Mechanical loading in osteocytes induces formation of a Src/Pyk2/MBD2 complex that suppresses anabolic gene expression.
Mechanical stimulation of the skeleton promotes bone gain and suppresses bone loss, ultimately resulting in improved bone strength and fracture resistance. The molecular mechanisms directing anabolic and/or anti-catabolic actions on the skeleton during loading are not fully understood. Identifying molecular mechanisms of mechanotransduction (MTD) signaling cascades could identify new therapeutic targets. Most research into MTD mechanisms is typically focused on understanding the signaling pathways that stimulate new bone formation in response to load. However, we investigated the structural, signaling and transcriptional molecules that suppress the stimulatory effects of loading. The high bone mass phenotype of mice with global deletion of either Pyk2 or Src suggests a role for these tyrosine kinases in repression of bone formation. We used fluid shear stress as a MTD stimulus to identify a novel Pyk2/Src-mediated MTD pathway that represses mechanically-induced bone formation. Our results suggest Pyk2 and Src function as molecular switches that inhibit MTD in our mechanically stimulated osteocyte culture experiments. Once activated by oscillatory fluid shear stress (OFSS), Pyk2 and Src translocate to and accumulate in the nucleus, where they associate with a protein involved in DNA methylation and the interpretation of DNA methylation patterns -methyl-CpG-binding domain protein 2 (MBD2). OFSS-induced Cox-2 and osteopontin expression was enhanced in Pyk2 KO osteoblasts, while inhibition of Src enhanced osteocalcin expression in response to OFSS. We found that Src kinase activity increased in the nucleus of osteocytes in response to OFSS and an interaction activated between Src (Y418) and Pyk2 (Y402) increased in response to OFSS. Thus, as a mechanism to prevent an over-reaction to physical stimulation, mechanical loading may induce the formation of a Src/Pyk2/MBD2 complex in the nucleus that functions to suppress anabolic gene expression.
Department of Cellular and Integrative Physiology, Indiana University School of Medicine, Indianapolis, Indiana, United States of America.
Julia M
JM
Hum
Department of Cellular and Integrative Physiology, Indiana University School of Medicine, Indianapolis, Indiana, United States of America.
Richard N
RN
Day
Department of Anatomy and Cell Biology, Indiana University School of Medicine, Indianapolis, Indiana, United States of America.
Joseph P
JP
Bidwell
School of Engineering, University of California San Diego, San Diego, California, United States of America.
Yingxiao
Y
Wang
Department of Cellular and Integrative Physiology, Indiana University School of Medicine, Indianapolis, Indiana, United States of America.
Fredrick M
FM
Pavalko
D000818:Animals; D000873:Anthracenes; D015153:Blotting, Western; D019175:DNA Methylation / Q000235:genetics; D017931:DNA Primers / Q000235:genetics; D004268:DNA-Binding Proteins / Q000378:metabolism; D031541:Fluorescence Resonance Energy Transfer; D005455:Fluorescent Antibody Technique; D051418:Focal Adhesion Kinase 2 / Q000378:metabolism; D005786:Gene Expression Regulation / Q000502:physiology*; D040542:Mechanotransduction, Cellular / Q000502:physiology*; D051379:Mice; D046912:Multiprotein Complexes / Q000096:biosynthesis / Q000378:metabolism*; D010011:Osteocytes / Q000378:metabolism / Q000502:physiology*; D060888:Real-Time Polymerase Chain Reaction; D033081:Shear Strength; D013314:Stress, Mechanical; D019061:src-Family Kinases / Q000378:metabolism
C546739:AnOV compound; D000873:Anthracenes; D017931:DNA Primers; D004268:DNA-Binding Proteins; C489326:Mbd2 protein, mouse; D046912:Multiprotein Complexes; D051418:Focal Adhesion Kinase 2; C496053:Ptk2b protein, mouse; D019061:src-Family Kinases