Publication Highlights: February 2018-April 2018 CFATG
Publication Highlights: February 2018-April 2018

Publication Highlights: Articles on autophagy research published by french laboratories and selected by CFATG.

In plant, autophagy is essential for nutrient recycling and plays a fundamental role in nitrogen remobilization from senescing leaves to seeds. During seed maturation, storage proteins (12S globulins and 2S albumins) accumulates in specific vacuoles (PSV for Protein Storage Vacuole) and will represent a nitrogen source for the plantlet during germination. In this study, we show that autophagy controls resource allocations from fruit to seed, embryo development and protein storage accumulation in Arabidopsis. It is the first study showing autophagosomes in Arabidopsis embryo.

Seeds of atg5 mutant exhibit a high abortion rate (Figure 1), probably due to a nutriment defect, an accelerated browning of the tegument and an early and differential accumulation of high weight proteins (> 37 kDa) compared to wild type seeds. In addition, storage protein content (12S and 2S) was 63% reduced in atg5 seeds, in comparison to wild type.

These results indicate that autophagy would not simply be a degradation process but would play un role in precursor maturation of the storage proteins, either directly via the transport in autophagosome of specific proteases (VPE for Vacuolar Processing Enzymes) responsible for precursor maturation, either indirectly by controlling the rate of oxidized protein that could affect the maturation or the transport of these precursors in the protein storage vacuole (PSV).

 

 

Journal of Experimental Botany, Volume 69, Issue 6, 14 March 2018, Pages 1403–1414.

p53 is a multifaceted tumor suppressor protein involved in key cellular responses that can both activate and repress autophagy. This dual effect is directly driven by its subcellular distribution. Thus, nuclear p53 elicits a pro-autophagy phenotype via the transcriptional activation of pro-autophagy genes while cytoplasmic p53 represses autophagy independently of its transcription factor function, via the activation of the AMPK-dependent inhibition of mTOR signaling cascade.
Our work completed this rather simplistic view by demonstrating that p53 can also repress mitophagy by controlling PINK1, a key pro-mitophagy protein that grants mitochondrial homeostasis. Thus, we demonstrated that nuclear p53 induces the transcriptional repression of PINK1 promoter activity, protein and mRNA levels, ex-vivo and in vivo. We showed that PINK1 behaves as a direct transcriptional target of p53 since the deletion of a p53-responsive element on PINK1 promoter impacts p53-mediated PINK1 transcriptional repression. This conclusion was further validated by DNA binding assay that demonstrated a physical interaction of p53 with PINK1 promoter. Accordingly, p53 knockout enhances LC3 maturation as well as optineurin and NDP52 autophagy receptors expression and down-regulates TIM23, TOM20 and HSP60 mitophagy markers, ex-vivo and mice brain. Finally, we show that the p53-mediated negative regulation of autophagy is PINK1-dependent. Hence, pifithrin-α-mediated blockade of p53 transcriptional activity like p53 genetic invalidation lead to increased mitophagy (augmentation of LC3 maturation and reduced p62, TIM23, TOM20 and HSP60 protein levels), a response that is fully abolished by PINK1 depletion. Overall, our data pinpoints to a novel signaling circuit by which nuclear p53 can repress autophagy/mitophagy. This pathway could well be impacted in both neurodegenerative and cancer, two pathological conditions where p53 expression and transcriptional activity are drastically altered.

Cell Death Differ. 2018 Jan 19

The multi-subunit V-ATPase acidifies intracellular organelles, thereby controlling a number of events in the secretory and endocytic pathway, such as proteolytic processing, protein degradation, autophagy and glycosylation. The biogenesis of the proton pump V-ATPase commences with the assembly of the proton pore sector V0 in the endoplasmic reticulum (ER). This process occurs under the control of a group of assembly factors whose mutations have recently been shown to cause glycosylation disorders with syndromic liver disease in humans. In our study, we used whole exome sequencing to screen for causative mutations in patients with similar glycosylation disorders. We identified X-linked mutations of the accessory V-ATPase subunit ATP6AP2 leading to steatohepatitis, serum lipid abnormalities, immunodeficiency and cognitive impairment in three patients from two unrelated families. We showed that ATP6AP2 interacts with members of the V0 assembly complex, and that its ER localization, mediated by a C-terminal ER retrieval motif, is crucial for V-ATPase activity. Using Drosophila, we demonstrated that ATP6AP2 mutations can cause developmental defects and steatotic phenotypes. Mechanistically, we showed that these phenotypes are the result of impaired V-ATPase assembly, defective lysosomal acidification, reduced mTOR signaling and autophagic misregulation. In the liver-like fat body tissue, this pathogenetic cascade leads to decreased lipophagy with subsequent accumulation of large cytoplasmic lipid droplets. X-linked ATP6AP2 deficiency thus belongs to a new class of hereditary hepatopathies where impaired V-ATPase function causes defects in glycosylation and autophagy.

J Exp Med. 2017 Dec 4;214(12):3707-3729

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