Isolation and characterization of recombinant human apolipoprotein C-II expressed in Escherichia coli

The effects of ezetimibe and/or orlistat on triglyceride-rich lipoprotein metabolism in obese hypercholesterolemic patients

We investigated the factors influencing triglycerides (TG) reduction during ezetimibe, alone or combined with orlistat, administration. Eighty-six obese hypercholesterolemic subjects were prescribed a low-fat diet and were randomized to ezetimibe (E group), orlistat (O group), or both (OE group) for 6 months. Plasma TG and apolipoprotein (apo) C-III reduction was significantly greater in the combination group compared with monotherapy. Multivariate analysis showed that in E group apoC-III reduction and baseline TG levels were independently positively correlated, whereas baseline apoC-II levels were negatively correlated, with TG lowering. In OE group apoC-III reduction was the only independent contributor to TG reduction.

Methionine oxidation inhibits assembly and promotes disassembly of apolipoprotein C-II amyloid fibrils

Methionine residues are linked to the pathogenicity of several amyloid diseases; however, the mechanism of this relationship is largely unknown. These diseases are characterized, in vivo, by the accumulation of insoluble proteinaceous plaques, of which the major constituents are amyloid fibrils. In vitro, methionine oxidation has been shown to modulate fibril assembly in several well-characterized amyloid systems. Human apolipoprotein (apo) C-II contains two methionine residues (Met-9 and Met-60) and readily self-assembles in vitro to form homogeneous amyloid fibrils, thus providing a convenient system to examine the effect of methionine oxidation on amyloid fibril formation and stability. Upon oxidation of the methionine residues of apoC-II with hydrogen peroxide, fibril formation was inhibited. Oxidized apoC-II molecules did not inhibit native apoC-II assembly, indicating that the oxidized molecules had a reduced ability to interact with the growing fibrils. Single Met-Val substitutions were performed and showed that oxidation of Met-60 had a more significant inhibitory effect than oxidation of Met-9. In addition, Met-Gln substitutions designed to mimic the effect of oxidation on side chain hydrophilicity showed that a change in hydrophobicity at position 60 within the core region of the fibril had a potent inhibitory effect. The oxidation of preformed apoC-II fibrils caused their dissociation; however, mutants in which the Met-60 was substituted with a valine were protected from this peroxide-induced dissociation. This work highlights an important role for methionine in the formation of amyloid fibril structure and gives new insight into how oxidation affects the stability of mature fibrils.

Serpin Acceleration of Amyloid Fibril Formation: A Role for Accessory Proteins

Protein aggregation underlies an increasing number of human diseases. Recent experiments have shown that the aggregation reaction is exquisitely specific involving particular interactions between non-native proteins. However, aggregation of certain proteins, for example beta-amyloid, in vivo leads to the recruitment of other proteins into the aggregate. Antichymotrypsin, a non-fibril forming protein, is always observed to be associated with beta-amyloid plaques in Alzheimer's sufferers. The role of antichymotrypsin is controversial with studies showing it can either accelerate or inhibit the aggregation reaction. To investigate the role of antichymotrypsin in fibrillogenesis we have studied its interaction with apolipoprotein C-II, a well characterized model system for the study of fibrillogenesis. Our data demonstrate that sub-stoichiometric amounts of antichymotrypsin and its alternate structural forms can dramatically accelerate the aggregation of apolipoprotein C-II, whereas the presence of alpha(1)-antitrypsin, a structural homologue of antichymotrypsin, cannot. Sedimentation velocity experiments show more apolipoprotein C-II fibrils were formed in the presence of antichymotrypsin. Using pull-down assays and immuno-gold labeling we demonstrate an interaction between antichymotrypsin and apolipoprotein C-II fibrils that specifically occurs during fibrillogenesis. Taken together these data demonstrate an interaction between antichymotrypsin and apolipoprotein C-II that accelerates fibrillogenesis and indicates a specific role for accessory proteins in protein aggregation.

Apolipoprotein C-II is a novel substrate for matrix metalloproteinases

We previously reported an efficient proteomic approach to identify matrix metalloproteinase (MMP) substrates from complex protein mixture. Using the proteomic approach, apolipoprotein C-II (apoC-II), which is a cofactor of lipoprotein lipase (LPL) and a component of very-low density lipoprotein and chylomicron, has been identified as a putative MMP-14 substrate. Cleavage of apoC-II, with various MMPs, demonstrated that apoC-II is cleaved most efficiently by MMP-14, and also by MMP-7, among the tested MMPs. The 79-amino acid residue apoC-II was cleaved between Asn35 and Leu36 by MMP-14, and between Phe14 and Leu15 and between Asn35 and Leu36 by MMP-7. Cleavage of apoC-II by MMP-14 markedly decreased LPL activity and would thus impair hydrolysis of triglycerides in plasma and transfer of fatty acids to tissues. Our result suggests that cleavage of apoC-II by MMPs would be important for development of pathophysiological situations of apoC-II deficiency such as atherosclerosis.

Optimization of the primary recovery of human interferon alpha2b from Escherichia coli inclusion bodies

The human interferon alpha2b (hu-IFNalpha2b) gene was cloned in Escherichia coli JM109(DE3) and the recombinant protein was expressed as cytoplasmic inclusion bodies (IB). The present work discusses the recovery of hu-IFNalpha2b IB from the E. coli cells. An optimized protocol is proposed based on the sequential evaluation of recovery steps and parameters: (i) cell disruption, (ii) IB recovery and separation from cell debris, (iii) IB washing, and (iv) IB solubilization. Parameters such as hu-IFNalpha2b purity and recovery yield were measured after each step. The optimized recovery protocol yielded 60% of hu-IFNalpha2b with a purity of up to 80%. The protein was renatured at high concentration after recovery and it was found to display biological activity.

The circularization of amyloid fibrils formed by apolipoprotein C-II

Amyloid fibrils have historically been characterized by diagnostic dye-binding assays, their fibrillar morphology, and a "cross-beta" x-ray diffraction pattern. Whereas the latter demonstrates that amyloid fibrils have a common beta-sheet core structure, they display a substantial degree of morphological variation. One striking example is the remarkable ability of human apolipoprotein C-II amyloid fibrils to circularize and form closed rings. Here we explore in detail the structure of apoC-II amyloid fibrils using electron microscopy, atomic force microscopy, and x-ray diffraction studies. Our results suggest a model for apoC-II fibrils as ribbons approximately 2.1-nm thick and 13-nm wide with a helical repeat distance of 53 nm +/- 12 nm. We propose that the ribbons are highly flexible with a persistence length of 36 nm. We use these observed biophysical properties to model the apoC-II amyloid fibrils either as wormlike chains or using a random-walk approach, and confirm that the probability of ring formation is critically dependent on the fibril flexibility. More generally, the ability of apoC-II fibrils to form rings also highlights the degree to which the common cross-beta superstructure can, as a function of the protein constituent, give rise to great variation in the physical properties of amyloid fibrils.

Non-fibrillar Components of Amyloid Deposits Mediate the Self-association and Tangling of Amyloid Fibrils

Amyloid deposits are proteinaceous extra-cellular aggregates associated with a diverse range of disease states. These deposits are composed predominantly of amyloid fibrils, the unbranched, beta-sheet rich structures that result from the misfolding and subsequent aggregation of many proteins. In addition, amyloid deposits contain a number of non-fibrillar components that interact with amyloid fibrils and are incorporated into the deposits in their native folded state. The influence of a number of the non-fibrillar components in amyloid-related diseases is well established; however, the mechanisms underlying these effects are poorly understood. Here we describe the effect of two of the most important non-fibrillar components, serum amyloid P component and apolipoprotein E, upon the solution behavior of amyloid fibrils in an in vitro model system. Using analytical ultracentrifugation, electron microscopy, and rheological measurements, we demonstrate that these non-fibrillar components cause soluble fibrils to condense into localized fibrillar aggregates with a greatly enhanced local density of fibril entanglements. These results suggest a possible mechanism for the observed role of non-fibrillar components as mediators of amyloid deposition and deposit stability.

Fibrillar amyloid protein present in atheroma activates CD36 signal transduction

The self-association of proteins to form amyloid fibrils has been implicated in the pathogenesis of a number of diseases including Alzheimer's, Parkinson's, and Creutzfeldt-Jakob diseases. We recently reported that the myeloid scavenger receptor CD36 initiates a signaling cascade upon binding to fibrillar beta-amyloid that stimulates recruitment of microglia in the brain and production of inflammatory mediators. This receptor plays a key role in the pathogenesis of atherosclerosis, prompting us to evaluate whether fibrillar proteins were present in atherosclerotic lesions that could initiate signaling via CD36. We show that apolipoprotein C-II, a component of very low and high density lipoproteins, readily forms amyloid fibrils that initiate macrophage inflammatory responses including reactive oxygen production and tumor necrosis factor alpha expression. Using macrophages derived from wild type and Cd36(-/-) mice to distinguish CD36-specific events, we show that fibrillar apolipoprotein C-II activates a signaling cascade downstream of this receptor that includes Lyn and p44/42 MAPKs. Interruption of this signaling pathway through targeted deletion of Cd36 or blocking of p44/42 MAPK activation inhibits macrophage tumor necrosis factor alpha gene expression. Finally, we demonstrate that apolipoprotein C-II in human atheroma co-localizes to regions positive for markers of amyloid and macrophage accumulation. Together, these data characterize a CD36-dependent signaling cascade initiated by fibrillar amyloid species that may promote atherogenesis.

Sedimentation velocity analysis of flexible macromolecules: self-association and tangling of amyloid fibrils

A novel bead modeling technique has been developed for the analysis of the sedimentation velocity behavior of flexible fibrils. The method involves the generation of a family of bead models representing a sample of the conformations available to the molecule and the calculation of the sedimentation coefficients of these models by established techniques. This approach has been used to investigate the size distribution of amyloid fibrils formed by human apolipoprotein C-II (apoC-II). ApoC-II fibrils have a simple and homogeneous ribbon morphology with no evidence of amorphous aggregation. Freshly prepared apoC-II forms fibrils with systematically larger sedimentation coefficients upon increasing protein concentration (modes of 100, 300, and 800 for apoC-II concentrations of 0.3, 0.7, and 1.0 mg/mL, respectively). The sedimentation coefficient distributions are not affected by rotor speed, and are not significantly changed by dilution once the fibrils are formed. The kinetics of aggregation (1 mg/mL apoC-II) as assessed using thioflavin T and preparative pelleting assays reveal that monomeric apoC-II is depleted after approximately 12 h incubation at room temperature. In contrast, the sedimentation coefficient distribution of fibrils continues to grow larger over a period of 48 h to an average value of 800 S. Calculations using the bead modeling procedure suggest maximum sedimentation coefficients for individual apoC-II fibrils to be around 100 S. The larger experimentally observed sedimentation coefficients for apoC-II fibrils indicate an extensive and time-dependent tangling or association of the fibrils to form specific networks.

Suppression of apolipoprotein C-II amyloid formation by the extracellular chaperone, clusterin

The effect of the extracellular chaperone, clusterin, on amyloid fibril formation by lipid-free human apolipoprotein C-II (apoC-II) was investigated. Sub-stoichiometric levels of clusterin, derived from either plasma or semen, potently inhibit amyloid formation by apoC-II. Inhibition is dependent on apoC-II concentration, with more effective inhibition by clusterin observed at lower concentrations of apoC-II. The average sedimentation coefficient of apoC-II fibrils formed from apoC-II (0.3 mg.mL-1) is reduced by coincubation with clusterin (10 microg x mL(-1)). In contrast, addition of clusterin (0.1 mg x mL(-1)) to preformed apoC-II amyloid fibrils (0.3 mg x mL(-1)) does not affect the size distribution after 2 days. This sedimentation velocity data suggests that clusterin inhibits fibril growth but does not promote fibril dissociation. Electron micrographs indicate similar morphologies for amyloid fibrils formed in the presence or absence of clusterin. The substoichiometric nature of the inhibition suggests that clusterin interacts with transient amyloid nuclei leading to dissociation of the monomeric subunits. We propose a general role for clusterin in suppressing the growth of extracellular amyloid.

Macromolecular crowding accelerates amyloid formation by human apolipoprotein C-II

Human apolipoprotein C-II (apoC-II) slowly forms amyloid fibers in lipid-free solutions at physiological pH and salt concentrations (Hatters, D. M., MacPhee, C. E., Lawrence, L. J., Sawyer, W. H., and Howlett, G. J. (2000) Biochemistry 39, 8276--8283). Measurements of the time dependence of solution turbidity, thioflavin T reactivity, and the amount of sedimentable aggregate reveal that the rate and extent of amyloid formation are significantly increased by the addition of an inert polymer, dextran T10, at concentrations exceeding 20 g/liter. High dextran concentrations do not alter the secondary structure of the protein, fiber morphology, or the thioflavin T and Congo Red binding capacity of apoC-II amyloid. Analytical ultracentrifugation studies show that monomeric apoC-II does not associate significantly with dextran. The observed dependence of the overall rate of amyloid formation on dextran concentration may be accounted for quantitatively by a simple model for nonspecific volume exclusion. The model predicts that an increase in the fractional volume occupancy of macromolecules in a physiological fluid can nonspecifically accelerate the formation of amyloid fibers by any amyloidogenic protein.

The molecular chaperone, alpha-crystallin, inhibits amyloid formation by apolipoprotein C-II

Under lipid-free conditions, human apolipoprotein C-II (apoC-II) exists in an unfolded conformation that over several days forms amyloid ribbons. We examined the influence of the molecular chaperone, alpha-crystallin, on amyloid formation by apoC-II. Time-dependent changes in apoC-II turbidity (at 0.3 mg/ml) were suppressed potently by substoichiometric subunit concentrations of alpha-crystallin (1-10 microg/ml). alpha-Crystallin also inhibits time-dependent changes in the CD spectra, thioflavin T binding, and sedimentation coefficient of apoC-II. This contrasts with stoichiometric concentrations of alpha-crystallin required to suppress the amorphous aggregation of stressed proteins such as reduced alpha-lactalbumin. Two pieces of evidence suggest that alpha-crystallin directly interacts with amyloidogenic intermediates. First, sedimentation equilibrium and velocity experiments exclude high affinity interactions between alpha-crystallin and unstructured monomeric apoC-II. Second, the addition of alpha-crystallin does not lead to the accumulation of intermediate sized apoC-II species between monomer and large aggregates as indicated by gel filtration and sedimentation velocity experiments, suggesting that alpha-crystallin does not inhibit the relatively rapid fibril elongation upon nucleation. We propose that alpha-crystallin interacts stoichiometrically with partly structured amyloidogenic precursors, inhibiting amyloid formation at nucleation rather than the elongation phase. In doing so, alpha-crystallin forms transient complexes with apoC-II, in contrast to its chaperone behavior with stressed proteins.

The interaction of human apolipoprotein C-I with sub-micellar phospholipid

Mature human apolipoprotein C-I (apoC-I), comprising 57 amino acids, is the smallest member of the plasma apolipoprotein family. Amphipathic helical regions within apoC-I, common to this class of proteins, are mediators of lipid binding, a process that underlies the functional properties of apoC-I, including the capacity to activate the plasma enzyme LCAT, to disrupt apoE mediated receptor interactions and to inhibit cholesterol ester transfer protein. To examine apoC-I/phospholipid interactions, we have developed an expression system in Escherichia coli to obtain purified apoC-I with yields of approximately 4-5 mg per L of culture. The purified product has properties similar to plasma-derived apoC-I including self-association in the lipid-free state and induced alpha-helical content in the presence of egg-yolk phosphatidylcholine and dimyristoylglycerophosphocholine vesicles. We chose the short-chain phospholipid, dihexanoylglycerophosphocholine (Hex2Gro-PCho), to examine the interaction of apoC-I with submicellar phospholipid. Circular dichroism spectroscopy and cross-linking experiments show that apoC-I acquires helical content and remains self-associated at submicellar concentrations of Hex2Gro-PCho (4 mM). Sedimentation equilibrium studies of apoC-I at submicellar levels of Hex2Gro-PCho and analysis of the effects of apoC-I on the 1H NMR spectrum of Hex2Gro-PCho indicate micelle induction by apoC-I, and establish the capacity of apoC-I to assemble individual phospholipid molecules.

Sub-micellar phospholipid accelerates amyloid formation by apolipoprotein C-II

Lipid-free human apolipoprotein C-II (apoC-II) forms amyloid fibrils with characteristic beta-structure. This conformation is distinct from the alpha-helical fold of lipid-bound apoC-II. We have investigated the effect of the short-chain phospholipid, dihexanoylphosphatidylcholine (DHPC) on amyloid formation by apoC-II. The alpha-helical content of apoC-II increases in the presence of micellar DHPC (16 mM) and amyloid formation is inhibited. However, at sub-micellar DHPC concentrations (below 8 mM) amyloid formation is accelerated 6 fold. These results suggest that individual phospholipid molecules in vivo may exert significant effects on amyloid folding pathways.

Use of generic fast gradient liquid chromatography-tandem mass spectroscopy in quantitative bioanalysis

Short narrow analytical HPLC columns have been used successfully with high linear flow-rates and combined with mass spectrometric detection to produce a generic approach to quantitative bioanalysis. The approach has been used to validate several assays in the low ng/ml region and an example is given in this paper. When combined with a simple solid-phase extraction process the need for complicated, time consuming method development has been removed for the majority of pharmaceutical compounds. The approach takes advantage of not only the extra selectivity of the MS-MS detector but the excellent resolution and peak shape produced by gradient elution.

Mass spectrometric characterization and glycosylation profile of bovine pancreatic bile salt-activated lipase

We developed a procedure for the large scale isolation of bovine bile salt-activated lipase (BAL) for its crystallization [Wang, X., et al. (1997) Structure 5, 1209-1218] and also carried out a study on the molecule's glycosylation profile for a better deduction of the structure of the enzyme. Mass spectrometric analysis of the CNBr-generated peptides indicated that only one (Asn-361) of the two potential N-glycosylation sites (Asn-187 and Asn-361) with NXT motif is glycosylated. The analysis of the isolated CNBr peptide containing Asn-361 showed that it existed in three glycoforms in a ratio of 1.0:2.8:1.0, with oligosaccharide moieties weighing 1900.1, 2045.2, and 2336.4 Da, respectively. The major oligosaccharide chain contained mannose: galactose:N-acetylglucosamine:fucose:sialic acid in a molar ratio of 2:2:4:2:1. It was also determined that the potential O-glycosylated peptide (CB13) is not O-glycosylated and, in addition, it was found that there was microheterogeneity in the C-terminus of the isolated bovine BAL. The results obtained from this mass spectrometric study combined with the X-ray crystallographic studies provide more precise structural information on BAL. The procedure described here for the mass spectrometric analysis of CNBr-generated peptides also has general applicability for analysis of the glycosylation profile of glycoproteins and the C-terminal peptide structure of proteins.