Disturbed Ca2+ Homeostasis Increases Glutaminyl Cyclase Expression; Connecting Two Early Pathogenic Events in Alzheimer’s Disease In Vitro

Line De Kimpe,Rob Zwart,Wiep Scheper,Jeroen J M Hoozemans,Anna Bennis,Elise S Van Haastert,Koichi M Iijima

doi:10.1371/journal.pone.0044674

Line De Kimpe, Rob Zwart + Show 5 more

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https://doi.org/10.1371/journal.pone.0044674

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Abstract

A major neuropathological hallmark of Alzheimer’s disease (AD) is the deposition of aggregated β amyloid (Aβ) peptide in the senile plaques. Aβ is a peptide of 38–43 amino acids and its accumulation and aggregation plays a key role early in the disease. A large fraction of β amyloid is N-terminally truncated rendering a glutamine that can subsequently be cyclized into pyroglutamate (pE). This makes the peptide more resistant to proteases, more prone to aggregation and increases its neurotoxicity. The enzyme glutaminyl cyclase (QC) catalyzes this conversion of glutamine to pE. In brains of AD patients, the expression of QC is increased in the earliest stages of pathology, which may be an important event in the pathogenesis. In this study we aimed to investigate the regulatory mechanism underlying the upregulation of QC expression in AD. Using differentiated SK-N-SH as a neuronal cell model, we found that neither the presence of Aβ peptides nor the unfolded protein response, two early events in AD, leads to increased QC levels. In contrast, we demonstrated increased QC mRNA levels and enzyme activity in response to another pathogenic factor in AD, perturbed intracellular Ca2+ homeostasis. The QC promoter contains a putative binding site for the Ca2+ dependent transcription factors c-fos and c-jun. C-fos and c-jun are induced by the same Ca2+-related stimuli as QC and their upregulation precedes QC expression. We show that in the human brain QC is predominantly expressed by neurons. Interestingly, the Ca2+- dependent regulation of both c-fos and QC is not observed in non-neuronal cells. Our results indicate that perturbed Ca2+ homeostasis results in upregulation of QC selectively in neuronal cells via Ca2+- dependent transcription factors. This suggests that disruption of Ca2+ homeostasis may contribute to the formation of the neurotoxic pE Aβ peptides in Alzheimer’s disease.

Highlights

The formation of Ab peptides from the amyloid precursor protein (APP) is a crucial event in Alzheimer’s disease (AD)
We observe a significant increase in mRNA levels of QC in the patients with the earliest stages of pathology (Fig. 1)
QC Expression is not Upregulated by Ab or the unfolded protein response (UPR) Because QC is increased very early in AD, we investigated if early events occurring in AD might be involved in the regulation

Summary

Introduction

The formation of Ab peptides from the amyloid precursor protein (APP) is a crucial event in Alzheimer’s disease (AD). Proteolytic processing of APP results in different forms of the Ab protein with different characteristics [1]. N-terminal truncations have been identified that expose a glutamic acid at position 3 or 11. This glutamate residue can be cyclized into a pyroglutamate (pE) N-terminus which leads to the formation of Ab3(pE)-x and Ab11(pE)-x in a reaction catalyzed by glutaminyl cyclases (QCs). The conversion of the glutamatic acid into a pE residue results in a loss of charge and the consequent increased hydrophobicity leads to an increase in aggregation propensity [4,5]. The change in physicochemical properties is accompanied by increased neurotoxicity of pE Ab peptides compared to unmodified Ab species [6,7,8]

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