Ingrid 't Hart

56 Chapter 3 3 Introduc�on Glycosphingolipids (GSLs) are ceramides modified by glycans and are the major glycolipid component of mammalian cell membranes. Ceramides consist of a sphingoid base with a fa�y acid at the C-2 amine. In mammals, the most common sphingoid base is sphingosine. The C-2 fa�y acids can vary in their carbon chain length (14 to >30) and are occasionally unsaturated or bear α-hydroxyl groups. The biosynthesis of ceramide starts in the ER and glycan extensions are performed in the Golgi apparatus. Glycosyla�ons are performed by various glycosyltransferases (GTs), which starts with the forma�on of lactosylceramide (Lac-Cer) and can then be extended into more complex glycan chains (Fig 1). 1,2,3 The first glycosyla�on of the globo-series GSLs is catalyzed by A4GalT, which adds α1,4-galactoside on Lac-Cer to provide Gb3-Cer (CD77, P k ). 4 Next, Gb3-Cer can be extended with acetylated β1,3-galactosamine by β3GalNAc-T1 to form Gb4-Cer. 5 The enzyme β3GalT5 adds β1,3-galactoside to Gb4-Cer to form Gb5-Cer. 6 Gb5-Cer can be decorated by α1,2-fucoside (Globo-H) or α2,3-neuraminic acid (MSGb5-Cer, SSEA-4) by FUT1 and FUT2 and ST3Gal2, respec�vely. 7,8 A second internal α2,6-neuraminic acid can be transferred by ST6GalNAc6 onto MSGb5-Cer to form DSGb5-Cer. 9 Upon comple�on, the newly synthesized GSLs are transported to the cell membrane. 1,2,3 GSLs mediate biological func�ons such as cell prolifera�on, apoptosis, adhesion and migra�on and the forma�on of endo- and exosomes. 1,2,3 Figure 1. Lactosylceramide (Lac-Cer) is the star�ng point for a high variety of glycosphingolipids (GSLs). The globo-series start with the addi�on of α1,4-galactose to form Gb3-Cer. Black: lactosylsphingosine; Grey : fa�y acid, where R 1 = a carbon chain varying from 14 to >30 and occasionally contains unsatura�ons or α-hydroxyl groups. R 2 = the glycosyla�on site to extend to more complex globo-series GSLs. Globo-series GSLs have been found on kidney �ssue, erythrocytes and are important biomarkers of embryonic and pluripotent stem cells. 10,11,12 Some of the globo GSLs are overexpressed by epithelial cancers, such breast, kidney and prostate cancers. 7,13,14 GSLs can interactwithcomponents residingon the same cell ( cis interac�ons) or other cells ( trans interac�ons). Since GSLs o�en cluster in lipid ra�s in the membrane, interac�ons take place in a mul�valent manner, resul�ng in high avidity of binding. 1,15 Important protein interac�ons have been a�ributed to the globo-series GSLs. As an example, Gb4 promotes EGFR auto-phosphoryla�on, ac�va�ng the downstream cascade and thereby promo�ng cell prolifera�on. Another example is the Fas (CD95) receptor, which binds Lac-Cer and Gb3, but not Gb4 or other gangliosides. Upon binding Gb3, ligand-bound Fas will undergo endocytosis and this results in a cell death signal. Furthermore, globo- series GSLs have been iden�fied as interac�ng partners for viral and bacterial proteins. Gb3 binds gp120 on the human immunodeficiency virus (HIV) and verotoxins 1 and 2 O O OH HO OH O HO OH HO OH HN OH (CH 2 ) 12 CH 3 O R 1 O O O OH HO OH O O OH HO OH HN OH (CH 2 ) 12 CH 3 O R 1 O GTs O HO OH R 2 O OH Lac-Cer Globo-series GSLs