Biphasic stage sensitivity to UV suppression of gastrulation in sea urchin embryos

Overview of the molecular defense systems used by sea urchin embryos to cope with UV radiation

The sea urchin embryo is a well-recognized developmental biology model and its use in toxicological studies has been widely appreciated. Many studies have focused on the evaluation of the effects of chemical stressors and their mixture in marine ecosystems using sea urchin embryos. These are well equipped with defense genes used to cope with chemical stressors. Recently, ultraviolet radiation (UVR), particularly UVB (280-315 nm), received more attention as a physical stressor. Mainly in the Polar Regions, but also at temperate latitudes, the penetration of UVB into the oceans increases as a consequence of the reduction of the Earth's ozone layer. In general, UVR induces oxidative stress in marine organisms affecting molecular targets such as DNA, proteins, and lipids. Depending on the UVR dose, developing sea urchin embryos show morphological perturbations affecting mainly the skeleton formation and patterning. Nevertheless, embryos are able to protect themselves against excessive UVR, using mechanisms acting at different levels: transcriptional, translational and post-translational. In this review, we recommend the sea urchin embryo as a suitable model for testing physical stressors such as UVR and summarize the mechanisms adopted to deal with UVR. Moreover, we review UV-induced apoptotic events and the combined effects of UVR and other stressors.

ATP-binding cassette transporters protect sea urchin gametes and embryonic cells against the harmful effects of ultraviolet light

Embryos of marine organisms whose development occurs externally are particularly sensitive to ultraviolet (UV) light (bands A and B, respectively, UVA and UVB). ATP-binding cassette (ABC) transporters are the first line of cellular defense against chemical or physical stress. The present work investigated the involvement of ABC transporters on UVA or UVB effects on eggs, spermatozoa, and embryonic cells of the sea urchin Echinometra lucunter. Gametes or embryos were exposed to UVA (3.6-14.4 kJ m(-2)) or UVB (0.112-14.4 kJ m(-2)), and embryonic development was monitored by optical microscopy at different developmental stages in the presence or absence of the ABC-transporter blockers reversin205 (ABCB1 blocker) or MK571 (ABCC1 blocker). E. lucunter eggs, spermatozoa and embryos were resistant to UVA exposure. Resistance to the harmful effects of UVB was strongly associated to ABC transporter activity (embryos > eggs > spermatozoa). ABCB1 or ABCC1 blockage promoted the injurious effects of UVA on spermatozoa. ABCC1 transporter blockage increased UVB-dependent damage in eggs while ABCB1 transporter inhibition increased harmful effects of UVB in embryonic cells. ABC-transporter activity was not, however, affected by UVB exposure. In conclusion, the present study is the first report on the protective role of ABC transporters against harmful effects of UVA and UVB on sea urchin eggs and embryonic cells.

Identification of specific malformations of sea urchin larvae for toxicity assessment: application to marine pisciculture effluents

Standard toxicity screening tests are useful tools in the management of impacted coastal ecosystems. To our knowledge, this is the first time that the sea urchin embryo development test has been used to evaluate the potential impact of effluents from land-based aquaculture farms in coastal areas. The toxicity of effluents from 8 land-based turbot farms was determined by calculating the percentage of abnormal larvae, according to two criteria: (a) standard, considering as normal pyramid-shaped larvae with differentiated components, and (b) skeletal, a new criterion that considers detailed skeletal characteristics. The skeletal criterion appeared to be more sensitive and enabled calculation of effective concentrations EC(5), EC(10), EC(20) and EC(50), unlike the classical criterion. Inclusion of the skeleton criterion in the sea urchin embryo development test may be useful for categorizing the relatively low toxicity of discharges from land-based marine fish farms. Further studies are encouraged to establish any causative relationships between pollutants and specific larval deformities.

Ultraviolet radiation and echinoderms: past, present and future perspectives

There is general consensus that solar ultraviolet radiation (UVR) negatively impacts many marine species. Echinoderms are ubiquitous within the marine environment, with members of the phyla often long-lived and numerically dominant within the benthic macrofauna, consequently the impact of UVR on the population dynamics of these organisms will influence marine communities and ecosystems. Research to date has shown that exposure of echinoderms to solar UVR can, affect reproduction and development, change behaviour, cause numerous biochemical and physiological changes and potentially cause increased mutation rates, by causing DNA damage. There is also considerable evidence that echinoderms utilise several different mechanisms to protect themselves against excessive UVR and subsequent UVR-induced damage. However, these protective mechanisms may pose conflicting selection pressures on echinoderms, as UVR is an additional stressor in oceans subjected to anthropogenic-induced climate change. This review summarises our knowledge of the effects of UVR on the Echinodermata. We outline the research conducted to date, highlight key studies on UVR that have utilised echinoderms and look to the future of UVR research in a rapidly changing ocean.

Gastrulation in the sea urchin embryo: a model system for analyzing the morphogenesis of a monolayered epithelium

Processes of gastrulation in the sea urchin embryo have been intensively studied to reveal the mechanisms involved in the invagination of a monolayered epithelium. It is widely accepted that the invagination proceeds in two steps (primary and secondary invagination) until the archenteron reaches the apical plate, and that the constituent cells of the resulting archenteron are exclusively derived from the veg2 tier of blastomeres formed at the 60-cell stage. However, recent studies have shown that the recruitment of the archenteron cells lasts as late as the late prism stage, and some descendants of veg1 blastomeres are also recruited into the archenteron. In this review, we first illustrate the current outline of sea urchin gastrulation. Second, several factors, such as cytoskeletons, cell contact and extracellular matrix, will be discussed in relation to the cellular and mechanical basis of gastrulation. Third, differences in the manner of gastrulation among sea urchin species will be described; in some species, the archenteron does not elongate stepwise but continuously. In those embryos, bottle cells are scarcely observed, and the archenteron cells are not rearranged during invagination unlike in typical sea urchins. Attention will be also paid to some other factors, such as the turgor pressure of blastocoele and the force generated by blastocoele wall. These factors, in spite of their significance, have been neglected in the analysis of sea urchin gastrulation. Lastly, we will discuss how behavior of pigment cells defines the manner of gastrulation, because pigment cells recently turned out to be the bottle cells that trigger the initial inward bending of the vegetal plate.

UVB radiation prevents skeleton growth and stimulates the expression of stress markers in sea urchin embryos

Ozone depletion results in an increased flux of biologically damaging radiations reaching the earth. Although ultraviolet (UV) penetration is attenuated by the seawater, harmful effects can be still observed at low depths where sea urchin embryos are living. We have used Paracentrotus lividus embryos to study the impacts of UV radiation on their development. Blastula cultures were exposed to different doses of UVB (312 nm) radiations and the resulting endpoint effects were evaluated in terms of embryonic morphological abnormalities, variations in specific gene expression, and changes in the levels of stress proteins. We found that embryos were moderately sensitive to 50 J/m2 UVB radiation; an increase in the number of developmentally delayed and malformed embryos was detected when increasing doses, up to 1000 J/m2, were used. Major developmental defects, observed 24 and 48 h after exposure, consisted in the failure of skeleton elongation and patterning. Accordingly, we found a reduction in the number of primary mesenchyme cells that expressed Pl-SM30, a gene coding for one of the specific matrix proteins of the skeleton. The morphological effects observed 1, 24, and 48 h after exposure were correlated with a dose-dependent increase in the level and in the activation of two recognized stress markers, namely hsp70 and p38 MAPk, respectively, consistent with their role in mediating cellular response to stress and suggesting a function in embryo survival.

Inhibition of mitogen activated protein kinase signaling affects gastrulation and spiculogenesis in the sea urchin embryo

The mitogen activated protein (MAP) kinase signaling cascade has been implicated in a wide variety of events during early embryonic development. We investigated the profile of MAP kinase activity during early development in the sea urchin, Strongylocentrotus purpuratus, and tested if disruption of the MAP kinase signaling cascade has any effect on developmental events. MAP kinase undergoes a rapid, transient activation at the early blastula stage. After returning to basal levels, the activity again peaks at early gastrula stage and remains high through the pluteus stage. Immunostaining of early blastula stage embryos using antibodies revealed that a small subset of cells forming a ring at the vegetal plate exhibited active MAP kinase. In gastrula stage embryos, no specific subset of cells expressed enhanced levels of active enzyme. If the signaling cascade was inhibited at any time between the one cell and early blastula stage, gastrulation was delayed, and a significant percentage of embryos underwent exogastrulation. In embryos treated with MAP kinase signaling inhibitors after the blastula stage, gastrulation was normal but spiculogenesis was affected. The data suggest that MAP kinase signaling plays a role in gastrulation and spiculogenesis in sea urchin embryos.