Ingrid 't Hart

108 Chapter 6 6 Chapter 1 The chemical structures and biological roles of glycosphingolipids (GSLs) and sulfoglycolipids (SGLs) are introduced in this chapter. Chemical and chemoenzyma�c synthe�c strategies to obtain GSL oligosaccharides and glycolipids are discussed. GSLs are composed of a glycan part, covering hundreds of different oligosaccharides, and a lipid part: ceramide. Due to their amphiphilic nature, they reside in the cell membrane, where the glycan moiety faces the extracellular milieu and the lipid is anchored in the membrane. GSLs can interact with both extracellular components ( trans interac�ons) and components in the same cell membrane ( cis interac�ons). The biosynthesis of GSLs starts with the forma�on of glucosylceramide (Glc-Cer) in the ER and early Golgi apparatus. Next, Glc-Cer is extended to Lac-Cer: the common precursor for both the globo- and ganglio-series of GSLs, which cover the structures discussed in this thesis. Various glycosyltransferases are involved in the biosynthesis of globo- and ganglio-series glycolipids. GSLs are involved in various processes in the cell, such as cell adhesion and transmembrane signaling. Furthermore, GSLs are involved in various diseases, which are mainly neurological (Tay Sachs, Sandhoff, Alzheimer’s, Parkinson’s, MS) or immune related (verotoxins, HIV). Other diseases involving GSLs are discussed more in depth: cancer and the auto-immune disease Guillain-Barré Syndrome (GBS). A common feature in carcinogenesis is the upregula�on of sialylated structures. Sialylated GSLs are known as gangliosides and play important roles in different types of cancer. A lesser inves�gated ganglioside, DSGb5, has been found in renal cell carcinomas. DSGb5 was reported to bind Siglec-7: an immune inhibitory receptor mainly found on NK-cells. If cancer cells bind receptors such as Siglec-7, they are able to evade an immune response. This makes DSGb5 an interes�ng target for cancer detec�on or treatment. Cancer specific an�bodies have been elicited towards various glycans. The an�-epiglycanin an�body HAE3, is known to bind mucin-type glycans and is selec�ve binds epithelial cancer cells. Surprisingly, HAE3 strongly bound to the sulfoglycolipid SM1a in a glycan-microarray. Other SGLs, such as GM3 and SB1a, have been associated with liver cancer, however li�le is known of the exact role of SM1a. Another important disease involving gangliosides, is the auto- immune disease Guillain-Barré syndrome GBS. In GBS, the peripheral nervous system is damaged and an�ganglioside an�bodies seem to be a leading cause. The forma�on of an�ganglioside an�bodies seems to be triggered by the gram-nega�ve bacterium Campylobacter jejuni . Ganglioside mimics can be found on lipooligosaccharides (LOS) in the outer membrane of C. jejuni and seem to trigger the forma�on of cross-reac�ve an�ganglioside an�bodies. In GBS serum samples, an�bodies towards gangliosides such as GM1a have been detected. These gangliosides are mainly present in the peripheral nervous system, explaining certain symptoms of the disease. To study the biological roles and protein interac�ons of GSLs more closely ex vivo , pure homogeneous GSLs are required. Isola�on of GSLs is a tedious job, especially for nega�vely charged compounds such as gangliosides and SGLs. Furthermore, isola�on o�en results in heterogeneous mixtures. Therefore, chemical or enzyma�c synthesis is the best op�on to obtain these GSLs. Different strategies have been designed for the synthesis of GSL glycolipids. Each approach has its advantages and limita�ons. The most resent approaches are the Glc-Cer casse�e approach, to solve the problem of the low reac�vity of a chemical glycosyla�on between an oligosaccharide donor and ceramide acceptor. Another promising approach is the chemoenzyma�c strategy, where chemically synthesized lactosylceramide (Lac-