The periosteum is the major source of cells involved in fracture healing. We sought to characterize progenitor cells and their contribution to bone fracture healing. The periosteum is highly enriched for progenitor cells, including Sca1+ cells, CFU-F and label-retaining cells compared to the endosteum and bone marrow. Using lineage tracing, we demonstrate that αSMA identifies long-term, slow-cycling, self-renewing osteochondroprogenitors in the adult periosteum that are functionally important for bone formation during fracture healing. In addition, Col2.3CreER-labeled osteoblast cells contribute around 10% of osteoblasts, but no chondrocytes in fracture calluses. Most periosteal osteochondroprogenitors following fracture, can be targeted by αSMACreER. Previously identified skeletal stem cell populations were common in periosteum, but contained high proportions of mature osteoblasts. We have demonstrated that the periosteum is highly enriched for skeletal progenitor cells and there is heterogeneity in the populations of cells that contribute to mature lineages during periosteal fracture healing.

BMPs are used in various clinical applications to promote bone formation. The limited success of the BMPs in clinical settings and supraphysiological doses required for their effects prompted us to evaluate the influence of other signaling molecules, specifically platelet‐derived growth factor (PDGF) on BMP2‐induced osteogenesis. Periosteal cells make a major contribution to fracture healing. We detected broad expression of PDGF receptor beta (PDGFRβ) within the intact periosteum and healing callus during fracture repair. In vitro, periosteum‐derived progenitor cells were highly responsive to PDGF as demonstrated by increased proliferation and decreased apoptosis. However, PDGF blocked BMP2‐induced osteogenesis by inhibiting the canonical BMP2/Smad pathway and downstream target gene expression. This effect is mediated via PDGFRβ and involves ERK1/2 MAPK and PI3K/AKT signaling pathways. Therapeutic targeting of the PDGFRβ pathway in periosteum‐mediated bone repair might have profound implications in the treatment of bone disease in the future. © 2018 The Authors JBMR Plus is published by Wiley Periodicals, Inc. on behalf of the American Society for Bone and Mineral Research.

Fracture healing involves interactions of different cell types, driven by various growth factors, and signaling cascades. Periosteal mesenchymal progenitor cells give rise to the majority of osteoblasts and chondrocytes in a fracture callus. Notch signaling has emerged as an important regulator of skeletal cell proliferation and differentiation. We investigated the effects of Notch signaling during the fracture healing process. Increased Notch signaling in osteochondroprogenitor cells driven by overexpression of Notch1 intracellular domain (NICD1) (αSMACreERT2 mice crossed with Rosa-NICD1) during fracture resulted in less cartilage, more mineralized callus tissue, and stronger and stiffer bones after 3 weeks. Periosteal cells overexpressing NICD1 showed increased proliferation and migration in vitro. In vivo data confirmed that increased Notch1 signaling caused expansion of alpha-smooth muscle actin (αSMA)-positive cells and their progeny including αSMA-derived osteoblasts in the callus without affecting osteoclast numbers. In contrast, anti-NRR1 antibody treatment to inhibit Notch1 signaling resulted in increased callus cartilage area, reduced callus bone mass, and reduced biomechanical strength. Our study shows a positive effect of induced Notch1 signaling on the fracture healing process, suggesting that stimulating the Notch pathway could be beneficial for fracture repair.

Reactive oxygen species (ROS) and nitrogen species have an indispensable role in regulating cell signalling pathways, including transcriptional control via hypoxia inducible factor-1α (HIF-1α). Hyperbaric oxygenation treatment (HBO2) increases tissue oxygen content and leads to enhanced ROS production. In the present study DSS-induced colitis has been employed in BALB/c mice as an experimental model of gut mucosa inflammation to investigate the effects of HBO2 on HIF-1α, antioxidative enzyme, and proinflammatory cytokine genes during the colonic inflammation. Here we report that HBO2 significantly reduces severity of DSS-induced colitis, as evidenced by the clinical features, histological assessment, impaired immune cell expansion and mobilization, and reversal of IL-1β, IL-2, and IL-6 gene expression. Gene expression and antioxidative enzyme activity were changed by the HBO2 and the inflammatory microenvironment in the gut mucosa. Strong correlation of HIF-1α mRNA level to GPx1, SOD1, and IL-6 mRNA expression suggests involvement of HIF-1α in transcriptional regulation of these genes during colonic inflammation and HBO2. This is further confirmed by a strong correlation of HIF-1α with known target genes VEGF and PGK1. Results demonstrate that HBO2 has an anti-inflammatory effect in DSS-induced colitis in mice, and this effect is at least partly dependent on expression of HIF-1α and antioxidative genes.

Previously, a facilitating effect of hyperbaric oxygenation (HBO₂) on aortic ring responses to angiotensin-(1-7) in healthy rats was reported, with epoxyeicosatrienoic acids (EETs) possibly playing an important role. The aim of this study was to assess whether HBO₂ exerts similar effects in diabetic rats and to further explore the role of specific cytochrome P450 (CYP) enzymes in changes induced by HBO₂. Aortic relaxation to angiotensin-(1-7) was significantly higher in HBO₂ diabetic rats compared to control diabetic rats, while HBO₂ had no effect on angiotensin II contraction. N-methylsulphonyl-6-(2-propargyloxyphenyl/hexanamide inhibited the facilitation of angiotensin-(1-7) responses in HBO₂ rats, suggesting an important role of EETs in this modulation. mRNA expression of CYP2J3 and protein expression of CYP2C11 were significantly upregulated in HBO₂ diabetic rats, whereas CYP4A1, CYP4A2 and CYP4A3 mRNA and CYP2J3 protein expression was similar between groups. Mean arterial pressure, ferric reducing ability of plasma and Thiobarbituric Acid Reactive Substances levels and serum angiotensin-(1-7) concentrations were not significantly changed.