The nervous system develops from a set of undifferentiated cells into an elaborate network of precisely interconnected neurons. The creation and organization of this network is directed and defined by a series of molecular guidance cues, leading axons from target to target until they have reached their specified destinations. Understanding how these guidance cues interact in the process of directing axons to their destinations brings us closer to a full understanding of how precise neural circuits are formed, which may, in turn, lead to better molecular drug targets and improved therapeutic strategies for encouraging the accurate guidance of reorganizing brain areas of neural grafts after brain injury. In topographic mapping, axons are sent to consistently ordered positions within the CNS based on certain qualities, such as frequency of sound or spatial position;these maps are often repeated site-by-site throughout brain circuits. Retinal ganglion cells (RGCs), for example, send axons from the retina to the optic tectum, forming orderly connections on the tectal surface based on their original retinal locations. While ephrins were initially thought to be the only molecules to play a role in generating this map, recently a repulsive Wnt gradient, countering the attractive ephrinB gradient on one mapping axis, was identified. Elucidating how these Wnt and ephrinB gradients, and more notably the gradients of their respective receptors in the RGCs, interact will shed light on how opposing molecular guidance cues direct and define the topographic map, both within the growth cone and through effects on branch morphogenesis. This proposal will examine the interaction of Ryk and EphB receptors in RGC growth cones through live imaging of electroporated receptor constructs and immunohistochemistry in retinal explant culture, with long-term retinal explant and dissociated retinal culture to observe changes in branch morphology, and with use gain- and loss-of-function studies with electroporated receptor constructs and RNAi constructs in embryonic chick retina to observe changes to mapping in vivo.
Accurate topographic mapping is required to correct sort incoming sensory signals and form the consistently organized circuit along which they are sent within the CNS. Thus, by understanding better how receptors crucial to mapping interact to create this order, we may gain insight into how many circuits in the brain are precisely generated. This information could then also be used to generate drugs to improve the accurate rewiring of the brain after stroke or other injury, as well as to encourage accurate new circuit formation in patients with neural grafts.
