Effect of chemically modified GM1 and neoglycolipid analogs of GM1 on liposome circulation time: evidence supporting the dysopsonin hypothesis.

The sugar group of the ganglioside GM1 has been modified by periodate oxidation, reduction or reductive amination. The negative charge of sialic acid of GM1 has also been removed by methylation or reductive hydrolysis. A series of neoglycolipid analogs of GM1 were synthesized by coupling the GM1 oligosaccharide (GM1OS) to dioleoylphosphatidylethanolamine (DOPE) via different spacer arms. The individual GM1 derivatives were incorporated into egg phosphatidylcholine/cholesterol liposomes and tested in mice in order to see whether they were effective in prolonging liposome circulation. The oxidized GM1 did not show any ability to prolong circulation. However, the lost activity after oxidation was completely recovered by the subsequent reduction step. A series of aminated GM1 derivatives were prepared via oxidation followed by reductive amination with various substituted amines. beta-Alanyl GM1 showed a comparable activity to the native GM1, while other aminated GM1S showed reduced activity in terms of prolonging circulation of liposomes. Blocking the negative charge of sialic acid by methylation did not greatly lessen the activity, and removing the carboxyl group of sialic acid by reductive hydrolysis reduced the activity by only approx. 20%. Among the neoglycolipid analogs of GM1 only GM1OS directly conjugated to DOPE was effective in prolonging the circulation, whereas conjugates with a spacer of various length were not effective. These results emphasize the importance of the molecular structure of GM1 for its functional ability to prolong the liposome circulation. Furthermore, modifications which abolish the cholera toxin-binding activity of GM1 also decrease the ability to prolong the circulation time of the liposomes, and vice versa. Such strong correlation further supports the idea that the specific recognition of GM1 oligosaccharide by putative dysopsonin(s) is responsible for the ability to prolong the liposomes circulation time.