Ganglioside Probes Enable Tracking of Membrane Lipid Rafts in Living Cells

A team of Japanese cell biologists developed a series of fluorescently labeled gangliosides that enabled them to visualize cell membrane lipid raft domains in living cells.

Combinations of glycosphingolipids and protein receptors are organized in glycolipoprotein microdomains known as lipid rafts within the cellular plasma membrane. These specialized membrane microdomains compartmentalize cellular processes by serving as organizing centers for the assembly of signaling molecules, influencing membrane fluidity and membrane protein trafficking, and regulating neurotransmission and receptor trafficking. Lipid rafts are more highly ordered and tightly packed than the surrounding bilayer, but float freely in the membrane bilayer. Although more common in the plasma membrane, lipid rafts have also been reported in other parts of the cell, such as the Golgi apparatus and lysosomes.

The exact functions of lipid rafts are poorly understood, primarily because of the scarcity of suitable fluorescent ganglioside analogs to serve as tracer molecules. To correct this deficit, investigators at Kyoto University (Japan) synthesized four complete ganglioside molecules with fluorescent markers attached at specific locations. They used these tracers in a single-fluorescent-molecule living-cell imaging system.

Results published in the April 4, 2016, online edition of the journal Nature Chemical Biology revealed that in the live-cell plasma membrane there could be seen clear but transient co-localization and co-diffusion of fluorescent ganglioside analogs with a fluorescently labeled glycosylphosphatidylinisotol (GPI)-anchored protein, human CD59. The ganglioside molecules were always mobile in quiescent cells with extremely short interactions taking place that last for about 12 milliseconds for CD59 monomers, 40 milliseconds for CD59's transient homodimer rafts, and 48 milliseconds for engaged-CD59-cluster rafts, in cholesterol- and GPI-anchoring-dependent manners.

"Such dynamic behaviors were difficult to find using normal techniques, and our findings were made possible by single-molecule tracking of new fluorescent ganglioside probes," said contributing author Dr. Kenichi Suzuki, professor of membrane biology at Kyoto University. "Our findings established the concept of dynamic raft domains: their constituent molecules assemble to form raft domains, do their jobs within several tens of milliseconds, and then move away for the next assembly to perform the next task."

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