Abstract: Despite the great advance of bone tissue engineering in the last few years, repair of bone defects remains a major problem. Low cell engraftment and dose-dependent side effects linked to the concomitant administration of bone morphogenetic proteins (BMPs) are the main problems currently hindering the clinical use of mesenchymal stem cell (MSC)-based therapies in this field. We have managed to bypass these drawbacks by combining the silencing the Smurf1 ubiquitin ligase in MSCs with the use of a scaffold that sustainably releases low doses of BMP-2. In this system, Smurf1 silencing is achieved by using GapmeRs, a clinically safe method that avoids the use of viral vectors, facilitating its translation to the clinic. Here, we show that a single transient transfection with a small quantity of a Smurf1-specific GapmeR is able to induce a significant level of silencing of the target gene, enough to prime MSCs for osteogenic differentiation. Smurf1 silencing highly increases MSCs responsiveness to BMP-2, allowing a dramatic reduction of the dose needed to achieve the desired therapeutic effect. The combination of these primed cells with alginate scaffolds designed to sustainably and locally release low doses of BMP-2 to the defect microenvironment is able to induce the formation of a mature bone matrix both in an osteoporotic rat calvaria system and in a mouse ectopic model. Importantly, this approach also enhances osteogenic differentiation in MSCs from osteoporotic patients, characterized by a reduced bone-forming potential, even at low BMP doses, underscoring the regenerative potential of this system.

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