Biologically potent analogues of salmon calcitonin which do not contain an N-terminal disulfide-bridged ring structure

A structural basis for amylin receptor phenotype

Amylin receptors (AMYRs) are heterodimers of the calcitonin (CT) receptor (CTR) and one of three receptor activity-modifying proteins (RAMPs), AMY₁R, AMY₂R, and AMY₃R. Selective AMYR agonists and dual AMYR/CTR agonists are being developed as obesity treatments; however, the molecular basis for peptide binding and selectivity is unknown. We determined the structure and dynamics of active AMYRs with amylin, AMY₁R with salmon CT (sCT), AMY₂R with sCT or human CT (hCT), and CTR with amylin, sCT, or hCT. The conformation of amylin-bound complexes was similar for all AMYRs, constrained by the RAMP, and an ordered midpeptide motif that we call the bypass motif. The CT-bound AMYR complexes were distinct, overlapping the CT-bound CTR complexes. Our findings indicate that activation of AMYRs by CT-based peptides is distinct from their activation by amylin-based peptides. This has important implications for the development of AMYR therapeutics.

Fibrillation of Human Calcitonin and Its Analogs: Effects of Phosphorylation and Disulfide Reduction

Some therapeutic peptides self-assemble in solution to form ordered, insoluble, β-sheet-rich amyloid fibrils. This physical instability can result in reduced potency, cause immunogenic side effects, and limit options for formulation. Understanding the mechanisms of fibrillation is key to developing rational mitigation strategies. Here, amide hydrogen-deuterium exchange with mass spectrometric analysis (HDX-MS) coupled with proteolytic digestion was used to identify the early stage interactions leading to fibrillation of human calcitonin (hCT), a peptide hormone important in calcium metabolism. hCT fibrillation kinetics was sigmoidal, with lag, growth, and plateau phases as shown by thioflavin T and turbidity measurements. HDX-MS of fibrillating hCT (pH 7.4; 25°C) suggested early involvement of the N-terminal (1-11) and central (12-19) fragments in interactions during the lag phase, whereas C-terminal fragments (20-32 and 26-32) showed limited involvement during this period. The residue-level information was used to develop phosphorylated hCT analogs that showed modified fibrillation that depended on phosphorylation site. Phosphorylation in the central region resulted in complete inhibition of fibrillation for the phospho-Thr-13 hCT analog, whereas phosphorylation in the N-terminal and C-terminal regions inhibited but did not prevent fibrillation. Reduction of the Cys1-Cys7 disulfide bond resulted in faster fibrillation with involvement of different hCT residues as indicated by pulsed HDX-MS. Together, the results demonstrate that small structural changes have significant effects on hCT fibrillation and that understanding these effects can inform the rational development of fibrillation-resistant hCT analogs.

Molecular Signature for Receptor Engagement in the Metabolic Peptide Hormone Amylin

The pancreatic peptide hormone, amylin, plays a critical role in the control of appetite, and synergizes with other key metabolic hormones such as glucagon-like peptide 1 (GLP-1). There is opportunity to develop potent and long-acting analogues of amylin or hybrids between these and GLP-1 mimetics for treating obesity. To achieve this, interrogation of how the 37 amino acid amylin peptide engages with its complex receptor system is required. We synthesized an extensive library of peptides to profile the human amylin sequence, determining the role of its disulfide loop, amidated C-terminus and receptor "capture" and "activation" regions in receptor signaling. We profiled four signaling pathways with different ligands at multiple receptor subtypes, in addition to exploring selectivity determinants between related receptors. Distinct roles for peptide subregions in receptor binding and activation were identified, resulting in peptides with greater activity than the native sequence. Enhanced peptide activity was preserved in the brainstem, the major biological target for amylin. Interpretation of our data using full-length active receptor models supported by molecular dynamics, metadynamics, and supervised molecular dynamics simulations guided the synthesis of a potent dual agonist of GLP-1 and amylin receptors. The data offer new insights into the function of peptide amidation, how allostery drives peptide-receptor interactions, and provide a valuable resource for the development of novel amylin agonists for treating diabetes and obesity.

Extracellular loops 2 and 3 of the calcitonin receptor selectively modify agonist binding and efficacy

Class B peptide hormone GPCRs are targets for the treatment of major chronic disease. Peptide ligands of these receptors display biased agonism and this may provide future therapeutic advantage. Recent active structures of the calcitonin (CT) and glucagon-like peptide-1 (GLP-1) receptors reveal distinct engagement of peptides with extracellular loops (ECLs) 2 and 3, and mutagenesis of the GLP-1R has implicated these loops in dynamics of receptor activation. In the current study, we have mutated ECLs 2 and 3 of the human CT receptor (CTR), to interrogate receptor expression, peptide affinity and efficacy. Integration of these data with insights from the CTR and GLP-1R active structures, revealed marked diversity in mechanisms of peptide engagement and receptor activation between the CTR and GLP-1R. While the CTR ECL2 played a key role in conformational propagation linked to Gs/cAMP signalling this was mechanistically distinct from that of GLP-1R ECL2. Moreover, ECL3 was a hotspot for distinct ligand- and pathway-specific effects, and this has implications for the future design of biased agonists of class B GPCRs.

Design, synthesis, characterization and in-vivo activity of a novel salmon calcitonin conjugate containing a novel PEG-lipid moiety

Objectives

The aim of the study was to explore (1) the synthesis of a novel poly(ethylene glycol) modified lipid (PEG-lipid, PL) containing a chemically active tri-block linker, ε-maleimido lysine (Mal), and its conjugation with salmon calcitonin (sCT), and (2) the biophysical properties and activity of the resulting conjugate, Mal-PL-sCT, relative to the control, 2PEG-Mal-sCT, which comprises sCT conjugated with α-palmitoyl-N-ε-maleimido-l-lysine at cysteine 1 and cysteine 7, and PEG moieties at lysine 11 and lysine 18 via a conventional stepwise method.

Methods

The PEG-lipid was obtained by condensing palmitic acid derivative of ε-maleimido lysine with methoxy poly(ethylene glycol) amine. Under reductive conditions, the PEG-lipid readily reacted with sCT to yield the resultant compound, Mal-PL-sCT.

Key findings

Dynamic light scattering analyses suggested that Mal-PL-sCT and 2PEG-Mal-sCT exhibited robust helical structures with a high tendency to aggregate in water. Both compounds were more stable against intestinal degradation than sCT, although Mal-PL-sCT was less stable than 2PEG-Mal-sCT. However, 2PEG-Mal-sCT did not possess hypocalcaemic activity while Mal-PL-sCT retained the hypocalcaemic activity of sCT when it was subcutaneously injected in the rat model. Multiple functional groups may be conjugated to a peptide via a tri-block linker without the risk of obliterating the intrinsic bioactivity of the peptide.

Conclusions

The resultant novel PEG-lipid has a potential role to optimize protein and peptide delivery.

Aqueous-Soluble, Non-Reversible Lipid Conjugate of Salmon Calcitonin: Synthesis, Characterization and In Vivo Activity

PURPOSE

A novel, non-reversible, aqueous-based lipidization strategy with palmitic acid as a model lipid was evaluated for conjugation with salmon calcitonin (sCT).

MATERIALS AND METHODS

A water-soluble epsilon-maleimido lysine derivative of palmitic acid was synthesized from reaction of palmitic acid N-succinimidyl ester and epsilon-maleimido lysine. The latter was generated from reaction of alpha-Boc-lysine and methylpyrrolecarboxylate, with subsequent deprotection of the Boc group. The palmitic derivative was further conjugated with sCT via a thio-ether bond to produce Mal-sCT in aqueous solution. The identity and purity of Mal-sCT was confirmed by Electrospray Ionisation Mass spectrometry (ESI-MS) and HPLC.

RESULTS

Yield of Mal-sCT was 83%. Dynamic light scattering and circular dichroism data suggested that Mal-sCT presented as a stable helical structure in aqueous solutions of varying polarity, with a propensity to aggregate at concentrations above 11 microM. Cellular uptake of Mal-sCT was twice that of sCT in the Caco-2 cell model, and the conjugate was more resistant to liver enzyme degradation. Mal-sCT exhibited comparable hypocalcemic activity to sCT when administered subcutaneously in the rat model at sCT equivalent dose of 0.114 mg/kg. Peroral Mal-sCT, however, produced variability in therapeutic outcome. While four out of six rats did not respond following intragastric gavage with Mal-sCT, two rats showed significantly suppressed plasma calcium levels (approximately 60% of baseline) for up to 10 h.

CONCLUSION

A novel non-reversible, water-soluble lipid conjugate of sCT was successfully synthesized that showed (1) different aggregation behavior and secondary structure, (2) improved enzymatic stability and cellular uptake, and (3) comparable hypocalcemic activity in vivo compared to sCT.

Conformational analysis of human calcitonin in solution

The solution conformation of human calcitonin in a mixture of 60% water and 40% trifluoroethanol has been determined by the combined use of 1H NMR spectroscopy and distance geometry calculations with a distributed computing technique. 1H NMR spectroscopy provided 195 distance constraints and 13 hydrogen bond constraints. The 20 best converged structures exhibit atomic rmsd of 0.43 A for the backbone atoms from the averaged coordinate position in the region of Asn3-Phe22. The conformation is characterized by a nearly amphiphilic alpha-helix domain that extends from Leu4 in the cyclic region to His20. There are no significant differences observed among the overall structures of a series of calcitonins obtained from ultimobranchial bodies, including those that possess 20- to 50-fold greater activity. Three aromatic amino acid residues, Tyr12, Phe16 and Phe19, form a hydrophobic surface of human calcitonin. Bulky side chains on the surface could interfere with the ligand-receptor interaction thereby causing its low activity, relative to those of other species.

Solution structure and biological activity of recombinant salmon calcitonin S-sulfonated analog

Salmon calcitonin S-sulfonated analog (abbreviated as [S-SO(3)(-)]rsCT) was prepared by introducing two sulfonic groups into the side chains of Cys1 and Cys7 of recombinant salmon calcitonin. The hypocalcemic potency of this open-chain analog is 5500IU/mg, which is about 30% higher than that (4500IU/mg) of the wild type. The solution conformation of [S-SO(3)(-)]rsCT was studied in aqueous trifluoroethanol solution by CD, 2D-NMR spectroscopy, and distance geometry calculations. In the mixture of 60% TFE and 40% water, the peptide assumes an amphipathic alpha-helix in the region of residues 4-22, which is one turn longer than that of the native sCT. The structural feature analysis of the peptide revealed the presence of hydrophobic surface composed of five hydrophobic side chains of residues Leu4, Leu9, Leu12, Leu16, and Leu19, and a network of salt-bridges that consisted of a tetrad of oppositely charged side chains (Cys7-SO(3)(-)-Lys11(+)-Glu15(-)-Lys18(+)). The multiple salt bridges resulted in the stabilization of the longer amphipathic alpha-helix. Meanwhile, the higher hypocalcemic potency of the peptide could be attributed to the array of hydrophobic side chains of five leucine residues of the amphipathic alpha-helix.

Cockroach diuretic hormones: characterization of a calcitonin-like peptide in insects

Insect diuretic hormones are crucial for control of water balance. We isolated from the cockroach Diploptera punctata two diuretic hormones (DH), Dippu-DH(31) and Dippu-DH(46), which increase cAMP production and fluid secretion in Malpighian tubules of several insect species. Dippu-DH(31) and -DH(46) contain 31 and 46 amino acids, respectively. Dippu-DH(46) belongs to the corticotropin-releasing factor (CRF)-like insect DH family, whereas Dippu-DH(31) has little sequence similarity to the CRF-like DH, but is similar to the calcitonin family. Dippu-DH(46) and -DH(31) have synergistic effects in D. punctata but have only additive effects in Locusta migratoria. Dippu-DH(31) represents a distinct type of insect DH with actions that differ from those of previously identified insect peptides with diuretic activity.

Conformational transitions and fibrillation mechanism of human calcitonin as studied by high-resolution solid-state 13C NMR

Conformational transitions of human calcitonin (hCT) during fibril formation in the acidic and neutral conditions were investigated by high-resolution solid-state 13C NMR spectroscopy. In aqueous acetic acid solution (pH 3.3), a local alpha-helical form is present around Gly10 whereas a random coil form is dominant as viewed from Phe22, Ala26, and Ala31 in the monomer form on the basis of the 13C chemical shifts. On the other hand, a local beta-sheet form as viewed from Gly10 and Phe22, and both beta-sheet and random coil as viewed from Ala26 and Ala31 were detected in the fibril at pH 3.3. The results indicate that conformational transitions from alpha-helix to beta-sheet, and from random coil to beta-sheet forms occurred in the central and C-terminus regions, respectively, during the fibril formation. The increased 13C resonance intensities of fibrils after a certain delay time suggests that the fibrillation can be explained by a two-step reaction mechanism in which the first step is a homogeneous association to form a nucleus, and the second step is an autocatalytic heterogeneous fibrillation. In contrast to the fibril at pH 3.3, the fibril at pH 7.5 formed a local beta-sheet conformation at the central region and exhibited a random coil at the C-terminus region. Not only a hydrophobic interaction among the amphiphilic alpha-helices, but also an electrostatic interaction between charged side chains can play an important role for the fibril formation at pH 7.5 and 3.3 acting as electrostatically favorable and unfavorable interactions, respectively. These results suggest that hCT fibrils are formed by stacking antiparallel beta-sheets at pH 7.5 and a mixture of antiparallel and parallel beta-sheets at pH 3.3.

Production of recombinant salmon calcitonin by in vitro amidation of an Escherichia coli produced precursor peptide

Salmon calcitonin (sCT) is a 32 amino acid peptide hormone that requires C-terminal amidation for full biological activity. We have produced salmon calcitonin by in vitro amidation of an E. coli produced precursor peptide. Glycine-extended sCT, the substrate for amidation, was produced in recombinant E. coli as part of a fusion with glutathione-S-transferase. The microbially produced soluble fusion protein was purified to near homogeneity by affinity chromatography. Following S-sulfonation of the fusion protein, the glycine-extended peptide was cleaved from the fusion by cyanogen bromide. The S-sulfonated peptide was recovered and enzymatically converted to the amidated peptide in a reaction with recombinant peptidylglycine alpha-amidating enzyme (alpha-AE) secreted from Chinese hamster ovary (CHO) cells. After reformation of the intramolecular disulfide bond, the sCT was purified with a step yield of 60%. The ease and speed of this recombinant process, as well as its potential for scale-up, make it adaptable to production demands for calcitonin, a proven useful agent for the treatment of post-menopausal osteoporosis. Moreover, the relaxed specificity of the recombinant alpha-AE for the penultimate amino acid which is amidated allows the basic process to be applied to the production of other amidated peptides.

Central nervous system binding sites for calcitonin and calcitonin gene-related peptide

Alternative splicing of the primary RNA transcript of the calcitonin gene leads to the generation of two distinct peptides, calcitonin (CT) and calcitonin gene-related peptide (CGRP). These peptides share only limited sequence homology and generally subserve different biological functions through their own distinct binding sites, which differ in specificity and distribution. Additionally, a binding site with high-affinity binding for both peptides that has a restricted pattern of distribution has been identified. The present article reviews the biochemical and morphological characteristics of centra CT and CGRP binding sites.

Two-dimensional NMR and structure determination of salmon calcitonin in methanol

The structure of the 32-residue peptide salmon calcitonin (sCT) in 90% MeOH-10% H2O has been investigated by two-dimensional NMR techniques and molecular modeling. Sequential assignments for nearly all of the 32 spin systems have been obtained, and results indicate that the heptaresidue loop formed by the disulfide bond between Cys-1 and Cys-7 is followed by an alpha-helical segment from Val-8 through Tyr-22. A region of conformational heterogeneity is observed for residues 20-25, resulting from the slow isomerism of the cis and trans forms of Pro-23. The C-terminal segment is found to exist in an extended conformation.

The calcitonin gene peptides: biology and clinical relevance

The calcitonin/CGRP multigene complex encodes a family of peptides: calcitonin, its C-terminal flanking peptide, katacalcin, and a third novel peptide, calcitonin gene-related peptide (CGRP). The 32-amino acid peptide calcitonin inhibits the osteoclast, thereby conserving skeletal mass during periods of potential calcium lack, such as pregnancy, growth, and lactation. This hormonal role is emphasized by observations that lower circulating calcitonin levels are associated with bone loss and that calcitonin replacement prevents further bone loss. Structurally, CGRP resembles calcitonin and has been implicated in neuromodulation and in the physiological regulation of blood flow. Here we review the molecular genetics, structure, and function of the calcitonin-gene peptides as analyzed in the laboratory and focus on more recent clinical studies relating to disorders and therapeutics.

Calcitonin inhibits the rise of intracellular calcium induced by thyrotropin-releasing hormone in GH3 cells

Both human and salmon calcitonins markedly inhibit the TRH-stimulated rise in intracellular [Ca2+] in GH3 cells. Calcitonin also inhibits prolactin release from these cells. Both [Ala] salmon calcitonin and salmon calcitonin (1-23) peptide amide also inhibit this rise in [Ca2+] and also inhibit TRH-stimulated prolactin release from GH3 cells as well as from primary pituitary cell cultures. It is likely that calcitonin inhibits prolactin release in the pituitary by decreasing the extent of the rise of intracellular calcium concentration. Neither an intact disulfide bond at the amino terminus nor residues 24-32 of the carboxyl terminus of salmon calcitonin are required for this inhibition.

Biologically active calcitonin analogs which have minimal interactions with phospholipids

A number of peptide hormones have been shown to contain amphipathic helical segments capable of binding to phospholipids. This conformational feature has been associated with increased biological activity of these hormones. We demonstrate, however, that two calcitonin analogs, [Gly8,Ala16]-des-Leu19 salmon calcitonin and des-1-amino-[Ala1,7,Gly8]-des-Leu19 salmon calcitonin have minimal interactions with phospholipids. Neither of these peptides acquire any increased helical content in the presence of dimyristoylphosphatidylglycerol and these peptides have only weak effects in altering the phase transition properties of this lipid. Therefore, although the presence of a phospholipid-induced amphipathic helical sequence may enhance the biological activity, it is not required for activity.

No requirements of cyclic conformation of antagonists in binding to vasopressin receptors

Early reports that acyclic analogues of oxytocin and vasopressin (AVP) have drastically reduced agonistic activities established as dogma that an intact hexapeptide ring structure is essential for the pharmacological activities of analogues of neurohypophysial hormones. Thus, virtually all the many hundreds of agonistic and antagonistic analogues of the neurohypophysial peptides that have been reported contain an intact ring. Here we report that an intact ring is not essential for binding of antagonistic AVP analogues to vasopressor (V1) or antidiuretic (V2) AVP receptors. In fact, one acyclic AVP analogue seems to be about as potent as any previously reported cyclic V2 antagonist. This finding suggests new possibilities for the design of AVP analogues as pharmacological probes and for therapeutic use. Similar modifications might be useful in the design of analogues of other cyclic peptides, such as calcitonin, somatostatin and the atrial natriuretic factors.