Jung Seung-Ryoung, Kim Mean-Hwan, Hille Bertil, Koh Duk-Su
Department of Physiology and Biophysics, University of Washington, Seattle, WA 98195-7290, USA.
Traffic. 2009 Apr;10(4):392-410. doi: 10.1111/j.1600-0854.2009.00884.x. Epub 2009 Jan 24.
Elevation of intracellular Ca(2+) concentration (Ca(2+)) triggers exocytosis of secretory granules in pancreatic duct epithelia. In this study, we find that the signal also controls granule movement. Motions of fluorescently labeled granules stopped abruptly after a Ca(2+) increase, kinetically coincident with formation of filamentous actin (F-actin) in the whole cytoplasm. At high resolution, the new F-actin meshwork was so dense that cellular structures of granule size appeared physically trapped in it. Depolymerization of F-actin with latrunculin B blocked both the F-actin formation and the arrest of granules. Interestingly, when monitored with total internal reflection fluorescence microscopy, the immobilized granules still moved slowly and concertedly toward the plasma membrane. This group translocation was abolished by blockers of myosin. Exocytosis measured by microamperometry suggested that formation of a dense F-actin meshwork inhibited exocytosis at small Ca(2+) rises <1 microm. Larger Ca(2+) rises increased exocytosis because of the co-ordinate translocation of granules and fusion to the membrane. We propose that the Ca(2+)-dependent freezing of granules filters out weak inputs but allows exocytosis under stronger inputs by controlling granule movements.
细胞内钙离子浓度（Ca(2+)）升高会触发胰腺导管上皮细胞中分泌颗粒的胞吐作用。在本研究中，我们发现该信号还控制颗粒的移动。Ca(2+)升高后，荧光标记颗粒的运动突然停止，在动力学上与整个细胞质中丝状肌动蛋白（F-肌动蛋白）的形成同时发生。在高分辨率下，新形成的F-肌动蛋白网络非常密集，以至于颗粒大小的细胞结构似乎被物理性地困在其中。用拉特罗毒素B使F-肌动蛋白解聚，可同时阻断F-肌动蛋白的形成和颗粒的停止移动。有趣的是，当用全内反射荧光显微镜监测时，固定的颗粒仍缓慢且协调地向质膜移动。这种成组移位被肌球蛋白阻滞剂消除。通过微电流测定法测量的胞吐作用表明，在小幅度Ca(2+)升高<1微摩尔时，致密F-肌动蛋白网络的形成会抑制胞吐作用。较大幅度的Ca(2+)升高会增加胞吐作用，这是由于颗粒的协同移位和与膜的融合。我们提出，Ca(2+)依赖性的颗粒冻结会滤除微弱的输入信号，但通过控制颗粒移动，在较强输入信号下允许胞吐作用发生。
