Starvation Induced Cell Death in Autophagy-Defective Yeast Mutants Is Caused by Mitochondria Dysfunction

Sho W Suzuki,Jun Onodera,Yoshinori Ohsumi,Dhyan Chandra

doi:10.1371/journal.pone.0017412

Sho W Suzuki, Jun Onodera + Show 2 more

Open Access

https://doi.org/10.1371/journal.pone.0017412

Copy DOI

Abstract

Autophagy is a highly-conserved cellular degradation and recycling system that is essential for cell survival during nutrient starvation. The loss of viability had been used as an initial screen to identify autophagy-defective (atg) mutants of the yeast Saccharomyces cerevisiae, but the mechanism of cell death in these mutants has remained unclear. When cells grown in a rich medium were transferred to a synthetic nitrogen starvation media, secreted metabolites lowered the extracellular pH below 3.0 and autophagy-defective mutants mostly died. We found that buffering of the starvation medium dramatically restored the viability of atg mutants. In response to starvation, wild-type (WT) cells were able to upregulate components of the respiratory pathway and ROS (reactive oxygen species) scavenging enzymes, but atg mutants lacked this synthetic capacity. Consequently, autophagy-defective mutants accumulated the high level of ROS, leading to deficient respiratory function, resulting in the loss of mitochondria DNA (mtDNA). We also showed that mtDNA deficient cells are subject to cell death under low pH starvation conditions. Taken together, under starvation conditions non-selective autophagy, rather than mitophagy, plays an essential role in preventing ROS accumulation, and thus in maintaining mitochondria function. The failure of response to starvation is the major cause of cell death in atg mutants.

Highlights

Unlike the ubiquitin-proteasome system that requires strict recognition of targets [1], autophagy is a non-selective degradation system induced under starvation conditions that mediates the recycling of cytoplasmic components to supply amino acids [2,3]
Autophagy is required for cell survival under conditions of nitrogen starvation in yeast [13], and to better understand the cellular events occurring during starvation we examined wild-type (WT) or autophagy-deficient atg1D cells during nitrogen starvation
93.4% of atg1D cells remained viable after 120 hours of nitrogen starvation (Figure 1A and 1B), and similar rates of viability were seen for other atg mutants (Figure 2C)

Summary

Introduction

Unlike the ubiquitin-proteasome system that requires strict recognition of targets [1], autophagy is a non-selective degradation system induced under starvation conditions that mediates the recycling of cytoplasmic components to supply amino acids [2,3]. Autophagy-deficient yeast cells have impaired protein translation during starvation due to inadequate supplies of amino acids [4]. Autophagy is critical for starvation-induced differentiation or development of yeast, nematodes, flies, and mice [5]. Mitochondria generate ATP through respiratory chain activity, but reactive oxygen species (ROS) are generated as by-products of cellular respiration. ROS induce damages to membrane, DNA, protein, and organelles, mechanisms regulating the function and quantity of mitochondria are essential for eukaryotic cell function. Autophagy contributes for mitochondria maintenance by their clearance [8], and this process is mediated by selective type of autophagy termed mitophagy [9,10,11,12]

Methods

Results

Conclusion