Biophysical factors generated during loading inhibit osteoclastogenesis through regulation of two molecules, RANKL and eNOS. This response requires activation of ERK1/2, indicating that strain must initiate proximal events in the MARK signaling cascade. Our study of upstream molecules has revealed that low-magnitude strains activate a single isoform of Ras, H-Ras; and further we show that RNA silencing of H-Ras prevents strain regulated gene expression. The selective Ras activation offers clues to the nature of the mechanotransducer: Ras isoforms have specific spatial distributions within the membrane. H-Ras is located within 'signaling centers' associated with organized membrane (lipid rafts or caveolae). Thus, for H-Ras, the membrane serves as a platform where MARK signaling events are regulated. Our data further shows that disruption of lipid rafts prevents strain activation of H-Ras. The requirement for an intact organized membrane leads to our hypothsis that membrane organization of signaling molecules is required for mechanical regulation of RANKL and eNOS expression and that the organized membrane may serve as the mechanotransducer. We propose to focus on the proximal requirements for converting a mechanical signal into an intracellular chemical signal. In SA#1 we will examine the requirement for H-Ras in the cellular strain response resulting in distal changes in RANKL and eNOS. SA#2 examines the association of H-Ras with the organized membrane, including the requirement for caveolin-1 in mechanotransduction.
These aims utilize 1 degree stromal murine cells challenged with substrate strain and measurements of specific Ras activation, RANKL and eNOS response, and silencing of key molecules through siRNA. SA#3 asks whether membrane organization is relevant for application of load to mice in vivo. We will apply load to tibia of wt C57/B6 as well as to H-Ras null and caveolin-1 null mice. Local expression of genes (RANKL, eNOS, Ras isoforms, caveolin) and bone histomorphometry will be analyzed after loading. With this work we will not only define the mechanisms involved in strain regulation of bone remodeling but also generate a new paradigm for a general mechanotransducer that converts mechanical information into intracellular signals through perturbation of the organized plasma membrane.
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