NMR relaxation and structural elucidation of peptides in the presence and absence of trifluoroethanol illuminates the critical molecular nature of integrin αvβ6 ligand specificity

Optimized serum stability and specificity of an αvβ6 integrin-binding peptide for tumor targeting

The integrin αvβ6 is an antigen expressed at low levels in healthy tissue but upregulated during tumorigenesis, which makes it a promising target for cancer imaging and therapy. A20FMDV2 is a 20-mer peptide derived from the foot-and-mouth disease virus that exhibits nanomolar and selective affinity for αvβ6 versus other integrins. Despite this selectivity, A20FMDV2 has had limited success in imaging and treating αvβ6+ tumors in vivo because of its poor serum stability. Here, we explore the cyclization and modification of the A20FMDV2 peptide to improve its serum stability without sacrificing its affinity and specificity for αvβ6. Using cysteine amino acid substitutions and cyclization by perfluoroarylation with decafluorobiphenyl, we synthesized six cyclized A20FMDV2 variants and discovered that two retained binding to αvβ6 with modestly improved serum stability. Further d-amino acid substitutions and C-terminal sequence optimization outside the cyclized region greatly prolonged peptide serum stability without reducing binding affinity. While the cyclized A20FMDV2 variants exhibited increased nonspecific integrin binding compared with the original peptide, additional modifications with the non-natural amino acids citrulline, hydroxyproline, and d-alanine were found to restore binding specificity, with some modifications leading to greater αvβ6 integrin selectivity than the original A20FMDV2 peptide. The peptide modifications detailed herein greatly improve the potential of utilizing A20FMDV2 to target αvβ6 in vivo, expanding opportunities for cancer targeting and therapy.

Tumour targeted contrast enhanced ultrasound imaging dual-modal microbubbles for diagnosis and treatment of triple negative breast cancer

At present, the treatment of triple negative breast cancer (TNBC) is a worldwide problem, urgently requiring early precise diagnosis and effective treatment methods. In our study, we designed a type of tumour targeted dual-modal microbubbles, paclitaxel (PTX)-loaded RGD-lipid microbubbles (PTX@RGD-MBs), combined with ultrasonic targeted microbubble destruction (UTMD) to precisely diagnose TNBC and to improve the curative effect. As the first-line drug, PTX, lacking specific tumour targeting and the ability to be effectively internalized by TNBC cells, is still not effective in killing TNBC cells. For this reason, we used the tumour active targeting peptide RGD to precisely guide MBs around TNBC cells through the high affinity to the integrin α_vβ₃ receptor, and use UTMD to generate shear stress on cells to open transient pores in the cellular membrane, so as to improve MB penetration into the cellular membrane. Sulfur hexafluoride (SF₆) as the internal gas of MBs can greatly improve the resolution and sensitivity of conventional US images through nonlinear harmonics to enhance the comparison between lesions and surrounding normal tissues. The results in our study demonstrated that RGD-MBs with UTMD were internalized by TNBC cells more effectively, leading to significant increase in intercellular drug concentrations of TNBC cells, thus achieving the best inhibitory effect on TNBC cells in vitro. Ultrasonic experiment showed that PTX@RGD-MBs produced high quality contrast enhanced ultrasound (CEUS) images in vitro and in vivo, providing a better method for diagnosis and evaluation of the TNBC. Therefore, we conclude the advantages of excellent anti-tumour effect and CEUS imaging of PTX@RGD-MBs provided a better application for diagnosis and treatment of TNBC.

Tumour targeted contrast-enhanced ultrasound imaging dual-modal microbubbles (PTX@RGD-MBs) combined with UTMD for diagnosing and treating triple negative breast cancer.

Stable Peptides Instead of Stapled Peptides: Highly Potent αvβ6-Selective Integrin Ligands

The αvβ6 integrin binds the RGD-containing peptide of the foot and mouth disease virus with high selectivity. In this study, the long binding helix of this ligand was downsized to an enzymatically stable cyclic peptide endowed with sub-nanomolar binding affinity toward the αvβ6 receptor and remarkable selectivity against other integrins. Computational studies were performed to disclose the molecular bases underlying the high binding affinity and receptor subtype selectivity of this peptide. Finally, the utility of the ligand for use in biomedical studies was also demonstrated here.

Q2DSTD NMR deciphers epitope-mapping variability for peptide recognition of integrin αvβ6

Integrin αvβ6 is a cell surface arginine-glycine-aspartic acid (RGD)-specific heterodimeric glycoprotein that is only expressed on epithelia during processes of tissue remodelling, including cancer. The specificity and molecular nature of interactions toward this integrin are poorly understood and new insights into such processes are important to cell biologists and pharmaceutical drug discovery. This study demonstrates the application of quantitative two-dimensional saturation transfer (Q2DSTD) NMR to obtain precise details of peptide interactions with integrin αvβ6 and their correlation to specificity for the integrin. This approach highlights subtle but significant differences in ligand contact by three related 21-mer peptides: FMDV2, an αvβ6 specific peptide and DBD1 and LAP2T1 peptides that bind many αv integrins in addition to αvβ6. FMDV2 and DBD1 differ only by the cyclisation of DBD1; a process that removes αvβ6 specificity. Q2DSTD NMR demonstrates these peptides experience significantly different interactions with the integrin; FMDV contacts primarily through four residues: 6Leu, 10Leu, 12Val and 13Leu, whereas DBD1 and LAP2T1 have more widespread contacts across their sequences. Q2DSTD NMR combined two-dimensional STD with quantitation by considering the relaxation of the ligand (CRL) to provide precise ligand contact information. This study also examines the role of CRL in the Q2DSTD process and how quantitation modifies STD data and unravels epitope-mapping variability to provide precise results that differentiate interactions at the atomic level for each peptide.

Solution structure of a bacterial microcompartment targeting peptide and its application in the construction of an ethanol bioreactor

Targeting of proteins to bacterial microcompartments (BMCs) is mediated by an 18-amino-acid peptide sequence. Herein, we report the solution structure of the N-terminal targeting peptide (P18) of PduP, the aldehyde dehydrogenase associated with the 1,2-propanediol utilization metabolosome from Citrobacter freundii. The solution structure reveals the peptide to have a well-defined helical conformation along its whole length. Saturation transfer difference and transferred NOE NMR has highlighted the observed interaction surface on the peptide with its main interacting shell protein, PduK. By tagging both a pyruvate decarboxylase and an alcohol dehydrogenase with targeting peptides, it has been possible to direct these enzymes to empty BMCs in vivo and to generate an ethanol bioreactor. Not only are the purified, redesigned BMCs able to transform pyruvate into ethanol efficiently, but the strains containing the modified BMCs produce elevated levels of alcohol.

NMR investigation of disulfide containing peptides and proteins

Peptides and proteins with disulfide bonds are abundant in all kingdoms and play essential role in many biological events. Because small disulfide-rich peptides (proteins) are usually difficult to crystallize, nuclear magnetic resonance (NMR) is by far one of the most powerful techniques for the determination of their solution structure. Besides the “static” three-dimensional structure, NMR has unique opportunities to acquire additional information about molecular dynamics and folding at atomic resolution. Nowadays it is becoming increasingly evident, that “excited”, “disordered” or “fuzzy” protein states may exhibit biological function and disulfide proteins are also promising targets for such studies. In this short two-three years overview those disulfide peptides and proteins were cited from the literature that were studied by NMR. Though we may have missed some, their structural diversity and complexity as well as their wide repertoire of biological functions is impressive. We emphasised especially antimicrobial peptides and peptide based toxins in addition to some biologically important other structures. Besides the general NMR methods we reviewed some contemporary techniques suitable for disclosing the peculiar properties of disulfide bonds. Interesting dynamics and folding studies of disulfide proteins were also mentioned. It is important to disclose the essential structure, dynamics, function aspects of disulfide proteins since this aids the design of new compounds with improved activity and reduced toxicity. Undoubtedly, NMR has the potential to accelerate the development of new disulfide peptides/proteins with pharmacological activity.