Department of Cell Biology, Molecular, Cell and Developmental Biology Program, University of Virginia, Charlottesville, VA 22908-0732, USA.
Traffic. 2011 May;12(5):600-14. doi: 10.1111/j.1600-0854.2011.01170.x. Epub 2011 Feb 25.
During exocytosis, neuroendocrine cells can achieve partial release of stored secretory products from dense core vesicles (DCVs) by coupling endocytosis directly at fusion sites and without full discharge. The physiological role of partial secretion is of substantial interest. Much is known about SNARE-mediated initiation of exocytosis and dynamin-mediated completion of endocytosis, but little is known about coupling events. We have used real-time microscopy to examine the role of secretory carrier membrane protein SCAMP1 in exo-endocytic coupling in PC12 cells. While reduced SCAMP1 expression is known to impede dilation of newly opened fusion pores during onset of DCV exocytosis, we now show that SCAMP1 deficiency also inhibits closure of fusion pores after they have opened. Inhibition causes accumulation of fusion figures at the plasma membrane. Closure is recovered by restoring expression and accelerated slightly by overexpression. Interestingly, inhibited pore closure resulting from loss of SCAMP1 appears to increase secondary fusion of DCVs to already-fused DCVs (compound exocytosis). Unexpectedly, reinternalization of expanded DCV membranes following compound exocytosis appears to proceed normally in SCAMP1-deficient cells. SCAMP1's apparent dual role in facilitating dilation and closure of fusion pores implicates its function in exo-endocytic coupling and in the regulation of partial secretion. Secondarily, SCAMP1 may serve to limit the extent of compound exocytosis.
在胞吐作用过程中，神经内分泌细胞可以通过在融合部位直接发生内吞作用而不进行完全释放，从而实现致密核心囊泡 (DCV) 中储存的分泌产物的部分释放。部分分泌的生理作用具有重要意义。虽然我们已经了解了 SNARE 介导的胞吐作用起始和动力蛋白介导的内吞作用完成，但对耦联事件知之甚少。我们使用实时显微镜研究了分泌载体膜蛋白 SCAMP1 在 PC12 细胞中外排-内吞耦联中的作用。虽然已知 SCAMP1 表达减少会阻碍新开放的融合孔在 DCV 胞吐作用开始时的扩张，但我们现在表明，SCAMP1 缺乏也会抑制融合孔在开放后的关闭。抑制导致融合图形在质膜上的积累。通过恢复表达可以恢复关闭，而过表达会略微加速。有趣的是，由于 SCAMP1 的缺失而导致的抑制的孔关闭似乎会增加 DCV 与已经融合的 DCV 的二次融合（复合胞吐作用）。出乎意料的是，在 SCAMP1 缺陷细胞中，复合胞吐作用后扩张的 DCV 膜的再内化似乎正常进行。SCAMP1 在促进融合孔扩张和关闭中的双重作用表明其在胞外-胞内耦联和部分分泌的调节中的功能。其次，SCAMP1 可能有助于限制复合胞吐作用的程度。
