Rodney J. O'Keefe

Parathyroid hormone (PTH) 1–34 is known to enhance fracture healing. Tendon repair is analogous to bone healing in its dependence on the proliferation and differentiation of mesenchymal stem cells, matrix formation, and tissue remodeling.^1,2,3 We hypothesized that PTH 1–34 enhances tendon healing in a flexor digitorum longus (FDL) tendon repair model. C57Bl/6J mice were treated with either intraperitoneal PTH 1–34 or vehicle‐control (PBS). Tendons were harvested at 3–28 days for histology, gene expression, and biomechanical testing. The metatarsophalangeal joint range of motion was reduced 1.5–2‐fold in PTH 1–34 mice compared to control mice. The gliding coefficient, a measure of adhesion formation, was 2–3.5‐fold higher in PTH 1–34 mice. At 14 days post‐repair, the tensile strength was twofold higher in PTH 1–34 specimens, but at 28 days there were no differences. PTH 1–34 mice had increased fibrous tissue deposition that correlated with elevated expression of collagens and fibronectin as seen on quantitative PCR. PTH 1–34 accelerated the deposition of reparative tissue but increased adhesion formation. © 2014 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 33:17–24, 2015.

: The complexity of fracture repair makes it an ideal process for studying the interplay between the molecular, cellular, tissue, and organ level events involved in tissue regeneration. Additionally, as fracture repair recapitulates many of the processes that occur during embryonic development, investigations of fracture repair provide insights regarding skeletal embryogenesis. Specifically, inflammation, signaling, gene expression, cellular proliferation and differentiation, osteogenesis, chondrogenesis, angiogenesis, and remodeling represent the complex array of interdependent biological events that occur during fracture repair. Here we review studies of bone regeneration in genetically modified mouse models, during aging, following environmental exposure, and in the setting of disease that provide insights regarding the role of multipotent cells and their regulation during fracture repair. Complementary animal models and ongoing scientific discoveries define an increasing number of molecular and cellular targets to reduce the morbidity and complications associated with fracture repair. Last, some new and exciting areas of stem cell research such as the contribution of mitochondria function, limb regeneration signaling, and microRNA (miRNA) posttranscriptional regulation are all likely to further contribute to our understanding of fracture repair as an active branch of regenerative medicine. © 2014 American Society for Bone and Mineral Research.

The ability to isolate primary chondrocytes from wild-type and genetically altered mice has provided tremendous advances in the understanding of signaling networks that regulate chondrocytes in health and disease. Isolation of chondrocytes from both growth plate tissues and articular cartilage has been challenging due to the cells being embedded within a highly organized tissue matrix. Here we describe highly reproducible methods for the isolation of pure populations of growth plate chondrocytes from the murine sternum and ribs and articular chondrocytes from the knee joint.

Bone repair is a complicated process that includes many types of cells, signaling molecules, and growth factors. Fracture healing involves a temporally and spatially regulated biologic process that involves recruitment of stem cells to the injury site, tissue specific differentiation, angiogenesis, and remodeling. In light of its proximity to bone and abundant vascularity, muscle is an important potential source of cells and signals for bone healing. More complete understanding of the role of muscle in bone formation and repair will provide new therapeutic approaches to enhance fracture healing. Recent studies establish that muscle-derived stem cells are able to differentiate into cartilage and bone and can directly participate in fracture healing. The role of muscle-derived stem cells is particularly important in fractures associated with more severe injury to the periosteum. Sarcopenia is a serious consequence of aging, and studies show a strong association between bone mass and lean muscle mass. Muscle anabolic agents may improve function and reduce the incidence of fracture with aging.

The pathogenesis of adhesions following primary tendon repair is poorly understood, but is thought to involve dysregulation of matrix metalloproteinases (Mmps). We have previously demonstrated that Mmp9 gene expression is increased during the inflammatory phase following murine flexor digitorum (FDL) tendon repair in association with increased adhesions. To further investigate the role of Mmp9, the cellular, molecular, and biomechanical features of healing were examined in WT and Mmp9^−/− mice using the FDL tendon repair model. Adhesions persisted in WT, but were reduced in Mmp9^−/− mice by 21 days without any decrease in strength. Deletion of Mmp9 resulted in accelerated expression of neo-tendon associated genes, Gdf5 and Smad8, and delayed expression of collagen I and collagen III. Furthermore, WT bone marrow cells (GFP⁺) migrated specifically to the tendon repair site. Transplanting myeloablated Mmp9^−/− mice with WT marrow cells resulted in greater adhesions than observed in Mmp9^−/− mice and similar to those seen in WT mice. These studies show that Mmp9 is primarily derived from bone marrow cells that migrate to the repair site, and mediates adhesion formation in injured tendons. Mmp9 is a potential target to limit adhesion formation in tendon healing.

A National Institutes of Health sponsored workshop “Bone Tissue Engineering and Regeneration: From Discovery to the Clinic” gathered thought leaders from medicine, science, and industry to determine the state of art in the field and to define the barriers to translating new technologies to novel therapies to treat bone defects. Tissue engineering holds enormous promise to improve human health through prevention of disease and the restoration of healthy tissue functions. Bone tissue engineering, similar to that for other tissues and organs, requires integration of multiple disciplines such as cell biology, stem cells, developmental and molecular biology, biomechanics, biomaterials science, and immunology and transplantation science. Although each of the research areas has undergone enormous advances in last decade, the translation to clinical care and the development of tissue engineering composites to replace human tissues has been limited. Bone, similar to other tissue and organs, has complex structure and functions and requires exquisite interactions between cells, matrices, biomechanical forces, and gene and protein regulatory factors for sustained function. The process of engineering bone, thus, requires a comprehensive approach with broad expertise. Although in vitro and preclinical animal studies have been pursued with a large and diverse collection of scaffolds, cells, and biomolecules, the field of bone tissue engineering remains fragmented up to the point that a clear translational roadmap has yet to emerge. Translation is particularly important for unmet clinical needs such as large segmental defects and medically compromised conditions such as tumor removal and infection sites. Collectively, manuscripts in this volume provide luminary examples toward identification of barriers and strategies for translation of fundamental discoveries into clinical therapeutics.

Osteosarcoma is a devastating tumor of bone, primarily affecting adolescents. Osteosarcoma tumors are notoriously radioresistant. Radioresistant cancers, including osteosarcoma, typically exhibit a considerable potential for relapse and development of metastases following treatment. Relapse and metastatic potential can, in part, be due to a specific radioresistant subpopulation of cells with stem‐like characteristics, cancer stem cells, which maintain the capacity to regenerate entire tumors. In the current study, we have investigated whether in vitro treatments with parthenolide, a naturally occurring small molecule that interferes with NF‐κB signaling and has various other effects, will re‐sensitize cancer stem cells and the entire cell population to radiotherapy in osteosarcoma. Our results indicate that parthenolide and ionizing radiation synergistically induce cell death in LM7 osteosarcoma cells. Importantly, the combination treatment results in a significant reduction in the viability of both the overall population of osteosarcoma cells and the cancer stem cell subpopulation. This effect is dependent on the ability of parthenolide to induce oxidative stress. Therefore, as a supplement to current multimodal therapy, parthenolide may sensitize osteosarcoma tumors to radiation and greatly reduce the prevalence of relapse and metastatic progression. J. Cell. Biochem. 113: 1282–1291, 2012.

As a downstream product of cyclooxygenase 2 (COX-2), prostaglandin E₂ (PGE₂) plays a crucial role in the regulation of bone formation. It has four different receptor subtypes (EP1 through EP4), each of which exerts different effects in bone. EP2 and EP4 induce bone formation through the protein kinase A (PKA) pathway, whereas EP3 inhibits bone formation in vitro. However, the effect of EP1 receptor signaling during bone formation remains unclear. Closed, stabilized femoral fractures were created in mice with EP1 receptor loss of function at 10 weeks of age. Healing was evaluated by radiographic imaging, histology, gene expression studies, micro–computed tomographic (µCT), and biomechanical measures. EP1^−/− mouse fractures have increased formation of cartilage, increased fracture callus, and more rapid completion of endochondral ossification. The fractures heal faster and with earlier fracture callus mineralization with an altered expression of genes involved in bone repair and remodeling. Fractures in EP1^−/− mice also had an earlier appearance of tartrate-resistant acid phosphatase (TRAcP)–positive osteoclasts, accelerated bone remodeling, and an earlier return to normal bone morphometry. EP1^−/− mesenchymal progenitor cells isolated from bone marrow have higher osteoblast differentiation capacity and accelerated bone nodule formation and mineralization in vitro. Loss of the EP1 receptor did not affect EP2 or EP4 signaling, suggesting that EP1 and its downstream signaling targets directly regulate fracture healing. We show that unlike the PGE₂ receptors EP2 and EP4, the EP1 receptor is a negative regulator that acts at multiple stages of the fracture healing process. Inhibition of EP1 signaling is a potential means to enhance fracture healing. © 2011 American Society for Bone and Mineral Research.