Differential count and time-course analysis of the cellular composition of coelomocyte aggregate of the Japanese sea cucumber Apostichopus japonicus

Physiological responses of Holothuria grisea during a wound healing event: An integrated approach combining tissue, cellular and humoral evidence

Due to their tissue structure similar to mammalian skin and their close evolutionary relationship with chordates, holothurians (Echinodermata: Holothuroidea) are particularly interesting for studies on wound healing. However, previous studies dealing with holothuroid wound healing have had limited approaches, being restricted to tissue repair or perivisceral immune response. In this study, we combined tissue, cellular and humoral parameters to study the wound healing process of Holothuria grisea. The immune responses of the perivisceral coelom were assessed by analyzing the number, proportion and viability of coelomocytes and the volume and protein concentration of the coelomic fluid. Additionally, the morphology of the healing tissue and number of coelomocytes in the connective tissue of different body wall layers were examined over 30 days. Our results showed that perivisceral reactions started 3 h after injury and decreased to baseline levels within 24 h. In contrast, tissue responses were delayed, beginning after 12 h and returning to baseline levels only after day 10. The number of coelomocytes in the connective tissue suggests a potential cooperation between these cells during wound healing: phagocytes and acidophilic spherulocytes act together in tissue clearance/homeostasis, whereas fibroblast-like and morula cells cooperate in tissue remodeling. Finally, our results indicate that the major phases observed in mammalian wound healing are also observed in H. grisea, despite occurring at a different timing, which might provide insights for future studies. Based on these data, we propose a model that explains the entire healing process in H. grisea.

Transcriptome analysis reveals key transcription factors and pathways of polian vesicle associated with cell proliferation in Vibrio splendidus-challenged Apostichopus japonicus

Polian vesicle is thought to produce coelomocytes and contribute to the sea cucumber's immune system. Our previous work has indicated that polian vesicle was responsible for cell proliferation at 72 h post pathogenic challenge. However, the transcription factors related to the activation of effector factors and the molecular process behind this remained unknown. In this study, to reveal the early functions of polian vesicle in response to the microbe, a comparative transcriptome sequencing of polian vesicle in V. splendidus-challenged Apostichopus japonicus, including normal group (PV 0 h), pathogen challenging for 6 h (PV 6 h) and 12 h (PV 12 h) was performed. Compared PV 0 h to PV 6 h, PV 0 h to PV 12 h, and PV 6 h to PV 12 h, we found 69, 211, and 175 differentially expressed genes (DEGs), respectively. KEGG enrichment analysis revealed the DEGs, including several transcription factors such as fos, FOS-FOX, ATF2, egr1, KLF2, and Notch3 between PV 6 h and PV 12 h were consistently enriched in MAPK, Apelin and Notch3 signaling pathways related to cell proliferation compared with that in PV 0 h. Important DEGs involved in cell growth were chosen, and their expression patterns were almost the same as the transcriptome profile analysis by qPCR. Protein interaction network analysis indicated that two DEGs of fos and egr1 were probably significant as key candidate genes controlling cell proliferation and differentiation in polian vesicle after pathogenic infection in A. japonicus. Overall, our analysis demonstrates that polian vesicles may play an essential role in regulating proliferation via transcription factors-mediated signaling pathway in A. japonicus and provide new insights into hematopoietic modulation of polian vesicles in response to pathogen infection.

Establishment of the immunological self in juvenile Patiria pectinifera post-metamorphosis

Ontogeny of the immune system is a fundamental immunology issue. One indicator of immune system maturation is the establishment of the immunological self, which describes the ability of the immune system to distinguish allogeneic individuals (allorecognition ability). However, the timing of immune system maturation during invertebrate ontogeny is poorly understood. In the sea star Patiria pectinifera, cells that have dissociated from the embryos and larvae are able to reconstruct larvae. This reconstruction phenomenon is possible because of a lack of allorecognition capability in the larval immune system, which facilitates the formation of an allogeneic chimera. In this study, we revealed that the adult immune cells of P. pectinifera (coelomocytes) have allorecognition ability. Based on a hypothesis that allorecognition ability is acquired before and after metamorphosis, we conducted detailed morphological observations and survival time analysis of metamorphosis-induced chimeric larvae. The results showed that all allogeneic chimeras died within approximately two weeks to one month of reaching the juvenile stage. In these chimeras, the majority of the epidermal cell layer was lost and the mesenchymal region expanded, but cell death appeared enhanced in the digestive tract. These results indicate that the immunological self of P. pectinifera is established post-metamorphosis during the juvenile stage. This is the first study to identify the timing of immune system maturation during echinodermal ontogenesis. As well as establishing P. pectinifera as an excellent model for studies on self- and non-self-recognition, this study enhances our understanding of the ontogeny of the immune system in invertebrates.

Rainbow bodies: Revisiting the diversity of coelomocyte aggregates and their synthesis in echinoderms

The innate immunity of echinoderms has been a research focus since the early twentieth century, consistently providing ever deeper knowledge of its complexity and evolutionary aspects. At its core are coelomocytes, which are diverse cells collectively known to respond in a variety of ways, including via movement, phagocytosis, and aggregation. However, features of cellular immunity have never been compared in echinoderms from phylogenetic and distributional perspectives, to provide insight into ecological and evolutionary patterns. The present study catalyzed and characterized the formation of coelomocyte aggregates in members of all five extant classes of echinoderms. The morphological characteristics of these aggregates (including their colour, shape, texture, size) were assessed, as well as the major cells composing them. Coelomocyte diversity (both as free and aggregated forms) was determined to be maximum in class Holothuroidea, followed by Echinoidea, with the other classes showing similar levels of diversity. The colours of coelomocyte aggregates appeared to be more closely linked to phylogeny (classes, orders) rather than geographic range, or external colour of the species themselves. Asteroids and ophiuroids displayed primarily light-coloured aggregates, from transparent to green; while holothuroids, echinoids and crinoids demonstrated more vivid variants, from red to deep purple. The kinetics of aggregate formation and expulsion were monitored in selected species, showing immediate cellular response to foreign particulate matter in the form of encapsulation and various methods of expulsion, including through the dermal papillae of asteroids and the anus (cloaca) of holothuroids. The findings support that coelomocyte aggregate formation is a conserved immune response across all five extant classes of echinoderms with variations in their cell catalysts, complexity, shape, colour, and size.

Galactose-Binding C-Type Lectin Promotes Cellular Aggregation of Coelomocytes in Sea Cucumber

Echinoderms have a large coelomic cavity containing coelomocytes. When the coelomic fluid is removed from the cavity, the cells aggregate immediately. We found that a fraction or an extract of the intestine of the sea cucumber, Apostichopus japonicus, markedly accelerated cellular movement and aggregation on a glass slide, and this effect was clearly inhibited by galactose. We successfully purified the aggregation-promoting factor, a 16 kDa protein, from the intestine. TOF-MS analysis followed by de novo sequencing revealed that the protein is a C-type lectin. RNA-seq data and cDNA cloning demonstrated the factor to be a novel lectin, named AjGBCL, consisting of 158 aa residues in the mature form. Microscopic observation revealed that most of the aggregating cells moved toward aggregates and not to an intestinal fragment, suggesting that AjGBCL is not a chemoattractant but a cellular aggregation-inducing factor that may induce aggregates to release chemoattractant. We report, for the first time, an endogenous molecule that promotes coelomocyte aggregation in echinoderms.

Inhibition of ultraviolet-induced sea cucumber (Stichopus japonicus) autolysis by maintaining coelomocyte intracellular calcium homeostasis

Apoptosis plays a critical role in sea cucumber autolysis. To investigate the ultraviolet (UV)-induced apoptosis, sea cucumbers with and without injection of BAPTA-AM (cytosolic calcium chelator) were exposed to UV (15 W/m²) for 30 min. The results showed that UV irradiation caused several changes in sea cucumber coelomocytes, including calcium imbalance, abnormal morphology of endoplasmic reticulum, upregulation of pro-apoptotic proteins CRT, CHOP, and caspases 9 and 3, and downregulation of anti-apoptotic protein Bcl-2. A comparison between the two groups showed that injection of the calcium chelator into sea cucumbers helped maintain coelomocyte intracellular calcium homeostasis and suppressed other abnormal changes caused by ER stress, indicating apoptosis in sea cucumbers is mediated by calcium imbalance and follows the activation of the ER stress pathway. Therefore, this study broadens understanding of the apoptotic mechanism involved in sea cucumber autolysis, which is helpful in developing preservative agents for sea cucumbers.

Transcriptome analysis reveals roles of polian vesicle in sea cucumber Apostichopus japonicus response to Vibrio splendidus infection

Polian vesicle is originally regarded as a hematopoietic and inflammatory response organ in sea cucumber by the operations of cell depletion and heterogeneous cells injection, respectively. In the present study, to reveal the role and immune mechanisms of polian vesicle in response to pathogen, Vibrio splendidus, we first performed a comparative transcriptome analysis for the cells from polian vesicle wall in V. splendidus-challenged Apostichopus japonicus through RNA high-throughput sequencing technology. Briefly, 465,356,848 clean reads were obtained after cleaning up low-quality reads in total. Approximately 73% of the sequenced reads could be aligned to the reference genome of A. japonicus. The DEGs of CG (control group) vs TG 24 h (24 h post-infection group), CG vs TG 72 h (72 h post-infection group) and TG 24 h vs TG 72 h were 3762, 1391 and 3258, respectively. Gene Ontology (GO) annotation assay revealed that those genes associated with the processes such as cell process, cell, binding and catalytic activity were significantly induced in all three groups post V. splendidus infection. KEGG enrichment analysis suggested the DEGs in TG 24 h were enriched in Toll-like receptor (TLR) signaling pathway, complement and coagulation cascades, antigen processing and presentation and IL-17 signaling pathway compared with that in CG, while the pathways including ribosome biogenesis in eukaryotes, DNA replication, and cell cycle related with cell proliferation were mainly enriched in TG 72 h than that of CG. Furthermore, six important DEGs were chosen and showed the consistent expression patterns with the results of RNA-seq by qPCR. Overall, our analysis towards the current data demonstrates that polian vesicle may play an essential role in the regulation of immune response in A. japonicus and provide new insights into hematopoietic function of polian vesicle in response to pathogen infection.

Stem Cells and Innate Immunity in Aquatic Invertebrates: Bridging Two Seemingly Disparate Disciplines for New Discoveries in Biology

The scopes related to the interplay between stem cells and the immune system are broad and range from the basic understanding of organism's physiology and ecology to translational studies, further contributing to (eco)toxicology, biotechnology, and medicine as well as regulatory and ethical aspects. Stem cells originate immune cells through hematopoiesis, and the interplay between the two cell types is required in processes like regeneration. In addition, stem and immune cell anomalies directly affect the organism's functions, its ability to cope with environmental changes and, indirectly, its role in ecosystem services. However, stem cells and immune cells continue to be considered parts of two branches of biological research with few interconnections between them. This review aims to bridge these two seemingly disparate disciplines towards much more integrative and transformative approaches with examples deriving mainly from aquatic invertebrates. We discuss the current understanding of cross-disciplinary collaborative and emerging issues, raising novel hypotheses and comments. We also discuss the problems and perspectives of the two disciplines and how to integrate their conceptual frameworks to address basic equations in biology in a new, innovative way.

Characterization of Coelomic Fluid Cell Types in the Starfish Marthasterias glacialis Using a Flow Cytometry/Imaging Combined Approach

Coelomocytes is the generic name for a collection of cellular morphotypes, present in many coelomate animals, and highly variable among echinoderm classes. The roles attributed to the major types of these free circulating cells present in the coelomic fluid of echinoderms include immune response, phagocytic digestion and clotting. Our main aim in this study was to characterize coelomocytes found in the coelomic fluid of Marthasterias glacialis (class Asteroidea) by using a combination of flow cytometry (FC), imaging flow cytometry (IFC) and fluorescence plus transmission electron microscopy (TEM). Two coelomocyte populations (P1 and P2) identified through flow cytometry were subsequently studied in terms of abundance, morphology, ultrastructure, cell viability and cell cycle profiles. Ultrastructurally, P2 diploid cells were present as two main morphotypes, similar to phagocytes and vertebrate thrombocytes, whereas the smaller P1 cellular population was characterized by low mitotic activity, a relatively undifferentiated cytotype and a high nucleus/cytoplasm ratio. In the present study we could not rule out possible similarities between haploid P1 cells and stem-cell types in other animals. Additionally, we report the presence of two other morphotypes in P2 that could only be detected by fluorescence microscopy, as well as a morphotype revealed via combined microscopy/FC. This integrative experimental workflow combined cells physical separation with different microscopic image capture technologies, enabling us to better tackle the characterization of the heterogeneous composition of coelomocytes populations.

Mannose-binding C-type lectins as defense molecules on the body surface of the sea urchin Pseudocentrotus depressus

We found that the extract of the body wall of the sea urchin, Pseudocentrotus depressus, agglutinate Escherichia coli and is inhibited by mannose. A mannose-binding protein of 22 kDa was purified via affinity chromatography using mannose-agarose. Amino acid sequences obtained by Edman degradation and liquid chromatography quadrupole time-of-flight mass spectrometry followed by de novo sequencing suggested that the protein is a C-type lectin. Products of PCR with a degenerate primer pair and of RACE PCR for the cDNA of the 22 kDa protein were sequenced and produced two full-length cDNA sequences encoding C-type lectins. These two lectins, named P. depressus mannose-binding C-type lectin (PdMBCL) 1 and 2 are composed of 187 and 189 amino acid residues, including signal peptides, respectively, and share 86% identity in their mature form. PdMBCLs agglutinated Lactococcus garvieae, a Gram-positive fish pathogen. Reverse transcription PCR showed that both the genes for the PdMBCLs were expressed in the body wall and in other tissues. Furthermore, the lectins were detected from a rinse of the body surface. Taken together, the present study showed that PdMBCLs function as anti-microbial agents on the body surface of P. depressus.

Transcriptome analysis of sea cucumber (Apostichopus japonicus) polian vesicles in response to evisceration

Polian vesicles are considered as the site of coelomocyte formation, and play crucial roles in the inflammatory reaction in sea cucumber. After evisceration, coelomocytes and internal organs except polian vesicles are excreted. Our previous study found that the total number of coelomocytes was rapidly recovered at 6 h post-evisceration in sea cucumber Apostichopus japonicus, and this regeneration of coelomocytes might be closely related to polian vesicles. To further investigate the related-gene expression pattern of the polian vesicles at 6 h post-evisceration, the transcriptome analysis of polian vesicles was carried out. A total of 2752 differentially expressed genes (DEGs) were identified, including 1,453 up-regulated genes and 1299 down-regulated genes. Gene Ontology (GO) enrichment showed that most of the DEGs were classified under Regulation of transcription, Regulation of RNA metabolic process, Regulation of nucleic acid-templated transcription. Meanwhile, 11 significantly enriched Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways were identified. Among them, Wnt, TGF-β and Endocytosis pathways are well-related with cell proliferation and differentiation, which may be involved in the regeneration of coelomocytes in A. japonicus after evisceration. In addition, FoxO signaling pathway plays important roles in immunoregulation, in which the expression levels of the DEGs were significantly up-regulated, inferring that polian vesicles could not only participate in the coelomocyte regeneration process, but also undertake a certain immune defense function in A. japonicus after evisceration. These findings will be beneficial for understanding the mechanisms of coelomocyte regeneration and immune defense of A. japonicus after evisceration.

Localization and characterization of hematopoietic tissues in adult sea cucumber, Apostichopus japonicus

Sea cucumber Apostichopus japonicus rely on the efficient innate immune mechanisms against invaders, in which the consumption and regeneration of coelomocytes take place at the same time. In the present study, histological features of putative hematopoietic tissues (HPTs) including the rete mirabile, the respiratory tree, the polian vesicle and the coelomic epithelium were characterized. The distribution of transcription factor GATA1 in coelomocytes and putative HPTs was examined by immunohistochemistry. In addition, cell proliferation using EdU labeling and coelomocyte distribution in different tissues using monoclonal antibody labeling were analyzed to further confirm the HPTs. The results showed that two homologs of GATA1 were detected with molecular weight of 43 and 90 kDa in coelomocytes, rete mirabile, respiratory tree and polian vesicle, whereas no signals were detected in the coelomic epithelium. A few cells were detected to be EdU-positive for coelomocytes, which accounted for approximately 9.5%. In the rete mirabile and the respiratory tree, the EdU signals were strong in cells of the tube wall. In the polian vesicle, numerous EdU-positive cells were detected in the cyst wall. In the coelomic epithelium, little EdU signaling was detected. Immunohistochemistry analysis by mAb 3F6 against A. japonicus coelomocytes showed that positive signals were observed in the tube wall of the rete mirabile, respiratory tree, cyst wall of the polian vesicle and in the coelomocyte antrum of coelomic epithelium. These results suggest that the rete mirabile, respiratory tree and polian vesicle are the HPTs of A. japonicus.