Mechanical loading of bone induces fluid shear stress (FSS), which stimulates Src and the mitogen- activated protein kinases Erk1/2 leading to increased osteoblast/cyte proliferation and survival. We found that FSS activates the nitric oxide (NO)/cGMP/cGMP-dependent protein kinase (PKG) pathway, and that PKG activation is necessary for shear-induced Src and Erk activation. NO donors and cGMP analogs mimicked the effect of FSS on Src/Erk in osteoblasts/cytes, while siRNA knock-down of membrane-bound PKG II abolished it. Src activation by FSS or cGMP occurred through de-phosphorylation of Src Tyr529 (an inhibitory site), which required the protein tyrosine phosphatases (PTP) Shp-1 and -2, and cell attachment through $3 integrins. PKG II, Src, and Shp-2 co-localized with $3 in focal adhesion complexes in FSS-stimulated osteoblasts, and PKG II phosphorylated Shp-1 and -2, but not Src. We hypothesize that PKG II activates Src through activation or recruitment of Shp-1/2, which de-phosphorylate Src Tyr529, and/or through inhibition or displacement of C- terminal Src kinase (CSK), which phosphorylates Tyr529. Consistent with the skeletal phenotype of NO synthase-deficient mice, and based on defective FSS-induced signaling in PKG II-null osteoblasts, PKG II deficiency may lead to decreased bone formation during growth and/or in response to skeletal loading.
The Specific Aims are: (i) to determine the mechanism(s) of Src activation by NO/cGMP/PKGII in FSS-stimulated osteoblasts/cytes;(ii) to characterize the Src-containing signaling complex activated by FSS and NO/cGMP/PKG II;and (iii) to define the role of PKG II in bone (re)modeling in mice. We will map Shp-1/2 phosphorylation by PKG II, and test the effects of PKG II on PTP and CSK activity and subcellular localization. We will assess PKG II, SHP-1/2, and $3 integrin functions in shear-induced Src activation using siRNA approaches with reconstitution of wild type and mutant proteins. We will use co-immunoprecipitation, immuno- fluorescence staining, and bimolecular fluorescence complementation to characterize the PKG-regulated Src signaling complex, and use a proteomics approach to identify novel PKG II substrates and interacting partners in osteoblast membranes. We will examine the skeletal phenotype of osteoblast/cyte-specific PKG II knockout mice during skeletal growth and aging, and under conditions of unloading and reloading, using micro-CT, histo- morphometry, and gene expression analysis. We will analyze FSS responses and differentiation of primary PKG-/- osteoblasts. These studies will provide new insights into NO/cGMP/PKG actions in bone, and could lead to improved therapies for osteoporosis.
Mechanical stimulation is a potent stimulus for bone cell (osteoblast) growth and differentiation, improving bone strength and preventing osteoporosis;however, the molecular mechanisms by which osteoblasts convert mechanical stimuli into biochemical changes (a process known as mechanotransduction) remain poorly understood. We recently defined a novel function of the nitric oxide/cGMP/cGMP-dependent protein kinase (PKG) signal transduction pathway in osteoblast mechanotransduction, and we now propose to study the mechanisms whereby PKG controls important down-stream signaling proteins in mechanically-stimulated osteoblasts and determine the role of PKG II in bone remodeling in genetically-modified mice. These studies will provide the foundation for novel and improved treatment strategies in osteoporosis.
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