Retinoic acid signaling and neurogenic niche regulation in the developing peripheral nervous system of the cephalochordate amphioxus

Insights into vitamin A in bladder cancer, lack of attention to gut microbiota?

Vitamin A has long been associated with bladder cancer, and many exogenous vitamin A supplements, vitamin A derivatives, and synthetic drugs have been investigated over the years. However, the effectiveness of these strategies in clinical practice has not met expectations, and they have not been widely adopted. Recent medical research on intestinal flora has revealed that bladder cancer patients exhibit reduced serum vitamin A levels and an imbalance of gut microbiota. In light of the close relationship between gut microbiota and vitamin A, one can speculate that a complex regulatory mechanism exists between the two in the development and occurrence of bladder cancer. As such, further exploration of their interaction in bladder cancer may help guide the use of vitamin A for preventive purposes. During the course of this review, attention is paid to the influence of intestinal microbiota on the vitamin A metabolism and the RA signaling pathway, as well as the mutual promotion relationships between them in the prevention of bladder cancer, In addition, it emphasizes the importance of intestinal microbiota for bladder cancer prevention and treatment.

Retinoic Acid and POU Genes in Developing Amphioxus: A Focus on Neural Development

POU genes are a family of evolutionarily conserved transcription factors with key functions in cell type specification and neurogenesis. In vitro experiments have indicated that the expression of some POU genes is controlled by the intercellular signaling molecule retinoic acid (RA). In this work, we aimed to characterize the roles of RA signaling in the regulation of POU genes in vivo. To do so, we studied POU genes during the development of the cephalochordate amphioxus, an animal model crucial for understanding the evolutionary origins of vertebrates. The expression patterns of amphioxus POU genes were assessed at different developmental stages by chromogenic in situ hybridization and hybridization chain reaction. Expression was further assessed in embryos subjected to pharmacological manipulation of endogenous RA signaling activity. In addition to a detailed description of the effects of these treatments on amphioxus POU gene expression, our survey included the first description of Pou2 and Pou6 expression in amphioxus embryos. We found that Pit-1, Pou2, Pou3l, and Pou6 expression are not affected by alterations of endogenous RA signaling levels. In contrast, our experiments indicated that Brn1/2/4 and Pou4 expression are regulated by RA signaling in the endoderm and the nerve cord, respectively. The effects of the treatments on Pou4 expression in the nerve cord revealed that, in developing amphioxus, RA signaling plays a dual role by (1) providing anteroposterior patterning information to neural cells and (2) specifying neural cell types. This finding is coherent with a terminal selector function of Pou4 for GABAergic neurons in amphioxus and represents the first description of RA-induced changes in POU gene expression in vivo.

Highly distinct genetic programs for peripheral nervous system formation in chordates

Background

Vertebrates develop their peripheral nervous system (PNS) from transient unique embryonic structures, the neural crest, and the ectodermal placodes that are located at the border of the forming central nervous system. By contrast, in the invertebrate chordates, amphioxus and ascidians, a large part of the PNS originates at the opposite of the embryo, in the ventral ectoderm. In both groups, a biphasic mechanism regulates ventral PNS formation: high BMP levels specify a neurogenic territory within which glutamatergic epidermal sensory neuron formation is controlled by the Notch pathway. Given these similarities and the phylogenetic relationships within chordates, it is likely that ventral PNS is an ancestral feature in chordates and that it has been lost in vertebrates.

Results

In order to get insights into the molecular control of ventral PNS formation and to test the hypothesis of their homology and potential contribution to the emergence of vertebrate PNS, we undertook a close comparison of ventral PNS formation in the ascidian Phallusia mammillata and the amphioxus Branchiostoma lanceolatum. Using timed RNA-seq series, we identified novel markers of the ventral PNS during different phases of its development in both species. By extensively determining the expression of paralogous and orthologous genes, we observed that only a minority of genes have a shared expression in the ventral PNS. However, a large fraction of ventral PNS orthologous genes are expressed in the dorsally forming PNS of vertebrates.

Conclusions

Our work has significantly increased the molecular characterization of ventral PNS formation in invertebrate chordates. The low observed conservation of gene expression in the ventral PNS suggests that the amphioxus and ascidian ventral PNS are either not homologous, or alternatively extensive drift has occurred in their regulatory mechanisms following a long period (600 My) of separate evolution and accelerated evolution in the ascidian lineage. The homology to genes expressed in the dorsally forming PNS of vertebrates suggests that ancestral sensory neurons gene networks have been redeployed in vertebrates.

Functional Conservation and Genetic Divergence of Chordate Glycinergic Neurotransmission: Insights from Amphioxus Glycine Transporters

Glycine is an important neurotransmitter in vertebrates, performing both excitatory and inhibitory actions. Synaptic levels of glycine are tightly controlled by the action of two glycine transporters, GlyT1 and GlyT2, located on the surface of glial cells and neurons, respectively. Only limited information is available on glycinergic neurotransmission in invertebrates, and the evolution of glycinergic neurotransmission is poorly understood. Here, by combining phylogenetic and gene expression analyses, we characterized the glycine transporter complement of amphioxus, an important invertebrate model for studying the evolution of chordates. We show that amphioxus possess three glycine transporter genes. Two of these (GlyT2.1 and GlyT2.2) are closely related to GlyT2 of vertebrates, whereas the third (GlyT) is a member of an ancestral clade of deuterostome glycine transporters. GlyT2.2 expression is predominantly non-neural, whereas GlyT and GlyT2.1 are widely expressed in the amphioxus nervous system and are differentially expressed, respectively, in neurons and glia. Vertebrate glycinergic neurons express GlyT2 and glia GlyT1, suggesting that the evolution of the chordate glycinergic system was accompanied by a paralog-specific inversion of gene expression. Despite this genetic divergence between amphioxus and vertebrates, we found strong evidence for conservation in the role glycinergic neurotransmission plays during larval swimming, the implication being that the neural networks controlling the rhythmic movement of chordate bodies may be homologous.

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.

Amphioxus neuroglia: Molecular characterization and evidence for early compartmentalization of the developing nerve cord

Glial cells play important roles in the development and homeostasis of metazoan nervous systems. However, while their involvement in the development and function in the central nervous system (CNS) of vertebrates is increasingly well understood, much less is known about invertebrate glia and the evolutionary history of glial cells more generally. An investigation into amphioxus glia is therefore timely, as this organism is the best living proxy for the last common ancestor of all chordates, and hence provides a window into the role of glial cell development and function at the transition of invertebrates and vertebrates. We report here our findings on amphioxus glia as characterized by molecular probes correlated with anatomical data at the transmission electron microscopy (TEM) level. The results show that amphioxus glial lineages express genes typical of vertebrate astroglia and radial glia, and that they segregate early in development, forming what appears to be a spatially separate cell proliferation zone positioned laterally, between the dorsal and ventral zones of neural cell proliferation. Our study provides strong evidence for the presence of vertebrate-type glial cells in amphioxus, while highlighting the role played by segregated progenitor cell pools in CNS development. There are implications also for our understanding of glial cells in a broader evolutionary context, and insights into patterns of precursor cell deployment in the chordate nerve cord.

High-content analysis of larval phenotypes for the screening of xenobiotic toxicity using Phallusia mammillata embryos

In recent years, pollution of surface waters with xenobiotic compounds became an issue of concern in society and has been the object of numerous studies. Most of these xenobiotic compounds are man-made molecules and some of them are qualified as endocrine disrupting chemicals (EDCs) when they interfere with hormones actions. Several studies have investigated the teratogenic impacts of EDCs in vertebrates (including marine vertebrates). However, the impact of such EDCs on marine invertebrates is much debated and still largely obscure. In addition, DNA-altering genotoxicants can induce embryonic malformations. The goal of this study is to develop a reliable and effective test for assessing toxicity of chemicals using embryos of the ascidian (Phallusia mammillata) in order to find phenotypic signatures associated with xenobiotics. We evaluated embryonic malformations with high-content analysis of larval phenotypes by scoring several quantitative and qualitative morphometric endpoints on a single image of Phallusia tadpole larvae with semi-automated image analysis. Using this approach we screened different classes of toxicants including genotoxicants, known or suspected EDCs and nuclear receptors (NRs) ligands. The screen presented here reveals a specific phenotypic signature for ligands of retinoic acid receptor/retinoid X receptor. Analysis of larval morphology combined with DNA staining revealed that embryos with DNA aberrations displayed severe malformations affecting multiple aspects of embryonic development. In contrast EDCs exposure induced no or little DNA aberrations and affected mainly neural development. Therefore the ascidian embryo/larval assay presented here can allow to distinguish the type of teratogenicity induced by different classes of toxicants.

The evolutionary origins of the vertebrate olfactory system

Vertebrates develop an olfactory system that detects odorants and pheromones through their interaction with specialized cell surface receptors on olfactory sensory neurons. During development, the olfactory system forms from the olfactory placodes, specialized areas of the anterior ectoderm that share cellular and molecular properties with placodes involved in the development of other cranial senses. The early-diverging chordate lineages amphioxus, tunicates, lampreys and hagfishes give insight into how this system evolved. Here, we review olfactory system development and cell types in these lineages alongside chemosensory receptor gene evolution, integrating these data into a description of how the vertebrate olfactory system evolved. Some olfactory system cell types predate the vertebrates, as do some of the mechanisms specifying placodes, and it is likely these two were already connected in the common ancestor of vertebrates and tunicates. In stem vertebrates, this evolved into an organ system integrating additional tissues and morphogenetic processes defining distinct olfactory and adenohypophyseal components, followed by splitting of the ancestral placode to produce the characteristic paired olfactory organs of most modern vertebrates.

Cephalochordates: A window into vertebrate origins

How vertebrates evolved from their invertebrate ancestors has long been a central topic of discussion in biology. Evolutionary developmental biology (evodevo) has provided a new tool-using gene expression patterns as phenotypic characters to infer homologies between body parts in distantly related organisms-to address this question. Combined with micro-anatomy and genomics, evodevo has provided convincing evidence that vertebrates evolved from an ancestral invertebrate chordate, in many respects resembling a modern amphioxus. The present review focuses on the role of evodevo in addressing two major questions of chordate evolution: (1) how the vertebrate brain evolved from the much simpler central nervous system (CNS) in of this ancestral chordate and (2) whether or not the head mesoderm of this ancestor was segmented.

Whole mount in situ hybridization and immunohistochemistry for studying retinoic acid signaling in developing amphioxus

Retinoic acid (RA) is a vitamin A-derived signaling molecule acting during development and in the adult. This chapter provides protocols to characterize the role of RA signaling during development of the invertebrate chordate amphioxus. As sister group to all other chordates and characterized by the most vertebrate-like RA signaling system of all invertebrates, amphioxus is an important model for studying the evolution of RA signaling. Focusing on the development of GABAergic neurons in the amphioxus central nervous system, we provide detailed protocols for maintaining and breeding adult animals, for performing pharmacological treatments of embryos and for analyzing the effects of these treatments by whole mount in situ hybridization and immunohistochemistry coupled to confocal microscopy.