MRI contrast for molecular and cellular imaging of the brain

National Institute Of Neurological Disorders And Stroke

Investigators

Linked publications & trials

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 measurement of these processes and are finding widespread application to imaging pre-clinical disease model. We have made progress in the specific aims. Aim 1: Develop iron oxide based contrast for labeling and imaging the migration of individual 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 ability to detect a single particle enables inefficient labeling strategies. In particular, over the past 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 groups of new neurons during unilateral nasal occlusion and recovery showing that there was exquisite coupling between bulb anatomy and cell migration both during nasal block and recovery. Over the past two years we have completed work that demonstrated the gain of using microfabricated iron particles rather than the micron particles we have used in the past to track cells migrating out the Rostral Migratory Stream (RMS) into the olfactory bulb. A manuscript is being prepared that has followed the trajectories of individual cells along the migratory pathway to the bulb in normal and naris occluded mice. The trajectories are quite complex with some cells migrating continuously to the bulb some stopping and going, and most interestingly some changing directions and some continuously migrating in the wrong direction! This is all affected by naris occlusion which leads to an overall slowing of migration consistent with our previous results that detected a slowing of bulk cell migration in the RMS. We have begun to discuss with potential collaborators whether we should tackle the challenging problems of attributing molecular identities and cues to the complex migratory behavior. Previously We have demonstrated unique potential for magnetocaloric materials for MRI contrast agents. Over the past few years we have gotten a unique magnetic field shifter working that will enable us to change fields up to 1 Tesla in the MRI magnet. This was accomplished in a project between Resonance Research Inc and the LFMI MRI Engineering Core. This allows us to switch these materials from low to high magnetic moment and back again in non-invasive manner which is better than our previous work using temperature. We have demonstrated that this approach can enable very specific detection of single micron scale magnetocaloric particles. This work usign field shifting used particles that were on the large size for biological applications. Fe-Rh particles at sizes that we know living cells will uptake are being produced for us. Over the next year we will determine if we can label cells and detect them specifically with the combination of the Tesla shifter and the microfabricated magnetocaloric particles. We have been using antibody labeled micron particles to sensitively detect vessel inflammation. We have the sensitivity to detect inflammation at the level of single vessels. We have established positive controls (transferrin receptor antibodies) and negative controls (IgG). We have completed a study showing that we can see vessel inflammation due to LPS with VCAM antibody labeled particles. In all cases where we detected antibody labeling of vessels by MRI we can compete with antibody alone and eliminate the MRI contrast indicating specificity. There are interesting issues related to concentration dependence and time courses that we will investigate in our next studies. Some antibodies do not work even though there targets are on brain vessels. We have determined that this is due to squelching of antibodies-particles at sites other than brain such as immune cells. A protocol to use free antibody to eliminate these sights and preserve brain binding looks very promising. We have applied the VCAM-MPIO to study inflammation during high fat diet as well as in mouse models of neurodegeneration. There is little data about regional brain and time course of inflammation of brain vessels in these models that MRI should be able to supply. A collaboration led by the Reich group is attempting to detect early vessel inflammation in animal models of MS. This is using novel micron iron oxide particles that degrade to begin to determine if there is translational potential. There is much interest in imaging neuroinflammation, and our hypothesis is that brain inflammation will be reflected in vessel inflammation even at the very early stages. Aim 2: Develop novel MRI contrast for imaging the brain. We have focused this work on developing a new approach to making an MRI gene expression reporter strategy similar to GFP for light microscopy. For over 30 years there have been numerous and clever approaches to solving this problem. Unfortunately, none have risen to widespread use in pre-clinical (or clinical) models. We have demonstrated that the transporter ZIP14 can be used to alter MRI contrast very significantly in a way that is consistent with it being a dominant Mn2+ transporter. Mn2+ is an excellent MRI contrast agent and changes in cellular concentrations are readily detected by MRI. A first paper was published this year demonstrating that expression of ZIP14 in neurons (and astrocytes) leads to readily detectable MRI contrast without the need to add any contrast agent. The sensitivity is sufficient to detect anterograde projections from specific sites of expression. Retrograde viruses lead to strong expression at sites that project to areas of injection. This is very exciting and should enable us to shift from using direct injection of Mn2+ or other chemical MRI neural tracers to follow changes in neural connectivity over time in individual animals. Sensitivity is further increased with systemic Mn2+, and we have optimized MRI sequences to begin to detect only changes in Mn2+. Another transporter ZIP8 works at least as well as ZIP14. We have been able to quantitate manganese changes in tissue sections to verify and quantify best strategies to improve this class of MRI gene expression reporters. We will test sensitivity and potential toxicity by controlling expression of ZIP14 as well as apply the strategy to determine if we are sensitive to sprouting and neural degeneration. This project bridges our other Z01 entitled "Functional Imaging of the Brain".