Aránzazu Mediero, Bhama Ramkhelawon, Miguel Perez-Aso, Kathryn J. Moore and Bruce N. Cronstein
Bone metabolism is a vital process that involves resoption by osteoclasts and formation by osteoblasts which is closely regulated by immune cells. The neuronal guidance protein Netrin-1 regulates immune cell migration and inflammatory reactions, but its role in bone metabolism is unknown. During osteoclast differentiation osteoclast precursors increase expression of Netrin-1 and its receptor Unc5b. Netrin-1 binds, in an autocrine and paracrine manner, to Unc5b to promote osteoclast differentiation in vitro and absence of Netrin-1 or antibody-mediated blockade of Netrin-1 or Unc5b prevents osteoclast differentiation of both murine and human precursors. We confirmed the functional relationship of Netrin-1 in osteoclast differentiation in vivo using Netrin-1-deficient (Ntn1-/-) or wildtype (WT) bone marrow transplanted mice. Notably, Ntn1-/- chimeras have markedly diminished osteoclasts, as well as increased cortical and trabecular bone density and volume compared to WT mice. Mechanistic studies revealed that Netrin-1 regulates osteoclast differentiation by altering cytoskeletal assembly. Netrin-1 increases LARG (regulator of Rho-GEF subfamily) and RGMa (repulsive guidance molecule) association with Unc5b, which increases expression and activation of cytoskeletal regulators RhoA and focal adhesion kinase (FAK). Netrin-1 and its receptor unc5b likely play a role in fusion of osteoclast precursors since Netrin-1 and DC-STAMP are tightly linked. These results identify Netrin-1 as a key regulator of osteoclast differentiation that may be a new target for bone therapies.