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

Susceptibility and modifier genes in Portuguese transthyretin V30M amyloid polyneuropathy: complexity in a single-gene disease

Familial amyloid polyneuropathy type I is an autosomal dominant disorder caused by mutations in the transthyretin (TTR) gene; however, carriers of the same mutation exhibit variability in penetrance and clinical expression. We analyzed alleles of candidate genes encoding non-fibrillar components of TTR amyloid deposits and a molecule metabolically interacting with TTR [retinol-binding protein (RBP)], for possible associations with age of disease onset and/or susceptibility in a Portuguese population sample with the TTR V30M mutation and unrelated controls. We show that the V30M carriers represent a distinct subset of the Portuguese population. Estimates of genetic distance indicated that the controls and the classical-onset group were furthest apart, whereas the late-onset group appeared to differ from both. Importantly, the data also indicate that genetic interactions among the multiple loci evaluated, rather than single-locus effects, are more likely to determine differences in the age of disease onset. Multifactor dimensionality reduction indicated that the best genetic model for classical onset group versus controls involved the APCS gene, whereas for late-onset cases, one APCS variant (APCSv1) and two RBP variants (RBPv1 and RBPv2) are involved. Thus, although the TTR V30M mutation is required for the disease in Portuguese patients, different genetic factors may govern the age of onset, as well as the occurrence of anticipation.

Hydroxylated Polychlorinated Biphenyls Selectively Bind Transthyretin in Blood and Inhibit Amyloidogenesis: Rationalizing Rodent PCB Toxicity

Polychlorinated biphenyls (PCBs) and their hydroxylated metabolites (OH-PCBs) are known to bind to transthyretin (TTR) in vitro, possibly explaining their bioaccumulation, rodent toxicity, and presumed human toxicity. Herein, we show that several OH-PCBs bind selectively to TTR in blood plasma; however, only one of the PCBs tested binds TTR in plasma. Some of the OH-PCBs displace thyroid hormone (T4) from TTR, rationalizing the toxicity observed in rodents, where TTR is the major T4 transporter. Thyroid binding globulin and albumin are the major T4 carriers in humans, making it unlikely that enough T4 could be displaced from TTR to be toxic. OH-PCBs are excellent TTR amyloidogenesis inhibitors in vitro because they bind to the TTR tetramer, imparting kinetic stability under amyloidogenic denaturing conditions. Four OH-PCB/TTR cocrystal structures provide further insight into inhibitor binding interactions.

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

The transthyretin (TTR) amyloidoses are human diseases in which the misfolded TTR protein aggregates in tissues with subsequent visceral, peripheral, and autonomic nerve dysfunction. Recent reports have stressed the importance of oligomeric intermediates as major cytotoxic species in various forms of amyloidogenesis. We have examined the cytotoxic effects of several quaternary structural states of wild-type and variant TTR proteins on cells of neural lineage. TTR amyloid fibrils and soluble aggregates >100 kDa were not toxic. Incubation of TTR under the conditions of the cell assay and analysis by size-exclusion chromatography and SDS/PAGE reveal that monomeric TTR or relatively small, rapidly formed aggregates of a maximum size of six subunits were the major cytotoxic species. Small molecules that stabilize the native tetrameric state were shown to prevent toxicity. The studies are consistent with a model in which the misfolded TTR monomer rapidly aggregates to form transient low molecular mass assemblies (<100 kDa) that are highly cytotoxic in tissue culture.

Silent Prions Lying in Wait: A Two-hit Model of Prion/Amyloid Formation and Infection

Diseases such as type 2 diabetes, Alzheimer's and Parkinson's are associated with the formation of amyloid. The transmissible spongiform encephalopathies, such as variant Creutzfeldt-Jakob disease, are believed to result from infectious forms of amyloid proteins termed prions. The ability of amyloid to initiate spontaneously and in the case of prions, to transfer successfully from one host to another, has been hard to fully rationalize. In this paper we use a mathematical model to explore the idea that it might be a combination of the presence of the prion/amyloid form and a change in the state of the host that allows the amyloid/prion to successfully initiate and propagate itself. We raise the intriguing possibility that potentially infectious amyloid may lie dormant in an apparently healthy individual awaiting a change in the state of the host or transmittal to a new more susceptible host. On this basis we make an analogy between prion/amyloid disease development and the two-hit model of cancer progression. We additionally raise the possibility that infectious amyloid strains may be characterized by a size distribution of length or radius.

Therapeutic approaches to protein-misfolding diseases

Several sporadic and genetic diseases are caused by protein misfolding. These include cystic fibrosis and other devastating diseases of childhood as well as Alzheimer's, Parkinson's and other debilitating maladies of the elderly. A unified view of the molecular and cellular pathogenesis of these conditions has led to the search for chemical chaperones that can slow, arrest or revert disease progression. Molecules are now emerging that link our biophysical insights with our therapeutic aspirations.

Reversible amyloid formation by the p53 tetramerization domain and a cancer-associated mutant

The tetramerization domain for wild-type p53 (p53tet-wt) and a p53 mutant, R337H (p53tet-R337H), associated with adrenocortical carcinoma (ACC) in children, can be converted from the soluble native state to amyloid-like fibrils under certain conditions. Circular dichroism, Fourier transform infrared spectroscopy and staining with Congo red and thioflavin T showed that p53tet-wt and p53tet-R337H adopt an alternative beta-sheet conformation (p53tet-wt-beta and p53tet-R337H-beta, respectively), characteristic of amyloid-like fibrils, when incubated at pH 4.0 and elevated temperatures. Electron micrographs showed that the alternative conformations for p53tet-wt (p53tet-wt-beta) and p53tet-R337H (p53tet-R337H-beta) were supramolecular structures best described as "molecular ribbons". FT-IR analysis demonstrated that the mechanism of amyloid-like fibril formation involved unfolding of the p53tet-wt beta-strands, followed by unfolding of the alpha-helices, followed finally by formation of beta-strand-containing structures that other methods showed were amyloid-like ribbons. The mutant, p53tet-R337H, had a significantly higher propensity to form amyloid-like fibrils. Both p53tet-wt (pH 4.0) and p53tet-R337H (pH 4.0 and 5.0), when incubated at room temperature (22 degrees C) for one month, were converted to molecular ribbons. In addition, p53tet-R337H, and not p53tet-wt, readily formed ribbons at pH 4.0 and 37 degrees C over 20 hours. Interestingly, unlike other amyloid-forming proteins, p53tet-wt-beta and p53tet-R337H-beta disassembled and refolded to the native tetramer conformation when the solution pH was raised from 4.0 to 8.5. Although fibril formation at pH 4.0 was concentration and temperature-dependent, fibril disassembly at pH 8.5 was independent of both. Finally, we propose that the significantly higher propensity of the mutant to form ribbons, compared to the wild-type, may provide a possible mechanism for the observed nuclear accumulation of p53 in ACC cells and other cancerous cells.

Stimulated interaction between and subunits of tryptophan synthase from hyperthermophile enhances its thermal stability

Tryptophan synthase from hyperthermophile, Pyrococcus furiosus, was found to be a tetrameric form (22) composed of and 2 subunits. To elucidate the relationship between the features of the subunit association and the thermal stability of the tryptophan synthase, the subunit association and thermal stability were examined by isothermal titration calorimetry and differential scanning calorimetry, respectively, in comparison with those of the counterpart from Escherichia coli. The association constants between the and subunits in the hyperthermophile protein were of the order of 108 M1, which were higher by two orders of magnitude than those in the mesophile one. The negative values of the heat capacity change and enthalpy change upon the subunit association were much lower in the hyperthermophile protein than in the mesophile one, indicating that the conformational change of the hyperthermophile protein coupled to the subunit association is slight. The denaturation temperature of the subunit from the hyperthermophile was enhanced by 17 degrees C due to the formation of the 22 complex. This increment in denaturation temperature due to complex formation could be quantitatively estimated by the increase in the association constant compared with that of the counterpart from E. coli.

Evidence for defective retinoid transport and function in late onset Alzheimer's disease

The hypothesis of this article is that late onset Alzheimer's disease (AD) is influenced by the availability in brain of retinoic acid (RA), the final product of the vitamin A (retinoid) metabolic cascade. Genetic, metabolic, and environmental/dietary evidence is cited supporting this hypothesis. Significant genetic linkages to AD are demonstrated for markers close to four of the six RA receptors, RA receptor G at 12q13, retinoid X receptor B at 6p21.3, retinoid X receptor G at 1q21, and RA receptor A at 17q21. Three of the four retinol-binding proteins at 3q23 and 10q23 and the RA-degrading cytochrome P450 enzymes at 10q23 and 2p13 map to AD linkages. Synthesis of the evidence supports retinoid hypofunction and impaired transport as contributing factors. These findings suggest testable experiments to determine whether increasing the availability of retinoid in brain, possibly through pharmacologic targeting of the RA receptors and the cytochrome P450 RA-inactivating enzymes, can prevent or decrease amyloid plaque formation.

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.

New class of inhibitors of amyloid-beta fibril formation. Implications for the mechanism of pathogenesis in Alzheimer's disease

The amyloid hypothesis suggests that the process of amyloid-beta protein (Abeta) fibrillogenesis is responsible for triggering a cascade of physiological events that contribute directly to the initiation and progression of Alzheimer's disease. Consequently, preventing this process might provide a viable therapeutic strategy for slowing and/or preventing the progression of this devastating disease. A promising strategy to achieve prevention of this disease is to discover compounds that inhibit Abeta polymerization and deposition. Herein, we describe a new class of small molecules that inhibit Abeta aggregation, which is based on the chemical structure of apomorphine. These molecules were found to interfere with Abeta1-40 fibrillization as determined by transmission electron microscopy, Thioflavin T fluorescence and velocity sedimentation analytical ultracentrifugation studies. Using electron microscopy, time-dependent studies demonstrate that apomorphine and its derivatives promote the oligomerization of Abeta but inhibit its fibrillization. Preliminary structural activity studies demonstrate that the 10,11-dihydroxy substitutions of the D-ring of apomorphine are required for the inhibitory effectiveness of these aporphines, and methylation of these hydroxyl groups reduces their inhibitory potency. The ability of these small molecules to inhibit Abeta amyloid fibril formation appears to be linked to their tendency to undergo rapid autoxidation, suggesting that autoxidation product(s) acts directly or indirectly on Abeta and inhibits its fibrillization. The inhibitory properties of the compounds presented suggest a new class of small molecules that could serve as a scaffold for the design of more efficient inhibitors of Abeta amyloidogenesis in vivo.

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.