Targeting angiogenesis for fracture nonunion treatment under inflammatory diseases

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ABSTRACT Fracture nonunion poses a significant clinical problem. In the United States, approximately 1.6 million bone fractures encounter prolonged healing or nonunion each year. Fracture nonunion treatment usually involves complicated and massive procedures in practice, and sometimes needs multiple surgeries, therefore increases the cost of health care and results in marked patient disability. The major population bearing with these clinical complications are patients with inflammatory conditions, e.g, elder patients, smoking, diabetic or rheumatoid arthritis (RA) patients, highlighting the potential deleterious role of chronic systemic inflammation in fracture r epair. The overarching hypothesis of this proposal is that chronic inflammation results in fracture nonunion through Dnmt3b downregulation mediated angiogenesis defect, and local delivery of OPN and CXCL12 restores angiogenesis and fracture repair under inflammatory conditions. This hypothesis is supported by our preliminary data wherein we show that Dnmt3b is highly expressed in fracture callus during fracture repair and Dnmt3b is the major DNA methyltransferase (Dnmt) responsive to cytokines in MPCs. Relevant to our proposal, we provide evidence that inflammatory signals inhibit Dnmt3b in an NF-κB-dependent manner. Consistently, mice with Dnmt3b loss-of-function (LOF) in chondrocytes display impaired angiogenesis and fracture repair; and Dnmt3b gain-of-function (GOF) in chondrocytes shows protective effect from inflammation in vitro and accelerates fracture repair in mice. Mechanistically, angiogenesis defect mediated by inflammation and Dnmt3b LOF coincide with downregulation of OPN (Osteopontin) and CXCL12 (C-X-C Motif Chemokine Ligand 12) and exogenous OPN and CXCL12 can restore angiogenesis capacity in vitro. To further examine the efficacy of local delivery of OPN and CXCL12 in vivo, we have developed an optimized biomaterial sheet loaded with OPN and CXCL12 and showed a robust angiogenesis process and a restoration of fracture union in RA mice. Three main Specific Aims are proposed. Specific Aim 1 will delineate the mechanism by which inflammation reduces angiogenesis via downregulating Dnmt3b during fracture repair. Specific Aim 2 will determine the optimal release kinetics of OPN and CXCL12 on angiogenesis. Specific Aim 3 will determine the therapeutic effect of sustained OPN and CXCL12 release on angiogenesis and fracture nonunion in mice. The proposed studies will enhance our understanding of mechanisms by which systemic inflammation (via the NF- κB pathway) affects the angiogenic process through Dnmt3b. This work will establish an important therapeutic option to improve the angiogenesis and fracture healing.