Abstract
BackgroundAlzheimer's disease (AD) is characterized by cerebral deposition of β-amyloid (Aβ) peptides. Aβ is released from ectodomain cleaved amyloid precursor protein (APP) via intramembranous proteolysis by γ-secretase, a complex consisting of presenilin and a few other proteins. p23/TMP21, a member of the p24 family type I transmembrane proteins, was recently identified as a presenilin complex component capable of modulating γ-secretase cleavage. The p24 family proteins form oligomeric complexes and regulate vesicular trafficking in the early secretory pathway, but their role in APP trafficking has not been investigated.ResultsHere, we report that siRNA-mediated depletion of p23 in N2a neuroblastoma and HeLa cells produces concomitant knockdown of additional p24 family proteins and increases secretion of sAPP. Furthermore, intact cell and cell-free Aβ production increases following p23 knockdown, similar to data reported earlier using HEK293 cells. However, we find that p23 is not present in mature γ-secretase complexes isolated using an active-site γ-secretase inhibitor. Depletion of p23 and expression of a familial AD-linked PS1 mutant have additive effects on Aβ42 production. Knockdown of p23 expression confers biosynthetic stability to nascent APP, allowing its efficient maturation and surface accumulation. Moreover, immunoisolation analyses show decrease in co-residence of APP and the APP adaptor Mint3. Thus, multiple lines of evidence indicate that p23 function influences APP trafficking and sAPP release independent of its reported role in γ-secretase modulation.ConclusionThese data assign significance to p24 family proteins in regulating APP trafficking in the continuum of bidirectional transport between the ER and Golgi, and ascribe new relevance to the regulation of early trafficking in AD pathogenesis.
Highlights
Alzheimer's disease (AD) is characterized by cerebral deposition of β-amyloid (Aβ) peptides
The C-terminal amyloid precursor protein (APP) stubs (APP C-terminal fragment (CTF)) resulting from α – and β-secretase cleavage serve as substrates for intramembranous proteolysis by γ-secretase, a multimeric complex made of presenilin (PS) 1 or 2, nicastrin, APH1 and PEN2 [4]
Our results are in accordance with the earlier reported role for p23 in modulating Aβ production, but since p23 knockdown invariably attenuates the expression of several p24 family proteins, our results suggest a broader role for p24 family proteins in regulating APP metabolism and secretion
Summary
Alzheimer's disease (AD) is characterized by cerebral deposition of β-amyloid (Aβ) peptides. Aβ is released from ectodomain cleaved amyloid precursor protein (APP) via intramembranous proteolysis by γ-secretase, a complex consisting of presenilin and a few other proteins. P23/TMP21, a member of the p24 family type I transmembrane proteins, was recently identified as a presenilin complex component capable of modulating γ-secretase cleavage. Cleavage of APP within the lumenal domain by BACE1, the major neuronal β-secretase, releases the APP ectodomain and generates the N-terminus of Aβ [2]. Following ligand binding at the cell surface, Notch is endocytosed and sequentially cleaved by ADAM family metalloproteases and γ-secretase [6]. Reducing p23 expression resulted in increased γ-secretase cleavage of APP, without affecting the proteolysis of Notch [8]
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