Investigation of the molecular basis underlying a multi-step model of axon injury responses

Open Access
Author:
Chen, Li
Graduate Program:
Cell and Developmental Biology
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
May 12, 2015
Committee Members:
  • Melissa Rolls, Dissertation Advisor
  • William O Hancock, Committee Member
  • Richard W Ordway, Committee Member
  • Douglas Cavener, Committee Member
  • Zhi Chun Lai, Committee Member
Keywords:
  • axon regeneration
  • neuroprotection
  • microtubules
  • gamma-tubulin
  • Nmnat
  • DLK
  • JNK
  • fos
  • caspases
  • mitochondria
Abstract:
Neurons are susceptible to a range of genetic and environmental insults. The ability to survive and recover from these insults is critical to maintaining a healthy and functional nervous system. The goal of my study is to identify the molecular and cellular mechanisms behind two axon injury responses, self-protection and axon regeneration, and study the relationship between the two. I have established a conditioning lesion assay in Drosophila larval sensory neurons to identify an endogenous protective response. I have discovered that traumatic axon injury protects dendrites against a secondary injury, and this protection requires DLK-JNK-fos-dependent upregulation of microtubule dynamics. Furthermore, the microtubule nucleation protein γTub23C is important for injury-induced microtubule dynamics and protection. Strikingly, this microtubule-based protection is activated in two types of chronic stresses including expressing poly-Q neurodegenerative disease proteins and compromising kinesin-3-mediated axonal transport. In both scenarios, microtubule dynamics antagonize long-term neurodegeneration. Therefore, neurons may utilize upregulated microtubule dynamics as a general survival strategy to resist a variety of acute and chronic neuronal insults. Nicotinamide mononucleotide adenylyltransferases (Nmnats) are well-established neuroprotective factors. I have found that endogenous Nmnat is required for axon injury-induced protection. Genetic analysis suggests that Nmnat is subject to positive regulation by the DLK-JNK-fos pathway and negative regulation by a caspase cascade downstream of mitochondrial fission. Together, these results reveal a high degree of signaling complexity in Nmnat regulation. Why is Nmnat so tightly regulated? Protection is an early axon injury response and axon regeneration occurs after its inactivation. Hyperactivation of Nmnat caused by JNK and fos overexpression or reduction of the initiator caspase Dronc, however, results in abnormalprotection in conjunction with axon regeneration defects. These defects can be rescued by reducing Nmnat. This suggests that (1) completion of neuroprotection is necessary for axon regeneration; (2) coordination between protection and axon regeneration relies on a tight control of Nmnat activity. In summary, my results suggest that axon injury responses can be divided into multiple steps, such as protection and regeneration, and each step involves distinct regulatory mechanisms. As a beginning effort to untangle the complex regulatory networks of this multi-step model, I have uncovered that the DLK pathway protects neurons through Nmnat and microtubules early after axon injury, and later inactivates Nmnat through mitochondrial fission and caspases to turn protection off and allow axons to regenerate.