Intervertebral disc degeneration is a common and significant health concern that often leads to low back pain and worker disability. Disc degeneration is often irreversible, and current treatments do not adequately restore cell health or biomechanical function. Tissue engineering strategies are a possible solution to regenerate disc matrix, but one challenge for regenerating tissue is the degenerative disc microenvironment, which is characterized by inflammation, hypoxia, low glucose, low pH, and high osmolality. This research focuses on the effect of design factors on tissue-engineered constructs in the context of the disc microenvironment.
To evaluate the effects of cell type and configuration on anabolic and catabolic performance, we compared MSC-only, NPC-only, and 50:50 coculture groups in individual cell and micropellet configurations in basal and degenerative (hypoxic and inflammatory) media conditions. NPC and coculture groups had the most anabolic gene expression and GAG synthesis, but inflammatory media conditions dramatically reduced anabolic activity in all groups. Inflammatory and hypoxic media also led to upregulation in catabolic gene expression in NPC-only groups, but MSC-only and coculture groups were more resistant to this upregulation. A micropellet configuration, which provides more direct cell-cell contact, also reduced catabolic induction, and the group combining a cocultured cell type with a micropellet configuration had the lowest catabolic gene expression, suggesting that both design factors contribute to the response to inflammation. The coculture micropellet group also self-organized into a bilayered pellet, which may mimic a structure and mechanical environment present during development.
In the latter part of this research, we evaluated the effect of cell type and hypoxic preconditioning in a diffusion chamber we created to simulate nutrient limitations in the disc microenvironment. We limited diffusion within an agarose gel, and tested NPC-only, MSConly, and 50:50 coculture groups, along with MSC-only and coculture groups that included MSCs expanded in hypoxia. Within the diffusion-limited system, cell type affected viability. Surprisingly, NPCs had the lowest viability and MSCs had the highest. Viability of the coculture group was close to the mean of its component groups, suggesting that there was no synergistic benefit. Hypoxic preconditioning of MSCs also did not significantly affect viability.
Our results emphasize the importance of considering the microenvironmental context when evaluating cell therapies. In addition, improved understanding of the effects of various design factors can improve the performance of tissue-engineered constructs.
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