SECRETED AMYLOID PRECURSOR PROTEIN-ALPHA REGULATES GLUTAMATE RECEPTOR TRAFFICKING IN THE HIPPOCAMPUS

Abstract

Proteolysis of the amyloid precursor protein produces by α-secretase activity liberates the secreted amyloid precursor protein-alpha (sAPPα). This molecule, unlike amyloid-β, has many long-lasting positive effects. These include neuroprotection and enhancement of neurogenesis and memory mechanisms such as long-term potentiation (LTP). While this mounting evidence highlights sAPPα as a possible therapeutic agent for the treatment of Alzheimer's disease, the underlying molecular mechanisms employed by sAPPα are not well elucidated. Since LTP is dependent on regulated trafficking of glutamate receptors we hypothesized that sAPPα may harness this mechanism to promote synaptic plasticity. Acute hippocampal slice, from young adult male Sprague-Dawley rats, were treated with recombinant sAPPα (1 nM, 30 min). Cell-surface proteins were isolated by reversible tagging with a membrane-impermeable biotin moiety and precipitated by addition of Neutravidin-agarose. Glutamate receptor subunit levels were assessed by Western blot and relative changes in cell-surface levels were determined by comparison with levels in sham-treated slices. Group data were subjected to one-way ANOVA followed by Tukey's post-hoc tests. sAPPα induced a significant increase in cell-surface levels of specific AMPA and NMDA-glutamate receptor subunits. While GluA2 surface expression was unaffected, sAPPα induced a significant increase in the early phase LTP-related subunit of the AMPA-subtype of glutamate receptor, GluA1 (1.55 ± 0.05, n=15, p<0.0001), as well as phosphorylation of GluA1 at serine 831 (1.34 ± 0.08, n=7, p=0.005), when compared to control untreated slices. sAPPa also increased cell-surface levels of the obligatory subunit of the NMDA receptor, GluN1 (1.32 ± 0.12, n=15, p=0.011). Co-application of the calmodulin-dependent kinase inhibitor KN62 (10 μM) completely blocked these effects. Further, the trafficking inhibitor Brefeldin A (35 μM) blocked the enhancement of GluA1 and GluN1 surface expression, but inhibition of either Protein Kinase G or MAP-kinase activity only partially blocked the effect. The translation inhibitor cycloheximide (60 μM) had no significant effect on the sAPPα-mediated increase in GluA1 cell-surface expression, but blocked the increase in GluN1 expression, suggesting that AMPA and NMDA receptor levels are regulated independently by sAPPα. These findings suggest that increased trafficking of glutamate receptor subunits is one mechanism which underpins the LTP-enhancing properties of sAPPα.