Human Platelet Membrane Lipid Dynamics: Agonist Stimulation Results in the Aggregation of Lipid Rafts and the Lateral Phase Separation of like Lipids in Intact Human Platelets.

Sphingomyelin and cholesterol rich platelet membrane domains are organized into lipid rafts, which are present in the liquid ordered state, and which have been suggested to be key membrane components both during cold-induced human platelet activation (Gousset et al. 2002 Evidence for a physiological role for membrane rafts in human platelets. J. Cellular Physiol. 190:117–128) as well as during agonist induced activation. We have previous demonstrated that platelets have two membrane phase transitions, a raft transition at 34–40°C (Gousset et al., 2002 ) and a phospholipid phase transition at 10–20°C (Tablin et al. 1996 The membrane phase transition of intact human platelets: correlation with cold-induced activation. J. Cellular Physiol. 168:305–313). Using Fourier transform infrared spectroscopy (FTIR) we are able to sample all the key phases adopted by intact platelet phospholipids and sphingolipids (gel, liquid crystalline, liquid ordered (raft ) , hexagonal). This method offers unique advantages for the study of phospholipid acyl chain structures and interactions, as it is non-invasive and can detect molecular vibrations produced by dipole moment oscillations at infrared frequencies. Equally important , these vibrational frequencies are well characterized for individual phospholipids, permitting unambiguous assignment of the transitions to membranes in intact cells (Crowe et al. 1999. Are lipid phase transitions responsible for chilling damage in human platelets? Cryobiology 38:180-101). Most recently, using FTIR we have examined platelet lipid membrane dynamics associated with agonist induced human platelet activation. FTIR analysis of thrombin ( 1U/ml and 0.5U/ml) activated human platelets demonstrates that agonist stimulation results in the development of multiple (four or more) phase transitions, strongly suggestive of phase separation of lipids into like-lipid domains, also referred to as lateral phase separation. These like-lipid domains have phase transition temperatures very similar to that of isolated single lipid species, which, we have previously demonstrated combined to form the main phospholipids phase transition at between 10–20°C (Crowe et al. 1999). Furthermore, when human platelets are repeatedly scanned by FTIR - despite the temperature excursions which might normally result in the remixing of pure lipid populations, the thrombin treated platelets maintained their multiple phase transitions, suggesting that the lateral phase separation is an irreversible event during agonist induced activation. In addition to FTIR studies of human platelet populations, we have also examined the distribution of diIC 18 in washed resting, and agonist stimulated human platelets. This dye, which preferentially partitions into liquid ordered or gel phases was uniformly distributed in resting platelets, but became aggregated when platelets were treated by either low concentrations of thrombin (0.01U/ml) or collagen (3.0ug/ml). In addition, platelets treated with higher concentrations of either agonist, demonstrated lipid rafts which further aggregated into larger liquid ordered domains. These results strongly suggest that platelet membrane lipid organization plays a key role in membrane function and further downstream signaling. Funded by NHLBI and DARPA.