RANKL and OPG activity is regulated by injury size in networks of osteocyte-like cells

Bone remodelling is an intricate process encompassing numerous paracrine and autocrine biochemical pathways and mechanical mechanisms. It is responsible for maintaining bone homeostasis, structural integrity and function. The RANKL-RANK-OPG cytokine system is one of the principal mediators in the maintenance of bone cell function and activation of bone remodelling by the Basic Multicellular Unit (BMU) which carries out remodelling. Theories surrounding the initiation of bone remodelling include mechanical loading, fluid flow and microdamage as potential stimuli. This study focused on microdamage. In an in vitro simulated bone environment, gel embedded MLO-Y4 cell networks were subjected to damage in the form of planar, crack-like defects of constant area and varying thickness. The biochemical response was determined by ELISA and luciferase assay. The results showed that RANKL release increased and OPG decreased in a manner which depended on injury size (i.e. thickness) and time following application of injury. The effect of microdamage on cell viability and apoptosis was also evaluated. This work demonstrates that injury alone, in the absence of imposed strain or fluid flow, is sufficient to initiate changes in cytokine concentrations of the type which are known to stimulate bone remodeling.