The immunomodulatory lipoglycans, lipoarabinomannan and lipomannan, are exposed at the mycobacterial cell surface

Abstract

By labeling surface carbohydrates we found that a pool of lipoglycans, cell wall associated, is exposed at the cell surface of mycobacteria and thus, most probably, inserted in the outer leaflet of the outer membrane. In contrast, plasma membrane anchored lipoglycans are not accessible to surface labeling. This result supports the role of lipoglycans as key immunomodulatory molecules but raises the question of their transport from the plasma membrane, where they are synthesized, to the outermost layers of the envelope, where they can act as modulins. The data are discussed in term of consequences for cell envelope organization. KeywordsMycobacterium; lipoglycan; localization; cell envelope; transporter The lipoglycans, lipoarabinomannan (LAM) and lipomannan (LM), are major immunomodulatory molecules of the mycobacterial cell envelope 1-4 . Mannose-capped LAM (ManLAM) is a potential ligand for the entry of Mycobacterium tuberculosis into phagocytic cells via the host C-lectins 5 . In addition, purified ManLAM reproduces several properties of M. tuberculosis that may contribute to the inhibition of the host defense response and define ManLAM as a major virulence factor of the pathogen 5 . In contrast, phosphoinositol-capped LAM (PILAM) and LM stimulate innate immunity via signaling through Toll-like receptor 2 (TLR2) 5 . Lipoglycans are delivered from infected macrophages, via exosomes or apoptotic vesicles, to non-infected bystander dendritic cells 6-8 and thus can modulate the functions of the latter, via binding C-lectins 9 or TLR-2, even though they are not necessarily receptor ligands on the whole bacterium 10 . Nevertheless, their role as mycobacterial cell surface adhesins or as soluble molecules released by phagocytic cells implies that they are exposed to Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. NIH Public AccessAuthor Manuscript Tuberculosis (Edinb) the cell surface or, at least, located in the outermost part of the cell envelope. However, their precise localization remains unclear. They are not covalently attached to the cell envelope, but they have never been found in culture supernatants or in the surface-exposed material obtained by gentle mechanical treatment of cells with glass beads and/or detergent treatment 11,12 , suggesting that they are instead imbedded in the cell wall.In the present study, the exposition of lipoglycans to the cell surface of mycobacteria was investigated by cell labeling with biotin. The validity of this approach relies on the assumption that labeling is indeed restricted to surface components. BCG cells were grown as ...

Mycobacterial outer membrane is a lipid bilayer and the inner membrane is unusually rich in diacyl phosphatidylinositol dimannosides

“…As described above, PI and its mannosylated forms containing up to six mannose residues were confined to the IM, although higher PIMs were seen in the RMS extract. Location of the hyper-mannosylated forms of PIMs, namely LMs and lipoarabinomannans (LAMs), has always been a subject of debate (18)(19)(20). Their location in OM was assumed on the basis of their being absent from an IM or spheroplast preparation (18,20).…”

“…Location of the hyper-mannosylated forms of PIMs, namely LMs and lipoarabinomannans (LAMs), has always been a subject of debate (18)(19)(20). Their location in OM was assumed on the basis of their being absent from an IM or spheroplast preparation (18,20). The poor solubility of these lipopolysaccharides in either aqueous or conventional organic solvents has also been an additional obstacle in their extraction.…”

Molecular Basis of Phosphatidyl-myo-inositol Mannoside Biosynthesis and Regulation in Mycobacteria

“…A reanalysis of PIMs from M. smegmatis in their deacylated form later revealed a structure based on that previously defined by Ballou et al but containing six Manp residues (PIM 6 ) (16). The complete chemical structures of the acylated native forms of PIM 2 and PIM 6 were later reinvestigated in M. bovis BCG and unequivocally established (Fig. 1A) (10,17).…”

“…Over the past 40 years, the structure of the complete family of PI mannosides (PIM 1 -PIM 6 ) in various Mycobacterium spp. and related Actinomycetes has been defined, first as deacylated glycerophosphoryl-myo-inositol mannosides and later as the fully acylated native molecules (4).…”

“…PIMs and metabolically related lipoglycans comprising lipomannan (LM) and lipoarabinomannan (LAM) are noncovalently anchored through their PI moiety to the inner and outer membranes of the cell envelope (5,6) and play various essential although poorly defined roles in mycobacterial physiology. They are also thought to be important virulence factors during the infection cycle of M. tuberculosis.…”