Stable Golgi-mitochondria complexes and formation of Golgi Ca2+ gradients in pancreatic acinar cells

We have determined the localization of the Golgi with respect to other organelles in living pancreatic acinar cells and the importance of this localization to the establishment of Ca2+ gradients over the Golgi. Using confocal microscopy and the Golgi- specific fluorescent probe 6-(( N-( 7- nitrobenz- 2- oxa- 1,3- diazol- 4- yl) amino) hexanoyl) sphingosine, we found Golgi structures localizing to the outer edge of the secretory granular region of individual acinar cells. We also assessed Golgi positioning in acinar cells located within intact pancreatic tissue using two- photon microscopy and found a similar localization. The mitochondria segregate the Golgi from lateral regions of the plasma membrane, the nucleus, and the basal part of the cytoplasm. The Golgi is therefore placed between the principal Ca2+ release sites in the apical region of the cell and the important Ca2+ sink formed by the peri- granular mitochondria. During acetylcholine-induced cytosolic Ca2+ signals in the apical region, large Ca2+ gradients form over the Golgi ( decreasing from trans- to cis- Golgi). We further describe a novel, close interaction of the peri- granular mitochondria and the Golgi apparatus. The mitochondria and the Golgi structures form very close contacts, and these contacts remain stable over time. When the cell is forced to swell, the Golgi and mitochondria remain juxtaposed up to the point of cell lysis. The strategic position of the Golgi ( closer to release sites than the bulk of the mitochondrial belt) makes this organelle receptive to local apical Ca2+ transients. In addition the Golgi is ideally placed to be preferentially supplied by ATP from adjacent mitochondria.