Pasteurella multocida Toxin Triggers RANKL-Independent Osteoclastogenesis

Abstract

Bone remodeling is a continuous process to retain the structural integrity and function of the skeleton. A tight coupling is maintained between osteoclast-mediated resorption of old or damaged bones and osteoblast-mediated formation of new bones for bone homeostasis. While osteoblasts differentiate from mesenchymal stem cells, osteoclasts are hematopoietic in origin and derived from myeloid precursor cells. Osteoclast differentiation is driven by two cytokines, cytokine receptor activator of NF-κB ligand (RANKL), and macrophage colony-stimulating factor. Imbalances in the activity of osteoblasts and osteoclasts result in the development of bone disorders. Bacterially caused porcine atrophic rhinitis is characterized by a loss of nasal ventral conche bones and a distortion of the snout. While Bordetella bronchiseptica strains cause mild and reversible symptoms, infection of pigs with toxigenic Pasteurella multocida strains causes a severe and irreversible decay. The responsible virulence factor Pasteurella multocida toxin (PMT) contains a deamidase activity in its catalytical domain that constitutively activates specific heterotrimeric G proteins to induce downstream signaling cascades. While osteoblasts are inhibited by the toxin, osteoclasts are activated, thus skewing bone remodeling toward excessive bone degradation. Still, the mechanism by which PMT interferes with bone homeostasis, and the reason for this unusual target tissue is not yet well understood. Here, we show that PMT has the potential to differentiate bone marrow-derived macrophages into functional osteoclasts. This toxin-mediated differentiation process is independent of RANKL, a cytokine believed to be indispensable for triggering osteoclastogenesis, as addition of osteoprotegerin to PMT-treated macrophages does not show any effect on PMT-induced osteoclast formation. Although RANKL is not a prerequisite, toxin-primed macrophages show enhanced responsiveness to low concentrations of RANKL, suggesting that the PMT-generated microenvironment offers conditions where low concentrations of RANKL lead to an increase in the number of osteoclasts resulting in increased resorption. PMT-mediated release of the osteoclastogenic cytokines such as IL-6 and TNF-α, but not IL-1, supports the differentiation process. Although the production of cytokines and the subsequent activation of signaling cascades are necessary for PMT-mediated differentiation into osteoclasts, they are not sufficient and PMT-induced activation of G protein signaling is essential for efficient osteoclastogenesis.

Plumbagin, a Biomolecule with (Anti)Osteoclastic Properties

“…Previously, we established bone marrow-derived macrophages (BMDMs) as a suitable model system for differentiation into osteoclasts [ 15 ]. Thus, we first determined whether plumbagin treatment would significantly change the phenotype of BMDMs ( Figure 1 C).…”

'Candidatus Ornithobacterium hominis': insights gained from draft genomes obtained from nasopharyngeal swabs

“…PMT is a toxin produced by some serovars of Pasteurella multocida that causes a range of host pathologies including nasal bone resorption [ 45 ], lower respiratory tract disease [ 46, 47 ] and dermonecrotic wound infections [ 48 ], and has also been shown experimentally to affect the heart [ 49 ], liver [ 50 ] and bladder [ 51 ]. It acts by deamidating the α subunits of several heterotrimeric G proteins, thereby activating mitogenic signalling pathways [ 52, 53 ].…”

Bacterial toxins in musculoskeletal infections

“…PMT‐mediated bone damage is independent of RANKL, a cytokine thought to be crucial for triggering osteoclastogenesis. When RANKL plus osteoprotegerin are added to PMT‐treated macrophages, no effect on PMT‐induced osteoclast formation is observed 67 . PMT mediates the release of osteoclastogenic cytokines such as IL‐6 and TNF‐α, but not IL‐1, which supports the differentiation process.…”

“…When RANKL plus osteoprotegerin are added to PMT-treated macrophages, no effect on PMT-induced osteoclast formation is observed. 67 PMT mediates the release of osteoclastogenic cytokines such as IL-6 and TNF-α, but not IL-1, which supports the differentiation process. An in vitro study using etanercept (an anti-TNF compound) strongly reduced PMT-induced osteoclast formation.…”