Role of the fourth intracellular loop of D1-like dopaminergic receptors in conferring subtype-specific signaling properties

We investigate whether the fourth intracellular loop (IL4) of D1 and D5 dopaminergic receptors (D1R, D5R) confers D1-like subtype-specific signaling properties. Using chimeric receptors (D1R-IL4B and D5R-IL4A), we show that swapping of IL4 leads to a switch in dopamine affinity and constitutive activity of D1R and D5R. Dopamine potency was reduced for both chimeras in comparison with wild-type receptors. Moreover, dopamine-mediated maximal activation was drastically increased in cells expressing D1R-IL4B when compared with those harboring D5R-IL4A or wild-type receptors. In conclusion, IL4 plays a pivotal role in imparting subtype-specific ligand binding and activation properties to highly homologous seven-transmembrane receptors.     

Synthesis and characterization of the cobalt(III) complexes of two pendant-arm cross-bridged cyclams

The cobalt(III) complexes of 4,11-diacetato-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (1), [Co(1)]PF₆, and 4,11-diacetamido-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (2), [Co(2)][PF₆]₃, have been synthesized and characterized. The crystal structure of [Co(1)]PF₆ consists of an octahedral cobalt(III) cation coordinated to all four ligand nitrogen donors in the macrobicycle’s cavity, as well as to the deprotonated carboxylate oxygen atoms of both pendant arms. Analytical and spectroscopic data indicates that the ligand in [Co(2)][PF₆]₃ is not deprotonated, suggesting coordination through the amide carbonyl oxygens. Study of the electronic spectra of these novel complexes and comparison with data from related cobalt(III) complexes characterizes the ligands as strong field with Δ₀=24,040 and Δ₀=24,250 cm⁻¹ for 1 and 2, respectively. Cyclic voltammograms were obtained for both complexes with large variations observed due to the differences in ligand charge and coordination.     

Oligonucleotide-displaced organic monolayer-protected silver nanoparticles and enhanced luminescence of their salted aggregates

N-(2-mercaptopropionyl)glycine (tiopronin) monolayer-protected silver particles were partially displaced by single-stranded oligonucleotides through ligand exchanges. The oligonucleotide-displaced particles could be hybridized with complementary fluorophore-labeled oligonucleotides. Both the oligonucleotide-displaced and hybridized particles could be aggregated by electrostatic interactions with salt in buffer solution, and the aggregates displayed enhanced luminescence from fluorophores. This result suggests the possible application of surface-enhanced fluorescence from metallic nanoparticle aggregation for DNA detection.     

Mutation of active site residues in the chitin-binding domain ChBDChiA1 from chitinase A1 of Bacillus circulans alters substrate specificity: use of a green fluorescent protein binding assay

A fluorescent binding assay was developed to investigate the effects of mutagenesis on the binding affinity and substrate specificity of the chitin-binding domain of chitinase A1 from Bacillus circulans WL-12. The chitin-binding domain was genetically fused to the N-terminus of a green fluorescent protein, and the polyhistidine-tagged hybrid protein was expressed in Escherichia coli. Residues likely to be involved in the binding site were mutated and their contributions to binding and substrate specificity were evaluated by affinity electrophoresis and depletion assays. The experimental binding isotherms were analyzed by non-linear regression using a modified Langmuir equation. Non-conservative substitution of tryptophan residue (W687) nearly abolished chitin-binding affinity and dramatically lowered chitosan binding while retaining the original level of curdlan binding. Double mutation E668K/P689A had altered specificity for several substrates and also impaired chitin binding significantly. Other substitutions in the binding site altered substrate specificity but had little effect on overall affinity for chitin. Interestingly, mutation T682A led to a higher specificity towards chitinous substrates than the wildtype. Furthermore, the ChBD-GFP hybrid protein was tested for use in diagnostic staining of cell walls of fungi and yeast and for the detection of fungal infections in tissue samples.     

Emodin-induced apoptosis through p53-dependent pathway in human hepatoma cells

Most of the commonly used cytotoxic anticancer drugs have been shown to induce apoptosis in susceptible cells. However, the signaling pathway of their apoptotic effects remains undefined. In this study, the cytotoxic effect of emodin on various human hepatoma cell lines was investigated. Results demonstrated that emodin exhibited strongly suppressing effect on HepG2/C3A, PLC/PRF/5, and SK-HEP-1 cells, with the IC(50) value of 42.5, 46.6, and 53.1 microM, respectively. Furthermore, emodin induced apoptosis in HepG2/C3A cells was clearly verified by the appearance of DNA fragmentation and sub-G(1) accumulation. Besides, HepG2/C3A cells were found to be arrested in G(2)/M phase after the cells were treated with 60 microM emodin for 48 h. Moreover, significant increase in the levels of apoptosis-related signals such as p53 (419.3 pg/ml), p21 (437.4 units/ml), Fas (6.6 units/ml), and caspase-3 (35.4 pmol/min) were observed in emodin treated HepG2/C3A cells. Taken together, emodin displays effective inhibitory effects on the growth of various human hepatoma cell lines and stimulates the expression of p53 and p21 that resulted in the cell cycle arrest of HepG2/C3A cells at G(2)/M phase. Results also suggest that emodin-induced apoptosis in HepG2/C3A cells were mediated through the activation of p53, p21, Fas/APO-1, and caspase-3. It implies that emodin could be a useful chemotherapeutical agent for treatment of hepatocellular carcinoma (HCC).     

Deciphering the Kinetic Binding Mechanism of Dimeric Ligands Using a Potent Plasma-stable Dimeric Inhibitor of Postsynaptic Density Protein-95 as an Example

Dimeric ligands can be potent inhibitors of protein-protein or enzyme-substrate interactions. They have increased affinity and specificity toward their targets due to their ability to bind two binding sites simultaneously and are therefore attractive in drug design. However, few studies have addressed the kinetic mechanism of interaction of such bivalent ligands. We have investigated the binding interaction of a recently identified potent plasma-stable dimeric pentapeptide and PDZ1–2 of postsynaptic density protein-95 (PSD-95) using protein engineering in combination with fluorescence polarization, isothermal titration calorimetry, and stopped-flow fluorimetry. We demonstrate that binding occurs via a two-step process, where an initial binding to either one of the two PDZ domains is followed by an intramolecular step, which produces the bidentate complex. We have determined all rate constants involved in the binding reaction and found evidence for a conformational transition of the complex. Our data demonstrate the importance of a slow dissociation for a successful dimeric ligand but also highlight the possibility of optimizing the intramolecular association rate. The results may therefore aid the design of dimeric inhibitors in general.     

“Pseudo” γ-Butyrolactone Receptors Respond to Antibiotic Signals to Coordinate Antibiotic Biosynthesis

In actinomycetes, the onset of secondary metabolite biosynthesis is often triggered by the quorum-sensing signal γ-butyrolactones (GBLs) via specific binding to their cognate receptors. However, the presence of multiple putative GBL receptor homologues in the genome suggests the existence of an alternative regulatory mechanism. Here, in the model streptomycete Streptomyces coelicolor, ScbR2 (SCO6286, a homologue of GBL receptor) is shown not to bind the endogenous GBL molecule SCB1, hence designated “pseudo” GBL receptor. Intriguingly, it could bind the endogenous antibiotics actinorhodin and undecylprodigiosin as ligands, leading to the derepression of KasO, an activator of a cryptic type I polyketide synthase gene cluster. Likewise, JadR2 is also a putative GBL receptor homologue in Streptomyces venezuelae, the producer of chloramphenicol and cryptic antibiotic jadomycin. It is shown to coordinate their biosynthesis via direct repression of JadR1, which activates jadomycin biosynthesis while repressing chloramphenicol biosynthesis directly. Like ScbR2, JadR2 could also bind these two disparate antibiotics, and the interactions lead to the derepression of jadR1. The antibiotic responding activities of these pseudo GBL receptors were further demonstrated in vivo using the lux reporter system. Overall, these results suggest that pseudo GBL receptors play a novel role to coordinate antibiotic biosynthesis by binding and responding to antibiotics signals. Such an antibiotic-mediated regulatory mechanism could be a general strategy to coordinate antibiotic biosynthesis in the producing bacteria.     

Tumor necrosis factor alpha-induced inflammation is increased but apoptosis is inhibited by common food additive carrageenan

Tumor necrosis factor (TNF)-α, a homotrimeric, pleiotropic cytokine, is secreted in response to inflammatory stimuli in diseases such as rheumatoid arthritis and inflammatory bowel disease. TNF-α mediates both apoptosis and inflammation, stimulating an inflammatory cascade through the non-canonical pathway of NF-κB activation, leading to increased nuclear RelB and p52. In contrast, the common food additive carrageenan (CGN) stimulates inflammation through both the canonical and non-canonical pathways of NF-κB activation and utilizes the adaptor molecule BCL10 (B-cell leukemia/lymphoma 10). In a series of experiments, colonic epithelial cells and mouse embryonic fibroblasts were treated with TNF-α and carrageenan in order to simulate the possible effects of exposure to dietary CGN in the setting of a TNF-α-mediated inflammatory disease process. A marked increase in secretion of IL-8 occurred, attributable to synergistic effects on phosphorylated NF-κB-inducing kinase (NIK) in the non-canonical pathway. TNF-α induced the ubiquitination of TRAF2 (TNF receptor-associated factor 2), which interacts with NIK, and CGN induced phosphorylation of BCL10, leading to increased NIK phosphorylation. These results suggest that TNF-α and CGN in combination act to increase NIK phosphorylation, thereby increasing activation of the non-canonical pathway of NF-κB activation. In contrast, the apoptotic effects of TNF-α, including activation of caspase-8 and PARP-1 (poly(ADP-ribose) polymerase 1) fragmentation, were markedly reduced in the presence of CGN, and CGN caused reduced expression of Fas. These findings demonstrate that exposure to CGN drives TNF-α-stimulated cells toward inflammation rather than toward apoptotic cell death and suggest that CGN exposure may compromise the effectiveness of anti-TNF-α therapy.     

Fas/Fas Ligand�Cmediated Apoptosis in Different Cell Lineages and Functional Compartments of Human Lymph Nodes

We have optimized an immunohistochemical double-staining method combining immunohistochemical lymphocyte lineage marker detection and apoptosis detection with terminal deoxyribonucleotidyl transferase–mediated dUTP nick end labeling. The method was used to trace Fas-mediated apoptosis in human reactive lymph nodes according to cell lineage and anatomical location. In addition to Fas, we also studied the expression of Fas ligand (FasL), CD3, CD20, CD19, CD23, and CD68 of apoptotic cells. The presence of simultaneous Fas and FasL positivity indicated involvement of activation-induced death in the induction of paracortical apoptosis. FasL expression in the high endothelial venules might be an inductor of apoptosis of Fas-positive lymphoid cells. In addition to B-lymphocyte apoptosis in the germinal centers, there was often a high apoptosis rate of CD23-expressing follicular dendritic cells. In summary, our double-staining method provides valuable new information about the occurrence and mechanisms of apoptosis of different immune cell types in the lymph node compartments. Among other things, we present support for the importance of Fas/FasL–mediated apoptosis in lymph node homeostasis. (J Histochem Cytochem 58:131–140, 2010)