β-hairpin peptide that targets vascular endothelial growth factor (VEGF) receptors: design, NMR characterization, and biological activity

Antibacterial and Angiogenic (2A) Bio-Heterojunctions Facilitate Infectious Ischemic Wound Regeneration via an Endogenous-Exogenous Bistimulatory Strategy

In infectious ischemic wounds, a lack of blood perfusion significantly worsens microbe-associated infection symptoms and frequently complicates healing. To overcome this daunting issue, antibacterial and angiogenic (2A) bio-heterojunctions (bio-HJs) consisting of CuS/MXene heterojunctions and a vascular endothelial growth factor (VEGF)-mimicking peptide (VMP) are devised and developed to accelerate infectious cutaneous regeneration by boosting angiogenesis via an endogenous-exogenous bistimulatory (EEB) strategy. Assisted by near-infrared irradiation, the bio-HJ platform exhibits versatile synergistic photothermal, photodynamic, and chemodynamic effects for robust antibacterial efficacy. In addition, copper ions liberated from 2A bio-HJs elevate VEGF secretion from fibroblasts, which provokes VEGF receptors (VEGFR) activation through an endogenous pathway, whereas VMP itself promotes an exogenous pathway to facilitate endothelial cell multiplication and tube formation by directly activating the VEGFR signaling pathway. Moreover, employing an in vivo model of infectious ischemic wounds, it is confirmed that the EEB strategy can considerably boost cutaneous regeneration through pathogen elimination, angiogenesis promotion, and collagen deposition. As envisaged, this work leads to the development of a powerful 2A bio-HJ platform that can serve as an effective remedy for bacterial invasion-induced ischemic wounds through the EEB strategy.

Structure-Based Design of Peptides Targeting VEGF/VEGFRs

Vascular endothelial growth factor (VEGF) and its receptors (VEGFRs) play a main role in the regulation of angiogenesis and lymphangiogenesis. Furthermore, they are implicated in the onset of several diseases such as rheumatoid arthritis, degenerative eye conditions, tumor growth, ulcers and ischemia. Therefore, molecules able to target the VEGF and its receptors are of great pharmaceutical interest. Several types of molecules have been reported so far. In this review, we focus on the structure-based design of peptides mimicking VEGF/VEGFR binding epitopes. The binding interface of the complex has been dissected and the different regions challenged for peptide design. All these trials furnished a better understanding of the molecular recognition process and provide us with a wealth of molecules that could be optimized to be exploited for pharmaceutical applications.

Light-Regulated Angiogenesis via a Phototriggerable VEGF Peptidomimetic

The application of growth factor based therapies in regenerative medicine is limited by the high cost, fast degradation kinetics, and the multiple functions of these molecules in the cell, which requires regulated delivery to minimize side effects. Here a photoactivatable peptidomimetic of the vascular endothelial growth factor (VEGF) that allows the light-controlled presentation of angiogenic signals to endothelial cells embedded in hydrogel matrices is presented. A photoresponsive analog of the 15-mer peptidomimetic Ac-KLTWQELYQLKYKGI-NH₂ (abbreviated ^P QK) is prepared by introducing a 3-(4,5-dimethoxy-2-nitrophenyl)-2-butyl (DMNPB) photoremovable protecting group at the Trp4 residue. This modification inhibits the angiogenic potential of the peptide temporally. Light exposure of ^P QK modified hydrogels provide instructive cues to embedded endothelial cells and promote angiogenesis at the illuminated sites of the 3D culture, with the possibility of spatial control. ^P QK modified photoresponsive biomaterials offer an attractive approach for the dosed delivery and spatial control of pro-angiogenic factors to support regulated vascular growth by just using light as an external trigger.

Metabolic and conformational stabilization of a VEGF-mimetic beta-hairpin peptide by click-chemistry

HPLW is a Vascular Endothelial Growth Factor (VEGF)-mimicking beta-hairpin peptide endowed of proangiogenic effect and showing valuable biomedical application in the proangiogenic therapy. However, the translational potential of HPLW is limited by its low metabolic stability, which would shorten the in vivo efficacy of the molecule. Here, we developed a peptide analog of HPLW, named HPLW2, that retains the structural and biological properties of the original peptide but features an impressive resistance to degradation by human serum proteases. HPLW2 was obtained by covalently modifying the chemical structure of the peptide with molecular tools known to impart protease resistance. Notably, the peptide was cyclized by installing an interstrand triazole bridge through Cu(I)-catalyzed azide-alkyne 1,3-dipolar cycloaddition (CuAAC) reaction. HPLW2 appears as a novel and promising drug candidate with potential biomedical application in the proangiogenic therapy as a low molecular weight drug, alternative to the use of VEGF. Our work points out the utility of the interstrand triazole bridge as effective chemical platform for the conformational and metabolic stabilization of beta-hairpin bioactive peptides.

Effect of the hairpin structure of peptide inhibitors on the blockade of PD-1/PD-L1 axis

Blockade of the PD-1/PD-L1 axis using antibody drugs has been a clinically efficacious immunotherapy in cancer treatment. However, studies on peptide inhibitors blocking the interaction between PD-1/PD-L1 in cancer treatment in clinical practice have not yet been reported. In this study, a series of peptide inhibitors were synthesized based on a continuous sequence of 14 amino acids from PD-L1 and suitable modifications to form a hairpin structure. The effect of inhibitors on the blockage of PD-1/PD-L1 by increasing the stability of the hairpin structure was determined using BLI and co-culture models. The results showed that increasing the stability of the hairpin improved the affinity of inhibitors to PD-1 and increased IL-2 secretion. Therefore, modifying the hairpin structure of peptide inhibitors may be a useful approach to block the interaction between PD-1 and PD-L1.

Peptide-Based Functional Biomaterials for Soft-Tissue Repair

Synthetically derived peptide-based biomaterials are in many instances capable of mimicking the structure and function of their full-length endogenous counterparts. Combine this with the fact that short mimetic peptides are easier to produce when compared to full length proteins, show enhanced processability and ease of modification, and have the ability to be prepared under well-defined and controlled conditions; it becomes obvious why there has been a recent push to develop regenerative biomaterials from these molecules. There is increasing evidence that the incorporation of peptides within regenerative scaffolds can result in the generation of structural recognition motifs that can enhance cell attachment or induce cell signaling pathways, improving cell infiltration or promote a variety of other modulatory biochemical responses. By highlighting the current approaches in the design and application of short mimetic peptides, we hope to demonstrate their potential in soft-tissue healing while at the same time drawing attention to the advances made to date and the problems which need to be overcome to advance these materials to the clinic for applications in heart, skin, and cornea repair.

Design and characterization of hydrogel nanoparticles with tunable network characteristics for sustained release of a VEGF-mimetic peptide

Peptides that mimic the bioactivity of growth factors are rapidly emerging as therapeutics for a variety of drug delivery applications including therapeutic neovascularization. Neovascularization requires controlled and sustained delivery of proangiogenic factors to stimulate reperfusion of ischemic tissues. To this end, hydrogel nanoparticles were designed to provide sustained and tunable diffusion-based release of a pro-angiogenic peptide, QK. Inverse phase mini-emulsion polymerization (IPMP) was used to generate crosslinked poly(ethylene) glycol (PEG) hydrogel nanoparticles entrapped with the QK peptide. Peptide release kinetics were tuned through adjustments in nanoparticle crosslink density. This was achieved by altering the mole fraction of the crosslinking agent which allowed for the synthesis of low crosslink density (0.754 mmol cm^-3) and high crosslink density (0.810 mmol cm^-3) nanoparticles. Nanoparticle tracking analysis revealed narrow particle size distributions and similar particle sizes regardless of crosslink density (225 ± 75 nm and 233 ± 73 nm, for low and high crosslink density nanoparticles, respectively). The zeta potential was found to be -26 mV for blank nanoparticles and +4 mV in the case of QK-loaded nanoparticles. The resulting nanoparticle crosslink density impacted both peptide loading as well as release kinetics. In terms of cumulative fractional release and weight of peptide released per mass of nanoparticle, higher crosslink density nanoparticles resulted in slower peptide release kinetics. The IPMP process preserved the QK secondary structure and its bioactivity as confirmed using circular dichroism spectroscopy and a Matrigel tubulogenesis assay, respectively, with released peptide. The presented nanoparticles hold great potential for use as drug delivery carriers for stimulation of therapeutic neovascularization of ischemic tissues.

Unveiling a VEGF-mimetic peptide sequence in the IQGAP1 protein

The ability to modulate angiogenesis by chemical tools has several important applications in different scientific fields. With the perspective of finding novel proangiogenic molecules, we searched peptide sequences with a chemical profile similar to that of the QK peptide, a well described VEGF mimetic peptide. We found that residues 1617-1627 of the IQGAP1 protein show molecular features similar to those of the QK peptide sequence. The IQGAP1-derived synthetic peptide was analyzed by NMR spectroscopy and its biological activity was characterized in endothelial cells. These studies showed that this IQGAP1-derived peptide has a biological activity similar to that of VEGF and could be considered as a novel tool for reparative angiogenesis.

Conformational stabilization of a β-hairpin through a triazole–tryptophan interaction

Triazole and indole rings stabilize a β-hairpin conformation through an aromatic–aromatic interaction.

Optimization of a β-sheet-cap for long loop closure

Protein loops make up a large portion of the secondary structure in nature. But very little is known concerning loop closure dynamics and the effects of loop composition on fold stability. We have designed a small system with stable β-sheet structures, including features that allow us to probe these questions. Using paired Trp residues that form aromatic clusters on folding, we are able to stabilize two β-strands connected by varying loop lengths and composition (an example sequence: RWITVTI - loop - KKIRVWE). Using NMR and CD, both fold stability and folding dynamics can be investigated for these systems. With the 16 residue loop peptide (sequence: RWITVTI-(GGGGKK)₂ GGGG-KKIRVWE) remaining folded (ΔG_U  = 1.6 kJ/mol at 295K). To increase stability and extend the series to longer loops, we added an additional Trp/Trp pair in the loop flanking position. With this addition to the strands, the 16 residue loop (sequence: RWITVRIW-(GGGGKK)₂ GGGG-WKTIRVWE) supports a remarkably stable β-sheet (ΔG_U  = 6.3 kJ/mol at 295 K, T_m  = ∼55°C). Given the abundance of loops in binding motifs and between secondary structures, these constructs can be powerful tools for peptide chemists to study loop effects; with the Trp/Trp pair providing spectroscopic probes for assessing both stability and dynamics by NMR.

Targeting kinase signaling pathways with constrained peptide scaffolds

Kinases are amongst the largest families in the human proteome and serve as critical mediators of a myriad of cell signaling pathways. Since altered kinase activity is implicated in a variety of pathological diseases, kinases have become a prominent class of proteins for targeted inhibition. Although numerous small molecule and antibody-based inhibitors have already received clinical approval, several challenges may still exist with these strategies including resistance, target selection, inhibitor potency and in vivo activity profiles. Constrained peptide inhibitors have emerged as an alternative strategy for kinase inhibition. Distinct from small molecule inhibitors, peptides can provide a large binding surface area that allows them to bind shallow protein surfaces rather than defined pockets within the target protein structure. By including chemical constraints within the peptide sequence, additional benefits can be bestowed onto the peptide scaffold such as improved target affinity and target selectivity, cell permeability and proteolytic resistance. In this review, we highlight examples of diverse chemistries that are being employed to constrain kinase-targeting peptide scaffolds and highlight their application to modulate kinase signaling as well as their potential clinical implications.

Miniaturizing VEGF: Peptides mimicking the discontinuous VEGF receptor-binding site modulate the angiogenic response

The angiogenic properties of VEGF are mediated through the binding of VEGF to its receptor VEGFR2. The VEGF/VEGFR interface is constituted by a discontinuous binding region distributed on both VEGF monomers. We attempted to reproduce this discontinuous binding site by covalently linking into a single molecular entity two VEGF segments involved in receptor recognition. We designed and synthesized by chemical ligation a set of peptides differing in length and flexibility of the molecular linker joining the two VEGF segments. The biological activity of the peptides was characterized in vitro and in vivo showing a VEGF-like activity. The most biologically active mini-VEGF was further analyzed by NMR to determine the atomic details of its interaction with the receptor.

Side-chain-to-tail cyclization of ribosomally derived peptides promoted by aryl and alkyl amino-functionalized unnatural amino acids

A strategy for the production of side-chain-to-tail cyclic peptides from ribosomally derived polypeptide precursors is reported. Two genetically encodable unnatural amino acids, bearing either an aryl or alkyl amino group, were investigated for their efficiency toward promoting the formation of medium to large-sized peptide macrocycles via intein-mediated side-chain-to-C-terminus cyclization. While only partial cyclization was observed with precursor proteins containing para-amino-phenylalanine, efficient peptide macrocyclization could be achieved using O-2-aminoethyl-tyrosine as the reactive moiety. Conveniently, the latter was generated upon quantitative, post-translational reduction of the azido-containing counterpart, O-2-azidoethyl-tyrosine, directly in E. coli cells. This methodology could be successfully applied for the production of a 12 mer cyclic peptide with enhanced binding affinity for the model target protein streptavidin as compared to the acyclic counterpart (KD: 5.1 μM vs. 22.4 μM), thus demonstrating its utility toward the creation and investigation of novel, functional macrocyclic peptides.

Long range Trp-Trp interaction initiates the folding pathway of a pro-angiogenic β-hairpin peptide

HPLW, a designed VEGF (Vascular Endothelium Growth Factor) receptor-binding peptide, assumes a well folded β-hairpin conformation in water and is able to induce angiogenesis in vivo. In this study, we investigated at atomic resolution the thermal folding/unfolding pathway of HPLW by means of an original multi-technique approach combining DSC, NMR, MD and mutagenesis analyses. In particular, careful NMR investigation of the single proton melting temperatures together with DSC analysis accurately delineate the peptide folding mechanism, which is corroborated by computational folding/unfolding simulations. The HPLW folding process consists of two main events, which are successive but do not superimpose. The first folding step initiates at 320 K upon the hydrophobic collapse of the Trp5 and Trp13 side-chains which stabilizes the concurrent β-turn formation, whose COi-HNi + 3 hydrogen bond (Asp10 → Arg7) appears particularly stable. At 316 K, once the β-turn is completely formed, the two β-strands pair, very likely starting by Trp5 and Trp13, which thus play a key role also in the final step of the β-hairpin folding. Overall, here we describe a multi-state hierarchical folding pathway of a highly structured β-hairpin, which can be classified as a broken-zipper mechanism.

Neuroprotective Effect of VEGF-Mimetic Peptide QK in Experimental Brain Ischemia Induced in Rat by Middle Cerebral Artery Occlusion

We investigated the effect of the VEGF-mimetic peptide, QK, on ischemic brain damage and on blood-brain barrier permeability in the rat. QK administered by the intracerebroventricular, intravenous, or intranasal route caused a 40% decrease in ischemic brain damage induced by permanent occlusion of the middle cerebral artery relative to that in controls. No increase in the volume of the ischemic hemisphere compared to that of the contralateral nonischemic hemisphere was observed in rats treated with QK, suggesting that this peptide did not cause brain edema. The effect of QK on vessel permeability was evaluated by intravital pial microvessel videoimaging, a technique that allows the pial vessels to be visualized through a surgically prepared open cranial window. The results showed that QK did not cause any leakage of intravenously injected fluorescein-dextran conjugates after intracarotid administration or topical application to the brain cortex. Collectively, these data suggest that QK may exert neuroprotective activity in the context of stroke without promoting any increase in vascular permeability. Because VEGF's neuroprotective activity may be overshadowed by the appearance of brain edema and microbleeds, QK could represent a significant step forward in stroke treatment.

Functional binding surface of a β-hairpin VEGF receptor targeting peptide determined by NMR spectroscopy in living cells

In this study, the functional interaction of HPLW peptide with VEGFR2 (Vascular Endothelial Growth Factor Receptor 2) was determined by using fast (15)N-edited NMR spectroscopic experiments. To this aim, (15)N uniformly labelled HPLW has been added to Porcine Aortic Endothelial Cells. The acquisition of isotope-edited NMR spectroscopic experiments, including (15)N relaxation measurements, allowed a precise characterization of the in-cell HPLW epitope recognized by VEGFR2.

Design and synthesis of C-terminal modified cyclic peptides as VEGFR1 antagonists

Previously designed cyclic peptide antagonist c[YYDEGLEE]-NH2 disrupts the interaction between vascular endothelial growth factor (VEGF) and its receptors (VEGFRs). It represents a promising tool in the fight against cancer and age-related macular degeneration. We described in this paper the optimization of the lead peptide by C-terminal modification. A new strategy for the synthesis of cyclic peptides is developed, improving the cyclisation efficiency. At 100 µM, several new peptides with an aromatic group flexibly linked at C-terminal end showed significantly increased receptor binding affinities in competition ELISA test. The most active peptide carrying a coumarin group may be a useful tool in anti-angiogenic biological studies.

Design of monomeric water-soluble β-hairpin and β-sheet peptides

Since the first report in 1993 (JACS 115, 5887-5888) of a peptide able to form a monomeric β-hairpin structure in aqueous solution, the design of peptides forming either β-hairpins (two-stranded antiparallel β-sheets) or three-stranded antiparallel β-sheets has become a field of growing interest and activity. These studies have yielded great insights into the principles governing the stability and folding of β-hairpins and antiparallel β-sheets. This chapter provides an overview of the reported β-hairpin/β-sheet peptides focussed on the applied design criteria, reviews briefly the factors contributing to β-hairpin/β-sheet stability, and describes a protocol for the de novo design of β-sheet-forming peptides based on them. Guidelines to select appropriate turn and strand residues and to avoid self-association are provided. The methods employed to check the success of new designed peptides are also summarized. Since NMR is the best technique to that end, NOEs and chemical shifts characteristic of β-hairpins and three-stranded antiparallel β-sheets are given.

Functional and pharmacological characterization of a VEGF mimetic peptide on reparative angiogenesis

Vascular endothelial growth factor (VEGF) is the main regulator of physiological and pathological angiogenesis. Low molecular weight molecules able to stimulate angiogenesis have interesting medical application for example in regenerative medicine, but at present none has reached the clinic. We reported that a VEGF mimetic helical peptide, QK, designed on the VEGF helix sequence 17-25, is able to bind and activate the VEGF receptors, producing angiogenesis. In this study we evaluate the pharmacological properties of peptide QK with the aim to propose it as a VEGF-mimetic drug to be employed in reparative angiogenesis. We show that the peptide QK is able to recapitulate all the biological activities of VEGF in vivo and on endothelial cells. In experiments evaluating sprouting from aortic ring and vessel formation in an in vivo angiogenesis model, the peptide QK showed biological effects comparable with VEGF. At endothelial level, the peptide up-regulates VEGF receptor expression, activates intracellular pathways depending on VEGFR2, and consistently it induces endothelial cell proliferation, survival and migration. When added to angiogenic factors (VEGF and/or FGF-2), QK produces an improved biological action, which resulted in reduced apoptosis and accelerated in vitro wound healing. The VEGF-like activity of the short peptide QK, characterized by lower cost of production and easier handling compared to the native glycoprotein, suggests that it is an attractive candidate to be further developed for application in therapeutic angiogenesis.