Low density lipoproteins bound to Schistosoma mansoni do not alter the rapid lateral diffusion or shedding of lipids in the outer surface membrane

Schistosomula of Schistosoma mansoni bind human low density lipoproteins (LDL) in a concentration-dependent and saturable manner. The bound LDL could provide phospholipids and sterol to the worm, which cannot synthesize sterol de novo and lacks acyl chain-modifying capability. Here we have used three phospholipid analogues to explore the effect of LDL binding on the parasite's outer tegumental membrane, i.e. the outer of the two membranes that cover its surface syncytium. Fluorescein phosphatidylethanolamine (Fl-PE) and rhodamine phosphatidylethanolamine (Rh-PE) bound to the parasite in a saturable manner and, as shown by fluorescence microscopy, were confined to the surface. Fl-PE fluorescence was completely quenched by Trypan Blue and Fl-PE was lost from the surface following single exponential decay kinetics (t1/2 = 12 h), further suggesting that the probe was confined to the outer membrane. 1,1'-Dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine perchlorate (DiI-C18(3); DiI) did not bind saturably and was seen in both the surface and the internal parasite membranes. Fluorescence photobleaching recovery was used to measure the lateral mobility of Fl-PE in the outer membrane. The lateral diffusion coefficient of Fl-PE was approximately 10(-7) cm2s-1. The fractional mobility of Fl-PE was 85% when measured using a laser beam of radius 1.8 microns and 45% using a beam of radius 4.3 microns. These measurements suggest that the outer membrane is composed of micron-scale liquid crystalline-phase lipid domains that lack significant amounts of transmembrane proteins. LDL binding to the parasite surface did not alter the lateral mobility of Fl-PE or the rate of loss of either Fl-PE or Rh-PE.