Nontuberculous mycobacteria induce mitochondrial dysfunction in macrophages

Abstract

Aims and Objectives: The incidence of nontuberculous mycobacterial (NTM) disease continues to rise, especially in developed countries. While NTM can affect almost any organ system, pulmonary infection is more prevalent than disseminated disease and can occur in patients with COPD, non-cystic fibrosis (CF) bronchiectasis, and CF. Mitochondria are the powerhouse of the cell, but it is unknown what role they play in the host response to NTM infection. Using electron microscopy, we have previously shown that mitochondrial morphology is altered upon infection with Mycobacterium avium intracellulare (MAI). Thus, we hypothesize that mitochondrial damage leading to changes in mitochondrial function and dynamics impair host response to infection with MAI and Mycobacterium abscessus (MAB). Methods: We infected MH-S cells (murine alveolar macrophages) and bone marrow-derived macrophages (BMDM) with clinical strains of MAI or MAB (multiplicity of infection [MOI] of 25) for 6 and 24 hours. We determined the expression of mitochondrial DNA fragments mt79 and mt230 that are associated with mitochondrial DNA damage using RT-qPCR; we calculated the fold change of these fragments relative to GAPDH and determined a ratio of these fragments. We also determined the expression of key mitochondrial quality control genes, including peroxisome proliferator activated receptor gamma coactivator (PGC)-1α, transcription factor A mitochondrial (TFAM), and mitofusin (MFN)-2, using RT-qPCR. Results: We found that in MH-S cells, the mt79:mt230 ratio was increased at 6 and 24 hours for both MAI and MAB relative to controls infected with PBS. Furthermore, expression of genes involved in mitochondrial biogenesis, PGC-1 α and TFAM, were both decreased at 6 and 24 hours. MFN2 was also decreased at both 6 and 24 hours for MAI and MAB. These data were consistent in BMDM, with a more significant decrease in these genes noted at 24 hours. Conclusions: These data suggest that there is an increase in mitochondrial damage as indicated by the mt79:230 ratio when macrophages are infected with NTM. The infected macrophages exhibit decreased mitochondrial biogenesis and also have impaired mitochondrial quality control, including mitochondrial fusion and fission. The functional impact of these impaired mitochondria on host response to NTM infection needs to be further explored as there may be a role for pharmacologic interventions, like PGC-1 α activators, that can augment bacterial killing in patients infected with NTM.