TNF-α promotes fracture repair by augmenting the recruitment and differentiation of muscle-derived stromal cells

Abstract

With an aging population, skeletal fractures are increasing in incidence, including the typical closed and the less common open fractures in normal bone, as well as fragility fractures in patients with osteoporosis. For the older age group, there is an urgent unmet need to induce predictable bone formation as well as improve implant fixation in situations such as hip joint replacement. Using a murine model of slow-healing fractures, we have previously shown that coverage of the fracture with muscle accelerated fracture healing and increased union strength. Here, we show that cells from muscle harvested after 3 d of exposure to an adjacent fracture differentiate into osteoblasts and form bone nodules in vitro. The osteogenic potential of these cells exceeds that of adipose and skinderived stromal cells and is equivalent to bone marrow stromal cells. Supernatants from human fractured tibial bone fragments promote osteogenesis and migration of muscle-derived stromal cells (MDSC) in vitro. The main factor responsible for this is TNF-α, which promotes first MDSC migration, then osteogenic differentiation at low concentrations. However, TNF-α is inhibitory at high concentrations. In our murine model, addition of TNF-α at 1 ng/mL at the fracture site accelerated healing. These data indicate that manipulating the local inflammatory environment to recruit, then differentiate adjacent MDSC, may be a simple yet effective way to enhance bone formation and accelerate fracture repair. Our findings are based on a combination of human specimens and an in vivo murine model and may, therefore, translate to clinical care.

Botulinum Toxin-induced Muscle Paralysis Inhibits Heterotopic Bone Formation

“…The simplicity of this model compared with a closed or open fracture model (eg, smaller and more confined callus formation, lack of gait defect) supports a direct role for muscle/bone crosstalk in fracture healing [12,19,21,28]. Previous studies using closed and open fracture models have reported conflicting findings over the potential benefit (or lack thereof) of muscle paralysis on fracture healing [3,13].…”

Glial cell line-derived neurotrophic factor influences proliferation of osteoblastic cells

“…Indeed, this study demonstrated that GDNF cooperated with the cytokine TNF-α to stimulate osteoblastic cell growth suggesting an interaction between GDNF and TNF-α signalling pathways in osteoblasts. TNF-α has been ascribed a multifunctional role in bone metabolism (22); TNF-α effects may involve a proresorptive (osteoclastic bone degradation) action during inflammatory conditions (21), but TNF-α has also been recognised as an anabolic cytokine stimulating osteogenic migration, proliferation and differentiation (46)(47)(48)(49)(50). Interestingly, TNF-α has been shown to have neuroprotective capabilities which in part may be dependent on induction of cytoprotective neutrotrophic growth factors such as GDNF (51-53).…”

Unraveling macrophage contributions to bone repair

“…Altered endochondral callus formation has been reported as a result of manipulation of macrophages and or macrophage-expressed molecules, but the experimental designs did not specifically target these molecules during the anabolic phase. 28,29,34,45 Therefore, altered inflammatory progression/resolution cannot be ruled out as a contributing mechanism to the observed compromised fracture repair. Carefully timed manipulation of macrophages and/or their products is required to elucidate the pro-anabolic contributions of macrophages to bone repair via endochondral ossification.…”

“…Experimentally, elevated systemic inflammation due to additional soft-tissue trauma injuries impairs fracture healing, 42,43 confirming that persistence of a proinflammatory environment is one mechanism responsible for detrimental fracture healing. 44 Interestingly, low-dose TNF administered to the fracture site at the time of surgery and 1 day post surgery improved fracture healing in mice, 45 suggesting that TNF, and potentially macrophage activation, has complex dose-and time-dependent outcomes on bone healing.…”