Age-related osteoporosis is characterized by reduced bone formation and accumulation of fat in the bone marrow compartment. Here, we report that the type 1 cannabinoid receptor (CB1) regulates this process. Mice with CB1 deficiency (CB1(-/-)) had increased peak bone mass due to reduced bone resorption, but developed age-related osteoporosis with reduced bone formation and accumulation of adipocytes in the bone marrow space. Marrow stromal cells from CB1(-/-) mice had an enhanced capacity for adipocyte differentiation, a reduced capacity for osteoblast differentiation, and increased expression of phosphorylated CREB (pCREB) and PPARgamma. Pharmacological blockade of CB1 receptors stimulated adipocyte differentiation, inhibited osteoblast differentiation, and increased cAMP and pCREB in osteoblast and adipocyte precursors. The CB1 receptor is therefore unique in that it regulates peak bone mass through an effect on osteoclast activity, but protects against age-related bone loss by regulating adipocyte and osteoblast differentiation of bone marrow stromal cells.

The endocannabinoid system has recently been shown to play a role in the regulation of bone metabolism. The type 2 cannabinoid receptor (CB2) has been reported to regulate bone mass, but conflicting results have been reported with regard to its effects on bone resorption and osteoclast function. Here we investigated the role that CB2 plays in regulating bone mass and osteoclast function using a combination of pharmacological and genetic approaches. The CB2-selective antagonist/inverse agonist AM630 inhibited osteoclast formation and activity in vitro, whereas the CB2-selective agonists JWH133 and HU308 stimulated osteoclast formation. Osteoclasts generated from CB2 knockout mice (CB2-/-) were resistant to the inhibitory effects of AM630 in vitro, consistent with a CB2-mediated effect. There was no significant difference in peak bone mass between CB2-/- mice and wild-type littermates, but after ovariectomy, bone was lost to a greater extent in wild-type compared with CB2-/- mice. Furthermore, AM630 protected against bone loss in wild-type mice, but the effect was blunted in CB2-/- mice. We conclude that CB2 regulates osteoclast formation and bone resorption in vitro and that under conditions of increased bone turnover, such as after ovariectomy, CB2 regulates bone loss. These observations indicate that CB2 regulates osteoclast formation and contributes to ovariectomy-induced bone loss and demonstrate that cannabinoid receptor antagonists/inverse agonists may be of value in the treatment of bone diseases characterized by increased osteoclast activity.

Identification of functional programmable mechanical stimulation (PMS) on tendon not only provides the insight of the tendon homeostasis under physical/pathological condition, but also guides a better engineering strategy for tendon regeneration. The aims of the study are to design a bioreactor system with PMS to mimic the in vivo loading conditions, and to define the impact of different cyclic tensile strain on tendon. Rabbit Achilles tendons were loaded in the bioreactor with/without cyclic tensile loading (0.25 Hz for 8 h/day, 0-9% for 6 days). Tendons without loading lost its structure integrity as evidenced by disorientated collagen fiber, increased type III collagen expression, and increased cell apoptosis. Tendons with 3% of cyclic tensile loading had moderate matrix deterioration and elevated expression levels of MMP-1, 3, and 12, whilst exceeded loading regime of 9% caused massive rupture of collagen bundle. However, 6% of cyclic tensile strain was able to maintain the structural integrity and cellular function. Our data indicated that an optimal PMS is required to maintain the tendon homeostasis and there is only a narrow range of tensile strain that can induce the anabolic action. The clinical impact of this study is that optimized eccentric training program is needed to achieve maximum beneficial effects on chronic tendinopathy management.