Ingrid 't Hart

9 Introduc�on 1 Figure 2. GSLs in the membrane and the types of interac�ons. ( A ) GSLs cluster in membrane microdomains with other proteins and lipids. ( B ) cis interac�ons of GSLs with components in the samecellmembrane.( C )crosslinkingofseveralGSLscaninducetransmembranesignaltransduc�on. (D) trans interac�ons with components on other cell membranes. (Image from Zhang et al., 2019 1 ) GSLs are divided into different subclasses based on their glycan structures, namely the: ganglio-, lacto-, neolacto-, globo-, isoglobo-, mollu- and arthroseries. A common component of these GSL subtypes is glucosylceramide (Glc-Cer) (Fig. 3). 3 For GSLs, the minimal glycolipid component is either Gal-Cer or Lac-Cer, which are 3-O-sulfated by galactose-3-O-sulfotransferases, which use PAPS (3’-phosphoadenosine-5’- phosphosulfate) as donor substrate. 3-O-sulfa�on of Gal-Cer results in sulfa�de (SM4), which is highly abundant in nervous �ssues, but will not be further discussed in this thesis. 8,9 GSL(-mimic)s of globo- and ganglio-series members represent the main targets of this thesis, therefore the biosynthesis of these subclasses will be discussed in more detail. Furthermore, their prevalence in healthy and diseased environments will be briefly discussed. The first step in the biosynthesis of GSLs is the a�achment of glucose to ceramide by UDP- glucose ceramide glucosyltransferase or glucosylceramide synthase (GlcCer-S) on the cytoplasmic face of the early (or cis-)Golgi membrane. Glc-Cer is next flipped to the non- cytoplasmic face by a lipid flippase to enable further extension of the glycosphingolipid saccharide moiety by luminal glycosyltransferases (GTs). 10 Two glycosyltransferases, B4GalT5 and B4GalT6, can add galactose to Glc-Cer to form lactosylceramide (Lac- Cer). 2,3,11