Oral Presentation 30th Australian and New Zealand Bone and Mineral Society Annual Scientific Meeting 2020

Interaction between osteocyte SOCS3-dependent signaling and the bone marrow microenvironment maintains cortical bone integrity (#4)

Tsuyoshi Isojima 1 , Emma C Walker 1 , Blessing Crimeen-Irwin 1 , Ingrid J Poulton 1 , Narelle E McGregor 1 , Jonathan H Gooi 1 2 3 , T. John Martin 1 2 , Natalie A Sims 1 2
  1. St Vincent's Institute of Medical Research, Fitzroy, VIC, Australia
  2. Department of Medicine at St. Vincent’s Hospital, The University of Melbourne, Fitzroy, VIC, Australia
  3. Bio21 Molecular Science and Biotechnology Institute, Parkville, VIC, Australia

Cortical structure is a crucial determinant of bone strength, yet the underlying mechanisms controlling its organization remain poorly understood. We recently reported that cortical bone develops through pore closure and accumulation of high-density bone by SOCS3-mediated suppression of gp130-STAT3 signaling in osteocytes. Since SOCS3 also suppresses G-CSFR signaling, we studied whether global G-CSFR (Csf3r) ablation could improve the structure of Dmp1Cre.Socs3f/f cortical bone.

Dmp1Cre.Socs3f/f.Csf3r-/- mice were generated by crossing Dmp1Cre.Socs3f/fmice with Csf3r null mice on a C57BL/6 background. Csf3r null mice exhibited no change in bone structure compared to wild type. Surprisingly, Dmp1Cre.Socs3f/f bone structure was worsened by Csf3r deletion. At 12 and 26 weeks of age Dmp1Cre.Socs3f/f.Csf3r-/- bone had a higher proportion of low-density bone than Dmp1Cre.Socs3f/f (increased by 17%), and very little high-density bone (6%, compared to 18% in Dmp1Cre.Socs3f/f). Despite this, femoral strength was not different. Histology revealed that Dmp1Cre.Socs3f/f.Csf3r-/- cortical bone contained a “double-shell” of lamellar bone separated by highly porous woven bone, containing 5x more osteoclasts and a 3-fold increase in blood vessel area compared to the already high levels in Dmp1Cre.Socs3f/f. qPCR of flushed femora in Dmp1Cre.Socs3f/f.Csf3r-/- showed extremely high mRNA levels of osteoclast markers (Dc-stamp, Acp5), RANKL (Tnfsf11) and angiogenesis markers (Endomucin, Tie-1). A significant increase in phospho-STAT1 (~15%) and phospho-STAT3 (~20%) positive osteocytes, and elevations in STAT1 and STAT3 target gene mRNAs (Socs1 and Bcl3), indicated that G-CSFR deletion further increased STAT signaling in Dmp1Cre.Socs3f/f bone.

Since G-CSFR is not expressed in osteocytes, we suggest that G-CSFR deficiency promotes STAT signaling in osteocytes through its influence on the bone marrow microenvironment. In the absence of SOCS3 negative feedback, this increases cortical porosity by promoting local vascularization and bone resorption. This suggests cortical bone integrity requires communication between osteocytes and G-CSFR-mediated signals in the bone marrow microenvironment.