Xiaoyan Ren

Osteogenesis on nanoparticulate mineralized collagen scaffolds via autogenous activation of the canonical BMP receptor signaling pathway

Skeletal regenerative medicine frequently incorporates deliverable growth factors to stimulate osteogenesis. However, the cost and side effects secondary to supraphysiologic dosages of growth factors warrant investigation of alternative methods of stimulating osteogenesis for clinical utilization. In this work, we describe growth factor independent osteogenic induction of human mesenchymal stem cells (hMSCs) on a novel nanoparticulate mineralized collagen glycosaminoglycan scaffold (MC-GAG). hMSCs demonstrated elevated osteogenic gene expression and mineralization on MC-GAG with minimal to no effect upon addition of BMP-2 when compared to non-mineralized scaffolds (Col-GAG). To investigate the intracellular pathways responsible for the increase in osteogenesis, we examined the canonical and non-canonical pathways downstream from BMP receptor activation. Constitutive Smad1/5 phosphorylation with nuclear translocation occurred on MC-GAG independent of BMP-2, whereas Smad1/5 phosphorylation depended on BMP-2 stimulation on Col-GAG. When non-canonical BMPR signaling molecules were examined, ERK1/2 phosphorylation was found to be decreased in MC-GAG but elevated in Col-GAG. No differences in Smad2/3 or p38 activation were detected. Collectively, these results demonstrated that MC-GAG scaffolds induce osteogenesis without exogenous BMP-2 addition via endogenous activation of the canonical BMP receptor signaling pathway.

Optimizing Collagen Scaffolds for Bone Engineering: Effects of Cross-linking and Mineral Content on Structural Contraction and Osteogenesis.

Introduction: Osseous defects of the craniofacial skeleton occur frequently in congenital, posttraumatic, and postoncologic deformities. The field of scaffold-based bone engineering emerged to address the limitations of using autologous bone for reconstruction of such circumstances. In this work, the authors evaluate 2 modifications of three-dimensional collagen-glycosaminoglycan scaffolds in an effort to optimize structural integrity and osteogenic induction. Methods: Human mesenchymal stem cells (hMSCs) were cultured in osteogenic media on nonmineralized collagen-glycosaminoglycan (C-GAG) and nanoparticulate mineralized collagen-glycosaminoglycan (MC-GAG) type I scaffolds, in the absence and presence of cross-linking. At 1, 7, and 14 days, mRNA expression was analyzed using quantitative real-time -reverse-transcriptase polymerase chain reaction for osteocalcin (OCN) and bone sialoprotein (BSP). Structural contraction was measured by the ability of the scaffolds to maintain their original dimensions. Mineralization was detected by microcomputed tomographic (micro-CT) imaging at 8 weeks. Statistical analyses were performed with Student t-test. Results: Nanoparticulate mineralization of collagen-glycosaminoglycan scaffolds increased expression of both OCN and BSP. Cross-linking of both C-GAG and MC-GAG resulted in decreased osteogenic gene expression; however, structural contraction was significantly decreased after cross-linking. Human mesenchymal stem cells-directed mineralization, detected by micro-CT, was increased in nanoparticulate mineralized scaffolds, although the density of mineralization was decreased in the presence of cross-linking. Conclusions: Optimization of scaffold material is an essential component of moving toward clinically translatable engineered bone. Our current study demonstrates that the combination of nanoparticulate mineralization and chemical cross-linking of C-GAG scaffolds generates a highly osteogenic and structurally stable scaffold.

Nanoparticulate Mineralized Collagen Scaffolds and BMP-9 Induce a Long-Term Bone Cartilage Construct in Human Mesenchymal Stem Cells

Engineering the osteochondral junction requires fabrication of a microenvironment that supports both osteogenesis and chondrogenesis. Multiphasic scaffold strategies utilizing a combination of soluble factors and extracellular matrix components are ideally suited for such applications. In this work, the contribution of an osteogenic nanoparticulate mineralized glycosaminoglycan scaffold (MC-GAG) and a dually chondrogenic and osteogenic growth factor, BMP-9, in the differentiation of primary human mesenchymal stem cells (hMSCs) is evaluated. Although 2D cultures demonstrate alkaline phosphatase activity and mineralization of hMSCs induced by BMP-9, MC-GAG scaffolds do not demonstrate significant differences in the collagen I expression, osteopontin expression, or mineralization. Instead, BMP-9 increases expression of collagen II, Sox9, aggrecan (ACAN), and cartilage oligomeric protein. However, the hypertrophic chondrocyte marker, collagen X, is not elevated with BMP-9 treatment. In addition, histologic analyses demonstrate that while BMP-9 does not increase mineralization, BMP-9 treatment results in an increase of sulfated glycosaminoglycans. Thus, the combination of BMP-9 and MC-GAG stimulates chondrocytic and osteogenic differentiation of hMSCs.

Nonmineralized and Mineralized Collagen Scaffolds Induce Differential Osteogenic Signaling Pathways in Human Mesenchymal Stem Cells

The instructive capabilities of extracellular matrix components in progenitor cell differentiation have recently generated significant interest in the development of bioinspired materials for regenerative applications. Previously, a correlation was described between the osteogenic capabilities of nanoparticulate mineralized collagen glycosaminoglycan scaffolds (MC-GAG) and an autogenous activation of small mothers against decapentaplegic ( Smad1/5) in the canonical bone morphogenetic protein receptor (BMPR) pathway with a diminished extracellular signal regulated kinase 1/2 (ERK1/2) activation when compared to nonmineralized collagen glycosaminoglycan scaffolds (Col-GAG). This work utilizes a canonical BMPR inhibitor (dorsomorphin homologue 1, DMH1) and an inhibitor of the mitogen activated protein kinase/ERK kinase (MEK)/(ERK) cascade (PD98059) to characterize the necessity of each pathway for osteogenesis. While DMH1 inhibits runt-related transcription factor 2 (Runx2) and bone sialoprotein II (BSPII) gene expression of primary human mesenchymal stem cells (hMSCs) on MC-GAG, PD98059 inhibits BSPII expression on Col-GAG independent of Runx2 expression. DMH1 inhibits mineralization on both Col-GAG and MC-GAG, however, PD98059 only inhibits mineralization on Col-GAG. DMH1 inhibits both Smad1/5 phosphorylation and Runx2 protein expression, whereas PD98059 inhibits ERK1/2 and c-Jun amino-terminal kinase 1/2 (JNK1/2) phosphorylation without affecting Runx2. Thus, activation of the canonical BMPR signaling is necessary for osteogenic differentiation and mineralization of hMSCs on Col-GAG or MC-GAG. The MEK/ERK cascade, intimately tied to JNK activation, is necessary for Runx2-independent osteogenesis on Col-GAG, while completely dispensable in osteogenesis on MC-GAG.