Steady-state Regulation of Secretory Cargo Export by Inositol Trisphosphate Receptors and Penta EF Hand Proteins

ABSTRACTER Ca2+ regulates ER-to-Golgi transport machinery. Sustained Ca2+ signaling by inositol trisphosphate receptors (IP3Rs) leads to depression of cargo export through activation of penta EF hand protein (PEF) ALG-2 which reduces outer COPII coat at ER exit sites (ERES). However, we do not know whether tonic Ca2+ signals during steady-state conditions affect ER export rates and if so by what mechanisms. Here we report that partial depletion of IP3 receptors from NRK epithelial cells causes a marked increase of basal ER export of the transmembrane glycoprotein cargo VSV-G. The increased ER-to-Golgi transport required ALG-2 and was actuated by decreased peflin and increased ALG-2 at ER exit sites (ERES) – a condition previously demonstrated to stimulate COPII-dependent ER export. Upon IP3R depletion the amount of outer coat at ERES increased, the opposite to what occurs during ALG-2-dependent inhibition of secretion during agonist-driven Ca2+ signaling. The increased ER export correlated with reduced spontaneous cytosolic Ca2+ oscillations caused by the reduced number of Ca2+ release channels. IP3R depletion also unexpectedly resulted in partial depletion of ER luminal Ca2+ stores. The low Ca2+ conditions appeared to decrease both ALG-2 and peflin expression levels somewhat, but these were the only detectable expression changes in COPII trafficking machinery and the Ca2+ decrease had no detectable impact on ER stress. We conclude that at steady state, IP3Rs produce tonic Ca2+ signals that suppress the basal rate of ER export by maintaining lower outer coat targeting to ERES.

IP3R and RyR channels are involved in traffic-related PM2.5-induced disorders of calcium homeostasis

Traffic-related PM2.5 can result in immune system damage and diseases; however, the possible mechanism of its effect remains unclear. Calcium (Ca2+) is a critical signaling molecule in a variety of cells. Indeed, Ca2+ is involved in numerous basic functions, including cell growth and death. In this study, Jurkat T cells were used to explore the possible mechanisms of PM2.5-elicited intracellular Ca2+signal responses. The results indicate that PM2.5 could raise the level of intracellular Ca2+ concentration ([Ca2+]i). The [Ca2+]i in Jurkat T cells significantly decreased after treatment with heparin as an inhibitor of inositol trisphosphate receptors (IP3 R), or procaine as an inhibitor of ryanodine receptors (RyR). The expression of calmodulin (CAM) protein decreased in a time-dependent manner after exposure to PM2.5, whereas the activity of Ca2+-Mg2+-ATPase seemed to show a slight drop trend after exposure to PM2.5. Our findings demonstrate that PM2.5 stimulation to Jurkat T cells would result in an increase in [Ca2+]i, which is modulated by IP3 R and RyR, as well as CAM.