Comparative study of the stabilities of synthetic in vitro and natural ex vivo transthyretin amyloid fibrils

Degradation versus fibrillogenesis, two alternative pathways modulated by seeds and glycosaminoglycans

The mechanism that converts native human transthyretin into amyloid fibrils in vivo is still a debated and controversial issue. Commonly, non-physiological conditions of pH, temperature, or organic solvents are used in in vitro models of fibrillogenesis of globular proteins. Transthyretin amyloid formation can be achieved under physiological conditions through a mechano-enzymatic mechanism involving specific serine proteases such as trypsin or plasmin. Here, we investigate S52P and L111M transthyretin variants, both causing a severe form of systemic amyloidosis mostly targeting the heart at a relatively young age with heterogeneous phenotype among patients. Our studies on thermodynamics show that both proteins are significantly less stable than other amyloidogenic variants. However, despite a similar thermodynamic stability, L111M variant seems to have enhanced susceptibility to cleavage and a lower tendency to form fibrils than S52P in the presence of specific proteases and biomechanical forces. Heparin strongly enhances the fibrillogenic capacity of L111M transthyretin, but has no effect on the S52P variant. Fibrillar seeds similarly affect the fibrillogenesis of both proteins, with a stronger effect on the L111M variant. According to our model of mechano-enzymatic fibrillogenesis, both full-length and truncated monomers, released after the first cleavage, can enter into fibrillogenesis or degradation pathways. Our findings show that the kinetics of the two processes can be affected by several factors, such as intrinsic amyloidogenicity due to the specific mutations, environmental factors including heparin and fibrillar seeds that significantly accelerate the fibrillogenic pathway.

Thermodynamic characterization of amyloid polymorphism by microfluidic transient incomplete separation

Amyloid fibrils of proteins such as α-synuclein are a hallmark of neurodegenerative diseases and much research has focused on their kinetics and mechanisms of formation. The question as to the thermodynamic stability of such structures has received much less attention. Here, we newly utilize the principle of transient incomplete separation of species in laminar flow in combination with chemical depolymerization for the quantification of amyloid fibril stability. The relative concentrations of fibrils and monomer at equilibrium are determined through an in situ separation of these species based on their different diffusivity inside a microfluidic capillary. The method is highly sample economical, using much less than a microliter of sample per data point and its only requirement is the presence of aromatic residues (W, Y) because of its label-free nature, which makes it widely applicable. Using this method, we investigate the differences in thermodynamic stability between different fibril polymorphs of α-synuclein and quantify these differences for the first time. Importantly, we show that fibril formation can be under kinetic or thermodynamic control and that a change in solution conditions can both stabilise and destabilise amyloid fibrils. Taken together, our results establish the thermodynamic stability as a well-defined and key parameter that can contribute towards a better understanding of the physiological roles of amyloid fibril polymorphism.

Oral Therapy for the Treatment of Transthyretin-Related Amyloid Cardiomyopathy

The care of systemic amyloidosis has improved dramatically due to improved awareness, accurate diagnostic tools, the development of powerful prognostic and companion biomarkers, and a continuous flow of innovative drugs, which translated into the blooming of phase 2/3 interventional studies for light chain (AL) and transthyretin (ATTR) amyloidosis. The unprecedented availability of effective drugs ignited great interest across various medical specialties, particularly among cardiologists who are now recognizing cardiac amyloidosis at an extraordinary pace. In all amyloidosis referral centers, we are observing a substantial increase in the prevalence of wild-type transthyretin (ATTRwt) cardiomyopathy, which is now becoming the most common form of cardiac amyloidosis. This review focuses on the oral drugs that have been recently introduced for the treatment of ATTR cardiac amyloidosis, for their ease of use in the clinic. They include both old repurposed drugs or fit-for-purpose designed compounds which bind and stabilize the TTR tetramer, thus reducing the formation of new amyloid fibrils, such as tafamidis, diflunisal, and acoramidis, as well as fibril disruptors which have the potential to promote the clearance of amyloid deposits, such as doxycycline. The development of novel therapies is based on the advances in the understanding of the molecular events underlying amyloid cardiomyopathy.

Toxic Misfolded Transthyretin Oligomers with Different Molecular Conformations Formed through Distinct Oligomerization Pathways

Protein aggregation is initiated by structural changes from native polypeptides to cytotoxic oligomers, which form cross-β structured amyloid. Identification and characterization of oligomeric intermediates are critically important for understanding not only the molecular mechanism of aggregation but also the cytotoxic nature of amyloid oligomers. Preparation of misfolded oligomers for structural characterization is, however, challenging because of their transient, heterogeneous nature. Here, we report two distinct misfolded transthyretin (TTR) oligomers formed through different oligomerization pathways. A pathogenic TTR variant with a strong aggregation propensity (L55P) was used to prepare misfolded oligomers at physiological pH. Our mechanistic studies showed that the full-length TTR initially forms small oligomers, which self-assemble into short protofibrils at later stages. Enzymatic cleavage of the CD loop was also used to induce the formation of N-terminally truncated oligomers, which was detected in ex vivo cardiac TTR aggregates extracted from the tissues of patients. Structural characterization of the oligomers using solid-state nuclear magnetic resonance and circular dichroism revealed that the two TTR misfolded oligomers have distinct molecular conformations. In addition, the proteolytically cleaved TTR oligomers exhibit a higher surface hydrophobicity, suggesting the presence of distinct oligomerization pathways for TTR oligomer formation. Cytotoxicity assays also revealed that the cytotoxicity of cleaved oligomers is stronger than that of the full-length TTR oligomers, indicating that hydrophobicity might be an important property of toxic oligomers. These comparative biophysical analyses suggest that the toxic cleaved TTR oligomers formed through a different misfoling pathway may adopt distinct structural features that produce higher surface hydrophobicity, leading to the stronger cytotoxic activities.

Plasmin activity promotes amyloid deposition in a transgenic model of human transthyretin amyloidosis

Cardiac ATTR amyloidosis, a serious but much under-diagnosed form of cardiomyopathy, is caused by deposition of amyloid fibrils derived from the plasma protein transthyretin (TTR), but its pathogenesis is poorly understood and informative in vivo models have proved elusive. Here we report the generation of a mouse model of cardiac ATTR amyloidosis with transgenic expression of human TTR^S52P. The model is characterised by substantial ATTR amyloid deposits in the heart and tongue. The amyloid fibrils contain both full-length human TTR protomers and the residue 49-127 cleavage fragment which are present in ATTR amyloidosis patients. Urokinase-type plasminogen activator (uPA) and plasmin are abundant within the cardiac and lingual amyloid deposits, which contain marked serine protease activity; knockout of α₂-antiplasmin, the physiological inhibitor of plasmin, enhances amyloid formation. Together, these findings indicate that cardiac ATTR amyloid deposition involves local uPA-mediated generation of plasmin and cleavage of TTR, consistent with the previously described mechano-enzymatic hypothesis for cardiac ATTR amyloid formation. This experimental model of ATTR cardiomyopathy has potential to allow further investigations of the factors that influence human ATTR amyloid deposition and the development of new treatments.

ATTR amyloidosis causes heart failure through the accumulation of misfolded transthyretin in cardiac muscle. Here the authors report a mouse model of ATTR amyloidosis and demonstrate the involvement of protease activity in ATTR amyloid deposition.

Repositioning of the Anthelmintic Drugs Bithionol and Triclabendazole as Transthyretin Amyloidogenesis Inhibitors

Transthyretin (TTR) is a causative protein of TTR amyloidosis (ATTR amyloidosis), a general term for diseases characterized by deposition of TTR amyloid fibrils in specific organs. ATTR amyloidosis can be ameliorated by stabilization of the TTR tetramer through the binding of small molecules. Here, we show that the clinical anthelmintic drugs bithionol (42) and triclabendazole (43) potently inhibit aggregation of the amyloidogenic variant V30M-TTR. A competitive binding assay using a fluorescence probe showed that the binding affinity of 42 with V30M-TTR was significantly higher than that of the first-in-class drug tafamidis (1), and the binding affinity of 43 was similar to that of 1. The crystallographic and thermodynamic analysis revealed that 42 efficiently occupied the halogen-binding grooves of TTR, resulting in the favorable binding entropy. Multifaceted in vitro studies of anthelmintic drugs have the potential to reposition these drugs as ATTR amyloidosis inhibitors.

Amyloid-type Protein Aggregation and Prion-like Properties of Amyloids

This review will focus on the process of amyloid-type protein aggregation. Amyloid fibrils are an important hallmark of protein misfolding diseases and therefore have been investigated for decades. Only recently, however, atomic or near-atomic resolution structures have been elucidated from various in vitro and ex vivo obtained fibrils. In parallel, the process of fibril formation has been studied in vitro under highly artificial but comparatively reproducible conditions. The review starts with a summary of what is known and speculated from artificial in vitro amyloid-type protein aggregation experiments. A partially hypothetic fibril selection model will be described that may be suitable to explain why amyloid fibrils look the way they do, in particular, why at least all so far reported high resolution cryo-electron microscopy obtained fibril structures are in register, parallel, cross-β-sheet fibrils that mostly consist of two protofilaments twisted around each other. An intrinsic feature of the model is the prion-like nature of all amyloid assemblies. Transferring the model from the in vitro point of view to the in vivo situation is not straightforward, highly hypothetic, and leaves many open questions that need to be addressed in the future.

Transthyretin: From Structural Stability to Osteoarticular and Cardiovascular Diseases

Transthyretin (TTR) is a tetrameric protein transporting hormones in the plasma and brain, which has many other activities that have not been fully acknowledged. TTR is a positive indicator of nutrition status and is negatively correlated with inflammation. TTR is a neuroprotective and oxidative-stress-suppressing factor. The TTR structure is destabilized by mutations, oxidative modifications, aging, proteolysis, and metal cations, including Ca2+. Destabilized TTR molecules form amyloid deposits, resulting in senile and familial amyloidopathies. This review links structural stability of TTR with the environmental factors, particularly oxidative stress and Ca2+, and the processes involved in the pathogenesis of TTR-related diseases. The roles of TTR in biomineralization, calcification, and osteoarticular and cardiovascular diseases are broadly discussed. The association of TTR-related diseases and vascular and ligament tissue calcification with TTR levels and TTR structure is presented. It is indicated that unaggregated TTR and TTR amyloid are bound by vicious cycles, and that TTR may have an as yet undetermined role(s) at the crossroads of calcification, blood coagulation, and immune response.

Methods to study the structure of misfolded protein states in systemic amyloidosis

Systemic amyloidosis is defined as a protein misfolding disease in which the amyloid is not necessarily deposited within the same organ that produces the fibril precursor protein. There are different types of systemic amyloidosis, depending on the protein constructing the fibrils. This review will focus on recent advances made in the understanding of the structural basis of three major forms of systemic amyloidosis: systemic AA, AL and ATTR amyloidosis. The three diseases arise from the misfolding of serum amyloid A protein, immunoglobulin light chains or transthyretin. The presented advances in understanding were enabled by recent progress in the methodology available to study amyloid structures and protein misfolding, in particular concerning cryo-electron microscopy (cryo-EM) and nuclear magnetic resonance (NMR) spectroscopy. An important observation made with these techniques is that the structures of previously described in vitro formed amyloid fibrils did not correlate with the structures of amyloid fibrils extracted from diseased tissue, and that in vitro fibrils were typically more protease sensitive. It is thus possible that ex vivo fibrils were selected in vivo by their proteolytic stability.

AA amyloid fibrils from diseased tissue are structurally different from in vitro formed SAA fibrils

Systemic AA amyloidosis is a world-wide occurring protein misfolding disease of humans and animals. It arises from the formation of amyloid fibrils from serum amyloid A (SAA) protein. Using cryo electron microscopy we here show that amyloid fibrils which were purified from AA amyloidotic mice are structurally different from fibrils formed from recombinant SAA protein in vitro. Ex vivo amyloid fibrils consist of fibril proteins that contain more residues within their ordered parts and possess a higher β-sheet content than in vitro fibril proteins. They are also more resistant to proteolysis than their in vitro formed counterparts. These data suggest that pathogenic amyloid fibrils may originate from proteolytic selection, allowing specific fibril morphologies to proliferate and to cause damage to the surrounding tissue.

Systemic AA amyloidosis is a protein misfolding disease caused by the formation of amyloid fibrils from serum amyloid A (SAA) protein. Here, the authors present the cryo-EM structures of AA amyloid fibrils isolated from mouse tissue and in vitro formed fibrils, which differ in their structures and they also show that the ex vivo fibrils are more resistant to proteolysis than the in vitro fibrils and propose that pathogenic amyloid fibrils might originate from proteolytic selection.