α-Helix or ß-Turn? An Investigation into N-Terminally Constrained Analogues of Glucagon-like Peptide 1 (GLP-1) and Exendin-4.

Peptide agonists acting on the glucagon-like peptide 1 receptor (GLP-1R) promote glucose-dependent insulin release and therefore represent important therapeutic agents for type 2 diabetes (T2D). Previous data indicated that an N-terminal type II ß-turn motif might be an important feature for agonists acting on the GLP-1R. In contrast, recent publications reporting the structure of the full-length GLP-1R have shown the N-terminus of receptor-bound agonists in an α-helical conformation. To reconcile these conflicting results, we prepared N-terminally constrained analogues of glucagon-like peptide 1 (GLP-1) and exendin-4 and evaluated their receptor affinity and functionality in vitro; we then examined their crystal structures in complex with the extracellular domain of the GLP-1R and used molecular modeling and molecular dynamics simulations for further investigations. We report that the peptides' N-termini in all determined crystal structures adopted a type II ß-turn conformation, but in vitro potency varied several thousand-fold across the series. Potency correlated better with α-helicity in our computational model, although we have found that the energy barrier between the two mentioned conformations is low in our most potent analogues and the flexibility of the N-terminus is highlighted by the dynamics simulations.

Target 2035 - an update on private sector contributions.

Target 2035, an international federation of biomedical scientists from the public and private sectors, is leveraging 'open' principles to develop a pharmacological tool for every human protein. These tools are important reagents for scientists studying human health and disease and will facilitate the development of new medicines. It is therefore not surprising that pharmaceutical companies are joining Target 2035, contributing both knowledge and reagents to study novel proteins. Here, we present a brief progress update on Target 2035 and highlight some of industry's contributions.

Solid-phase route to Fmoc-protected cationic amino acid building blocks.

Diamino acids are commonly found in bioactive compounds, yet only few are commercially available as building blocks for solid-phase peptide synthesis. In the present work a convenient, inexpensive route to multiple-charged amino acid building blocks with varying degree of hydrophobicity was developed. A versatile solid-phase protocol leading to selectively protected amino alcohol intermediates was followed by oxidation to yield the desired di- or polycationic amino acid building blocks in gram-scale amounts. The synthetic sequence comprises loading of (S)-1-(p-nosyl)aziridine-2-methanol onto a freshly prepared trityl bromide resin, followed by ring opening with an appropriate primary amine, on-resin N(ß)-Boc protection of the resulting secondary amine, exchange of the N(α)-protecting group, cleavage from the resin, and finally oxidation in solution to yield the target γ-aza substituted building blocks having an Fmoc/Boc protection scheme. This strategy facilitates incorporation of multiple positive charges into the building blocks provided that the corresponding partially protected di- or polyamines are available. An array of compounds covering a wide variety of γ-aza substituted analogs of simple neutral amino acids as well as analogs displaying high bulkiness or polycationic side chains was prepared. Two building blocks were incorporated into peptide sequences using microwave-assisted solid-phase peptide synthesis confirming their general utility.

Mechanistic study of the sPLA2-mediated hydrolysis of a thio-ester pro anticancer ether lipid.

Secretory phospholipase A(2) (sPLA(2)) is an interesting enzyme for triggered liposomal drug delivery to tumor tissue due the overexpression of sPLA(2) in cancerous tissue. A drug delivery system based on the triggered release of therapeutics from sPLA(2)-sensitive liposomes constituted of pro anticancer ether lipids, which become cytotoxic upon sPLA(2)-catalyzed hydrolysis has previously been established. To optimize the hydrolysis rate of the lipids and thereby optimizing the release profile of the drugs from the liposomes, we have synthesized a thio-ester pro anticancer ether lipid. Liposomes constituted of this lipid showed an altered rate of hydrolysis by sPLA(2). We have tested the cytotoxicity of the thio-ester pro anticancer ether lipids toward cancer cells, and the results showed that the cytotoxicity is indeed maintained upon sPLA(2) exposure. To further understand the origin for the observed different hydrolysis rates for the esters, we have applied molecular dynamics simulations and density functional theory. The combination of these theoretical methods has given valuable insight into the molecular mechanism for sPLA(2) action on sulfur-containing phospholipids. It appears that the enzyme-catalyzed hydrolysis of thio-esters follow a different pathway compared to the hydrolysis pathway of the free thio-ester.

Drug delivery by an enzyme-mediated cyclization of a lipid prodrug with unique bilayer-formation properties.

Special delivery: Liposomal drug-delivery systems in which prodrugs are activated specifically by disease-associated enzymes have great potential for the treatment of severe diseases, such as cancer. A new type of phospholipid-based prodrug has the ability to form stable small unilamellar vesicles (see picture). Activation of the prodrug vesicles by the enzyme sPLA(2) initiates a cyclization reaction, which leads to the release of the drug.

Molecular basis of phospholipase A2 activity toward phospholipids with sn-1 substitutions.

We studied secretory phospholipase A(2) type IIA (sPLA(2)) activity toward phospholipids that are derivatized in the sn-1 position of the glycerol backbone. We explored what type of side group (small versus bulky groups, hydrophobic versus polar groups) can be introduced at the sn-1 position of the glycerol backbone of glycerophospholipids and at the same time be hydrolyzed by sPLA(2). The biophysical characterization revealed that the modified phospholipids can form multilamellar vesicles, and several of the synthesized sn-1 functionalized phospholipids were hydrolyzed by sPLA(2). Molecular dynamics simulations provided detailed insight on an atomic level that can explain the observed sPLA(2) activity toward the different phospholipid analogs. The simulations revealed that, depending on the nature of the side chain located at the sn-1 position, the group may interfere with an incoming water molecule that acts as the nucleophile in the enzymatic reaction. The simulation results are in agreement with the experimentally observed sPLA(2) activity toward the different phospholipid analogs.

Selective pressurized liquid extraction of polychlorinated biphenyls in sediment.

A selective pressurized liquid extraction procedure (SPLE) was developed for a fast determination of polychlorinated biphenyls in sediment. The final method was performed at 100 degrees C with heptane/dichloromethane (90:10, v/v) as extraction solvent for 2x5 min. Sulfuric acid impregnated silica was placed downstream of the sample in the extraction cell to remove interfering components. This simultaneous extraction/clean-up was performed in 20 min, with an average congener recovery of 92% compared to a classical 24 h Soxhlet methodology and 2 h of external manual clean-up.

Synthesis of sn-1 functionalized phospholipids as substrates for secretory phospholipase A2.

Secretory phospholipase A2 (sPLA2) represents a family of small water-soluble enzymes that catalyze the hydrolysis of phospholipids in the sn-2 position liberating free fatty acids and lysophospholipids. Herein we report the synthesis of two new phospholipids (1 and 2) with bulky allyl-substituents attached to the sn-1 position of the glycerol backbone. The synthesis of phospholipids 1 and 2 is based upon the construction of a key aldehyde intermediate 3 which locks the stereochemistry in the sn-2 position of the final phospholipids. The aldehyde functionality serves as the site for insertion of the allyl-substituents by a zinc mediated allylation. Small unilamellar liposomes composed of phospholipids 1 and 2 were subjected to sPLA2 activity measurements. Our results show that only phospholipid 1 is hydrolyzed by the enzyme. Molecular dynamics simulations revealed that the lack of hydrolysis of phospholipid 2 is due to steric hindrance caused by the bulky side chain of the substrate allowing only limited access of water molecules to the active site.

Acylation of salmon calcitonin modulates in vitro intestinal peptide flux through membrane permeability enhancement.

Acylation of peptide drugs with fatty acid chains has proven beneficial for prolonging systemic circulation, as well as increasing enzymatic stability and interactions with lipid cell membranes. Thus, acylation offers several potential benefits for oral delivery of therapeutic peptides, and we hypothesize that tailoring the acylation may be used to optimize intestinal translocation. This work aims to characterize acylated analogues of the therapeutic peptide salmon calcitonin (sCT), which lowers blood calcium, by systematically increasing acyl chain length at two positions, in order to elucidate its influence on intestinal cell translocation and membrane interaction. We find that acylation drastically increases in vitro intestinal peptide flux and confers a transient permeability enhancing effect on the cell layer. The analogues permeabilize model lipid membranes, indicating that the effect is due to a solubilization of the cell membrane, similar to transcellular oral permeation enhancers. The effect is dependent on pH, with larger effect at lower pH, and is impacted by acylation chain length and position. Compared to the unacylated peptide backbone, N-terminal acylation with a short chain provides 6- or 9-fold increase in peptide translocation at pH 7.4 and 5.5, respectively. Prolonging the chain length appears to hamper translocation, possibly due to self-association or aggregation, although the long chain acylated analogues remain superior to the unacylated peptide. For K(18)-acylation a short chain provides a moderate improvement, whereas medium and long chain analogues are highly efficient, with a 12-fold increase in permeability compared to the unacylated peptide backbone, on par with currently employed oral permeation enhancers. For K(18)-acylation the medium chain acylation appears to be optimal, as elongating the chain causes greater binding to the cell membrane but similar permeability, and we speculate that increasing the chain length further may decrease the permeability. In conclusion, acylated sCT acts as its own in vitro intestinal permeation enhancer, with reversible effects on Caco-2 cells, indicating that acylation of sCT may represent a promising tool to increase intestinal permeability without adding oral permeation enhancers.

Acylation of Glucagon-like peptide-2: interaction with lipid membranes and in vitro intestinal permeability.

BACKGROUND: Acylation of peptide drugs with fatty acid chains has proven beneficial for prolonging systemic circulation as well as increasing enzymatic stability without disrupting biological potency. Acylation has furthermore been shown to increase interactions with the lipid membranes of mammalian cells. The extent to which such interactions hinder or benefit delivery of acylated peptide drugs across cellular barriers such as the intestinal epithelia is currently unknown. The present study investigates the effect of acylating peptide drugs from a drug delivery perspective. PURPOSE: We hypothesize that the membrane interaction is an important parameter for intestinal translocation, which may be used to optimize the acylation chain length for intestinal permeation. This work aims to characterize acylated analogues of the intestinotrophic Glucagon-like peptide-2 by systematically increasing acyl chain length, in order to elucidate its influence on membrane interaction and intestinal cell translocation in vitro. RESULTS: Peptide self-association and binding to both model lipid and cell membranes was found to increase gradually with acyl chain length, whereas translocation across Caco-2 cells depended non-linearly on chain length. Short and medium acyl chains increased translocation compared to the native peptide, but long chain acylation displayed no improvement in translocation. Co-administration of a paracellular absorption enhancer was found to increase translocation irrespective of acyl chain length, whereas a transcellular enhancer displayed increased synergy with the long chain acylation. CONCLUSIONS: These results show that membrane interactions play a prominent role during intestinal translocation of an acylated peptide. Acylation benefits permeation for shorter and medium chains due to increased membrane interactions, however, for longer chains insertion in the membrane becomes dominant and hinders translocation, i.e. the peptides get 'stuck' in the cell membrane. Applying a transcellular absorption enhancer increases the dynamics of membrane insertion and detachment by fluidizing the membrane, thus facilitating its effects primarily on membrane associated peptides.

Secretory phospholipase A2 activity toward diverse substrates.

We have studied secretory phospholipase A(2)-IIA (sPLA(2)) activity toward different phospholipid analogues by performing biophysical characterizations and molecular dynamics simulations. The phospholipids were natural substrates, triple alkyl phospholipids, a prodrug anticancer etherlipid, and an inverted ester. The latter were included to study head group-enzyme interactions. Our simulation results show that the lipids are optimally placed into the binding cleft and that water molecules can freely reach the active site through a well-defined pathway; both are indicative that these substrates are efficiently hydrolyzed, which is in good agreement with our experimental data. The phospholipid analogue with three alkyl side chains forms aggregates of different shapes with no well-defined sizes due to its cone-shape structure. Phosphatidylglycerol and phosphatidylcholine head groups interact with specific charged residues, but relatively large fluctuations are observed, suggesting that these interactions are not necessarily important for stabilizing substrate binding to the enzyme.

Synthesis and biophysical characterization of chlorambucil anticancer ether lipid prodrugs.

The synthesis and biophysical characterization of four prodrug ether phospholipid conjugates are described. The lipids are prepared from the anticancer drug chlorambucil and have C16 and C18 ether chains with phosphatidylcholine or phosphatidylglycerol headgroups. All four prodrugs have the ability to form unilamellar liposomes (86-125 nm) and are hydrolyzed by phospholipase A(2), resulting in chlorambucil release. Liposomal formulations of prodrug lipids displayed cytotoxicity toward HT-29, MT-3, and ES-2 cancer cell lines in the presence of phospholipase A(2), with IC(50) values in the 8-36 microM range.

Domain-induced activation of human phospholipase A2 type IIA: local versus global lipid composition.

Secretory human phospholipase A2 type IIA (PLA2-IIA) catalyzes the hydrolysis of the sn-2 ester bond in glycerolipids to produce fatty acids and lysolipids. The enzyme is coupled to the inflammatory response, and its specificity toward anionic membrane interfaces suggests a role as a bactericidal agent. PLA2-IIA may also target perturbed native cell membranes that expose anionic lipids to the extracellular face. However, anionic lipid contents in native cells appear lower than the threshold levels necessary for activation. By using phosphatidylcholine/phosphatidylglycerol model systems, we show that local enrichment of anionic lipids into fluid domains triggers PLA2-IIA activity. In addition, the compositional range of enzyme activity is shown to be related to the underlying lipid phase diagram. A comparison is done between PLA2-IIA and snake venom PLA2, which in contrast to PLA2-IIA hydrolyzes both anionic and zwitterionic membranes. In general, this work shows that PLA2-IIA activation can be accomplished through local enrichment of anionic lipids into domains, indicating a mechanism for PLA2-IIA to target perturbed native membranes with low global anionic lipid contents. The results also show that the underlying lipid phase diagram, which determines the lipid composition at a local level, can be used to predict PLA2-IIA activity.