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1.  EPR assessment of protein sites for incorporation of Gd(III) MRI contrast labels 
We have engineered Apolipoprotein A-I (apoA-I), a major protein constituent of high-density lipoprotein (HDL), to contain DOTA-chelated Gd(III) as an MRI contrast agent for the purpose of imaging reconstituted HDL (rHDL) biodistribution, metabolism, and regulation in vivo. This protein contrast agent was obtained by reacting the thiol-reactive Gd[MTS-ADO3A] label with Cys engineered at four distinct positions (52, 55, 76 and 80) in apoA-I. MRI of infused mice previously showed that the Gd-labeled apoA-I migrates to both the liver and the kidney, the organs responsible for HDL catabolism; however, the contrast properties of apoA-I are superior when the ADO3A moiety is located at position 55, compared to the protein labeled at positions 52, 76 or 80. It is shown here that continuous wave X-band (9 GHz) EPR spectroscopy is capable of detecting differences in the Gd(III) signal when comparing the labeled protein in the lipid-free to the rHDL state. Furthermore, the values of NMR relaxivity obtained for labeled variants in both the lipid-free and rHDL states correlate to the product of the X-band Gd(III) spectral width and the collision frequency between a nitroxide spin label and a polar relaxation agent. Consistent with its superior relaxivity measured by NMR, the rHDL-associated apoA-I containing the Gd[MTS-ADO3A] probe attached to position 55 displays favorable dynamic and water accessibility properties as determined by X-band EPR. While room temperature EPR requires >1 mM Gd(III)-labeled and only >10 μM nitroxide-labeled protein to resolve the spectrum, the volume requirement is exceptionally low (~5μL). Thus, X-band EPR provides a practical assessment for the suitability of imaging candidates containing the site-directed ADO3A contrast probe.
PMCID: PMC3633150  PMID: 23606429
apolipoprotein A-I; magnetic resonance imaging (MRI); electron paramagnetic resonance (EPR) spectroscopy; protein contrast agent; high-density lipoprotein (HDL); site-directed MRI label
2.  Secondary Structure Changes in ApoA-I Milano (R173C) Are Not Accompanied by a Decrease in Protein Stability or Solubility 
PLoS ONE  2014;9(4):e96150.
Apolipoprotein A-I (apoA-I) is the main protein of high-density lipoprotein (HDL) and a principal mediator of the reverse cholesterol transfer pathway. Variants of apoA-I have been shown to be associated with hereditary amyloidosis. We previously characterized the G26R and L178H variants that both possess decreased stability and increased fibril formation propensity. Here we investigate the Milano variant of apoAI (R173C; apoAI-M), which despite association with low plasma levels of HDL leads to low prevalence of cardiovascular disease in carriers of this mutation. The R173C substitution is located to a region (residues 170 to 178) that contains several fibrillogenic apoA-I variants, including the L178H variant, and therefore we investigated a potential fibrillogenic property of the apoAI-M protein. Despite the fact that apoAI-M shared several features with the L178H variant regarding increased helical content and low degree of ThT binding during prolonged incubation in physiological buffer, our electron microscopy analysis revealed no formation of fibrils. These results suggest that mutations inducing secondary structural changes may be beneficial in cases where fibril formation does not occur.
PMCID: PMC3995965  PMID: 24755625
3.  Single injections of apoA-I acutely improve in vivo glucose tolerance in insulin-resistant mice 
Diabetologia  2014;57:797-800.
Apolipoprotein A-I (apoA-I), the main protein constituent of HDL, has a central role in the reverse cholesterol-transport pathway, which together with the anti-inflammatory properties of apoA-I/HDL provide cardioprotection. Recent findings of direct stimulation of glucose uptake in muscle by apoA-I/HDL suggest that altered apoA-I and HDL functionality may be a contributing factor to the development of diabetes. We have studied the in vivo effects of short treatments with human apoA-I in a high-fat diet fed mouse model. In addition to native apoA-I, we investigated the effects of the cardioprotective Milano variant (Arg173Cys).
Male C57Bl6 mice on a high-fat diet for 2 weeks that received a single injection of human apoA-I proteins (wild-type and Milano) were analysed for blood glucose and insulin levels during a 3 h incubation followed by glucose tolerance tests. Incorporation of injected human apoA-I protein into HDLs was analysed by native gel electrophoresis.
ApoA-I treatment significantly improved insulin secretion and blood glucose clearance in the glucose tolerance test, with an efficiency exceeding that of lean control animals, and led to decreased basal glucose during the 3 h incubation. Notably, the two apoA-I variants triggered insulin secretion and glucose clearance to the same extent.
ApoA-I treatment leads to insulin- and non-insulin-dependent effects on glucose homeostasis. The experimental model of short-term (2 weeks) feeding of a high-fat diet to C57Bl6 mice provides a suitable and time-efficient system to unravel the resulting tissue-specific mechanisms of acute apoA-I treatment that lead to improved glucose homeostasis.
PMCID: PMC3940850  PMID: 24442447
ApoA-I; Apolipoprotein A-I; Diabetes; Glucose metabolism; HDL; Insulin; Insulin resistance
4.  Imaging apolipoprotein AI in vivo 
NMR in biomedicine  2011;24(7):916-924.
Coronary disease risk increases inversely with high-density lipoprotein (HDL) level. The measurement of the biodistribution and clearance of HDL in vivo, however, has posed a technical challenge. This study presents an approach to the development of a lipoprotein MRI agent by linking gadolinium methanethiosulfonate (Gd[MTS-ADO3A]) to a selective cysteine mutation in position 55 of apo AI, the major protein of HDL. The contrast agent targets both liver and kidney, the sites of HDL catabolism, whereas the standard MRI contrast agent, gadolinium-diethylenetriaminepentaacetic acid-bismethylamide (GdDTPA-BMA, gadodiamide), enhances only the kidney image. Using a modified apolipoprotein AI to create an HDL contrast agent provides a new approach to investigate HDL biodistribution, metabolism and regulation in vivo.
PMCID: PMC3726305  PMID: 21264979
NMR; apolipoprotein; HDL; contrast agent; MRI; gadolinium; cardiovascular risk
Biochemistry  2007;46(34):9693-9699.
The single amino acid mutation G26R in human apolipoprotein A-I (apoA-IIOWA) leads to the formation of β-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 pro-amyloidogenic 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 native apo-protein separates a preceding β-strand structure (residues 20–25) from a downstream largely α-helical region. The current study demonstrates that the G26R variant promotes a structural transition of positions 27–56 to a mixture of coil and β-strand secondary structure. Microscopy and staining by amyloidophilic dyes suggest that this alteration extends throughout the protein within one 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 β-strand propagation and the promotion of β structure when an Arg is introduced adjacent to succeeding residue of identical charge and size, Arg27.
PMCID: PMC3650831  PMID: 17665932
6.  Discoidal HDL and apoA-I-derived peptides improve glucose uptake in skeletal muscle 
Journal of Lipid Research  2013;54(5):1275-1282.
Lipid-free apoA-I and mature spherical HDL have been shown to induce glucose uptake in skeletal muscle. To exploit apoA-I and HDL states for diabetes therapy, further understanding of interaction between muscle and apoA-I is required. This study has examined whether nascent discoidal HDL, in which apoA-I attains a different conformation from mature HDL and lipid-free states, could induce muscle glucose uptake and whether a specific domain of apoA-I can mediate this effect. Using L6 myotubes stimulated with synthetic reconstituted discoidal HDL (rHDL), we show a glucose uptake effect comparable to insulin. Increased plasma membrane GLUT4 levels in ex vivo rHDL-stimulated myofibers from HA-GLUT4-GFP transgenic mice support this observation. rHDL increased phosphorylation of AMP kinase (AMPK) and acetyl-coA carboxylase (ACC) but not Akt. A survey of domain-specific peptides of apoA-I showed that the lipid-free C-terminal 190–243 fragment increases plasma membrane GLUT4, promotes glucose uptake, and activates AMPK signaling but not Akt. This may be explained by changes in α-helical content of 190–243 fragment versus full-length lipid-free apoA-I as assessed by circular dichroism spectroscopy. Discoidal HDL and the 190–243 peptide of apoA-I are potent agonists of glucose uptake in skeletal muscle, and the C-terminal α-helical content of apoA-I may be an important determinant of this effect.
PMCID: PMC3653404  PMID: 23471027
muscle fiber; GLUT4 transporter; diabetes; insulin resistance; apolipoprotein A-I; high density lipoprotein
7.  The fibrillogenic L178H variant of apolipoprotein A-I forms helical fibrils 
Journal of Lipid Research  2012;53(3):390-398.
A number of amyloidogenic variants of apoA-I have been discovered but most have not been analyzed. Previously, we showed that the G26R mutation of apoA-I leads to increased β-strand structure, increased N-terminal protease susceptibility, and increased fibril formation after several days of incubation. In vivo, this and other variants mutated in the N-terminal domain (residues 26 to ∼90) lead to renal and hepatic accumulation. In contrast, several mutations identified within residues 170 to 178 lead to cardiac, laryngeal, and cutaneous protein deposition. Here, we describe the structural changes in the fibrillogenic variant L178H. Like G26R, the initial structure of the protein exhibits altered tertiary conformation relative to wild-type protein along with decreased stability and an altered lipid binding profile. However, in contrast to G26R, L178H undergoes an increase in helical structure upon incubation at 37°C with a half time (t1/2) of about 12 days. Upon prolonged incubation, the L178H mutant forms fibrils of a diameter of 10 nm that ranges in length from 30 to 120 nm. These results show that apoA-I, known for its dynamic properties, has the ability to form multiple fibrillar conformations, which may play a role in the tissue-specific deposition of the individual variants.
PMCID: PMC3276462  PMID: 22184756
high density lipoprotein; apoA-I; fibril formation; amyloid; protein stability
8.  The “Beta-Clasp” model of apolipoprotein A-I - a lipid-free solution structure determined by electron paramagnetic resonance spectroscopy 
Biochimica et Biophysica Acta  2012;1821(3):448-455.
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.
PMCID: PMC3402359  PMID: 22245143
9.  Postprandial apoE Isoform and Conformational Changes Associated with VLDL Lipolysis Products Modulate Monocyte Inflammation 
PLoS ONE  2012;7(11):e50513.
Postprandial hyperlipemia, characterized by increased circulating very low-density lipoproteins (VLDL) and circulating lipopolysaccharide (LPS), has been proposed as a mechanism of vascular injury. Our goal was to examine the interactions between postprandial lipoproteins, LPS, and apoE3 and apoE4 on monocyte activation.
Methods and Results
We showed that apoE3 complexed to phospholipid vesicles attenuates LPS-induced THP-1 monocyte cytokine expression, while apoE4 increases expression. ELISA revealed that apoE3 binds to LPS with higher affinity than apoE4. Electron paramagnetic resonance (EPR) spectroscopy of site-directed spin labels placed on specific amino acids of apoE3 showed that LPS interferes with conformational changes normally associated with lipid binding. Specifically, compared to apoE4, apoE bearing the E3-like R112→Ser mutation displays increased self association when exposed to LPS, consistent with a stronger apoE3-LPS interaction. Additionally, lipolysis of fasting VLDL from normal human donors attenuated LPS-induced TNFα secretion from monocytes to a greater extent than postprandial VLDL, an effect partially reversed by blocking apoE. This effect was reproduced using fasting VLDL lipolysis products from e3/e3 donors, but not from e4/e4 subjects, suggesting that apoE3 on fasting VLDL prevents LPS-induced inflammation more readily than apoE4.
Postprandial apoE isoform and conformational changes associated with VLDL dramatically modulate vascular inflammation.
PMCID: PMC3509065  PMID: 23209766
10.  The intrinsic GTPase activity of the Gtr1 protein from Saccharomyces cerevisiae 
BMC Biochemistry  2012;13:11.
The Gtr1 protein of Saccharomyces cerevisiae is a member of the RagA subfamily of the Ras-like small GTPase superfamily. Gtr1 has been implicated in various cellular processes. Particularly, the Switch regions in the GTPase domain of Gtr1 are essential for TORC1 activation and amino acid signaling. Therefore, knowledge about the biochemical activity of Gtr1 is required to understand its mode of action and regulation.
By employing tryptophan fluorescence analysis and radioactive GTPase assays, we demonstrate that Gtr1 can adopt two distinct GDP- and GTP-bound conformations, and that it hydrolyses GTP much slower than Ras proteins. Using cysteine mutagenesis of Arginine-37 and Valine-67, residues at the Switch I and II regions, respectively, we show altered GTPase activity and associated conformational changes as compared to the wild type protein and the cysteine-less mutant.
The extremely low intrinsic GTPase activity of Gtr1 implies requirement for interaction with activating proteins to support its physiological function. These findings as well as the altered properties obtained by mutagenesis in the Switch regions provide insights into the function of Gtr1 and its homologues in yeast and mammals.
PMCID: PMC3477016  PMID: 22726655
Gtr1; GTPase; Intrinsic tryptophan fluorescence; Rag GTPase; Cysteine mutagenesis; Switch region
11.  The secondary structure of apolipoprotein A-I on 9.6-nm reconstituted high-density lipoprotein determined by EPR spectroscopy 
The Febs Journal  2013;280(14):3416-3424.
Apolipoprotein A-I (ApoA-I) is the major protein component of high-density lipoprotein (HDL), and is critical for maintenance of cholesterol homeostasis. During reverse cholesterol transport, HDL transitions between an array of subclasses, differing in size and composition. This process requires ApoA-I to adapt to changes in the shape of the HDL particle, transiting from an apolipoprotein to a myriad of HDL subclass-specific conformations. Changes in ApoA-I structure cause alterations in HDL-specific enzyme and receptor-binding properties, and thereby direct the HDL particle through the reverse cholesterol transport pathway. In this study, we used site-directed spin label spectroscopy to examine the conformational details of the ApoA-I central domain on HDL. The motional dynamics and accessibility to hydrophobic/hydrophilic relaxation agents of ApoA-I residues 99–163 on 9.6-nm reconstituted HDL was analyzed by EPR. In previous analyses, we examined residues 6–98 and 164–238 (of ApoA-I's 243 residues), and combining these findings with the current results, we have generated a full-length map of the backbone structure of reconstituted HDL-associated ApoA-I. Remarkably, given that the majority of ApoA-I's length is composed of amphipathic helices, we have identified nonhelical residues, specifically the presence of a β-strand (residues 149–157). The significance of these nonhelical residues is discussed, along with the other features, in the context of ApoA-I function in contrast to recent models derived by other methods.
PMCID: PMC3906832  PMID: 23668303
apolipoprotein A-I (ApoA-I); cardiovascular; cholesterol; EPR spectroscopy; high-density lipoprotein (HDL)

Results 1-11 (11)