Peptide receptors and immune-related proteins expressed in the digestive system of a urochordate, Ciona intestinalis

A Novel Hemocyte-Derived Peptide and Its Possible Roles in Immune Response of Ciona intestinalis Type A

A wide variety of bioactive peptides have been identified in the central nervous system and several peripheral tissues in the ascidian Ciona intestinalis type A (Ciona robusta). However, hemocyte endocrine peptides have yet to be explored. Here, we report a novel 14-amino-acid peptide, CiEMa, that is predominant in the granular hemocytes and unilocular refractile granulocytes of Ciona. RNA-seq and qRT-PCR revealed the high CiEma expression in the adult pharynx and stomach. Immunohistochemistry further revealed the highly concentrated CiEMa in the hemolymph of the pharynx and epithelial cells of the stomach, suggesting biological roles in the immune response. Notably, bacterial lipopolysaccharide stimulation of isolated hemocytes for 1-4 h resulted in 1.9- to 2.4-fold increased CiEMa secretion. Furthermore, CiEMa-stimulated pharynx exhibited mRNA upregulation of the growth factor (Fgf3/7/10/22), vanadium binding proteins (CiVanabin1 and CiVanabin3), and forkhead and homeobox transcription factors (Foxl2, Hox3, and Dbx) but not antimicrobial peptides (CrPap-a and CrMam-a) or immune-related genes (Tgfbtun3, Tnfa, and Il17-2). Collectively, these results suggest that CiEMa plays roles in signal transduction involving tissue development or repair in the immune response, rather than in the direct regulation of immune response genes. The present study identified a novel Ciona hemocyte peptide, CiEMa, which paves the way for research on the biological roles of hemocyte peptides in chordates.

Ciona spp. and ascidians as bioindicator organisms for evaluating effects of endocrine disrupting chemicals: A discussion paper

In context of testing, screening and monitoring of endocrine-disrupting (ED) type of environmental pollutants, tunicates could possibly represent a particularly interesting group of bioindicator organisms. These primitive chordates are already important model organisms within developmental and genomics research due to their central position in evolution and close relationship to vertebrates. The solitary ascidians, such as the genus Ciona spp. (vase tunicates), could possibly be extra feasible as ED bioindicators. They have a free-swimming, tadpole-like larval stage that develops extremely quickly (<20 h under favorable conditions), has a short life cycle (typically 2-3 months), are relatively easy to maintain in laboratory culture, have fully sequenced genomes, and transgenic embryos with 3D course data of the embryo ontogeny are available. In this article, we discuss possible roles of Ciona spp. (and other solitary ascidians) as ecotoxicological bioindicator organisms in general but perhaps especially for effect studies of contaminants with presumed endocrine disrupting modes of action.

Kobayashi Award 2021: Neuropeptides, receptors, and follicle development in the ascidian, Ciona intestinalis Type A: New clues to the evolution of chordate neuropeptidergic systems from biological niches

Ciona intestinalis Type A (Ciona robusta) is a cosmopolitan species belonging to the phylum Urochordata, invertebrate chordates that are phylogenetically the most closely related to the vertebrates. Therefore, this species is of interest for investigation of the evolution and comparative physiology of endocrine, neuroendocrine, and nervous systems in chordates. Our group has identified>30 Ciona neuropeptides (80% of all identified ascidian neuropeptides) primarily using peptidomic approaches combined with reference to genome sequences. These neuropeptides are classified into two groups: homologs or prototypes of vertebrate neuropeptides and novel (Ciona-specific) neuropeptides. We have also identified the cognate receptors for these peptides. In particular, we elucidated multiple receptors for Ciona-specific neuropeptides by a combination of a novel machine learning system and experimental validation of the specific interaction of the predicted neuropeptide-receptor pairs, and verified unprecedented phylogenies of receptors for neuropeptides. Moreover, several neuropeptides were found to play major roles in the regulation of ovarian follicle development. Ciona tachykinin facilitates the growth of vitellogenic follicles via up-regulation of the enzymatic activities of proteases. Ciona vasopressin stimulates oocyte maturation and ovulation via up-regulation of maturation-promoting factor- and matrix metalloproteinase-directed collagen degradation, respectively. Ciona cholecystokinin also triggers ovulation via up-regulation of receptor tyrosine kinase signaling and the subsequent activation of matrix metalloproteinase. These studies revealed that the neuropeptidergic system plays major roles in ovarian follicle growth, maturation, and ovulation in Ciona, thus paving the way for investigation of the biological roles for neuropeptides in the endocrine, neuroendocrine, nervous systems of Ciona, and studies of the evolutionary processes of various neuropeptidergic systems in chordates.

Impact of Machine Learning-Associated Research Strategies on the Identification of Peptide-Receptor Interactions in the Post-Omics Era

BACKGROUNDS

Elucidation of peptide-receptor pairs is a prerequisite for many studies in the neuroendocrine, endocrine, and neuroscience fields. Recent omics analyses have provided vast amounts of peptide and G protein-coupled receptor (GPCR) sequence data. GPCRs for homologous peptides are easily characterized based on homology searching, and the relevant peptide-GPCR interactions are also detected by typical signaling assays. In contrast, conventional evaluation or prediction methods, including high-throughput reverse-pharmacological assays and tertiary structure-based computational analyses, are not useful for identifying interactions between novel and omics-derived peptides and GPCRs.

SUMMARY

Recently, an approach combining machine learning-based prediction of novel peptide-GPCR pairs and experimental validation of the predicted pairs have been shown to breakthrough this bottleneck. A machine learning method, logistic regression for human class A GPCRs and the multiple subsequent signaling assays led to the deorphanization of human class A orphan GPCRs, namely, the identification of 18 peptide-GPCR pairs. Furthermore, using another machine learning algorithm, the support vector machine (SVM), the peptide descriptor-incorporated SVM was originally developed and employed to predict GPCRs for novel peptides characterized from the closest relative of vertebrates, Ciona intestinalis Type A (Ciona robusta). Experimental validation of the predicted pairs eventually led to the identification of 11 novel peptide-GPCR pairs. Of particular interest is that these newly identified GPCRs displayed neither significant sequence similarity nor molecular phylogenetic relatedness to known GPCRs for peptides.

KEY MESSAGES

These recent studies highlight the usefulness and versatility of machine learning for enabling the efficient, reliable, and systematic identification of novel peptide-GPCR interactions.

Micro and Nano Plastics Distribution in Fish as Model Organisms: Histopathology, Blood Response and Bioaccumulation in Different Organs

Micro- and nano-plastic (MP/NP) pollution represents a threat not only to marine organisms and ecosystems, but also a danger for humans. The effects of these small particles resulting from the fragmentation of waste of various types have been well documented in mammals, although the consequences of acute and chronic exposure are not fully known yet. In this review, we summarize the recent results related to effects of MPs/NPs in different species of fish, both saltwater and freshwater, including zebrafish, used as model organisms for the evaluation of human health risk posed by MNPs. The expectation is that discoveries made in the model will provide insight regarding the risks of plastic particle toxicity to human health, with a focus on the effect of long-term exposure at different levels of biological complexity in various tissues and organs, including the brain. The current scientific evidence shows that plastic particle toxicity depends not only on factors such as particle size, concentration, exposure time, shape, and polymer type, but also on co-factors, which make the issue extremely complex. We describe and discuss the possible entry pathways of these particles into the fish body, as well as their uptake mechanisms and bioaccumulation in different organs and the role of blood response (hematochemical and hematological parameters) as biomarkers of micro- and nano-plastic water pollution.

Sensing the world and its dangers: An evolutionary perspective in neuroimmunology

Detecting danger is key to the survival and success of all species. Animal nervous and immune systems cooperate to optimize danger detection. Preceding studies have highlighted the benefits of bringing neurons into the defense game, including regulation of immune responses, wound healing, pathogen control, and survival. Here, we summarize the body of knowledge in neuroimmune communication and assert that neuronal participation in the immune response is deeply beneficial in each step of combating infection, from inception to resolution. Despite the documented tight association between the immune and nervous systems in mammals or invertebrate model organisms, interdependence of these two systems is largely unexplored across metazoans. This review brings a phylogenetic perspective of the nervous and immune systems in the context of danger detection and advocates for the use of non-model organisms to diversify the field of neuroimmunology. We identify key taxa that are ripe for investigation due to the emergence of key evolutionary innovations in their immune and nervous systems. This novel perspective will help define the primordial principles that govern neuroimmune communication across taxa.

Reflections on the Use of an Invertebrate Chordate Model System for Studies of Gut Microbial Immune Interactions

The functional ecology of the gastrointestinal tract impacts host physiology, and its dysregulation is at the center of various diseases. The immune system, and specifically innate immunity, plays a fundamental role in modulating the interface of host and microbes in the gut. While humans remain a primary focus of research in this field, the use of diverse model systems help inform us of the fundamental principles legislating homeostasis in the gut. Invertebrates, which lack vertebrate-style adaptive immunity, can help define conserved features of innate immunity that shape the gut ecosystem. In this context, we previously proposed the use of a marine invertebrate, the protochordate Ciona robusta, as a novel tractable model system for studies of host-microbiome interactions. Significant progress, reviewed herein, has been made to fulfill that vision. We examine and review discoveries from Ciona that include roles for a secreted immune effector interacting with elements of the microbiota, as well as chitin-rich mucus lining the gut epithelium, the gut-associated microbiome of adults, and the establishment of a large catalog of cultured isolates with which juveniles can be colonized. Also discussed is the establishment of methods to rear the animals germ-free, an essential technology for dissecting the symbiotic interactions at play. As the foundation is now set to extend these studies into the future, broadening our comprehension of how host effectors shape the ecology of these microbial communities in ways that establish and maintain homeostasis will require full utilization of "multi-omics" approaches to merge computational sciences, modeling, and experimental biology in hypothesis-driven investigations.

Invertebrate Gonadotropin-Releasing Hormone Receptor Signaling and Its Relevant Biological Actions

Gonadotropin-releasing hormones (GnRHs) play pivotal roles in reproduction via the hypothalamus-pituitary-gonad axis (HPG axis) in vertebrates. GnRHs and their receptors (GnRHRs) are also conserved in invertebrates lacking the HPG axis, indicating that invertebrate GnRHs do not serve as “gonadotropin-releasing factors” but, rather, function as neuropeptides that directly regulate target tissues. All vertebrate and urochordate GnRHs comprise 10 amino acids, whereas amphioxus, echinoderm, and protostome GnRH-like peptides are 11- or 12-residue peptides. Intracellular calcium mobilization is the major second messenger for GnRH signaling in cephalochordates, echinoderms, and protostomes, while urochordate GnRHRs also stimulate cAMP production pathways. Moreover, the ligand-specific modulation of signal transduction via heterodimerization between GnRHR paralogs indicates species-specific evolution in Ciona intestinalis. The characterization of authentic or putative invertebrate GnRHRs in various tissues and their in vitro and in vivo activities indicate that invertebrate GnRHs are responsible for the regulation of both reproductive and nonreproductive functions. In this review, we examine our current understanding of and perspectives on the primary sequences, tissue distribution of mRNA expression, signal transduction, and biological functions of invertebrate GnRHs and their receptors.

An indoor study of the combined effect of industrial pollution and turbulence events on the gut environment in a marine invertebrate

Natural storms are able to determine reworking of seabed up to considerable depths and favour suspension of sediment-associated chemicals. Yet, a direct link between exposure to resuspended contaminants and the biological effects on marine organisms have to be fully established. We exposed adults of a suspension feeder, the ascidian Ciona robusta, to polluted sediment (e.g., containing mixtures of polycyclic aromatic hydrocarbons and heavy metals) from the industrial area of Bagnoli-Coroglio under two temporal patterns ('aggregated' vs. 'spaced') of turbulence events. Then, we assessed the impact of resuspended pollutants on the ascidian gut environment via four broad categories: oxidative stress, innate immunity, host-microbiota interactions, and epithelium. An early oxidative stress response was seen after a week of exposure to static sediment. Instead, water turbulence had no effect on the antioxidant defence. The first episode of turbulent suspension induced a minimal pro-inflammatory response in the 'spaced' pattern. Mucus overproduction and a complete occlusion of the crypt lumen were found following sediment reworking. This study suggests a protective response of the gut environment in marine invertebrates exposed to environmental extremes, leading to increased susceptibility to disease and to concerns on the combined effects of chronic environmental contamination and acute disturbance events possibly associated with climate change.

The ventral peptidergic system of the adult ascidian Ciona robusta (Ciona intestinalis Type A) insights from a transgenic animal model

Ascidians are the sister group of vertebrates and occupy a critical position in explorations of the evolution of the endocrine and nervous systems of chordates. Here, we describe the complete ventral peptidergic system in adult transgenic Ciona robusta (Ciona intestinalis Type A) which expresses the Kaede reporter gene driven by the prohormone convertase 2 (PC2) gene promoter. Numerous PC2 promoter-driven fluorescent (Kaede-positive) non-neural cells were distributed in the blood sinus located at the anterior end of the pharynx, suggesting the acquisition of a peptidergic circulatory system in Ciona. Kaede-positive ciliated columnar cells, rounded cells, and tall ciliated cells were observed in the alimentary organs, including the endostyle, pharynx, esophagus, stomach, and intestine, suggesting that digestive functions are regulated by multiple peptidergic systems. In the heart, Kaede-positive neurons were located in the ring-shaped plexus at both ends of the myocardium. Nerve fiber-like tracts ran along the raphe and appeared to be connected with the plexuses. Such unique structures suggest a role for the peptidergic system in cardiac function. Collectively, the present anatomic analysis revealed the major framework of the ventral peptidergic system of adult Ciona, which could facilitate investigations of peptidergic regulation of the pharynx, endostyle, alimentary tissues, and heart.

Assessing Immunological Memory in the Solitary Ascidian Ciona robusta

The immune defensive mechanisms active in the solitary ascidian Ciona robusta include phagocytic and encapsulating activity, largely brought about by phagocytic cells within the haemocyte population, the presence of complement components, which have been molecularly and functionally identified, and expression of a number of immune-related genes and pathways, identified by genome-based homology with vertebrate counterparts. Since C. robusta only displays highly conserved innate immune mechanisms, being devoid of an adaptive immune system, this organism is an excellent model for studying the features of innate memory, i.e., the capacity of the innate immune system to re-programming its responsiveness to potentially dangerous agents upon repeated exposure. In this study, we have developed an in vivo model for assessing the establishment and molecular/functional features of innate memory, by sequentially exposing C. robusta to a priming stimulus (microbial molecules), followed by a period of resting to return to basal conditions, and a challenge with microbial agents in homologous or cross-stimulation. The endpoints of immune activation were a functional activity (phagocytosis) and the molecular profiles of immune-related gene expression. The results show that exposure of C. robusta to microbial agents induces a reaction that primes animals for developing a different (expectedly more protective) response to subsequent challenges, showing the effective establishment of an immune memory. This immune memory relies on the modulation of a number of different mechanisms, some of which are priming-specific, others that are challenge-specific, and others that are non-specific, i.e., are common to all priming/challenge combinations (e.g., up-regulation of the Tnf and Lbp genes). Memory-dependent expression of the humoral immunity-related gene C3ar inversely correlates with memory-dependent variations of phagocytic rate, suggesting that complement activation and phagocytosis are alternative defensive mechanisms in C. robusta. Conversely, memory-dependent expression of the cellular immunity-related gene Cd36 directly correlates with variations of phagocytic rate, suggesting a direct involvement of this gene in the functional regulation of phagocytosis.