Outgrowth of pseudopodial cables induced by all-trans retinoic acid in micromere-derived cells isolated from sea urchin embryos

Thyroid hormone membrane receptor binding and transcriptional regulation in the sea urchin Strongylocentrotus purpuratus

Thyroid hormones (THs) are small amino acid derived signaling molecules with broad physiological and developmental functions in animals. Specifically, their function in metamorphic development, ion regulation, angiogenesis and many others have been studied in detail in mammals and some other vertebrates. Despite extensive reports showing pharmacological responses of invertebrate species to THs, little is known about TH signaling mechanisms outside of vertebrates. Previous work in sea urchins suggests that non-genomic mechanisms are activated by TH ligands. Here we show that several THs bind to sea urchin (Strongylocentrotus purpuratus) cell membrane extracts and are displaced by ligands of RGD-binding integrins. A transcriptional analysis across sea urchin developmental stages shows activation of genomic and non-genomic pathways in response to TH exposure, suggesting that both pathways are activated by THs in sea urchin embryos and larvae. We also provide evidence associating TH regulation of gene expression with TH response elements in the genome. In ontogeny, we found more differentially expressed genes in older larvae compared to gastrula stages. In contrast to gastrula stages, the acceleration of skeletogenesis by thyroxine in older larvae is not fully inhibited by competitive ligands or inhibitors of the integrin membrane receptor pathway, suggesting that THs likely activate multiple pathways. Our data confirms a signaling function of THs in sea urchin development and suggests that both genomic and non-genomic mechanisms play a role, with genomic signaling being more prominent during later stages of larval development.

Metabolomic changes and polyunsaturated fatty acid biosynthesis during gonadal growth and development in the sea urchin Strongylocentrotus intermedius

Strongylocentrotus intermedius is an important commercial species of sea urchin distributed in the coastal waters of China. However, the metabolomic changes that accompany its gonadal growth and development stages remain unclear. In this study, we have histologically observed gonad growth stages, analyzed the fatty acid composition, and employed an untargeted metabolomic approach to investigate the metabolites associated with the gonadal growth and development of S. intermedius, as well as the biosynthesis and metabolism of polyunsaturated fatty acids (PUFAs) at different stages and in different sexes. The gonad mass of sea urchin increased from 0.70 ± 0.18 g in January (at the recovering stage) to 8.78 ± 2.89 g in July (the reproductive stage), with the GSI increasing from 4.02 ± 0.88% to 16.86 ± 2.79%. We have analyzed 34 types of fatty acids, such as arachidonic acid, eicosapentaenoic acid, etc., of which PUFAs were the dominant fatty acid class in this species, accounting for >48.55% of the total. In the metabolomic analysis, linolelaidic acid, sciadonic acid, cis-8,11,14,17-eicosatetraenoic acid, adrenic acid, docosapentaenoic acid, and tetracosapentaenoic acid were detected in the differentially expressed metabolites of the unsaturated fatty acids biosynthesis pathway. We found that the most significant functional pathways during gonadal growth and development were "arachidonic acid metabolism", "alpha-linolenic acid metabolism" and "linoleic acid metabolism", which are all related to fatty acid metabolism. These results will provide valuable information on the possible presence of both exogenous and endogenous fatty acids in sea urchin gonads and the metabolomic changes in S. intermedius during gonadal growth periods, and will further our understanding of the intermediary metabolism and the molecular bases of growth traits in this species.

The role of retinoic acid signaling in starfish metamorphosis

Background

Although retinoic acid (RA) signaling plays a crucial role in the body patterning of chordates, its function in non-chordate invertebrates, other than its mediation of environmental cues triggering metamorphosis in cnidarians, is largely unknown. We investigated the role of RA signaling in the metamorphosis of starfish (Echinodermata).

Results

We found that exogenous RA treatment induced metamorphosis in starfish larvae. In contrast, inhibitors of RA synthesis and RA receptors suppressed metamorphosis triggered by attachment to a substrate. Gene expressions of the RA signaling component were detected in competent larvae.

Conclusions

This study provides insight into the ancestral function of RA signaling, which is conserved in the metamorphosis of cnidarians and starfish.

Electronic supplementary material

The online version of this article (10.1186/s13227-018-0098-x) contains supplementary material, which is available to authorized users.

Evolution of retinoic acid receptors and retinoic acid signaling

Retinoic acid (RA) is a vitamin A-derived morphogen controlling important developmental processes in vertebrates, and more generally in chordates, including axial patterning and tissue formation and differentiation. In the embryo, endogenous RA levels are controlled by RA synthesizing and degrading enzymes and the RA signal is transduced by two retinoid receptors: the retinoic acid receptor (RAR) and the retinoid X receptor (RXR). Both RAR and RXR are members of the nuclear receptor superfamily of ligand-activated transcription factors and mainly act as heterodimers to activate the transcription of target genes in the presence of their ligand, all-trans RA. This signaling pathway was long thought to be a chordate innovation, however, recent findings of gene homologs involved in RA signaling in the genomes of a wide variety of non-chordate animals, including ambulacrarians (sea urchins and acorn worms) and lophotrochozoans (annelids and mollusks), challenged this traditional view and suggested that the RA signaling pathway might have a more ancient evolutionary origin than previously thought. In this chapter, we discuss the evolutionary history of the RA signaling pathway, and more particularly of the RARs, which might have experienced independent gene losses and duplications in different animal lineages. In sum, the available data reveal novel insights into the origin of the RA signaling pathway as well as into the evolutionary history of the RARs.

The search for non-chordate retinoic acid signaling: lessons from chordates

Signaling by retinoic acid (RA) is an important pathway in the development and homeostasis of vertebrate and invertebrate chordates, with a critical role in mesoderm patterning. Classical studies on the distribution of nuclear receptors of animals suggested that the family of RA receptors (RARs/NR1B) was restricted to chordates, while the family of RA X receptors (RXR/NR2B) was distributed from cnidarians to chordates. However, the accumulation of data from genome projects and studies in non-model species is questioning this traditional view. Here we discuss the evidence for non-chordate RA signaling systems in the light of recent advances in our understanding of carotene (pro-Vitamin A) metabolism and of the identification of potential RARs and members of the NR1 family in echinoderms and lophotrochozoan trematodes, respectively. We conclude, as have others before (Bertrand et al., 2004. Mol Biol Evol 21(10):1923-1937), that signaling by RA is more likely an ancestral feature of bilaterians than a chordate innovation.

Retinoic acid signaling and the evolution of chordates

In chordates, which comprise urochordates, cephalochordates and vertebrates, the vitamin A-derived morphogen retinoic acid (RA) has a pivotal role during development. Altering levels of endogenous RA signaling during early embryology leads to severe malformations, mainly due to incorrect positional codes specifying the embryonic anteroposterior body axis. In this review, we present our current understanding of the RA signaling pathway and its roles during chordate development. In particular, we focus on the conserved roles of RA and its downstream mediators, the Hox genes, in conveying positional patterning information to different embryonic tissues, such as the endoderm and the central nervous system. We find that some of the control mechanisms governing RA-mediated patterning are well conserved between vertebrates and invertebrate chordates, such as the cephalochordate amphioxus. In contrast, outside the chordates, evidence for roles of RA signaling is scarce and the evolutionary origin of the RA pathway itself thus remains elusive. In sum, to fully understand the evolutionary history of the RA pathway, future research should focus on identification and study of components of the RA signaling cascade in non-chordate deuterostomes (such as hemichordates and echinoderms) and other invertebrates, such as insects, mollusks and cnidarians.

PI3K inhibitors block skeletogenesis but not patterning in sea urchin embryos

Skeletogenesis in the sea urchin embryo is a simple model of biomineralization, pattern formation, and cell-cell communication during embryonic development. The calcium carbonate skeletal spicules are secreted by primary mesenchyme cells (PMCs), but the skeletal pattern is dictated by the embryonic ectoderm. Although the process of skeletogenesis is well characterized, there is little molecular understanding of the basis of patterning within this system. In this study, we examined the contribution of phosphatidylinositide 3-kinase (PI3K)-mediated signaling to the skeletogenic process in sea urchin embryos by using the well-established PI3K inhibitors LY294002 and wortmannin. Our results show that PI3K inhibitors specifically and reversibly block skeletogenesis, and that this blockade occurs within the PMCs rather than in the ectoderm, because the inhibitors block spiculogenesis in cultured micromeres. Our results are consistent with a model in which PI3K signaling is required, not for pattern sensing or interpretation but rather for the biomineralization process itself in the sea urchin embryo.