Neurons have the capability to activate pathways that cause degeneration of either the entire cell by apoptosis or only the axons. Axon-specific degeneration is physiologically important as it allows neurons to remove excessive or misguided axon branches and permit plasticity in neuronal connections. However, exactly how a neuron can activate and compartmentalize this degenerative pathway to destroy its axon without putting the rest of the cell at risk is unclear. This is particularly interesting since recent studies idenified Bax, a key protein in the apoptosis pathway, to also regulate axon-specific degeneration. Our goal is to focus on the molecular intersection between apoptosis and axon-specific degeneration pathways. First, we established a microfluidic chamber-based model of sympathetic neurons where deprivation of nerve growth factor (NGF) from the axon compartment only induces axon-specific degeneration whereas NGF deprivation from the axon and soma compartments induces apoptosis. Second, we found substantial overlap between apoptosis and axon-specific degeneration but also identified distinct and unexpected differences. For example, while apoptosis required both Apaf-1 and Caspase-9 (Casp9), we found axon degeneration to require Casp9 but, surprisingly, not Apaf-1. Our results also show that neurons are exquisitely capable of selectively engaging apoptosis or axon-specific degeneration as mature neurons completely restrict apoptosis but remain permissive for axon degeneration. In this proposal, we will investigate several novel mechanisms by which neurons regulate axon-specific degeneration. We will focus specifically on determining how Bax is activated in this pathway (Aim 1), examining the mechanism by which Casp9 is activated independently of Apaf-1 (Aim 2), and probing the mechanism by which mature neurons selectively restrict apoptosis but remain permissive for axon degeneration (Aim 3). These studies will undoubtedly uncover critical aspects of how neurons utilize many of the same components for apoptosis and axon-selective degeneration but engage distinct mechanisms to allow precise spatial and temporal control over the activation of these pathways.
Our main goal is to understand the molecular mechanism by which neurons activate pathways that induce axon degeneration. Axon-specific degeneration is physiologically important as it allows for the removal of excessive or misguided axon branches and permits the establishment of new neuronal connections. Importantly, axon degeneration is also seen in many neurodegenerative diseases (e.g. Alzheimer's, Parkinson's, Huntington's diseases) where it impairs normal neuronal functions. The results from this study will help identify key mechanisms by which axon degeneration can be controlled for potential therapeutic benefit.