytes in tick, and culture of ovary in vitro silenced Rh-Chc, the development of oocytes in ticks also slowed down. Overall, the results of this study indicated that Rh-Chc is a vital gene in the tick R. haemaphysaloides that plays an important role in its growth, development, and reproduction.

Tissue mechanics, an important regulator of development and disease

A growing body of work describes how physical forces in and around cells affect their growth, proliferation, migration, function and differentiation into specialized types. How cells receive and respond biochemically to mechanical signals is a process termed mechanotransduction. Disease may arise if a disruption occurs within this mechanism of sensing and interpreting mechanics. Cancer, cardiovascular diseases and developmental defects, such as during the process of neural tube formation, are linked to changes in cell and tissue mechanics. A breakdown in normal tissue and cellular forces activates mechanosignalling pathways that affect their function and can promote disease progression. The recent advent of high-resolution techniques enables quantitative measurements of mechanical properties of the cell and its extracellular matrix, providing insight into how mechanotransduction is regulated. In this review, we will address the standard methods and new technologies available to properly measure mechanical properties, highlighting the challenges and limitations of probing different length-scales. We will focus on the unique environment present throughout the development and maintenance of the central nervous system and discuss cases where disease, such as brain cancer, arises in response to changes in the mechanical properties of the microenvironment that disrupt homeostasis. This article is part of a discussion meeting issue 'Forces in cancer: interdisciplinary approaches in tumour mechanobiology'.

A Subtractive FACS Method for Isolation of Planarian Stem Cells and Neural Cells

Planarians are among the metazoan organisms with the greatest regenerative abilities. This ability is based on their pluripotent stem cells, called neoblasts, which constitute 10-20% of the cells in their body. Elucidating the molecular mechanisms of the planarian stem cell system, for example, the maintenance of stem cell homeostasis and orchestration of lineage choices, contributes powerfully to the advancement of regenerative biology. Our group has developed fluorescence activated cell sorting (FACS) methodologies for the reliable isolation of planarian stem cells, which constitutes an important experimental asset in the field. Here, we describe detailed protocols for the isolation of (1) planarian stem cells and (2) neural cells. Planarian stem cells are isolated by subtraction of the FACS profiles of intact and γ-ray-irradiated (= stem cell depleted) animals stained with Hoechst 33342 and Calcein AM. The neural cells are isolated by subtracting the FACS profiles of head and tail fragments stained with Hoechst 33258 and Merocyanine 540.

Draft genome of Dugesia japonica provides insights into conserved regulatory elements of the brain restriction gene nou-darake in planarians

BACKGROUND

Planarians are non-parasitic Platyhelminthes (flatworms) famous for their regeneration ability and for having a well-organized brain. Dugesia japonica is a typical planarian species that is widely distributed in the East Asia. Extensive cellular and molecular experimental methods have been developed to identify the functions of thousands of genes in this species, making this planarian a good experimental model for regeneration biology and neurobiology. However, no genome-level information is available for D. japonica, and few gene regulatory networks have been identified thus far.

RESULTS

To obtain whole-genome information on this species and to study its gene regulatory networks, we extracted genomic DNA from 200 planarians derived from a laboratory-bred asexual clonal strain, and sequenced 476 Gb of data by second-generation sequencing. Kmer frequency graphing and fosmid sequence analysis indicated a complex genome that would be difficult to assemble using second-generation sequencing short reads. To address this challenge, we developed a new assembly strategy and improved the de novo genome assembly, producing a 1.56 Gb genome sequence (DjGenome ver1.0, including 202,925 scaffolds and N50 length 27,741 bp) that covers 99.4% of all 19,543 genes in the assembled transcriptome, although the genome is fragmented as 80% of the genome consists of repeated sequences (genomic frequency ≥ 2). By genome comparison between two planarian genera, we identified conserved non-coding elements (CNEs), which are indicative of gene regulatory elements. Transgenic experiments using Xenopus laevis indicated that one of the CNEs in the Djndk gene may be a regulatory element, suggesting that the regulation of the ndk gene and the brain formation mechanism may be conserved between vertebrates and invertebrates.

CONCLUSION

This draft genome and CNE analysis will contribute to resolving gene regulatory networks in planarians. The genome database is available at: http://www.planarian.jp.

Multiple Neuropeptide-Coding Genes Involved in Planarian Pharynx Extension

Planarian feeding behavior involves three steps: moving toward food, extending the pharynx from their planarian's ventral side after arriving at the food, and ingesting the food through the pharynx. Although pharynx extension is a remarkable behavior, it remains unknown what neuronal cell types are involved in its regulation. To identify neurons involved in regulating pharynx extension, we quantitatively analyzed pharynx extension and sought to identify these neurons by RNA interference (RNAi) and in situ hybridization. This assay, when performed using planarians with amputation of various body parts, clearly showed that the head portion is indispensable for inducing pharynx extension. We thus tested the effects of knockdown of brain neurons such as serotonergic, GABAergic, and dopaminergic neurons by RNAi, but did not observe any effects on pharynx extension behavior. However, animals with RNAi of the Prohormone Convertase 2 (PC2, a neuropeptide processing enzyme) gene did not perform the pharynx extension behavior, suggesting the possible involvement of neuropeptide(s in the regulation of pharynx extension. We screened 24 neuropeptide-coding genes, analyzed their functions by RNAi using the pharynx extension assay system, and identified at least five neuropeptide genes involved in pharynx extension. These was expressed in different cells or neurons, and some of them were expressed in the brain, suggesting complex regulation of planarian feeding behavior by the nervous system.

Spontaneous Behaviors and Wall-Curvature Lead to Apparent Wall Preference in Planarian

The planarian Dugesia japonica tends to stay near the walls of its breeding containers and experimental dishes in the laboratory, a phenomenon called “wall preference”. This behavior is thought to be important for environmental adaptation, such as hiding by planarians in nature. However, the mechanisms regulating wall-preference behavior are not well understood, since this behavior occurs in the absence of any particular stimulation. Here we show the mechanisms of wall-preference behavior. Surprisingly, planarian wall-preference behavior was also shown even by the head alone and by headless planarians. These results indicate that planarian “wall-preference” behavior only appears to be a “preference” behavior, and is actually an outcome of spontaneous behaviors, rather than of brain function. We found that in the absence of environmental cues planarians moved basically straight ahead until they reached a wall, and that after reaching a wall, they changed their direction of movement to one tangential to the wall, suggesting that this spontaneous behavior may play a critical role in the wall preference. When we tested another spontaneous behavior, the wigwag movement of the planarian head, using computer simulation with various wigwag angles and wigwag intervals, large wigwag angle and short wigwag interval reduced wall-preference behavior. This indicated that wigwag movement may determine the probability of staying near the wall or leaving the wall. Furthermore, in accord with this simulation, when we tested planarian wall-preference behavior using several assay fields with different curvature of the wall, we found that concavity and sharp curvature of walls negatively impacted wall preference by affecting the permissible angle of the wigwag movement. Together, these results indicate that planarian wall preference may be involuntarily caused by the combination of two spontaneous planarian behaviors: moving straight ahead until reaching a wall and then moving along it in the absence of environmental cues, and wigwag movements of the head.

SCYL2 Protects CA3 Pyramidal Neurons from Excitotoxicity during Functional Maturation of the Mouse Hippocampus

Neuronal death caused by excessive excitatory signaling, excitotoxicity, plays a central role in neurodegenerative disorders. The mechanisms regulating this process, however, are still incompletely understood. Here we show that the coated vesicle-associated kinase SCYL2/CVAK104 plays a critical role for the normal functioning of the nervous system and for suppressing excitotoxicity in the developing hippocampus. Targeted disruption of Scyl2 in mice caused perinatal lethality in the vast majority of newborn mice and severe sensory-motor deficits in mice that survived to adulthood. Consistent with a neurogenic origin of these phenotypes, neuron-specific deletion of Scyl2 also caused perinatal lethality in the majority of newborn mice and severe neurological defects in adult mice. The neurological deficits in these mice were associated with the degeneration of several neuronal populations, most notably CA3 pyramidal neurons of the hippocampus, which we analyzed in more detail. The loss of CA3 neurons occurred during the functional maturation of the hippocampus and was the result of a BAX-dependent apoptotic process. Excessive excitatory signaling was present at the onset of degeneration, and inhibition of excitatory signaling prevented the degeneration of CA3 neurons. Biochemical fractionation reveals that Scyl2-deficient mice have an altered composition of excitatory receptors at synapses. Our findings demonstrate an essential role for SCYL2 in regulating neuronal function and survival and suggest a role for SCYL2 in regulating excitatory signaling in the developing brain. Significance statement: Here we examine the in vivo function of SCYL2, an evolutionarily conserved and ubiquitously expressed protein pseudokinase thought to regulate protein trafficking along the secretory pathway, and demonstrate its importance for the normal functioning of the nervous system and for suppressing excitatory signaling in the developing brain. Together with recent studies demonstrating a role of SCYL1 in preventing motor neuron degeneration, our findings clearly establish the SCY1-like family of protein pseudokinases as key regulators of neuronal function and survival.

Membrane indentation triggers clathrin lattice reorganization and fluidization

Clathrin-mediated endocytosis involves the coordinated assembly of clathrin cages around membrane indentations, necessitating fluid-like reorganization followed by solid-like stabilization. This apparent duality in clathrin's in vivo behavior provides some indication that the physical interactions between clathrin triskelia and the membrane effect a local response that triggers fluid-solid transformations within the clathrin lattice. We develop a computational model to study the response of clathrin protein lattices to spherical deformations of the underlying flexible membrane. These deformations are similar to the shapes assumed during intracellular trafficking of nanoparticles. Through Monte Carlo simulations of clathrin-on-membrane systems, we observe that these membrane indentations give rise to a greater than normal defect density within the overlaid clathrin lattice. In many cases, the bulk surrounding lattice remains in a crystalline phase, and the extra defects are localized to the regions of large curvature. This can be explained by the fact that the in-plane elastic stress in the clathrin lattice are reduced by coupling defects to highly curved regions. The presence of defects brought about by indentation can result in the fluidization of a lattice that would otherwise be crystalline, resulting in an indentation-driven, defect-mediated phase transition. Altering subunit elasticity or membrane properties is shown to drive a similar transition, and we present phase diagrams that map out the combined effects of these parameters on clathrin lattice properties.

Thermosensory signaling by TRPM is processed by brain serotonergic neurons to produce planarian thermotaxis

For most organisms, sensitive recognition of even slight changes in environmental temperature is essential for adjusting their behavioral strategies to ensure homeostasis and survival. However, much remains to be understood about the molecular and cellular processes that regulate thermosensation and the corresponding behavioral responses. Planarians display clear thermotaxis, although they have a relatively simple brain. Here, we devised a quantitative thermotaxis assay and unraveled a neural pathway involved in planarian thermotaxis by combinatory behavioral assays and RNAi analysis. We found that thermosensory neurons that expressed a planarian Dugesia japonica homolog of the Transient Receptor Potential Melastatin family a (DjTRPMa) gene were required for the thermotaxis. Interestingly, although these thermosensory neurons are distributed throughout their body, planarians with a dysfunctional brain due to regeneration-dependent conditional gene knockdown (Readyknock) of the synaptotagmin gene completely lost their thermotactic behavior. These results suggest that brain function is required as a central processor for the thermosensory response. Therefore, we investigated the type(s) of brain neurons involved in processing the thermal signals by gene knockdown of limiting enzymes for neurotransmitter biosynthesis in the brain. We found that serotonergic neurons with dendrites that were elongated toward DjTRPMa-expressing thermosensory neurons might be required for the processing of signals from thermosensory neurons that results in thermotaxis. These results suggest that serotonergic neurons in the brain may interact with thermosensory neurons activated by TRPM ion channels to produce thermotaxis 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.

Membrane fluctuations destabilize clathrin protein lattice order

We develop a theoretical model of a clathrin protein lattice on a flexible cell membrane. The clathrin subunit is modeled as a three-legged pinwheel with elastic deformation modes and intersubunit binding interactions. The pinwheels are constrained to lie on the surface of an elastic sheet that opposes bending deformation and is subjected to tension. Through Monte Carlo simulations, we predict the equilibrium phase behavior of clathrin lattices at various levels of tension. High membrane tensions, which correspond to suppressed membrane fluctuations, tend to stabilize large, flat crystalline structures similar to plaques that have been observed in vivo on cell membranes that are adhered to rigid surfaces. Low tensions, on the other hand, give rise to disordered, defect-ridden lattices that behave in a fluidlike manner. The principles of two-dimensional melting theory are applied to our model system to further clarify how high tensions can stabilize crystalline order on flexible membranes. These results demonstrate the importance of environmental physical cues in dictating the collective behavior of self-assembled protein structures.

The history and enduring contributions of planarians to the study of animal regeneration

Having an almost unlimited capacity to regenerate tissues lost to age and injury, planarians have long fascinated naturalists. In the Western hemisphere alone, their documented history spans more than 200 years. Planarians were described in the early 19th century as being 'immortal under the edge of the knife', and initial investigation of these remarkable animals was significantly influenced by studies of regeneration in other organisms and from the flourishing field of experimental embryology in the late 19th and early 20th centuries. This review strives to place the study of planarian regeneration into a broader historical context by focusing on the significance and evolution of knowledge in this field. It also synthesizes our current molecular understanding of the mechanisms of planarian regeneration uncovered since this animal's relatively recent entrance into the molecular-genetic age.

An RNAi screen reveals intestinal regulators of branching morphogenesis, differentiation, and stem cell proliferation in planarians

Planarians grow and regenerate organs by coordinating proliferation and differentiation of pluripotent stem cells with remodeling of postmitotic tissues. Understanding how these processes are orchestrated requires characterizing cell-type-specific gene expression programs and their regulation during regeneration and homeostasis. To this end, we analyzed the expression profile of planarian intestinal phagocytes, cells responsible for digestion and nutrient storage/distribution. Utilizing RNA interference, we identified cytoskeletal regulators required for intestinal branching morphogenesis and a modulator of bioactive sphingolipid metabolism, ceramide synthase, required for the production of functional phagocytes. Additionally, we found that a gut-enriched homeobox transcription factor, nkx-2.2, is required for somatic stem cell proliferation, suggesting a niche-like role for phagocytes. Identification of evolutionarily conserved regulators of intestinal branching, differentiation, and stem cell dynamics demonstrates the utility of the planarian digestive system as a model for elucidating the mechanisms controlling postembryonic organogenesis.

Optic chiasm formation in planarian I: Cooperative netrin- and robo-mediated signals are required for the early stage of optic chiasm formation

Freshwater planarians can regenerate a brain, including eyes, from the anterior blastema, and coordinately form an optic chiasm during eye and brain regeneration. To investigate the role of the netrin- and slit-signaling systems during optic chiasm formation, we cloned three receptor genes (Djunc5A, Djdcc and DjroboA) expressed in visual neurons and their ligand genes (DjnetB and Djslit) and analyzed their functions by RNA interference (RNAi). Although each of DjroboA(RNAi), Djunc5A(RNAi) and DjnetB(RNAi) showed a weak phenotype and Djslit(RNAi) showed a severe defect of eye formation, we did not observe any defect of crossing of visual axons over the midline among single knockdown planarians. However, among double knockdown planarians, some of DjnetB(RNAi);DjroboA(RNAi) and Djunc5A(RNAi);DjroboA(RNAi) showed complete disconnection between the visual axons from the two sides, suggesting that some combination of netrin- and robo-mediated signals may be required for crossing over the midline. Finally, we carefully investigated the distribution patterns of cells expressing DjNetB protein, DjnetB, and Djslit at the early stage of regeneration, and found that visual axons projected along a path sandwiched between DjNetB protein and Djslit-positive cells. These results suggest that two different collaborative or combinatory signals may be required for midline crossing at the early stage of chiasm formation during eye and brain regeneration.

Autophagy and apoptosis in planarians

Adult planarians are capable of undergoing regeneration and body remodelling in order to adapt to physical damage or extreme environmental conditions. Moreover, most planarians can tolerate long periods of starvation and during this time, they shrink from an adult size to, and sometimes beyond, the initial size at hatching. Indeed, these properties have made them a classic model to study stem cells and regeneration. Under such stressful conditions, food reserves from the gastrodermis and parenchyma are first used up and later the testes, copulatory organs and ovaries are digested. More surprisingly, when food is again made available to shrunken individuals, they grow back to adult size and all their reproductive structures reappear. These cycles of growth and shrinkage may occur over long periods without any apparent impairment to the individual, or to its future maturation and breeding capacities. This plasticity resides in a mesoderm tissue known as the parenchyma, which is formed by several differentiated non-proliferating cell types and only one mitotically active cell type, the neoblasts, which represent approximately 20-30% of the cells in the parenchyma. Neoblasts are generally thought to be somatic stem-cells that participate in the normal continuous turnover of all cell types in planarians. Hence, planarians are organisms that continuously adapt their bodies (morphallaxis) to different environmental stresses (i.e.: injury or starvation). This adaptation involves a variety of processes including proliferation, differentiation, apoptosis and autophagy, all of which are perfectly orchestrated and tightly regulated to remodel or restore the body pattern. While neoblast biology and body re-patterning are currently the subject of intense research, apoptosis and autophagy remain much less studied. In this review we will summarize our current understanding and hypotheses regarding where and when apoptosis and autophagy occur and fulfil an essential role in planarians.

Cellular and molecular dissection of pluripotent adult somatic stem cells in planarians

Freshwater planarians, Plathelminthes, have been an intriguing model animal of regeneration studies for more than 100 years. Their robust regenerative ability is one of asexual reproductive capacity, in which complete animals develop from tiny body fragments within a week. Pluripotent adult somatic stem cells, called neoblasts, assure this regenerative ability. Neoblasts give rise to not only all types of somatic cells, but also germline cells. During the last decade, several experimental techniques for the analysis of planarian neoblasts at the molecular level, such as in situ hybridization, RNAi and fluorescence activated cell sorting, have been established. Moreover, information about genes involved in maintenance and differentiation of neoblasts has been accumulated. One of the molecular features of neoblasts is the expression of many RNA regulators, which are involved in germline development in other animals, such as vasa and piwi family genes. In this review, we introduce physiological and molecular features of the neoblast, and discuss how germline genes regulate planarian neoblasts and what differences exist between neoblasts and germline cells.

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.

Cell death and tissue remodeling in planarian regeneration

Many long-lived organisms, including humans, can regenerate some adult tissues lost to physical injury or disease. Much of the previous research on mechanisms of regeneration has focused on adult stem cells, which give rise to new tissue necessary for the replacement of missing body parts. Here we report that apoptosis of differentiated cells complements stem cell division during regeneration in the planarian Schmidtea mediterranea. Specifically, we developed a whole-mount TUNEL assay that allowed us to document two dramatic increases in the rate of apoptosis following amputation-an initial localized response near the wound site and a subsequent systemic response that varies in magnitude depending on the type of fragment examined. The latter cell death response can be induced in uninjured organs, occurs in the absence of planarian stem cells, and can also be triggered by prolonged starvation. Taken together, our results implicate apoptosis in the restoration of proper anatomical scale and proportion through remodeling of existing tissues. We also report results from initial mechanistic studies of apoptosis in planarians, which revealed that a S. mediterranea homolog of the antiapoptotic gene BCL2 is required for cell survival in adult animals. We propose that apoptosis is a central mechanism working in concert with stem cell division to restore anatomical form and function during metazoan regeneration.

Molecular actions guiding neural regeneration in planarian

Planarian is among the simplest animals that possess a centralized nervous system (CNS), and its neural regeneration involves the replacement of cells lost to normal 'wear and tear' (cell turnover), and/or injury. In this review, we state and discuss the recent studies on molecular control of neural regeneration in planarians. The spatial and temporal expression patterns of genes in intact and regenerating planarian CNS have already been described relatively clearly. The bone morphogenetic protein (BMP) and Wnt signaling pathways are identified to regulate neural regeneration. During neural regeneration, conserved axon guidance mechanisms are necessary for proper wiring of the nervous system. In addition, apoptosis may play an important role in controlling cell numbers, eliminating unnecessary tissues or cells and remodeling the old tissues for regenerating CNS. The bilateral symmetry is established by determination of anterior-posterior (A-P) and dorsal-ventral (D-V) patterns. Moreover, neurons positive to dopamine, serotonin (5-HT), and gamma-aminobutyric acid (GABA) have been detected in planarians. Therefore, planarians present us with new, experimentally accessible contexts to study the molecular actions guiding neural regeneration.

Brain regeneration from pluripotent stem cells in planarian

How can planarians regenerate their brain? Recently we have identified many genes critical for this process. Brain regeneration can be divided into five steps: (1) anterior blastema formation, (2) brain rudiment formation, (3) pattern formation, (4) neural network formation, and (5) functional recovery. Here we will describe the structure and process of regeneration of the planarian brain in the first part, and then introduce genes involved in brain regeneration in the second part. Especially, we will speculate about molecular events during the early steps of brain regeneration in this review. The finding providing the greatest insight thus far is the discovery of the nou-darake (ndk; 'brains everywhere' in Japanese) gene, since brain neurons are formed throughout the entire body as a result of loss of function of the ndk gene. This finding provides a clue for elucidating the molecular and cellular mechanisms underlying brain regeneration. Here we describe the molecular action of the nou-darake gene and propose a new model to explain brain regeneration and restriction in the head region of the planarians.

Organization of the nervous system in the model planarian Schmidtea mediterranea: an immunocytochemical study

Freshwater planarians are an emerging model in which to study regeneration at the molecular level. These animals can regenerate a complete central nervous system (CNS) in only a few days. In recent years, hundreds of genes expressed in the nervous system have been identified in two popular planarian species used by several laboratories: Dugesia japonica and Schmidtea mediterranea. Functional analyses of some of those neural genes have allowed the process of CNS regeneration to begin to be elucidated in those animals. However, additional work is required to characterize the different neuronal populations. Thus, the identification or generation of antibodies that act as markers for specific neuronal cell types would be extremely useful not only in obtaining a more detailed characterization of the planarian nervous system but also for the analysis of phenotypes obtained by RNA interference. Here, I have used five different antibodies to describe different neuronal populations in the freshwater planarian S. mediterranea. This study represents a first step in characterizing the organization of the nervous system of this species at the cellular level.

Identification of glutamic acid decarboxylase gene and distribution of GABAergic nervous system in the planarian Dugesia japonica

The planarian Dugesia japonica has a relatively well-organized CNS that includes the brain and the ventral nerve cords, and also has high regenerative capacity derived from pluripotent stem cells present in the mesenchymal space throughout the body. Glutamic acid decarboxylase (GAD) is the enzyme that converts glutamic acid into GABA, a major inhibitory neurotransmitter. In this study, we first identified a full-length GAD gene (DjGAD, D. japonica glutamic acid decarboxylase) in the planarian D. japonica. Whole-mount in situ hybridization revealed that a few cells expressed DjGAD mRNA, and these cells were located in both the head and pharynx regions. In order to examine the distribution pattern of DjGAD protein, we generated a mouse monoclonal anti-DjGAD antibody. The distribution pattern of DjGAD protein was very similar to that of DjGAD mRNA. A neural network of DjGAD-immunopositive cells was also clearly observed. In addition, we examined the immunofluorescence during the process of regeneration of the head from the tail piece. At day 3 of regeneration, we could detect newly formed DjGAD-immunopositive neurons in the anterior region. During day 5-7 of regeneration, reconstruction of the neural network of DjGAD-immunopositive cells occurred. DjGAD-immunoreactivity was lost in DjGAD-knockdown planarians obtained by RNA interference. The amount of GABA was significantly decreased in DjGAD-knockdown planarians, which lost negative phototaxis but not locomotion activity. These results suggest that DjGAD is clearly required for GABA biosynthesis and photosensitivity in planarians, and expression of DjGAD as detected by anti-DjGAD antibody is a useful marker for GABAergic neurons.

DjCBC-1, a conserved DEAD box RNA helicase of the RCK/p54/Me31B family, is a component of RNA-protein complexes in planarian stem cells and neurons

The stem cells of planarians, known as neoblasts, can give rise to all cell types in planarians. Neoblasts can be identified by electron microscopy as cells with electron-dense chromatoid bodies, which are large RNP (ribonucleoprotein) complexes, in their cytoplasm. However, the components and function of chromatoid bodies are still relatively unknown. Here we identified a DEAD box RNA helicase gene of the RCK/p54/Me31B family from a planarian EST database and showed the localization of its product in chromatoid bodies by immunoelectron microscopy. We named this gene Djcbc-1 (Dugesia japonica chromatoid body component 1). Djcbc-1 was also strongly expressed in the brain and in the germline stem cells of sexualized planarians. We observed chromatoid body-like electron-dense bodies in brain neurons, where DjCBC-1 was also expressed. These observations suggest that common molecular components of RNP complexes may be involved in the regulation of somatic and germline stem cells, and neurons in planarians.

Regenerating the central nervous system: how easy for planarians!

The regenerative capabilities of freshwater planarians (Platyhelminthes) are very difficult to match. A fragment as tiny as 1/279th of the planarian body is able to regenerate a whole animal within very few days [Morgan. Arch Entwm 7:364-397 (1898)]. Although the planarian central nervous system (CNS) may appear quite morphologically simple, recent studies have shown it to be more complex at the molecular level, revealing a high degree of molecular compartmentalization in planarian cephalic ganglia. Planarian neural genes include homologues of well-known transcription factors and genes involved in human diseases, neurotransmission, axon guidance, signaling pathways, and RNA metabolism. The availability of hundreds of genes expressed in planarian neurons coupled with the ability to silence them through the use of RNA interference makes it possible to start unraveling the molecular mechanisms underlying CNS regeneration. In this review, I discuss current knowledge on the planarian nervous system and the genes involved in its regeneration, and I discuss some of the important questions that remain to be answered.

Wnt signaling is required for antero-posterior patterning of the planarian brain

Wnt signaling functions in axis formation and morphogenesis in various animals and organs. Here we report that Wnt signaling is required for proper brain patterning during planarian brain regeneration. We showed here that one of the Wnt homologues in the planarian Dugesia japonica, DjwntA, was expressed in the posterior region of the brain. When DjwntA-knockdown planarians were produced by RNAi, they could regenerate their heads at the anterior ends of the fragments, but formed ectopic eyes with irregular posterior lateral branches and brain expansion. This suggests that the Wnt signal may be involved in antero-posterior (A-P) patterning of the planarian brain, as in vertebrates. We also investigated the relationship between the DjwntA and nou-darake/FGFR signal systems, as knockdown planarians of these genes showed similar phenotypes. Double-knockdown planarians of these genes did not show any synergistic effects, suggesting that the two signal systems function independently in the process of brain regeneration, which accords with the fact that nou-darake was expressed earlier than DjwntA during brain regeneration. These observations suggest that the nou-darake/FGFR signal may be involved in brain rudiment formation during the early stage of head regeneration, and subsequently the DjwntA signal may function in A-P patterning of the brain rudiment.