Regulated O-glycosylation of the Alzheimer beta-A4 amyloid precursor protein in thyrocytes

In thyrocytes, the beta-amyloid precursor protein (beta-APP) is expressed, proteolytically cleaved and released into the extracellular space in a TSH-dependent fashion. Immunocytochemically, beta-APP was detectable mainly in the stacked Golgi cisternae indicating the accumulation in this organelle. Because this unusual immunoreactivity might be related to the Golgi-specific posttranslational processing we studied the glycosylation of beta-APP and the possible regulation of this process. For this purpose we used FRTL-5 cells which showed that the degree of glycosylation was also TSH dependent. Glycosidase digestion experiments revealed that only the O-glycans, not the N-glycans, of beta-APP were regulated by TSH. Using enzyme digestion and lectin precipitation analyses we showed that O-glycosylation involved mainly alpha 2,6-sialylated Gal 1-3 GalNAc-alpha-core glycans (approximately 85%) whereas the 2,3 linked sialic acids amounted to only approximately 15% of total sialic acid residues. Upon stimulation with TSH, O-glycosylation as measured by the degree of sialylation increased by a factor of approximately 1.7 thereby raising the molecular mass of mature beta-APP by 4 to 5 kDa above that from control cells. This process coincided with the accumulation of a proteolytically derived 8.5 kDa C-terminal beta-APP fragment indicating that the proteolytic processing of mature beta-APP was not inhibited by its O-glycosylation. When cells were stimulated with TSH in the presence of cycloheximide, the Golgi cisternae lost their predominant immunoreactivity for beta-APP and were rapidly emptied (within 30 min). Hence, under the conditions of normal protein synthesis, the Golgi cisternae may operate as a storage compartment for beta-APP.