Down-regulation of N-methyl D-aspartate receptor in rat-modeled disuse osteopenia

Abstract

Lack of mechanical stress may result in osteoporosis; however, the underlying mechanisms of disuse osteoporosis remain unclear. It has been indicated that mechanical loading causes extracellular glutamate accumulation in osteoblasts. We hypothesized that the glutamate receptor mediation on bone cells might also be involved in mechanically stimulated osteogenesis. In this study, we investigated the changes of bone formation and the expressions of osteogenic genes and N-methyl D-aspartate (NMDA) receptors, the major glutamate receptors, in disused bones. Rat modeled disuse osteopenia in hind limbs was induced by a 3-week tail suspension in Sprague-Dawley rats. Bone mineral density and trabecular bone volume of distal femurs were measured to verify the osteopenia of disused bones. The mRNA expressions of cbfa1/Runx2, type I collagen, alkaline phosphatase (ALP) and osteocalcin (OC) in bones were measured as osteogenic markers. The influences of mechanical unloading on the expressions of NMDA receptors (NR1 and NR2D) in bones were also examined. The effects of NMDA mediation on osteogenesis were tested by a treatment of MK-801, a non-competitive NMDA receptor antagonist, in cultured osteoblasts and bone marrow stroma cells. Our result showed that mRNA expressions of cbfa1/Runx2, type I collagen, ALP and OC were significantly decreased in disused bones. The mRNA and protein expressions of NR1 and NR2D were significantly decreased in disused bones; furthermore, immunolocalization of both receptors showed decreases in osteoblasts, but not in osteoclasts. The results from the in vitro study showed that MK-801 inhibited mRNA expression of cbfa1/Runx2 in bone marrow stroma cells and also inhibited those of collagen type I, ALP and OC of osteoblasts in a dose-dependent manner. These results suggest that NMDA receptor mediation may play an important role in transmitting mechanical loading in bones, and decreases of the expressions of NMDA receptors in disused bones, especially in osteoblasts, may contribute to the decrease of osteogenesis.

One-year soy isoflavone supplementation prevents early postmenopausal bone loss but without a dose-dependent effect

“…Taiwanese women used to be known to have femoral neck bone mineral density (BMD) values of 0%-15% lower than those of Caucasians. The vertebral fracture rate was 18% for women, whereas the hip fracture rate was 0.203% in 1996, which was lower than that in Caucasians and similar to those in Chinese and other Asian women [2][3][4][5]. However, the incidence appeared to be as high as 505 per 100,000 in a recent nationwide survey in Taiwan [6].…”

Shear stress-induced Ca2+ elevation is mediated by autocrine-acting glutamate in osteoblastic MC3T3-E1 cells

“…It has been reported that in vivo mechanical loading decreased the expression of ionotropic glutamate receptors in osteoclasts and bone lining cells (41). Meanwhile, mechanical unloading also decreased NMDA receptor in disuse osteopenia (42). These effects on the expression of glutamate receptors may cause changes in the sensitivity to mechanical loading.…”

Extracellular glutamate alters mature osteoclast and osteoblast functions

“…Specifically blocking NMDA receptors with (+)-MK 801 reduced Ocdependent bone resorption and actin ring formation in vitro (Chenu et al 1998;Itzstein et al 2000), but this antagonist was ineffective at altering mature Oc activity when added after 7 days of culture (Peet et al 1999). Furthermore, one study reported that (+)-MK 801 inhibited alkaline phosphatase (ALP) activity, an enzyme expressed by Ob during new bone formation (Ho et al 2005). Although suggestive of a more important role for NMDA receptors in differentiation rather than mature cell functions, the potential for indirect effects due to other cells in mixed culture models advocates for a more cautious interpretation.…”