Skip to main content


Src activation by cGMP/PKG II in osteoblasts: characterization of a mechano-sensitive signalling complex


Mechanical stimulation of bone cells is critical for maintaining bone mass and strength, and a better understanding of how mechanical stimuli are converted into intracellular signals to activate an anabolic program in osteoblasts/cytes is fundamental to improving treatments for osteo-degenerative diseases [1, 2]. Weight bearing and locomotion stimulate interstitial fluid flow through the bone canalicular system, and the resultant shear stress is thought to be a major mechanism whereby mechanical forces stimulate osteoblast/osteocyte growth and differentiation [1, 2]. In primary osteoblasts and osteoblast/cyte-like cell lines, fluid shear stress induces rapid expression of c-fos, fra-1, fra-2, and fosB/ΔfosB mRNAs [3]; these genes encode transcriptional regulators important for osteoblast proliferation and differentiation, as demonstrated by the phenotypes of mice that over-express or lack these proteins, respectively [4]. We have previously shown that fluid shear stress increases osteoblast/cyte nitric oxide (NO) production, leading to increased cGMP synthesis and activation of cGMP-dependent protein kinases (PKGs). The NO/cGMP/PKG signaling pathway is required for shear-induced expression of all four fos family genes, and induction of these genes is mediated through activation of the mitogen-activated protein kinases Erk1/2 [3]. However, molecular mechanisms leading to Erk activation in shear-stressed osteoblasts are largely unknown.


We have now defined the events leading from shear stress activation of NO/cGMP/PKG II to the activation of Src [5]; we show that this pathway is required for Erk activation, and controls osteoblast proliferation and survival. We found a novel link between NO/cGMP/PKG and β3 integrins, the key mechano-sensors in bone, and show that PKG II activates β3-associated Src by activating the tyrosine phosphatase Shp-1. PKG II directly phosphorylates and stimulates Shp-1 activity, which de-phosphorylates a C-terminal, inhibitory phosphorylation site on Src, leading to Src activation. Fluid shear stress triggers PKG II, Src, and Shp-1/2 recruitment to a mechano-sensitive complex containing β3 integrins, defining a novel “mechanosome”. We found that PKG II-null mice have defective osteoblast Src/Erk signaling, decreased Erk-dependent gene expression in bone, and impaired osteoblast-dependent, membranous bone formation. These results complement previous studies in PKG II-deficient mice, which showed defective chondroblast differentiation and endochondral bone formation, and studies in NO synthase-deficient mice, which demonstrated an important role of NO in osteoblast biology [6, 7].


Our findings reveal crosstalk between NO/cGMP/PKG and integrins, establish a new mechanism of Src activation, and fill a gap in our understanding of how mechanical forces acting on cell-matrix adhesions are translated into cellular responses. Since Src controls Erk, a key regulator of osteoblast growth and survival, our results support using PKG-activating drugs as mechano-mimetics for treating osteoporosis.


  1. 1.

    Ehrlich PJ, Lanyon LE: Mechanical strain and bone cell function: a review. Osteoporos. Int. 2002, 13: 688-700. 10.1007/s001980200095.

  2. 2.

    Ozcivici E, Luu YK, Adler B, Qin YX, Rubin J, Judex S, Rubin CT: Mechanical signals as anabolic agents in bone. Nat Rev Rheumatol. 2010, 6: 50-59. 10.1038/nrrheum.2009.239.

  3. 3.

    Rangaswami H, Marathe N, Zhuang S, Chen Y, Yeh JC, Frangos JA, Boss GR, Pilz RB: Type II cGMP-dependent protein kinase mediates osteoblast mechanotransduction. J Biol Chem. 2009, 284: 14796-14808. 10.1074/jbc.M806486200.

  4. 4.

    Wagner EF, Eferl R: Fos/AP-1 proteins in bone and the immune system. Immunol Rev. 2005, 208: 126-140. 10.1111/j.0105-2896.2005.00332.x.

  5. 5.

    Rangaswami H, Schwappacher R, Marathe N, Zhuang S, Casteel DE, Haas B, Chen Y, Pfeifer A, Kato H, Shattil S, Boss GB, Pilz RB: Cyclic GMP and protein kinase G control a Src-containing mechanosome in osteoblasts. Sci Signal. 2010, 3: ra91-10.1126/scisignal.2001423.

  6. 6.

    Pfeifer A, Aszòdi A, Seidler U, Ruth P, Hofmann F, Fässler R: Intestinal secretory defects and dwarfism in mice lacking cGMP-dependent protein kinase II. Science. 1996, 274: 2082-2086. 10.1126/science.274.5295.2082.

  7. 7.

    Armour KE, Armour KJ, Gallagher ME, Godecke A, Helfrich MH, Reid DM, Ralston SH: Defective bone formation and anabolic response to exogenous estrogen in mice with targeted disruption of endothelial nitric oxide synthase. Endocrinology. 2001, 142: 760-766. 10.1210/en.142.2.760.

Download references

Author information

Correspondence to Renate B Pilz.

Rights and permissions

Reprints and Permissions

About this article


  • Nitric Oxide
  • Fluid Shear Stress
  • Osteoblast Proliferation
  • Endochondral Bone Formation
  • Interstitial Fluid Flow