Author: Jiefu Li

Publisher:

Published: 2020

Total Pages:

ISBN-13:

In the evolutionary transition from unicellular to multicellular organisms, single cells assemble into highly organized tissues and cooperatively carry out physiological functions. To act as an integrated system, individual cells communicate with each other extensively through signaling at the cellular interface. Cell-surface signaling thus controls almost every aspect of the development and physiology of multicellular organisms. Taking the nervous system as an example, cell-surface wiring molecules dictate the precise assembly of the neural network during development, while neurotransmitter receptors and ion channels mediate synaptic transmission and plasticity in adults. Delineating the cell-surface signaling is therefore crucial for understanding the organizing principles and operating mechanisms of multicellular systems. My dissertation research integrates method development and mechanistic interrogation to investigate cell-surface signaling in neural circuit assembly. My collaborators and I developed, for the first time to my knowledge, a method for quantitatively profiling cell-surface proteomes in intact tissues with cell-type and spatiotemporal specificities. Applying this method to Drosophila olfactory projection neurons (PNs), I captured cell-surface proteomes of developing and mature PNs and observed globally coordinated dynamics of PN surface proteins corresponding to the wiring and functional stages of the olfactory circuit, providing the first systemic view on how neuronal surface evolves in development. A proteome-instructed in vivo screen identified 20 new cell-surface molecules regulating neural circuit assembly, many of which belong to evolutionarily conserved protein families not previously linked to neural development. These discoveries highlight the power of this new method in uncovering new regulators of brain wiring. Notably, this method should be readily applicable for profiling proteins on the surface of other cell types, tissues, and organisms, far beyond the fly olfactory circuit. In a complementary set of studies on the Plexin B receptor, I developed a generalizable protein tagging approach that reveals cell-type-specific endogenous protein distribution in vivo. Combining this tagging approach and genetic analyses, we found that the Plexin B receptor uses multiple molecular strategies: temporally-regulated protein distribution, level-dependent signaling, and divergent engagement of signaling motifs, to sequentially instruct several distinct steps of axon targeting. These include axon-axon interaction, axon guidance, and synaptic partner selection. Our findings illustrate how a single molecule achieves multi-functionality while preserving its signal specificity in brain wiring. In summary, this dissertation combines quantitative multi-omic profiling and in vivo mechanistic interrogation to elucidate the systemic properties and molecular bases of cell-surface signaling underlying the precise wiring of neural circuits.