Fusion-activated Ca2+ entry via vesicular P2X4 receptors promotes fusion pore opening and exocytotic content release in pneumocytes

Ca2+ is considered a key element in multiple steps during regulated exocytosis. During the postfusion phase, an elevated cytoplasmic Ca2+ concentration ([Ca2+])(c) leads to fusion pore dilation. In neurons and neuroendocrine cells, this results from activation of voltage-gated Ca2+ channels in the plasma membrane. However, these channels are activated in the prefusion stage, and little is known about Ca2+ entry mechanisms during the postfusion stage. This may be particularly important for slow and nonexcitable secretory cells. We recently described a fusion-activated Ca2+ entry (FACE) mechanism in alveolar type II (ATII) epithelial cells. FACE follows initial fusion pore opening with a delay of 200-500 ms. The site, molecular mechanisms, and functions of this mechanism remain unknown, however. Here we show that vesicle-associated Ca2+ channels mediate FACE. Using RT-PCR, Western blot analysis, and immunofluorescence, we demonstrate that P2X(4) receptors are expressed on exocytotic vesicles known as lamellar bodies (LBs). Electrophysiological, pharmacological, and genetic data confirm that FACE is mediated via these vesicular P2X(4) receptors. Furthermore, analysis of fluorophore diffusion into and out of individual vesicles after exocytotic fusion provides evidence that FACE regulates postfusion events of LB exocytosis via P2X(4). Fusion pore dilation was clearly correlated with the amplitude of FACE, and content release from fused LBs was accelerated in fusions followed by FACE. Based on these findings, we propose a model for regulation of the exocytotic postfusion phase in nonexcitable cells in which Ca2+ influx via vesicular Ca2+ channels regulates fusion pore expansion and vesicle content release.