Ultrastructural organization of ex vivo amyloid fibrils formed by the apolipoprotein A-I Leu174Ser variant: an atomic force microscopy study

Improved folding of recombinant protein via co-expression of exogenous chaperones

Expression of heterologous genes in Escherichia coli is a routine technology for recombinant protein production, but the predictable recovery of properly folded and uniformly bioactive material remains a challenge. Misfolded proteins typically accumulate as insoluble inclusion bodies, and a variety of strategies have been employed in efforts to increase the yield of soluble product. One technique is the overexpression of E. coli protein chaperones during recombinant protein induction, in an effort to increase the folding capacity of the bacterial host. We have developed an alternative approach, by supplementing the host protein folding machinery with chaperones from other species. Extremophiles have evolved under conditions (extremes of temperature, salinity, pressure, and/or pH) that make them attractive candidates for possessing chaperones with novel folding activities. The green fluorescent protein (GFP) of Aequorea victoria, which is predominantly insoluble under typical recombinant expression culture conditions, was employed as an in vivo indicator of protein folding activity for chaperone homologs from a variety of extremophiles. For a subset of the chaperones tested, co-expression with GFP promoted an increase in both fluorescence signal intensity as well as the amount of GFP recovered in the soluble protein fraction. Several archaeal chaperones were also found to be able to refold soluble Lyt_Orn C40 peptidase from inclusion bodies in vitro. In particular, Pf Cpn(MA), a mutant chaperonin which exhibited significant refolding activity, is also shown to deconstruct the morphology and structure of inclusion bodies (Kurouski et al., 2012). Hence, the simple and rapid GFP assay provides a tool to screen for extremophilic chaperones that exhibit folding activity under E. coli growth conditions, and suggests that increasing the repertoire of heterologous chaperones might provide a partial but general solution to the problem of recombinant protein insolubility.

Protein Fibrillar Nanopolymers: Molecular-Level Insights into Their Structural, Physical and Mechanical Properties

Amyloid fibrils represent a generic class of mechanically strong and stable biomaterials with extremely advantageous properties. Although amyloids were initially associated only with severe neurological disorders, the role of these structures nowadays is shifting from health debilitating to highly beneficial both in biomedical and technological aspects. Intensive involvement of fibrillar assemblies into the wide range of pathogenic and functional processes strongly necessitate the molecular level characterization of the structural, physical and elastic features of protein nanofibrils. In the present contribution, we made an attempt to highlight the up-to-date progress in the understanding of amyloid properties from the polymer physics standpoint. The fundamental insights into protein fibril behavior are essential not only for development of therapeutic strategies to combat the protein misfolding disorders but also for rational and precise design of novel biodegradable protein-based nanopolymers.

Atomic force microscopy of ex vivo amyloid fibrils

Here, we report a study of ex vivo amyloid fibrils formed, respectively, by the Leu174Ser Apolipoprotein A-I (ApoA-I-LS) variant and by β2-microglobulin (β2-m) (Relini et al., J. Biol. Chem. 281:16521-16529, 2006; Relini et al., Biochim. Biophys. Acta 1690:33-41, 2004). In the work on ApoA-I-LS, the AFM has been used to characterize and compare the morphologies of amyloid fibrils isolated from two different patients, while in the study on β2-m our investigation provided important information about the factors that can promote the aggregation in vivo.

Methionine oxidation induces amyloid fibril formation by full-length apolipoprotein A-I

Apolipoprotein A-I (apoA-I) is the major protein component of HDL, where it plays an important role in cholesterol transport. The deposition of apoA-I derived amyloid is associated with various hereditary systemic amyloidoses and atherosclerosis; however, very little is known about the mechanism of apoA-I amyloid formation. Methionine residues in apoA-I are oxidized via several mechanisms in vivo to form methionine sulfoxide (MetO), and significant levels of methionine oxidized apoA-I (MetO-apoA-I) are present in normal human serum. We investigated the effect of methionine oxidation on the structure, stability, and aggregation of full-length, lipid-free apoA-I. Circular dichrosim spectroscopy showed that oxidation of all three methionine residues in apoA-I caused partial unfolding of the protein and decreased its thermal stability, reducing the melting temperature (T(m)) from 58.7 degrees C for native apoA-I to 48.2 degrees C for MetO-apoA-I. Analytical ultracentrifugation revealed that methionine oxidation inhibited the native self association of apoA-I to form dimers and tetramers. Incubation of MetO-apoA-I for extended periods resulted in aggregation of the protein, and these aggregates bound Thioflavin T and Congo Red. Inspection of the aggregates by electron microscopy revealed fibrillar structures with a ribbon-like morphology, widths of approximately 11 nm, and lengths of up to several microns. X-ray fibre diffraction studies of the fibrils revealed a diffraction pattern with orthogonal peaks at spacings of 4.64 A and 9.92 A, indicating a cross-beta amyloid structure. This systematic study of fibril formation by full-length apoA-I represents the first demonstration that methionine oxidation can induce amyloid fibril formation.

The (1-63) region of the p53 transactivation domain aggregates in vitro into cytotoxic amyloid assemblies

The transcriptional regulator p53 plays an essential role in tumor suppression. Accordingly, it is found mutated, and its activity is reduced, in many human cancers. Recent reports show that some cancers are characterized by a loss of function of wild-type p53, which, in several cases, accumulates in intracellular aggregates. Although the nature of such aggregates is still unclear, recent evidence indicates that the p53 C-terminal and core domains can undergo amyloid aggregation in vitro under mild denaturing conditions, although no information is available on the largely unstructured N-terminal transactivation domain. We therefore decided to investigate the amyloid propensity of the acidic unfolded 1-63 fragment of the transactivation domain, cloned, expressed, and purified from a bacterial strain. Here we show that, when exposed to acidic pH, the 1-63 fragment forms thioflavine T-positive aggregates whose amyloid nature was confirmed by Fourier transform infrared spectroscopy analysis, atomic force microscopy, and x-ray diffraction. These aggregates were shown to be cytotoxic to human SH-SY5Y cells by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide reduction, lactate dehydrogenase release, and caspase-3 activity assays. These results add new significant details to the picture describing the propensity of single domains of p53 to aggregate, further suggesting that, under suitable destabilizing conditions, the whole protein may aggregate into amyloid assemblies in vivo.

Nonnative protein polymers: structure, morphology, and relation to nucleation and growth

Thermally induced aggregates of alpha-chymotrypsinogen A and bovine granulocyte-colony stimulating factor in acidic solutions were characterized by a combination of static and dynamic light scattering, spectroscopy, transmission electron microscopy, and monomer loss kinetics. The resulting soluble, high-molecular weight aggregates (approximately 10(3)-10(5) kDa) are linear, semiflexible polymer chains that do not appreciably associate with one another under the conditions at which they were formed, with classic power-law scaling of the radius of gyration and hydrodynamic radius with weight-average molecular weight (M(w)). Aggregates in both systems are composed of nonnative monomers with elevated levels of beta-sheet secondary structure, and bind thioflavine T. In general, the aggregate size distributions showed low polydispersity by light scattering. Together with the inverse scaling of M(w) with protein concentration, the results clearly indicate that aggregation proceeds via nucleated (chain) polymerization. For alpha-chymotrypsinogen A, the scaling behavior is combined with the kinetics of aggregation to deduce separate values for the characteristic timescales for nucleation (tau(n)) and growth (tau(g)), as well as the stoichiometry of the nucleus (x). The analysis illustrates a general procedure to noninvasively and quantitatively determine tau(n), tau(g), and x for soluble (chain polymer) aggregates, as well as the relationship between tau(n)/tau(g) and aggregate M(w).

The workings of the amyloid diseases

The amyloidoses constitute a large group of diseases caused by an alteration in the conformation and metabolism of several globular proteins which, under particular conditions, deposit in tissues as insoluble fibrillar aggregates. To date, at least 24 different proteins have been recognized as causative agents of amyloid diseases. Despite a high heterogeneity in amino acid sequence, three-dimensional structure, and biological function, all amyloidogenic proteins share a reduced folding stability, a strong propensity to acquire more than one conformation, and the capacity to form almost indistinguishable amyloid fibrils. In some cases, the generation of an aggregation-prone state can be triggered or enhanced by the occurrence of mutations, a proteolytic cleavage, or a seeding process. The interaction between the amyloidogenic precursor, some common components of amyloid deposits, and the extra-cellular environment also plays a role in fibrillogenesis and in particular in the organ tropism of amyloid deposition. The process of amyloid fibril formation exerts a cytotoxic effect, resulting in tissue damage and organ dysfunction. Prefibrillar aggregates are thought to have an active part in this process. Due to the pathogenic complexity of amyloid diseases, the integration of several therapeutic interventions involving different critical levels of the amyloidogenic cascade is envisaged.

Amyloid fibrils formation and amorphous aggregation in concanavalin A

We here report an experimental study on the thermal aggregation process of concanavalin A, a protein belonging to the legume lectins family. The aggregation process and the involved conformational changes of the protein molecules were followed by means of fluorescence techniques, light scattering, circular dichroism, zeta potential measurements and atomic force microscopy. Our results show that the aggregation process of concanavalin A may evolve through two distinct pathways leading, respectively, to the formation of amyloids or amorphous aggregates. The relative extent of the two pathways is determined by pH, as amyloid aggregation is favored at high pH values ( approximately 9), while the formation of amorphous aggregates is favored at low pH ( approximately 5). At difference from amorphous aggregation, the formation of amyloid fibrils requires significant conformational changes on the protein, both at secondary and tertiary structural level. To our knowledge, this is the first observation of amyloid fibrils from concanavalin A.

Recombinant amyloidogenic domain of ApoA-I: analysis of its fibrillogenic potential

A variety of amyloid diseases are associated with fibrillar aggregates from N-terminal fragments of ApoA-I generated through a largely unexplored multi-step process. The understanding of the molecular mechanism is impaired by the lack of suitable amounts of the fibrillogenic polypeptides that could not be produced by recombinant methods so far. We report the production and the conformational analysis of recombinant ApoA-I 1-93 fragment. Similarly to the polypeptide isolated ex vivo, a pH switch from 7 to 4 induces a fast and reversible conformational transition to a helical state and leads to the identification of a key intermediate in the fibrillogenesis process. Limited proteolysis experiments suggested that the C-terminal region is involved in helix formation. The recombinant polypeptide generates fibrils at pH 4 on a time scale comparable with that of the native fragment. These findings open the way to studies on structural, thermodynamic, and kinetic aspects of ApoA-I fibrillogenesis.

Structure, function and amyloidogenic propensity of apolipoprotein A-I

Apolipoprotein A-I, the major structural apolipoprotein of high-density lipoproteins, efficiently protects humans from cholesterol accumulation in tissues; however, it can cause systemic amyloidosis in the presence of peculiar amino acid replacements. The wild-type molecule also has an intrinsic tendency to generate amyloid fibrils that localise within the atherosclerotic plaques. The structure, folding and metabolism of normal apolipoprotein A-I are extremely complex and as yet not completely clarified, but their understanding appears essential for the elucidation of the amyloid transition. We reviewed present knowledge on the structure, function and amyloidogenic propensity of apolipoprotein A-I with the aim of highlighting the possible molecular mechanisms that might contribute to the pathogenesis of this disease. Important clues on apolipoprotein A-I amyloidogenesis may be obtained from classical comparative studies of the properties of the wild-type versus the amyloidogenic counterpart. Additionally, in the case of apoA-I, further insights on the molecular mechanisms underlying its amyloidogenic propensity may derive from comparative studies between amyloidogenic variants and other mutations associated with hypoalphalipoproteinemia without amyloidosis.

Collagen Plays an Active Role in the Aggregation of β2-Microglobulin under Physiopathological Conditions of Dialysis-related Amyloidosis

Dialysis-related amyloidosis is characterized by the deposition of insoluble fibrils of beta(2)-microglobulin (beta(2)-m) in the musculoskeletal system. Atomic force microscopy inspection of ex vivo amyloid material reveals the presence of bundles of fibrils often associated to collagen fibrils. Aggregation experiments were undertaken in vitro with the aim of reproducing the physiopathological fibrillation process. To this purpose, atomic force microscopy, fluorescence techniques, and NMR were employed. We found that in temperature and pH conditions similar to those occurring in periarticular tissues in the presence of flogistic processes, beta(2)-m fibrillogenesis takes place in the presence of fibrillar collagen, whereas no fibrils are obtained without collagen. Moreover, the morphology of beta(2)-m fibrils obtained in vitro in the presence of collagen is extremely similar to that observed in the ex vivo sample. This result indicates that collagen plays a crucial role in beta(2)-m amyloid deposition under physiopathological conditions and suggests an explanation for the strict specificity of dialysis-related amyloidosis for the tissues of the skeletal system. We hypothesize that positively charged regions along the collagen fiber could play a direct role in beta(2)-m fibrillogenesis. This hypothesis is sustained by aggregation experiments performed by replacing collagen with a poly-L-lysine-coated mica surface. As shown by NMR measurements, no similar process occurs when poly-L-lysine is dissolved in solution with beta(2)-m. Overall, the findings are consistent with the estimates resulting from a simplified collagen model whereby electrostatic effects can lead to high local concentrations of oppositely charged species, such as beta(2)-m, that decay on moving away from the fiber surface.

Apolipoprotein structure and dynamics

PURPOSE OF REVIEW

This review highlights recent advances in structural studies of exchangeable human apolipoproteins and the insights they provide into lipoprotein action in cardiovascular and amyloid diseases.

RECENT FINDINGS

The high-resolution X-ray crystal structure of free apoA-II reveals a parallel helical array that may represent other lipid-poor apolipoproteins, and the structure in complex with detergent substantiates the belt model for the protein arrangement on lipoproteins. Nuclear magnetic resonance structures of apolipoprotein-detergent complexes show a repertoire of curved helical conformations, suggesting multiple helical arrangements on the lipid. Low-resolution spectroscopic analyses, interface studies and molecular modeling provide new insights into the 'hinge-domain' mechanism of apolipoprotein adaptation at variable lipoprotein surfaces. A kinetic mechanism for lipoprotein stabilization is proposed.

SUMMARY

Cumulative evidence supports the belt model that provides a general structural basis for understanding the molecular mechanisms of functional apolipoprotein reactions, such as binding to lipoprotein receptors, lipid transporters, and the activation of lipophilic enzymes. However, the detailed protein and lipid conformations on lipoproteins and the underlying molecular interactions are unclear. New insights will hopefully emerge once the first detailed lipoprotein structure is solved.