Biology Reference
In-Depth Information
approach for one condition may, thus, have unforeseen consequences in other tis-
sues. In the case of miRNA-146a, for example, its overexpression alters haematopoi-
esis [
78
]. Local injections may lead to fewer side-effects and avoid a number of
delivery barriers, but serum nucleases and the local immune system still have to be
considered. In one study, a cocktail of miR-1, miR-133 and miR-206, all miRNAs
that have roles in muscle development, was shown to promote skeletal muscle dif-
ferentiation and fibre formation in vitro [
2
]. Subsequent in vivo local injections of
atelocollagen complexes containing the miRNA cocktail directly into damaged
muscle tissue accelerated muscle regeneration.
3.4.1
MicroRNA Delivery in Combination with Implantable Cells
When combining delivery and cell therapy, the implanted cells are typically transfected
prior to transplantation, allowing unbound miRNA transfection reagent to be
removed prior to injection. The advantage of this approach is the lowered risk of
targeting undesired cells. In one study, for example, embryonic stem cells were
transfected with a plasmid-encoding miR-1, which promotes cardiomyocyte dif-
ferentiation, and afterwards implanted in infarcted myocardium where the trans-
fected cells led to improved cardiac function [
79
] .
In another study, mesenchymal stem cells were transfected with an anti-miR
directed against miR-138, a repressor of osteogenesis [
5
] . The transfected cells were
then transferred onto a hydroxyapatite/tricalcium phosphate ceramic powder, which
was implanted subcutaneously in mice. Interestingly, the anti-miR-138-transfected
cells produced more bone than the cells containing a control miRNA and much
more than pre-miR-138-transfected cells. In another study, when miR-148b and an
antagonist against miR-489 were transfected into mesenchymal stem cells, which
were encapsulated into PEG/peptide hydrogels, osteogenic differentiation of hMSCs
was enhanced in vitro [
80
] .
In many cases, however, it is beneficial to deliver the miRNA vector from the
implant itself. Benefits include the following (a) the possibility of producing a cell-
free bioactive implant, which may be easier to use and to get regulatory approval for;
(b) pre-synthesis of miRNA-enhanced scaffolds simplifying handling by the end user
to cell seeding and implantation; (c) controlled release of miRNA from an implant,
prolonging its activity without resorting to viral vectors (d) tailoring of implants to
deliver different miRNAs in multiple temporal phases and/or spatial locations.
3.4.2
Scaffold-Directed MicroRNA Delivery
There are two main ways in which miRNAs can be incorporated into scaffolds:
They can be adsorbed onto the surface or be embedded within the scaffold polymer.
The release rate of nucleic acids from implants depends on the method of incorpora-
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