N-terminus FITC labeling of peptides on solid support: the truth behind the spacer

High cell selectivity and low-level antibacterial resistance of designed amphiphilic peptide G(IIKK)(3)I-NH(2)

On the basis of cell cultures involving bacterial strains (Escherichia coli 5α and Bacillus subtilis 168) and a mammalian cell line (NIH 3T3), the potent antibacterial activity and distinct selectivity from designed amphiphilic peptides G(IIKK)nI-NH2 (n = 2-4) have been demonstrated. This work extends these studies to multidrug resistant pathogens (ESBL-producing E. coli) and primary human cells (HDFa), followed by the in vivo mouse model investigation of ESBL-producing bacterial infection. G(IIKK)3I-NH2 exhibits high antibacterial activity against the pathogenic strain both in vitro and in vivo while displaying low toxicity toward the primary cells and the mice. Peptide molecules can kill bacteria by selectively interacting with bacterial membranes, causing structural disruptions. Furthermore, multidrug resistant ESBL-producing bacteria do not develop resistance after multiple treatments with G(IIKK)3I-NH2. The high cellular selectivity, low toxicity toward mammalian hosts and noninducing bacterial resistance indicate great potential for developing the peptides as anti-infection agents.

Integration of surface-active, periodically sequenced peptides into lipid-based microbubbles

The development of microbubbles toward functional, "theranostic" particles requires the incorporation of constituents with high binding specificity and therapeutic efficacy. Integrating peptides or proteins into the shell of lipid-based microbubbles can provide a means to access both receptor-ligand interactions and therapeutic properties. Simultaneously, peptides or proteins can define the characteristic monolayer mechanics of lipid bubbles and eliminate the need for post-bubble generation modification. The ability to engineer peptide sequences de novo that effectively partition into the bubble monolayer remains parametrically daunting. This work contributes to this effort using two simple amphipathic helical peptides that examine the role of local electrostatics and secondary structure. The two periodically sequenced peptides both have three positive charges, but peptide "K-2.5" spaces those charges 2.5 amino acids apart, while peptide "K-6.0" spaces the charges six amino acids apart. Size populations were determined for bubbles containing each peptide species using light scattering, and a quantitative method was developed to clearly define the fraction of peptides binding onto the microbubble monolayer. The impact of both the initial peptide concentration and the zwitterionic:anionic lipid ratio on peptide binding was also evaluated. Our results indicate that the lipid ratio affected only K-6.0 binding, which appears to be an outcome of the greater ensemble average α-helical population of the K-6.0. These findings provide further insights into the role of charge separation on peptide secondary structure, establishing a simple design metric for peptide binding onto microbubble systems.

Targeting folded RNA: a branched peptide boronic acid that binds to a large surface area of HIV-1 RRE RNA

On-bead high-throughput screening of a medium-sized (1000-2000 Da) branched peptide boronic acid (BPBA) library consisting of 46,656 unique sequences against HIV-1 RRE RNA generated peptides with binding affinities in the low micromolar range. In particular, BPBA1 had a K(d) of 1.4 μM with RRE IIB, preference for RNA over DNA (27 fold), and selectivity of up to >75 fold against a panel of RRE IIB variants. Structure-activity studies suggest that the boronic acid moiety and "branching" in peptides are key structural features for efficient binding and selectivity for the folded RNA target. BPBA1 was efficiently taken up by HeLa and A2780 cells. RNA-footprinting studies revealed that the BPBA1 binding site encompasses a large surface area that spans both the upper stem as well as the internal loop regions of RRE IIB.

Dipeptide mimic oligomer transporter mediates intracellular delivery of Cathepsin D inhibitors: a potential target for cancer therapy

Implication of the intracellular proteolytic activity of Cathepsin D (CathD), a lysosomal aspartyl-protease overexpressed in numerous solid tumors, has been evidenced on tumor growth. Its intracellular inhibition by potent inhibitors such as pepstatin constitutes a relevant but challenging molecular target. Indeed the potential of pepstatin as a therapeutic molecule is hampered by its too low intracellular penetration. We addressed this limitation by designing and developing a bioconjugate combining a pepstatin derivative with a new vector of cell penetration (CPNP) specifically targeting the endolysosomal compartment. We showed that this pepstatin conjugate (JMV4463) exhibited high anti-proliferative effect on tumor cell cultures via intracellular CathD inhibition and altered cell cycle associated with apoptotic events in vitro. When tested in mice xenografted with breast cancer cells, JMV4463 delayed tumor emergence and growth.

Peptide release, side-chain deprotection, work-up, and isolation

After having successfully synthesized a peptide, it has to be released from the solid support, unless it is being used for on-resin display. The linker and, in some cases, the cleavage mixture determine the C-terminal functionality of the released peptide. In most cases, the peptide is released with concomitant removal of side-chain protecting groups. However, some combinations of linkers and side-chain protecting groups enable a two-stage procedure, either using orthogonal chemistry or graduated labilities. Herein, we describe protocols for the release of peptides from the most commonly used linker types providing a variety of different C-terminal functionalities, including acids, amides, amines, and aldehydes. Moreover, suggestions for determination of peptide purity and for storage conditions are provided.

Biochemical and pharmacological characterization of nuclear urotensin-II binding sites in rat heart

BACKGROUND AND PURPOSE During the past decade, a few GPCRs have been characterized at the nuclear membrane where they exert complementary physiological functions. In this study, we investigated (1) the presence of a functional urotensin-II (U-II) receptor (UT) in rat heart nuclear extracts and (2) the propensity of U-II and U-II-related peptide (URP) to cross the plasma membrane in a receptor-independent manner. EXPERIMENTAL APPROACH Biochemical and pharmacological methods including competitive binding assays, photoaffinity labelling, immunoblotting as well as de novo RNA synthesis were used to characterize the presence of functional UT receptors in rat heart nuclei. In addition, confocal microscopy and flow cytometry analysis were used to investigate the cellular uptake of fluorescent U-II and URP derivatives. KEY RESULTS The presence of specific U-II binding sites was demonstrated in rat heart nuclear extracts. Moreover, such subcellular localization was also observed in monkey heart extracts. In vitro transcription initiation assays on rat, freshly isolated, heart nuclei suggested that nuclear UT receptors are functional, and that U-II, but not URP, participates in nuclear UT-associated gene expression. Surprisingly, hU-II and URP efficiently crossed the plasma membrane in a receptor-independent mechanism involving endocytosis through caveolin-coated pits; this uptake of hU-II, but not that of URP, was dependent on extracellular pH. CONCLUSION Our results suggest that (1) U-II and URP can differentially modulate nuclear UT functions such as gene expression, and (2) both ligands can reach the internal cellular space through a receptor-independent mechanism.

Peptide-drug conjugate linked via a disulfide bond for kidney targeted drug delivery

Chronic kidney disease (CKD) is a worldwide public health problem, and unfortunately, the therapeutic index of clinically available drugs is limited. Thus, there is a great need to exploit effective treatment strategies, and the carrier-drug approach is an attractive method to improve the kidney specificity of the therapeutic agents. The aim of this present study is to develop a peptide-drug conjugate for the kidney targeted delivery of angiotensin-converting enzyme (ACE) inhibitor captopril (CAP), since G3-C12 peptide (ANTPCGPYTHDCPVKR) could specifically accumulate in the kidney after intravenous injection. Therefore, FITC labeled G3-C12 peptide (G3-C12-FITC) and peptide-drug conjugate (G3-C12-CAP) with a disulfide bond which can be cleaved by reduced glutathione in the kidney were prepared by solid-phase peptide synthesis. The fluorescence imaging of G3-C12-FITC revealed that the labeled peptide specifically accumulated in the kidney soon after i.v. injection to mice, and the accumulation is due largely to the reabsorption of the peptide by the proximal renal tubule cells. Furthermore, in comparison with the corresponding nonconjugated form, a 2.7-fold increase in renal area under concentration-time curve produced by the conjugate was observed in mice. Interestingly, the CAP entirely released in the kidney even at 0.05 h postinjection through disulfide reduction. As a consequence, the in vivo renal ACE inhibition was significantly increased. In conclusion, these findings suggest the potential of G3-C12 peptide serving as a suitable candidate carrier for kidney-targeted drug delivery.

Site-specific fluorescein labeling of human insulin

Three fluorescein derivatives of human insulin (HI, 1) labeled at positions N(αA1) , N(αB1) and N(εB29) respectively, were synthesized using an N-trifluoroacetyl-based protecting group scheme. The Tfa protecting group introduced by reaction with ethyl trifluoroacetate was found to be stable in aqueous and organic media and efficiently removed under mild basic conditions.

Toward targeting RNA structure: branched peptides as cell-permeable ligands to TAR RNA

Rational design of RNA ligands continues to be a formidable challenge, but the potential powerful applications in biology and medicine catapults it to the forefront of chemical research. Indeed, small molecule and macromolecular intervention are attractive approaches, but selectivity and cell permeability can be a hurdle. An alternative strategy is to use molecules of intermediate molecular weight that possess large enough surface area to maximize interaction with the RNA structure but are small enough to be cell-permeable. Herein, we report the discovery of nontoxic and cell-permeable branched peptide (BP) ligands that bind to TAR RNA in the low micromolar range from on-bead high-throughput screening of 4,096 compounds. TAR is a short RNA motif in the 5'-UTR of HIV-1 that is responsible for efficient generation of full RNA transcripts. We demonstrate that BPs are selective for the native TAR RNA structure and that "branching" in peptides provides multivalent interaction, which increases binding affinity to RNA.

Stapled peptides for intracellular drug targets

Proteins that engage in intracellular interactions with other proteins are widely considered among the most biologically appealing yet chemically intractable targets for drug discovery. The critical interaction surfaces of these proteins typically lack the deep hydrophobic involutions that enable potent, selective targeting by small organic molecules, and their localization within the cell puts them beyond the reach of protein therapeutics. Considerable interest has therefore arisen in next-generation targeting molecules that combine the broad target recognition capabilities of protein therapeutics with the robust cell-penetrating ability of small molecules. One type that has shown promise in early-stage studies is hydrocarbon-stapled α-helical peptides, a novel class of synthetic miniproteins locked into their bioactive α-helical fold through the site-specific introduction of a chemical brace, an all-hydrocarbon staple. Stapling can greatly improve the pharmacologic performance of peptides, increasing their target affinity, proteolytic resistance, and serum half-life while conferring on them high levels of cell penetration through endocytic vesicle trafficking. Here, we discuss considerations crucial to the successful design and evaluation of potent stapled peptide interactions, our intention being to facilitate the broad application of this technology to intractable targets of both basic biologic interest and potential therapeutic value.

Optimization of fluorescently labeled Nrf2 peptide probes and the development of a fluorescence polarization assay for the discovery of inhibitors of Keap1-Nrf2 interaction

Activation of the antioxidant response element (ARE) upregulates enzymes involved in detoxification of electrophiles and reactive oxygen species. The induction of ARE genes is regulated by the interaction between redox sensor protein Keap1 and the transcription factor Nrf2. Fluorescently labeled Nrf2 peptides containing the ETGE motif were synthesized and optimized as tracers in the development of a fluorescence polarization (FP) assay to identify small-molecule inhibitors of the Keap1-Nrf2 interaction. The tracers were optimized to increase the dynamic range of the assay and their binding affinities to the Keap1 Kelch domain. The binding affinities of Nrf2 peptide inhibitors obtained in our FP assay using FITC-9mer Nrf2 peptide amide as the probe were in good agreement with those obtained previously by a surface plasmon resonance assay. The FP assay exhibits considerable tolerance toward DMSO and produced a Z' factor greater than 0.6 in a 384-well format. Further optimization of the probe led to cyanine-labeled 9mer Nrf2 peptide amide, which can be used along with the FITC-9mer Nrf2 peptide amide in a high-throughput screening assay to discover small-molecule inhibitors of Keap1-Nrf2 interaction.

Designed antimicrobial and antitumor peptides with high selectivity

We report a new class of cationic amphiphilic peptides with short sequences, G(IIKK)(n)I-NH(2) (n = 1-4), that can kill Gram-positive and Gram-negative bacteria as effectively as several well-known antimicrobial peptides and antibiotics. In addition, some of these peptides possess potent antitumor activities against cancer cell lines. Moreover, their hemolytic activities against human red blood cells (hRBCs) remain remarkably low even at some 10-fold bactericidal minimum inhibitory concentrations (MICs). When bacteria or tumor cells are cocultured with NIH 3T3 fibroblast cells, G(IIKK)(3)I-NH(2) showed fast and strong selectivity against microbial or tumor cells, without any adverse effect on NIH 3T3 cells. The high selectivity and associated features are attributed to two design tactics: the use of Ile residues rather than Leu and the perturbation of the hydrophobic face of the helical structure with the insertion of a positively charged Lys residue. This class of simple peptides hence offers new opportunities in the development of cost-effective and highly selective antimicrobial and antitumor peptide-based treatments.

Structure-activity relationship of conformationally constrained peptidomimetics for antiproliferative activity in HER2-overexpressing breast cancer cell lines

Human epidermal growth factor receptor 2 (HER2) is a member of the human epidermal growth factor receptor kinases and is involved in a signaling cascade for cell growth and differentiation. It is well established that HER2-mediated heterodimerization has important implications in cancer. Deregulation of signaling pathways and overexpression of HER2 is known to occur in cancer cells, indicating the role of HER2 in tumorigenesis. Therefore, blocking HER2-mediated signaling has potential therapeutic value. We have designed several peptidomimetics to inhibit HER2-mediated signaling for cell growth. One of the compounds (compound 5, Arg-[3-amino-3(1-napthyl)-propionic acid]-Phe) exhibited antiproliferative activity with IC(50) values in the nanomolar to micromolar range in breast cancer cell lines. To further investigate the structure-activity relationship of the compounds, various analogs of compound 5 were designed. Conformational constraints were initiated in the peptidomimetic with introduction of a Pro residue in the peptidomimetic sequence. Results of antiproliferative activity indicated that analogs of compound 5 with C-and N-terminal ends capped (compound 16) and compound 9 with Asp at the C-terminal exhibited antiproliferative activity in the lower micromolar range against breast cancer cell lines. Introduction of conformational constraints such as Pro residue in the sequence or cyclization did not enhance the activity of the peptidomimetic. Competitive binding studies were carried out to evaluate the binding of potent peptidomimetics to HER2-overexpressing cancer cell lines. Results indicated that compounds exhibiting antiproliferative activity in breast cancer cell lines bind to the cells that overexpress HER2 protein.

Amphiphilic CCK peptides assembled in supramolecular aggregates: structural investigations and in vitro studies

Supramolecular aggregates obtained by self-aggregation of five new cationic amphiphilic CCK8 peptides have been obtained in water solution and characterized for: (i) aggregate structure and stability; (ii) CCK8 peptide conformation and bioavailability on the external aggregate surface; and (iii) for their cell binding properties. The cationic amphiphilic CCK8 peptides self-aggregate giving a combination of liposomal and micelle structures, with radii ranging between ~60 nm and ~90 nm, and between ~5 and ~10 nm, respectively. The presence of CCK8 peptide well-exposed on the aggregate surface is demonstrated by fluorescence measurements. Peptide conformation changes in the five supramolecular aggregates: the CCK8 conformational behaviour is probably induced by the presence of three charged lysine residues close to the bioactive peptide sequence. Only aggregates in which the CCK8 peptide presents a structural arrangement similar to that found for the same peptide in DPC micelles give promising binding properties to CCK2-R receptors overexpressed by transfected A431 cells. Chemical modifications on the CCK8 N-terminus seem to play an important role in stabilizing the peptide active conformation, either when the peptide derivative is in monomeric or in aggregate form. For their easy preparation procedures and their binding properties, supramolecular aggregates based on cationic peptide amphiphiles can be considered as promising candidates for target selective drug carriers on cancer cells.

Superhydrophilic Surfaces for Antifoqging and Antifouling Microfluidic Devices

Superhydrophilic surfaces are investigated for their potential to provide antifogging and antifouling properties for microfluidic devices. Two types of exemplary superhydrophilic surfaces are prepared, including polyester films treated by oxygen plasma and indium tin oxide-coated glasses treated by an electrochemical method. The superhydrophilicity of the treated surfaces presented herein is confirmed by their near-zero water contact angles. Their corresponding antifogging and antifouling capability is examined. The fluorescence microscopic study has confirmed the significantly reduced adhesion of the fluorescein and fluorescent proteins after the surfaces are treated to be superhydrophilic, indicating their potential for antifouling applications. The degradation of the superhydrophilicity under different humidity conditions is also investigated.