Evidence That Intramolecular Associations between Presenilin Domains Are Obligatory for Endoproteolytic Processing

Carlos A Saura,Taisuke Tomita,Frances Davenport,Christie L Harris,Takeshi Iwatsubo,Gopal Thinakaran

doi:10.1074/jbc.274.20.13818

Abstract

Mutations in genes encoding presenilins (PS1 and PS2) cosegregate with the majority of early onset cases of familial Alzheimer's disease. PS1 and PS2 are polytopic membrane proteins that undergo endoproteolytic cleavage to generate stable NH2- and COOH-terminal derivatives (NTF and CTF, respectively). Several lines of evidence suggest that the endoproteolytic derivatives are likely the functional units of PS in vivo. In the present report, we examine the disposition of PS NTF and CTF assemblies in stable mouse N2a neuroblastoma cell lines expressing human PS polypeptides. We show that exogenous expression of PS1 NTFs neither assemble with endogenous CTF nor exhibit dominant negative inhibitory effects on the endogenous PS1 cleavage and the accumulation of derivatives. In cells co-expressing PS1 and PS2, PS1- and PS2-derived fragments do not form mixed assemblies. In contrast, cells expressing a chimeric PS1/PS2 polypeptide form stable PS1 NTF-PS2 CTF assemblies. Moreover, expression of chimeric PS1/PS2 polypeptides harboring a familial early onset AD-linked mutation (M146L) elevates the production of Abeta42 peptides. Our results provide evidence that assembly of structural domains contained within NH2- and COOH-terminal regions of PS occur prior to endoproteolytic cleavage.

Highlights

Alzheimer’s disease (AD),1 a progressive neurodegenerative disorder, is the most common cause of dementia in the elderly
Transgene-derived Human presenilin 1 (PS1) NTF Fails to Assemble with Endogenous PS1 COOH-terminal fragments (CTF)—In previous studies, we and others reported that the NTF and CTF derived from endoproteolysis of PS1 are noncovalently associated to each other in vivo (26, 36, 37)
We reported that the expression of human full-length PS1 in mouse N2a cells resulted in a decrease in the levels of murine PS1-derived NTF and CTF (21)

Summary

Introduction

Alzheimer’s disease (AD), a progressive neurodegenerative disorder, is the most common cause of dementia in the elderly. Arguing against this notion, Citron et al (27) reported that expression of a truncated PS1 species corresponding to the human PS1 NTF with a FAD mutation failed to increase A␤42 production; the authors suggested that the pathogenic effect of mutant PS is elicited by the full-length molecule and not the endoproteolytic derivatives. Expressing PS1 and PS2, processed fragments derived from full-length PS polypeptides form assemblies consisting entirely of either PS1 NTF1⁄7CTF or PS2 NTF1⁄7CTF Based on these results, we conclude that the association of NH2 and COOH domains of the precursor PS polypeptide precedes endoproteolytic cleavage and that PS1 and PS2 do not form heteromultimeric assemblies. Our results are consistent with a model where stabilization of intramolecular interaction(s) between domains within the NH2- and COOH-terminal regions of PS1 is a prerequisite for endoproteolytic processing

Methods

Results

Conclusion