Proteolytic Degradation of Amyloidogenic Proteins: Initial Discovery for Transthyretin Amyloidosis

Transthyretin (TTR) is a highly conserved transport protein in the human thyroid-hormone pathway¹. Following translation, TTR naturally folds into a homotetrameric protein². However, mutations in the ttr gene and age-related wild-type degradation result in misfolded tetrameric TTR. This instability causes the tetramer to break down and form free monomers. These monomeric units can misfold into aggregates, oligomerize, and form fibril structures which deposit in various organs, including the brain and the heart¹. Once accumulation in the heart advances, transthyretin amyloidosis (aTTR) can occur and lead to the progressive and deadly disease Transthyretin Amyloid Cardiomyopathy (ATTR-CM). Often under and misdiagnosed, ATTR-CM is characterized by the formation of insoluble TTR fibrils in areas of the heart³ and is responsible for approximately 20% of heart failure deaths in the United States⁴. The fatal disease occurs most commonly in wild-type TTR amyloidosis (aTTRwt) compared to the genetic variant TTR amyloidosis (aTTRv), and individuals over 60 years of age⁵.

Current clinical treatments for ATTR-CM focus on preventing TTR aggregation via protein stabilization, liver transplantation and, more recently, gene editing and silencing⁶. Despite these advances, limited strategies exist to remove misfolded TTR species, including monomers, oligomers and fibrils. Proteases present a promising and novel approach to disaggregation due to their high catalytic activity in vivo. The downside to utilizing proteases is their promiscuous nature, which could result in unwanted side effects. As a result, this research aims to identify and characterize candidate proteases to degrade transthyretin aggregates. Currently, we are focusing on the protease HTRA1 as previous research has shown its ability to disaggregate α-synuclein amyloids⁷, cleaving after the amino acid valine⁸. Intriguingly, TTR β-strands F and H, which were shown to be responsible for driving TTR aggregation⁹, contain two valine amino acids, respectively, introducing the possibility of HTRA1-mediated proteolysis at these sites. Understanding this possible degradation mechanism will allow for broader research into engineering proteases specific to certain amyloids, eliminating the threat of indiscriminate cleaving. Success in this research will open the door to a new novel strategy for treating various deadly diseases caused by aggregating proteins.

References

(1) Liz, M. A.; Coelho, T.; Bellotti, V.; Fernandez-Arias, M. I.; Mallaina, P.; Obici, L. A Narrative Review of the Role of Transthyretin in Health and Disease. Neurology and Therapy 2020, 9 (2), 395–402. https://doi.org/10.1007/s40120-020-00217-0.

(2) Sanguinetti, C.; Minniti, M.; Susini, V.; Caponi, L.; Panichella, G.; Castiglione, V.; Aimo, A.; Emdin, M.; Vergaro, G.; Franzini, M. The Journey of Human Transthyretin: Synthesis, Structure Stability, and Catabolism. Biomedicines 2022, 10 (8), 1906. https://doi.org/10.3390/biomedicines10081906.

(3) Obi, C. A.; Mostertz, W. C.; Griffin, J. M.; Judge, D. P. ATTR Epidemiology, Genetics, and Prognostic Factors. Methodist DeBakey Cardiovascular Journal 2022, 18 (2), 17–26. https://doi.org/10.14797/mdcvj.1066.

(4) Jain, A.; Zahra, F. Transthyretin Amyloid Cardiomyopathy (ATTR-CM). In StatPearls; StatPearls Publishing LLC: Treasure Island, FL, 2019.

(5) Nativi-Nicolau, J.; Siu, A.; Dispenzieri, A.; Maurer, M. S.; Rapezzi, C.; Kristen, A. V.; Garcia-Pavia, P.; LoRusso, S.; Waddington-Cruz, M.; Lairez, O.; Witteles, R.; Chapman, D.; Amass, L.; Grogan, M. Temporal Trends of Wild-Type Transthyretin Amyloid Cardiomyopathy in the Transthyretin Amyloidosis Outcomes Survey. JACC: CardioOncology 2021, 3 (4), 537–546. https://doi.org/10.1016/j.jaccao.2021.08.009.

(6) Teng, C.; Li, P.; Bae, J. Y.; Pan, S.; Dixon, R. A. F.; Liu, Q. Diagnosis and Treatment of Transthyretin‐Related Amyloidosis Cardiomyopathy. Clinical Cardiology 2020, 43 (11), 1223–1231. https://doi.org/10.1002/clc.23434.

(7) Chen, S.; Puri, A.; Bell, B.; Fritsche, J.; Palacios, H. H.; Balch, M.; Sprunger, M. L.; Howard, M. K.; Ryan, J. J.; Haines, J. N.; Patti, G. J.; Davis, A. A.; Jackrel, M. E. HTRA1 Disaggregates α-Synuclein Amyloid Fibrils and Converts Them into Non-Toxic and Seeding Incompetent Species. Nature Communications 2024, 15 (1). https://doi.org/10.1038/s41467-024-46538-8.

(8) Grau, S.; Baldi, A.; Bussani, R.; Tian, X.; Stefanescu, R.; Przybylski, M.; Richards, P.; Jones, S. A.; Shridhar, V.; Clausen, T.; Ehrmann, M. Implications of the Serine Protease HtrA1 in Amyloid Precursor Protein Processing. Proceedings of the National Academy of Sciences 2005, 102 (17), 6021–6026. https://doi.org/10.1073/pnas.0501823102.

(9) Saelices, L.; Johnson, L. M.; Liang, W. Y.; Sawaya, M. R.; Cascio, D.; Ruchala, P.; Whitelegge, J.; Jiang, L.; Riek, R.; Eisenberg, D. S. Uncovering the Mechanism of Aggregation of Human Transthyretin. Journal of Biological Chemistry 2015, 290 (48), 28932–28943. https://doi.org/10.1074/jbc.m115.659912.