GM2 activator protein is a protein cofactor which has been found to be intimately related to the catabolism of GM2-ganglioside. The physiological importance of GM2 activator protein has been shown by the fact that a deficiency of this activator protein results in a massive cerebral accumulation of GM2. However, the mechanism of action and the biological function of this activator protein are still not well understood. The purpose of this essay is to briefly recollect the personal accounts of our involvement with GM2 activator protein and also to discuss four unsettled questions pertaining to the specificity and the mechanism of action of this intriguing protein cofactor.
Model surfaces in a water environment suitable for the study of membrane ganglioside properties have been developed. They, together with ganglioside micelles of large molecular mass, consist of ganglioside-phosphatidylcholine mixed micelles of small molecular mass and ceramide-modified ganglioside micelles of intermediate molecular mass. The experimental information on the conformational and geometrical properties of ganglio-series gangliosides inserted into these model surfaces is discussed, compared and integrated with that obtained using multilamellar aggregate models. The results show that the conformational properties of the chain are determined by interresidual interactions between the sialic acid and N-acetylgalactosamine units, regardless of the total oligosaccharide structure, the structure of the ganglioside ceramide, possible phospholipid head group or Ca++ interactions, a highly enriched ganglioside environment, the presence of cholesterol in the membrane or temperature variation. Furthermore, the oligosaccharide chain seems to be well extended beyond the bilayer surface with a well defined average conformation and motional order, common to all the investigated gangliosides.
Sphingolipids are complex ubiquitous lipids that have been relegated to serving a structural role in membranes. Over the last few years, sphingolipid derivatives have been identified as intracellular signal transducing molecules (1-3). Sequential catabolic metabolites of sphingomyelin induce opposing biological effects; ceramides suppress mitogenesis and sphingosines enhance cell growth. The intracellular signaling pathways transducing these effects currently remain elusive. In this review, I will focus upon recent advances in the field of sphingolipid signaling with particular emphasis on the regulation of intracellular kinase cascades by sphingolipid-derived second messengers. Selective activation of distinct mitogen-activated protein kinase (MAPK) cascades by sphingolipid metabolites may, in part, determine the cellular phenotype.
The aim of liver glycotargeting of antiviral nucleoside analogues (NAs) is to reduce the extrahepatic side effects of these drugs in the treatment of chronic viral hepatitis. For this purpose NAs are conjugated with galactosyl-terminating macromolecules. The conjugates selectively enter hepatocytes after interaction of the carrier galactose residues with a lectin (the asialoglycoprotein receptor) present in large amounts and high affinity only on these cells. Within hepatocytes the conjugates are delivered to lysosomes where enzymes split the bond between the carrier and the drug, allowing the latter to become concentrated in the liver. The majority of experiments of liver glycotargeting of NAs have been performed employing a conjugate of lactosaminated human albumin with adenine arabinoside monophosphate (ara-AMP), a phosphorylated NA active against hepatitis B virus. This conjugate, administered to patients with chronic hepatitis B, exerted the same antiviral activity as the free drug without producing any clinical side effect, including the severe neurotoxicity caused by free ara-AMP. Pre-clinical studies are now underway with conjugates obtained using lactosaminated poly-L-lysine [Lac-poly(LYS)] as the hepatotropic carrier. These new conjugates have some advantages over those prepared with L-HSA: (a) they can be administered by the intramuscular route; (b) they are obtained entirely by chemical synthesis, thus eliminating the problems involved in the use of hemoderivatives; (c) they have a heavy drug load, enabling administration of smaller quantities of conjugate, more easily digested in lysosomes; (d) they allow higher quantities of drug to be introduced into hepatocytes. The results of experiments with two Lac-poly (LYS) conjugates, one with ara-AMP and one with ribavirin (a drug active in treatment of type C hepatitis), further support the validity of the liver targeting approach through the asialoglycoprotein receptor. In conclusion, coupling to galactosyl-terminating carriers appears to be a way of obtaining higher concentrations of antiviral NAs within hepatocytes and permitting the administration of drugs, whose use would otherwise be prevented by extrahepatic side effects.