MRI contrast for molecular and cellular imaging of the brain

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There continues to be increasing interest in developing molecular imaging approaches that enable traditional radiological imaging techniques to obtain a wide range of information about molecular and cellular processes. A range of information is considered important such as the ability to monitor cell migration, the development of reporters that enable imaging of gene expression, the development of robust strategies to image receptors, and the development of environmentally sensitive agents that can be used to detect the presence of specific enzymes or monitor changes in ion status. The long term goals of this work are to develop strategies that enable MRI contrast that is sensitive to a wide range of molecular and cellular processes. This work builds on over 30 years of work where we have demonstrated the first MRI strategy for detecting gene expression, the first MRI approach for monitoring a surrogate of calcium influx, the first MRI approach for performing neuronal track tracing, and the first MRI approach for monitoring the migration of single cells in vivo. These all represented initial reports by any radiological imaging technique which enabled these processes to be measured and are finding widespread application to imaging pre-clinical models disease. We have made progress in the specific aims. Aim 1: Develop iron oxide based contrast for labeling and imaging the migration of endogenous neural stem cells. Over the past few years we have demonstrated the unique advantages of micron sized iron oxide particles for MRI of specific cells. Single cells can be detected and indeed, single particles within single cells can be detected. The main paradigm for MRI of cell migration is to label cells ex vivo and monitor migration after transplantation into an animal. The ability to detect a single particle enables inefficient labeling strategies. In particular, over the past few years we have demonstrated that injection of particles into the ventricles of the rat brain enables particles to be taken up by neural precursors in the sub-ventricular zone and MRI can monitor the migration of cells to the olfactory bulb. Previously, we have measured the changes in migration of new neurons during unilateral nasal occlusion and recovery showing that the was exquisite coupling between bulb anatomy and cell migration both during nasal block and recovery. Studies to determine whether introduction of specific odors after naris occlusion alters the pattern of migration of the cells into the bulb have been delayed due to improvements in cell labeling (Aim 2). In a new project inspired by studying these endogenous new neurons we have shown that cortical and mid-brain precursor cells can be grown in the adult CSF to form fully integrated and normally appearing brain tissue. Over the past year we have almost completed studies that show this new tissue can integrate into the motor pathway and olfactory pathways if placed in appropriate places in the CSF. These studies shave led to us to return to making MRI neural circuit tracers and we have increased our ability to detect a classical tracer, CTB by about five fold (Aim 4). This new tracer should allow us to determine how the tissue in CSF connect to the host brain. A second major project images the entire brain to study immune brain interactions. We are close to completing studies to detect T cell migration into the brain at single cell level during virus infection in mouse models. Interesting T cell accumulation is associated with small bleeding opening the issue of whether T cell infiltration into brain parenchyma is associated with BBB breakdown in these viral models even when very few cells are involved. Aim 2: Apply microfabrication techniques to manufacture unique metal structures that may be valuable for MRI contrast. Iron oxide particles commonly used for MRI are very potent contrast agents enabling detection of single micron sized particles. However, due to bulk phase manufacture of particles they are not very uniform and they do not contain very high content of metal. A solution to this problem is to use modern microfabrication techniques to manufacture metal based, micron sized contrast agents. Over the past few years we have shown that double doughnut, cylinders, and ellipsoid structures offer unique advantages for distinguishing particles and that these structures can be turned into sensors for pH. Over the past year we are completing a study that demonstrates that simple microfabricated gold coated iron discs can be used for both our new neuron and immune cell tracking projects. Finally, our collaborator at NIST, Gary Zabow (former fellow)has developed novel ways to manufacture this class of MRI contrast that are being tested here at NIH. Finally, over the past couple of years, in collaboration with M. Barbic, we have demonstrated that magnetocaloric materials have unique properties that have potential to be very useful for MRI studies of cell tracking. these proof of principle studies set the stage for future work attempting to make these materials in small particle formulations. Aim 3: Develop novel delivery mechanisms to extend the applicability of manganese enhanced MRI. Over the past ten years we have demonstrated the remarkable utility of the manganese ion for MRI contrast. Manganese ion enters cells on ligand or voltage gated calcium channels and so can be used as an MRI agent to monitor calcium influx. Once inside of neurons, manganese will move in an anterograde direction and cross functional synapses enabling neuronal networks to be imaged with MRI. Finally, manganese given systemically gives cytoarchitectural information about the rodent brain. These successes have us interested in broadening the ways in which manganese ion can be delivered to cells. A major limitation of manganese enhanced MRI are the concentrations required. This limits translation for use in human imaging. Over the past couple of years we have tackled this problem in two ways, we have published an initial study that demonstrates that manganese positron emitting isotopes will enable PET to obtain similar information that can be obtained with manganese enhanced MRI, including neural tracing and functional activation of tissue. Discussion have begun to decide if to translate this to human studies. We completed our first study in collaboration with Daniel Reich to test if an FDA approved agent that releases Mn might be useful for disease detection. The first manuscript on normal volunteers was published. Studies on MS patients were completed this past year and studies of patients with epilepsy are underway in collaboration with Sara Inati. Please note all human studies are done under the protocols of our collaborators and that is where the relevant human use data is reported for this work. Aim 4: Develop strategies that enable cellular processes to alter the relaxivity of MRI contrast agents. We continue to develop the microfabricated particles produced under Aim 2 as sensors. Over the past few years we have completed a study that demonstrates that the microfabricated particles can be made into a sensors. Over the past year we have sought ways to make the manufacture of this very interesting class of sensors more robust. In addition, we have developed a novel polymer of Gd chelates that has high relaxivity but relatively low molecular weight. This has been attached to a class of classical neural tracers (CTB and dextrans) to enable MRI neural tracers. This platform should enable us to make new classes of physiologically responsive MRI agents. When opportunities arise we make specialized MRI detectors for specific projects This includes an interventional pituitary coil that is being tested by Dr. Chittiboina, a coil for improving perfusion MRI, and approaches to make wireless detectors.