ECM Remodeling in Angiogenesis - Mechanical and Chemical Signaling in Angiogenesis - page 197

5 Linking ECM Mechanical Properties and Remodeling
The ECM's ability to regulate angiogenesis is complex and multivariate. Not only
does it impact capillary morphogenesis via biochemical regulatory mechanisms,
including through growth factor sequestration, integrin-mediated adhesion, and
protease susceptibility, it also acts as an instructive structural framework to
support sprouting and nascent capillary functionality. An additional feature of the
ECM that has been proposed as a regulator of angiogenesis is its mechanical
resistance to cell-generated tractional forces [ 122 ]. Evidence from several studies
corroborates the idea that mechanical cues directly impact tubulogenesis
[ 123 - 130 ]. Vailhe et al. demonstrated that human umbilical vein ECs seeded on
top of 2-D fibrin gels varying in concentration from 0.5 to 8 mg/ml only formed
capillary-like structures on the softest of the gels. The authors concluded that the
ECs do not form capillary-like structures on the more rigid gels because the cells
are unable to generate the necessary contractile forces to remodel the more rigid
substrate [ 125 ]. Another study conducted by Deroanne et al. showed that ECs
seeded on collagen-functionalized polyacrylamide gels of different stiffness
change morphologies from a monolayer to a tube-like phenotype as substrate
rigidity decreases [ 123 ]. In 3-D culture, Urech et al. investigated angiogenic
process extension in 3-D fibrin gels and manipulated their mechanical properties
by adding exogenous factor XIII to form additional cross-links [ 127 ]. Sieminski
et al. also studied the 3-D formation of capillary-like structures by two different
types of ECs in freely-floating versus. mechanically-constrained (attached)
collagen gels, and concluded that changing the collagen concentration modulates
the formation of these structures by regulating the amount of traction force exerted
by the cells [ 124 ]. Further evidence links EC-generated traction forces with
branching [ 128 ], the formation of capillary-like structures [ 123 , 124 , 131 ], and the
transcriptional control of soluble pro-angiogenic molecules [ 129 ]. A more detailed
discussion of the role of ECM mechanics and EC-generated tractional forces is
found elsewhere in this topic.
In the scope of this chapter, it is important to recognize that the mechanical
properties of the ECM are highly dynamic due to active remodeling induced both
by the ECs and stromal cells. A study by Lee et al. used second harmonic
generation and two-photon excited fluorescence to show that ECs induce quanti-
fiable alterations in local collagen matrix density via a process that involved cell-
generated forces [ 132 ]. Another study by Krishnan et al. tracked changes to the
ECM during the process of angiogenesis using a 3-D culture model [ 133 ]. The
authors reported an overall softening of the ECM as MMP activity increased
during the initial sprouting phase, and then a slow stiffening as the MMP activity
held steady and sprouts increased in length during tubulogenesis. Other studies
have suggested that matrix stiffness may also indirectly modulate MMP activity in
ECs [ 134 - 136 ]; however, the underlying mechanisms linking ECM mechanical
properties and protease expression and/or activity remain to be elucidated.