94
powerful features that can be exploited to further direct vascularization. Changes in
ECM mechanics can lead to changes in GF availability [40, 110], drive capillary
morphogenesis [109], and stimulate angiogenesis in vivo [122]. By altering matrix
adhesive characteristics and mechanics, Ingber and Folkman illustrated how bFGFstimulated ECs can be switched between growth and differentiation during angiogenesis [110]. Recently, biomechanical cues from the ECM and signals from GF
receptors have been implicated in regulating the balance of activity between TFII-I
and GATA2 transcription factors, which govern the expression of VEGFR2 to instigate angiogenesis [158]. Matrix stiffness regulates not only the cell’s response to
soluble GFs but also cell morphogenesis during angiogenic sprouting. Primarily
due to MMP activity, the tip of a new capillary sprout becomes thinner, locally
degrading the basement membrane proteins. This region, with its high rate of ECM
turnover and thin basement membrane, becomes more compliant and stretches more
than the neighboring tissue. Consequently, the decrease in matrix stiffness changes
the balance of forces across the cell integrin receptors, increases cell tension, and
results in cytoskeletal arrangement to form branching patterns that are characteristic
of all growing vascular networks [109].
The pioneering work by Deroanne et al. showed that a decrease of matrix stiffness increased capillary branching and the elongation of tubes. A reduced tension
between ECs and ECM, accompanied by a profound remodeling of the actin-FAP
complex, is sufficient to trigger an intracellular signaling cascade leading to tubulogenesis [52]. This observation has been further confirmed in collagen gels [52, 200],
fibrin gels [211], self-assembling peptides, and HA-gelatin hydrogels.
Although ECM-based gels, such as collagen, fibrin, and Matrigel, have been
widely used in angiogenesis assays, their inherent physical properties have limited
their usage when studying the effects of matrix mechanics on angiogenesis. The
stiffness of ECM-based gels can be increased either by increasing their concentration, which also alters their ligand and fibril density [189], or by altering the crosslinking of ECM proteins in a narrow range using a microbial transglutaminase
[244]. Therefore, examining the effects of matrix stiffness alone on angiogenesis
requires the use of synthetic hydrogels, the stiffness of which can be easily adjusted
over a wide range of moduli without altering other chemical properties. Unlike naturally available ECM-based gels, the elasticity of which is limited to their inherent
cross-linking density, synthetic HA hydrogels can be used to study a physiologically relevant range of matrix elasticity [88]. When the cross-linking density of the
HA-gelatin hydrogels was further reduced, the matrix elasticity became relatively
compliant, resulting in an increase of capillary branching, elongated tubes, and
enlarged lumen structures [88]. On a relatively compliant matrix, EPCs can produce
fewer MMPs than a stiffer matrix would require and still degrade, exert mechanical
tension on, and contract the matrix to enable vascular morphogenesis. On the other
hand, EPCs must produce more MMPs on a stiffer matrix, to overcome the extra
mechanical barriers; even then, this local decrease in substrate stiffness cannot support vascular morphogenesis (Fig. 4.4). This model also explains the rapid appearance of large functional vessels in granulation tissue, as a response to the
wound-healing mechanism [122].
M. R. Blatchley et al.
powerful features that can be exploited to further direct vascularization. Changes in
ECM mechanics can lead to changes in GF availability [40, 110], drive capillary
morphogenesis [109], and stimulate angiogenesis in vivo [122]. By altering matrix
adhesive characteristics and mechanics, Ingber and Folkman illustrated how bFGFstimulated ECs can be switched between growth and differentiation during angiogenesis [110]. Recently, biomechanical cues from the ECM and signals from GF
receptors have been implicated in regulating the balance of activity between TFII-I
and GATA2 transcription factors, which govern the expression of VEGFR2 to instigate angiogenesis [158]. Matrix stiffness regulates not only the cell’s response to
soluble GFs but also cell morphogenesis during angiogenic sprouting. Primarily
due to MMP activity, the tip of a new capillary sprout becomes thinner, locally
degrading the basement membrane proteins. This region, with its high rate of ECM
turnover and thin basement membrane, becomes more compliant and stretches more
than the neighboring tissue. Consequently, the decrease in matrix stiffness changes
the balance of forces across the cell integrin receptors, increases cell tension, and
results in cytoskeletal arrangement to form branching patterns that are characteristic
of all growing vascular networks [109].
The pioneering work by Deroanne et al. showed that a decrease of matrix stiffness increased capillary branching and the elongation of tubes. A reduced tension
between ECs and ECM, accompanied by a profound remodeling of the actin-FAP
complex, is sufficient to trigger an intracellular signaling cascade leading to tubulogenesis [52]. This observation has been further confirmed in collagen gels [52, 200],
fibrin gels [211], self-assembling peptides, and HA-gelatin hydrogels.
Although ECM-based gels, such as collagen, fibrin, and Matrigel, have been
widely used in angiogenesis assays, their inherent physical properties have limited
their usage when studying the effects of matrix mechanics on angiogenesis. The
stiffness of ECM-based gels can be increased either by increasing their concentration, which also alters their ligand and fibril density [189], or by altering the crosslinking of ECM proteins in a narrow range using a microbial transglutaminase
[244]. Therefore, examining the effects of matrix stiffness alone on angiogenesis
requires the use of synthetic hydrogels, the stiffness of which can be easily adjusted
over a wide range of moduli without altering other chemical properties. Unlike naturally available ECM-based gels, the elasticity of which is limited to their inherent
cross-linking density, synthetic HA hydrogels can be used to study a physiologically relevant range of matrix elasticity [88]. When the cross-linking density of the
HA-gelatin hydrogels was further reduced, the matrix elasticity became relatively
compliant, resulting in an increase of capillary branching, elongated tubes, and
enlarged lumen structures [88]. On a relatively compliant matrix, EPCs can produce
fewer MMPs than a stiffer matrix would require and still degrade, exert mechanical
tension on, and contract the matrix to enable vascular morphogenesis. On the other
hand, EPCs must produce more MMPs on a stiffer matrix, to overcome the extra
mechanical barriers; even then, this local decrease in substrate stiffness cannot support vascular morphogenesis (Fig. 4.4). This model also explains the rapid appearance of large functional vessels in granulation tissue, as a response to the
wound-healing mechanism [122].
M. R. Blatchley et al.