J Biol Chem. 2013 May 3;288(18):12920-31.

Transcriptional regulation of insulin-degrading enzyme modulates mitochondrial amyloid β (Aβ) peptide catabolism and functionality.

María C. Leal, Natalia Magnani; Sergio Villordo, Cristina Marino Buslje, Pablo Evelson, Eduardo M. Castaño and Laura Morelli.

Fundación Instituto Leloir, Instituto de Investigaciones Bioquímicas de Buenos Aires (Consejo Nacional de Investigaciones Científicas y Tecnológicas (CONICET)), Avenida Patricias Argentinas 435, Ciudad Autónoma de Buenos Aires C1405BWE, Argentina.

 

Abstract

Studies of postmortem brains from Alzheimer’s disease (AD) patients suggest that oxidative damage induced by mitochondrial Aβ (mitAβ) accumulation is associated with mitochondrial dysfunction. However, the regulation of mitAβ metabolism is unknown. One of the proteases involved in mitAβ catabolism is long Insulin-Degrading Enzyme (IDE) isoform (IDE-Met1) . Yet, the mechanisms of its expression are unknown and its presence in brain uncertain. We detected IDE-Met1 in brain and showed that its expression is regulated by mitochondrial biogenesis pathway (PGC-1α/NRF- 1). A strong positive correlation between PGC-1α or NRF-1 and L-IDE transcripts was found in non-demented brains. This correlation was weaker in AD. In vitro inhibition of IDE increased mitAβ and impaired mitochondrial respiration. These changes were restored by inhibition of γ-secretase or promotion of mitochondrial biogenesis. Our results suggest that IDE-Met1 links mitochondrial biogenesis pathway with mitAβ levels and organella functionality.

PMID: 23525105

 

Supplements

By using DAVID data base, which allows in silico analysis of regulatory sequences of 14821 human genes, we realized that 471 (3.17 %) have the NRF-1 DNA binding motif in the proximal region (-250/1) and 9 of them belongs to mitochondrial genes suggesting that at least part of hIDE transcriptional regulation is similar to this specific sub-set of genes. The NRF-1 site in IDE promoter is located in a CpG rich island. In normal cells methylation occurs predominantly in CG-poor regions, while CpG-island remains unmethylated. The exceptions are the extensive methylation of CpG islands associated with transcriptional inactivation of regulatory regions of genes. Methylation of DNA is an important regulator of gene expression and early studies of genomic DNA methylation reported it to decrease with age. However recent evidence indicates that this relationship is more complex. A recent study reported positive correlations between CpG island methylation and ageing. The change in the epigenetic signature with age is likely to be significantly determined by environmental exposures, including diet, tobacco smoke, alcohol and previous illnesses and therapies. The biological consequence of the methylation status in the cluster of NRF-1 binding site is not clear. Although the reporter gene experiments underlined the potential importance of promoter methylation on IDE expression the assay determined the effects of methylation of the whole promoter fragment rather than individual or clusters of CpGs. However one of the CpG analyzed corresponds to the polymorphism C/T at position -51 on IDE promoter which haplotype with the G/T on position -1002 was described as a protective (-1002G/-51T) or risk (1002T/-51C) factor for sporadic AD in Chinese population. In this context, methylation of C-51 may result in a downregulation of IDE transcription and a potential risk factor for AD.

We described in AD brains a poor correlation between the expression of genes involved in mitochondrial biogenesis and L-IDE supporting the concept that impairments on mitochondrial biogenesis, due to repression of PCG-1α–dependent genes, preclude mitAβ degradation and that this mechanism may be an upstream event responsible of the mitochondrial dysfunction, a prominent and early feature of AD brain mainly characterized by reduced energy metabolism, lower expression of mitochondrial DNA, excessive mitochondrial fragmentation and abnormal mitochondrial distribution.

The link described here among mitochondrial biogenesis, IDE promoter activation and L-IDE expression supports a concert action of NRF-1 together with PPARγ (co-activated by PGC-1α) to enhance hIDE transcription. The role of PCG-1α in gluconeogenesis and insulin resistance, the relevance of insulin -PI3K-akt pathway in PPARγ, NRF-1 activity and IDE expression and the metabolic phenotype of insulin resistance described in AD and Diabetes Mellitus patients stand out the relevance of our results in the understanding the role of hIDE in the pathogenesis of both human diseases (Fig. 1).

This study favors the notion that IDE has an “eclipsed” distribution with huge amounts of the cytosolic isoform (IDE-Met42) that mask mitochondrial IDE (IDE-Met1). The high levels of cytosolic IDE may be explained by the multiplicity and abundance of the “short” transcripts. Taking into account that mitochondrial accumulation of Aβ may be not a physiologic process, the role of IDE-Met1 may be to degrade free mitochondrial targeting peptides and also other small peptide substrates that may be toxic to organellar function.

The identification that IDE- Met1 isoform is expressed in human brain, that PGC-1α/NRF-1 pathway modulates mitAβ through the regulation of L-IDE mRNA expression and that this pathway has a functional significance in mitochondrial function may provide a starting point for examining novel prophylactic strategies in the treatment of mitochondrial dysfunction in AD.

Laura Morelli-1

FIGURE 1. Schematic representation of the effect of mitochondrial biogenesis on hIDE transcriptional activation and mitochondrial functionality. Activation of PGC1-α by effect of mitochondrial biogenesis stimuli promotes NRF-1 expression which impact on proximal hIDE specific DNA binding domains promoting the transcription of L-IDE and S-IDE mRNAs which further translate the long (IDE-Met1) and the short (IDE-Met42) IDE isoforms. Mitochondrial Aβ can be degraded by IDEMet1 originating non-toxic peptides or it can interact with cyclophilin D, Aβ- binding alcohol dehydrogenase (ABAD) or Cytochrome c oxidase (COX) causing elevated reactive oxygen species (ROS). The balance between both processes results in functional or dysfunctional mitochondria, respectively.

Laura Morelli-2

Laura Morelli (left), María Celeste Leal and Eduardo Miguel Castaño

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