Biophysical Characterization of Human Protamine‑1 as a Responsive CEST MR Contrast Agent

The protamines are a low-molecular-weight, arginine-rich family of nuclear proteins that protect chromosomal DNA in germ cells by packing it densely using electrostatic interactions. Human protamine-1 (hPRM1) has been developed as a magnetic resonance imaging (MRI) chemical exchange saturation transfer (CEST) reporter gene, based on a sequence that is approximately 50% arginine, which has a side chain with rapidly exchanging protons. In this study, we have synthesized hPRM1 and determined how its CEST MRI contrast varies as a function of pH, phosphorylation state, and upon noncovalent interaction with nucleic acids and heparin (as antagonist). CEST contrast was found to be highly sensitive to phosphorylation on serine residues, intra- and intermolecular disulfide bridge formation, and the binding of negatively charged nucleotides and heparin. In addition, the nucleotide binding constants (<i>K</i>_eq) for the protamines were determined through plotting the molar concentration of heparin versus CEST contrast and compared between hPRM1 and salmon protamine. Taken together, these findings are important for explaining the CEST contrast of existing arginine-rich probes as well as serving as a guideline for designing new genetic or synthetic probes

Metal Ion Sensing Using Ion Chemical Exchange Saturation Transfer ¹⁹F Magnetic Resonance Imaging

Although metal ions are involved in a myriad of biological processes, noninvasive detection of free metal ions in deep tissue remains a formidable challenge. We present an approach for specific sensing of the presence of Ca²⁺ in which the amplification strategy of chemical exchange saturation transfer (CEST) is combined with the broad range of chemical shifts found in ¹⁹F NMR spectroscopy to obtain magnetic resonance images of Ca²⁺. We exploited the chemical shift change (Δω) of ¹⁹F upon binding of Ca²⁺ to the 5,5′-difluoro derivative of 1,2-bis­(<i>o</i>-amino­phenoxy)­ethane-<i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-tetra­acetic acid (5F-BAPTA) by radiofrequency labeling at the Ca²⁺-bound ¹⁹F frequency and detection of the label transfer to the Ca²⁺-free ¹⁹F frequency. Through the substrate binding kinetics we were able to amplify the signal of Ca²⁺ onto free 5F-BAPTA and thus indirectly detect low Ca²⁺ concentrations with high sensitivity

Mesoporous Silica-Coated Hollow Manganese Oxide Nanoparticles as Positive <i>T</i>₁ Contrast Agents for Labeling and MRI Tracking of Adipose-Derived Mesenchymal Stem Cells

Mesoporous silica-coated hollow manganese oxide (HMnO@mSiO₂) nanoparticles were developed as a novel <i>T</i>₁ magnetic resonance imaging (MRI) contrast agent. We hypothesized that the mesoporous structure of the nanoparticle shell enables optimal access of water molecules to the magnetic core, and consequently, an effective longitudinal (<i>R</i>₁) relaxation enhancement of water protons, which value was measured to be 0.99 (mM⁻¹s⁻¹) at 11.7 T. Adipose-derived mesenchymal stem cells (MSCs) were efficiently labeled using electroporation, with much shorter <i>T</i>₁ values as compared to direct incubation without electroporation, which was also evidenced by signal enhancement on <i>T</i>₁-weighted MR images in vitro. Intracranial grafting of HMnO@mSiO₂-labeled MSCs enabled serial MR monitoring of cell transplants over 14 days. These novel nanoparticles may extend the arsenal of currently available nanoparticle MR contrast agents by providing positive contrast on <i>T</i>₁-weighted images at high magnetic field strengths