Substrate-Guided Design of a Potent and Selective Kallikrein-Related Peptidase Inhibitor for Kallikrein 4

Engineering the Cyclization Loop of MCoTI-II Generates Targeted Cyclotides that Potently Inhibit Factor XIIa

Factor XIIa (FXIIa) is a promising target for developing new drugs that prevent thrombosis without causing bleeding complications. A native cyclotide (MCoTI-II) is gaining interest for engineering FXIIa-targeted anticoagulants as this peptide inhibits FXIIa but not other coagulation proteases. Here, we engineered the native biosynthetic cyclization loop of MCoTI-II (loop 6) to generate improved FXIIa inhibitors. Decreasing the loop length led to gains in potency up to 7.7-fold, with the most potent variant having five residues in loop 6 (K_i = 25 nM). We subsequently examined sequence changes within loop 6 and an adjacent loop, with substitutions at P4 and P2' producing a potent FXIIa inhibitor (K_i = 2 nM) that displayed more than 700-fold selectivity, was stable in human serum, and blocked the intrinsic coagulation pathway in human plasma. These findings demonstrate that engineering the biosynthetic cyclization loop can generate improved cyclotide variants, expanding their potential for drug discovery.

Rational Design of Plant Hairpin-like Peptide EcAMP1: Structural-Functional Correlations to Reveal Antibacterial and Antifungal Activity

Plant antimicrobial peptides from the α-hairpinins family (hairpin-like peptides) are known to possess a wide range of biological activities. However, less is known about the structural determinants of their antimicrobial activity. Here, we suggest that spatial structure as well as surface charge and hydrophobicity level contribute to the antimicrobial properties of α-hairpinin EcAMP1 from barnyard grass (Echinochloa cruss-galli) seeds. To examine the role of the peptide spatial structure, two truncated forms of EcAMP1 restricted by inner and outer cysteine pairs were synthesized. It was shown that both truncated forms of EcAMP1 lost their antibacterial activity. In addition, their antifungal activity became weaker. To review the contribution of surface charge and hydrophobicity, another two peptides were designed. One of them carried single amino acid substitution from tryptophan to alanine residue at the 20th position. The second one represented a truncated form of the native EcAMP1 lacking six C-terminal residues. But the α-helix was kept intact. It was shown that the antifungal activity of both modified peptides weakened. Thereby we can conclude that the secondary structural integrity, hydrophobic properties, and surface charge all play roles in the antimicrobial properties of α-hairpinins. In addition, the antibacterial activity of cereal α-hairpinins against Gram-positive bacteria was described for the first time. This study expands on the knowledge of structure–function interactions in antimicrobial α-hairpinins.

Discrimination of Methionine Sulfoxide and Sulfone by Human Neutrophil Elastase

Human neutrophil elastase (HNE) is a uniquely destructive serine protease with the ability to unleash a wave of proteolytic activity by destroying the inhibitors of other proteases. Although this phenomenon forms an important part of the innate immune response to invading pathogens, it is responsible for the collateral host tissue damage observed in chronic conditions such as chronic obstructive pulmonary disease (COPD), and in more acute disorders such as the lung injuries associated with COVID-19 infection. Previously, a combinatorially selected activity-based probe revealed an unexpected substrate preference for oxidised methionine, which suggests a link to oxidative pathogen clearance by neutrophils. Here we use oxidised model substrates and inhibitors to confirm this observation and to show that neutrophil elastase is specifically selective for the di-oxygenated methionine sulfone rather than the mono-oxygenated methionine sulfoxide. We also posit a critical role for ordered solvent in the mechanism of HNE discrimination between the two oxidised forms methionine residue. Preference for the sulfone form of oxidised methionine is especially significant. While both host and pathogens have the ability to reduce methionine sulfoxide back to methionine, a biological pathway to reduce methionine sulfone is not known. Taken together, these data suggest that the oxidative activity of neutrophils may create rapidly cleaved elastase "super substrates" that directly damage tissue, while initiating a cycle of neutrophil oxidation that increases elastase tissue damage and further neutrophil recruitment.

Binding loop of sunflower trypsin inhibitor 1 serves as a design motif for proteolysis-resistant antimicrobial peptides

Although antimicrobial peptides (AMPs) have become powerful drug candidates in the post-antibiotic era, but their low protease stability hinders their clinical application. In the present study, the natural sunflower trypsin inhibitor 1 (SFTI-1) binding loop (CTKSIPPIC) was used to design and synthesize a specific anti-proteolytic sequence template ((RX)_n W (RX)_n CTKSIPPIC (n = 2, 3; X represents A, I, L, V, F, and W)). After several antibacterial, bactericidal, and toxicity tests, RV3 stood out from the variants and had the highest average selectivity index (SI _all = 156.03). It is highly stable in serum, varying pH, temperature, and salt ions as well as under high trypsin, pepsin, or papain concentrations. In a mouse skin inflammation model, established by Pseudomonas aeruginosa infection, RV3 could effectively kill the pathogen, promote wound healing, inhibit inflammatory cell infiltration, and inhibit mRNA and protein expression of TNF-α, IL-6, and IL-1β inflammatory factors. The antibacterial mechanisms of RV3 include combining with lipopolysaccharides and increasing cell membrane permeability, leading to cell membrane rupture and death. These findings indicate that RV3 has great potential for the treatment of bacterial infections.

Exploring and exploiting plant cyclic peptides for drug discovery and development

Ever since the discovery of insulin, natural peptides have become an important resource for therapeutic development. Decades of research has led to the discovery of a long list of peptide drugs with broad applications in clinics, from antibiotics to hypertension treatment to pain management. Many of these US FDA-approved peptide drugs are derived from microorganisms and animals. By contrast, the great potential of plant cyclic peptides as therapeutics remains largely unexplored. These macrocyclic peptides typically have rigid structures, good bioavailability and membrane permeability, making them appealing candidates for drug development and engineering. In this review, we introduce the three major classes of plant cyclic peptides and summarize their potential medical applications. We discuss how we can leverage the genome information of many different plants to quickly search for new cyclic peptides and how we can take advantage of the insights gained from their biosynthetic pathways to transform the process of production and drug development. These recent developments have provided a new angle for exploring and exploiting plant cyclic peptides, and we believe that many more peptide drugs derived from plants are about to come.

Sunflower Trypsin Inhibitor-1 (SFTI-1): Sowing Seeds in the Fields of Chemistry and Biology

Nature-derived cyclic peptides have proven to be a vast source of inspiration for advancing modern pharmaceutical design and synthetic chemistry. The focus of this Review is sunflower trypsin inhibitor-1 (SFTI-1), one of the smallest disulfide-bridged cyclic peptides found in nature. SFTI-1 has an unusual biosynthetic pathway that begins with a dual-purpose albumin precursor and ends with the production of a high-affinity serine protease inhibitor that rivals other inhibitors much larger in size. Investigations on the molecular basis for SFTI-1's rigid structure and adaptable function have planted seeds for thought that have now blossomed in several different fields. Here we survey these applications to highlight the growing potential of SFTI-1 as a versatile template for engineering inhibitors, a prototypic peptide for studying inhibitory mechanisms, a stable scaffold for grafting bioactive peptides, and a model peptide for evaluating peptidomimetic motifs and platform technologies.

Bowman-Birk Inhibitors: Insights into Family of Multifunctional Proteins and Peptides with Potential Therapeutical Applications

Bowman-Birk inhibitors (BBIs) are found primarily in seeds of legumes and in cereal grains. These canonical inhibitors share a highly conserved nine-amino acids binding loop motif CTP1SXPPXC (where P1 is the inhibitory active site, while X stands for various amino acids). They are natural controllers of plants' endogenous proteases, but they are also inhibitors of exogenous proteases present in microbials and insects. They are considered as plants' protective agents, as their elevated levels are observed during injury, presence of pathogens, or abiotic stress, i.a. Similar properties are observed for peptides isolated from amphibians' skin containing 11-amino acids disulfide-bridged loop CWTP1SXPPXPC. They are classified as Bowman-Birk like trypsin inhibitors (BBLTIs). These inhibitors are resistant to proteolysis and not toxic, and they are reported to be beneficial in the treatment of various pathological states. In this review, we summarize up-to-date research results regarding BBIs' and BBLTIs' inhibitory activity, immunomodulatory and anti-inflammatory activity, antimicrobial and insecticidal strength, as well as chemopreventive properties.

KLK4 Induces Anti-Tumor Effects in Human Xenograft Mouse Models of Orthotopic and Metastatic Prostate Cancer

Simple Summary

The serine protease kallikrein-related peptidase 4 (KLK4) has been reported to potentially play a role in the progression of prostate cancer and other cancer types. However, most of these reports have been limited to in vitro studies. In vivo cancer models offer greater complexity to mimic the characteristics of cancer growth and metastasis in humans. In this study, we used in vivo models of prostate cancer and demonstrated that KLK4 can strongly inhibit the growth of primary prostate tumors as well as bone metastases. To our knowledge, this is the first report of an anti-tumor effect of KLK4 in prostate cancer in vivo.

Abstract

Recent reports have suggested the role of kallikrein-related peptidase 4 (KLK4) to be that of remodeling the tumor microenvironment in many cancers, including prostate cancer. Notably, these studies have suggested a pro-tumorigenic role for KLK4, especially in prostate cancer. However, these have been primarily in vitro studies, with limited in vivo studies performed to date. Herein, we employed an orthotopic inoculation xenograft model to mimic the growth of primary tumors, and an intracardiac injection to induce metastatic dissemination to determine the in vivo tumorigenic effects of KLK4 overexpressed in PC3 prostate cancer cells. Notably, we found that these KLK4-expressing cells gave rise to smaller localized tumors and decreased metastases than the parent PC-3 cells. To our knowledge, this is the first report of an anti-tumorigenic effect of KLK4, particularly in prostate cancer. These findings also provide a cautionary tale of the need for in vivo analyses to substantiate in vitro experimental data.

Cysteine-Rich Peptides: Hyperstable Scaffolds for Protein Engineering

Cysteine-rich peptides (CRPs) are small proteins of less than 100 amino acids in length characterized by the presence of disulfide bridges and common end-to-end macrocyclization. These properties confer hyperstability against high temperatures, salt concentration, serum presence, and protease degradation to CRPs. Moreover, their intercysteine domains (loops) are susceptible to residue hypervariability. CRPs have been successfully applied as stable scaffolds for molecular grafting, a protein engineering process in which cysteine-rich structures provide higher thermodynamic and metabolic stability to an epitope and acquire new biological function(s). This review describes the successes and limitations of seven cysteine-rich scaffolds, their bioactive epitopes, and the resulting grafted peptides.

Truncation of Huia versabilis Bowman-Birk inhibitor increases its selectivity, matriptase-1 inhibitory activity and proteolytic stability

A gradual truncation of the primary structure of frog skin-derived Huia versabilis Bowman-Birk peptidic inhibitor (HV-BBI) resulted in 18-times stronger inhibitor of matriptase-1 (peptide 6, K_i = 8 nm) in comparison to the full-length HV-BBI (K_i = 155 nm). Analogous increase in the inhibitory activity in correlation with the peptide length reduction was not observed in case of other serine proteases, bovine trypsin (K_i = 151 nm for peptide 6 and K_i = 120 nm for HV-BBI) and plasmin (K_i = 120 nm for peptide 6 and 82 nm for HV-BBI). Weaker binding affinity to these enzymes emphasized an inhibitory specificity of peptide 6. Molecular dynamic analysis revealed that the observed variations in the binding affinity of peptide 6 and HV-BBI with matriptase-1 are associated with the entropic differences of the unbound peptides. Moreover, several aspects explaining differences in the inhibition of matriptase-1 by peptide 6 (bearing the C-terminal amide group) and its two analogues, peptide 6∗ (having the C-terminal carboxyl group, K_i = 473 nm) and cyclic peptide 6∗∗ (K_i = 533 nm), both exhibiting more than 50-fold reduced inhibitory potency, were discovered. It was also shown that peptide 6 presented significantly higher resistance to proteolytic degradation in human serum than HV-BBI. Additional investigations revealed that, in contrast to some amphibian-derived inhibitors, HV-BBI and its truncated analogues do not possess bactericidal activity, thus they cannot be considered as bifunctional agents.

Cell surface-anchored serine proteases in cancer progression and metastasis

Over the last two decades, a novel subgroup of serine proteases, the cell surface-anchored serine proteases, has emerged as an important component of the human degradome, and several members have garnered significant attention for their roles in cancer progression and metastasis. A large body of literature describes that cell surface-anchored serine proteases are deregulated in cancer and that they contribute to both tumor formation and metastasis through diverse molecular mechanisms. The loss of precise regulation of cell surface-anchored serine protease expression and/or catalytic activity may be contributing to the etiology of several cancer types. There is therefore a strong impetus to understand the events that lead to deregulation at the gene and protein levels, how these precipitate in various stages of tumorigenesis, and whether targeting of selected proteases can lead to novel cancer intervention strategies. This review summarizes current knowledge about cell surface-anchored serine proteases and their role in cancer based on biochemical characterization, cell culture-based studies, expression studies, and in vivo experiments. Efforts to develop inhibitors to target cell surface-anchored serine proteases in cancer therapy will also be summarized.

Binding Loop Substitutions in the Cyclic Peptide SFTI-1 Generate Potent and Selective Chymase Inhibitors

Chymase is a serine protease that is predominantly expressed by mast cells and has key roles in immune defense and the cardiovascular system. This enzyme has also emerged as a therapeutic target for cardiovascular disease due to its ability to remodel cardiac tissue and generate angiotensin II. Here, we used the nature-derived cyclic peptide sunflower trypsin inhibitor-1 (SFTI-1) as a template for designing novel chymase inhibitors. The key binding contacts of SFTI-1 were optimized by combining a peptide substrate library screen with structure-based design, which yielded several variants with potent activity. The lead variant was further modified by replacing the P1 Tyr residue with para-substituted Phe derivatives, generating new inhibitors with improved potency (K_i = 1.8 nM) and higher selectivity over closely related enzymes. Several variants were shown to block angiotensin I cleavage in vitro, highlighting their potential for further development and future evaluation as pharmaceutical leads.

A Versatile and Robust Serine Protease Inhibitor Scaffold from Actinia tenebrosa

Serine proteases play pivotal roles in normal physiology and a spectrum of patho-physiological processes. Accordingly, there is considerable interest in the discovery and design of potent serine protease inhibitors for therapeutic applications. This led to concerted efforts to discover versatile and robust molecular scaffolds for inhibitor design. This investigation is a bioprospecting study that aims to isolate and identify protease inhibitors from the cnidarian Actinia tenebrosa. The study isolated two Kunitz-type protease inhibitors with very similar sequences but quite divergent inhibitory potencies when assayed against bovine trypsin, chymostrypsin, and a selection of human sequence-related peptidases. Homology modeling and molecular dynamics simulations of these inhibitors in complex with their targets were carried out and, collectively, these methodologies enabled the definition of a versatile scaffold for inhibitor design. Thermal denaturation studies showed that the inhibitors were remarkably robust. To gain a fine-grained map of the residues responsible for this stability, we conducted in silico alanine scanning and quantified individual residue contributions to the inhibitor's stability. Sequences of these inhibitors were then used to search for Kunitz homologs in an A. tenebrosa transcriptome library, resulting in the discovery of a further 14 related sequences. Consensus analysis of these variants identified a rich molecular diversity of Kunitz domains and expanded the palette of potential residue substitutions for rational inhibitor design using this domain.

Buried treasure: biosynthesis, structures and applications of cyclic peptides hidden in seed storage albumins

Covering: 1999 up to the end of 2017The small cyclic peptide SunFlower Trypsin Inhibitor-1 (SFTI-1) from sunflower seeds is the prototypic member of a novel family of natural products. The biosynthesis of these peptides is intriguing as their gene-encoded peptide backbone emerges from a precursor protein that also contains a seed storage albumin. The peptide sequence is cleaved out from the precursor and cyclised by the albumin-maturing enzymatic machinery. Three-dimensional solution NMR structures of a number of these peptides, and of the intact precursor protein preproalbumin with SFTI-1, have now been elucidated. Furthermore, the evolution of the family has been described and a detailed understanding of the biosynthetic steps, which are necessary to produce cyclic SFTI-1, is emerging. Macrocyclisation provides peptide stability and thus represents a key strategy in peptide drug development. Consequently the constrained structure of SFTI-1 has been explored as a template for protein engineering, for tuning selectivity towards clinically relevant proteases and for grafting in sequences with completely novel functions. Here we review the discovery of the SFTI-1 peptide family, their evolution, biosynthetic origin, and structural features, as well as highlight the potential applications of this unique class of natural products.

Crystal structure of the inhibitor-free form of the serine protease kallikrein-4

Kallikrein 4 (KLK4) is a serine protease that is predominantly expressed in the prostate and is overexpressed in prostate cancer. As such, it has gained attention as an attractive target for prostate cancer therapeutics. Currently, only liganded structures of KLK4 exist in the Protein Data Bank. Until now, inferences about the subtle structural changes in KLK4 upon ligand binding have been made by comparison to other liganded forms, rather than to an apo form. In this study, an inhibitor-free form of KLK4 was crystallized. The crystals obtained belonged to space group P1, contained four molecules in the asymmetric unit and diffracted to 1.64 Å resolution. Interestingly, a nonstandard rotamer of the specificity-determining residue Asp189 was observed in all chains. This model will provide a useful unliganded structure for the future structure-guided design of KLK4 inhibitors.

Rapid and Scalable Plant-Based Production of a Potent Plasmin Inhibitor Peptide

The backbone cyclic and disulfide bridged sunflower trypsin inhibitor-1 (SFTI-1) peptide is a proven effective scaffold for a range of peptide therapeutics. For production at laboratory scale, solid phase peptide synthesis techniques are widely used, but these synthetic approaches are costly and environmentally taxing at large scale. Here, we developed a plant-based approach for the recombinant production of SFTI-1-based peptide drugs. We show that transient expression in Nicotiana benthamiana allows for rapid peptide production, provided that asparaginyl endopeptidase enzymes with peptide-ligase functionality are co-expressed with the substrate peptide gene. Without co-expression, no target cyclic peptides are detected, reflecting rapid in planta degradation of non-cyclized substrate. We test this recombinant production system by expressing a SFTI-1-based therapeutic candidate that displays potent and selective inhibition of human plasmin. By using an innovative multi-unit peptide expression cassette, we show that in planta yields reach ~60 μg/g dry weight at 6 days post leaf infiltration. Using nuclear magnetic resonance structural analysis and functional in vitro assays, we demonstrate the equivalence of plant and synthetically derived plasmin inhibitor peptide. The methods and insights gained in this study provide opportunities for the large scale, cost effective production of SFTI-1-based therapeutics.

KLK4 Inhibition by Cyclic and Acyclic Peptides: Structural and Dynamical Insights into Standard-Mechanism Protease Inhibitors

Sunflower trypsin inhibitor (SFTI-1) is a 14 amino acid serine protease inhibitor. The dual antiparallel β-sheet arrangement of SFTI-1 is stabilized by an N-terminal-C-terminal backbone cyclization and a further disulfide bridge to form a final bicyclic structure. This constrained structure is further rigidified by an extensive network of internal hydrogen bonds. Thus, the structure of SFTI-1 in solution resembles the protease-bound structure, reducing the entropic penalty upon protease binding. When cleaved at the scissile bond, it is thought that the rigidifying features of SFTI-1 maintain its structure, allowing the scissile bond to be reformed. The lack of structural plasticity for SFTI-1 is proposed to favor initial protease binding and continued occupancy in the protease active site, resulting in an equilibrium between the cleaved and uncleaved inhibitor in the presence of a protease. We have determined, at 1.15 Å resolution, the X-ray crystal structures of complexes between human kallikrein-related peptidase 4 (KLK4) and SFTI-FCQR(Asn14) and between KLK4 and an acyclic form of the same inhibitor, SFTI-FCQR(Asn14)[1,14], with the latter displaying a cleaved scissile bond. Structural analysis and MD simulations together reveal the roles of the altered contact sequence, intramolecular hydrogen bonding network, and backbone cyclization in altering the state of SFTI's scissile bond ligation at the protease active site. Taken together, the data presented reveal insights into the role of dynamics in the standard-mechanism inhibition and suggest that modifications on the non-contact strand may be a useful, underexplored approach for generating further potent or selective SFTI-based inhibitors against members of the serine protease family.

Amino Acid Scanning at P5' within the Bowman-Birk Inhibitory Loop Reveals Specificity Trends for Diverse Serine Proteases

Sunflower trypsin inhibitor-1 (SFTI-1) is a 14-amino acid cyclic peptide that shares an inhibitory loop with a sequence and structure similar to a larger family of serine protease inhibitors, the Bowman-Birk inhibitors. Here, we focus on the P5' residue in the Bowman-Birk inhibitory loop and produce a library of SFTI variants to characterize the P5' specificity of 11 different proteases. We identify seven amino acids that are generally preferred by these enzymes and also correlate with P5' sequence diversity in naturally occurring Bowman-Birk inhibitors. Additionally, we show that several enzymes have divergent specificities that can be harnessed in engineering studies. By optimizing the P5' residue, we improve the potency or selectivity of existing inhibitors for kallikrein-related peptidase 5 and show that a variant with substitutions at 7 of the scaffold's 14 residues retains a similar structure to SFTI-1. These findings provide new insights into P5' specificity requirements for the Bowman-Birk inhibitory loop.

Potent, multi-target serine protease inhibition achieved by a simplified β-sheet motif

Engagement of an extended β-sheet is a common substrate/inhibitor interaction at the active site of serine proteases and is an important feature of Laskowski mechanism inhibitors that present a substrate-like loop to a target protease. This loop is cleaved but subsequently relegated forming a stable inhibitor/protease complex. Laskowski inhibitors are ubiquitous in nature and are used extensively in serine protease inhibitor design. However, most studies concentrate on introducing new sidechain interactions rather than the direct contributions of the substrate-like β-sheet to enzyme inhibition. Here we report the crystal structure of an simplified β-sheet inhibitory motif within the Sunflower Trypsin Inhibitor (SFTI) in complex with trypsin. We show that the intramolecular hydrogen bond network of this SFTI variant (SFTI-TCTR) engages the inhibitor sidechains that would normally interact with a target protease, giving mainchain interactions a more prominent role in complex formation. Despite having reduced sidechain interactions, this SFTI variant is remarkably potent and inhibits a diverse range of serine proteases. Crystal structural analysis and molecular modelling of SFTI-TCTR complexes again indicates an interface dominated by β–sheet interactions, highlighting the importance of this motif and the adaptability of SFTI as a scaffold for inhibitor design.

Using backbone-cyclized Cys-rich polypeptides as molecular scaffolds to target protein-protein interactions

The use of disulfide-rich backbone-cyclized polypeptides, as molecular scaffolds to design a new generation of bioimaging tools and drugs that are potent and specific, and thus might have fewer side effects than traditional small-molecule drugs, is gaining increasing interest among the scientific and in the pharmaceutical industries. Highly constrained macrocyclic polypeptides are exceptionally more stable to chemical, thermal and biological degradation and show better biological activity when compared with their linear counterparts. Many of these relatively new scaffolds have been also found to be highly tolerant to sequence variability, aside from the conserved residues forming the disulfide bonds, able to cross cellular membranes and modulate intracellular protein-protein interactions both in vitro and in vivo These properties make them ideal tools for many biotechnological applications. The present study provides an overview of the new developments on the use of several disulfide-rich backbone-cyclized polypeptides, including cyclotides, θ-defensins and sunflower trypsin inhibitor peptides, in the development of novel bioimaging reagents and therapeutic leads.

Potent, Selective, and Cell-Penetrating Inhibitors of Kallikrein-Related Peptidase 4 Based on the Cyclic Peptide MCoTI-II

Kallikrein-related peptidase 4 (KLK4) is a serine protease that has putative intracellular and extracellular functions in prostate cancer progression. Here we show that MCoTI-II, a 34-amino acid cyclic peptide found in the seeds of red gac (Momordica cochinchinensis), is an inhibitor of KLK4. By grafting a preferred KLK4 cleavage sequence into MCoTI-II, we produced a highly potent KLK4 inhibitor (K _i = 0.1 nM) that displayed 100,000-fold selectivity over related KLKs and the ability to penetrate cells. Additionally, by substituting positively charged noncontact residues in this compound, we produced a potent and selective KLK4 inhibitor that does not penetrate cells. The inhibitors were shown to be nontoxic to human cells and stable in human serum. These KLK4 inhibitors provide useful chemical tools to further define the role(s) of both intracellular and extracellular KLK4 in prostate cancer cell lines and disease models.

Small Peptides as Modulators of Serine Proteases

Serine proteases play critical roles in many physiological and pathological processes, and are proven diagnostic and therapeutic targets in a number of clinical indications. Suppression of the aberrant proteolytic activities of these proteases has been clinically used for the treatments of relevant diseases. Polypeptides with 10-20 residues are of great interests as medicinal modulators of serine proteases, because these peptides demonstrate the characteristics of both small molecule drugs and macromolecular drugs. In this review, we summarized the recent development of peptide-based inhibitors against serine proteases with potent inhibitory and high specificity comparable to monoclonal antibodies. In addition, we also discussed the strategies of enhancing plasma half-life and bioavailability of peptides in vivo, which is the main hurdle that limits the clinical translation of peptide-based drugs. This review advocates new avenue for the development of effective serine protease inhibitors and highlights the prospect of the medicinal use of these inhibitors.

Identification and pharmaceutical evaluation of novel frog skin-derived serine proteinase inhibitor peptide-PE-BBI (Pelophylax esculentus Bowman-Birk inhibitor) for the potential treatment of cancer

Amphibian venom-derived peptides have high potential in the field of anticancer drug discovery. We have isolated a novel Bowman-Birk proteinase inhibitor (BBI)-type peptide from the skin secretion of Pelophylax esculentus (PE) named PE-BBI, and evaluated its bio-functions and anti-cancer activity in vitro. PE-BBI is a heptadecapeptide with C-terminal amidation. The mRNA sequence and primary structure of PE-BBI were identified using RT-PCR and LC/MS, respectively. A trypsin inhibitory assay was used to characterize the serine proteinase inhibitory activity of synthetic PE-BBI. PE-BBI’s myotropic activity was analyzed using isolated rat bladder and rat-tail artery smooth muscle tissues, and the anti-cancer ability of PE-BBI using human colorectal cancer cells. PE-BBI’s mechanism of action was investigated using Discovery studio software. PE-BBI showed trypsin inhibitory activity (K_i = 310 ± 72 nM), strong myotropic activity, and cytotoxicity that were specific to cancer cells, and no side effect to normal epithelial cells. The docking stimulation showed that PE-BBI had high affinity to several members of human kallikrein related peptidase (KLK) family. This finding helps to enrich our understanding of BBI peptides’ mode of action. Moreover, the data presented here validates frog secretions as sources of potential novel proteinase inhibitors for cancer treatment.

Bile salt hydrolases: Structure and function, substrate preference, and inhibitor development

The worldwide trend of limiting the use of antibiotic growth promoters (AGPs) in animal production creates challenges for the animal feed industry, thus necessitating the development of effective non-antibiotic alternatives to improve animal performance. Increasing evidence has shown that the growth-promoting effect of AGPs is highly correlated with the reduced activity of bile salt hydrolase (BSH, EC 3.5.1.24), an intestinal bacteria-producing enzyme that has a negative impact on host fat digestion and energy harvest. Therefore, BSH inhibitors may become novel, attractive alternatives to AGPs. Detailed knowledge of BSH substrate preferences and the wealth of structural data on BSHs provide a solid foundation for rationally tailored BSH inhibitor design. This review focuses on the relationship between structure and function of BSHs based on the crystal structure, kinetic data, molecular docking and comparative structural analyses. The molecular basis for BSH substrate recognition is also discussed. Finally, recent advances and future prospectives in the development of potent, safe, and cost-effective BSH inhibitors are described.

Engineering potent mesotrypsin inhibitors based on the plant-derived cyclic peptide, sunflower trypsin inhibitor-1

Plants produce a diverse range of peptides and proteins that inhibit the activity of different serine proteases. The value of these inhibitors not only stems from their native role(s) in planta, but they are also regarded as promising templates for inhibitor engineering. Interest in this field has grown rapidly in recent years, particularly for therapeutic applications. The serine protease mesotrypsin has been implicated in several cancers, but is a challenging target for inhibitor engineering as a number of serine protease inhibitors that typically display broad-range activity show limited activity against mesotrypsin. In this study, we use a cyclic peptide isolated from sunflower seeds, sunflower trypsin inhibitor-1 (SFTI-1), as a scaffold for engineering potent mesotrypsin inhibitors. SFTI-1 comprises 14-amino acids and is a potent inhibitor of human cationic trypsin (K_i = 30 ± 0.8 pM) but shows 165,000-fold weaker activity against mesotrypsin (K_i = 4.96 ± 0.2 μM). Using an inhibitor library based on SFTI-1, we show that the inhibitor's P2' residue (Ile) is a key contributor to SFTI-1's limited activity against mesotrypsin. Substituting P2' Ile with chemically diverse amino acids, including non-canonical aromatic residues, produced new inhibitor variants that maintained a similar structure to SFTI-1 and showed marked improvements in activity (exceeding 100-fold). An assessment of the activity of the new inhibitors against closely-related trypsin paralogs revealed that the improved activity against mesotrypsin was accompanied by a loss in activity against off-target proteases, such that several engineered variants showed comparable activity against mesotrypsin and human cationic trypsin. Together, these findings identify potent mesotrypsin inhibitors that are suitable for further optimisation studies and demonstrate the potential gains in activity and selectivity that can be achieved by optimising the P2' residue, particularly for engineered SFTI-based inhibitors.

Development of Novel Melanocortin Receptor Agonists Based on the Cyclic Peptide Framework of Sunflower Trypsin Inhibitor-1

Ultrastable cyclic peptide frameworks offer great potential for drug design due to their improved bioavailability compared to their linear analogues. Using the sunflower trypsin inhibitor-1 (SFTI-1) peptide scaffold in combination with systematic N-methylation of the grafted pharmacophore led to the identification of novel subtype selective melanocortin receptor (MCR) agonists. Multiple bicyclic peptides were synthesized and tested toward their activity at MC1R and MC3-5R. Double N-methylated compound 18 showed a p K_i of 8.73 ± 0.08 ( K_i = 1.92 ± 0.34 nM) and a pEC₅₀ of 9.13 ± 0.04 (EC₅₀ = 0.75 ± 0.08 nM) at the human MC1R and was over 100 times more selective for MC1R. Nuclear magnetic resonance structural analysis of 18 emphasized the role of peptide bond N-methylation in shaping the conformation of the grafted pharmacophore. More broadly, this study highlights the potential of cyclic peptide scaffolds for epitope grafting in combination with N-methylation to introduce receptor subtype selectivity in the context of peptide-based drug discovery.

Co-expression of a cyclizing asparaginyl endopeptidase enables efficient production of cyclic peptides in planta

Cyclotides are ultra-stable, backbone-cyclized plant defence peptides that have attracted considerable interest in the pharmaceutical industry. This is due to their range of native bioactivities as well as their ability to stabilize other bioactive peptides within their framework. However, a hindrance to their widespread application is the lack of scalable, cost-effective production strategies. Plant-based production is an attractive, benign option since all biosynthetic steps are performed in planta. Nonetheless, cyclization in non-cyclotide-producing plants is poor. Here, we show that cyclic peptides can be produced efficiently in Nicotiana benthamiana, one of the leading plant-based protein production platforms, by co-expressing cyclotide precursors with asparaginyl endopeptidases that catalyse peptide backbone cyclization. This approach was successful in a range of other plants (tobacco, bush bean, lettuce, and canola), either transiently or stably expressed, and was applicable to both native and engineered cyclic peptides. We also describe the use of the transgenic system to rapidly identify new asparaginyl endopeptidase cyclases and interrogate their substrate sequence requirements. Our results pave the way for exploiting cyclotides for pest protection in transgenic crops as well as large-scale production of cyclic peptide pharmaceuticals in plants.

Mass spectrometry based proteomics analyses in kallikrein-related peptidase research: implications for cancer research and therapy

INTRODUCTION

Kallikrein-related peptidases (KLKs) are a family of serine peptidases that are deregulated in numerous pathological conditions, with a multitude of KLK-mediated functional roles implicated in the progression of cancer. Advances in multidimensional mass spectrometry (MS)-based proteomics have facilitated the quantitative measurement of deregulated KLK expression in cancer, identifying certain KLKs, as well as their substrates, as potential cancer biomarkers. Areas covered: In this review, we discuss how these approaches have been utilized for KLK biomarker discovery and unbiased substrate determination in complex protein pools that mimic the in vivo extracellular microenvironment. Expert commentary: Although a limited number of studies have been performed, the quantity of information generated has greatly improved our understanding of the functional roles of KLKs in cancer progression. In addition, these data suggest additional means through which deregulated KLK expression may be targeted in cancer treatment, highlighting the potential therapeutic value of these state-of-the-art MS-based studies.

Ribosomally-synthesised cyclic peptides from plants as drug leads and pharmaceutical scaffolds

Owing to their exceptional stability and favourable pharmacokinetic properties, plant-derived cyclic peptides have recently attracted significant attention in the field of peptide-based drug design. This article describes the three major classes of ribosomally-synthesised plant peptides - the cyclotides, the PawS-derived peptides and the orbitides - and reviews their applications as leads or scaffolds in drug design. These ribosomally-produced peptides have a range of biological activities, including anti-HIV, cytotoxic and immunomodulatory activity. In addition, recent interest has focused on their use as scaffolds to stabilise bioactive peptide sequences, thereby enhancing their biopharmaceutical properties. There are now more than 30 published papers on such 'grafting' applications, most of which have been reported only in the last few years, and several such studies have reported in vivo activity of orally delivered cyclic peptides. In this article, we describe approaches to the synthesis of cyclic peptides and their pharmaceutically-grafted derivatives as well as outlining their biosynthetic routes. Finally, we describe possible bioproduction routes for pharmaceutically active cyclic peptides, involving plants and plant suspension cultures.

An engineered cyclic peptide alleviates symptoms of inflammation in a murine model of inflammatory bowel disease

Inflammatory bowel diseases (IBDs) are a set of complex and debilitating diseases for which there is no satisfactory treatment. Recent studies have shown that small peptides show promise for reducing inflammation in models of IBD. However, these small peptides are likely to be unstable and rapidly cleared from the circulation, and therefore, if not modified for better stability, represent non-viable drug leads. We hypothesized that improving the stability of these peptides by grafting them into a stable cyclic peptide scaffold may enhance their therapeutic potential. Using this approach, we have designed a novel cyclic peptide that comprises a small bioactive peptide from the annexin A1 protein grafted into a sunflower trypsin inhibitor cyclic scaffold. We used native chemical ligation to synthesize the grafted cyclic peptide. This engineered cyclic peptide maintained the overall fold of the naturally occurring cyclic peptide, was more effective at reducing inflammation in a mouse model of acute colitis than the bioactive peptide alone, and showed enhanced stability in human serum. Our findings suggest that the use of cyclic peptides as structural backbones offers a promising approach for the treatment of IBD and potentially other chronic inflammatory conditions.

Efficient recombinant expression of SFTI-1 in bacterial cells using intein-mediated protein trans-splicing

We report for the first time the recombinant expression of bioactive wild-type sunflower trypsin inhibitor 1 (SFTI-1) inside E. coli cells by making use of intracellular protein trans-splicing in combination with a high efficient split-intein. SFTI-1 is a small backbone-cyclized polypeptide with a single disulfide bridge and potent trypsin inhibitory activity. Recombinantly produced SFTI-1 was fully characterized by NMR and was observed to actively inhibit trypsin. The in-cell expression of SFTI-1 was very efficient reaching intracellular concentration ≈ 40 µM. This study clearly demonstrates the possibility of generating genetically encoded SFTI-based peptide libraries in live E. coli cells, and is a critical first step for developing in-cell screening and directed evolution technologies using the cyclic peptide SFTI-1 as a molecular scaffold. © 2016 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 106: 818-824, 2016.

Diverse cyclic seed peptides in the Mexican zinnia (Zinnia haageana)

A new family of small plant peptides was recently described and found to be widespread throughout the Millereae and Heliantheae tribes of the sunflower family Asteraceae. These peptides originate from the post-translational processing of unusual seed-storage albumin genes, and have been termed PawS-derived peptides (PDPs). The prototypic family member is a 14-residue cyclic peptide with potent trypsin inhibitory activity named SunFlower Trypsin Inhibitor (SFTI-1). In this study we present the features of three new PDPs discovered in the seeds of the sunflower species Zinnia haageana by a combination of de novo transcriptomics and liquid chromatography-mass spectrometry. Two-dimensional solution NMR spectroscopy was used to elucidate their structural characteristics. All three Z. haageana peptides have well-defined folds with a head-to-tail cyclized peptide backbone and a single disulfide bond. Although two possess an anti-parallel β-sheet structure, like SFTI-1, the Z. haageana peptide PDP-21 has a more irregular backbone structure. Despite structural similarities with SFTI-1, PDP-20 was not able to inhibit trypsin, thus the functional roles of these peptides is yet to be discovered. Defining the structural features of the small cyclic peptides found in the sunflower family will be useful for guiding the exploitation of these peptides as scaffolds for grafting and protein engineering applications.

Design of Potent and Selective Cathepsin G Inhibitors Based on the Sunflower Trypsin Inhibitor-1 Scaffold

Neutrophils are directly responsible for destroying invading pathogens via reactive oxygen species, antimicrobial peptides, and neutrophil serine proteases (NSPs). Imbalance between NSP activity and endogenous protease inhibitors is associated with chronic inflammatory disorders, and engineered inhibitors of NSPs are a potential therapeutic pathway. In this study we characterized the extended substrate specificity (P4-P1) of the NSP cathepsin G using a peptide substrate library. Substituting preferred cathepsin G substrate sequences into sunflower trypsin inhibitor-1 (SFTI-1) produced a potent cathepsin G inhibitor (K_i = 0.89 nM). Cathepsin G's P2' preference was determined by screening against a P2' diverse SFTI-based library, and the most preferred residue at P2' was combined in SFTI-1 with a preferred substrate sequence (P4-P2) and a nonproteinogenic P1 residue (4-guanidyl-l-phenylalanine) to produce a potent (K_i = 1.6 nM) and the most selective (≥360-fold) engineered cathepsin G inhibitor reported to date. This compound is a promising lead for further development of cathepsin G inhibitors targeting chronic inflammatory disorders.

Exploring the active site binding specificity of kallikrein-related peptidase 5 (KLK5) guides the design of new peptide substrates and inhibitors

Kallikrein-related peptidase 5 (KLK5) is a promising therapeutic target in several skin diseases, including Netherton syndrome, and is emerging as a potential target in various cancers. In this study, we used a sparse matrix library of 125 individually synthesized peptide substrates to characterize the binding specificity of KLK5. The sequences most favored by KLK5 were GRSR, YRSR and GRNR, and we identified sequence-specific interactions involving the peptide N-terminus by analyzing kinetic constants (kcat and KM) and performing molecular dynamics simulations. KLK5 inhibitors were subsequently engineered by substituting substrate sequences into the binding loop (P1, P2 and P4 residues) of sunflower trypsin inhibitor-1 (SFTI-1). These inhibitors were effective against KLK5 but showed limited selectivity, and performing a further substitution at P2′ led to the design of a new variant that displayed improved activity against KLK5 (Ki=4.2±0.2 nm), weak activity against KLK7 and 12-fold selectivity over KLK14. Collectively, these findings provide new insight into the design of highly favored binding sequences for KLK5 and reveal several opportunities for modulating inhibitor selectivity over closely related proteases that will be useful for future studies aiming to develop therapeutic molecules targeting KLK5.

Tissue Kallikrein Inhibitors Based on the Sunflower Trypsin Inhibitor Scaffold - A Potential Therapeutic Intervention for Skin Diseases

Tissue kallikreins (KLKs), in particular KLK5, 7 and 14 are the major serine proteases in the skin responsible for skin shedding and activation of inflammatory cell signaling. In the normal skin, their activities are controlled by an endogenous protein protease inhibitor encoded by the SPINK5 gene. Loss-of-function mutations in SPINK5 leads to enhanced skin kallikrein activities and cause the skin disease Netherton Syndrome (NS). We have been developing inhibitors based on the Sunflower Trypsin Inhibitor 1 (SFTI-1) scaffold, a 14 amino acids head-to-tail bicyclic peptide with a disulfide bond. To optimize a previously reported SFTI-1 analogue (I10H), we made five analogues with additional substitutions, two of which showed improved inhibition. We then combined those substitutions and discovered a variant (Analogue 6) that displayed dual inhibition of KLK5 (tryptic) and KLK7 (chymotryptic). Analogue 6 attained a tenfold increase in KLK5 inhibition potency with an Isothermal Titration Calorimetry (ITC) K_d of 20nM. Furthermore, it selectively inhibits KLK5 and KLK14 over seven other serine proteases. Its biological function was ascertained by full suppression of KLK5-induced Protease-Activated Receptor 2 (PAR-2) dependent intracellular calcium mobilization and postponement of Interleukin-8 (IL-8) secretion in cell model. Moreover, Analogue 6 permeates through the cornified layer of in vitro organotypic skin equivalent culture and inhibits protease activities therein, providing a potential drug lead for the treatment of NS.

Review seed biopharmaceutical cyclic peptides: From discovery to applications

Mini-proteins (or peptides) with disulfide bond/s and a cyclic backbone offer exciting opportunities for applications in medicine, as these ribosomally synthesized and posttranslationally modified peptides are exceptionally stable and amenable to grafting epitopes with desirable activities. Here I discuss important aspects of the discovery and applications of disulfide-bonded cyclic peptides from seeds, i.e., the trypsin inhibitor cyclotides and the preproalbumin with sunflower trypsin inhibitor-derived peptides, focusing on bioanalytical methods for and insights generated from their discovery as well as their potential use as engineering scaffolds for peptide-based drug design. The recent discovery of their precursors and processing enzymes could potentially enable in planta production of designer disulfide-bonded cyclic peptides, preferably in edible seeds, and address the demand for new biopharmaceutical peptides in a cost-effective manner.

A current perspective on applications of macrocyclic-peptide-based high-affinity ligands

Monoclonal antibodies can bind with high affinity and high selectivity to their targets. As a tool in therapeutics or diagnostics, however, their large size (∼150 kDa) reduces penetration into tissue and prevents passive cellular uptake. To overcome these and other problems, minimized protein scaffolds have been chosen or engineered, with care taken to not compromise binding affinity or specificity. An alternate approach is to begin with a minimal non-antibody scaffold and select functional ligands from a de novo library. We will discuss the structure, production, applications, strengths, and weaknesses of several classes of antibody-derived ligands, that is, antibodies, intrabodies, and nanobodies, and nonantibody-derived ligands, that is, monobodies, affibodies, and macrocyclic peptides. In particular, this review is focussed on macrocyclic peptides produced by the Random non-standard Peptides Integrated Discovery (RaPID) system that are small in size (typically ∼2 kDa), but are able to perform tasks typically handled by larger proteinaceous ligands.

Direct and indirect mechanisms of KLK4 inhibition revealed by structure and dynamics

The kallikrein-related peptidase (KLK) family of proteases is involved in many aspects of human health and disease. One member of this family, KLK4, has been implicated in cancer development and metastasis. Understanding mechanisms of inactivation are critical to developing selective KLK4 inhibitors. We have determined the X-ray crystal structures of KLK4 in complex with both sunflower trypsin inhibitor-1 (SFTI-1) and a rationally designed SFTI-1 derivative to atomic (~1 Å) resolution, as well as with bound nickel. These structures offer a structural rationalization for the potency and selectivity of these inhibitors, and together with MD simulation and computational analysis, reveal a dynamic pathway between the metal binding exosite and the active site, providing key details of a previously proposed allosteric mode of inhibition. Collectively, this work provides insight into both direct and indirect mechanisms of inhibition for KLK4 that have broad implications for the enzymology of the serine protease superfamily, and may potentially be exploited for the design of therapeutic inhibitors.

Natural structural diversity within a conserved cyclic peptide scaffold

We recently isolated and described the evolutionary origin of a diverse class of small single-disulfide bonded peptides derived from Preproalbumin with SFTI-1 (PawS1) proteins in the seeds of flowering plants (Asteraceae). The founding member of the PawS derived peptide (PDP) family is the potent trypsin inhibitor SFTI-1 (sunflower trypsin inhibitor-1) from Helianthus annuus, the common sunflower. Here we provide additional structures and describe the structural diversity of this new class of small peptides, derived from solution NMR studies, in detail. We show that although most have a similar backbone framework with a single disulfide bond and in many cases a head-to-tail cyclized backbone, they all have their own characteristics in terms of projections of side-chains, flexibility and physiochemical properties, attributed to the variety of their sequences. Small cyclic and constrained peptides are popular as drug scaffolds in the pharmaceutical industry and our data highlight how amino acid side-chains can fine-tune conformations in these promising peptides.

Selective Substrates and Inhibitors for Kallikrein-Related Peptidase 7 (KLK7) Shed Light on KLK Proteolytic Activity in the Stratum Corneum

Proteases have pivotal roles in the skin's outermost layer, the epidermis. In the stratum corneum, serine proteases from the kallikrein-related peptidase (KLK) family have been implicated in several key homeostatic processes, including desquamation. However, the precise contribution of specific KLKs to each process remains unclear. To address this, we used a chemical biology approach and designed selective substrates and inhibitors for KLK7, the most abundant KLK protease in the stratum corneum. The resulting KLK7 inhibitor is the most potent inhibitor of this protease reported to date (K_i = 140 pM), and displays at least 1,000-fold selectivity over several proteases that are related by function (KLK5 and KLK14) or specificity (chymotrypsin). We then used substrates and inhibitors for KLK5, KLK7, and KLK14 to explore the activity of each protease in the stratum corneum using casein zymography and an ex vivo desquamation assay. These experiments provide the most detailed assessment of each KLK's contribution to corneocyte shedding in the plantar stratum corneum, revealing that inhibition of KLK7 alone is sufficient to block shedding, whereas KLK5 is also a major contributor. Collectively, these findings unveil chemical tools for studying KLK activity and demonstrate their potential for characterizing KLK biological functions in epidermal homeostasis.

Targeting protein-protein interfaces using macrocyclic peptides

Protein-protein interactions (PPIs) are critical in numerous biological processes including signaling transduction, function regulations, and disease development. To regulate PPIs has been thought to be challenging due to their highly dynamic and expansive interfacial areas. Nonetheless, successful examples have been reported of targeting PPIs using small molecules, peptides, and proteins. Peptides, especially macrocyclic peptides have proven to be a particularly useful tool to inhibit PPIs for their exquisite potency, stability and selectivity. Herein we review the recent developments of this area of research, focusing on the macrocyclic peptides isolated from natural products, identified from library screening, and rationally designed based on structures, as PPI regulators.

Mechanism-based selection of a potent kallikrein-related peptidase 7 inhibitor from a versatile library based on the sunflower trypsin inhibitor SFTI-1

Potent and specific enzyme inhibition is a key goal in the development of therapeutic inhibitors targeting proteolytic activity. The backbone-cyclized peptide, Sunflower Trypsin Inhibitor (SFTI-1) affords a scaffold that can be engineered to achieve both these aims. SFTI-1's mechanism of inhibition is unusual in that it shows fast-on/slow-off kinetics driven by cleavage and religation of a scissile bond. This phenomenon was used to select a nanomolar inhibitor of kallikrein-related peptidase 7 (KLK7) from a versatile library of SFTI variants with diversity tailored to exploit distinctive surfaces present in the active site of serine proteases. Inhibitor selection was achieved through the use of size exclusion chromatography to separate protease/inhibitor complexes from unbound inhibitors followed by inhibitor identification according to molecular mass ascertained by mass spectrometry. This approach identified a single dominant inhibitor species with molecular weight of 1562.4 Da, which is consistent with the SFTI variant SFTI-WCTF. Once synthesized individually this inhibitor showed an IC50 of 173.9 ± 7.6 nM against chromogenic substrates and could block protein proteolysis. Molecular modeling analysis suggested that selection of SFTI-WCTF was driven by specific aromatic interactions and stabilized by an enhanced internal hydrogen bonding network. This approach provides a robust and rapid route to inhibitor selection and design.

Inhibition of Kallikrein-Related Peptidases 7 and 5 by Grafting Serpin Reactive-Center Loop Sequences onto Sunflower Trypsin Inhibitor-1 (SFTI-1)

Serpin proteins irreversibly inhibit serine proteases, but only a small part of the serpin reactive-center loop (RCL) is responsible for the initial protein-protein interaction (PPI). To develop peptidic protease inhibitors, kallikrein-related peptidases 7 (KLK7) and 5 (KLK5) were chosen. Firstly, we demonstrated that short peptides derived from RCL sequences can be cleaved by KLK7 in a substrate-like manner. Next, these substrates were grafted onto the protease-binding loop of sunflower trypsin inhibitor-1 (SFTI-1). Peptides based on kallistatin, α1 -antichymotrypsin, and protein C inhibitor (PCI) inhibited KLK7 with Ki =0.4, 0.5, and 0.7 μm, respectively. In contrast, the trypsin-like KLK5 was only blocked by the peptide derived from PCI (Ki =0.6 μm). Thus, serpin function can be mimicked by introducing its PPI site into the rigid structure of the SFTI-1 scaffold. This approach might be applicable not only to KLKs but also to other serine protease members, thus opening up new therapeutic fields.