Secreted amyloid-beta precursor protein functions as a GABABR1a ligand to modulate synaptic transmission.

INTRODUCTION More than 30 years have passed since the amyloid-β precursor protein (APP) was identified. Although the role of APP in Alzheimer’s disease has been studied widely, its normal physiological function in the brain has remained elusive. APP undergoes ectodomain shedding by α-, β-, or η-secretase to release secreted APP (sAPPα, sAPPβ, or sAPPη, respectively). sAPPα affects synaptic transmission and plasticity and is sufficient to rescue synaptic defects in App knockout mice. This has led to speculation of a yet unidentified cell-surface receptor for sAPPα. RATIONALE To elucidate the physiological function of APP, we sought to identify the cell-surface receptor mediating its effects on synaptic function. To identify candidate synaptic interactors for sAPPα, we performed affinity-purification experiments using recombinant sAPPα to pull down interacting proteins from synaptosome extracts, followed by mass spectrometric analysis of bound proteins. We identified the γ-aminobutyric acid type B receptor (GABA B R), the metabotropic receptor for the inhibitory neurotransmitter γ-aminobutyric acid (GABA), as the leading candidate for a synaptic, cell-surface receptor for sAPPα. We then performed a combination of cell-surface binding assays and in vitro biophysical techniques to determine the interacting domains and structural consequences of binding. We investigated whether sAPPα can modulate GABA B R function by assessing miniature excitatory and inhibitory postsynaptic currents (mEPSCs and mIPSCs, respectively) and synaptic vesicle recycling in mouse hippocampal neuron cultures, short-term plasticity in acute hippocampal slices from mice, and in vivo neuronal activity in the hippocampus of anesthetized mice. RESULTS Recombinant sAPPα selectively bound to GABA B R subunit 1a (GABA B R1a)–expressing cells. Binding was mediated by the flexible, partially structured extension domain in the linker region of sAPP and the natively unstructured sushi 1 domain specific to GABA B R1a. sAPPβ and sAPPη, which both contain the extension domain, also bound to GABA B R1a-expressing cells. Conversely, APP family members APP-like proteins 1 and 2, which lack a conserved extension domain, failed to bind GABA B R1a-expressing cells. Acute application of sAPPα reduced the frequency of mEPSCs and mIPSCs and decreased synaptic vesicle recycling in cultured mouse hippocampal neurons. In addition, sAPPα enhanced short-term facilitation in acute hippocampal slices from mice. Together, these findings demonstrate that sAPP reduces the release probability of synaptic vesicles. These effects were dependent on the presence of the extension domain in sAPP and were occluded by a GABA B R antagonist. A short APP peptide corresponding to the GABA B R1a binding region within APP stabilized the natively unstructured sushi 1 domain of GABA B R1a, allowing determination of its solution structure using nuclear magnetic resonance spectroscopy and the generation of a structural model of the APP–sushi 1 complex. Application of a 17–amino acid APP peptide mimicked the effects of sAPPα on GABA B R1a-dependent inhibition of synaptic vesicle release and reversibly suppressed spontaneous neuronal activity in vivo. CONCLUSION We identified GABA B R1a as a synaptic receptor for sAPP and revealed a physiological role for sAPP in regulating GABA B R1a function to modulate synaptic transmission and plasticity. Our findings provide a potential target for the development of GABA B R1a isoform–specific therapeutics, which is relevant to a number of neurological disorders in which GABA B R signaling is implicated.