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The recently elucidated proresolving conjugates in tissue regeneration (CTR) maresin-CTR (MCTR), protectin-CTR (PCTR), and resolvin-CTR (RCTR), termed cysteinyl-specialized proresolving mediators (cys-SPMs) each promotes regeneration, controls infection, and accelerates resolution of inflammation. Here, we sought evidence for cys-SPM activation of primordial pathways in planaria ( Dugesia japonica ) regeneration that might link resolution of inflammation and regeneration. On surgical resection, planaria regeneration was enhanced with MCTR3, PCTR3, or RCTR3 (10 nM), each used for RNA sequencing. The three cys-SPMs shared up-regulation of 175 known transcripts with fold-change > 1.25 and combined false discovery rate (FDR) < 0.002, and 199 canonical pathways (FDR < 0.25), including NF-κB pathways and an ortholog of human TRAF3 (TNFR-associated factor 3). Three separate pathway analyses converged on TRAF3 up-regulation by cys-SPMs. With human macrophages, three cys-SPMs each dose-dependently increased TRAF3 expression in a cAMP-PKA-dependent manner. TRAF3 overexpression in macrophages enhanced Interleukin-10 (IL-10) and phagocytosis of Escherichia coli IL-10 also increased phagocytosis in a dose-dependent manner. Silencing of mouse TRAF3 in vivo significantly reduced IL-10 and macrophage phagocytosis. TRAF3 silencing in vivo also relieved cys-SPMs' actions in limiting polymorphonuclear neutrophil in E. coli exudates. These results identify cys-SPM-regulated pathways in planaria regeneration, uncovering a role for TRAF3/IL-10 in regulating mammalian phagocyte functions in resolution. Cys-SPM activation of TRAF3 signaling is a molecular component of both regeneration and resolution of infectious inflammation., Competing Interests: The authors declare no competing interest.

The powerful regenerative ability of planarians has long been a concern of scientists, but recently, their efficient immune system has attracted more and more attention from researchers. Gamma-interferon-inducible lysosomal thiol reductase (GILT) is related not only to antigen presentation but also to bacteria invasions. But the systematic studies are not yet to be conducted on the relationship between bacterial infection. Our study reveals for the first time that GILT of planarian (DjGILT) plays an essential role in the clearance of Gram-negative bacteria by conducting H 2 O 2 concentration in planarians. In animals that DjGILT was silenced, it persisted for up to 9 days before all bacteria were cleared, compared with 6 days of the control group. When infected with E. coli and V. anguillarum, the level of H 2 O 2 was significantly increased in DjGILT-silenced planarians, and concomitantly, mRNA level of C-type lectin DjCTL, which modulates agglutination and clearance efficiency of invading bacteria, was decreased. Further study showed that the decrease of H 2 O 2 level led to a significant increase in DjCTL transcripts. Collectively, we proposed a mechanism model for the involvement of GILT gene in bacterial elimination. We have for the first time revealed the specific mechanism of GILT in innate immune response against bacterial infection., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Candida albicans is one of the most common fungal pathogens of humans. Currently, there are limitations in the evaluation of C. albicans infection in existing animal models, especially in terms of understanding the influence of specific infectious stages of the fungal pathogen on the host. We show that C. albicans infects, grows and invades tissues in the planarian flatworm Schmidtea mediterranea, and that the planarian responds to infection by activating components of the host innate immune system to clear and repair host tissues. We study different stages of C. albicans infection and demonstrate that planarian stem cells increase division in response to fungal infection, a process that is likely evolutionarily conserved in metazoans. Our results implicate MORN2 and TAK1/p38 signaling pathways as possible mediators of the host innate immune response to fungal infection. We propose the use of planarians as a model system to investigate host-pathogen interactions during fungal infections., (Copyright © 2018. Published by Elsevier Ltd.)

Retinoic acid-inducible gene I (RIG-I), an RNA sensor with a conserved structure, activates the host interferon (IFN) system to produce IFNs and cytokines for eliminating pathogens upon recognizing PAMPs. However, the biological functions and the mechanism by which RIG-I regulates the innate immunity response in invertebrates are still unknown at present. Here we identified an atypical RIG-I in planarian Dugesia japonica. Sequence analysis, 3D structure modeling and phylogenetic analysis showed that this atypical protein was clustered into a single clade at the base of the tree in invertebrates, suggesting that DjRIG-I is an ancient and unique protein of the RIG-I-like receptors (RLRs). In situ hybridization analysis revealed that the DjRIG-I mRNAs were predominantly expressed in the pharynx and head of the adult and regenerative planarians. Stimulation with PAMPs induced the over-expression of DjRIG-I in planarians. The molecular simulation demonstrated that DjRIG-I formed a large hole-structure for the docking of dsRNAs, and the pull-down assay confirmed the interaction between DjRIG-I and viral analog poly(I:C). Importantly, some representative antiviral/antibacterial genes in the RIG-I-mediated IFN and P38 signaling pathway, TBK1, IRF-3, Mx, and P38, were significantly upregulated in planarians stimulated with PAMPs. Interference of the DjRIG-I expression by RNAi, inhibited the PAMPs-induced over-expression, suggesting that DjRIG-I is a key player for downstream signaling events. These results indicate that DjRIG-I triggered the intracellular signaling cascades independent of the classical CARD domains and played an essential role in the virus/bacteria-induced innate immunity of planarian., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Studies of tissue regeneration and host-pathogen interactions using the model planarian Schmidtea mediterranea have been performed at an experimental temperature of 19 °C. S. mediterranea planarians exposed to 19 °C-32 °C were observed for survival, mobility, feeding and regeneration for three months and elimination of the Staphylococcus aureus pathogen over six days. S. mediterranea planarians died at 30 °C-32 °C after 18 days of observation but tolerated temperatures of 19 °C up to 28 °C with non-significant differences in mobility and feeding behavior. Genetic malleability tested by RNAi feeding was still efficient at 26 °C and 28 °C. Concerning the immune capacity of planarians, we reported an exacerbation of the immune response in worms infected by S. aureus at 26 °C and 28 °C. These observations suggest a temperature modulation of planarian stem cells and illustrate the importance of modulating experimental temperature when using planarians as model organisms to study regeneration and immune response.

Planarians, which are non-parasitic flatworms, are highly resistant to bacterial infections. To better understand the mechanisms underlying this resistance, we investigated the role of the circadian machinery in the anti-bacterial response of the freshwater planarian Schmidtea mediterranea. We identified Smed-Tim from S. mediterranea as a homolog of the mammalian clock gene Tim. We showed via RNA interference that Smed-Tim is required for the anti-microbial activities of Schmidtea mediterranea against Staphylococcus aureus infection during the light/dark cycle. Indeed, S. aureus infection leads to the expression of Smed-Tim, which in turn promotes Smed-Traf6 and Smed-morn2, but not Smed-p38 MAPK expression, 2 master regulators of planarian anti-microbial responses.

14-3-3 proteins are a family of highly conserved acidic proteins that regulate cellular processes. They act as a kind of important signaling molecules taking part in many crucial decisions throughout the development process. We have isolated and characterized two members of the 14-3-3 family, namely, Dj14-3-3 α and Dj14-3-3 ζ in the planarian Dugesia japonica. The Dj14-3-3 α and ζ genes encode polypeptides of 260 and 255 amino acids respectively. We have proved that the Dj14-3-3 α and ζ genes were especially expressed in the pharynx in adult and regenerating planarians by in situ hybridization and they were not involved in regeneration process. Besides, Dj14-3-3 α and ζ genes can compensate each other in planarians by RNA interference. The Dj14-3-3 α and ζ were significantly up-regulated expression when planarians were stimulated with the pathogen-associated molecular patterns including lipopolysaccharide (LPS), peptidoglycan (PGN), β-Glu and Poly (I:C), indicating that the Dj14-3-3 α and ζ may be involved in the immune responses., (Copyright © 2017 Elsevier B.V. All rights reserved.)

Planarian, a representative of platyhelminthes, has strong regeneration ability and less complicated innate immune system. However, planarian immune system remains poorly understood. In this paper, a novel C-type lectin-like protein, namely, DjCTL was identified and characterized in Dugesia japonica. DjCTL was mainly expressed in the pharyngeal and epidermis and up-regulated upon the induction of lipopolysaccharide (LPS), peptidoglycan (PGN), Gram-positive and Gram-negative bacteria indicating that DjCTL may be involved in the immune responses. Recombination DjCTL protein agglomerated rabbit red blood cells and interacted with LPS, PGN, mannose and galactose as well as both Gram-positive and Gram-negative bacteria, but it can only cause the agglutination of Gram-negative bacteria. Importantly, in the early periods of regeneration, DjCTL had a significantly high expression and was mainly expressed in early blastemas. RNA interference of DjCTL by dsRNA-DjCTL led to a slow wound healing during regeneration. These findings suggest that DjCTL participates in the innate immune response and plays an important role in early stages of regeneration., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

Background: The freshwater planarian Dugesia japonica (D. japonica) possesses extraordinary ability to regenerate lost organs or body parts. Interestingly, in the process of regeneration, there is little wound infection, suggesting that D. japonica has a formidable innate immune system. The importance of immune system prompted us to search for immune-related genes and RIG-I-like receptor signaling pathways., Results: Transcriptome sequencing of D. japonica was performed on an IlluminaHiSeq2000 platform. A total of 27,180 transcripts were obtained by Trinity assembler. CEGMA analysis and mapping of all trimmed reads back to the assembly result showed that our transcriptome assembly covered most of the whole transcriptome. 23,888 out of 27,180 transcripts contained ORF (open reading fragment), and were highly similar to those in Schistosoma mansoni using BLASTX analysis. 8,079 transcripts (29.7%) and 8,668 (31.9%) were annotated by Blast2GO and KEGG respectively. A DYNLRB-like gene was cloned to verify its roles in the immune response. Finally, the expression patterns of 4 genes (RIG-I, TRAF3, TRAF6, P38) in the RIG-I-like receptor signaling pathway were detected, and the results showed they are very likely to be involved in planarian immune response., Conclusion: RNA-Seq analysis based on the next-generation sequencing technology was an efficient approach to discover critical genes and to understand their corresponding biological functions. Through GO and KEGG analysis, several critical and conserved signaling pathways and genes related to RIG-I-like receptor signaling pathway were identified. Four candidate genes were selected to identify their expression dynamics in the process of pathogen stimulation. These annotated transcripts of D. japonica provide a useful resource for subsequent investigation of other important pathways.

Background: Efforts to elucidate the cellular and molecular mechanisms of regeneration have required the application of methods to detect specific cell types and tissues in a growing cohort of experimental animal models. For example, in the planarian Schmidtea mediterranea, substantial improvements to nucleic acid hybridization and electron microscopy protocols have facilitated the visualization of regenerative events at the cellular level. By contrast, immunological resources have been slower to emerge. Specifically, the repertoire of antibodies recognizing planarian antigens remains limited, and a more systematic approach is needed to evaluate the effects of processing steps required during sample preparation for immunolabeling., Results: To address these issues and to facilitate studies of planarian digestive system regeneration, we conducted a monoclonal antibody (mAb) screen using phagocytic intestinal cells purified from the digestive tracts of living planarians as immunogens. This approach yielded ten antibodies that recognized intestinal epitopes, as well as markers for the central nervous system, musculature, secretory cells, and epidermis. In order to improve signal intensity and reduce non-specific background for a subset of mAbs, we evaluated the effects of fixation and other steps during sample processing. We found that fixative choice, treatments to remove mucus and bleach pigment, as well as methods for tissue permeabilization and antigen retrieval profoundly influenced labeling by individual antibodies. These experiments led to the development of a step-by-step workflow for determining optimal specimen preparation for labeling whole planarians as well as unbleached histological sections., Conclusions: We generated a collection of monoclonal antibodies recognizing the planarian intestine and other tissues; these antibodies will facilitate studies of planarian tissue morphogenesis. We also developed a protocol for optimizing specimen processing that will accelerate future efforts to generate planarian-specific antibodies, and to extend functional genetic studies of regeneration to post-transcriptional aspects of gene expression, such as protein localization or modification. Our efforts demonstrate the importance of systematically testing multiple approaches to species-specific idiosyncracies, such as mucus removal and pigment bleaching, and may serve as a template for the development of immunological resources in other emerging model organisms.

Planarians famously can regenerate after decapitation. In this issue, Abnave et al. (2014) find they resist infection by multiple bacterial species pathogenic to humans, Drosophila and C. elegans, including M. tuberculosis. These results identify a conserved gene controlling phagocytosis and establish planarians as a powerful system for analyzing host-pathogen interactions., (Copyright © 2014 Elsevier Inc. All rights reserved.)

Dugesia japonica planarian flatworms are naturally exposed to various microbes but typically survive this challenge. We show that planarians eliminate bacteria pathogenic to Homo sapiens, Caenorhabditis elegans, and/or Drosophila melanogaster and thus represent a model to identify innate resistance mechanisms. Whole-transcriptome analysis coupled with RNAi screening of worms infected with Staphylococcus aureus or Legionella pneumophila identified 18 resistance genes with nine human orthologs, of which we examined the function of MORN2. Human MORN2 facilitates phagocytosis-mediated restriction of Mycobacterium tuberculosis, L. pneumophila, and S. aureus in macrophages. MORN2 promotes the recruitment of LC3, an autophagy protein also involved in phagocytosis, to M. tuberculosis-containing phagosomes and subsequent maturation to degradative phagolysosomes. MORN2-driven trafficking of M. tuberculosis to single-membrane, LC3-positive compartments requires autophagy-related proteins Atg5 and Beclin-1, but not Ulk-1 and Atg13, highlighting the importance of MORN2 in LC3-associated phagocytosis. These findings underscore the value of studying planarian defenses to identify immune factors., (Copyright © 2014 Elsevier Inc. All rights reserved.)

The immune system has been implicated as an important modulator of tissue regeneration. However, the mechanisms driving injury-induced immune response and tissue repair remain poorly understood. For over 200 years, planarians have been a classical model for studies on tissue regeneration, but the planarian immune system and its potential role in repair is largely unknown. We found through comparative genomic analysis and data mining that planarians contain many potential homologs of the innate immune system that are activated during injury and repair of adult tissues. These findings support the notion that the relationship between adult tissue repair and the immune system is an ancient feature of basal Bilateria. Further analysis of the planarian immune system during regeneration could potentially add to our understanding of how the innate immune system and inflammatory responses interplay with regenerative signals to induce scar-less tissue repair in the context of the adult organism., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

GRP78 (78 kDa glucose-regulated protein) has ubiquitously existed in nearly all organisms from yeast to humans, reflecting the central roles it plays in cell survival. In this report, we isolated and sequenced the full-length cDNA of GRP78 (designated DjGRP78) from the planarian Dugesia japonica. The cDNA is 2121 bp, including an open reading frame (ORF) of 1983 bp encoding a polypeptide of 660 amino acids with three HSP70 family signatures. DjGRP78 contains signal peptides at the N-terminus and a KTEL peptide motif at the C-terminus, which suggests that it localizes in the endoplasmic reticulum (ER). Fluorescent real time RT-PCR was employed to detect the expression pattern of Djgrp78 in response to different stressors. Our results show that heat shock and heavy metals (Hg(2+) and Pb(2+)) induce Djgrp78 expression, but starvation does not. Interestingly, we found that Djgrp78 was up-regulated in planarians with septic tissues, and also verified that it was up-regulated in response to bacterial challenge. Our data indicate that Djgrp78 may be a multifunctional gene, and play important roles in physiological and pathological stress in planarians., (Copyright © 2014 Elsevier Inc. All rights reserved.)

Trypsin-like serine proteases are involved in large number of processes, especially in digestive degradation and immune responses. Here, we identify the characterization of a trypsin-like serine protease in planarian, Djtry, which interestingly has the incompletely conserved catalytic triad (K, D, and S). Phylogenetic analysis suggests that Djtry is an ancient type of trypsin-like serine proteases. The spatial and temporal expression patterns of Djtry are shown during regenerating and embryonic development by whole-mount in situ hybridization. Djtry is found to display a tissue specific expression pattern, with a predominant expression detected in whole gut region of intact and regenerating planarian. While the tissue- and stage-specific expression patterns during the embryonic development imply the roles of Djtry involve in yolk degradation and gut formation. Quantitative real-time PCR was carried out to analyze the function of this protease in vivo after planarians were stimulated to a bacterial challenge and food. The results showed that Djtry increased after a bacterial challenge and was basically stable for food. Therefore, the trypsin-like serine protease might be involved in the innate defense reactions against bacterial infection.

The planarian Dugesia japonica has two genes encoding myosin heavy chain, DjMHC-A and B (Kobayashi et al., 1998). We produced antibodies specifically recognizing each myosin heavy chain protein using their carboxyl terminal regions expressed in E. coli as antigens. Immunohistochemical analyses of sections and whole-mount specimens revealed the detailed structure and distribution of each type of muscle fiber in the planarian. In general, the MHC-A muscle fibers were distributed beneath the epithelial layers, namely, they were observable in the pharynx, the mouth, the intestine, the eyes and the body wall. In the pharynx, only MHC-A muscle fibers were present. In contrast, the MHC-B muscle fibers were distributed in the mesenchyme as dorso-ventral and transverse muscles, and in the body wall. The body-wall muscles were composed of an outer layer of circular MHC-A muscles and inner longitudinal and intermediate diagonal MHC-B muscle layers. Thus, two types of muscle fibers were distinguished by their distribution in the planarian.

To obtain specific immunological probes for studying molecular mechanisms involved in cell renewal, cell differentiation, and pattern formation in intact and regenerating planarians, we have produced a hybridoma library specific for the asexual race of the fresh-water planarian Dugesia (Girardia) tigrina. Among the 276 monoclonal antibodies showing tissue-, cell-, cell subtype-, subcellular- and position-specific staining, we have found monoclonal antibodies against all tissues and cell types with the exception of neoblasts, the undifferentiated totipotent stem-cells in planarians. We have also detected position-specific antigens that label anterior, central, and posterior regions. Patterns of expression uncovered an unexpected heterogeneity among previously thought single cell types, as well as interesting cross-reactivities that deserve further study. Characterization of some of these monoclonal antibodies suggests they may be extremely useful as molecular markers for studying cell renewal and cell differentiation in the intact and regenerating organism, tracing the origin, lineage, and differentiation of blastema cells, and characterizing the stages and mechanisms of early pattern formation. Moreover, two position-specific monoclonals, the first ones isolated in planarians, will be instrumental in describing in molecular terms how the new pattern unfolds during regeneration and in devising the pattern formation model that best fits classical data on regeneration in planarians.

A transient and apparently orderly re-expression of embryonal antigens in planarian regenerates has been demonstrated. This finding seems to corroborate the hypothesis that regeneration in planarians is based on a recapitulation of mechanisms that operate the embryogenesis of these animals, andgives some support to the concept of cancer as a misprogrammed regeneration when contemplated against the background of facts pointed out in the literature.