Phase transition of FUS protein causes amyotrophic lateral sclerosis

Fused in sarcoma (FUS) is a multifunctional RNA-binding protein that regulates transcription and mRNA processing inside cells. It is mainly present in the nucleus with traces in the cytoplasm. Alteration of the FUS protein and its abnormal aggregation has been associated with the development of neurodegenerative diseases like familial amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration.

FUS, like many other proteins, has been noted to undergo liquid-liquid phase separation (LLPS) both in artificial lab conditions and in its natural conditions in the body, to form liquid-like droplets containing ribonucleoprotein aggregates. LLPS separates solutions of macromolecules like protein and RNA into two distinct liquid phases that have various biological functions. Although LLPS of FUS, leading to aberrant liquid-solid transition, is suggested to cause ALS, the underlying mechanism of the process remains unknown.

To this end, a group of researchers from the Ritsumeikan University in Japan have investigated the dynamics of the FUS-LLPS liquid condensates. The team led by Prof. Ryo Kitahara discovered how FUS undergoes phase transition, and also identified a new therapeutic target for ALS. The study was recently published in the journal Physical Chemistry Chemical Physics. While talking about the rationale behind their work, Prof. Kitahara explains, “The mechanism of liquid-to-solid phase transition of FUS must be understood in order to comprehend the onset of ALS and develop therapeutic agents targeting FUS”.

Previous studies have established that LLPS of FUS forms two distinct, reversible types of liquid condensates in equilibrium, the LP-LLPS (normal type) and the HP-LLPS (aberrant type), each with different partial molar volumes. This information provided the team with a great base to build upon. They examined the FUS solution using fluorescence recovery after photobleaching (FRAP), UV-Vis spectroscopy, and microscopy combined with hydrostatic pressure variations.

The result of FRAP experiments provided some interesting insights into the dynamics of FUS phase transitions. Apparently, HP-LLPS accelerated the ‘aging of droplets’, which is the time-dependent transition of liquid condensates into solid aggregates. Motivated by this discovery, the researchers used UV-Vis spectroscopy to investigate the reversibility of the phases. They found that the time-dependent formation of irreversible FUS aggregates was greater with HP-LLPS when compared to LP-LLPS. In other words, HP-LLPS promoted formation of irreversible solid aggregates with time, which could possibly be the reason behind the aging or degeneration of nerve cells.

In a nutshell, the above results uncovered the missing key in understanding the mystery behind liquid condensates transitioning to solid phase. As corroborated by Prof. Kitahara, “We strongly suggest that the formation of HP-LLPS via one-phase or LP-LLPS might be the physiological pathway underlying the aberrant liquid-to-solid conversion of FUS liquid condensates”.

Upon confirming that LLPS dynamics is the key to neurodegenerative disorders, the authors wanted to search for potential therapeutic agents. Since FUS-LLPS is governed by interactions between tyrosine and arginine residues, the team evaluated the effect of small molecules like arginine, dopamine, and pyrocatechol on LLPS and droplet formation. Interestingly, they found that all three suppressed HP-LLPS formation more strongly than LP-LLPS. They were thrilled to find out that arginine suppressed FUS aggregation even at very low concentrations. “Nerve cells gradually age due to abnormal protein aggregation but can remain healthy up on arginine consumption. Therefore, arginine could be a plausible drug candidate for ALS-causing FUS”, Prof. Kitahara explains.

While arginine is already marketed as a supplement, its effects on ALS prevention and progression delay should be confirmed by future clinical trials.

Discussing their long-term goals, Prof. Kitahara enthusiastically shares, “This study is the first to identify aberrant LLPS as a therapeutic target. We hope that our findings will not only help in identifying new treatments for ALS, but also stimulate further studies into the relevance of aberrant LLPS in various human diseases”.

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Reference

DOI: https://doi.org/10.1039/D2CP02171D

About Ritsumeikan University, Japan

Ritsumeikan University is one of the most prestigious private universities in Japan. Its main campus is in Kyoto, where inspiring settings await researchers. With an unwavering objective to generate social symbiotic values and emergent talents, it aims to emerge as a next-generation research university. It will enhance researcher potential by providing support best suited to the needs of young and leading researchers, according to their career stage. Ritsumeikan University also endeavors to build a global research network as a “knowledge node” and disseminate achievements internationally, thereby contributing to the resolution of social/humanistic issues through interdisciplinary research and social implementation.

Website: http://en.ritsumei.ac.jp/

About Professor Ryo Kitahara from Ritsumeikan University, Japan

Dr. Ryo Kitahara is a Professor at the Department of Pharmaceutical Sciences, College of Pharmaceutical Sciences, Ritsumeikan University in Japan. He received his Ph.D. degree in 2002-03 form the Kobe University. Prof. Kitahara specializes in Biophysics and Structural Biochemistry with special emphasis on high pressure structural biology. He has more than 50 publications to his credit and is a member of prestigious societies like the American Chemical Society, Biophysical Society and the NMR Society of Japan. He is currently exploring new design strategies of functional biomolecules based on high-energy conformers of proteins.  

Funding information

This work was supported by research grants from the Naito Science & Engineering Foundation and the institute for Chemical Fibers, Japan to R. K., Grant JSPS KAKEN JP19K16060, 21H05042, and Grant AMED JP21ek0109558 to T. Y.