The convergence of fracture repair and stem cells: interplay of genes, aging, environmental factors and disease

Abstract

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.

Keywords: AGING; ANIMAL MODELS; CELL/TISSUE SIGNALING; CELLS OF BONE; GENETICALLY ALTERED MICE; INJURY/FRACTURE HEALING; ORTHOPAEDICS; PARACRINE PATHWAYS; STEM AND PROGENITOR CELLS.

Fig. 1

The regenerative response to fracture. (A) Bone is a mineralized tissue that resists tension, compression, and torsional stresses. However, if the deforming energy surpasses a critical threshold structural failure of bone results in a fracture. (B) The initial response to the injury involves development of a hematoma and formation of a fibrin clot at the site of the fracture. Inflammatory cytokines, chemokines, and growth factors, such as TNF-alpha, CXCR4, SDF1, and PDGF, are released into the fracture hematoma. (C) Signals initiated by the injury activate stem cell progenitors located primarily in the periosteum and in the bone marrow. This results in proliferation and expansion of the pool of progenitor cells necessary for repair. Key signals involved in these early events include BMP, COX-2/PGE2, Wnt/β-catenin, and Notch signaling pathways. Marked changes in gene expressions occur in these cell populations and there is a shift in cell metabolism from glycolysis to oxidative phosphorylation. Evidence supporting a role for progenitor cells from the circulation has only been demonstrated in preclinical models, and a potential role in humans has not been established. (D) The expanded stem cell population undergoes differentiation with the expression of chondrocyte lineage genes (Sox9, Col2a1, ColXa1) and osteoblast lineage genes (Runx2, Osterix, Col1a1). Osteoblasts form bone directly through intramembranous ossification along the surface of the bone atadjacent ends ofthe fracture where injury is less severe. Cartilage is formed in the central, more hypoxic area of the fracture where the tissue injury is maximal. Undifferentiated mesenchyme persists longest in the most central region of the fracture and is flanked by cartilage and bone tissues at either end of the fracture. (E) The cartilage callus tissue undergoes calcification and is invaded by blood vessels that bring in osteoprogenitors and chondroprogenitors that initiate secondary bone formation and remodeling. Terminal differentiation, calcification, angiogenesis, and remodeling are associated with the expression of Osteocalcin, VEGF, MMP13, and RANKL. Pericytesin the vasculature provide arobust sourceof osteoprogenitors for secondary bone remodeling. (F) Fracture repair occurs with bridging of the fracture with calcified tissues, but is completed with remodeling of the fracture, which restores the bone to its normal size and shape. (G) Complete healing. SC = stem cell;

Fig. 2

Schematic of progenitor cell lineage. Multipotent stem cells have self-renewal capacity. A key feature of these cell populations is the expression of telomerase and the maintenance of telomere length. Multipotent cells are maintained in a quiescent state with inactive mitochondrial function until needed for tissue maintenance or regeneration. The multipotent cells have capacity to differentiate into different cell and tissue types. The various progenitor cell populations are present within specific tissues and begin to express lineage-specific transcription factors. For example, the mesodermal progenitor cell in muscle tissue expresses Pax7 and is referred to as a satellite cell. Upon tissue injury, progenitor cells proliferate and provide a pool of immature tissue lineage cells necessary for tissue regeneration. Immature tissue lineage cells have limited self-renewal but readily differentiate with the expression genes and transcription factors associated with mature cell and tissue function. The progression through the differentiation process is associated with loss of telomere length and reduced self-renewal capacity. The factors that stimulate progenitor cells to progress through this process, and the manner in which aging, disease, environmental factors, and genetics modulates these processes are focus areas in the study of tissue regeneration.

Fig. 3

Regeneration of the fingertip in mammals. (A) The fingertip is a complex tissue that includes bone, tendon, nerve, and the nail matrix, which is a specialized epithelial tissue. Amputation of the fingertip with preservation of the proximal nail matrix is the only example of an appendage that undergoes complete regeneration in humans and other mammals. (B) Amputation of the fingertip results in activation of nail bed stem cells and induction of Wnt/β-catenin signaling. Particularly important is the proximal nail matrix where the progenitor cell (SC) population is located. In the distal nail matrix, progenitor cells are a transient cell population that is undergoing amplification and subsequent differentiation. Wnt/β-catenin signaling in the nail bed leads to proliferation of mesenchymal stem cells and formation of a regenerative blastema distal to the bone and soft tissue injury. (C) The undifferentiated mesenchymal cells in the blastema have abundant expression the osteoprogenitor gene, Runx2. The blastema osteoprogenitor cell population expresses Fgfr1. The regenerative blastema is further enhanced by the invading regenerating nerve tissues that express FGF-2. (D) The interactions between the various tissues eventually results in complete regeneration of the fingertip. SC = progenitor cell; Fgfr = fibroblast growth factor receptor; FGF = fibroblast growth factor; MSC = mesenchymal stem cell.