Molecular Mechanism of Cyclodextrin Mediated Cholesterol Extraction

The depletion of cholesterol from membranes, mediated by β-cyclodextrin (β-CD) is well known and documented, but the molecular details of this process are largely unknown. Using molecular dynamics simulations, we have been able to study the CD mediated extraction of cholesterol from model membranes, in particular from a pure cholesterol monolayer, at atomic resolution. Our results show that efficient cholesterol extraction depends on the structural distribution of the CDs on the surface of the monolayer. With a suitably oriented dimer, cholesterol is extracted spontaneously on a nanosecond time scale. Additional free energy calculations reveal that the CDs have a strong affinity to bind to the membrane surface, and, by doing so, destabilize the local packing of cholesterol molecules making their extraction favorable. Our results have implications for the interpretation of experimental measurements, and may help in the rational design of efficient CD based nano-carriers

Cyclodextrins in Drug Delivery Systems and Their Effects on Biological Barriers

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Isoprenoids determine Th1/Th2 fate in pathogenic T cells, providing a mechanism of modulation of autoimmunity by atorvastatin.

3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase is a critical enzyme in the mevalonate pathway that regulates the biosynthesis of cholesterol as well as isoprenoids that mediate the membrane association of certain GTPases. Blockade of this enzyme by atorvastatin (AT) inhibits the destructive proinflammatory T helper cell (Th)1 response during experimental autoimmune encephalomyelitis and may be beneficial in the treatment of multiple sclerosis and other Th1-mediated autoimmune diseases. Here we present evidence linking specific isoprenoid intermediates of the mevalonate pathway to signaling pathways that regulate T cell autoimmunity. We demonstrate that the isoprenoid geranylgeranyl-pyrophosphate (GGPP) mediates proliferation, whereas both GGPP and its precursor, farnesyl-PP, regulate the Th1 differentiation of myelin-reactive T cells. Depletion of these isoprenoid intermediates in vivo via oral AT administration hindered these T cell responses by decreasing geranylgeranylated RhoA and farnesylated Ras at the plasma membrane. This was associated with reduced extracellular signal-regulated kinase (ERK) and p38 phosphorylation and DNA binding of their cotarget c-fos in response to T cell receptor activation. Inhibition of ERK and p38 mimicked the effects of AT and induced a Th2 cytokine shift. Thus, by connecting isoprenoid availability to regulation of Th1/Th2 fate, we have elucidated a mechanism by which AT may suppress Th1-mediated central nervous system autoimmune disease

Cholesterol activates the G-protein coupled receptor Smoothened to promote Hedgehog signaling

Cholesterol is necessary for the function of many G-protein coupled receptors (GPCRs). We find that cholesterol is not just necessary but also sufficient to activate signaling by the Hedgehog (Hh) pathway, a prominent cell-cell communication system in development. Cholesterol influences Hh signaling by directly activating Smoothened (SMO), an orphan GPCR that transmits the Hh signal across the membrane in all animals. Unlike most GPCRs, which are regulated by cholesterol through their heptahelical transmembrane domains, SMO is activated by cholesterol through its extracellular cysteine-rich domain (CRD). Residues shown to mediate cholesterol binding to the CRD in a recent structural analysis also dictate SMO activation, both in response to cholesterol and to native Hh ligands. Our results show that cholesterol can initiate signaling from the cell surface by engaging the extracellular domain of a GPCR and suggest that SMO activity may be regulated by local changes in cholesterol abundance or accessibility

Cholesterol-Independent Effects of Methyl-β-Cyclodextrin on Chemical Synapses

The cholesterol chelating agent, methyl-β-cyclodextrin (MβCD), alters synaptic function in many systems. At crayfish neuromuscular junctions, MβCD is reported to reduce excitatory junctional potentials (EJPs) by impairing impulse propagation to synaptic terminals, and to have no postsynaptic effects. We examined the degree to which physiological effects of MβCD correlate with its ability to reduce cholesterol, and used thermal acclimatization as an alternative method to modify cholesterol levels. MβCD impaired impulse propagation and decreased EJP amplitude by 40% (P<0.05) in preparations from crayfish acclimatized to 14°C but not from those acclimatized to 21°C. The reduction in EJP amplitude in the cold-acclimatized group was associated with a 49% reduction in quantal content (P<0.05). MβCD had no effect on input resistance in muscle fibers but decreased sensitivity to the neurotransmitter L-glutamate in both warm- and cold-acclimatized groups. This effect was less pronounced and reversible in the warm-acclimatized group (90% reduction in cold, P<0.05; 50% reduction in warm, P<0.05). MβCD reduced cholesterol in isolated nerve and muscle from cold- and warm-acclimatized groups by comparable amounts (nerve: 29% cold, 25% warm; muscle: 20% cold, 18% warm; P<0.05). This effect was reversed by cholesterol loading, but only in the warm-acclimatized group. Thus, effects of MβCD on glutamate-sensitivity correlated with its ability to reduce cholesterol, but effects on impulse propagation and resulting EJP amplitude did not. Our results indicate that MβCD can affect both presynaptic and postsynaptic properties, and that some effects of MβCD are unrelated to cholesterol chelation

Cholesterol-directed nanoparticle assemblies based on single amino acid peptide mutations activate cellular uptake and decrease tumor volume.

Peptide drugs have been difficult to translate into effective therapies due to their low in vivo stability. Here, we report a strategy to develop peptide-based therapeutic nanoparticles by screening a peptide library differing by single-site amino acid mutations of lysine-modified cholesterol. Certain cholesterol-modified peptides are found to promote and stabilize peptide α-helix formation, resulting in selectively cell-permeable peptides. One cholesterol-modified peptide self-assembles into stable nanoparticles with considerable α-helix propensity stabilized by intermolecular van der Waals interactions between inter-peptide cholesterol molecules, and shows 68.3% stability after incubation with serum for 16 h. The nanoparticles in turn interact with cell membrane cholesterols that are disproportionately present in cancer cell membranes, inducing lipid raft-mediated endocytosis and cancer cell death. Our results introduce a strategy to identify peptide nanoparticles that can effectively reduce tumor volumes when administered to in in vivo mice models. Our results also provide a simple platform for developing peptide-based anticancer drugs

Protein sorting to the apical membrane of epithelial cells

The structure and functions of lipid rafts and the mechanisms of intracellular membrane trafficking are major topics in current cell biological research. Rafts have been proposed to act as sorting platforms during biosynthetic transport, especially along pathways that deliver proteins to the apical membrane of polarised cells. Based on this, the aim of this work was to contribute to the understanding of apical sorting in epithelial cells. The study of how lipid rafts are structured has been hampered by the scarcity of techniques for their purification. Rafts are thought to be partially resistant to solubilisation by mild detergents, which has made the isolation of detergent-resistant membranes (DRMs) the primary method to characterise them biochemically. While a growing number of detergents is being used to prepare DRMs, it is not clear what can be inferred about the native structure of cell membranes from the composition of different DRMs. This issue was addressed by an analysis of DRMs prepared with a variety of mild detergents. The protein and lipid content of different DRMs from two cell lines, Madin-Darby canine kidney (MDCK) and Jurkat cells, was compared. It was shown that the detergents differed considerably in their ability to selectively solubilise membrane proteins and lipids. These results make it unlikely that different DRMs reflect the same underlying principle of membrane organisation. Another obstacle for understanding apical sorting is that the evidence implicating certain proteins in this process has come from various disparate approaches. It would be helpful to re-examine the putative components of the apical sorting machinery in a single experimental system. To this end, a retroviral system for RNA interference (RNAi) in MDCK cells was established. Efficient suppression of thirteen genes was achieved by retroviral co-expression of short hairpin RNAs and a selectable marker. In addition, the system was extended to simultaneously target two genes, giving rise to double knockdowns.Retroviral RNAi was applied to deplete proteins implicated in apical sorting. Surprisingly, none of the knockdowns analysed caused defects in surface delivery of influenza virus hemagglutinin, a common marker protein for apical transport. Therefore, none of the proteins examined is absolutely required for transport to the apical membrane of MDCK cells. Cells may adapt to the depletion of proteins involved in membrane trafficking by activating alternative pathways. To avoid such adaptation, a visual transport assay was established. It is based on the adenoviral expression of fluorescent marker proteins whose surface transport can be followed microscopically as soon as RNAi has become effective. With this assay, it should now be possible to screen the knockdowns for defects in surface transport. Taken together, this work has provided a number of experimental tools for the study of membrane trafficking in epithelial cells. First, the biochemical analysis of DRMs highlighted that DRMs obtained with different detergents are unlikely to correspond to distinct types of membrane microdomains in cell membranes. Second, the retroviral RNAi system should be valuable for defining the function of proteins, not only in membrane transport, but also in processes like epithelial polarisation. Third, the visual assay for monitoring the surface transport of adenovirally expressed marker proteins should be suitable to detect defects in polarised sorting