Amyloid-ß Protein Precursor Regulates Depolarization-Induced Calcium-Mediated Synaptic Signaling in Brain Slices.

BACKGROUND: Coordinated calcium influx upon neuronal depolarization activates pathways that phosphorylate CaMKII, ERKs, and the transcription factor CREB and, therefore, expression of pro-survival and neuroprotective genes. Recent evidence indicates that amyloid-ß protein precursor (AßPP) is trafficked to synapses and promotes their formation. At the synapse, AßPP interacts with synaptic proteins involved in vesicle exocytosis and affects calcium channel function. OBJECTIVE: Herein, we examined the role of AßPP in depolarization-induced calcium-mediated signaling using acute cerebral slices from wild-type C57bl/6 mice and AßPP-/- C57bl/6 mice. METHODS: Depolarization of acute cerebral slices from wild-type C57bl/6 and AßPP-/- C57bl/6 mice was used to induce synaptic signaling. Protein levels were examined by western blot and calcium dynamics were assessed using primary neuronal cultures. RESULTS: In the absence of AßPP, decreased pCaMKII and pERKs levels were observed. This decrease was sensitive to the inhibition of N- and P/Q-type Voltage Gated Calcium Channels (N- and P/Q-VGCCs) by ω-conotoxin GVIA and ω-conotoxin MVIIC, respectively, but not to inhibition of L-type VGCCs by nifedipine. However, the absence of AßPP did not result in a statistically significant decrease of pCREB, which is a known substrate of pERKs. Finally, using calcium imaging, we found that down regulation of AßPP in cortical neurons results in a decreased response to depolarization and altered kinetics of calcium response. CONCLUSION: AßPP regulates synaptic activity-mediated neuronal signaling by affecting N- and P/Q-VGCCs.

Downregulation of AßPP enhances both calcium content of endoplasmic reticulum and acidic stores and the dynamics of store operated calcium channel activity.

The amyloid-ß protein precursor (AßPP) is a type-1 transmembrane protein involved in Alzheimer's disease (AD). It has become increasingly evident that AßPP, its protein-protein interactions, and its proteolytical fragments may affect calcium homeostasis and vice versa. In addition, there is evidence that calcium dysregulation contributes to AD. To study the role of AßPP in calcium homeostasis, we downregulated its expression in SH-SY5Y cells using shRNA (SH-SY5Y/AßPP-) or increased expression of AßPP695 by transfection (SH-SY5Y/AßPP+). The levels of cytosolic Ca2+ after treatment with thapsigargin, monensin, activation of capacitative calcium entry (CCE), and treatment with SKF, a store operated channel (SOCs) inhibitor, were measured by fura-2AM fluorimetry. SH-SY5Y/AßPP+ cells show reduced response to thapsigargin and reduced CCE, although this reduction is not statistically significant. On the other hand, we found that, relative to SH-SY5Y, SH-SY5Y/AßPP- cells show a significant increase in the response to thapsigargin but not in CCE and their SOCs were more susceptible to SKF inhibition. Additionally, downregulation of AßPP resulted in increased response to monensin that induces calcium release from acidic stores. The increase of calcium release from the endoplasmic reticulum and the acidic stores, when AßPP is downregulated, could be attributed to elevated Ca2+ content or to a dysregulation of Ca2+ transfer through their membranes. These data, along with already existing evidence regarding the role of AßPP in calcium homeostasis and the early occurring structural and functional abnormalities of endosomes, further substantiate the role of AßPP in calcium homeostasis and in AD.