Emerging roles of ATG7 in human health and disease

The cardinal stages of macroautophagy are driven by core autophagy-related (ATG) proteins, whose ablation largely abolishes intracellular turnover. Disrupting ATG genes is paradigmatic of studying autophagy deficiency, yet emerging data suggest that ATG proteins have extensive biological importance beyond autophagic elimination. An important example is ATG7, an essential autophagy effector enzyme that in concert with other ATG proteins, also regulates immunity, cell death and protein secretion, and independently regulates the cell cycle and apoptosis. Recently, a direct association between ATG7 dysfunction and disease was established in patients with biallelic ATG7 variants and childhood-onset neuropathology. Moreover, a prodigious body of evidence supports a role for ATG7 in protecting against complex disease states in model organisms, although how dysfunctional ATG7 contributes to manifestation of these diseases, including cancer, neurodegeneration and infection, in humans remains unclear. Here, we systematically review the biological functions of ATG7, discussing the impact of its impairment on signalling pathways and human pathology. Future studies illuminating the molecular relationship between ATG7 dysfunction and disease will expedite therapies for disorders involving ATG7 deficiency and/or impaired autophagy.Peer reviewe

Avoiding unscheduled transcription in shared promoters: Saccharomyces cerevisiae Sum1p represses the divergent gene pair SPS18-SPS19 through a midsporulation element (MSE)

The sporulation-specific gene SPS18 shares a common promoter region with the oleic acid-inducible gene SPS19. Both genes are transcribed in sporulating diploid cells, albeit unevenly in favour of SPS18, whereas in haploid cells grown on fatty acids only SPS19 is highly activated. Here, SPS19 oleate-response element (ORE) conferred activation on a basal CYC1-lacZ reporter gene equally in both orientations, but promoter analysis using SPS18-lacZ reporter constructs with deletions identified a repressing fragment containing a midsporulation element (MSE) that could be involved in imposing directionality towards SPS19 in oleic acid-induced cells. In sporulating diploids, MSEs recruit the Ndt80p transcription factor for activation, whereas under vegetative conditions, certain MSEs are targeted by the Sum1p repressor in association with Hst1p and Rfm1p. Quantitative real-time PCR demonstrated that in haploid sum1Δ, hst1Δ, or rfm1Δ cells, oleic acid-dependent expression of SPS18 was higher compared with the situation in wild-type cells, but in the sum1Δ mutant, this effect was diminished in the absence of Oaf1p or Pip2p. We conclude that SPS18 MSE is a functional element repressing the expression of both SPS18 and SPS19, and is a component of a stricture mechanism shielding SPS18 from the dramatic increase in ORE-dependent transcription of SPS19 in oleic acid-grown cells

DGAT1 activity synchronises with mitophagy to protect cells from metabolic rewiring by iron depletion

Mitophagy removes defective mitochondria via lysosomal elimination. Increased mitophagy coincides with metabolic reprogramming, yet it remains unknown whether mitophagy is a cause or consequence of such state changes. The signalling pathways that integrate with mitophagy to sustain cell and tissue integrity also remain poorly defined. We performed temporal metabolomics on mammalian cells treated with deferiprone, a therapeutic iron chelator that stimulates PINK1/PARKIN-independent mitophagy. Iron depletion profoundly rewired the metabolome, hallmarked by remodelling of lipid metabolism within minutes of treatment. DGAT1-dependent lipid droplet biosynthesis occurred several hours before mitochondrial clearance, with lipid droplets bordering mitochondria upon iron chelation. We demonstrate that DGAT1 inhibition restricts mitophagy in vitro, with impaired lysosomal homeostasis and cell viability. Importantly, genetic depletion of DGAT1 in vivo significantly impaired neuronal mitophagy and locomotor function in Drosophila. Our data define iron depletion as a potent signal that rapidly reshapes metabolism and establishes an unexpected synergy between lipid homeostasis and mitophagy that safeguards cell and tissue integrity.Peer reviewe

Mitochondrial stress response triggered by defects in protein synthesis quality control

Mitochondria have a compartmentalized gene expression system dedicated to the synthesis of membrane proteins essential for oxidative phosphorylation. Responsive quality control mechanisms are needed to ensure that aberrant protein synthesis does not disrupt mitochondrial function. Pathogenic mutations that impede the function of the mitochondrial matrix quality control protease complex composed of AFG3L2 and paraplegin cause a multifaceted clinical syndrome. At the cell and molecular level, defects to this quality control complex are defined by impairment to mitochondrial form and function. Here, we establish the etiology of these phenotypes. We show how disruptions to the quality control of mitochondrial protein synthesis trigger a sequential stress response characterized first by OMA1 activation followed by loss of mitochondrial ribosomes and by remodelling of mitochondrial inner membrane ultrastructure. Inhibiting mitochondrial protein synthesis with chloramphenicol completely blocks this stress response. Together, our data establish a mechanism linking major cell biological phenotypes of AFG3L2 pathogenesis and show how modulation of mitochondrial protein synthesis can exert a beneficial effect on organelle homeostasis.Peer reviewe

A novel human leiomyoma tissue derived matrix for cell culture studies

Background: The composition of the matrix molecules is important in in vitro cell culture experiments of e.g. human cancer invasion and vessel formation. Currently, the mouse Engelbreth-Holm-Swarm (EHS) sarcoma -derived products, such as Matrigel (R), are the most commonly used tumor microenvironment (TME) mimicking matrices for experimental studies. However, since Matrigel (R) is non-human in origin, its molecular composition does not accurately simulate human TME. We have previously described a solid 3D organotypic myoma disc invasion assay, which is derived from human uterus benign leiomyoma tumor. Here, we describe the preparation and analyses of a processed, gelatinous leiomyoma matrix, named Myogel. Methods: A total protein extract, Myogel, was formulated from myoma. The protein contents of Myogel were characterized and its composition and properties compared with a commercial mouse Matrigel (R). Myogel was tested and compared to Matrigel (R) in human cell adhesion, migration, invasion, colony formation, spheroid culture and vessel formation experiments, as well as in a 3D hanging drop video image analysis. Results: We demonstrated that only 34 % of Myogel's molecular content was similar to Matrigel (R). All test results showed that Myogel was comparable with Matrigel (R), and when mixed with low-melting agarose (Myogel-LMA) it was superior to Matrigel (R) in in vitro Transwell (R) invasion and capillary formation assays. Conclusions: In conclusion, we have developed a novel Myogel TME matrix, which is recommended for in vitro human cell culture experiments since it closely mimics the human tumor microenvironment of solid cancers.Peer reviewe

Autophagy in the mammalian nervous system: a primer for neuroscientists

Autophagy refers to the lysosomal degradation of damaged or superfluous components and is essential for metabolic plasticity and tissue integrity. This evolutionarily conserved process is particularly vital to mammalian post-mitotic cells such as neurons, which face unique logistical challenges and must sustain homoeostasis over decades. Defective autophagy has pathophysiological importance, especially for human neurodegeneration. The present-day definition of autophagy broadly encompasses two distinct yet related phenomena: non-selective and selective autophagy. In this minireview, we focus on established and emerging concepts in the field, paying particular attention to the physiological significance of macroautophagy and the burgeoning world of selective autophagy pathways in the context of the vertebrate nervous system. By highlighting established basics and recent breakthroughs, we aim to provide a useful conceptual framework for neuroscientists interested in autophagy, in addition to autophagy enthusiasts with an eye on the nervous system.Peer reviewe

A conserved mammalian mitochondrial isoform of acetyl-CoA carboxylase ACC1 provides the malonyl-CoA essential for mitochondrial biogenesis in tandem with ACSF3

Abstract Mitochondrial fatty acid synthesis (mtFAS) is a highly conserved pathway essential for mitochondrial biogenesis. The mtFAS process is required for mitochondrial respiratory chain assembly and function, synthesis of the lipoic acid cofactor indispensable for the function of several mitochondrial enzyme complexes and essential for embryonic development in mice. Mutations in human mtFAS have been reported to lead to neurodegenerative disease. The source of malonyl-CoA for mtFAS in mammals has remained unclear. We report the identification of a conserved vertebrate mitochondrial isoform of ACC1 expressed from an ACACA transcript splicing variant. A specific knockdown (KD) of the corresponding transcript in mouse cells, or CRISPR/Cas9-mediated inactivation of the putative mitochondrial targeting sequence in human cells, leads to decreased lipoylation and mitochondrial fragmentation. Simultaneous KD of ACSF3, encoding a mitochondrial malonyl-CoA synthetase previously implicated in the mtFAS process, resulted in almost complete ablation of protein lipoylation, indicating that these enzymes have a redundant function in mtFAS. The discovery of a mitochondrial isoform of ACC1 required for lipoic acid synthesis has intriguing consequences for our understanding of mitochondrial disorders, metabolic regulation of mitochondrial biogenesis and cancer

Expression and Evolution of the Non-Canonically Translated Yeast Mitochondrial Acetyl-CoA Carboxylase Hfa1p

<div><p>The <i>Saccharomyces cerevisiae</i> genome encodes two sequence related acetyl-CoA carboxylases, the cytosolic Acc1p and the mitochondrial Hfa1p, required for respiratory function. Several aspects of expression of the <i>HFA1</i> gene and its evolutionary origin have remained unclear. Here, we determined the <i>HFA1</i> transcription initiation sites by 5′ RACE analysis. Using a novel “Stop codon scanning” approach, we mapped the location of the <i>HFA1</i> translation initiation site to an upstream AUU codon at position −372 relative to the annotated start codon. This upstream initiation leads to production of a mitochondrial targeting sequence preceding the ACC domains of the protein. <i>In silico</i> analyses of fungal <i>ACC</i> genes revealed conserved “cryptic” upstream mitochondrial targeting sequences in yeast species that have not undergone a whole genome duplication. Our Δ<i>hfa1</i> baker's yeast mutant phenotype rescue studies using the protoploid <i>Kluyveromyces lactis ACC</i> confirmed functionality of the cryptic upstream mitochondrial targeting signal. These results lend strong experimental support to the hypothesis that the mitochondrial and cytosolic acetyl-CoA carboxylases in <i>S. cerevisiae</i> have evolved from a single gene encoding both the mitochondrial and cytosolic isoforms. Leaning on a cursory survey of a group of genes of our interest, we propose that cryptic 5′ upstream mitochondrial targeting sequences may be more abundant in eukaryotes than anticipated thus far.</p></div

The W1536 8B Δ<i>hfa1</i> strain carrying plasmids with inserted stop codon mutation at position downstream of −372 resulted in unchanged lactate deficiency.

<p>Only stop codon mutations relevant to define the putative translation initiation site and controls are shown. The yeast cells were grown on media containing Glucose (SCD) or lactate (Lactate) as the sole carbon source at 33°C. Strains used for this study are W1536 8B, W1536 8B Δ<i>hfa1</i> or W1536 8B Δ<i>htd2</i> (respiratory deficient control) and the plasmids carried by the strains are indicated at the left side of the panels. YCp33: empty plasmid; HFA1: YCp33 <i>HFA1</i>; −381: YCp33 <i>HFA1</i> −381; −372: YCp33 <i>HFA1</i> −372; −363: YCp33<i>HFA1</i> −363. Only stop codon mutations relevant to define the putative translation initiation site and controls are shown. The results for other mutants shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0114738#pone-0114738-g002" target="_blank">Fig. 2</a> can be found as supplementary data.</p