New research explains how the protein TDP43 normally concentrates into droplets and how ALS-related mutations disrupt that, leading to them to form more problematic aggregates that afflict cells. Credit: Gül Zerze, Lehigh University
Structural biologists provide a new explanation for how ALS-associated genetic flaws interfere with the proper function and behavior of the protein TDP-43. In amyotrophic lateral sclerosis, aggregates of the protein TDP-43 are almost always found in afflicted neurons and glial cells. Meanwhile, about 50 ALS-linked mutations are known to affect a particular region of TDP-43. Yet scientists have never understood how those two associations connect. A new study shows how ALS mutations disrupt the protein at the atomic level, preventing it from executing its proper function and instead leading to those aggregates.
“We knew that part of TDP-43 builds up in aggregates and that there are 50 mutations in that domain, but we didn’t know the job of that domain, how it goes wrong and why it aggregates,” said Assistant Prof. Nicolas Fawzi. In general, TDP-43 acts like a chaperone for RNA in a cell, binding to it, guiding its processing, transporting it to where it needs to go and regulating it, so that other proteins can be expressed properly. Using NMR, computer simulations and microscopy, Fawzi et al at Lehigh University were able to show that under normal circumstances, TDP-43 molecules concentrate into little droplets, a process called “liquid-liquid phase separation.” It’s within these droplets that they could process and ferry RNA.
ALS-causing point mutations remodel the energy landscape of the self-assembly of the TDP-43 prion-like domain. (A) Energy landscape of the self-association of the wild-type prion-like domain; and (B) its assembly of the dynamic oligomers by self-association or interacting with nucleic acids, which is characteristic of the presence of a small portion of disordered regions. (C) Energy landscape of the self-association of the ALS-causing mutants; and (D) formation of the amyloid oligomers by self-association or aggregates upon interacting with nucleic acids. The dashed arrows are used to indicate the pathways which are likely to be inaccessible.
The team’s focus was on a particular region of TDP-43, “C-terminal domain,” which appeared to be crucial in the concentration of molecules that leads to phase separation. “We were looking for a functional role for this part of the protein,” Fawzi said.
Characterization of the wild-type TDP-43 prion-like domain.
The interactions and resulting concentration of TDP-43 molecules depend on the helix of protein’s C-terminal domain. The same sequence of DNA specifying that corkscrew shape has been exactly preserved by evolution in many vertebrate animals suggesting a biological function. What they found is that as one TDP-43 molecule meets another, the corkscrews stabilize and lengthen, promoting a bond between them. Various ALS mutations disrupt this process, either by upsetting the formation of the corkscrews or their ability to lengthen and stabilize. The result is that the concentration and phase separation does not occur. Instead the proteins can combine in a more potentially harmful way – in the aggregates seen in diseased neurons.
“That might be one mechanism by which ALS mutation cause ALS – by disrupting TDP-43’s normal function,” he said. However, only about 10% of ALS cases are traceable to a genetic cause. It remains unclear what’s happening to disrupt TDP-43 in many cases when a known mutation is not the cause.
But now scientists have new a new set of data and an explanation of how TDP-43 appears to work and what can make it fail. That’s also important, Fawzi noted, because TDP-43 is implicated in other degenerative neural diseases as well. “Given the recent evidence that TDP-43 also accumulates in Alzheimer’s disease, understanding the role of TDP-43 is all the more urgent,” he said. http://www.eurekalert.org/pub_releases/2016-08/bu-ssh081116.php
http://journals.plos.org/plosbiology/article?id=10.1371%2Fjournal.pbio.1002338
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