Novel EphB Receptor Forward Signaling Pathways in the Development of Major White Matter Tracts of the Mammalian Brain

The development of the vertebrate central nervous system (CNS), including the brain and spinal cord, progresses in a stepwise and patterned fashion that involves the function of thousands of genes. The complex processes of neurogenesis and early cell migration mark the earliest stages CNS development, and subsequent developmental steps, including axon guidance, establish the functional circuitry between young neurons. Axon guidance is characterized by the growth of axonal processes that sprout from neuronal cell bodies and navigate within the developing brain to reach target synaptic zones. This precise navigation is accomplished by the recruitment of numerous axon guidance molecules that activate essential signal transduction pathways to mediate morphological changes in developing axon. In this dissertation, I collected research data on EphB receptor forward signaling mechanisms and on the impact of these signaling pathways during the development of the major white matter tracts of the mouse brain. These major tracts include the optic nerve and optic tracts, the anterior commissure, the corpus callosum, and the reciprocal corticothalamic and thalamocortical axon pathways. I specifically present data in which I demonstrate that Vav2 and Vav3 GEF (guanine nucleotide exchange factor) molecules form a specific forward signaling interaction with the EphB1 receptor that is essential for the proper axon guidance of retinal ganglion cell axons in vivo. Similarly, I demonstrated that Pak1, a signaling kinase, forms a novel interaction with the EphB2 receptor that is essential for the ephrinB2-stimulated growth cone repulsion of cortical axons. I also present in vivo data using EphB mutant mouse lines to test EphB forward signaling events during the development of mouse forebrain circuitry. I show that forward signaling pathways are necessary for the proper development of the anterior commissure in vivo, as this tract is significantly misprojected off course in EphB deficient brains. Next, I present findings that characterize a novel role of EphB receptor forward signaling during thalamocortical axon pathfinding, where I demonstrate that, in the absence of proper EphB forward signaling, an early thalamocortical axon guidance error leads to major postnatal axon guidance defects. This data also presents strong evidence for the co-mingling of thalamocortical axons with reciprocal corticothalamic axons during their development through a selective fasciculation event. Together, these findings represent a major step forward in the understanding of axon connectivity in specific regions of the mammalian brain and also highlight the widespread function and importance of EphB receptors and EphB-mediated forward signaling during the axon guidance stage of brain development.