UNDERSTANDING CELL SECRETION AND MEMBRANE FUSION PROCESSES ON THE NANOSCALE USING THE ATOMIC FORCE MICROSCOPE - Life at the Nanoscale: Atomic Force Microscopy of Live Cells - page 100

The other major breakthrough in our understanding of cell secretion came
with the discovery of a new cellular structure, the “porosome”, using AFM. 6-13
In the past 12 years, the porosome has been determined to be the universal
secretory machinery in cells. Porosomes are supramolecular lipoprotein
structures at the cell plasma membrane, where membrane-bound secretory
vesicles transiently dock and fuse to release intravesicular contents to the
outside during cell secretion. The mouth of the porosome opening to the
outside ranges in size from 150 nm in diameter in acinar cells of the exocrine
pancreas to 12 nm in neurons, which dilates during cell secretion, returning
to its resting size following the completion of the process. In the past decade,
the composition of the porosome, its structure and dynamics at nanometre
resolution and in real time, and its functional reconstitution into artiicial
lipid membrane, have all been elucidated. Since porosomes in exocrine and
neuroendocrine cells measure 100-180 nm, and only 20-35% increase in
porosome diameter is demonstrated following the docking and fusion of 0.2-
1.2 μm in diameter secretory vesicles, it is concluded that secretory vesicles
“transiently” dock and fuse at the base of the porosome complex to release
their contents to the outside ( Fig. 5.2 ) .

5.2 DISCOVERY OF THE “POROSOME”

Porosomes were irst discovered in acinar cells of the exocrine pancreas.

Exocrine pancreatic acinar cells are polarized secretory cells possessing
an apical and a basolateral end. This well-characterized cell of the exocrine
pancreas synthesizes digestive enzymes, which is stored within 0.2-1.2 μm
diameter of apically located membranous sacs or secretory vesicles referred
to as zymogen granules. Following a secretory stimulus, ZGs dock and fuse
with the apical plasma membrane to release their contents to the outside.
Contrary to neurons, where secretion of neurotransmitters occurs in the
millisecond time regime, the pancreatic acinar cells secrete digestive enzymes
over minutes following a secretory stimulus. As pancreatic acinar cells are
slow secretory cells, they were ideal for investigation of the molecular steps
involved in cell secretion. In the mid 1990s, AFM studies were undertaken
on live pancreatic acinar cells to evaluate at high resolution the structure
and dynamics of the apical plasma membrane in both resting and following
stimulation of cell secretion. To our surprise, isolated live pancreatic acinar
cells in physiological buffer, when imaged using AFM,

6

reveal new cellular
structures. At the apical plasma membrane, a group of circular “pits”
measuring 0.4-1.2 μm in diameter, containing smaller “depressions”, were
observed.

6