Normal brain function is dependent on the correct positioning and connectivity of neurons established during development. The Reelin signaling pathway plays a crucial role in cortical lamination. Reelin is a secreted glycoprotein that exerts its function by binding to lipoprotein receptors and inducing tyrosine phosphorylation of the intracellular adaptor protein Dab1. Mutations in genes of the Reelin signaling pathway lead to profound defects in neuronal positioning during brain development in both mice and humans. However, the molecular mechanisms by which Reelin controls neuronal morphology and migration are unknown. We have used a systems analysis approach to identify genes perturbed in the Reelin signaling pathway and identified microtubule stabilizing CLIP-associated protein 2 (CLASP2) as a key cytoskeletal modifier of Reelin mutant phenotypes. Currently, little is known about the role of CLASP2 in the developing brain. We propose that CLASP2 is a key effector in the Reelin signaling pathway controlling basic aspects of cortical layering, neuronal morphology, and function. CLASP2 is a plus-end tracking protein and this localization places CLASP2 in a strategic position to control neurite outgrowth, directionality, and responsiveness to extracellular cues. Our results demonstrate that CLASP2 expression correlates with neurite length and synaptic activity in primary neuron cultures; however, the role of CLASP2 during brain development was unknown. In this dissertation, we have characterized the role of CLASP2 during cortical development by in utero electroporation of shRNA plasmids and found that silencing CLASP2 in migrating neurons leads to mislocalized cells at deeper cortical layers, abnormal positioning of the centrosome-Golgi complex, and aberrant length/orientation of the leading process. In addition, we found that GSK3β-mediated phosphorylation of CLASP2 controls Dab1 binding and is required for regulating CLASP2 effects on neuron morphology and migration. This dissertation provides the first steps in gaining insight into how Reelin signaling affects cytoskeletal reorganization to regulate fundamental features of neuronal migration, positioning and morphogenesis.

ABSTRACT The Golgi apparatus has always been an interesting organelle of study because of its unique morphology as well as the critical roles it plays in cell biology. It is situated next to the endoplasmic reticulum and secreted proteins must pass through the Golgi vesicular pathway for modifications and targeting. In addition, the Golgi apparatus plays an essential role in establishing cellular polarity. Cell polarity refers to difference in orientation of cell structures spatially, and is involved in establishing functionality. The Golgi apparatus establishes cell polarity in various ways including orienting itself spatially, biasing vesicular trafficking within the cell, and most importantly through its role as a microtubule organizing center. The cytoskeleton provides the structural framework for cells. Microtubules nucleated from the Golgi-dependent microtubule organizing center result in an asymmetric cytoskeleton. An asymmetric cytoskeleton is essential to establishing cell polarity. Neurons require cell polarity to establish the essential structures such as the axon and dendrites. The Golgi apparatus establishes neuronal polarity through its extensive network of associated proteins. In this review, we will discuss the growing evidence supporting the role of the Golgi apparatus in establishing neuronal polarity.