Simulations of sea urchin early development delineate the role of oriented cell division in the morula-to-blastula transition

Characterization of a novel short-type peptidoglycan recognition protein from the sea cucumber Apostichopus japonicus

Peptidoglycan recognition proteins (PGRPs) represent a key component of the family of pattern recognition receptors (PRRs). The functional mechanisms of PGRPs in innate immunity are poorly understood. In this study, we identified a novel short-type PGRP, AjPGRP-S2, from the sea cucumber Apostichopus japonicus. Our data showed that AjPGRP-S2 encoded an extracellular protein that possessed a signal peptide, a complete zinc (Zn2+) binding site, and a disulfide bond. A recombinant AjPGRP-S2 (rAjPGRP-S2) lacking the signal peptide was generated and demonstrated to exhibit amidase activity. Tissue expression analysis revealed that AjPGRP-S2 was highly expressed in coelomocytes and tube feet. Immune-responsive analysis indicated that AjPGRP-S2 was able to bind to various pathogen-associated molecular patterns (PAMPs) from bacteria and fungi, as well as to Gram-positive and -negative bacteria, and was majorly induced by DAP-PGN challenge. Basing on RNA-Seq and Pearson's correlation testing, RNA interference, and pull-down analysis, AjPGRP-S2 was found to be involved in transducing immune signals to the complement system and other PRRs, such as fibrinogen, by protein interactions to further recognize and kill pathogens. To respond comprehensively against pathogenic invasion, AjPGRP-S2 may also have the potential in transducing immune signals to key processes, such as cell adhesion, nerve conduction, apoptosis, and transcription by complex pathways that have yet to be elucidated. Our findings not only promote our understanding of the immune-related function and mechanisms of the PGRP family in A. japonicus, but also provide important data that will facilitate the identification of key evolutionary characteristics associated with invertebrate PGRPs.

A novel short-type peptidoglycan recognition protein with unique polysaccharide recognition specificity in sea cucumber, Apostichopus japonicus

Pattern recognition receptors (PRRs) are the first line of immune defense in invertebrates against pathogen infection; they recognize pathogens and transmit signals to downstream immune pathways. Among these, peptidoglycan recognition proteins (PGRPs) are an important family in invertebrates that generally comprise of complicated isoforms. A comprehensive understanding of PGRPs in evolutionarily and economically important marine invertebrates, such as the sea cucumber, Apostichopus japonicus, is crucial. Previous studies have identified two PGRPs in sea cucumber, AjPGRP-S and AjPGRP-S1, and another novel short-type PGRP, AjPGRP-S3, was additionally identified here. The full-length cDNA sequence of AjPGRP-S3 was obtained here by PCR-RACE, followed by which showed its gene expression analyses by in situ hybridization that showed it to be relatively highly expressed in coelomocytes and tube feet. Based on an analysis of the recombinant protein, rAjPGRP-S3, a board-spectrum pathogen recognition ability was noted that covered diverse Gram-negative and -positive bacteria, and fungi. Moreover, according to the results of yeast two-hybridization, it was suggested that rAJPGRP-S3 interacted with multiple immune-related factors, including proteins involved in the complement system, extracellular matrix, vesicle trafficking, and antioxidant system. These findings prove the important functions of AjPGRP-S3 in the transduction of pathogen signals to downstream immune effectors and help explore the functional differences in the AjPGRP isoforms.

Ecdysone signal pathway participates in shell formation in pearl oysters Pinctada fucata martensii

Ecdysone exists in arthropods, Mollusca and other invertebrates and plays vital roles in exoskeleton formation of Ecdysozoa. However, little is known about its functions in bivalve species. Herein, we identified ecdysone from the serum of pearl oyster Pinctada fucata martensii and obtained the coding sequence of ecdysone receptor (PmEcR) and homologue of its heterodimer protein retinoid X receptor (PmRXR). The deduced amino acid sequences of PmEcR and PmRXR contained a DNA-binding and ligand-binding domain and were very similar to the orthologs of other species. Moreover, PmEcR and PmRXR were located in the nuclei and cytoplasm of HEK-293T cells. PmEcR and PmRXR were highly expressed in early embryos and biomineralized mantle tissue. Moreover, the serum concentration of ecdysone significantly increased at 2, 4, 6, and 8 h post-shell notching. The expression of PmEcR in the mantle tissue was significantly induced at the corresponding time points, while that of PmRXR was significantly induced at 6 h. Ecdysone stimulation remarkably induced the expression of growth factors (BMP2 and BMP7), transcription factors (PmRunt and AP-1), and shell matrix protein genes (chitinase, lysine-rich matrix protein (KRMP), TYR2, and PmCOLVI), which indicated that ecdysone signaling plays important roles in shell repair. However, yeast two-hybrid assay and bimolecular fluorescence complementation showed that PmEcR and PmRXR did not form dimers, suggesting the different molecular interactions of EcR in bivalves. These findings provide insights into the function of ecdysone and its regulation pathway in bivalve species.

Blastocoel morphogenesis: A biophysics perspective

The blastocoel is a fluid-filled cavity characteristic of animal embryos at the blastula stage. Its emergence is commonly described as the result of cleavage patterning, but this historical view conceals a large diversity of mechanisms and overlooks many unsolved questions from a biophysics perspective. In this review, we describe generic mechanisms for blastocoel morphogenesis, rooted in biological literature and simple physical principles. We propose novel directions of study and emphasize the importance to study blastocoel morphogenesis as an evolutionary and physical continuum.

A hydro-osmotic coarsening theory of biological cavity formation

Fluid-filled biological cavities are ubiquitous, but their collective dynamics has remained largely unexplored from a physical perspective. Based on experimental observations in early embryos, we propose a model where a cavity forms through the coarsening of myriad of pressurized micrometric lumens, that interact by ion and fluid exchanges through the intercellular space. Performing extensive numerical simulations, we find that hydraulic fluxes lead to a self-similar coarsening of lumens in time, characterized by a robust dynamic scaling exponent. The collective dynamics is primarily controlled by hydraulic fluxes, which stem from lumen pressures differences and are dampened by water permeation through the membrane. Passive osmotic heterogeneities play, on the contrary, a minor role on cavity formation but active ion pumping can largely modify the coarsening dynamics: it prevents the lumen network from a collective collapse and gives rise to a novel coalescence-dominated regime exhibiting a distinct scaling law. Interestingly, we prove numerically that spatially biasing ion pumping may be sufficient to position the cavity, suggesting a novel mode of symmetry breaking to control tissue patterning. Providing generic testable predictions, our model forms a comprehensive theoretical basis for hydro-osmotic interaction between biological cavities, that shall find wide applications in embryo and tissue morphogenesis.

Physical constraints on early blastomere packings

At very early embryonic stages, when embryos are composed of just a few cells, establishing the correct packing arrangements (contacts) between cells is essential for the proper development of the organism. As early as the 4-cell stage, the observed cellular packings in different species are distinct and, in many cases, differ from the equilibrium packings expected for simple adherent and deformable particles. It is unclear what are the specific roles that different physical parameters, such as the forces between blastomeres, their division times, orientation of cell division and embryonic confinement, play in the control of these packing configurations. Here we simulate the non-equilibrium dynamics of cells in early embryos and systematically study how these different parameters affect embryonic packings at the 4-cell stage. In the absence of embryo confinement, we find that cellular packings are not robust, with multiple packing configurations simultaneously possible and very sensitive to parameter changes. Our results indicate that the geometry of the embryo confinement determines the packing configurations at the 4-cell stage, removing degeneracy in the possible packing configurations and overriding division rules in most cases. Overall, these results indicate that physical confinement of the embryo is essential to robustly specify proper cellular arrangements at very early developmental stages.

At the initial stages of embryogenesis, the precise arrangement of cells in the embryo is critical to ensure that each cell gets the right chemical and physical signals to guide the formation of the organism. Even when the embryo is made of only four cells, different species feature varying cellular arrangements: cells in mouse embryos arrange as a tetrahedron, in the nematode worm C. elegans cells make a diamond and in sea urchins cells arrange in a square configuration. How do cells in embryos of different species control their arrangements? Using computer simulations, we studied how cell divisions, physical contacts between cells and the confinement of the embryo by an eggshell affect the arrangements of cells when the embryos have only 4 cells. We find that the shape of the confining eggshell plays a key role in controlling the cell arrangements, removing unwanted arrangements and robustly specifying the proper contacts between cells. Our results highlight the important roles of embryonic confinement in establishing the proper cell-cell contacts as the embryo starts to develop.