NO signaling in mechanical adaptation of bone

Abstract

Intercellular signaling is an important physiological phenomenon involved in maintaining homeostasis in all tissues. In bone, intercellular communication via chemical signals like nitric oxide (NO) plays a critical role in the dynamic process of bone remodeling. It is believed that osteocytes are the mechanosensors in bone. If bones are mechanically loaded, the resulting shear stress caused by strain-derived interstitial fluid flow can activate them. Activated osteocytes produce signaling molecules like NO, which modulate the activity of the bone forming osteoblasts, and the bone resorbing osteoclasts, thereby orchestrating bone adaptation to mechanical loading. This single-cell level mechanosensing and intercellular signaling is essential for bone adaptation to mechanical loading. Since NO production is essential for loading-induced bone formation in vivo, the NO production in response to mechanical stimulation is a meaningful parameter for measuring bone cell activation. Several studies have shown that NO production is rapidly increased in response to mechanical stress in bone cells, including isolated osteocytes. How a mechanically-stimulated single osteocyte functions in a social context via intercellular communication, was studied by monitoring changes in intracellular NO production, using DAR-4M AM chromophore, in the surrounding osteocytes. A single mechanically-stimulated osteocyte activates its surrounding osteocytes via direct and/or indirect intercellular signaling. In single osteocytes, mechanical stimulation of both cell body and cell process results in upregulation of intracellular NO production, indicating that both cell body and cell process might play a role in mechanosensing and mechanotransduction in bone. Osteocytes with round-suspended morphology require lower force stimulation to show an increase in NO production, even though they are an order of magnitude more elastic compared to flat-adherent cells. Apparently, elastic osteocytes seem to require less mechanical forces in order to respond with increased NO production than stiffer cells. Bone cells release increased amounts of NO in relation to the rate of fluid shear stress as demonstrated by bone cells in a broad range of frequencies induced by vibration stress. This supports the notion that bone formation in vivo is stimulated by dynamic rather than static loads. This rate-dependent response occurs provided that the cells initially experience a stress-kick. The bone cell response to stress is enhanced by the addition of noise, i.e. the NO released by bone cells reaches a maximum at the application of an optimum noise-level by fluid shear stress. Since mechanical stimuli and NO production enhance the fracture resistance of bone, these data may facilitate the development of novel osteoporosis treatments.