Effects of subunit 8 deficiency of the conserved oligomeric Golgi complex on proteoglycan synthesis

Abstract

Abstract The conserved oligomeric Golgi (COG) complex is a hetero-octameric complex involved in retrograde transport of Golgi resident enzymes along the secretory pathway. Its presumed function is in the tethering of vesicles to the membrane of Golgi cisternae. Double mutations of COG subunits have been shown to have severe effects on both Golgi organization and whole organism phenotype. Disturbance of the finely-tuned regulation of Golgi organization causes disruption of its function. This has been shown in several patients with congenital disorders of glycosylation (CDG). One of them is COG8-CDG in which patient fibroblasts show a disturbed metabolism of N-linked and O-linked glycosylation. N-linked glycosylation and O-linked glycosylation are two major functions of the Golgi apparatus. A third one being polymerization of glycosaminoglycan (GAG) chains on proteoglycans (PG). These are proteins with at least one attached GAG chain. PGs are involved in many important physiological functions, some of which are, immune defense, bone formation, and nervous tissue development. But despite their importance no report has been published that explores the effects of COG8 deficiency on PG synthesis. In this thesis PGs and especially their glycosaminoglycan (GAG) chains from COG8 deficient fibroblast have been studied. Radioactively labeled sulfate is almost exclusively incorporated into GAG chains. These can then be separated by SDS-PAGE, gel filtration and ion-exchange chromatography and analyzed. The obtained results point to an increased mass for PGs from COG8 deficient fibroblasts. Selective degradation of different GAG chain types then allows the quantification of these types. Additional separation of GAGs from their protein cores allows the isolated analysis of a single GAG chain type. The results of these experiments show an increased chain length for GAGs produced by patient fibroblast. These results support the notion of a disturbed glycosylation machinery in the Golgi apparatus, especially of the late Golgi cisternae and the trans-Golgi network (TGN) because the enzymes catalyzing GAG chain synthesis are thought to be located there.