EXPLORING MECHANISMS UNDERLYING AXON REGENERATION USING DROSOPHILA SENSORY NEURONS
Open Access
- Author:
- Rao, Kavitha S
- Graduate Program:
- Biochemistry, Microbiology, and Molecular Biology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 22, 2016
- Committee Members:
- Melissa Rolls, Dissertation Advisor/Co-Advisor
Melissa Rolls, Committee Chair/Co-Chair
Wendy Hanna-Rose, Committee Member
Lorraine C Santy, Committee Member
Richard W Ordway, Outside Member
Graham Hugh Thomas, Committee Member - Keywords:
- neuron
injury
neuronal injury
axon regeneration
drosophila
zebrafish
axotomy
dendrite
microtubule
spastin
atlastin
hereditary spastic paraplegia - Abstract:
- Axons and dendrites form two morphologically and functionally distinct compartments of a polarized neuron, and thus require different sets of proteins and organelles. Microtubules are at the crux of neuronal polarity, as they facilitate directional trafficking within the neuron. Moreover, microtubule orientation within each compartment is considered to be one of the key determinants of axonal and dendritic identity. Neurons are vulnerable to various types of injury and it is critical for these cells to repair, as most neurons cannot be replaced. The overarching theme of the upcoming sections is neuronal responses to injury and the underlying molecular mechanisms. Loss of an axon is a major challenge to a neuron. Neurons in both vertebrates and invertebrates respond to axon loss by converting a dendrite to an axon. In Chapter 2, we show that the nociceptive Class IV sensory neurons in Drosophila, also respond to complete axon loss or proximal axotomy by converting a dendrite to a new axon. However, distal axotomy frequently led to formation of two axons, one from the axon stump and another by converting a dendrite. Using microtubule polarity changes as a read-out, we describe a novel feedback mechanism between the two regenerating axons, such that when growth from the stump is blocked, the decision to convert a dendrite becomes accelerated. Previous studies have discovered that axon regeneration in Drosophila sensory neurons requires spastin, a microtubule-severing enzyme. However, it is not known how loss of spastin leads to this defect. In Chapter 3, we present evidence that loss of atlastin, a protein that interacts with spastin, also results in defective axon regeneration. Interestingly, impaired axon regeneration was observed only when reduced levels of spastin or atlastin were combined with the presence of a dominant-negative microtubule regulator. Further, impaired axon regeneration due to loss of spastin or atlastin was associated with defects in localization of the endoplasmic reticulum (ER) at regenerating axon tips. Thus, we propose a model where, spastin and atlastin are involved in ER-microtubule co-ordination during axon regeneration. In comparison to axon regeneration, very little is known about dendrite regeneration. Previous studies in the dendrites of Drosophila peripheral neurons have revealed that dendrite regeneration does not require the dual leucine-zipper kinase (DLK) signaling pathway, which is critical during axon regeneration. In order to extend these studies to vertebrates, somatosensory neurons in zebrafish can be used as a model. However, the input-receiving sensory branches of these neurons have been considered as axons, and not dendrites. In Chapter 4, microtubule polarity of the sensory endings in zebrafish Rohon-Beard neurons is investigated by live imaging techniques, in an effort to resolve their identity. Also, tools to test the requirement of the DLK pathway in the regeneration of zebrafish sensory endings are also described.