Effect of bilayer phospholipid composition and curvature on ligand transfer by the alpha-tocopherol transfer protein

The tocopherol transfer protein TTP mediates Vitamin Vitamin E trafficking between cerebellar astrocytes and neurons

Alpha-tocopherol (vitamin E) is an essential nutrient that functions as a major lipid-soluble antioxidant in humans. The tocopherol transfer protein (TTP) binds α-tocopherol with high affinity and selectivity and regulates whole-body distribution of the vitamin. Heritable mutations in the TTPA gene result in familial vitamin E deficiency, elevated indices of oxidative stress, and progressive neurodegeneration that manifest primarily in spinocerebellar ataxia. Although the essential role of vitamin E in neurological health has been recognized for over 50 years, the mechanisms by which this essential nutrient is transported in the central nervous system are poorly understood. Here we found that, in the murine cerebellum, TTP is selectively expressed in GFAP-positive astrocytes, where it facilitates efflux of vitamin E to neighboring neurons. We also show that induction of oxidative stress enhances the transcription of the TtpA gene in cultured cerebellar astrocytes. Furthermore, secretion of vitamin E from astrocytes is mediated by an ABC-type transporter, and uptake of the vitamin into neurons involves the low-density lipoprotein receptor-related protein 1 (LRP1) receptor. Taken together, our data indicate that TTP-expressing astrocytes control the delivery of vitamin E from astrocytes to neurons, and that this process is homeostatically responsive to oxidative stress. These are the first observations that address the detailed molecular mechanisms of vitamin E transport in the central nervous system, and these results have important implications for understanding the molecular underpinnings of oxidative stress-related neurodegenerative diseases.

Engineering of a functional γ-tocopherol transfer protein

α-tocopherol transfer protein (TTP) was previously reported to self-aggregate into 24-meric spheres (α-TTP_S) and to possess transcytotic potency across mono-layers of human umbilical vein endothelial cells (HUVECs). In this work, we describe the characterisation of a functional TTP variant with its vitamer selectivity shifted towards γ-tocopherol. The shift was obtained by introducing an alanine to leucine substitution into the substrate-binding pocket at position 156 through site directed mutagenesis. We report here the X-ray crystal structure of the γ-tocopherol specific particle (γ-TTP_S) at 2.24 Å resolution. γ-TTP_S features full functionality compared to its α-tocopherol specific parent including self-aggregation potency and transcytotic activity in trans-well experiments using primary HUVEC cells. The impact of the A156L mutation on TTP function is quantified in vitro by measuring the affinity towards γ-tocopherol through micro-differential scanning calorimetry and by determining its ligand-transfer activity. Finally, cell culture experiments using adherently grown HUVEC cells indicate that the protomers of γ-TTP, in contrast to α-TTP, do not counteract cytokine-mediated inflammation at a transcriptional level. Our results suggest that the A156L substitution in TTP is fully functional and has the potential to pave the way for further experiments towards the understanding of α-tocopherol homeostasis in humans.

In vitro lipid transfer assays of phosphatidylinositol transfer proteins provide insight into the in vivo mechanism of ligand transfer

Fluorescence resonance energy transfer (FRET) assays and membrane binding determinations were performed using three phosphatidylinositol transfer proteins, including the yeast Sec14 and two mammalian proteins PITPα and PITPβ. These proteins were able to specifically bind the fluorescent phosphatidylcholine analogue NBD-PC ((2-(12-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)dodecanoyl-1-hexadecanoyl-sn-glycero-3-phosphocholine)) and to transfer it to small unilamellar vesicles (SUVs). Rate constants for transfer to vesicles comprising 100% PC were slower for all proteins than when increasing percentages of phosphatidylinositol were incorporated into the same SUVs. The rates of ligand transfer by Sec14 were insensitive to the inclusion of equimolar amounts of another anionic phospholipid phosphatidylserine (PS), but the rates of ligand transfer by both mammalian PITPs were strikingly enhanced by the inclusion of phosphatidic acid (PA) in the receptor SUV. Binding of Sec14 to immobilized bilayers was substantial, while that of PITPα and PITPβ was 3-7 times weaker than Sec14 depending on phospholipid composition. When small proportions of the phosphoinositide PI(4)P were included in receptor SUVs (either with PI or not), Sec14 showed substantially increased rates of NBD-PC pick-up, whereas the PITPs were unaffected. The data are supportive of a role for PITPβ as functional PI transfer protein in vivo, but that Sec14 likely has a more elaborate function.

Lipid and membrane recognition by the oxysterol binding protein and its phosphomimetic mutant using dual polarization interferometry

OSBP binds, extracts and transfers sterols and phosphatidylinositol-4-phosphate (PI(4)P between liposomes, but the sequence of steps at the membrane surface leading to ligand removal is poorly characterized. In this study, we used dual polarization interferometry (DPI), a label-free surface analytical technique, to characterize the interaction of recombinant, purified OSBP as it flows over immobilized dioleoyl-phosphatidylcholine (DOPC) bilayers containing PI(4)P, cholesterol or 25-hydroxycholesterol. Kinetics of membrane interaction were analyzed for PI(4)P-binding and phosphorylation mutants of OSBP. Wild-type OSBP demonstrated a distinctive association with immobilized DOPC bilayers containing 1-8 mol% PI(4)P that was characterized by initial saturable binding followed by desorption, indicative of PI(4)P extraction. In support of this conclusion, an OSBP mutant with impaired binding and extraction of PI(4)P was stably absorbed to PI(4)P-containing membranes, while a pleckstrin homology domain mutant did not associate with PI(4)P-containing membranes. The inclusion of >2 mol% cholesterol, but not 25-hydroxycholesterol, in membranes, enhanced the absorption of the wild-type OSBP. A phosphomimetic of OSBP with enhanced in vitro sterol binding activity displayed membrane interaction properties similar to wild-type. These real-time flow studies allow us to dissect the association of OSBP with PI(4)P into discrete components; initial recruitment to PI(4)P membranes by the PH domain, detection and extraction of PI(4)P, and desorption due to ligand depletion.

Ligand and membrane-binding behavior of the phosphatidylinositol transfer proteins PITPα and PITPβ

Phosphatidylinositol transfer proteins (PITPs) are believed to be lipid transfer proteins because of their ability to transfer either phosphatidylinositol (PI) or phosphatidylcholine (PC) between membrane compartments, in vitro. However, the detailed mechanism of this transfer process is not fully established. To further understand the transfer mechanism of PITPs we examined the interaction of PITPs with membranes using dual polarization interferometry (DPI), which measures protein binding affinity on a flat immobilized lipid surface. In addition, a fluorescence resonance energy transfer (FRET)-based assay was also employed to monitor how quickly PITPs transfer their ligands to lipid vesicles. DPI analysis revealed that PITPβ had a higher affinity to membranes compared with PITPα. Furthermore, the FRET-based transfer assay revealed that PITPβ has a higher ligand transfer rate compared with PITPα. However, both PITPα and PITPβ demonstrated a preference for highly curved membrane surfaces during ligand transfer. In other words, ligand transfer rate was higher when the accepting vesicles were highly curved.

Vitamin E and Phosphoinositides Regulate the Intracellular Localization of the Hepatic α-Tocopherol Transfer Protein

α-Tocopherol (vitamin E) is an essential nutrient for all vertebrates. From the eight naturally occurring members of the vitamin E family, α-tocopherol is the most biologically active species and is selectively retained in tissues. The hepatic α-tocopherol transfer protein (TTP) preferentially selects dietary α-tocopherol and facilitates its transport through the hepatocyte and its secretion to the circulation. In doing so, TTP regulates body-wide levels of α-tocopherol. The mechanisms by which TTP facilitates α-tocopherol trafficking in hepatocytes are poorly understood. We found that the intracellular localization of TTP in hepatocytes is dynamic and responds to the presence of α-tocopherol. In the absence of the vitamin, TTP is localized to perinuclear vesicles that harbor CD71, transferrin, and Rab8, markers of the recycling endosomes. Upon treatment with α-tocopherol, TTP- and α-tocopherol-containing vesicles translocate to the plasma membrane, prior to secretion of the vitamin to the exterior of the cells. The change in TTP localization is specific to α-tocopherol and is time- and dose-dependent. The aberrant intracellular localization patterns of lipid binding-defective TTP mutants highlight the importance of protein-lipid interaction in the transport of α-tocopherol. These findings provide the basis for a proposed mechanistic model that describes TTP-facilitated trafficking of α-tocopherol through hepatocytes.

Synthesis and characterization of a fluorescent probe for α-tocopherol suitable for fluorescence microscopy

Previously prepared fluorescent derivatives of α-tocopherol have shown tremendous utility in both in vitro exploration of the mechanism of ligand transfer by the α-tocopherol transfer protein (α-TTP) and the intracellular transport of α-tocopherol in cells and tissues. We report here the synthesis of a 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) containing α-tocopherol analog having extended conjugation with an alkenyl thiophene group that extends the absorption and emission maxima to longer wavelengths (λex=571nm and λem=583nm). The final fluorophore thienyl-ene-BODIPY-α-tocopherol, 2, binds to recombinant human α-TTP with a Kd=8.7±1.1nM and is a suitable probe for monitoring the secretion of α-tocopherol from cultured Mcf7#189 cells.

Complexity of vitamin E metabolism

Bioavailability of vitamin E is influenced by several factors, most are highlighted in this review. While gender, age and genetic constitution influence vitamin E bioavailability but cannot be modified, life-style and intake of vitamin E can be. Numerous factors must be taken into account however, i.e., when vitamin E is orally administrated, the food matrix may contain competing nutrients. The complex metabolic processes comprise intestinal absorption, vascular transport, hepatic sorting by intracellular binding proteins, such as the significant α-tocopherol-transfer protein, and hepatic metabolism. The coordinated changes involved in the hepatic metabolism of vitamin E provide an effective physiological pathway to protect tissues against the excessive accumulation of, in particular, non-α-tocopherol forms. Metabolism of vitamin E begins with one cycle of CYP4F2/CYP3A4-dependent ω-hydroxylation followed by five cycles of subsequent β-oxidation, and forms the water-soluble end-product carboxyethylhydroxychroman. All known hepatic metabolites can be conjugated and are excreted, depending on the length of their side-chain, either via urine or feces. The physiological handling of vitamin E underlies kinetics which vary between the different vitamin E forms. Here, saturation of the side-chain and also substitution of the chromanol ring system are important. Most of the metabolic reactions and processes that are involved with vitamin E are also shared by other fat soluble vitamins. Influencing interactions with other nutrients such as vitamin K or pharmaceuticals are also covered by this review. All these processes modulate the formation of vitamin E metabolites and their concentrations in tissues and body fluids. Differences in metabolism might be responsible for the discrepancies that have been observed in studies performed in vivo and in vitro using vitamin E as a supplement or nutrient. To evaluate individual vitamin E status, the analytical procedures used for detecting and quantifying vitamin E and its metabolites are crucial. The latest methods in analytics are presented.

The orchestra of lipid-transfer proteins at the crossroads between metabolism and signaling

Within the eukaryotic cell, more than 1000 species of lipids define a series of membranes essential for cell function. Tightly controlled systems of lipid transport underlie the proper spatiotemporal distribution of membrane lipids, the coordination of spatially separated lipid metabolic pathways, and lipid signaling mediated by soluble proteins that may be localized some distance away from membranes. Alongside the well-established vesicular transport of lipids, non-vesicular transport mediated by a group of proteins referred to as lipid-transfer proteins (LTPs) is emerging as a key mechanism of lipid transport in a broad range of biological processes. More than a hundred LTPs exist in humans and these can be divided into at least ten protein families. LTPs are widely distributed in tissues, organelles and membrane contact sites (MCSs), as well as in the extracellular space. They all possess a soluble and globular domain that encapsulates a lipid monomer and they specifically bind and transport a wide range of lipids. Here, we present the most recent discoveries in the functions and physiological roles of LTPs, which have expanded the playground of lipids into the aqueous spaces of cells.

Mechanisms of recognition and binding of α-TTP to the plasma membrane by multi-scale molecular dynamics simulations

We used multiple sets of simulations both at the atomistic and coarse-grained level of resolution to investigate interaction and binding of α-tochoperol transfer protein (α-TTP) to phosphatidylinositol phosphate lipids (PIPs). Our calculations indicate that enrichment of membranes with such lipids facilitate membrane anchoring. Atomistic models suggest that PIP can be incorporated into the binding cavity of α-TTP and therefore confirm that such protein can work as lipid exchanger between the endosome and the plasma membrane. Comparison of the atomistic models of the α-TTP-PIPs complex with membrane-bound α-TTP revealed different roles for the various basic residues composing the basic patch that is key for the protein/ligand interaction. Such residues are of critical importance as several point mutations at their position lead to severe forms of ataxia with vitamin E deficiency (AVED) phenotypes. Specifically, R221 is main residue responsible for the stabilization of the complex. R68 and R192 exchange strong interactions in the protein or in the membrane complex only, suggesting that the two residues alternate contact formation, thus facilitating lipid flipping from the membrane into the protein cavity during the lipid exchange process. Finally, R59 shows weaker interactions with PIPs anyway with a clear preference for specific phosphorylation positions, hinting a role in early membrane selectivity for the protein. Altogether, our simulations reveal significant aspects at the atomistic scale of interactions of α-TTP with the plasma membrane and with PIP, providing clarifications on the mechanism of intracellular vitamin E trafficking and helping establishing the role of key residue for the functionality of α-TTP.

α-Tocopherol's Location in Membranes Is Not Affected by Their Composition

To this day, α-tocopherol's (aToc) role in humans is not well known. In previous studies, we have tried to connect aToc's biological function with its location in a lipid bilayer. In the present study, we have determined, by means of small-angle neutron diffraction, that not only is aToc's hydroxyl group located high in the membrane but its tail also resides far from the center of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayers. In addition, we located aToc's hydroxyl group above the lipid backbone in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE), 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-L-serine (POPS), and sphingomyelin bilayers, suggesting that aToc's location near the lipid-water interface may be a universal property of vitamin E. In light of these data, how aToc efficiently terminates lipid hydroperoxy radicals at the membrane center remains an open question.

Intracellular transport of fat-soluble vitamins A and E

Vitamins are compounds that are essential for the normal growth, reproduction and functioning of the human body. Of the 13 known vitamins, vitamins A, D, E and K are lipophilic compounds and are therefore called fat-soluble vitamins. Because of their lipophilicity, fat-soluble vitamins are solubilized and transported by intracellular carrier proteins to exert their actions and to be metabolized properly. Vitamin A and its derivatives, collectively called retinoids, are solubilized by intracellular retinoid-binding proteins such as cellular retinol-binding protein (CRBP), cellular retinoic acid-binding protein (CRABP) and cellular retinal-binding protein (CRALBP). These proteins act as chaperones that regulate the metabolism, signaling and transport of retinoids. CRALBP-mediated intracellular retinoid transport is essential for vision in human. α-Tocopherol, the main form of vitamin E found in the body, is transported by α-tocopherol transfer protein (α-TTP) in hepatic cells. Defects of α-TTP cause vitamin E deficiency and neurological disorders in humans. Recently, it has been shown that the interaction of α-TTP with phosphoinositides plays a critical role in the intracellular transport of α-tocopherol and is associated with familial vitamin E deficiency. In this review, we summarize the mechanisms and biological significance of the intracellular transport of vitamins A and E.

Mechanisms for the prevention of vitamin E excess

The liver is at the nexus of the regulation of lipoprotein uptake, synthesis, and secretion, and it is the site of xenobiotic detoxification by cytochrome P450 oxidation systems (phase I), conjugation systems (phase II), and transporters (phase III). These two major liver systems control vitamin E status. The mechanisms for the preference for α-tocopherol relative to the eight naturally occurring vitamin E forms largely depend upon the liver and include both a preferential secretion of α-tocopherol from the liver into the plasma for its transport in circulating lipoproteins for subsequent uptake by tissues, as well as the preferential hepatic metabolism of non-α-tocopherol forms. These mechanisms are the focus of this review.

Altered vitamin E status in Niemann-Pick type C disease

Vitamin E (α-tocopherol) is the major lipid-soluble antioxidant in many species. Niemann-Pick type C (NPC) disease is a lysosomal storage disorder caused by mutations in the NPC1 or NPC2 gene, which regulates lipid transport through the endocytic pathway. NPC disease is characterized by massive intracellular accumulation of unesterified cholesterol and other lipids in lysosomal vesicles. We examined the roles that NPC1/2 proteins play in the intracellular trafficking of tocopherol. Reduction of NPC1 or NPC2 expression or function in cultured cells caused a marked lysosomal accumulation of vitamin E in cultured cells. In vivo, tocopherol significantly accumulated in murine Npc1-null and Npc2-null livers, Npc2-null cerebella, and Npc1-null cerebral cortices. Plasma tocopherol levels were within the normal range in Npc1-null and Npc2-null mice, and in plasma samples from human NPC patients. The binding affinity of tocopherol to the purified sterol-binding domain of NPC1 and to purified NPC2 was significantly weaker than that of cholesterol (measurements kindly performed by R. Infante, University of Texas Southwestern Medical Center, Dallas, TX). Taken together, our observations indicate that functionality of NPC1/2 proteins is necessary for proper bioavailability of vitamin E and that the NPC pathology might involve tissue-specific perturbations of vitamin E status.

Mammalian diseases of phosphatidylinositol transfer proteins and their homologs

Inositol and phosphoinositide signaling pathways represent major regulatory systems in eukaryotes. The physiological importance of these pathways is amply demonstrated by the variety of diseases that involve derangements in individual steps in inositide and phosphoinositide production and degradation. These diseases include numerous cancers, lipodystrophies and neurological syndromes. Phosphatidylinositol transfer proteins (PITPs) are emerging as fascinating regulators of phosphoinositide metabolism. Recent advances identify PITPs (and PITP-like proteins) to be coincidence detectors, which spatially and temporally coordinate the activities of diverse aspects of the cellular lipid metabolome with phosphoinositide signaling. These insights are providing new ideas regarding mechanisms of inherited mammalian diseases associated with derangements in the activities of PITPs and PITP-like proteins.

The contribution of surface residues to membrane binding and ligand transfer by the α-tocopherol transfer protein (α-TTP)

Previous work has shown that the α-tocopherol transfer protein (α-TTP) can bind to vesicular or immobilized phospholipid membranes. Revealing the molecular mechanisms by which α-TTP associates with membranes is thought to be critical to understanding its function and role in the secretion of tocopherol from hepatocytes into the circulation. Calculations presented in the Orientations of Proteins in Membranes database have provided a testable model for the spatial arrangement of α-TTP and other CRAL-TRIO family proteins with respect to the lipid bilayer. These calculations predicted that a hydrophobic surface mediates the interaction of α-TTP with lipid membranes. To test the validity of these predictions, we used site-directed mutagenesis and examined the substituted mutants with regard to intermembrane ligand transfer, association with lipid layers and biological activity in cultured hepatocytes. Substitution of residues in helices A8 (F165A and F169A) and A10 (I202A, V206A and M209A) decreased the rate of intermembrane ligand transfer as well as protein adsorption to phospholipid bilayers. The largest impairment was observed upon mutation of residues that are predicted to be fully immersed in the lipid bilayer in both apo (open) and holo (closed) conformations such as Phe165 and Phe169. Mutation F169A, and especially F169D, significantly impaired α-TTP-assisted secretion of α-tocopherol outside cultured hepatocytes. Mutation of selected basic residues (R192H, K211A, and K217A) had little effect on transfer rates, indicating no significant involvement of nonspecific electrostatic interactions with membranes.

The location and behavior of alpha-tocopherol in membranes

Vitamin E (alpha-tocopherol) has long been recognized as the major antioxidant in biological membranes, and yet many structurally related questions persist of how the vitamin functions. For example, the very low levels of alpha-tocopherol reported for whole cell extracts question how this molecule can successfully protect the comparatively enormous quantities of PUFA-containing phospholipids found in membranes that are highly susceptible to oxidative attack. The contemporary realization that membranes laterally segregate into regions of distinct lipid composition (domains), we propose, provides the answer. We hypothesize alpha-tocopherol partitions into domains that are enriched in polyunsaturated phospholipids, amplifying the concentration of the vitamin in the place where it is most needed. These highly disordered domains depleted in cholesterol are analogous, but organizationally antithetical, to the well-studied lipid rafts. We review here the ideas that led to our hypothesis. Experimental evidence in support of the formation of PUFA-rich domains in model membranes is presented, focusing upon docosahexaenoic acid that is the most unsaturated fatty acid commonly found. Physical methodologies are then described to elucidate the nature of the interaction of alpha-tocopherol with PUFA and to establish that the vitamin and PUFA-containing phospholipids co-localize in non-raft domains.

Synthesis and characterization of BODIPY-alpha-tocopherol: a fluorescent form of vitamin E

Fluorescent nitrobenzoxadiazole analogues of alpha-tocopherol (NBD-alpha-Tocs; lambda(ex) = 468 nm, lambda(em) = 527 nm) have been made previously to aid study of the intracellular location and transfer of vitamin E. However, these analogues are susceptible to photobleaching while under illumination for confocal microscopy as well as in in vitro FRET transfer assays. Here we report the synthesis of three fluorescent analogues of alpha-tocopherol incorporating the more robust dipyrrometheneboron difluoride (BODIPY) fluorophore. A BODIPY-linked chromanol should have no intervening polar functional groups that might interfere with binding to the hydrophobic binding site of the tocopherol transfer protein (alpha-TTP). A key step in bringing the two ring systems together was a metathesis reaction of vinyl chromanol and an alkenyl BODIPY. An o-tolyl containing second generation Grubbs catalyst was identified as the best catalyst for effecting the metathesis without detectable alkene isomerization, which when it occurred produced a mixture of chain lengths in the alkyl linker. C8-BODIPY-alpha-Toc 10c (lambda(ex) = 507 nm, lambda(em) = 511 nm, epsilon(507) = 83,000 M(-1) cm(-1)) having an eight-carbon chain between the chromanol and fluorophore, had the highest affinity for alpha-TTP (K(d) = 94 +/- 3 nM) and bound specifically as it could not be displaced with cholesterol.