Lipid droplets and the transcriptome of Mycobacterium tuberculosis from direct sputa: a literature review

Abstract

Mycobacterium tuberculosis (Mtb), the main etiology of tuberculosis (TB), is predominantly an intracellular pathogen that has caused infection, disease and death in humans for centuries. Lipid droplets (LDs) are dynamic intracellular organelles that are found across the evolutionary tree of life. This review is an evaluation of the current state of knowledge regarding Mtb-LD formation and associated Mtb transcriptome directly from sputa.Based on the LD content, Mtb in sputum may be classified into three groups: LD positive, LD negative and LD borderline. However, the clinical and evolutionary importance of each state is not well elaborated. Mounting evidence supports the view that the presence of LD positive Mtb bacilli in sputum is a biomarker of slow growth, low energy state, towards lipid degradation, and drug tolerance. In Mtb, LD may serve as a source of chemical energy, scavenger of toxic compounds, prevent destruction of Mtb through autophagy, delay trafficking of lysosomes towards the phagosome, and contribute to Mtb persistence. It is suggest that LD is a key player in the induction of a spectrum of phenotypic and metabolic states of Mtb in the macrophage, granuloma and extracellular sputum microenvironment. Tuberculosis patients with high proportion of LD positive Mtb in pretreatment sputum was associated with higher rate of poor treatment outcome, indicating that LD may have a clinical application in predicting treatment outcome.The propensity for LD formation among Mtb lineages is largely unknown. The role of LD on Mtb transmission and disease phenotype (pulmonary TB vs extra-pulmonary TB) is not well understood. Thus, further studies are needed to understand the relationships between LD positivity and Mtb lineage, Mtb transmission and clinical types.

Keywords: Host-pathogen interaction; Lineage; Lipid droplet; Mycobacterium; Sputum; Transcriptome; Transmission; Treatment outcome; Tuberculosis.

Fig. 1

A:Necrotic granuloma, B: Foamy macrophages that contain LD-positive Mtb in granuloma tissue, phagocytosed Mtb C: Lipid droplet. A: The necrotic granuloma is a cryptic infectious immunopathological architecture and compacted collection phagocytic cells. It is the hallmark of tuberculosis [24]. Evidence showed that, except macrophages which serve as a feeder for new Mtb infection, innate immunity has only a little role in the initiation of granuloma formation and bacterial virulence factors such as trehalosdimycolate and ESX-1 are the driving factors for priming granuloma formation [25]. Once it is primed, dendritic cells migrate to regional lymph nodes, activate Th cells making the granuloma mature through layering of cells (macrophage, foamy macrophage, epithelioid, T cells and fibroblasts) [25]. The macrophage is the predominant phagocytic cell which occurs in differentiated forms. These are epithelioid, multinucleated giant cells, foamy macrophages and ruffled membrane macrophages [24]. Mtb might be found in the granuloma microenvironment due to rupture of phagosome and foamy macrophages. When the granuloma ruptures Mtb will be seeded to the environment through coughing, sneezing and talking. The metabolism and the level of stress in each microenvironment is different, driving Mtb into at least three distinct phenotypic and metabolic states; actively replicating (green), Lipid droplets (LD) loaded persister phenotype (red) and borderline between the two states (yellow). B: A macrophage that ingests Mtb through phagocytosis may harbor multiple Mtb phenotypes and may become a warehouse of lipid and serving as an energy reserve. These lipid-loaded macrophages are called foamy macrophages). C: Lipid droplets are composed of a hydrophobic core of neutral lipids (triacylglycerol, TAG and cholesterol ester, CE) surrounded by a phospholipid monolayer (phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylinositol (PI) and lyso-phospholipids) decorated with different proteins. LD is an efficient energy storage organelle, as the most compacted and efficient means to store excess lipid in cells. Figures are created with BioRender.com

Fig. 2

Survival strategies of M. tuberculosis inside the phagosome environment. This figure illustrates the mechanisms proposed to allow Mtb to survive inside the phagosome or Mtb escaping mechanisms from host defense. (1) Mtb survives inside the hostile phagosome by expressing Mycobacterial acid resistance Protein (MarP), a protein that buffers the acidic milieu. (2) Mtb survives inside the phagosome and evades the host immune response by residing apposition to the host lipid droplet. (3) Mtb avoids phagosome maturation and phagolysosome fusion by tagging early endosome markers (Rab5, Rab11, coronin1/TACO) and avoiding attachment and activation of several others (Rab7, CD63, lysosomal hydrolase, cathepsin D), which inhibits the proton–ATPase activity. Mtb accomplishes this by expressing various virulent factor lipoproteins (Man LAM, secreted phosphatase, lipid phosphatidylinositol 3 phosphate, phosphatase ptpA, TDM). (4) Mtb exits the phagosome and replicates inside the cytoplasm by rupturing the phagosome expressing ESX-1, DIM/PDIM, and phospho lipase A2 [46]. This phagosomal escape is advantageous to the pathogen for acquiring essential amino acids (arginine, methionine, or leucine), replication and dissemination [47]. Mtb: M. tuberculosis; Man LAM: Mannosylated lipoarabinomannan; TDM: Trehalose-6,6′-dimycolate; ESX-1: Early secretary antigenic target 6 (ESAT6) secretion system like protein; TACO: tryptophan aspartate containing coat protein, also named P57, Coronin1; DIM/PDIM: phthiocerol dimycocerosates. Figure is created with BioRender.com