Ecto-5'-nucleotidase (CD73) regulates bone formation and remodeling during intramembranous bone repair in aging mice

Ecto-5'-nucleotidase (CD73) generates adenosine, an osteoblast activator and key regulator of skeletal growth. It is unknown, however, if CD73 regulates osteogenic differentiation during fracture healing in adulthood, and in particular how CD73 activity regulates intramembranous bone repair in the elderly. Monocortical tibial defects were created in 46-52-week-old wild type (WT) and CD73 knock-out mice (CD73^-/-) mice. Injury repair was analyzed at post-operative days 5, 7, 14 and 21 by micro-computed tomography (micro-CT), histomorphometry, proliferating cell nuclear antigen (PCNA) immunostaining, alkaline phosphatase (ALP) and tartrate-resistant acid phosphatase (TRAP) histochemistry. Middle-aged CD73 knock-out mice exhibited delayed bone regeneration and significantly reduced bone matrix deposition detected by histomorphometry and micro-CT. Cell proliferation, ALP activity and osteoclast number were reduced in the CD73^-/- mice, suggesting a combined defect in bone formation and resorption due the absence of CD73 activity in this model of intramembranous bone repair. Results from this study demonstrate that osteoblast activation through CD73 activity is essential during bone repair in aging mice, and it may present a drugable target for future biomimetic therapeutic approaches that aim at enhancing bone formation in the elderly patients.