Biomedical Engineering Reference
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factors that trigger expression of a portfolio of pro-survival and angiogenic cues.
Under normoxia, cells hydroxylate HIFs, marking them for degradation [
31
].
While HIFs are active, modified gene expression leads to secretion of VEGF, a
chemotactic agent of sub-nanomolar potency [
32
], and formation of a paracrine
diffusion gradient. Moreover, interaction between protease secretion and intersti-
tial flow intensifies the gradient [
33
]. VEGF stimulates human microvascular
endothelial cells (hMVECs) to secrete proteases such as matrix metalloproteinase-
9 (MMP9) [
34
] and urokinase plasminogen activator [
35
], which cleave the
extracellular matrix and release sequestered VEGF [
36
], adding to the already
present paracrine VEGF. Interstitial flow, which is increased during sprouting
angiogenesis due to loosening of intercellular junctions in the sprouting vessel
[
37
], adds directionality to protease-released VEGF, creating a gradient as it
sweeps growth factor and protease away from the sprout's origin [
38
]. Cells sense
these soluble gradients through the spatial mechanism of competitive receptor
clustering (Fig.
1
)[
39
]. As activated receptors cluster, the pool of potentially
active receptors is depleted. Since clusters will form more quickly, on average, on
the side of the cell facing the higher concentration, the distribution of receptor
signaling complexes acquires a spatial bias in response to a gradient. Overall,
hypoxic cells biophysically communicate their need for perfusion by establishing
self-reinforcing soluble gradients of VEGF that impart directionality to targeted
hMVECs.
Among the soluble factors involved in sprouting angiogenesis, VEGF is the
most well-known for its capacity to elicit sprouts from existing blood vessels.
Upon stimulation by a soluble VEGF gradient, quiescent hMVECs polarize and
transition to a migratory phenotype, as marked by lamellipod extension [
40
] and
cytoskeletal polarization [
41
]. In addition, soluble VEGF gradients induce asym-
metry in the distribution of caveolin-1, partitioning it to the side of the cell facing
the higher concentration [
42
]. Caveolin-1 anchors caveolae, protein- and choles-
terol-rich invaginations in the plasma membrane, to the cytoskeleton. Organization
of caveolae facilitates integrin turnover [
43
] and migration [
44
]. In addition to
caveolae, actin-anchored filopodia segregate in the direction of the gradient in
activated hMVECs. Filopodia enhance path-finding by presentation of receptors
[
45
] and membrane-bound proteases [
46
]. Protease activity in response to a soluble
gradient can amplify directional signaling by selectively degrading the basal
lamina in addition to releasing sequestered VEGF. ECM components of the basal
lamina signal for vascular quiescence [
47
], so their removal may facilitate pro-
liferation and migration. The overall result is that soluble VEGF gradients polarize
hMVECs and contribute to their phenotypic transition from quiescent to invasive
during sprouting angiogenesis.
To resolve hypoxia, the VEGF gradient continues to impart directionality to
hMVECs as they form sprouts, tubular structures that precede nascent vessels, and
undergo columnar migration. After detecting a gradient and transitioning to a
migratory phenotype, activated hMVECs, known as tip cells, suppress tip spe-
cialization and encourage stalk specialization in neighboring cells [
48
]. Tip cells
remodel
the
ECM
and
chemotax,
while
the
main
activity of
stalk
cells
is
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