Aims

Dysregulation of the Endoplasmic Reticulum (ER) Calcium homeostasis^1-4, and subsequent ER stress activation occur in Alzheimer’s disease (AD). The human truncated isoform of the Sarco-endoplasmic reticulum Calcium ATPase 1 (S1T)⁵ triggers and amplifies ER stress response, leading to subsequent cell commitment to apoptosis through the control of Calcium mobilization from ER to mitochondria⁶. We examined S1T expression in AD and investigated the mutual link between S1T expression and APP processing and neuroinflammation.

Methods

We used SH-SY5Y cells overproducing beta-amyloid precursor protein-derived fragments (APPswe) or treated with oligomeric Amyloid beta (Abeta) peptides, 3xTg-AD transgenic mice and human brains. We used biochemical, quantitative RT-PCR, and sterotaxic injection in 3xTg-AD mice brains.

Results

S1T expression is increased in SH-SY5Y cells expressing APPswe or treated with Abeta oligomers and in sporadic AD brains and is correlated with Abeta load and key ER stress proteins. Overexpression of S1T enhances in return the production of APP C-terminal fragments and Abeta through specific increases of beta-secretase expression and activity. Elevated S1T expression also triggers neuroinflammation in vitro and in vivo⁷.

Conclusions

We describe a molecular interplay between S1T-dependent ER Calcium leak, ER stress and APP processing that could contribute to AD pathogenesis.

1-Oules B,et al. 2012, J Neurosci.

2-Del Prete D, et al. 2014, Mol Neurodegener .

3-Lacampagne A, et al. 2017, Acta Neuropathol.

4-Bussiere R, et al. 2017, J Biol Chem .

5-Chami M, et al. 2001, J Cell Biol.

6-Chami M, et al. 2008, Mol Cell.

7-Bussiere R, et al. 2019, Cells.

Abstract Body

Aims

Parkinson’s disease (PD)-affected brains show endoplasmic reticulum (ER) stress and mitophagic dysfunctions. How these processes are altered and whether they are directly linked remain a matter of questions. XBP1 is a transcription factor activated upon ER stress after unconventional splicing by the nuclease IREa thereby yielding XBP1S. PINK1 is a kinase considered as the sensor of mitochondrial physiology and a master gatekeeper of mitophagy process. Our work aims at delineating the putative link between XBP1S and PINK1 that could explain ER stress alteration and mitophagic defects in Parkinsons disease.

Methods

We have used cell biology, molecular biology and pharmacological approaches to unravel a forward loop regulation between XBP1 and PINK1 in various neuronal models and in vivo. Extensive ex-vivo and in vivo mitophagy analysis by biochemical approaches and evaluation of mitochondrial function (mitochondrial potential, Δψmit) and morphology were assessed by combined procedures including, flow cytometry, live imaging analysis and electronic microcopy.

Results

We show that XBP1S transactivates PINK1 in human cells, primary cultured neurons and mice brain, and triggers a pro-mitophagic phenotype that is fully dependent of endogenous PINK1. We also unravel a PINK1-dependent phosphorylation of XBP1S that conditions its nuclear localization and thereby, governs its transcriptional activity. PINK1-induced XBP1S phosphorylation occurs at residues reminiscent of those phosphorylated in substantia nigra of PD-affected brains.

Conclusions

Overall, our study delineates for the first time a functional forward loop between XBP1S and PINK1 governing a molecular link between ER-stress and mitophagy that could be disrupted in PD condition