Insights into the Role of Cell–Cell Junctions in Physiology and Disease - International Review of Cell and Molecular Biology - page 206

adhesion differences are only present in sparsely plated cultures; under conflu-
ent conditions, such differences appear diminished or lost.
Plakoglobin, a desmosomal protein, regulates migratory speed and cell-
cell adhesiveness as well as apoptosis ( Dusek et al., 2007; Huang et al., 2008;
Todorovic et al., 2010 ). Curiously, mutations in plakoglobin can be associ-
ated with disease, yet many (perhaps most) in vitro studies use variants of
knockouts rather than the mutant plakoglobin. Avoiding mutant forms
may lead to misleading results. For example, diminishing plakoglobin, either
by siRNA or by isolating cells from transgenic models generally leads to
diminished cell-cell adhesion ( Dusek et al., 2007; Wei et al., 2011 ).
However, expressing disease-causing mutant plakoglobin only sometimes
leads to altered cell-cell adhesion. Thus, it is not only the amount of
plakoglobin that is important, but also the function of different domains that
determine the exact role plakoglobin plays in disease. Further, in diseases
such as ARVC, certain mutations in any of the desmosomal proteins lead
to alteration of plakoglobin distribution, underscoring the interactive nature
of the junctional structures. Thus, while plakoglobin may be an “active”
component of signaling, several other proteins potentially regulate
plakoglobin in turn. Similar roles may exist for other junctional proteins,
such as b -catenin, a related protein (plakoglobin being a member of the cat-
enins, after all).
Cell sheet engineering is a new area of research that focuses on charac-
terizing how cells respond when substrate interactions are limited or elim-
inated ( Harris et al., 2012 ). This area developed from a desire to use cell
patches in order to repair or replace damaged tissue. Cell sheets could be
dissociated as intact monolayers and then attached to tissues immediately.
While this method is promising, it is limited by the fragility of the sheets
as well as the lack of information regarding the characteristics of such sheets.
Since it is difficult to manipulate a fragile, floating layer of cells, progress in
this area has been slow. However, such a sheet offers an unprecedented way
to study cell junctions when substrate interactions are no longer dominant.
A recent development by Harris et al. used novel plating configuration
whereby cells were plated between two pipettes on a collagen scaffold,
which was subsequently enzymatically digested after the cells had established
a sheet. By separating the pipettes, they could determine the response of the
cell sheet to being stretched under a variety of conditions ( Fig. 5.5 ). Studies
taking a similar approach may hold promise for developing a comprehensive
picture of cellular regulation by junctions.