Objectives Our aim was to develop and validate a non-invasive imaging tool to visualize HDL’s in vivo behavior by positron emission tomography (PET), with an emphasis on its plaque targeting abilities. Background High-density lipoprotein (HDL) is a natural nanoparticle that interacts with atherosclerotic plaque macrophages to facilitate reverse cholesterol transport. HDL-cholesterol concentration in blood is inversely associated with risk of coronary heart disease and remains one of the strongest independent predictors of incident cardiovascular events. Methods Discoidal HDL nanoparticles were prepared by reconstitution of its components apolipoprotein A-I (APOA1) and the phospholipid DMPC. For radiolabeling with Zirconium-89 (89Zr), the chelator DFO was introduced by conjugation to APOA1 or as a phospholipid-chelator (DSPE-DFO). Radiolabeled HDL’s biodistribution and plaque targeting was studied in established murine, rabbit and porcine atherosclerosis models by PET combined with computed tomography (PET/CT) or with magnetic resonance imaging (PET/MRI). Ex vivo validation was conducted by radioactivity counting, autoradiography and near infrared fluorescence imaging. Flow cytometric assessment of cellular specificity in different tissues was performed in the murine model. Results We observed distinct pharmacokinetic profiles for the two 89Zr-HDL nanoparticles. Both APOA1- and phospholipid-labeled HDL mainly accumulated in kidneys, liver and spleen with some marked quantitative differences in radioactivity uptake values. Radioactivity concentrations in rabbit atherosclerotic aortas were 3–4-fold higher than in controls at 5 days p.i. for both 89Zr-HDL nanoparticles. In the porcine model, we observed increased accumulation of radioactivity in lesions by in vivo PET imaging. Irrespective of the radiolabel’s location we found HDL nanoparticles to preferentially target plaque macrophages and monocytes. Conclusions 89Zr labeling of HDL allows studying its in vivo behavior by non-invasive PET imaging, including visualization of its accumulation in advanced atherosclerotic lesions. The different labeling strategies provide insight on the pharmacokinetics and biodistribution of HDL’s main components, i.e. phospholipids and APOA1.
Synopsis With the introduction of clinical PET/MR systems, novel attenuation correction methods are needed, as there are no direct or indirect MR methods to measure the attenuation of the objects in the FOV. A unique challenge for PET/MR attenuation correction is that coils for MR data acquisition are located in the FOV of the PET detector and could induce significant quantitative errors. In this review, we summarize and evaluate current methods and techniques to correct for the attenuation of a variety of coils.
Bacteriophage T7 gene 2.5 protein (gp2.5) is a single-stranded DNA (ssDNA)-binding protein that has essential roles in DNA replication, recombination and repair. However, it differs from other ssDNA-binding proteins by its weaker binding to ssDNA and lack of cooperative ssDNA binding. By studying the rate-dependent DNA melting force in the presence of gp2.5 and its deletion mutant lacking 26 C-terminal residues, we probe the kinetics and thermodynamics of gp2.5 binding to ssDNA and double-stranded DNA (dsDNA). These force measurements allow us to determine the binding rate of both proteins to ssDNA, as well as their equilibrium association constants to dsDNA. The salt dependence of dsDNA binding parallels that of ssDNA binding. We attribute the four orders of magnitude salt-independent differences between ssDNA and dsDNA binding to nonelectrostatic interactions involved only in ssDNA binding, in contrast to T4 gene 32 protein, which achieves preferential ssDNA binding primarily through cooperative interactions. The results support a model in which dimerization interactions must be broken for DNA binding, and gp2.5 monomers search dsDNA by 1D diffusion to bind ssDNA. We also quantitatively compare the salt-dependent ssDNA- and dsDNA-binding properties of the T4 and T7 ssDNA-binding proteins for the first time.
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