Niemann-Pick C2 protein regulates sterol transport between plasma membrane and late endosomes in human fibroblasts.

Niemann-Pick disease type C2 is a lipid storage disorder in which mutations in the NPC2 protein cause accumulation of lipoprotein-derived cholesterol in late endosomes and lysosomes (LE/LYSs). Whether cholesterol delivered by other means to NPC2 deficient cells also accumulates in LE/LYSs is currently unknown. We show that the close cholesterol analog dehydroergosterol (DHE), when delivered to the plasma membrane (PM) accumulates in LE/LYSs of human fibroblasts lacking functional NPC2. We measured two different time scales of sterol diffusion; while DHE rich LE/LYSs moved by slow anomalous diffusion in disease cells (D ∼ 4.6∙10-4 µm2/sec; α∼0.76), a small pool of sterol could exchange rapidly with D ∼ 3 µm2/s between LE/LYSs, as shown by fluorescence recovery after photobleaching (FRAP). By quantitative lipid mass spectrometry we found that esterification of 13C-labeled cholesterol but not of DHE is reduced 10-fold in disease fibroblasts compared to control cells. Internalized NPC2 rescued the sterol storage phenotype and strongly expanded the dynamic sterol pool seen in FRAP experiments. Together, our study shows that cholesterol esterification and trafficking of sterols between the PM and LE/LYSs depends on a functional NPC2 protein. NPC2 likely acts inside LE/LYSs from where it increases non-vesicular sterol exchange with other organelles.

Ergosterol is mainly located in the cytoplasmic leaflet of the yeast plasma membrane.

Transbilayer lipid asymmetry is a fundamental characteristic of the eukaryotic cell plasma membrane (PM). While PM phospholipid asymmetry is well documented, the transbilayer distribution of PM sterols such as mammalian cholesterol and yeast ergosterol is not reliably known. We now report that sterols are asymmetrically distributed across the yeast PM, with the majority (~80%) located in the cytoplasmic leaflet. By exploiting the sterol-auxotrophic hem1Δ yeast strain we obtained cells in which endogenous ergosterol was quantitatively replaced with dehydroergosterol (DHE), a closely related fluorescent sterol that functionally and accurately substitutes for ergosterol in vivo. Using fluorescence spectrophotometry and microscopy we found that <20% of DHE fluorescence was quenched when the DHE-containing cells were exposed to membrane-impermeant collisional quenchers (spin-labeled phosphatidylcholine and trinitrobenzene sulfonic acid). Efficient quenching was seen only after the cells were disrupted by glass-bead lysis or repeated freeze-thaw to allow quenchers access to the cell interior. The extent of quenching was unaffected by treatments that deplete cellular ATP levels, collapse the PM electrochemical gradient or affect the actin cytoskeleton. However, alterations in PM phospholipid asymmetry in cells lacking phospholipid flippases resulted in a more symmetric transbilayer distribution of sterol. Similarly, an increase in the quenchable pool of DHE was observed when PM sphingolipid levels were reduced by treating cells with myriocin. We deduce that sterols comprise up to ~45% of all inner leaflet lipids in the PM, a result that necessitates revision of current models of the architecture of the PM lipid bilayer.

Fluorescent Sterols and Cholesteryl Esters as Probes for Intracellular Cholesterol Transport.

Cholesterol transport between cellular organelles comprised vesicular trafficking and nonvesicular exchange; these processes are often studied by quantitative fluorescence microscopy. A major challenge for using this approach is producing analogs of cholesterol with suitable brightness and structural and chemical properties comparable with those of cholesterol. This review surveys currently used fluorescent sterols with respect to their behavior in model membranes, their photophysical properties, as well as their transport and metabolism in cells. In the first part, several intrinsically fluorescent sterols, such as dehydroergosterol or cholestatrienol, are discussed. These polyene sterols (P-sterols) contain three conjugated double bonds in the steroid ring system, giving them slight fluorescence in ultraviolet light. We discuss the properties of P-sterols relative to cholesterol, outline their chemical synthesis, and explain how to image them in living cells and organisms. In particular, we show that P-sterol esters inserted into low-density lipoprotein can be tracked in the fibroblasts of Niemann-Pick disease using high-resolution deconvolution microscopy. We also describe fluorophore-tagged cholesterol probes, such as BODIPY-, NBD-, Dansyl-, or Pyrene-tagged cholesterol, and eventual esters of these analogs. Finally, we survey the latest developments in the synthesis and use of alkyne cholesterol analogs to be labeled with fluorophores by click chemistry and discuss the potential of all approaches for future applications.

Felodipine-diazabicyclo[2.2.2]octane-water (1/1/1).

The title compound, C(18)H(19)Cl(2)NO(4)·C(6)H(12)N(2)·H(2)O, is a cocrystal hydrate containing the active pharmaceutical ingredient felodipine and diazabicyclo[2.2.2]octane (DABCO). The DABCO and water molecules are linked through O-H···N hydrogen bonds into chains around 2(1) screw axes, while the felodipine molecules form N-H···O hydrogen bonds to the water molecules. The felodipine molecules adopt centrosymmetric back-to-back arrangements that are similar to those present in all of its four reported polymorphs. The dichlorophenyl rings also form π-stacking interactions. The inclusion of water molecules in the cocrystal, rather than formation of N-H···N hydrogen bonds between felodipine and DABCO, may be associated with steric hindrance that would arise between DABCO and the methyl groups of felodipine if they were directly involved in hydrogen bonding.

2-[(2-Acet-oxy-benzo-yl)-oxy]benzoic acid.

The title compound, C(16)H(12)O(6), is a common impurity of ortho-acetyl-salicylic acid (aspirin). The benzene rings form a dihedral angle of 81.9 (1)° while the acetyl and carboxyl groups form dihedral angles of 74.0 (1) and 26.4 (2)°, respectively, with the benzene rings to which they are bound. In the crystal, mol-ecules are linked by pairs of O-H⋯O hydrogen bonds between the carboxyl groups, forming inversion dimers.

Intermolecular interactions and unexpected isostructurality in the crystal structures of the dichlorobenzaldehyde isomers.

The crystal structures of the six dichlorobenzaldehyde isomers, four of them newly determined, are analyzed in terms of the geometry and energies of their intermolecular interactions, quantified using the semi-classical density sums (SCDS-PIXEL) method. A consistent feature in all six structures is molecular stacks propagating along a short crystallographic axis of ca 3.8 Å. The stacks have a closely comparable geometry in each isomer, but the interaction energies between stacked molecules are variable on account of the differing relative positions of the Cl substituents. In the majority of the isomers the stacking interactions are the most stabilizing in the structure. Exceptions are the 2,4- and 3,5-isomers, where more stabilizing interactions are made between stacks. In general, the most stabilizing non-stacking intermolecular interactions in the structures are those involving C-H...O contacts. Observed motifs based on Cl...Cl interactions appear to be largely imposed by the constraints of other more stabilizing intermolecular interactions. The isomeric series displays the following noteworthy features: (i) the 2,3- and 2,6-isomers are isostructural despite having different orientations of the Cl and aldehyde functionalities; (ii) the 2,5-isomer exhibits whole-molecule disorder; (iii) the 2,5- and 3,5-isomers have more than one molecule in the crystallographic asymmetric unit (Z' > 1). These features in particular are considered on the basis of the intermolecular interaction energies.

2,6-Bis(2,4-dimethyl-benzyl-idene)cyclo-hexa-none.

In the crystal structure of the title compound, C(24)H(6)O, the mol-ecule exhibits point symmetry m but the mirror plane is not utilized as part of the space-group symmetry. The structure contains face-to-face inter-actions between the 2,4-dimethyl-benzyl-idene substituents in which the methyl groups lie directly above the centroids of adjacent benzene rings.