Ultrasound-mediated rapamycin delivery for promoting osseointegration of 3D printed prosthetic interfaces via autophagy regulation in osteoporosis

Osteoporosis is a common disease characterised by a systemic impairment of bone mass and microarchitecture that results in fragility fractures. With an ageing population, the medical and socioeconomic effect of osteoporosis, particularly postmenopausal osteoporosis, will increase further. A detailed knowledge of bone biology with molecular insights into the communication between bone-forming osteoblasts and bone-resorbing osteoclasts and the orchestrating signalling network has led to the identification of novel therapeutic targets. Novel treatment strategies have been developed that aim to inhibit excessive bone resorption and increase bone formation. The most promising novel treatments include: denosumab, a monoclonal antibody for receptor activator of NF-κB ligand, a key osteoclast cytokine; odanacatib, a specific inhibitor of the osteoclast protease cathepsin K; and antibodies against the proteins sclerostin and dickkopf-1, two endogenous inhibitors of bone formation. This overview discusses these novel therapies and explains their underlying physiology. Copyright © 2011 Elsevier Ltd. All rights reserved.

Summary Bone marrow‐derived mesenchymal stem cells (BMMSCs) exhibit degenerative changes, including imbalanced differentiation and reduced proliferation during aging, that contribute to age‐related bone loss. We demonstrate here that autophagy is significantly reduced in aged BMMSCs compared with young BMMSCs. The autophagy inhibitor 3‐methyladenine (3‐MA) could turn young BMMSCs into a relatively aged state by reducing their osteogenic differentiation and proliferation capacity and enhancing their adipogenic differentiation capacity. Accordingly, the autophagy activator rapamycin could restore the biological properties of aged BMMSCs by increasing osteogenic differentiation and proliferation capacity and decreasing adipogenic differentiation capacity. Possible underlying mechanisms were explored, and the analysis revealed that autophagy could affect reactive oxygen species and p53 levels, thus regulating biological properties of BMMSCs. In an in vivo study, we found that activation of autophagy restored bone loss in aged mice. In conclusion, our results suggest that autophagy plays a pivotal role in the aging of BMMSCs, and activation of autophagy could partially reverse this aging and may represent a potential therapeutic avenue to clinically treat age‐related bone loss.

Osteoblasts and adipocytes originate from a common progenitor, which arises from bone marrow mesenchymal stroma/stem cells (mMSC). Aging causes a decrease in the number of bone-forming osteoblasts and an increase in the number of marrow adipocytes. Here, we demonstrate that, during aging, the status of mMSC changes with respect to both their intrinsic differentiation potential and production of signaling molecules, which contributes to the formation of a specific marrow microenvironment necessary for maintenance of bone homeostasis. Aging causes a decrease in the commitment of mMSC to the osteoblast lineage and an increase in the commitment to the adipocyte lineage. This is reflected by changes in the expression of phenotype-specific gene markers. The expression of osteoblast-specific transcription factors, Runx2 and Dlx5, and osteoblast markers, collagen and osteocalcin, is decreased in aged mMSC. Conversely, the expression of adipocyte-specific transcription factor PPAR-gamma2, shown previously to regulate osteoblast development and bone formation negatively and to regulate marrow adipocyte differentiation positively, is increased, as is a gene marker of adipocyte phenotype, fatty acid binding protein aP2. Furthermore, production of an endogenous PPAR-gamma activator(s) that stimulates adipocyte differentiation and production of autocrine/paracrine factor(s) that suppresses the osteoblastic phenotype are also increased. In addition, expression of different components of TGF-beta and BMP2/4 signaling pathways is altered, suggesting that activities of these two cytokines essential for bone homeostasis change with aging.