Extracellular Regulation and Scaling of Sensory Dendrites and Tissue-specific Mutagenesis Tools in Drosophila

Abstract

The diverse and intricate dendritic branching patterns of neurons determine their ability to collect synaptic and sensory information. Defects in the formation and maintenance of dendritic patterns underlie many neurological disorders. Therefore, it is essential to understand how neurons develop dendritic arbors with the proper branching patterns in a complex nervous system. During animal development, many neurons establish dendritic territories early and then expand dendritic arbors proportionally to the body size in a process called scaling. Among those neurons, some are capable of filling empty space in the receptive field with highly dynamic dendritic branches. Dendritic scaling and space filling require an integration of intrinsic mechanisms, extracellular signals, and information about the organism’s nutritional environment. To understand the multi-level mechanisms controlling dendrite branching, my research used Drosophila class IV dendritic arborization (da) neurons. For the first part of my thesis, I identified the epidermal cell-derived heparan sulfate proteoglycans (HSPGs), Dally and Sdc, as permissive signals for the space filling of class IV da neuron. We demonstrated that HSPGs stabilize dendrites by promoting microtubule stabilization. These data uncover novel pathways through which extracellular signals regulate dendritic space-filling in sensory neurons. In the second part of my thesis, I examined the relationship between sensory neuron and larval body growth under normal and nutrient stress conditions. My research revealed distinct cellular responses of sensory neurons and epidermal cells to nutrient restriction, with sensory dendrites growing preferentially under nutrient stress. Interestingly, autophagy and the expression of the transcription factor FoxO is suppressed in sensory neurons, making neurons insensitive to nutrient restriction. These experiments reveal distinctive molecular mechanisms regulating neuron/non-neural cell growth. Finally, we developed and optimized a tissue-specific gene loss-of-function (LOF) strategy using the CRISPR/Cas9 system. This system allowed us to generate efficient tissue-specific gene knockout and to analyze redundantly acting genes in neural development. Using this strategy, we discovered the redundant and perdurant functions of SNARE components in dendrite morphogenesis. Together, my research reveals novel mechanisms in which extracellular signals, the nutritional environment, and redundantly acting genes regulate dendrite branching pattern formation.