Epigenetic modulation of Foam cell generation during Mycobacterium tuberculosis/ Cryptococcus neoformans infection

Abstract

Foamy macrophages or Foam cells are a critical cellular component of the granuloma formed during pulmonary infection. These lipid rich cells generally contain neutral lipids, Cholesteryl Esters (CE) and/or Triglycerides (TAGs) which not only serve as the carbon source for the pathogen to thrive in the hostile environment of host, but also act as a substrate for various immunomodulatory enzymes such as COX-2 etc. Lipid accretion is a result of various cellular processes involved in lipid uptake and synthesis that are fine-tuned by various transcription factors and chromatin modifiers. The thesis submitted is focused around the epigenetics of foam cell formation during pulmonary infection stemming from Mycobacterium tuberculosis and Cryptococcus neoformans. Mycobacterium tuberculosis, which is the causative agent of tuberculosis induces the formation of foamy macrophages in the host to permit its own growth. In the present context, we have highlighted the role of WNT-responsive epigenetic modifiers, G9a (H3K9 methyltransferase) and SIRT6 (H3K9 deacetylase) in fine-tuning the expression level of genes involved in cholesterol biosynthesis and efflux. Moreover, augmented levels of cholesterol was observed to fuel anti-oxidative response as depletion of cholesterol or G9a/SIRT6 elevated the oxidative response and eventually reduced bacterial survival. Similarly, LncRNAs have recently been showed to play a cardinal role in regulating the gene expression via modulating the activity of various transcription factors and chromatin remodellers. In the present study, we have attempted to underpin the role of Malat1 in governing Mtb pathogenesis. High throughput analysis including RNA sequencing and ATAC sequencing revealed Malat1-dependent global change in transcriptome and chromatin accessibility, respectively. Furthermore, In vivo study utilising both WT and Malat1 KO mice study ascertained the pivotal role of Malat1 in regulating Mtb burden as Malat1 KO mice showed reduced bacillary burden and improved lung pathology. Moreover, macrophages devoid of Malat1 showed reduced lipid content and enhanced necroptosis and elevated extracellular burden. In addition to above, thesis submitted also elucidates the molecular players involved in perturbing lipid content during C. neoformans infection. Enhanced lipid content has been demonstrated to support intracellular growth of C. neoformans. In this respect, Pyk2- cRaf axis-driven WNT signalling was found to be critical in regulating the levels of Lysine Specific Demethylase-1 (LSD1). LSD1 owing to its demethylase activity removes H3K9me2 repressive marks over the promoters of the genes involved in lipid homeostasis, leading to their enhanced transcription. Moreover, LSD1 sustains elevated lipid levels by inhibiting host lipophagy and hence preventing lipid turnover.