Secretory-granule dynamics visualized in vivo with a phogrin–green fluorescent protein chimaera

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Abstract
To image the behaviour in real time of single secretory granules in neuroendocrine cells we have expressed cDNA encoding a fusion construct between the dense-core secretory-granule-membrane glycoprotein, phogrin (phosphatase on the granule of insulinoma cells), and enhanced green fluorescent protein (EGFP). Expressed in INS-1 β-cells and pheochromocytoma PC12 cells, the chimaera was localized efficiently (up to 95%) to dense-core secretory granules (diameter 200–1000 nm), identified by co-immunolocalization with anti-(pro-)insulin antibodies in INS-1 cells and dopamine β-hydroxylase in PC12 cells. Using laser-scanning confocal microscopy and digital image analysis, we have used this chimaera to monitor the effects of secretagogues on the dynamics of secretory granules in single living cells. In unstimulated INS-1 β-cells, granule movement was confined to oscillatory movement (dithering) with period of oscillation 5–10 s and mean displacement < 1 µm. Both elevated glucose concentrations (30 mM), and depolarization of the plasma membrane with K+, provoked large (5–10 µm) saltatory excursions of granules across the cell, which were never observed in cells maintained at low glucose concentration. By contrast, long excursions of granules occurred in PC12 cells without stimulation, and occurred predominantly from the cell body towards the cell periphery and neurite extensions. Purinergic-receptor activation with ATP provoked granule movement towards the membrane of PC12 cells, resulting in the transfer of fluorescence to the plasma membrane consistent with fusion of the granule and diffusion of the chimaera in the plasma membrane. These results illustrate the potential use of phogrin–EGFP chimeras in the study of secretory-granule dynamics, the regulation of granule–cytoskeletal interactions and the trafficking of a granule-specific transmembrane protein during the cycle of exocytosis and endocytosis.