Noncanonical G-Protein-Dependent Modulation of Osteoclast Differentiation and Bone Resorption Mediated by Pasteurella multocida Toxin

Pasteurella multocida toxin (PMT) induces atrophic rhinitis in animals, which is characterized by a degradation of nasal turbinate bones, indicating an effect of the toxin on bone cells such as osteoblasts and osteoclasts. The underlying molecular mechanism of PMT was defined as a persistent activation of heterotrimeric G proteins by deamidation of a specific glutamine residue. Here, we show that PMT acts directly on osteoclast precursor cells such as bone marrow-derived CD14(+) monocytes and RAW246.7 cells to induce osteoclastogenesis as measured by expression of osteoclast-specific markers such as tartrate-resistant acid phosphatase and bone resorption activity. Treatment performed solely with PMT stimulates osteoclast differentiation, showing a receptor activator of nuclear factor-κB ligand (RANKL)-independent action of the toxin. The underlying signal transduction pathway was defined as activation of the heterotrimeric G proteins Gα(q/11) leading to the transactivation of Ras and the mitogen-activated protein kinase pathway. Gα(q/11) transactivates Ras via its effector phospholipase Cβ-protein kinase C (PKC) involving proline-rich tyrosine kinase 2 (Pyk2). PMT-induced activation of the mitogen-activated protein kinase pathway results in stimulation of the osteoclastogenic transcription factors AP-1, NF-κB, and NFATc1. In addition, Ca(2+)-dependent calcineurin activation of NFAT is crucial for PMT-induced osteoclastogenesis. The data not only elucidate a rationale for PMT-dependent bone loss during atrophic rhinitis but also highlight a noncanonical, G-protein-dependent pathway toward bone resorption that is distinct from the RANKL-RANK pathway but mimics it. We define heterotrimeric G proteins as as-yet-underestimated entities/players in the maturation of osteoclasts which might be of pharmacological relevance.