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 that occur in normal and diseased tissue. 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 20 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 of newly born neurons, 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. These techniques are finding widespread application to imaging pre-clinical models of a broad range of diseases. Over the past year we have made progress in all of 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 subventricular zone and MRI can monitor the migration of cells to the olfactory bulb. A paper got published that measured the changes in migration of new neurons during unilateral nasal occlusion and recovery There was exquisite coupling between bulb anatomy and cell migration both during nasal block and recovery. New neurons were required for bulb re-growth after naris occlusion. We are now studying whether introduction of specific odors after naris oclusion alters the pattern of migration of the cells into the bulb. 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. A second major project images the entire brain to study immune brain interactions. We have succeeded in detecting T cell migration into the brain at single cell level during virus infection in mouse models. Over the next year we will quantitate sites of entry and determine the earliest time point that we can detect T cell infiltration. These results open the possibility of using migration of a very few T cells to help detect neurological disorders.
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 have continued to demonstrate the sensor properties with an aim to detecting force. We have also begun to use routinely simple microfabricated gold coated iron discs for cell tracing since they are eight times better than classical iron oxide particles. 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.
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. Over the past year we have published an initial study that is demonstrating 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. We continue our collaboration with the human imaging groups to test if an FDA approved agent that releases Mn might be useful for disease detection. Finally, in collaboration with Dorian McGavern we are pursuing novel approaches to add Mn2+ to the brain without disruption of the skull using the intricate vessel/marrow system of the skull.
Aim 4 : Develop strategies that enable cellular processes to alter the relaxivity of MRI contrast agents.
In specific aim 3 we demonstrated a way in which a normal biological process (endocytosis of transferrin-Mn or MnO particles) can alter the effectiveness of an MRI contrast agent. It would be very exciting to find ways in which this can occur which are sensitive to other biological processes. To this end we have begun to explore ways in which the microfabricated particles produced under Aim 2 can be modulated. Over the past few years we have completed a study that demonstrates that the microfabricated particles can be made into a pH sensor. This was accomplished by embedding a pH sensitive gel between the discs in our double disc microfabricated structure. Shrinking and swelling of the gel changes the disc spacing which in turn leads to a large change in MRI properties. The strategy used is generalizable to sense many other processes and we will extend this over the coming year. Finally, over the last few years, a novel, miniature wireless MRI detector that shows much promise for use where it may be possible to implant an MRI detector has been developed. Two engineering solutions have been demonstrated. Over the past year we have collaborated with a group at NCI to demonstrate the usefulness of this approach for in vivo EPR experiments.
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