Building a Magnetosome-like Nanoparticle for Magnetic Resonance Imaging
Abstract
With its superb spatial and temporal resolution, magnetic resonance imaging (MRI) has great potential to track cellular activities that define early stages of disease. To improve molecular imaging techniques, we are developing MRI reporter gene expression based on the magnetosome. In magnetotactic bacteria (MTB), magnetosome formation compartmentalizes iron biominerals in membrane-enclosed vesicles. We hypothesize that essential magnetosome proteins interact in any cell type to form rudimentary magnetosome-like nanoparticles, providing a genetically-controlled contrast agent for molecular MRI.
MTB genes mamE, mamB, mamI, and mamL were cloned from M. magneticum sp. AMB-1 genomic DNA by PCR and inserted into fluorescent vectors to create Mam fusion proteins then stably expressed in human MDA-MB-435 melanoma cells. Cines of fluorescent elements detected in intact cells were captured with confocal microscopy (Nikon A1R) and analyzed using both ImageJ and Mathematica for Brownian motion and velocity. To obtain longitudinal and transverse relaxation rates, cells stably expressing magnetosome proteins were supplemented with 250 µM ferric nitrate, harvested, mounted in a gelatin phantom, and scanned at 3 Tesla (Biograph mMR).
Tomato-MamL, Tomato-MamL/GFP-MamI, and Tomato-MamB all express punctate, mobile fluorescence, while GFP-MamE expresses punctate but stationary fluorescence. Analysis of motility revealed that magnetosome proteins have variable diffusion coefficients due to their variable sizes, but all magnetosome proteins travel at a velocity of around 0.2 µm/s. Relaxation rates of iron-supplemented cells expressing Tomato-MamB, GFP-MamI, or Tomato-MamL have significantly higher R2 and R2* than non-supplemented cells. Interestingly, iron-supplemented cells expressing GFP-MamE or co-expressing FLAG-MamL/GFP-MamI had relaxation rates comparable to unsupplemented cell types.
This is the first report characterizing essential magnetosome proteins MamE, MamB, MamI, and MamL in mammalian cells. Analysis of motion shows that magnetosome proteins travel at velocities comparable to the mammalian motor protein myosin. Expression of either MamB, MamI or MamL increases transverse relaxation rates; however, co-expression of MamI and MamL reduces them again, suggesting a regulatory effect of magnetosome gene combinations. Biosynthesis of magnetosome-like nanoparticles in mammalian cells would provide an endogenous magnetic resonance (MR) contrast agent under genetic control. This patented technology would provide long-term molecular imaging for tracking cellular and molecular activities throughout the cell's life cycle.