Zygotic LvBMP5-8 is required for skeletal patterning and for left–right but not dorsal–ventral specification in the sea urchin embryo

Polychrome labeling reveals skeletal triradiate and elongation dynamics and abnormalities in patterning cue-perturbed embryos

The larval skeleton of the sea urchin Lytechinus variegatus is an ideal model system for studying skeletal patterning; however, our understanding of the etiology of skeletal patterning in sea urchin larvae is limited due to the lack of approaches to live-image skeleton formation. Calcium-binding fluorochromes have been used to study the temporal dynamics of bone growth and healing. To date, only calcein green has been used in sea urchin larvae to fluorescently label the larval skeleton. Here, we optimize labeling protocols for two additional calcium-binding fluorochromes: xylenol orange and calcein blue- and demonstrate that these fluorochromes can be used individually or in nested pulse-chase experiments to understand the temporal dynamics of skeletogenesis and patterning. Using a pulse-chase approach, we show that the initiation of skeletogenesis begins around 15 h post fertilization. We also assess the timing of triradiate formation in embryos treated with a range of patterning perturbagens and demonstrate that triradiate formation is delayed and asynchronous in embryos ventralized via treatment with either nickel or chlorate. Finally, we measure the extent of fluorochrome incorporation in triple-labeled embryos to determine the elongation rate of numerous skeletal elements throughout early skeletal patterning and compare this to the rate of skeletal growth in embryos treated with axitinib to inhibit VEGFR. We find that skeletal elements elongate much more slowly in axitinib-treated embryos, and that axitinib treatment is sufficient to induce abnormal orientation of the triradiates.

Ethanol exposure perturbs sea urchin development and disrupts developmental timing

Ethanol is a known vertebrate teratogen that causes craniofacial defects as a component of fetal alcohol syndrome (FAS). Our results show that sea urchin embryos treated with ethanol similarly show broad skeletal patterning defects, potentially analogous to the defects associated with FAS. The sea urchin larval skeleton is a simple patterning system that involves only two cell types: the primary mesenchymal cells (PMCs) that secrete the calcium carbonate skeleton and the ectodermal cells that provide migratory, positional, and differentiation cues for the PMCs. Perturbations in RA biosynthesis and Hh signaling pathways are thought to be causal for the FAS phenotype in vertebrates. Surprisingly, our results indicate that these pathways are not functionally relevant for the teratogenic effects of ethanol in developing sea urchins. We found that developmental morphology as well as the expression of some ectodermal and PMC genes was delayed by ethanol exposure. Temporal transcriptome analysis revealed significant impacts of ethanol on signaling and metabolic gene expression, and a disruption in the timing of GRN gene expression that includes both delayed and precocious gene expression throughout the specification network. We conclude that the skeletal patterning perturbations in ethanol-treated embryos likely arise from a loss of temporal synchrony within and between the instructive and responsive tissues.

The developmental transcriptome for Lytechinus variegatus exhibits temporally punctuated gene expression changes

Embryonic development is arguably the most complex process an organism undergoes during its lifetime, and understanding this complexity is best approached with a systems-level perspective. The sea urchin has become a highly valuable model organism for understanding developmental specification, morphogenesis, and evolution. As a non-chordate deuterostome, the sea urchin occupies an important evolutionary niche between protostomes and vertebrates. Lytechinus variegatus (Lv) is an Atlantic species that has been well studied, and which has provided important insights into signal transduction, patterning, and morphogenetic changes during embryonic and larval development. The Pacific species, Strongylocentrotus purpuratus (Sp), is another well-studied sea urchin, particularly for gene regulatory networks (GRNs) and cis-regulatory analyses. A well-annotated genome and transcriptome for Sp are available, but similar resources have not been developed for Lv. Here, we provide an analysis of the Lv transcriptome at 11 timepoints during embryonic and larval development. Temporal analysis suggests that the gene regulatory networks that underlie specification are well-conserved among sea urchin species. We show that the major transitions in variation of embryonic transcription divide the developmental time series into four distinct, temporally sequential phases. Our work shows that sea urchin development occurs via sequential intervals of relatively stable gene expression states that are punctuated by abrupt transitions.

Gastrulation in the sea urchin

Gastrulation is arguably the most important evolutionary innovation in the animal kingdom. This process provides the basic embryonic architecture, an inner layer separated from an outer layer, from which all animal forms arise. An extraordinarily simple and elegant process of gastrulation is observed in the sea urchin embryo. The cells participating in sea urchin gastrulation are specified early during cleavage. One outcome of that specification is the expression of transcription factors that control each of the many subsequent morphogenetic changes. The first of these movements is an epithelial-mesenchymal transition (EMT) of skeletogenic mesenchyme cells, then EMT of pigment cell progenitors. Shortly thereafter, invagination of the archenteron occurs. At the end of archenteron extension, a second wave of EMT occurs to release immune cells into the blastocoel and primordial germ cells that will home to the coelomic pouches. The archenteron then remodels to establish the three parts of the gut, and at the anterior end, the gut fuses with the stomodaeum to form the through-gut. As part of the anterior remodeling, mesodermal coelomic pouches bud off the lateral sides of the archenteron tip. Multiple cell biological processes conduct each of these movements and in some cases the upstream transcription factors controlling this process have been identified. Remarkably, each event seamlessly occurs at the right time to orchestrate formation of the primitive body plan. This review covers progress toward understanding many of the molecular mechanisms underlying this sequence of morphogenetic events.

The Bmp signaling pathway regulates development of left-right asymmetry in amphioxus

Establishment of asymmetry along the left-right (LR) body axis in vertebrates requires interplay between Nodal and Bmp signaling pathways. In the basal chordate amphioxus, the left-sided activity of the Nodal signaling has been attributed to the asymmetric morphogenesis of paraxial structures and pharyngeal organs, however the role of Bmp signaling in LR asymmetry establishment has not been addressed to date. Here, we show that Bmp signaling is necessary for the development of LR asymmetric morphogenesis of amphioxus larvae through regulation of Nodal signaling. Loss of Bmp signaling results in loss of the left-sided expression of Nodal, Gdf1/3, Lefty and Pitx and in gain of ectopic expression of Cerberus on the left side. As a consequence, the larvae display loss of the offset arrangement of axial structures, loss of the left-sided pharyngeal organs including the mouth, and ectopic development of the right-sided organs on the left side. Bmp inhibition thus phenocopies inhibition of Nodal signaling and results in the right isomerism. We conclude that Bmp and Nodal pathways act in concert to specify the left side and that Bmp signaling plays a fundamental role during LR development in amphioxus.

Developmental effects of the protein kinase inhibitor kenpaullone on the sea urchin embryo

The selection and validation of bioactive compounds require multiple approaches, including in-depth analyses of their biological activity in a whole-animal context. We exploited the sea urchin embryo in a rapid, medium-scale range screening to test the effects of the small synthetic kinase inhibitor kenpaullone. We show that sea urchin embryos specifically respond to this molecule depending on both dose and timing of administration. Phenotypic effects of kenpaullone are not immediately visible, since this molecule affects neither the fertilization nor the spatial arrangement of blastomeres at early developmental stages. Nevertheless, kenpaullone exposure from the beginning of embryogenesis profoundly perturbs specification, detachment from the epithelium, and migration of the primary mesenchyme cells, thus affecting the whole embryonic epithelial mesenchymal transition process. Our results reaffirm the sea urchin embryo as an excellent and sensitive in vivo system, which provides straightforward and rapid response to external stimuli.

TGF-β sensu stricto signaling regulates skeletal morphogenesis in the sea urchin embryo

Cell-cell signaling plays a prominent role in the formation of the embryonic skeleton of sea urchins, but the mechanisms are poorly understood. In the present study, we uncover an essential role for TGF-β sensu stricto signaling in this process. We show that TgfbrtII, a type II receptor dedicated to signaling through TGF-β sensu stricto, is expressed selectively in skeletogenic primary mesenchyme cells (PMCs) during skeleton formation. Morpholino (MO) knockdowns and studies with a specific TgfbrtII inhibitor (ITD-1) in both S. purpuratus and Lytechinus variegatus embryos show that this receptor is required for biomineral deposition. We provide pharmacological evidence that Alk4/5/7 is the cognate TGF-β type I receptor that pairs with TgfbrtII and show by inhibitor treatments of isolated micromeres cultured in vitro that both Alk4/5/7 and TgfbrtII function cell-autonomously in PMCs. Gene expression and gene knockdown studies suggest that TGF-β sensu stricto may be the ligand that interacts with TgfbrtII and support the view that this TGF-β superfamily ligand provides an essential, permissive cue for skeletogenesis, although it is unlikely to provide spatial patterning information. Taken together, our findings reveal that this model morphogenetic process involves an even more diverse suite of cell signaling pathways than previously appreciated and show that PMCs integrate a complex set of both generalized and spatially localized cues in assembling the endoskeleton.

Function and regulation of microRNA-31 in development and disease

MicroRNAs (miRNAs) are small noncoding RNAs that orchestrate numerous cellular processes both under normal physiological conditions as well as in diseases. This review summarizes the functional roles and transcriptional regulation of the highly evolutionarily conserved miRNA, microRNA-31 (miR-31). miR-31 is an important regulator of embryonic implantation, development, bone and muscle homeostasis, and immune system function. Its own regulation is disrupted during the onset and progression of cancer and autoimmune disorders such as psoriasis and systemic lupus erythematosus. Limited studies suggest that miR-31 is transcriptionally regulated by epigenetics, such as methylation and acetylation, as well as by a number of transcription factors. Overall, miR-31 regulates diverse cellular and developmental processes by targeting genes involved in cell proliferation, apoptosis, cell differentiation, and cell motility. Mol. Reprod. Dev. 83: 654-674, 2016 © 2016 Wiley Periodicals, Inc.