Suppression of glypican-1 autodegradation by NO-deprivation correlates with nuclear accumulation of amyloid beta in normal fibroblasts

Abstract

Heparan sulfate (HS)-containing, S-nitrosylated (SNO) glypican-1 (Gpc-1) releases anhydromannose-containing HS (anMan-HS) by SNO-catalyzed autodegradation in endosomes. Transport of anMan-HS to the nucleus requires processing of the amyloid precursor protein (APP) to amyloid beta peptides (Aβ). To further examine the relationship between APP and Gpc-1 processing in normal fibroblasts we have suppressed Gpc-1 autodegradation by aminoguanidine inhibition of NO synthesis and prevented lysosomal degradation of anMan-HS by using chloroquine. Deconvolution immunofluorescence microscopy and SDS-PAGE using anMan- and APP/Aβ-specific antibodies and markers for nuclei and autophagosomes were used to identify subcellular localization of Aβ and its oligomeric state. Wild-type mouse embryonic fibroblasts (WT MEF) grown during NO-deprivation accumulated 95–98 % of Aβ as oligomers in the nucleus. WT MEF treated with chloroquine accumulated both anMan-HS and Aβ, first in the nucleus then in autophagosomes. Maximal nuclear anMan-HS and Aβ accumulation was obtained after 4 and 7 h of growth, respectively. Both yielded similar banding patterns on SDS-PAGE which were also similar to the Aβ oligomers obtained after NO-deprivation. Nuclear Aβ accumulation was marginally increased (from 54 to 58 %) by suppression of both release and degradation of anMan-HS. Nuclear exit of Aβ, accumulated during growth in aminoguanidine, was enhanced by ascorbate-induced reactivation of anMan-HS production. Transgenic Alzheimer disease mouse (Tg2576) MEF, which produces excess amount of Aβ was used for comparison. Overall, nuclear Aβ exit and lysosomal degradation was compromised by inhibition of the autophagosome-lysosome pathway in both WT and Tg2576 MEF, while only WT MEF was sensitive to suppression of Gpc-1 autodegradation.