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Polyhedral 3D structure of human plasma very low density lipoproteins by individual particle cryo-electron tomography 1 [S]

View Article: PubMed Central - PubMed

ABSTRACT

Human VLDLs assembled in the liver and secreted into the circulation supply energy to peripheral tissues. VLDL lipolysis yields atherogenic LDLs and VLDL remnants that strongly correlate with CVD. Although the composition of VLDL particles has been well-characterized, their 3D structure is elusive because of their variations in size, heterogeneity in composition, structural flexibility, and mobility in solution. Here, we employed cryo-electron microscopy and individual-particle electron tomography to study the 3D structure of individual VLDL particles (without averaging) at both below and above their lipid phase transition temperatures. The 3D reconstructions of VLDL and VLDL bound to antibodies revealed an unexpected polyhedral shape, in contrast to the generally accepted model of a spherical emulsion-like particle. The smaller curvature of surface lipids compared with HDL may also reduce surface hydrophobicity, resulting in lower binding affinity to the hydrophobic distal end of the N-terminal β-barrel domain of cholesteryl ester transfer protein (CETP) compared with HDL. The directional binding of CETP to HDL and VLDL may explain the function of CETP in transferring TGs and cholesteryl esters between these particles. This first visualization of the 3D structure of VLDL could improve our understanding of the role of VLDL in atherogenesis.

No MeSH data available.


Related in: MedlinePlus

The 3D morphology of VLDL particles by cryo-ET. A: Three representative views of the single-axis tilt series of frozen hydrated VLDLs. B–G: Refinement procedures and results for one VLDL particle (image contrast reversed). B: IPET 3D reconstruction procedures. C, D: Two orthogonal views of refined particles low-pass filtered at 50 Å shown as an iso-surface representation (top) or a reprojection (bottom). The reader faces the Z-axis of the 3D reconstruction when viewing the left panel. E: Resolution was estimated by FSC between two models built from odd- and even-numbered views, respectively. F, G: The XY slices of the 3D maps at different heights are displayed as projection (top) and iso-surface (bottom) views. H–M: IPET 3D reconstruction procedures of another VLDL particle are shown. N: The 3D density maps of six representative VLDL particles are reconstructed and displayed from two perpendicular directions. The reader faces the Z-axis of the 3D reconstruction when viewing the left panel. Each map was low-pass filtered to 50 Å. O: The same maps as in (N) marked with the vertices and edges. P, Q: The 3D density maps of two representative VLDLs are displayed at a high contour level (cyan color) and a low contour level (gray color). The high contour map shows the structure of the high density components, e.g., apolipoproteins of VLDL. Scale bars: 50 nm (A–O); 25 nm (P, Q).
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f2: The 3D morphology of VLDL particles by cryo-ET. A: Three representative views of the single-axis tilt series of frozen hydrated VLDLs. B–G: Refinement procedures and results for one VLDL particle (image contrast reversed). B: IPET 3D reconstruction procedures. C, D: Two orthogonal views of refined particles low-pass filtered at 50 Å shown as an iso-surface representation (top) or a reprojection (bottom). The reader faces the Z-axis of the 3D reconstruction when viewing the left panel. E: Resolution was estimated by FSC between two models built from odd- and even-numbered views, respectively. F, G: The XY slices of the 3D maps at different heights are displayed as projection (top) and iso-surface (bottom) views. H–M: IPET 3D reconstruction procedures of another VLDL particle are shown. N: The 3D density maps of six representative VLDL particles are reconstructed and displayed from two perpendicular directions. The reader faces the Z-axis of the 3D reconstruction when viewing the left panel. Each map was low-pass filtered to 50 Å. O: The same maps as in (N) marked with the vertices and edges. P, Q: The 3D density maps of two representative VLDLs are displayed at a high contour level (cyan color) and a low contour level (gray color). The high contour map shows the structure of the high density components, e.g., apolipoproteins of VLDL. Scale bars: 50 nm (A–O); 25 nm (P, Q).

Mentions: To confirm the observation of the angular shape of VLDL in 3D, we imaged the samples from a series of tilting angles by cryo-ET under low-dose mode with a total dose of ∼150 e−/Å2 and a magnification of 50 k× (corresponding to 2.4 Å/pixel) (supplemental Video S1). The survey of cryo-ET micrographs at three representative angles displayed eight VLDL particles whose diameters and shapes differed substantially from each other, such that they could not be averaged for 3D reconstruction (Fig. 2A); therefore, we used the IPET reconstruction method we developed to reconstruct the 3D density maps for single VLDL particles.


Polyhedral 3D structure of human plasma very low density lipoproteins by individual particle cryo-electron tomography 1 [S]
The 3D morphology of VLDL particles by cryo-ET. A: Three representative views of the single-axis tilt series of frozen hydrated VLDLs. B–G: Refinement procedures and results for one VLDL particle (image contrast reversed). B: IPET 3D reconstruction procedures. C, D: Two orthogonal views of refined particles low-pass filtered at 50 Å shown as an iso-surface representation (top) or a reprojection (bottom). The reader faces the Z-axis of the 3D reconstruction when viewing the left panel. E: Resolution was estimated by FSC between two models built from odd- and even-numbered views, respectively. F, G: The XY slices of the 3D maps at different heights are displayed as projection (top) and iso-surface (bottom) views. H–M: IPET 3D reconstruction procedures of another VLDL particle are shown. N: The 3D density maps of six representative VLDL particles are reconstructed and displayed from two perpendicular directions. The reader faces the Z-axis of the 3D reconstruction when viewing the left panel. Each map was low-pass filtered to 50 Å. O: The same maps as in (N) marked with the vertices and edges. P, Q: The 3D density maps of two representative VLDLs are displayed at a high contour level (cyan color) and a low contour level (gray color). The high contour map shows the structure of the high density components, e.g., apolipoproteins of VLDL. Scale bars: 50 nm (A–O); 25 nm (P, Q).
© Copyright Policy - open-access
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC5036368&req=5

f2: The 3D morphology of VLDL particles by cryo-ET. A: Three representative views of the single-axis tilt series of frozen hydrated VLDLs. B–G: Refinement procedures and results for one VLDL particle (image contrast reversed). B: IPET 3D reconstruction procedures. C, D: Two orthogonal views of refined particles low-pass filtered at 50 Å shown as an iso-surface representation (top) or a reprojection (bottom). The reader faces the Z-axis of the 3D reconstruction when viewing the left panel. E: Resolution was estimated by FSC between two models built from odd- and even-numbered views, respectively. F, G: The XY slices of the 3D maps at different heights are displayed as projection (top) and iso-surface (bottom) views. H–M: IPET 3D reconstruction procedures of another VLDL particle are shown. N: The 3D density maps of six representative VLDL particles are reconstructed and displayed from two perpendicular directions. The reader faces the Z-axis of the 3D reconstruction when viewing the left panel. Each map was low-pass filtered to 50 Å. O: The same maps as in (N) marked with the vertices and edges. P, Q: The 3D density maps of two representative VLDLs are displayed at a high contour level (cyan color) and a low contour level (gray color). The high contour map shows the structure of the high density components, e.g., apolipoproteins of VLDL. Scale bars: 50 nm (A–O); 25 nm (P, Q).
Mentions: To confirm the observation of the angular shape of VLDL in 3D, we imaged the samples from a series of tilting angles by cryo-ET under low-dose mode with a total dose of ∼150 e−/Å2 and a magnification of 50 k× (corresponding to 2.4 Å/pixel) (supplemental Video S1). The survey of cryo-ET micrographs at three representative angles displayed eight VLDL particles whose diameters and shapes differed substantially from each other, such that they could not be averaged for 3D reconstruction (Fig. 2A); therefore, we used the IPET reconstruction method we developed to reconstruct the 3D density maps for single VLDL particles.

View Article: PubMed Central - PubMed

ABSTRACT

Human VLDLs assembled in the liver and secreted into the circulation supply energy to peripheral tissues. VLDL lipolysis yields atherogenic LDLs and VLDL remnants that strongly correlate with CVD. Although the composition of VLDL particles has been well-characterized, their 3D structure is elusive because of their variations in size, heterogeneity in composition, structural flexibility, and mobility in solution. Here, we employed cryo-electron microscopy and individual-particle electron tomography to study the 3D structure of individual VLDL particles (without averaging) at both below and above their lipid phase transition temperatures. The 3D reconstructions of VLDL and VLDL bound to antibodies revealed an unexpected polyhedral shape, in contrast to the generally accepted model of a spherical emulsion-like particle. The smaller curvature of surface lipids compared with HDL may also reduce surface hydrophobicity, resulting in lower binding affinity to the hydrophobic distal end of the N-terminal β-barrel domain of cholesteryl ester transfer protein (CETP) compared with HDL. The directional binding of CETP to HDL and VLDL may explain the function of CETP in transferring TGs and cholesteryl esters between these particles. This first visualization of the 3D structure of VLDL could improve our understanding of the role of VLDL in atherogenesis.

No MeSH data available.


Related in: MedlinePlus