Fluorescent glucagon derivatives. I. Synthesis and characterisation of fluorescent glucagon derivatives

Chemical Synthesis and Characterization of a Nonfibrillating Glycoglucagon

The current commercially available glucagon formulations for the treatment of severe hypoglycemia must be reconstituted immediately prior to use, owing to the susceptibility of glucagon to fibrillation and aggregation in an aqueous solution. This results in the inconvenience of handling, misuse, and wastage of this drug. To address these issues, we synthesized a glycosylated glucagon analogue in which the 25th residue (Trp) was replaced with a cysteine (Cys) and a Br-disialyloligosaccharide was conjugated at the Cys thiol moiety. The resulting analogue, glycoglucagon, is a highly potent full agonist at the glucagon receptor. Importantly, glycoglucagon exhibits markedly reduced propensity for fibrillation and enhanced thermal and metabolic stability. This novel analogue is thus a valuable lead for producing stable liquid glucagon formulations that will improve patient compliance and minimize wastage.

Monitoring drug–lipid membrane interactions via a molecular rotor probe

A label-free sensing method based on membrane viscosity changes to study the interactions between small drug molecules and lipid bilayers.

Fluorescent approaches for understanding interactions of ligands with G protein coupled receptors

G protein coupled receptors are responsible for a wide variety of signaling responses in diverse cell types. Despite major advances in the determination of structures of this class of receptors, the underlying mechanisms by which binding of different types of ligands specifically elicits particular signaling responses remain unclear. The use of fluorescence spectroscopy can provide important information about the process of ligand binding and ligand dependent conformational changes in receptors, especially kinetic aspects of these processes that can be difficult to extract from X-ray structures. We present an overview of the extensive array of fluorescent ligands that have been used in studies of G protein coupled receptors and describe spectroscopic approaches for assaying binding and probing the environment of receptor-bound ligands with particular attention to examples involving yeast pheromone receptors. In addition, we discuss the use of fluorescence spectroscopy for detecting and characterizing conformational changes in receptors induced by the binding of ligands. Such studies have provided strong evidence for diversity of receptor conformations elicited by different ligands, consistent with the idea that GPCRs are not simple on and off switches. This diversity of states constitutes an underlying mechanistic basis for biased agonism, the observation that different stimuli can produce different responses from a single receptor. It is likely that continued technical advances will allow fluorescence spectroscopy to play an important role in continued probing of structural transitions in G protein coupled receptors. This article is part of a Special Issue entitled: Structural and biophysical characterisation of membrane protein-ligand binding.

High-throughput screening of drug-lipid membrane interactions via counter-propagating second harmonic generation imaging

Here we report the use of counter-propagating second harmonic generation (SHG) to image the interactions between the local anesthetic tetracaine and a multicomponent planar supported lipid bilayer array in a label-free manner. The lipid bilayer arrays, prepared using a 3D continuous flow microspotter, allow the effects of lipid phase and cholesterol content on tetracaine binding to be examined simultaneously. SHG images show that tetracaine has a higher binding affinity to liquid-crystalline phase lipids than to solid-gel phase lipids. The presence of 28 mol % cholesterol decreased the binding affinity of tetracaine to bilayers composed of the mixed chain lipid, 1-steroyl-2-oleoyl-sn-glycero-3-phophocholine (SOPC), and the saturated lipids 1,2-dimyristoyl-sn-glycero-3-phophocholine (DMPC) and 1,2-dipamitoyl-sn-glycero-3-phophocholine (DPPC) while having no effect on diunsaturated 1,2-dioleoyl-sn-glycero-3-phophocholine (DOPC). The maximum surface excess of tetracaine increases with the degree of unsaturation of the phospholipids and decreases with cholesterol in the lipid bilayers. The paper demonstrates that SHG imaging is a sensitive technique that can directly image and quantitatively measure the association of a drug to a multicomponent lipid bilayer array, providing a high-throughput means to assess drug-membrane interactions.

In vivo labeling of L-type Ca2+ channels by fluorescent dihydropyridines: evidence for a functional, extracellular heparin-binding site

We have synthesized and characterized fluorescently labeled dihydropyridines (DHPs) as probes for L-type Ca2+ channels. Racemic as well as (+)- and (-)-1,4-dihydro- 2,6-dimethyl-4-(2-trifluoromethylphenyl)-3,5-pyridinecarboxylic acid 2-(aminoethyl)ethyl ester hydrochlorides were coupled to boron dipyrromethane (Bodipy) derivatives. (4,4-Difluoro-5,7-dimethyl-4-bora-3a,4a-diaza)-3- (s-indacene)propionic acid (DMBodipy)-DHP and (4,4-difluoro-7-styryl-4-bora-3a,4a-diaza)-3-(s-indacene+ ++)propionic acid (STBodipy)-DHP have Kd values in the nanomolar range for membrane-bound or partially purified skeletal muscle and for neuronal L-type Ca2+ channels. (-)- and (+)-STBodipy-DHPs block 45Ca2+ uptake through L-type Ca2+ channels into GH3 cells with IC50 values of 14.8 and 562 nM, respectively. The measurement of bound fluorescence after removal of free DMBodipy-DHP with charcoal shows that the probes can substitute for radioactive ligands to study the properties (equilibrium binding, kinetics, allosteric regulation) of partially purified L-type Ca2+ channels from skeletal muscle. L-type Ca2+ channels on GH3 cells were steroselectively visualized by using the optical enantiomers of STBodipy-DHP. Heparin inhibited GH3 cell labeling by (-)-STBodipy-DHP with an IC50 value of 9.7 micrograms/ml and blocked L-type Ca(2+)-channel-mediated 45Ca2+ uptake with an IC50 value of 32 micrograms/ml. These findings argue for an extracellular orientation of the heparin-binding domain of the Ca2+ channel that is coupled to the DHP receptor.

Studies of ligand binding to Escherichia coli adenylosuccinate synthetase

Dissociation constants of Escherichia coli adenylosuccinate synthetase with IMP, GTP, adenylosuccinate, and AMP (a competitive inhibitor for IMP) were determined by measuring the extent of quenching of the intrinsic tryptophan fluorescence of the enzyme. The enzyme has one binding site for each of these ligands. Aspartate and GDP did not quench the fluorescence to any great extent, and their dissociation constants could not be determined. These ligand binding studies were generally supportive of the kinetic mechanism proposed earlier for the enzyme. Cys291 was modified with the fluorescent chromophores N-(iodoacetylaminoethyl)-5-naphthylamine-1-sulfonate and tetramethylrhodamine maleimide in order to measure enzyme conformational changes attending ligand binding. The excitation and emission spectra of these fluorophores are not altered by the addition of active site binding ligands. TbGTP and TbGDP were used as native reporter groups, and changes in their fluorescence on complexing with the enzyme and various ligands made it possible to detect conformational changes occurring at the active site. Evidence is presented for abortive complexes of the type: enzyme-TbGTP-adenylosuccinate and enzyme-TbGTP-adenylosuccinate-aspartate. These results suggest that the IMP and aspartate binding sites are spatially separated.

Fluorescent glucagon derivatives. II. The use of fluorescent glucagon derivatives for the study of receptor disposition in membranes

When monolayer cultured hepatocytes were incubated with 1 nM [125I]glucagon at 30 degrees C, equilibrium was reached after 10 min, whereas at 4 degrees C, equilibrium was reached after 60 min. At the higher temperature, 11.2% of the bound ligand was broken down after 60 min, at the lower temperature, the amount of degradation was negligible. At 30 degrees C, acid-washing did not remove specifically bound ligand; thus, it was assumed that the ligand was internalised at this temperature, since some of the specifically bound ligand could be washed off at lower temperatures. This was confirmed in experiments when monolayer cultures of hepatocytes were incubated with fluorescein-labelled derivatives of glucagon. The distribution of specific binding on the cell surface was studied at both 30 and 4 degrees C using video intensification microscopic techniques. In keeping with studies using radiolabelled glucagon, more fluorescence was detected following incubation at 4 degrees C than at 30 degrees C and it could be removed by washing the cells. Video intensification microscopy indicated that at the lower temperature, the bound ligand was distributed all over the cell surface. At the higher temperature, ligand-derived fluorescence could only be detected in mobile intracellular vesicles.