The complex fate in plasma of gadolinium incorporated into high-density lipoproteins used for magnetic imaging of atherosclerotic plaques

We have previously reported enhancing the imaging of atherosclerotic plaques in mice using reconstituted high density lipoproteins (HDL) as nanocarriers for the MRI contrast agent gadolinium (Gd). This study focuses on the underlying mechanisms of Gd delivery to atherosclerotic plaques. HDL, LDL, and VLDL particles containing Gd chelated to phosphatidyl ethanolamine (DTPA-DMPE) and a lipidic fluorophore were used to demonstrate the transfer of Gd-phospholipids among plasma lipoproteins in vitro and in vivo. To determine the basis of this transfer, the roles of phospholipid transfer protein (PLTP) and lipoprotein lipase (LpL) in mediating the migration of Gd-DTPA-DMPE among lipoproteins were investigated. The results indicated that neither was an important factor, suggesting that spontaneous transfer of Gd-DTPA-DMPE was the most probable mechanism. Finally, two independent mouse models were used to quantify the relative contributions of HDL and LDL reconstituted with Gd-DTPA-DMPE to plaque imaging enhancement by MR. Both sets of results suggested that Gd-DTPA-DMPE originally associated with LDL was about twice as effective as that injected in the form of Gd-HDL, and that some of Gd-HDL's effectiveness in vivo is indirect through transfer of the imaging agent to LDL. In conclusion, the fate of Gd-DTPA-DMPE associated with a particular type of lipoprotein is complex, and includes its transfer to other lipoprotein species that are then cleared from the plasma into tissues.

Comparison of synthetic high density lipoprotein (HDL) contrast agents for MR imaging of atherosclerosis

Determining arterial macrophage expression is an important goal in the molecular imaging of atherosclerosis. Here, we compare the efficacy of two synthetic, high density lipoprotein (HDL) based contrast agents for magnetic resonance imaging (MRI) of macrophage burden. Each form of HDL was labeled with gadolinium and rhodamine to allow MRI and fluorescence microscopy. Either the 37 or 18 amino acid peptide replaced the apolipoprotein A-I in these agents, which were termed 37pA-Gd or 18A-Gd. The diameters of 37pA-Gd and 18A-Gd are 7.6 and 8.0 nm, respectively, while the longitudinal relaxivities are 9.8 and 10.0 (mM s)(-1). 37pA has better lipid binding properties. In vitro tests with J774A.1 macrophages proved the particles possessed the functionality of HDL by eliciting cholesterol efflux and were taken up in a receptor-like fashion by the cells. Both agents produced enhancements in atherosclerotic plaques of apolipoprotein E knockout mice of approximately 90% (n = 7 per agent) and are macrophage specific as evidenced by confocal microscopy on aortic sections. The half-lives of 37pA-Gd and 18A-Gd are 2.6 and 2.1 h, respectively. Despite the more favorable lipid interactions of 37pA, both agents gave similar, excellent contrast for the detection of atherosclerotic macrophages using MRI.

HDL as a contrast agent for medical imaging

Contrast-enhanced MRI of atherosclerosis can provide valuable additional information on a patient's disease state. As a result of the interactions of HDL with atherosclerotic plaque and the flexibility of its reconstitution, it is a versatile candidate for the delivery of contrast-generating materials to this pathogenic lesion. We herein discuss the reports of HDL modified with gadolinium to act as an MRI contrast agent for atherosclerosis. Furthermore, HDL has been modified with fluorophores and nanocrystals, allowing it to act as a contrast agent for fluorescent imaging techniques and for computed tomography. Such modified HDL has been found to be macrophage specific, and, therefore, can provide macrophage density information via noninvasive MRI. As such, modified HDL is currently a valuable contrast agent for probing preclinical atherosclerosis. Future developments may allow the application of this particle to further diseases and pathological or physiological processes in both preclinical models as well as in patients.

High-relaxivity gadolinium-modified high-density lipoproteins as magnetic resonance imaging contrast agents

There is an ongoing desire to produce high-relaxivity, Gd-based magnetic resonance imaging (MRI) contrast agents. These may allow for lower doses to be used, which is especially important in view of the current safety concerns surrounding Gd in patients. Here we report the synthesis of a high-relaxivity MRI contrast agent, by incorporating Gd-chelating lipids that coordinate two water molecules into high-density lipoprotein (q = 2 HDL). We compared the properties of q = 2 HDL with those of an analogous HDL particle labeled with Gd-chelating lipids that coordinate only one water molecule (q = 1 HDL). We found that the q = 2 HDL possessed an elevated r(1) of 41 mM(-1) s(-1) compared to 9 mM(-1) s(-1) for q = 1 HDL at 20 MHz, but the q = 2 HDL exhibited high R(2)* values at high fields, precluding imaging above 128 MHz. While carrying out this investigation we observed that enlarged, disrupted particles were formed when the synthesis was carried out above the lipid critical micelle concentration (cmc), indicating the importance of synthesis below the cmc when modifying lipoproteins in this manner. The high relaxivity of q = 2 HDL means it will be an efficacious contrast agent for future MR imaging studies.

Incorporation of an apoE-derived lipopeptide in high-density lipoprotein MRI contrast agents for enhanced imaging of macrophages in atherosclerosis

Magnetic resonance (MR) imaging is becoming a pivotal diagnostic method to identify and characterize vulnerable atherosclerotic plaques. We previously reported a reconstituted high-density lipoprotein (rHDL) nanoparticle platform enriched with Gd-based amphiphiles as a plaque-specific MR imaging contrast agent. Further modification can be accomplished by inserting targeting moieties into this platform to potentially allow for improved intraplaque macrophage uptake. Since studies have indicated that intraplaque macrophage density is directly correlated to plaque vulnerability, modification of the rHDL platform may allow for better detection of vulnerable plaques. In the current study we incorporated a carboxyfluoresceine-labeled apolipoprotein E-derived lipopeptide, P2fA2, into rHDL. The in vitro macrophage uptake and in vivo MR efficacy were demonstrated using murine J774A.1 macrophages and the apolipoprotein E knock-out (apoE(-/-)) mouse model of atherosclerosis. The in vitro studies indicated enhanced association of murine macrophages to P2fA2 enriched rHDL (rHDL-P2A2) nanoparticles, relative to rHDL, using optical techniques and MR imaging. The in vivo studies showed a more pronounced and significantly higher signal enhancement of the atherosclerotic wall 24 h after the 50 micromol Gd/kg injection of rHDL-P2A2 relative to administration of rHDL. The normalized enhancement ratio for atherosclerotic wall of rHDL-P2A2 contrast agent injection was 90%, while that of rHDL was 53% 24 h post-injection. Confocal laser scanning microscopy revealed that rHDL-P2A2 nanoparticles co-localized primarily with intraplaque macrophages. The results of the current study confirm the hypothesis that intraplaque macrophage uptake of rHDL may be enhanced by the incorporation of the P2fA2 peptide into the modified HDL particle.

Nanocrystal core high-density lipoproteins: a multimodality contrast agent platform

High density lipoprotein (HDL) is an important natural nanoparticle that may be modified for biomedical imaging purposes. Here we developed a novel technique to create unique multimodality HDL mimicking nanoparticles by incorporation of gold, iron oxide, or quantum dot nanocrystals for computed tomography, magnetic resonance, and fluorescence imaging, respectively. By including additional labels in the corona of the particles, they were made multifunctional. The characteristics of these nanoparticles, as well as their in vitro and in vivo behavior, revealed that they closely mimic native HDL.

An ApoA-I mimetic peptide high-density-lipoprotein-based MRI contrast agent for atherosclerotic plaque composition detection

Cardiovascular disease is one of the prime causes of mortality throughout the world and there is a need for targeted and effective contrast agents to allow noninvasive imaging of the cholesterol-rich atherosclerotic plaques in arteries. A new, fully synthetic, high-density lipoprotein (HDL)-mimicking MRI contrast agent is developed, which enhances macrophage-rich areas of plaque in a mouse model of atherosclerosis by 94%. Confirmation of the targeting of this nanoparticulate agent is achieved using confocal microscopy by tracking a fluorescent lipid incorporated into the nanoparticle.

The Mid-Region of Parathyroid Hormone (1−34) Serves as a Functional Docking Domain in Receptor Activation

Elucidating the bimolecular interface between parathyroid hormone (PTH) and its cognate G protein-coupled receptor (PTHR1) should yield insights into the basis of molecular recognition and the mechanism of ligand-mediated intracellular signaling for a system that is critically important in regulating calcium levels in blood. We used photoaffinity scanning (PAS) to identify key ligand-receptor interactions for residues from the unstructured mid-region domain of PTH-(1-34). Four PTH analogues, containing a single photoreactive p-benzoylphenylalanine (Bpa) residue in position 11, 15, 18, or 21, were found to photo-cross-link within receptor regions [165-176], [183-189], [190-298], and [165-176], respectively. Addition of these mid-region contacts as constraints to our previously proposed model of the PTH-PTHR1 complex and extensive molecular simulation experiments enables substantial refinement of the model. Specifically, (1) the overall receptor-bound conformation of the hormone is not extended, but bent; (2) helix [169-176] of the N-terminal extracellular domain (N-ECD) of the receptor is redirected toward the heptahelical bundle; and (3) the hormone traverses between the top of transmembrane (TM) helices 1 and 2, rather than between TM-7 and TM-1. This significantly alters the model of both the receptor-bound tertiary structure of the hormone and the topological orientation of the C-terminus of the N-ECD in the hormone-receptor bimolecular complex. We propose that the mid-region of PTH-(1-34) has a role in fixing, by extensive contacts with the receptor, the entry of the N-terminal helix of the hormone into the heptahelical bundle between TM-1 and TM-2. This anchorage would orient the amino terminus into position to activate the receptor.

Bioactive N-terminal undecapeptides derived from parathyroid hormone: the role of alpha-helicity

The N-terminal 1-34 segment of parathyroid hormone (PTH) is fully active in vitro and in vivo and it can reproduce all biological responses in bone characteristic of the native intact PTH. Recent studies have demonstrated that N-terminal fragments presenting the principal activating domain such as PTH(1-11) and PTH(1-14) with helicity-enhancing substitutions yield potent analogues with PTH(1-34)-like activity. To further investigate the role of alpha-helicity on biological potency, we designed and synthesized by solid-phase methodology the following hPTH(1-11) analogues substituted at positions 1 and/or 3 by the sterically hindered and helix-promoting C(alpha)-tetrasubstituted alpha-amino acids alpha-amino isobutyric acid (Aib), 1-aminocyclopentane-1-carboxylic acid (Ac(5)c) and 1-aminocyclohexane-1-carboxylic acid (Ac(6)c): Ac(5)c-V-Aib-E-I-Q-L-M-H-Q-R-NH(2) (I); Aib-V-Ac(5)c-E-I-Q-L-M-H-Q-R-NH(2) (II); Ac(6)c-V-Aib-E-I-Q-L-M-H-Q-R-NH(2) (III); Aib-V-Ac(6)c-E-I-Q-L-M-H-Q-R-NH(2) (IV); Aib-V-Aib-E-I-Q-L-M-H-Q-R-NH(2) (V); S-V-Aib-E-I-Q-L-M-H-Q-R-NH(2) (VI), S-V-Ac(5)c-E-I-Q-L-M-H-Q-R-NH(2) (VII); Ac(5)c-V-S-E-I-Q-L-M-H-Q-R-NH(2) (VIII); Ac(6)c-V-S-E-I-Q-L-M-H-Q-R-NH(2) (IX); Ac(5)c-V-Ac(5)c-E-I-Q-L-M-H-Q-R-NH(2) (X); Ac(6)c-V-Ac(6)c-E-I-Q-L-M-H-Q-R-NH(2) (XI). All analogues were biologically evaluated and conformationally characterized in 2,2,2-trifluoroethanol (TFE) solution by circular dichroism (CD). Analogues I-V, which cover the full range of biological activity observed in the present study, were further conformationally characterized in detail by nuclear magnetic resonance (NMR) and computer simulations studies. The results of ligand-stimulated cAMP accumulation experiments indicated that analogues I and II are active, analogues III, VI and VII are very weakly active and analogues IV, V, VIII-XI are inactive. The most potent analogue, I exhibits biological activity 3500-fold higher than that of the native PTH(1-11) and only 15-fold weaker than that of the native sequence hPTH(1-34). Remarkably, the two most potent analogues, I and II, and the very weakly active analogues, VI and VII, exhibit similar helix contents. These results indicate that the presence of a stable N-terminal helical sequence is an important but not sufficient condition for biological activity.

Role of Secondary Structure in the Asymmetric Acylation Reaction Catalyzed by Peptides Based on Chiral Cα-Tetrasubstituted α-Amino Acids

In a recent series of papers, Miller and co-workers were able to show that His(pi-Me)-based, terminally protected peptides are potent catalysts of the asymmetric acyl transfer reaction, useful for the kinetic resolution of alcohols. In a structure-supporting solvent, one of the most active compounds, an Aib-containing tetrapeptide, is folded in a doubly intramolecularly H-bonded beta-hairpin motif incorporating a type-II' beta-turn conformation. In this work, we have expanded the study of the Miller tetrapeptide by examining a set of analogues and shorter sequences (dipeptide amides), characterized by chiral C(alpha)-tetrasubstituted alpha-amino acids of diverging bulkiness and optical configuration. Peptide synthesis in solution, conformational analysis by FT-IR absorption and (1)H NMR techniques, and screening of catalytic activity as well have been performed. Our results confirm the close relationship between the beta-hairpin 3D-structure and the catalytic activity of the peptides. A tetrapeptide analogue slightly more selective than the Miller compound has been found. However, the terminally protected, industrially more appealing, dipeptide amides are poorly effective.

Nitroxyl peptides as catalysts of enantioselective oxidations

The achiral, nitroxyl-containing alpha-amino acid TOAC (TOAC = 2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid), in combination with the chiral alpha-amino acid C(alpha)-methyl valine [(alphaMe)Val], was used to prepare short peptides (from di- to hexa-) that induced the enantioselective oxidation of racemic 1-phenylethanol to acetophenone. The best catalyst was an N(alpha)-acylated dipeptide alkylamide with the -TOAC-(alphaMe)Val- sequence folded in a stable, intramolecularly hydrogen-bonded beta-turn conformation with large, lipophilic (hydrophobic) N- and C-terminal blocking groups. We rationalized our findings by proposing models for the diastereomeric intermediates between (R)-[and (S)]-1-phenylethanol and the catalyst Fmoc-TOAC-L-(alphaMe)Val-NHiPr, based on the X-ray diffraction structure of the latter.