Assembly of neuronal networks with distinct patterns of connectivity during nervous system development involves the growth, extension and branching of axons and dendrites. Over the years genetic and biochemical studies in model organisms have contributed significantly in identifying mechanisms regulating axon growth and extension. However the molecular mechanisms underlying axon branching remain unclear.
The egg-laying neuronal circuitry in C. elegans has proven to be a robust system for identifying and characterizing novel genes involved in neuronal morphology. This circuitry which mediates egg-laying behavior in nematodes is composed of two families of motorneurons, HSNs and VCs, which are among the most branched neurons in C. elegans. A genetic screen for axon branch defects in the egg-laying neurons identified png-1 to disrupt neuronal morphology including axon branching. png-1 encodes a Peptide: N-glycanase (PNGase), a conserved cytosolic enzyme that removes N-linked sugar moieties from glycoproteins. In this thesis I present my work characterizing and examining png-1 and its role in mediating axon branching. Mutations in png-1 resulted in excessive ectopic axon branching in the VC4 and VC5 egg-laying neurons as well as branching in the normally unbranched AVL and DVB neurons. Behavioral analysis in these mutants revealed defects in egg-laying behavior and mild in-utero egg retention phenotypes. Cellular characterization shows ubiquitous expression of png-1 in many tissues including vulva cells, muscles, gonads, and neurons. My analysis also shows that png-1 acts both cell-autonomously and cell non-autonomously from neurons and epithelial cells to restrict axon branching around the vulva. Using a candidate gene approach I identified a deletion allele of the DNA repair gene, rad-23, to display axon branching defects and interact with png-1 within a common pathway to regulate axon branching. Additionally, through a genetic modifier screen for enhancers and suppressors of VC4-5 branching defects in png-1, I identified a new allele of sax-2 as an enhancer mutation. sax-2 encodes a scaffolding protein that regulates the activity and localization of sax-1, an NDR kinase. Examination of neuronal phenotypes in sax-1 and sax-2 mutants revealed similar png-1 like defects in VC4-5. Genetic analysis of the double mutants png-1;sax-1 and png-1;sax-2 revealed strong synergistic phenotypes suggesting that png-1 and sax-1/sax-2 function in parallel pathways to regulate axon branching. In summary, this thesis reveals novel components and pathways in the regulation of neuronal branching.
| Identifer | oai:union.ndltd.org:uottawa.ca/oai:ruor.uottawa.ca:10393/30699 |
| Date | 25 March 2014 |
| Creators | Habibi-Babadi, Nasrin |
| Contributors | Colavita, Antonio |
| Publisher | Université d'Ottawa / University of Ottawa |
| Source Sets | Université d’Ottawa |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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