Elucidating the interlinked roles of spastin and protrudin in axonal degeneration and regeneration

Axons are long threadlike parts of nerve cells (neurons) and they conduct electrical signals from one neuron to another. Degeneration of axons contributes to both rare and common neurological conditions, while axons in the brain and spinal cord have a poor capacity to regrow after injury, explaining the devastating effects of, for example, paralysis after spinal cord injury. This means that it is important that we understand in molecular detail what axons need to stay healthy, as this might reveal strategies to prevent axonal degeneration and promote regrowth after injury.

This project examines the molecular biology of two proteins related, spastin and protrudin, that work together in cells, and which have been implicated respectively in axon degeneration and regrowth. Neurons lacking sufficient amounts of spastin for a long period develop axonal degeneration and this underlies one cause of a condition called hereditary spastic paraplegia, in which affected people become paralysed. In contrast, neurons in which the amounts of protrudin have been experimentally increased develop a better capacity to regrow after injury. In our recent work in non-neuronal cells, we have found that one function of spastin is to inhibit protrudin. In this project, we will test whether this same relationship between the proteins exists in nerve cells, and if so will determine in detail how spastin inhibits protrudin. As decreasing the activity of spastin enhances the activity of protrudin, we will ask whether this increases the potential for axon regrowth, which could potentially represent a treatment strategy after nerve damage in the brain or spinal cord. Finally, we will test whether over-activation of protrudin is responsible for some of the pathological effects of lack of spastin. If this is that case it would suggesting that blocking the effects of protrudin could be a treatment for hereditary spastic paraplegia.

Overall, this project will enhance our understanding of the fundamental biology that underpins axon health and potential for regrowth after injury, could uncover unsuspected links between the mechanisms of axonal degeneration and regeneration, as well as potentially identifying new treatment strategies for neuro-degenerative conditions and nerve injury in the brain and spinal cord.

This project will characterise in neurons the functional relationship between spastin and protrudin, two proteins implicated in CNS axonal degeneration and regeneration. Mutations in spastin are the leading cause of the axonal degenerative condition hereditary spastic paraplegia, while protrudin enhances CNS axonal regeneration. These proteins interact at the endoplasmic reticulum (ER) to regulate membrane traffic processes that depend on ER-endosome contacts. The functional significance of this interaction was unknown until our recent work demonstrated that spastin inhibits protrudin in non-neuronal cells. Cells lacking spastin or its binding partners IST1 or CHMP1B (both atypical endosomal sorting complex required for transport-III [ESCRT-III] proteins) develop highly polarised phenotypes identical to those observed on protrudin expression and which require protrudin for their formation. However, important questions remain regarding the role and molecular mechanisms of action of this pathway in human neurons, HSP pathogenesis and CNS axonal regeneration:

1. Is the inhibitory relationship that we have discovered between spastin, ESCRT-III proteins and protrudin present in neurons? Specifically, what functions of protrudin are affected and by what mechanisms?

2. Do spastin and the ESCRT proteins inhibit protrudin in the special context of its role in promoting axonal regeneration, suggesting acute inhibition of these proteins as a novel strategy to promote regeneration?

3. Are pathological effects of spastin abnormality in neurons mediated by increased activity of protrudin, suggesting protrudin inhibition as a novel therapeutic strategy for spastin-HSP?

We will answer these questions, predominantly using human induced pluripotent stem cell-derived neuronal systems. In doing so we will elucidate molecular cellular mechanisms that link axonal degeneration and regeneration, and identify potential treatment strategies for both.