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Jeffrey Beekman - Improving CF Treatment
Ineke Braakman - Protein Folding


Ineke Braakman, July 3, 2021

Our Quest for Helping Hands for CFTR

We consume proteins because we recycle the building blocks of those proteins (amino acids) and need them to continuously create new proteins. This occurs in all cells of our body, where proteins perform almost all the work. They can only perform this work when they have attained the correct shape, and this is impossible without assistance. This assistance is provided by so-called 'helper proteins,' referred to as molecular chaperones in the research field.

Cystic Fibrosis and CFTR

In cystic fibrosis patients, the CFTR protein is defective. Normally, it functions as a salt channel with a built-in gate, and for this, it must be "folded" into the correct shape (as shown in the picture). Similar to a real lock, numerous malfunctions can occur.

CFTR consists of 1480 different components that are connected like a string of beads. It is, therefore, quite complex to fold this long chain into a working gate and channel!

It helps that CFTR initially folds into 5 components. Then, it becomes a chain of 5 interconnected puzzle pieces (T1, N1, R, T2, N2, as shown in the drawing) that need to fit together. But even then, this process isn't achievable without assistance, and CFTR is just one of approximately 20,000 different proteins in our body that require folding.

Ivacaftor, Orkambi, and Trikafta

The unique medications currently available for CF patients address the foundation of the disease: the defects in the mutant CFTR protein. For instance, Orkambi contains Lumacaftor, a 'corrector' that improves the folding of the CFTR protein and supports CFTR like crutches, allowing the protein to walk again and become more stable. Orkambi also includes a 'potentiator,' a substance that stimulates the weak protein, similar to a strong cup of coffee for us. The fact that improving the CFTR protein works for patients has thus been proven.

Every patient is different, and the available treatments are not yet sufficient to free all CF patients. That's why we have chosen a different approach, also focused on the folding problem of CFTR mutants.

Helper Proteins

The cell addresses the folding problem with a large collection of helper proteins that work in teams, with a 'head helper,' chaperone, surrounded by 'assistant helpers,' co-chaperones. Without these helper proteins, healthy CFTR cannot attain its active form, and the defective, diseased forms of CFTR require this assistance even more. Despite this extra help, there are still patients who do not benefit because the CFTR mutants cannot manage.

We have previously found that healthy CFTR protein requires assistance from the chaperone Hsp90. Without Hsp90, the healthy CFTR protein folds as poorly as the most common diseased variant, F508del-CFTR. Hsp90 is like a spider in a web of chaperones and co-chaperones, all of which help regulate and improve the function of Hsp90.

Together with the laboratory of Professor Georgios Karras in Houston (USA), we have now discovered that yet another chaperone is required for the folding of CFTR, namely Hsp70. Furthermore, the assistance of Hsp90 and Hsp70 seems to be specific to the CFTR protein, as family members of the CFTR protein are much less dependent on Hsp90. We have also determined that Hsp90 and Hsp70 primarily bind to the N1 portion (the purple section in the image). This is exactly where the F508del mutant is located. N1 appears to be the foundation of a well-functioning CFTR channel, and many mutations in N1 lead to the strongest assistance from the helper proteins and yet the most severe defects.

What's Next?

We have found which two chaperones are important for the folding and proper functioning of the CFTR protein, and we have identified which part of CFTR they bind to. What we still don't know is how assistance from the helper proteins affects the other CFTR components. Do they perform better or worse when all the assistance is directed toward one component? Or do those other parts also receive assistance, but perhaps less or differently?

As described above, the chaperones work in large teams with co-chaperones. Because we now know so much about the chaperones and how the CFTR protein folds into its active form, we will focus on the assistant helpers, the co-chaperones, and the customization that these teams provide for each patient mutant. We expect to discover what each helper team looks like: which (assistant) helpers bind to the healthy CFTR and to each mutant CFTR protein. The next step is to investigate how manipulating these teams changes the CFTR mutants. This provides a basis for developing a new type of drug for CF that addresses the defective protein through the chaperone teams.