Biomedical Engineering Reference
In-Depth Information
signaling from GPR124/forebrain paracrine factors (Fig.
11
c). In agreement with
the chemotaxis data, significant sprout alignment with the gradient only occurred
when cells expressed GPR124 and did not occur for cells treated with GPR124
shRNA (Fig.
11
d). As a control, sprouts exposed to gradients of hindbrain-con-
ditioned medium displayed significantly less directionality (Fig.
11
d). These data
demonstrate the powerful use of microfluidic devices as reductive screening assays
to discover new regulators of organ-specific vascularization, which have been
hypothesized to be critical during embryonic development and may also prove to
be beneficial in regenerative medicine therapies [
73
].
In summary, the case studies presented here illustrate how this type of
''restricted-flow'' device can be utilized in two different angiogenic models. First,
these devices enable 2D chemotactic studies of shear-sensitive endothelial cells in
a reductionist manner. Second, these devices enable the reductionist study of
sprouting morphogenesis, sprout pathfinding, and sprout maturation within 3D
matrices. These devices have been used both in quantitative studies to evaluate the
concentration profile requirements to induce endothelial cell polarization of known
chemotactic cues (i.e., VEGF) as well as in discovery-driven experiments to
identify novel regulators of organ-specific endothelial cell chemotaxis. Our results
highlight the fact that endothelial responses to a specific gradient of soluble factors
is context-dependent. Identical gradients can result in different cellular responses
depending on the matrix microenvironment and potentially other biophysical and
biochemical cues. Given the inherent complexity in the processes that regulate
angiogenesis, these restricted-flow microfluidic devices are especially well suited
for systematic experimental studies due to their quantitative control, substrate
independence, and stability of the generated gradients.
5 Potential Impact and Future Opportunities
Building on the successful use of these devices to study 2D endothelial cell
chemotaxis and 3D sprouting morphogenesis, future opportunities exist to further
utilize this experimental platform in the development of clinically translatable
therapies.
The controlled delivery of soluble factors from hydrogels [
74
], nanoparticles
[
75
], or coacervates [
76
], to treat pathological angiogenesis such as chronic
ischemia and cancer is a highly active area of research. A variety of innovative
encapsulation and release strategies have been developed to enable delivery of
specific concentration profiles; however, often the optimal profile required to elicit
the desired cellular response is unknown. Due to the small volume of reagents
required, microfluidic devices are ideal platforms to quantitatively screen multiple
concentration profiles to identify conditions that promote or inhibit processes such
as sprouting morphogenesis. These studies are expected to enable more efficient,
goal-driven design of the delivery system. Moreover, the timed delivery of
multiple factors is also known to affect therapeutic outcome. For example, a
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