Neurophysiological abnormalities and COG defects
Since 2004, defects in seven out of the eight COG subunits have been associated with human Congenital Disorders of Glycosylation (CDG)-type II, a growing family of diseases involving malfunctions in the processing of N- and O-linked glycans and resulting from mutations in proteins involved with glycosylation (Wu et al., 2004; Spaapen et al., 2005; Foulquier et al., 2006, 2007; Kranz et al., 2007; Ng et al., 2007, 2011; Paesold-Burda et al., 2009; Reynders et al., 2009; Richardson et al., 2009; Lübbehusen et al., 2010; Fung et al., 2012; Huybrechts et al., 2012; Rymen et al., 2012, 2015; Kodera et al., 2015; Table 1). Glycosylation is a highly dynamic process that occurs in the ER and Golgi which requires an estimated two percent of the human genome to encode the enzymes and trafficking components for the proper maturation of newly formed glycan chains (Freeze et al., 2014). COG deficiency can cause a redistribution of COG-dependent Golgi resident proteins, including glycosylation enzymes. Most COG-CDG patients have defects in sialylation and galactosylation, as indicated by fluorescent lectin staining of plasma membrane glycoconjugates from patient fibroblasts and MALDI-TOF mass spectrometry of serum glycoproteins (Foulquier et al., 2007; Kranz et al., 2007; Paesold-Burda et al., 2009; Reynders et al., 2009; Zeevaert et al., 2009a,b). Along with other multi-system pathologies, COG-CDG patients display mild to severe neurological defects including hypotonia, intellectual disability, developmental delays, epilepsy, and ataxia (Table 1). Specific symptoms and the severity of condition appears to relate to the COG subunit that is deficient with COG6 and COG7 patients demonstrating the most severe phenotypes (Rymen et al., 2015). Additionaly, COG defects have not yet been attributed to any other subtype of CDG.
Several CDGs result from mutated COG subunits that are either severely truncated or rapidly degraded. Loss of one COG subunit can destabilize the remaining subunits and reduce their expression and association with the Golgi. Early studies of the COG3 subunit invoked participation of the COG complex in the proper distribution of Golgi enzymes. COG3 depletion by siRNA in HeLa cells causes extensive Golgi fragmentation and destabilization of the COG complex (Zolov and Lupashin, 2005). COG3 and COG7 knockdown generates an accumulation of COG complex-dependent (CCD) vesicles carrying the SNAREs GS15 and GS28, and Golgi enzymes MAN2A1 and GALNT2 (Zolov and Lupashin, 2005; Shestakova et al., 2006). The accumulation of CCD vesicles suggests that in COG deficient cells a significant fraction of Golgi glycosylation enzymes are separated from the proteins they need to modify. COG8-CDG patient fibroblasts have decreased levels of the other lobe B subunits (COG5, COG6, and COG7) all of which have lost their association with the Golgi (Foulquier et al., 2007; Kranz et al., 2007). COG lobe B destabilization was also seen in COG7-CDG patient fibroblasts, resulting in the loss of COG6 association with the Golgi (Kudlyk et al., 2013). The loss of COG also challenges the function of interacting SNAREs. The endosome-to-trans-Golgi Network (TGN) SNARE protein STX16 was mislocalized in COG8-CDG patient fibroblasts (Willett et al., 2013a), and the STX5/GS28/Ykt6/GS15 and STX6/STX16/Vti1a/VAMP4 SNARE complexes were destabilized in both COG7- and COG8-CDG patient fibroblasts (Laufman et al., 2013a). In a non-CDG patient presenting intellectual disability, Shaheen et al. identified a mutation in COG6 which resulted in reduced COG6 and STX6 protein expression (Shaheen et al., 2013). Anterograde trafficking does not appear to be affected in cells with COG mutations, but retrograde trafficking is affected as indicated by a partial resistance to treatment with the transport inhibitor brefeldin A (Steet and Kornfeld, 2006; Foulquier et al., 2007; Kranz et al., 2007; Ng et al., 2007; Paesold-Burda et al., 2009; Reynders et al., 2009) and by endosome-to-TGN trafficking defects elucidated by application of Shiga toxin and SubAB toxin (Zolov and Lupashin, 2005; Smith et al., 2009). Therefore, retrograde intra-Golgi and endosome-to-TGN sorting are particularly impaired by COG deficiency.