A survival factor for axons: roles in disease and downstream mechanism

Our nervous system cannot function without axons, the long ?wires? conducting electrical signals from one nerve cell (neuron) to another. Even if other parts of the neuron (the cell body and dendrites) survive, a neuron without an axon is functionally dead. Axons are very vulnerable because of their immense length (up to one metre in man) and their need to deliver essential components from cell bodies to all locations along their lengths using a sophisticated process known as ?axonal transport?. Consequently, axon degeneration makes critical contributions to symptoms in many neurodegenerative conditions, including multiple sclerosis, glaucoma, diabetic neuropathy, motor neuron disease and Alzheimer?s disease. Once lost, axons in our brain and spinal cord do not regenerate so it is essential to preserve them in ageing and disease.

We recently identified an enzyme (Nmnat2) as an axon survival factor using a neuronal culture system. We propose that failure to deliver Nmnat2 could be responsible for axon death in diseases where axonal transport fails. This is based on experiments where substituting with a similar but longer-lasting enzyme named WldS increases axon survival and alleviates disease. WldS is not present in people whereas Nmnat2 is, which makes this new development particularly exciting. Because we can now regulate the degenerative process by manipulating a single molecule that humans do have, we can make rapid progress in understanding this type of degeneration and working out the best way to block it pharmacologically.

To get the full picture, we also need to study this process in the context of a mammalian nervous system and its roles in neurodegenerative disorders. We will genetically modify mice to block or reduce the production of Nmnat2 in neurons. When its production stops altogether we expect that axons will die through a mechanism called ?Wallerian-like degeneration?, a pathway we have studied for many years and can test for using the WldS gene. When production of the proposed survival factor is reduced by around 50%, we expect that axons may initially survive but become more susceptible to other stresses such as neurotoxins, physical pressure, inherited defects that ?clog up? our axons and possibly even normal ageing. By testing whether axonal Nmnat2 levels are reduced in axonal transport disorders, and whether the degree of reduction is related to the severity of the disease, we aim to understand the molecular steps leading to axon degeneration and ultimately target them therapeutically.

In neurodegenerative disorders axons typically degenerate before neuronal cell death. This sequence of events, and particularly the early loss of distal axons, is known as ?dying back? degeneration. The causes of axon degeneration include protein aggregation, inflammation, neurotoxicity and ischaemia, and many of these diverse stresses converge on a common degenerative pathway involving axonal transport impairment. Axonal transport is the bidirectional trafficking of molecules and organelles along axons for huge cellular distances. It is essential for axon survival but deficient in multiple sclerosis, glaucoma, motor neuron disease and many other disorders.

Despite the prevalence of axonal transport impairment, the specific molecular changes leading to axon degeneration are poorly understood. Cutting axons, which causes Wallerian degeneration, is a useful experimental model that can help identify the key molecular events. A mutant protein named Wallerian degeneration slow (WldS) delays Wallerian degeneration by tenfold and alleviates some ?dying back? disorders, showing that the mechanisms are related. Thus, axons do not die by passive wasting when isolated from cell bodies but by a specific and regulatable process.

WldS is an aberrant protein that occurs naturally in only one strain of mouse, so until now it has been largely unclear how we might use it to protect axons in human disease. Recently, we identified the NAD+ synthesising enzyme Nmnat2 as an endogenous regulator of the same pathway in primary neuronal cultures. Nmnat2 is an unstable protein, so if axonal transport fails to replenish it, continual protein turnover in axons takes Nmnat2 below a threshold level that triggers Wallerian degeneration.

Nmnat2 is now the key to understanding the degenerative mechanism and thereby identifying suitable steps to target pharmacologically, but for the full picture it must also be studied in vivo. We hypothesise that depleting Nmnat2 is sufficient to initiate Wallerian-like degeneration in vivo and that failure to deliver it to distal axons in some axonopathies is the direct cause of ?dying back? axon loss. We also hypothesise that Nmnat2 and WldS control a common downstream pathway, which we can activate very specifically by removing Nmnat2. Thus, we can now factor out the many non-specific consequences of cutting axons or of blocking axonal transport, and focus specifically on events leading to axon degeneration. This is a unique opportunity to move towards translation for axonal transport disorders and for significant progress in understanding how axon survival and degeneration are controlled at the molecular level.