New Important Insights on How TDP-43 Impacts Intracellular Transport in ALS

New research published recently in the journal Nature Neuroscience provides a detailed analysis of TDP-43 aggregate composition and sheds important new insight on intracellular transport defects in ALS and other neurodegenerative diseases.

TDP-43 protein is a pathological hallmark of both ALS and other neurodegenerative diseases that is mislocalized in the cell’s cytoplasm, the area of the cell that surrounds the nucleus, and accumulates into aggregates — protein clumps. Through funding by The ALS Association and others, Dr. Wilfried Rossoll and colleagues at Mayo Clinic in Jacksonville and Emory University took a deep dive in understanding how TDP-43 is involved in disease.

Their analysis of TDP-43 aggregate composition showed that TDP-43 mislocalization and aggregation causes direct defects in the transport of proteins between the nucleus and cytoplasm though nuclear pores, called nucleocytoplasmic transport. Importantly, they found TDP-43 aggregates in postmortem brains from people who lived with ALS show nuclear pore defects that are comparable to their findings in cells, suggesting a similar disease pathway in humans. Read more to learn about these exciting, novel findings.

Why do we care about TDP-43?

TDP-43 or TAR DNA-binding protein-43 is a protein that is so commonly found in both people living with ALS and frontotemporal dementia (FTD) that is it is called a hallmark of disease. Mutations in TDP-43 are also found in some inherited or familial ALS cases. TDP-43 is normally located in the nucleus of a cell. Under disease conditions, TDP-43 crosses the nuclear pore — the gateway between the nucleus and cytoplasm — and accumulates — aggregates — in the cell’s cytoplasm. Environmental stress conditions and aging may also encourage TDP-43 cytoplasmic aggregation.

This pathology is observed in approximately 97 percent of ALS and 50 percent of reported FTD cases. It is also commonly present in other neurodegenerative diseases. The exact mechanism of how this TDP-43 pathology causes neurodegeneration and the exact composition of the TDP-43 aggregates are currently unknown. Insight into this disease pathway will be imperative for ALS and other neurodegenerative disease therapeutic development.

The study goals

Dr. Wilfried Rossoll at the Mayo Clinic Jacksonville in Florida and his colleagues (lab team pictured at top) set out to further understand the TDP-43 disease pathway and its aggregate composition using novel methods and building on information discovered in the past.

Dr. Rossoll with paper first author Dr. Ching-Chieh (Ian) Chou

Numerous researchers have discovered that TDP-43 is mislocalized to the cell’s cytoplasm, where it forms pathological aggregates. Previously, it has been demonstrated that the C9orf72 repeat expansion, the most commonly inherited mutation in ALS, also induces defects in nucleocytoplasmic transport and causes cytoplasmic accumulation of TDP-43. This study takes a deeper dive in understanding this ALS disease pathway and suggests that nucleocytoplasmic transport defects are not limited to C9orf72 cases, but a pathological hallmark of the vast majority of sporadic and familial forms of ALS and other neurodegenerative diseases.

The specific composition of TDP-43 aggregates have remained elusive, most likely due to the technical limitations in preserving protein interactions within the aggregates. Lysis methods used to break the aggregates apart to explore its contents normally prevent such a detailed analysis. Here, the researchers modified a comprehensive protein labeling method, called “proximity-dependent biotin identification” (BioID), to characterize TDP43 aggregate protein composition in detail.

Results

The researchers discovered evidence for several key points:

Proteins involved in nucleocytoplasmic transport are major components of TDP-43 aggregates.
Aggregated, mutated TDP-43 causes mislocalization and/or aggregation of nucleoporins components of the nuclear pore complex — and transport factors.
Specific structural domains that are present in TDP-43 and some nucleoporins called low-complexity domains and prion-like domains are involved in their co-aggregation.
Pathological forms of TDP-43 also trigger morphological changes in the nuclear pore complexes and the nuclear membrane, in addition to functional defects in protein import and messenger RNA export in and out of the cell’s nucleus.
These defects may contribute to the disease process in the vast majority of ALS cases that are characterized by TDP-43 pathology, including sporadic and most familial forms of ALS, and potentially also other neurodegenerative diseases.
Several nucleoporin genes act as genetic modifiers of TDP-43 toxicity in fly models, meaning that they can reduce toxicity when mutated.
Pharmacological and molecular inhibition of TDP-43 toxicity could block the observed nucleocytoplasmic transport defects.
Modulators of nucleocytoplasmic transport, which have previously shown neuroprotective effect in pre-clinical models of C9orf72-linked ALS and Huntington’s disease, can also rescue disease models of TDP-43 proteinopathy.

Importantly, the researchers looked at the pathology in the postmortem brain tissue of people who lived with non-inherited ALS and inherited ALS that both carried TDP-43 aggregates compared to controls. They specifically observed TDP-43 aggregates in the brain’s motor cortex, which is responsible for movement, and frontal cortex, which is responsible for cognition. They discovered nuclear pore pathology associated with in TDP-43 aggregates in people with ALS and not in controls. This suggests defects in nucleocytoplasmic transport are a common hallmark in people living with ALS and potentially in people living with other neurodegenerative diseases that also present with TDP-43 aggregates in the brain.

Conclusions

Together, the researchers have given the most detailed view of TDP-43 pathology to date using a comprehensive protein labeling method. In doing so, they greatly furthered the understanding of defects in the nucleocytoplasmic transport pathway in ALS. Not only does TDP-43 mislocalization to the cytoplasm and aggregation exacerbate nuclear-cytoplasm transport defects involving the C9orf72 repeat expansion, but TDP-43 cytoplasmic aggregates themselves can directly disrupt nucleocytoplasmic defects by changing the normal localization of components of the nuclear pore complex and transport factors. This better understanding of TDP-43’s impact on nucleocytoplasmic transport gives further evidence that this pathway should be and could be therapeutically targeted as a potential treatment for ALS in the future.

“Building on earlier findings from our colleagues at Stanford, St. Jude, and Johns Hopkins that were specific to C9orf72-linked ALS, our new study establishes nuclear pore complex defects and nucleocytoplasmic transport dysfunction as a common feature of sporadic and familial ALS. This suggests that the development of drugs that specifically target intracellular transport defects may be beneficial for a large number of ALS patients.” Dr. Rossoll

This study was supported by awards from The ALS Association to Dr. Wilfried Rossoll at the Mayo Clinic Jacksonville and co-author Dr. Rita Sattler at Barrow Neurological Institute in Phoenix.

Paper Citation

TDP-43 pathology disrupts nuclear pore complexes and nucleocytoplasmic transport in ALS/FTD

Ching-Chieh Chou, Yi Zhang, Mfon E. Umoh, Spencer W. Vaughan, Ileana Lorenzini, Feilin Liu, Melissa Sayegh, Paul G. Donlin-Asp, Yu Han Chen, Duc M. Duong, Nicholas T. Seyfried, Maureen A. Powers, Thomas Kukar, Chadwick M. Hales, Marla Gearing, Nigel J. Cairns, Kevin B. Boylan, Dennis W. Dickson, Rosa Rademakers, Yong-Jie Zhang, Leonard Petrucelli, Rita Sattler, Daniela C. Zarnescu, Jonathan D. Glass & Wilfried Rossoll

Nature Neuroscience (2018), doi:10.1038/s41593-017-0047-3

Published online: January 8, 2018

For more information about Dr. Rossoll’s funded project click here.