Unexpected timely fracture union in matrix metalloproteinase 9 deficient mice

<div><p>Immediately following a fracture, a fibrin laden hematoma is formed to prevent bleeding and infection. Subsequently, the organized removal of fibrin, via the protease plasmin, is essential to permit fracture repair through angiogenesis and ossification. Yet, when plasmin activity is lost, the depletion of fibrin alone is insufficient to fully restore fracture repair, suggesting the existence of additional plasmin targets important for fracture repair. Previously, activated matrix metalloproteinase 9 (MMP-9) was demonstrated to function in fracture repair by promoting angiogenesis. Given that MMP-9 is a defined plasmin target, it was hypothesized that pro-MMP-9, following plasmin activation, promotes fracture repair. This hypothesis was tested in a fixed murine femur fracture model with serial assessment of fracture healing. Contrary to previous findings, a complete loss of MMP-9 failed to affect fracture healing and union through 28 days post injury. Therefore, these results demonstrated that MMP-9 is dispensable for timely fracture union and cartilage transition to bone in fixed femur fractures. Pro-MMP-9 is therefore not a significant target of plasmin in fracture repair and future studies assessing additional plasmin targets associated with angiogenesis are warranted.</p></div

Revascularization of the fracture is unaffected by a loss of MMP-9.

<p>Visual representation of microfil based angiography overlaid with radiographic images. No differences in vascularity were observed between wild type and MMP-9 deficient mice at any time point.</p

Vascular quantification of healing fractures of WT and MMP-9 deficient mice.

<p>Quantitative assessment of microfil based angiography surrounding the healing fracture. No statistical differences in vascularity were identified between wild type and MMP-9 deficient mice at any time point for either vascular volume (mm³) or vessel thickness (mm). 7d- WT: N = 4, MMP-9: N = 3; 10d- WT: N = 4, MMP-9: N = 3; 14d- WT: N = 5, MMP-9: N = 6; 21d- WT: N = 4, MMP-9: N = 6; 28d- WT: N = 7, MMP-9: N = 6. Individual time points were assessed by non-parametric t-tests. Alpha = 0.05.</p

MMP-9 is not required for endochondral fracture healing.

<p>A) Both MMP-9 deficient and WT littermates possessed abundant soft cartilage at 7 and 10 dpf. At 21 dpf, hard tissue callus dominated in MMP-9 deficient and WT littermates. B) Quantification of the total callus volume and % soft tissue fracture callus demonstrated that there was no significant difference between MMP-9 deficient mice and WT littermates at any time assessed. While the % soft tissue callus area appears to trend higher in MMP-9 deficient mice at 14 DPI as previously described [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0198088#pone.0198088.ref005" target="_blank">5</a>], there is marked variability between MMP-9 deficient mice.</p

Skeletal healing of stabilized femur fracture in MMP-9 deficient mice.

<p>An open femur fracture model with stabilization by at 23-gauge intramedullary pin was used to compare key aspects of fracture healing in mice with and without MMP-9. To assess temporal development and subsequent remodeling of the hard tissue callus, we performed A) serial x-rays of the fractured femurs (yellow arrow indicates fracture site. Hard tissue callus is evident by X-ray proximal and distal to the fracture in mice with or without MMP-9 by 10 dpf. From 14 to 21 dpf, the proximal and distal hard tissue callus expands and coalesces at the fracture. At this point the hard tissue callus is at its maximum size and subsequently it starts remodeling through day 28. B) <i>Mmp-9 -/-</i> mice did not demonstrate any significant differences in fracture healing compared to WT littermates, based on scores of the bone formation, bone union, and bone remodeling. Data displayed represent the mean ± SD. Statistical significance between groups at each time point was determined using a non-parametric Mann-Whitney Test. Number of mice per genotype: 7dpf: WT- 22, MMP-9 KO- 35; 10d: WT-13, MMP-9 KO- 17; 14dpf: WT- 17, MMP-9 KO- 25; 21dpf: WT- 13, MMP-9 KO- 19; 28dpf: WT- 12, MMP-9 KO- 17.</p