Pannexin‐1 and P2X7‐Receptor Are Required for Apoptotic Osteocytes in Fatigued Bone to Trigger RANKL Production in Neighboring Bystander Osteocytes |
| |
Authors: | Wing Yee Cheung J Christopher Fritton Stacy Ann Morgan Zeynep Seref‐Ferlengez Jelena Basta‐Pljakic Mia M Thi Sylvia O Suadicani David C Spray Robert J Majeska Mitchell B Schaffler |
| |
Institution: | 1. Department of Biomedical Engineering, City College of New York, New York, NY, USA;2. Department of Orthopaedic Surgery, New Jersey Medical School, Rutgers University, Newark, NJ, USA;3. Departments of Orthopaedic Surgery, Urology, and Neuroscience, Albert Einstein College of Medicine and Montefiore Medical Center, Bronx, NY, USA |
| |
Abstract: | Osteocyte apoptosis is required to induce intracortical bone remodeling after microdamage in animal models, but how apoptotic osteocytes signal neighboring “bystander” cells to initiate the remodeling process is unknown. Apoptosis has been shown to open pannexin‐1 (Panx1) channels to release adenosine diphosphate (ATP) as a “find‐me” signal for phagocytic cells. To address whether apoptotic osteocytes use this signaling mechanism, we adapted the rat ulnar fatigue‐loading model to reproducibly introduce microdamage into mouse cortical bone and measured subsequent changes in osteocyte apoptosis, receptor activator of NF‐κB ligand (RANKL) expression and osteoclastic bone resorption in wild‐type (WT; C57Bl/6) mice and in mice genetically deficient in Panx1 (Panx1KO). Mouse ulnar loading produced linear microcracks comparable in number and location to the rat model. WT mice showed increased osteocyte apoptosis and RANKL expression at microdamage sites at 3 days after loading and increased intracortical remodeling and endocortical tunneling at day 14. With fatigue, Panx1KO mice exhibited levels of microdamage and osteocyte apoptosis identical to WT mice. However, they did not upregulate RANKL in bystander osteocytes or initiate resorption. Panx1 interacts with P2X7R in ATP release; thus, we examined P2X7R‐deficient mice and WT mice treated with P2X7R antagonist Brilliant Blue G (BBG) to test the possible role of ATP as a find‐me signal. P2X7RKO mice failed to upregulate RANKL in osteocytes or induce resorption despite normally elevated osteocyte apoptosis after fatigue loading. Similarly, treatment of fatigued C57Bl/6 mice with BBG mimicked behavior of both Panx1KO and P2X7RKO mice; BBG had no effect on osteocyte apoptosis in fatigued bone but completely prevented increases in bystander osteocyte RANKL expression and attenuated activation of resorption by more than 50%. These results indicate that activation of Panx1 and P2X7R are required for apoptotic osteocytes in fatigued bone to trigger RANKL production in neighboring bystander osteocytes and implicate ATP as an essential signal mediating this process. © 2016 American Society for Bone and Mineral Research. |
| |
Keywords: | OSTEOCYTE APOPTOSIS PANNEXIN 1 P2X7 RECEPTOR BYSTANDER SIGNALING RANKL FATIGUE MICRODAMAGE |
|
|