Trevor Douglas

Magnetodendrimers allow endosomal magnetic labeling and in vivo tracking of stem cells.

Magnetic resonance (MR) tracking of magnetically labeled stem and progenitor cells is an emerging technology, leading to an urgent need for magnetic probes that can make cells highly magnetic during their normal expansion in culture. We have developed magnetodendrimers as a versatile class of magnetic tags that can efficiently label mammalian cells, including human neural stem cells (NSCs) and mesenchymal stem cells (MSCs), through a nonspecific membrane adsorption process with subsequent intracellular (non-nuclear) localization in endosomes. The superparamagnetic iron oxide nanocomposites have been optimized to exhibit superior magnetic properties and to induce sufficient MR cell contrast at incubated doses as low as 1 μg iron/ml culture medium. When containing between 9 and 14 pg iron/cell, labeled cells exhibit an ex vivo nuclear magnetic resonance (NMR) relaxation rate (1/T2) as high as 24–39 s−1/mM iron. Labeled cells are unaffected in their viability and proliferating capacity, and labeled human NSCs differentiate normally into neurons. Furthermore, we show here that NSC-derived (and LacZ-transfected), magnetically labeled oligodendroglial progenitors can be readily detected in vivo at least as long as six weeks after transplantation, with an excellent correlation between the obtained MR contrast and staining for β-galactosidase expression. The availability of magnetodendrimers opens up the possibility of MR tracking of a wide variety of (stem) cell transplants.

Host–guest encapsulation of materials by assembled virus protein cages

Self-assembled cage structures of nanometre dimensions can be used as constrained environments for the preparation of nanostructured materials, and the encapsulation of guest molecules, with potential applications in drug delivery and catalysis. In synthetic systems the number of subunits contributing to cage structures is typically rather small,. But the protein coats of viruses (virions) commonly comprise hundreds of subunits that self-assemble into a cage for transporting viral nucleic acids. Many virions, moreover, can undergo reversible structural changes that open or close gated pores to allow switchable access to their interior. Here we show that such a virion — that of the cowpea chlorotic mottle virus — can be used as a host for the synthesis of materials. We report the mineralization of two polyoxometalate species (paratungstate and decavanadate) and the encapsulation of an anionic polymer inside this virion, controlled by pH-dependent gating of the virions pores. The diversity in size and shape of such virus particles make this a versatile strategy for materials synthesis and molecular entrapment.

Crystallization at Inorganic-organic Interfaces: Biominerals and Biomimetic Synthesis

Crystallization is an important process in a wide range of scientific disciplines including chemistry, physics, biology, geology, and materials science. Recent investigations of biomineralization indicate that specific molecular interactions at inorganic-organic interfaces can result in the controlled nucleation and growth of inorganic crystals. Synthetic systems have highlighted the importance of electrostatic binding or association, geometric matching (epitaxis), and stereochemical correspondence in these recognition processes. Similarly, organic molecules in solution can influence the morphology of inorganic crystals if there is molecular complementarity at the crystal-additive interface. A biomimetic approach based on these principles could lead to the development of new strategies in the controlled synthesis of inorganic nanophases, the crystal engineering of bulk solids, and the assembly of organized composite and ceramic materials.

Inorganic–Organic Nanotube Composites from Template Mineralization of Tobacco Mosaic Virus

The use of biological molecules, assemblies and systems in the development of inorganic materials synthesis continues to offer new and exciting alternatives to conventional synthetic strategies. Biological templates, such as protein cages, viroid capsules, bacterial rhapidosomes, S-layers, multicellular superstructures, biolipid cylinders, and DNA, have been utilized to direct the deposition, assembly, and patterning of inorganic nanoparticles and microstructures. In this paper, we report a new approach to the template-directed synthesis of inorganic±organic nanotubes using tobacco mosaic virus (TMV). TMV is a remarkably stable virion, remaining intact at temperatures up to 60 C and at pH values between 2 and 10. Each viral particle consists of 2130 identical protein subunits arranged in a helical motif around a single strand of RNA to produce a hollow protein tube, 300 18 nm in size, with a 4 nm-wide central channel. The internal and external surfaces of the protein consist of repeated patterns of charged amino acid residues, such as glutamate, aspartate, arginine, and lysine. In principle, these functionalities should offer a wide variety of nucleation sites for surface-controlled inorganic deposition, which, in association with the high thermal and pH stability, could be exploited in the synthesis of unusual materials such as high-aspect-ratio composites and protein-confined inorganic nanowires. Here we show that TMV is a suitable template for reactions such as co-crystallization (CdS and PbS), oxidative hydrolysis (iron oxides), and sol-gel condensation (SiO2) (Fig. 1).

Synthesis and Structure of an Iron(III) Sulfide-Ferritin Bioinorganic Nanocomposite.

Amorphous iron sulfide minerals containing either 500 or 3000 iron atoms in each cluster have been synthesized in situ within the nanodimensional cavity of horse spleen ferritin. Iron-57 M�ssbauer spectroscopy indicated that most of the iron atoms in the 3000-iron atom cores are trivalent, whereas in the 500-iron atom clusters, approximately 50 percent of the iron atoms are Fe(III), with the remaining atoms having an effective oxidation state of about +2.5. Iron K-edge extended x-ray absorption fine structure data for the 500-iron atom nanocomposite are consistent with a disordered array of edge-shared FeS4 tetrahedra, connected by Fe(S)2Fe bridges with bond lengths similar to those of the cubane-type motif of iron-sulfur clusters. The approach used here for the controlled synthesis of bioinorganic nanocomposites could be useful for the nanoscale engineering of dispersed materials with biocompatible and bioactive properties.

Plant Viruses as Biotemplates for Materials and Their Use in Nanotechnology

In recent years, plant virus capsids, the protein shells that form the surface of a typical plant virus particle, have emerged as useful biotemplates for material synthesis. All virus capsids are assembled from virus-coded protein subunits. Many plant viruses assemble capsids with precise 3D structures providing nanoscale architectures that are highly homogeneous and can be produced in large quantities. Capsids are amenable to both genetic and chemical modifications allowing new functions to be incorporated into their structure by design. The three capsid surfaces, the interior surface, the exterior surface, or the interface between coat protein subunits, can be independently functionalized to produce multifunctional biotemplates. In this review, we examine the recent advances in using plant virus capsids as biotemplates for nanomaterials and their potential for applications in nanotechnology, especially medicine.

Preparation of Magnetically Labeled Cells for Cell Tracking by Magnetic Resonance Imaging

Publisher Summary This chapter describes the preparation of magnetically labeled cells for cell tracking by magnetic resonance imaging (MRI). Magnetic resonance (MR) tracking of magnetically labeled cells following transplantation or transfusion is a rapidly evolving new field. At one hand, MR cell tracking with its excellent spatial resolution can be used as a noninvasive tool to provide unique information on the dynamics of cell movement within and from tissues in animal disease models. As for MR contrast agents, gadolinium is the most effective paramagnetic contrast agent, owing to its seven unpaired electrons, but its relaxivity is far lower than the so-called superparamagnetic iron oxides. A significant improvement of labeling of nonphagocytic cells has been achieved by linking the particles to the human immunodeficiency virus (HIV) tat peptide. The distribution of magnetic microsphere-labeled porcine mesenchymal stem cells has been studied in a swine myocardial infarct model using MRI. Control cells are run side-by-side to determine the percent increased reactive oxygen species (ROS) production in the labeled cells. A transient increase in ROS production is also observed for the labeled cells.

Paramagnetic viral nanoparticles as potential high-relaxivity magnetic resonance contrast agents

In order to compensate for the inherent high threshold of detectability of MR contrast agents, there has been an active interest in the development of paramagnetic nanoparticles incorporating high payloads of Gd3+ with high molecular relaxivities. Toward this end, the protein cage of Cowpea chlorotic mottle virus (CCMV), having 180 metal binding sites, is being explored. In vivo CCMV binds Ca2+ at specific metal binding sites; however, Gd3+ can also bind at these sites. Using fluorescence resonance energy transfer we have characterized the binding affinity of Gd3+ to the metal binding sites by competition experiments with Tb3+. The measured dissociation constant (Kd) for Gd3+ bound to the virus is 31 μM. The T1 and T2 relaxivities of solvent water protons in the presence of Gd3+‐bound CCMV were 202 and 376 mM−1 s−1, respectively, at 61 MHz Larmor frequency. The unusually high relaxivity values of the Gd3+–CCMV are largely a result of the nanoparticle virus size and the large number of Gd3+ ions bound to the virus. These preliminary results should encourage further investigations into the use of viral protein cages as a new platform for MR contrast agents. Magn Reson Med, 2005. Published 2005 Wiley‐Liss, Inc.

Virus Particles as Templates for Materials Synthesis

The interface between biology, chemistry, and materialsscience has provided inspiration for novel approaches to theformationofmaterials.Increasingly,organizedbiomoleculararchitectures are being used as templates for the precisepatterning of inorganic materials in a biomimetic approachto materials synthesis. For example bacterial S-layers,

The Ferritin Superfamily: Supramolecular Templates for Materials Synthesis

Members of the ferritin superfamily are multi-subunit cage-like proteins with a hollow interior cavity. These proteins possess three distinct surfaces, i.e. interior and exterior surfaces of the cages and interface between subunits. The interior cavity provides a unique reaction environment in which the interior reaction is separated from the external environment. In biology the cavity is utilized for sequestration of irons and biomineralization as a mechanism to render Fe inert and sequester it from the external environment. Material scientists have been inspired by this system and exploited a range of ferritin superfamily proteins as supramolecular templates to encapsulate nanoparticles and/or as well-defined building blocks for fabrication of higher order assembly. Besides the interior cavity, the exterior surface of the protein cages can be modified without altering the interior characteristics. This allows us to deliver the protein cages to a targeted tissue in vivo or to achieve controlled assembly on a solid substrate to fabricate higher order structures. Furthermore, the interface between subunits is utilized for manipulating chimeric self-assembly of the protein cages and in the generation of symmetry-broken Janus particles. Utilizing these ideas, the ferritin superfamily has been exploited for development of a broad range of materials with applications from biomedicine to electronics.