Role of Nbr1 in bone remodeling

Abstract

The growth and differentiation of bone cells is controlled by various factors which can be modulated by heparan sulfates. Here, we investigated the effects of a low-molecular-weight “heparin-like”, an oversulfated exopolysaccharide (OS-EPS) produced by a mesophilic strain found in deep-sea hydrothermal vents, on bone cells in regard to their differentiation, proliferation, and function in vitro. We compared this effect with that of a non-oversulfated exopolysaccharide (EPS). Cell viability was assessed by trypan blue exclusion, and alive and dead cells were manually counted. Besides, by time-lapse experiments, the cell divisions were manually scored. Mesenchymal stem cells (MSCs) isolated and propagated previously from the rat bone marrow were tested for their capabilities to differentiate into osteoblasts in vitro. We showed that exogenous application of OS-EPS to cultures of primary MSCs during in vitro differentiation inhibited MSC mineralization. In addition, OS-EPS strongly reduces the proliferation of differentiating osteoblasts MSCs as we observed for the rat bone marrow cells. This proliferation inhibition was not due to an induction of apoptosis. These results suggest that OS-EPS reduces the differentiated cells proliferation, leading to strong inhibition of mineralized nodule formation. In contrast, OS-EPS enhanced no differenciated MSCs proliferation. EPS had no effect in any of the experimental conditions, underlying the importance of sulfatation in the proliferation and differentiation effects. Bone remodelling is regulated by osteogenic cells which act individually through cellular and molecular interaction. Osteoclasts are bone-resorptive cells that differentiate from hematopoietic precursors upon receptor activator of nuclear factor kappaB ligand (RANKL) activation. To investigate the OS-EPS effect in osteoclastic differentiation, we used several models: murine RAW264.7 cells and human CD14+ monocytes. In the two models used, the addition of OS-EPS inhibited RANKL-induced osteoclastogenesis. In contrast to EPS or other glycosaminoglycan, surface plasmon resonance experiments revealed that RANKL and OS-EPS are able to form a complex. In addition, a strong decrease of the osteoclastic precursor cells adhesion was observed. These results suggest that OS-EPS is a strongly inhibitor of osteoclastogenesis. It also underlines the importance of OS-EPS as a co-receptor for the signaling of RANKL. Conflict of interest: None declared.