The Role of Down Syndrome Cell Adhesion Molecule in Development and Diseases
Liu, Hao
2020
Abstract
How neurons elaborate dendrites and axons and how they form synapses with targeted cells are fundamental questions in neurodevelopment. Although a number of genes have been identified to play a role in these processes, the following two aspects are largely unknown and interesting to study. First, while most genes go through alternative splicing to generate different protein isoforms, how each protein isoform contributes to the function of this gene in neurodevelopment remains a challenging question, especially at the cell-type specific and endogenous levels. Second, in neurodevelopmental diseases, such as those in Down syndrome, the expression levels of hundreds or thousands of genes are altered. The dysregulation of which gene cause the neurite and synaptic defects in these brain disorders remains largely unknown. My dissertation aims to answer these two questions with the focus of one gene, Down syndrome cell adhesion molecule (Dscam). Dscam is an evolutionarily conserved single-pass transmembrane protein that plays critical roles in multiple aspects of neuronal wiring, including dendritic/axonal growth and synapse formation. In Drosophila, alternative splicing of Dscam produces two mutually exclusive isoforms, Dscam[TM1] and [TM2], which differ in their transmembrane and juxtamembrane region. By developing a novel genetic method, termed isoTarget, which enables the investigation of isoform-specific function and endogenous localization in specific cells, I report differential function and localization of Dscam isoforms in axons versus dendrites. In addition, I uncovered an isoform-specific signaling pathway that involves DLK/Wallenda, Dock, and Dscam[TM2], but not Dscam[TM1], at the axon terminals of Drosophila sensory neurons. I provide evidence showing that Dscam[TM2]-specific function and signaling in axon terminals are caused by its isoform-specific localization, rather than by biochemical differences between Dscam[TM1] and [TM2]. These findings not only demonstrate the specific function, localization and signaling of Dscam isoforms but also demonstrate the critical role of subcellular localization in expanding isoform diversity. In human, the Dscam gene is localized in the Down syndrome critical region on chromosome 21. By taking advantage of sparse neuron labeling for morphological analysis and whole-cell patch-clamp for functional assay, I found that Dscam regulates inhibitory neuron development in the neocortex in a dose-dependent fashion. Loss of Dscam impairs the presynaptic growth in chandelier and basket cells, two major types of inhibitory neurons in the neocortex. In the Ts65Dn mouse model for Down syndrome, where Dscam is overexpressed, GABAergic inhibition of pyramidal neurons (PyNs), the main excitatory neurons in the cortex, is increased. Genetic normalization of Dscam expression rescues the excessive GABAergic synapse formation and inhibition of PyNs. These findings reveal abnormal GABAergic innervation in the neocortex of Down syndrome mouse model and identify Dscam overexpression as the cause. They also implicate dysregulated Dscam levels as a potential pathogenic driver in related neurological disorders. In summary, by using the novel genetic method isoTarget, this dissertation uncovers Dscam isoform-specific subcellular signaling cascade in neurodevelopment; using genetic normalization in Down syndrome mouse model, it establishes a causal effect between dysregulated Dscam expression levels and cortical defects in Down syndrome pathology. These findings are expected to advance our understanding of the role of Dscam in neurodevelopment and neurological disorders.Subjects
Neuron Development Isoform Dscam Down syndrome
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