Regulated exocytosis per partes

This Special Issue (SI) of Cell Calcium focuses on regulated exocytosis, a recent evolutionary invention of eukaryotic cells. This essential cellular process consists of several stages: (i) the delivery of membrane bound vesicles to specific plasma membrane sites, (ii) where the merger between the vesicle and the plasma membranes occurs, (iii) leading to the formation of an aqueous channel through which vesicle content starts to be discharged to the cell exterior, (iv) after the full incorporation of the vesicle membrane into the plasma membrane, the added vesicle membrane is retrieved back into the cytoplasm by endocytosis. (v) When a fusion pore opens it may close again, a process known as transient fusion pore opening (also kiss-and-run exocytosis). In some cell types these stages are extremely shortlived, as in some neurons, and thus relatively inaccessible to experimentation. In other cell types the transition between these stages is orders of magnitude slower and can be studied in more detail. However, despite the intense investigations of this critical biological process over the last decades, the molecular mechanisms underlying regulated exocytosis have yet to be fully resolved. We thus still lack a comprehensive physiological insight into the nature of the progressive and coupled stages of exocytosis. Such a molecular-level understanding would help to fully reconstruct this process in vitro, as well as identify potential therapeutic targets for a range of diseases and dysfunctions. There are 18 papers in this SI which have been organized into three sections: Rapid regulated exocytosis and calcium homeostasis with an introduction by Erwin Neher, Molecular mechanisms of regulated exocytosis, and Cell models for regulated exocytosis. Here we briefly outline and integrate the messages of these sections.