Perfused 3D angiogenic sprouting in a high-throughput in vitro platform

Angiogenic sprouting, the growth of new blood vessels from pre-existing vessels, is orchestrated by cues from within the cellular microenvironment, such as biochemical gradients and perfusion. However, many of these cues are missing in current in vitro models of angiogenic sprouting. We here describe an in vitro platform that integrates both perfusion and the generation of stable biomolecular gradients and demonstrate its potential to study more physiologically relevant angiogenic sprouting and microvascular stabilization. The platform consists of an array of 40 individually addressable microfluidic units that enable the culture of perfused microvessels against a three-dimensional collagen-1 matrix. Upon the introduction of a gradient of pro-angiogenic factors, the endothelial cells differentiated into tip cells that invaded the matrix. Continuous exposure resulted in continuous migration and the formation of lumen by stalk cells. A combination of vascular endothelial growth factor-165 (VEGF-165), phorbol 12-myristate 13-acetate (PMA), and sphingosine-1-phosphate (S1P) was the most optimal cocktail to trigger robust, directional angiogenesis with S1P being crucial for guidance and repetitive sprout formation. Prolonged exposure forces the angiogenic sprouts to anastomose through the collagen to the other channel. This resulted in remodeling of the angiogenic sprouts within the collagen: angiogenic sprouts that anastomosed with the other perfusion channel remained stable, while those who did not retracted and degraded. Furthermore, perfusion with 150 kDa FITC-Dextran revealed that while the angiogenic sprouts were initially leaky, once they fully crossed the collagen lane they became leak tight. This demonstrates that once anastomosis occurred, the sprouts matured and suggests that perfusion can act as an important survival and stabilization factor for the angiogenic microvessels. The robustness of this platform in combination with the possibility to include a more physiological relevant three-dimensional microenvironment makes our platform uniquely suited to study angiogenesis in vitro.

Quantitative Imaging-Based Examination of Pericytes Controlling Endothelial Growth Dynamics and Angiogenesis

Microvascular endothelial cell-mural cell interactions are instrumental in modulating both physiological and pathologic angiogenesis. Pericyte-endothelial cell communication through direct physical associations and secreted effectors comprises a bidirectional signal array that regulates vascular maturation and integrity. As endothelial cell proliferation, migration, and morphogenesis are key elements of vascular growth and remodeling during angiogenesis, we have developed novel preclinical systems for studying the roles of endothelial-mural cell dynamics on cell cycle entry and angiogenic activity in vitro. These coculture models not only enable evaluation of endothelial cell-pericyte "cross talk" but also allow for the quantitative analysis of both heterotypic contact-dependent and contact-independent cell cycle progression in either cell population, as well as angiogenic sprouting in three-dimensional vascular networks. Cells actively proliferating in two-dimensional assays can be labeled via incorporation of 5-ethynyl-2'-deoxyuridine (EdU) into their DNA. Additionally, each cell population can be vitally labeled with a variety of cell-specific and/or membrane-permeant lipophilic dyes prior to coculture, such as DiO, or through immunofluorescence of mural or endothelial cell-specific markers after cellular fixation and/or permeabilization. Ultimately, this experimental approach can be used to investigate cellular contact-dependent and soluble mechanisms mediating mural-endothelial cell interactions, which may be instrumental in microvascular development and remodeling in vivo.