The Mycobacterium tuberculosis Virulence Factor Trehalose Dimycolate Imparts Desiccation Resistance to Model Mycobacterial Membranes

Abstract

Mycobacteria, including persistent pathogens like Mycobacterium tuberculosis, have an unusual membrane structure in which, outside the plasma membrane, a nonfluid hydrophobic fatty acid layer supports a fluid monolayer rich in glycolipids such as trehalose 6,6'-dimycolate (TDM; cord factor). Given the abilities of mycobacteria to survive desiccation and trehalose in solution to protect biomolecules and whole organisms during freezing, drying, and other stresses, we hypothesized that TDM alone may suffice to confer dehydration resistance to the membranes of which it is a constituent. We devised an experimental model that mimics the structure of mycobacterial envelopes in which an immobile hydrophobic layer supports a TDM-rich, two-dimensionally fluid leaflet. We have found that TDM monolayers, in stark contrast to phospholipid membranes, can be dehydrated and rehydrated without loss of integrity, as assessed by fluidity and protein binding. Strikingly, this protection from dehydration extends to TDM-phospholipid mixtures with as little as 25 mol % TDM. The dependence of the recovery of membrane mobility upon rehydration on TDM fraction shows a functional form indicative of spatial percolation, implying that the connectivity of TDM plays a crucial role in membrane preservation. Our observations are the first reported instance of dehydration resistance provided by a membrane glycolipid.

Synthetic Trehalose Glycolipids Confer Desiccation Resistance to Supported Lipid Monolayers

“…Supported monolayers consisting of 0.99-X:X:0.01, DOPC:trehalose glycolipid:Texas Red DHPE were deposited using the supported monolayer architecture used for earlier investigations of TDM (12), described also in Materials and Methods. (See Materials and Methods for lipid abbreviations.)…”

“…Motivated by this, and by the uniqueness of trehalose glycolipids in the mycobacteria, we recently examined whether TDM can confer stress resistance to lipid membranes of which it is a component. Using an experimental model of mycobacterial membrane structure that allows control of molecular composition (Figure 2), we found that TDM does confer dehydration resistance to membranes, and can do so even as a minority membrane component (12). To the best of our knowledge, this was the first reported discovery of a dehydration-resistant lipid.…”

“…Though the trehalose headgroup is the likely candidate for stabilizing desiccated membranes, one may suspect a role for the molecules' four large hydrophobic chains as well, mediating interactions in the hydrophobic membrane interior. The observed sharp onset of TDM-derived protection above a critical TDM concentration in the membrane suggests a mapping of protection onto a geometric percolation transition (12), but the appropriateness of this model requires further quantitative tests. Beyond its importance for microbiology, the desiccation resistance observed with TDM points out a previously unrealized physical capability of lipids.…”

“…The only persistent structural feature among these glycolipids is the trehalose headgroup. We incorporated these trehalose glycolipids into two-dimensionally fluid supported lipid monolayers that structurally resemble the outer envelope of mycobacteria, as in previous studies of TDM (12). We find that the synthetic trehalose glycolipids confer desiccation resistance to membranes of which they are a component.…”

Maradolipids: Diacyltrehalose Glycolipids Specific to Dauer Larva in Caenorhabditis elegans

“…The cord factor is a constituent of the outer lipid layer of the bacterial cell wall [12] . One of the presumable functions of the cord factor is to protect the bacteria from desiccation by stabilizing the physicochemical properties of the lipid layer and keeping it intact upon drying [13] . We have investigated the kinetics of the synthesis of maradolipids over the course of the development of dauer larva.…”