Oral Presentation 29th Australian and New Zealand Bone and Mineral Society Annual Scientific Meeting 2019

Tram2 acts in osteoblasts and osteoclasts to regulate bone mass and strength (#78)

Victoria D Leitch 1 2 , Penny Sparkes 2 , Elena Ghirardello 2 , John G Logan 2 , Agnes Swiatkowska 3 , Natalie C Butterfield 2 , Ryan Chai 4 , Scott Youlten 4 , Peter I Croucher 4 , Christopher J Lelliott 3 , JH Duncan Bassett 2 , Graham R Williams 2
  1. RMIT Centre for Additive Manufacturing, RMIT, Melbourne, VIC, Australia
  2. Department of Medicine, Imperial College London, London, UK
  3. Mouse Pipelines, Wellcome Trust Sanger Institute, Cambridge, UK
  4. Bone Biology Laboratory, Garvan Institute of Medical Research, Sydney, NSW, Australia

Translocation Associated Membrane Protein 2 (Tram2) is a component of the translocon, which traffics proteins through the endoplasmic reticulum. Despite no known skeletal role for Tram2, global knockout in mice (Tram2KO) results in significant skeletal defects including short stature, grossly reduced bone mineral content (BMC) and mineralisation, profoundly reduced cortical and trabecular bone mass, and decreased bone strength and stiffness. This extreme phenotype indicates that Tram2 plays a pivotal role in the regulation of bone mass and strength, and thus may present a novel therapeutic target for the treatment of osteoporosis. Histomorphometry and primary culture experiments led us to hypothesise that Tram2 regulates bone mass via actions in both osteoblasts and osteoclasts. Cell-specific deletion of Tram2 and RNA-seq were used to test this hypothesis. Detailed analysis (X-ray microradiography, micro-CT, biomechanical testing, n=4 males, per genotype) of both osteoblast (Ocn-Cre) and osteoclast (LysM-Cre) specific Tram2KO mice had profoundly reduced BMC at P70 (p<0.0001, Kolmogorov-Smirnov test), reduced cortical bone mass (P<0.001, t-test); and reduced yield, maximum and fracture loads (P<0.001 and P<0.01 respectively, t-test), stiffness (P<0.001 and P<0.01 respectively, t-test) and energy dissipated prior to fracture (P< 0.05, t-test). BMC was also reduced at P21 in LysM-Cre mice (P<0.05, Kolmogorov-Smirnov test). RNAseq identified 2790 differentially expressed genes between WT and Tram2KO osteocytes and 125 in osteoclasts, with in-depth analysis of gene ontology terms implicating key pathways that may underlie the mechanisms of bone loss in Tram2-deficiency. These data confirm Tram2 is a key determinant of bone mass and strength, and the extreme skeletal phenotype in global Tram2KO mice results from defects in the function of both osteoblasts and osteoclasts. Identification of signalling pathways that are responsible for bone loss in Tram2-deficient mice will advance understanding of bone formation and turnover and may identify novel tractable therapeutic targets for osteoporosis prevention and treatment.