Folding A Shirt With 12-Foot Arms: Understanding Protein Folding in Huntington’s Disease

Imagine a busy clothing factory. Proteins are like newly sewn shirts coming off the machines. They’re floppy, wrinkled, and can create a giant mess if they’re left unfolded. Normally, the cellular clothing factory employs “chaperone proteins” that act like skilled pressers. They grab each shirt, apply energy from a hot iron, and carefully fold it into the right shape so everything stays neat and tidy. 

But Huntington’s disease (HD) creates abnormally shaped clothes that can throw the chaperone folders off. When that happens, proteins, like irregular shirts, tend to be folded incorrectly, get tangled up, and create a huge mess. In new work published in Nature Communications, a team of researchers dove into how they could improve the protein folding process in HD by engineering extra chaperone folders. So what did they find, and what does this mean for HD?

Folding a shirt with 12-foot arms

HD is caused by an expanded stretch of genetic letters in the huntingtin gene that produce an extra-long protein, called expanded huntingtin. That extra-long protein creates harmful clumps that form inside cells. If the cell is a busy clothing factory, expanded huntingtin is like a shirt with 12 foot arms – hard to fold and store properly, causing a big mess.

Usually, there are helper proteins, called “chaperones,” that work throughout the cell to make sure other proteins fold correctly and prevent them from clumping. However, in diseases like HD, this chaperone system can become overwhelmed. There’s just too much expanded huntingtin for them to keep up with.

PEX19: solo folding machine

Chaperones need a constant supply of energy in the form of a molecule called ATP, and a team of other helper proteins, to do their job correctly. Since they depend on this support system, it makes chaperones complicated to turn into medicines – they require too much energy and teamwork. It’s like trying to use a whole factory instead of just one machine to do a job. This study focuses on a specific type of chaperone called PEX19, which is unique because it works without the need of energy (ATP) or any helper protein. 

PEX19 normally helps certain proteins get to a part of the cell called the peroxisome. Peroxisomes are little recycling and detox factories inside your cells, working to break down waste and harmful substances, such as fats and poisons, to keep the cell healthy by converting them into harmless water and oxygen. Because PEX19 works by itself and does not require ATP, it could potentially be harnessed to address protein clump formation without the need to engineer a treatment that incorporates partner proteins. 

Clearing clumps in multiple models of HD

The main goal of this research was to find a way to prevent the clumping of the expanded mutant huntingtin protein that contributes to the onset of HD symptoms. The researchers thought that, given its unique characteristics, they could modify PEX19 at the molecular level to target and remove harmful huntingtin clumps. To this end, they produced different modified versions of the PEX19 chaperone and tested which version prevented the formation of protein clumps in different organisms that model HD.

First, they engineered tiny little yeast cells that produce the huntingtin protein. If left untreated, protein clumps form that make the yeast sick. The researchers added to the yeast cells two specially designed versions of PEX19, and observed that this treatment stopped huntingtin clumps from making the yeast sick. 

Next, they added these two versions of PEX19 to human HD cells grown in a lab dish, and observed that huntingtin protein clumps formed much more slowly. Finally, they treated HD fruit flies with their most effective version of PEX19 and observed that these sick flies lived longer and could climb better, a skill that deteriorates in flies with the gene for HD. They think this was because the sticky clumps in their brains were reduced.

Stopping the mess before it starts

These interesting observations suggest that an energy-independent chaperone, PEX19, can be engineered to target and prevent the formation of huntingtin protein clumps in yeast, human cells, and fruit fly brains. This suggests that the two special versions of PEX19 may have therapeutic potential against HD in humans. 

In addition to these takeaways, the researchers identified the specific mechanism by which these two PEX19 versions inhibit protein clumps, an insight that is crucial for the development of drugs that mimic the actions of PEX19. The team observed that man-made variations of PEX19 can prevent the initial formation of protein clumps, but they cannot break apart clumps that had already formed, because this requires significant energy.  

It’s the difference between neatly folding each unwieldy, floppy-sleeved shirt right after it’s sewn, versus tackling a huge, messy mountain of them. Although this is a limitation of engineered PEX19, the fact that the chaperone is energy-independent and does not require many helper proteins still makes it an attractive candidate for therapeutic development.

Tweaking the folds for a better fit

This research makes a significant contribution by introducing a novel strategy for developing therapies for Huntington’s disease and potentially other protein aggregation disorders. It demonstrates the feasibility of engineering an ATP-independent chaperone to target and reduce the clumping of a disease-causing protein. This approach offers an alternative to traditional chaperones that rely on complex cellular machinery and energy, which can limit their effectiveness in diseased cells.

Future work should focus on further optimizing the engineered PEX19 variants to enhance their specificity and potency by fine-tuning PEX19 architecture to bind better to the huntingtin protein. Additionally, testing these variants in more complex mammalian models and eventually in clinical trials would be crucial for their development as a therapeutic agent for Huntington’s disease.

Summary

• The clumping of an expanded huntingtin protein inside cells contributes to Huntington’s disease (HD). 
• Traditional protein chaperones that could prevent formation of protein clumps require significant energy (ATP) and helper proteins, making them unsuitable as simple treatments.
• This study analyzed many different versions of PEX19, a unique chaperone that functions independently and does not use ATP, for their ability to inhibit the formation of huntingtin protein clumps in human and animal cells. 
• The research team identified two engineered versions of PEX19 capable of preventing huntingtin protein clumps in yeast, human cells, and fruit flies.
• The engineered PEX19 variants can prevent new clumps but are unable to break down existing ones. 
• The results offer a simplified approach for potential HD therapies and may inspire future research into optimizing these variants and testing them in mammalian models and clinical trials.

Learn MoreOriginal research article, “Engineering a membrane protein chaperone to ameliorate the proteotoxicity of mutant huntingtin” (open access).