Effect of chemically modified G M1 and neoglycolipid analogs of G M1 on liposome circulation time: evidence supporting the dysopsonin hypothesis

Abstract

The sugar group of the ganglioside G M1 has been modified by periodate oxidation, reduction or reductive amination. The negative charge of sialic acid of G M1 has also been removed by methylation or reductive hydrolysis. A series of neoglycolipid analogs of G M1 were synthesized by coupling the G M1 oligosaccharide (G M1OS) to dioleoylphosphatidylethanolamine (DOPE) via different spacer arms. The individual G M1 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 G M1 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 G M1 derivatives were prepared via oxidation followed by reductive amination with various substituted amines. β-Alanyl G M1 showed a comparable activity to the native G M1, while other aminated G M1s 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 G M1 only G M1OS 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 G M1 for its functional ability to prolong the liposome circulation. Furthermore, modifications which abolish the cholera toxin-binding activity of G M1 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 G M1 oligosaccharide by putative dysopsonin(s) is responsible for the ability to prolong the liposomes circulation time.