MRI contrast for molecular and cellular imaging of the brain

人们对开发分子成像方法的兴趣不断增加，这些方法使传统的放射成像技术能够获得有关分子和细胞过程的广泛信息。一系列的信息被认为是重要的，如监测细胞迁移的能力，使成像的基因表达的报告的发展，强大的战略，以图像受体的发展，和环境敏感剂的发展，可用于检测特定酶的存在或监测离子状态的变化。这项工作的长期目标是开发策略，使MRI对比度对广泛的分子和细胞过程敏感。这项工作建立在30多年的工作基础上，我们已经证明了第一种用于检测基因表达的MRI策略，第一种用于监测钙内流替代物的MRI方法，第一种用于执行神经元追踪的MRI方法，以及第一种用于监测体内单细胞迁移的MRI方法。这些都代表了任何放射成像技术的初始报告，这些技术能够测量这些过程，并广泛应用于临床前疾病模型的成像。我们在具体目标方面取得了进展。 目的1：开发用于标记和成像细胞迁移的氧化铁基造影剂。在过去的几年里，我们已经证明了微米级氧化铁颗粒用于特定细胞MRI的独特优势。可以检测单个细胞，并且实际上可以检测单个细胞内的单个颗粒。细胞迁移的MRI的主要范例是离体标记细胞并在移植到动物中后监测迁移。一个对整个大脑进行成像以研究免疫大脑相互作用的项目，使用MRI检测小的微出血和标记的T细胞，以跟踪VSV感染大脑后的免疫细胞浸润。这项研究表明，VSV引起微出血独立于外周免疫反应;病毒特异性T细胞减少微出血;病毒感染后早期，在微出血部位以及微出血前产生病毒的远端部位检测到病毒特异性T细胞。我们已经能够在出血之前对病毒特异性T细胞的初始流入进行成像，这应该使我们能够对导致出血的过程进行成像。检测单个颗粒的能力使得低效的标记策略成为可能。特别是，在过去的几年中，我们已经证明，注射颗粒到脑室的大鼠大脑，使颗粒被采取的神经前体在脑室下区和MRI可以监测细胞迁移到嗅球。以前，我们已经测量了在单侧鼻阻塞和恢复过程中新神经元迁移的变化，表明在鼻阻塞和恢复过程中，球解剖结构和细胞迁移之间存在微妙的耦合。在过去的一年里，我们已经证明了使用微加工铁颗粒而不是我们过去使用的微粒的好处。这些微制造的颗粒能够灵敏地检测单细胞，因为它们从脑室下区迁移到啮齿动物的嗅球中，从而开启了一系列有趣的实验。这些颗粒将用于免疫细胞追踪，并可能用于标记脑血管炎症。我们已经证明了磁热材料用于MRI造影剂的独特潜力。我们已经获得了一个磁场移位器，这将使我们能够充分改变磁场，将这些材料从高磁矩切换到低磁矩。初步的测量正在进行中，我们希望证明这种方法对磁热材料的强大使用。我们还建立了合作，看看我们是否可以获得微米级的磁热材料，使细胞成像。 目的2：开发用于脑部成像的新型MRI对比剂。在过去的几年里，我们的工作重点是使用MRI可检测的神经示踪剂来增加神经连接测量的灵敏度。我们已经证明，经典的神经示踪剂霍乱毒素B（CT B）可以跟踪与少量的MRI对比附加。增加更多的造影剂抑制了示踪剂。因此，我们增加了我们可以使用设计为携带五个Gd螯合物的肽连接的对比度的量，并证明了CTB示踪剂的可检测性增加。通常，该领域要么向大分子量蛋白质添加更多的对比度，要么制造纳米颗粒造影剂以获得更多的对比度有效载荷。这种肽策略打开了一扇门，增加对比有效载荷，同时保持小尺寸。我们将继续增加肽，看看我们抑制CTB示踪的大小，以继续增加灵敏度。如果我们发现较大的铁颗粒抑制抗体抗原识别，这项工作将对我们的目标是重要的抗体靶向血管标记物。

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 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. A project that 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. We have been able to image the initial influx of virus specific T cells prior to bleeding which should enable us to image the processes that lead to bleeding going forward. 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. Over the past year we have demonstrated the gain of using microfabricated iron particles rather than the micro particles we have used in the past. These microfabricated particles are enabling sensitive detection of single cells as they migrate from the sub-ventricular zone into the rodent olfactory bulb opening a range of interesting experiments. These particles will be used for immune cell tracking and potentially for labeling brain vascular inflammation. We have demonstrated unique potential for magnetocaloric materials for MRI contrast agents. We have acquired a magnetic field shifter that will enable us to change fields sufficiently to switch these materials from high to low magnetic moment. Preliminary measurements are being made and we hope to prove this approach for robust use of magnetocaloric materials. We have also established collaborations to see if we can get micron scale magnetocaloric materials made to enable cellular imaging. Aim 2: Develop novel MRI contrast for imaging the brain. Over the past couple of years we have focused this work on increasing the sensitivity of measures of neural connectivity using MRI detectable neural tracing agents. We had shown that the classical neural tracer Cholera Toxin B (CTB) can trace with a small amount of MRI contrast attached. Adding more contrast inhibited the tracer. Therefore we increased the amount of contrast we could attach using a peptide designed to carry five Gd chelates and demonstrated increased detectability of the CTB tracer. Typically the field either adds more contrast to large MW proteins or makes nanoparticle contrast agents to get more contrast payload. This peptide strategy opens a door to increasing contrast payload while maintaining small sizes. We will continue to grow the peptide to see at what size we inhibit CTB tracing to continue to increase sensitivity. This work will be important for our goals of antibody targeting to vascular markers if we find the larger iron particles inhibit antibody antigen recognition.