Effects of Aging on Osteogenic Response and Heterotopic Ossification Following Burn Injury in Mice

Abstract

Heterotopic ossification (HO) is a common and debilitating complication of burns, traumatic brain injuries, and musculoskeletal trauma and surgery. Although the exact mechanism of ectopic bone formation is unknown, mesenchymal stem cells (MSCs) capable of osteogenic differentiation are known to play an essential role. Interestingly, the prevalence of HO in the elderly population is low despite the high overall occurrence of musculoskeletal injury and orthopedic procedures. We hypothesized that a lower osteogenicity of MSCs would be associated with blunted HO formation in old compared with young mice. In vitro osteogenic differentiation of adipose-derived MSCs from old (18-20 months) and young (6-8 weeks) C57/BL6 mice was assessed, with or without preceding burn injury. In vivo studies were then performed using an Achilles tenotomy with concurrent burn injury HO model. HO formation was quantified using mCT scans, Raman spectroscopy, and histology. MSCs from young mice had more in vitro bone formation, upregulation of bone formation pathways, and higher activation of Smad and nuclear factor kappa B (NF-kB) signaling following burn injury. This effect was absent or blunted in cells from old mice. In young mice, burn injury significantly increased HO formation, NF-kB activation, and osteoclast activity at the tenotomy site. This blunted, reactive osteogenic response in old mice follows trends seen clinically and may be related to differences in the ability to mount acute inflammatory responses. This unique characterization of HO and MSC osteogenic differentiation following inflammatory insult establishes differences between age populations and suggests potential pathways that could be targeted in the future with therapeutics.

The Systemic Effect of Burn Injury and Trauma on Muscle and Bone Mass and Composition

“…26 Our laboratory has also demonstrated that trauma induced by Achilles tenotomy produces local elevation in NF-κB. 19 Our present results demonstrate similar findings, with significantly increased expression of NF-κB and resultant activation and nuclear translocation of p-NF-κB in burn plus tenotomy mice compared with tenotomy alone. Furthermore, the activation of NF-κB is sustained in both burn plus tenotomy and tenotomy at 3 weeks following injury, suggesting the prominent role NF-κB signaling has in unloading atrophy.…”

“…To assess the systemic effects of burn on local myopathy, we used the modified burned mouse model as described previously 19,20 Dorsal hair was removed using clippers under isoflurane anesthesia to expose approximately 30 percent of the total body surface area. The exposed dorsal skin was immersed in a water bath (60°C; n = 16) for 18 seconds to produce a partial-thickness dermal burn.…”

“…All mice underwent an Achilles tenotomy at the tendon midpoint on the left leg after burn or sham/nonburn injury as described previously 19 to represent a model of muscle unloading. Specifically, the Achilles tenotomy was performed using a 1-cm incision on the lateral aspect of the Achilles tendon with subsequent exposure of tendon from its origin to insertion.…”

Pilot study for detection of early changes in tissue associated with heterotopic ossification: moving toward clinical use of Raman spectroscopy

“…Raman spectroscopy has been shown to be an effective means of identifying early HO lesions (21)(22)(23)(24)(25). It has also become a useful tool in many areas of research, including mineralization analysis, biomedical assays, cellular analysis, cancer detection and other areas (26)(27)(28)(29)(30)(31)(32).…”

Inhibition of connexin 43 prevents trauma-induced heterotopic ossification

“…Although the exact mechanism of ectopic bone formation is still unknown, progenitor cells capable of osteogenic differentiation are known to play an essential role14. In this study, we leveraged our knowledge to demonstrate that Cx43/ERK signaling represents a route for traumatic ectopic bone formation.…”