5-HT₄ Receptors Constitutively Promote the Non-Amyloidogenic Pathway of APP Cleavage and Interact with ADAM10

Abstract

In addition to the amyloidogenic pathway, amyloid precursor protein (APP) can be cleaved by α-secretases, producing soluble and neuroprotective APP alpha (sAPPα) (nonamyloidogenic pathway) and thus preventing the generation of pathogenic amyloid-β. However, the mechanisms regulating APP cleavage by α-secretases remain poorly understood. Here, we showed that expression of serotonin type 4 receptors (5-HT 4 Rs) constitutively (without agonist stimulation) induced APP cleavage by the α-secretase ADAM10 and the release of neuroprotective sAPPα in HEK-293 cells and cortical neurons. This effect was independent of cAMP production. Interestingly, we demonstrated that 5-HT 4 receptors physically interacted with the mature form of ADAM10. Stimulation of 5-HT 4 receptors by an agonist further increased sAPPα secretion, and this effect was mediated by cAMP/Epac signaling. These findings describe a new mechanism whereby a GPCR constitutively stimulates the cleavage of APP by α-secretase and promotes the nonamyloidogenic pathway of APP processing. KEYWORDS: Alpha-secretase, Alzheimer's disease, sAPP alpha, serotonin A ccording to the amyloid hypothesis, alteration in synaptic transmission and neuronal loss observed in Alzheimer's disease (AD) mainly result from the formation of toxic amyloid-β (Aβ) oligomers followed by the extracellular accumulation of Aβ aggregates. Aβ is produced by the successive cleavage of a transmembrane amyloid precursor protein (APP) by β-secretase and γ-secretase. 1 In addition to this amyloidogenic pathway, APP can be cleaved by α-secretases, a set of membrane-bound proteases of the ADAM (A disintegrin and metalloprotease) family, generating the soluble APP ectodomain (sAPPα) and a membrane-bound carboxy-terminal fragment (nonamyloidogenic pathway). As α-secretases cleave APP within the Aβ sequence, APP shedding by α-secretases prevents the generation of the pathogenic Aβ peptide. 2,3 Therefore, enhancing α-secretase expression or activity has been considered as a valuable strategy for inhibiting Aβ formation. For instance, it has recently been shown that activation of the transcription of the gene encoding the α-secretase ADAM10, by retinoic acid and sirtuin 1 (SIRT1), reduces Aβ production. 4−6 Previous reports have shown that G protein-coupled receptors (GPCRs) can differentially affect Aβ peptide production by either modulating the cellular trafficking of APP or by influencing the activity and trafficking of α-, β-and γ-secretases. Moreover, both the expression and the stimulation of GPCRs can affect APP metabolism. 7 GPCRs that enhance sAPPα production by stimulating α-secretase activities include the muscarinic M 1 -M 3 receptors, mGlu2 metabotropic glutamate receptor, serotonin 2A (5-HT 2A ) and 5-HT 2C receptors, corticotropin-releasing factor (CRF) receptor 1, purinergic receptor P2X 7 and pituitary adenylate cyclase-activating polypeptide (PACAP) type 1 (PAC 1 ) receptor. 7 These GPCRs are supposed to have a beneficial effect, because sAPPα exerts neuroprotective and neurotro...

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