The regulation of axon degeneration by SARM1

Lead Research Organisation: University of Cambridge
Department Name: Clinical Neurosciences

Abstract

The aim of this Industrial Partnership is to understand a preventable and widely-occurring mechanism of axon degeneration, known as Wallerian degeneration, underpinning a rapidly increasing interest in drug development. When activated by nerve injury, blocking Wallerian degeneration can rescue axons for several weeks. When activated by disrupting one specific protein, we recently showed that blocking it rescues axons indefinitely. Animal and cell culture studies indicate the Wallerian degeneration mechanism contributes widely to disease so it has become an important goal in the Pharmaceutical industry. Together with our industrial partner, AstraZeneca, we will focus on crucial step in the pathway generating new basic knowledge that will support future drug development.

Axons are the long fibres that connect one nerve cell with another, relaying electrical information around our nervous system. They are essential for many normal body functions, not only those we typically associate with nervous system such as thinking, memory, pain and movement, but also for vision, hearing, gut function, bladder control and breathing. In short, without fully functional axons there is no normal life.

Unfortunately, axons are the most vulnerable parts of our nerve cells. Many of them are very long, up to one meter compared to typical cellular dimensions of a fraction of a millimeter. Other axons are highly branched, posing a significant challenge for the far smaller cell body to support it. Like any remote structure dependent on central support, axons die first when things go wrong, for example in ageing, injury and disease.

Our research opened up an entirely new field in understanding why axons degenerate and how we can prevent it. We identified the first gene known to preserve injured axons and showed that it also protects axons that are compromised in other ways without physical injury. Stemming from this finding, disruption of other genes has been found to block Wallerian degeneration too. This project focusses on one of them, SARM1, a protein required for axons to undergo Wallerian degeneration. In some circumstances, blocking SARM1 confers lifelong rescue of axons.

SARM1 has become an area of considerable interest to the Pharma industry, including to our industrial partner AstraZeneca, following identification of an enzyme activity associated with it. In order to maximise the chance of success, it is vital to understand SARM1 function more than we currently do. In particular, we need
to understand how it is regulated and the molecular consequences of its activation that lead to axon death. Our group has already made significant progress by identifying steps in the Wallerian degeneration mechanism that precede SARM1 activation. Here we present new hypotheses regarding these next steps together with strategies to test them.

These studies should identify new strategies to block SARM1 activation and rescue axons from degeneration. Thus, our industrial partner AstraZeneca is co-sponsoring this proposal and will contribute significant additional expertise in molecular neuroscience, chemistry and mass spectrometry. Together with our long-term collaborators Giuseppe Orsomando and colleagues, experts in NAD metabolism, we make a highly effective team able to drive this important topic forward and underpin future drug development.

Technical Summary

This Industrial Partnership proposal is a three-way collaboration between experts in axon degeneration (Coleman, Cambridge), NAD metabolism (Orsomando, Ancona) and drug discovery (Wang, AstroZeneca). We seek to understand the mechanism of activation of the SARM1 protein, a closely-regulated NAD degrading enzyme required for axons to degenerate in many circumstances. Deletion of the SARM1 gene blocks the Wallerian degeneration of axons distal to an injury for several weeks. In axons specifically lacking the NAD synthesising enzyme NMNAT2 it confers a lifelong rescue of their survival and function.

This striking, indefinite rescue of axons has led to significant interest in targeting SARM1 for drug development in axonal disorders such as peripheral neuropathies, Parkinson's disease, multiple sclerosis, glaucoma and others. However, several fundamental aspects of SARM1 function remain unclear, most notably how it is regulated, how it causes axon degeneration and its subcellular location. We will use unique resources and expertise available to our three collaborating laboratories to address these questions, thereby underpinning future drug development strategies.

We will test the hypothesis that one or more activators of SARM1 accumulate/s in the axoplasm of NMNAT2-/-;SARM1-/- double mutant mice, a healthy strain in which Wallerian degeneration has been first constitutively activated by removing NMNAT2 and then prevented from completing by also removing SARM1. We will test the hypothesis that SARM1 causes axon death not by NAD depletion but by signalling for calcium mobilisation through the products of its NADase activity. Finally, we will identify the subcellular location where SARM1 resides by colocalisation and cosegregation studies and test whether this changes during axon degeneration.

Planned Impact

The actual and potential non-academic beneficiaries of this project will be:

(1) Our industrial partner, AstraZeneca, who will gain from intellectual and practical collaboration particularly in the field of Wallerian degeneration and longer-term with the potential to license and develop new intellectual outputs from this project taking it towards drug discovery. However, it is highly likely that our discussions will be even more wide-ranging as the project goes forward and other areas of potential interaction emerge. This will be an extremely important collaboration for both sides of this partnership.

(2) Other Pharma/biotech companies targeting axon degeneration disorders. We will of course pay full regard to the commercial sensitivities of our industrial partner and adhere to the agreement that we make. Nevertheless, in the general field of axon degeneration the increased understanding we will gain from this research, including after any period for patenting and publication, will be of significant interest to the wider Pharma and biotech community. We are aware of a significant number of companies who are targeting the Wallerian degeneration pathway who will closely watch this research.

(3) Patients with axonal disorders. There is no treatment for axon degeneration in neurodegenerative disease, nor indeed in ageing or metabolic disorders such as diabetes. This work has significant potential for later translation into drug discovery with applications particularly in chemotherapy-induced peripheral neuropathy, multiple sclerosis, stroke and other acute or relapsing/remitting conditions where even temporary axon protection is useful. As the human genetics of axonal disorders becomes clearer, it is likely that the few cases of NMNAT2 mutation we are already aware of (papers in preparation) will grow and these individuals could benefit significantly from blocking SARM1 activity, considering the lifelong rescue of axons we found in mice (Gilley et al, Cell Rep 2017).

(4) Lay public interested in nervous system function and dysfunction. In our public engagement activities we find a strong interest in the nervous system among the lay public. While there is only a limited amount that individuals can do about the health of their nervous system, understanding what goes wrong can at least help some patients to come to terms with it, and motivates some healthy individuals to ensure they avoid a lifestyle that places strains on axon survival (e.g., excessive alcohol consumption, obesity, drug abuse).

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