CEST MRI to study BrAin MetaBolism and Function: insights from OptO fMRI and MR spectroscopy

化学交换饱和转移（CEST）和化学交换自旋锁（CESL） MRI是代谢成像的有力工具。使用CEST/CESL MRI可以检测特定的神经代谢物，与NMR波谱（MRS）相比，CEST/CESL MRI具有更高的灵敏度和时间和空间分辨率，通过成像应用专用的预饱和或自旋锁定模块后大量水信号的减少。已经开发了用于大量不同代谢物成像的CEST方案，并且已经证明了在不同学科（例如肿瘤学，肌肉骨骼成像，生理学或神经科学）中的有前途的应用。最近，我们利用CEST MRI来成像葡萄糖（glucoCEST）在感觉刺激下的变化，并表明CEST功能MRI （CEST- fmri）是检测神经激活过程中糖酵解活性增加的一种很有前途的替代方法。CEST-fMRI的一个普遍限制是信号变化不能明确地只分配给一种代谢物。这是神经影像学特别关注的问题，因为涉及能量代谢物和神经递质水平的一些短暂变化。在本项目中，我们旨在阐明葡萄糖、乳酸和谷氨酸三种主要的神经代谢物对CEST/CESL功能实验中检测到的对比度的贡献，并开发和优化这些新的CEST/CESL- fmri技术，以研究神经激活过程中能量代谢和神经传递之间的耦合。我们的CEST/CESL-fMRI方案将在法国和德国的两台小动物扫描仪上以高磁场强度联合实施，并用于绘制这些代谢物在大鼠大脑中的浓度变化。为了验证CEST/CESL-fMRI的特异性并分离个体代谢物的贡献，法国合作伙伴将采用局部1H和间接1H-{13C} MRS技术。为了进一步评估细胞类型特异性对CEST/CESL信号的贡献，德国合作伙伴将同时使用光遗传学方法和MRI。病毒转导将用于在特定的细胞类型（神经元、星形胶质细胞）中表达钙、乳酸或谷氨酸的视蛋白和荧光报告蛋白。在感觉或细胞类型特异性（光遗传学）刺激下读出荧光信号的细胞类型特异性将使我们能够识别对观察到的功能性CEST/CESL信号的个体贡献。这项工作的结果将包括健全和经过验证的采集协议，具有在其他器官中使用的强大潜力。即将到来的高磁场临床系统可能会促进人类的应用。在更基本的层面上，使用CEST/CESL-fMRI方法可以帮助探索在正常和病理条件下，或在药物挑战下，在特定设计的激活范式中，神经胶质-神经元-血管功能单元内的代谢耦合。

Chemical exchange saturation transfer (CEST) and chemical exchange spin-lock (CESL) MRI are powerful tools for metabolic imaging. Detection of specific neurometabolites can be performed using CEST/CESL MRI with enhanced sensitivity, and temporal and spatial resolution compared to NMR Spectroscopy (MRS), by imaging the reduction in bulk water signal after application of dedicated presaturation or spin-locking modules. CEST protocols for imaging of a large number of different metabolites have been developed, and promising applications in different disciplines (e.g. oncology, musculoskeletal imaging, physiology or neuroscience) have been demonstrated. Recently, we employed CEST MRI to image glucose (glucoCEST) changes induced by sensory stimulation, and have shown that CEST functional MRI (CEST-fMRI) is a promising surrogate for detecting increased glycolytic activity during neural activation. One general limitation for CEST-fMRI is that signal changes cannot be unambiguously assigned to exclusively one metabolite. This is of particular concern for neuroimaging, since a number of transient changes in energy metabolites and neurotransmitter levels are involved.In this project, we aim to elucidate the contributions of three major neurometabolites, glucose, lactate and glutamate, to the contrast detected during functional CEST/CESL experiments, and to develop and optimize these new CEST/CESL-fMRI techniques to study the coupling between energy metabolism and neurotransmission during neural activation. Our CEST/CESL-fMRI protocols will be implemented conjointly at high magnetic field strength on two small animal scanners, in France and Germany, and used to map concentration changes of these metabolites in the rat brain. To verify the specificity of CEST/CESL-fMRI and to separate the contributions of individual metabolites, localized 1H and indirect 1H-{13C} MRS techniques will be employed by the French partners. To further assess cell type-specific contributions to the CEST/CESL signal, optogenetic methods will be used simultaneously with MRI by the German partners. Viral transduction will be used to express both opsins and fluorescence reporter proteins for calcium, lactate, or glutamate in defined cell types (neurons, astrocytes). Cell type-specific read out of fluorescence signal upon sensory or cell type-specific (optogenetic) stimulation will enable us to identify individual contributions to the observed functional CEST/CESL signal.The outcome of this work will consist in robust and validated acquisition protocols with a strong potential for use in other organs. Application in humans may be facilitated by the upcoming high magnetic field clinical systems. At a more fundamental level, the use of CEST/CESL-fMRI methods could help exploring the metabolic coupling within the glial-neuronal-vascular functional unit during specifically designed activation paradigms in normal and pathological conditions, or following pharmacological challenges.