Fertilization regulates apoptosis of Ciona intestinalis extra-embryonic cells through thyroxine (T4)-dependent NF-κB pathway activation during early embryonic development

Characterization of a novel species-specific 51-amino acid peptide, PEP51, as a caspase-3/7 activator in ovarian follicles of the ascidian, Ciona intestinalis Type A

Invertebrates lack hypothalamic-pituitary-gonadal axis, and have acquired species-specific regulatory systems for ovarian follicle development. Ascidians are marine invertebrates that are the phylogenetically closest living relatives to vertebrates, and we have thus far substantiated the molecular mechanisms underlying neuropeptidergic follicle development of the cosmopolitan species, Ciona intestinalis Type A. However, no ovarian factor has so far been identified in Ciona. In the present study, we identified a novel Ciona-specific peptide, termed PEP51, in the ovary. Immunohistochemical analysis demonstrated the specific expression of PEP51 in oocyte-associated accessory cells, test cells, of post-vitellogenic (stage III) follicles. Immunoelectron microscopy revealed that PEP51 was localized in the cytosol of test cells in early stage III follicles, which lack secretory granules. These results indicate that PEP51 acts as an intracellular factor within test cells rather than as a secretory peptide. Confocal laser microscopy verified that activation of caspase-3/7, the canonical apoptosis marker, was detected in most PEP51-positive test cells of early stage III. This colocalization of PEP51 and the apoptosis marker was consistent with immunoelectron microscopy observations demonstrating that a few normal (PEP51-negative) test cells reside in the aggregates of PEP51-positive apoptotic test cells of early stage III follicles. Furthermore, transfection of the PEP51 gene into COS-7 cells and HEK293MSR cells resulted in activation of caspase-3/7, providing evidence that PEP51 induces apoptotic signaling. Collectively, these results showed the existence of species-specific ovarian peptide-driven cell metabolism in Ciona follicle development. Consistent with the phylogenetic position of Ciona as the closest sister group of vertebrates, the present study sheds new light on the molecular and functional diversity of the regulatory systems of follicle development in the Chordata.

Topological properties and shape of proliferative and nonproliferative cell monolayers

During embryonic development, structures with complex geometry can emerge from planar epithelial monolayers; studying these shape transitions is of key importance for revealing the biophysical laws involved in the morphogenesis of biological systems. Here, using the example of normal proliferative monkey kidney (COS) cell monolayers, we investigate global and local topological characteristics of this model system in dependence on its shape. The obtained distributions of cells by their valence demonstrate a difference between the spherical and planar monolayers. In addition, in both spherical and planar monolayers, the probability of observing a pair of neighboring cells with certain valences depends on the topological charge of the pair. The zero topological charge of the cell pair can increase the probability for the cells to be the nearest neighbors. We then test and confirm that analogous relationships take place in a more ordered spherical system with a larger fraction of 6-valent cells, namely, in the nonproliferative epithelium (follicular system) of ascidian species oocytes. However, unlike spherical COS cell monolayers, ascidian monolayers are prone to nonrandom agglomeration of 6-valent cells and have linear topological defects called scars and pleats. The reasons for this difference in morphology are discussed. The morphological peculiarities found are compared with predictions of the widely used vertex model of epithelium.

Crystal-like order and defects in metazoan epithelia with spherical geometry

Since Robert Hooke studied cork cell patterns in 1665, scientists have been puzzled by why cells form such ordered structures. The laws underlying this type of organization are universal, and we study them comparing the living and non-living two-dimensional systems self-organizing at the spherical surface. Such-type physical systems often possess trigonal order with specific elongated defects, scars and pleats, where the 5-valence and 7-valence vertices alternate. In spite of the fact that the same physical and topological rules are involved in the structural organization of biological systems, such topological defects were never reported in epithelia. We have discovered them in the follicular spherical epithelium of ascidians that are emerging models in developmental biology. Surprisingly, the considered defects appear in the epithelium even when the number of cells in it is significantly less than the previously known threshold value. We explain this result by differences in the cell sizes and check our hypothesis considering the self-assembly of different random size particles on the spherical surface. Scars, pleats and other complex defects found in ascidian samples can play an unexpected and decisive role in the permanent renewal and reorganization of epithelia, which forms or lines many tissues and organs in metazoans.

IAP genes partake weighty roles in the astogeny and whole body regeneration in the colonial urochordate Botryllus schlosseri

Inhibitors of Apoptosis Protein (IAP) genes participate in processes like apoptosis, proliferation, innate immunity, inflammation, cell motility, differentiation and in malignancies. Here we reveal 25 IAP genes in the tunicate Botryllus schlosseri's genome and their functions in two developmental biology phenomena, a new mode of whole body regeneration (WBR) induced by budectomy, and blastogenesis, the four-staged cycles of botryllid ascidian astogeny. IAP genes that were specifically upregulated during these developmental phenomena were identified, and protein expression patterns of one of these genes, IAP28, were followed. Most of the IAP genes upregulation recorded at blastogenetic stages C/D was in concert with the upregulation at 100 μM H₂O₂ apoptotic-induced treatment and in parallel to expressions of AIF1, Bax, Mcl1, caspase 2 and two orthologues of caspase 7. Wnt agonist altered the takeover duration along with reduced IAP expressions, and displacement of IAP28⁺ phagocytes. WBR was initiated solely at blastogenetic stage D, where zooidal absorption was attenuated and regeneration centers were formed either from remains of partially absorbed zooids or from deformed ampullae. Subsequently, bud-bearing zooids developed, in concert with a massive IAP28-dependent phagocytic wave that eliminated the old zooids, then proceeded with the establishment of morphologically normal-looking colonies. IAP4, IAP14 and IAP28 were also involved in WBR, in conjunction with the expression of the pro-survival PI3K-Akt pathway. IAPs function deregulation by Smac mimetics resulted in severe morphological damages, attenuation in bud growth and differentiation, and in destabilization of colonial coordination. Longtime knockdown of IAP functions prior to the budectomy, resulted in colonial death.

The non-proliferative nature of ascidian folliculogenesis as a model of highly ordered cellular topology distinct from proliferative epithelia

Previous studies have addressed why and how mono-stratified epithelia adopt a polygonal topology. One major additional, and yet unanswered question is how the frequency of different cell shapes is achieved and whether the same distribution applies between non-proliferative and proliferative epithelia. We compared different proliferative and non-proliferative epithelia from a range of organisms as well as Drosophila melanogaster mutants, deficient for apoptosis or hyperproliferative. We show that the distribution of cell shapes in non-proliferative epithelia (follicular cells of five species of tunicates) is distinctly, and more stringently organized than proliferative ones (cultured epithelial cells and Drosophila melanogaster imaginal discs). The discrepancy is not supported by geometrical constraints (spherical versus flat monolayers), number of cells, or apoptosis events. We have developed a theoretical model of epithelial morphogenesis, based on the physics of divided media, that takes into account biological parameters such as cell-cell contact adhesions and tensions, cell and tissue growth, and which reproduces the effects of proliferation by increasing the topological heterogeneity observed experimentally. We therefore present a model for the morphogenesis of epithelia where, in a proliferative context, an extended distribution of cell shapes (range of 4 to 10 neighbors per cell) contrasts with the narrower range of 5-7 neighbors per cell that characterizes non proliferative epithelia.

Evolution and phylogeny of the corticotropin-releasing factor (CRF) family of peptides: expansion and specialization in the vertebrates

New sequence data on CRF family members from a number of genomes has led to the modification of our understanding of CRF evolution in the Metazoa. The corticotropin-releasing factor (CRF) family of peptides include four paralogous lineages in jawed vertebrates; CRF, urotensin-I/urocortin/sauvagine, urocortin 2 (Ucn2) and urocortin 3 (Ucn3). CRF and the urotensin-I/urocortin/sauvagine group represent a gene duplication from one lineage, whereas Ucns 2 and 3 are the result of a gene duplication in the other paralogous lineage. Both paralogous lineages are the result of a gene duplication from a single ancestral peptide that occurred after the divergence of the tunicates from the ancestor that led to the evolution of chordates and vertebrates. The presence of a single CRF-like peptide in tunicates and insects suggests that a single CRF-like ancestor was present before the separation of deuterostomes and protostomes. Currently there is no strong evidence that indicates that CRF-like peptides were present in metazoan taxa that evolved before this time although the structural similarity between some CRF peptides in insects, tunicates and vertebrates with the calcitonin family of peptides hints that prior to the formation of deuterostomes and protostomes the ancestral peptide possessed both CRF and calcitonin-like structural attributes. Here, we show evidences of conservation of CRF-like function dating back to early prokaryotes. This ancestral CRF-calcitonin-like peptide may have initially resulted from a horizontal gene transfer event from prokaryotes to a protistan species that later gave rise to the metazoans.

[Would D'Arcy Thompson have predicted a topological control of apoptosis?]

The laws that drive morphogenesis remain a major biological question. Today's views emphasize molecular autonomous processes rather than physical and mechanical constraints proposed by d'Arcy Thompson earlier on. In Ciona intestinalis oocyte, follicular cells formed by two distinct sets of geometrically-ordered epithelial monolayers positioned over the egg control apoptosis, implying that physically-predetermined shapes play a role in the control of cell determinism. In follicular cells ideally positioned over the spherical geometry of the egg, a drastic, optimized and polarized inward apoptosis sequence directly results from this positioning, suggesting the existence of some apoptotic master cells which control the destiny of neighboring cells. This concept could shed a new light on the origin of massive apoptosis phases that take place during embryogenesis in vertebrates (e.g., cavitation, inter-digitation). It could also be applied to specific therapeutic strategies to fight cancer.

Systems approaches to genomic and epigenetic inter-regulation of peptide hormones in stress and reproduction

The evolution of the organismal stress response and fertility are two of the most important aspects that drive the fitness of a species. However, the integrated regulation of the hypothalamic pituitary adrenal (HPA) and hypothalamic-pituitary-gonadal (HPG) axes has been traditionally thwarted by the complexity of these systems. Pepidergic signalling systems have emerged as critical integrating systems for stress and reproduction. Current high throughput systems approaches are now providing a detailed understanding of peptide signalling in stress and reproduction. These approaches were dependent upon a long history of discovery aimed at the structural characterization of the associated molecular components. The combination of comparative genomics, microarray and epigenetic studies has led not only to a much greater understanding of the integration of stress and reproduction but also to the discovery of novel physiological systems. Recent epigenomic approaches have similarly yielded a new level of complexity in the interaction of these physiological systems. Together, such studies have provided a greater understanding of the effects of stress and reproduction.

Characterization of a corticotropin-releasing factor (CRF)/diuretic hormone-like peptide from tunicates: insight into the origins of the vertebrate CRF family

The corticotropin-releasing factor (CRF) superfamily of peptides includes the four paralogous vertebrate peptide lineages of CRF, urotensin-1/urocortin/sauvagine, urocortin 2 and urocortin 3, as well as the diuretic hormones (DH) and peptides of the arthropods. However, there are considerable sequence differences between the group of vertebrate peptides and those of the arthropods, notably insects. Because of the likely incidence of the formation of paralogous forms within the insects and the great variation in primary structures among these peptides, establishing the structure of the ancestral vertebrate version has not been possible. We screened the genomes of the tunicates, Ciona intestinalis and Ciona savignii, in silico, using the various conserved motifs of both the vertebrate CRF paralogues and the insect diuretic hormone sequences to identify the structure of the Ciona CRF/DH-like peptide genes. A single peptide gene was found in both genomes that possessed motifs reflective of both groups of peptides. These structures suggest a single CRF-like peptide was inherited by vertebrates and possibly chordates. Moreover, the conserved structure of the CRF peptide may have become constrained once it became associated with the regulation of the hypothalamus-pituitary-adrenal/interrenal axis.

Structural evolution of urotensin-I: reflections of life before corticotropin releasing factor

Peptides have a long evolutionary history that predates the appearance of metazoans. The corticotropin releasing factor (CRF) family of peptides is among the most ancient peptide lineages. The identification and characterization of urotensin-I and related orthologues led the way for the elucidation of the entire CRF peptide family. A comparative analysis of the CRF paralogue sequences suggest that CRF is the most derived of these peptides and has lost many of its ancestral characteristics after it became associated with the hypothalamic-pituitary-adrenal/interrenal (HPA/I axis). In vertebrates, the urotensin-I group of orthologues, which includes sauvagine and urocortin, possess a number of shared characteristics that may be indicative of the ancestral peptide. Given the early origin of the CRF family peptides, it is likely that other peptide lineages are distantly related to the CRF family. In silico or cDNA library screening using probes based on urotensin-I/urocortin characteristics have been used to identify novel CRF family and related sequences that provide clues the evolutionary origin of the CRF family.

Topological control of life and death in non-proliferative epithelia

Programmed cell death is one of the most fascinating demonstrations of the plasticity of biological systems. It is classically described to act upstream of and govern major developmental patterning processes (e.g. inter-digitations in vertebrates, ommatidia in Drosophila). We show here the first evidence that massive apoptosis can also be controlled and coordinated by a pre-established pattern of a specific ‘master cell’ population. This new concept is supported by the development and validation of an original model of cell patterning. Ciona intestinalis eggs are surrounded by a three-layered follicular organization composed of 60 elongated floating extensions made of as many outer and inner cells, and indirectly spread through an extracellular matrix over 1200 test cells. Experimental and selective ablation of outer and inner cells results in the abrogation of apoptosis in respective remaining neighbouring test cells. In addition incubation of outer/inner follicular cell-depleted eggs with a soluble extract of apoptotic outer/inner cells partially restores apoptosis to apoptotic-defective test cells. The 60 inner follicular cells were thus identified as ‘apoptotic master’ cells which collectively are induction sites for programmed cell death of the underlying test cells. The position of apoptotic master cells is controlled by topological constraints exhibiting a tetrahedral symmetry, and each cell spreads over and can control the destiny of 20 smaller test cells, which leads to optimized apoptosis signalling.

Using Ciona to study developmental programmed cell death

Ciona intestinalis, a member of Tunicates, the closest group to vertebrates, has emerged as an appropriate organism for the study of developmentally regulated programmed cell death. First, because massive phases of apoptosis occur all along embryogenesis. Second, because the lecithotrophic mode of development is associated with autophagic process occurring during juvenile formation. Third, because the biochemical cell death machinery is close to that found in mammals. Altogether, the Ciona system contributes to identify new specific regulatory pathways and to explain how molecular mechanisms of programmed cell death evolved from invertebrates to vertebrates.