Barrier Maintenance in Neovessels - Mechanical and Chemical Signaling in Angiogenesis - page 209

bridges formed by filopodia-like protrusions connected by VE-cadherin-rich
junctions. Subsequently, bridge bundles recruited nonmuscle myosin II and mature
into stress fibers. Myosin II activity was important for bridge formation and
accumulation of VE-cadherin in nascent adherens junctions. Thus, a novel
lamellipodia-to-filopodia transition seems to be a mechanism of cell-cell junction
formation in endothelial cells, where lamellipodia act as the initial protrusive
contact, subsequently transforming into filopodia-like bridges connected through
adherens junctions. The lamellipodia not only play a role in initial formation of
endothelial junctions, but are also important in the maintenance of cell-cell
interactions.
3 Endothelial Barrier Maintenance
Many studies have indicated the important contribution of the actin cytoskeleton to
determining junctional integrity [ 4 , 8 , 13 - 15 ]. A cortical actin belt is thought to be
important for the maintenance of stable junctions. In contrast, actin stress fibers are
thought to generate centripetal tension within endothelial cells that weakens
junctions. Many of these concepts have been based on studies in which endothelial
cells are treated with inflammatory mediators known to increase endothelial per-
meability, and then fixing the cells and labeling F-actin for microscopic obser-
vation. These studies have missed essential information on the dynamic nature of
the endothelial barrier under resting conditions. Recent live-cell imaging allows
incorporation of the dynamic nature of the endothelial barrier into the studies of
the mechanisms determining endothelial barrier integrity [ 13 , 16 ]. Time-lapse
images of fluorescently-labeled proteins in monolayers of confluent endothelial
cells indicate that there exists time-dependent changes in endothelial barrier
integrity.
These studies not only provide important information on processes involved
both in endothelial barrier disruption and restoration under inflammatory condi-
tions, but also highlight the dynamic nature of the maintenance of endothelial
barrier integrity [ 13 ]. A remarkable feature of a quiescent endothelial monolayer is
the rapid opening and closure of interendothelial gaps (see Fig. 1 ). Local lamel-
lipodia all along the edges of endothelial cells are forming and regressing all the
time in both nonconfluent and confluent cell monolayers.
In the quiescent endothelium, VE-cadherin-based junctions are subjected to
continuous reorganization, which renders endothelial junctions highly dynamic and
sensitive to extracellular stimuli. In fact, during the process of new vessel forma-
tion, endothelial cells undergo dynamic rearrangement upon angiogenic stimuli
while continuously and simultaneously reorganizing cell-cell junctions and
maintaining barrier function. This coordination is largely regulated by adhesion
molecules at intercellular junctions and is particularly important for tube stabil-
ization and maintenance or even restoration of the full barrier function. These subtle
changes of the morphology at the cellular periphery are known as micromotion.