Serotonin 5-HT2a and 5-HT2c Receptors Stimulate Amyloid Precursor Protein Ectodomain Secretion

Roger M Nitsch,Meihua Deng,John H Growdon,Richard J Wurtman

doi:10.1074/jbc.271.8.4188

Abstract

Alzheimer's disease amyloid consists of amyloid beta-peptides (Abeta) derived from the larger precursor amyloid precursor protein (APP). Non-amyloidogenic APP processing involves regulated cleavage within the Abeta domain followed by secretion of the ectodomain (APPs). APPs secretion can be stimulated by muscarinic acetylcholine receptors coupled to phospholipases and kinases. To determine whether other receptor classes can regulate APP processing, we examined the relation between serotonin receptors and APPs secretion. Serotonin increased APPs release 3-4-fold in 3T3 cells stably overexpressing 5-HT2aR or 5-HT2cR. The increase was dose-dependent and was blocked by serotoninergic antagonists. Phorbol esters also increased APPs secretion, but neither kinase inhibitors nor down-regulation of PKC blocked the serotonin-induced increase in APPs secretion. Thus PKC is not necessary to stimulate APPs secretion. Phospholipase A2 (PLA2) inhibitors blocked the 5-HT2aR-mediated increase in APPs secretion, suggesting a role of PLA2 in coupling 5-HT2aR to APP processing. In contrast, coupling of 5-HT2cR to APPs secretion involved both PKC and PLA2. Serotonin also stimulated the release of the APLP2 ectodomain, suggesting that additional members of the APP multigene family are processed via similar regulated pathways. Inasmuch as generation of APPs precludes the formation of amyloidogenic derivatives, serotonin receptors provide a novel pharmacological target to reduce these derivatives in Alzheimer's disease.

Highlights

The amyloid precursor protein (APP)1 is an ubiquitous membrane-spanning glycoprotein (Kang et al, 1987; Weidemann et al, 1989) that is present at high levels in brain cells
The increase in APPs secretion caused by 5-HT2aR stimulation was blocked by the serotoninergic antagonists ketanserin, mianserin, and ritanserin (Fig. 3), and the 5-HT2cR-mediated increase was blocked by mianserin (Fig. 3), indicating that the effects on APP processing in both cell lines were mediated by activation of the overexpressed 5-HT2 receptor subtypes
In 5-HT2aR-expressing cells, the 5-HT-mediated increase in APPs secretion was effectively blocked by the Phospholipase A2 (PLA2)-inhibitors manoalide (ML), dimethyleicosadienoic acid (DEDA), and oleyloxyethyl phosphorylcholine (OPC) (Fig. 4B), indicating that PLA2 may be involved in the cellular signaling cascade that couples 5-HT2aR activation to APPs secretion

Summary

Introduction

The amyloid precursor protein (APP) is an ubiquitous membrane-spanning glycoprotein (Kang et al, 1987; Weidemann et al, 1989) that is present at high levels in brain cells. APP or its derivatives may be involved in the pathogenesis of Alzheimer’s disease because several familial forms of the disease are linked to APP mutations within or close to the ␤-amyloid domain In cell culture, these mutations cause misprocessing of APP and related increases in the formation of A␤ (Cai et al, 1993; Citron et al, 1992, 1994) or the generation of longer than normal A␤ molecules (Suzuki et al, 1994). Secretory APP processing and the formation of APPs can be readily accelerated by an unusual mechanism coupled to a variety of extra- and intracellular signals These include muscarinic acetylcholine receptors (Buxbaum et al, 1992; Nitsch et al, 1992; Wolf et al, 1995), metabotropic glutamate receptors (Lee et al, 1995), protein kinase C (PKC) (Caporaso et al, 1992; Slack et al, 1993), tyrosine kinases (Slack et al, 1995), and arachidonic acid (Emmerling et al, 1993). They provide a valid model system to study the effects of specific transfected serotonin receptor subtypes on signaling as well as on APPs secretion

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Conclusion