The stability and folding process of amyloidogenic mutant human lysozymes

Molecular insights into the phase transition of lysozyme into amyloid nanostructures: Implications of therapeutic strategies in diverse pathological conditions

Lysozyme, a well-known bacteriolytic enzyme, exhibits a fascinating yet complex behavior when it comes to protein aggregation. Under certain conditions, this enzyme undergoes flexible transformation, transitioning from partially unfolded intermediate units of native conformers into complex cross-β-rich nano fibrillar amyloid architectures. Formation of such lysozyme amyloids has been implicated in a multitude of pathological and medical severities, like hepatic dysfunction, hepatomegaly, splenic rupture as well as spleen dysfunction, nephropathy, sicca syndrome, renal dysfunction, renal amyloidosis, and systemic amyloidosis. In this comprehensive review, we have attempted to provide in-depth insights into the aggregating behavior of lysozyme across a spectrum of variables, including concentrations, temperatures, pH levels, and mutations. Our objective is to elucidate the underlying mechanisms that govern lysozyme's aggregation process and to unravel the complex interplay between its structural attributes. Moreover, this work has critically examined the latest advancements in the field, focusing specifically on novel strategies and systems, that have been implemented to delay or inhibit the lysozyme amyloidogenesis. Apart from this, we have tried to explore and advance our fundamental understanding of the complex processes involved in lysozyme aggregation. This will help the research community to lay a robust foundation for screening, designing, and formulating targeted anti-amyloid therapeutics offering improved treatment modalities and interventions not only for lysozyme-linked amyloidopathy but for a wide range of amyloid-related disorders.

Amyloids, amorphous aggregates and assemblies of peptides - Assessing aggregation

Amyloid and amorphous aggregates represent the two major categories of aggregates associated with diseases, and although exhibiting distinct features, researchers often treat them as equivalent, which demonstrates the need for more thorough characterization. Here, we compare amyloid and amorphous aggregates based on their biochemical properties, kinetics, and morphological features. To further decipher this issue, we propose the use of peptide self-assemblies as minimalistic models for understanding the aggregation process. Peptide building blocks are significantly smaller than proteins that participate in aggregation, however, they make a plausible means to bridge the gap in discerning the aggregation process at the more complex, protein level. Additionally, we explore the potential use of peptide-inspired models to research the liquid-liquid phase separation as a feasible mechanism preceding amyloid formation. Connecting these concepts can help clarify our understanding of aggregation-related disorders and potentially provide novel drug targets to impede and reverse these serious illnesses.

Two physics‐based models for pH ‐dependent calculations of protein solubility

When engineering a protein for its biological function, many physicochemical properties are also optimized throughout the engineering process, and the protein's solubility is among the most important properties to consider. Here, we report two novel computational methods to calculate the pH-dependent protein solubility, and to rank the solubility of mutants. The first is an empirical method developed for fast ranking of the solubility of a large number of mutants of a protein. It takes into account electrostatic solvation energy term calculated using Generalized Born approximation, hydrophobic patches, protein charge, and charge asymmetry, as well as the changes of protein stability upon mutation. This method has been tested on over 100 mutations for 17 globular proteins, as well as on 44 variants of five different antibodies. The prediction rate is over 80%. The antibody tests showed a Pearson correlation coefficient, R, with experimental data from .83 to .91. The second method is based on a novel, completely force-field-based approach using CHARMm program modules to calculate the binding energy of the protein to a part of the crystal lattice, generated from X-ray structure. The method predicted with very high accuracy the solubility of Ribonuclease SA and its 3K and 5K mutants as a function of pH without any parameter adjustments of the existing BIOVIA Discovery Studio binding affinity model. Our methods can be used for rapid screening of large numbers of design candidates based on solubility, and to guide the design of solution conditions for antibody formulation.

Characterisation of the structural, dynamic and aggregation properties of the W64R amyloidogenic variant of human lysozyme

The accumulation in vital organs of amyloid fibrils made of mutational variants of lysozyme (HuL) is associated with a human systemic amyloid disease. The detailed comparison of the in vitro properties of the I56T and D67H amyloidogenic variants to those of the T70N non-amyloidogenic variant and the wild-type (WT) protein suggested that the deposition of large amounts of aggregated disease-related lysozyme variants is initiated by the formation of transient intermediate species. The ability to populate such intermediates is essentially due to the destabilisation of the protein and the loss of the global structural cooperativity under physiologically relevant conditions. Here, we report the characterisation of a third naturally occurring amyloidogenic lysozyme variant, W64R, in comparison with the I56T and WT proteins. The X-ray crystal structure of the W64R variant at 1.15 Å resolution is very similar to that of the WT protein; a few interactions within the β-domain and at the interface between the α- and β-domains differ, however, from those in the WT protein. Consequently, the W64R mutation destabilizes the protein to an extent that is similar to that observed for the I56T and D67H mutations. The ΔG°_NU(H₂O) is reduced by 24 kJ·mol^-1 and the T_m is about 12 °C lower than that of the WT protein. Under native conditions, the W64R and I56T proteins are readily digested by proteinase K, while the WT protein remains intact. These results suggest that the two variant proteins transiently populate similar partially unfolded states in which proteinase K cleavage sites are accessible to the protease. Moreover, the in vitro aggregation properties of the W64R protein are similar to those of the I56T variant. Altogether, these results indicate that the properties of the W64R protein are astonishingly similar to those of the I56T variant. They further corroborate the idea that HuL variants associated with the disease are those whose stability and global structural cooperativity are sufficiently reduced to allow the formation of aggregation prone partially folded intermediates under physiological conditions.

M. Brito R. Structure and Aggregation Mechanisms in Amyloids

The aggregation of a polypeptide chain into amyloid fibrils and their accumulation and deposition into insoluble plaques and intracellular inclusions is the hallmark of several misfolding diseases known as amyloidoses. Alzheimer′s, Parkinson′s and Huntington’s diseases are some of the approximately 50 amyloid diseases described to date. The identification and characterization of the molecular species critical for amyloid formation and disease development have been the focus of intense scrutiny. Methods such as X-ray and electron diffraction, solid-state nuclear magnetic resonance spectroscopy (ssNMR) and cryo-electron microscopy (cryo-EM) have been extensively used and they have contributed to shed a new light onto the structure of amyloid, revealing a multiplicity of polymorphic structures that generally fit the cross-β amyloid motif. The development of rational therapeutic approaches against these debilitating and increasingly frequent misfolding diseases requires a thorough understanding of the molecular mechanisms underlying the amyloid cascade. Here, we review the current knowledge on amyloid fibril formation for several proteins and peptides from a kinetic and thermodynamic point of view, the structure of the molecular species involved in the amyloidogenic process, and the origin of their cytotoxicity.

Amyloidogenic lysozymes accumulate in the endoplasmic reticulum accompanied by the augmentation of ER stress signals

BACKGROUND

Naturally occurring single mutants, I56T, F57I, W64R and D67H of lysozyme in human, have been known to form abnormal protein aggregates (amyloid fibrils) and to accumulate in several organs, including the liver, spleen and kidney, resulting in familial systemic amyloidosis. These human pathogenic lysozyme variants are considered to raise subtle conformational changes compared to the wild type.

METHODS

Here we examined the effects of the aberrant mutant lysozymes I56T, F57I, W64R and D67H, each of which possesses a point mutation in its molecule, on a cultured human cell line, HEK293, in which the genes were individually integrated and overexpressed.

RESULTS

Western blot analyses showed lesser amounts of these variant proteins in the medium compared to the wild type, but they were abundant in the cell pellets, indicating that the modified lysozyme proteins were scarcely secreted into the medium but were retained in the cells. Immunocytochemistry revealed that these proteins resided in restricted regions which were stained by an endoplasmic reticulum (ER) marker. Moreover, the overexpression of the mutant lysozymes were accompanied by marked increases in XBP-1s and GRP78/BiP, which are downstream agents of the IRE1α signaling pathway responding to the unfolded protein response (UPR) upon ER stress. RNAi for the mutant lysozymes' expression greatly suppressed the increases of these agents.

CONCLUSIONS AND GENERAL SIGNIFICANCE

Our results suggest that the accumulation of pathogenic lysozymes in the ER caused ER stress and the UPR response mainly via the IRE1α pathway.

A nanobody binding to non-amyloidogenic regions of the protein human lysozyme enhances partial unfolding but inhibits amyloid fibril formation

We report the effects of the interaction of two camelid antibody fragments, generally called nanobodies, namely cAb-HuL5 and a stabilized and more aggregation-resistant variant cAb-HuL5G obtained by protein engineering, on the properties of two amyloidogenic variants of human lysozyme, I56T and D67H, whose deposition in vital organs including the liver, kidney, and spleen is associated with a familial non-neuropathic systemic amyloidosis. Both NMR spectroscopy and X-ray crystallographic studies reveal that cAb-HuL5 binds to the α-domain, one of the two lobes of the native lysozyme structure. The binding of cAb-HuL5/cAb-HuL5G strongly inhibits fibril formation by the amyloidogenic variants; it does not, however, suppress the locally transient cooperative unfolding transitions, characteristic of these variants, in which the β-domain and the C-helix unfold and which represents key early intermediate species in the formation of amyloid fibrils. Therefore, unlike two other nanobodies previously described, cAb-HuL5/cAb-HuL5G does not inhibit fibril formation via the restoration of the global cooperativity of the native structure of the lysozyme variants to that characteristic of the wild-type protein. Instead, it inhibits a subsequent step in the assembly of the fibrils, involving the unfolding and structural reorganization of the α-domain. These results show that nanobodies can protect against the formation of pathogenic aggregates at different stages in the structural transition of a protein from the soluble native state into amyloid fibrils, illustrating their value as structural probes to study the molecular mechanisms of amyloid fibril formation. Combined with their amenability to protein engineering techniques to improve their stability and solubility, these findings support the suggestion that nanobodies can potentially be developed as therapeutics to combat protein misfolding diseases.

Disease-related amyloidogenic variants of human lysozyme trigger the unfolded protein response and disturb eye development in Drosophila melanogaster

We have created a Drosophila model of lysozyme amyloidosis to investigate the in vivo behavior of disease-associated variants. To achieve this objective, wild-type (WT) protein and the amyloidogenic variants F57I and D67H were expressed in Drosophila melanogaster using the UAS-gal4 system and both the ubiquitous and retinal expression drivers Act5C-gal4 and gmr-gal4. The nontransgenic w(1118) Drosophila line was used as a control throughout. We utilized ELISA experiments to probe lysozyme protein levels, scanning electron microscopy for eye phenotype classification, and immunohistochemistry to detect the unfolded protein response (UPR) activation. We observed that expressing the destabilized F57I and D67H lysozymes triggers UPR activation, resulting in degradation of these variants, whereas the WT lysozyme is secreted into the fly hemolymph. Indeed, the level of WT was up to 17 times more abundant than the variant proteins. In addition, the F57I variant gave rise to a significant disruption of the eye development, and this correlated to pronounced UPR activation. These results support the concept that the onset of familial amyloid disease is linked to an inability of the UPR to degrade completely the amyloidogenic lysozymes prior to secretion, resulting in secretion of these destabilized variants, thereby leading to deposition and associated organ damage.

Population of nonnative states of lysozyme variants drives amyloid fibril formation

The propensity of protein molecules to self-assemble into highly ordered, fibrillar aggregates lies at the heart of understanding many disorders ranging from Alzheimer's disease to systemic lysozyme amyloidosis. In this paper we use highly accurate kinetic measurements of amyloid fibril growth in combination with spectroscopic tools to quantify the effect of modifications in solution conditions and in the amino acid sequence of human lysozyme on its propensity to form amyloid fibrils under acidic conditions. We elucidate and quantify the correlation between the rate of amyloid growth and the population of nonnative states, and we show that changes in amyloidogenicity are almost entirely due to alterations in the stability of the native state, while other regions of the global free-energy surface remain largely unmodified. These results provide insight into the complex dynamics of a macromolecule on a multidimensional energy landscape and point the way for a better understanding of amyloid diseases.

A non-natural variant of human lysozyme (I59T) mimics the in vitro behaviour of the I56T variant that is responsible for a form of familial amyloidosis

We report here the detailed characterisation of a non-naturally occurring variant of human lysozyme, I59T, which possesses a destabilising point mutation at the interface of the alpha- and beta-domains. Although more stable in its native structure than the naturally occurring variants that give rise to a familial form of systemic amyloidosis, I59T possesses many attributes that are similar to these disease-associated species. In particular, under physiologically relevant conditions, I59T populates transiently an intermediate in which a region of the structure unfolds cooperatively; this loss of global cooperativity has been suggested to be a critical feature underlying the amyloidogenic nature of the disease-associated lysozyme variants. In the present study, we have utilised this variant to provide direct evidence for the generic nature of the conformational transition that precedes the ready formation of the fibrils responsible for lysozyme-associated amyloid disease. This non-natural variant can be expressed at higher levels than the natural amyloidogenic variants, enabling, for example, singly isotopically labelled protein to be generated much more easily for detailed structural studies by multidimensional NMR spectroscopy. Moreover, we demonstrate that the I59T variant can readily form fibrils in vitro, similar in nature to those of the amyloidogenic I56T variant, under significantly milder conditions than are needed for the wild-type protein.

Impact of the native-state stability of human lysozyme variants on protein secretion by Pichia pastoris

We report the secreted expression by Pichia pastoris of two human lysozyme variants F57I and W64R, associated with systemic amyloid disease, and describe their characterization by biophysical methods. Both variants have a substantially decreased thermostability compared with wild-type human lysozyme, a finding that suggests an explanation for their increased propensity to form fibrillar aggregates and generate disease. The secreted yields of the F57I and W64R variants from P. pastoris are 200- and 30-fold lower, respectively, than that of wild-type human lysozyme. More comprehensive analysis of the secretion levels of 10 lysozyme variants shows that the low yields of these secreted proteins, under controlled conditions, can be directly correlated with a reduction in the thermostability of their native states. Analysis of mRNA levels in this selection of variants suggests that the lower levels of secretion are due to post-transcriptional processes, and that the reduction in secreted protein is a result of degradation of partially folded or misfolded protein via the yeast quality control system. Importantly, our results show that the human disease-associated mutations do not have levels of expression that are out of line with destabilizing mutations at other sites. These findings indicate that a complex interplay between reduced native-state stability, lower secretion levels, and protein aggregation propensity influences the types of mutation that give rise to familial forms of amyloid disease.

Structure of amyloid beta fragments in aqueous environments

Conformational studies on amyloid beta peptide (Abeta) in aqueous solution are complicated by its tendency to aggregate. In this study, we determined the atomic-level structure of Abeta(28-42) in an aqueous environment. We fused fragments of Abeta, residues 10-24 (Abeta(10-24)) or 28-42 (Abeta(28-42)), to three positions in the C-terminal region of ribonuclease HII from a hyperthermophile, Thermococcus kodakaraensis (Tk-RNase HII). We then examined the structural properties in an aqueous environment. The host protein, Tk-RNase HII, is highly stable and the C-terminal region has relatively little interaction with other parts. CD spectroscopy and thermal denaturation experiments demonstrated that the guest amyloidogenic sequences did not affect the overall structure of the Tk-RNase HII. Crystal structure analysis of Tk-RNase HII(1-197)-Abeta(28-42) revealed that Abeta(28-42) forms a beta conformation, whereas the original structure in Tk-RNase HII(1-213) was alpha helix, suggesting beta-structure formation of Abeta(28-42) within full-length Abeta in aqueous solution. Abeta(28-42) enhanced aggregation of the host protein more strongly than Abeta(10-24). These results and other reports suggest that after proteolytic cleavage, the C-terminal region of Abeta adopts a beta conformation in an aqueous environment and induces aggregation, and that the central region of Abeta plays a critical role in fibril formation. This study also indicates that this fusion technique is useful for obtaining structural information with atomic resolution for amyloidogenic peptides in aqueous environments.

Reduced global cooperativity is a common feature underlying the amyloidogenicity of pathogenic lysozyme mutations

One of the 20 or so human amyloid diseases is associated with the deposition in vital organs of full-length mutational variants of the antibacterial protein lysozyme. Here, we report experimental data that permit a detailed comparison to be made of the behaviour of two of these amyloidogenic variants, I56T and D67H, under identical conditions. Hydrogen/deuterium exchange experiments monitored by NMR and mass spectrometry reveal that, despite their different locations and the different effects of the two mutations on the structure of the native state of lysozyme, both mutations cause a cooperative destabilisation of a remarkably similar segment of the structure, comprising in both cases the beta-domain and the adjacent C-helix. As a result, both variant proteins populate transiently a closely similar, partially unstructured intermediate in which the beta-domain and the adjacent C-helix are substantially and simultaneously unfolded, whereas the three remaining alpha-helices that form the core of the alpha-domain still have their native-like structure. We show, in addition, that the binding of a camel antibody fragment, cAb-HuL6, which was raised against wild-type lysozyme, restores to both variant proteins the stability and cooperativity characteristic of the wild-type protein; as a consequence, it inhibits the formation of amyloid fibrils by both variants. These results indicate that the reduction in global cooperativity, and the associated ability to populate transiently a specific, partly unfolded intermediate state under physiologically relevant conditions, is a common feature underlying the behaviour of these two pathogenic mutations. The formation of intermolecular interactions between lysozyme molecules that are in this partially unfolded state is therefore likely to be the fundamental trigger of the aggregation process that ultimately leads to the formation and deposition in tissue of amyloid fibrils.

Structures of wild-type and P28L/Y173F tryptophan synthase alpha-subunits from Escherichia coli

The alpha-subunit of tryptophan synthase (alphaTS) catalyzes the cleavage of indole-3-glycerol phosphate to glyceraldehyde-3-phosphate and indole, which is used to yield the amino acid tryptophan in tryptophan biosynthesis. Here, we report the first crystal structures of wild-type and double-mutant P28L/Y173F alpha-subunit of tryptophan synthase from Escherichia coli at 2.8 and 1.8A resolution, respectively. The structure of wild-type alphaTS from E. coli was similar to that of the alpha(2)beta(2) complex structure from Salmonella typhimurium. As compared with both structures, the conformational changes are mostly in the interface of alpha- and beta-subunits, and the substrate binding region. Two sulfate ions and two glycerol molecules per asymmetric unit bind with the residues in the active sites of the wild-type structure. Contrarily, double-mutant P28L/Y173F structure is highly closed at the window for the substrate binding by the conformational changes. The P28L substitution induces the exposure of hydrophobic amino acids and decreases the secondary structure that causes the aggregation. The Y173F suppresses to transfer a signal from the alpha-subunit core to the alpha-subunit surface involved in interactions with the beta-subunit and increases structural stability.

Rapid unfolding of a domain populates an aggregation-prone intermediate that can be recognized by GroEL

Some amino acid substitutions in phage P22 coat protein cause a temperature-sensitive folding (tsf) phenotype. In vivo, these tsf amino acid substitutions cause coat protein to aggregate and form intracellular inclusion bodies when folded at high temperatures, but at low temperatures the proteins fold properly. Here the effects of tsf amino acid substitutions on folding and unfolding kinetics and the stability of coat protein in vitro have been investigated to determine how the substitutions change the ability of coat protein to fold properly. The equilibrium unfolding transitions of the tsf variants were best fit to a three-state model, N if I if U, where all species concerned were monomeric, a result confirmed by velocity sedimentation analytical ultracentrifugation. The primary effect of the tsf amino acid substitutions on the equilibrium unfolding pathway was to decrease the stability (DeltaG) and the solvent accessibility (m-value) of the N if I transition. The kinetics of folding and unfolding of the tsf coat proteins were investigated using tryptophan fluorescence and circular dichroism (CD) at 222 nm. The tsf amino acid substitutions increased the rate of unfolding by 8-14-fold, with little effect on the rate of folding, when monitored by tryptophan fluorescence. In contrast, when folding or unfolding reactions were monitored by CD, the reactions were too fast to be observed. The tsf coat proteins are natural substrates for the molecular chaperones, GroEL/S. When native tsf coat protein monomers were incubated with GroEL, they bound efficiently, indicating that a folding intermediate was significantly populated even without denaturant. Thus, the tsf coat proteins aggregate in vivo because of an increased propensity to populate this unfolding intermediate.

Amyloid protofilaments from the calcium-binding protein equine lysozyme: formation of ring and linear structures depends on pH and metal ion concentration

The calcium-binding equine lysozyme has been found to undergo conversion into amyloid fibrils during incubation in solution at acidic pH. At pH 4.5 and 57 degrees C, where equine lysozyme forms a partially unfolded molten globule state, the protein forms protofilaments with a width of ca. 2 nm. In the absence of Ca(2+) the protofilaments are present as annular structures with a diameter of 40-50 nm. In the presence of 10 mM CaCl(2) the protofilaments of equine lysozyme are straight or curved; they can assemble into thicker threads, but they do not appear to undergo circularisation. At pH 2.0, where the protein is more destabilised compared to pH 4.5, fibril formation occurs at 37 degrees C and 57 degrees C. At pH 2.0, both ring-shaped and linear protofilaments are formed, in which periodic repeats of ca 35 nm can be distinguished clearly. The rings constitute about 10% of all fibrillar species under these conditions and they are characterised by a larger diameter of 70-80 nm. All the structures bind Congo red and thioflavine T in a manner similar to fibrils associated with a variety of amyloid diseases. At pH 2.0, fibril formation is accompanied by some acidic hydrolysis, producing specific fragmentation of the protein, leading to the accumulation of two peptides in particular, consisting of residues 1-80 and 54-125. At the initial stages of incubation, however, full-length equine lysozyme represents the dominant species within the fibrils. We propose that the ring-shaped structures observed here, and in the case of disease-associated proteins such as alpha-synuclein, could be a second generic type of amyloid structure in addition to the more common linear fibrils.

Structural and folding dynamic properties of the T70N variant of human lysozyme

Definition of the transition mechanism from the native globular protein into fibrillar polymer was greatly improved by the biochemical and biophysical studies carried out on the two amyloidogenic variants of human lysozyme, I56T and D67H. Here we report thermodynamic and kinetic data on folding as well as structural features of a naturally occurring variant of human lysozyme, T70N, which is present in the British population at an allele frequency of 5% and, according to clinical and histopathological data, is not amyloidogenic. This variant is less stable than the wild-type protein by 3.7 kcal/mol, but more stable than the pathological, amyloidogenic variants. Unfolding kinetics in guanidine are six times faster than in the wild-type, but three and twenty times slower than in the amyloidogenic variants. Enzyme catalytic parameters, such as maximal velocity and affinity, are reduced in comparison to the wild-type. The solution structure, determined by 1H NMR and modeling calculations, exhibits a more compact arrangement at the interface between the beta-sheet domain and the subsequent loop on one side and part of the alpha domain on the other side, compared with the wild-type protein. This is the opposite of the conformational variation shown by the amyloidogenic variant D67H, but it accounts for the reduced stability and catalytic performance of T70N.

Mutations in the B1 domain of protein G that delay the onset of amyloid fibril formation in vitro

We previously reported that under certain experimental conditions, many variants of the B1 domain of IgG-binding protein G from Streptococcus form fibrils reproducibly. The variant I6T53 was the focus of the present study because the lag phase in the kinetics of fibril formation by this variant is significantly longer than that of other variants. This lag phase is distinguished by changes in both intrinsic fluorescence intensity and in light scattering of the protein. NMR diffusion measurements suggest that the soluble protein during the lag phase is monomeric. The kinetic profiles of fibril formation are found to depend on experimental conditions. The first kinetic phase diminishes almost completely when the reaction is seeded with preformed amyloid fibrils.

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.

Structure, stability, and aggregation of paired helical filaments from tau protein and FTDP-17 mutants probed by tryptophan scanning mutagenesis

By using tryptophan scanning mutagenesis, we observed the kinetics and structure of the polymerization of tau into paired helical filaments (PHFs) independently of exogenous reporter dyes. The fluorescence exhibits pronounced blue shifts due to burial of the residue inside PHFs, depending on Trp position. The effect is greatest near the center of the repeat domain, showing that the packing is tightest near the beta-structure inducing hexapeptide motifs. The tryptophan response allows measurement of PHF stability made by different tau isoforms and mutants. Unexpectedly, the stability of PHFs is quite low (denaturation half-points approximately 1.0 m GdnHCl), implying that incipient aggregation should be reversible and that the observed high stability of Alzheimer PHFs is due to other factors. The stability increases with the number of repeats and with tau mutants promoting beta-structure, arguing for a gain of toxic function in frontotemporal dementias. Fluorescence resonance energy transfer (FRET) was used to analyze the distances of Tyr(310) to tryptophans in different positions. The degree of FRET in the soluble protein was position-dependent, with highest signals within the second and third repeats but low or no signals further away. In PHFs most mutants showed FRET, indicating that tight packing results from assembly of tau into PHFs.

High hydrostatic pressure as a tool to study protein aggregation and amyloidosis

Aggregation of proteins is a serious problem, affecting both industrial production of proteins and human health. Despite recent advances in the theories and experimental techniques available to address understanding of protein aggregation processes, mechanisms of aggregate formation have proved challenging to study. This is in part because the typical irreversibility of protein aggregation processes at atmospheric conditions complicates analysis of their kinetics and thermodynamics. Because high hydrostatic pressures act to disfavor the hydrophobic and electrostatic interactions that cause protein aggregation, studies conducted under high hydrostatic pressures may allow protein aggregates to be formed reversibly, enabling thermodynamic and kinetic parameters to be measured in greater detail. Although application of high hydrostatic pressures to protein aggregation problems is rather recent, a growing literature, reviewed herein, suggests that high pressure may be a useful tool for both understanding protein aggregation and reversing it in industrial applications.