Glycosylation and stem cells: Regulatory roles and application of iPSCs in the study of glycosylation-related disorders

Abstract

Glycosylation refers to the co- and post-translational modification of protein and lipids by monosaccharides or oligosaccharide chains. The surface of mammalian cells is decorated by a heterogeneous and highly complex array of protein and lipid linked glycan structures that vary significantly between different cell types, raising questions about their roles in development and disease pathogenesis. This review will begin by focusing on recent findings that define roles for cell surface protein and lipid glycosylation in pluripotent stem cells and their functional impact during normal development. Then, we will describe how patient derived induced pluripotent stem cells are being used to model human diseases such as congenital disorders of glycosylation. Collectively, these studies indicate that cell surface glycans perform critical roles in human development and disease.

Keywords: congenital disorders of glycosylation; glycosylation; pluripotent stem cells.

Figure 1

Biosynthesis of the lipid-linked oligosaccharide precursor for N-linked glycosylation and known defects in this pathway. Scheme depicting the enzymes and proteins involved in the biosynthesis of the 14-sugar lipid-linked oligosaccharide (LLO) precursor for N-glycosylation. This precursor is then transferred to nascent polypeptides by the oligosaccharyltransferase (OST) enzyme complex in a co- and post-translational manner. Red lines under the gene names denote those genes associated with known CDG defects. Mutations in the OST genes STT3A, STT3B, DDOST, TUSC3 and MAGT1 have been identified in human patients.

Figure 2

Advances in glycomics allow the rapid profiling of cell-associated glycans. The surfaces of all cells are coated with a rich diversity of glycans including glycoproteins, glycolipids and glycosaminoglycans. This coat is called the glycocalyx and serves multiple vital functions for the cell. New advances in glycomics such as tandem mass spectrometry (MS_n), capillary electrophoresis (CE) and high performance and ultra performance liquid chromatography (HPLC and UPLC) provide the opportunity to rapidly profile the diversity and abundance of different glycan structures. This profiling has facilitated the identification of glycan signatures in pluripotent and differentiated lineages.

Figure 3

Nomenclature and structures of common glycan-based epitopes used as pluripotency markers for mouse and human ESCs. SSEA-1 and SSEA-5 belong to the lactoseries (Lewis, ABH antigens) and are most commonly found on glycolipids but can also be present on glycoproteins. SSEA-3 and SSEA-4 belong to the globoseries (P blood group antigens). The carbohydrate structures of TRA-1-60 and TRA-1-81 antigens are defined as repeating lactosamine units and are most commonly present on glycolipids and mucin-type O-glycans.

Figure 4

Increased polysialylation is a hallmark of PSC differentiation. A: PSCs bear unique glycan signatures such as a high relative abundance of high mannose glycans that decrease in abundance upon differentiation. Other signatures such as polysialic acid (PSA) chains increase at the cell surface as cells move from the pluripotent state towards differentiated lineages. B: The increase in PSA expression upon differentiation is driven by increased expression of the polysialyltransferase PST and transcription factor GOOSECOID (GSC) along with a corresponding decrease in NANOG and BRACHYURY (T) expression.

Figure 5

iPSCs and iPSC-derived cell types as tools for disease modeling. Recent efforts have lead to the development of induced pluripotent stem cells (iPSCs) for several glycosylation-related disorders including the congenital disorders of glycosylation (CDGs), congenital muscular dystrophies (CMDs), and lysosomal storage disorders (LSDs). iPSCs are generated by reprogramming patient fibroblasts with factors that drive pluripotency. These iPSCs can then be directed to differentiate into various disease-relevant specialized cell types such as neurons or muscle precursors.