Activation of the MAPKs ERK1/2 by cell swelling in turbot hepatocytes

Caveolar remodeling is a critical mechanotransduction mechanism of the stretch-induced L-type Ca2+ channel activation in vascular myocytes

Activation of L-type voltage-dependent Ca²⁺ channels (VDCC_L) by membrane stretch contributes to many biological responses such as myogenic contraction of arteries. However, mechanism for the stretch-induced VDCC_L activation is unclear. In this study, we examined the hypothesis that caveolar remodeling and its related signaling cascade contribute to the stretch-induced activation of VDCC_L in rat mesenteric arterial smooth muscle cells. The VDCC_L currents were recorded with nystatin-perforated or with conventional whole-cell patch-clamp technique. Hypotonic (~230 mOsm) swelling-induced membrane stretch reversibly increased the VDCC_L currents. Electron microscope and confocal imaging analysis revealed that both hypotonic swelling and cholesterol depletion by methyl-β-cychlodextrin (MβCD) similarly disrupted the caveolae structure and translocated caveolin-1 (Cav-1) from membrane to cytosolic space. Accordingly, MβCD also increased VDCC_L currents. Moreover, subsequent hypotonic swelling after MβCD treatment failed to increase the VDCC_L currents further. Western blotting experiments revealed that hypotonic swelling phosphorylated Cav-1 and JNK. Inhibitors of tyrosine kinases (genistein) and JNK (SP00125) prevented the swelling-induced facilitation of VDCC_L currents. Knockdown of Cav-1 by small interfering RNA blocked both the VDCC_L current facilitation by stretch and the related phosphorylation of JNK. Taken together, the results suggest that membrane stretch is transduced to the facilitation of VDCC_L currents via caveolar structure-dependent tyrosine phosphorylation of Cav-1 and subsequent activation of JNK in rat mesenteric arterial myocytes.

Evidence for Extracellular ATP as a Stress Signal in a Single-Celled Organism

ATP is omnipresent in biology and acts as an extracellular signaling molecule in mammals. Information regarding the signaling function of extracellular ATP in single-celled eukaryotes is lacking. Here, we explore the role of extracellular ATP in cell volume recovery during osmotic swelling in the amoeba Dictyostelium. Release of micromolar ATP could be detected during cell swelling and regulatory cell volume decrease (RVD) phases during hypotonic challenge. Scavenging ATP with apyrase caused profound cell swelling and loss of RVD. Apyrase-induced swelling could be rescued by 100 μM βγ-imidoATP. N-Ethylmalemide (NEM), an inhibitor of vesicular exocytosis, caused heightened cell swelling, loss of RVD, and inhibition of ATP release. Amoebas with impaired contractile vacuole (CV) fusion (drainin knockout [KO] cells) displayed increased swelling but intact ATP release. One hundred micromolar Gd(3+) caused cell swelling while blocking any recovery by βγ-imidoATP. ATP release was 4-fold higher in the presence of Gd(3+). Cell swelling was associated with an increase in intracellular nitric oxide (NO), with NO-scavenging agents causing cell swelling. Swelling-induced NO production was inhibited by both apyrase and Gd(3+), while NO donors rescued apyrase- and Gd(3+)-induced swelling. These data suggest extracellular ATP released during cell swelling is an important signal that elicits RVD. Though the cell surface receptor for ATP in Dictyostelium remains elusive, we suggest ATP operates through a Gd(3+)-sensitive receptor that is coupled with intracellular NO production.