Sphingosine-induced alterations in membrane biophysical properties: biological relevance in the pathophysiology of human disease

The study of biological and model membrane systems currently represents an important area of scientific research. Lipids are involved in the regulation of multiple cellular processes, being fundamental for the mantainance of cell homeostasis. Sphingosine (Sph) belongs to this group of biologically active lipids and is an important signaling molecule. When abnormally accumulated in the lysosomes and late endosomes (LE), Sph is associated to one of the most complex lysosomal storage diseases (LSD), Niemann-Pick type C (NPC). Despite this, little is known about its role in the lysosome, in particular with respect to the biophysical effects of its accumulation. By understanding the interactions of Sph with other lipids and their effect on the physical state of model and cell membranes, new insights into its mode of action may arise. Using complementary established techniques (fluorescence spectroscopy, dynamic (DLS) and electrophoretic (ELS) light scattering), a thorough biophysical characterization of membranes containing Sph was performed. This study revealed that Sph is able to decrease membrane fluidity both in fluid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and lipid raft-mimicking (POPC/SM/Chol) membrane models in a concentration dependent way. Sph-induced changes on membrane fluidity are highly dependent on pH and membrane lipid composition. It was observed that Sph has a more dramatic impact on membrane organization and permeability in vesicles with a pH gradient resembling the lysosome - the lysosome mimicking vesicles - LMVs (pH 5.0in/7.4out) - particularly in those with a lipid composition mimicking NPC1 conditions (i.e. higher Chol, SM and Sph content), compared to physiological-like situations. In the biological context, it was shown that cells displaying the NPC phenotype have an altered membrane fluidity when compared with the wild-type (WT) cells and that these changes are complex and cell type dependent. Moreover, it was observed that Sph has the ability to decrease the fluidity of biological membranes in accordance with model membrane data. Overall the results suggest that Sph abnormal accumulation in cells is associated with alterations in membrane biophysical properties, likely affecting different membrane associated cellular processes. These changes could urderly some Sph biological actions. In particular, Sph-induced biophysical alterations might affect the endocytic trafficking and consequently the normal cell function in NPC disease