Spatial and temporal patterns of gene expression during neurogenesis in the sea urchin Lytechinus variegatus

Echinobase: a resource to support the echinoderm research community

Echinobase (www.echinobase.org) is a model organism knowledgebase serving as a resource for the community that studies echinoderms, a phylum of marine invertebrates that includes sea urchins and sea stars. Echinoderms have been important experimental models for over 100 years and continue to make important contributions to environmental, evolutionary, and developmental studies, including research on developmental gene regulatory networks. As a centralized resource, Echinobase hosts genomes and collects functional genomic data, reagents, literature, and other information for the community. This third-generation site is based on the Xenbase knowledgebase design and utilizes gene-centric pages to minimize the time and effort required to access genomic information. Summary gene pages display gene symbols and names, functional data, links to the JBrowse genome browser, and orthology to other organisms and reagents, and tabs from the Summary gene page contain more detailed information concerning mRNAs, proteins, diseases, and protein-protein interactions. The gene pages also display 1:1 orthologs between the fully supported species Strongylocentrotus purpuratus (purple sea urchin), Lytechinus variegatus (green sea urchin), Patiria miniata (bat star), and Acanthaster planci (crown-of-thorns sea star). JBrowse tracks are available for visualization of functional genomic data from both fully supported species and the partially supported species Anneissia japonica (feather star), Asterias rubens (sugar star), and L. pictus (painted sea urchin). Echinobase serves a vital role by providing researchers with annotated genomes including orthology, functional genomic data aligned to the genomes, and curated reagents and data. The Echinoderm Anatomical Ontology provides a framework for standardizing developmental data across the phylum, and knowledgebase content is formatted to be findable, accessible, interoperable, and reusable by the research community.

Solute carrier (SLC) expression reveals skeletogenic cell diversity

Within the developing embryo is a microcosm of cell type diversity. Single cell RNA-sequencing (scRNA-seq) is used to reveal cell types, typically by grouping cells according to their gene regulatory states. However, both across and within these regulatory states are additional layers of cellular diversity represented by the differential expression of genes that govern cell function. Here, we analyzed scRNA-seq data representing the late gastrula stage of Strongylocentrotus purpuratus (purple sea urchin) to understand the patterning of transporters belonging to the ABC and SLC families. These transporters handle diverse substrates from amino acids to signaling molecules, nutrients and xenobiotics. Using transporter-based clustering, we identified unique transporter patterns that are both shared across cell lineages, as well as those that were unique to known cell types. We further explored three patterns of transporter expression in mesodermal cells including secondary mesenchyme cells (pigment cells and blastocoelar cells) and skeletogenic cells (primary mesenchyme cells). The results revealed the enrichment of SMTs potentially involved in nutrient absorption (SLC5A9, SLC7A11, SLC35F3, and SLC52A3) and skeletogenesis (SLC9A3, SLC13A2/3/5, and SLC39A13) in pigment cells and blastocoelar cells respectively. The results indicated that the strategy of clustering by cellular activity can be useful for discovering cellular populations that would otherwise remain obscured.

Genome-wide identification and comparative analysis of Dmrt genes in echinoderms

The Dmrt (Doublesex-mab3-related transcription factor) gene family is a class of crucial transcription factors characterized by one or several conserved DM (Doublesex/Mab-3) domains. Dmrt family genes can participate in various physiological developmental processes, especially in sex determination/differentiation. Echinoderms are extremely important research objects in various fields, such as sex determination/differentiation and neuroscience. However, to date, the genome-wide characterization and analysis of Dmrt genes in echinoderms have not been investigated. In this study, the identification and analysis of Dmrt genes in 11 representative echinoderms were performed using bioinformatics methods. A total of 43 Dmrt genes have been found in the studied echinoderms, and the number of Dmrt genes in different species ranges from 2 to 5. The phylogenetic tree showed that all Dmrt genes from echinoderms can be subdivided into 5 classes, the Dmrt2-like class, Dmrt3-like class, Dmrt4/5-like class, Dsx-like class, and a novel Dmrt (starfish-specific) class. Furthermore, selective pressure assessment suggested that the Dmrt genes underwent purifying selection pressure. In general, this study provides a molecular basis for echinoderm Dmrt genes and may serve as a reference for in-depth phylogenomics.

Early expression onset of tissue-specific effector genes during the specification process in sea urchin embryos

BACKGROUND

In the course of animal developmental processes, various tissues are differentiated through complex interactions within the gene regulatory network. As a general concept, differentiation has been considered to be the endpoint of specification processes. Previous works followed this view and provided a genetic control scheme of differentiation in sea urchin embryos: early specification genes generate distinct regulatory territories in an embryo to express a small set of differentiation driver genes; these genes eventually stimulate the expression of tissue-specific effector genes, which provide biological identity to differentiated cells, in each region. However, some tissue-specific effector genes begin to be expressed in parallel with the expression onset of early specification genes, raising questions about the simplistic regulatory scheme of tissue-specific effector gene expression and the current concept of differentiation itself.

RESULTS

Here, we examined the dynamics of effector gene expression patterns during sea urchin embryogenesis. Our transcriptome-based analysis indicated that many tissue-specific effector genes begin to be expressed and accumulated along with the advancing specification GRN in the distinct cell lineages of embryos. Moreover, we found that the expression of some of the tissue-specific effector genes commences before cell lineage segregation occurs.

CONCLUSIONS

Based on this finding, we propose that the expression onset of tissue-specific effector genes is controlled more dynamically than suggested in the previously proposed simplistic regulation scheme. Thus, we suggest that differentiation should be conceptualized as a seamless process of accumulation of effector expression along with the advancing specification GRN. This pattern of effector gene expression may have interesting implications for the evolution of novel cell types.

Prohibitins in neurodegeneration and mitochondrial homeostasis

The incidence of age-related neurodegenerative disorders has risen with the increase of life expectancy. Unfortunately, the diagnosis of such disorders is in most cases only possible when the neurodegeneration status is already advanced, and symptoms are evident. Although age-related neurodegeneration is a common phenomenon in living animals, the cellular and molecular mechanisms behind remain poorly understood. Pathways leading to neurodegeneration usually diverge from a common starting point, mitochondrial stress, which can serve as a potential target for early diagnosis and treatments. Interestingly, the evolutionarily conserved mitochondrial prohibitin (PHB) complex is a key regulator of ageing and metabolism that has been associated with neurodegenerative diseases. However, its role in neurodegeneration is still not well characterized. The PHB complex shows protective or toxic effects in different genetic and physiological contexts, while mitochondrial and cellular stress promote both up and downregulation of PHB expression. With this review we aim to shed light into the complex world of PHB’s function in neurodegeneration by putting together the latest advances in neurodegeneration and mitochondrial homeostasis associated with PHB. A better understanding of the role of PHB in neurodegeneration will add knowledge to neuron deterioration during ageing and help to identify early molecular markers of mitochondrial stress. This review will deepen our understanding of age-related neurodegeneration and provide questions to be addressed, relevant to human health and to improve the life quality of the elderly.

Development of a larval nervous system in the sea urchin

This review reports recent findings on the specification and patterning of neurons that establish the larval nervous system of the sea urchin embryo. Neurons originate in three regions of the embryo. Perturbation analyses enabled construction of gene regulatory networks controlling the several neural cell types. Many of the mechanisms described reflect shared features of all metazoans and others are conserved among deuterostomes. This nervous system with a very small number of neurons supports the feeding and swimming behaviors of the larva until metamorphosis when an adult nervous system replaces that system.

The evolution of the metazoan Toll receptor family and its expression during protostome development

BACKGROUND

Toll-like receptors (TLRs) play a crucial role in immunity and development. They contain leucine-rich repeat domains, one transmembrane domain, and one Toll/IL-1 receptor domain. TLRs have been classified into V-type/scc and P-type/mcc TLRs, based on differences in the leucine-rich repeat domain region. Although TLRs are widespread in animals, detailed phylogenetic studies of this gene family are lacking. Here we aim to uncover TLR evolution by conducting a survey and a phylogenetic analysis in species across Bilateria. To discriminate between their role in development and immunity we furthermore analyzed stage-specific transcriptomes of the ecdysozoans Priapulus caudatus and Hypsibius exemplaris, and the spiralians Crassostrea gigas and Terebratalia transversa.

RESULTS

We detected a low number of TLRs in ecdysozoan species, and multiple independent radiations within the Spiralia. V-type/scc and P-type/mcc type-receptors are present in cnidarians, protostomes and deuterostomes, and therefore they emerged early in TLR evolution, followed by a loss in xenacoelomorphs. Our phylogenetic analysis shows that TLRs cluster into three major clades: clade α is present in cnidarians, ecdysozoans, and spiralians; clade β in deuterostomes, ecdysozoans, and spiralians; and clade γ is only found in spiralians. Our stage-specific transcriptome and in situ hybridization analyses show that TLRs are expressed during development in all species analyzed, which indicates a broad role of TLRs during animal development.

CONCLUSIONS

Our findings suggest that a clade α TLR gene (TLR-Ca) and a clade β/γ TLR gene (TLR-Cβ/γ) were already present in the cnidarian-bilaterian common ancestor. However, although TLR-Ca was conserved in cnidarians, TLR-Cβ/γ was lost during the early evolution of these taxa. Moreover, TLR-Cβ/γ duplicated to generate TLR-Cβ and TLR-Cγ in the lineage to the last common protostome-deuterostome ancestor. TLR-Ca, TLR-Cβ and TLR-Cγ further expanded generating the three major TLR clades. While all three clades radiated in several spiralian lineages, specific TLRs clades have been presumably lost in other lineages. Furthermore, the expression of the majority of these genes during protostome ontogeny suggests a likely role in development.

Involvement of Netrin/Unc-5 Interaction in Ciliary Beating and in Pattern Formation of the Ciliary Band-Associated Strand (CBAS) in the Sea Urchin, Hemicentrotus pulcherrimus

The GABAergic neural circuit is involved in the motile activities of both larval and juvenile sea urchins. Therefore, its function is inherited beyond metamorphosis, despite large scale remodeling of larval organs during that period. However, the initial neural circuit formation mechanism is not well understood, including how glutamate decarboxylase-expressing blastocoelar cells (GADCs) construct the neural circuit along the circumoral ciliary band (a ciliary band-associated strand, CBAS) on the larval body surface. In this study, using whole-mount immunohistochemistry and 3D reconstructed imaging, the ontogenic process of CBAS patterning was studied by focusing on Netrin and the interaction with its receptor, Unc-5. During the early 2-arm pluteus stage, a small number of GADCs egress onto the apical surface of the larval ectoderm. Then, they line up on the circumoral side of the ciliary band, and by being inserted by a further number of GADCs, form longer multicellular strands along the Netrin stripe. Application of a synthetic peptide, CRFNMELYKLSGRKSGGVC of Hp-Netrin, that binds to the immunoglobulin domain of Unc-5 during the prism stage, causes stunted CBAS formation due to inhibition of GADC egression. This also results in reduced ciliary beating. Thus, the Netrin/Unc-5 interaction is involved in the construction and function of the CBAS.

Developmental origin of peripheral ciliary band neurons in the sea urchin embryo

In the sea urchin larva, most neurons lie within an ectodermal region called the ciliary band. Our understanding of the mechanisms of specification and patterning of these peripheral ciliary band neurons is incomplete. Here, we first examine the gene regulatory landscape from which this population of neural progenitors arise in the neuroectoderm. We show that ciliary band neural progenitors first appear in a bilaterally symmetric pattern on the lateral edges of chordin expression in the neuroectoderm. Later in development, these progenitors appear in a salt-and-pepper pattern in the ciliary band where they express soxC, and prox, which are markers of neural specification, and begin to express synaptotagminB, a marker of differentiated neurons. We show that the ciliary band expresses the acid sensing ion channel gene asicl, which suggests that ciliary band neurons control the larva's ability to discern touch sensitivity. Using a chemical inhibitor of MAPK signaling, we show that this signaling pathway is required for proper specification and patterning of ciliary band neurons. Using live imaging, we show that these neural progenitors undergo small distance migrations in the embryo. We then show that the normal swimming behavior of the larvae is compromised if the neurogenesis pathway is perturbed. The developmental sequence of ciliary band neurons is very similar to that of neural crest-derived sensory neurons in vertebrates and may provide insights into the evolution of sensory neurons in deuterostomes.

Diversity of cilia-based mechanosensory systems and their functions in marine animal behaviour

Sensory cells that detect mechanical forces usually have one or more specialized cilia. These mechanosensory cells underlie hearing, proprioception or gravity sensation. To date, it is unclear how cilia contribute to detecting mechanical forces and what is the relationship between mechanosensory ciliated cells in different animal groups and sensory systems. Here, we review examples of ciliated sensory cells with a focus on marine invertebrate animals. We discuss how various ciliated cells mediate mechanosensory responses during feeding, tactic responses or predator-prey interactions. We also highlight some of these systems as interesting and accessible models for future in-depth behavioural, functional and molecular studies. We envisage that embracing a broader diversity of organisms could lead to a more complete view of cilia-based mechanosensation. This article is part of the Theo Murphy meeting issue 'Unity and diversity of cilia in locomotion and transport'.

Phylogenetic analysis and embryonic expression of panarthropod Dmrt genes

Background

One set of the developmentally important Doublesex and Male-abnormal-3 Related Transcription factors (Dmrt) is subject of intense research, because of their role in sex-determination and sexual differentiation. This likely non-monophyletic group of Dmrt genes is represented by the Drosophila melanogaster gene Doublesex (Dsx), the Caenorhabditis elegans Male-abnormal-3 (Mab-3) gene, and vertebrate Dmrt1 genes. However, other members of the Dmrt family are much less well studied, and in arthropods, including the model organism Drosophila melanogaster, data on these genes are virtually absent with respect to their embryonic expression and function.

Results

Here we investigate the complete set of Dmrt genes in members of all main groups of Arthropoda and a member of Onychophora, extending our data to Panarthropoda as a whole. We confirm the presence of at least four families of Dmrt genes (including Dsx-like genes) in Panarthropoda and study their expression profiles during embryogenesis. Our work shows that the expression patterns of Dmrt11E, Dmrt93B, and Dmrt99B orthologs are highly conserved among panarthropods. Embryonic expression of Dsx-like genes, however, is more derived, likely as a result of neo-functionalization after duplication.

Conclusions

Our data suggest deep homology of most of the panarthropod Dmrt genes with respect to their function that likely dates back to their last common ancestor. The function of Dsx and Dsx-like genes which are critical for sexual differentiation in animals, however, appears to be much less conserved.

Electronic supplementary material

The online version of this article (10.1186/s12983-019-0322-0) contains supplementary material, which is available to authorized users.