G-CSF Receptor Deletion Amplifies Cortical Bone Dysfunction in Mice With STAT3 Hyperactivation in Osteocytes

Abstract

Bone strength is determined by the structure and composition of its thickened outer shell (cortical bone), yet the mechanisms controlling cortical consolidation are poorly understood. Cortical bone maturation depends on SOCS3-mediated suppression of IL-6 cytokine-induced STAT3 phosphorylation in osteocytes, the cellular network embedded in bone matrix. Because SOCS3 also suppresses granulocyte-colony-stimulating factor receptor (G-CSFR) signaling, we here tested whether global G-CSFR (Csf3r) ablation altereed bone structure in male and female mice lacking SOCS3 in osteocytes, (Dmp1 Cre :Socs3 f/f mice). Dmp1 Cre :Socs3 f/f :Csf3r À/À mice were generated by crossing Dmp1 Cre :Socs3 f/f mice with Csf3r À/À mice. Although G-CSFR is not expressed in osteocytes, Csf3r deletion further delayed cortical consolidation in Dmp1 Cre :Socs3 f/f mice. Micro-CT images revealed extensive, highly porous low-density bone, with little true cortex in the diaphysis, even at 26 weeks of age; including more low-density bone and less high-density bone in Dmp1 Cre :Socs3 f/f :Csf3r À/À mice than controls. By histology, the area where cortical bone would normally be found contained immature compressed trabecular bone in Dmp1 Cre :Socs3 f/f :Csf3r À/À mice and greater than normal levels of intracortical osteoclasts, extensive new woven bone formation, and the presence of more intracortical blood vessels than the already high levels observed in Dmp1 Cre :Socs3 f/f controls. qRT-PCR of cortical bone from Dmp1 Cre :Socs3 f/f :Csf3r À/À mice also showed more than a doubling of mRNA levels for osteoclasts, osteoblasts, RANKL, and angiogenesis markers. The further delay in cortical bone maturation was associated with significantly more phospho-STAT1 and phospho-STAT3-positive osteocytes, and a threefold increase in STAT1 and STAT3 target gene mRNA levels, suggesting G-CSFR deletion further increases STAT signaling beyond that of Dmp1 Cre :Socs3 f/f bone. G-CSFR deficiency therefore promotes STAT1/3 signaling in osteocytes, and when SOCS3 negative feedback is absent, elevated local angiogenesis, bone resorption, and bone formation delays cortical bone consolidation. This points to a critical role of G-CSF in replacing condensed trabecular bone with lamellar bone during cortical bone formation.

Survivors of polymicrobial sepsis are refractory to G-CSF-induced emergency myelopoiesis and hematopoietic stem and progenitor cell mobilization

“…SOCS3 can also modulate responsiveness to other receptors and loss of Socs3 results in hyperinflammation ( Wong, 2006 ). Similarly, SOCS3 was found to dampen inflammation in macrophage cultures and in lipopolysaccharide (LPS)-induced inflammation ( Qin et al., 2012 ), and G-CSF signaling via STAT3 and induction of SOCS3 limit inflammation in bone remodeling ( Isojima et al., 2022 ). Therefore, we reason that failed G-CSF signaling and induction of SOCS3 expression in sepsis survivors may also contribute to the enhanced responses to LPS stimulation observed in sepsis survivors ( Denstaedt et al., 2021 ).…”

Oncostatin M: Dual Regulator of the Skeletal and Hematopoietic Systems

“…It must also be noted that neutrophils are also an important source of OSM [ 23 ••]. However, whether OSM produced by neutrophils contributes to bone maintenance is unclear since mice rendered neutropenic through deletion of the G-CSF receptor exhibit no bone phenotype, unless the STAT3 inhibitor SOCS3 is also removed [ 18 ]. Additional studies using mice with a floxed Osm gene crossed with transgenic mice expressing Cre recombinase controlled by a monocyte/macrophage- or neutrophil-specific promoter will be necessary to clarify a role of neutrophil-produced OSM in regulating skeletal bone homeostasis or repair.…”

“…G-CSF is a hematopoietic growth factor whose main function is to promote the development and maturation of granulocyte progenitors in the BM in steady-state conditions and in response to infections when hematopoiesis shifts from balanced lymphopoiesis, erythropoiesis, and myelopoiesis to mostly myelopoiesis. However, G-CSF has little role in the maintenance of the bone tissue in steady-state, with no bone phenotype being detected in G-CSF or G-CSFR null mice [ 18 ]. Beyond G-CSF, infections also induce the expression of a wide array of inflammatory cytokines by myeloid cells including IL-1α and IL-1β, interferons, tumor necrosis factor (TNF), and IL-6, the roles of which on bone have been recently reviewed [ 19 •].…”