Ca-dependent Nonsecretory Vesicle Fusion in a Secretory Cell

We have compared Ca-dependent exocytosis in excised giant membrane patches and in whole-cell patch clamp with emphasis on the rat secretory cell line, RBL. Stable patches of 2-4 pF are easily excised from RBL cells after partially disrupting actin cytoskeleton with latrunculin A. Membrane fusion is triggered by switching the patch to a cytoplasmic solution containing 100-200 microM free Ca. Capacitance and amperometric recording show that large secretory granules (SGs) containing serotonin are mostly lost from patches. Small vesicles that are retained (non-SGs) do not release serotonin or other substances detected by amperometry, although their fusion is reduced by tetanus toxin light chain. Non-SG fusion is unaffected by N-ethylmaleimide, phosphatidylinositol-4,5-bis-phosphate (PI(4,5)P(2)) ligands, such as neomycin, a PI-transfer protein that can remove PI from membranes, the PI(3)-kinase inhibitor LY294002 and PI(4,5)P(2), PI(3)P, and PI(4)P antibodies. In patch recordings, but not whole-cell recordings, fusion can be strongly reduced by ATP removal and by the nonspecific PI-kinase inhibitors wortmannin and adenosine. In whole-cell recording, non-SG fusion is strongly reduced by osmotically induced cell swelling, and subsequent recovery after shrinkage is then inhibited by wortmannin. Thus, membrane stretch that occurs during patch formation may be a major cause of differences between excised patch and whole-cell fusion responses. Regarding Ca sensors for non-SG fusion, fusion remains robust in synaptotagmin (Syt) VII-/- mouse embryonic fibroblasts (MEFs), as well as in PLCdelta1, PLC delta1/delta4, and PLCgamma1-/- MEFs. Thus, Syt VII and several PLCs are not required. Furthermore, the Ca dependence of non-SG fusion reflects a lower Ca affinity (K(D) approximately 71 microM) than expected for these C2 domain-containing proteins. In summary, we find that non-SG membrane fusion behaves and is regulated substantially differently from SG fusion, and we have identified an ATP-dependent process that restores non-SG fusion capability after it is perturbed by membrane stretch or cell dilation.