Overlapping expression of amphioxus homologs of the thyroid transcription factor-1 gene and thyroid peroxidase gene in the endostyle: insight into evolution of the thyroid gland

Neuropeptides and receptors in the cephalochordate: a crucial model for understanding the origin and evolution of vertebrate neuropeptide systems

Genomes and transcriptomes from diverse organisms are providing a wealth of data to explore the evolution and origin of neuropeptides and their receptors in metazoans. While most neuropeptide-receptor systems have been extensively studied in vertebrates, there is still a considerable lack of understanding regarding their functions in invertebrates, an extraordinarily diverse group that account for the majority of animal species on Earth. Cephalochordates, commonly known as amphioxus or lancelets, serve as the evolutionary proxy of the chordate ancestor. Their key evolutionary position, bridging the invertebrate to vertebrate transition, has been explored to uncover the origin, evolution, and function of vertebrate neuropeptide systems. Amphioxus genomes exhibit a high degree of sequence and structural conservation with vertebrates, and sequence and functional homologues of several vertebrate neuropeptide families are present in cephalochordates. This review aims to provide a comprehensively overview of the recent findings on neuropeptides and their receptors in cephalochordates, highlighting their significance as a model for understanding the complex evolution of neuropeptide signaling in vertebrates.

Spatially resolved single-cell atlas of ascidian endostyle provides insight into the origin of vertebrate pharyngeal organs

The pharyngeal endoderm, an innovation of deuterostome ancestors, contributes to pharyngeal development by influencing the patterning and differentiation of pharyngeal structures in vertebrates; however, the evolutionary origin of the pharyngeal organs in vertebrates is largely unknown. The endostyle, a distinct pharyngeal organ exclusively present in basal chordates, represents a good model for understanding pharyngeal organ origins. Using Stereo-seq and single-cell RNA sequencing, we constructed aspatially resolved single-cell atlas for the endostyle of the ascidian Styela clava. We determined the cell composition of the hemolymphoid region, which illuminates a mixed ancestral structure for the blood and lymphoid system. In addition, we discovered a cluster of hair cell-like cells in zone 3, which has transcriptomic similarity with the hair cells of the vertebrate acoustico-lateralis system. These findings reshape our understanding of the pharynx of the basal chordate and provide insights into the evolutionary origin of multiplexed pharyngeal organs.

Hierarchies in the energy budget: Thyroid hormones and the evolution of human life history patterns

The evolution of human life history characteristics required dramatic shifts in energy allocation mechanisms compared with our primate ancestors. Thyroid hormones, such as thyroxine (T4) and triiodothyronine (T3), are sensitive to energy balance, and are significant determinants for both tissue-specific and whole-body metabolic rate. Thus, thyroid hormones are in part responsible for setting the body's overall energy budget and likely played an important role in the evolution of human life history patterns. We propose that the dynamics of mammalian T3 production, uptake, and action have evolved so that energy allocation prioritizes the high demands of brain development and functioning, often at the expense of growth and reproduction. This paper explores the role of thyroid hormone dynamics in the evolution of human encephalization, prolonged childhood and adolescence, long lifespans, reproduction, and human aging.

Transcriptional Analysis of the Endostyle Reveals Pharyngeal Organ Functions in Ascidian

The endostyle is a pharyngeal organ with an opening groove and cilia in invertebrate chordates (amphioxus and ascidian) and cyclostomate (lamprey), serving as a filter-feeding tract and thyroid-secreting location. Emerging evidence implies its complex cellular composition and potentially versatile functions. Multiple cell types in the endostyle have been thought to be progenitors of complex organs in advanced vertebrates. To describe the expression profile and the potential functions, bulk RNA sequencing on the endostyle in ascidian Styela clava was conducted and distinct markers were selected by multileveled comparative analysis. Transcriptional data assay and qRT-PCR-verified results showed the regional expression patterns of Hox genes in the longitudinal axis. Organ-specific markers of the endostyle was proposed by comparing expression with the main organs of the ascidian. A cross-species transcriptional profile projection between the endostyle and organs from Danio rerio and Homo sapiens indicates a robust homogenous relationship to the thyroid and digestive system of the endostyle. The high similarity between the endostyle and the head kidney in zebrafish/the bone marrow in human implies uniquely profound functions of the pharyngeal organ in proto-vertebrates. Our result revealed that the transcriptional profile of the human parathyroid gland was similar to the ascidian endostyle, indicating the evolutionary origin of vertebrate hormone secretion organs.

Molecular and Cellular Characterization of the TH Pathway in the Sea Urchin Strongylocentrotus purpuratus

Thyroid Hormones (THs) are a class of signaling molecules produced by coupling iodine with tyrosine residues. In vertebrates, extensive data support their important role in a variety of processes such as metabolism, development and metamorphosis. On the other hand, in invertebrates, the synthesis and role of the THs have been, so far, poorly investigated, thus limiting our understanding of the function and evolution of this important animal signaling pathway. In sea urchins, for example, while several studies focused on the availability and function of external sources of iodotyrosines, preliminary evidence suggests that an endogenous TH pathway might be in place. Here, integrating available literature with an in silico analysis, various homologous genes of the vertebrate TH molecular toolkit have been identified in the sea urchin Strongylocentrotus purpuratus. They include genes involved in the synthesis (Sp-Pxdn), metabolism (Sp-Dios), transport (Sp-Ttrl, Sp-Mct7/8/10) and response (Sp-Thr, Sp-Rxr and Sp-Integrin αP) to thyroid hormones. To understand the cell type(s) involved in TH synthesis and/or response, we studied the spatial expression of the TH toolkit during urchin development. Exploiting single-cell transcriptomics data in conjunction with in situ hybridization and immunohistochemistry, we identified cell types that are potentially producing or responding to THs in the sea urchin. Finally, growing sea urchin embryos until the larva stage with and without a source of inorganic iodine, we provided evidence that iodine organification is important for larval skeleton growth.

Thyroid-like hormone signaling in invertebrates and its potential role in initial screening of thyroid hormone system disrupting chemicals

This review examines the presence and evolution of thyroid-like systems in selected aquatic invertebrates to determine the potential use of these organisms in screens for vertebrate thyroid hormone axis disrupting chemicals (THADCs). Such a screen might support the phasing out of some vertebrate testing. Although arthropods including crustaceans do not contain a functional thyroid signaling system, elements of such a system exist in the aquatic phyla mollusks, echinoderms, tunicates, and cephalochordates. These phyla can synthesize thyroid hormone, which has been demonstrated in some groups to induce the nuclear thyroid hormone receptor (THR). Thyroid hormone may act in these phyla through interaction with a membrane integrin receptor. Thyroid hormone regulates inter alia metamorphosis but, unlike in vertebrates, this does not occur via receptor activation by the ligands triiodothyronine (T3) and thyroxine (T4). Instead, the unliganded nuclear receptor itself controls metamorphosis in mollusks, echinoderms, and tunicates, whereas the T3 derivative tri-iodothyroacetic acid (TRIAC) acts as a THR ligand in cephalochordates. In view of this, it may be possible to develop an invertebrate-based screen that is sensitive to vertebrate THADCs that interfere with thyroid hormone synthesis or metabolism along with interaction with membrane receptors. The review makes some recommendations for the need to develop an appropriate test method. Integr Environ Assess Manag 2023;19:63-82. © 2022 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

Differentiation of endostyle cells by Nkx2-1 and FoxE in the ascidian Ciona intestinalis type A: insights into shared gene regulation in glandular- and thyroid-equivalent elements of the chordate endostyle

Due to similarities in iodine concentrations and peroxidase activities, the thyroid in vertebrates is considered to originate from the endostyle of invertebrate chordates even though it is a glandular (mucus-producing) organ for aquatic suspension feeding. Among chordates with an endostyle, urochordates are useful evolutionary research models for the study of vertebrate traits. The ascidian Ciona intestinalis forms an endostyle with specific components of glandular- and thyroid-related elements, and molecular markers have been identified for these components. Since we previously examined a simple endostyle in the larvacean Oikopleura dioica, the expression of the thyroid-related transcription factor genes, Ciona Nkx2-1 and FoxE, was perturbed by TALEN-mediated gene knockout in the present study to elucidate the shared and/or divergent features of a complex ascidian endostyle. The knockout of Ciona Nkx2-1 and FoxE exerted different effects on the morphology of the developing endostyle. The knockout of Nkx2-1 eliminated the expression of both glandular and thyroidal differentiation marker genes, e.g., vWFL1, vWFL2, CiEnds1, TPO, and Duox, while that of FoxE eliminated the expression of the differentiation marker genes, TPO and CiEnds1. The supporting element-related expression of Pax2/5/8a, Pax2/5/8b, FoxQ1, and β-tubulin persisted in the hypoplastic endostyles of Nkx2-1- and FoxE-knockout juveniles. Although the gene regulation of ascidian-specific CiEnds1 remains unclear, these results provide insights into the evolution of the vertebrate thyroid as well as the urochordate endostyle.

Thyroid and endostyle development in cyclostomes provides new insights into the evolutionary history of vertebrates

Background

The endostyle is an epithelial exocrine gland found in non-vertebrate chordates (amphioxi and tunicates) and the larvae of modern lampreys. It is generally considered to be an evolutionary precursor of the thyroid gland of vertebrates. Transformation of the endostyle into the thyroid gland during the metamorphosis of lampreys is thus deemed to be a recapitulation of a past event in vertebrate evolution. In 1906, Stockard reported that the thyroid gland in hagfish, the sister cyclostome group of lampreys, develops through an endostyle-like primordium, strongly supporting the plesiomorphy of the lamprey endostyle. However, the findings in hagfish thyroid development were solely based on this single study, and these have not been confirmed by modern molecular, genetic, and morphological data pertaining to hagfish thyroid development over the last century.

Results

Here, we showed that the thyroid gland of hagfish undergoes direct development from the ventrorostral pharyngeal endoderm, where the previously described endostyle-like primordium was not found. The developmental pattern of the hagfish thyroid, including histological features and regulatory gene expression profiles, closely resembles that found in modern jawed vertebrates (gnathostomes). Meanwhile, as opposed to gnathostomes but similar to non-vertebrate chordates, lamprey and hagfish share a broad expression domain of Nkx2-1/2-4, a key regulatory gene, in the pharyngeal epithelium during early developmental stages.

Conclusions

Based on the direct development of the thyroid gland both in hagfish and gnathostomes, and the shared expression profile of thyroid-related transcription factors in the cyclostomes, we challenge the plesiomorphic status of the lamprey endostyle and propose an alternative hypothesis where the lamprey endostyle could be obtained secondarily in crown lampreys.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01282-7.

Comparative analysis of transcriptomic profiles among ascidians, zebrafish, and mice: Insights from tissue-specific gene expression

Tissue/organ-specific genes (TSGs) are important not only for understanding organ development and function, but also for investigating the evolutionary lineages of organs in animals. Here, we investigate the TSGs of 9 adult tissues of an ascidian, Ciona intestinalis Type A (Ciona robusta), which lies in the important position of being the sister group of vertebrates. RNA-seq and qRT-PCR identified the Ciona TSGs in each tissue, and BLAST searches identified their homologs in zebrafish and mice. Tissue distributions of the vertebrate homologs were analyzed and clustered using public RNA-seq data for 12 zebrafish and 30 mouse tissues. Among the vertebrate homologs of the Ciona TSGs in the neural complex, 48% and 63% showed high expression in the zebrafish and mouse brain, respectively, suggesting that the central nervous system is evolutionarily conserved in chordates. In contrast, vertebrate homologs of Ciona TSGs in the ovary, pharynx, and intestine were not consistently highly expressed in the corresponding tissues of vertebrates, suggesting that these organs have evolved in Ciona-specific lineages. Intriguingly, more TSG homologs of the Ciona stomach were highly expressed in the vertebrate liver (17-29%) and intestine (22-33%) than in the mouse stomach (5%). Expression profiles for these genes suggest that the biological roles of the Ciona stomach are distinct from those of their vertebrate counterparts. Collectively, Ciona tissues were categorized into 3 groups: i) high similarity to the corresponding vertebrate tissues (neural complex and heart), ii) low similarity to the corresponding vertebrate tissues (ovary, pharynx, and intestine), and iii) low similarity to the corresponding vertebrate tissues, but high similarity to other vertebrate tissues (stomach, endostyle, and siphons). The present study provides transcriptomic catalogs of adult ascidian tissues and significant insights into the evolutionary lineages of the brain, heart, and digestive tract of chordates.

Nkx2-1 and FoxE regionalize glandular (mucus-producing) and thyroid-equivalent traits in the endostyle of the chordate Oikopleura dioica

The endostyle is a ventral pharyngeal organ used for internal filter feeding of basal chordates and is considered homologous to the follicular thyroid of vertebrates. It contains mucus-producing (glandular) and thyroid-equivalent regions organized along the dorsoventral (DV) axis. Although thyroid-related genes (Nkx2-1, FoxE, and thyroid peroxidase (TPO)) are known to be expressed in the endostyle, their roles in establishing regionalization within the organ have not been demonstrated. We report that Nkx2-1 and FoxE are essential for establishing DV axial identity in the endostyle of Oikopleura dioica. Genome and expression analyses showed von Willebrand factor-like (vWFL) and TPO/dual oxidase (Duox)/Nkx2-1/FoxE as orthologs of glandular and thyroid-related genes, respectively. Knockdown experiments showed that Nkx2-1 is necessary for the expression of glandular and thyroid-related genes, whereas FoxE is necessary only for thyroid-related genes. Moreover, Nkx2-1 expression is necessary for FoxE expression in larvae during organogenesis. The results demonstrate the essential roles of Nkx2-1 and FoxE in establishing regionalization in the endostyle, including (1) the Nkx2-1-dependent glandular region, and (2) the Nkx2-1/FoxE-dependent thyroid-equivalent region. DV axial regionalization may be responsible for organizing glandular and thyroid-equivalent traits of the pharynx along the DV axis.

An Updated Staging System for Cephalochordate Development: One Table Suits Them All

Chordates are divided into three subphyla: Vertebrata, Tunicata, and Cephalochordata. Phylogenetically, the Cephalochordata, more commonly known as lancelets or amphioxus, constitute the sister group of Vertebrata and Tunicata. Lancelets are small, benthic, marine filter feeders, and their roughly three dozen described species are divided into three genera: Branchiostoma, Epigonichthys, and Asymmetron. Due to their phylogenetic position and their stereotypical chordate morphology and genome architecture, lancelets are key models for understanding the evolutionary history of chordates. Lancelets have thus been studied by generations of scientists, with the first descriptions of adult anatomy and developmental morphology dating back to the 19th century. Today, several different lancelet species are used as laboratory models, predominantly for developmental, molecular and genomic studies. Surprisingly, however, a universal staging system and an unambiguous nomenclature for developing lancelets have not yet been adopted by the scientific community. In this work, we characterized the development of the European lancelet (Branchiostoma lanceolatum) using confocal microscopy and compiled a streamlined developmental staging system, from fertilization through larval life, including an unambiguous stage nomenclature. By tracing growth curves of the European lancelet reared at different temperatures, we were able to show that our staging system permitted an easy conversion of any developmental time into a specific stage name. Furthermore, comparisons of embryos and larvae from the European lancelet (B. lanceolatum), the Florida lancelet (Branchiostoma floridae), two Asian lancelets (Branchiostoma belcheri and Branchiostoma japonicum), and the Bahamas lancelet (Asymmetron lucayanum) demonstrated that our staging system could readily be applied to other lancelet species. Although the detailed staging description was carried out on developing B. lanceolatum, the comparisons with other lancelet species thus strongly suggested that both staging and nomenclature are applicable to all extant lancelets. We conclude that this description of embryonic and larval development will be of great use for the scientific community and that it should be adopted as the new standard for defining and naming developing lancelets. More generally, we anticipate that this work will facilitate future studies comparing representatives from different chordate lineages.

On the origin of vertebrate body plan: Insights from the endoderm using the hourglass model

The vertebrate body plan is thought to be derived during the early Cambrian from a worm-like chordate ancestor. While all three germ layers were clearly involved in this innovation, the role of the endoderm remains elusive. According to the hourglass model, the optimal window for investigating the evolution of vertebrate endoderm-derived structures during cephalochordate development is from the Spemann's organizer stage to the opening of the mouth (Stages 1-7, described herein). Regulatory gene expression, examined during these stages, illustrate that the cephalochordate endoderm is patterned into 12 organ primordia. Early vertebrates inherited at least a portion of 6 of these primordia, while the remainder were lost. Of those that were preserved, we demonstrate that the vertebrate symmetric mouth was built on a vestige of the anterior pre-oral pit, that the pre-existing pharyngeal pouch in this chordate ancestor laid the foundation for the new neural crest cell (NCC)-derived vertebrate-type pharyngeal arches, that the thyroid evolved from the posterior endostyle primordim, that the pancreas was derived from the Pdx1-expressing diverticulum primordium, and the small and large intestines originated with the Cdx1-expressing hindgut rudiments. This investigation uncovers the evolutionary foundations of vertebrate endoderm-derived structures, and demonstrates that the number of organ primordia were reduced during evolution.

Molecular and evolutionary aspects of the protochordate digestive system

The digestive system is a functional unit consisting of an endodermal tubular structure (alimentary canal) and accessory organs that function in nutrition processing in most triploblastic animals. Various morphologies and apparatuses are formed depending on the phylogenetical relationship and food habits of the specific species. Nutrition processing and morphogenesis of the alimentary canal and accessory organs have both been investigated in vertebrates, mainly humans and mammals. When attempting to understand the evolutionary processes that led to the vertebrate digestive system, however, it is useful to examine other chordates, specifically protochordates, which share fundamental functional and morphogenetic molecules with vertebrates, which also possess non-duplicated genomes. In protochordates, basic anatomical and physiological studies have mainly described the characteristic traits of suspension feeders. Recent progress in genome sequencing has allowed researchers to comprehensively detail protochordate genes and has compared the genetic backgrounds among chordate nutrition processing and alimentary canal/accessory organ systems based on genomic information. Gene expression analyses have revealed spatiotemporal gene expression profiles in protochordate alimentary canals. Additionally, to investigate the basis of morphological diversity in the chordate alimentary canal and accessory organs, evolutionary developmental research has examined developmental transcription factors related to morphogenesis and anterior-posterior pattering of the alimentary canal and accessory organs. In this review, we summarize the current knowledge of molecules involved in nutrition processing and the development of the alimentary canal and accessory organs with innate immune and endocrine roles in protochordates and we explore the molecular basis for understanding the evolution of the chordate digestive system.

My Favorite Animal, Amphioxus: Unparalleled for Studying Early Vertebrate Evolution

Amphioxus represents the most basally divergent group in chordates and probably the best extant proxy to the ancestor of all chordates including vertebrates. The amphioxus, or lancelets, are benthic filter feeding marine animals and their interest as a model in research is due to their phylogenetic position and their anatomical and genetic stasis throughout their evolutionary history. From the first works in the 19th century to the present day, enormous progress is made mainly favored by technical development at different levels, from spawning induction and husbandry techniques, through techniques for studies of gene function or of the role of different signalling pathways through embryonic development, to functional genomics techniques. Together, these advances foretell a plethora of interesting developments in the world of research with the amphioxus model. Here, the discovery and development of amphioxus as a superb model organism in evolutionary and evolutionary-developmental biology are reviewed.

Functional Characterization of Thyrostimulin in Amphioxus Suggests an Ancestral Origin of the TH Signaling Pathway

Thyrostimulin, consisting of GpA2 and GpB5 subunits, has been identified in amphioxus, but to date, little is known about the roles of GPA2/GPB5‒type hormone in this evolutionarily important animal. We showed here that amphioxus GpA2, GpB5, and TSH receptor (TSHR) represent the archetypes of vertebrate TSHα, TSHβ, and TSHR, respectively, and both gpa2 and gpb5 were coexpressed in the Hatschek pit, a homolog of the vertebrate pituitary, in amphioxus. We also showed that recombinant amphioxus GpA2 and GpB5, like zebrafish TSHα and TSHβ, bound to both amphioxus and zebrafish TSHR and that tethered amphioxus thyrostimulin activated both protein kinase A and protein kinase C pathways in the cells expressing amphioxus TSHR. Moreover, we demonstrated that recombinant amphioxus thyrostimulin induced the production of thyroid hormone (TH) T4. Because genuine TSH is absent in amphioxus and thyrostimulin is the only and sole glycoprotein hormone, our data likely provide evidence that amphioxus thyrostimulin is a functional glycoprotein hormone that plays a role as TSH does in vertebrates. The data also suggest that the TH signaling pathway evolved in the basal chordate more than 500 million years ago.

Evolution of thyroid hormone signaling in animals: Non-genomic and genomic modes of action

Much research has focused on vertebrate thyroid hormone (TH) synthesis and their function in development and metabolism. While important differences in TH synthesis and signaling exist, comparative studies between vertebrates fail to explain the evolutionary origins of this important regulatory axis. For that, one needs to make sense out of the diverse TH effects which have been described in invertebrate phyla but for which a mechanistic understanding is largely missing. Almost every major group of non-vertebrate animals possesses the capability to synthesize and metabolize thyroid hormones and there is evidence for a nuclear thyroid hormone receptor mediated mechanism in the bilateria, especially in molluscs, echinoderms, cephalochordates and ascidians. Still, genomic pathways cannot fully explain many observed effects of thyroid hormones in groups such as cnidarians, molluscs, and echinoderms and it is therefore possible that TH may signal via other mechanisms, such as non-genomic signaling systems via membrane bound or cytoplasmic receptors. Here we provide a brief review of TH actions in selected invertebrate species and discuss the hypothesis that non-genomic TH action may have played a critical role in TH signaling throughout animal evolution.

Non-mammalian models reveal the role of alternative ligands for thyroid hormone receptors

Thyroid hormones, or THs, are well-known regulators of a wide range of biological processes that occur throughout the lifespan of all vertebrates. THs act through genomic mechanisms mediated by thyroid hormone receptors (TRs). The main product of the thyroid gland is thyroxine or T4, which can be further transformed by different biochemical pathways to produce at least 15 active or inactive molecules. T3, a product of T4 outer-ring deiodination, has been recognized as the main bioactive TH. However, growing evidence has shown that other TH derivatives are able to bind to, and/or activate TRs, to induce thyromimetic effects. The compiled data in this review points to at least two of these TR alternative ligands: TRIAC and T2. Taking this into account, non-mammalian models have proven to be advantageous to explore new TH derivatives with potential novel actions, prompting a re-evaluation of the role and mechanism of action of TR alternative ligands that were previously believed to be inactive. The functional implications of these ligands across different vertebrates may require us to reconsider current established notions of thyroid physiology.

Independent evolution of genomic characters during major metazoan transitions

Metazoan evolution encompasses a vast evolutionary time scale spanning over 600 million years. Our ability to infer ancestral metazoan characters, both morphological and functional, is limited by our understanding of the nature and evolutionary dynamics of the underlying regulatory networks. Increasing coverage of metazoan genomes enables us to identify the evolutionary changes of the relevant genomic characters such as the loss or gain of coding sequences, gene duplications, micro- and macro-synteny, and non-coding element evolution in different lineages. In this review we describe recent advances in our understanding of ancestral metazoan coding and non-coding features, as deduced from genomic comparisons. Some genomic changes such as innovations in gene and linkage content occur at different rates across metazoan clades, suggesting some level of independence among genomic characters. While their contribution to biological innovation remains largely unclear, we review recent literature about certain genomic changes that do correlate with changes to specific developmental pathways and metazoan innovations. In particular, we discuss the origins of the recently described pharyngeal cluster which is conserved across deuterostome genomes, and highlight different genomic features that have contributed to the evolution of this group. We also assess our current capacity to infer ancestral metazoan states from gene models and comparative genomics tools and elaborate on the future directions of metazoan comparative genomics relevant to evo-devo studies.

Identification of Thyroid Hormones and Functional Characterization of Thyroid Hormone Receptor in the Pacific Oyster Crassostrea gigas Provide Insight into Evolution of the Thyroid Hormone System

Thyroid hormones (THs) play important roles in development, metamorphosis, and metabolism in vertebrates. During the past century, TH functions were regarded as a synapomorphy of vertebrates. More recently, accumulating evidence has gradually convinced us that TH functions also occur in invertebrate chordates. To date, however, TH-related studies in non-chordate invertebrates have been limited. In this study, THs were qualitatively detected by two reliable methods (HPLC and LC/MS) in a well-studied molluscan species, the Pacific oyster Crassostrea gigas. Quantitative measurement of THs during the development of C. gigas showed high TH contents during embryogenesis and that oyster embryos may synthesize THs endogenously. As a first step in elucidating the TH signaling cascade, an ortholog of vertebrate TH receptor (TR), the most critical gene mediating TH effects, was cloned in C. gigas. The sequence of CgTR has conserved DNA-binding and ligand-binding domains that normally characterize these receptors. Experimental results demonstrated that CgTR can repress gene expression through binding to promoters of target genes and can interact with oyster retinoid X receptor. Moreover, CgTR mRNA expression was activated by T4 and the transcriptional activity of CgTR promoter was repressed by unliganded CgTR protein. An atypical thyroid hormone response element (CgDR5) was found in the promoter of CgTR, which was verified by electrophoretic mobility shift assay (EMSA). These results indicated that some of the CgTR function is conserved. However, the EMSA assay showed that DNA binding specificity of CgTR was different from that of the vertebrate TR and experiments with two dual-luciferase reporter systems indicated that l-thyroxine, 3,3′,5-triiodothyronine, and triiodothyroacetic acid failed to activate the transcriptional activity of CgTR. This is the first study to functionally characterize TR in mollusks. The presence of THs and the functions of CgTR in mollusks contribute to better understanding of the evolution of the TH system.

The Nodal signaling pathway controls left-right asymmetric development in amphioxus

Background

Nodal is an important determinant of the left-right (LR) body axis in bilaterians, specifying the right side in protostomes and non-chordate deuterostomes as opposed to the left side in chordates. Amphioxus represents an early-branching chordate group, rendering it especially useful for studying the character states that predate the origin of vertebrates. However, its anatomy, involving offset arrangement of axial structures, marked asymmetry of the oropharyngeal region, and, most notably, a mouth positioned on the left side, contrasts with the symmetric arrangement of the corresponding regions in other chordates.

Results

We show that the Nodal signaling pathway acts to specify the LR axis in the cephalochordate amphioxus in a similar way as in vertebrates. At early neurula stages, Nodal switches from initial bilateral to the left-sided expression and subsequently specifies the left embryonic side. Perturbation of Nodal signaling with small chemical inhibitors (SB505124 and SB431542) alters expression of other members of the pathway and of left/right-sided, organ-specific genes. Upon inhibition, larvae display loss of the innate alternation of both somites and axons of peripheral nerves and loss of left-sided pharyngeal structures, such as the mouth, the preoral pit, and the duct of the club-shaped gland. Concomitantly, the left side displays ectopic expression of otherwise right-sided genes, and the larvae exhibit bilaterally symmetrical morphology, with duplicated endostyle and club-shaped gland structures.

Conclusions

We demonstrate that Nodal signaling is necessary for establishing the LR embryonic axis and for developing profound asymmetry in amphioxus. Our data suggest that initial symmetry breaking in amphioxus and propagation of the pathway on the left side correspond with the situation in vertebrates. However, the organs that become targets of the pathway differ between amphioxus and vertebrates, which may explain the pronounced asymmetry of its oropharyngeal and axial structures and the left-sided position of the mouth.

Electronic supplementary material

The online version of this article (doi:10.1186/2041-9139-6-5) contains supplementary material, which is available to authorized users.

Larval body patterning and apical organs are conserved in animal evolution

Background

Planktonic ciliated larvae are characteristic for the life cycle of marine invertebrates. Their most prominent feature is the apical organ harboring sensory cells and neurons of largely undetermined function. An elucidation of the relationships between various forms of primary larvae and apical organs is key to understanding the evolution of animal life cycles. These relationships have remained enigmatic due to the scarcity of comparative molecular data.

Results

To compare apical organs and larval body patterning, we have studied regionalization of the episphere, the upper hemisphere of the trochophore larva of the marine annelid Platynereis dumerilii. We examined the spatial distribution of transcription factors and of Wnt signaling components previously implicated in anterior neural development. Pharmacological activation of Wnt signaling with Gsk3β antagonists abolishes expression of apical markers, consistent with a repressive role of Wnt signaling in the specification of apical tissue. We refer to this Wnt-sensitive, six3- and foxq2-expressing part of the episphere as the ‘apical plate’. We also unraveled a molecular signature of the apical organ - devoid of six3 but expressing foxj, irx, nkx3 and hox - that is shared with other marine phyla including cnidarians. Finally, we characterized the cell types that form part of the apical organ by profiling by image registration, which allows parallel expression profiling of multiple cells. Besides the hox-expressing apical tuft cells, this revealed the presence of putative light- and mechanosensory as well as multiple peptidergic cell types that we compared to apical organ cell types of other animal phyla.

Conclusions

The similar formation of a six3+, foxq2+ apical plate, sensitive to Wnt activity and with an apical tuft in its six3-free center, is most parsimoniously explained by evolutionary conservation. We propose that a simple apical organ - comprising an apical tuft and a basal plexus innervated by sensory-neurosecretory apical plate cells - was present in the last common ancestors of cnidarians and bilaterians. One of its ancient functions would have been the control of metamorphosis. Various types of apical plate cells would then have subsequently been added to the apical organ in the divergent bilaterian lineages. Our findings support an ancient and common origin of primary ciliated larvae.

Ciona intestinalis peroxinectin is a novel component of the peroxidase-cyclooxygenase gene superfamily upregulated by LPS

Peroxinectins function as hemoperoxidase and cell adhesion factor involved in invertebrate immune reaction. In this study, the ascidian (Ciona intestinalis) peroxinectin gene (CiPxt) and its expression during the inflammatory response have been examined. CiPxt is a new member of the peroxidase-cyclooxygenase gene superfamily that contains both the peroxidase domain and the integrin KGD (Lys-Gly-Asp) binding motif. A phylogenetic tree showed that CiPxt is very close to the chordate group and appears to be the outgroup of mammalian MPO, EPO and TPO clades. The CiPxt molecular structure model resulted superimposable to the human myeloperoxidase. The CiPxt mRNA expression is upregulated by LPS inoculation suggesting it is involved in C. intestinalis inflammatory response. The CiPxt was expressed in hemocytes (compartment/morula cells), vessel epithelium, and unilocular refractile granulocytes populating the inflamed tunic matrix and in the zones 7, 8 and 9 of the endostyle, a special pharynx organs homolog to the vertebrate thyroid gland.

Identification and expression analysis of BMP signaling inhibitors genes of the DAN family in amphioxus

Bone morphogenetic proteins (BMPs) are members of the Transforming Growth Factor-β (TGF-β) family implicated in many developmental processes in metazoans such as embryo axes specification. Their wide variety of actions is in part controlled by inhibitors that impede the interaction of BMPs with their specific receptors. Here, we focused our attention on the Differential screening-selected gene Aberrative in Neuroblastoma (DAN) family of inhibitors. Although they are well-characterized in vertebrates, few data are available for this family in other metazoan species. In order to understand the evolution of potential developmental roles of these inhibitors in chordates, we identified the members of this family in the cephalochordate amphioxus, and characterized their expression patterns during embryonic development. Our data suggest that the function of Cerberus/Dand5 subfamily genes is conserved among chordates, whereas Gremlin1/2 and NBL1 subfamily genes seem to have acquired divergent expression patterns in each chordate lineage. On the other hand, the expression of Gremlin in the amphioxus neural plate border during early neurulation strengthens the hypothesis of a conserved neural plate border gene network in chordates.

Recent insights into the cell biology of thyroid angiofollicular units

In thyrocytes, cell polarity is of crucial importance for proper thyroid function. Many intrinsic mechanisms of self-regulation control how the key players involved in thyroid hormone (TH) biosynthesis interact in apical microvilli, so that hazardous biochemical processes may occur without detriment to the cell. In some pathological conditions, this enzymatic complex is disrupted, with some components abnormally activated into the cytoplasm, which can lead to further morphological and functional breakdown. When iodine intake is altered, autoregulatory mechanisms outside the thyrocytes are activated. They involve adjacent capillaries that, together with thyrocytes, form the angiofollicular units (AFUs) that can be considered as the functional and morphological units of the thyroid. In response to iodine shortage, a rapid expansion of the microvasculature occurs, which, in addition to nutrients and oxygen, optimizes iodide supply. These changes are triggered by angiogenic signals released from thyrocytes via a reactive oxygen species/hypoxia-inducible factor/vascular endothelial growth factor pathway. When intra- and extrathyrocyte autoregulation fails, other forms of adaptation arise, such as euthyroid goiters. From onset, goiters are morphologically and functionally heterogeneous due to the polyclonal nature of the cells, with nodules distributed around areas of quiescent AFUs containing globules of compact thyroglobulin (Tg) and surrounded by a hypotrophic microvasculature. Upon TSH stimulation, quiescent AFUs are activated with Tg globules undergoing fragmentation into soluble Tg, proteins involved in TH biosynthesis being expressed and the local microvascular network extending. Over time and depending on physiological needs, AFUs may undergo repetitive phases of high, moderate, or low cell and tissue activity, which may ultimately culminate in multinodular goiters.

Identification, expression of a glycoprotein hormone receptor homolog in the amphioxus Branchiostoma belcheri with implications for origin of vertebrate GpHRs

Amphioxus is phylogenetically located at the most primitive position of the chordate clade. Its endocrine system has been extensively studied; however, key information about the glycoprotein hormone (GpH) and its receptor (GpHR) in the endocrine system remained to be elucidated. In this study, a GpHR homologous gene, BbGpHR-like, has been identified in the amphioxus Branchiostoma belcheri. It contains a 1377bp open reading frame that corresponds to a deduced protein of 458 amino acid residues with a predicted molecular mass of approximately 50kDa. Phylogenetic analysis shows that BbGpHR-like is located at the position between vertebrate and invertebrate, indicating amphioxus B. belcheri is in an evolutionary transitional state between vertebrate and invertebrate in the GpHR lineage. BbGpHR-like has been successfully expressed in Pichia pastoris. Western blot analysis confirmed that the receptor produced a cross-immunoreactivity with human thyrotropin receptor (TSHR), the luteinizing hormone receptor (LHR) and the follicle-stimulating hormone receptor (FSHR). In situ hybridization and immunohistochemical analyses revealed that the BbGpHR-like transcripts and proteins were strongly distributed in the ovaries, testes and/or endostyle. These findings suggest that BbGpHR-like possibly plays an ancient and fundamental role in the control of thyroid hormone synthesis and gametogenesis in chordates. Presumably, the amphioxus BbGpHR-like represents the ancestral form of the GpHR gene prior to its split to the vertebrate paralogs gonadotropin receptor and thyrotropin receptor.

Mechanisms of thyroid development and dysgenesis: an analysis based on developmental stages and concurrent embryonic anatomy

Thyroid dysgenesis is the most common cause of congenital hypothyroidism that affects 1 in 3000 newborns. Although a number of pathogenetic mutations in thyroid developmental genes have been identified, the molecular mechanism of disease is unknown in most cases. This chapter summarizes the current knowledge of normal thyroid development and puts the different developmental stages in perspective, from the time of foregut endoderm patterning to the final shaping of pharyngeal anatomy, for understanding how specific malformations may arise. At the cellular level, we will also discuss fate determination of follicular and C-cell progenitors and their subsequent embryonic growth, migration, and differentiation as the different thyroid primordia evolve and merge to establish the final size and shape of the gland.

Expression and regulation by thyroid hormone (TH) of zebrafish IGF-I gene and amphioxus IGFl gene with implication of the origin of TH/IGF signaling pathway

Thyroid hormone (TH)/insulin-like growth factor (IGF) signaling pathway has been identified in all the vertebrates, but its evolutionary origin remains elusive. In this study we examined the expression profiles in vitro as well as in vivo of the IGF-I gene of fish Danio rerio (vertebrate) and the IGF-like gene (IGFl) of amphioxus Branchiostoma japonicum (protochordate) following T(3) treatment. Our results showed that T(3) was able to enhance hepatic IGF-I/IGFl gene expression in vitro in both zebrafish and amphioxus in a dose-dependent manner. This T(3)-induced hepatic expression of IGF-I/IGFl genes in both species was significantly inhibited by the T(3)-specific inhibitor DEA, indicating the specificity of IGF-I/IGFl gene regulation by T(3). At 100nM T(3), in both the long (42h) and short (8h) time course experiments, the IGF-I/IGFl gene expression profiles following T(3) treatment in the tissue cultures of both species exhibited closely similar pattern and trend. Moreover, exposure of zebrafish and amphioxus to T(3)in vivo for 72h induced a significant increase in the expression of IGF-I/IGFl genes in both the liver and the hepatic caecum. These data together suggest that amphioxus and zebrafish both share a similar regulatory mechanism of IGF gene expression in response to T(3), providing an evidence for the presence of a vertebrate-like TH/IGF signaling pathway in the protochordate amphioxus.

Minireview: Intrinsic and extrinsic factors in thyroid gland development: an update

In vertebrates the portion of the thyroid gland synthesizing the thyroid hormones develops from a small group of endodermal cells in the foregut. The nature of the signals that lead to the biochemical and morphogenetic events responsible for the organization of these cells into the adult thyroid gland has only recently become evident. In this review we summarize recent developments in the understanding of these processes, derived from evidence collected in several organisms.

Ancient homeobox gene loss and the evolution of chordate brain and pharynx development: deductions from amphioxus gene expression

Homeobox genes encode a large superclass of transcription factors with widespread roles in animal development. Within chordates there are over 100 homeobox genes in the invertebrate cephalochordate amphioxus and over 200 in humans. Set against this general trend of increasing gene number in vertebrate evolution, some ancient homeobox genes that were present in the last common ancestor of chordates have been lost from vertebrates. Here, we describe the embryonic expression of four amphioxus descendants of these genes--AmphiNedxa, AmphiNedxb, AmphiMsxlx and AmphiNKx7. All four genes are expressed with a striking asymmetry about the left-right axis in the pharyngeal region of neurula embryos, mirroring the pronounced asymmetry of amphioxus embryogenesis. AmphiMsxlx and AmphiNKx7 are also transiently expressed in an anterior neural tube region destined to become the cerebral vesicle. These findings suggest significant rewiring of developmental gene regulatory networks occurred during chordate evolution, coincident with homeobox gene loss. We propose that loss of otherwise widely conserved genes is possible when these genes function in a confined role in development that is subsequently lost or significantly modified during evolution. In the case of these homeobox genes, we propose that this has occurred in relation to the evolution of the chordate pharynx and brain.

Active metabolism of thyroid hormone during metamorphosis of amphioxus

Thyroid hormones (THs), and more precisely the 3,3',5-triiodo-l-thyronine (T(3)) acetic derivative 3,3',5-triiodothyroacetic acid (TRIAC), have been shown to activate metamorphosis in amphioxus. However, it remains unknown whether TRIAC is endogenously synthesized in amphioxus and more generally whether an active TH metabolism is regulating metamorphosis. Here we show that amphioxus naturally produces TRIAC from its precursors T(3) and l-thyroxine (T(4)), supporting its possible role as the active TH in amphioxus larvae. In addition, we show that blocking TH production inhibits metamorphosis and that this effect is compensated by exogenous T(3), suggesting that a peak of TH production is important for advancement of proper metamorphosis. Moreover, several amphioxus genes encoding proteins previously proposed to be involved in the TH signaling pathway display expression profiles correlated with metamorphosis. In particular, thyroid hormone receptor (TR) and deiodinases gene expressions are either up- or down-regulated during metamorphosis and by TH treatments. Overall, these results suggest that an active TH metabolism controls metamorphosis in amphioxus, and that endogenous TH production and metabolism as well as TH-regulated metamorphosis are ancestral in the chordate lineage.

Cephalochordates (amphioxus or lancelets): a model for understanding the evolution of chordate characters

Cephalochordates, commonly called amphioxus or lancelets, are marine invertebrate chordates. They were traditionally held as the closest living relatives of vertebrates and have long occupied a key phylogenetic position in our discussions on the origin of vertebrates. However, recent molecular phylogenetic analyses have placed tunicates as the sister taxon of vertebrates and cephalochordates as the most basal chordate group. Therefore, studies on cephalochordates can also provide important insights for understanding the conserved patterning mechanisms for all chordates. Cephalochordates and vertebrates share several key chordate features, namely, the dorsal nerve cord, notochord, segmented somites, and pharyngeal gill slits. However, vertebrates have ~25% more genes than amphioxus and several features not present in cephalochordates, including a more elaborate head and forebrain, migratory neural crest, and neurogenic placodes. During the last two decades, expression patterns of important developmental genes have been used extensively to infer homologies between cephalochordate and vertebrate embryos. These studies have answered some long-standing questions concerning the evolution of vertebrates from their invertebrate ancestors and have also generated interesting hypotheses for further investigations. With the completion of a cephalochordate genome project in 2008, comparison between cephalochordate and vertebrate genomes has allowed us to postulate a picture for the genome of the last common ancestor of all chordates. Such comparisons are also providing important biological insights into the functional biology of cephalochordates and the evolutionary origin of developmental mechanisms that led to the emergence of the vertebrate body plan.

The amphioxus genome enlightens the evolution of the thyroid hormone signaling pathway

Thyroid hormones (THs) have pleiotropic effects on vertebrate development, with amphibian metamorphosis as the most spectacular example. However, developmental functions of THs in non-vertebrate chordates are largely hypothetical and even TH endogenous production has been poorly investigated. In order to get better insight into the evolution of the thyroid hormone signaling pathway in chordates, we have taken advantage of the recent release of the amphioxus genome. We found amphioxus homologous sequences to most of the genes encoding proteins involved in thyroid hormone signaling in vertebrates, except the fast-evolving thyroglobulin: sodium iodide symporter, thyroid peroxidase, deiodinases, thyroid hormone receptor, TBG, and CTHBP. As only some genes encoding proteins involved in TH synthesis regulation were retrieved (TRH, TSH receptor, and CRH receptor but not their corresponding receptors and ligands), there may be another mode of upstream regulation of TH synthesis in amphioxus. In accord with the notion that two whole genome duplications took place at the base of the vertebrate tree, one amphioxus gene often corresponded to several vertebrate homologs. However, some amphioxus specific duplications occurred, suggesting that several steps of the TH pathway were independently elaborated in the cephalochordate and vertebrate lineages. The present results therefore indicate that amphioxus is capable of producing THs. As several genes of the TH signaling pathway were also found in the sea urchin genome, we propose that the thyroid hormone signaling pathway is of ancestral origin in chordates, if not in deuterostomes, with specific elaborations in each lineage, including amphioxus.

The evolution of the ligand/receptor couple: a long road from comparative endocrinology to comparative genomics

Comparative endocrinology considers the evolution of bioregulatory systems and the anatomical structures and molecules that constitute the neuroendocrine and endocrine systems. One aim of comparative endocrinology is to trace the origins of the main endocrine systems. The understanding of the evolution of the ligand/receptor couple is central to this objective. One classical approach to tackle this question is the characterization of receptors and ligands in various types of non-model organisms using as a starting point the knowledge accumulated on classical models such as mammals (mainly human and mouse) and arthropods (with Drosophila among other insects). In this review we discuss the potential caveats associated to this two-by-two comparison between a classical model and non-model organisms. We suggest that the use of an evolutionary approach involving comparisons of several organisms in a coherent framework permits reconstruction of the most probable scenarios. The use of the vast amount of genomic data now available, coupled to functional experiments, offers unprecedented possibilities to trace back the origins of the main ligand/receptor couples.

Brain segmentation and trigeminal projections in the lamprey; with reference to vertebrate brain evolution

Vertebrate brains exhibit remarkable diversity in each animal group, reflecting evolutionary changes at the molecular-level developmental program of the nervous system that vertebrates have experienced. We focused on the developmental morphological plan of the brain to understand the evolutionary scenario that led to the above diversity. By comparing the organization of the brain of non-vertebrate chordates, cyclostomes and gnathostomes, a step-wise modification of brain patterning programs becomes apparent. Furthermore, by labeling the lamprey oral region, the somatotopic projections of the trigeminal nerve that enter into the hindbrain become visible. Finally, by combining the knowledge on rhombomere segments and neuronal projections, the evolutionary relationship between somatotopy and brain segmentation are discussed.