Knockdown of Amyloid Precursor Protein Increases Ion Channel Expression and Alters Ca Signaling Pathways.

Although the physiological role of the full-length Amyloid Precursor Protein (APP) and its proteolytic fragments remains unclear, they are definitively crucial for normal synaptic function. Herein, we report that the downregulation of APP in SH-SY5Y cells, using short hairpin RNA (shRNA), alters the expression pattern of several ion channels and signaling proteins that are involved in synaptic and Ca signaling. Specifically, the levels of GluR2 and GluR4 subunits of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid glutamate receptors (AMPAR) were significantly increased with knockdown. Similarly, the expression of the majority of endoplasmic reticulum (ER) residing proteins, such as the ER Ca channels IPR (Inositol 1,4,5-triphosphate Receptor) and RyR (Ryanodine Receptor), the Ca pump SERCA2 (Sarco/endoplasmic reticulum Ca ATPase 2) and the ER Ca sensor STIM1 (Stromal Interaction Molecule 1) was upregulated. A shift towards the upregulation of p-AKT, p-PP2A, and p-CaMKIV and the downregulation of p-GSK, p-ERK1/2, p-CaMKII, and p-CREB was observed, interconnecting Ca signal transduction from the plasma membrane and ER to the nucleus. Interestingly, we detected reduced responses to several physiological stimuli, with the most prominent being the ineffectiveness of SH-SY5Y/APP- cells to mobilize Ca from the ER upon carbachol-induced Ca release through IPRs and RyRs. Our data further support an emerging yet perplexing role of APP within a functional molecular network of membrane and cytoplasmic proteins implicated in Ca signaling.