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  • Book
    William Jinsoo Joo.
    Proper wiring of the nervous system is crucial for behavior, and defects in nervous system connectivity have been associated with cognitive impairment and neurological disease. How do neurons acquire their diverse morphologies, and how do they interact during development to form the intricate "maps" and circuits of the nervous system? While neural circuit assembly requires the intricate choreography of diverse processes, the specific placement of axons and dendrites is particularly important in determining how circuits process information. Using two well-characterized model circuits in Drosophila and mouse, I will discuss the genetic and molecular mechanisms that regulate how axons find their targets, and how dendrites adopt specific morphologies. The Drosophila olfactory system is an excellent model of wiring specificity, with stereotyped 1:1 connectivity between 50 classes of peripheral olfactory receptor neurons (ORNs) and 50 classes of central projection neurons (PNs). While studying how this connectivity pattern emerges during development, I discovered that a family of guidance factors called semaphorins regulates axon and dendrite development through multiple mechanisms. Specifically, secreted Semaphorin-2b acts both cell-autonomously and non-autonomously to specify developing axon trajectory. Indeed, secreted semaphorins mediate both axon-axon interactions and axon-target interactions. Furthermore, Sema-2b is negatively regulated by the Notch pathway during ORN development, and thus inextricably links cell fate determination to axon trajectory choice. Developmental trajectory defects have devastating consequences for the final targeting of ORN axons. Together, these findings reveal how reiterative use of the same molecules can seamlessly pattern neural circuits during successive developmental stages, and highlight novel mechanisms of semaphorin signaling. To study dendrite morphogenesis, I turned to the mouse cerebellum, another well-characterized neural circuit. This project arose as part of a larger effort to explore how neurotrophins regulate central brain development. Neurotrophins are well known for their roles in regulating the survival, differentiation, and plasticity of central and peripheral neurons. However, their functions in neural circuit assembly remain mysterious. Using a sparse mosaic genetic technique, I discovered that the neurotrophin receptor TrkC is specifically required for cerebellar Purkinje cell dendrite arborization. TrkC mutant Purkinje cells exhibited stunted dendritic trees with decreased complexity and length. Interestingly, removing TrkC from all Purkinje cells did not cause dendrite defects, raising the possibility of a competitive mechanism. Indeed, a series of conditional knockout and virus-based experiments suggest that TrkC and its ligand NT-3 drive competitive interactions between Purkinje cells. As functionally important NT-3 comes from the presynaptic partners of Purkinje cells, such "dendritic competition" contrasts with the classic target-derived "neurotrophic hypothesis." Together, these studies highlight the usefulness of mosaic genetic approaches in revealing the cellular mechanisms of neural circuit assembly. They also uncover surprising new roles for two historic signaling systems, and demonstrate how cells integrate both cell-intrinsic and environmental cues to establish the exquisite architecture of the nervous system.