Intracellular organelle dynamics at nm resolution

In the past 15 years, the dynamics of intracellular membrane-bound secretory vesicles ranging in size from 200 to 1200 nm in pancreatic acinar cells to 30-50 nm in neurons, have been extensively studied, providing for the first time the molecular process involved in vesicular discharge during cell secretion. Live pancreatic acinar cells in near physiological buffer, when imaged using the atomic force microscope (AFM), reveal at nanometer resolution the size of secretory vesicles called zymogen granules (ZGs) lying immediately below the surface of the apical plasma membrane. Within 2.5 min of exposure to a secretory stimulus, majority of ZGs within cells swell, followed by a decrease in ZG size, and a concomitant release of secretory products. These studies directly demonstrated intracellular swelling of secretory vesicles following stimulation of cell secretion in live cells, and vesicle deflation following partial discharge of vesicular contents. Furthermore, a direct estimation of vesicle size dynamics at nm resolution under various experimental conditions, have enabled the determination of the molecular mechanism of secretory vesicle swelling. Atomic force microscopy and photon correlation spectroscopy have been major players in these studies.