Research Article

Autosomal dominant VCP hypomorph mutation impairs disaggregation of PHF-tau

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Science  20 Nov 2020:
Vol. 370, Issue 6519, eaay8826
DOI: 10.1126/science.aay8826

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Two ways to get tangled?

Neurodegeneration in Alzheimer's disease dementia is associated with neurofibrillary tangles composed of aggregated tau protein. Darwich et al. describe an additional form of autosomal-dominant dementia with neurofibrillary tangles linked to a hypomorph mutation in valosin-containing protein (VCP). VCP was found to disaggregate pathologic tau, and the hypomorph mutation increased tau accumulation in cells and mice. These findings highlight the role of protein turnover in maintaining neuronal health and suggest that VCP may provide a therapeutic target for Alzheimer's disease.

Science, this issue p. eaay8826

Structured Abstract

INTRODUCTION

Alzheimer’s disease (AD) is a fatal neurodegenerative disease in which progressive brain degeneration compromises cognitive function. AD neurodegeneration is tightly associated with abnormal neuronal inclusions called neurofibrillary tangles, which are composed of aggregated tau protein. The importance of tau protein in dementia is highlighted by various known autosomal-dominant mutations in MAPT (microtubule-associated protein tau), the gene that encodes for tau, that are associated with frontotemporal lobar degeneration with tau inclusions (FTLD-tau). Identifying additional disease-causing genes that affect tau accumulation, including genes involved in protein quality control and tau clearance, has the potential to reveal previously unknown mechanisms that maintain neuronal health.

RATIONALE

Two families were identified with autosomal-dominant dementia linked to a p.Asp395Gly mutation in VCP. Valosin-containing protein (VCP) is a AAA+ [adenosine triphosphatases (ATPases) associated with diverse cellular activities] protein and uses energy from adenosine 5′-triphosphate (ATP) hydrolysis to unfold substrates to assist the dismantling of macromolecular complexes. Other VCP mutations have been identified in a disease called multisystem proteinopathy (MSP), which is associated with neuronal TDP-43 [TAR DNA-binding protein 43] protein inclusions. The mechanisms by which p.Asp395Gly VCP leads to neurodegeneration are unknown.

RESULTS

Two kindred were identified with an autosomal-dominant inheritance pattern of frontotemporal degeneration linked to a p.Asp395Gly VCP mutation. We have named this disease vacuolar tauopathy because of the presence of neuronal vacuoles and tau aggregates. Tau aggregates were morphologically and biochemically similar to AD neurofibrillary tangles. Moreover, the presence of vacuoles and neurofibrillary tangles in the brain were inversely correlated. Degenerating brain regions such as the frontal neocortex exhibited tau aggregation, whereas nondegenerating brain regions such as the visual cortex exhibited vacuolization. To further characterize the p.Asp395Gly VCP mutation, we assessed recombinant VCP proteins for ATPase activity in an in vitro assay. This approach demonstrated that p.Asp395Gly VCP exhibited a partial loss of ATPase activity, in contrast with MSP mutations, which increase ATPase activity. Given that VCP unfolds protein substrates, we hypothesized that VCP may disaggregate pathologic tau aggregates. Indeed, VCP appeared to partially disaggregate pathologic tau aggregates derived from AD human brain tissues, and the p.Asp395Gly mutation impaired this activity. VCP activity against pathologic tau was energy (ATP) dependent and required polyubiquitination of the tau substrate. In addition, expression of p.Asp395Gly VCP in a cell culture model of tau aggregation was associated with enhanced accumulation of cellular tau aggregates. Last, we generated mice in which the p.Asp395Gly mutation was knocked in, which exhibited a minimal phenotype when unchallenged. However, upon initiating tau aggregation through microinjection of pathologic AD tau extracts into the mouse brain, mutant VCP mice showed an increase in tau accumulation compared with that of wild-type animals.

CONCLUSION

We describe a partial loss-of-function mutation in VCP that was associated with a neurodegenerative disease, which we named vacuolar tauopathy. VCP appeared to exhibit activity that promoted the disruption of tau aggregates in an energy- and polyubiquitin-dependent manner. Furthermore, the p.Asp395Gly VCP mutation enhanced tau aggregation in a cell culture model system and in a knock-in mouse model. These results highlight a potential role for protein disaggregation in the maintenance of neuronal health. Furthermore, our findings suggest that VCP may provide a potential therapeutic target in the development of future AD therapies.

Vacuolar tauopathy and VCP function.

Model of tau aggregation, highlighting different mechanisms that lead to neurodegeneration in AD and FTLD-tau. Tau is a soluble protein (green) that forms highly structured fibrils (purple) in AD. Tau aggregates are also a hallmark of FTLD-tau. Autosomal dominant FTLD-tau mutations in MAPT, the gene that encodes tau protein, can alter splicing or enhance tau protein aggregation, which results in the accumulation of insoluble tau inclusions in neurons and glia. VCP (blue) appears to exhibit disaggregase activity against pathologic tau. This activity is dependent on the availability of energy (ATP) and the presence of polyubiquitin on tau protein aggregates. Vacuolar tauopathy is an autosomal-dominant form of dementia linked to a p.Asp395Gly VCP mutation (orange-red). Neurodegeneration in vacuolar tauopathy was associated with tau aggregates of similar biochemical composition and morphology as those seen in Alzheimer’s disease. The accumulation of insoluble tau aggregates in vacuolar tauopathy appeared to be in part due to a partial loss of tau disaggregase function associated with the p.Asp395Gly VCP mutation. WT, wild-type.

Abstract

Neurodegeneration in Alzheimer’s disease (AD) is closely associated with the accumulation of pathologic tau aggregates in the form of neurofibrillary tangles. We found that a p.Asp395Gly mutation in VCP (valosin-containing protein) was associated with dementia characterized neuropathologically by neuronal vacuoles and neurofibrillary tangles. Moreover, VCP appeared to exhibit tau disaggregase activity in vitro, which was impaired by the p.Asp395Gly mutation. Additionally, intracerebral microinjection of pathologic tau led to increased tau aggregates in mice in which p.Asp395Gly VCP mice was knocked in, as compared with injected wild-type mice. These findings suggest that p.Asp395Gly VCP is an autosomal-dominant genetic mutation associated with neurofibrillary degeneration in part owing to reduced tau disaggregation, raising the possibility that VCP may represent a therapeutic target for the treatment of AD.

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