Matrix Mechanics and Cell Contractility in Angiogenesis - Mechanical and Chemical Signaling in Angiogenesis - page 148

inhibition recovers the sensitivity of cytoskeletal organization to strain application
and normalizes capillary network formation. These data indicate that the mediators
of cell contractility involved in mechanosensing are an important switch between
normal and abnormal function during angiogenesis.
Taken together, these data indicate that changes in the external environment
(matrix stiffness or exogenous forces) may ultimately alter the intrinsic mechanical
properties of ECs, and suggest that these changes in turn affect mechanosensing
and cell function. This is of fundamental importance in understanding the rela-
tionship between the mechanical environment and angiogenesis, in both the design
of suitable scaffolds for vascular tissue engineering constructs, and in disease
states with changing mechanical environments (e.g. tumor angiogenesis).
4 Endothelial Cell Network Assembly
An important event during angiogenesis is the spatial reorganization of ECs into a
capillary morphology. In 1980, Dr. Judah Folkman published seminal work
describing the intrinsic ability of ECs to reorganize into a capillary-like phenotype
in vitro [ 56 ]. Since then, the mechanical properties of the ECM have emerged as a
critical regulator of the development of such capillary-like morphologies [ 57 ].
4.1 Matrix Stiffness Regulates Endothelial Cell-Cell Assembly
and Sprouting
In general, compliant ECMs support the formation of networks of ECs, a mor-
phology reminiscent of the capillary beds seen in vivo [ 58 ] (Fig. 4 a). Early
experiments indicated that capillary tube formation occurs on compliant ECM or
when cells retract and elevate above stiff culture dishes [ 59 ]. In 2D studies, EC
network formation decreases as substrate stiffness is increased on fibrin gel [ 60 ]
(Fig. 4 b), collagen gel [ 58 ], or matrigel substrates [ 61 , 62 ]. Similarly, capillary-
like network formation in 3D decreases with increasing stiffness of fibrin [ 63 ] and
collagen gels [ 64 ] (Fig. 4 c). These studies indicate that ECs have a propensity to
self-assembly in sufficiently compliant mechanical environments.
Work in our lab with variably compliant polyacrylamide gels has determined
that compliant substrates (E \ 1kPa) promote the spontaneous assembly of EC
networks, and the propensity of endothelial cells to assemble into networks
decreases with increasing matrix stiffness (E = 2.5-10 kPa) [ 11 ] (Fig. 4 d). In
contrast to previous work utilizing collagen or fibrin gels, experimental systems
where gel stiffness is coupled to changes in protein concentration, the stiffness of
polyacrylamide substrates is tuned independently of the protein ligand concen-
tration available to adherent cells. This system allowed us to determine that