Neurotoxins as Tools in Dissecting the Exocytic Machinery - Fusion of Biological Membranes and Related Problems - page 61

Still, the possibility that all secretory pathway eventually will turn
to be SNARE-dependent (either containing toxin-sensitive or -insensitive
SNAREs) is open. Such cases were proposed in systems outside the nervous
system (Fiedler et al., 1995; Ikonen et al., 1995).
5.4. Studying the Diversity of Secretory Systems
The use of a battery of TeNT and BoNTs allows us to study to what
extent SNAREs are involved in secretion and trafficking of many cell
lines and systems. Studies in PC12 and in bovine adrenal chromaffin
primary cells revealed a difference in term of the ATP dependency of
the release. In both types of cells exocytosis can be triggered by µM
amounts of Ca 2+ , but chromaffin cells in addition require ATP. In per-
meabilized PC12 cells TeNT-LC blocks exocytosis in the absence of
ATP similar to the effect obtained with permeabilized bovine adrenal chro-
maffin cells, in the presence of ATP. By contrast, BoNT/A-LC, which is
highly potent in permeabilized bovine adrenal chromaffin cells, causes only
a weak inhibition in PC12 cells. According to these studies it may be that
while TeNT and BoNT/B probably block a common step during exocytosis
from both PC12 cells and adrenal chromaffin cells, the accessibility of
SNAP-25 to the action of BoNT/A is different between these two model
systems.
The set of clostridial toxins and TeNT was applied in exocrine cells,
platelets, mast cells, kidney cells, cholecystokinin-secreting cell lines and
more. In the last case, the three neuronal SNAREs were detected. Intro-
ducing either of the TeNT or BoNTs to the streptolysin-O-permeabilized
cells abolished Ca 2+ -induced cholecystokinin secretion (Nemoz-Gaillard et
al., 1998). Similar results were also obtained in the exocrine pancreas and
in the parotid cells (Fujita-Yoshigaki et al., 1998; Gaisano et al., 1994). In
such systems, it is possible to evaluate the level of inhibition of secretion
following treatment by the toxins. Partial inhibition is indicative for paral-
lel secretory pathways or to the presence of unique SNAREs with differ-
ent sensitivities to the toxins (Rossi et al., 1997).
The unified view on secretion is extended to phagocytosis by the use
of clostridial toxins. This process is triggered by close apposition of the
leukocytes membrane onto an invading microorganism. Phagocytosis
was shown to be associated with increase in cell surface area by exocytosis.
Selective cleavage of components of the secretory machinery by microin-
jection or transfection of bacterial neurotoxins (see 4.2) induced an inhibi-
tion of phagocytosis (Hackam et al., 1998). These observations indicate that
SNARE proteins participate not only in trafficking and exocytosis but also
in particle internalization during phagocytosis.