Coordination of Axon Degeneration in the Peripheral Nervous System

Neuronal axon degeneration is a hallmark of all neurodegenerative disorders including Alzheimer’s, Luo Gehrig’s, and Huntington’s disease, as well as neural injury (e.g. stroke, traumatic brain injury and spinal cord injury). Moreover, loss of axons and synapses via degeneration is thought to be a main cause of cognitive decline, movement disorders, and paralysis associated with these pathologies. Despite their importance as therapeutic targets, very little is known about the mechanisms underlying axon loss. Beyond pathological situations, axon degeneration is also an important aspect of nervous system refinement during development. One particular aspect of developmental degeneration that has yet to be discovered in injury-induced degeneration, is the ability to respond to axon derived extrinsic factors. My thesis work addressed this question and establishes that axon degeneration is promoted by TNFR family members and is also coordinated via extrinsic factors.

Recent studies have revealed that members of the Tumor Necrosis Factor Receptor (TNFR) superfamily are required for developmental degeneration in Peripheral Nervous System (PNS) and Central Nervous System (CNS). However, the involvement of a cell surface receptor such as a TNFR family member has not been described previously for Wallerian Degeneration. The pathways underlying Wallerian degeneration have also long been obscured in part because of a series of long held and incorrect assumptions including that axon degeneration occurs in an axon autonomous manner.

In this thesis, I sought to determine whether the TNFR family members are involved in injury-induced degeneration. Using an in vitro microfluidic injury paradigm we assessed the neurons lacking various TNFR family members. We found that loss of TNFR family member Death Receptor 6 (DR6) delay Wallerian degeneration. I also discuss my in vivo findings, which employ electron microscopy to study the degeneration of transected sciatic nerves in different TNFR family member knockout mice. Further, I show that DR6 feeds in to the same axon degeneration downstream pathway by phosphorylation of MAPK member JNK. Additionally, I demonstrate that abberant myelination in DR6 knockout mice and full protection of axon degeneration in DR6-/-;p75NTR-/- mice, indicating uncoupling of axon and myelin degeneration processes. This work demonstrates the first described receptor involved in Wallerian degeneration and it’s downstream pathway. Furthermore, this finding indicates that there are other signaling pathways, which would involve these cell surface receptors to coordinate axon degneration.

To identify the coordination factors in Wallerian degeneration, I employed a conditioned media experiment with injured axons. My experiments demonstrate that calcium, secreted from injured axons, functions as an extrinsic factor, which coordinates axonal degeneration of neighbors in a DR6-dependent autocrine/paracrine manner. Interestingly, in the event of an injury, the calcium stores in the axons (ER and mitochondria) release calcium to the cytoplasm, which in turn gets released to the outside milieu. Taken together, these findings suggest that injured nerves coordinate their degeneration via calcium release and death receptor signaling.

In conclusion, this work establishes the notion that DR6 promotes Wallerian degeneration and MAPK/JNK signaling is involved in the downstream pathway. Furthermore, axon degeneration is coordinated by extrinsic calcium via DR6. However, several open questions remains to be addressed such as, the mechanism of calcium entry and release, the upstream/downstream components of Wallerian Degeneration pathway with respect to DR6 and other factors/ligands that affect DR6.