A new study, led by scientists at UCL and the Francis Crick Institute, has found that the inhibition of an ATPase is able to reverse a key feature of motor neurone disease. Their findings have important implications for future therapies.
Motor neurone disease
Amyotrophic lateral sclerosis (ALS), also known as motor neurone disease, is a devastating and fatal neurological disease. It is characterised by a selective and progressive degeneration of motor neurones (MNs). This causes patients to gradually lose their muscle control, which eventually leads to them losing the ability to speak, eat and breathe.
An important pathological hallmark of ALS is the subcellular mislocalisation of RNA-binding proteins (RBPs). In 97% of all ALS cases, RBPs abnormally localise in the cytoplasm rather than the nucleus of a MN. A single RBP can bind to thousands of RNA targets. Therefore, disruption to just one RBP can potentially lead to a broad impact on RNA metabolism, which in turn can cause disease.
Mutated motor neurone cells
In this study, published in Brain Communications, researchers explored MNs that contained mutations in valosin-contain protein (VCP). VCP is an abundant ATPase that has a large variety of intracellular functions, including protein homeostasis and apoptosis.
Pathogenic mutations in VCP have been identified in multiple neurological diseases. These include familial ALS, of which VCP mutations account for 1-2% of cases. Here, the researchers studied the distribution of key RBPs in VCP mutant MNs compared to controls.
VCP mutations cause selective mislocalisation
The VCP mutant MNs were used to comprehensively investigate the subcellular localisation of ALS-related RBPs. Single-cell analysis of more than 70,000 neurones revealed a selective decrease in the nuclear-to-cytoplasmic ratio of two RBPs, TDP-43 and SFPQ, in the VCP mutant MNs. These findings build on a recent study by the same group, which found that another key RBP called FUS also had a reduced nuclear-to-cytoplasmic ratio in VCP mutant MNs.
In addition to accumulating in the cytoplasm, it was discovered that TDP-43, FUS and SFPQ were also localising in neurites, which are projections from the cell body of neurons. Recent studies revealed that these RBPs may play a role in axonal mRNA viability. This suggests that impaired axonal RNA processing could contribute to ALS pathophysiology in MNs.
VCP inhibition in healthy motor neurones
Although it was known that VCP mutations affected human MNs, the mechanism by which the mutations led to RBP mislocalisation was unknown. Therefore, the researchers selectively inhibited an ATPase domain in VCP in healthy MNs to see if this would induce ALS phenotypes. The VCP inhibition increased the cytoplasmic localisation of TDP-43 but had no effect on the nuclear-to-neurite ratio. Conversely, there were no changes in the nuclear-to-cytoplasmic ratio of FUS, but its nuclear-to-neurite ratio increased by a small but statistically significant amount.
From these findings, the researchers concluded that loss-of-function in the VCP ATPase domain in healthy cells is not a mechanism for ALS phenotypes.
Reversal of ALS phenotype
Next, the researchers inhibited VCP in VCP mutant MNs. Amazingly, this reversed the mislocalisation of TDP-43 and FUS. In fact, the distribution of both RBPs returned to normal, healthy levels. For SFPQ, VCP inhibition had a partial effect. Specifically, neurite mislocalisation was reversed, but reversal of cytoplasm mislocalisation was not large enough to be statistically significant.
First author Dr Jasmine Harley said: “Demonstrating proof-of-concept for how a chemical can reverse one of the key hallmarks of ALS is incredibly exciting. We showed this worked on three key RNA binding proteins, which is important as it suggests it could work on other disease phenotypes too.”
The molecular mechanism of VCP’s interaction with TDP-43, FUS and SFPQ is still unknown. Therefore, this should be a point of focus for future studies. However, the main finding that VCP inhibition can reverse RBP mislocalisation in VCP mutant cells is incredibly exciting. Further studies will be able to build on this to see the degree of therapeutic potential VCP inhibitors hold.
Dr Jasmine Harley said: “More research is needed to investigate this further. We need to see if this might reverse other pathological hallmarks of ALS and also, in other ALS disease models.”