Molecular mechanisms that control dendrite branching in Caenorhabditis elegans

Abstract

Dendritic arbors or networks are composed of specialized neuronal compartments called dendrites that play a critical role in receiving and processing synaptic or sensory input. Accordingly, cell-type specific dendritic arborization patterns help define the specific inputs and functional outputs associated with a given neuronal subtype. Dendrite branching represents a key stage in the formation of dendritic arbors. Although studies in Drosophila and vertebrate systems have identified a variety of factors that regulate dendrite branch formation, the molecular mechanisms that control this process remain ill-defined. Within the last decade, RNA Interference (RNAi) has become a well-established, powerful tool for identifying novel genes involved in wide-ranging biological phenomena. In this thesis, I use the C. elegans PVD neurons, a pair of sensory neurons which elaborate complex dendritic arbors, as a tractable model for conducting high-throughput RNAi screens aimed at identifying key regulators of dendrite branch formation. Notably, these screens are the first large-scale RNAi screens that aim to reveal novel regulators of dendritic development in C. elegans. By carrying out two separate RNAi screens; a small-scale, candidate-based and a large-scale screen of the approximately 3000 genes on Chromosome IV, I retrieved eleven genes that either promote or suppress the formation of PVD-associated dendrites. I then present a detailed functional characterization of one of the identified genes, bicd-1, a microtubule-associated protein previously shown to modulate the transport of mRNAs and organelles in a variety of organisms. Specifically, I describe a novel role for bicd-1 in regulating PVD dendrite branch formation, and show that bicd-1 is likely expressed, and primarily required, in PVD neurons to control dendrite branching. I also provide evidence that bicd-1 acts in a common genetic pathway with the dynein and kinesin motor complexes to control PVD dendritic development. Furthermore, I report a novel role for the repulsive guidance receptor, unc-5, in the establishment of the PVD dendrite branching pattern. I show that bicd-1 and unc-5 genetically interact in this context, and that this interaction may require the cytoplasmic tail of UNC-5. Lastly, I demonstrate that unc-5 function in PVD dendrite development may be independent of the canonical ligand, unc-6..