Genetic aspects of amyloidosis

Thyroxine metabolite-derived 3-iodothyronamine (T1AM) and synthetic analogs as efficient suppressors of transthyretin amyloidosis

Aggregation and fibrillization of transthyretin (TTR) is a fatal pathogenic process that can cause cardiomyopathic and polyneuropathic diseases in humans. Although several therapeutic strategies have been designed to prevent and treat related pathological events, there is still an urgent need to develop better strategies to improve potency and wider applicability. Here, we present our study demonstrating that 3-iodothyronamine (T1AM) and selected thyronamine-like compounds can effectively prevent TTR aggregation. T1AM is one of the thyroid hormone (TH) metabolites, and T1AM and its analogs, such as SG2, SG6, and SG12, are notable molecules for their beneficial activities against metabolic disorders and neurodegeneration. Using nuclear magnetic resonance (NMR) spectroscopy and biochemical analysis, we confirmed that T1AM analogs could bind to and suppress acid-induced aggregation of TTR. In addition, we employed computational approaches to further understand the detailed mechanisms of the interaction between T1AM analogs and TTR. This study demonstrates that T1AM analogs, whose beneficial effects against several pathological processes have already been proven, may have additional benefits against TTR aggregation and fibrillization. Moreover, we believe that our work provides invaluable insights to enhance the pleiotropic activity of T1AM and structurally related analogs, relevant for their therapeutic potential, with particular reference to the ability to prevent TTR aggregation.

Modulating the Effects of the Bacterial Chaperonin GroEL on Fibrillogenic Polypeptides through Modification of Domain Hinge Architecture

The isolated apical domain of the Escherichia coli GroEL subunit displays the ability to suppress the irreversible fibrillation of numerous amyloid-forming polypeptides. In previous experiments, we have shown that mutating Gly-192 (located at hinge II that connects the apical domain and the intermediate domain) to a tryptophan results in an inactive chaperonin whose apical domain is disoriented. In this study, we have utilized this disruptive effect of Gly-192 mutation to our advantage, by substituting this residue with amino acid residues of varying van der Waals volumes with the intent to modulate the affinity of GroEL toward fibrillogenic peptides. The affinities of GroEL toward fibrillogenic polypeptides such as Aβ(1-40) (amyloid-β(1-40)) peptide and α-synuclein increased in accordance to the larger van der Waals volume of the substituent amino acid side chain in the G192X mutants. When we compared the effects of wild-type GroEL and selected GroEL G192X mutants on α-synuclein fibril formation, we found that the effects of the chaperonin on α-synuclein fibrillation were different; the wild-type chaperonin caused changes in both the initial lag phase and the rate of fibril extension, whereas the effects of the G192X mutants were more specific toward the nucleus-forming lag phase. The chaperonins also displayed differential effects on α-synuclein fibril morphology, suggesting that through mutation of Gly-192, we may induce changes to the intermolecular affinities between GroEL and α-synuclein, leading to more efficient fibril suppression, and in specific cases, modulation of fibril morphology.

Suppression of amyloid fibrils using the GroEL apical domain

In E. coli cells, rescue of non-native proteins and promotion of native state structure is assisted by the chaperonin GroEL. An important key to this activity lies in the structure of the apical domain of GroEL (GroEL-AD) (residue 191–376), which recognizes and binds non-native protein molecules through hydrophobic interactions. In this study, we investigated the effects of GroEL-AD on the aggregation of various client proteins (α-Synuclein, Aβ42, and GroES) that lead to the formation of distinct protein fibrils in vitro. We found that GroEL-AD effectively inhibited the fibril formation of these three proteins when added at concentrations above a critical threshold; the specific ratio differed for each client protein, reflecting the relative affinities. The effect of GroEL-AD in all three cases was to decrease the concentration of aggregate-forming unfolded client protein or its early intermediates in solution, thereby preventing aggregation and fibrillation. Binding affinity assays revealed some differences in the binding mechanisms of GroEL-AD toward each client. Our findings suggest a possible applicability of this minimal functioning derivative of the chaperonins (the “minichaperones”) as protein fibrillation modulators and detectors.

Pharmacological Chaperones: Design and Development of New Therapeutic Strategies for the Treatment of Conformational Diseases

Errors in protein folding may result in premature clearance of structurally aberrant proteins, or in the accumulation of toxic misfolded species or protein aggregates. These pathological events lead to a large range of conditions known as conformational diseases. Several research groups have presented possible therapeutic solutions for their treatment by developing novel compounds, known as pharmacological chaperones. These cell-permeable molecules selectively provide a molecular scaffold around which misfolded proteins can recover their native folding and, thus, their biological activities. Here, we review therapeutic strategies, clinical potentials, and cost-benefit impacts of several classes of pharmacological chaperones for the treatment of a series of conformational diseases.

Cardiac amyloidosis: approaches to diagnosis and management

Amyloidosis is a clinical disorder caused by the extracellular deposition of misfolded, insoluble aggregated protein with a characteristic ss pleated sheet configuration that produces apple-green birefringence under polarized light when stained with Congo red dye. The spectrum of organ involvement can include the kidneys, heart, blood vessels, central and peripheral nervous systems, liver, intestines, lungs, eyes, skin, and bones. Cardiovascular amyloidosis can be primary, a part of systemic amyloidosis, or the result of chronic systemic disease elsewhere in the body. The most common presentations are congestive heart failure because of restrictive cardiomyopathy and conduction abnormalities. Recent developments in imaging techniques and extracardiac tissue sampling have minimized the need for invasive endomyocardial biopsy for amyloidosis. Cardiac amyloidosis management will vary depending on the subtype but consists of supportive treatment of cardiac related symptoms and reducing the amyloid fibrils formation attacking the underlying disease. Despite advances in treatment, the prognosis for patients with amyloidosis is still poor and depends on the underlying disease type. Early diagnosis of cardiac amyloidosis may improve outcomes but requires heightened suspicion and a systematic clinical approach to evaluation. Delays in diagnosis, uncertainties about the relative merits of available therapies, and difficulties in mounting large-scale clinical trials in rare disorders combine to keep cardiac amyloidosis a challenging problem. This review outlines current approaches to diagnosis, assessment of disease severity, and treatment of cardiac amyloidosis.

FAP mutations destabilize transthyretin facilitating conformational changes required for amyloid formation

Functional transthyretin (TTR) can be transformed into amyloid by partial acid denaturation yielding a monomeric amyloidogenic intermediate which self-associates. The amyloidogenic intermediate has substantial beta-sheet structure with non-native but defined tertiary structure. pH-dependent proteolysis sensitivity studies have identified portions of TTR which become disordered and solvent-exposed in the amyloidogenic intermediate. These include the C-strand-loop D-strand portion of TTR which moves away from the core of the beta-sandwich fold. Mutations that are associated with early onset-amyloid disease (familial amyloidotic polyneuropathy; FAP) function by destabilizing tetrameric TTR in favour of the monomeric amyloidogenic intermediate which has a rearranged C-strand-loop D-strand region. In most cases the FAP mutations do not significantly alter the native folded structure, but instead act on the denaturation pathway by a mechanism that is not completely understood. Interestingly, mutations have also been characterized which strongly stabilize tetrameric TTR and make amyloid formation very difficult at pHs accessible in vivo.

Interaction of transthyretin with amyloid beta-protein: binding and inhibition of amyloid formation

Aggregated amyloid beta-protein (A beta) is a key component of the amyloid depositions found in the brains of patients with Alzheimer's disease. In contrast, in cerebrospinal fluid (CSF), A beta is found in a soluble form. The analysis of complexes of A beta with CSF proteins in a KBr gradient revealed an association of A beta only with free proteins and not with lipoprotein particles. Transthyretin (TTR), a second major CSF protein, formed SDS-stable complexes with A beta and significantly decreased the rate of A beta fibril formation. In physiological buffers and CSF, TTR exclusively decreased the level of A beta pentamers. Endogenous TTR-A beta complexes were detected in human CSF by immunoprecipitation. Using site-directed mutagenesis and computer-assisted modelling, we identified amino acid residues on the surface of the TTR monomer that interact with A beta. Specific TTR immunoreactivity was detected in multiple cortical neurons and astrocytes in the human brain. We propose that A beta binding proteins play a key role in the modulation of A beta aggregation and normally prevent amyloid formation in biological fluids and in the brain.

The genetics of the amyloidoses: interactions with immunity and inflammation

Historically, the amyloidoses have been associated with inflammation and the immune response. From Virchow's original description in human pathologic inflammatory states through their identification in horses used to produce antitoxin to their frequent occurrence in the course of multiple myeloma and a somewhat abortive designation as 'gammaloid', the disorders were felt to have an inflammatory origin. These presumptive associations antedated the availability of a reliable method for tissue extraction that would allow chemical analysis of the major deposited molecules. With the identification of the multiple precursors and the realization that most were not intrinsic elements of immune/inflammatory pathways, the investigative emphasis shifted to the analysis of the biophysical events involved in aggregation and fibril formation. As more in vivo models and better tools for examination of tissues have become available, it appears as if inflammation may participate as both a response to, and an amplifier of, the effects of the fibrillar aggregates. Hence, while only a limited number of amyloid protein precursors are involved in immunity and inflammation per se, host defense, in its broadest sense, is likely to be involved in the clinically relevant amyloidoses. Further it now appears that harnessing the immune response in an appropriate fashion may be able to play a role in treatment.

Protein Denaturation and Aggregation: Cellular Responses to Denatured and Aggregated Proteins

Protein aggregation is a prominent feature of many neurodegenerative diseases, such as Alzheimer's, Huntington's, and Parkinson's diseases, as well as spongiform encephalopathies and systemic amyloidoses. These diseases are sometimes called protein misfolding diseases, but the latter term begs the question of what is the "folded" state of proteins for which normal structure and function are unknown. Amyloid consists of linear, unbranched protein or peptide fibrils of approximately 100 A diameter. These fibrils are composed of a wide variety of proteins that have no sequence homology, and no similarity in three-dimensional structures--and yet, as fibrils, they share a common secondary structure, the beta-sheet. Because of the prominence of amyloid deposits in many of these diseases, much effort has gone into elucidation of fibril structure. Recent advances in solid-state NMR spectroscopy and other biophysical techniques have led to the partial elucidation of fibril structure. Surprisingly at the time, for beta-amyloid, a set of 39-43-amino-acid peptides believed to play a pathogenic role in Alzheimer's disease, the beta-sheets are parallel with all amino acids of the sheets in-register. Since the time of those observations, however, it has become clear that there is no universal structure for amyloid fibrils. While many of the amyloid fibrils described thus far have a parallel beta-sheet structure, some have antiparallel beta-sheets, and other, more subtle structural differences among amyloids exist as well. Amyloids demonstrate conformational plasticity, the ability to adopt more than one stable tertiary fold. Conformational plasticity could account for "strain" differences in prions, and for the fact that a single polypeptide can form different fibril types with conformational differences at the atomic level. More recent data now indicate that the fibrils may not be the most potent or proximate mediators of cyto- and neurotoxicity. This damage is not confined to cell death, but also includes more subtle forms of damage, such as disruption of synaptic plasticity in the central nervous system. Rather than fibrils, prefibrillar aggregates, variously called "micelles," "protofibrils," or ADDLs (beta-amyloid-derived diffusible ligands in the case of beta-amyloid) may be the more proximate mediators of cell damage. These are soluble oligomers of aggregating peptides or proteins, but their structure is very challenging to study, because they are generally difficult to obtain in large enough quantities for high-resolution structural techniques, and they are temporally unstable, rapidly changing into more mature, and eventually fibrillar forms. Consequently, the mechanisms by which they disrupt cellular function are also not well understood. Nevertheless, three broad, overlapping, nonexclusive sets of mechanisms have been proposed as responsible for the cellular damage caused by soluble, oligomeric protein aggregates. These are: (1) disruption of cell membranes and their functions [e.g., by inserting into membranes and disrupting normal ion gradients]; (2) inactivation of normally folded, functional proteins [e.g., by sequestering or localizing transcription factors to the wrong cellular compartment]; and (3) "gumming up the works," by binding to and inactivating components of the quality-control system of cells, such as the proteasome or chaperone proteins.

beta-Sheet mediated self-assembly of dipeptides of omega-amino acids and remarkable fibrillation in the solid state

Single crystal X-ray diffraction studies show that the extended structure of dipeptide Boc-beta-Ala-m-ABA-OMe (m-ABA: meta-aminobenzoic acid) self-assembles in the solid state by intermolecular hydrogen bonding to create an infinite parallel beta-sheet structure. In dipeptide Boc-gamma-Abu-m-ABA-OMe (gamma-Abu: gamma-aminobutyric acid), two such parallel beta-sheets are further cross-linked by intermolecular hydrogen bonding through m-aminobenzoic acid moieties. SEM (scanning electron microscopy) studies reveal that both the peptides and form amyloid-like fibrils in the solid state. The fibrils are also found to be stained readily by Congo red, a characteristic feature of the amyloid fiber whose accumulation causes several fatal diseases such as Alzheimer's, prion-protein etc.

Amyloid-β Binds Cu2+ in a Mononuclear Metal Ion Binding Site

Amyloid-beta (Abeta) peptide is the principal constituent of plaques associated with Alzheimer's disease and is thought to be responsible for the neurotoxicity associated with the disease. Metal ions have been hypothesized to play a role in the formation and neurotoxicity of aggregates associated with Alzheimer's disease (Bush, A. I.; et al. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 11934). Elucidation of the chemistry through which transition-metal ions participate in the assembly and toxicity of Abeta oligomers is important to drug design efforts if inhibition of Abeta containing bound metal ions becomes a treatment for Alzheimer's disease. In this paper, we report electron paramagnetic resonance (EPR) spectroscopic characterization of Cu(2+) bound to soluble and fibrillar Abeta. Addition of stoichiometric amounts of Cu(2+) to soluble Abeta produces an EPR signal at 10 K with observable Cu(2+) hyperfine lines. A nearly identical spectrum is observed for Abetafibrils assembled in the presence of Cu(2+). The EPR parameters are consistent with a Type 2 Cu(2+) center with three nitrogen donor atoms and one oxygen donor atom in the coordination sphere of Cu(2+): g( parallel) = 2.26 and A( parallel) = 174 +/- 4 G for soluble Abeta with Cu(2+), and g( parallel) = 2.26 and A( parallel) = 175 +/- 1 G for Abeta fibrils assembled with Cu(2+). Investigation of the temperature dependence of the EPR signal for Cu(2+) bound to soluble Abetaor Cu(2+) in fibrillar Abeta shows that the Cu(2+) center displays normal Curie behavior, indicating that the site is a mononuclear Cu(2+) site. Fibrils assembled in the presence of Cu(2+) contain one Cu(2+) ion per peptide. These results show that the ligand donor atom set to Cu(2+) does not change during organization of Abetamonomers into fibrils and that neither soluble nor fibrillar forms of Abeta(1-40) with Cu(2+) contain antiferromagnetically exchange-coupled binuclear Cu(2+) sites in which two Cu(2+) ions are bridged by an intervening ligand.

Amyloidogenic and anti-amyloidogenic properties of recombinant transthyretin variants

Most transthyretin (TTR) mutations lead to TTR amyloid depositions in patients with familial amyloidotic polyneuropathy and familial amyloidotic cardiomyopathy. However, though an amyloidogenic protein itself, TTR inhibits aggregation of Alzheimer's amyloid beta protein (A beta) in vitro and in vivo. The pathogenic relationship between two amyloidogenic processes remains unclear. To understand how TTR mutations influence the ability of TTR to inhibit A beta amyloidosis, forty-seven recombinant TTR variants were produced and analyzed. We showed that all recombinant proteins formed tetramers and were functional in thyroxine binding. Acid denaturation at pH 3.8 resulted in aggregation and fibril formation of all TTR variants. However, only TTR G42 and TTR P55 formed fibrils at pH 6.8. Most TTR variants bound to A beta and inhibited A beta aggregation in vitro. TTR variants S64, A71, Q89, V107, H114 and I122 revealed decreased binding to A beta and decreased inhibition of A beta aggregation. Only TTR G42 and TTR P55 completely failed to bind A beta and to inhibit A beta aggregation. We suggest that TTR variants characterized by decreased binding to A beta or by decreased inhibition of A beta aggregation in vitro may contribute to A beta amyloid formation in vivo. These TTR variants might be important targets for epidemiological studies in Alzheimer's disease.

An outline of inherited disorders of the thyroid hormone generating system

To date, various genetic defects impairing the biosynthesis of thyroid hormone have been identified. These congenital heterogeneous disorders result from mutations of genes involved in many steps of thyroid hormone synthesis, storage, secretion, delivery, or utilization. In contrast to thyroid dyshormonogenesis, the elucidation of the underlying etiology of most cases of thyroid dysgenesis is much less understood. It is suggested that genetic factors might play a role in some cases of thyroid dysgenesis and the best candidate genes involved are those encoding transcription factors known to play a role in the embryonic development of the thyroid gland. Moreover, discordance for thyroid dysgenesis is the rule for monozygotic twins as recently reported and this may result from epigenetic phenomena, early somatic mutations, or postzygotic events. In the final part of this review the molecular defects involved in proteins that transport thyroid hormone in the circulation are described: thyroxine-binding globulin (TBG), transtiretin and albumin, that may be associated with altered thyroid function tests and other pathologic conditions such as amyloidotic polyneuropathy.

Amyloid deposition in the oral cavity: a retrospective study and review of the literature

OBJECTIVE

A retrospective study was conducted to investigate the anatomic location and characteristics of amyloid deposition in the oral cavity.

STUDY DESIGN

Seventeen biopsy specimens that were conclusive for a diagnosis of amyloidosis were assessed in terms of their anatomic location and histopathologic characteristics.

RESULTS

Biopsy specimens were received from 13 patients-9 females and 4 males. Six specimens were taken from the buccal mucosa, 4 from the tongue, 3 from the palate, 2 from the gingiva, and 2 from the floor of the mouth. Fifteen of 17 specimens (88%) had amyloid deposition in the subepithelial connective tissue in all locations.

CONCLUSION

On the basis of our pilot data, a previously overlooked intraoral anatomic location, the buccal mucosa, may prove to be of higher diagnostic value than others previously reported in terms of obtaining a diagnosis of amyloidosis. In addition, the manner in which tissue biopsies are conducted for amyloid detection may be altered to create less morbidity.

Prevention of transthyretin amyloid disease by changing protein misfolding energetics

Genetic evidence suggests that inhibition of amyloid fibril formation by small molecules should be effective against amyloid diseases. Known amyloid inhibitors appear to function by shifting the aggregation equilibrium away from the amyloid state. Here, we describe a series of transthyretin amyloidosis inhibitors that functioned by increasing the kinetic barrier associated with misfolding, preventing amyloidogenesis by stabilizing the native state. The trans-suppressor mutation, threonine 119 --> methionine 119, which is known to ameliorate familial amyloid disease, also functioned through kinetic stabilization, implying that this small-molecule strategy should be effective in treating amyloid diseases.

Sequence-dependent denaturation energetics: A major determinant in amyloid disease diversity

Several misfolding diseases commence when a secreted folded protein encounters a partially denaturing microenvironment, enabling its self assembly into amyloid. Although amyloidosis is modulated by numerous environmental and genetic factors, single point mutations within the amyloidogenic protein can dramatically influence disease phenotype. Mutations that destabilize the native state predispose an individual to disease; however, thermodynamic stability alone does not reliably predict disease severity. Here we show that the rate of transthyretin (TTR) tetramer dissociation required for amyloid formation is strongly influenced by mutation (V30M, L55P, T119M, V122I), with rapid rates exacerbating and slow rates reducing amyloidogenicity. Although these rates are difficult to predict a priori, they notably influence disease penetrance and age of onset. L55P TTR exhibits severe pathology because the tetramer both dissociates quickly and is highly destabilized. Even though V30M and L55P TTR are similarly destabilized, the V30M disease phenotype is milder because V30M dissociates more slowly, even slower than wild type (WT). Although WT and V122I TTR have nearly equivalent tetramer stabilities, V122I cardiomyopathy, unlike WT cardiomyopathy, has nearly complete penetrance-presumably because of its 2-fold increase in dissociation rate. We show that the T119M homotetramer exhibits kinetic stabilization and therefore dissociates exceedingly slowly, likely explaining how it functions to protect V30MT119M compound heterozygotes from disease. An understanding of how mutations influence both the kinetics and thermodynamics of misfolding allows us to rationalize the phenotypic diversity of amyloid diseases, especially when considered in concert with other genetic and environmental data.

Synthesis and structural analysis of the N-terminal domain of the thyroid hormone-binding protein transthyretin

Transthyretin (TTR) is a 55 kDa protein responsible for the transport of thyroid hormones and retinol in human serum. Misfolded forms of the protein are implicated in the amyloid diseases familial amyloidotic polyneuropathy and senile systemic amyloidosis. Its folding properties and stabilization by ligands are of current interest due to their importance in understanding and combating these diseases. To assist in such studies we developed a method for the solid phase synthesis of the monomeric unit of a TTR analogue and its folding to form a functional 55 kDa tetramer. The monomeric unit of the protein was chemically synthesized in three parts, comprising amino acid residues 1-51, 54-99 and 102-127, and ligated using chemoselective thioether ligation chemistry. The synthetic protein was folded and assembled to a tetrameric structure in the presence of the TTR's native ligand, thyroxine, as shown by gel filtration chromatography, native gel electrophoresis, TTR antibody recognition and thyroid hormone binding. In the current study the solution structure of the first of these fragment peptides, TTR(1-51) is examined to determine its intrinsic propensity to form beta-sheet structure, potentially involved in amyloid fibril formation by TTR. Despite the presence of extensive beta-structure in the native form of the protein, the N-terminal fragment adopts an essentially random coil conformation in solution.

Altered aggregation properties of mutant gamma-crystallins cause inherited cataract

Protein inclusions are associated with a diverse group of human diseases ranging from localized neurological disorders through to systemic non-neuropathic diseases. Here, we present evidence that the formation of intranuclear inclusions is a key event in cataract formation involving altered gamma-crystallins that are un likely to adopt their native fold. In three different inherited murine cataracts involving this type of gamma-crystallin mutation, large inclusions containing the altered gamma-crystallins were found in the nuclei of the primary lens fibre cells. Their formation preceded not only the first gross morphological changes in the lens, but also the first signs of cataract. The inclusions contained filamentous material that could be stained with the amyloid-detecting dye, Congo red. In vitro, recombinant mutant gammaB-crystallin readily formed amyloid fibrils under physiological buffer conditions, unlike wild-type protein. These data suggest that this type of cataract is caused by a mechanism involving the nuclear targeting and deposition of amyloid-like inclusions. The mutant gamma-crystallins initially disrupt nuclear function, but then this progresses to a full cataract phenotype.

Therapeutic strategies for human amyloid diseases

Amyloid diseases are a large group of a much larger family of misfolding diseases. This group includes pathologies as diverse as Alzheimer's disease, immunoglobulin-light-chain disease, reactive amyloid disease and the familial amyloid polyneuropathies. These diseases are generally incurable at present, although some drugs are known to transiently slow the progression of Alzheimer's disease. As we increase our understanding of the causative mechanisms of these disorders, the likelihood of success for a given therapeutic strategy will become clearer. This review will look at small-molecule and macromolecular approaches for intervention in amyloid diseases other than Alzheimer's disease, although select examples from Alzheimer's disease will be discussed.

The V122I cardiomyopathy variant of transthyretin increases the velocity of rate-limiting tetramer dissociation, resulting in accelerated amyloidosis

The transthyretin (TTR) amyloid diseases are of keen interest, because there are >80 mutations that cause, and a few mutations that suppress, disease. The V122I variant is the most common amyloidogenic mutation worldwide, producing familial amyloidotic cardiomyopathy primarily in individuals of African descent. The substitution shifts the tetramer-folded monomer equilibrium toward monomer (lowers tetramer stability) and lowers the kinetic barrier associated with rate-limiting tetramer dissociation (pH 7; relative to wild-type TTR) required for amyloid fibril formation. Fibril formation is also accelerated because the folded monomer resulting from the tetramer-folded monomer equilibrium rapidly undergoes partial denaturation and self-assembles into amyloid (in vitro) when subjected to a mild denaturation stress (e.g., pH 4.8). Incorporation of the V122I mutation into a folded monomeric variant of transthyretin reveals that this mutation does not destabilize the tertiary structure or alter the rate of amyloidogenesis relative to the wild-type monomer. The increase in the velocity of rate-limiting tetramer dissociation coupled with the lowered tetramer stability (increasing the mol fraction of folded monomer present at equilibrium) may explain why V122I confers an apparent absolute anatomic risk for cardiac amyloid deposition.

Support for the multigenic hypothesis of amyloidosis: the binding stoichiometry of retinol-binding protein, vitamin A, and thyroid hormone influences transthyretin amyloidogenicity in vitro

The amyloidoses are a large group of protein misfolding diseases. Genetic and biochemical evidence support the hypothesis that amyloid formation from wild-type or 1 of 80 sequence variants of transthyretin causes the human amyloid diseases senile systemic amyloidosis or familial amyloid polyneuropathy, respectively. The late onset and variable penetrance of these diseases has led to their designation as multigenic--implying that the expression levels and alleles of multiple gene products influence the course of pathology. Here we show that the binding stoichiometry of three interacting molecules, retinol-binding protein, vitamin A, and L-thyroxine, notably influenced transthyretin amyloidogenicity in vitro. At least 70 genes control retinol-binding protein, vitamin A, and L-thyroxine levels in plasma and have the potential to modulate the course of senile systemic amyloidosis or familial amyloid polyneuropathy.

Synthesis of an Analog of the Thyroid Hormone-binding Protein Transthyretin via Regioselective Chemical Ligation

Transthyretin is an essential protein responsible for the transport of thyroid hormones and retinol in human serum and is also implicated in the amyloid diseases familial amyloidotic polyneuropathy and senile systemic amyloidosis. Its folding properties and stabilization by ligands are of current interest due to their importance in understanding and combating these diseases. Here we report the solid phase synthesis of the monomeric unit of a transthyretin analog (equivalent to 127 amino acids) using t-Boc chemistry and peptide ligation and its folding to form a functional 54-kDa tetramer. The monomeric unit of the protein was chemically synthesized in three parts (positions 1--51, 54--99, and 102--127) and ligated using a chemoselective thioether ligation chemistry. The synthetic protein was folded and assembled to a tetrameric structure in the presence of transthyretin's native ligand, thyroxine, as shown by gel filtration chromatography, native gel electrophoresis, transthyretin antibody recognition, and thyroid hormone binding. Other folding products included a high molecular weight aggregate as well as a transient dimeric species. This represents one of the largest macromolecules chemically synthesized to date and demonstrates the potential of protein chemical synthesis for investigations of protein-ligand interactions.

Protein misfolding and disease; protein refolding and therapy

Diverse human disorders, including several neurodegenerative diseases and systemic amyloidosis, are thought to arise from the misfolding and aggregation of an underlying protein. Recent findings strongly support this hypothesis and have increased our understanding of the molecular mechanism of protein conformational disorders. Many questions are still pending, but the data overall suggest that correction of protein misfolding constitutes a viable therapeutic strategy for conformational diseases.

Evaluating the binding selectivity of transthyretin amyloid fibril inhibitors in blood plasma

Transthyretin (TTR) tetramer dissociation and misfolding facilitate assembly into amyloid fibrils that putatively cause senile systemic amyloidosis and familial amyloid polyneuropathy. We have previously discovered more than 50 small molecules that bind to and stabilize tetrameric TTR, inhibiting amyloid fibril formation in vitro. A method is presented here to evaluate the binding selectivity of these inhibitors to TTR in human plasma, a complex biological fluid composed of more than 60 proteins and numerous small molecules. Our immunoprecipitation approach isolates TTR and bound small molecules from a biological fluid such as plasma, and quantifies the amount of small molecules bound to the protein by HPLC analysis. This approach demonstrates that only a small subset of the inhibitors that saturate the TTR binding sites in vitro do so in plasma. These selective inhibitors can now be tested in animal models of TTR amyloid disease to probe the validity of the amyloid hypothesis. This method could be easily extended to evaluate small molecule binding selectivity to any protein in a given biological fluid without the necessity of determining or guessing which other protein components may be competitors. This is a central issue to understanding the distribution, metabolism, activity, and toxicity of potential drugs.

Destabilization of Ca2+-free gelsolin may not be responsible for proteolysis in Familial Amyloidosis of Finnish Type

Mutations at position 187 in secreted gelsolin enable aberrant proteolysis at the 172-173 and 243-244 amide bonds, affording the 71-residue amyloidogenic peptide deposited in Familial Amyloidosis of Finnish Type (FAF). Thermodynamic comparisons of two different domain 2 constructs were carried out to study possible effects of the mutations on proteolytic susceptibility. In the construct we consider to be most representative of domain 2 in the context of the full-length protein (134-266), the D187N FAF variant is slightly destabilized relative to wild type (WT) under the conditions of urea denaturation, but exhibits a T(m) identical to WT. The D187Y variant is less stable to intermediate urea concentrations and exhibits a T(m) that is estimated to be approximately 5 degrees C lower than WT (pH 7.4, Ca(2+)-free). Although the thermodynamic data indicate that the FAF mutations may slightly destabilize domain 2, these changes are probably not sufficient to shift the native to denatured state equilibrium enough to enable the proteolysis leading to FAF. Biophysical data indicate that these two FAF variants may have different native state structures and possibly different pathways of amyloidosis.

New transthyretin variants SER 91 and SER 116 associated with familial amyloidotic polyneuropathy. Mutations in brief no. 151. Online

Mutations of the transthyretin (TTR) gene are associated with familial amyloidotic polyneuropathy (FAP). Two new mutations were detected in French patients with TTR amyloidosis. The first patient was a 72 year old man who presented with severe and rapidly evolving sensory motor polyneuropathy of the 4 limbs, a bilateral carpal tunnel syndrome and a restrictive cardiomyopathy. His father died after a clinical history suggestive in retrospect of TTR amyloidosis. The second patient was a 75 year old man who presented with axonal sensory neuropathy of the 4 limbs and a bilateral carpal tunnel syndrome. In both cases immunohistochemistry performed on a nerve biopsy reveled TTR positive amyloid. Direct genomic sequencing of the full TTR gene coding region indicated two heterozygous transversions encoding Ser for Ala 91 substitution in the third exon of the gene in patient 1 and Ser for Tyr 116 substitution in the fourth exon of the gene in patient 2. The mutations were confirmed by digesting PCR products with restriction enzymes and were not found in a control population of 100 unrelated individuals. The Ser 116 substitution was also detected in the daughter and the 70 year old sister of the proband. However the absence of symptomatology suggestive of TTR amyloidosis may be related to the late onset of the disease. The clinical immunohistochemical and molecular studies in both patients are highly suggestive of an association between the Ser 91 and Ser 116 TTR variants with amyloidosis.

Synthesis and evaluation of inhibitors of transthyretin amyloid formation based on the non-steroidal anti-inflammatory drug, flufenamic acid

A light scattering-based amyloid fibril formation assay was employed to evaluate potential inhibitors of transthyretin (TTR) amyloid fibril formation in vitro. Twenty nine aromatic small molecules, some with homology to flufenamic acid (a known non-steroidal anti-inflammatory drug) were tested to identify important structural features for inhibitor efficacy. The results of these experiments and earlier data suggest that likely inhibitors will have aromatic-based structures with at least two aromatic rings. The ring or fused ring system occupying the outermost TTR binding pocket needs to be substituted with an acidic functional group (e.g. a carboxylic acid) to interact with complimentary charges in the TTR binding site. The promising TTR amyloid fibril inhibitors ranked in order of efficacy are: 2 > 4 approximately 7 > 3 > 9 > 6 > 21.

Diet, amyloid enhancing factor (AEF) and amyloidogenesis: an hypothesis

At least two forms of amyloidosis, amyloid A (AA) and prion protein (PrP), can be transmitted by dietary ingestion of an agent(s) present in crude mammalian tissues. Although the incubation time for PrP or scrapie-induced diseases to develop in experimental animals extends over months or years, AA or secondary amyloidosis in mice is inducible within a week. In response to inflammatory stimuli we hypothesize that dietary factor(s) modulate the rate at which beta-pleated sheet fibrils accumulate in most forms of amyloidosis. The critical importance of precursor protein polymorphism, cell surface proteoglycans (PG), lipids and apolipoprotein metabolism has also been addressed in this hypothesis.

Synthesis and evaluation of anthranilic acid-based transthyretin amyloid fibril inhibitors

Eight small molecules were synthesized to evaluate the structure activity relationships (SAR) of N-substituted anthranilic acids. The molecules were synthesized by benzylation or arylation of methyl anthranilate. A light scattering-based amyloid fibril formation assay was used to evaluate potential inhibitors of transthyretin (TTR) amyloid fibril formation in vitro. The m-carboxyphenylated and o-trifluoromethylphenylated anthranilic acids are potent inhibitors that will be subjected to further SAR and structural analysis.

Familial amyloid with a transthyretin leucine 33 mutation presenting with ascites

A 65-year-old female presented with symptomatic ascites. Light and electron microscopy examination of omental and peritoneal tissue obtained at exploratory laparotomy revealed amyloidosis. Immunochemical studies of the amyloid tissue showed positive staining with antibodies to transthyretin. Polymerase chain reaction (PCR), single strand conformation polymorphism analysis, and direct DNA sequencing demonstrated a transthyretin phenylalanine to leucine substitution at codon 33. This is only the second reported case of a transthyretin leucine 33 mutation. Moreover, this patient is unique among cases of transthyretin-associated amyloidosis with the clinical presentation of ascites.

Inhibiting transthyretin conformational changes that lead to amyloid fibril formation

Insoluble protein fibrils resulting from the self-assembly of a conformational intermediate are implicated as the causative agent in several severe human amyloid diseases, including Alzheimer's disease, familial amyloid polyneuropathy, and senile systemic amyloidosis. The latter two diseases are associated with transthyretin (TTR) amyloid fibrils, which appear to form in the acidic partial denaturing environment of the lysosome. Here we demonstrate that flufenamic acid (Flu) inhibits the conformational changes of TTR associated with amyloid fibril formation. The crystal structure of TTR complexed with Flu demonstrates that Flu mediates intersubunit hydrophobic interactions and intersubunit hydrogen bonds that stabilize the normal tetrameric fold of TTR. A small-molecule inhibitor that stabilizes the normal conformation of a protein is desirable as a possible approach to treat amyloid diseases. Molecules such as Flu also provide the means to rigorously test the amyloid hypothesis, i.e., the apparent causative role of amyloid fibrils in amyloid disease.

Discovering transthyretin amyloid fibril inhibitors by limited screening

Insoluble protein fibrils, resulting from the self-assembly of a conformational intermediate are implicated to be the causative agent in several human amyloid diseases including familial amyloid polyneuropathy (FAP) and senile systemic amyloidosis (SSA). These diseases are associated with transthyretin (TTR) amyloid fibrils, which appear to form in the acidic partial denaturing environment of a lysosome or endosome. Here we identify several structural classes of small molecules that are capable of inhibiting the TTR conformational changes facilitating amyloid fibril formation. A small molecule inhibitor that stabilizes the normal conformation of a protein is desirable as a promising approach to treat amyloid diseases and to rigorously test the amyloid hypothesis, the apparent causative role of amyloid fibrils in amyloid disease.

Amyloid myopathy: an underdiagnosed entity

Amyloidosis can involve multiple organs, including kidney, heart, peripheral nerve, skin, joints, and skeletal muscle, but rarely presents as a myopathy. We studied 13 adults with muscle weakness for between 3 months and 4 years in whom the diagnosis of systemic amyloidosis was unsuspected before or until just before the time of the muscle biopsy. All muscle specimens demonstrated congophilic deposits around blood vessels and muscle fibers, some necrotic and regenerating fibers, and signs of mild denervation. Immunostains in 10 patients revealed immunoglobulin amyloidosis in 7 and gelsolin amyloidosis in 1. Apolipoprotein E co-localized with the congophilic deposits in all 10, and a C-terminal epitope of the beta-amyloid precursor protein was detected in 6. The frequency of the diagnosis of amyloid myopathy increased 10-fold when we adopted the fluorescent Congo red stain as a routine procedure in assessing muscle biopsy specimens.

The alternative conformations of amyloidogenic proteins and their multi-step assembly pathways

The conformational change hypothesis postulates that tertiary structural changes under partially denaturing conditions convert one of 17 normally soluble and functional human proteins into an alternative conformation that subsequently undergoes self-assembly into an amyloid fibril, the putative causative agent in amyloid disease. This hypothesis is consistent with Anfinsen's view that the tertiary structure of a protein is determined both by its sequence and the aqueous environment; the latter does not always favor the normally folded state. Unlike sickle cell hemoglobin assembly, where owing to a surface mutation, hemoglobin polymerizes in its normally folded conformation, amyloid proteins self-assemble as a result of the formation of an alternative tertiary structure-a conformational intermediate formed under partially denaturing conditions. The pathway by which an amyloidogenic protein assembles into amyloid fibrils appears to involve quaternary structural intermediates that assemble into increasingly complex quaternary structures, including amyloid protofilaments, which ultimately assemble into amyloid fibrils. Several recent studies have discussed the multi-step assembly pathway(s) characterizing amyloid fibril formation.

Amyloid fibril formation and protein misassembly: a structural quest for insights into amyloid and prion diseases

The assembly and misassembly of normally soluble proteins into fibrilar structures is thought to be a causative agent in a variety of human amyloid and prion diseases. Structural and mechanistic studies of this process are beginning to elucidate the conformational changes required for the conversion of a normally soluble and functional protein into a defined quaternary structure.

Variant-sequence transthyretin (isoleucine 122) in late-onset cardiac amyloidosis in black Americans

BACKGROUND

After the age of 60, isolated cardiac amyloidosis is four times more common among blacks than whites in the United States; 3.9 percent of blacks are heterozygous for an amyloidogenic allele of the normal serum carrier protein transthyretin in which isoleucine is substituted for valine at position 122 (Ile 122). We hypothesized that the high prevalence of transthyretin Ile 122 is at least partially responsible for the increased frequency of senile cardiac amyloidosis among blacks.

METHODS

Paraffin blocks of cardiac tissue were obtained from an earlier study of 52,370 autopsies in Los Angeles and were examined by immunohistochemical and DNA analyses. Samples were available from 32 of 55 blacks and 20 of 78 whites over 60 years of age with isolated cardiac amyloidosis and from two control groups (228 cases).

RESULTS

Transthyretin amyloidosis was identified in 31 of the 32 cardiac-tissue samples from the black patients and in 19 of the 20 samples from the white patients. Six of the 26 analyzable DNA samples (23 percent) from the black patients and none of the 19 samples from the white patients were heterozygous for the Ile 122 variant. Four of 125 DNA samples obtained at autopsy (3.2 percent) from a second, more recent, age-matched cohort of blacks without amyloidosis at the same institution were heterozygous for the transthyretin Ile 122 allele. On reexamination the cardiac tissue from these four patients contained small amounts of amyloid not detected at the initial autopsies. All subjects with the Ile 122 variant had ventricular amyloid.

CONCLUSIONS

The assessment of elderly black patients with unexplained heart disease should include a consideration of transthyretin amyloidosis, particularly that related to the Ile 122 allele.

Inhibiting transthyretin amyloid fibril formation via protein stabilization

Transthyretin (TTR) amyloid fibril formation is observed systemically in familial amyloid polyneuropathy and senile systemic amyloidosis and appears to be the causative agent in these diseases. Herein, we demonstrate conclusively that thyroxine (10.8 microM) inhibits TTR fibril formation efficiently in vitro and does so by stabilizing the tetramer against dissociation and the subsequent conformational changes required for amyloid fibril formation. In addition, the nonnative ligand 2,4,6-triiodophenol, which binds to TTR with slightly increased affinity also inhibits TTR fibril formation by this mechanism. Sedimentation velocity experiments were employed to show that TTR undergoes dissociation (linked to a conformational change) to form the monomeric amyloidogenic intermediate, which self-assembles into amyloid in the absence, but not in the presence of thyroxine. These results demonstrate the feasibility of using small molecules to stabilize the native fold of a potentially amyloidogenic human protein, thus preventing the conformational changes, which appear to be the common link in several human amyloid diseases. This strategy and the compounds resulting from further development should prove useful for critically evaluating the amyloid hypothesis--i.e., the putative cause-and-effect relationship between TTR amyloid deposition and the onset of familial amyloid polyneuropathy and senile systemic amyloidosis.

Brain amyloid--a physicochemical perspective

The ability to form stable cross-beta fibrils is an intrinsic physicochemical characteristic of the human beta-amyloid peptide (A beta), which forms the brain amyloid of Alzheimer's disease (AD). The high amyloidogenicity and low solubility of this hydrophobic approximately 40-mer have been barriers to its study in the past, but the availability of synthetic peptide and new physical methods has enabled many novel approaches in recent years. Model systems for A beta aggregation (relevant to initial nidus formation) and A beta deposition (relevant to plaque growth and maturation) in vitro have allowed structure/activity relationships and kinetics to be explored quantitatively, and established that these processes are biochemically distinct. Different forms of the peptide, with different physiochemical characteristics, are found in vascular and parenchymal amyloid. Various spectroscopic methods have been used to explore the three-dimensional conformation of A beta both in solution and in solid phase, and demonstrated that the peptide adopts a different configuration in each state. A significant conformational transition is essential to the transformation of A beta from solution to fibril. These observations suggest new therapeutic targets for the treatment of AD.

Molecular diagnosis of transthyretin Met30 mutation in an Italian family with familial amyloidotic polyneuropathy

We report the molecular analysis of the transthyretin gene in a large Italian pedigree with familial amyloidotic polyneuropathy and demonstrate the presence of a Met30 mutation. The usefulness of the genetic analysis in the identification of presymptomatic persons and the diagnosis of individuals with partial symptoms is discussed.

Homology of calcitonin with the amyloid-related proteins

We present evidence for a structural homology between the amino acid sequence of calcitonin (CT)--the fibrillar protein of the amyloid deposits of medullary thyroid cancer--and that of other 12 amyloid-related proteins (ARP). Seven of the 32 residues of CT are conserved in at least five ARP, and five of these seven amino acids belong to the stretch Gly2-Gln14. Gln14 is conserved in all 12 ARP and Cys7 in all eight ARP containing cysteine. The concentration of the homology in the N-terminal half of CT goes along with the knowledge that is the C-terminal region the one more important for the hormonal action of CT. Since an imbalance between synthesis and catabolism of a given ARP is believed to be the general pathogenetic mechanism of amyloidosis, the intratumoral deposition of CT in the form of amyloid fibrils would be due to the overproduction of a protein structurally similar to the ARP.

Amyloidosis

The biochemistry of amyloidosis as it relates to clinical medicine and experimental pathology is presented. Amyloidoses are complex disorders in which normally soluble precursors undergo pathological conformational changes and polymerize as insoluble fibrils with the beta-pleated sheet conformation. Over the past 20 years, 16 biochemically diverse proteins have been identified as fibrillar constituents of amyloid deposits; in all cases the protein-protein interactions that result in amyloid fibril formation appear to be stabilized both by the structure and the microenvironment of the precursor protein. Either genetic predisposition or dysfunctions of the immune system favor amyloid fibril formation. In particular, macrophage function is a factor in the pathogenesis of many of the amyloidoses. The diagnosis of amyloidosis involves acquisition of a tissue biopsy, staining of the specimen with Congo red, and observation of classic green birefringence on polarization microscopy. The subdiagnosis of the systemic amyloidoses involves characterization of variant or monoclonal plasma amyloid precursor proteins in the context of clinical symptoms. Treatment is generally supportive, with the use of antiinflammatory therapy, dialysis, or transplantation and genetic counseling where indicated.

A double-variant transthyretin allele (Ser 6, Ile 33) in the Israeli patient "SKO" with familial amyloidotic polyneuropathy

Transthyretin (TTR) isolated from amyloid fibrils from an Israeli patient ("SKO") with familial amyloidotic polyneuropathy has been studied by two groups of investigators. Originally, a position 49 Thr-->Gly substitution was reported; subsequently, a position 33 Phe-->Ile substitution was found instead. We have studied DNA from this patient by single strand conformation polymorphism analysis, restriction analysis, and DNA sequencing. On one allele, exon 2 contained both a T-->A transversion at the first position of codon 33, encoding the previously described Phe-->Ile substitution, and a G-->A transition at the first position of codon 6, encoding a Gly-->Ser substitution. The originally reported position 49 mutation was not encoded in the genomic DNA. This is the first report of a TTR double-variant allele in a patient with TTR amyloidosis.

Three autosomal dominant corneal dystrophies map to chromosome 5q

The two most common autosomal dominant dystrophies of the corneal stroma are lattice corneal dystrophy type I and granular dystrophy. A third autosomal dominant stromal dystrophy (Avellino) has also been recognized. Chromosome linkage analysis of four families with Avellino dystrophy mapped the disease-causing gene to chromosome 5q. Subsequent linkage analysis of two families with typical lattice dystrophy and two with typical granular dystrophy also revealed significant linkage with the same markers. Thus, each of three clinically and histopathologically distinct phenotypes is independently linked to 5q. The maximum combined lod score using all 114 affected patients was 28.6 with marker D5S393. None of the 14 known human amyloid-associated genes map to chromosome 5.