Ca²⁺-regulated secretory granule exocytosis in pancreatic and parotid acinar cells

Abstract

Protein secretion from acinar cells of the pancreas and parotid glands is controlled by G-protein coupled receptor activation and generation of the cellular messengers Ca(2+), diacylglycerol and cAMP. Secretory granule (SG) exocytosis shares some common characteristics with nerve, neuroendocrine and endocrine cells which are regulated mainly by elevated cell Ca(2+). However, in addition to diverse signaling pathways, acinar cells have large ∼1 μm diameter SGs (∼30 fold larger diameter than synaptic vesicles), respond to stimulation at slower rates (seconds versus milliseconds), demonstrate significant constitutive secretion, and in isolated acini, undergo sequential compound SG-SG exocytosis at the apical membrane. Exocytosis proceeds as an initial rapid phase that peaks and declines over 3 min followed by a prolonged phase that decays to near basal levels over 20-30 min. Studies indicate the early phase is triggered by Ca(2+) and involves the SG proteins VAMP2 (vesicle associated membrane protein2), Ca(2+)-sensing protein synatotagmin 1 (syt1) and the accessory protein complexin 2. The molecular details for regulation of VAMP8-mediated SG exocytosis and the prolonged phase of secretion are still emerging. Here we review the known regulatory molecules that impact the sequential exocytic process of SG tethering, docking, priming and fusion in acinar cells.

Keywords: Acinar exocytosis; Ca(2+); Complexin; Docking; Fusion; Membrane trafficking; Pancreatic parotid acinar secretion; SNARE; Secretory granules; Synaptotagmin; Tethering; Zymogen granules; cAMP.

Figure 1

Secretory granule exocytosis proceeds through tethering, docking, priming and fusion steps. Putative acinar SG tethering factors include Rab3D, Rab27B and Slps. Docking and priming involves SNAREpin formation and in acinar cells is facilitated by Munc18-b and Munc 18-c interactions with syntaxins. Synaptotagmin 1 and complexin 2 (not shown) trigger VAMP2-mediated granule fusion in response to elevated Ca²⁺.

Figure 2

Secretion from pancreatic acinar cells is marked by an initial rapid phase which peaks and declines over 2-3 min followed by a second prolonged phase that decays toward basal levels over 20 min. Evidence supports that the initial phase involves a post-priming step and is therefore independent of ATP. The early phase is Ca²⁺-dependent and regulated by synaptotagmin 1 (syt1) and complexin 2. Molecular details of regulatory control of exocytosis in the second phase involving both VAMP2 and VAMP8 are unclear.

Figure 3

Acinar secretion is mediated by G-protein coupled receptor activation resulting adenylyl cyclase (AC) mediated cAMP production and/or elevation of Ca²⁺ and DAG levels via phospholipase C (PLC) activation. The small G-protein Rap1 is present on secretory granules and activated by the guanine exchange factors EPAC1 and CalDAG-GEFIII which respond to cAMP and Ca²⁺/DAG, respectively. Rap1 inhibition was shown to significantly inhibit both cAMP- and Ca²⁺-stimulated amylase secretion.