MRI contrast for molecular and cellular imaging of the brain

National Institute Of Neurological Disorders And Stroke

Investigators

Linked publications, trials & patents

Abstract

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 efforts to improve cell labeling (Aim 2). A second major project images the entire brain to study immune brain interactions using MRI to detect small microbleeds and labeled T cells to follow immune cell infiltration after infection of the brain with VSV. This study has shown that VSV causes microbleeds independent of peripheral immune responses; that virus-specific T cells decrease microbleeds; and that early after viral infection, viral-specific T cells are detected at the site of microbleeds as well as remote sites where virus is produced prior to microbleeds. Future work will determine if the entry of the T Cells is via the microbleeds followed by migration to remote sites or entry via other sites. The developments in Aim 2 are critical for distinguishing T Cells from microbleeds unambiguously. Our ability to sensitively detect microbleeds and iron led to two collaborations to detect bleeds in human olfactory bulb and to demonstrate that single oligodendrocytes can be detected in human tissue due to their high iron content. 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 several years we have begun to translate these structures to routine use, starting with simple microfabricated gold coated iron discs. These are about 8x more potent than our previous labeling strategies and are being 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. Over the past few 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. We have begun to work to make these materials in particle formulations useful for cell labeling. In addition we have designed and contracted to get an insert that will allow us to shift the MRI field by as much as 1 Tesla allowing us more flexibility in detection strategies for these magnetocaloric materials. Aim 3: Develop novel delivery mechanisms to extend the applicability of manganese enhanced MRI. 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. We have begun to translate Manganese Enhanced MRI to humans using an FDA approved agent, Teslascan, as a slow release delivery mechanism. The first paper using this in humans with MS was completed ,showing that areas of BBB disruption can be detected and the development to new lesions could be detected. Teslascan is no longer available and we are searching for a source that can be safely administered to humans for other applications. Very recent results indicate that we can express specific manganese transporters that can alter the levels of manganese in a specific tell type, opening the way towards a robust MRI gene reporter strategy as well as a way to control cell specificity that should be useful for a number of studies. 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 robustly. In addition, we have developed a novel polymer of Gd chelates in collaboration with the NIH Imaging Probe Development Center that has high relaxivity but relatively low molecular weight. This allows us to link this agent to biomolecules in a manner that has minimal effects on function. Over the past year this strategy has been demonstrated by making MRI detectable neural tracers based on classical tracers that require sacrificing animals to assay. This will complement our long-standing work using manganese ion as a neural tracer and allow minimally invasive studies to determine neural connectivity in living brains.