Apolipoprotein A-I induced amyloidosis

Leveraging knowledge of HDLs major protein ApoA1: Structure, function, mutations, and potential therapeutics

Apolipoprotein A1 (ApoA1) is a member of the Apolipoprotein family of proteins. It's a vital protein that helps in the production of high-density lipoprotein (HDL) particles, which are crucial for reverse cholesterol transport (RCT). It also has anti-inflammatory, anti-atherogenic, anti-apoptotic, and anti-thrombotic properties. These functions interact to give HDL particles their cardioprotective characteristics. ApoA1 has recently been investigated for its potential role in atherosclerosis, diabetes, neurological diseases, cancer, and certain infectious diseases. Since ApoA1's discovery, numerous mutations have been reported that affect its structural integrity and alter its function. Hence these insights have led to the development of clinically relevant peptides and synthetic reconstituted HDL (rHDL) that mimics the function of ApoA1. As a result, this review has aimed to provide an organized explanation of our understanding of the ApoA1 protein structure and its role in various essential pathways. Furthermore, we have comprehensively reviewed the important ApoA1 mutations (24 mutations) that are reported to be involved in various diseases. Finally, we've focused on the therapeutic potentials of some of the beneficial mutations, small peptides, and synthetic rHDL that are currently being researched or developed, since these will aid in the development of novel therapeutics in the future.

On the Aggregation of Apolipoprotein A-I

In vivo, apolipoprotein A-I (ApoA-I) is commonly found together with lipids in so-called lipoprotein particles. The protein has also been associated with several diseases—such as atherosclerosis and amyloidosis—where insoluble aggregates containing ApoA-I are deposited in various organs or arteries. The deposited ApoA-I has been found in the form of amyloid fibrils, suggesting that amyloid formation may be involved in the development of these diseases. In the present study we investigated ApoA-I aggregation into amyloid fibrils and other aggregate morphologies. We studied the aggregation of wildtype ApoA-I as well as a disease-associated mutant, ApoA-I K107Δ, under different solution conditions. The aggregation was followed using thioflavin T fluorescence intensity. For selected samples the aggregates formed were characterized in terms of size, secondary structure content, and morphology using circular dichroism spectroscopy, dynamic light scattering, atomic force microscopy and cryo transmission electron microscopy. We find that ApoA-I may form globular protein-only condensates, in which the α-helical conformation of the protein is retained. The protein in its unmodified form appears resistant to amyloid formation; however, the conversion into amyloid fibrils rich in β-sheet is facilitated by oxidation or mutation. In particular, the K107Δ mutant shows higher amyloid formation propensity, and the end state appears to be a co-existence of β-sheet rich amyloid fibrils and α-helix-rich condensates.

Colocalization of Apolipoprotein AI in Various Kinds of Systemic Amyloidosis

Apolipoprotein AI (apoAI), a major component of high-density lipoproteins, is one of the major amyloid fibril proteins and a minor constituent of the senile plaques observed in Alzheimer's disease. We examined colocalization of apoAI in various kinds of systemic amyloidosis in this study. Forty-three of 48 formalin-fixed paraffin-embedded heart specimens with various forms of systemic amyloidosis reacted immunohistochemically with anti-human apoAI antibody. ApoAI was also detected in water-extracted amyloid material by immunoblotting. In addition, we observed colocalization of apoAI and murine amyloid A (AA) amyloidosis in human apoAI transgenic mice. This is the first report of colocalization of apoAI with amyloid deposits in various forms of human systemic amyloidosis and murine AA amyloidosis in human apoAI transgenic mice. ApoAI may not always be a major component of amyloid fibrils, even when it is present in systemic amyloid deposits.

Vascular alterations in apolipoprotein A-I amyloidosis (Leu75Pro). A case-control study

BACKGROUND

Among hereditary amyloidoses, apolipoprotein A-I (APO A-I) amyloidosis (Leu75Pro) is a rare, autosomal dominant condition in which renal, hepatic, and testicular involvement has been demonstrated.

OBJECTIVE

To investigate vascular structural as well as functional alterations.

METHODS

In 131 carriers of the amyloidogenic Leu75Pro APO A-I mutation (mean age 52 + 16 years, 56 women) and in 131 subjects matched for age, sex, body mass index and clinic blood pressure (BP), arterial stiffness (pulse wave velocity, PWV) and carotid intima-media thickness (IMT) were measured.

RESULTS

By definition no differences for age, sex, body mass index, and BP were observed. Meanmax IMT (Mmax-IMT) in the common (CC), bifurcation (BIF) and internal (ICA) carotid artery were comparable in the two groups. After adjustment for high-density lipoprotein cholesterol and renal function differences between the two groups, a lower meanmax-IMT was observed in APO A-I Leu75Pro mutation carriers than in controls (CC Mmax-IMT 0.87 ± 0.21 versus 0.93 ± 0.2 mm, p = 0.07; BIF Mmax-IMT 1.19 ± 0.48 versus 1.36 ± 0.46 mm, p = 0.025; ICA Mmax-IMT 0.9 ± 0.37 versus 1.02 ± 0.35 mm, p = 0.028). On the other hand, aortic stiffness was significantly greater in patients with APO A-I amyloidosis than controls (PWV 11.5 ± 2.9 and 10.7 ± 2.3 m/s, p < 0.05), even after adjusting for confounders.

CONCLUSIONS

In carriers of the amyloidogenic Leu75Pro APO A-I mutation, a significant increase in arterial stiffness is observed; on the contrary, carotid artery IMT is comparable to that of control subjects. These results may add significant information to the clinical features of this rare genetic disorder.

Microenvironmentally controlled secondary structure motifs of apolipoprotein A-I derived peptides

The structure of apolipoprotein A-I (apoA-I), the major protein of HDL, has been extensively studied in past years. Nevertheless, its corresponding three-dimensional structure has been difficult to obtain due to the frequent conformational changes observed depending on the microenvironment. Although the function of each helical segment of this protein remains unclear, it has been observed that the apoA-I amino (N) and carboxy-end (C) domains are directly involved in receptor-recognition, processes that determine the diameter for HDL particles. In addition, it has been observed that the high structural plasticity of these segments might be related to several amyloidogenic processes. In this work, we studied a series of peptides derived from the N- and C-terminal domains representing the most hydrophobic segments of apoA-I. Measurements carried out using circular dichroism in all tested peptides evidenced that the lipid environment promotes the formation of α-helical structures, whereas an aqueous environment facilitates a strong tendency to adopt β-sheet/disordered conformations. Electron microscopy observations showed the formation of amyloid-like structures similar to those found in other well-defined amyloidogenic proteins. Interestingly, when the apoA-I peptides were incubated under conditions that promote stable globular structures, two of the peptides studied were cytotoxic to microglia and mouse macrophage cells. Our findings provide an insight into the physicochemical properties of key segments contained in apoA-I which may be implicated in disorder-to-order transitions that in turn maintain the delicate equilibrium between both, native and abnormal conformations, and therefore control its propensity to become involved in pathological processes.

Dual role of an N-terminal amyloidogenic mutation in apolipoprotein A-I: destabilization of helix bundle and enhancement of fibril formation

A number of naturally occurring mutations of apolipoprotein (apo) A-I, the major protein of HDL, are known to be associated with hereditary amyloidosis and atherosclerosis. Here, we examined the effects of the G26R point mutation in apoA-I (apoA-I(Iowa)) on the structure, stability, and aggregation propensity to form amyloid fibril of full-length apoA-I and the N-terminal fragment of apoA-I. Circular dichroism and fluorescence measurements demonstrated that the G26R mutation destabilizes the N-terminal helix bundle domain of full-length protein, leading to increased hydrophobic surface exposure, whereas it has no effect on the initial structure of the N-terminal 1-83 fragment, which is predominantly a random coil structure. Upon incubation for extended periods at neutral pH, the N-terminal 1-83 variants undergo a conformational change to β-sheet-rich structure with a great increase in thioflavin T fluorescence, whereas no structural change is observed in full-length proteins. Comparison of fibril-forming propensity among substituted mutants at Gly-26 position of 1-83 fragments demonstrated that the G26R mutation enhances the nucleation step of fibril formation, whereas G26K and G26E mutations have small or inhibiting effects on the formation of fibrils. These fibrils of the 1-83 variants have long and straight morphology as revealed by atomic force microscopy and exhibited significant toxicity with HEK293 cells. Our results indicate dual critical roles of the arginine residue at position 26 in apoA-I(Iowa): destabilization of the N-terminal helix bundle structure in full-length protein and enhancement of amyloid fibril formation by the N-terminal 1-83 fragment.

The "beta-clasp" model of apolipoprotein A-I--a lipid-free solution structure determined by electron paramagnetic resonance spectroscopy

Apolipoprotein A-I (apoA-I) is the major protein component of high density lipoproteins (HDL) and plays a central role in cholesterol metabolism. The lipid-free/lipid-poor form of apoA-I is the preferred substrate for the ATP-binding cassette transporter A1 (ABCA1). The interaction of apoA-I with ABCA1 leads to the formation of cholesterol laden high density lipoprotein (HDL) particles, a key step in reverse cholesterol transport and the maintenance of cholesterol homeostasis. Knowledge of the structure of lipid-free apoA-I is essential to understanding its critical interaction with ABCA1 and the molecular mechanisms underlying HDL biogenesis. We therefore examined the structure of lipid-free apoA-I by electron paramagnetic resonance spectroscopy (EPR). Through site directed spin label EPR, we mapped the secondary structure of apoA-I and identified sites of spin coupling as residues 26, 44, 64, 167, 217 and 226. We capitalize on the fact that lipid-free apoA-I self-associates in an anti-parallel manner in solution. We employed these sites of spin coupling to define the central plane in the dimeric apoA-I complex. Applying both the constraints of dipolar coupling with the EPR-derived pattern of solvent accessibility, we assembled the secondary structure into a tertiary context, providing a solution structure for lipid-free apoA-I. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).

Apolipoproteins and amyloid fibril formation in atherosclerosis

Amyloid fibrils arise from the aggregation of misfolded proteins into highly-ordered structures. The accumulation of these fibrils along with some non-fibrillar constituents within amyloid plaques is associated with the pathogenesis of several human degenerative diseases. A number of plasma apolipoproteins, including apolipoprotein (apo) A-I, apoA-II, apoC-II and apoE are implicated in amyloid formation or influence amyloid formation by other proteins. We review present knowledge of amyloid formation by apolipoproteins in disease, with particular focus on atherosclerosis. Further insights into the molecular mechanisms underlying their amyloidogenic propensity are obtained from in vitro studies which describe factors affecting apolipoprotein amyloid fibril formation and interactions. Additionally, we outline the evidence that amyloid fibril formation by apolipoproteins might play a role in the development and progression of atherosclerosis, and highlight possible molecular mechanisms that could contribute to the pathogenesis of this disease.

Novel N-terminal mutation of human apolipoprotein A-I reduces self-association and impairs LCAT activation

We have identified a novel mutation in apoA-I (serine 36 to alanine; S36A) in a human subject with severe hypoalphalipoproteinemia. The mutation is located in the N-terminal region of the protein, which has been implicated in several functions, including lipid binding and lecithin:cholesterol acyltransferase (LCAT) activity. In the present study, the S36A protein was produced recombinantly and characterized both structurally and functionally. While the helical content of the mutant protein was lower compared with wild-type (WT) apoA-I, it retained its helical character. The protein stability, measured as the resistance to guanidine-induced denaturation, decreased significantly. Interestingly, native gel electrophoresis, cross-linking, and sedimentation equilibrium analysis showed that the S36A mutant was primarily present as a monomer, notably different from the WT protein, which showed considerable oligomeric forms. Although the ability of S36A apoA-I to solubilize phosphatidylcholine vesicles and bind to lipoprotein surfaces was not altered, a significantly impaired LCAT activation compared with the WT protein was observed. These results implicate a region around S36 in apoA-I self-association, independent of the intact C terminus. Furthermore, the region around S36 in the N-terminus of human apoA-I is necessary for LCAT activation.

Interaction between the N- and C-terminal domains modulates the stability and lipid binding of apolipoprotein A-I

The tertiary structures of human and mouse apolipoprotein A-I (apoA-I) are comprised of an N-terminal helix bundle and a separate C-terminal domain. To define the possible intramolecular interaction between the N- and the C-terminal domains, we examined the effects on protein stability and lipid-binding properties of exchanging either the C-terminal domain or helix between human and mouse apoA-I. Chemical denaturation experiments demonstrated that replacement of the C-terminal domain or helical segment in human apoA-I with the mouse counterparts largely destabilizes the N-terminal helix bundle. Removal of the C-terminal domain or alpha-helix in human apoA-I had a similar effect on the destabilization of the helix bundle against urea denaturation, indicating that the C-terminal helical segment mainly contributes to stabilizing the N-terminal helix bundle structure in the apoA-I molecule. Consistent with this, KI quenching experiments indicated that removal or replacement of the C-terminal domain or helix in human apoA-I causes Trp residues in the N-terminal domain to become exposed to solvent. Measurements of the heats of binding to egg phosphatidylcholine (PC) vesicles and the kinetics of solubilization of dimyristoyl PC vesicles demonstrated that the destabilized human N-terminal helix bundle can strongly interact with lipids without the hydrophobic C-terminal helix. In addition, site-specific labeling of the N- and C-terminal helices by acrylodan to probe the conformational stability and the spatial proximity of the two domains indicated that the C-terminal helix is located near the N-terminal helix bundle, leading to a relatively less solvent-exposed, more organized conformation of the C-terminal domain. Taken together, these results suggest that interaction between the N- and C-terminal tertiary structure domains in apoA-I modulates the stability and lipid-binding properties of the N-terminal helix bundle.

Molecular and cellular aspects of protein misfolding and disease

Proteins are essential elements for life. They are building blocks of all organisms and the operators of cellular functions. Humans produce a repertoire of at least 30,000 different proteins, each with a different role. Each protein has its own unique sequence and shape (native conformation) to fulfill its specific function. The appearance of incorrectly shaped (misfolded) proteins occurs on exposure to environmental changes. Protein misfolding and the subsequent aggregation is associated with various, often highly debilitating, diseases for which no sufficient cure is available yet. In the first part of this review we summarize the structural composition of proteins and the current knowledge of underlying forces that lead proteins to lose their native structure. In the second and third parts we describe the molecular and cellular mechanisms that are associated with protein misfolding in disease. Finally, in the last part we portray recent efforts to develop treatments for protein misfolding diseases.

Phospholipid interaction induces molecular-level polymorphism in apolipoprotein C-II amyloid fibrils via alternative assembly pathways

A common feature of many of the most important and prominent amyloid-forming proteins is their ability to bind lipids and lipid complexes. Lipids are ubiquitous components of disease-associated amyloid plaques and deposits in humans, yet the specific roles of lipid in the process of amyloid fibril formation are poorly understood. This study investigated the effect of phospholipids on amyloid fibril formation by human apolipoprotein (apo) C-II using phosphatidylcholine derivatives comprising acyl chains of up to 14 carbon atoms. Submicellar concentrations of short-chain phospholipids increase the rate of apoC-II fibril formation in an acyl-chain-length- and concentration-dependent fashion, while high micellar concentrations of phospholipids completely inhibited amyloid formation. At lower concentrations of soluble phospholipid complexes, fibril formation by apoC-II was only partially inhibited, and under these conditions, aggregation followed a two-phase process. Electron microscopy showed that the fibrils resulting from the second phase of aggregation were straight, cablelike, and about 13 nm wide, in contrast to the homogeneous twisted-ribbon morphology of apoC-II fibrils formed under lipid-free conditions. Seeding experiments showed that this alternative fibril structure could be templated both in the presence and in the absence of lipid complex, suggesting that the two morphologies result from distinct assembly pathways. Circular dichroism spectroscopy studies indicated that the secondary structural conformation within the straight-type and ribbon-type fibrils were distinct, further suggesting divergent assembly pathways. These studies show that phospholipid complexes can change the structural architecture of mature fibrils and generate new fibril morphologies with the potential to alter the in vivo behaviour of amyloid. Such lipid interactions may play a role in defining the structural features of fibrils formed by diverse amyloidogenic proteins.

Mapping the structural transition in an amyloidogenic apolipoprotein A-I

The single amino acid mutation G26R in human apolipoprotein A-I (apoA-IIOWA) leads to the formation of beta-secondary structure rich amyloid fibrils in vivo. Here we show that full-length apoA-IIOWA has a decreased lipid-binding capability, an increased amino-terminal sensitivity to protease, and a propensity to form annular protofibrils visible by electron microscopy. The molecular basis for the conversion of apolipoprotein A-I to a proamyloidogenic form was examined by electron paramagnetic resonance spectroscopy. Our recent findings [Lagerstedt, J. O., Budamagunta, M. S., Oda, M. N., and Voss, J. C. (2007) J. Biol. Chem. 282, 9143-9149] indicate that Gly26 in the native apoprotein separates a preceding beta-strand structure (residues 20-25) from a downstream largely alpha-helical region. The current study demonstrates that the G26R variant promotes a structural transition of positions 27-56 to a mixture of coil and beta-strand secondary structure. Microscopy and staining by amyloidophilic dyes suggest that this alteration extends throughout the protein within 1 week of incubation in vitro, leading to insoluble aggregates of distinct morphology. The severe consequences of the Iowa mutation likely arise from the combination of losing the contribution of the native Gly residue in terminating beta-strand propagation and the promotion of beta-structure when an Arg is introduced adjacent to the succeeding residue of identical charge and size, Arg27.

A structural core within apolipoprotein C-II amyloid fibrils identified using hydrogen exchange and proteolysis

Plasma apolipoproteins show alpha-helical structure in the lipid-bound state and limited conformational stability in the absence of lipid. This structural instability of lipid-free apolipoproteins may account for the high propensity of apolipoproteins to aggregate and accumulate in disease-related amyloid deposits. Here, we explore the properties of amyloid fibrils formed by apolipoproteins using human apolipoprotein (apo) C-II as a model system. Hydrogen-deuterium exchange and NMR spectroscopy of apoC-II fibrils revealed core regions between residues 19-37 and 57-74 with reduced amide proton exchange rates compared to monomeric apoC-II. The C-terminal core region was also identified by partial proteolysis of apoC-II amyloid fibrils using endoproteinase GluC and proteinase K. Complete tryptic hydrolysis of apoC-II fibrils followed by centrifugation yielded a single peptide in the pellet fraction identified using mass spectrometry as apoC-II(56-76). Synthetic apoC-II(56-76) readily formed fibrils, albeit with a different morphology and thioflavinT fluorescence yield compared to full-length apoC-II. Studies with smaller peptides narrowed this fibril-forming core to a region within residues 60-70. We postulate that the ability of apoC-II(60-70) to independently form amyloid fibrils drives fibril formation by apoC-II. These specific amyloid-forming regions within apolipoproteins may underlie the propensity of apolipoproteins and their peptide derivatives to accumulate in amyloid deposits in vivo.

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.

Contributions of the N- and C-Terminal Helical Segments to the Lipid-Free Structure and Lipid Interaction of Apolipoprotein A-I†

The tertiary structure of lipid-free apolipoprotein (apo) A-I in the monomeric state comprises two domains: a N-terminal alpha-helix bundle and a less organized C-terminal domain. This study examined how the N- and C-terminal segments of apoA-I (residues 1-43 and 223-243), which contain the most hydrophobic regions in the molecule and are located in opposite structural domains, contribute to the lipid-free conformation and lipid interaction. Measurements of circular dichroism in conjunction with tryptophan and 8-anilino-1-naphthalenesulfonic acid fluorescence data demonstrated that single (L230P) or triple (L230P/L233P/Y236P) proline insertions into the C-terminal alpha helix disrupted the organization of the C-terminal domain without affecting the stability of the N-terminal helix bundle. In contrast, proline insertion into the N terminus (Y18P) disrupted the bundle structure in the N-terminal domain, indicating that the alpha-helical segment in this region is part of the helix bundle. Calorimetric and gel-filtration measurements showed that disruption of the C-terminal alpha helix significantly reduced the enthalpy and free energy of binding of apoA-I to lipids, whereas disruption of the N-terminal alpha helix had only a small effect on lipid binding. Significantly, the presence of the Y18P mutation offset the negative effects of disruption/removal of the C-terminal helical domain on lipid binding, suggesting that the alpha helix around Y18 concealed a potential lipid-binding region in the N-terminal domain, which was exposed by the disruption of the helix-bundle structure. When these results are taken together, they indicate that the alpha-helical segment in the N terminus of apoA-I modulates the lipid-free structure and lipid interaction in concert with the C-terminal domain.

Renal apolipoprotein A-I amyloidosis associated with a novel mutant Leu64Pro

Apolipoprotein A-I amyloidosis (Apo A-I) is an inherited systemic disease that results from the pathologic deposition in tissues throughout the body of fibrils composed of Apo A-I-related molecules. This disorder has been linked to mutations occurring within the coding region of the Apo A-I gene and, to date, 11 such substitutions have been documented. In 4 of these cases, the kidney was the target organ of the disease process. The authors report their studies on a patient with renal amyloidosis and a heretofore undescribed alteration in the amyloidogenic precursor protein. Analyses of genomic DNA evidenced a transition in the second base of codon 64 (T-->C) in one Apo A-I allele that resulted in the replacement of leucine by proline at position 64 (Leu64Pro). Additionally, fibrils extracted from the kidney and characterized chemically were found to be composed almost exclusively of an approximately 96-residue N-terminal Apo A-I fragment that contained the Leu64Pro substitution. These studies have provided further evidence for Apo A-I amyloidogenicity and the propensity of certain mutants to deposit in renal parenchyma.

Serum amyloid A protein forms a complex with a fragment of apolipoprotein A-I in the domestic blue fox: a protective mechanism against AA amyloidosis?

The spontaneous occurrence of protein AA-type of amyloidosis varies among animal species. As reactive AA-type of amyloidosis has never been detected in the blue fox, we obtained acute phase sera to search for amyloid-protective elements. The purified SAA fraction was characterized by mass and sequence analyses to disclose any unique domains in the amino acid sequence. The data revealed an SAA protein with heterogeneities in several positions, and showed the typical insertion between positions 69 and 70. By comparing the amino acid sequence with that from other mammals, no unique sequence could be observed. However, a C-terminal fragment of apolipoprotein A-I (ApoA-I) was found attached to the SAA. The amino acid sequence of the ApoA-I fragment revealed a partially blocked and ragged N-terminus. A comparison of the amino acid sequence of ApoA-I with that from the dog showed that the ApoA-I fragment started about position 190, had an intact C-terminus, and showed an identical sequence in all positions, except one. Based on the data, we suggest an interaction of the C-terminal fragment of ApoA-I with the SAA protein that inhibits the AA fibrillogenesis in the blue fox.

Liver biopsy discloses a new apolipoprotein A-I hereditary amyloidosis in several unrelated Italian families

BACKGROUND & AIMS

Hereditary systemic amyloidoses are autosomal dominant, late-onset disorders caused by mutations in the genes for a group of plasma proteins including transthyretin, lysozyme, fibrinogen Aalpha chain, gelsolin, apolipoprotein A-I, and apolipoprotein A-II. We investigated both phenotypic and genotypic aspects of apolipoprotein A-I amyloidosis unexpectedly disclosed by liver biopsy in 13 unrelated individuals with asymptomatic, persistent elevation of alkaline phosphatase and gamma-glutamyltransferase levels.

METHODS

Immunoelectron microscopy was used for in situ characterization of amyloid deposits on liver biopsy specimens. Mutation analysis was performed by sequencing of the apolipoprotein A-I gene in all patients. Wild-type/variant apolipoprotein A-I ratio in plasma high-density lipoproteins was assessed by a peptide mass fingerprinting approach after purification of total apolipoprotein A-I of 2 patients.

RESULTS

Family history was informative in 5 cases. Renal failure developed in 9 cases. Hypogonadism due to testicular involvement was observed. Amyloid fibrils specifically stained with anti-apolipoprotein A-I antibody. A novel (Leu75Pro) heterozygous mutation in the apolipoprotein A-I gene was present in affected individuals but not in controls. Variant apolipoprotein A-I was about 10% of the total protein in high-density lipoproteins.

CONCLUSIONS

The high number of individuals with apparently sporadic disease might reflect widespread occurrence of this mutation in the population and a milder phenotype of this variant compared with other apolipoprotein A-I amyloidogenic mutants. These findings suggest that specific staining for amyloid should be performed on liver biopsy of individuals with asymptomatic chronic elevation of alkaline phosphatase and gamma-glutamyltransferase levels.

Hepatic amyloidosis resulting from deposition of the apolipoprotein A-I variant Leu75Pro

Apolipoprotein A-I amyloidosis (AApo A-I) is an inherited systemic disease that results from pathologic deposition in tissues of fibrils composed of Apo A-I-related molecules. This disorder has been linked to mutations occurring within the coding region of the Apo A-I gene and heretofore, nine such variants had been described. Recently, a tenth alteration was found in an Italian population where the substitution of proline for leucine at position 75 (Leu75Pro) was associated with amyloid deposits in the liver. We now report our studies on a patient of different ethnicity who has hepatic amyloidosis and a similar mutation in the amyloidogenic precursor protein, as evidenced from analyses of genomic Apo A-I-encoding DNA. Additionally, fibrils extracted from the liver and characterized chemically were found to be composed almost exclusively of a approximately 96 residue N-terminal Apo A-I fragment that contained the Leu75Pro substitution. RFLP analyses revealed that the patient was heterozygous for this mutation; however, < 10% of the plasma Apo A-I consisted of the aberrant protein while the remainder had the normal (wild-type) sequence. Our findings provide further evidence that the Leu75Pro variant is associated with a predominant hepatic phenotype and can occur in individuals of diverse ethnic backgrounds.

The N-terminal globular domain and the first class A amphipathic helix of apolipoprotein A-I are important for lecithin:cholesterol acyltransferase activation and the maturation of high density lipoprotein in vivo

To investigate the role of the N terminus of apolipoprotein A-I (apoA-I) in the maturation of high density lipoproteins (HDL), two N-terminal mutants with deletions of residues 1-43 and 1-65 (referred to as Delta 1-43 and Delta 1-65 apoA-I) were studied. In vitro, these deletions had little effect on cellular cholesterol efflux from macrophages but LCAT activation was reduced by 50 and 70% for the Delta 1-43 and Delta 1-65 apoA-I mutants, respectively, relative to wild-type (Wt) apoA-I. To further define the role of the N terminus of apoA-I in HDL maturation, we constructed recombinant adenoviruses containing Wt apoA-I and two similar mutants with deletions of residues 7-43 and 7-65 (referred to as Delta 7-43 and Delta 7-65 apoA-I, respectively). Residues 1-6 were not removed in these mutants to allow proper cleavage of the pro-sequence in vivo. Following injection of these adenoviruses into apoA-I-deficient mice, plasma concentrations of both Delta 7-43 and Delta 7-65 apoA-I were reduced 4-fold relative to Wt apoA-I. The N-terminal deletion mutants, in particular Delta 7-65 apoA-I, were associated with greater proportions of pre beta-HDL and accumulated fewer HDL cholesteryl esters relative to Wt apoA-I. Wt and Delta 7-43 apoA-I formed predominantly alpha-migrating and spherical HDL, whereas Delta 7-65 apoA-I formed only pre beta-HDL of discoidal morphology. This demonstrates that deletion of the first class A amphipathic alpha-helix has a profound additive effect in vivo over the deletion of the globular domain alone (amino acids 1-43) indicating its important role in the production of mature alpha-migrating HDL. In summary, the combined in vitro and in vivo studies demonstrate a role for the N terminus of apoA-I in lecithin:cholesterol acyltransferase activation and the requirement of the first class A amphipathic alpha-helix for the maturation of HDL in vivo.

A study of apolipoproteins E and A-I in cutaneous amyloids

BACKGROUND

Apolipoprotein E (apoE) is present in a variety of biochemically different amyloid deposits, including Alzheimer's disease, systemic amyloidosis and primary cutaneous amyloidosis (PCA). Among the three closely related alleleic forms of apoE, the epsilon4 allele is linked to Alzheimer's disease. Apolipoprotein A-I (apoA-I), another apolipoprotein, is also found in senile plaques of Alzheimer's disease and in amyloid of aortic atherosclerotic plaques. Furthermore, apoA-I has recently been found to be associated with hereditary cutaneous and cardiac amyloidosis.

OBJECTIVES

To determine whether the apoE epsilon4 allele is associated with increased risk of PCA and whether apoE and apoA-I are present in PCA and common secondary cutaneous amyloidosis (SCA) (i.e. basal cell carcinoma, Bowen's disease and seborrhoeic keratosis).

METHODS

We examined the apoE genotype in 57 Chinese patients with PCA and 58 normal healthy control subjects of similar age. In addition, immunohistochemical staining was performed to determine the localization of apoE and apoA-I in skin tissues from 15 patients with SCA and 15 with PCA.

RESULTS

The frequency of the epsilon4 allele in the PCA group was not significantly higher than that in the control group (8.8% vs. 6.9%, P > 0.05). ApoE was present in amyloid deposits in both PCA and SCA, but apoA-I was not detected in these cutaneous amyloid deposits.

CONCLUSIONS

ApoE is also a component of amyloid deposits in SCA. Although the genetic susceptibility of certain apoE isoforms may not be a crucial factor in the development of PCA and, although apoA-I is not associated with amyloid deposits of PCA and SCA, the role of apolipoproteins in amyloidogenesis deserves further scrutiny.

Structural diversity of ex vivo amyloid fibrils studied by cryo-electron microscopy

Cryo-electron microscopy studies are presented on amyloid fibrils isolated from amyloidotic organs of two patients with different forms of hereditary non-neuropathic systemic amyloidosis, caused, respectively, by Leu60Arg apolipoprotein AI and Asp67His lysozyme. Although ex vivo amyloid fibrils were thought to be more uniform in structure than those assembled in vitro, our findings show that these fibrils are also quite variable in structure. Structural disorder and variability of the fibrils have precluded three-dimensional reconstruction, but averaged cryo-electron microscopy images suggest models for protofilament packing in the lysozyme fibrils. We conclude that ex vivo amyloid fibrils, although variable, assemble as characteristic structures according to the identity of the precursor protein.

Livers from patients with apolipoprotein A-I amyloidosis are not suitable as "domino" donors

Orthotopic liver transplantation, by eliminating the major site of amyloidogenic protein synthesis, is currently the only definitive treatment of most hereditary amyloidoses. Because of the minimal parenchymal involvement, the explanted livers from familial amyloidotic polyneuropathy (FAP) patients have been transplanted into non-FAP patients in a "domino" fashion. The aim of this study was to evaluate the extent of amyloid deposits in explanted livers from two patients with apolipoprotein A-I amyloidosis, with the Arg26 mutation, to determine their suitability as domino donors. A detailed histologic review of the explanted livers from two patients was performed and assessed for the extent of amyloid deposition by routine and Congo red stains. Both patients had identical histopathologic features. The liver parenchymal involvement was strikingly severe. Large patches of amyloid separated hepatic cords, with accentuation around the central veins. All portal triads were consistently and markedly involved with amorphous eosinophilic deposits within the connective tissue compressing the bile ducts and vascular structures. Hilar vessels had patchy deposits. No involvement of hilar nerve branches was seen. The hepatic parenchyma is extensively involved in hereditary Apolipoprotein A-I amyloidosis, with the Arg26 mutation. These livers, removed at orthotopic liver transplantation, are not suitable for domino donation.

Plasma factors controlling atrial natriuretic peptide (ANP) aggregation: role of lipoproteins

We have previously shown that human plasma atrial alpha-natriuretic peptide (alpha-hANP) sequestering is a protective phenomenon against amyloid aggregation. In the present work, the possible role of lipoproteins as alpha-hANP binding factors has been investigated in vitro using an experimental model, developed in our laboratory, that allows to work at physiological concentrations. This approach consists of gel filtration on Sephacryl S-300 HR of big alpha-[(125)I]hANP generated in phosphate buffered saline or in human normal plasma supplemented or not with lipoproteins. The results of these experiments indicate that high density lipoproteins (HDL) are responsible for the ANP binding phenomenon observed in vitro, while low density lipoproteins and very low density lipoproteins do not directly interact with ANP. Moreover, the HDL remodeling process occurring in vitro has been analyzed during plasma incubation by monitoring the redistribution of lipids and apolipoproteins among the HDL subclasses. The changes in HDL size and composition observed in incubated plasma were compared with the redistribution of endogenous and labeled big ANP. The obtained results revealed that both tend to follow the molecular rearrangement in plasma of apolipoprotein A-I containing particles and suggested that, among HDL species, the small particles are mainly involved in the ANP binding phenomenon. This hypothesis was further demonstrated by ligand blotting experiments that confirmed the existence of differences in the ability of HDL particles to bind alpha-[(125)I]hANP.

Structural models of human apolipoprotein A-I: a critical analysis and review

Human apolipoprotein (apo) A-I has been the subject of intense investigation because of its well-documented anti-atherogenic properties. About 70% of the protein found in high density lipoprotein complexes is apo A-I, a molecule that contains a series of highly homologous amphipathic alpha-helices. A number of significant experimental observations have allowed increasing sophisticated structural models for both the lipid-bound and the lipid-free forms of the apo A-I molecule to be tested critically. It seems clear, for example, that interactions between amphipathic domains in apo A-I may be crucial to understanding the dynamic nature of the molecule and the pathways by which the lipid-free molecule binds to lipid, both in a discoidal and a spherical particle. The state of the art of these structural studies is discussed and placed in context with current models and concepts of the physiological role of apo A-I and high-density lipoprotein in atherosclerosis and lipid metabolism.

Apolipoprotein A-I-derived amyloid in atherosclerosis. Its association with plasma levels of apolipoprotein A-I and cholesterol

Wild-type apolipoprotein A-I (apo A-I)-derived amyloid commonly occurs in atherosclerotic plaques. To clarify apo A-I amyloid formation, plasma levels of apo A-I and cholesterol were related to the presence of amyloid in atherosclerotic plaques in 15 patients with peripheral atherosclerosis, subjected to arterial reconstruction. Plasma levels of apo A-I and high-density lipoprotein (HDL) cholesterol were slightly higher in patients with apo A-I-derived amyloid than in those without, but the difference was not significant. Levels of low-density lipoprotein cholesterol and total cholesterol were significantly higher in the group with amyloid. High concentrations of apo A-I in the arterial intima are probably of greater importance to amyloid formation than high plasma levels of the protein. During atherosclerosis, the acute phase reactant serum amyloid A may displace apo A-I from HDL, leading to increased concentration of lipid-free apo A-I in the intima and conformational changes of apo A-I, which make it more fibrillogenic. Some forms of amyloid fibrils have been shown to be cytotoxic. Apo A-I-derived amyloid is possibly a pathogenically important factor in atherosclerosis.

Amyloid fibrils derived from the apolipoprotein A1 Leu174Ser variant contain elements of ordered helical structure

We recently described a new apolipoprotein A1 variant presenting a Leu174Ser replacement mutation that is associated with a familial form of systemic amyloidosis displaying predominant heart involvement. We have now identified a second unrelated patient with very similar clinical presentation and carrying the identical apolipoprotein A1 mutation. In this new patient the main protein constituent of the amyloid fibrils is the polypeptide derived from the first 93 residues of the protein, the identical fragment to that found in the patient previously described to carry this mutation. The X-ray fiber diffraction pattern obtained from preparations of partially aligned fibrils displays the cross-beta reflections characteristic of all amyloid fibrils. In addition to these cross-beta reflections, other reflections suggest the presence of well-defined coiled-coil helical structure arranged with a defined orientation within the fibrils. In both cases the fibrils contain a trace amount of full-length apolipoprotein A1 with an apparent prevalence of the wild-type species over the variant protein. We have found a ratio of full-length wild-type to mutant protein in plasma HDL of three to one. The polypeptide 1--93 purified from natural fibrils can be solubilized in aqueous solutions containing denaturants, and after removal of denaturants it acquires a monomeric state that, based on CD and NMR studies, has a predominantly random coil structure. The addition of phospholipids to the monomeric form induces the formation of some helical structure, thought most likely to occur at the C-terminal end of the polypeptide.

The protofilament substructure of amyloid fibrils

Tissue deposition of normally soluble proteins, or their fragments, as insoluble amyloid fibrils causes the usually fatal, acquired and hereditary systemic amyloidoses and is associated with the pathology of Alzheimer's disease, type 2 diabetes and the transmissible spongiform encephalopathies. Although each type of amyloidosis is characterised by a specific amyloid fibril protein, the deposits share pathognomonic histochemical properties and the structural morphology of all amyloid fibrils is very similar. We have previously demonstrated that transthyretin amyloid fibrils contain four constituent protofilaments packed in a square array. Here, we have used cross-correlation techniques to average electron microscopy images of multiple cross-sections in order to reconstruct the sub-structure of ex vivo amyloid fibrils composed of amyloid A protein, monoclonal immunoglobulin lambda light chain, Leu60Arg variant apolipoprotein AI, and Asp67His variant lysozyme, as well as synthetic fibrils derived from a ten-residue peptide corresponding to the A-strand of transthyretin. All the fibrils had an electron-lucent core but the packing arrangement comprised five or six protofilaments rather than four. The structural similarity that defines amyloid fibres thus exists principally at the level of beta-sheet folding of the polypeptides within the protofilament, while the different types vary in the supramolecular assembly of their protofilaments.

Human apolipoprotein C-II forms twisted amyloid ribbons and closed loops

Human apolipoprotein C-II (apoC-II) self-associates in solution to form aggregates with the characteristics of amyloid including red-green birefringence in the presence of Congo Red under cross-polarized light, increased fluorescence in the presence of thioflavin T, and a fibrous structure when examined by electron microscopy. ApoC-II was expressed and purified from Escherichia coli and rapidly exchanged from 5 M guanidine hydrochloride into 100 mM sodium phosphate, pH 7.4, to a final concentration of 0.3 mg/mL. This apoC-II was initially soluble, eluting as low molecular weight species in gel filtration experiments using Sephadex G-50. Circular dichroism (CD) spectroscopy indicated predominantly unordered structure. Upon incubation for 24 h, apoC-II self-associated into high molecular weight aggregates as indicated by elution in the void volume of a Sephadex G-50 column, by rapid sedimentation in an analytical ultracentrifuge, and by increased light scattering. CD spectroscopy indicated an increase in beta-sheet content, while fluorescence emission spectroscopy of the single tryptophan revealed a blue shift and an increase in maximum intensity, suggesting repositioning of the tryptophan into a less polar environment. Electron microscopy of apoC-II aggregates revealed a novel looped-ribbon morphology (width 12 nm) and several isolated closed loops. Like all of the conserved plasma apolipoproteins, apoC-II contains amphipathic helical regions that account for the increase in alpha-helix content on lipid binding. The increase in beta-structure accompanying apoC-II fibril formation points to an alternative folding pathway and an in vitro system to explore the general tendency of apolipoproteins to form amyloid in vivo.

The new apolipoprotein A-I variant leu(174) --> Ser causes hereditary cardiac amyloidosis, and the amyloid fibrils are constituted by the 93-residue N-terminal polypeptide

We identified a novel missense mutation in the apolipoprotein A-I gene, T2069C Leu(174) --> Ser, in a patient affected by familial systemic nonneuropathic amyloidosis. The amyloid deposits mostly affected the heart of the proband, who underwent transplantation for end-stage congestive heart failure. Amyloid fibrils of myocardial and periumbilical fat samples immunoreacted exclusively with anti-ApoA-I antibodies. Amyloid fibrils extracted from the heart were constituted, according to amino acid sequencing and mass spectrometry analysis, by an amino-terminal polypeptide ending at Val(93) of apolipoprotein A-I (apoA-I); no other significant fragments were detected. The mutation segregates with the disease; it was demonstrated in the proband and in an affected uncle and excluded in three healthy siblings. The plasma levels of high-density lipoprotein and apoA-I were significantly lower in the patient than in unaffected individuals. This represents the first case of familial apoA-I amyloidosis in which the mutation is outside the polypeptide fragment deposited as fibrils. Visualization of the mutation in the three-dimensional structure of lipid-free apoA-I, composed of four identical polypeptide chains, indicates that position 174 of one chain is located near position 93 of an adjacent chain and suggests that the amino acid replacement in position 174 is permissive for a proteolytic split at the C-terminal of Val(93).