An AP-3-dependent mechanism drives synaptic-like microvesicle biogenesis in pancreatic islet beta-cells

• Published in: American journal of physiology: endocrinology and metabolism Vol. 299; no. 1; pp. E23 - E32
• Main Authors: Suckow, Arthur T, Craige, Branch, Faundez, Victor, Cain, William J, Chessler, Steven D
• Format: Journal Article
• Language: English
• Published: United States American Physiological Society 01.07.2010
• Subjects: Animals; Blotting, Western; Brefeldin A - pharmacology; DNA-Binding Proteins - genetics; DNA-Binding Proteins - physiology; Immunohistochemistry; Insulin - physiology; Insulin-Secreting Cells - physiology; Microscopy, Confocal; PC12 Cells; Protein Synthesis Inhibitors - pharmacology; Rats; Reverse Transcriptase Polymerase Chain Reaction; RNA - chemistry; RNA - genetics; Signal Transduction; Synaptic Vesicles - physiology; Transcription Factors - genetics; Transcription Factors - physiology; Vesicular Inhibitory Amino Acid Transport Proteins - physiology
• ISSN: 0193-1849; 1522-1555
• DOI: 10.1152/ajpendo.00664.2009

Pancreatic islet beta-cells contain synaptic-like microvesicles (SLMVs). The origin, trafficking, and role of these SLMVs are poorly understood. In neurons, synaptic vesicle (SV) biogenesis is mediated by two different cytosolic adaptor protein complexes, a ubiquitous AP-2 complex and the neuron-specific AP-3B complex. Mice lacking AP-3B subunits exhibit impaired GABAergic (inhibitory) neurotransmission and reduced neuronal vesicular GABA transporter (VGAT) content. Since beta-cell maturation and exocytotic function seem to parallel that of the inhibitory synapse, we predicted that AP-3B-associated vesicles would be present in beta-cells. Here, we test the hypothesis that AP-3B is expressed in islets and mediates beta-cell SLMV biogenesis. A secondary aim was to test whether the sedimentation properties of INS-1 beta-cell microvesicles are identical to those of bona fide SLMVs isolated from PC12 cells. Our results show that the two neuron-specific AP-3 subunits beta3B and mu3B are expressed in beta-cells, the first time these proteins have been found to be expressed outside the nervous system. We found that beta-cell SLMVs share the same sedimentation properties as PC12 SLMVs and contain SV proteins that sort specifically to AP-3B-associated vesicles in the brain. Brefeldin A, a drug that interferes with AP-3-mediated SV biogenesis, inhibits the delivery of AP-3 cargoes to beta-cell SLMVs. Consistent with a role for AP-3 in the biogenesis of GABAergic SLMV in beta-cells, INS-1 cell VGAT content decreases upon inhibition of AP-3 delta-subunit expression. Our findings suggest that beta-cells and neurons share molecules and mechanisms important for mediating the neuron-specific membrane trafficking pathways that underlie synaptic vesicle formation.