In vivo oxygen detection using exogenous hemoglobin as a contrast agent in magnetic resonance microscopy

Photoluminescent oxygen-release microspheres to image the oxygen release process in vivo

Cell therapy is a promising strategy to treat ischemic diseases, but the efficacy is limited due to high rate of cell death under low oxygen environment of the ischemic tissues. Sustained release of oxygen to continuously oxygenate the transplanted cells may augment cell survival and improve therapeutic efficacy. We have shown previously that oxygen released from oxygen-release microspheres stimulated cell survival in ischemic tissue [1]. To understand how oxygen is released in vivo and duration of release, it is attractive to image the process of oxygen release. Herein, we have developed photoluminenscent oxygen-release microspheres where the in vivo oxygen release can be non-invasively and real-time monitored by an In Vivo Imaging System (IVIS). In the oxygen-release microspheres, a complex of polyvinylpyrrolidone, H₂O₂ and a fluorescent drug hypericin (HYP) was used as core, and poly(N-isopropylacrylamide-co-acrylate-oligolactide-co-hydroxyethyl methacrylate-co-N-acryloxysuccinimide) conjugated with catalase was used as shell. To distinguish fluorescent signal change for different oxygen release kinetics, the microspheres with various release profiles were developed by using the shell with different degradation rates. In vitro, the fluorescent intensity gradually decreased during the 21-day oxygen release period, consistent with oxygen release kinetics. The released oxygen significantly augmented mesenchymal stem cell (MSC) survival under hypoxic condition. In vivo, the oxygen release rate was faster. The fluorescent signal can be detected for 17 days for the microspheres with the slowest oxygen release kinetics. The implanted microspheres did not induce substantial inflammation. The above results demonstrate that the developed microspheres have potential to monitor the in vivo oxygen release.

Biomolecular MRI reporters: Evolution of new mechanisms

Magnetic resonance imaging (MRI) is a powerful technique for observing the function of specific cells and molecules inside living organisms. However, compared to optical microscopy, in which fluorescent protein reporters are available to visualize hundreds of cellular functions ranging from gene expression and chemical signaling to biomechanics, to date relatively few such reporters are available for MRI. Efforts to develop MRI-detectable biomolecules have mainly focused on proteins transporting paramagnetic metals for T1 and T2 relaxation enhancement or containing large numbers of exchangeable protons for chemical exchange saturation transfer. While these pioneering developments established several key uses of biomolecular MRI, such as imaging of gene expression and functional biosensing, they also revealed that low molecular sensitivity poses a major challenge for broader adoption in biology and medicine. Recently, new classes of biomolecular reporters have been developed based on alternative contrast mechanisms, including enhancement of spin diffusivity, interactions with hyperpolarized nuclei, and modulation of blood flow. These novel reporters promise to improve sensitivity and enable new forms of multiplexed and functional imaging.

Metalloprotein-based MRI probes

Metalloproteins have long been recognized as key determinants of endogenous contrast in magnetic resonance imaging (MRI) of biological subjects. More recently, both natural and engineered metalloproteins have been harnessed as biotechnological tools to probe gene expression, enzyme activity, and analyte concentrations by MRI. Metalloprotein MRI probes are paramagnetic and function by analogous mechanisms to conventional gadolinium or iron oxide-based MRI contrast agents. Compared with synthetic agents, metalloproteins typically offer worse sensitivity, but the possibilities of using protein engineering and targeted gene expression approaches in conjunction with metalloprotein contrast agents are powerful and sometimes definitive strengths. This review summarizes theoretical and practical aspects of metalloprotein-based contrast agents, and discusses progress in the exploitation of these proteins for molecular imaging applications.

Bioengineered probes for molecular magnetic resonance imaging in the nervous system

The development of molecular imaging probes has changed the nature of neurobiological research. Some of the most notable successes have involved the use of biological engineering techniques for the creation of fluorescent protein derivatives for optical imaging, but recent work has also led to a number of bioengineered probes for magnetic resonance imaging (MRI), the preeminent technique for noninvasive investigation of brain structure and function. Molecular MRI agents are beginning to be applied for experiments in the nervous system, where they have the potential to bridge from molecular to systems or organismic levels of analysis. Compared with canonical synthetic small molecule agents, biomolecular or semibiosynthetic MRI contrast agents offer special advantages due to their amenability to molecular engineering approaches, their properties in some cases as catalysts, and their specificity in targeting and ligand binding. Here, we discuss an expanding list of instances where biological engineering techniques have aided in the design of MRI contrast agents and reporter systems, examining both advantages and limitations of these types of probes for studies in the central nervous system.

Metal-substituted protein MRI contrast agents engineered for enhanced relaxivity and ligand sensitivity

Engineered metalloproteins constitute a flexible new class of analyte-sensitive molecular imaging agents detectable by magnetic resonance imaging (MRI), but their contrast effects are generally weaker than synthetic agents. To augment the proton relaxivity of agents derived from the heme domain of cytochrome P450 BM3 (BM3h), we formed manganese(III)-containing proteins that have higher electron spin than their native ferric iron counterparts. Metal substitution was achieved by coexpressing BM3h variants with the bacterial heme transporter ChuA in Escherichia coli and supplementing the growth medium with Mn3+-protoporphyrin IX. Manganic BM3h variants exhibited up to 2.6-fold higher T1 relaxivities relative to native BM3h at 4.7 T. Application of ChuA-mediated porphyrin substitution to a collection of thermostable chimeric P450 domains resulted in a stable, high-relaxivity BM3h derivative displaying a 63% relaxivity change upon binding of arachidonic acid, a natural ligand for the P450 enzyme and an important component of biological signaling pathways. This work demonstrates that protein-based MRI sensors with robust ligand sensitivity may be created with ease by including metal substitution among the toolkit of methods available to the protein engineer.

Activatable T₁ and T₂ magnetic resonance imaging contrast agents

Magnetic resonance imaging (MRI) has become one of the most important diagnosis tools available in medicine. Typically MRI is not capable of sensing biochemical activities. However, recently emerged activatable MRI contrast agents (CAs), whose relaxivity is variable in response to a specific parameter change in the surrounding physiological microenvironment, potentially allow for MRI to indicate biological processes. Among the various factors influencing the relaxivity of a CA, the number of inner-sphere water molecules (q) directly coordinated to the metal center, the residence time of the coordinated water molecule (τ (m)), and the rotational correlation time representing the molecular tumbling time of a complex (τ (R)) contribute strongly to the relaxivity of an activatable CA. Tuning the ligand structure and properties has been the subject of intensive research for activatable MR CA designs. This review summarizes a variety of activatable MRI CAs sensitive to common variables in microenvironment in vivo, i.e., pH, luminescence, metal ions, redox, and enzymes, etc., with emphasis on the influence of ligand design on parameters q, τ (m), and τ (R).

Bioresponsive, cell-penetrating, and multimeric MR contrast agents

Magnetic resonance imaging (MRI) has become increasingly popular in experimental molecular imaging and clinical radiology because it is non-invasive and capable of producing three-dimensional representations of opaque organisms with high spatial and temporal resolution. Approximately 35% of all clinical MR scans utilize contrast media, however a primary limitation of MR imaging is the sensitivity of contrast agents that require high concentrations (0.1-0.6 mM). A number of strategies have been employed to amplify the observed in vivo signal of MR contrast agents. Approaches include attachment of Gd(III) chelates to polymers, proteins and particles, encapsulation into micelles and caged structures, and targeting to receptors. While each of these approaches has yielded significant increases in the relaxivity of MR contrast agents (and therefore sensitivity), all of these classes of complexes possess intrinsic background signal and function solely as anatomical reporters. In order to reduce the background signal and simultaneously create probes that are modulated by biochemical processes, caged complexes were designed to coordinatively saturate the paramagnetic ion. Coupled with amplification strategies, these agents represent a means to selectively modulate the observed MR signal and function as in vivo biochemical reporters. For example, to create an in vivo MR assay of enzymatic activities and secondary messengers, agents have been designed and synthesized with removable protection groups that largely prevent access of water to a paramagnetic center. By limiting the access of bulk water (q-modulation) the unprocessed agent is designed to be an ineffective contrast agent, and hence serves as a reliable marker for regions of enzyme activity or the presence of secondary messengers. Further, we have focused on designing multimodal contrast agents that are simultaneously detectable by more than one imaging technique. For example, attaching an optical probe to a MR contrast agent provides the means to detect the probe in a whole animal and subsequently validate the results by histological methods. Finally, we describe strategies for signal amplification, and cell delivery vehicles attached to imaging probes for in vivo long-term fate mapping experiments.

MRI contrast agents for functional molecular imaging of brain activity

Functional imaging with MRI contrast agents is an emerging experimental approach that can combine the specificity of cellular neural recording techniques with noninvasive whole-brain coverage. A variety of contrast agents sensitive to aspects of brain activity have recently been introduced. These include new probes for calcium and other metal ions that offer high sensitivity and membrane permeability, as well as imaging agents for high-resolution pH and metabolic mapping in living animals. Genetically encoded MRI contrast agents have also been described. Several of the new probes have been validated in the brain; in vivo use of other agents remains a challenge. This review outlines advantages and disadvantages of specific molecular imaging approaches and discusses current or potential applications in neurobiology.

Towards extracellular Ca2+ sensing by MRI: synthesis and calcium-dependent 1H and 17O relaxation studies of two novel bismacrocyclic Gd3+ complexes

Two new bismacrocyclic Gd3+ chelates containing a specific Ca2+ binding site were synthesized as potential MRI contrast agents for the detection of Ca2+ concentration changes at the millimolar level in the extracellular space. In the ligands, the Ca2+-sensitive BAPTA-bisamide central part is separated from the DO3A macrocycles either by an ethylene (L1) or by a propylene (L2) unit [H4BAPTA is 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid; H3DO3A is 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid]. The sensitivity of the Gd3+ complexes towards Ca2+ and Mg2+ was studied by (1)H relaxometric titrations. A maximum relaxivity increase of 15 and 10% was observed upon Ca2+ binding to Gd2L1 and Gd2L2, respectively, with a distinct selectivity of Gd2L1 towards Ca2+ compared with Mg2+. For Ca2+ binding, association constants of log K = 1.9 (Gd2L1) and log K = 2.7 (Gd2L2) were determined by relaxometry. Luminescence lifetime measurements and UV-vis spectrophotometry on the corresponding Eu3+ analogues proved that the complexes exist in the form of monohydrated and nonhydrated species; Ca2+ binding in the central part of the ligand induces the formation of the monohydrated state. The increasing hydration number accounts for the relaxivity increase observed on Ca2+ addition. A 1H nuclear magnetic relaxation dispersion and 17O NMR study on Gd2L1 in the absence and in the presence of Ca2+ was performed to assess the microscopic parameters influencing relaxivity. On Ca2+ binding, the water exchange is slightly accelerated, which is likely related to the increased steric demand of the central part leading to a destabilization of the Ln-water binding interaction.

Genetically controlled MRI contrast mechanisms and their prospects in systems neuroscience research

Application of MRI contrast agents to neural systems research is complicated by the need to deliver agents past the blood-brain barrier or into cells, and the difficulty of targeting agents to specific brain structures or cell types. In the future, these barriers may be wholly or partially overcome using genetic methods for producing and directing MRI contrast. Here we review MRI contrast mechanisms that have used gene expression to manipulate MRI signal in cultured cells or in living animals. We discuss both fully genetic systems involving endogenous biosynthesis of contrast agents, and semi-genetic systems in which expressed proteins influence the localization or activity of exogenous contrast agents. We close by considering which contrast-generating mechanisms might be most suitable for applications in neuroscience, and we ask how genetic control machinery could be productively combined with existing molecular agents to enable next-generation neuroimaging experiments.

Dynamic imaging with MRI contrast agents: quantitative considerations

Time-resolved MRI has had enormous impact in cognitive science and may become a significant tool in basic biological research with the application of new molecular imaging agents. In this paper, we examine the temporal characteristics of MRI contrast agents that could be used in dynamic studies. We consider "smart" T1 contrast agents, T2 agents based on reversible aggregation of superparamagnetic nanoparticles and sensors that produce changes in saturation transfer effects (chemical exchange saturation transfer, CEST). We discuss response properties of several agents with reference to available experimental data, and we develop a new theoretical model that predicts the response rates and relaxivity changes of aggregation-based sensors. We also perform calculations to define the extent to which constraints on temporal resolution are imposed by the imaging methods themselves. Our analysis confirms that some small T1 agents may be compatible with MRI temporal resolution on the order of 100 ms. Nanoparticle aggregation T2 sensors are applicable at much lower concentrations, but are likely to respond on a single second or slower timescale. CEST agents work at high concentrations and temporal resolutions of 1-10 s, limited by a requirement for long presaturation periods in the MRI pulse sequence.

Functional MRI using molecular imaging agents

Contrast agents for magnetic resonance imaging (MRI) have recently been used as cellular-level probes of neural function. New in vivo labeling strategies now enable researchers to follow plasticity of brain activation patterns and cellular structure over time. On the horizon is the prospect that molecular imaging agents specifically designed for functional imaging (fMRI) on a relatively fast timescale could offer an alternative to conventional hemodynamics-based approaches. Development of several MRI sensors has defined principles by which imaging agents for "molecular fMRI" can be constructed; application of engineered sensors for cellular-level correlates of neuronal activity would allow researchers to combine the noninvasiveness of MRI with spatial resolution of tens of microns and temporal resolution of 100ms or less. Facilitated by advances in imaging-agent delivery methods and model systems appropriate for high-resolution neuroimaging, novel molecular imaging strategies continue to potentiate MRI as a tool for mechanistic investigation of neural systems.

In vivo 3D MRI of insect brain: cerebral development during metamorphosis of Manduca sexta

High-resolution 3D MRI of male pupae of Manduca sexta was performed at 2.35 T in order to evaluate its potential for an in vivo characterization of insect brain during metamorphosis. T1-weighted 3D FLASH (TR/TE = 20/7.8 ms, 25 degrees flip angle) and T2-weighted 3D fast SE MRI data sets (TR/TEeff = 3000/100 ms) were acquired at different developmental stages with an isotropic resolution of 100 microm. Both T1- and T2-weighted 3D MRI allowed for the identification of cerebral structures such as the antennal nerve, antennal and optical lobe, and central brain. Pronounced developmental alterations of the morphology were observed during metamorphosis. The results demonstrate the feasibility of 3D MRI at nanoliter resolution to identify major brain systems of M. sexta and respective changes during pupal development from caterpillar to sphinx moth. Together with the use of suitable contrast agents, this approach may provide new ways for studying the axonal connectivity and neural function of the developing insect brain.