Between Life and Death: Sea Urchin Embryos Undergo Peculiar DNA Fragmentation after Exposure to Vanadium, Cadmium, Gadolinium, and Selenium

Abstract

Exogenous DNA damage represents one of the most harmful outcomes produced by environmental, physical, or chemical agents. Here, a comparative analysis of DNA fragmentation was carried out on Paracentrotus lividus sea urchin embryos exposed to four common pollutants of the marine environment: vanadium, cadmium, gadolinium and selenium. Using the terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay, fragmented DNA was quantified and localized in apoptotic cells mapping whole-mount embryos. This is the first study reporting how different chemicals are able to activate distinctive apoptotic features in sea urchin embryos, categorized as follows: (i) cell-selective apoptosis, showing DNA fragmentation restricted to a subset of extremely damaged cells, acting as an embryo survival mechanism; or (ii) total apoptosis, with fragmented DNA widespread throughout the cells of the entire embryo, leading to its death. Also, this is the first report of the effects of Se exposure on P. lividus sea urchin embryos. These data confirm the TUNEL assay as the most suitable test to study DNA fragmentation in the sea urchin embryo model system. Taken together, this research highlights embryos' ability to find alternative pathways and set physiological limits for development under stress conditions.

Keywords: DNA fragmentation; TUNEL assay; apoptosis; marine ecotoxicology; metal pollution; sea urchin embryo.

Figure 1

Location of the sampling points for Paracentrotus lividus sea urchins in the Favignana island MPA, west coast of Sicily.

Figure 2

Representative images of 48 h larvae: (A) controls; embryos exposed to 100 μM of (B) vanadium (V); (C) cadmium (Cd); (D) gadolinium (Gd); (E) selenium (Se). Bar: 100 μm.

Figure 3

Larval body length (BL) detected after 48 h of development (n = 9 ± SD). Treatments with the same letter do not differ (Tukey HSD).

Figure 4

Fluorescent terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay on whole-mount sea urchin embryos. Images of representative control and treated embryos at 48 h, observed under a fluorescence microscope: fragmented DNA (A1–G1); nuclei stained with propidium iodide (A2–G2); merging of both signals (A3–G3). Control embryo (A1–A3); V-treated embryo (B1–B3); Cd-treated embryo (C1–C3); Gd-treated embryo (D1–D3); Se-treated embryo (E1–E3); negative-control embryo (F1–F3); and Positive-control embryo (G1–G3). Bar = 100 μm.

Figure 5

Histogram showing fluorescence optical densitometry analysis related to fragmented DNA signals. Data report the quantification of green fluorescence for the entire morphological population (n = 9 ± SD). Treatments with the same letter do not differ (Tukey HSD).