Endothelial nitric oxide synthase gene-deficient mice demonstrate marked retardation in postnatal bone formation, reduced bone volume, and defects inosteoblast maturation and activity

Nitric oxide (NO) has been implicated in the local regulation of bone metab olism. However, the contribution made by specific NO synthase (NOS) enzymes is unclear. Here we show that endothelial NOS gene knockout mice (eNOS-/-) have marked abnormalities in bone formation. Histomorphometric analysis of eNOS-/- femurs showed bone volume and bone formation rate was reduced by u p to 45% (P < 0.01) and 52% (P < 0.01),respectively. These abnormalities we re prevalent in young (6 to 9 weeks old) adults but by 12 to 18 weeks bone phenotype was restored toward wild-type. Dual energy X-ray absorptiometry a nalysis confirmed the age-related bone abnormalities revealing significant reductions in femoral(P < 0.05) and spinal bone mineral densities (P < 0.01 ) at 8 weeks that were normalized at 12 weeks. Reduction in bone formation and volume was not related to increased osteoclast numbers or activity but rather to dysfunctional osteoblasts. Osteoblast numbers and mineralizing ac tivity were reduced in eNOS-/- mice. In vitro, osteoblasts from calvarial e xplants showed retarded proliferation and differentiation (alkaline phospha tase activity and mineral deposition) that could be restored by exogenous a dministration of a NO donor. These cells were also unresponsive to 17 beta -estradiol and had an attenuated chemotactic response to transforming growt h factor-p, In conclusion, eNOS is involved in the postnatal regulation of bone mass and lack of eNOS gene results in reduced bone formation and volum e and this is related to impaired osteoblast function.