Discovery of a new cellular structure--the porosome: elucidation of the molecular mechanism of secretion.

Invention of the light microscope some 300 years ago allowed discovery of the unit of life, the cell. Up until the early 1940s, the light microscope, so efficient in the 19th century, reached the limits of its resolving power, of some 200 nanometer in lateral resolution and much less in its depth resolution. With the invention of the electron microscope and its use in biological research, a new era dawned for cell biology, when major subcellular organelles, and their organization and function, were elucidated. Pioneering cell biologists—George E. Palade, Keith Porter, and Albert Claude—made stellar contributions during this period. With a lateral resolving power of nearer a nanometer, the electron microscope matured into the ultimate imaging tool in the study of the cell. Unlike the light microscope, the electron microscope sacrificed live cell imaging for a considerable gain in resolution. Although, the electron microscope was capable of imaging biological samples at nanometer resolution, it required sample processing that included freezing, fixation, dehydration, and staining with heavy metal compounds to increase contrast. Hence, morphological changes due to freezing, tissue fixation, and processing for electron microscopy have always been a major concern. None the less, using electron microscopy, it appeared we knew just about all there is to know of cellular structure. After nearly half a century, nature has surprised us once more, with the discovery of a new cellular structure, the ‘porosome’, by Bhanu Jena and his research team (Jena et al., 2003; Jeremic et al., 2003; see Figs. 1 and 2). During 1995–96, Jena’s group circumvented the problems faced by both light and electron microscopy in their pioneering studies, by using the Atomic Force Microscope (AFM), which had been developed in the mid 1980s. In a series of elegant studies on live cells, his research team utilized the power and scope of the AFM to discover a new structure at the plasma membrane where membrane-bounded secretory vesicles dock and fuse to release their vesicular contents. The structure was found to be located in the plasma membrane of cells where secretion occurs. Circular ‘pits’ containing 150 nm diameter ‘depressions’ (now identified as porosomes) were present at the apical plasma membrane in living pancreatic acinar cells (Schneider et al., 1997). Stimulation of secretion caused the ‘depressions’ to dilate and then return to their resting size following completion of the process. Exposure of cells to actin depolymerizing agents resulted in collapse of ‘depressions’ and a consequent loss in secretion. Zymogen granules, the membrane-bounded secretory vesicles in exocrine pancreas, contain the starch digesting enzyme amylase. Using amylase-specific immunogold-AFM studies, localization of amylase at ‘depressions’ following stimulation of secretion was demonstrated (Cho et al., 2002c; Jena, 2002). These studies demonstrate that these ‘depressions’ are the fusion pores of pancreatic acinar cells, where membranebound secretory vesicles dock and fuse to release their vesicular contents. Subsequently, a similar presence and operation of fusion pores was confirmed in growth hormone secreting cells of the pituitary gland (Cho et al., 2002a) and in chromaffin cells (Cho et al., 2002e), suggesting their universal presence in secretory cells. Recently, the detailed morphology of the porosome was also revealed using electron microscopy, showing a cup-shaped basket-like morphology (Jena et al., 2003; Jeremic et al., 2003). Using immuno-AFM and biochemical approaches, further studies revealed the composition of the fusion pores, and demonstrated t-SNAREs associated with the base of the cup (Jeremic * Corresponding author. Tel.: +1-515-294-5540; fax: +1-515-294-4471 E-mail address: llanders@iastate.edu (L.L. Anderson). Cell Biology International 28 (2004) 3–5 Cell Biology International