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Abstract
In regenerative medicine, hydrogels are employed to fill defects and support the infiltration
of cells that can ultimately regenerate tissue. Gene delivery within hydrogels targeting
infiltrating cells has the potential to promote tissue formation, but the delivery
efficiency of non-viral vectors within hydrogels is low, hindering their applicability
in tissue regeneration. To improve their functionality, we have conducted a mechanistic
study to investigate the contribution of cell migration and matrix degradation on
gene delivery. In this report, lipoplexes were entrapped within hydrogels based on
poly(ethylene glycol) (PEG) crosslinked with peptides containing matrix metalloproteinase
degradable sequences. The mesh size of these hydrogels is substantially less than
the size of the entrapped lipoplexes, which can function to retain vectors. Cell migration
and transfection were simultaneously measured within hydrogels with varying density
of cell adhesion sites (Arg-Gly-Asp peptides) and solids content. Increasing RGD density
increased expression levels up to 100-fold, while greater solids content sustained
expression levels for 16days. Increasing RGD density and decreasing solids content
increased cell migration, which indicates expression levels increase with increased
cell migration. Initially exposing cells to vector resulted in transient expression
that declined after 2days, verifying the requirement of migration to sustain expression.
Transfected cells were predominantly located within the population of migrating cells
for hydrogels that supported cell migration. Although the small mesh size retained
at least 70% of the lipoplexes in the absence of cells after 32days, the presence
of cells decreased retention to 10% after 16days. These results indicate that vectors
retained within hydrogels contact migrating cells, and that persistent cell migration
can maintain elevated expression levels. Thus, matrix degradation and cell migration
are fundamental design parameters for maximizing gene delivery within hydrogels.