Designing biomaterials for the delivery of RNA therapeutics to stimulate bone healing

Abstract

Ribonucleic acids (small interfering RNA, microRNA, and messenger RNA) have been emerging as a promising new class of therapeutics for bone regeneration. So far, however, research has mostly focused on stability and complexation of these oligonucleotides for systemic delivery. By comparison, delivery of RNA nanocomplexes from biomaterial carriers can facilitate a spatiotemporally controlled local delivery of osteogenic oligonucleotides. This review provides an overview of the state-of-the-art in the design of biomaterials which allow for temporal and spatial control over RNA delivery. We correlate this concept of spatiotemporally controlled RNA delivery to the most relevant events that govern bone regeneration to evaluate to which extent tuning of release kinetics is required. In addition, inspired by the physiological principles of bone regeneration, potential new RNA targets are presented. Finally, considerations for clinical translation and upscaled production are summarized to stimulate the design of clinically relevant RNA-releasing biomaterials.

Keywords: Bone regeneration; Controlled release; Oligonucleotide delivery; RNA delivery; mRNA.

Fig. 1

RNA mechanisms of action. Left: A gene is transcribed into mRNA. After export through the nuclear pores into the cytosol, the mRNA is translated into the corresponding protein by the ribosome. In particular along the secretory route, the protein will undergo posttranslational modifications. Right: Single-stranded miRNA is transcribed in the nucleus and gets exported into the cytosol where it associates with the RNA-induced silencing complex (RISC). In contrast, siRNA is a double-stranded RNA of exogenous origin, which gets processed into a single-strand molecule in the cytosol before association with RISC. The miRNA or siRNA strand guide the RISC complex to the target mRNA by (partial) sequence complementarity. miRNA either leads to degradation of the target mRNA or inhibits its translation, whereas siRNA usually leads to the degradation of the target mRNA.

Fig. 2

Stages of bone healing and involved signaling molecules. Stages of bone healing and main cell types involved are depicted in the top row. The expression of signaling molecules is shown in continuous lines for the different stages of bone healing. Dashed lines represent time spans where expression profiles vary between the various studies. The time scale of regeneration is based on bone healing in rodents. Abbreviations: IL, interleukin; TNF-α, tumor necrosis factor alpha; PDGFs: platelet-derived growth factors; TGF-β, transforming growth factor beta; BMP, bone morphogenetic protein; Wnt, proteins involved in Wnt signaling; VEGF, vascular endothelial growth factor; Ang, angiopoietin; MMP, matrix metalloproteinase; Dkk3, Dickkopf-related protein 3. Data based on [1,40,45,77,123,129,130].

Fig. 3

Design criteria for RNA-delivering biomaterials.

Fig. 4

Loading of RNA into biomaterial carriers. Schematic illustration of loading strategies for RNA into biomaterials. Left: mRNA, miRNA or siRNA is complexed with a complexation agent before loading into the biomaterial. Right: Double-stranded siRNA is modified with linker molecules before covalent bonding to the biomaterial.