Oral Presentation Annual Meetings of the Endocrine Society of Australia and Society for Reproductive Biology and Australia and New Zealand Bone and Mineral Society 2016

Regulation of mitochondrial transfer and mitophagy in osteocytes (#92)

Junjie Gao 1 , Tak Cheng 1 , Rui Ruan 1 , Tao Wang 1 , Nathan J Pavlos 1 , An Qin 2 , James H Steer 3 , Qing Jiang 4 , Hiroshi Takayanagi 1 5 , Ming-Hao Zheng 1
  1. Centre for Orthopaedic Research, School of Surgery, University of Western Australia, Perth, WA, Australia
  2. Shanghai Key Laboratory of Orthopedic Implants, Department of Orthopedic Surgery, Shanghai Ninth People’s Hospital, Shanghai Jiao Tong University School of Medicine, Shanghai, China
  3. Pharmacology Unit, School of Medicine and Pharmacology, University of Western Australia, Perth, WA, Australia
  4. Department of Sports Medicine and Adult Reconstruction Surgery, Drum Tower Hospital Affiliated to Medical School of Nanjing University, Nanjing, Jiangsu, China
  5. Department of Immunology, Graduate School of Medicine and Faculty of Medicine, The University of Tokyo, Tokyo, Japan

Maintenance of mitochondrial function and energy homeostasis is essential for mammalian cells to adapt to various stressors throughout their life. It is not clear, however, how osteocytes imprisoned in the mineralised bone matrix maintain their mitochondrial function in response to various pharmacological and mechanical stress in bone. Here using dual florescence live cell confocal imaging, we provide evidence of dynamic mitochondrial transfer between cultured MLO-Y4 osteocytes along the dendritic process. By TOM20 and F-actin co-staining we further confirmed mitochondria transfer along the dendrites of mouse calvarial primary osteocyte. Interestingly, administration of glucocorticoid (GC) significantly impedes mitochondria movement in both MLO-Y4 osteocytes and primary osteocytes. The reduction in mitochondrial trafficking between osteocytes by GC is a result of reduced mitochondrial transmembrane potential, induction of mitochondrial fragmentation ultimately leading to mitophagic recycling. To identify the molecular pathway involved in GC-induced mitochondrial stress in osteocytes, we employed mitochondrial microarray analyses and identified dusp1 encoding the MAP kinase phosphatase-1 (MKP1), as a GC responsive gene that regulates mitochondrial function. Mitochondrial membrane sub-fractionation assay demonstrated localization of MKP1 to both inner and outer membranes of mitochondria. Upon the administration of GC, MKP1 was found to be translocated from the outer membrane to inner membrane. This correspondingly led to membrane depolarization and the accumulation of PINK1 on the outer membrane, resulting in the induction of mitochondrial fragmentation and subsequent removal of damaged mitochondria by mitophagy. Silencing of MKP1 using siRNA gene knockdown protected against this GC-induced mitochondrial stress events. Collectively our data demonstrated for the first time that dynamic mitochondrial transfer occurs between osteocytes within their dendritic network. It suggests that osteocytes share their energy consumption in the network. This process is inhibited by GC mediated MKP-1 induced mitochondrial stress, which leads to induction of mitophagy.