The role of serum amyloid A3 protein in bone loss during Mycobacterium avium infection

• Autor(a) principal: Fonseca, Óscar Manuel Ferreira
• Data de Publicação: 2022
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10773/38135
• topic: Mycobacterial infection; Bone turnover; TNFα; IFNγ; SAA3; Osteoclasts; Osteoblasts

Mycobacterial infections can lead to the loss of bone mass without direct colonization of the bone. Osteopenia arises from a dysregulation of the bone turnover, due to enhanced bone degradation and/or impaired new bone formation. The indirect bone loss during infection is due to the production of immune mediators by the host upon pathogen invasion. Previous studies using a murine model of M. avium disseminated infection indicated that bone mass was decreased in an IFNγ and TNFα-dependent manner, and that Saa3 was the gene whose expression in the bone increases the most with infection. Therefore, this thesis aims to further understand the molecular mechanisms of bone loss associated to chronic mycobacterial infection and dissect the role of the serum amyloid A3 protein (SAA3) in bone degradation and bone formation. Here, we show that M. avium-infected macrophages produce soluble pro-osteoclastogenic factors (SPOFs) that increase bone resorption by osteoclasts. We identified TNFα as a SPOF produced by infected macrophages and whose production is potentiated by IFNγ. Moreover, we assessed that SAA3 is another SPOF produced by M. avium-infected macrophages in vitro and TNFα enhances Saa3 expression in infected macrophages. Likewise, we determined that in vitro differentiated osteoclasts produce SAA3 upon TNFα stimulation. Furthermore, the SAA protein stimulates osteoclastogenesis, while not enhancing their resorptive activity. However, TNFα addition increases the proportion of trenches. We further found that the conditioned media from infected macrophages had no effect on the mineralization capacity of osteoblasts and that SAA diminished it, indicating that SAA impairs osteoblastic differentiation and activity. Overall, our results highlight a possible molecular mechanism for bone loss during chronic infection, in which IFNγ, TNFα, and SAA3 are the key molecules. M. avium infection leads to the production of IFNγ, which will instruct macrophages to produce SPOFs like TNFα and SAA3. Chronic infection will increase bone destruction, as SAA and TNFα enhance the formation and activity of osteoclasts, respectively. Simultaneously, SAA protein impairs osteoblastic differentiation and activity. Our results emphasize the role of SAA proteins, particularly the SAA3 protein, as key proteins in the bone turnover process during mycobacterial infection.