REGULATED O-GLYCOSYLATION OF THE ALZHEIMER BETA-A4 AMYLOID PRECURSOR PROTEIN IN THYROCYTES

In thyrocytes, the beta-amyloid precursor protein (beta-APP) is expres sed, proteolytically cleaved and released into the extracellular space in a TSH-dependent fashion. Immunocytochemically, beta-APP was detect able mainly in the stacked Golgi cisternae indicating the accumulation in this organelle. Because this unusual immunoreactivity might be rel ated 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 o f glycosylation was also TSH dependent. Glycosidase digestion experime nts revealed that only the O-glycans, not the N-glycans, of beta-APP w ere regulated by TSH. Using enzyme digestion and lectin precipitation analyses we showed that O-glycosylation involved mainly a 2,6-sialylat ed Gal 1-3 GalNAc-alpha-core glycans (similar to 85%) whereas the 2,3 linked sialic acids amounted to only similar to 15% of total sialic ac id residues. Upon stimulation with TSH, O-glycosylation as measured by the degree of sialylation increased by a factor of similar to 1.7 the reby 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 ind icating that the proteolytic processing of mature beta-APP was not inh ibited by its O-glycosylation. When cells were stimulated with TSH in the presence of cycloheximide, the Golgi cisternae lost their predomin ant immunoreactivity for beta-APP and were rapidly emptied (within 30 min). Hence, under the conditions of normal protein synthesis, the Gol gi cisternae may operate as a storage compartment for beta-APP.