The anti-HIV drug abacavir stimulates β-catenin activity in osteoblast lineage cells

Bone mineral density (BMD) loss in people living with HIV occurs with the initiation of combined antiretroviral therapy (cART), particularly with tenofovir disoproxil fumarate (TDF) containing cART. Switching from TDF to abacavir (ABC) or dolutegravir (DTG) leads to increased BMD. Whether BMD gains are due to cessation of TDF or anabolic effects of ABC or DTG is unclear. We investigated the effects of ABC and DTG on osteoblast lineage cells in vitro and in vivo. Primary human osteoblasts and male C57BL/6 mice were treated with individual antiretrovirals (ARVs) or a combination of ABC/DTG/lamivudine (3TC). Nearly all ARVs and cART inhibited osteogenic activity in vitro. Due to the importance of Wnt/β-catenin in bone formation, we further investigated ARV effects on the Wnt/β-catenin pathway. ABC, alone and as part of ABC/DTG/3TC, increased osteoblastic β-catenin activity as indicated by increased TOPFlash activity, hypo-phosphorylated (active) β-catenin staining, and β-catenin targeted gene expression. Mice treated with TDF had decreased lumbar spine BMD and trabecular connectivity density in the vertebrae, while those treated with ABC/DTG/3TC reduced cortical area and thickness in the femur. Mice treated with ABC alone had no bone structural changes, increased circulating levels of the bone formation marker, P1NP, and elevated expression of the Wnt/β-catenin target gene, Lef1, in osteocyte enriched samples. Further, bones from ARV-treated mice were isolated to evaluate ARV distribution. All ARVs were detected in the bone tissue, which was inclusive of bone marrow, but when bone marrow was removed, only TDF, ABC, and DTG were detected at ~0.1% of the circulating levels. Overall, our findings demonstrate that ABC activates Wnt/β-catenin signaling, but whether this leads to increased bone formation requires further study. Assessing the impact of ARVs on bone is critical to informing ARV selection and/or discovery of regimens that do not negatively impact the skeleton.

EGFR-dependent endocytosis of Wnt9a and Fzd9b promotes β-catenin signaling during hematopoietic stem cell development in zebrafish

Cell-to-cell communication through secreted Wnt ligands that bind to members of the Frizzled (Fzd) family of transmembrane receptors is critical for development and homeostasis. Wnt9a signals through Fzd9b, the co-receptor LRP5 or LRP6 (LRP5/6), and the epidermal growth factor receptor (EGFR) to promote early proliferation of zebrafish and human hematopoietic stem cells during development. Here, we developed fluorescently labeled, biologically active Wnt9a and Fzd9b fusion proteins to demonstrate that EGFR-dependent endocytosis of the ligand-receptor complex was required for signaling. In human cells, the Wnt9a-Fzd9b complex was rapidly endocytosed and trafficked through early and late endosomes, lysosomes, and the endoplasmic reticulum. Using small-molecule inhibitors and genetic and knockdown approaches, we found that Wnt9a-Fzd9b endocytosis required EGFR-mediated phosphorylation of the Fzd9b tail, caveolin, and the scaffolding protein EGFR protein substrate 15 (EPS15). LRP5/6 and the downstream signaling component AXIN were required for Wnt9a-Fzd9b signaling but not for endocytosis. Knockdown or loss of EPS15 impaired hematopoietic stem cell development in zebrafish. Other Wnt ligands do not require endocytosis for signaling activity, implying that specific modes of endocytosis and trafficking may represent a method by which Wnt-Fzd specificity is established.

Sclerostin antibody improves alveolar bone quality in the Hyp mouse model of X-linked hypophosphatemia (XLH)

X-linked hypophosphatemia (XLH) is a rare disease of elevated fibroblast growth factor 23 (FGF23) production that leads to hypophosphatemia and impaired mineralization of bone and teeth. The clinical manifestations of XLH include a high prevalence of dental abscesses and periodontal disease, likely driven by poorly formed structures of the dentoalveolar complex, including the alveolar bone, cementum, dentin, and periodontal ligament. Our previous studies have demonstrated that sclerostin antibody (Scl-Ab) treatment improves phosphate homeostasis, and increases long bone mass, strength, and mineralization in the Hyp mouse model of XLH. In the current study, we investigated whether Scl-Ab impacts the dentoalveolar structures of Hyp mice. Male and female wild-type and Hyp littermates were injected with 25 mg·kg-1 of vehicle or Scl-Ab twice weekly beginning at 12 weeks of age and euthanized at 20 weeks of age. Scl-Ab increased alveolar bone mass in both male and female mice and alveolar tissue mineral density in the male mice. The positive effects of Scl-Ab were consistent with an increase in the fraction of active (nonphosphorylated) β-catenin, dentin matrix protein 1 (DMP1) and osteopontin stained alveolar osteocytes. Scl-Ab had no effect on the mass and mineralization of dentin, enamel, acellular or cellular cementum. There was a nonsignificant trend toward increased periodontal ligament (PDL) attachment fraction within the Hyp mice. Additional PDL fiber structural parameters were not affected by Scl-Ab. The current study demonstrates that Scl-Ab can improve alveolar bone in adult Hyp mice.

Wnt regulation of hematopoietic stem cell development and disease

Hematopoietic stem cells (HSCs) are multipotent stem cells that give rise to all cells of the blood and most immune cells. Due to their capacity for unlimited self-renewal, long-term HSCs replenish the blood and immune cells of an organism throughout its life. HSC development, maintenance, and differentiation are all tightly regulated by cell signaling pathways, including the Wnt pathway. Wnt signaling is initiated extracellularly by secreted ligands which bind to cell surface receptors and give rise to several different downstream signaling cascades. These are classically categorized either β-catenin dependent (BCD) or β-catenin independent (BCI) signaling, depending on their reliance on the β-catenin transcriptional activator. HSC development, homeostasis, and differentiation is influenced by both BCD and BCI, with a high degree of sensitivity to the timing and dosage of Wnt signaling. Importantly, dysregulated Wnt signals can result in hematological malignancies such as leukemia, lymphoma, and myeloma. Here, we review how Wnt signaling impacts HSCs during development and in disease.

Bone Quality in Relation to HIV and Antiretroviral Drugs

PURPOSE OF REVIEW

People living with HIV (PLWH) are at an increased risk for osteoporosis, a disease defined by the loss of bone mineral density (BMD) and deterioration of bone quality, both of which independently contribute to an increased risk of skeletal fractures. While there is an emerging body of literature focusing on the factors that contribute to BMD loss in PLWH, the contribution of these factors to bone quality changes are less understood. The current review summarizes and critically reviews the data describing the effects of HIV, HIV disease-related factors, and antiretroviral drugs (ARVs) on bone quality.

RECENT FINDINGS

The increased availability of high-resolution peripheral quantitative computed tomography has confirmed that both HIV infection and ARVs negatively affect bone architecture. There is considerably less data on their effects on bone remodeling or the composition of bone matrix. Whether changes in bone quality independently predict fracture risk, as seen in HIV-uninfected populations, is largely unknown. The available data suggests that bone quality deterioration occurs in PLWH. Future studies are needed to define which factors, viral or ARVs, contribute to loss of bone quality and which bone quality factors are most associated with increased fracture risk.

Sclerostin antibody improves phosphate metabolism hormones, bone formation rates, and bone mass in adult Hyp mice

X-linked hypophosphatemia (XLH) is caused by a loss-of-function mutation in the phosphate regulating gene with homology to endopeptidase located on the X chromosome (PHEX). Loss of functional PHEX results in elevated fibroblast growth factor 23 (FGF23), impaired phosphate reabsorption, and inhibited skeletal mineralization. Sclerostin, a protein produced primarily by osteocytes, suppresses bone formation by antagonizing canonical Wnt-signaling and is reported to be elevated in XLH patients. Our previous study reported that a monoclonal antibody to sclerostin (Scl-Ab) decreases FGF23 and increases phosphate and bone mass in growing Hyp mice (XLH murine model). In the current study, we investigated the efficacy of Scl-Ab in treating XLH pathophysiology in adult Hyp mice that are past the period of rapid skeletal growth (12 and 20-weeks old). We hypothesized that Scl-Ab would not only increase bone formation, bone strength and bone mass, but would also normalize phosphate regulating hormones, FGF23, parathyroid hormone (PTH), and vitamin 1,25(OH)2D. Scl-Ab treatment increased cortical area, trabecular bone volume fraction, trabecular bone formation rate, and the bending moment in both sexes of both age groups. Scl-Ab treatment suppressed circulating levels of intact FGF23 and c-term FGF23 in treated male and female wild-type and Hyp mice of both age groups and improved both vitamin 1,25(OH)2D and PTH. Scl-Ab treated Hyp mice also showed evidence of increased renal expression of the sodium-phosphate co-transporter, NPT2a, specifically in the female Hyp mice. Our study suggests that Scl-Ab treatment can improve several skeletal and metabolic pathologies associated with XLH, further establishes the role of sclerostin in the regulation of FGF23 and provides evidence that Scl-Ab can improve phosphate regulation by targeting the bone-renal axis.

Sclerostin Antibody Treatment Increases Bone Mass and Normalizes Circulating Phosphate Levels in Growing Hyp Mice

X-linked hypophosphatemia (XLH), caused by a loss-of-function mutation in the phosphate regulating gene with homology to endopeptidase located on the X chromosome (PHEX), is the most common form of vitamin D-resistant rickets. Loss of functional PHEX results in elevated fibroblast growth factor 23 (FGF23) levels, impaired phosphate reabsorption, and inhibited skeletal mineralization. Sclerostin, a protein produced primarily in osteocytes, suppresses bone formation by antagonizing Wnt signaling and is reported to be elevated in XLH patients. This study used the Hyp mouse model to investigate sclerostin's role in the pathophysiology of XLH by evaluating the use of a monoclonal antibody to sclerostin in a mouse model of XLH, the Hyp mouse. Male and female wild-type and Hyp littermates were injected with 25 mg/kg of vehicle or sclerostin antibody (Scl-Ab) twice weekly, beginning at 4 weeks of age and euthanized at 8 weeks of age. Scl-Ab treatment increased serum phosphate levels and suppressed circulating levels of intact FGF23 in treated wild-type and Hyp mice of both sexes. Cortical area, trabecular bone volume fraction (BV/TV), metaphyseal apparent density, and the peak load increased with Scl-Ab treatment in both sexes. This short-term treatment study suggests that Scl-Ab treatment can effectively improve some of the pathologies associated with XLH, including normalization of phosphate, and that sclerostin may play a role in regulating FGF23 and phosphate metabolism in XLH. © 2019 American Society for Bone and Mineral Research.

Bronchoalveolar lavage fluid concentrations of transforming growth factor (TGF)-beta1, TGF-beta2, interleukin (IL)-4 and IL-13 after segmental allergen challenge and their effects on alpha-smooth muscle actin and collagen III synthesis by primary human lung fibroblasts

RATIONALE

Asthmatic airway remodelling is characterized by myofibroblast hyperplasia and subbasement membrane collagen deposition. We hypothesized that cytokines and growth factors implicated in asthmatic airway remodelling are increased in bronchoalveolar lavage (BAL) fluid of asthmatics after segmental allergen challenge (SAC), and that these growth factors and cytokines increase alpha-smooth muscle actin (alpha-SMA) and collagen III synthesis by human lung fibroblasts (HLFs).

METHODS

Transforming growth factor (TGF)-beta1, TGF-beta2, IL-4 and IL-13 levels were measured in BAL fluid from 10 asthmatics and 9 non-asthmatic controls at baseline and then 1 day, 1 week and 2 weeks after SAC. Confluent cultures of HLFs were stimulated by exogenous addition of TGF-beta1, TGF-beta2, IL-4 or IL-13 (concentration range 0.01-10 ng/mL) over 48 h. Collagen III was measured in culture supernates and alpha-SMA in cell lysates by Western blot.

RESULTS

At baseline, there was no difference in BAL fluid concentrations of TGF-beta1, IL-4 and IL-13 between asthmatics and controls; however, non-asthmatics had higher concentrations of total TGF-beta2. In asthmatics, BAL fluid concentrations of all four factors increased significantly 1 day after SAC. TGF-beta1, TGF-beta2 and IL-13 concentrations returned to baseline by 1 week after SAC, but BAL fluid IL-4 concentration remained elevated for at least 2 weeks. TGF-beta1, TGF-beta2 and IL-4 significantly increased alpha-SMA in fibroblasts, but only IL-4 caused corresponding increases in collagen III synthesis. IL-13 had no direct effects on collagen III synthesis and alpha-SMA expression.

CONCLUSIONS

Because IL-4 caused a dose-dependent increase in alpha-SMA and collagen III synthesis, it may be an important cytokine mediating asthmatic airway remodelling. TGF-beta1 and TGF-beta2 may also play a role in airway remodelling by stimulating phenotypic change of fibroblasts to myofibroblasts. Additionally, collagen III synthesis appears to be independent of myofibroblast phenotype and is apparently regulated by different growth factors and cytokines.

Association between dietary factors and calcium oxalate and magnesium ammonium phosphate urolithiasis in cats

OBJECTIVE

To identify dietary factors associated with the increase in occurrence of calcium oxalate (CaOx) uroliths and the decrease in occurrence of magnesium ammonium phosphate (MAP) uroliths in cats.

DESIGN

Case-control study.

ANIMALS

173 cats with CaOx uroliths, 290 cats with MAP uroliths, and 827 cats without any urinary tract diseases.

PROCEDURE

Univariate and multivariate logistic regression were performed.

RESULTS

Cats fed diets low in sodium or potassium or formulated to maximize urine acidity had an increased risk of developing CaOx uroliths but a decreased risk of developing MAP uroliths. Additionally, compared with the lowest contents, diets with the highest moisture or protein contents and with moderate magnesium, phosphorus, or calcium contents were associated with decreased risk of CaOx urolith formation. In contrast, diets with moderate fat or carbohydrate contents were associated with increased risk of CaOx urolith formation. Diets with the highest magnesium, phosphorus, calcium, chloride, or fiber contents and moderate protein content were associated with increased risk of MAP urolith formation. On the other hand, diets with the highest fat content were associated with decreased risk of MAP urolith formation.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggest that diets formulated to contain higher protein, sodium, potassium, moisture, calcium, phosphorus, and magnesium contents and with decreased urine acidifying potential may minimize formation of CaOx uroliths in cats. Diets formulated to contain higher fat content and lower protein and potassium contents and with increased urine acidifying potential may minimize formation of MAP uroliths.

Turn structures in CGRP C-terminal analogues promote stable arrangements of key residue side chains

The 37-amino acid calcitonin gene-related peptide (CGRP) is a potent endogenous vasodilator thought to be implicated in the genesis of migraine attack. CGRP antagonists may thus have therapeutic value for the treatment of migraine. The CGRP C-terminally derived peptide [D(31),P(34),F(35)]CGRP(27-37)-NH(2) was recently identified as a high-affinity hCGRP(1) receptor selective antagonist. Reasonable CGRP(1) affinity has also been demonstrated for several related analogues, including [D(31),A(34),F(35)]CGRP(27-37)-NH(2). In the study presented here, conformational and structural features in CGRP(27-37)-NH(2) analogues that are important for hCGRP(1) receptor binding were explored. Structure-activity studies carried out on [D(31),P(34),F(35)]CGRP(27-37)-NH(2) resulted in [D(31),P(34),F(35)]CGRP(30-37)-NH(2), the shortest reported CGRP C-terminal peptide analogue exhibiting reasonable hCGRP(1) receptor affinity (K(i) = 29.6 nM). Further removal of T(30) from the peptide's N-terminus greatly reduced receptor affinity from the nanomolar to micromolar range. Additional residues deemed critical for hCGRP(1) receptor binding were identified from an alanine scan of [A(34),F(35)]CGRP(28-37)-NH(2) and included V(32) and F(37). Replacement of the C-terminal amide in this same peptide with a carboxyl, furthermore, resulted in a greater than 50-fold reduction in hCGRP(1) affinity, thus suggesting a direct role for the amide moiety in receptor binding. The conformational properties of two classes of CGRP(27-37)-NH(2) peptides, [D(31),X(34),F(35)]CGRP(27-37)-NH(2) (X is A or P), were examined by NMR spectroscopy and molecular modeling. A beta-turn centered on P(29) was a notable feature consistently observed among active peptides in both series. This turn led to exposure of the critical T(30) residue to the surrounding environment. Peptides in the A(34) series were additionally characterized by a stable C-terminal helical turn that resulted in the three important residues (T(30), V(32), and F(37)) adopting consistent interspatial positions with respect to one another. Peptides in the P(34) series were comparatively more flexible at the C-terminus, although a large proportion of the [D(31),P(34),F(35)]CGRP(27-37)-NH(2) calculated conformers contained a gamma-turn centered on P(34). These results collectively suggest that turn structures at both the C-terminus and N-terminus of CGRP(27-37)-NH(2) analogues may help to appropriately orient critical residues (T(30), V(32), and F(37)) for hCGRP(1) receptor binding.

Patient and environmental factors associated with calcium oxalate urolithiasis in dogs

OBJECTIVE

To test the hypothesis that breed, age, sex, body condition, and environment are risk factors for development of calcium oxalate uroliths in dogs.

DESIGN

Case-control study.

ANIMALS

1,074 dogs that formed calcium oxalate uroliths and 1,724 control dogs that did not have uroliths.

PROCEDURE

A validated multiple-choice questionnaire was designed to collect information from veterinarians and owners within 1 year of the date of urolith detection concerning signalment and environment of the dogs. Univariate and multivariate analyses were performed to calculate odds ratios to assess whether breed, age, sex, body condition, and environment were risk factors for calcium oxalate urolith formation.

RESULTS

Middle-aged (8- to 12-year-old) castrated male dogs had increased risk for formation of calcium oxalate uroliths. Urolith formation was also associated with increasing age. Dogs of certain breeds, including Miniature and Standard Schnauzer, Lhasa Apso, Yorkshire Terrier, Bichon Frise, Shih Tzu, and Miniature and Toy Poodle, had increased risk for developing calcium oxalate uroliths. Overweight dogs also had increased risk.

CONCLUSIONS AND CLINICAL RELEVANCE

Knowledge of patient and environmental risk factors for development of calcium oxalate uroliths may facilitate development of surveillance strategies that result in earlier detection of this disease. Modification of environmental factors and body weight may minimize calcium oxalate urolith formation and recurrence.

Association between patient-related factors and risk of calcium oxalate and magnesium ammonium phosphate urolithiasis in cats

OBJECTIVE

To determine whether breed, age, sex, or reproductive status (i.e., neutered versus sexually intact) was associated with the apparent increase in prevalence of calcium oxalate (CaOx) uroliths and the decrease in prevalence of magnesium ammonium phosphate (MAP) uroliths in cats over time.

DESIGN

Case-control study.

ANIMALS

Case cats consisted of cats with CaOx (n = 7,895) or MAP (7,334) uroliths evaluated at the Minnesota Urolith Center between 1981 and 1997. Control cats consisted of cats without urinary tract disease admitted to veterinary teaching hospitals in the United States and Canada during the same period (150,482).

PROCEDURE

Univariate and multivariate logistic regression were performed.

RESULTS

British Shorthair, Exotic Shorthair, Foreign Shorthair, Havana Brown, Himalayan, Persian, Ragdoll, and Scottish Fold cats had an increased risk of developing CaOx uroliths, as did male cats and neutered cats. Chartreux, domestic shorthair, Foreign Shorthair, Himalayan, Oriental Shorthair, and Ragdoll cats had an increased risk of developing MAP uroliths, as did female cats and neutered cats. Cats with CaOx uroliths were significantly older than cats with MAP uroliths.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggest that changes in breed, age, sex, or reproductive status did not contribute to the apparent reciprocal relationship between prevalences of CaOx and MAP uroliths in cats during a 17-year period. However, cats of particular breeds, ages, sex, and reproductive status had an increased risk of developing CaOx and MAP uroliths.

Hydrophobic forces are responsible for the folding of a highly potent natriuretic peptide analogue at a membrane mimetic surface: an NMR study

A conformational study by nmr spectroscopy was performed with the highly active 28 residue hybrid natriuretic peptide analogue pBNP1 [M. Mimeault, A. De Léan, M. Lafleur, D. Bonenfant, and A. Fournier (1995) Biochemistry, Vol. 34, pp. 955-964], which consists of the cyclic peptide core of pBNP32 and the N- and C-terminal exocyclic segments of rANP (99-126). In purely aqueous solution pBNP1 exhibits random coil behavior as evidenced by the almost complete absence of structurally significant nmr observables. By contrast, elements of secondary structure emerged upon the addition of dodecylphosphocholine micelles to the aqueous sample. Nuclear Overhauser effect distance-restrained molecular dynamics simulations in conjunction with torsional angle determinations permitted the generation of reasonable model of the lipid-bound conformation of pBNP1. According to this model, pBNP1 adopts turn-like features in the cyclic and C-terminal regions of the peptide, but remains quite flexible in the N-terminal segment. Two hydrophobic cores separated by a hydrophilic cleft were also evident in the generated structure. A mechanism is proposed whereby the hydrophobic interactions necessary to stabilize a folded structure of pBNP1 are facilitated by the presence of the membrane-like polar/apolar interface provided by the phospholipid micelles.

The glycine residue in cyclic lactam analogues of galanin(1-16)-NH2 is important for stabilizing an N-terminal helix

The neuropeptide galanin is a 29- or 30-residue peptide whose physiological functions are mediated by G-protein-coupled receptors. Galanin's agonist activity has been shown to be associated with the N-terminal sequence, galanin(1-16). Conformational investigations previously carried out on full-length galanin have, furthermore, indicated the presence of a helical conformation in the neuropeptide's N-terminal domain. Several cyclic lactam analogues of galanin(1-16)-NH2 were prepared in an attempt to stabilize an N-terminal helix in the peptide. Here we describe and compare the solution conformational properties of these analogues in the presence of SDS micelles as determined by NMR, CD, and fluorescence spectroscopy. Differences in CD spectral profiles were observed among the compounds that were studied. Both c[D4, K8]Gal(1-16)-NH2 and c[D4,K8]Gal(1-12)-NH2 adopted stable helical conformations in the micelle solution. On the basis of the analyses of their respective alpha H chemical shifts and NOE patterns, this helix was localized to the first 10 residues. The distance between the aromatic rings of Trp2 and Tyr9 in c[D4, K8]Gal(1-16)-NH2 was determined to be 10.8 +/- 3 A from fluorescence resonance energy transfer measurements. This interchromophore spacing was found to be more consistent with a helical structure than an extended one. Removal of the Gly1 residue in compounds c[D4,K8]Gal(1-16)-NH2 and c[D4, K8]Gal(1-12)-NH2 resulted in a loss of helical conformation and a concomitant reduction in binding potency at the GalR1 receptor but not at the GalR2 receptor. The nuclear Overhauser enhancements obtained for the Gly1 deficient analogues did, however, reveal the presence of nascent helical structures within the N-terminal sequence. Decreasing the ring structure size in c[D4, K8]Gal(1-16)-NH2 by replacing Lys8 with an ornithine residue or by changing the position of the single lysine residue from eight to seven was accompanied by a complete loss of helical structure and dramatically reduced receptor affinity. It is concluded from the data obtained for the series of cyclic galanin(1-16)-NH2 analogues that both the ring structure size and the presence of an N-terminal glycine residue are important for stabilizing an N-terminal helix in these compounds. However, although an N-terminal helix constitutes a predominant portion of the conformational ensemble for compounds c[D4,K8]Gal(1-16)-NH2 and c[D4, K8]Gal(1-12)-NH2, these peptides nevertheless are able to adopt other conformations in solution. Consequently, the correlation between the ability of the cyclic galanin analogues to adopt an N-terminal helix and bind to the GalR1 receptor may be considered as a working hypothesis.

Subtleties of structure-agonist versus antagonist relationships of opioid peptides and peptidomimetics

The development of novel delta opioid antagonists and delta opioid agonists structurally derived from the prototype delta antagonist TIPP (H-Tyr-Tic-Phe-Phe-OH), is reviewed. Both delta antagonists and delta agonists with extraordinary potency and unprecedented delta receptor selectivity were discovered. Some of them are already widely used as pharmacological tools and are also of interest as potential therapeutic agents for use in analgesia. The results of the performed structure-activity studies revealed that the delta antagonist versus delta agonist behavior of this class of compounds depended on very subtle structural differences in diverse locations of the molecule. These observations can be best explained with a receptor model involving a number of different inactive and active receptor conformations.

Aggregation behaviour and Zn2+ binding properties of secretin

Dynamic light scattering measurements were carried out on secretin in aqueous solution (2 mM; pH 5.0). The results indicated that the molecule exists as a fairly compact hexamer under these solution conditions. Secondary structural properties of the secretin hexameric complex were evaluated using CD and NMR spectroscopy. Specifically, the spectral properties of secretin in water were examined as a function of peptide concentration. Results from the analyses indicated a 2-fold increase (17-32%) in alpha-helical content within the region Ser11-Arg21 as the peptide concentration was increased from 0.1 to 2 mM. Displacement of the alphaH proton chemical shifts relative to random coil values did not alter significantly with increasing peptide concentration. This observation confirmed that the length of the helical segment is independent of peptide concentration between 0.1 and 2 mM. The nature of the helix was furthermore determined as amphipathic, and thus the potential for a cooperative intermolecular association through the apolar helical face of individual monomers was indicated. These findings suggest that secretin aggregates into symmetric hexamers at millimolar concentrations and, furthermore, that the helical domain is stabilized through this intermolecular association. The potential for secretin to bind divalent cations, including Ca2+ and Zn2+, was also examined by CD1 and NMR spectroscopy. The results revealed that Zn2+ specifically coordinates to the His1 and Asp3 residues of each secretin monomer without disrupting the peptide's helical structure, whereas Ca2+ did not exhibit any interaction with the peptide hormone. It was concluded from these studies that secretin may be stored in a hexameric form within its secretory tissues and that zinc may play a role in the storage of secretin through a specific interaction with the N-terminal histidine and aspartic acid residues.

The receptor-bound conformation of H-Tyr-Tic-(Phe-Phe)-OH-related delta-opioid antagonists contains all trans peptide bonds

Two different models for the receptor-bound conformation of delta-opioid peptide antagonists containing the N-terminal dipeptide segment H-Tyr-Tic (Tic = 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid) have been proposed. Both models are based on spatial overlap of the Tyr1 and Tic2 aromatic rings and N-terminal amino group with the corresponding aromatic rings and nitrogen atom of the nonpeptide delta-antagonist naltrindole. However, in one model the peptide bond between the Tyr1 and Tic2 residues assumes the trans conformation, whereas in the other it is in the cis conformation. To distinguish between these two models, we prepared the two peptides H-Tyr(psi)[CH2NH]Tic-Phe-Phe-OH and H-Tyr(psi)[CH2NH]MeTic-Phe-Phe-OH (MeTic = 3-methyl-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid) in which a cis peptide bond between the Tyr and Tic (or MeTic) residues is sterically forbidden. Both compounds turned out to be moderately potent delta-opioid antagonists in the mouse vas deferens assay. A molecular mechanics study performed with both peptides resulted in low-energy conformations in which the torsional angle ("omega1") of the reduced peptide bond between Tyr and Tic (or MeTic) had a value of 180 degrees (trans conformation) and which were in good agreement with the proposed model with all trans peptide bonds. Furthermore, this study confirmed that neither of these two peptides could assume low-energy conformations in which "omega1" had a value of 0 degrees (cis conformation). Conformers with that same bond in the gauche conformation ("omega1" = -60 degrees) were also identified, but were higher in energy and showed no spatial overlap with naltrindole. On the basis of these results it is concluded that the receptor-bound conformation of delta-peptide antagonists containing an N-terminal H-Tyr-Tic-dipeptide segment must have all trans peptide bonds.

The octapeptide angiotensin II adopts a well-defined structure in a phospholipid environment

The conformational properties adopted by angiotensin II in a phospholipid micelle solution were studied by NMR spectroscopy and molecular modelling. The octapeptide was found to assume a well-defined hairpin structure with its C- and N-termini approaching to within 0.76 nm of each other. Three of the residues had fixed side chain configurations; Tyr4 (g+), His6 (g-) and Val3 (g-). Consequently, the His6 and Tyr4 aromatic rings were consistently close together. Conformers containing a cis His6-Pro7 peptide bond were observed for the peptide in a purely aqueous sample but completely disappeared when lipid vesicles were added to the sample. This result is explained by the existence of a very stable hydrogen bond between the Phe8 NH and the His6 carbonyl group of the lipid-solvated trans isomer, resulting in the formation of an inverse gamma turn centered on Pro7. 1H-NMR selective line broadening was apparent for several of the angiotensin II protons upon titration of an aqueous sample with less than stoichiometric amounts of 1,2-dimyristoyl-sn-glycero-3-phosphocholine bilayer vesicles. The data obtained were consistent with the structure derived for micelle-bound angiotensin II, indicating that conformations adopted by the peptide hormone in the presence of micelles and lipid-bilayer vesicles are similar.

Role of hydrophobic substituents in the interaction of opioid Tyr-Tic dipeptide analogs with dodecylphosphocholine micelles. Molecular partitioning in model membrane systems

The conformational properties of three Tyr-Tic-NH-R dipeptide analogs [where R = (CH2)2-Ph, (CH2)3-Ph or (CH2)2-cHx; Ph = phenyl; cHx = cyclohexyl and Tic = tetrahydroisoquinoline-3-carboxylic acid] have been investigated in purely aqueous solution and in the presence of fully deuterated dodecylphosphocholine micelles. H-Tyr-Tic-NH-(CH2)2-Ph is an opioid delta-agonist, whereas H-Tyr-Tic-NH-(CH2)3-Ph is a fairly potent delta-antagonist. H-Tyr-Tic-NH-(CH2)2-cHx is a less potent delta-antagonist. 1H-NMR spectra revealed that conformers containing cis and trans configurations of the Tyr-Tic peptide bond were present in all compounds in H2O and the H2O/lipid solvent. Analyses of the NMR data for the compounds in H2O indicate that in all three dipeptides the C-terminal substituent is flexible and the Tyr-side-chain adopts a trans orientation in most of the conformations. This promotes a compact Tyr-Tic structure. NOE patterns observed for the compounds in the micelle solution indicate that Tyr has an even greater tendency to assume a trans side chain configuration in the biphasic-solvent system. This feature was more pronounced in the trans conformers than in the cis conformers. Specific lipid-peptide interactions were indicated by NOESY spectra acquired for micelle samples incorporating 20% (by mass) protonated lipid. According to the obtained NOE data, Tyr and Tic form an aromatic cluster which preferentially inserts into the lipid interior of the micelle for the trans conformers of all three dipeptides and for the cis conformer of H-Tyr-Tic-NH-(CH2)2-Ph. For the cis isomers, partitioning of the C-terminal substituents into the lipid phase exhibited more diverse behaviour. The cis conformers of H-Tyr-Tic-NH-(CH2)3-Ph and H-Tyr-Tic-NH-(CH2)2-cHx preferentially anchor to the micelle via their C-terminal substituent, while the corresponding region in H-Tyr-Tic-NH-(CH2)2-Ph remains flexible and immersed in the aqueous phase. The inconsistent mode of peptide-micelle interaction observed for cis conformers of the three compounds studied is explained in terms of differences in their dipeptide-substituent hydrophobicities. The more apolar the substituent, the greater its tendency to preferentially insert into the lipid core of the micelle. Amide-proton temperature coefficients measured for the three peptides revealed differences amongst the cis and trans isomers. The amide proton in the trans conformer of each compound is highly exposed to the aqueous phase in both solvent systems studied, whereas the cis NH proton of each peptide is only partially exposed. These results demonstrate that a subtle structural modification of an active peptide analog can result in dramatic changes of its biological activity and its mode of partitioning at a membrane surface.

Distinct conformational preferences of three cyclic beta-casomorphin-5 analogs determined using NMR spectroscopy and theoretical analysis

The conformational properties of three cyclic beta-casomorphin analogs based on the general formula H-Tyr-c[-D-Orn-2-Nal-D-Pro-Xaa-] (2-Nal = 2-naphthylalanine; Xaa = D-Ala, Sar or NMe-Ala) in DMSO solution were investigated using NMR spectroscopy in conjunction with molecular modeling techniques. The D-Ala5- and Sar5-analogs (compounds 1 and 2 respectively) are potent mixed mu-agonist/delta-antagonists with high mu- and delta-opioid receptor affinities, whereas the NMe-Ala5-analog (compound 3) is a potent mu-agonist and a weak partial delta-agonist. Distinct conformational differences emerged for the three compounds studied. Flexibility in the bare ring structures was found to increase in the order 3 < 2 < 1. The increased structural rigidity of 3 may be responsible for its low delta-receptor affinity as compared to the two other analogs. A low fractional population of conformers containing two cis peptide bonds was found for compound 2 but not for analog 1 or 3. Initial evidence for this observation was obtained from NMR differential line-broadening experiments and later confirmed by molecular mechanics simulations. Comparison of the temperature dependence of amide proton chemical shifts acquired for the three cyclic analogs indicate a large degree of intramolecular hydrogen bonding for 1 but not for the other two peptides.

Stabilization of an isolated helical capping box in solution by hydrophobic interactions: evidence from the NMR study of bioactive peptides from the C-terminus of human C5a anaphylatoxin

Synthetic analogues of the C-terminal portion of C5a were designed and found to be agonists of the C5a receptor [J. A. Ember et al. (1992) Jounral of Immunology, Vol. 148, p. 3165]. Nuclear magnetic resonance experiments were carried out to determine the solution conformation of the most potent analogue, the peptide C5a 65-74 (Tyr65, Phe67) (Tyr65-Ser66-Phe67-Lys68-Asp69-Met70 -Gln71- Leu72-Gly73-Arg74). Medium-range nuclear Overhauser effects (NOEs) were observed for residues 65-70 of this C5a peptide, suggesting that this region adopts a folded conformation in a significant population of the solution conformational ensemble. Quantitative analyses of (3)J(NH-alphaH) coupling constants and sequential NOE cross peaks gave an estimated helical population of 65% in the region Ser66-Met70. Additional evidence supporting the presence of a helical turn includes reduced amide-proton temperature coefficients and lowered (3)J(NH-alphaH) coupling constants in the region of Phe67-Met70. Conformational behavior of this C5a analogue peptide was studied using molecular modeling incorporating observed NOEs as constraints. The side chains of Tyr65, Phe67, and Met70 consistently form a hydrophobic cluster in all the model structures. The side chains of residues Ser66 and Asp69 can form reciprocal hydrogen bonds with the backbone NH groups of these two residues, indicating that residues Ser66-Phe67-Lys68-Asp69 (or SFKD) form a helix-stabilizing capping box (E. T. Harper and G. D. Rose (1993) Biochemistry, Vol. 32, p. 7605; H. X. Zhou et al. (1994) Proteins: Structure, Function and Genetics, Vol. 18, p. 1] even within the single turn of helical structure found in the analogue C5a peptide. A comparison of nmr results obtained for the analogue peptide and the natural decapeptide C5a 65-74 (Ile65-Ser66-His67-Lys68-Asp-69- Met70-Gln71-Leu72-Gly73-Arg74) indicated that incorporation of residues Tyr65 and Phe67 helps stabilize an isolated capping box involving residues Ser66-Asp69 in the C5a peptides through more extensive hydrophobic/aromatic interactions between residues Tyr65, Phe67, and Met70 in the analogue peptide C5a 65-74 (Tyr65, Phe67). These results constitute the first experimental demonstration of hydrophobic determinants in helical capping-box interactions, proposed recently by a statistical analysis of protein structures [J. W. Seale et al. (1994) Protein Science, Vol. 3, pp. 1741-1745]. The stabilized helical turn may also account for the greater potency of the analogue peptide C5a65-74 (Tyr65, Phe67) in receptor-binding assays.

Influence of sample pH on the conformational backbone dynamics of a pseudotripeptide (H-Tyr-Tic psi [CH2-NH]Phe-OH) incorporating a reduced peptide bond: an NMR investigation

In the present paper we investigate the influence of sample pH on the conformational and dynamical properties of the pseudotripeptide H-Tyr-Tic psi [CH2-NH]Phe-OH (TIP[psi]; Tic: 1,2,3,4,-tetrahydroisoquinoline-3-carboxylic acid) using various one- and two-dimensional nmr techniques in conjunction with molecular modeling. Studies were conducted at three different pH levels corresponding to the zwitterionic peptide containing a formal positive charge (pH 3.1), the deprotonated molecule (pH 9.1), and a situation at neutral pH (pH 7.2) involving both protonated and deprotonated states of the reduced peptide bond. Analysis of the one-dimensional1H-nmr spectra reveals that in solution TIP[psi] is in slow dynamic exchange between conformations containing cis and trans configurations of the Tyr-Tic bond. An nmr pH dependence study of the cis:trans ratio indicated that the exchange process was governed by the protonation state of the reduced bone amine. From the nmr data, reduced peptide bond pK alpha values of 6.5 and 7.5 were determined for the cis and trans conformers, respectively. It was concluded that conformations containing a trans Tyr-Tic bond are stabilized at low pH by an intramolecular hydrogen bond between the Tyr carbonyl and the reduced peptide bond protonated amine. This observation was corroborated by molecular mechanics investigations that revealed low energy trans structures compatible with nmr structural data, and furthermore, were consistently characterized by the existence of a strong N+ H ... O = C interaction closing a seven-membered cycle. The dynamics of cis-trans isomerization about the Tyr-Tic peptide bond were probed by nmr exchange experiments. The selective presaturation of exchanging resonances carried out at several temperatures between 50 and 70 degrees C allowed the determination of isomerization rate constants as well as thermodynamic activation parameters. delta G not equal to values were in close agreement with the cis-->trans energy barrier found in X-Pro peptide fragments (approximately 83 kJ/mol). A large entropic barrier determined for the trans-->cis conversion of TIP[psi] (5.7 JK-1 mol-1 at pH 3.1;6.5 J K-1 mol-1 at pH 9.1) is discussed in terms of decreased solvent molecular ordering around the conformers possessing a trans Tyr-Tic bond. Evidence that the neutral form of the reduced peptide bond gains rigidity upon protonation was obtained from relaxation measurements in the rotating frame. T1 rho measurements of several protons in the vicinity of the reduced peptide bond were made as a function of spin-lock field.(ABSTRACT TRUNCATED AT 400 WORDS)

The effects of positive and negative pre-sleep stimuli on dream experiences

In this study, the emotional and physical content of dreams was examined with post-sleep reports. In the first phase of the study, 45 students were asked to keep a dream diary for a week. The next week, the students were asked to look at a picture with positive, neutral, or negative affect in the evening before going to bed and then to record their dreams the following morning. Results showed that the pictures produced corresponding affect in morning dream reports, though physical elements of dreams and pictorial stimuli were not related to affect.