Manipulation of the endocytic pathway and phagocyte functions by Mycobacterium tuberculosis lipoarabinomannan

Abstract

Lipoarabinomannan is a major immunomodulatory lipoglycan found in the cell envelope of Mycobacterium tuberculosis and related human pathogens. It reproduces several salient properties of M. tuberculosis in phagocytic cells, including inhibition of pro-inflammatory cytokine production, inhibition of phagolysosome biogenesis, and inhibition of apoptosis as well as autophagy. In this review, we present our current knowledge on lipoarabinomannan structure and ability to manipulate the endocytic pathway as well as phagocyte functions. A special focus is put on the molecular mechanisms employed and the signaling pathways hijacked. Available information is discussed in the context of M. tuberculosis pathogenesis.

Keywords: Mycobacterium; apoptosis; autophagy; cytokine; lipoarabinomannan; phagosome.

Figure 1

Structural model of and cell signaling pathways triggered by ManLAM and its biosynthetic precursors LM and PIMs. (A) ManLAM is a 17 kDa heterogenous macromolecule exhibiting a tripartite structure: (i) a MPI anchor, which can be mono- to tetra-acylated, (ii) a polysaccharide backbone composed of D-Mannan and D-Arabinan, and (iii) mannose caps, which are mono-, (α1→2)-di- and (α1→2)-tri-mannoside units. PIMs and LM are biosynthetic precursors of LAM. Their structure is based on the MPI anchor, glycosylated by one to six mannose units (PIMs) or the full mannan domain (LM). To our present knowledge, the MPI anchor and the Mannose caps are the main structural determinants of ManLAM biological properties. (B) LM, and to a much lesser extent PIMs, PILAM and ManLAM, induce pro-inflammatory cytokines production in DCs and macrophages via the recognition of tri- or tetra-acylated MPI anchor by TLR2/TLR1 heterodimer. ManLAM elicits cytokines in bone marrow-derived DCs via mannose caps binding to Dectin-2. But it also inhibits the production of pro-inflammatory cytokines IL-12, TNF-α, and IL-6, and induces IL-10 by LPS-stimulated human DCs through DC-SIGN ligation. The signaling pathway involves activation of Raf-1, which results in the phosphorylation of the p65 subunit of NF-κB at Ser276, leading to the acetylation of p65 by two histone acetyltransferases. Translocation of NF-κB in response to TLR activation, and initially dedicated to the transcription of the pro-inflammatory cytokine-coding genes, is reoriented on anti-inflammatory promoter targets, resulting in the decrease of these cytokines to the benefit of IL-10. ManLAM also inhibits IL-12 and TNF-α in macrophages independently of IL-10 production, by directly acting on the TLR4 signaling cascade through induced expression of IRAK-M, which can compete with IRAK1 for binding to TRAF6 and thus inhibit NF-κB activation. ManLAM also promotes tyrosine dephosphorylation of multiple proteins including MAPK, an effect that might be explained by an increased activity of tyrosine phosphatase SHP-1. MR is likely to mediate ManLAM immunosuppressive activities in macrophages, although it has no signaling motif in its cytoplasmic domain, raising the intriguing question as to whether it associates with adapter molecules to transduce signals. The ability of ManLAM to bind MR might in part determine its other inhibitory properties as detailed in Figure 2.

Figure 2

Schematic representation of ManLAM and PIM action on phagosome maturation and autophagy. Right part: After phagocytosis, mycobacteria reside in a vacuole, called phagosome. Phagosome maturation consists in a series of fusion events with exocytic and endocytic pathways. One key step is the delivery of Cathepsin D and H⁺-ATPase subunit Vo from Trans-Golgi-Network (TGN) to the phagosome. This step is mediated by tethering protein EEA1 which is recruited to the phagosome by small GTPase Rab5 and phosphatidylinositol 3-phosphate (PI3P). ManLAM blocks phagosome maturation through inhibition of Ca²⁺/CaM/CaMKII signaling pathway involved in PI3P production by type III PI3Kinase hVPS34. ManLAM can also block phagosome maturation by engaging mannose receptor and disrupting membrane microdomains, rafts, however, the link with Ca²⁺/CaM signaling has not been studied. PIMs, ManLAM precursor, impair phagosome maturation by stimulating fusion between phagosome and early endosomes. Mycobacterium tuberculosis recruits Rab14 to phagosome to promote early endosome fusion thus impairs phagosome maturation. It remains to be established whether PIMs promotes early endosome fusion through Rab14 recruitment. Left part: Mammalian target of rapamycin (mTOR) kinase, activated by Ser/Thr kinase Akt and inhibited by AMP-activated Protein Kinase (AMPK), is a master repressor of autophagy. Beclin-1, an autophagy-related protein in complex with hVPS34, is essential for autophagy. Beclin-1/hVPS34 complex is activated by AMPK and repressed by Bcl-2. ULK1, another important autophagy-related protein, is activated by AMPK and inhibited by mTOR. Ca²⁺ influx has been shown to activate AMPK, hVPS34, and represses Bcl-2 expression. IFNγ induces autophagy. Based on known effects of ManLAM on Ca²⁺ influx, Bcl-2, Akt and IFNγ signaling, we postulate that ManLAM might inhibit autophagy by targeting Beclin-1/hVPS34 complex, Akt/mTOR or IFNγ pathways. The relationship between effects of ManLAM on these different signaling pathways and autophagy awaits investigation. Arrows and characters are represented in gray to indicate that the molecular mechanisms of LAM action on autophagy are hypothetical.