TMP21 Transmembrane Domain Regulates γ-Secretase Cleavage

Raphaëlle Pardossi-Piquard,Christopher Böhm,Fusheng Chen,Soshi Kanemoto,Frédéric Checler,Gerold Schmitt-Ulms,Peter St George-Hyslop,Paul E Fraser

doi:10.1074/jbc.m109.059345

Abstract

TMP21 has been shown to be associated with the γ-secretase complex and can specifically regulate γ-cleavage without affecting ϵ-mediated proteolysis. To explore the basis of this activity, TMP21 modulation of γ-secretase activity was investigated independent of ϵ-cleavage using an amyloid-β precursor proteinϵ (APPϵ) construct which lacks the amyloid intracellular domain domain. The APPϵ construct behaves similarly to the full-length precursor protein with respect to α- and β-cleavages and is able to undergo normal γ-processing. Co-expression of APPϵ and TMP21 resulted in the accumulation of membrane-embedded higher molecular weight Aβ-positive fragments, consistent with an inhibition of γ-secretase cleavage. The APPϵ system was used to examine the functional domains of TMP21 through the investigation of a series of TMP21-p24a chimera proteins. It was found that chimeras containing the transmembrane domain bound to the γ-secretase complex and could decrease γ-secretase proteolytic processing. This was confirmed though investigation of a synthetic peptide corresponding to the TMP21 transmembrane helix. The isolated TMP21 TM peptide but not the homologous p24a domain was able to reduce Aβ production in a dose-dependent fashion. These observations suggest that the TMP21 transmembrane domain promotes its association with the presenilin complex that results in decreased γ-cleavage activity.

Highlights

Of the main histopathological hallmarks in Alzheimer disease is senile plaque, an extracellular protein deposit composed primarily of the amyloid ␤-peptide (A␤),5 which is generated by sequential cleavages of the amyloid-␤ precursor protein (APP) by ␤- and ␥-secretase activities [1]
Cleavage by ␥-secretase occurs within the APP transmembrane domain at several sequences, such as the ␥-site, to produce A␤40/42, and the ⑀-site, which results in the release of the cytosolic amyloid intracellular domain (AICD) [2]
Cated APP construct terminating at the ⑀-site was employed to (Fig. 1B) and undergoes normal processing by ␣, ␤, and allow investigation of TMP21 activity independent of ⑀-medi- ␥-secretase to produce similar levels of A␤ peptides and soluble

Summary

Introduction

Of the main histopathological hallmarks in Alzheimer disease is senile plaque, an extracellular protein deposit composed primarily of the amyloid ␤-peptide (A␤),5 which is generated by sequential cleavages of the amyloid-␤ precursor protein (APP) by ␤- and ␥-secretase activities [1]. The effects of TMP21 on ␥-secretase activity were investigated independent of ⑀-cleavage using an APP⑀ construct which lacks the AICD domain (terminates at A␤49). Co-expression of APP⑀ and TMP21 resulted in the accumulation of membrane-embedded higher molecular weight A␤-positive fragments, consistent with an inhibition of ␥-secretase cleavage.

Results

Conclusion