Stimulation of a shorter duration in the state of anergy by an invariant natural killer T cell agonist enhances its efficiency of protection from type 1 diabetes

We have reported previously that treatment of non-obese diabetic (NOD) mice with the invariant natural killer T (iNK T) cell agonist α-galactosylceramide C26:0 (α-GalCer) or its T helper type 2 (Th2)-biasing derivative α-GalCer C20:2 (C20:2) protects against type 1 diabetes (T1D), with C20:2 yielding greater protection. After an initial response to α-GalCer, iNK T cells become anergic upon restimulation. While such anergic iNK T cells can induce tolerogenic dendritic cells (DCs) that mediate protection from T1D, chronic administration of α-GalCer also results in long-lasting anergy accompanied by significantly reduced iNK T cell frequencies, which raises concerns about its long-term therapeutic use. In this study, our objective was to understand more clearly the roles of anergy and induction of tolerogenic DCs in iNK T cell-mediated protection from T1D and to circumvent potential complications associated with α-GalCer. We demonstrate that NOD iNK T cells activated during multi-dose (MD) treatment in vivo with C20:2 enter into and exit from anergy more rapidly than after activation by α-GalCer. Importantly, this shorter duration of iNK T cells in the anergic state promotes the more rapid induction of tolerogenic DCs and reduced iNK T cell death, and enables C20:2 stimulated iNK T cells to elicit enhanced protection from T1D. Our findings further that suggest C20:2 is a more effective therapeutic drug than α-GalCer for protection from T1D. Moreover, the characteristics of C20:2 provide a basis of selection of next-generation iNK T cell agonists for the prevention of T1D.

An alpha-galactosylceramide C20:2 N-acyl variant enhances anti-inflammatory and regulatory T cell-independent responses that prevent type 1 diabetes

Protection from type 1 diabetes (T1D), a T helper type 1 (Th1)-mediated disease, is achievable in non-obese diabetic (NOD) mice by treatment with alpha-galactosylceramide (alpha-GalCer) glycolipids that stimulate CD1d-restricted invariant natural killer T (iNK T) cells. While we have reported previously that the C20:2 N-acyl variant of alpha-GalCer elicits a Th2-biased cytokine response and protects NOD mice from T1D more effectively than a form of alpha-GalCer that induces mixed Th1 and Th2 responses, it remained to determine whether this protection is accompanied by heightened anti-inflammatory responses. We show that treatment of NOD mice with C20:2 diminished the activation of 'inflammatory' interleukin (IL)-12 producing CD11c(high)CD8+ myeloid dendritic cells (mDCs) and augmented the function of 'tolerogenic' DCs more effectively than treatment with the prototypical iNKT cell activator KRN7000 (alpha-GalCer C26:0) that induces Th1- and Th2-type responses. These findings correlate with a reduced capacity of C20:2 to sustain the early transactivation of T, B and NK cells. They may also explain our observation that C20:2 activated iNK T cells depend less than KRN7000 activated iNK T cells upon regulation by regulatory T cells for cytokine secretion and protection from T1D. The enhanced anti-inflammatory properties of C20:2 relative to KRN7000 suggest that C20:2 should be evaluated further as a drug to induce iNK T cell-mediated protection from T1D in humans.

Glycosylation of the phosphate binding protein, PstS, in Streptomyces coelicolor by a pathway that resembles protein O-mannosylation in eukaryotes

Previously mutations in a putative protein O-mannosyltransferase (SCO3154, Pmt) and a polyprenol phosphate mannose synthase (SCO1423, Ppm1) were found to cause resistance to phage, phiC31, in the antibiotic producing bacteria Streptomyces coelicolor A3(2). It was proposed that these two enzymes were part of a protein O-glycosylation pathway that was necessary for synthesis of the phage receptor. Here we provide the evidence that Pmt and Ppm1 are indeed both required for protein O-glycosylation. The phosphate binding protein PstS was found to be glycosylated with a trihexose in the S. coelicolor parent strain, J1929, but not in the pmt(-) derivative, DT1025. Ppm1 was necessary for the transfer of mannose to endogenous polyprenol phosphate in membrane preparations of S. coelicolor. A mutation in ppm1 that conferred an E218V substitution in Ppm1 abolished mannose transfer and glycosylation of PstS. Mass spectrometry analysis of extracted lipids showed the presence of a glycosylated polyprenol phosphate (PP) containing nine repeated isoprenyl units (C(45)-PP). S. coelicolor membranes were also able to catalyse the transfer of mannose to peptides derived from PstS, indicating that these could be targets for Pmt in vivo.

Inactivation of polyketide synthase and related genes results in the loss of complex lipids in Mycobacterium tuberculosis H37Rv

AIMS

Phthiocerol dimycocerosate (PDIM) waxes and other lipids are necessary for successful Mycobacterium tuberculosis infection, although the exact role of PDIM in host-pathogen interactions remains unclear. In this study, we investigated the contribution of tesA, drrB, pks6 and pks11 genes in complex lipid biosynthesis in M. tuberculosis.

METHODS AND RESULTS

Four mutants were selected from M. tuberculosis H37Rv transposon mutant library. The transposon insertion sites were confirmed to be within the M. tuberculosis open reading frames for tesA (a probable thioesterase), drrB (predicted ABC transporter), pks11 (putative chalcone synthase) and pks6 (polyketide synthase). The first three of these transposon mutants were unable to generate PDIM and the fourth lacked novel polar lipids.

CONCLUSIONS

Mycobacterium tuberculosis can be cultivated in vitro without the involvement of certain lipid synthesis genes, which may be necessary for in vivo pathogenicity.

SIGNIFICANCE AND IMPACT OF THE STUDY

The use of transposon mutants is a new functional genomic approach for the eventual definition of the mycobacterial 'lipidome'.

The complete genome sequence and analysis of Corynebacterium diphtheriae NCTC13129

Corynebacterium diphtheriae is a Gram-positive, non-spore forming, non-motile, pleomorphic rod belonging to the genus Corynebacterium and the actinomycete group of organisms. The organism produces a potent bacteriophage-encoded protein exotoxin, diphtheria toxin (DT), which causes the symptoms of diphtheria. This potentially fatal infectious disease is controlled in many developed countries by an effective immunisation programme. However, the disease has made a dramatic return in recent years, in particular within the Eastern European region. The largest, and still on-going, outbreak since the advent of mass immunisation started within Russia and the newly independent states of the former Soviet Union in the 1990s. We have sequenced the genome of a UK clinical isolate (biotype gravis strain NCTC13129), representative of the clone responsible for this outbreak. The genome consists of a single circular chromosome of 2 488 635 bp, with no plasmids. It provides evidence that recent acquisition of pathogenicity factors goes beyond the toxin itself, and includes iron-uptake systems, adhesins and fimbrial proteins. This is in contrast to Corynebacterium's nearest sequenced pathogenic relative, Mycobacterium tuberculosis, where there is little evidence of recent horizontal DNA acquisition. The genome itself shows an unusually extreme large-scale compositional bias, being noticeably higher in G+C near the origin than at the terminus.

The M. tuberculosis antigen 85 complex and mycolyltransferase activity

AIMS

The antigen 85 complex (Ag85) from Mycobacterium tuberculosis consists of three abundantly secreted proteins (FbpA, FbpB and FbpC2) which play a key role in the pathogenesis of tuberculosis and also exhibit cell wall mycolyltransferase activity. A related protein with similarity to the Ag85 complex was recently annotated in the M. tuberculosis genome as FbpC1. An investigation was carried out to determine whether FbpC1 may also possess mycolyltransferase activity, a characteristic feature of the Ag85 complex.

METHODS AND RESULTS

Heterologous expression of FbpA, FbpC1 and FbpC2 was performed in Escherichia coli. Recombinant proteins were purified under non-denaturating conditions and used in an in vitro mycolyltransferase assay.

CONCLUSIONS

In contrast to FbpA and FbpC2, recombinant FbpC1 did not possess in vitro mycolyltransferase activity and was not recognized by two monoclonal antibodies to the native Ag85.

SIGNIFICANCE AND IMPACT OF THE STUDY

Mycolyltransferase activity is restricted to FbpA, FbpbB and FbpC2 only; the actual function of FbpC1 remains to be established.

Studies on (beta,1-->5) and (beta,1-->6) linked octyl Gal(f) disaccharides as substrates for mycobacterial galactosyltransferase activity

The emergence of multi-drug resistant (MDR) strains of Mycobacterium tuberculosis (MTB) and the continuing pandemic of tuberculosis emphasizes the urgent need for the development of new anti-tubercular agents with novel drug targets. The recent structural elucidation of the mycobacterial cell wall highlights a large variety of structurally unique components that may be a basis for new drug development. This publication describes the synthesis, characterization, and screening of several octyl Galf(beta,1-->5)Galf and octyl Galf(beta,1-->6)Galf derivatives. A cell-free assay system has been utilized for galactosyltransferase activity using UDP[14C]Galf as the glycosyl donor, and in vitro inhibitory activity has been determined in a colorimetric broth microdilution assay system against MTB H37Ra and three clinical isolates of Mycobacterium avium complex (MAC). Certain derivatives showed moderate activities against MTB and MAC. The biological evaluation of these disaccharides suggests that more hydrophobic analogues with a blocked reducing end showed better activity as compared to totally deprotected disaccharides that more closely resemble the natural substrates in cell wall biosynthesis.

Studies on alpha(1-->5) linked octyl arabinofuranosyl disaccharides for mycobacterial arabinosyl transferase activity

The appearance multi-drug resistant Mycobacterium tuberculosis (MTB) throughout the world has prompted a search for new, safer and more active agents against tuberculosis. Based on studies of the biosynthesis of mycobacterial cell wall polysaccharides, octyl 5-O-(alpha-D-arabinofuranosyl)-alpha-D-arabinofuranoside analogues were synthesized and evaluated as inhibitors for M. tuberculosis and Mycobacterium avium. A cell free assay system has been used for the evaluation of these disaccharides as substrates for mycobacterial arabinosyltransferase activity.

Glycolipid targets of CD1-mediated T-cell responses

Members of the CD1 family of antigen-presenting molecules bind and present a variety of mammalian and microbial glycolipids for specific recognition by T cells. CD1 proteins accomplish their antigen-presenting function by binding the alkyl chains of the antigens within a deep, hydrophobic groove on the membrane distal surface of CD1, making the hydrophilic elements of the antigen available for contact with the variable regions of antigen-specific T-cell receptors. Most models of CD1-restricted T cells function in infectious, neoplastic, or autoimmune diseases and are based on the premise that CD1-restricted T-cell responses are initiated by alterations in cellular glycolipid content. Although a growing number of self, altered self and foreign glycolipid antigens have been identified, the cellular mechanisms that could lead to the generation of antigenic glycolipids within cells, or control the presentation of particular classes of altered self or microbial glycolipids in disease states have only recently come under investigation. Here we review the structures of known glycolipid antigens for T cells and discuss how the chemical nature of these antigens, which is quite different from that of peptides, influences their recognition by T cells.

Galactan biosynthesis in Mycobacterium tuberculosis. Identification of a bifunctional UDP-galactofuranosyltransferase

The cell wall of Mycobacterium tuberculosis and related genera is unique among prokaryotes, consisting of a covalently bound complex of mycolic acids, D-arabinan and D-galactan, which is linked to peptidoglycan via a special linkage unit consisting of Rhap-(1-->3)-GlcNAc-P. Information concerning the biosynthesis of this entire polymer is now emerging with the promise of new drug targets against tuberculosis. Accordingly, we have developed a galactosyltransferase assay that utilizes the disaccharide neoglycolipid acceptors beta-d-Galf-(1-->5)-beta-D-Galf-O-C(10:1) and beta-D-Galf-(1-->6)-beta-D-Galf-O-C(10:1), with UDP-Gal in conjunction with isolated membranes. Chemical analysis of the subsequent reaction products established that the enzymatically synthesized products contained both beta-D-Galf linkages ((1-->5) and (1-->6)) found within the mycobacterial cell, as well as in an alternating (1-->5) and (1-->6) fashion consistent with the established structure of the cell wall. Furthermore, through a detailed examination of the M. tuberculosis genome, we have shown that the gene product of Rv3808c, now termed glfT, is a novel UDP-galactofuranosyltransferase. This enzyme possesses dual functionality in performing both (1-->5) and (1-->6) galactofuranosyltransferase reactions with the above neoglycolipid acceptors, using membranes isolated from the heterologous host Escherichia coli expressing Rv3808c. Thus, at a biochemical and genetic level, the polymerization of the galactan region of the mycolyl-arabinogalactan complex has been defined, allowing the possibility of further studies toward substrate recognition and catalysis and assay development. Ultimately, this may also lead to a more rational approach to drug design to be explored in the context of mycobacterial infections.

Biochemical characterization of acyl carrier protein (AcpM) and malonyl-CoA:AcpM transacylase (mtFabD), two major components of Mycobacterium tuberculosis fatty acid synthase II

Malonyl coenzyme A (CoA)-acyl carrier protein (ACP) transacylase (MCAT) is an essential enzyme in the biosynthesis of fatty acids in all bacteria, including Mycobacterium tuberculosis. MCAT catalyzes the transacylation of malonate from malonyl-CoA to activated holo-ACP, to generate malonyl-ACP, which is an elongation substrate in fatty acid biosynthesis. To clarify the roles of the mycobacterial acyl carrier protein (AcpM) and MCAT in fatty acid and mycolic acid biosynthesis, we have cloned, expressed, and purified acpM and mtfabD (malonyl-CoA:AcpM transacylase) from M. tuberculosis. According to the culture conditions used, AcpM was produced in Escherichia coli in two or three different forms: apo-AcpM, holo-AcpM, and palmitoylated-AcpM, as revealed by electrospray mass spectrometry. The mtfabD gene encoding a putative MCAT was used to complement a thermosensitive E. coli fabD mutant. Expression and purification of mtFabD resulted in an active enzyme displaying strong MCAT activity in vitro. Enzymatic studies using different ACP substrates established that holo-AcpM constitutes the preferred substrate for mtFabD. In order to provide further insight into the structure-function relationship of mtFabD, different mutant proteins were generated. All mutations (Q9A, R116A, H194A, Q243A, S91T, and S91A) completely abrogated MCAT activity in vitro, thus underlining the importance of these residues in transacylation. The generation and characterization of the AcpM forms and mtFabD opens the way for further studies relating to fatty acid and mycolic acid biosynthesis to be explored in M. tuberculosis. Since a specific type of FabD is found in mycobacterial species, it represents an attractive new drug target waiting to be exploited.

Synthetic mannosides act as acceptors for mycobacterial α1-6 mannosyltransferase

A series of synthetic mannosides was screened in a cell-free system for their ability to act as acceptor substrates for mycobacterial mannosyltransferases. Evaluation of these compounds demonstrated the incorporation of [14C]Man from GDP-[14C]Man into a radiolabeled organic-soluble fraction and analysis by thin layer chromatography and autoradiography revealed the formation of two radiolabeled products. Each synthetic acceptor was capable of accepting one or two mannose residues, resulting in a major and a minor mannosylated product. Both products from each acceptor were isolated and their mass was confirmed by fast-atom bombardment-mass spectrometry (FABMS). Characterization of each mannosylated product by exo-glycosidase digestion. acetolysis and linkage analysis by gas chromatography mass spectrometry of partially per-O-methylated alditols, revealed only alpha1-6-linked products. In addition. the antibiotic amphomycin selectively inhibited the formation of mannosylated products suggesting polyprenolmonophosphate-mannose (C15 50-P-Man) was the immediate mannose donor in all mannosylation reactions observed. The ability of synthetic disaccharides to act as acceptor substrates in this system, is most likely due to the action of a mycobacterial polyprenol-P-Man:mannan alpha1-6 mannosyltransferase involved in the biosynthesis of linear alpha1-6-linked lipomannan.

Biosynthesis of the galactan component of the mycobacterial cell wall

The structural core of the cell walls of Mycobacterium spp. consists of peptidoglycan bound by a linker unit (-alpha-L-Rhap-(1-->3)-D-GlcNAc-P-) to a galactofuran, which in turn is attached to arabinofuran and mycolic acids. The sequence of reactions leading to the biogenesis of this complex starts with the formation of the linker unit on a polyprenyl-P to produce polyprenyl-P-P-GlcNAc-Rha (Mikusová, K., Mikus, M., Besra, G. S., Hancock, I., and Brennan, P. J. (1996) J. Biol. Chem. 271, 7820-7828). We now establish that formation of the galactofuran takes place on this intermediate with UDP-Galf as the Galf donor presented in the form of UDP-Galp and UDP-Galp mutase (the glf gene product) and is catalyzed by galactofuranosyl transferases, one of which, the Mycobacterium tuberculosis H37Rv3808c gene product, has been identified. Evidence is also presented for the growth of the arabinofuran on this polyprenyl-P-P-linker unit-galactan intermediate catalyzed by unidentified arabinosyl transferases, with decaprenyl-P-Araf or 5-P-ribosyl-PP as the Araf donor. The product of these steps, the lipid-linked-LU-galactan-arabinan has been partially characterized in terms of its heterogeneity, size, and composition. Biosynthesis of the major components of mycobacterial cell walls is proving to be extremely complex. However, partial definition of arabinogalactan synthesis, the site of action of several major anti-tuberculosis drugs, facilitates the present day thrust for new drugs to counteract multiple drug-resistant tuberculosis.

CD1b-mediated T cell recognition of a glycolipid antigen generated from mycobacterial lipid and host carbohydrate during infection

T cells recognize microbial glycolipids presented by CD1 proteins, but there is no information regarding the generation of natural glycolipid antigens within infected tissues. Therefore, we determined the molecular basis of CD1b-restricted T cell recognition of mycobacterial glycosylated mycolates, including those produced during tissue infection in vivo. Transfection of the T cell receptor (TCR) alpha and beta chains from a glucose monomycolate (GMM)-specific T cell line reconstituted GMM recognition in TCR-deficient T lymphoblastoma cells. This TCR-mediated response was highly specific for natural mycobacterial glucose-6-O-(2R, 3R) monomycolate, including the precise structure of the glucose moiety, the stereochemistry of the mycolate lipid, and the linkage between the carbohydrate and the lipid. Mycobacterial production of antigenic GMM absolutely required a nonmycobacterial source of glucose that could be supplied by adding glucose to media at concentrations found in mammalian tissues or by infecting tissue in vivo. These results indicate that mycobacteria synthesized antigenic GMM by coupling mycobacterial mycolates to host-derived glucose. Specific T cell recognition of an epitope formed by interaction of host and pathogen biosynthetic pathways provides a mechanism for immune response to those pathogenic mycobacteria that have productively infected tissues, as distinguished from ubiquitous, but innocuous, environmental mycobacteria.

Polyprenyl phosphate biosynthesis in Mycobacterium tuberculosis and Mycobacterium smegmatis

Mycobacterium smegmatis has been shown to contain two forms of polyprenyl phosphate (Pol-P), while Mycobacterium tuberculosis contains only one. Utilizing subcellular fractions from M. smegmatis and M. tuberculosis, we show that Pol-P synthesis is different in these species. The specific activities of the prenyl diphosphate synthases in M. tuberculosis are 10- to 100-fold lower than those in M. smegmatis. In M. smegmatis decaprenyl diphosphate and heptaprenyl diphosphate were the main products synthesized in vitro, whereas in M. tuberculosis only decaprenyl diphosphate was synthesized. The data from both organisms suggest that geranyl diphosphate is the allylic substrate for two distinct prenyl diphosphate synthases, one located in the cell membrane that synthesizes omega,E,Z-farnesyl diphosphate and the other present in the cytosol that synthesizes omega,E,E,E-geranylgeranyl diphosphate. In M. smegmatis, the omega,E, Z-farnesyl diphosphate is utilized by a membrane-associated prenyl diphosphate synthase activity to generate decaprenyl diphosphate, and the omega,E,E,E-geranylgeranyl diphosphate is utilized by a membrane-associated activity for the synthesis of the heptaprenyl diphosphate. In M. tuberculosis, however, omega,E,E,E-geranylgeranyl diphosphate is not utilized for the synthesis of heptaprenyl diphosphate. Thus, the difference in the compositions of the Pol-P of M. smegmatis and M. tuberculosis can be attributed to distinct enzymatic differences between these two organisms.

Activation of the pro-drug ethionamide is regulated in mycobacteria

The anti-tuberculosis drug ethionamide (ETH), which is a structural analog of isoniazid (INH), is known to strongly inhibit mycolic acid synthesis in Mycobacterium tuberculosis. Although several targets have been identified for INH, only speculative information is available concerning ETH. Mutations within the promoter and the coding region of enoyl-acyl carrier protein reductase (InhA) were found to confer resistance to both drugs, thus leading to the impression that INH and ETH may share a common mode of action. However, a notable distinction between the two drugs lies in the lack of cross-resistance in clinical isolates. This may be attributed in part to the fact that the pro-drug INH must be activated via KatG, and no activation step for ETH has yet been described. Here we report the identification of an activator for ETH. The ETH activator (Rv3854c), which we have termed EthA, was found to be homologous to various monooxygenases and induced ETH sensitivity when overexpressed in mycobacteria. Interestingly, the neighboring open reading frame (Rv3855), which was found homologous to transcriptional repressors of the tetR family, led to ETH resistance when overexpressed. In addition, chromosomal inactivation of this gene by transposition led to ETH hypersensitivity. These data strongly suggest that Rv3855, which we have termed EthR, regulates the production of EthA, which subsequently activates the pro-drug ETH. This study opens up new avenues of research relating to ETH activation in mycobacteria, possibly leading to an improved efficacy of ETH and to the generation of new anti-mycobacterial agents.

Identification and substrate specificity of beta -ketoacyl (acyl carrier protein) synthase III (mtFabH) from Mycobacterium tuberculosis

The long-chain alpha-alkyl-beta-hydroxy fatty acids, termed mycolic acids, which are characteristic components of the mycobacterial cell wall are produced by successive rounds of elongation catalyzed by a multifunctional (type I) fatty acid synthase complex followed by a dissociated (type II) fatty acid synthase. In bacterial type II systems, the first initiation step in elongation is the condensation of acetyl-CoA with malonyl-acyl carrier protein (ACP) catalyzed by beta-ketoacyl-ACP III (FabH). An open reading frame in the Mycobacterium tuberculosis genome (Rv0533c), now termed mtfabH, was 37.3% identical to Escherichia coli ecFabH and contained the Cys-His-Asn catalytic triad signature. However, the purified recombinant mtFabH clearly preferred long-chain acyl-CoA substrates rather than acyl-ACP primers and did not utilize acetyl-CoA as a primer in comparison to ecFabH. In addition, purified mtFabH was sensitive to thiolactomycin and resistant to cerulenin in an in vitro assay. However, mtFabH overexpression in Mycobacterium bovis BCG did not confer thiolactomycin resistance, suggesting that mtFabH may not be the primary target of thiolactomycin inhibition in vivo and led to several changes in the lipid composition of the bacilli. The data presented is consistent with a role for mtFabH as the pivotal link between the type I and type II fatty acid elongation systems in M. tuberculosis. This study opens up new avenues for the development of selective and novel anti-mycobacterial agents targeted against mtFabH.

Thiolactomycin and related analogues as novel anti-mycobacterial agents targeting KasA and KasB condensing enzymes in Mycobacterium tuberculosis

Prevention efforts and control of tuberculosis are seriously hampered by the appearance of multidrug-resistant strains of Mycobacterium tuberculosis, dictating new approaches to the treatment of the disease. Thiolactomycin (TLM) is a unique thiolactone that has been shown to exhibit anti-mycobacterial activity by specifically inhibiting fatty acid and mycolic acid biosynthesis. In this study, we present evidence that TLM targets two beta-ketoacyl-acyl-carrier protein synthases, KasA and KasB, consistent with the fact that both enzymes belong to the fatty-acid synthase type II system involved in fatty acid and mycolic acid biosynthesis. Overexpression of KasA, KasB, and KasAB in Mycobacterium bovis BCG increased in vivo and in vitro resistance against TLM. In addition, a multidrug-resistant clinical isolate was also found to be highly sensitive to TLM, indicating promise in counteracting multidrug-resistant strains of M. tuberculosis. The design and synthesis of several TLM derivatives have led to compounds more potent both in vitro against fatty acid and mycolic acid biosynthesis and in vivo against M. tuberculosis. Finally, a three-dimensional structural model of KasA has also been generated to improve understanding of the catalytic site of mycobacterial Kas proteins and to provide a more rational approach to the design of new drugs.

CD1c-mediated T-cell recognition of isoprenoid glycolipids in Mycobacterium tuberculosis infection

The discovery of the CD1 antigen presentation pathway has expanded the spectrum of T-cell antigens to include lipids, but the range of natural lipid antigens and functions of CD1-restricted T cells in vivo remain poorly understood. Here we show that the T-cell antigen receptor and the CD1c protein mediate recognition of an evolutionarily conserved family of isoprenoid glycolipids whose members include essential components of protein glycosylation and cell-wall synthesis pathways. A CD1c-restricted, mycobacteria-specific T-cell line recognized two previously unknown mycobacterial hexosyl-1-phosphoisoprenoids and structurally related mannosyl-beta1-phosphodolichols. Responses to mannosyl-beta1-phosphodolichols were common among CD1c-restricted T-cell lines and peripheral blood T lymphocytes of human subjects recently infected with M. tuberculosis, but were not seen in naive control subjects. These results define a new class of broadly distributed lipid antigens presented by the CD1 system during infection in vivo and suggest an immune mechanism for recognition of senescent or transformed cells that are known to have altered dolichol lipids.

Crystal structure of the secreted form of antigen 85C reveals potential targets for mycobacterial drugs and vaccines

The antigen 85 (ag85) complex, composed of three proteins (ag85A, B and C), is a major protein component of the Mycobacterium tuberculosis cell wall. Each protein possesses a mycolyltransferase activity required for the biogenesis of trehalose dimycolate (cord factor), a dominant structure necessary for maintaining cell wall integrity. The crystal structure of recombinant ag85C from M. tuberculosis, refined to a resolution of 1.5 A, reveals an alpha/beta-hydrolase polypeptide fold, and a catalytic triad formed by Ser 124, Glu 228 and His 260. ag85C complexed with a covalent inhibitor implicates residues Leu 40 and Met 125 as components of the oxyanion hole. A hydrophobic pocket and tunnel extending 21 A into the core of the protein indicates the location of a probable trehalose monomycolate binding site. Also, a large region of conserved surface residues among ag85A, B and C is a probable site for the interaction of ag85 proteins with human fibronectin.

The molecular basis of CD1-mediated presentation of lipid antigens

The CD1 family of proteins mediates a newly described pathway for presentation of lipids and glycolipids for specific recognition by T cells. All four of the known human CD1 proteins (CD1a, CD1b, CD1c and CD1d) as well as murine CD1d have now been shown to mediate T-cell recognition of lipid or glycolipid antigens. These antigens include naturally occurring foreign glycolipids from intracellular pathogens or synthetic glycolipids that are related in structure to mammalian glycolipids. The CD1b and CD1d-presented antigens differ in their fine structures but reveal a general motif in which a rigid hydrophilic cap is bound to two aliphatic hydrocarbon chains. Different T-cell populations recognize individual antigens without cross-reactivity to closely related antigen structures or CD1 isoforms, documenting the complexity and fine specificity of CD1-mediated T-cell responses. Mapping of the molecular determinants of recognition for CD1b and CD1d-presented antigens reveals that T cells discriminate the fine structure of the hydrophilic cap of the antigen, but both the length and structure of the lipid chains may be altered without loss of recognition. This pattern of lipid antigen recognition may be accounted for by a simple molecular mechanism of presentation that parallels the known mechanism for presentation of peptides, but solves the special problems related to the hydrophobic chemical nature of the lipid antigens. We propose that CD1 binds antigen by accommodating the two lipid tails within the hydrophobic groove of its two membrane distal domains, positioning the rigid hydrophilic cap of the antigen on the solvent-exposed surface of the CD1 protein, where it can directly contact the T-cell antigen receptor. This model provides a molecular basis for recognition of a new and diverse set of T-cell antigens contained within the lipid bilayers of cellular membranes.

Studies on beta-D-Gal(f)-(1-->4)-alpha-L-Rha(p) octyl analogues as substrates for mycobacterial galactosyl transferase activity

The biochemically unique structures of sugar residues in the outer cell wall of Mycobacterium tuberculosis (MTB) make the pathways for their biosynthesis and utilization attractive targets for the development of new and selective anti-tubercular agents. A cell-free assay system for galactosyltransferase activity using UDP[14C]Gal as the glycosyl donor, as well as an in vitro colorimetric broth micro-dilution assay system, were used to determine the activities of three beta-D-gal(f)(1-->4)-alpha-L-rham(p) octyl disaccharides as substrates and antimycobacterial agents respectively. The cell-free enzymatic studies using compounds 8 and 10 suggested that these disaccharides bind to and are effective substrates for a putative mycobacterial galactosyltransferase. The modified acceptor 8 was found to be a slower but prolonged binder as compared to the less substituted analogue 10 as evidenced by their Km and Vmax values. Moderate antimycobacterial activity was observed with compounds 8 and 9 against MTB H37Ra and three clinical isolates of Mycobacterium avium complex (MAC).

The pimB gene of Mycobacterium tuberculosis encodes a mannosyltransferase involved in lipoarabinomannan biosynthesis

The biosynthesis of lipoarabinomannan (LAM), a key mycobacterial lipoglycan that has been implicated in numerous immunoregulatory functions, was examined utilizing D-mannosamine (ManN) as a tool to identify mannosyltransferase genes involved in LAM synthesis. Cell-free reactions utilizing cellular membranes of mycobacteria as the enzyme source indicated that ManN inhibited the synthesis of phosphatidylinositol mannosides, early precursors to LAM. A selection strategy was devised to screen a Mycobacterium tuberculosis genomic library in Mycobacterium smegmatis for clones conferring conditional resistance to ManN, with the rationale that overexpression of the gene(s) encoding a target of ManN would impart a ManN-resistant phenotype under these conditions. This strategy led to the identification of pimB, whose deduced amino acid sequence shows similarity to mannosyltransferases and other glycosyltransferases. Partially purified recombinant PimB protein from Escherichia coli or membranes from M. smegmatis overexpressing the pimB gene were used in cell-free assays to show that PimB catalyzes the formation of triacylphosphatidylinositol dimannoside from GDP-mannose and triacylphosphatidylinositol monomannoside.

Antimycobacterial activities of isoxyl and new derivatives through the inhibition of mycolic acid synthesis

Isoxyl (ISO), a thiourea (thiocarlide; 4, 4'-diisoamyloxythiocarbanilide), demonstrated potent activity against Mycobacterium tuberculosis H37Rv (MIC, 2.5 micrograms/ml), Mycobacterium bovis BCG (MIC, 0.5 microgram/ml), Mycobacterium avium (MIC, 2.0 microgram/ml), and Mycobacterium aurum A+ (MIC, 2.0 microgram/ml), resulting in complete inhibition of mycobacteria grown on solid media. Importantly, a panel of clinical isolates of M. tuberculosis from different geographical areas with various drug resistance patterns were all sensitive to ISO in the range of 1 to 10 microgram/ml. In a murine macrophage model, ISO exhibited bactericidal killing of viable intracellular M. tuberculosis in a dose-dependent manner (0.05 to 2.50 microgram/ml). The selective action of ISO on mycolic acid synthesis was studied through the use of [1, 2-14C]acetate labeling of M. tuberculosis H37Rv, M. bovis BCG, and M. aurum A+. At its MIC for M. tuberculosis, ISO inhibited the synthesis of both fatty acids and mycolic acids (alpha-mycolates by 91.6%, methoxymycolates by 94.3%, and ketomycolates by 91.1%); at its MIC in M. bovis BCG, ISO inhibited the synthesis of alpha-mycolates by 87.2% and that of ketomycolates by 88.5%; and the corresponding inhibitions for M. aurum A+ were 87.1% for alpha-mycolates, 87.2% for ketomycolates, and 86.5% for the wax-ester mycolates. A comparison with isoniazid (INH) and ethionamide (ETH) demonstrated marked similarity in action, i.e., inhibition of the synthesis of all kinds of mycolic acids. However, unlike INH and ETH, ISO also inhibited the synthesis of shorter-chain fatty acids. ISO showed no acute toxicity against primary macrophage cell cultures as demonstrated by diminution of redox activity. A homologous series of ISO derivatives were synthesized. Most derivatives were as effective or more effective than the parent compound in the agar proportion assay. Thus, these thioureas, like INH and ETH, specifically inhibit mycolic acid synthesis and show promise in counteracting a wide variety of drug-sensitive and -resistant strains of M. tuberculosis.

Phosphatidylinositol synthesis in mycobacteria

The metabolism and synthesis of an important mycobacterial lipid component, phosphatidylinositol (PI), and its metabolites, was studied in Mycobacterium smegmatis and M. smegmatis subcellular fractions. Little is known about the synthesis of PI in prokaryotic cells. Only a cell wall fraction (P60) in M. smegmatis was shown to possess PI synthase activity. Product was identified as PI by migration on TLC, treatment with phospholipase C and ion exchange chromatography. PI was the only major product (92.3%) when both cells and P60 fraction were labeled with [3H]inositol. Also, a neutral lipid inositol-containing product (4.1% of the total label) was identified in the P60 preparations. Strangely, PI synthase substrates, CDP-dipalmitoyl-DAG and CDP-NBD-DAG, added to the assay did not stimulate [3H]PI and NBD-PI yield by M. smegmatis. At the same time, addition of both substrates to rat liver and Saccharomyces cerevisiae PI synthase assays resulted in an increase in the product yield. Upon addition of CHAPS to the mycobacterial PI synthase assay, both substrates were utilized in a dose-dependent manner for the synthesis of NBD-PI and [3H]PI. These results demonstrate a strict substrate specificity of mycobacterial PI synthase toward endogenous substrates. K(m) of the enzyme toward inositol was shown to be 25 microM; Mg2+ stimulated the enzyme to a greater degree than Mn2+. Structural analogs of myo-inositol, epi-inositol and scyllo-inositol and Zn2+ were shown to be more potent inhibitors of mycobacterial PI synthase than of mammalian analogs. Lack of sequence homology with mammalian PI synthases, different kinetic characteristics, existence of selective inhibitors and an important physiological role in mycobacteria, suggest that PI synthase may be a good potential target for antituberculosis therapy.

Uptake and processing of glycosylated mycolates for presentation to CD1b-restricted T cells

Antigen presenting cells (APCs) expressing CD1b mediate the specific T cell recognition of mycobacterial lipid antigens. These lipid antigens require internalization by APCs prior to presentation, but the detailed mechanisms of uptake and intracellular processing are not known. Here we have examined several steps in the presentation of two related classes of CD1b-presented antigens, free and glycosylated mycolates. T cell recognition of glucose monomycolate (GMM) was blocked by agents that fix APC membranes or neutralize the pH of endosomes, indicating a requirement for GMM uptake into an acidic compartment prior to recognition. Different T cell lines responded to free mycolate or GMM without crossreactivity, yet both antigens were taken up by APCs at the same rate. This demonstrated that differential recognition of these antigens resulted from T cell specificity for their hydrophilic caps and that APCs were unable to interconvert these antigens by enzymatic or chemical deglycosylation or glycosylation. APCs were also unable to cleave mycobacterial trehalose dimycolate (TDM) at its most chemically labile linkages to yield antigenic free mycolates or GMM. Our results indicate that these mycolate-containing antigens are resistant to chemical or enzymatic cleavage by APCs, suggesting that molecular trimming is not a universal feature of lipid antigen processing.

Nuclear-encoded proteins target to the plastid in Toxoplasma gondii and Plasmodium falciparum

A vestigial, nonphotosynthetic plastid has been identified recently in protozoan parasites of the phylum Apicomplexa. The apicomplexan plastid, or "apicoplast," is indispensable, but the complete sequence of both the Plasmodium falciparum and Toxoplasma gondii apicoplast genomes has offered no clue as to what essential metabolic function(s) this organelle might perform in parasites. To investigate possible functions of the apicoplast, we sought to identify nuclear-encoded genes whose products are targeted to the apicoplast in Plasmodium and Toxoplasma. We describe here nuclear genes encoding ribosomal proteins S9 and L28 and the fatty acid biosynthetic enzymes acyl carrier protein (ACP), beta-ketoacyl-ACP synthase III (FabH), and beta-hydroxyacyl-ACP dehydratase (FabZ). These genes show high similarity to plastid homologues, and immunolocalization of S9 and ACP verifies that the proteins accumulate in the plastid. All the putatively apicoplast-targeted proteins bear N-terminal presequences consistent with plastid targeting, and the ACP presequence is shown to be sufficient to target a recombinant green fluorescent protein reporter to the apicoplast in transgenic T. gondii. Localization of ACP, and very probably FabH and FabZ, in the apicoplast implicates fatty acid biosynthesis as a likely function of the apicoplast. Moreover, inhibition of P. falciparum growth by thiolactomycin, an inhibitor of FabH, indicates a vital role for apicoplast fatty acid biosynthesis. Because the fatty acid biosynthesis genes identified here are of a plastid/bacterial type, and distinct from those of the equivalent pathway in animals, fatty acid biosynthesis is potentially an excellent target for therapeutics directed against malaria, toxoplasmosis, and other apicomplexan-mediated diseases.

Biosynthetic origin of mycobacterial cell wall galactofuranosyl residues

SETTING

Mycobacterial galactofuran is essential to the linking of the peptidoglycan and mycolic acid cell wall layers. Galactofuran biosynthesis should thus be essential for viability.

OBJECTIVE

The objective was to determine the pathway of galactofuranosyl biosynthesis and to clone a gene encoding an essential enzyme necessary for its formation.

DESIGN

Specific enzymatic conversions involved in formation of galactopyranose and galactofuranose residues in other bacteria were tested for in Mycobacterium smegmatis. M. tuberculosis deoxyribonucleic acid (DNA) was identified by homology.

RESULTS

It was shown that the de novo synthesis of the galactose carbon skeleton occurred in M. smegmatis by the transformation of UDP-glucopyranose to UDP-galactopyranose via the enzyme UDP-glucose 4-epimerase (E.C. 5.1.3.2). The N-terminal sequence of this enzyme was obtained after purification. The galactose salvage pathway enzyme, UDP-glucose-galactose-1-phosphate uridylyltransferase (E.C. 2.7.7.12), was also shown to be present. The critical biosynthetic transformation of the galactopyranose to galactofuranose ring form was shown to occur at the sugar nucleotide level via the enzyme UDP-galactopyranose mutase (E.C. 5.4.99.9). The M. tuberculosis DNA encoding this enzyme was sequenced, the gene expressed in Escherichia coli, and the expected enzymatic activity demonstrated.

CONCLUSION

Galactofuranose biosynthesis can now be pursued as a potential drug target in M. tuberculosis.

Synthesis of beta-D-arabinofuranosyl-1-monophosphoryl polyprenols: examination of their function as mycobacterial arabinosyl transferase donors

A convenient synthetic strategy has been developed to produce libraries of beta-D-arabinofuranosyl-monophosphorylpolyprenol. Those containing C50 and C55 polyprenols were the most active as donors for the cell-free synthesis of the arabinans of mycobacterial cell walls.

Synthetic arabinofuranosyl oligosaccharides as mycobacterial arabinosyltransferase substrates

A series of arabinofuranosyl oligosaccharides found as constituent parts of the polysaccharide portion of the cell wall of Mycobacterium tuberculosis have been chemically synthesized. Screening of these oligosaccharides as substrates for arabinosyltransferases present in mycobacterial membrane preparations suggests that modified oligosaccharide analogs as small as disaccharides may be inhibitors of glycan biosynthesis. Such inhibitors would be of potential utility as lead compounds in the identification of new drugs for the treatment of mycobacterial infections.

Molecular interaction of CD1b with lipoglycan antigens

The ability of human CD1b molecules to present nonpeptide antigens is suggested by the T cell recognition of microbial lipids and lipoglycans in the presence of CD1b-expressing antigen-presenting cells. We demonstrate the high-affinity interaction of CD1b molecules with the acyl side chains of known T cell antigens, lipoarabinomannan, phosphatidylinositol mannoside, and glucose monomycolate. Furthermore, CD1b-antigen binding was optimal at acidic pH, consistent with the known requirement for endosomal acidification in CD1b-restricted antigen presentation. The mechanism for CD1b-ligand interaction involves the partial unfolding of the alpha helices of CD1b at acidic pH, revealing a hydrophobic binding site that could accommodate lipid. These data provide direct evidence that the CD1b molecule has evolved unique biochemical properties that enable the binding of lipid-containing antigens from intracellular pathogens.

Mycobacterial arabinan biosynthesis: the use of synthetic arabinoside acceptors in the development of an arabinosyl transfer assay

Information on the biosynthesis of the D-arabinans of the cell wall of Mycobacterium tuberculosis is rapidly emerging, with the promise of new targets for drug development against tuberculosis. Accordingly, arabinosyl transferase assays were developed utilizing synthesized [1-14C]-beta-D-arabinofuranosyl-1-monophosphoryldecaprenol as donor and a variety of O- and S-alkyl arabinosides as acceptors. These were: alpha-D-Araf-(1-->5)-alpha-D-Araf-O- and -S-alkyl diarabinosides and alpha-D-Araf-(1-->5)-alpha-D-Araf-(1-->5)-alpha-D-Araf -O- and -S-alkyl triarabinosides. Whereas the O- and S-alkyl monosaccharide acceptors were inactive, the O- and S-alkyl disaccharide and the O- and S-alkyl trisaccharide acceptors (<C12) possessed considerable acceptor activity, and the trisaccharide acceptors were more potent than the corresponding disaccharides. The O-alkyl disaccharide acceptors with a C8 alkyl chain were more active than those containing the C6 or C10 analogs. Chemical analysis of the enzymatically synthesized products of the reactions demonstrated that beta-D-arabinofuranosyl-1-monophosphoryldecaprenol was an effective donor for two of the three potential arabinosyl transferases: beta-D-arabinofuranosyl-1-monophosphoryldecaprenol: arabinan alpha(1-->5) arabinosyl transferase and beta-D-arabinofuranosyl-1-monophosphoryldecaprenol: arabinan beta(1-->2) arabinosyl transferase. The beta(1-->2) arabinosyl transferase activity was more in evidence in the presence of the O-alkyl disaccharide acceptor, whereas both transferases were about equivalent in the presence of the S-alkyl trisaccharide acceptor. The tuberculosis drug, ethambutol, a known mycobacterial arabinosyl transferase inhibitor, was inactive within these arabinosyl transferase/acceptor based assay systems, supporting other evidence that a third activity, responsible for the formation of alpha1-->3 linkage, is the drug target.

Structural requirements for glycolipid antigen recognition by CD1b-restricted T cells

The human CD1b protein presents lipid antigens to T cells, but the molecular mechanism is unknown. Identification of mycobacterial glucose monomycolate (GMM) as a CD1b-presented glycolipid allowed determination of the structural requirements for its recognition by T cells. Presentation of GMM to CD1b-restricted T cells was not affected by substantial variations in its lipid tails, but was extremely sensitive to chemical alterations in its carbohydrate or other polar substituents. These findings support the view that the recently demonstrated hydrophobic CD1 groove binds the acyl chains of lipid antigens relatively nonspecifically, thereby positioning the hydrophilic components for highly specific interactions with T cell antigen receptors.

A novel pathogenic taxon of the Mycobacterium tuberculosis complex, Canetti: characterization of an exceptional isolate from Africa

In an attempt to characterize an unusual mycobacterial strain isolated from a 2-year-old Somali patient with lymphadenitis, we applied various molecular methods not previously used for the taxonomic classification of mycobacteria. This isolate, designated So93, did not differ from Mycobacterium tuberculosis in the biochemical tests and in its 16S rRNA sequence, but produced smooth and glossy colonies, which is highly exceptional for this species. This smooth phenotype was unstable and switched nonreversibly to a rough colony morphology with a low frequency. The two colony types were equally virulent for the guinea pig, exhibiting characteristic tuberculous disease. Both morphotypes had shorter generation times than the M. tuberculosis reference laboratory strain H37Rv and clinical isolates of M. tuberculosis and Mycobacterium bovis. Furthermore, the So93 isolate differed from all M. tuberculosis complex strains described thus far by having only a single copy of insertion sequence IS1081, an unusual composition of the direct repeat cluster, and a characteristic phenolic glycolipid and lipooligosaccharide. This glycolipid had previously been observed only in a smooth isolate of M. tuberculosis obtained in 1969 by Canetti in France. Analysis of the Canetti strain showed that it shared virtually all genetic properties characteristic of So93, distinguishing these two strains from the known M. tuberculosis complex taxa, M. tuberculosis, Mycobacterium africanum, M. bovis, and Mycobacterium microti. The natural reservoir, host range, and mode of transmission of the group of bacteria described in this paper are presently unknown. This study, partly based on not previously used molecular criteria, supports the idea that the established members within the M. tuberculosis complex and the newly described Canetti grouping should be regarded as a single species, which likely will be designated "M. tuberculosis".

Biosynthesis of mycobacterial lipoarabinomannan

The mycobacterial lipoglycans, lipomannan (LM) and lipoarabinomannan (LAM), are potent immunomodulators in tuberculosis and leprosy. Little is known of their biosynthesis, other than being based on phosphatidylinositol (PI), and they probably originate in the phosphatidylinositol mannosides (PIMs; PIMans). A novel form of cell-free incubation involving in vitro and in situ labeling with GDP-[14C]Man of the polyprenyl-P-mannoses (C35/C50-P-Man) and the simpler PIMs of mycobacterial membranes, reisolation of the [14C]Man-labeled membranes, and in situ chase demonstrated the synthesis of a novel alpha(1-->6)-linked linear form of LM at the expense of the C35/C50-P-Man. There was little or no synthesis under these conditions of PIMan5 with its terminal alpha(1-->2)Man unit or the mature LM or LAM with copious alpha(1-->2)Man branching. Synthesis of the linear LM, but not of the simpler PIMan2, was susceptible to amphomycin, a lipopeptide antibiotic that specifically inhibits polyprenyl-P-requiring translocases. A mixture of P[3H]I and P[3H]IMan2 was incorporated into the linear LM, supporting other evidence that, like the PIMs, LM and LAM, it is a lipid-linked mannooligosaccharide and a new member of the mycobacterial glycosylphosphatidylinositol lipoglycan/glycolipid class. Hence, the simpler PIMs originate in PI and GDP-Man, but further growth of the linear backbone emanates from C35-/C50-P-Man and is amphomycin-sensitive. The origin of the alpha(1-->2)Man branches of mature PIMan5, LM, and LAM is not known at this time but is probably GDP-Man.

Mycolic acid biosynthesis: definition and targeting of the Claisen condensation step

Through the use of 2,2-[2H]palmitic acid pulse labeling of the whole cells of C. matruchotti and analysis by gas chromatography-mass spectrometry of the non-labeled and [2H]-labeled corynomycolates, we established a new mechanism for palmitate condensation devoid of the postulated carboxylation step. This evidence allowed the design and synthesis of several structurally related antagonists against the condensation reactions which were shown to possess potent in vivo activity against C. matruchotti with complete inhibition of growth on solid media at concentrations between 1-10 microg/ml. In addition, a cell-free in vitro assay of corynomycolate synthesis was developed to allow the screening of these and other antagonists.

Characterization of the in vitro synthesized arabinan of mycobacterial cell walls

Previous studies have shown that polymerized [14C]arabinan can be synthesized from polyprenylphosphate-[14C]arabinose by the particulate enzymes of Mycobacterium smegmatis [R.E. Lee, K. Mikusová, P.J. Brennan and G.S Besra (1995) J. Am. Chem. Soc. 117, 11829-11832]. In the present investigation, the [14C]arabinan product was biochemically characterized. Sizing chromatography revealed a molecular weight consistent with that expected from mature arabinan. Digestion of the [14C]arabinan with a mixture of arabinases produced oligo[14C]arabinoside fragments including hexa[14C]arabinoside and tetra[14C]arabinoside which originated from the non-reducing terminal regions of the polymer, and di[14C]arabinoside from the internal regions of the polymer. These arabinoside fragments represent the major known structural motifs that comprise the arabinan segment of arabinogalactan and lipoarabinomannan. The presence of [14C]arabinose in both the internal and external regions of the [14C]arabinan suggests that polyprenylphosphate-arabinose is the major, and perhaps the only, donor of arabinosyl residues in mycobacteria.

Role of the major antigen of Mycobacterium tuberculosis in cell wall biogenesis

The dominant exported proteins and protective antigens of Mycobacterium tuberculosis are a triad of related gene products called the antigen 85 (Ag85) complex. Each has also been implicated in disease pathogenesis through its fibronectin-binding capacities. A carboxylesterase domain was found within the amino acid sequences of Ag85A, B, and C, and each protein acted as a mycolyltransferase involved in the final stages of mycobacterial cell wall assembly, as shown by direct enzyme assay and site-directed mutagenesis. Furthermore, the use of an antagonist (6-azido-6-deoxy-alpha, alpha'-trehalose) of this activity demonstrates that these proteins are essential and potential targets for new antimycobacterial drugs.

Antimycobacterial action of thiolactomycin: an inhibitor of fatty acid and mycolic acid synthesis

Thiolactomycin (TLM) possesses in vivo antimycobacterial activity against the saprophytic strain Mycobacterium smegmatis mc2155 and the virulent strain M. tuberculosis Erdman, resulting in complete inhibition of growth on solid media at 75 and 25 micrograms/ml, respectively. Use of an in vitro murine macrophage model also demonstrated the killing of viable intracellular M. tuberculosis in a dose-dependent manner. Through the use of in vivo [1,2-14C]acetate labeling of M. smegmatis, TLM was shown to inhibit the synthesis of both fatty acids and mycolic acids. However, synthesis of the shorter-chain alpha'-mycolates of M. smegmatis was not inhibited by TLM, whereas synthesis of the characteristic longer-chain alpha-mycolates and epoxymycolates was almost completely inhibited at 75 micrograms/ml. The use of M. smegmatis cell extracts demonstrated that TLM specifically inhibited the mycobacterial acyl carrier protein-dependent type II fatty acid synthase (FAS-II) but not the multifunctional type I fatty acid synthase (FAS-I). In addition, selective inhibition of long-chain mycolate synthesis by TLM was demonstrated in a dose-response manner in purified, cell wall-containing extracts of M. smegmatis cells. The in vivo and in vitro data and knowledge of the mechanism of TLM resistance in Escherichia coli suggest that two distinct TLM targets exist in mycobacteria, the beta-ketoacyl-acyl carrier protein synthases involved in FAS-II and the elongation steps leading to the synthesis of the alpha-mycolates and oxygenated mycolates. The efficacy of TLM against M. smegmatis and M. tuberculosis provides the prospects of identifying fatty acid and mycolic acid biosynthetic genes and revealing a novel range of chemotherapeutic agents directed against M. tuberculosis.

Mycolic acid structure determines the fluidity of the mycobacterial cell wall

The low permeability of the mycobacterial cell wall is thought to contribute to the well known resistance of mycobacteria to antibiotics and chemotherapeutic agents. We have used differential scanning calorimetry to demonstrate that the high temperature phase transition observed in purified cell walls, usually in the 60-70 degrees C range, suggestive of a lipid environment of extremely low fluidity, can also be observed in whole organisms and in cell walls from which much of the free lipids was removed by extraction with Triton X-114. A survey of seven mycobacterial species demonstrated that this high temperature transition was a general property of these organisms. Cell walls isolated from two Corynebacterium species, which contain much shorter corynemycolic acids, displayed a much lower temperature transition, suggesting that the transition temperature was directly correlated to the length of mycolic acid. Methyl esters of mycolic acids were found to have a phase transition temperature that was linearly related to the amount of trans-mycolate. Both Mycobacterium avium and M. smegmatis responded to increasing growth temperature by increasing the proportion of trans-mycolate and displaying a correspondingly higher melting temperature. Whole cells of M. smegmatis grown at higher temperature allowed a less rapid influx of two lipophilic agents, norfloxacin and chenodeoxycholate. These results provide strong evidence that the nature of mycolic acid plays a crucial role in determining the fluidity and permeability of mycobacterial cell wall.

Truncated structural variants of lipoarabinomannan in ethambutol drug-resistant strains of Mycobacterium smegmatis. Inhibition of arabinan biosynthesis by ethambutol

The anti-tuberculosis drug, ethambutol (Emb), was previously shown to inhibit the synthesis of arabinans of both the cell wall arabinogalactan (AG) and lipoarabinomannan (LAM) of Mycobacterium tuberculosis and other mycobacteria. However, an Emb-resistant mutant, isolated by consecutive passage of the Mycobacterium smegmatis parent strain in media containing increasing concentrations of Emb, while synthesizing a normal version of AG, produced truncated forms of LAM when maintained on 10 microg/ml Emb (Mikusová, K., Slayden, R. A., Besra, G. S., and Brennan, P. J. (1995) Antimicrob. Agents Chemother. 39, 2482-2489). We have now isolated and characterized the truncated LAMs made by both the resistant mutant and a recombinant strain transfected with a plasmid containing the emb region from Mycobacterium avium which encodes for Emb resistance. By chemical analysis, endoarabinanase digestion, high pH anion exchange chromatography, and mass spectrometry analyses, truncation was demonstrated as primarily a consequence of selective and partial inhibition of the synthesis of the linear arabinan terminal motif, which constitutes a substantial portion of the arabinan termini in LAM but not of AG. However, at higher concentrations, Emb also affected the general biosynthesis of arabinan destined for both AG and LAM, resulting in severely truncated LAM as well as AG with a reduced Ara:Gal ratio. The results suggested that Emb exerts its antimycobacterial effect by inhibiting an array of arabinosyltransferases involved in the biosynthesis of arabinans unique to the mycobacterial cell wall. It was further concluded that the uniquely branched terminal Ara6 motif common to both AG and LAM is an essential structural entity for a functional cell wall and, consequently, that the biosynthetic machinery responsible for its synthesis is the effective target of Emb in its role as a potent anti-tuberculosis drug.

The embAB genes of Mycobacterium avium encode an arabinosyl transferase involved in cell wall arabinan biosynthesis that is the target for the antimycobacterial drug ethambutol

The antimycobacterial compound ethambutol [Emb; dextro-2,2'-(ethylenediimino)-di-1-butanol] is used to treat tuberculosis as well as disseminated infections caused by Mycobacterium avium. The critical target for Emb lies in the pathway for the biosynthesis of cell wall arabinogalactan, but the molecular mechanisms for drug action and resistance are unknown. The cellular target for Emb was sought using drug resistance, via target overexpression by a plasmid vector, as a selection tool. This strategy led to the cloning of the M. avium emb region which rendered the otherwise susceptible Mycobacterium smegmatis host resistant to Emb. This region contains three complete open reading frames (ORFs), embR, embA, and embB. The translationally coupled embA and embB genes are necessary and sufficient for an Emb-resistant phenotype which depends on gene copy number, and their putative novel membrane proteins are homologous to each other. The predicted protein encoded by embR, which is related to known transcriptional activators from Streptomyces, is expendable for the phenotypic expression of Emb resistance, but an intact divergent promoter region between embR and embAB is required. An Emb-sensitive cell-free assay for arabinan biosynthesis shows that overexpression of embAB is associated with high-level Emb-resistant arabinosyl transferase activity, and that embR appears to modulate the in vitro level of this activity. These data suggest that embAB encode the drug target of Emb, the arabinosyl transferase responsible for the polymerization of arabinose into the arabinan of arabinogalactan, and that overproduction of this Emb-sensitive target leads to Emb resistance.

Chemistry of the lyxose-containing mycobacteriophage receptors of Mycobacterium phlei/Mycobacterium smegmatis

Mycobacterium phlei (strain Timothy) (Mycobacterium smegmatis ATCC 19249) is characterized by the presence of a family of alkali-labile glycolipids, reminiscent of the trehalose-containing lipooligosaccharide class of antigens but lacking the nonreducing trehalose core. Through a combination of methylation analyses, 1H and 13C NMR, two-dimensional 1H/1H and 1H/13C NMR, fast atom bombardment-mass spectrometry, gas chromatography-mass spectrometry, and other analytical techniques, these new structures were shown to possess three distinct features. Firstly, they contained the pentose D-lyxose (Lyx), rarely found in biology, but an epimer of D-arabinose, a key component of the mycobacterial cell wall arabinogalactan and lipoarabinomannnan. Thus, it was apparent that these glycolipids are the same as those described by Bisso et al. and attributed with phage receptor properties [Bisso, G., Castelnuovo, G., Nardelli, M.-G., Orefici, G., Arancia, G., Lanéelle, G., Asselineau, C., & Asselineau, J. (1976) Biochemie 58, 87-97]. Secondly, the complex oligosaccharides within the glycolipids contain the repeating units Lyxn(6-O-CH3-Glc)m and Lyxn(6-O-CH3-Glc)mMan1, where n+m equal to approximately 16 glycosyl residues. Thirdly, the M. phlei glycolipids were found to be heavily O-acylated, such that every D-Lyx residue invariably possesses an acyl function at position -2 and, in some instances, at both positions -2 and -4. The chemical characterization of these glycolipids, not feasible 20 years ago, clearly demonstrates that they are distinct from the type- and species-specific glycopeptidolipids, lipooligosaccharides, phenolic glycolipids, and the genus-specific phosphatidylinositol-based lipoglycans of mycobacteria. This present and previous studies begin to define the precise structural requirements responsible for the attachment of mycobacteriophage to the host cell wall.