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...

The 5-HT(7) receptor is the most recently described member of the serotonin receptor family. This receptor is mainly expressed in the thalamus, hypothalamus as well as in the hippocampus and cortex. In the present study, we demonstrate that the mouse 5-hydroxytryptamine 5-HT(7(a)) receptor undergoes post-translational modification by the palmitate, which is covalently attached to the protein through a thioester-type bond. Analysis of protein-bound fatty acids revealed that the 5-HT(7(a)) receptor predominantly contains palmitic acid. Labelling experiments performed in the presence of agonists show that the 5-HT(7(a)) receptor is dynamically palmitoylated in an agonist-dependent manner and that previously synthesized receptors may be subjected to repeated cycles of palmitoylation/depalmitoylation. Mutation analysis revealed that cysteine residues 404 and 438/441 located in the C-terminal receptor domain are the main palmitoylation sites responsible for the attachment of 90% of the receptor-bound palmitate. Analysis of acylation-deficient mutants revealed that non-palmitoylated 5-HT(7(a)) receptors were indistinguishable from the wild-type for their ability to interact with G(s)- and G(12)-proteins after agonist stimulation. However, mutation of the proximal palmitoylation site Cys404-Ser (either alone or in combination with Cys438/441-Ser) significantly increased the agonist-independent, G(s)-mediated constitutive 5-HT(7(a)) receptor activity, while the activation of Galpha(12)-protein was not affected. This demonstrates a functional importance of 5-HT(7(a)) dynamic palmitoylation for the fine tuning of receptor-mediated signaling.

Alzheimer's disease (AD) is the primary cause of dementia in the elderly and one of the major health problems worldwide. Since its first description by Alois Alzheimer in 1907, noticeable but insufficient scientific comprehension of this complex pathology has been achieved. All the research that has been pursued takes origin from the identification of the pathological hallmarks in the forms of amyloid-β (Aβ) deposits (plaques), and aggregated hyperphosphorylated tau protein filaments (named neurofibrillary tangles). Since this discovery, many hypotheses have been proposed to explain the origin of the pathology. The "amyloid cascade hypothesis" is the most accredited theory. The mechanism suggested to be one of the initial causes of AD is an imbalance between the production and the clearance of Aβ peptides. Therefore, Amyloid Precursor Protein (APP) synthesis, trafficking and metabolism producing either the toxic Aβ peptide via the amyloidogenic pathway or the sAPPα fragment via the non amyloidogenic pathway have become appealing subjects of study. Being able to reduce the formation of the toxic Aβ peptides is obviously an immediate approach in the trial to prevent AD. The following review summarizes the most relevant discoveries in the field of the last decades.

G protein-coupled receptors (GPCRs) can activate simultaneously multiple signaling pathways upon agonist binding. The combined use of engineered GPCRs, such as the receptors activated solely by synthetic ligands (RASSLs), and of biased ligands that activate only one pathway at a time might help deciphering the physiological role of each G protein signaling. In order to find serotonin type 4 receptor (5-HT₄R) biased ligands, we analyzed the ability of several compounds to activate the Gs and G(q/11) pathways in COS-7 cells that transiently express wild type 5-HT₄R, the 5-HT₄R-D(100)A mutant (known also as 5-HT₄-RASSL, or Rs1) or the 5-HT₄R-T(104)A mutant, which modifies agonist-induced 5-HT₄R activation. This analysis allowed completing the pharmacological profile of the two mutant 5-HT₄Rs, but we did not find any biased ligand for the mutant receptors. Conversely, we identified the first biased agonists for wild type 5-HT₄R. Indeed, RS 67333 and prucalopride acted as partial agonists to induce cAMP accumulation, but as antagonists on inositol phosphate production. Moreover, they showed very different antagonist potencies that could be exploited to study the activation of the G(s) pathway, with or without concomitant block of G(q/11) signaling. This article is part of a Special Issue entitled Optogenetics (7th BRES).