The Skaggs Institute
for Chemical Biology

Scientific Report 2007

Chemical Tools for Improving Human Health

K.D. Janda, J. Ashley, A. Brogan, C. Carney, K. Capkova, C. Chung, S. De Lamo Marin, J. Denery, T. Dickerson, L. Eubanks, K. Fukuchi, C. Hernandez, A. Hoyt, A. Ino, G. Kaufmann, J. Liu, Y. Liu, C. Lowery, S. Mahajan, A. Mayorov, G. McElhaney, K. McKenzie, J. Mee, A. Moreno, Y. Nakai, J. Park, N. Salzameda, S. Steiniger, J. Treweek, A. Willis, Y. Xu, Y. Yoneda, B. Zhou, H. Zhou

Immunotherapeutic Interference of Quorum Sensing by Targeting N-(3-oxo-acyl) Homoserine Lactones

In 1993, the American Society for Microbiology acknowledged the role of biofilm formation in the growth of bacterial colonies. This assertion was a radical departure from the traditional view that individual autonomous cells made up bacterial colonies. Instead, bacteria coordinate their behavior relative to other community members, whether of the same or different species, through an advanced system of cell-to-cell signaling termed quorum sensing. Because quorum sensing regulates many aspects of bacterial growth and responsiveness to the environment, it can be critically important in the formation of biofilms, in which a community of cells interact to form a cell density–dependent, protective matrix.

Bacterial biofilms make their composite members resistant to antimicrobial measures, generating a significant clinical hurdle for the clearance of a given bacterial infection. Two classic examples are the antibiotic tolerance of Staphylococcus aureus infections and the chronic lung infections predominantly caused by Pseudomonas aeruginosa in patients with cystic fibrosis. Because quorum sensing mediates the formation of biofilms, it follows that therapies that interrupt cell-to-cell communication may be effective as novel antiinfective strategies.

N-acyl homoserine lactones (AHLs) are a class of secreted molecules that participate in quorum sensing of gram-negative bacteria. Upon reaching a threshold concentration, AHLs bind to their cognate receptors and induce the expression of target genes, which then mediate the course of the bacterial infection. The acylation pattern of specific AHLs, which is conferred by the activity of the autoinducer enzymes during AHL synthesis, lends specificity to the quorum-sensing molecules. For example, the pathogenicity of P aeruginosa is governed by 2 AHLs: N-(3-oxo-acyl) homoserine lactone and N-butyryl homoserine lactone. In order to combat bacterial pathogenicity and circumvent the development of antibiotic-resistant bacterial strains, research has been conducted in the synthesis of AHL mimics to function as quorum-sensing antagonists and in the engineering of enzymes that can degrade AHLs.

In recent investigations of antibody-mediated inhibition of AHL-based quorum sensing, we designed several haptens based on the highly conserved molecular scaffold of AHLs and the stability of the AHLs under physiologic conditions. A panel of antibodies was generated, and 5 candidates were screened for their ability to disrupt N-(3-oxo-acyl) homoserine lactone–based quorum sensing in P aeruginosa. Gratifyingly, the monoclonal antibody RS2-1G9 prevented normal quorum sensing, and, in turn, expression of the virulence factor pyocyanin was similarly inhibited. Subsequently, we began research into the development of antibodies capable of spurring AHL degradation. We investigated the potential of antibodies obtained from a squaric monoester monoamide hapten (Fig. 1) to catalyze the hydrolysis of homoserine lactone 3-oxo-C₁₂-AHL to its ring-opened product. From this initial screen, we selected 17 antibodies. Among the 17, the monoclonal antibody XYD-11G2 catalyzed the degradation of 3-oxo-C₁₂-AHL, as assessed by detection of the parent compound and hydrolysis product by liquid chromatography/mass spectrometry.

Fig. 1. Reactive hapten based on a squaric monoester monoamide motif.

To further examine the catalytic activity of XYD-11G2, we measured the ability of the hapten shown in Figure 1 to inhibit XYD-11G2–mediated hydrolysis of the homoserine lactone. The hapten inhibited the enzymatic activity of XYD-11G2, and kinetic analysis confirmed the hapten's moderate catalytic efficiency. To examine the potential of XYD-11G2 to disrupt AHL-based quorum sensing, we assayed cultures of P aeruginosa to determine concentrations of the virulence factor pyocyanin, whose production by this bacterium is regulated by AHL-based signaling. When applied to cultures of P aeruginosa for 12 hours, XYD-11G2 did not alter growth of the bacteria but did suppress synthesis of pyocyanin. In conclusion, using our reactive immunization strategy with the squaric monoester monoamide hapten (Fig. 1), we successfully generated catalytic antibodies specific to AHLs, and these antibodies were able to curtail bacterial infectivity by disrupting quorum sensing.

Molecular Link Between the Active Component of Marijuana and Pathologic Changes in Alzheimer's Disease

Alzheimer's disease is the leading cause of dementia among the elderly, and cases of this disease are expected to triple during the next 50 years as the baby boomer population matures. Consequently, development of treatments that slow or halt the disease progression have become imperative to both improve the quality of life for patients and reduce the health care costs attributable to Alzheimer's disease.

The primary neuropathologic features of Alzheimer's disease include accumulation of abnormally hyperphosphorylated tau to form intracellular neurofibrillary tangles, degeneration of cholinergic neurons in the basal forebrain, and deposition of amyloid β-peptide (Aβ) into amyloid plaques in areas of the brain important for memory and cognition. However, the exact factors that give rise to these neuropathologic changes and the extent to which these features are related remain elusive. As a result, current pharmacotherapy provides symptomatic relief; acetylcholinesterase inhibitors provide the greatest benefit.

Inhibition of acetylcholinesterase appears to address both the cholinergic hypothesis and the amyloid hypothesis for neurodegeneration in Alzheimer's disease. Whereas acetylcholinesterase inhibitors improve cholinergic transmission by increasing the synaptic content of acetylcholine, the inhibitors also seem to stabilize cognitive function by affecting the processing and deposition of Aβ. This latter function may be related to the ability of acetylcholinesterase inhibitors to block the peripheral anionic site (PAS) of the enzyme rather than bind the catalytic triad of the active site. Thus, research continues on the discovery and mechanism of action of novel drugs that mediate acetylcholinesterase inhibition by interacting with key residues of either site.

We hypothesized that Δ⁹-tetrahydrocannabinol (Δ⁹-THC), the active component of marijuana, was an ideal candidate for binding the allosteric PAS of acetylcholinesterase because of the lipophilic nature and tricyclic structure of the active component. This theory arose from our previous studies of compounds whose rigid, aromatic scaffolds enabled them to disrupt protein-protein interactions. By modeling Δ⁹-THC binding to acetylcholinesterase in silico in collaboration with A.J. Olson, Scripps Research, we determined that the ABC-fused ring of the Δ⁹-THC scaffold could interact with the tryptophan 86 indole side chain and backbone carbonyls phenylalanine 123 and serine 125 of acetylcholinesterase. Although Δ⁹-THC competitively inhibited the catalytic activity of acetylcholinesterase only moderately (K_i = 10.2 μM), the potential for Δ⁹-THC to block substrate entry into the active site of the catalytic triad and thereby inhibit acetylcholinesterase activity is enhanced by its superior binding affinity to PAS in comparison with known PAS binders. In addition, we found that Δ⁹-THC not only completely blocked the effect of acetylcholinesterase on Aβ aggregation but also had significantly greater potency than propidium, one of the most effective aggregation inhibitors reported to date (Fig. 2).

Fig. 2. Inhibition of acetylcholinesterase (AChE)-induced Aβ aggregation by Δ9-THC and propidium. * P < .05 vs Aβ only; # P < .05 vs Aβ + propidium.

In total, we have shown that Δ⁹-THC competitively inhibits acetylcholinesterase and prevents acetylcholinesterase-induced aggregation of Aβ. Although the psychoactive properties of Δ⁹-THC preclude its application as drug for treatment of Alzheimer's disease, its superior efficacy in inhibiting aggregation of Aβ, as compared with medications approved by the Food and Drug Administration, support study of Δ⁹-THC analogs as drug targets for the treatment of the signs, symptoms, and progression of Alzheimer's disease.

Publications

Brogan, A.P., Dickerson, T.J., Janda, K.D. Catalytic antibodies: past, present, and future. In: Wiley Encyclopedia of Chemical Biology. Wiley, New York, in press.

Brogan, A.P., Eubanks, L.M., Koob, G.F., Dickerson, T.J., Janda, K.D. Antibody-catalyzed oxidation of Δ⁹-tetrahydrocannabinol. J. Am. Chem. Soc. 129:3698, 2007.

De Lamo Marin, S., Xu, Y., Meijler, M.M., Janda, K.D. Antibody catalyzed hydrolysis of a quorum sensing signal found in gram-negative bacteria. Bioorg. Med. Chem. Lett. 17:1549, 2007.

Debler, E.W., Kaufmann, G.F., Kirchdoerfer, R.N., Mee, J.M., Janda, K.D., Wilson, I.A. Crystal structures of a quorum-quenching antibody. J. Mol. Biol. 368:1392, 2007.

Dickerson, T.J., Janda, K.D. The use of small molecules to investigate molecular mechanisms and therapeutic targets for treatment of botulinum neurotoxin A intoxication. ACS Chem. Biol. 2:359, 2007.

Dickerson, T.J., McKenzie, K.M, Hoyt, A.S., Wood, M.R., Janda, K.D., Brenner, S.B., Lerner, R.A. Phage escape libraries for checkmate analysis. Proc. Natl. Acad. Sci. U. S. A. 104:12703, 2007.

Eubanks, L.M., Dickerson, T.J., Janda, K.D. Technological advancements for the detection of and protection against biological and chemical warfare agents. Chem. Soc. Rev. 36:458, 2007.

Eubanks, L.M., Hixon, M.S., Jin, W., Hong, S., Clancy, C.M., Tepp, W.H., Baldwin, M.R., Malizio, C.J., Goodnough, M.C., Barbieri, J.T., Johnson, E.A., Boger, D.L., Dickerson, T.J., Janda, K.D. An in vitro and in vivo disconnect uncovered through high-throughput identification of botulinum neurotoxin A antagonists. Proc. Natl. Acad. Sci. U. S. A. 104:2602, 2007.

Ino, A., Dickerson, T.J., Janda, K.D. Positional linker effects in haptens for cocaine immunopharmacotherapy. Bioorg. Med. Chem. Lett. 17:4280, 2007.

Liu, Y., Steiniger, S.C., Kim, Y., Kaufmann, G.F., Felding-Habermann, B., Janda, K.D. Mechanistic studies of a peptidic GRP78 ligand for cancer cell-specific drug delivery. Mol. Pharm. 4:435, 2007.

McAllister, L.A., Hixon, M.S., Schwartz, R., Kubitz, D.S., Janda, K.D. Synthesis and application of a novel ligand for affinity chromatography based removal of endotoxin from antibodies. Bioconjug. Chem. 18:559, 2007.

McKenzie, K.M., Mee, J.M., Rogers, C.J., Hixon, M.S., Kaufmann, G.F., Janda, K.D. Identification and characterization of single chain anti-cocaine catalytic antibodies. J. Mol. Biol. 365:722, 2006.

Rogers, C.J., Eubanks, L.M., Dickerson, T.J., Janda, K.D. Unexpected acetylcholinesterase activity of cocaine esterases. J. Am. Chem. Soc. 128:15364, 2006.

Shigenaga, A., Moss, J.A., Janda, K.D. New synthetic methodology for the synthesis of cyclic depsipeptides employing acylphenyldiazene activation. In: Peptide Science 2005: Proceedings of the 42nd Japanese Peptide Symposium. Wakamiya, T. (Ed.). Protein Research Foundation, Osaska, Japan, 2006, p. 137.

Silvaggi, N.R., Boldt, G.E., Hixon, M.S., Kennedy, J.P., Tzipori, S., Janda, K.D., Allen, K.N. Structures of Clostridium botulinum neurotoxin serotype A light chain complexed with small-molecule inhibitors highlight active-site flexibility. Chem. Biol. 14:533, 2007.

Steiniger, S.C., Altobell, L.J. III, Zhou, B., Janda, K.D. Selection of human antibodies against cell surface-associated oligomeric anthrax protective antigen. Mol. Immunol. 44:2749, 2007.

Treweek, J., Wee, S., Koob, G.F., Dickerson, T.J., Janda, K.D. Self-vaccination by methamphetamine glycation products chemically links chronic drug abuse and cardiovascular disease. Proc. Natl. Acad. Sci. U. S. A. 104:11580, 2007.

Xu, Y., Hixon, M.S., Yamamoto, N., McAllister, L.A., Wentworth, A.D., Wentworth, P., Jr., Janda, K.D. Antibody-catalyzed anaerobic destruction of methamphetamine. Proc. Natl. Acad. Sci. U. S. A. 104:3681, 2007.