Influence of apolipoprotein A-V on the metabolic fate of triacylglycerol

PURPOSE OF REVIEW

Apolipoprotein (apo) A-V functions to modulate intracellular and extracellular triacylglycerol metabolism. The present review addresses molecular mechanisms underlying these effects. The relevance of apoA-V to human disease conditions is illustrated by the strong correlation between single nucleotide polymorphisms in APOA5, elevated plasma triacylglycerol and dyslipidemic disease.

RECENT FINDINGS

Despite undergoing processing for secretion from hepatocytes, a portion of apoA-V escapes this destiny and accumulates as a component of cytosolic lipid droplets. Expression of recombinant apoA-V in hepatocarcinoma cells results in increased lipid droplet size and number at the expense of triacylglycerol secretion.ApoA-V modulates atherosclerosis in hypercholesterolemic apoE null mice. ApoE null/human apoA-V transgenic mice had reduced levels of triacylglycerol and cholesterol in plasma along with decreased aortic lesion size.

SUMMARY

ApoA-V modulates triacylglycerol metabolic fate. Following its synthesis, apoA-V enters the endoplasmic reticulum and associates with membrane defects created by triacylglycerol accumulation. Association of apoA-V with endoplasmic reticulum membrane defects promotes nascent lipid droplets budding toward the cytosol. Despite its low concentration in plasma (∼150 ng/ml), apoA-V modulates lipoprotein metabolism by binding to glycosylphosphatidylinositol-anchored high-density lipoprotein binding protein 1. This interaction effectively localizes triacylglycerol-rich lipoproteins in the vicinity of glycosylphosphatidylinositol-anchored high-density lipoprotein binding protein1's other ligand, lipoprotein lipase.

Gene transfer of apolipoprotein A-V improves the hypertriglyceridemic phenotype of apoa5 (-/-) mice

OBJECTIVE

Apolipoprotein (apo) A-V is a low abundance protein with a profound influence on plasma triacylglycerol levels. In human populations, single nucleotide polymorphisms and mutations in APOA5 positively correlate with hypertriglyceridemia. As an approach to preventing the deleterious effects of chronic hypertriglyceridemia, apoA-V gene therapy has been pursued.

METHODS AND RESULTS

Recombinant adeno-associated virus (AAV) 2/8 harboring the coding sequence for human apoA-V or a control AAV2/8 was transduced into hypertriglyceridemic apoa5 (-/-) mice. After injection of 1×10(12) viral genome AAV2/8-apoA-V, maximal plasma levels of apoA-V protein were achieved at 3 to 4 weeks, after which the concentration slowly declined. Complementing the appearance of apoA-V was a decrease (50±6%) in plasma triacylglycerol content compared with apoa5 (-/-) mice treated with AAV2/8-β-galactosidase. After 8 weeks the mice were euthanized and plasma lipoproteins separated. AAV2/8-apoA-V-transduced mice displayed a dramatic reduction in very low-density lipoprotein triacylglycerol content. Vector generated apoA-V in plasma associated with both very low-density lipoprotein and high-density lipoprotein fractions.

CONCLUSIONS

Taken together, the data show that gene transfer of apoA-V improves the severe hypertriglyceridemia phenotype of apoa5 (-/-) mice. Given the prevalence of hypertriglyceridemia, apoA-V gene therapy offers a potential strategy for maintenance of plasma triacylglycerol homeostasis.

Influence of apolipoprotein A-V on hepatocyte lipid droplet formation

Apolipoprotein A-V (apoA-V) is postulated to modulate intra-hepatic triglyceride (TG) trafficking. Stably transfected McA-RH7777 hepatocarcinoma cells expressing human apoA-V displayed enhanced neutral lipid staining while conditioned media from these cells had 40±8% less TG than cells transfected with a control vector. To obtain homogeneous cell lines expressing different amounts of apoA-V, a strategy of clonal selection was pursued. Immunoblot analysis of two distinct apoA-V stable cell lines yielded one that expresses low amounts of apoA-V and another that expresses higher amounts. Confocal fluorescence microscopy of control cells and cells expressing low levels of apoA-V had similar numbers of lipid droplets while cells expressing higher amounts of apoA-V had twice as many lipid droplets, on average. Thus, apoA-V expression promotes lipid droplet accumulation in these cells.

Tweaking the cholesterol efflux capacity of reconstituted HDL

Mechanisms to increase plasma high-density lipoprotein (HDL) or to promote egress of cholesterol from cholesterol-loaded cells (e.g., foam cells from atherosclerotic lesions) remain an important target to regress heart disease. Reconstituted HDL (rHDL) serves as a valuable vehicle to promote cellular cholesterol efflux in vitro and in vivo. rHDL were prepared with wild type apolipoprotein (apo) A-I and the rare variant, apoA-I Milano (M), and each apolipoprotein was reconstituted with phosphatidylcholine (PC) or sphingomyelin (SM). The four distinct rHDL generated were incubated with CHO cells, J774 macrophages, and BHK cells in cellular cholesterol efflux assays. In each cell type, apoA-I(M) SM-rHDL promoted the greatest cholesterol efflux. In BHK cells, the cholesterol efflux capacities of all four distinct rHDL were greatly enhanced by increased expression of ABCG1. Efflux to PC-containing rHDL was stimulated by transfection of a nonfunctional ABCA1 mutant (W590S), suggesting that binding to ABCA1 represents a competing interaction. This interpretation was confirmed by binding experiments. The data show that cholesterol efflux activity is dependent upon the apoA-I protein employed, as well as the phospholipid constituent of the rHDL. Future studies designed to optimize the efflux capacity of therapeutic rHDL may improve the value of this emerging intervention strategy.

Expressed protein ligation-mediated template protein extension

Expressed protein ligation (EPL) was performed to investigate sequence requirements for a variant human apolipoprotein A-I (apoA-I) to adopt a folded structure. A C-terminal truncated apoA-I, corresponding to residues 1-172, was expressed and isolated from Escherichia coli. Compared to full length apoA-I (243 amino acids), apoA-I(1-172) displayed less α-helix secondary structure and lower stability in solution. To determine if extension of this polypeptide would confer secondary structure content and/or stability, 20 residues were added to the C-terminus of apoA-I(1-172) by EPL, creating apoA-I(Milano)(1-192). The EPL product displayed biophysical properties similar to full-length apoA-I(Milano). The results provide a general protein engineering strategy to modify the length of a recombinant template polypeptide using synthetic peptides as well as a convenient, cost effective way to investigate the structure/function relations in apolipoprotein fragments or domains of different size.

Effect of amphotericin B nanodisks on plant fungal diseases

BACKGROUND

The development of water-soluble nanodevices extends the potential use of compounds developed for other purposes (e.g. antifungal drugs or antibiotics) for applications in agriculture. For example, the broad-spectrum, water-insoluble, macrolide polyene antibiotic amphotericin B (AMB) could be used to inhibit phytopathogenic fungi. A new formulation embedding AMB in nanodisks (NDs) enhances antibiotic solubility and confers protection against environmental damage. In the present study, AMB-NDs were tested for efficacy against several phytopathogenic fungi in vitro and on infected living plants (chickpea and wheat).

RESULTS

Compared with AMB in dimethylsulfoxide (DMSO), AMB-NDs increased the sensitivity of several fungal species to this antimycotic in vitro. Sensitivity varied with fungal species as well as with the forma specialis. Phytophthora cinnamomi, previously reported as insensitive to other polyene antimycotics, remained unaffected at the doses examined. Some effect against disease symptoms were obtained with AMB-NDs against fusarium wilt in chickpea, whereas the results were highly variable in wheat, depending on both the species and treatment regimen.

CONCLUSION

The results confirm that formulation of AMB into ND increases its effectiveness against phytopathogenic fungi in vitro, opening the possibility for its use on infected plants in the field.

Apolipoprotein A-V dependent modulation of plasma triacylglycerol: a puzzlement

The discovery of apolipoprotein A-V (apoA-V) in 2001 has raised a number of intriguing questions about its role in lipid transport and triglyceride (TG) homeostasis. Genome wide association studies (GWAS) have consistently identified APOA5 as a contributor to plasma TG levels. Single nucleotide polymorphisms (SNP) within the APOA5 gene locus have been shown to correlate with elevated plasma TG. Furthermore, transgenic and knockout mouse models support the view that apoA-V plays a critical role in maintenance of plasma TG levels. The present review describes recent concepts pertaining to apoA-V SNP analysis and their association with elevated plasma TG. The interaction of apoA-V with glycosylphosphatidylinositol-anchored high-density lipoprotein binding protein 1 (GPIHBP1) is discussed relative to its postulated role in TG-rich lipoprotein catabolism. The potential role of intracellular apoA-V in regulation of TG homeostasis, as a function of its ability to associate with cytosolic lipid droplets, is reviewed. While some answers are emerging, numerous mysteries remain with regard to this low abundance, yet potent, modulator of TG homeostasis. Given the strong correlation between elevated plasma TG and heart disease, there is great scientific and public interest in deciphering the numerous biological riddles presented by apoA-V. This article is part of a Special Issue entitled Triglyceride Metabolism and Disease.

Curcumin nanodisk-induced apoptosis in mantle cell lymphoma

Mantle cell lymphoma (MCL) is a pre-germinal center neoplasm characterized by cyclin D1 overexpression resulting from t(11;14)(q13;q32). Since MCL is incurable with standard lymphoma therapies, new treatment approaches are needed that target specific biologic pathways. In the present study, we investigated a novel drug delivery nanovehicle enriched with the bioactive polyphenol, curcumin (curcumin nanodisks; curcumin-ND). Cells treated with curcumin-ND showed a dose-dependent increase in apoptosis. This was accompanied by enhanced generation of reactive oxygen species (ROS). The antioxidant, N-acetylcysteine, inhibited curcumin-ND induced apoptosis, suggesting that ROS generation plays a role in curcumin action on MCL cells. Curcumin-ND decreased cyclin D1, pAkt, pIκBα, and Bcl(2) protein. In addition, enhanced FoxO3a and p27 expression as well as caspase-9, -3, and poly(ADP-ribose) polymerase (PARP) cleavage were observed. Curcumin-ND treatment led to enhanced G(1) arrest in two cultured cell models of MCL.

A two-step binding model of PCSK9 interaction with the low density lipoprotein receptor

PCSK9 (proprotein convertase subtilisin-like/kexin type 9) is an emerging target for pharmaceutical intervention. This multidomain protein interacts with the LDL receptor (LDLR), promoting receptor degradation. Insofar as PCSK9 inhibition induces a decrease in plasma cholesterol levels, understanding the nature of the binding interaction between PCSK9 and the LDLR is of critical importance. In this study, the ability of PCSK9 to compete with apoE3 N-terminal domain-containing reconstituted HDL for receptor binding was examined. Whereas full-length PCSK9 was an effective competitor, the N-terminal domain (composed of the prodomain and catalytic domain) was not. Surprisingly, the C-terminal domain (CT domain) of PCSK9 was able to compete. Using a direct binding interaction assay, we show that the PCSK9 CT domain bound to the LDLR in a calcium-dependent manner and that co-incubation with the prodomain and catalytic domain had no effect on this binding. To further characterize this interaction, two LDLR fragments, the classical ligand-binding domain (LBD) and the EGF precursor homology domain, were expressed in stably transfected HEK 293 cells and isolated. Binding assays showed that the PCSK9 CT domain bound to the LBD at pH 5.4. Thus, CT domain interaction with the LBD of the LDLR at endosomal pH constitutes a second step in the PCSK9-mediated LDLR binding that leads to receptor degradation.

Nanobiotechnology applications of reconstituted high density lipoprotein

High-density lipoprotein (HDL) plays a fundamental role in the Reverse Cholesterol Transport pathway. Prior to maturation, nascent HDL exist as disk-shaped phospholipid bilayers whose perimeter is stabilized by amphipathic apolipoproteins. Methods have been developed to generate reconstituted (rHDL) in vitro and these particles have been used in a variety of novel ways. To differentiate between physiological HDL particles and non-natural rHDL that have been engineered to possess additional components/functions, the term nanodisk (ND) is used. In this review, various applications of ND technology are described, such as their use as miniature membranes for solubilization and characterization of integral membrane proteins in a native like conformation. In other work, ND harboring hydrophobic biomolecules/drugs have been generated and used as transport/delivery vehicles. In vitro and in vivo studies show that drug loaded ND are stable and possess potent biological activity. A third application of ND is their use as a platform for incorporation of amphiphilic chelators of contrast agents, such as gadolinium, used in magnetic resonance imaging. Thus, it is demonstrated that the basic building block of plasma HDL can be repurposed for alternate functions.

Binding preferences for GPIHBP1, a glycosylphosphatidylinositol-anchored protein of capillary endothelial cells

OBJECTIVE

To define the ability of GPIHBP1 to bind other lipase family members and other apolipoproteins (apos) and lipoproteins.

METHODS AND RESULTS

GPIHBP1, a GPI-anchored lymphocyte antigen (Ly)6 protein of capillary endothelial cells, binds lipoprotein lipase (LPL) avidly, but its ability to bind related lipase family members has never been evaluated. As judged by cell-based and cell-free binding assays, LPL binds to GPIHBP1, but other members of the lipase family do not. We also examined the binding of apoAV-phospholipid disks to GPIHBP1. ApoAV binds avidly to GPIHBP1-transfected cells; this binding requires GPIHBP1's amino-terminal acidic domain and is independent of its cysteine-rich Ly6 domain (the latter domain is essential for LPL binding). GPIHBP1-transfected cells did not bind high-density lipoprotein. Chylomicrons bind avidly to GPIHBP1-transfected Chinese hamster ovary cells, but this binding is dependent on GPIHBP1's ability to bind LPL within the cell culture medium.

CONCLUSIONS

GPIHBP1 binds LPL but does not bind other lipase family members. GPIHBP1 binds apoAV but does not bind apoAI or high-density lipoprotein. The ability of GPIHBP1-transfected Chinese hamster ovary cells to bind chylomicrons is mediated by LPL; chylomicron binding does not occur unless GPIHBP1 first captures LPL from the cell culture medium.

Intravenous injection of apolipoprotein A-V reconstituted high-density lipoprotein decreases hypertriglyceridemia in apoav-/- mice and requires glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1

OBJECTIVE

Apolipoprotein A-V (apoA-V), a minor protein associated with lipoproteins, has a major effect on triacylglycerol (TG) metabolism. We investigated whether apoA-V complexed with phospholipid in the form of a reconstituted high-density lipoprotein (rHDL) has potential utility as a therapeutic agent for treatment of hypertriglyceridemia (HTG) when delivered intravenously.

METHODS AND RESULTS

Intravenous injection studies were performed in genetically engineered mouse models of severe HTG, including apoav-/- and gpihbp1-/- mice. Administration of apoA-V rHDL to hypertriglyceridemic apoav-/- mice resulted in a 60% reduction in plasma TG concentration after 4 hours. This decline can be attributed to enhanced catabolism/clearance of very-low-density lipoprotein (VLDL), where VLDL TG and cholesterol were reduced ≈60%. ApoA-V that associated with VLDL after injection was also rapidly cleared. Site-specific mutations in the heparin-binding region of apoA-V (amino acids 186 to 227) attenuated apoA-V rHDL TG-lowering activity by 50%, suggesting that this sequence element is required for optimal TG-lowering activity in vivo. Unlike apoav-/- mice, injection of apoA-V rHDL into gpihbp1-/- mice had no effect on plasma TG levels, and apoA-V remained associated with plasma VLDL.

CONCLUSIONS

Intravenously injected apoA-V rHDL significantly lowers plasma TG in an apoA-V deficient mouse model. Its intravenous administration may have therapeutic benefit in human subjects with severe HTG, especially in cases involving apoA-V variants associated with HTG.

Curcumin nanodisks: formulation and characterization

Nanodisks (NDs) are nanoscale, disk-shaped phospholipid bilayers whose edge is stabilized by apolipoproteins. In the present study, NDs were formulated with the bioactive polyphenol curcumin at a 6:1 phospholipid-to-curcumin molar ratio. Atomic force microscopy revealed that curcumin-NDs are particles with diameters <50 nm and the thickness of a phospholipid bilayer. When formulated in NDs, curcumin is water soluble and gives rise to a characteristic absorbance spectrum with a peak centered at 420 nm. Fluorescence spectroscopy of curcumin-NDs provided evidence of self-quenching. Incubation of curcumin-NDs with empty NDs relieved the self-quenching, indicating redistribution of curcumin between curcumin-loaded and empty NDs. In HepG2 cells, curcumin-NDs mediated enhanced cell growth inhibition as compared with free curcumin. In a cell culture model of mantle cell lymphoma, curcumin-NDs were a more potent inducer of apoptosis than free curcumin. The nanoscale size of the complexes, combined with their ability to solubilize curcumin, indicates NDs may have in vivo therapeutic applications.

FROM THE CLINICAL EDITOR

Nanodisks (NDs), disk-shaped phospholipid bilayers stabilized by apolipoproteins, are shown entrap curcumin and improve its delivery to HepG2 and mantle cell lymphoma cells in culture. These novel nanocomplexes demonstrate interesting therapeutic application potentials.

Apolipoprotein E: from lipid transport to neurobiology

Apolipoprotein (apo) E has a storied history as a lipid transport protein. The integral association between cholesterol homeostasis and lipoprotein clearance from circulation are intimately related to apoE's function as a ligand for cell-surface receptors of the low-density lipoprotein receptor family. The receptor binding properties of apoE are strongly influenced by isoform specific amino acid differences as well as the lipidation state of the protein. As understanding of apoE as a structural component of circulating plasma lipoproteins has evolved, exciting developments in neurobiology have revitalized interest in apoE. The strong and enduring correlation between the apoE4 isoform and age of onset and increased risk of Alzheimer's disease has catapulted apoE to the forefront of neurobiology. Using genetic tools generated for study of apoE lipoprotein metabolism, transgenic "knock-in" and gene-disrupted mice are now favored models for study of its role in a variety of neurodegenerative diseases. Key structural knowledge of apoE and isoform-specific differences is driving research activity designed to elucidate how a single amino acid change can manifest such profoundly significant pathological consequences. This review describes apoE through a lens of structure-based knowledge that leads to hypotheses that attempt to explain the functions of apoE and isoform-specific effects relating to disease mechanism.

The carboxyl-terminal segment of apolipoprotein A-V undergoes a lipid-induced conformational change

Apolipoprotein (apo) A-V is a 343-residue, multidomain protein that plays an important role in regulation of plasma triglyceride homeostasis. Primary sequence analysis revealed a unique tetraproline sequence (Pro293-Pro296) near the carboxyl terminus of the protein. A peptide corresponding to the 48-residue segment beyond the tetraproline motif was generated from a recombinant apoA-V precursor wherein Pro295 was replaced by Met. Cyanogen bromide cleavage of the precursor protein, followed by negative affinity chromatography, yielded a purified peptide. Nondenaturing polyacrylamide gel electrophoresis verified that apoA-V(296-343) solubilizes phospholipid vesicles, forming a relatively heterogeneous population of reconstituted high-density lipoprotein with Stokes' diameters >17 nm. At the same time, apoA-V(296-343) failed to bind a spherical lipoprotein substrate in vitro. Far-UV circular dichroism spectroscopy revealed the peptide is unstructured in buffer yet adopts significant alpha-helical secondary structure in the presence of the lipid mimetic solvent trifluoroethanol (TFE; 50% v/v). Heteronuclear multidemensional NMR spectroscopy experiments were conducted with uniformly (15)N- and (15)N/(13)C-labeled peptide in 50% TFE. Peptide backbone assignment and secondary structure prediction using TALOS+ reveal the peptide adopts alpha-helix secondary structure from residues 309 to 334. In TFE, apoA-V(296-343) adopts an extended amphipathic alpha-helix, consistent with a role in lipoprotein binding as a component of full-length apoA-V.

All trans retinoic acid nanodisks enhance retinoic acid receptor mediated apoptosis and cell cycle arrest in mantle cell lymphoma

Mantle cell lymphoma (MCL) is characterized by translocation t(11;14)(q13;q32), aggressive clinical behaviour, and poor patient outcomes following conventional chemotherapy. New treatment approaches are needed that target novel biological pathways. All trans retinoic acid (ATRA) is a key retinoid that acts through nuclear receptors that function as ligand-inducible transcription factors. The present study evaluated cell killing effects of ATRA-enriched nanoscale delivery particles, termed nanodisks (ND), on MCL cell lines. Results show that ATRA-ND induced cell death more effectively than naked ATRA (dimethyl sulphoxide) or empty ND. ATRA-ND induced reactive oxygen species (ROS) generation to a greater extent than naked ATRA. The antioxidant, N-acetylcysteine, inhibited ATRA-ND induced apoptosis. Compared to naked ATRA, ATRA-ND enhanced G1 growth arrest, up-regulated p21and p27, and down regulated cyclin D1. At ATRA concentrations that induced apoptosis, expression levels of retinoic acid receptor-alpha (RARalpha) and retinoid X receptor-gamma (RXRgamma) were increased. Compared to naked ATRA, ATRA-ND significantly stimulated transcriptional activity of RARA in a model carcinoma cell line. Furthermore, the RAR antagonist, Ro 41-5253, inhibited ATRA-ND induced ROS generation and prevented ATRA-ND induced cell growth arrest and apoptosis. In summary, incorporation of ATRA into ND enhanced the biological activity of this retinoid in cell culture models of MCL.

Apolipoprotein A-V associates with intrahepatic lipid droplets and influences triglyceride accumulation

Apolipoprotein A-V (apoA-V), secreted solely by the liver, is a low abundance protein that strongly influences plasma triglyceride (TG) levels. In vitro, in transfected hepatoma cell lines apoA-V is largely retained within the cell in association with cytosolic lipid droplets (LD). To evaluate if this is true in vivo, in the present study the amount of apoA-V in the plasma compartment versus liver tissue was determined in APOA5 transgenic (Tg) mice. The majority of total apoA-V ( approximately 80%) was in the plasma compartment. Injection of APOA5 Tg mice with heparin increased plasma apoA-V protein levels by approximately 25% indicating the existence of a heparin-releasable pool. Intrahepatic apoA-V was associated with LD isolated from livers of wild type (WT) and APOA5 Tg mice. Furthermore, livers from APOA5 Tg mice contained significantly higher amounts of TG than livers from WT or apoa5 knockout mice suggesting that apoA-V influences intrahepatic TG levels.

Apolipoprotein A-V N-terminal domain lipid interaction properties in vitro explain the hypertriglyceridemic phenotype associated with natural truncation mutants

The N-terminal 146 residues of apolipoprotein (apo) A-V adopt a helix bundle conformation in the absence of lipid. Because similarly sized truncation mutants in human subjects correlate with severe hypertriglyceridemia, the lipid binding properties of apoA-V(1-146) were studied. Upon incubation with phospholipid in vitro, apoA-V(1-146) forms reconstituted high density lipoproteins 15-17 nm in diameter. Far UV circular dichroism spectroscopy analyses of lipid-bound apoA-V(1-146) yielded an alpha-helix secondary structure content of 60%. Fourier transformed infrared spectroscopy analysis revealed that apoA-V(1-146) alpha-helix segments align perpendicular with respect to particle phospholipid fatty acyl chains. Fluorescence spectroscopy of single Trp variant apoA-V(1-146) indicates that lipid interaction is accompanied by a conformational change. The data are consistent with a model wherein apoA-V(1-146) alpha-helices circumscribe the perimeter of a disk-shaped bilayer. The ability of apoA-V(1-146) to solubilize dimyristoylphosphatidylcholine vesicles at a rate faster than full-length apoA-V suggests that N- and C-terminal interactions in the full-length protein modulate its lipid binding properties. Preferential association of apoA-V(1-146) with murine plasma HDL, but not with VLDL, suggests that particle size is a determinant of its lipoprotein binding specificity. It may be concluded that defective lipoprotein binding of truncated apoA-V contributes to the hypertriglyceridemia phenotype associated with truncation mutations in human subjects.

Local suppression of T cell responses by arginase-induced L-arginine depletion in nonhealing leishmaniasis

The balance between T helper (Th) 1 and Th2 cell responses is a major determinant of the outcome of experimental leishmaniasis, but polarized Th1 or Th2 responses are not sufficient to account for healing or nonhealing. Here we show that high arginase activity, a hallmark of nonhealing disease, is primarily expressed locally at the site of pathology. The high arginase activity causes local depletion of L-arginine, which impairs the capacity of T cells in the lesion to proliferate and to produce interferon-γ, while T cells in the local draining lymph nodes respond normally. Healing, induced by chemotherapy, resulted in control of arginase activity and reversal of local immunosuppression. Moreover, competitive inhibition of arginase as well as supplementation with L-arginine restored T cell effector functions and reduced pathology and parasite growth at the site of lesions. These results demonstrate that in nonhealing leishmaniasis, arginase-induced L-arginine depletion results in impaired T cell responses. Our results identify a novel mechanism in leishmaniasis that contributes to the failure to heal persistent lesions and suggest new approaches to therapy.

Leishmania parasites are obligate intracellular pathogens that predominantly invade macrophages. Instruction of macrophages by T cell-derived signals is required to control parasite growth. Here we show that arginase, an enzyme induced in Leishmania-infected macrophages, is highly expressed at the site of pathology in nonhealing lesions and causes local depletion of L-arginine, an amino acid that is essential for efficient T cell responses. This local reduction in L-arginine impairs the capacity of T cells in the lesion to proliferate and to produce interferon-γ, one of the signals required for parasite killing. Cure of Leishmania infection by drug treatment is accompanied by a reduction in arginase activity and restoration of T cell effector functions. Furthermore, inhibition of arginase, as well as injection of L-arginine, reverses immunosuppression and results in more efficient control of parasite replication. Our results identify a novel mechanism accounting for ineffective T cell responses in nonhealing leishmaniasis.

Nanodisks protect amphotericin B from ultraviolet light and oxidation-induced damage

BACKGROUND

Macrolide polyene antibiotics possess potent broad-spectrum antifungal properties. Use of these agents in the field or in controlled environments is impeded by their poor water solubility and susceptibility to oxidation- and/or light-induced degradation. While typically used for human disease therapy, there is potential to expand the utility of polyene macrolide antibiotics, such as amphotericin B, for control of fungal disease infestation in agricultural settings. Thus, the susceptibility of this antibiotic to exposure-induced activity loss was evaluated.

RESULTS

Incubation of the prototype polyene amphotericin B (AMB) with phospholipid vesicles and apolipoprotein A-I results in the formation of nanoscale complexes, termed nanodisks (NDs), capable of solubilizing significant quantities of AMB. To evaluate whether AMB incorporation into NDs conferred protection against light- or oxidation-induced damage, yeast growth inhibition assays were conducted. Compared with AMB solubilized in detergent micelles, AMB incorporated into NDs was protected from damage caused by exposure to UV light as well as by KMnO(4)-induced oxidation. Furthermore, AMB-NDs inhibited growth of the turfgrass fungus Marasmius oreades Fr.

CONCLUSION

Results suggest that this water-soluble formulation of a natural, biodegradable, antifungal agent represents a potential cost-effective, non-toxic and environmentally friendly substitute for chemical agents currently employed to control a range of fungal infestations.

Domain swapping reveals that low density lipoprotein (LDL) type A repeat order affects ligand binding to the LDL receptor

The low density lipoprotein receptor (LDLR) plays a key role in plasma cholesterol homeostasis by binding and internalizing lipoprotein ligands. Studies have revealed that one or more of the seven LDL type A repeats (LA1-LA7) in the receptor are responsible for apolipoprotein binding. In the present study, protein engineering was performed to swap or replace key LA repeats in a recombinant soluble LDLR (sLDLR). Although wild type sLDLR showed strong ligand binding activity, an sLDLR variant in which LA repeat 5 was replaced by a second copy of LA repeat 2 showed low binding activity. Likewise, a variant wherein LA repeats 2 and 5 were swapped displayed low binding activity. At the same time, substitution of LA repeat 2 with a second a copy of repeat 5 resulted in a receptor with ligand binding activity similar to wild type LDLR. When binding assays were conducted with human low density lipoprotein as ligand, LA repeat order was a less important determinant of binding activity. Taken together, the data indicate that the sequential order of LA repeats plays a key role in ligand binding properties of LDLR.

Nanodisks derived from amphotericin B lipid complex

The goal of this study was to determine the effect of apolipoproteins on Amphotericin B lipid complex (ABLC). We report that incubation of ABLC with recombinant human apolipoprotein A-I (apoA-I) induces solubilization of ABLC by transforming the micron sized phospholipid/AMB assemblies into discrete nanoscale disk-shaped complexes termed nanodisks (ND). ApoA-I induced changes in ABLC solubility and morphology were monitored by spectroscopy and electron microscopy. AMB efficacy was evaluated in yeast and pathogenic fungi growth inhibition assays and the effect of AMB formulation on cell toxicity was assessed in cultured Hep3B cells. AMB associated with ND were more efficiently nebulized than AMB associated with ABLC. Thus, transformation of ABLC into ND preserves the potent biological activity of AMB as well as the reduced toxicity of the ABLC formulation. ABLC derived AMB-ND offer advantages over conventional ABLC in terms of stability, storage, nebulization efficiency and provides an intrinsic "handle" for tissue specific targeting via genetic engineering of its protein component.

Targeting nanodisks via a single chain variable antibody--apolipoprotein chimera

Nanodisks (ND) are nanometer scale complexes of phospholipid and apolipoprotein that have been shown to function as drug delivery vehicles. ND harboring significant quantities of the antifungal agent, amphotericin B, or the bioactive isoprenoid, all trans retinoic acid, have been generated and characterized. As currently formulated, ND possess limited targeting capability. In this study, we constructed a single chain variable antibody (scFv).apolipoprotein chimera and assessed the ability of this fusion protein to form ND and recognize the antigen to which the scFv is directed. Data obtained revealed that alpha-vimentin scFv.apolipoprotein A-I is functional in ND formation and antigen recognition, opening the door to the use of such chimeras in targeting drug-enriched ND to specific tissues.

Semisynthesis and segmental isotope labeling of the apoE3 N-terminal domain using expressed protein ligation

Apolipoprotein E (apoE) is an exchangeable apolipoprotein that functions as a ligand for members of the LDL receptor family, promoting lipoprotein clearance from the circulation. Productive receptor binding requires that apoE adopt an LDL receptor-active conformation through lipid association, and studies have shown that the 22 kDa N-terminal (NT) domain (residues 1-183) of apoE is both necessary and sufficient for receptor interaction. Using intein-mediated expressed protein ligation (EPL), a semisynthetic apoE3 NT has been generated for use in structure-function studies designed to probe the nature of the lipid-associated conformation of the protein. Circular dichroism spectroscopy of EPL-generated apoE3 NT revealed a secondary structure content similar to wild-type apoE3 NT. Likewise, lipid and LDL receptor binding studies revealed that EPL-generated apoE3 NT is functional. Subsequently, EPL was used to construct an apoE3 NT enriched with 15N solely and specifically in residues 112-183. 1H-15N heteronuclear single quantum correlation spectroscopy experiments revealed that the ligation product is correctly folded in solution, adopting a conformation similar to wild-type apoE3-NT. The results indicate that segmental isotope labeling can be used to define the lipid bound conformation of the receptor binding element of apoE as well as molecular details of its interaction with the LDL receptor.

The ins (cell) and outs (plasma) of apolipoprotein A-V

Apolipoprotein A-V (apoA-V) has a close interrelationship with plasma triglyceride (TG). Since the discovery of the apoA-V gene in 2001, we have learned that single nucleotide polymorphisms in this gene correlate with altered plasma TG levels in humans, while genetically engineered mice manifest unique TG phenotypes. Studies of recombinant apoA-V protein have revealed that it is composed of two independently folded structural domains. The C-terminal domain possesses high lipid binding affinity, while the N-terminal domain adopts a helix bundle molecular architecture. A sequence element with high positive charge character, between residues 185 and 228, functions in binding of apoA-V to heparan sulfate proteoglycans as well as to members of the low-density lipoprotein receptor family and glycosylphosphatidylinositol high-density lipoprotein binding protein1. These interactions may be related to the capacity of this protein to regulate TG levels. ApoA-V is poorly secreted from transfected cultured hepatoma cell lines and is present in plasma at exceedingly low levels. Studies of apoA-V intracellular trafficking revealed an association with cytosolic lipid droplets. Thus, it is conceivable that apoA-V may also modulate TG metabolism within the cell. Much remains to be learned about this fascinating yet confounding member of the class of exchangeable apolipoproteins.