Metabolic switching and cell wall remodelling of Mycobacterium tuberculosis during bone tuberculosis

Abstract

Bone tuberculosis is widely characterized by irreversible bone destruction caused by Mycobacterium tuberculosis. Mycobacterium has the ability to adapt to various environmental stresses by altering its transcriptome in order to establish infection in the host. Thus, it is of critical importance to understand the transcriptional profile of M. tuberculosis during infection in the bone environment compared to axenic cultures of exponentially growing M.tb. In the current study, we characterized the in vivo transcriptome of M. tuberculosis within abscesses or necrotic specimens obtained from patients with bone TB using whole genome microarrays in order to gain insight into the M. tuberculosis adaptive response within this host microenvironment. A total of 914 mycobacterial genes were found to be significantly over-expressed and 1688 were repressed (fold change>2; p-value [≤]0.05) in human bone TB specimens. Overall, the mycobacteria displayed a hypo-metabolic state with significant (p[≤]0.05) downregulation of major pathways involved in translational machinery, cellular and protein metabolism and response to hypoxia. However, significant enrichment (p [≤]0.05) of amino-sugar metabolic processes, membrane glycolipid biosynthesis, amino acid biosynthesis (serine, glycine, arginine and cysteine) and accumulation of mycolyl-arabinogalactan-peptidoglycan complex suggests possible mycobacterial survival strategies within the bone lesions by strengthening its cell wall and cellular integrity. Data were also screened for M.tb virulence proteins using Virulent Pred and VICM Pred tools, which revealed five genes (Rv1046c, Rv1230c, DppD, PE_PGRS26 and PE_PGRS43) with a possible role in the pathogenesis of bone TB. Next, an osteoblast cell line model for bone TB was developed allowing for significant intracellular multiplication of M.tb. Interestingly, three virulence genes (Rv1046c, DppD and PE_PGRS26) identified from human bone TB microarray data were also found to be overexpressed by intracellular M. tuberculosis in osteoblast cell lines. Overall, these data demonstrate that M. tuberculosis alters its transcriptome as an adaptive strategy to survive in the host and establish infection in bone. Additionally, the in vitro osteoblast model we describe may facilitate our understanding of the pathogenesis of bone TB. Author SummaryMusculoskeletal tuberculosis is the third most common manifestation of extra-pulmonary tuberculosis and massive bone destruction along with vertebral discs are one of the hallmarks of this disease. Mycobacterium tuberculosis, the causative agent, has the tremendous potential to adapt itself to different host environments due to its ability to alter the expression of genes/proteins belonging to different pathways. This study shows that the mycobacterial infection in bone is driven by the increased expression of genes belonging to cell wall remodelling and DNA damage repair pathways important for its survival. Further data analysis showed that some of these genes are coding for proteins possessing virulence potential that may be essential for survival of M. tuberculosis under such hostile environment of bone. We also developed an in vitro model of bone tuberculosis using an osteoblast cell line and validated the expression of these virulence factors. Identification of such virulence factors in the bone environment by M. tuberculosis may aid to identify new therapeutic targets for bone TB. Further, development of cell line model for bone TB is important to understand some unknown facets of this disease.