The researchers demonstrated that fibrils are the globally stable states of driver proteins, whereas condensates are metastable sinks. They also showed that disease-linked mutations diminish condensate metastability, thereby enhancing fibril formation, the pathological hallmark of key neurodegenerative diseases. Amyloid fibrils formed by stress granule proteins, which resemble structures formed in other neurodegenerative disorders, have been previously suggested to originate within stress granules. However, the researchers showed that while fibril formation can be initiated on condensates’ surfaces, the condensates’ interiors actually suppress fibril formation. This means that condensates are not crucibles of ALS or FTD. Mutations that stabilize stress granules reversed the effects of disease-causing mutations in test tubes and cells, pointing to a protective role of stress granules in neurodegenerative diseases.

“It’s important to know whether stress granules are crucibles for fibril formation or protective,” said the study’s co-corresponding author Tanja Mittag, PhD, St. Jude Department of Structural Biology. “This information will aid in deciding how to develop potential treatments against a whole spectrum of neurodegenerative diseases.”

Mittag led the work alongside co-corresponding author Rohit Pappu, PhD, the Gene K. Beare Distinguished Professor of Biomedical Engineering and Director of the Center for Biomolecular Condensates at Washington University in St. Louis’s McKelvey School of Engineering, as part of the successful St. Jude Research Collaborative on the Biology and Biophysics of RNP Granules.

“This work, anchored in principles of physical chemistry, shows two things: Condensates are kinetically accessible thermodynamic ground states that detour proteins from the slow-growing, pathological fibrillar solids. And the interactions that drive condensation versus fibril formation were separable, which augurs well for therapeutic interventions that enhance the metastability of condensates,” said Pappu.

Disease fibrils form with or without stress granules

Under stress conditions such as heat, cells form stress granules to temporarily halt energy-intensive processes such as protein production. This is akin to a ship lowering its sails in a storm. When the stress is gone, the granules disassemble, and normal processes resume. Pathogenic mutations in key stress granule proteins such as hNRNPA1 prolong the lifetime of stress granules and drive the formation of insoluble fibril threads, which accumulate over time, causing neurodegeneration.

Mittag, Pappu, and their teams examined hNRNPA1 to better understand the relationship between stress granules and fibril formation. They found that disease-linked mutations drive proteins away from condensate interiors more rapidly than the “wild-type” proteins, thus enabling the formation of fibrils as they exit the condensate.

“We found that condensates are ‘metastable’ with respect to fibrils, meaning that they act as a sink for soluble proteins,” explained co-first author Fatima Zaidi, PhD, St. Jude Department of Structural Biology. “Eventually, however, proteins are drawn out of the condensate to form the globally stable fibrils.”

The authors further showed that while fibrils begin growing on condensates’ surfaces, proteins eventually incorporated into these fibrils stem from the outside, not from the inside of the condensates. Fibrils could also form in the complete absence of condensates.

Building on these foundational discoveries made jointly in the Mittag and Pappu labs, the researchers designed protein mutants which could suppress the process of fibril formation in favor of condensate formation. Remarkably, this approach also restored normal stress granule dynamics in cells bearing ALS-causing mutations.

“Collectively, this suggests that stress granules should be looked at not as a crucible, but rather a potential protective barrier to disease,” said co-first author Tapojyoti Das, PhD, St. Jude Department of Structural Biology.

These findings illuminate the role of stress granules in pathogenic fibril formation and provide an important foundation for investigating novel therapeutic approaches for neurodegenerative diseases.