The chemical modification of the C-2 substituent in the sugar moiety of N-acetylmuramoyl-l-alanyl-d-isoglutamine, and the immunoadjuvant activities

Recent Advances in the Synthesis and Biological Applications of Peptidoglycan Fragments

The biosynthesis, breakdown, and modification of peptidoglycan (PG) play vital roles in both bacterial viability and in the response of human physiology to bacterial infection. Studies on PG biochemistry are hampered by the fact that PG is an inhomogeneous insoluble macromolecule. Chemical synthesis is therefore an important means to obtain PG fragments that may serve as enzyme substrates and elicitors of the human immune response. This review outlines the recent advances in the synthesis and biochemical studies of PG fragments, PG biosynthetic intermediates (such as Park's nucleotides and PG lipids), and PG breakdown products (such as muramyl dipeptides and anhydro-muramic acid-containing fragments). A rich variety of synthetic approaches has been applied to preparing such compounds since carbohydrate, peptide, and phospholipid chemical methodologies must all be applied.

Convergent Synthesis of Novel Muramyl Dipeptide Analogues: Inhibition of Porphyromonas gingivalis-Induced Pro-inflammatory Effects by High Doses of Muramyl Dipeptide

Porphyromonas gingivalis (P.g.)-induced TNF-α can be affected by muramyl dipeptide (MDP) in a biphasic concentration-dependent manner. We found that in P.g.-exposed macrophages, treatment with 10 μg/mL of MDP (MDP-low) up-regulated TNF-α by 29%, while 100 μg/mL or higher (MDP-high) significantly decreased it (16% to 38%). MDP-high was found to affect the ubiquitin-editing enzyme A20 and activator protein 1 (AP1). An AP1 binding site was found in the promoter region of A20. A20 promoter activity was up-regulated after transfection of AP1 cDNA in cells. Four analogues of MDP (3-6) were prepared through a convergent strategy involving the synthesis of two unique carbohydrate fragments, 7a and 7b, using the peptide coupling reagents, EDCI and HOAt. Analogue 4 improved MDP function and P.g.-induced activities. We propose a new signaling pathway for TNF-α induction activated after exposing macrophages to both P.g. and MDP-high or analogue 4.

Peptidoglycan Modifications Tune the Stability and Function of the Innate Immune Receptor Nod2

Natural modifications of peptidoglycan modulate the innate immune response. Peptidoglycan derivatives activate this response via the intracellular innate immune receptor, Nod2. To probe how these modifications alter the response, a novel and efficient carbohydrate synthesis was developed to allow for late-stage modification of the amine at the 2-position. Modification of the carbohydrate was found to be important for stabilizing Nod2 and generating the proper response. The native Nod2 ligands demonstrate a significant increase in the cellular stability of Nod2. Moreover, changing the identity of the natural ligands at the carbohydrate 2-position allows for the Nod2-dependent immune response to be either up-regulated or down-regulated. The ligand structure can be adjusted to tune the Nod2 response, suggesting that other innate immune receptors and their ligands could use a similar strategy.

Synthesis and biological activities of N-acetyl-1-thiomuramoyl-L-alanyl-D-isoglutamine and some of its lipophilic derivatives

N-Acetyl-1-thiomuramoyl-L-alanyl-D-isoglutamine and some lipophilic analogs were synthesized from benzyl 2-acetamido-2-deoxy-4,6-O-isopropylidene-3-O-[D-1-(methoxycarbonyl)ethyl ]- alpha-D-glucopyranoside (1). O-Debenzoylation of 2, derived from 1 by oxidation, gave 2-acetamido-2-deoxy-4,6-O-isopropylidene-3-O-[D-1-(methoxycarbonyl)ethyl ]-D-glucopyranose (3). Condensation of the alkoxy-tris(dimethylamino)phosphonium chloride (4), formed from 3 by the action of carbon tetrachloride and tris(dimethylamino)phosphine, with potassium thioacetate afforded 2-acetamido-1-S-acetyl-2-deoxy-4,6-O-isopropylidene-3-O-[ D-1-(methoxycarbonyl)ethyl]-1-thio-beta-D-glucopyranose (8). Coupling of the acid 9, obtained from 8 by hydrolysis and subsequent S-acetylation, with the methyl ester of L-alanyl-D-isoglutamine gave N-[2-O-(2-acetamido-1-S-acetyl-2,3-dideoxy-4,6-O- isopropylidene-1-thio-beta-D-glucopyranose-3-yl)-D-lactoyl]-L-alan yl-D- isoglutamine methyl ester (10), which was converted, via O-deisopropylidenation, S-deacetylation, and de-esterification, into the N-acetyl-1-thiomuramoyl dipeptide. Condensation of 11 (derived from 10 by S-deacetylation) and of 12 (obtained from 10 by S-deacetylation and de-esterification) with various acyl chlorides yielded the corresponding 1-S-acyl-N-acetylmuramoyl-L-alanyl-D-isoglutamine derivatives, which were converted into the desired, lipophilic 1-thiomuramoyl dipeptides by cleavage of the isopropylidene group. Condensation of 11 with the alkyl bromides yielded the 1-S-alkyl derivatives, which were also converted, via O-deisopropylidenation and de-esterification, into the corresponding 1-S-alkylmuramoyl dipeptides. The biological activities were examined in guinea-pigs and mice.

Chemical modification of the C-6 substituent in the carbohydrate moiety of N-acetylmuramoyl-L-alanyl-D-isoglutamine (MDP), and the immunoadjuvant activity

N-Acetyl-6-O-mesyl-, -6-O-methyl-, and -4,6-di-O-methyl-muramoyl-L-alanyl-D-isoglutamine and N-acetyl-6-chloro-, -6-bromo-, and -6-azido-6-deoxymuramoyl-L-alanyl-D-isoglutamine were synthesized from benzyl 2-acetamido-2-deoxy-3-O-[D-1-(methoxycarbonyl) ethyl]-alpha-D-glucopyranoside and its 6-O-mesyl derivative. The immunoadjuvant activity of the products was examined, in order to clarify the structural requirements for the activity of the carbohydrate moiety in N-acetylmuramoyl-L-alanyl-D-isoglutamine.

Synthesis and immunoadjuvant activities of the repeating, disaccharide-dipeptide unit of the bacterial, cell-wall peptidoglycan and of some carbohydrate analogs

The repeating disaccharide-dipeptide units of the bacterial, cell-wall peptidoglycan, one being O-(N-acetyl-beta-muramoyl-L-alanyl-D-isoglutamine)-(1 leads to 4)-2-acetamido-2-deoxy-D-glucose, and the other, O-(2-acetamido-2-deoxy-beta-D-glucosyl)-(1 leads to 4)-N-acetyl-muramoyl-L-alanyl-D-isoglutamine, have been synthesized. Some carbohydrate analogs, such as O-(N-acetyl-beta-muramoyl-L-alanyl-D-isoglutamine)-(1 leads to 4)-N-acetylmuramoyl-L-alanyl-D-isoglutamine, O-beta-D-glucosyl-(1 leads to 4)-N-acetylmuramoyl-L-alanyl-D-isoglutamine, and O-(6-acetamido-6-deoxy-beta-D-glucosyl)-(1 leads to 4)-N-acetylmuramoyl-L-alanyl-D-isoglutamine, were also synthesized. Their immunoadjuvant activities were examined in guinea-pigs.