Biology and therapeutic potential of cannabinoid CB2 receptor inverse agonists

Evidence has emerged suggesting a role for the cannabinoid CB2 receptor in immune cell motility. This provides a rationale for a novel and generalized immunoregulatory role for cannabinoid CB2 receptor-specific compounds. In support of this possibility, we will review the biology of a class of cannabinoid CB2 receptor-specific inverse agonist, the triaryl bis-sulfones. We will show that one candidate, Sch.414319, is potent and selective for the cannabinoid CB2 receptor, based on profiling studies using biochemical assays for 45 enzymes and 80 G-protein coupled receptors and ion channels. We will describe initial mechanistic studies using this optimized triaryl bis-sulfone, showing that the compound exerts a broad effect on cellular protein phosphorylations in human monocytes. This profile includes the down regulation of a required phosphorylation of the monocyte-specific actin bundling protein L-plastin. We suggest that this observation may provide a mechanism for the observed activity of Sch.414319 in vivo. Our continued analysis of the in vivo efficacy of this compound in diverse disease models shows that Sch.414319 is a potent modulator of immune cell mobility in vivo, can modulate bone damage in antigen-induced mono-articular arthritis in the rat, and is uniquely potent at blocking experimental autoimmune encephalomyelitis in the rat.

Sch35966 is a potent, selective agonist at the peripheral cannabinoid receptor (CB2) in rodents and primates

BACKGROUND AND PURPOSE

The peripheral cannabinoid receptor (CB(2)) is expressed on peripheral immune cells and is thought to have a role in the immunosuppressive effects of cannabinoids. Historically, there have been few potent, CB(2)-selective agonists to assess the contribution of CB(2) to this phenomenon. The studies presented here describe the synthesis of 8,10-bis[(2,2-dimethyl-1-oxopropyl)oxy]-11-methyl-1234-tetrahydro-6H-benzo[beta]quinolizin-6-one (Sch35966), which binds with low nanomolar potency to CB(2) in both primates and rodents.

EXPERIMENTAL APPROACH

The affinity, potency and efficacy of Sch35966 and other cannabinoid ligands at CB(2) was assessed using competition binding assays vs [(3)H]CP55,940, [(35)S]GTPgammaS exchange, cAMP accumulation and cell chemotaxis assays.

KEY RESULTS

We showed that Sch35966 has >450-fold selectivity for CB(2) binding vs the central cannabinoid receptor (CB(1)) in primates (humans and cynomolgus monkeys) and rodents (rats and mice). Sch35966 is an agonist as it effectively inhibited forskolin-stimulated cAMP synthesis in CHO-hCB(2) cells, stimulated [(35)S]GTPgammaS exchange and directed chemotaxis in cell membranes expressing CB(2). In all species examined, Sch35966 was more potent, more efficacious and more selective than JWH-015 (a commonly used CB(2)-selective agonist).

CONCLUSIONS AND IMPLICATIONS

Taken together, the data show that Sch35966 is a potent and efficacious CB(2)-selective agonist in rodents and primates.

Signaling by covalent heterodimers of interferon-gamma. Evidence for one-sided signaling in the active tetrameric receptor complex

Interferon-gamma (IFN-gamma) and its receptor complex are dimeric and bilaterally symmetric. We created mutants of IFN-gamma that bind only one IFN-gammaR1 chain per dimer molecule (called a monovalent IFN-gamma) to see if the interaction of IFN-gamma with one-half of the receptor complex is sufficient for bioactivity. Mutating a receptor-binding sequence in either AB loop of a covalent dimer of IFN-gamma yielded two monovalent IFN-gammas, gamma(m)-gamma and gamma-gamma(m), which cross-link to only a single soluble IFN-gammaR1 molecule in solution and on the cell surface. Monovalent IFN-gamma competes fully with wild type IFN-gamma for binding to U937 cells but only at a greater than 100-fold higher concentration than wild type IFN-gamma. Monovalent IFN-gamma had anti-vesicular stomatitis virus activity and antiproliferative activity, and it induced major histocompatibility complex class I and class II (HLA-DR) expression. In contrast, the maximal levels of activated Stat1alpha produced by monovalent IFN-gammas after 15 min were never more than half of those produced by either wild type or covalent IFN-gammas in human cell lines. These data indicate that while monovalent IFN-gamma activates only one-half of a four-chain receptor complex, this is sufficient for Stat1alpha activation, major histocompatibility complex class I surface antigen induction, and antiviral and antiproliferative activities. Thus, while interaction with both halves of the receptor complex is required for high affinity binding of IFN-gamma and efficient signal transduction, interaction with only one-half of the receptor complex is sufficient to initiate signal transduction.

Structural characterization of nitric oxide synthase isoforms reveals striking active-site conservation

Crystal structures of human endothelial nitric oxide synthase (eNOS) and human inducible NOS (iNOS) catalytic domains were solved in complex with the arginine substrate and an inhibitor S-ethylisothiourea (SEITU), respectively. The small molecules bind in a narrow cleft within the larger active-site cavity containing heme and tetrahydrobiopterin. Both are hydrogen-bonded to a conserved glutamate (eNOS E361, iNOS E377). The active-site residues of iNOS and eNOS are nearly identical. Nevertheless, structural comparisons provide a basis for design of isozyme-selective inhibitors. The high-resolution, refined structures of eNOS (2.4 A resolution) and iNOS (2.25 A resolution) reveal an unexpected structural zinc situated at the intermolecular interface and coordinated by four cysteines, two from each monomer.

Purification of ADAM 10 from bovine spleen as a TNFalpha convertase

We have purified a protease with characteristics of TNFalpha convertase from bovine spleen membranes. Peptide sequencing of the purified protein identified it as ADAM 10 (Genbank accession no. Z21961). This metalloprotease cleaves a recombinant proTNFalpha substrate to mature TNFalpha, and can cleave a synthetic peptide substrate to yield the mature TNFalpha amino terminus in vitro. The enzyme is sensitive to a hydroxamate inhibitor of MMPs, but insensitive to phosphoramidon. In addition, cloned ADAM 10 mediates proTNFalpha processing in a processing-incompetent cell line.

Expression of human inducible nitric oxide synthase in Escherichia coli

We have expressed active full-length human inducible nitric oxide synthase (iNOS) in E. coli. Expression required co-expression with calmodulin, a particularly tight-binding cofactor. The extracts also required tetrahydrobiopterin to display activity. Specific activity of the purified recombinant iNOS was similar to iNOS purified from murine macrophages. This result indicates that no special processing events unique to eucaryotic cells are necessary for iNOS activity.

Crystal structure of a complex between interferon-gamma and its soluble high-affinity receptor

The crystal structure of interferon-gamma bound to the extracellular fragment of its high-affinity cell-surface receptor reveals the first view of a class-2 cytokine receptor-ligand complex. In the complex, one interferon-gamma homodimer binds two receptor molecules. Unlike the class-1 growth hormone receptor complex, the two interferon-gamma receptors do not interact with one another and are separated by 27 A. Upon receptor binding, the flexible AB loop of interferon-gamma undergoes a conformational change that includes the formation of a 3(10) helix.

Importance of the loop connecting A and B helices of human interferon-gamma in recognition by interferon-gamma receptor

Characterization of murine-human hybrid interferon-gamma (IFN-gamma) molecules suggests that substitution of the peptide connecting the A and B helices in human IFN-gamma with the murine sequence significantly blocks the protein's binding to the human interferon-gamma receptor. Mutagenesis showed that this effect is localized to the central part of this A-B loop peptide, particularly Ser20, Asp21, Val22, and Ala23. One mutant, IFN-gamma/A23E,D24E,N25K, was examined by NMR. This "EEK" mutation does not significantly alter the conformation of interferon-gamma, suggesting that the effects of these mutations are not the result of global conformational changes. The A-B loop is near histidine 111, a residue previously shown to be important in receptor-ligand interaction (Lunn, C. A., Fossetta, J., Dalgarno, D., Murgolo, N., Windsor, W., Zavodny, P. J., Narula, S. K., and Lundell, D. (1992) Protein Eng. 5, 253-257). We show that copper forms a complex between histidine 19 in the A-B loop and histidine 111. This metal complex lacks the ability to interact with the interferon-gamma receptor. These results suggest that the A-B loop contains important structural information needed for receptor-ligand binding and hence biological activity of human interferon-gamma.

An active covalently linked dimer of human interferon-gamma. Subunit orientation in the native protein

We have constructed and expressed a covalently linked head to tail dimer of human interferon-gamma (IFN-gamma) in which two monomers are joined head to tail via a rigid peptide hinge using genetic engineering techniques. The hinge was derived from the human immunoglobin IgA1 sequence (Hallewell, R.A., Laria, I., Tabrizi, A., Carlin, G., Getzoff, E.D., Tainer, J.A., Cousens, L.S., and Mullenbach, G.T. (1989) J. Biol. Chem. 264, 5260-5268). Analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis shows that the polypeptide produced by this construction migrates as a 30,000 polypeptide species. The protein elutes as a single species by molecular sieve chromatography under native conditions. The covalently linked dimer exhibits one-half the antiviral activity of native dimeric IFN-gamma; receptor binding assays show the covalently linked dimer binds to the IFN-gamma receptor with one-half the avidity of native IFN-gamma. This difference is not due to conformational differences between the two molecules, as the aromatic region of the NMR spectrum of the purified covalently linked dimer is identical with that of the wild type protein. From these data, we suggest that human IFN-gamma associates in a head to tail dimer in its active configuration. Regions of IFN-gamma are contiguous with the amino and carboxyl termini and are obscured by the hinge peptide in the covalently linked dimer. Our studies demonstrate that these regions may be important for receptor-ligand interaction.

A point mutation that decreases the thermal stability of human interferon gamma

We have identified a mutation of human gamma-interferon (IFN gamma) causing a temperature-sensitive phenotype. We used a randomized oligonucleotide to mutagenize a synthetic human IFN gamma gene, then screened the resulting mutants produced in Escherichia coli for proteins with altered biological activity. One mutant protein selected for detailed characterization exhibited less than 0.3% of the specific biological activity of native IFN gamma in an antiviral activity assay performed at 37 degrees C. However, the protein bound the human IFN gamma receptor with native efficiency at 4 degrees C. Sequencing the plasmid DNA encoding this protein showed that the mutation changed the lysine residue at amino acid 43 to glutamic acid (IFN gamma/K43E). Site-specific mutagenesis at amino acid 43 showed that this protein's phenotype resulted from positioning a negative charge at position 43. Structural characterization of IFN gamma/K43E using CD demonstrated that the protein had native conformation at 25 degrees C, but assumed an altered conformation at 37 degrees C. IFN gamma/K43E in this altered conformation bound poorly to the IFN gamma receptor at 37 degrees C, providing a rationale for the mutant's decreased antiviral activity.

A point mutation of human interferon gamma abolishes receptor recognition

We identified a single amino acid mutation that abolished the bioactivity of human IFN gamma. The mutation was identified by screening a mutagenized IFN gamma expression library for molecules with altered biological activity. The mutant protein was expressed at high levels in Escherichia coli, and remained soluble upon purification. However, the protein was completely inactive in all IFN gamma assays investigated, exhibiting less than 0.0006% of the specific activity of native IFN gamma antiviral activity. Sequencing the plasmid DNA encoding this mutant protein showed that the histidine at position 111 of native human IFN gamma is changed to aspartic acid (IFN gamma/H111D). Other mutations at this site showed that only hydrophobic amino acids at position 111 maintain significant, though low, biological activity. Structural characterization of the IFN gamma/H111D protein by NMR as well as CD spectroscopy demonstrated that the protein has limited conformational differences from native IFN gamma. Models of the X-ray crystal structure of human IFN gamma [Ealick, P.E., W.J. Cook, S. Vijay-Kumar, M. Carson, T.L. Nagabhushan, P.P. Trotta and C.E. Bugg (1991) Science, 252, 698-702] suggest that this histidine residue is located at a severe 55 degrees bend in the C-terminal F helix. We conclude that H111 lies within or affects the receptor binding domain of human IFN gamma.

Cytoplasmic and periplasmic expression of a highly basic protein, human interleukin 4, inEscherichia coli

Human IL-4 (hIL-4) has been cloned from a human T cell line based on its homology to the murine IL-4 cDNA sequence. We have compared cytoplasmic and extra-cytoplasmic expression of this basic protein in Escherichia coli using various combinations of promoters, replicons and host strains. Strains producing a cytoplasmic product were most successful at heterologous protein expression, producing up to 500 mg/l of an inactive aggregated form of the protein. The biological activity of the protein could be restored by refolding the protein with guanidine hydrochloride and glutathione giving a specific activity identical to that of IL-4 derived from CHO cell lines stably transformed with an hIL-4 expression plasmid. Strains designed to secrete human IL-4 into the periplasmic space produced far less protein (approximately 5 mg/l). However, a significant fraction of this protein was detected in the culture medium. This fraction appeared to be soluble after ultracentrifugation, and demonstrated high specific activity without refolding. Leakage of heterologous protein into the culture medium may be a viable way to recover biologically active products without relying on the denaturation and refolding in vitro that can, at times, yield incorrectly folded gene product.

Defective Escherichia coli signal peptides function in yeast

To investigate structural characteristics important for eukaryotic signal peptide function in vivo, a hybrid gene with interchangeable signal peptides was cloned into yeast. The hybrid gene encoded nine residues from the amino terminus of the major Escherichia coli lipoprotein, attached to the amino terminus of the entire mature E. coli beta-lactamase sequence. To this sequence were attached sequences encoding the nonmutant E. coli lipoprotein signal peptide, or lipoprotein signal peptide mutants lacking an amino-terminal cationic charge, with shortened hydrophobic core, with altered potential helicity, or with an altered signal-peptide cleavage site. These signal-peptide mutants exhibited altered processing and secretion in E. coli. Using the GAL10 promoter, production of all hybrid proteins was induced to constitute 4-5% of the total yeast protein. Hybrid proteins with mutant signal peptides that show altered processing and secretion in E. coli, were processed and translocated to a similar degree as the non-mutant hybrid protein in yeast (approximately 36% of the total hybrid protein). Both non-mutant and mutant signal peptides appeared to be removed at the same unique site between cysteine 21 and serine 22, one residue from the E. coli signal peptidase II processing site. The mature lipo-beta-lactamase was translocated across the cytoplasmic membrane into the yeast periplasm. Thus the protein secretion apparatus in yeast recognizes the lipoprotein signal sequence in vivo but displays a specificity towards altered signal sequences which differs from that of E. coli.

Effects of prolipoprotein signal peptide mutations on secretion of hybrid prolipo-beta-lactamase in Escherichia coli

Hybrid proteins were constructed by coupling beta-lactamase to the signal sequence (plus nine amino acids) of selected mutant prolipoproteins of Escherichia coli. The mutant prolipoprotein signal peptides contained lesions in two structural domains of the signal peptide, the basic amino-terminal domain and the hydrophobic core domain. We then compared the processing and localization of the mutant prolipo-beta-lactamases to the processing and localization of the comparable mutant prolipoproteins. We show that a mutant signal sequence with an anionic amino terminus exhibits similar limitations in the processing of prolipo-beta-lactamase as previously observed in prolipoprotein. Deletion of four hydrophobic residues from hydrophobic core results in a signal peptide which slowly translocates a fraction of the total mutant hybrid protein synthesized. This signal peptide was previously shown to translocate lipoprotein efficiently. Alteration of this hydrophobic core, which stimulated synthesis of mutant prolipoproteins, does not stimulate synthesis of prolipo-beta-lactamase. Finally mutations that slowed processing of prolipoprotein by affecting the proposed helical structure of the signal peptide had no significant effect on the processing of prolipo-beta-lactamase. These results suggest that the positively charged amino-terminal domain of the signal peptide has a common role in protein secretion regardless of the secretory protein. On the other hand, other domains of the signal peptide exhibit different phenotypes when the secretory protein is changed.

Association of thioredoxin with the inner membrane and adhesion sites in Escherichia coli

The intracellular localization of thioredoxin in Escherichia coli was determined by immunoelectron microscopy and correlated to previous biochemical data which had suggested that thioredoxin resides at inner-outer membrane adhesion sites. Since a considerable amount of thioredoxin was lost during preparation of cells for electron microscopy, we immobilized the protein with the heterobifunctional photoactivatable cross-linker p-azidophenacylbromide before the cells were fixed with aldehyde and embedded in Lowicryl K4M. Thin sections were labeled with affinity-purified antithioredoxin antiserum and protein A-gold complexes. Densities of immunolabel in a designated membrane-associated area and in the rest of the cytoplasm were compared and the data were statistically evaluated. Wild-type strain W3110 and strain SK3981, an overproducer of thioredoxin, exhibited increased labeling at the inner membrane and its adjacent cytoplasmic area. In contrast, the more centrally located cytoplasm of both strains showed much lower label density. This label distribution did not change with cell growth or in the stationary phase. Immunolabel was often found at bridges between the inner and outer membranes; this result is consistent with a model which places at least a portion of the thioredoxin at membrane adhesion sites, corresponding to an osmotically sensitive cytoplasmic compartment bounded by a hybrid inner-outer membrane (C.A. Lunn and V. Pigiet, J. Biol. Chem. 257:11424-11430, 1982; C.A. Lunn and V. Pigiet, J. Biol. Chem. 261:832-838, 1986). Specific label was absent in the periplasmic space.

The effect of thioredoxin on the radiosensitivity of bacteria

The ability of Escherichia coli thioredoxin to protect cells from lethal amounts of gamma radiation was tested using bacterial strains engineered to contain different amounts of thioredoxin per cell. Cells grown to late stationary phase demonstrated a decreasing sensitivity to gamma-radiation with increasing amounts of thioredoxin per cell. Exponentially growing cells were equally sensitive to the gamma-radiation regardless of the intracellular concentration of thioredoxin. Cells exhibiting the radiation-resistant phenotype in the stationary phase reverted to the radiation-sensitive phenotype when diluted into fresh growth medium. These results suggest that thioredoxin can protect cells from gamma-radiation under certain metabolic conditions.

The catalytic active site of thioredoxin: conformation and homology with bovine pancreatic trypsin inhibitor

Rabbit polyclonal antibody was raised to a chemically synthesized nonapeptide (Trp-Ala-Glu-Trp-Cys-Gly-Pro-Cys-Lys) corresponding to the active-site sequence of Escherichia coli thioredoxin. The antiserum efficiently inhibited thioredoxin activity in the standard thioredoxin reductase/NADPH coupled assay. This inhibition was blocked by preincubation of the antiserum with the nonapeptide. Tight association of the E. coli thioredoxin to the active-site antibody required SDS denaturation. These results suggest that thioredoxin reductase (NADPH: oxidized-thioredoxin oxidoreductase, EC 1.6.4.5) alters the conformation of thioredoxin sufficiently to permit binding to the antibody. The antiserum bound to plant and liver thioredoxins. Bovine pancreatic trypsin inhibitor, whose active site (Gly-Pro-Cys-Lys) is homologous to that of thioredoxin, also competes for the active-site antibody. This result led to experiments showing that thioredoxin can inhibit the digestion of cytochrome c by trypsin. The ability of thioredoxin to act as a trypsin inhibitor analogue provides a rationale for thioredoxin's resistance to digestion by trypsin.

Chemical cross-linking of thioredoxin to hybrid membrane fraction in Escherichia coli

Thioredoxin was cross-linked to a membrane fraction in vivo using the heterobifunctional photoreactive cross-linking reagent p-azidophenacyl bromide, chosen to couple thioredoxin via its highly reactive thiol. Under mild reaction conditions, a significant amount of thioredoxin (30%) was rapidly cross-linked to the crude membrane fraction. The cross-linking reaction was selective, with thioredoxin purified 15-fold in the cross-linked membrane fraction. Membrane fractionation studies showed that thioredoxin associated with the inner membrane and with a hybrid membrane fraction. This hybrid membrane fraction banded at a density between the inner and outer membranes. This result is consistent with the localization of thioredoxin in association with the bacterial membrane adhesion sites first described by Bayer (Bayer, M. (1968) J. Gen. Microbiol. 53, 395-404). Association of thioredoxin with the membrane adhesion sites defines a structure corresponding to the osmotically sensitive cytoplasmic compartment (Lunn, C. A., and Pigiet, V. (1982) J. Biol. Chem. 257, 11424-11430).

An alternate pathway for the processing of the prolipoprotein signal peptide in Escherichia coli

Previous studies showed that when the signal sequence plus 9 amino acid residues from the amino terminus of the major lipoprotein of Escherichia coli was fused to beta-lactamase, the resulting hybrid protein was modified, proteolytically processed, and assembled into the outer membrane as was the wild-type lipoprotein (Ghrayeb, J., and Inouye, M. (1983) J. Biol. Chem. 259, 463-467). We have constructed several hybrid proteins with mutations at the cleavage site of the prolipoprotein signal peptide. These mutations are known to block the lipid modification of the lipoprotein at the cysteine residue, resulting in the accumulation of unprocessed, unmodified prolipoprotein in the outer membrane. The mutations blocked the lipid modification of the hybrid protein. However, in contrast to the mutant lipoproteins, the cleavage of the signal peptides for the mutant hybrid proteins did occur, although less efficiently than the unaltered prolipo-beta-lactamase. The mutant prolipo-beta-lactamase proteins were cleaved at a site 5 amino acid residues downstream of the prolipoprotein signal peptide cleavage site. This new cleavage between alanine and lysine residues was resistant to globomycin, a specific inhibitor for signal peptidase II. This indicates that signal peptidase II, the signal peptidase which cleaves the unaltered prolipo-beta-lactamase, is not responsible for the new cleavage. The results demonstrate that the cleavage of the signal peptide is a flexible process that can occur by an alternative pathway when the normal processing pathway is blocked.

Amplification and purification of plasmid-encoded thioredoxin from Escherichia coli K12

The thioredoxin gene (trxA) from Escherichia coli K12 has been cloned on a 3-kilobase pair PvuII fragment in a derivative of pBR325 (pBHK8). Thioredoxin protein production was amplified 150-200-fold in a strain containing pBHK8 (SK3981), with the greatest increase/cell observed after cultures reached stationary phase. A simple purification procedure, involving DEAE and AcA-54 column chromatography, yielded homogeneous protein with approximately 70% yield. The high amplification of thioredoxin in these cells (i.e. 10(6) copies/cell representing 40% of total cell protein) approaches the maximum yields seen in genetically constructed cloning vehicles (Bernard, H.U., and Helinski, D.R. (1980) in Genetic Engineering (Setlow, J. K., and Hollaender, A., eds) Vol. 2, pp. 133-167, Plenum Press, New York). This tremendous overproduction of thioredoxin protein is attributed to the high plasmid copy number observed in SK3981 (1700/cell). These results suggest a role for thioredoxin in plasmid DNA replication.

Isolation and characterization of a protease-nicked thioredoxin

An altered form of thioredoxin, composed of two peptides with molecular weights of 7 and 5 X 10(3) has been isolated from Escherichia coli B after chromatography on Ultrogel AcA54. The position of the clip site, determined by amino acid sequencing to lie between Pro(64) and Gly(65), is consistent with a cleavage within the hinge region connecting the two prominent folding domains of thioredoxin. The NH2-terminal domain containing the active site, and the carboxyl-terminal domain correspond to the 7 and 5 X 10(3) fragments, respectively. The two peptides combine to form a tight complex, T'12. At concentrations below 10(-6) M, the clipped species was not efficiently reduced by thioredoxin reductase, showing only half the activity of native thioredoxin at a protein concentration of 9 X 10(-8) M. The Km of the clipped species for thioredoxin reductase (2.9 X 10(-6) M) was nearly equivalent to that of native thioredoxin (2.2 X 10(-6) M), and the Vmax for both native and clipped thioredoxins were nearly identical. These data are taken to imply a reversible association of the fragments to produce a thioredoxin equivalent to the intact species in its interaction with thioredoxin reductase. Some evidence exists for an unfolding of the 7 X 10(3) fragment upon dissociation of the complex. A functional role for the two-domain structure of thioredoxin is proposed in which the carboxyl domain serves to stabilize the active site within the amino domain and confer the species specificity in the binding of thioredoxin to thioredoxin reductase.