Leibniz-Institute for Age Research, Fritz-Lipmann-Institute (FLI), Beutenbergstrasse 11, 07745 Jena, Germany.
Neuroimage. 2012 Jan 2;59(1):363-76. doi: 10.1016/j.neuroimage.2011.07.069. Epub 2011 Jul 30.
Traditionally, depiction of isolated CNS fiber tracts is achieved by histological post mortem studies. As a tracer-dependent strategy, the calcium analog manganese has proved valuable for in vivo imaging of CNS trajectories, particularly in rats. However, adequate protocols in mice are still rare. To take advantage of the numerous genetic mouse mutants that are available to study axonal de- and regeneration processes, a MnCl2-based protocol for high-resolution contrast-enhanced MRI (MEMRI) of the visual pathway in mice acquired on a widely used clinical 3 Tesla scanner was established. Intravitreal application of MnCl2 significantly enhanced T1-weighted contrast and signal intensity along the retino-petal projection enabling its reconstruction in a 3D mode from a maximum intensity projection (MIP) calculated dataset. In response to crush injury of the optic nerve, axonal transport of MnCl2 was diminished and completely blocked proximal and distal to the lesion site, respectively. Conditions of Wallerian degeneration after acute optic nerve injury accelerated Mn2+-enhanced signal fading in axotomized projection areas between 12 and 24 h post-injury. In long-term regeneration studies 12 months after optic nerve injury, the MRI protocol proved highly sensitive and discriminated animals with rare spontaneous axonal regrowth from non-regenerating specimens. Also, structural MRI aspects shared high correlation with histological results in identical animals. Moreover, in a model of chronic neurodegeneration in p50/NF-κB-deficient mice, MnCl2-based neuron-axonal tracing supported by heat map imaging indicated neuropathy of the visual pathway due to atrophy of optic nerve fiber projections. Toxic effects of MnCl2 at MRI contrast-relevant dosages in repetitive administration protocols were ruled out by histological and optometric examinations. At higher dosages, photoreceptors, not retinal ganglion cells, turned out as most susceptible to the well-known toxicity of MnCl2. Our data accentuate in vivo MEMRI of the murine visual system as a highly specific and sensitive strategy to uncover axonal degeneration and restoration processes, even in a functional latent state. We expect MEMRI to be promising for future applications in longitudinal studies on development, aging, or regeneration of CNS projections in mouse models mimicking human CNS pathologies.
传统上，孤立的中枢神经系统纤维束的描绘是通过组织学死后研究来实现的。作为一种依赖示踪剂的策略，钙类似物锰已被证明对中枢神经系统轨迹的体内成像很有价值，特别是在大鼠中。然而，在小鼠中仍然很少有足够的方案。为了利用可用于研究轴突去和再生过程的大量遗传小鼠突变体，建立了一种基于 MnCl2 的方案，用于在广泛使用的临床 3T 扫描仪上对小鼠的视觉通路进行高分辨率对比增强 MRI（MEMRI）。眼内注射 MnCl2 可显著增强沿视网膜向视神经的 T1 加权对比度和信号强度，从而能够从计算出的最大强度投影（MIP）数据集重建 3D 模式。在视神经受压损伤后，MnCl2 的轴突转运减少，分别在损伤部位的近端和远端完全受阻。急性视神经损伤后的 Wallerian 变性条件加速了损伤后 12 至 24 小时之间轴突投射区 Mn2+增强信号的消退。在视神经损伤后 12 个月的长期再生研究中，该 MRI 方案被证明非常敏感，并能够区分罕见的自发轴突再生动物和非再生标本。此外，在 p50/NF-κB 缺陷型小鼠慢性神经退行性变模型中，基于 MnCl2 的神经元-轴突示踪通过热图成像得到证实，这表明视神经纤维投射的萎缩导致视觉通路的神经病。通过组织学和视力检查排除了在重复给药方案中与 MRI 对比相关剂量下 MnCl2 的毒性作用。在更高的剂量下，光感受器而不是视网膜神经节细胞，被证明对 MnCl2 的已知毒性最敏感。我们的数据强调了活体 MEMRI 在揭示轴突变性和恢复过程中的高度特异性和敏感性策略，即使在功能潜伏状态下也是如此。我们预计 MEMRI 将有望在模拟人类中枢神经系统病理学的小鼠模型中用于对中枢神经系统投射的发育、衰老或再生的纵向研究。
