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front where they can be reutilized for the formation of new adhesions. One can
envision that adhesion molecules, such as integrins, may tra c in endocytic
vesicles either directly from the rear to the front of the cell or through
endocytic recycling pathways. We and others have observed integrin
containing vesicles moving from the rear of the cell to a perinuclear region
(Bretscher, 1984; Bretscher and Aguado-Velasco, 1998; Laukaitis et al., 2001;
Lawson and Maxfield, 1995; Palecek et al., 1996; Pierini et al., 2000; Regen
and Horwitz, 1992). However, we have not yet detected a5 integrin vesicles
moving directly from the rear to the front of the cell. This may be due to our
inability to track the integrin vesicles once they enter the perinuclear region
where the volume of the cell and the large number of vesicular structures
preclude detection. Recently, we observed a fraction of the cytoplasmic
paxillin, like a5 integrin, tra cking in vesicles that co-localized with an
endocytic marker, FM 4-64. In migrating fibroblasts, in which we saw
minimal membrane ru ing, a5 containing vesicles moved from the peri-
nuclear region to the base, but not into the lamellipodium. In CHO and WI38
cells, where membrane ru ing was apparent, we also observed a5 integrin-
GFP vesicles that emanated from the leading edge in protrusions and
concentrated in a perinuclear region (Figure 19.2).
There is good evidence for the tra cking of adhesion molecules in vesicles
from both the rear and leading edge of cells and from the perinuclear region to
the base of the lamellipodia (Bretscher, 1984; Bretscher and Aguado-Velasco,
1998; Laukaitis et al., 2001; Lawson and Maxfield, 1995; Palecek et al., 1996;
Pierini et al., 2000; Regen and Horwitz, 1992). However, it is currently
unknown whether tra cking represents a significant mechanism for supplying
materials to the front of the cell. To clarify the role that tracking plays in
regulating cellular processes, additional questions need to be addressed. For
example, what fraction of the integrins tracking from the cell rear or front
are recycled versus degraded in a lysosomal compartment and what is the role
of de novo biosynthesis versus recycling in supplying material
to new
adhesions?
These tracking questions may be more readily addressed using a highly
polarized cell type, such as a neuron, since it would avoid the high
concentration of vesicles in the perinuclear region and the tracking can be
easily seen and assayed. When a5 integrin-GFP is expressed in hippocampal
neurons, we observed tra cking of the integrin in vesicle-like structures,
analogous to those seen in the fibroblasts. These vesicles moved from the soma
along the neurites toward the growth cones (Figure 19.3). As the neurites
extended, the integrin containing structures also moved from the growth cones
toward the soma. Most importantly, tracking individual integrin-containing
vesicles in these cells is quite feasible (Figure 19.3). Thus, this provides us with
a system to address the mechanisms of intracellular tra cking of adhesion
components.
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