Secretion Mechanisms

Release of neurotransmitters, hormones, and neuropeptides from specialized vesicles from neuroendocrine cells maintain daily homeostatic tone, autonomic control and underlies adaptation and survival under conditions of stress. The gathered reports highlight the series of molecular interactions and signalling events that initiates the process of secretion and controls the modes of vesicular release (full-collapse and kiss-and-run membrane fusion). Although the molecules of secretion are largely identified, new and improved high resolution imaging has revealed new insights into how these molecules interact during secretion.

The programmed differentiation and development of secretory cells cannot take place without early transcriptional control of clusters of genes involved in this process. D’Alessandro and Meldolesi highlight the importance of the levels of the transcriptional repressor REST/NRSF during the development of a neurosecretory cell. This was clearly demonstrated by a subclone of PC12 cells that contained higher levels of REST compared to wild type PC12 and was devoid of releasable Large Dense Cores Vesicles (LDCVs). Reductions in the levels of the REST that normally silence gene transcription through direct (conserved RE-1 promoter sequence) or indirect mechanisms controls the expression of multiple proteins such as synaptotagmin and SNAP25 required for secretion.

Kögel and Gerdes reveal how actin filament associated myosin-Va, together with interacting partners such as the Rab GTPase family member Rab27, not only control transport of newly assembled vesicles to the plasma membrane (PM) but also the maturation of vesicles by controlling processing of intraluminal proteins. Impairment of myosin-Va results in retention and accumulation of the endoprotease furin that is usually excluded from mature vesicles. This function is shared with Rab3D which also facilitates the processing of luminal proteins such as secretogranin II. Both myosin-Va and Rab family members resume different roles at successive stages of secretion.

The interaction between PM proteins SNAP25 and syntaxin and the vesicle membrane protein synaptobrevin leads to the formation of a pore required for exocytosis. Smyth and co-workers bring to our attention how Munc18 physically interacts at multiple sites and chaperones syntaxin to the PM and promotes interaction with SNAP25. As highlighted by Torregrosa et al., SNAP25 and syntaxin (t-SNARE intermediate acceptor complex) are arranged into dynamic clusters at cytoskeletal cage borders of the PM that promotes fast coupling of Ca²⁺ with exocytosis. Dunn and colleagues demonstrate through in vitro lipid bilayer binding assays, that these acceptor complexes can exist in two homogeneous populations of partially or fully zippered heterodimers that could be highly relevant to the mechanisms of secretion.

Momboisse et al. point to the importance of effector proteins in secretion and highlight the upstream role of the guanine nucleotide exchange factor (GEF), Intersectin-1L in activating Cdc42 required for actin rearrangements and βPix in facilitating Rac1-mediated, phospholipase D-dependent phosphatidic acid (PA) generation. Chasserot-Golaz and collaborators review the complex lipid–lipid and lipid–protein interactions required for secretion and demonstrate how depletion of cholesterol-rich membrane domains or inhibition of enzymes that prevent the activity-dependent turnover of lipids such as 3′-phosphorylated phosphoinositols or PA inhibit secretion. Lipids such as PA also control the number of primed secretory granules docked at the PM, presumably by participating in SNARE complex assembly, membrane fusion, and controlling membrane topology. Anantharam et al. reveal how such changes could possibly be detected by movement of the PM toward the inside of the cell prior to a fusion event.

Vesicular release can occur through full-collapse mode, in which the vesicle membrane completely fuses with the PM to release its entire contents and kiss-and-run mode which selectively release its contents through the formation of a narrow transient fusion pore while maintaining full integrity. Chan and colleagues highlight how activity-dependent calcineurin-dependent dephosphorylation of dynamin I, can recruit myosin II and possible binding partner’s syndapin-N-WASP-Arp2/3-F-actin to facilitate the full-collapse mode of secretion. Borges et al. review how changes in the intravesicular environment also influences release kinetics. LDCVs, containing acidic chromogranins, bind amines, nucleotides, ions, and peptide hormones to form the characteristic “dense-core” that slows the rate of release of small molecules during exocytosis. Endogenous regulators of alkalinization, such as nitric oxide and cAMP, can also affect amine diffusion from LDCVs, as can weak bases including false transmitters that can accumulate in vesicles and displace endogenous amines.

Exocytosis is followed by fast and slow modes of endocytosis for retrieval and recycling of fully collapsed membranes and for the removal of transiently attached vesicles. As Cárdenas and Marengo have demonstrated, fully collapsed vesicles are retrieved from the PM by the formation of clathrin-coated pits that are excised by the actions of dynamin and re-directed to Golgi for new vesicle biosynthesis. In a similar fashion, the membrane of several fused vesicles can be recycled by bulk endocytosis, following high or prolonged levels of secretion. During the kiss-and-run mode, dynamin, independent of clathrin, rapidly catalyses the removal of the transiently docked vesicle from the PM for replenishment and repriming. All these models of endocytosis ensure that all vesicular pools are replenished appropriately to meet demand.

In summary, the reports gathered in this section provide molecular details about the vesicle life cycle and kinetic regulation at each step of secretion. New findings highlight the multiple conformations proteins assume to perform their roles at different stages of secretion. The next advance will likely come from determining how these molecular details translate to secretory behavior in different neuroendocrine cell types.