Structural determinants of DTI observations in developing cortex and white matter

描述(由申请人提供)：使用名为扩散张量成像(DTI)的磁共振成像(MRI)技术的研究发现，受遗传性疾病(如Rett综合征、苯丙酮尿症)、环境因素(如胎儿酒精谱系障碍、早产)或多因素(如自闭症、精神分裂症)影响的成熟个体的大脑白质(WM)中的分数各向异性(FA)降低。虽然这些结果表明，在受影响的个体和年龄匹配的对照组之间存在神经结构差异，但这些细胞水平上的结构差异的性质仍不清楚。我们建议建立一个实验模型来研究由神经发育障碍引起的脑白质改变和大脑皮质改变的细胞学基础。我们选择研究的障碍是新生儿双侧眼球摘除，因为众所周知，这种操作会在啮齿动物和雪貂等非生物物种的视觉皮质中诱导神经连接的异常模式。我们选择雪貂是因为它们的大脑WM与灰质(GM)的比率相对较大，这使得应用与人类研究相关的扩散各向异性数据分析程序成为可能。根据对新生去核雪貂WM和大脑皮质FA变化的初步观察，我们推测WM FA的变化源于髓鞘/神经胶质细胞的异常，或轴突投射的发散/会聚的变化，或两者兼而有之。为了验证这一假设，将对对照组和去核成年动物的死后大脑进行DTI测量。来自视觉WM的分数各向异性值将与基于甲苯胺蓝的髓鞘形成的测量以及基于逆行示踪剂的轴突会聚/发散的测量进行比较。此外，我们假设皮质GM-FA的变化反映了神经束内轴突和树突分布的变化。这一假说源于我们最近开发的一个分析模型，该模型将大脑皮层的扩散各向异性(表示为FA)与神经束中轴突和树突的分布各向异性(表示为FAN)联系起来。我们将通过比较从出生后6天到45天的五个年龄段的死后雪貂大脑的FA测量，以及应用高尔基体和解剖学追踪程序在同一组织中标记的树突和轴突的扇形来测试这一模型。我们还将描述早期摘除后FA和FAN之间的关系的发展概况。这一结果有望证明DTI可以用来检测大脑皮层神经发育障碍的早期表现，可能是在中枢神经系统失去可塑性之前。所提出的实验将促进我们对WM和GM中扩散各向异性异常背后的细胞变化的理解。这一知识不仅有助于监测发育中的神经投射的完整性，而且还有助于制定治疗策略，以抵消神经发育障碍对大脑发育的有害影响。 公共卫生相关性：磁共振成像(MRI)可以用来检测受各种神经发育障碍影响的个体的大脑解剖异常，但它不能详细描述这些异常。因此，我们建议在神经发育障碍的动物模型上结合MRI实验和传统的细胞解剖学测量，为解释MRI结果提供一个框架。我们还将使用这一实验策略将MRI的能力扩展到对大脑发育早期阶段的研究。

DESCRIPTION (provided by applicant): Studies using the magnetic resonance imaging (MRI) technique termed diffusion tensor imaging (DTI) have found that fractional anisotropy (FA) is reduced in cerebral white matter (WM) of mature individuals affected by disorders either of genetic (e.g. Rett syndrome, phenylketonuria), environmental (e.g. fetal alcohol spectrum disorder, premature birth), or multifactorial (e.g. autism, schizophrenia) origin. While these results imply that there are neural structural differences between affected individuals and age-matched controls, the nature of these structural differences at the cellular level remains unknown. We propose to develop an experimental model to investigate the cellular bases of both WM and cerebral cortical changes in FA that are induced by neurodevelopmental disorders. The disorder we chose to study is neonatal bilateral enucleation because this manipulation is known to induce abnormal patterns of neural connections in the visual cortex of altricial species such as rodents and ferrets. We chose ferrets because they have a relatively large cerebral WM to gray matter (GM) ratio, which makes it possible to apply diffusion anisotropy data analysis procedures of relevance to human studies. Based on preliminary observations of changes in both WM and cerebral cortex FA in neonatally enucleated ferrets, we hypothesize that changes in WM FA arise from abnormalities of myelin/glia, or changes in the divergence/convergence of axonal projections, or both. To test this hypothesis, DTI measurements will be performed on post-mortem brains of control and enucleated adult animals. Fractional anisotropy values from visual WM will be compared to Toluidine blue-based measurements of myelination, and to retrograde tracer-based measurements of axonal convergence/divergence. In addition, we hypothesize that changes in cortical GM FA reflect changes in the distribution of axonal and dendritic neuronal processes in the neuropil. This hypothesis stems from an analytical model that we recently developed to relate diffusion anisotropy in the cerebral cortex (expressed as FA) to anisotropy in the distribution of axonal and dendritic neuronal processes in the neuropil (expressed as FAN). We will test this model by comparing FA measurements taken from post-mortem ferret brains at five ages spanning postnatal day (P)6 to P45, to measurements of FAN of dendrites and axons labeled in the same tissue by the application of Golgi and anatomical tracing procedures. We will also characterize the developmental profile of the relationship between FA and FAN following early enucleation. The results are expected to demonstrate that DTI can be used to detect early manifestations of neurodevelopmental disorders in cerebral cortex, potentially prior to the loss of central nervous system plasticity. The proposed experiments will advance our understanding of the cellular changes underlying abnormalities in diffusion anisotropy in both WM and GM. This knowledge will not only facilitate monitoring the integrity of developing neural projections, but also the formulation of therapeutic strategies to counteract deleterious effects that neurodevelopmental disorders on brain development. PUBLIC HEALTH RELEVANCE: Magnetic resonance imaging (MRI) can be used to detect brain anatomical abnormalities in individuals affected by a diverse array of neurodevelopmental disorders, but it is not capable of characterizing these abnormalities in detail. We therefore propose to combine MRI experiments with traditional cellular-anatomical measurements on an animal model of neurodevelopmental disorders to provide a framework for interpreting MRI findings. We will also use this experimental strategy to expand the capabilities of MRI to studies of earlier stages of brain development.