Cortical bone maturation in mice requires SOCS3 suppression of gp130/STAT3 signalling in osteocytes.

Emma C Walker,Natalie A Sims,Tsuyoshi Isojima,Kim Truong,Jonathan H Gooi,Narelle E Mcgregor,Ingrid J Poulton,T John Martin

doi:10.7554/elife.56666

Abstract

Bone strength is determined by its dense cortical shell, generated by unknown mechanisms. Here we use the Dmp1Cre:Socs3f/f mouse, with delayed cortical bone consolidation, to characterise cortical maturation and identify control signals. We show that cortical maturation requires a reduction in cortical porosity, and a transition from low to high density bone, which continues even after cortical shape is established. Both processes were delayed in Dmp1Cre:Socs3f/f mice. SOCS3 (suppressor of cytokine signalling 3) inhibits signalling by leptin, G-CSF, and IL-6 family cytokines (gp130). In Dmp1Cre:Socs3f/f bone, STAT3 phosphorylation was prolonged in response to gp130-signalling cytokines, but not G-CSF or leptin. Deletion of gp130 in Dmp1Cre:Socs3f/f mice suppressed STAT3 phosphorylation in osteocytes and osteoclastic resorption within cortical bone, leading to rescue of the corticalisation defect, and restoration of compromised bone strength. We conclude that cortical bone development includes both pore closure and accumulation of high density bone, and that these processes require suppression of gp130-STAT3 signalling in osteocytes.

Highlights

The characteristic external structure of bone forms during development and continues to consolidate as bones grow in length
In our earlier study we realised the limitations of morphological analyses of cortical bone, and here we develop unbiased micro-computed tomography methods to track the changes in tissue mineral content during cortical bone development; these methods are applicable to a wide range of applications in human and animal biology
We previously reported that the Dmp1Cre:Socs3f/f phenotype is not rescued by interleukin 6 (IL-6) deletion (Cho et al, 2017), and since all other gp130-binding cytokines tested induced prolonged phosphorylation, we determined whether gp130 deletion in osteocytes might resolve the phenotype

Summary

Introduction

The characteristic external structure of bone (cortical bone) forms during development and continues to consolidate as bones grow in length. Bone growth in length occurs at the growth plate, and measurement of a cortical region of interest in the developing cortex in animals and humans during bone lengthening is difficult because a fixed position on the bone cannot be reliably identified due to the non-linear pattern of growth. Differences in positioning of a region of interest by even 1 to 2 mm may alter the analysis of any structural changes in bone (Seeman and Ghasem-Zadeh, 2016) Adding to this challenge, multiple scans of children with developing bones would be required to measure the process of corticalisation in humans, which raises safety concerns about radiation exposure effects on growing bone (Williams and Davies, 2006; Brenner and Hall, 2007). We show that IL-6 family cytokines have amplified and extended STAT3 phosphorylation responses in bone in the absence of SOCS3 and that deletion of gp130 in osteocytes rescues the features of delayed corticalisation in Dmp1Cre:Socs3f/f mice

Results

A Figure 6

Discussion

B NORMAL cytokines gp130

Materials and methods