Structural determinants of DTI observations in developing cortex and white matter

描述（申请人提供）：使用称为扩散张量成像（DTI）的磁共振成像（MRI）技术的研究已经发现，受遗传性或非遗传性疾病影响的成熟个体的大脑白色物质（WM）中的分数各向异性（FA）降低。（例如Rett综合征、苯丙酮尿症）、环境（例如胎儿酒精谱系障碍、早产）或多因素（例如自闭症、精神分裂症）起源。虽然这些结果意味着受影响的个体和年龄匹配的对照之间存在神经结构差异，但这些结构差异在细胞水平上的性质仍然未知。我们建议开发一个实验模型，以调查的WM和大脑皮层的变化，在FA的神经发育障碍引起的细胞基础。我们选择研究的疾病是新生儿双侧眼球摘除，因为这种操作是已知的诱导异常模式的神经连接在altricial物种，如啮齿动物和雪貂的视觉皮层。我们选择雪貂，因为他们有一个相对较大的大脑WM灰质（GM）的比例，这使得它有可能适用于扩散各向异性数据分析程序相关的人类研究。基于初步观察的WM和大脑皮层FA在apneatally去核雪貂的变化，我们假设WM FA的变化产生异常的髓鞘/胶质细胞，或轴突投射的发散/收敛的变化，或两者兼而有之。为了检验这一假设，将对对照和去核成年动物的死后大脑进行DTI测量。将视觉WM的各向异性分数值与基于甲苯胺蓝的髓鞘形成测量值以及基于逆行示踪剂的轴突会聚/发散测量值进行比较。此外，我们假设皮质GM FA的变化反映了神经元轴突和树突状神经元过程分布的变化。这一假设源于我们最近开发的一个分析模型，该模型将大脑皮层中的扩散各向异性（表示为FA）与神经元中轴突和树突状神经元过程的分布各向异性（表示为FAN）联系起来。我们将测试这个模型进行比较FA测量从死后雪貂大脑在五个年龄跨度出生后第6天（P）P45，测量FAN标记在同一组织中的树突和轴突的应用高尔基体和解剖跟踪程序。我们还将描述早期去核后FA和FAN之间关系的发展概况。结果表明，DTI可用于检测大脑皮层神经发育障碍的早期表现，可能在中枢神经系统可塑性丧失之前。拟议的实验将推进我们的理解，在WM和GM的扩散各向异性异常的细胞变化。这些知识不仅有助于监测发育中神经投射的完整性，而且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）可用于检测受各种神经发育障碍影响的个体的大脑解剖异常，但无法详细描述这些异常。因此，我们建议结合联合收割机的MRI实验与传统的细胞解剖测量的动物模型的神经发育障碍，提供一个框架来解释MRI的结果。我们还将使用这种实验策略来扩展MRI的能力，以研究大脑发育的早期阶段。

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.