Vascular Cell Physiology Under Shear Flow: Role of Cell Mechanics and Mechanotransduction - Mechanical and Chemical Signaling in Angiogenesis - page 135

actin mediation of cellular stiffness [ 58 , 59 ]. Therefore, the highly dynamic
cytoskeleton not only provides cell structure, but is also integral in cell signaling.
Anchor points, including focal adhesions and cell-cell junctions, are the major
sites for fluid shear stress-induced tension on the cytoskeleton [ 53 , 54 ]. Focal
adhesions are locations where a cell interacts with the ECM through integrin
clusters. Focal adhesions are also linked to the cytoskeleton through linker pro-
teins, such as vinculin. Under high, unidirectional fluid shear stress, ECs remodel
and align their focal adhesions in the direction of fluid shear stress, demonstrating
that focal adhesions are responsive to mechanical stimuli [ 60 ]. ECs elongated on
micropatterned lanes have also demonstrated alignment of focal adhesions [ 61 ].
The alignment of focal adhesions and the cytoskeleton may promote mechano-
transduction due to internal cellular tension imposed by the elongated EC, inde-
pendent of fluid shear stress. Furthermore, the type of ECM associated with the
focal adhesions contributes to the type of signal cascade triggered by tension due
to fluid shear stress [ 62 ]. Interestingly, the focal adhesions comprised of integrin
a v b 3 , predominantly formed on fibronectin, are responsible for tension-induced
upregulation of IjB kinase, which phosphorylates and degrades IjB[ 62 ].
Cell-cell junctions act as anchor points between neighboring cells through
molecules such as cadherins and cell adhesion molecules. Cell-cell junctions also
are connected to the cytoskeleton through linker molecules. High, unidirectional
fluid shear stress modulates expression and phosphorylation of cell-cell junctions
including VECAD, catenins, and tight junctions [ 55 ]. While the signaling cascades
induced by tension on cell-cell junctions are largely unknown, MAP kinase
pathways including MEK and ERK1/2 have been implicated [ 63 ]. Functionally,
the upregulation of cell-cell junctions, e.g. VECAD, is often associated with
decreased EC permeability [ 64 ]. Recent data also suggests that the formation of
cell-cell junctions regulates EC proliferation through tension-induced signaling
via the actin cytoskeleton [ 65 ].
ECs elongated on micropatterned lanes have limited surface area that can
facilitate the formation of cell-cell contacts (Fig. 8 )[ 66 ]. This is due to the nature
of micropatterned surface, such that an entire side of the cell is freely exposed to a
passivated BSA-coated region. Cell-cell junctions can form end-to-end, with a
maximum contact area of 25 lm (the width of the micropatterned lanes). Multiple
cells can adhere and spread across the 25 lm wide lane, providing a neighboring
cell for the formation of cell-cell contacts, but these cells usually lack cell junc-
tional anchor points at the opposing cell surface. Thus, internal mechanical tension
derived from cell-cell junctions of MPECs is likely to be minimal compared to
confluent ECs. The large number of cell-cell contacts formed under high, unidi-
rectional fluid shear stress may offer additional tension-induced signaling mech-
anism that act to either: ( 1 ) enhance cytoskeletal-alignment dependent signaling or
( 2 ) possibly generate additional cascades that stimulate gene expression solely
dependent on fluid shear stress.
Fluid shear stress is a mechanical stimuli, and mechanotransduction is mediated
by the cytoskeleton [ 53 , 67 ]. Thus, fluid shear stress mechanical stimulation, and not
cytoskeletal alignment, may be required to trigger cytoskeletal-dependent signaling