Selective deletion of the receptor for CSF1, c-fms, in osteoclasts results in a high bone mass phenotype, smaller osteoclasts in vivo and an impaired response to an anabolic PTH regimen.

Meiling Zhu,Ben-Hua Sun,Erin Nevius,Jared Kaplan,João Pereira,Karl Insogna,Brenda Smith

doi:10.1371/journal.pone.0247199

Abstract

The receptor for Colony Stimulating Factor 1 (CSF1), c-fms, is highly expressed on mature osteoclasts suggesting a role for this cytokine in regulating the function of these cells. Consistent with this idea, in vitro studies have documented a variety of effects of CSF1 in mature osteoclasts. To better define the role of CSF1 in these cells, we conditionally deleted c-fms in osteoclasts (c-fms-OC-/-) by crossing c-fmsflox/flox mice with mice expressing Cre under the control of the cathepsin K promoter. The c-fms-OC-/- mice were of normal weight and had normal tooth eruption. However, when quantified by DXA, bone mass was significantly higher in the spine and femur of female knock out mice and in the femurs of male knock out mice. MicroCT analyses of femurs showed that female c-fms-OC-/- mice had significantly increased trabecular bone mass with a similar trend in males and both sexes demonstrated significantly increased trabecular number and reduced trabecular spacing. Histomorphometric analysis of the femoral trabecular bone compartment demonstrated a trend towards increased numbers of osteoclasts, +26% in Noc/BPm and +22% in OcS/BS in the k/o animals but this change was not significant. However, when the cellular volume of osteoclasts was quantified, the c-fms-OC-/- cells were found to be significantly smaller than controls. Mature osteoclasts show a marked spreading response when exposed to CSF1 in a non-gradient fashion. However, osteoclasts freshly isolated from c-fms-OC-/- mice had a near complete abrogation of this response. C-fms-OC-/- mice treated with (1–34)hPTH 80 ng/kg/d in single daily subcutaneous doses for 29 days showed an attenuated anabolic response in trabecular bone compared to wild-type animals. Taken together, these data indicate an important non-redundant role for c-fms in regulating mature osteoclast function in vivo.

Highlights

Colony Stimulating Factor-1 is one of two cytokines required for normal osteoclastogenesis
One of the earliest events in osteoclastogenesis is an upregulation in expression of the receptor for Colony Stimulating Factor 1 (CSF1), c-fms, which is subsequently followed by upregulation of RANK [2]
The genetic absence of c-fms in these cells resulted in an increased bone mass as measured by dual-energy absorptiometry

Summary

Introduction

Colony Stimulating Factor-1 is one of two cytokines required for normal osteoclastogenesis. Osteoclastogenesis fails, leading in mice to an osteopetrotic phenotype in which no osteoclasts are present in bone [1]. One of the earliest events in osteoclastogenesis is an upregulation in expression of the receptor for CSF1, c-fms, which is subsequently followed by upregulation of RANK [2]. It is likely that CSF1 engagement of c-fms plays a role in the initial expression of RANK on osteoclast precursors. CSF1 induces expression of c-fos [3], which is absolutely required for normal osteoclastogenesis since in its genetic absence osteoclasts fail to form [4]. Several pro-survival pathways are regulated by CSF1 in osteoclasts, including the ubiquitination of the proapoptotic factor Bim [5] and stimulation of the electroneutral sodium bicarbonate exchanger [6]

Methods

Results

Conclusion