Scientists have uncovered a new way in which the transthyretin (TTR) protein can unfold, a discovery that could provide new insights into treating Transthyretin amyloid disease (ATTR). ATTR is a rare, progressive condition that can affect the heart and nervous system, and in some cases, the brain. The study, published in the Proceedings of the National Academy of Sciences, shows that TTR can break apart through a second, previously unknown pathway, which may contribute to the formation of harmful protein clumps in certain genetic variants of the disease. TTR is a protein responsible for transporting vitamin A and the thyroid hormone thyroxine in the blood, cerebrospinal fluid, and the eye. Normally, it exists as a stable four-unit structure. However, over time, it can break down into smaller pieces that may misfold and form amyloid deposits, leading to ATTR. These deposits are particularly damaging to organs like the heart and nervous system, causing symptoms such as nerve pain, heart failure, and even death. To discover the alternative unfolding pathway, researchers tested over 100 different TTR variants under various conditions, including different levels of urea and acidity. Using advanced imaging techniques like cryogenic-electron microscopy and existing X-ray data, they found that some TTR variants can unfold more directly, bypassing the two-unit intermediate stage. This alternative route is especially relevant in variants linked to brain-related amyloidosis and is more active in acidic environments, such as lysosomes—organelles that break down cellular waste. The study suggests that while the new unfolding pathway is not typically active under normal conditions, certain genetic mutations may make it more prominent even in the bloodstream. This could have implications for patients with rare mutations who are currently treated with drugs like tafamidis, which help stabilize TTR. Researchers are now exploring whether this alternative pathway contributes to amyloid buildup in the cerebrospinal fluid, potentially affecting the effectiveness of existing treatments. The discovery also highlights the complexity of protein folding and unfolding, as TTR is the first oligomeric protein shown to have multiple unfolding pathways in solution without external force. This finding supports theoretical models that suggest proteins can fold and unfold through multiple routes, although proving this experimentally has been challenging. The research opens new possibilities for developing targeted therapies that address specific pathways involved in amyloidosis, potentially improving treatment outcomes for patients with rare genetic forms of the disease.