Age-related CXC chemokine receptor-4-deficiency impairs osteogenic differentiation potency of mouse bone marrow mesenchymal stromal stem cells.

Cysteine (C)-X-C chemokine receptor-4 (CXCR4) is the primary transmembrane receptor for stromal cell-derived factor-1 (SDF-1). We previously reported in mouse or human bone marrow-derived mesenchymal stromal stem cells (BMSCs) that deleting or antagonizing CXCR4 inhibits bone morphogenetic protein-2 (BMP2)-induced osteogenic differentiation. The goal of this study was to determine whether CXCR4-deficiency in BMSCs is an age-related effect in association with impaired osteogenic differentiation potency of aged BMSCs. Using BMSCs derived from C57BL/6J wild type mice at ages ranging from 3 to 23 months old, we detected decreased CXCR4 mRNA and protein expression as well as SDF-1 secretion with advancing aging. Moreover, CXCR4-deficient BMSCs from elderly vs. young mice exhibited impaired osteogenic differentiation in response to BMP2 stimulation or when cultured in dexamethasone (Dex)-containing osteogenic medium, evidenced by decreased alkaline phosphatase activity, osteocalcin synthesis, and calcium deposition (markers for immature and mature osteoblasts). Mechanistically, impaired BMP2- or Dex-osteoinduction in BMSCs of elderly mice was mediated by inhibited phosphorylation of intracellular R-Smads and Erk1/2 or Erk1/2 and p38 proteins, and decreased Runx2 and Osx expression (osteogenesis "master" regulators) were also detected. Furthermore, adenovirus-mediated repair of CXCR4 expression in BMSCs of elderly mice restored their osteogenic differentiation potentials to both BMP2 treatment and osteogenic medium. Collectively, our results demonstrate for the first time that CXCR4 expression in mouse BMSCs declines with aging, and this CXCR4-deficiency impairs osteogenic differentiation potency of aged BMSCs. These findings provide novel insights into mechanisms underlying age-related changes in BMSC-osteogenesis, and will potentiate CXCR4 as a therapeutic target to improve BMSC-based bone repair and regeneration in broad orthopedic situations.

Regulatory role of stromal cell-derived factor-1 in bone morphogenetic protein-2-induced chondrogenic differentiation in vitro.

Chemokine stromal cell-derived factor-1 (SDF-1) signals via binding to its primary transmembrane receptor, cysteine (C)-X-C chemokine receptor-4 (CXCR4). We previously reported that SDF-1 regulates osteogenic differentiation of mesenchymal stem/progenitor cells (MPCs) induced by bone morphogenetic protein-2 (BMP2). Although BMP2 is also capable of inducing chondrogenic differentiation of MPCs, the involvement of SDF-1 signaling in this function of BMP2 remains unknown. In this study, we aimed to test the role of SDF-1 signaling involved in BMP2-induced chondrogenic differentiation, using ATDC5 chondroprogenitors and mouse bone marrow-derived mesenchymal stromal cells (BMSCs). Our data showed that blocking of the SDF-1/CXCR4 pathway inhibits the differentiation of these cells into the chondrocytic lineages in response to BMP2 stimulation, evidenced by the reduced expression of type II collagen α1 (Col2α1), aggrecan, and type X collagen α1 (Col10α1), markers for chondrogenic differentiation. This effect of blocking SDF-1 signaling on BMP2-chondrogenic differentiation is associated with suppressed Sox9 and Runx2 expression (key transcription factors required for early and late stages of chondrogenic differentiation, respectively) and mediated via inhibiting intracellular Smad and Erk activation. Moreover, we found that addition of exogenous SDF-1 protein synergistically enhances the BMP2-induced chondrogenic differentiation in a dose-dependent manner. Collectively, our results demonstrated a novel role of SDF-1 signaling in regulating BMP2-induced chondrogenic differentiation in vitro. These data provide new insights into molecular mechanisms underlying BMP2-osteo/chondrogenesis, and may lead to the identification of new therapeutic targets and strategies to improve cartilage repair and regeneration in broad orthopaedic situations.