Evidence for Aquaporin-mediated Water Transport in Nematocytes of the Jellyfish <i>Pelagia noctiluca</i>

Abstract

Nematocytes, the stinging cells of Cnidarians, have a cytoplasm confined to a thin rim. The main cell body is occupied by an organoid, the nematocyst, containing the stinging tubule and venom. Exposed to hypotonic shock, nematocytes initially swell during an osmotic phase (OP) and then undergo regulatory volume decrease (RVD) driven by K⁺, Cl^- and obligatory water extrusion mechanisms. The purpose of this report is to characterize the OP. Nematocytes were isolated by the NaSCN/Ca²⁺ method from tentacles of the jellyfish Pelagia noctiluca, collected in the Strait of Messina, Italy. Isolated nematocytes were subjected to hyposmotic shock in 65% artificial seawater (ASW) for 15 min. The selective aquaporin water channel inhibitor HgCl₂ (0.1-25 µM) applied prior to osmotic shock prevented the OP and thus RVD. These effects were attenuated in the presence of 1mM dithiothreitol (DTT), a mercaptide bond reducing agent. AgNO₃ (1 µM) and TEA (tetraethylammonium, 100 µM), also reported to inhibit water transport, did not alter the OP but significantly diminished RVD, suggesting different modes of action for the inhibitors tested. Based on estimates of the nematocyte surface area and volume, and OP duration, a relative water permeability of ∼10^-7 cm/sec was calculated and the number of putative aquaporin molecules mediating the OP was estimated. This water permeability is 3-4 orders of magnitude lower in comparison to higher order animals and may constitute an evolutionary advantage for Cnidarian survival.

Cadmium affects osmotic phase and regulatory volume decrease in cultured human embryonic kidney cells

“…Furthermore, both 1 and 10 µM Cd 2+ -treated HEK 293 cells did not exhibit a peak value comparable to that observed in untreated cells. This result provides evidence for an inhibition of OP inhibition presumptively due to aquaporins, 18,34,35 rather than an inhibition of RVD mechanisms, suggesting thus a possible action of the metal on water flux. On the other hand, OP was preserved in 100 µM Cd 2+ -treated HEK 293 cells, while RVD inhibited, suggesting a possible action of the metal on ion transport, namely both K + and Cl -flux, playing a major role in RVD response.…”

“…32,33 Our results show that HEK 293 cells exhibit the expected OP within 4 min of 15% hyposmotic challenge, leading to water influx, as shown in other cell types. 18 After OP, RVD phase occurs and is completed within 30 min of hyposmotic shock application, with cell volume recovering, due to both K + and Cl -outflow, resulting in water efflux. 18,30 Here we demonstrate that time exposure comprised between 30 min and overnight did not induce cell death under either 1 or 10 or 100 µM Cd 2+ .…”

“…18 After OP, RVD phase occurs and is completed within 30 min of hyposmotic shock application, with cell volume recovering, due to both K + and Cl -outflow, resulting in water efflux. 18,30 Here we demonstrate that time exposure comprised between 30 min and overnight did not induce cell death under either 1 or 10 or 100 µM Cd 2+ . Furthermore, both 1 and 10 µM Cd 2+ -treated HEK 293 cells did not exhibit a peak value comparable to that observed in untreated cells.…”

“…13,15 RVD mechanisms are highly conserved and common to many cells from evolutionary distant species. 14,[16][17][18] Similarly, the uptake of osmolytes, followed by gain of osmotically obliged water occurs under hypertonic shrinkage, leading to regulatory volume increase (RVI), a process exhibited by many cell types and mainly due to Na + influx. 13,14,19 Studies on the volume sensor transduction pathway for volume regulation 20,21 provide evidence for intracellular Ca 2+ involvement, protein phosphorylation, arachidonic acid and phosphoinositide turnover, as well as mechanical sensors, such as stretch-activated channels and N o n -c o m m e r c i a l u s e o n l y cytoskeleton.…”

The venom and the toxicity of Pelagia noctiluca (Cnidaria: Scyphozoa). A review of three decades of research in Italian laboratories and future perspectives

“…OP and of the consequent cell swelling with inhibition of RVD was found to be mediated by a HgCl2 -sensitive transport mechanism with effective inhibitory concentrations of 0.1-25 μM HgCl2. 32 Other studies aimed to verify if crude venom (CV) from Pelagia noctiluca may affect RVD of isolated nematocytes from Aiptasia mutabilis (Anthozoan) have shown that the morphology of exposed nematocytes was not damaged by CV, but they did not exhibit RVD. This suggested the occurrence of inhibition on cell membrane ion transport mechanisms involved in RVD induced by CV.…”