SpCoel1: a sea urchin profilin gene expressed specifically in coelomocytes in response to injury

Coelomocyte populations in the sea urchin, Strongylocentrotus purpuratus, undergo dynamic changes in response to immune challenge

The sea urchin, Strongylocentrotus purpuratus has seven described populations of distinct coelomocytes in the coelomic fluid that are defined by morphology, size, and for some types, by known functions. Of these subtypes, the large phagocytes are thought to be key to the sea urchin cellular innate immune response. The concentration of total coelomocytes in the coelomic fluid increases in response to pathogen challenge. However, there is no quantitative analysis of how the respective coelomocyte populations change over time in response to immune challenge. Accordingly, coelomocytes collected from immunoquiescent, healthy sea urchins were evaluated by flow cytometry for responses to injury and to challenge with either heat-killed Vibrio diazotrophicus, zymosan A, or artificial coelomic fluid, which served as the vehicle control. Responses to the initial injury of coelomic fluid collection or to injection of V. diazotrophicus show significant increases in the concentration of large phagocytes, small phagocytes, and red spherule cells after one day. Responses to zymosan A show decreases in the concentration of large phagocytes and increases in the concentration of small phagocytes. In contrast, responses to injections of vehicle result in decreased concentration of large phagocytes. When these changes in coelomocytes are evaluated based on proportions rather than concentration, the respective coelomocyte proportions are generally maintained in response to injection with V. diazotrophicus and vehicle. However, this is not observed in response to zymosan A and this lack of correspondence between proportions and concentrations may be an outcome of clearing these large particles by the large phagocytes. Variations in coelomocyte populations are also noted for individual sea urchins evaluated at different times for their responses to immune challenge compared to the vehicle. Together, these results demonstrate that the cell populations in sea urchin immune cell populations undergo dynamic changes in vivo in response to distinct immune stimuli and to injury and that these changes are driven by the responses of the large phagocyte populations.

A flow cytometry based approach to identify distinct coelomocyte subsets of the purple sea urchin, Strongylocentrotus purpuratus

The sea urchin, Strongylocentrotus purpuratus, possesses at least seven distinguishable cell populations in the coelomic fluid, which vary in morphology, size, and function. Of these, the large phagocytes, small phagocytes, and red spherule cells are thought to be key to the echinoid immune response. Because there are currently no effective and rapid means of evaluating sea urchin coelomocytes, we developed a flow cytometry based approach to identify these subsets from unseparated, unstained, live cells. In particular our gating strategy distinguishes between the large phagocytes, small phagocytes, red spherule cells, and a mixed population of vibratile cells and colorless spherule cells. This flow cytometry based analysis increases the speed and improves the reliability of coelomocyte analysis compared to differential cell counts by microscopy.

Comparative study of coelomocytes from Arbacia lixula and Lythechinus variegatus: Cell characterization and in vivo evidence of the physiological function of vibratile cells

The knowledge on echinoderm coelomocytes has increased in recent years, but researchers still face a complex problem: how to obtain purified cells. Even flow cytometry being useful to address coelomocytes in suspension, the need for a method able to provide isolated cells is still noteworthy. Here, we use Imaging Flow Cytometry (IFC) to characterize the coelomocytes of two sea urchin species - Arbacia lixula and Lytechinus variegatus - and obtain gates to isolate cell populations. Then, we used these gates to study the physiological response of A. lixula coelomocytes during an induced immune challenge with Escherichia coli. An analysis of area and aspect ratio parameters of the flow cytometer allowed the identification of two main cell populations in the coelomic fluid: circular and elongated cells. A combination of this method with nucleus labeling using propidium iodide allowed the determination of gates containing isolated subpopulations of vibratile cells, red spherulocytes, and two phagocytes subpopulations in both species. We observed that during an induced bacterial immune challenge, A. lixula was able to modulate coelomocyte frequencies, increasing the phagocytes and decreasing red spherulocytes and vibratile cells. These results indicate that vibratile cells and red spherulocytes act by immobilizing and stoping bacterial growth, respectively, cooperating with phagocytes in the immune response. The use of IFC was fundamental not only to identify specific gates for the main coelomic subpopulations but also allowed the investigation on how echinoids modulate their physiological responses during immune challenges. Furthermore, we provide the first experimental evidence about the role of vibratile cells, corroborating its involvement with the immune system.

Methods for collection, handling, and analysis of sea urchin coelomocytes

Sea urchin coelomocytes can be collected in large numbers from adult sea urchins of the species, Strongylocentrotus purpuratus, which typically has 12-40mL of coelomic fluid. Coelomocytes are used for analysis of immune reactions and immune gene expression in addition to basic functions of cells, in particular for understanding structure and modifications of the cytoskeleton in phagocytes. The methods described here include coelomocyte isolation, blocking the clotting reaction, establishing and maintaining primary cultures, separation of different types of coelomocytes into fractions, processing live coelomocytes for light microscopy, fixation and staining for light and electron microscopy, analysis of coelomocyte populations by flow cytometry, and sorting single cells for more detailed follow-up analyses including transcriptomics or genomic characteristics. These methods are provided to make working with coelomocytes accessible to researchers who are unfamiliar with these cells and perhaps to aid others who have worked extensively with invertebrate cells.

Lectins identify distinct populations of coelomocytes in Strongylocentrotus purpuratus

Coelomocytes represent the immune cells of echinoderms, but detailed knowledge about their roles during immune responses is very limited. One major challenge for studying coelomocyte biology is the lack of reagents to identify and purify distinct populations defined by objective molecular markers rather than by morphology-based classifications that are subjective at times. Glycosylation patterns are known to differ significantly between cell types in vertebrates, and furthermore they can vary depending on the developmental stage and activation states within a given lineage. Thus fluorescently labeled lectins that recognize distinct glycan structures on cell surface proteins are routinely used to identify discrete cell populations in the vertebrate immune system. Here we now employed a panel of fifteen fluorescently-labeled lectins to determine differences in the glycosylation features on the surface of Strongylocentrotus purpuratus coelomocytes by fluorescence microscopy and flow cytometry. Eight of the lectins (succinylated wheat germ agglutinin, Len culinaris lectin, Pisum sativum agglutinin, Saphora japonica agglutinin, Solanum tuberosum lectin, Lycopersicon esculentum lectin, Datura stramonium lectin, Vicia villosa lectin) showed distinct binding patterns to fixed and live cells of three major coelomocyte classes: phagocytic cells, red spherule cells, and vibratile cells. Importantly, almost all lectins bound only to a subgroup of cells within each cell type. Lastly, we established fluorescently-labeled lectin-based fluorescence activated cell sorting as a strategy to purify distinct S. purpuratus coelomocyte (sub-)populations based on molecular markers. We anticipate that this will become a routine approach in future studies focused on dissecting the roles of different coelomocytes in echinoderm immunity.

Extraordinary Diversity of Immune Response Proteins among Sea Urchins: Nickel-Isolated Sp185/333 Proteins Show Broad Variations in Size and Charge

Effective protection against pathogens requires the host to produce a wide range of immune effector proteins. The Sp185/333 gene family, which is expressed by the California purple sea urchin Strongylocentrotus purpuratus in response to bacterial infection, encodes a highly diverse repertoire of anti-pathogen proteins. A subset of these proteins can be isolated by affinity to metal ions based on multiple histidines, resulting in one to four bands of unique molecular weight on standard Western blots, which vary depending on the individual sea urchin. Two dimensional gel electrophoresis (2DE) of nickel-isolated protein samples followed by Western blot was employed to detect nickel-isolated Sp185/333 (Ni-Sp185/333) proteins and to evaluate protein diversity in animals before and after immune challenge with marine bacteria. Ni-Sp185/333 proteins of the same molecular weight on standard Western blots appear as a broad complex of variants that differ in pI on 2DE Western blots. The Ni-Sp185/333 protein repertoire is variable among animals, and shows a variety of changes among individual sea urchins in response to immune challenges with both the same and different species of bacteria. The extraordinary diversity of the Ni-Sp185/333 proteins may provide significant anti-pathogen capabilities for sea urchins that survive solely on innate immunity.

A member of the Tlr family is involved in dsRNA innate immune response in Paracentrotus lividus sea urchin

The innate immune response involves proteins such as the membrane receptors of the Toll-like family (TLRs), which trigger different intracellular signalling pathways that are dependent on specific stimulating molecules. In sea urchins, TLR proteins are encoded by members of a large multigenic family composed of 60-250 genes in different species. Here, we report a newly identified mRNA sequence encoding a TLR protein (referred to as Pl-Tlr) isolated from Paracentrotus lividus immune cells. The partial protein sequence contained the conserved Toll/IL-1 receptor (TIR) domain, the transmembrane domain and part of the leucine repeats. Phylogenetic analysis of the Pl-Tlr protein was accomplished by comparing its sequence with those of TLRs from different classes of vertebrates and invertebrates. This analysis was suggestive of an evolutionary path that most likely represented the course of millions of years, starting from simple organisms and extending to humans. Challenge of the sea urchin immune system with poly-I:C, a chemical compound that mimics dsRNA, caused time-dependent Pl-Tlr mRNA up-regulation that was detected by QPCR. In contrast, bacterial LPS injury did not affect Pl-Tlr transcription. The study of the Tlr genes in the sea urchin model system may provide new perspectives on the role of Tlrs in the invertebrate immune response and clues concerning their evolution in a changing world.

Single sea urchin phagocytes express messages of a single sequence from the diverse Sp185/333 gene family in response to bacterial challenge

Immune systems in animals rely on fast and efficient responses to a wide variety of pathogens. The Sp185/333 gene family in the purple sea urchin, Strongylocentrotus purpuratus, consists of an estimated 50 (±10) members per genome that share a basic gene structure but show high sequence diversity, primarily due to the mosaic appearance of short blocks of sequence called elements. The genes show significantly elevated expression in three subpopulations of phagocytes responding to marine bacteria. The encoded Sp185/333 proteins are highly diverse and have central effector functions in the immune system. In this study we report the Sp185/333 gene expression in single sea urchin phagocytes. Sea urchins challenged with heat-killed marine bacteria resulted in a typical increase in coelomocyte concentration within 24 h, which included an increased proportion of phagocytes expressing Sp185/333 proteins. Phagocyte fractions enriched from coelomocytes were used in limiting dilutions to obtain samples of single cells that were evaluated for Sp185/333 gene expression by nested RT-PCR. Amplicon sequences showed identical or nearly identical Sp185/333 amplicon sequences in single phagocytes with matches to six known Sp185/333 element patterns, including both common and rare element patterns. This suggested that single phagocytes show restricted expression from the Sp185/333 gene family and infers a diverse, flexible, and efficient response to pathogens. This type of expression pattern from a family of immune response genes in single cells has not been identified previously in other invertebrates.

Expression analysis of immune related genes identified from the coelomocytes of sea cucumber (Apostichopus japonicus) in response to LPS challenge

The sea cucumber (Apostichopus japonicus) occupies a basal position during the evolution of deuterostomes and is also an important aquaculture species. In order to identify more immune effectors, transcriptome sequencing of A. japonicus coelomocytes in response to lipopolysaccharide (LPS) challenge was performed using the Illumina HiSeq™ 2000 platform. One hundred and seven differentially expressed genes were selected and divided into four functional categories including pathogen recognition (25 genes), reorganization of cytoskeleton (27 genes), inflammation (41 genes) and apoptosis (14 genes). They were analyzed to elucidate the mechanisms of host-pathogen interactions and downstream signaling transduction. Quantitative real-time polymerase chain reactions (qRT-PCRs) of 10 representative genes validated the accuracy and reliability of RNA sequencing results with the correlation coefficients from 0.88 to 0.98 and p-value <0.05. Expression analysis of immune-related genes after LPS challenge will be useful in understanding the immune response mechanisms of A. japonicus against pathogen invasion and developing strategies for resistant markers selection.

Expression of antimicrobial peptides in coelomocytes and embryos of the green sea urchin (Strongylocentrotus droebachiensis)

Antimicrobial peptides (AMPs) play a crucial role in innate immunity. We have previously reported the isolation and characterization of the AMPs, strongylocins 1 and 2, and centrocin 1, from coelomocyte extracts of Strongylocentrotus droebachiensis. Here we show that these AMPs were expressed in phagocytes. In addition, transcripts of strongylocin 1 were detected in vibratile cells and/or colorless spherule cells, while transcripts of strongylocin 2 were found in red spherule cells. Results from immunoblotting and immunocytochemistry studies showed that centrocin 1 was produced by phagocytes and stored in granular vesicles. Co-localization of centrocin 1 and phagocytosed bacteria suggests that the granular vesicles containing centrocin 1 may be involved in the formation of phagolysosomes. We also analyzed the temporal and spatial expression of AMPs throughout larval development. Strongylocins were expressed in the early pluteus stage, while centrocin 1 was expressed in the mid pluteus stage. The spatial expression pattern showed that centrocin 1 was mainly located in blastocoelar cells (BCs) around the stomach and the esophagus. In addition, a few patrolling BCs were detected in some larval arms. Together, these results suggest that AMPs are expressed in different types of coelomocytes and that centrocin 1 is involved in response against bacteria. Furthermore, the expression of AMPs in larval pluteus stage, especially in BCs, indicates that AMPs and BCs are engaged in the larval immune system.

Molecular and cellular characterization of the tomato pollen profilin, LePro1

Profilin is an actin-binding protein involved in the dynamic turnover and restructuring of the actin cytoskeleton in all eukaryotic cells. We previously cloned a profilin gene, designated as LePro1 from tomato pollen. To understand its biological role, in the present study, we investigated the temporal and spatial expression of LePro1 during pollen development and found that the transcript was only detected at late stages during microsporogenesis and pollen maturation. Using antisense RNA, we successfully knocked down the expression of LePro1 in tomato plants using stable transformation, and obtained two antisense lines, A2 and A3 showing significant down-regulation of LePro1 in pollen resulting in poor pollen germination and abnormal pollen tube growth. A disorganized F-actin distribution was observed in the antisense pollen. Down-regulation of LePro1 also appeared to affect hydration of pollen deposited on the stigma and arrested pollen tube elongation in the style, thereby affecting fertilization. Our results suggest that LePro1 in conjunction with perhaps other cytoskeletal proteins, plays a regulatory role in the proper organization of F-actin in tomato pollen tubes through promoting actin assembly. Down-regulation of LePro1 leads to interruption of actin assembly and disorganization of the actin cytoskeleton thus arresting pollen tube growth. Based on the present and previous studies, it is likely that a single transcript of profilin gives rise to multiple forms displaying multifunctionality in tomato pollen.

Time course proteomic profiling of cellular responses to immunological challenge in the sea urchin, Heliocidaris erythrogramma

Genome sequences and high diversity cDNA arrays have provided a detailed molecular understanding of immune responses in a number of invertebrates, including sea urchins. However, complementary analyses have not been undertaken at the level of proteins. Here, we use shotgun proteomics to describe changes in the abundance of proteins from coelomocytes of sea urchins after immunological challenge and wounding. The relative abundance of 345 reproducibly identified proteins were measured 6, 24 and 48 h after injection. Significant changes in the relative abundance of 188 proteins were detected. These included pathogen-binding proteins, such as the complement component C3 and scavenger receptor cysteine rich proteins, as well as proteins responsible for cytoskeletal remodeling, endocytosis and intracellular signaling. An initial systemic reaction to wounding was followed by a more specific response to immunological challenge involving proteins such as apolipophorin, dual oxidase, fibrocystin L, aminopeptidase N and α-2-macroglobulin.

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.

Localization and diversity of 185/333 proteins from the purple sea urchin--unexpected protein-size range and protein expression in a new coelomocyte type

The current paradigm proposes that the innate immune systems of invertebrates are much more complex than previously thought. The highly diverse 185/333 gene family in the purple sea urchin encodes a family of closely related proteins of varying length and sequence composition. Subsets of small phagocytes and polygonal cells express 185/333 proteins with localization on the surface of the small phagocytes and within perinuclear vesicles in both cell types. In short-term cultures, coelomocytes form small aggregates that progress to syncytia that are thought to be equivalent to encapsulation in vivo. These aggregates were found to be enriched for 185/333-positive (185/333(+)) small phagocytes. In response to lipopolysaccharide challenge, coelomocytes transiently increased, including frequencies of both 185/333(+) and 185/333-negative (185/333(-)) small phagocytes and 185/333(+) polygonal cells. The 185/333 proteins were present in a broad array of sizes, most of which were larger than that predicted from the cDNAs. Recombinant 185/333 proteins expressed in bacteria and insect cells were also larger than expected, suggesting that the proteins dimerize and multimerize. The diversity of the 185/333 proteins, their expression in response to immune challenge, and their cellular localization suggests this protein family and the small phagocytes have an important immunological role in the sea urchin.

Structural features and functional domains of amassin-1, a cell-binding olfactomedin protein

Amassin-1 mediates a rapid cell adhesion that tightly adheres sea urchin coelomocytes (body cavity immunocytes) together. Three major structural regions exist in amassin-1: a short beta region, 3 coiled coils, and an olfactomedin domain. Amassin-1 contains 8 disulfide-bonded cysteines that, upon reduction, render it inactive. Truncated forms of recombinant amassin-1 were expressed and purified from Pichia pastoris and their disulfide bonding and biological activities investigated. Expressed alone, the olfactomedin domain contained 2 intramolecular disulfide bonds, existed in a monomeric state, and inhibited amassin-1-mediated clotting of coelomocytes by a calcium-dependent cell-binding activity. The N-terminal beta region, containing 3 cysteines, was not required for clotting activity. The coiled coils may dimerize amassin-1 in a parallel orientation through a homodimerizing disulfide bond. Neither amassin-1 fragments that were disulfide-linked as dimers or that were engineered to exist as dimers induced coelomocytes clotting. Clotting required higher multimeric states of amassin-1, possibly tetramers, which occurred through the N-terminal beta region and (or) the first segment of coiled coils.

Extracellular heat shock protein 70 has novel functional effects on sea urchin eggs and coelomocytes

Numerous reports document that the 70 kDa heat shock proteins are not only intracellular proteins but are also present in blood and other extracellular compartments. How they affect cell function from the extracellular space remains unclear. Using two well-characterized cell types from the sea urchin, we show that extracellular mixtures of the constitutive and inducible forms of the 70 kDa heat shock proteins (Hsc70 and Hsp70, respectively) have dramatic effects on initiation of cell division in fertilized eggs and on the clotting reaction of hypotonically stressed coelomocytes. In suspensions of fertilized eggs to which Hsc70 or a 2:3 mixture of Hsc and Hsp70 was added, progression to the first mitotic division was accelerated. Evidence is provided that the extracellular Hsc70 passes into the egg cells in an unconventional manner, being distributed through the cytoplasm, and that it may alter the intracellular signaling cascade initiated by sperm penetration. In coelomocytes that were stimulated by hypotonic shock to mimic injury, the spreading reaction of the clotting response was significantly inhibited when either Hsp70 or Hsc70 was in the medium. These results suggest that the presence of Hsc and/or Hsp70 in the extracellular fluid may promote mitosis of dividing cells and suppress the reactivity of immune system cells.

Analysis of cytoskeletal and motility proteins in the sea urchin genome assembly

The sea urchin embryo is a classical model system for studying the role of the cytoskeleton in such events as fertilization, mitosis, cleavage, cell migration and gastrulation. We have conducted an analysis of gene models derived from the Strongylocentrotus purpuratus genome assembly and have gathered strong evidence for the existence of multiple gene families encoding cytoskeletal proteins and their regulators in sea urchin. While many cytoskeletal genes have been cloned from sea urchin with sequences already existing in public databases, genome analysis reveals a significantly higher degree of diversity within certain gene families. Furthermore, genes are described corresponding to homologs of cytoskeletal proteins not previously documented in sea urchins. To illustrate the varying degree of sequence diversity that exists within cytoskeletal gene families, we conducted an analysis of genes encoding actins, specific actin-binding proteins, myosins, tubulins, kinesins, dyneins, specific microtubule-associated proteins, and intermediate filaments. We conducted ontological analysis of select genes to better understand the relatedness of urchin cytoskeletal genes to those of other deuterostomes. We analyzed developmental expression (EST) data to confirm the existence of select gene models and to understand their differential expression during various stages of early development.

Monitoring chemical and physical stress using sea urchin immune cells

Coelomocytes are the cells freely circulating in the body fluid contained in echinoderm coelom and constitute the defence system, which, in response to injuries, host invasion, and adverse conditions, is capable of chemotaxis, phagocytosis, and production of cytotoxic metabolites. Red and colourless amoebocytes, petaloid and philopodial phagocytes, and vibratile cells are the cell types that, in different proportions, constitute the mixed coelomocyte cell population found in sea urchins. Advances in cellular and molecular biology have made it possible to identify a number of specific proteins expressed in coelomocytes under resting conditions or when activated by experimentally induced stress. Only recently, coelomocytes have been used for pollution studies with the aim of introducing a new biosensor for detection of stress at both cellular and molecular levels, as sentinel of sea health. In this chapter, we briefly review the important features of these valuable cells and describe studies on their use in the laboratory and in the field for the assessment of chemical and physical pollution of the sea.

Macroarray analysis of coelomocyte gene expression in response to LPS in the sea urchin. Identification of unexpected immune diversity in an invertebrate

The purple sea urchin, Strongylocentrotus purpuratus, is a member of the phylum Echinodermata, which is basal to the phylum Chordata within the deuterostome lineage of the animal kingdom. This relationship makes the analysis of the sea urchin immune system relevant to understanding the evolution of the deuterostome immune system leading to the Vertebrata. Subtractive suppression hybridization was employed to generate cDNA probes for screening high-density arrayed, conventional cDNA libraries to identify genes that were upregulated in coelomocytes responding to lipopolysaccharide. Results from 1,247 expressed sequence tags (ESTs) were used to infer that coelomocytes upregulated genes involved in RNA splicing, protein processing and targeting, secretion, endosomal activities, cell signaling, and alterations to the cytoskeletal architecture including interactions with the extracellular matrix. Of particular note was a set of transcripts represented by 60% of the ESTs analyzed, which encoded a previously uncharacterized family of closely related proteins, provisionally designated as 185/333. These transcripts exhibited a significant level of variation in their nucleotide sequence and evidence of putative alternative splicing that could yield up to 15 translatable elements. On the basis of the striking increase in gene expression in response to lipopolysaccharide and the unexpected level of diversity of the 185/333 messages, we propose that this set of transcripts encodes a family of putative immune response proteins that may represent a major component of an immunological response to bacterial challenge.

The sea urchin complement homologue, SpC3, functions as an opsonin

The purple sea urchin Strongylocentrotus purpuratus expresses a homologue of complement component C3 (SpC3), which acts as a humoral opsonin. Significantly increased phagocytic activity was evident when yeast target cells were opsonized after incubation with coelomic fluid containing SpC3. SpC3 could be detected on the surface of yeast, and phagocytic activity could be inhibited by an anti-SpC3 antibody. This indicates that SpC3 promotes phagocytosis by physically tagging target cells for ingestion. Confocal microscopy showed that opsonized yeast were phagocytosed by a single coelomocyte type (polygonal phagocytes), presumably because these cells express SpC3 receptors. Overall, these data indicate that SpC3 is a major humoral opsonin in S. purpuratus coelomic fluid.

Two cDNAs from the purple sea urchin, Strongylocentrotus purpuratus , encoding mosaic proteins with domains found in factor H, factor I, and complement components C6 and C7

The vertebrate complement system is composed of about 30 serum and cell surface proteins that make up three activation pathways, a lytic pathway, and a set of proteins that regulate complement. Regulatory proteins are required for host protection against autologous complement attack and to control the amplification feedback loop of the alternative pathway. Purple sea urchin, Strongylocentrotus purpuratus, homologues of complement C3 (SpC3) and factor B (SpBf) have been identified, suggesting the presence of an alternative complement pathway. This implies that echinoderms require a complement regulatory system for the same reasons that it is required in higher vertebrates. Two cDNAs, Sp5 and Sp5013, have been characterized from coelomocytes and the deduced structures of the encoded mosaic proteins, SpCRL ( S. p urpuratus complement related protein, long form) and SpCRS ( short form), have domains that are also found in regulatory proteins such as factor H and factor I and the terminal pathway components C6 and C7. These domains include multiple short consensus repeats, a fucolectin domain, Ser/Thr/Pro-rich regions, a Cys-rich region, and a factor I-membrane attack complex domain. The genes are constitutively expressed in all tissues of the sea urchin and are not induced in response to immune challenge. Multiple bands of varying intensity on both genome blots and RNA blots suggest that Sp5 and Sp5013 are members of a small gene family and that they might undergo alternative splicing. Based on the domains present in SpCRL and SpCRS, they might be either examples of complement regulatory proteins or members of the terminal pathway of complement.

Isolation of pigment cell specific genes in the sea urchin embryo by differential macroarray screening

New secondary mesenchyme specific genes, expressed exclusively in pigment cells, were isolated from sea urchin embryos using a differential screening of a macroarray cDNA library. The comparison was performed between mRNA populations of embryos having an expansion of the endo-mesodermal territory and embryos blocked in secondary mesenchyme specification. To be able to isolate transcripts with a prevalence down to five copies per cell, a subtractive hybridization procedure was employed. About 400 putative positive clones were identified and sequenced from the 5' end. Gene expression analysis was carried out on a subset of 66 clones with real time quantitative PCR and 40 clones were positive. This group of clones contained sequences highly similar to: the transcription factor glial cells missing (gcm); the polyketide synthase gene cluster (pks-gc); three different members of the flavin-containing monooxygenase gene family (fmo); and a sulfotransferase gene (sult). Using whole mount in situ hybridization, it was shown that these genes are specifically expressed in pigment cells. A functional analysis of the S. purpuratus pks and of one S. purpuratus fmo was carried out using antisense technology and it was shown that their expression is necessary for the biosynthesis of the sea urchin pigment echinochrome. The results suggest that S. purpuratus pks, fmo and sult could belong to a differentiation gene battery of pigment cells.

The gene encoding the sea urchin complement protein, SpC3, is expressed in embryos and can be upregulated by bacteria

Sea urchins have an innate immune response that functions in the absence of adaptive capabilities. It is mediated, in part, by components of the complement system, an important subsystem of the innate response in mammals. A homologue of complement C3, SpC3, has been identified in adult Strongylocentrotus purpuratus and is expressed in coelomocytes. In this study, transcript levels from the gene, Sp064, which encodes SpC3, were examined in developing embryos and found to be present in unfertilized eggs and throughout embryogenesis with a peak in transcript levels just prior to and during gastrulation. In addition, continuous exposure of embryos, beginning with the hatched blastula stage, to heat killed Vibrio diazatrophicus, a marine pathogen of sea urchins, significantly increased Sp064 message content in plutei compared to unexposed controls. These results suggest that sea urchin embryos may use a complement-based immune system for defense against pathogens in their aquatic environment.

Amassin, an olfactomedin protein, mediates the massive intercellular adhesion of sea urchin coelomocytes

Sea urchins have a fluid-filled body cavity, the coelom, containing four types of immunocytes called coelomocytes. Within minutes after coelomic fluid is removed from the body cavity, a massive cell-cell adhesion of coelomocytes occurs. This event is referred to as clotting. Clotting is thought to be a defense mechanism against loss of coelomic fluid if the body wall is punctured, and it may also function in the cellular encapsulation of foreign material and microbes. Here we show that this intercoelomocyte adhesion is mediated by amassin, a coelomic plasma protein with a relative molecular mass (Mr) of 75 kD. Amassin forms large disulfide-bonded aggregates that adhere coelomocytes to each other. One half of the amassin protein comprises an olfactomedin (OLF) domain. Structural predictions show that amassin and other OLF domain-containing vertebrate proteins share a common architecture. This suggests that other proteins of the OLF family may function in intercellular adhesion. These findings are the first to demonstrate a function for a protein of the OLF family.

Lipopolysaccharides induce intestinal serum amyloid A expression in the sea cucumber Holothuria glaberrima

We have previously characterized the first invertebrate homolog of serum amyloid A (SAA) proteins in the sea cucumber Holothuria glaberrima, where its expression is associated with intestinal regeneration, suggesting a possible involvement of SAA proteins in intestinal morphogenesis. Here we show that bacterial lipopolysaccharides (LPS) trigger a coelomocyte-mediated immune response in H. glaberrima, inducing an approximately threefold increase in coelomocyte phagocytic activity. Furthermore, LPS induces an approximately fourfold increase in SAA mRNA levels in non-regenerating intestines. These results show that in H. glaberrima, LPS act as an immune activator and that SAA expression can be modulated by immune-associated processes.

Thioester function is conserved in SpC3, the sea urchin homologue of the complement component C3

The amino acid sequence of the thioester site in the alpha chain of SpC3, the sea urchin homologue of C3, is conserved. This implies a conserved function of covalent bond formation with amine or hydroxyl groups on target molecules. When coelomic fluid (CF) was incubated with 14C-methylamine, a classic assay for thioester binding function, the alpha chain became labeled. When CF was treated to induce autolysis, peptide bond cleavage occurred at the thioester site. Autolysis could be blocked or reduced by pre-treating CF with either methylamine or yeast, both of which are known to bind to thioester sites C3 proteins from other organisms. The data suggest that SpC3 can bind to target cell surfaces, constituting indirect evidence that it can covalently bind to pathogen surfaces and function as an opsonin in vivo. This activity may be an important aspect of host defense in the sea urchin.

Differentiation of immune cells challenged by bacteria in the common European starfish, Asterias rubens (Echinodermata)

Amoebocytes are the main effector cells of the echinoderm immune system. In starfishes, a taxon in which bacterial diseases have been rarely reported, amoebocytes are considered to be the only circulating and immune cell type. The present paper addresses the question of amoebocyte differentiation in the starfish Asterias rubens when challenged by bacteria. Starfishes were injected with FITC-coupled bacteria (Micrococcus luteus). Amoebocytes were collected at regular time intervals for 24 h. The cytometric characteristics and the phagocytic activity were studied by flow cytometry. Three amoebocyte groups of different size were identified. The cell concentrations of the two largest and more numerous of these groups (G2 and G3) were modulated by immune stimulation while the group of smallest, less numerous, cells (G1) was unaffected. All of these cell groups were phagocytic but their kinetics of cell activation and bacteria ingestion differed. G1 cells showed the lowest phagocytic activity while G3 cells had the highest and fastest phagocytic activity. Starfish amoebocytes appear to be segregated in three groups, two of them (G2 and G3) being immunomodulated and one of them presenting a very fast reaction to bacteria. It is suggested that the high efficiency of the immune system in starfishes is related to this fast reaction.

Cellular and biochemical responses to environmental and experimentally induced stress in sea urchin coelomocytes

Coelomocytes are considered to be immune effectors of sea urchins. Subpopulations of coelomocytes can be purified from a total cell suspension. The proportion of each cell type can vary not only among species, but also between individuals of the same species, according to their size and physiological conditions. We tested the hypothesis that coelomocytes play a role in defense mechanisms activated by adverse external conditions. Total coelomocytes from control and stressed (temperature, pollution, and injuries) sea urchins were analyzed for their expression of the 70 kDa heat shock protein (hsp70), a well recognized stress marker. Further analysis was performed by separation of coelomocytes into subpopulations by step gradients. We demonstrated that sea urchin coelomocytes respond to temperature shock and to polluted seawater by the upregulation of hsp70. Among coelomocytes certain cells, known as red spherula cells, showed a great increase in number in animals collected from polluted seawaters or subjected to "accidental" injury. The present study confirms the immunological function of sea urchin coelomocytes, as indicated by the upregulation of the hsp70 molecular marker, and suggests that sea urchin coelomocytes can be utilized as sensitive bio-indicators of environmental stress.

Dynamic expression of multiple scavenger receptor cysteine-rich genes in coelomocytes of the purple sea urchin

Coelomocytes, the heterogeneous population of sea urchin putative immune cells, were found to express a complex set of transcripts featuring scavenger receptor cysteine-rich (SRCR) repeats. SRCR domains define a metazoan superfamily of proteins, many of which are implicated in development and regulation of the immune system of vertebrates. Coelomocytes transcribe multiple SRCR genes from among a multigene family encoding an estimated number of 1,200 SRCR domains in specific patterns particular to each individual. Transcription levels for given SRCR genes may range from pronounced to undetectable, yet all tested animals harbor the genomic loci encoding these genes. Analysis of several SRCR genes revealed multiple loci corresponding to each type. In the case of one SRCR type, a cluster of at least three genes was detected within a 133-kb bacterial artificial chromosome insert, and conserved as well as unique regions were identified in sequences of three genomic clones derived from a single animal. Array hybridizations with repeated samples of coelomocyte messages revealed substantial alterations in levels of expression of many SRCR genes, with fluctuations of up to 10-fold in 1 week and up to 30-fold over a period of 3 months. This report is the first demonstration of genomic and transcriptional complexity in molecules expressed by invertebrate coelomocytes. The mechanisms controlling SRCR gene expression and the functional significance of this dynamic system await elucidation.

Cellular and biochemical responses to environmental and experimentally induced stress in sea urchin coelomocytes

Coelomocytes are considered to be immune effectors of sea urchins. Subpopulations of coelomocytes can be purified from a total cell suspension. The proportion of each cell type can vary not only among species, but also between individuals of the same species, according to their size and physiological conditions. We tested the hypothesis that coelomocytes play a role in defense mechanisms activated by adverse external conditions. Total coelomocytes from control and stressed (temperature, pollution, and injuries) sea urchins were analyzed for their expression of the 70 kDa heat shock protein (hsp70), a well recognized stress marker. Further analysis was performed by separation of coelomocytes into subpopulations by step gradients. We demonstrated that sea urchin coelomocytes respond to temperature shock and to polluted seawater by the upregulation of hsp70. Among coelomocytes certain cells, known as red spherula cells, showed a great increase in number in animals collected from polluted seawaters or subjected to "accidental" injury. The present study confirms the immunological function of sea urchin coelomocytes, as indicated by the upregulation of the hsp70 molecular marker, and suggests that sea urchin coelomocytes can be utilized as sensitive bio-indicators of environmental stress.

Cellular and biochemical responses to environmental and experimentally induced stress in sea urchin coelomocytes

Coelomocytes are considered to be immune effectors of sea urchins. Subpopulations of coelomocytes can be purified from a total cell suspension. The proportion of each cell type can vary not only among species, but also between individuals of the same species, according to their size and physiological conditions. We tested the hypothesis that coelomocytes play a role in defense mechanisms activated by adverse external conditions. Total coelomocytes from control and stressed (temperature, pollution, and injuries) sea urchins were analyzed for their expression of the 70 kDa heat shock protein (hsp70), a well recognized stress marker. Further analysis was performed by separation of coelomocytes into subpopulations by step gradients. We demonstrated that sea urchin coelomocytes respond to temperature shock and to polluted seawater by the upregulation of hsp70. Among coelomocytes certain cells, known as red spherula cells, showed a great increase in number in animals collected from polluted seawaters or subjected to "accidental" injury. The present study confirms the immunological function of sea urchin coelomocytes, as indicated by the upregulation of the hsp70 molecular marker, and suggests that sea urchin coelomocytes can be utilized as sensitive bio-indicators of environmental stress.

Increased gene expression of a cytokine-related molecule and profilin after activation of Suberites domuncula cells with xenogeneic sponge molecule(s)

Porifera (sponges) constitute the lowest metazoan phylum. Experiments examined whether sponges can recognize self/nonself molecules. Cells from the marine sponge Suberites domuncula were incubated with membranes from either S. domuncula or another marine sponge, Geodia cydonium, as well as with recombinant alpha-integrin from G. cydonium. The cells responded immediately with a rise of intracellular Ca2+ ([Ca2+i]) if they were treated with membranes from G. cydonium but not after treatment by those from S. domuncula. This change of [Ca2+i] was also recorded with G. cydonium alpha-integrin. In parallel, the expression of two genes was strongly upregulated; one codes for a cytokine-related molecule, pre-B-cell colony-enhancing factor, and the other for profilin. These genes have previously been found to be highly expressed in human or echinoderm cells in the presence of xenogeneic proteins. Our data support the hypothesis that a primordial immune response system is present in sponges.

Echinoderm immunity and the evolution of the complement system

Our understanding of inflammatory responses in humans has its roots in the comparative approach to immunology. In the late 1900s, research on echinoderms provided the initial evidence for the importance of phagocytic cells in reactions to foreign material. Studies of allograft rejection kinetics have shown that echinoderms have a non-adaptive, activation type of immune response. Coelomocytes mediate the cellular responses to immune challenges through phagocytosis, encapsulation, cytotoxicity, and the production of antimicrobial agents. In addition, a variety of humoral factors found in the coelomic fluid, including lectins, agglutinins, and lysins, are important in host defense against pathogens and other foreign substances. Recently, a simple complement system has been identified in the purple sea urchin that is homologous to the alternative pathway in vertebrates. The sea urchin [corrected] homologue of C3, is inducible by challenge with lipopolysaccharide, which is known to activate coelomocytes. Complement components have been identified in all vertebrate classes, and now have been characterized in protochordates and echinoderms indicating the primordial nature of the complement system. Because it is thought that the complement system evolved from a few primordial genes by gene duplication and divergence, the origin of this system appears to have occurred within the common ancestor of the deuterostomes.

Coelomocytes Express SpBf, a Homologue of Factor B, the Second Component in the Sea Urchin Complement System

A homologue of factor B, SpBf, has been cloned and sequenced from an LPS-activated coelomocyte cDNA library from the purple sea urchin, Strongylocentrotus purpuratus. The deduced amino acid sequence and domain structure show significant similarity to the vertebrate Bf/C2 family proteins. SpBf is a mosaic protein, composed of five short consensus repeats, a von Willebrand Factor domain, and a serine protease domain. It has a deduced molecular mass of 91 kDa, with a conserved cleavage site for a putative factor D protease. It has ten consensus recognition sites for N-linked glycosylation. Amino acids involved in both Mg2+ binding and in serine protease activity in the vertebrate C2/Bf proteins are conserved in SpBf. Phylogenetic analysis of SpBf indicates that it is the most ancient member of the vertebrate Bf/C2 family. Additional phylogenetic analysis of the SCRs indicates that five SCRs in SpBf may be ancestral to three SCRs, which is the typical pattern in the vertebrate Bf/C2 proteins. RNA gel blots show that SpBf transcripts are 5.5 kb and are specifically expressed in coelomocytes. Genome blots suggest that the SpBf gene (Sp152) is single copy gene per haploid genome. This is the second complement component to be identified from the sea urchin, and, with the sea urchin C3 homologue, these two components may be part of a simple complement system that is homologous to the alternative pathway in higher vertebrates.

Modifications in protein binding to upstream sequences of the sea urchin cytoplasmic actin gene CyIIa in comparison to its linked neighbors, CyI and CyIIb

The sequences corresponding to regions upstream of the ATG and transcription start site of the CyIIa cytoplasmic actin gene of the sea urchin Strongylocentrotus purpuratus were determined and compared to the genomically linked CyI and CyIIb actin genes. Sites of protein-DNA interaction in the CyIIa upstream sequences were identified by DNase I footprinting. The similarity between CyIIa and CyI (and CyIIb) upstream sequences was limited and included a consensus octamer sequence, serum response element (SRE) and some short sequences within the proximal promoter region. The octamer sequence was found to bind protein. A single DNase I hypersensitive site was detected within the SRE and to two flanking nucleotides, but otherwise, the SRE did not appear to be protected. This is in contrast to strong protein binding to the CyIIb SRE. A region in the CyIIa gene with limited identity to the functionally significant protein binding site D in CyI also did not bind protein. Four additional sites of protein-DNA interaction were identified in CyIIa upstream sequences. One of these is similar to a protein binding site previously located in the CyI upstream sequences, whereas the others appear to be unique. These data indicate that the CyIIa upstream sequences differ extensively from those of CyI. The pattern of CyIIa expression is likely a consequence of these alternations in DNA sequence and protein-DNA interactions.

Sea Urchin Coelomocytes Specifically Express a Homologue of the Complement Component C3

A homologue of complement component C3 (SpC3) has been cloned and sequenced from the purple sea urchin, Strongylocentrotus purpuratus. The preprocessed, deduced protein size is estimated to be 186 kDa with a short leader and two chains, α and β. There are cysteines in conserved positions for interchain disulfide bonding, and there is a conserved thioester site in the α-chain with an associated histidine. There are five consensus N-linked glycosylation sites, and putative cleavage sites for factor I and C3 convertase. Partially purified SpC3 on protein gels shows a nonreduced size of 210 kDa and, under reducing conditions, reveals an α-chain of 130 kDa and a β-chain of 80 kDa. These sizes are larger than the deduced sizes, suggesting that the protein has carbohydrates added to most of the consensus N-linked glycosylation sites. Phylogenetic analysis of SpC3 compared with other members of the thioester protein family, which includes C3, C4, C5, and α2-macroglobulin, shows that SpC3 is the first divergent complement protein, falling at the base of the complement protein clade. Transcripts from the SpC3 gene (Sp064) are 9 kb, and the gene is expressed specifically in coelomocytes, which are the immunocytes in the sea urchin. Genome blots suggest that SpC3 is encoded by a single copy gene per haploid genome. This is the first identification of a complement component in an invertebrate, and suggests homology of the innate immune system within the deuterostome lineage of animals.

Motile areas of leech neurites are rich in microfilaments and two actin-binding proteins: gelsolin and profilin

Cell motility is produced by changes in the dynamics and organization of actin filaments. The aim of the experiments described here was to test whether growing neurites contain two actin-binding proteins, gelsolin and profilin, that regulate polymerization of actin and affect non-neuronal cell motility. The distribution of gelsolin, profilin and the microfilaments was compared by immunocytochemistry of leech neurons growing in culture. We observed that microfilaments are enriched in the peripheral motile areas of the neurites. Both gelsolin and profilin are also concentrated in these regions. Gelsolin is abundant in filopodia and is associated with single identifiable microfilament bundles in lamellipodia. Profilin is not prominent in filopodia and shows a diffuse staining pattern in lamellipodia. The colocalization of gelsolin and profilin in motile, microfilament-rich areas supports the hypothesis that they synergistically regulate the actin dynamics that underlie neurite growth.

Isolation and characterization of the Candida albicans PFY1 gene for profilin

We have isolated the Candida albicans gene for profilin, PFY1. Degenerate oligonucleotide primers based on regions of high homology were utilized to obtain a polymerase chain reaction-amplified copy of the gene. This was then used as a probe to isolate the gene from a C. albicans genomic library. Our studies indicate that the full-length gene is unstable in Escherichia coli. Several clones were sequenced, and the predicted amino acid sequence demonstrated homology with profilin proteins from other organisms, most notably Saccharomyces cerevisiae. Northern analysis revealed that the gene is expressed in C. albicans. Attempts to express the gene in S. cerevisiae cells were unsuccessful until the C. albicans promoter was replaced with an S. cerevisiae promoter. Functional complementation of the gene was demonstrated in S. cerevisiae profilin-requiring cells. Antibodies raised to isolated C. albicans profilin protein recognized a protein of the predicted molecular weight when the gene was expressed in S. cerevisiae cells.

Surface localization of the sea urchin egg receptor for sperm

The sea urchin egg receptor for sperm is thought to be involved in species-specific sperm-egg interactions at the egg surface. Recent revisions in the deduced amino acid sequence of the cloned cDNAs indicate that the protein encoded does not possess the common structural hallmarks of a membrane protein. Thus, investigation of the localization and association of the protein with the egg surface is crucial. We describe and characterize a new monoclonal antibody raised against recombinant sperm receptor protein. This antibody, in conjunction with several polyclonal antibodies, was used to study the receptor protein in eggs. Immunoprecipitation studies indicated that the antibodies recognize the high Mr (ca. 350 K) sperm receptor protein which copurified with egg plasma membrane-vitelline layer complexes. The sperm receptor protein was solubilized only by detergents and not by treatments designed to solubilize peripherally associated or lipid-anchored membrane proteins, suggesting a tight association with the membrane fraction. Confocal immunofluorescence microscopy of live eggs indicated surface staining. Finally, lysylendoproteinase C treatment of live eggs resulted in a loss of the high Mr receptor protein epitopes, and the concomitant release of a 70-kDa proteolytic fragment, which correlated with a reduced ability of the eggs to be fertilized. Taken together, these data indicate that at least some fraction of the sperm receptor protein is present on the egg surface, a requisite locale for a sperm binding protein.

Lipopolysaccharide activates the sea urchin immune system

Profilin is a small, actin-binding protein that functions at the intersection of signal transduction and cytoskeletal modifications. Increases in the number of profilin messages per cell correlate with sea urchin coelomocytes activation in response to injury. Here we show that coelomocytes respond to immune challenge from lipopolysaccharide with significant elevations in profilin transcripts per coelomyocyte.

The sea urchin profilin gene is specifically expressed in mesenchyme cells during gastrulation

Eggs and embryos of the purple sea urchin (Strongylocentrotus purpuratus) contain profilin that is partly supplied from maternal sources and partly produced by the gastrula. The maternal profilin protein content is about 13 microM and it persists in the embryo at least through gastrulation. Transcript quantitation from probe excess titrations show that very few profilin gene transcripts are present in the embryo during cleavage, but that they increase at the onset of gastrulation. By in situ hybridization, the newly synthesized profilin transcripts are localized in mesenchyme cells. Profilin gene expression increases when mesenchyme cells initiate migration and filopodial extension and retraction. We show that there are three isoforms of maternal profilin protein produced from the single copy gene during oogenesis. However, the blastula stage embryo only produces the major isoform, whereas the acidic isoform is produced in the early stages of gastrulation and the basic isoform appears by the end of gastrulation. Based on transcript prevalence and protein production rates, our calculations indicate that the amount of new protein produced in the mesenchyme cells in 12 hr is at maximum < 2% of that supplied from maternal sources. Because of the large amount of maternally supplied profilin present in the egg and embryo, we suggest that it may be used in the cytokinetic processes of cleavage. Alternatively, because of the small amount of embryonically produced profilin, we suggest that it may function in the cytoskeletal shape changes required for filopodial extension and motility in the mesenchyme cells during gastrulation.