Bone Abstracts

Background: Bone accrual during adolescence is the main contributor to peak bone mass. Bone remodeling is a balance between bone formation and resorption as directed by osteoblast and osteoclast activity, respectively. nitric oxide (NO) is a potent regulator of bone remodeling via mediating effects of cytokines, estrogen and mechanical strain. NO is synthesized from the conversion of L-arginine to L-citrulline by the enzyme nitric oxide synthase (NOS). Citrulline can be reconverted to arginine by the enzymes argininosuccinate synthase (ASS) and argininosuccinate lyase (ASL). ASL is the sole mammalian enzyme responsible for endogenous L-arginine production and routing exogenous arginine to NOS for NO synthesis. Deletion of ASL abolishes arginine dependent NO production in the cell. Multiple studies have attempted to determine the role(s) of NO in bone remodeling. However, as of yet no study has been able to dissect its role in a cell specific manner.

Hypothesis: NO produced by osteoblasts stimulates osteoblast proliferation and increases bone mass in homeostasis and under hormonal stress.

Research design and methods: Using Cre-Lox technology, we created an osteoblast specific knockout of Asl in the mouse model. Bilateral ovariectomy was performed at 12 weeks of age to induce estrogen deficiency. Osteoblast activity and bone remodeling was evaluated at 12, 20 and 24 weeks using bone turnover markers, μCT and histomorphometry. Asl cKO female mice were compared to their sham-operated and Asl ^Flox/Flox littermates at each time point.

Preliminary results: At 12 weeks of age, Asl cKO female mice have significantly lower trabecular bone density (BV/TV), trabecular number (Tb.N.) and higher trabecular separation (Tb.Sp.) in lumbar spine when compared to Asl^Flox/Flox littermate female controls (P<0.05).

Discussion: Our preliminary results show that Asl cKO female mice have lower bone mass in lumbar spine at baseline suggesting an inherent role of NO in normal bone mineralization. Evaluation of bone remodeling in the Asl cKO female mice under an estrogen deficient state is ongoing. The translational value of the Asl cKO model is potential development of pharmacologic and genetic manipulation of ASL as an effective regulator of NO metabolism, ultimately leading to improved bone mineralization.