The toposome, essential for sea urchin cell adhesion and development, is a modified iron-less calcium-binding transferrin

Toposome: Source of antimicrobial molecules in the gonads of the sea urchin Lytechinus variegatus (Lamarck, 1816)

Sea urchins live in a challenging environment that requires rapid and efficient responses against pathogens and invaders. This response may be also important in reproductive processes once males and females release their gametes into water. In addition, the gonads are organs with dual function: reproductive organ and nutrient reserve, therefore it needs efficient protective mechanisms to preserve the nutrients as well as the reproductive cells. The aim of this study was to evaluate the presence and characterize antimicrobial molecules in the male and female gonads of the sea urchin Lytechinus variegatus. Through HPLC purification, antimicrobial activity test and mass spectrometry several antimicrobial molecules were found in the gonads of both gender. Computational in silico analyses showed that they are fragments of a glycoprotein called toposome, also known as major yolk protein (MYP) which is one of the major proteins found in the gonads. Although different functions have been reported for this protein, this is the first description of a direct antimicrobial activity in Lytechinus variegatus. The results indicate that when undergoing proteolysis the toposome generates different fragments with antimicrobial activity which may indicate the importance of a rapid defense response strategy against invading microorganisms in the gonads used by both males and females sea urchins.

Sea Urchin Extracellular Proteins Design a Complex Protein Corona on Titanium Dioxide Nanoparticle Surface Influencing Immune Cell Behavior

Extensive exploitation of titanium dioxide nanoparticles (TiO₂NPs) augments rapid release into the marine environment. When in contact with the body fluids of marine invertebrates, TiO₂NPs undergo a transformation and adhere various organic molecules that shape a complex protein corona prior to contacting cells and tissues. To elucidate the potential extracellular signals that may be involved in the particle recognition by immune cells of the sea urchin Paracentrotus lividus, we investigated the behavior of TiO₂NPs in contact with extracellular proteins in vitro. Our findings indicate that TiO₂NPs are able to interact with sea urchin proteins in both cell-free and cell-conditioned media. The two-dimensional proteome analysis of the protein corona bound to TiO₂NP revealed that negatively charged proteins bound preferentially to the particles. The main constituents shaping the sea urchin cell-conditioned TiO₂NP protein corona were proteins involved in cellular adhesion (Pl-toposome, Pl-galectin-8, Pl-nectin) and cytoskeletal organization (actin and tubulin). Immune cells (phagocytes) aggregated TiO₂NPs on the outer cell surface and within well-organized vesicles without eliciting harmful effects on the biological activities of the cells. Cells showed an active metabolism, no oxidative stress or caspase activation. These results provide a new level of understanding of the extracellular proteins involved in the immune-TiO₂NP recognition and interaction in vitro, confirming that primary immune cell cultures from P. lividus can be an optional model for swift and efficient immune-toxicological investigations.

Uranium Uptake in Paracentrotus lividus Sea Urchin, Accumulation and Speciation

Uranium speciation and bioaccumulation were investigated in the sea urchin Paracentrotus lividus. Through accumulation experiments in a well-controlled aquarium followed by ICP-OES analysis, the quantification of uranium in the different compartments of the sea urchin was performed. Uranium is mainly distributed in the test (skeletal components), as it is the major constituent of the sea urchin, but in terms of quantity of uranium per gram of compartment, the following rating: intestinal tract > gonads ≫ test, was obtained. Combining both extended X-ray Absorption Spectroscopy and time-resolved laser-induced fluorescence spectroscopic analysis, it was possible to identify two different forms of uranium in the sea urchin, one in the test, as a carbonato-calcium complex, and the second one in the gonads and intestinal tract, as a protein complex. Toposome is a major calcium-binding transferrin-like protein contained within the sea urchin. EXAFS data fitting of both contaminated organs in vivo and the uranium-toposome complex from protein purified out of the gonads revealed that it is suspected to complex uranium in gonads and intestinal tract. This hypothesis is also supported by the results from two imaging techniques, i.e., Transmission Electron Microscopy and Scanning Transmission X-ray Microscopy. This thorough investigation of uranium uptake in sea urchin is one of the few attempts to assess the speciation in a living marine organism in vivo.

Sea urchin Paracentrotus lividus immune cells in culture: formulation of the appropriate harvesting and culture media and maintenance conditions

The sea urchin is an emergent model system for studying basic and translational immunology. Here we report a new method for the harvesting and maintenance of primary immune cells isolated from adult Paracentrotus lividus, a common Mediterranean sea urchin species. This optimised method uses coelomocyte culture medium, containing a high-affinity Ca²⁺ chelator, as the ideal harvesting and anti-clotting vehicle and short-term culture medium (≤48 h), and artificial seawater as the master medium that maintains cell survival and in vitro-ex vivo physiological homeostasis over 2 weeks. Gradually reducing the amount of anticoagulant solution in the medium and regularly replacing the medium led to improved culture viability. Access to a robust and straightforward in vitro-ex vivo system will expedite our understanding of deuterostome immunity as well as underscore the potential of sea urchin with respect to biomedicine and regulatory testing.

This article has an associated First Person interview with the first author of the paper.

Summary: Appropriate culture methods for sea urchin immune cells provide an invaluable and amenable model for answering immunological questions while limiting the use of mammalian organisms.

Biotic and environmental stress induces nitration and changes in structure and function of the sea urchin major yolk protein toposome

The major yolk protein toposome plays crucial roles during gametogenesis and development of sea urchins. We previously found that nitration of toposome increases in the gonads of a Paracentrotus lividus population living in a marine protected area affected by toxic blooms of Ostreospsis cf. ovata, compared to control populations. This modification is associated with ovatoxin accumulation, high levels of nitric oxide in the gonads, and a remarkable impairment of progeny development. However, nothing is known about the environmental-mediated-regulation of the structure and biological function of toposome. Here, we characterize through wide-ranging biochemical and structural analyses the nitrated toposome of sea urchins exposed to the bloom, and subsequently detoxified. The increased number of nitrated tyrosines in toposome of sea urchins collected during algal bloom induced structural changes and improvement of the Ca²⁺-binding affinity of the protein. After 3 months’ detoxification, ovatoxin was undetectable, and the number of nitric oxide-modified tyrosines was reduced. However, the nitration of specific residues was irreversible and occurred also in embryos treated with metals, used as a proxy of environmental pollutants. The structural and functional changes of toposome caused by nitration under adverse environmental conditions may be related to the defective development of sea urchins’ progeny.

Subtle reproductive impairment through nitric oxide-mediated mechanisms in sea urchins from an area affected by harmful algal blooms

The health of the sea urchin Paracentrotus lividus, a key species in the Mediterranean Sea, is menaced by several pressures in coastal environments. Here, we aimed at assessing the reproductive ability of apparently healthy P. lividus population in a marine protected area affected by toxic blooms of Ostreospsis cf. ovata. Wide-ranging analyses were performed in animals collected prior to and during the bloom, as well as at several times thereafter, during the reproductive season. Adults showed a low fertilization rate, along with high nitric oxide (NO) levels in the gonads and the nitration of the major yolk protein toposome, which is an important player in sea urchin development. Serious developmental anomalies were observed in the progeny, which persist several months after the bloom. NO levels were high in the different developmental stages, which also showed variations in the transcription of several genes that were found to be directly or indirectly modulated by NO. These results highlight subtle but important reproductive flaws transmitted from the female gonads to the offspring with the NO involvement. Despite a recovery along time after the bloom, insidious damages can be envisaged in the local sea urchin population, with possible reverberation on the whole benthic system.

Deciphering the molecular mechanisms underlying sea urchin reversible adhesion: A quantitative proteomics approach

Marine bioadhesives have unmatched performances in wet environments, being an inspiration for biomedical applications. In sea urchins specialized adhesive organs, tube feet, mediate reversible adhesion, being composed by a disc, producing adhesive and de-adhesive secretions, and a motile stem. After tube foot detachment, the secreted adhesive remains bound to the substratum as a footprint. Sea urchin adhesive is composed by proteins and sugars, but so far only one protein, Nectin, was shown to be over-expressed as a transcript in tube feet discs, suggesting its involvement in sea urchin adhesion. Here we use high-resolution quantitative mass-spectrometry to perform the first study combining the analysis of the differential proteome of an adhesive organ, with the proteome of its secreted adhesive. This strategy allowed us to identify 163 highly over-expressed disc proteins, specifically involved in sea urchin reversible adhesion; to find that 70% of the secreted adhesive components fall within five protein groups, involved in exocytosis and microbial protection; and to provide evidences that Nectin is not only highly expressed in tube feet discs but is an actual component of the adhesive. These results give an unprecedented insight into the molecular mechanisms underlying sea urchin adhesion, and opening new doors to develop wet-reliable, reversible, and ecological biomimetic adhesives.

SIGNIFICANCE

Sea urchins attach strongly but in a reversible manner to substratum, being a valuable source of inspiration for industrial and biomedical applications. Yet, the molecular mechanisms governing reversible adhesion are still poorly studied delaying the engineering of biomimetic adhesives. We used the latest mass spectrometry techniques to analyze the differential proteome of an adhesive organ and the proteome of its secreted adhesive, allowing us to uncover the key players in sea urchin reversible adhesion. We demonstrate, that Nectin, a protein previously pointed out as potentially involved in sea urchin adhesion, is not only highly expressed in tube feet discs, but is a genuine component of the secreted adhesive.

Primary cell cultures from sea urchin ovaries: a new experimental tool

In the present work, primary cell cultures from ovaries of the edible sea urchin Paracentrotus lividus were developed in order to provide a simple and versatile experimental tool for researches in echinoderm reproductive biology. Ovary cell phenotypes were identified and characterized by different microscopic techniques. Although cell cultures could be produced from ovaries at all stages of maturation, the cells appeared healthier and viable, displaying a higher survival rate, when ovaries at early stages of gametogenesis were used. In terms of culture medium, ovarian cells were successfully cultured in modified Leibovitz-15 medium, whereas poor results were obtained in minimum essential medium Eagle and medium 199. Different substrates were tested, but ovarian cells completely adhered only on poly-L-lysine. To improve in vitro conditions and stimulate cell proliferation, different serum-supplements were tested. Fetal calf serum and an originally developed pluteus extract were detrimental to cell survival, apparently accelerating processes of cell death. In contrast, cells cultured with sea urchin egg extract appeared larger and healthier, displaying an increased longevity that allowed maintaining them for up to 1 month. Overall, our study provides new experimental bases and procedures for producing successfully long-term primary cell cultures from sea urchin ovaries offering a good potential to study echinoid oogenesis in a controlled system and to investigate different aspects of echinoderm endocrinology and reproductive biology.

Evolution of yolk protein genes in the Echinodermata

Vitellogenin genes (vtg) encode large lipid transfer proteins (LLTPs) that are typically female-specific, functioning as precursors to major yolk proteins (MYPs). Within the phylum Echinodermata, however, the MYP of the Echinozoa (Echinoidea + Holothuroidea) is expressed by an unrelated transferrin-like gene that has a reproductive function in both sexes. We investigated egg proteins in the Asterozoa (Asteroidea + Ophiuroidea), a sister clade to the Echinozoa, showing that eggs of the asteroid Parvulastra exigua contain a vitellogenin protein (Vtg). vtg is expressed by P. exigua, a species with large eggs and nonfeeding larvae, and by the related asterinid Patiriella regularis which has small eggs and feeding larvae. In the Asteroidea, therefore, the reproductive function of vtg is conserved despite significant life history evolution. Like the echinozoan MYP gene, asteroid vtg is expressed in both sexes and may play a role in the development of both ovaries and testes. Phylogenetic analysis indicated that a putative Vtg from the sea urchin genome, a likely pseudogene, does not clade with asteroid Vtg. We propose the following sequence as a potential pathway for the evolution of YP genes in the Echinodermata: (1) the ancestral echinoderm produced YPs derived from Vtg, (2) bisexual vtg expression subsequently evolved in the echinoderm lineage, (3) the reproductive function of vtg was assumed by a transferrin-like gene in the ancestral echinozoan, and (4) redundant echinozoan vtg was released from stabilizing selection.

Proteomic profiles reveal age-related changes in coelomic fluid of sea urchin species with different life spans

Sea urchins have a different life history from humans and traditional model organisms used to study the process of aging. Sea urchins grow indeterminately, reproduce throughout their life span and some species have been shown to exhibit negligible senescence with no increase in mortality rate at advanced ages. Despite these properties, different species of sea urchins are reported to have very different natural life spans providing a unique model to investigate cellular mechanisms underlying life span determination and negligible senescence. To gain insight into the biological changes that accompany aging in these animals, proteomic profiles were examined in coelomic fluid from young and old sea urchins of three species with different life spans: short-lived Lytechinus variegatus, long-lived Strongylocentrotus franciscanus and Strongylocentrotus purpuratus which has an intermediate life span. The proteomic profiles of cell-free coelomic fluid were complex with many proteins exhibiting different forms and extensive post-translational modifications. Approximately 20% of the protein spots on 2-D gels showed more than two-fold change with age in each of the species. Changes that are consistent with age in all three species may prove to be useful biomarkers for age-determination for these commercially fished marine invertebrates and also may provide clues to mechanisms of negligible senescence. Among the proteins that change with age, the ectodomain of low-density lipoprotein receptor-related protein 4 (LRP4) was significantly increased in the coelomic fluid of all three sea urchin species suggesting that the Wnt signaling pathway should be further investigated for its role in negligible senescence.

Exploration of the sea urchin coelomic fluid via combinatorial peptide ligand libraries

The urchin Paracentrotus lividus has been characterized via previous capture and enhancement of low-abundance proteins with combinatorial peptide ligand libraries (CPLL, ProteoMiner). Whereas in the control only 26 unique gene products could be identified, 82 species could be detected after CPLL treatment. Due to the overwhelming presence of two major proteins-the toposome (a highly glycosylated, modified calcium-binding, iron-less transferrin) and the major yolk proteins, belonging to the class of cell adhesion proteins-which constituted about 70% of the proteome of this biological fluid and strongly interfered with the capture of the minority proteome, no additional proteins could be detected. Yet, at present, this constitutes the most thorough investigation of the proteome of this biological fluid.

Beyond bilobal: transferrin homologs having unusual domain architectures

BACKGROUND

Most transferrin family proteins have a familiar bilobal structure, the result of an ancient gene duplication, with an iron binding site in each of two homologous lobes. Scattered throughout the evolutionary tree from algae to mammals, though, are transferrin homologs having other kinds of domain architectures.

SCOPE OF REVIEW

This review covers a variety of unusual transferrin forms, including monolobals, bilobals with one or both iron-binding sites abrogated, bilobals accessorized with long insertions or with membrane anchors, and even trilobals. The monolobal transferrin homologs from marine invertebrate ascidians are especially highlighted here.

MAJOR CONCLUSIONS

Unusual transferrin homologs appear scattered through much of the evolutionary tree. For some of these proteins, iron binding and/or iron transport appear to be the primary roles; for others they clearly are not. Many are incompletely or not at all studied.

GENERAL SIGNIFICANCE

Taken together, these proteins begin to offer a glimpse into how the transferrin architecture has been repurposed for a diversity of applications. This article is part of a Special Issue entitled Transferrins: Molecular mechanisms of iron transport and disorders.

Accumulation of the major yolk protein and zinc in the agametogenic sea urchin gonad

Sea urchins of both sexes store the nutrients necessary for gametogenesis in nutritive phagocytes of the agametogenic gonad. A zinc-binding protein termed the major yolk protein (MYP) is stored here as two isoforms: the egg-type (predominant in egg yolk granules) and the coelomic fluid-type (a precursor with greater zinc-binding capacity). MYP is used during gametogenesis as material for synthesizing gametic proteins and other components. We investigated its accumulation and relationship to zinc contents in gonads during the non-reproductive season in Pseudocentrotus depressus. MYP constituted most of the protein in coelomic fluid and gonads. Both ovaries and testes grew gradually, accumulating MYP and zinc during the year. Total zinc contents and the ratio of coelomic fluid-type to egg-type protein were higher in ovaries than in testes as gametogenesis approached. Most of the zinc in the coelomic fluid was bound to MYP, and the concentrations of MYP and zinc were elevated toward the onset of oogenesis in the female coelomic fluid. Thus, MYP accumulates in the agametogenic ovaries and testes during the non-reproductive season, playing a role as a carrier to transport zinc to the gonad. Transportation of zinc by MYP is more active in females than in males.

X-ray structures of transferrins and related proteins

BACKGROUND

Transferrins are a group of iron-binding proteins including serum transferrin, lactoferrin and ovotransferrin.

SCOPE OF REVIEW

The structures of transferrins are discussed.

GENERAL SIGNIFICANCE

The typical transferrin molecules are folded into two homologous lobes. X-ray crystallography revealed that each lobe is further divided into two similarly sized domains, and that an iron-binding site is contained within the inter-domain cleft. The six iron coordination sites are occupied by four residues and a bidentate carbonate anion.

MAJOR CONCLUSIONS

The structures of the apo- and holo-forms revealed that the transferrins undergo a large-scale conformational change upon the uptake and release of irons: domains rotate as rigid bodies around a screw axis passing through inter-domain contacts. The iron-release mechanism of transferrin N-lobe is also revealed by X-ray crystallography; two basic residues in two domains form an unusual hydrogen bond in neutral pH, and the bond should be broken and facilitate iron release at a low pH of the endosome. For ovotransferrin, the iron release kinetics of two lobes correspond well with the numbers of anion binding sites found in crystal structures. The structures of transferrins bound to other metals revealed that the flexibility of the transferrin structure allows the ability to bind to other metals. This article is part of a Special Issue entitled Transferrins: Molecular mechanisms of iron transport and disorders.

Molecular evolution of the transferrin family and associated receptors

BACKGROUND

In vertebrates, serum transferrins are essential iron transporters that have bind and release Fe(III) in response to receptor binding and changes in pH. Some family members such as lactoferrin and melanotransferrin can also bind iron while others have lost this ability and have gained other functions, e.g., inhibitor of carbonic anhydrase (mammals), saxiphilin (frogs) and otolith matrix protein 1 (fish).

SCOPE OF REVIEW

This article provides an overview of the known transferrin family members and their associated receptors and interacting partners.

MAJOR CONCLUSIONS

The number of transferrin genes has proliferated as a result of multiple duplication events, and the resulting paralogs have developed a wide array of new functions. Some homologs in the most primitive metazoan groups resemble both serum and melanotransferrins, but the major yolk proteins show considerable divergence from the rest of the family. Among the transferrin receptors, the lack of TFR2 in birds and reptiles, and the lack of any TFR homologs among the insects draw attention to the differences in iron transport and regulation in those groups.

GENERAL SIGNIFICANCE

The transferrin family members are important because of their clinical significance, interesting biochemical properties, and evolutionary history. More work is needed to better understand the functions and evolution of the non-vertebrate family members. This article is part of a Special Issue entitled Molecular Mechanisms of Iron Transport and Disorders.

Sea urchin eggs in the acid reign

Sea urchin eggs have been an indispensable model system for studying egg activation and ionic signalling for at least a century. Instrumental in the discovery of two Ca(2+)-mobilizing second messengers, cyclic ADP-ribose and NAADP, the sea urchin has revolutionized cell biology for all phyla. This review attempts to summarize what we currently know about egg acidic vesicles in the context of Ca(2+) signalling. The dynamics of Ca(2+) storage, Ca(2+) mobilization, proton fluxes and two-pore channels will be discussed.

The major yolk protein is synthesized in the digestive tract and secreted into the body cavities in sea urchin larvae

Major yolk protein (MYP), a transferrin superfamily protein contained in yolk granules of sea urchin eggs, also occurs in the coelomic fluid of male and female adult sea urchins regardless of their reproductive cycle. MYP in the coelomic fluid (CFMYP; 180 kDa) has a zinc-binding capacity and has a higher molecular mass than MYP in eggs (EGMYP; 170 kDa). CFMYP is thought to be synthesized in the digestive tract and secreted into the coelomic fluid where it is involved in the transport of zinc derived from food. To clarify when and where MYP synthesis starts, we investigated the expression of MYP during larval development and growth in Pseudocentrotus depressus. MYP mRNA was detected using RT-PCR in the early 8-arm pluteus stage and its expression persisted until after metamorphosis. Real-time RT-PCR revealed that MYP mRNA increased exponentially from the early 8-arm stage to metamorphosis. Western blotting showed that maternal EGMYP disappeared by the 4-arm stage and that newly synthesized CFMYP was present at and after the mid 8-arm stage. In the late 8-arm larvae, MYP mRNA was detected in the digestive tract using in situ hybridization, and the protein was found in the somatocoel and the blastocoel-derived space between the somatocoel and epidermis using immunohistochemistry. These results suggest that CFMYP is synthesized in the digestive tract and secreted into the body cavities at and after the early 8-arm stage. We assume that in larvae, CFMYP transports zinc derived from food via the body cavities to various tissues, as suggested for adults.