br Introduction Developmental myelination and

Introduction Developmental myelination and myelin repair after demyelination are mediated by oligodendrocyte precursor cells (OPCs). OPCs are characterized by markers such as PDGFRα and the chondroitin sulfate proteoglycan NG2 (Dubois-Dalcq et al., 2008; Franklin and Ffrench-Constant, 2008; Nishiyama et al., 2009; Trotter et al., 2010). Differentiation protocols for the production of human embryonic stem cell-derived (hESCd) OPCs have been described and successful myelination was achieved by grafting of hESCd-OPCs into brain or spinal cord of myelin-deficient shiverer mice (Nistor et al., 2005; Hu et al., 2009a,b). Remyelination and improved functional recovery after transplantation of hESCd-OPCs have also been demonstrated in rat spinal cord injury (Keirstead et al., 2005; Sharp et al., 2010) and recently, myelination by human induced pluripotent stem cell (iPSC)-derived OPCs has been demonstrated in the mouse model (Wang et al., 2013; Douvaras et al., 2014). Thus, human stem cell-derived OPCs are on the way to be used in replacement therapies for treating demyelinating diseases (Goldman et al., 2012; Fox et al., 2014). Besides environmental and axonal cues, the efficiency of myelination is determined by the capacity of OPCs to migrate and to interact with non-myelinated axons. A regulatory factor of particularly these OPC properties is the modification of the cell surface with the unique glycan polysialic order Zalcitabine (polySia or PSA). The major carrier of polySia is the neural cell adhesion molecule NCAM and its main function is the modulation of cell surface interactions during brain development and plasticity (Rutishauser, 2008; Schnaar et al., 2014). PolySia on OPCs supports migration towards demyelinated lesions (Wang et al., 1994; Nait-Oumesmar et al., 1999, 2007; Zhang et al., 2004). Accordingly, enhanced polysialylation improved the migration capacity of mouse ESC- and human iPSC-derived OPCs (Glaser et al., 2007; Czepiel et al., 2014). In the course of oligodendrocyte maturation and myelin formation, polySia on OPCs is down-regulated and myelination is impairedwhen the down-regulation of polySia is prevented (Fewou et al., 2007). Moreover, polySia on axons inhibits myelination in vitro (Charles et al., 2000) and chronically demyelinated axons in multiple sclerosis lesions re-express polySia, whereas shadow plaques with partial remyelination are polySia-negative (Charles et al., 2002). Thus, polySia on OPCs facilitates OPC recruitment and the presence of polySia on OPCs and on axons may act as a negative regulator of myelin formation. PolySia synthesis is mediated by two independently regulated polysialyltransferases (polySTs), ST8SIA2 and ST8SIA4. Both enzymes show high acceptor specificity and therefore polySia is restricted to only few carrier molecules (Mühlenhoff et al., 2013). In addition to the major polySia carrier NCAM, we recently described the ST8SIA2-mediated polysialylation of a fraction of the synaptic cell adhesion molecule SynCAM 1 within a subset of NG2 cells of the early postnatal mouse brain (Galuska et al., 2010; Rollenhagen et al., 2012). NG2 cells are able to communicate with neurons via specialized neuron-NG2 cell synapses (Bergles et al., 2000; Karadottir et al., 2005; Kukley et al., 2007; Etxeberria et al., 2010). This axo-glial signaling may be involved in the onset of myelination (De Biase et al., 2010; Kukley et al., 2010). As SynCAM 1 is critical for assembly, organization and maintenance of neuronal synapses (Biederer et al., 2002; Stagi et al., 2010; Fogel et al., 2011) it may also contribute to the formation of synapses between neurons and NG2 cells. And because polySia attenuates adhesive interactions, it has been proposed that polysialylation of SynCAM 1 modulates the assembly or disassembly of neuron-NG2 cell synapses (Galuska et al., 2010). However, many aspects of OPC biology differ fundamentally between mice and humans (Sim et al., 2009) and so far, the polySia modification of SynCAM 1 has only been shown in mice (Galuska et al., 2010; Rollenhagen et al., 2012). Therefore, the present study was designed to assess polySia acceptors in human OPCs generated from ESCs.