SOLUBLE AMYLOID PRECURSOR PROTEIN IS AN ISOFORM-SPECIFIC GABA(B) RECEPTOR LIGAND THAT SUPPRESSES SYNAPTIC RELEASE PROBABILITY

Abstract

Amyloid Precursor Protein has been extensively studied for its generation of the amyloid-beta peptide implicated in Alzheimer's disease. However, the physiological function of APP and its processing products remains elusive. Soluble APP (sAPP), the product of APP ectodomain shedding, is sufficient to restore synaptic deficits in APP knockout mice, suggesting the presence of a synaptic receptor for sAPP. However, the putative synaptic receptor for sAPP has yet to be identified. To identify candidate synaptic interactors for sAPP, we performed a shotgun proteomics screen using purified sAPP-Fc as bait and synaptosome extracts from rat brain as prey. We performed cell surface binding assays and isothermal titration calorimetry to validate binding of sAPP to GABA(B) receptors. To access if sAPP modulates GABA(B)R function, we analyzed both miniature excitatory postsynaptic currents (mEPSCs) and exocytosis of FM1–43 labeled vesicles in hippocampal cultures in response to acute application of sAPP and GABA(B)R antagonists. Finally, we performed extracellular recordings on acute hippocampal slices to analyze GABA(B)R-dependent effects of sAPP on synaptic transmission at the CA3-CA1 pathway. Our proteomics screen led to the identification of GABA(B) receptor as a synaptic APP interactor. A conserved 17aa peptide within the extension domain of sAPP directly interacts with the sushi-1 domain specific to the GABA(B)R1a isoform. sAPP reduces both the frequency of mEPSCs and synaptic vesicle exocytosis in hippocampal cultures. The 17aa GABA(B)R1a-specific binding region of sAPP is both required and sufficient to suppress synaptic release. Finally, sAPP decreases the strength of basal synaptic transmission and enhances short-term synaptic facilitation in hippocampal slices, implying a reduced probability of release. The effect of sAPP on synaptic release both in cultures and in slices is blocked by pretreatment with a GABA(B)R antagonist, demonstrating that sAPP decreases the probability of release via presynaptic GABA(B) receptors. sAPP directly interacts with GABA(B)R1a and acts a positive modulator of GABA(B) receptor signaling to reduce synaptic release probability.