Ks. Graebert et al., REGULATED O-GLYCOSYLATION OF THE ALZHEIMER BETA-A4 AMYLOID PRECURSOR PROTEIN IN THYROCYTES, European journal of cell biology, 66(1), 1995, pp. 39-46
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.