Tissue damage in the amyloidoses: Transthyretin monomers and nonnative oligomers are the major cytotoxic species in tissue culture

Aqueous humor erythropoietin levels in open-angle glaucoma patients with and without TTR V30M familial amyloid polyneuropathy

PURPOSE

Glaucoma is the leading cause of irreversible blindness in familial amyloidotic polyneuropathy (FAP) patients. Erythropoietin (EPO) is a cytokine that has been shown to play a role in neuroprotection and is endogenously produced in the eye. EPO levels in the aqueous humor are increased in eyes with glaucoma. In this study, we evaluated the EPO concentration in the aqueous humor of FAP and non-FAP patients, with and without glaucoma.

METHODS

Undiluted aqueous humor samples were obtained from 42 eyes that underwent glaucoma surgery, phacoemulsification, or vitrectomy. EPO concentration in the aqueous humor and blood were measured using the Immulite 2000 Xpi using an automatic analyzer (Siemens Healthcare Diagnostics).

RESULTS

The mean EPO concentration in the aqueous humor of non-FAP glaucoma eyes group 2 (75.73±13.25 mU/ml) was significantly higher than non-FAP cataract eyes (17.22±5.33 mU/ml; p<0.001), FAP glaucoma eyes (18.82±10.16 mU/ml; p<0.001), and FAP nonglaucoma eyes (20.62±6.22 mU/ml; p<0.001). There was no statistically significant difference between FAP nonglaucoma eyes versus non-FAP cataract eyes (p = 0.23) and FAP glaucoma eyes versus FAP nonglaucoma eyes (p = 0.29). In the glaucoma groups, there was no correlation between the aqueous humor EPO concentration and the ocular pressure (p = 0.95) and mean deviation (p = 0.41). There was no correlation between the EPO serum concentration and EPO aqueous humor concentration in our patients (p = 0.77).

CONCLUSIONS

Unlike other glaucomatous patients, FAP patients with glaucoma do not show increased and potentially neuroprotective endocular EPO production in the aqueous humor and may need more aggressive glaucoma management.

Physiological IgM class catalytic antibodies selective for transthyretin amyloid

Peptide bond-hydrolyzing catalytic antibodies (catabodies) could degrade toxic proteins, but acquired immunity principles have not provided evidence for beneficial catabodies. Transthyretin (TTR) forms misfolded β-sheet aggregates responsible for age-associated amyloidosis. We describe nucleophilic catabodies from healthy humans without amyloidosis that degraded misfolded TTR (misTTR) without reactivity to the physiological tetrameric TTR (phyTTR). IgM class B cell receptors specifically recognized the electrophilic analog of misTTR but not phyTTR. IgM but not IgG class antibodies hydrolyzed the particulate and soluble misTTR species. No misTTR-IgM binding was detected. The IgMs accounted for essentially all of the misTTR hydrolytic activity of unfractionated human serum. The IgMs did not degrade non-amyloidogenic, non-superantigenic proteins. Individual monoclonal IgMs (mIgMs) expressed variable misTTR hydrolytic rates and differing oligoreactivity directed to amyloid β peptide and microbial superantigen proteins. A subset of the mIgMs was monoreactive for misTTR. Excess misTTR was dissolved by a hydrolytic mIgM. The studies reveal a novel antibody property, the innate ability of IgMs to selectively degrade and dissolve toxic misTTR species as a first line immune function.

Mechanisms of transthyretin aggregation and toxicity

Amyloidoses are characterised by the deposition of insoluble protein that occurs in the extracellular compartment of various tissues. One form of amyloidosis is caused by transthyretin (TTR) misfolding and deposition in target tissues. It is clear that many amyloidoses share common features of fibrillogenesis and toxicity. This chapter examines the mechanisms of TTR aggregation with a view to understanding the possible therapeutic interventions in amyloid disease.

Transthyretin as a novel candidate biomarker for preeclampsia

Preeclampsia (PE) is considered to be a potentially fatal complication during pregnancy. However, no effective laboratory assessment has been developed to enable early diagnosis and monitoring of PE. The present study aimed to identify differentially expressed transthyretin (TTR) during severe PE and evaluate TTR as a possible biomarker of this disease. TTR levels were determined in the different gestational weeks of normal pregnancy (before 20 weeks, n=41; after 20 weeks, n=39) using enzyme-linked immunosorbent assay (ELISA). TTR concentrations in pregnant females with severe PE (n=43) were compared with those in healthy matched control subjects (n=37) using western blot analysis and ELISA. The median TTR concentration during severe PE in each month of gestation was significantly lower than the concentrations recorded during normal pregnancy. TTR levels in females with severe PE were significantly downregulated compared with the control subjects (P<0.001; area under the curve, 0.834-0.967). Thus, TTR may be used as a potential biomarker of PE.

Insights into the role of the beta-2 microglobulin D-strand in amyloid propensity revealed by mass spectrometry

Characterising the differences between oligomers formed from the amyloidogenic protein β2-microglobulin and its mutant H51A using ESI-IMS-MS.

In vivo beta-2 microglobulin (β₂m) forms amyloid fibrils that are associated with the disease dialysis-related amyloidosis. Here, electrospray ionisation-ion mobility spectrometry-mass spectrometry has been used to compare the oligomers formed from wild-type β₂m with those formed from a variant of the protein containing a single point mutation in the D strand, H51A, during in vitro fibril assembly. Using the amyloid-binding fluorescent dye, Thioflavin T, to monitor fibrillation kinetics, H51A was shown to exhibit a two-fold increase in the lag-time of fibril formation. Despite this, comparison of the oligomeric species observed during the lag-time of self-aggregation indicated that H51A had a higher population of oligomers, and formed oligomers of higher order, than wild-type β₂m. The cross-sectional areas of the oligomers arising from H51A and wild-type protein were indistinguishable, although the H51A oligomers were shown to have a significantly higher kinetic stability on account of their reluctance to undergo sub-unit exchange when mixed with 15N-labelled protein. Together the data reveal a significant effect of His51, and thus that of the D-strand sequence, on amyloid formation. The results also highlight the power of mass spectrometry in probing complex biochemical mechanisms in real-time.

Induced pluripotent stem cell modeling of multisystemic, hereditary transthyretin amyloidosis

Familial transthyretin amyloidosis (ATTR) is an autosomal-dominant protein-folding disorder caused by over 100 distinct mutations in the transthyretin (TTR) gene. In ATTR, protein secreted from the liver aggregates and forms fibrils in target organs, chiefly the heart and peripheral nervous system, highlighting the need for a model capable of recapitulating the multisystem complexity of this clinically variable disease. Here, we describe the directed differentiation of ATTR patient-specific iPSCs into hepatocytes that produce mutant TTR, and the cardiomyocytes and neurons normally targeted in the disease. We demonstrate that iPSC-derived neuronal and cardiac cells display oxidative stress and an increased level of cell death when exposed to mutant TTR produced by the patient-matched iPSC-derived hepatocytes, recapitulating essential aspects of the disease in vitro. Furthermore, small molecule stabilizers of TTR show efficacy in this model, validating this iPSC-based, patient-specific in vitro system as a platform for testing therapeutic strategies.

Transthyretin suppresses the toxicity of oligomers formed by misfolded proteins in vitro

Although human transthyretin (TTR) is associated with systemic amyloidoses, an anti-amyloidogenic effect that prevents Aβ fibril formation in vitro and in animal models has been observed. Here we studied the ability of three different types of TTR, namely human tetramers (hTTR), mouse tetramers (muTTR) and an engineered monomer of the human protein (M-TTR), to suppress the toxicity of oligomers formed by two different amyloidogenic peptides/proteins (HypF-N and Aβ42). muTTR is the most stable homotetramer, hTTR can dissociate into partially unfolded monomers, whereas M-TTR maintains a monomeric state. Preformed toxic HypF-N and Aβ42 oligomers were incubated in the presence of each TTR then added to cell culture media. hTTR, and to a greater extent M-TTR, were found to protect human neuroblastoma cells and rat primary neurons against oligomer-induced toxicity, whereas muTTR had no protective effect. The thioflavin T assay and site-directed labeling experiments using pyrene ruled out disaggregation and structural reorganization within the discrete oligomers following incubation with TTRs, while confocal microscopy, SDS-PAGE, and intrinsic fluorescence measurements indicated tight binding between oligomers and hTTR, particularly M-TTR. Moreover, atomic force microscopy (AFM), light scattering and turbidimetry analyses indicated that larger assemblies of oligomers are formed in the presence of M-TTR and, to a lesser extent, with hTTR. Overall, the data suggest a generic capacity of TTR to efficiently neutralize the toxicity of oligomers formed by misfolded proteins and reveal that such neutralization occurs through a mechanism of TTR-mediated assembly of protein oligomers into larger species, with an efficiency that correlates inversely with TTR tetramer stability.

Transthyretin proteins regulate angiogenesis by conferring different molecular identities to endothelial cells

Familial amyloidotic polyneuropathy (FAP) has a high prevalence in Portugal, and the most common form of hereditary amyloidosis is caused by an amyloidogenic variant of transthyretin (TTR) with a substitution of methionine for valine at position 30 (V30M). Until now, the available efficient therapy is liver transplantation, when performed in an early phase of the onset of the disease symptoms. However, transplanted FAP patients have a significantly higher incidence of early hepatic artery thrombosis compared with non-FAP transplanted patients. Because FAP was described as an independent risk factor for early hepatic artery thrombosis, more studies to understand the underlying mechanisms involved in this outcome are of the utmost importance. Knowing that the liver is the major site for TTR production, we investigated the biological effects of TTR proteins in the vasculature and on angiogenesis. In this study, we identified genes differentially expressed in endothelial cells exposed to the WT or V30M tetramer. We found that endothelial cells may acquire different molecular identities when exposed to these proteins, and consequently TTR could regulate angiogenesis. Moreover, we show that V30M decreases endothelial survival by inducing apoptosis, and it inhibits migration. These findings provide new knowledge that may have critical implications in the prevention of early hepatic artery thrombosis in FAP patients after liver transplantation.

Amyloid formation in light chain amyloidosis

Light chain amyloidosis is one of the unique examples within amyloid diseases where the amyloidogenic precursor is a protein that escapes the quality control machinery and is secreted from the cells to be circulated in the bloodstream. The immunoglobulin light chains are produced by an abnormally proliferative monoclonal population of plasma cells that under normal conditions produce immunoglobulin molecules such as IgG, IgM or IgA. Once the light chains are in circulation, the proteins misfold and deposit as amyloid fibrils in numerous tissues and organs, causing organ failure and death. While there is a correlation between the thermodynamic stability of the protein and the kinetics of amyloid formation, we have recently found that this correlation applies within a thermodynamic range, and it is only a helpful correlation when comparing mutants from the same protein. Light chain amyloidosis poses unique challenges because each patient has a unique protein sequence as a result of the selection of a germline gene and the incorporation of somatic mutations. The exact location of the misfolding process is unknown as well as the full characterization of all of the toxic species populated during the amyloid formation process in light chain amyloidosis.

Bifunctional coumarin derivatives that inhibit transthyretin amyloidogenesis and serve as fluorescent transthyretin folding sensors

We describe coumarin derivatives that are non-fluorescent in aqueous buffers and that very selectively bind to transthyretin (TTR) in complex biological environments potently inhibiting TTR amyloidogenesis while also exhibiting sensitive off-on fluorescent sensing of the properly folded quaternary structure of TTR.

Mutation p.G83R in the transthyretin gene is associated with hereditary vitreous amyloidosis in Han Chinese families

PURPOSE

Hereditary vitreous amyloidosis (HVA) is a genetic ophthalmological disorder. The purpose of this study was to investigate whether a mutation in the transthyretin (TTR) gene is associated with HVA in Han Chinese families.

METHODS

We performed clinical evaluation of three Han Chinese families with HVA and sequenced the entire exon of the TTR gene in probands and normal individuals from the families. The identified mutation was further genotyped in 196 unrelated healthy controls. Evolutionary conservation analysis and structural prediction were used to infer the potential pathogenicity of the mutation.

RESULTS

Clinical penetrance of HVA varied in the three families (11/30 in Family A, 8/83 in Family B, and 7/47 in Family C). A comprehensive medical examination of the patients showed no signs of abnormality except ophthalmologic symptoms, in which floccular turbidity and high echo in both vitreous bodies were observed in all probands. Further histochemical examination of the vitrectomy specimen with Congo red staining identified amyloid deposits. A heterozygous mutation c.307G>C (p.G83R) in exon 3 of the TTR gene was identified in all patients, but not in some unaffected family members. Screening of 196 unrelated normal controls revealed no presence of this mutation. This mutation changed the highly conserved glycine to arginine in the 83(rd) position and altered the tertiary structure of the TTR protein.

CONCLUSIONS

Mutation p.G83R in the TTR protein is associated with HVA in Chinese families. The seemingly specific distribution of this mutation in Han Chinese may be used for clinical diagnosis.

Prion-like propagation of protein aggregation and related therapeutic strategies

Many neurodegenerative diseases are characterized by the progressive accumulation of aggregated protein. Recent evidence suggests the prion-like propagation of protein misfolding underlies the spread of pathology observed in these diseases. This review traces our understanding of the mechanisms that underlie this phenomenon and discusses related therapeutic strategies that derive from it.

Protein folding and aggregation into amyloid: the interference by natural phenolic compounds

Amyloid aggregation is a hallmark of several degenerative diseases affecting the brain or peripheral tissues, whose intermediates (oligomers, protofibrils) and final mature fibrils display different toxicity. Consequently, compounds counteracting amyloid aggregation have been investigated for their ability (i) to stabilize toxic amyloid precursors; (ii) to prevent the growth of toxic oligomers or speed that of fibrils; (iii) to inhibit fibril growth and deposition; (iv) to disassemble preformed fibrils; and (v) to favor amyloid clearance. Natural phenols, a wide panel of plant molecules, are one of the most actively investigated categories of potential amyloid inhibitors. They are considered responsible for the beneficial effects of several traditional diets being present in green tea, extra virgin olive oil, red wine, spices, berries and aromatic herbs. Accordingly, it has been proposed that some natural phenols could be exploited to prevent and to treat amyloid diseases, and recent studies have provided significant information on their ability to inhibit peptide/protein aggregation in various ways and to stimulate cell defenses, leading to identify shared or specific mechanisms. In the first part of this review, we will overview the significance and mechanisms of amyloid aggregation and aggregate toxicity; then, we will summarize the recent achievements on protection against amyloid diseases by many natural phenols.

The amyloid-cell membrane system. The interplay between the biophysical features of oligomers/fibrils and cell membrane defines amyloid toxicity

Amyloid cytotoxicity, structure and polymorphisms are themes of increasing importance. Present knowledge considers any peptide/protein able to undergo misfolding and aggregation generating intrinsically cytotoxic amyloids. It also describes growth and structure of amyloid fibrils and their possible disassembly, whereas reduced information is available on oligomer structure. Recent research has highlighted the importance of the environmental conditions as determinants of the amyloid polymorphisms and cytotoxicity. Another body of evidence describes chemical or biological surfaces as key sites of protein misfolding and aggregation or of interaction with amyloids and the resulting biochemical modifications inducing cell functional/viability impairment. In particular, the membrane lipid composition appears to modulate cell response to toxic amyloids, thus contributing to explain the variable vulnerability to the same amyloids of different cell types. Finally, a recent view describes amyloid toxicity as an emerging property dependent on a complex interplay between the biophysical features of early aggregates and the interacting cell membranes taken as a whole system.

Inhibition of human transthyretin aggregation by non-steroidal anti-inflammatory compounds: a structural and thermodynamic analysis

Transthyretin (TTR) is a homotetrameric protein that circulates in plasma and cerebral spinal fluid (CSF) whose aggregation into amyloid fibrils has been associated with at least two different amyloid diseases: senile systemic amyloidosis (SSA) and familial amyloid polyneuropathy (FAP). In SSA aggregates are composed of WT-TTR, while in FAP more than 100 already-described variants have been found in deposits. Until now, TTR-related diseases have been untreatable, although a new drug called Tafamidis has been approved only in Europe to specifically treat V30M patients. Thus, new strategies are still necessary to treat FAP caused by other variants of TTR. TTR has two channels in the dimer interface that bind to the hormone thyroxin and that have been used to accommodate anti-amyloidogenic compounds. These compounds stabilize the tetramers, rendering TTR less amyloidogenic. Here, we investigated the effects of three non-steroidal anti-inflammatory compounds—sulindac (SUL), indomethacin (IND) and lumiracoxib (LUM)—as tetramer stabilizers and aggregation inhibitors. WT-TTR and the very aggressive TTR variant L55P were used as models. These compounds were able to stabilize TTR against high hydrostatic pressure (HHP), increasing the ΔG_f by several kcal. They were also effective in inhibiting WT-TTR and L55P acid- or HHP-induced aggregation; in particular, LUM and IND were very effective, inhibiting almost 100% of the aggregation of both proteins under certain conditions. The species formed when aggregation was performed in the presence of these compounds were much less toxic to cells in culture. The crystal structures of WT-TTR bound to the three compounds were solved at high resolution, allowing the identification of the relevant protein:drug interactions. We discuss here the ligand-binding features of LUM, IND and SUL to TTR, emphasizing the critical interactions that render the protein more stable and less amyloidogenic.

Age-related oxidative modifications of transthyretin modulate its amyloidogenicity

The transthyretin amyloidoses are diseases of protein misfolding characterized by the extracellular deposition of fibrils and other aggregates of the homotetrameric protein transthyretin (TTR) in peripheral nerves, heart, and other tissues. Age is the major risk factor for the development of these diseases. We hypothesized that an age-associated increase in the level of protein oxidation could be involved in the onset of the senile forms of the TTR amyloidoses. To test this hypothesis, we have produced and characterized relevant age-related oxidative modifications of the wild type (WT) and the Val122Ile (V122I) TTR variant, both involved in cardiac TTR deposition in the elderly. Our studies show that methionine/cysteine-oxidized TTR and carbonylated TTR from either the WT or the V122I variant are thermodynamically less stable than their nonoxidized counterparts. Moreover, carbonylated WT and carbonylated V122I TTR have a stronger propensity to form aggregates and fibrils than WT and V122I TTR, respectively, at physiologically attainable pH values. It is well-known that TTR tetramer dissociation, the limiting step for aggregation and amyloid fibril formation, can be prevented by small molecules that bind the TTR tetramer interface. Here, we report that carbonylated WT TTR is less amenable to resveratrol-mediated tetramer stabilization than WT TTR. All the oxidized forms of TTR tested are cytotoxic to a human cardiomyocyte cell line known to be a target for cardiac-specific TTR variants. Overall, these studies demonstrate that age-related oxidative modifications of TTR can contribute to the onset of the senile forms of the TTR amyloidoses.

Inhibition of TTR aggregation-induced cell death--a new role for serum amyloid P component

Background

Serum amyloid P component (SAP) is a glycoprotein that is universally found associated with different types of amyloid deposits. It has been suggested that it stabilizes amyloid fibrils and therefore protects them from proteolytic degradation.

Methodology/Principal Findings

In this paper, we show that SAP binds not only to mature amyloid fibrils but also to early aggregates of amyloidogenic mutants of the plasma protein transthyretin (TTR). It does not inhibit fibril formation of TTR mutants, which spontaneously form amyloid in vitro at physiological pH. We found that SAP prevents cell death induced by mutant TTR, while several other molecules that are also known to decorate amyloid fibrils do not have such effect. Using a Drosophila model for TTR-associated amyloidosis, we found a new role for SAP as a protective factor in inhibition of TTR-induced toxicity. Overexpression of mutated TTR leads to a neurological phenotype with changes in wing posture. SAP-transgenic flies were crossed with mutated TTR-expressing flies and the results clearly confirmed a protective effect of SAP on TTR-induced phenotype, with an almost complete reduction in abnormal wing posture. Furthermore, we found in vivo that binding of SAP to mutated TTR counteracts the otherwise detrimental effects of aggregation of amyloidogenic TTR on retinal structure.

Conclusions/Significance

Together, these two approaches firmly establish the protective effect of SAP on TTR-induced cell death and degenerative phenotypes, and suggest a novel role for SAP through which the toxicity of early amyloidogenic aggregates is attenuated.

Amyloid neuropathies

Peripheral neuropathy is a common complication of many of the systemic amyloidoses. Although the cause of neuropathy is not entirely clear, it is likely related to amyloid deposition within the nerve. This may lead to focal, multifocal, or diffuse neuropathies involving sensory, motor and/or autonomic fibers. The presenting symptoms depend on the distribution of nerves affected. One of the most common phenotypes is sensorimotor polyneuropathy, which is characterized by symptoms of neuropathic pain, numbness, and in advanced cases weakness. Symptoms begin in the feet and ultimately progress to the proximal legs and hands. The most common focal neuropathy is a median neuropathy at the wrist, clinically known as carpal tunnel syndrome. Carpal tunnel symptoms may include pain and sensory disturbances in the lateral palm and fingers; hand weakness may ensue if the focal neuropathy is severe. Autonomic neuropathy may affect a variety of organ systems such as the cardiovascular, gastrointestinal, and genitourinary systems. Symptoms may be non-specific making the diagnosis of autonomic neuropathy more difficult to identify. However, it is important to recognize and distinguish autonomic neuropathy from diseases of the end-organs themselves. This article reviews the inherited and acquired amyloidoses that affect the peripheral nervous system including familial amyloid polyneuropathy, and primary, secondary and senile amyloidosis. We emphasize the clinical presentation of the neurologic aspects of these diseases, physical examination findings, appropriate diagnostic evaluation, treatment and prognosis.

Evaluation of tafamidis as first-line therapeutic agent for transthyretin familial amyloidotic polyneuropathy

Almost 100 mutations in the human transthyretin (TTR) gene cause the autosomal dominant disorders of familial amyloidotic polyneuropathy (FAP) and familial amyloidotic cardiomyopathy. While these have been clinically classified as separate disorders, the peripheral and autonomic nervous systems and the heart are frequently involved in the same patient. Deposition of amyloid derived from a kinetically or thermodynamically unstable mutant TTR precursor produces an ascending sensorimotor polyneuropathy with marked autonomic involvement. Since 1990, treatment has been liver transplantation from a donor carrying two wild-type TTR genes, providing a crude form of gene therapy. Multiple studies have shown that small molecules fitting in the T₄-binding pocket of TTR can stabilize the molecule, reducing its capacity to release the fibril precursor. Tafamidis is the first molecule to be tested in a placebo-controlled trial in patients with TTR-associated FAP. While the trial did not achieve its primary endpoints, it did stabilize TTR in vivo and had a favorable effect on some aspects of disease progression, particularly when administered early in the course. It may represent an alternative to liver transplantation, particularly in patients with early disease related to the V30M mutation. Longer-term studies are required to determine whether it represents a stabilizing or remittive form of treatment.

Distinct annular oligomers captured along the assembly and disassembly pathways of transthyretin amyloid protofibrils

Background

Defects in protein folding may lead to severe degenerative diseases characterized by the appearance of amyloid fibril deposits. Cytotoxicity in amyloidoses has been linked to poration of the cell membrane that may involve interactions with amyloid intermediates of annular shape. Although annular oligomers have been detected in many amyloidogenic systems, their universality, function and molecular mechanisms of appearance are debated.

Methodology/Principal Findings

We investigated with high-resolution in situ atomic force microscopy the assembly and disassembly of transthyretin (TTR) amyloid protofibrils formed of the native protein by pH shift. Annular oligomers were the first morphologically distinct intermediates observed in the TTR aggregation pathway. Morphological analysis suggests that they can assemble into a double-stack of octameric rings with a 16±2 nm diameter, and displaying the tendency to form linear structures. According to light scattering data coupled to AFM imaging, annular oligomers appeared to undergo a collapse type of structural transition into spheroid oligomers containing 8–16 monomers. Disassembly of TTR amyloid protofibrils also resulted in the rapid appearance of annular oligomers but with a morphology quite distinct from that observed in the assembly pathway.

Conclusions/Significance

Our observations indicate that annular oligomers are key dynamic intermediates not only in the assembly but also in the disassembly of TTR protofibrils. The balance between annular and more compact forms of aggregation could be relevant for cytotoxicity in amyloidogenic disorders.

The transthyretin amyloidoses: from delineating the molecular mechanism of aggregation linked to pathology to a regulatory-agency-approved drug

Transthyretin (TTR) is one of the many proteins that are known to misfold and aggregate (i.e., undergo amyloidogenesis) in vivo. The process of TTR amyloidogenesis causes nervous system and/or heart pathology. While several of these maladies are associated with mutations that destabilize the native TTR quaternary and/or tertiary structure, wild-type TTR amyloidogenesis also leads to the degeneration of postmitotic tissue. Over the past 20 years, much has been learned about the factors that influence the propensity of TTR to aggregate. This biophysical information led to the development of a therapeutic strategy, termed "kinetic stabilization," to prevent TTR amyloidogenesis. This strategy afforded the drug tafamidis which was recently approved by the European Medicines Agency for the treatment of TTR familial amyloid polyneuropathy, the most common familial TTR amyloid disease. Tafamidis is the first and currently the only medication approved to treat TTR familial amyloid polyneuropathy. Here we review the biophysical basis for the kinetic stabilization strategy and the structure-based drug design effort that led to this first-in-class pharmacologic agent.

Human placental transthyretin in fetal growth restriction in combination with preeclampsia and the HELLP syndrome

Fetal growth restriction is a serious, still poorly understood pregnancy-related pathology often associated with preeclampsia. Recent studies speculate on the role of human transthyretin, a carrier protein for thyroxin and retinol binding protein, in the etiology of both pregnancy pathologies. Objective was to investigate the localization and abundance of transthyretin (TTR) in placentas of pregnancies suffering from fetal growth restriction with and without preeclampsia and HELLP. This was a retrospective case control study on human paraffin-embedded placentas from pregnancies with a gestational age at delivery between the 24th and 34th week of gestation. 16 placentas were included in this study, 11 cases and 5 from normotensive pregnancies as controls. Cases were divided into three groups: four from early onset idiopathic intrauterine growth restriction (IUGR), four from early-onset severe preeclampsia (PE), and three from early-onset IUGR with preeclampsia plus HELLP syndrome. Distribution and abundance of TTR were investigated by means of immunohistochemistry. Semi quantitative analysis of TTR staining of placental sections revealed that TTR was mostly expressed in the villous trophoblast covering placental villi. Only weak staining of TTR in villous stroma could be detected. The comparison of placentas revealed that in pure IUGR and severe PE there is a much stronger TTR reactivity compared to controls and cases with IUGR + PE + HELLP. Concluding, the study showed that TTR is dysregulated in cases of IUGR and severe early onset preeclampsia. Interestingly, TTR expression is not affected in cases with HELLP syndrome that reveal the same staining intensities as age-matched controls.

Role of prion protein aggregation in neurotoxicity

In several neurodegenerative diseases, such as Parkinson, Alzheimer’s, Huntington, and prion diseases, the deposition of aggregated misfolded proteins is believed to be responsible for the neurotoxicity that characterizes these diseases. Prion protein (PrP), the protein responsible of prion diseases, has been deeply studied for the peculiar feature of its misfolded oligomers that are able to propagate within affected brains, inducing the conversion of the natively folded PrP into the pathological conformation. In this review, we summarize the available experimental evidence concerning the relationship between aggregation status of misfolded PrP and neuronal death in the course of prion diseases. In particular, we describe the main findings resulting from the use of different synthetic (mainly PrP106-126) and recombinant PrP-derived peptides, as far as mechanisms of aggregation and amyloid formation, and how these different spatial conformations can affect neuronal death. In particular, most data support the involvement of non-fibrillar oligomers rather than actual amyloid fibers as the determinant of neuronal death.

Detection of high-molecular-weight amyloid serum protein complexes using biological on-line tracer sedimentation

The systemic amyloidoses are a rare but deadly class of protein folding disorders with significant unmet diagnostic and therapeutic needs. The current model for symptomatic amyloid progression includes a causative role for soluble toxic aggregates as well as for the fibrillar tissue deposits. Although much research is focused on elucidating the potential mechanism of aggregate toxicity, evidence to support their existence in vivo has been limited. We report the use of a technique we have termed biological on-line tracer sedimentation (BOLTS) to detect abnormal high-molecular-weight complexes (HMWCs) in serum samples from individuals with systemic amyloidosis due to aggregation and deposition of wild-type transthyretin (senile systemic amyloidosis, SSA) or monoclonal immunoglobulin light chain (AL amyloidosis). In this proof-of-concept study, HMWCs were observed in 31 of 77 amyloid samples (40.3%). HMWCs were not detected in any of the 17 nonamyloid control samples subjected to BOLTS analyses. These findings support the existence of potentially toxic amyloid aggregates and suggest that BOLTS may be a useful analytic and diagnostic platform in the study of the amyloidoses or other diseases where abnormal molecular complexes are formed in serum.

Tafamidis, a potent and selective transthyretin kinetic stabilizer that inhibits the amyloid cascade

The transthyretin amyloidoses (ATTR) are invariably fatal diseases characterized by progressive neuropathy and/or cardiomyopathy. ATTR are caused by aggregation of transthyretin (TTR), a natively tetrameric protein involved in the transport of thyroxine and the vitamin A-retinol-binding protein complex. Mutations within TTR that cause autosomal dominant forms of disease facilitate tetramer dissociation, monomer misfolding, and aggregation, although wild-type TTR can also form amyloid fibrils in elderly patients. Because tetramer dissociation is the rate-limiting step in TTR amyloidogenesis, targeted therapies have focused on small molecules that kinetically stabilize the tetramer, inhibiting TTR amyloid fibril formation. One such compound, tafamidis meglumine (Fx-1006A), has recently completed Phase II/III trials for the treatment of Transthyretin Type Familial Amyloid Polyneuropathy (TTR-FAP) and demonstrated a slowing of disease progression in patients heterozygous for the V30M TTR mutation. Herein we describe the molecular and structural basis of TTR tetramer stabilization by tafamidis. Tafamidis binds selectively and with negative cooperativity (K(d)s ~2 nM and ~200 nM) to the two normally unoccupied thyroxine-binding sites of the tetramer, and kinetically stabilizes TTR. Patient-derived amyloidogenic variants of TTR, including kinetically and thermodynamically less stable mutants, are also stabilized by tafamidis binding. The crystal structure of tafamidis-bound TTR suggests that binding stabilizes the weaker dimer-dimer interface against dissociation, the rate-limiting step of amyloidogenesis.

Adsorption and disruption of lipid bilayers by nanoscale protein aggregates

Nanoparticles taken into biological systems can have biological impacts through their interactions with cell membranes, accompanied by protein adsorption onto the nanoparticle surfaces, forming a so-called protein corona. Our current research aims to demonstrate that nanoscale protein aggregates behave like such nanoparticles with regard to the interaction with lipid membranes. In this study, the adsorption and disruption of the lipid membranes by protein aggregates were investigated using amyloid fibrils and nanoscale thermal aggregates of lysozyme. Both types of protein aggregates had disruptive effects on the negatively charged liposomes, similar to polycationic nanoparticles. Interestingly, adsorption of liposomes on the amyloid fibrils preceding disruption occurred even if the net charge of the liposome was zero, suggesting the importance of hydrophobic interactions in addition to electrostatic interactions. The results of the present study provide new insights into the biological impacts of nanoparticles in vivo.

Differential proteomic distribution of TTR (pre-albumin) forms in serum and HDL of patients with high cardiovascular risk

Inflammation is a common condition contributing to cardiovascular disease progression which leads to clinical manifestations such as acute myocardial infarction (AMI). By applying a proteomic expression profiling approach we have investigated changes in transthyretin (TTR) in AMI-patients and its distribution patterns in HDL samples of patients with high cardiovascular risk, such as those with familiar hypercholesterolemia (FH).

METHODS AND RESULTS

The characterization by bidimensional electrophoresis (2-DE), followed by mass-spectrometry (MALDI-TOF) of serum samples revealed changes in the intensity of the TTR spot with a pI of 5.6 and a Mw of 42kDa (tTTR) between AMI-patients in association to diabetic dyslipemia. Serum TTR levels, determined by commercial ELISA, were significantly lower (p<0.0001) in AMI-patients (n=39) and FH-patients (n=100) than in healthy controls (n=60). Western blot and 2-DE analysis showed a differential distribution profile of TTR forms between serum, where 3 TTR forms of 42 (tTTR), 28 (dTTR), and 14kDa (mTTR) were detected, and HDL samples, where only mTTR was present.

CONCLUSIONS

Our results demonstrate alterations in TTR proteomic profile in relation to the clustering of risk factors which seems to highlight the implication of TTR in cardiovascular risk. The significant differences in TTR between serum (tTTR) and HDL (mTTR) underscore the importance of TTR-forms in the circulation and deserve further investigation to understand their function.

Potent kinetic stabilizers that prevent transthyretin-mediated cardiomyocyte proteotoxicity

A valine-to-isoleucine mutation at position 122 of the serum protein transthyretin (TTR), found in 3 to 4% of African Americans, alters its stability, leading to amyloidogenesis and cardiomyopathy. In addition, 10 to 15% of individuals older than 65 years develop senile systemic amyloidosis and cardiac TTR deposits because of wild-type TTR amyloidogenesis. Although several drugs are in development, no approved therapies for TTR amyloid cardiomyopathy are yet available, so the identification of additional compounds that prevent amyloid-mediated cardiotoxicity is needed. To this aim, we developed a fluorescence polarization-based high-throughput screen and used it to identify several new chemical scaffolds that target TTR. These compounds were potent kinetic stabilizers of TTR and prevented TTR tetramer dissociation, partial unfolding, and aggregation of both wild type and the most common cardiomyopathy-associated TTR mutant, V122I-TTR. High-resolution co-crystal structures and characterization of the binding energetics revealed how these diverse structures bound to tetrameric TTR. These compounds effectively inhibited the proteotoxicity of V122I-TTR toward human cardiomyocytes. Several of these ligands stabilized TTR in human serum more effectively than diflunisal, which is a well-studied inhibitor of TTR aggregation, and may be promising leads for the treatment or prevention of TTR-mediated cardiomyopathy.

A molecular history of the amyloidoses

The molecular investigation of the amyloidoses began in the mid-19th century with the observation of areas in human tissues obtained at autopsy that were homogeneous and eosinophilic with conventional stains but became blue when exposed to mixtures of iodine and sulfuric acid. The foci corresponded to regions formerly identified as "waxy" or lardaceous. Subsequent identification of the characteristic staining of the same tissues with metachromatic dyes such as crystal violet or with the cotton dye Congo red (particularly under polarized light) and thioflavins allowed the pathological classification of those tissues as belonging to a set of disorders known as the amyloidoses. Not unexpectedly, progress has reflected evolving technology and parallel advances in all fields of biological science. Investigation using contemporary methods has expanded our notions of amyloid proteins from being simply agents or manifestations of systemic, largely extracellular diseases to include "protein-only infection," the concept that "normal" functional amyloids might exist in eukaryotes and prokaryotes and that aggregatability may be an intrinsic structural price to be paid for some functional protein domains. We now distinguish between the amyloidoses, that is, diseases caused by the deposition of amyloid fibrils and amyloid proteins (i.e., purified or recombinant proteins that form amyloid fibrils in vitro), which may or may not be associated with disease in vivo.

Familial amyloidotic polyneuropathy and transthyretin

There has been much progress in our understanding of transthyretin (TTR)-related amyloidosis including familial amyloidotic polyneuropathy (FAP), senile systemic amyloidosis and its related disorders from many clinical and experimental aspects. FAP is an inherited severe systemic amyloidosis caused by mutated TTR, and characterized by amyloid deposition mainly in the peripheral nervous system and the heart. Liver transplantation is the only available treatment for the disease. FAP is now recognized not to be a rare disease, and to have many variations based on genetical and biochemical variations of TTR. This chapter covers the recent advances in the clinical and pathological aspects of, and therapeutic approaches to FAP, and the trend as to the molecular pathogenesis of TTR.

Doxycycline reduces fibril formation in a transgenic mouse model of AL amyloidosis

Systemic AL amyloidosis results from the aggregation of an amyloidogenic immunoglobulin (Ig) light chain (LC) usually produced by a plasma cell clone in the bone marrow. AL is the most rapidly fatal of the systemic amyloidoses, as amyloid fibrils can rapidly accumulate in tissues including the heart, kidneys, autonomic or peripheral nervous systems, gastrointestinal tract, and liver. Chemotherapy is used to eradicate the cellular source of the amyloidogenic precursor. Currently, there are no therapies that target the process of LC aggregation, fibril formation, or organ damage. We developed transgenic mice expressing an amyloidogenic λ6 LC using the cytomegalovirus (CMV) promoter to circumvent the disruption of B cell development by premature expression of recombined LC. The CMV-λ6 transgenic mice develop neurologic dysfunction and Congophilic amyloid deposits in the stomach. Amyloid deposition was inhibited in vivo by the antibiotic doxycycline. In vitro studies demonstrated that doxycycline directly disrupted the formation of recombinant LC fibrils. Furthermore, treatment of ex vivo LC amyloid fibrils with doxycycline reduced the number of intact fibrils and led to the formation of large disordered aggregates. The CMV-λ6 transgenic model replicates the process of AL amyloidosis and is useful for testing the antifibril potential of orally available agents.

Amitriptyline-mediated cognitive enhancement in aged 3×Tg Alzheimer's disease mice is associated with neurogenesis and neurotrophic activity

Approximately 35 million people worldwide suffer from Alzheimer's disease (AD). Existing therapeutics, while moderately effective, are currently unable to stem the widespread rise in AD prevalence. AD is associated with an increase in amyloid beta (Aβ) oligomers and hyperphosphorylated tau, along with cognitive impairment and neurodegeneration. Several antidepressants have shown promise in improving cognition and alleviating oxidative stress in AD but have failed as long-term therapeutics. In this study, amitriptyline, an FDA-approved tricyclic antidepressant, was administered orally to aged and cognitively impaired transgenic AD mice (3×TgAD). After amitriptyline treatment, cognitive behavior testing demonstrated that there was a significant improvement in both long- and short-term memory retention. Amitriptyline treatment also caused a significant potentiation of non-toxic Aβ monomer with a concomitant decrease in cytotoxic dimer Aβ load, compared to vehicle-treated 3×TgAD controls. In addition, amitriptyline administration caused a significant increase in dentate gyrus neurogenesis as well as increases in expression of neurosynaptic marker proteins. Amitriptyline treatment resulted in increases in hippocampal brain-derived neurotrophic factor protein as well as increased tyrosine phosphorylation of its cognate receptor (TrkB). These results indicate that amitriptyline has significant beneficial actions in aged and damaged AD brains and that it shows promise as a tolerable novel therapeutic for the treatment of AD.

Amyloidosis: pathogenesis and new therapeutic options

The systemic amyloidoses are a group of complex diseases caused by tissue deposition of misfolded proteins that results in progressive organ damage. The most common type, immunoglobulin light chain amyloidosis (AL), is caused by clonal plasma cells that produce misfolded light chains. The purpose of this review is to provide up-to-date information on diagnosis and treatment options for AL amyloidosis. Early, accurate diagnosis is the key to effective therapy, and unequivocal identification of the amyloidogenic protein may require advanced technologies and expertise. Prognosis is dominated by the extent of cardiac involvement, and cardiac staging directs the choice of therapy. Treatment for AL amyloidosis is highly individualized, determined on the basis of age, organ dysfunction, and regimen toxicities, and should be guided by biomarkers of hematologic and cardiac response. Alkylator-based chemotherapy is effective in almost two thirds of patients. Novel agents are also active, and trials are ongoing to establish their optimal use. Treatment algorithms will continue to be refined through controlled trials. Advances in basic research have led to the identification of new drug targets and therapeutic approaches, which will be integrated with chemotherapy in the future.

Mechanisms of transthyretin cardiomyocyte toxicity inhibition by resveratrol analogs

The transthyretin amyloidoses are a subset of protein misfolding diseases characterized by the extracellular deposition of aggregates derived from the plasma homotetrameric protein transthyretin (TTR) in peripheral nerves and the heart. We have established a robust disease-relevant human cardiac tissue culture system to explore the cytotoxic effects of amyloidogenic TTR variants. We have employed this cardiac amyloidosis tissue culture model to screen 23 resveratrol analogs as inhibitors of amyloidogenic TTR-induced cytotoxicity and to investigate their mechanisms of protection. Resveratrol and its analogs kinetically stabilize the native tetramer preventing the formation of cytotoxic species. In addition, we demonstrate that resveratrol can accelerate the formation of soluble non-toxic aggregates and that the resveratrol analogs tested can bring together monomeric TTR subunits to form non-toxic native tetrameric TTR.

TRPM8 and Nav1.8 sodium channels are required for transthyretin-induced calcium influx in growth cones of small-diameter TrkA-positive sensory neurons

Background

Familial amyloidotic polyneuropathy (FAP) is a peripheral neuropathy caused by the extracellular accumulation and deposition of insoluble transthyretin (TTR) aggregates. However the molecular mechanism that underlies TTR toxicity in peripheral nerves is unclear. Previous studies have suggested that amyloidogenic proteins can aggregate into oligomers which disrupt intracellular calcium homeostasis by increasing the permeability of the plasma membrane to extracellular calcium. The aim of the present study was to examine the effect of TTR on calcium influx in dorsal root ganglion neurons.

Results

Levels of intracellular cytosolic calcium were monitored in dorsal root ganglion (DRG) neurons isolated from embryonic rats using the calcium-sensitive fluorescent indicator Fluo4. An amyloidogenic mutant form of TTR, L55P, induced calcium influx into the growth cones of DRG neurons, whereas wild-type TTR had no significant effect. Atomic force microscopy and dynamic light scattering studies confirmed that the L55P TTR contained oligomeric species of TTR. The effect of L55P TTR was decreased by blockers of voltage-gated calcium channels (VGCC), as well as by blockers of Na_v1.8 voltage-gated sodium channels and transient receptor potential M8 (TRPM8) channels. siRNA knockdown of TRPM8 channels using three different TRPM8 siRNAs strongly inhibited calcium influx in DRG growth cones.

Conclusions

These data suggest that activation of TRPM8 channels triggers the activation of Na_v1.8 channels which leads to calcium influx through VGCC. We suggest that TTR-induced calcium influx into DRG neurons may contribute to the pathophysiology of FAP. Furthermore, we speculate that similar mechanisms may mediate the toxic effects of other amyloidogenic proteins such as the β-amyloid protein of Alzheimer's disease.

The lipid peroxidation products 4-oxo-2-nonenal and 4-hydroxy-2-nonenal promote the formation of α-synuclein oligomers with distinct biochemical, morphological, and functional properties

Oxidative stress has been implicated in the etiology of neurodegenerative disorders with α-synuclein pathology. Lipid peroxidation products such as 4-oxo-2-nonenal (ONE) and 4-hydroxy-2-nonenal (HNE) can covalently modify and structurally alter proteins. Herein, we have characterized ONE- or HNE-induced α-synuclein oligomers. Our results demonstrate that both oligomers are rich in β-sheet structure and have a molecular weight of about 2000 kDa. Atomic force microscopy analysis revealed that ONE-induced α-synuclein oligomers were relatively amorphous, with a diameter of 40-80 nm and a height of 4-8 nm. In contrast, the HNE-induced α-synuclein oligomers had a protofibril-like morphology with a width of 100-200 nm and a height of 2-4 nm. Furthermore, neither oligomer type polymerized into amyloid-like fibrils despite prolonged incubation. Although more SDS and urea stable, because of a higher degree of cross-linking, ONE-induced α-synuclein oligomers were less compact and more sensitive to proteinase K treatment. Finally, both ONE- and HNE-induced α-synuclein oligomers were cytotoxic when added exogenously to a neuroblastoma cell line, but HNE-induced α-synuclein oligomers were taken up by the cells to a significantly higher degree. Despite nearly identical chemical structures, ONE and HNE induce the formation of off-pathway α-synuclein oligomers with distinct biochemical, morphological, and functional properties.

Sulfated glycosaminoglycans accelerate transthyretin amyloidogenesis by quaternary structural conversion

Glycosaminoglycans (GAGs), which are found in association with all extracellular amyloid deposits in humans, are known to accelerate the aggregation of various amyloidogenic proteins in vitro. However, the precise molecular mechanism(s) by which GAGs accelerate amyloidogenesis remains elusive. Herein, we show that sulfated GAGs, especially heparin, accelerate transthyretin (TTR) amyloidogenesis by quaternary structural conversion. The clustering of sulfate groups on heparin and its polymeric nature are essential features for accelerating TTR amyloidogenesis. Heparin does not influence TTR tetramer stability or TTR dissociation kinetics, nor does it alter the folded monomer-misfolded monomer equilibrium directly. Instead, heparin accelerates the conversion of preformed TTR oligomers into larger aggregates. The more rapid disappearance of monomeric TTR in the presence of heparin likely reflects the fact that the monomer-misfolded amyloidogenic monomer-oligomer-TTR fibril equilibria are all linked, a hypothesis that is strongly supported by the light scattering data. TTR aggregates prepared in the presence of heparin exhibit a higher resistance to trypsin and proteinase K proteolysis and a lower exposure of hydrophobic side chains comprising hydrophobic clusters, suggesting an active role for heparin in amyloidogenesis. Our data suggest that heparin accelerates TTR aggregation by a scaffold-based mechanism, in which the sulfate groups comprising GAGs interact primarily with TTR oligomers through electrostatic interactions, concentrating and orienting the oligomers, facilitating the formation of higher molecular weight aggregates. This model raises the possibility that GAGs may play a protective role in human amyloid diseases by interacting with proteotoxic oligomers and promoting their association into less toxic amyloid fibrils.

Sedimentation velocity analysis of amyloid fibrils

Analytical ultracentrifugation is a classical technique used to study the solution behavior of proteins. Experimentally determined sedimentation coefficients provide information regarding the size, shape, and interactions of biological macromolecules. Sedimentation velocity methods have been used to characterize the different aggregation states of amyloid oligomers and fibrils. This chapter first describes the theoretical background for sedimentation velocity analysis. It then provides experimental protocols for sedimentation velocity experiments using the analytical ultracentrifuge. Finally, this chapter describes the procedure used to analyze sedimentation velocity data to obtain the size distribution of amyloid fibrils and their oligomeric precursors.