Research: Professor Steve Wrenn

Wrenn Group – Biological Colloids

Gallstone Disease - Cholesterol Nucleation from Small, Uni-lamellar Vesicles (SUVs):  We are examining mechanisms by which various proteins and enzymes influence cholesterol nucleation kinetics in solutions of model bile.  Above are Cryo-TEM images small, uni-lamellar model hepatic vesicles under control conditions (a) and after exposure to phospholipase  (b). The otherwise metastable vesicles (~50-250 nm in diameter with a clearly discernible bilayer) undergo rapid conversion to an oil phase upon exposure to the pro-aggregating enzyme (Cryo-TEM images provided by (a) Dganit Danino [Department of Food Engineering and Biotechnology, Technion City, Haifa, Israel] and (b) Dr. Yamaira González [Department of Chemical Engineering, University of Delaware]).

Measuring Sizes of Lipid Rafts:  Ternary phase diagrams are becoming available that reveal where liquid-ordered (raft-like) domains occur in membrane composition space (see S. Veatch and S. Keller, 2005, Biophys. Acta.  1746:172-185).  However, sizes of the rafts – and what determines size – is not known.  We recently developed a fluorescence assay that enables determination of raft size from the dependence of Förster resonance energy transfer (FRET) efficiency on quencher concentration.  For example, increasing the sphingomyelin composition – at a fixed cholesterol mole fraction of 40 mole% -
increases average raft size.

Cholesterol Nanodomains:  Cholesterol crystals are precursors to gallstones and are considered a hallmark of advanced atherosclerotic plaques.  We are investigating the formation of cholesterol nanodomains in lipid bilayers and the mechanism by which these domains serve as nuclei for cholesterol crystallization. Above are results from simulations of cholesterol nanodomain formation in model phospholipid membranes.  We use such simulations to analyze sensitivity and resolution of experimental assays used to measure nanodomain formation in real systems. 

Ultrasound for Drug Delivery:  We are collaborating with the Polish Academy of Sciences to develop novel technologies involving interactions between ultrasound and lipid bilayers.  One project involves using an ultrasound transducer as a remote trigger for  targeted drug delivery from liposomes.  Shown above is an ultrasound field obtained from a custom-designed transducer, showing two crossing beams, each of which corresponds to different frequency in the MHz range.  Mixing of the two, high frequencies yields a low frequency of nominally 20 kHZ.  As a result, one obtains the (cavitation) benefits of low frequency plus the (safety) benefits of high frequency. 
Image courtesy of Prof. Andrzej Nowicki, IPPT, Warsaw.

Giant Uni-lamellarVesicles (GUVs): We recently became interested in Giant vesicles.  Shown above GUVs prepared by the method of electro-poration.  The GUVs are labeled with a fluorescent phospholipid, which makes the lipid bilayer clearly visible in a fluorescent microscope.  In some cases, small vesicles are encapsulated inside the Giant vesicles, giving rise to a structure sometimes referred to as a “vesosome.”  We are using GUVs as models for cell membranes, to examine lipid raft formation, and to investigate ultrasound-induced drug release from liposomes.