Pex3 and Atg37 compete to regulate the interaction between the pexophagy receptor, Atg30, and the Hrr25 kinase

ATG and ESCRT control multiple modes of microautophagy

The discovery of microautophagy, the direct engulfment of cytoplasmic material by the lysosome, dates back to 1966 in a morphological study of mammalian cells by Christian de Duve. Since then, studies on microautophagy have shifted toward the elucidation of the physiological significance of the process. However, in contrast to macroautophagy, studies on the molecular mechanisms of microautophagy have been limited. Only recent studies revealed that ATG proteins involved in macroautophagy are also operative in several types of microautophagy and that ESCRT proteins, responsible for the multivesicular body pathway, play a central role in most microautophagy processes. In this review, we summarize our current knowledge on the function of ATG and ESCRT proteins in microautophagy.

The subset of peroxisomal tail-anchored proteins do not reach peroxisomes via ER, instead mitochondria can be involved

Peroxisomes are membrane-enclosed organelles with important roles in fatty acid breakdown, bile acid synthesis and biosynthesis of sterols and ether lipids. Defects in peroxisomes result in severe genetic diseases, such as Zellweger syndrome and neonatal adrenoleukodystrophy. However, many aspects of peroxisomal biogenesis are not well understood. Here we investigated delivery of tail-anchored (TA) proteins to peroxisomes in mammalian cells. Using glycosylation assays we showed that peroxisomal TA proteins do not enter the endoplasmic reticulum (ER) in both wild type (WT) and peroxisome-lacking cells. We observed that in cells lacking the essential peroxisome biogenesis factor, PEX19, peroxisomal TA proteins localize mainly to mitochondria. Finally, to investigate peroxisomal TA protein targeting in cells with fully functional peroxisomes we used a proximity biotinylation approach. We showed that while ER-targeted TA construct was exclusively inserted into the ER, peroxisome-targeted TA construct was inserted to both peroxisomes and mitochondria. Thus, in contrast to previous studies, our data suggest that some peroxisomal TA proteins do not insert to the ER prior to their delivery to peroxisomes, instead, mitochondria can be involved.

HO-1/autophagic flux axis alleviated sepsis-induced acute lung injury via inhibiting NLRP3 inflammasome

Among the multiple organ injuries induced by sepsis, acute lung injury (ALI) triggered by an excessive inflammatory response is one of the main causes contributing to patient death, and inhibition of the inflammation cascade is the key therapeutic strategy to improve prognosis. The NLRP3 inflammasome complex is considered an intracellular signaling molecule closely associated with the uncontrolled inflammatory response in sepsis-induced ALI. Therefore, exploring new targets to repress its activation is regarded as a potential therapeutic strategy. Growing evidence demonstrated that heme oxygenase-1 (HO-1) contributed to general anti-inflammation and exerted a protective role in ALI, but its underlying mechanisms have not been clarified completely. Herein, we investigated HO-1 was elevated in alveolar macrophages isolated from bronchoalveolar lavage fluid (BALF) of sepsis mice. HO-1 abundance suppressed NLRP3 inflammasome complex activation and attenuated pro-inflammatory cytokines release, thereby alleviating sepsis-induced ALI. Whereas inhibition of HO-1 reached the opposite effect. Meanwhile, HO-1 is an effective and functionally relevant regulator of autophagic flux. HO-1 activator decreased the expression of P62 and enhanced the LC3 II/LC3 I ratio, resulting in autophagic flux activation. In addition, the protective effects HO-1 exerted in sepsis-induced ALI could be abolished by autophagic flux inhibitor. Autophagic flux activator could suppress NLRP3 inflammasome activation and attenuate ALI, while autophagic flux inhibitor had the opposite effect. In conclusion, our study revealed increased HO-1 expression inhibited the level of NLRP3 inflammasome via regulating the activation of autophagic flux, thus attenuating inflammatory response and alleviating sepsis-induced ALI.

Cargo receptors and adaptors for selective autophagy in plant cells

Plant selective (macro)autophagy is a highly regulated process whereby eukaryotic cells spatiotemporally degrade some of their constituents that have become superfluous or harmful. The identification and characterization of the factors determining this selectivity make it possible to integrate selective (macro)autophagy into plant cell physiology and homeostasis. The specific cargo receptors and/or scaffold proteins involved in this pathway are generally not structurally conserved, as are the biochemical mechanisms underlying recognition and integration of a given cargo into the autophagosome in different cell types. This review discusses the few specific cargo receptors described in plant cells to highlight key features of selective autophagy in the plant kingdom and its integration with plant physiology, so as to identify evolutionary convergence and knowledge gaps to be filled by future research.

Regulation of Peroxisome Homeostasis by Post-Translational Modification in the Methylotrophic Yeast Komagataella phaffii

The methylotrophic yeast Komagataella phaffii (synoym Pichia pastoris) can grow on methanol with an associated proliferation of peroxisomes, which are subsequently degraded by pexophagy upon depletion of methanol. Two cell wall integrity and stress response component (WSC) family proteins (Wsc1 and Wsc3) sense the extracellular methanol concentration and transmit the methanol signal to Rom2. This stimulates the activation of transcription factors (Mxr1, Trm1, and Mit1 etc.), leading to the induction of methanol-metabolizing enzymes (methanol-induced gene expression) and synthesis of huge peroxisomes. Methanol-induced gene expression is repressed by the addition of ethanol (ethanol repression). This repression is not conducted directly by ethanol but rather by acetyl-CoA synthesized from ethanol by sequential reactions, including alcohol and aldehyde dehydrogenases, and acetyl-CoA synthetase. During ethanol repression, Mxr1 is inactivated by phosphorylation. Peroxisomes are degraded by pexophagy on depletion of methanol and this event is triggered by phosphorylation of Atg30 located at the peroxisome membrane. In the presence of methanol, Wsc1 and Wsc3 repress pexophagy by transmitting the methanol signal via the MAPK cascade to the transcription factor Rlm1, which induces phosphatases involved in dephosphorylation of Atg30. Upon methanol consumption, repression of Atg30 phosphorylation is released, resulting in initiation of pexophagy. Physiological significance of these machineries involved in peroxisome homeostasis and their post-translational modification is also discussed in association with the lifestyle of methylotrophic yeast in the phyllosphere.

ER-phagy: mechanisms, regulation and diseases connected to the lysosomal clearance of the endoplasmic reticulum

ER-phagy (reticulo-phagy) defines the degradation of portions of the endoplasmic reticulum (ER) within lysosomes or vacuoles. It is part of the self-digestion (i.e., auto-phagic) programs recycling cytoplasmic material and organelles, which rapidly mobilize metabolites in cells confronted with nutrient shortage. Moreover, selective clearance of ER subdomains participates to the control of ER size and activity during ER stress, the re-establishment of ER homeostasis after ER stress resolution and the removal of ER parts, in which aberrant and potentially cytotoxic material has been segregated. ER-phagy relies on the individual and/or concerted activation of the ER-phagy receptors, ER peripheral or integral membrane proteins that share the presence of LC3/Atg8-binding motifs in their cytosolic domains. ER-phagy involves the physical separation of portions of the ER from the bulk ER network, and their delivery to the endolysosomal/vacuolar catabolic district. This last step is accomplished by a variety of mechanisms including macro-ER-phagy (in which ER fragments are sequestered by double-membrane autophagosomes that eventually fuse with lysosomes/vacuoles), micro-ER-phagy (in which ER fragments are directly engulfed by endosomes/lysosomes/vacuoles), or direct fusion of ER-derived vesicles with lysosomes/vacuoles. ER-phagy is dysfunctional in specific human diseases and its regulators are subverted by pathogens, highlighting its crucial role for cell and organism life.

The methanol sensor Wsc1 and MAPK Mpk1 suppress degradation of methanol-induced peroxisomes in methylotrophic yeast

In nature, methanol is produced during the hydrolysis of pectin in plant cell walls. Methanol on plant leaves shows circadian dynamics, to which methanol-utilizing phyllosphere microorganisms adapt. In the methylotrophic yeast Komagataella phaffii (Kp; also known as Pichia pastoris), the plasma membrane protein KpWsc1 senses environmental methanol concentrations and transmits this information to induce the expression of genes for methanol metabolism and the formation of huge peroxisomes. In this study, we show that KpWsc1 and its downstream MAPK, KpMpk1, negatively regulate pexophagy in the presence of methanol concentrations greater than 0.15%. Although KpMpk1 was not necessary for expression of methanol-inducible genes and peroxisome biogenesis, KpMpk1, the transcription factor KpRlm1 and phosphatases were found to suppress pexophagy by controlling phosphorylation of KpAtg30, the key factor in regulation of pexophagy. We reveal at the molecular level how the single methanol sensor KpWsc1 commits the cell to peroxisome synthesis and degradation according to the methanol concentration, and we discuss the physiological significance of regulating pexophagy for survival in the phyllosphere. This article has an associated First Person interview with Shin Ohsawa, joint first author of the paper.

Guidelines for the use and interpretation of assays for monitoring autophagy (4th edition)1

In 2008, we published the first set of guidelines for standardizing research in autophagy. Since then, this topic has received increasing attention, and many scientists have entered the field. Our knowledge base and relevant new technologies have also been expanding. Thus, it is important to formulate on a regular basis updated guidelines for monitoring autophagy in different organisms. Despite numerous reviews, there continues to be confusion regarding acceptable methods to evaluate autophagy, especially in multicellular eukaryotes. Here, we present a set of guidelines for investigators to select and interpret methods to examine autophagy and related processes, and for reviewers to provide realistic and reasonable critiques of reports that are focused on these processes. These guidelines are not meant to be a dogmatic set of rules, because the appropriateness of any assay largely depends on the question being asked and the system being used. Moreover, no individual assay is perfect for every situation, calling for the use of multiple techniques to properly monitor autophagy in each experimental setting. Finally, several core components of the autophagy machinery have been implicated in distinct autophagic processes (canonical and noncanonical autophagy), implying that genetic approaches to block autophagy should rely on targeting two or more autophagy-related genes that ideally participate in distinct steps of the pathway. Along similar lines, because multiple proteins involved in autophagy also regulate other cellular pathways including apoptosis, not all of them can be used as a specific marker for bona fide autophagic responses. Here, we critically discuss current methods of assessing autophagy and the information they can, or cannot, provide. Our ultimate goal is to encourage intellectual and technical innovation in the field.

Selective autophagy of intracellular organelles: recent research advances

Macroautophagy (hereafter called autophagy) is a highly conserved physiological process that degrades over-abundant or damaged organelles, large protein aggregates and invading pathogens via the lysosomal system (the vacuole in plants and yeast). Autophagy is generally induced by stress, such as oxygen-, energy- or amino acid-deprivation, irradiation, drugs, etc. In addition to non-selective bulk degradation, autophagy also occurs in a selective manner, recycling specific organelles, such as mitochondria, peroxisomes, ribosomes, endoplasmic reticulum (ER), lysosomes, nuclei, proteasomes and lipid droplets (LDs). This capability makes selective autophagy a major process in maintaining cellular homeostasis. The dysfunction of selective autophagy is implicated in neurodegenerative diseases (NDDs), tumorigenesis, metabolic disorders, heart failure, etc. Considering the importance of selective autophagy in cell biology, we systemically review the recent advances in our understanding of this process and its regulatory mechanisms. We emphasize the 'cargo-ligand-receptor' model in selective autophagy for specific organelles or cellular components in yeast and mammals, with a focus on mitophagy and ER-phagy, which are finely described as types of selective autophagy. Additionally, we highlight unanswered questions in the field, helping readers focus on the research blind spots that need to be broken.

Mitophagy Improves Ethanol Tolerance in Yeast: Regulation by Mitochondrial Reactive Oxygen Species in Saccharomyces cerevisiae

Ethanol often accumulates during the process of wine fermentation, and mitophagy has critical role in ethanol output. However, the relationship between mitophagy and ethanol stress is still unclear. In this study, the expression of ATG11 and ATG32 genes exposed to ethanol stress was accessed by real-time quantitative reverse transcription polymerase chain reaction (qRT-PCR). The result indicated that ethanol stress induced expression of the ATG11 and ATG32 genes. The colony sizes and the alcohol yield of atg11 and atg32 were also smaller and lower than those of wild type strain under ethanol whereas the mortality of mutants is higher. Furthermore, compared with wild type, the membrane integrity and the mitochondrial membrane potential of atg11 and atg32 exhibited greater damage following ethanol stress. In addition, a greater proportion of mutant cells were arrested at the G1/G0 cell cycle. There was more aggregation of peroxide hydrogen (H₂O₂) and superoxide anion (O₂^•-) in mutants. These changes in H₂O₂ and O₂^•- in yeasts were altered by reductants or inhibitors of scavenging enzyme by means of regulating the expression of ATG11 and ATG32 genes. Inhibitors of the mitochondrial electron transport chain (mtETC) also increased production of H₂O₂ and O₂^•- by enhancing expression of the ATG11 and ATG32 genes. Further results showed that activator or inhibitor of autophagy also activated or inhibited mitophagy by altering production of H₂O₂ and O₂^•. Therefore, ethanol stress induces mitophagy which improves yeast the tolerance to ethanol and the level of mitophagy during ethanol stress is regulated by ROS derived from mtETC.

Pex3 confines pexophagy receptor activity of Atg36 to peroxisomes by regulating Hrr25-mediated phosphorylation and proteasomal degradation

In macroautophagy (hereafter autophagy), cytoplasmic molecules and organelles are randomly or selectively sequestered within double-membrane vesicles called autophagosomes and delivered to lysosomes or vacuoles for degradation. In selective autophagy, the specificity of degradation targets is determined by autophagy receptors. In the budding yeast Saccharomyces cerevisiae, autophagy receptors interact with specific targets and Atg11, resulting in the recruitment of a protein complex that initiates autophagosome formation. Previous studies have revealed that autophagy receptors are regulated by posttranslational modifications. In selective autophagy of peroxisomes (pexophagy), the receptor Atg36 localizes to peroxisomes by binding to the peroxisomal membrane protein Pex3. We previously reported that Atg36 is phosphorylated by Hrr25 (casein kinase 1δ), increasing the Atg36-Atg11 interaction and thereby stimulating pexophagy initiation. However, the regulatory mechanisms underlying Atg36 phosphorylation are unknown. Here, we show that Atg36 phosphorylation is abolished in cells lacking Pex3 or expressing a Pex3 mutant defective in the interaction with Atg36, suggesting that the interaction with Pex3 is essential for the Hrr25-mediated phosphorylation of Atg36. Using recombinant proteins, we further demonstrated that Pex3 directly promotes Atg36 phosphorylation by Hrr25. A co-immunoprecipitation analysis revealed that the interaction of Atg36 with Hrr25 depends on Pex3. These results suggest that Pex3 increases the Atg36-Hrr25 interaction and thereby stimulates Atg36 phosphorylation on the peroxisomal membrane. In addition, we found that Pex3 binding protects Atg36 from proteasomal degradation. Thus, Pex3 confines Atg36 activity to the peroxisome by enhancing its phosphorylation and stability on this organelle.

Balancing the Opposing Principles That Govern Peroxisome Homeostasis

Despite major advances in our understanding of players and mechanisms involved in peroxisome biogenesis and peroxisome degradation, very few studies have focused on unraveling the multi-layered connections between, and the coordination of, these two opposing processes that regulate peroxisome homeostasis. The intersection between these processes also provides exciting avenues for future research. This review highlights the links between peroxisome biogenesis and degradation, incorporating an integrative approach that is critical not only for a mechanistic understanding, but also for manipulating the balance between these processes in relevant disease models.

Deletion of PIN4 Suppresses the Protein Transport Defects Caused by sec12-4 Mutation in Saccharomyces cerevisiae

Newly synthesized secretory proteins are released into the lumen of the endoplasmic reticulum (ER). The secretory proteins are surrounded by coat protein complex II (COPII) vesicles, and transported from the ER and reach their destinations through the Golgi apparatus. Sec12p is a guanine nucleotide exchange factor for Sar1p, which initiates COPII vesicle budding from the ER. The activation of Sar1p by Sec12p and the subsequent COPII coat assembly have been well characterized, but the events that take place upstream of Sec12p remain unclear. In this study, we isolated the novel extragenic suppressor of sec12-4, PIN4/MDT1, a cell cycle checkpoint target. A yeast two-hybrid screening was used to identify Pin4/Mdt1p as a binding partner of the casein kinase I isoform Hrr25p, which we have previously identified as a modulator of Sec12p function. Deletion of PIN4 suppressed both defects of temperature-sensitive growth and the partial protein transport observed in sec12-4 mutants. The results of this study suggest that Pin4p provides novel aspects of Sec12p modulations.

Multiple Functions of ATG8 Family Proteins in Plant Autophagy

Autophagy is a major degradation process of cytoplasmic components in eukaryotes, and executes both bulk and selective degradation of targeted cargos. A set of autophagy-related (ATG) proteins participate in various stages of the autophagic process. Among ATGs, ubiquitin-like protein ATG8 plays a central role in autophagy. The ATG8 protein is conjugated to the membrane lipid phosphatidylethanolamine in a ubiquitin-like conjugation reaction that is essential for autophagosome formation. In addition, ATG8 interacts with various adaptor/receptor proteins to recruit specific cargos for degradation by selective autophagy. The ATG8-interacting proteins usually contain the ATG8-interacting motif (AIM) or the ubiquitin-interacting motif (UIM) for ATG8 binding. Unlike a single ATG8 gene in yeast, multiple ATG8 orthologs have been identified in the plant kingdom. The large diversity within the ATG8 family may explain the various functions of selective autophagy in plants. Here, we discuss and summarize the current view of the structure and function of ATG8 proteins in plants.

Mitochondrial Biogenesis Is Positively Regulated by Casein Kinase I Hrr25 Through Phosphorylation of Puf3 in Saccharomyces cerevisiae

Mitochondrial biogenesis requires coordinated expression of genes encoding mitochondrial proteins, which in Saccharomyces cerevisiae is achieved in part via post-transcriptional control by the Pumilio RNA-binding domain protein Puf3 Puf3 binds to the 3'-UTR of many messenger RNAs (mRNAs) that encode mitochondrial proteins, regulating their turnover, translation, and/or mitochondrial targeting. Puf3 hyperphosphorylation correlates with increased mitochondrial biogenesis; however, the kinase responsible for Puf3 phosphorylation is unclear. Here, we show that the casein kinase I protein Hrr25 negatively regulates Puf3 by mediating its phosphorylation. An hrr25 mutation results in reduced phosphorylation of Puf3 in vivo and a puf3 deletion mutation reverses growth defects of hrr25 mutant cells grown on medium with a nonfermentable carbon source. We show that Hrr25 directly phosphorylates Puf3, and that the interaction between Puf3 and Hrr25 is mediated through the N-terminal domain of Puf3 and the kinase domain of Hrr25 We further found that an hrr25 mutation reduces GFP expression from GFP reporter constructs carrying the 3'-UTR of Puf3 targets. Downregulation of GFP expression due to an hrr25 mutation can be reversed either by puf3Δ or by mutations to the Puf3-binding sites in the 3'-UTR of the GFP reporter constructs. Together, our data indicate that Hrr25 is a positive regulator of mitochondrial biogenesis by phosphorylating Puf3 and inhibiting its function in downregulating target mRNAs encoding mitochondrial proteins.

The diversity of ACBD proteins - From lipid binding to protein modulators and organelle tethers

Members of the large multigene family of acyl-CoA binding domain containing proteins (ACBDs) share a conserved motif required for binding of Coenzyme A esterified fatty acids of various chain length. These proteins are present in the three kingdoms of life, and despite their predicted roles in cellular lipid metabolism, knowledge about the precise functions of many ACBD proteins remains scarce. Interestingly, several ACBD proteins are now suggested to function at organelle contact sites, and are recognized as host interaction proteins for different pathogens including viruses and bacteria. Here, we present a thorough phylogenetic analysis of the ACBD family and discuss their structure and evolution. We summarize recent findings on the various functions of animal and fungal ACBDs with particular focus on peroxisomes, the role of ACBD proteins at organelle membranes, and their increasing recognition as targets for pathogens.

Pexophagy: A Model for Selective Autophagy

The removal of damaged or superfluous organelles from the cytosol by selective autophagy is required to maintain organelle function, quality control and overall cellular homeostasis. Precisely how substrate selectivity is achieved, and how individual substrates are degraded during selective autophagy in response to both extracellular and intracellular cues is not well understood. The aim of this review is to highlight pexophagy, the autophagic degradation of peroxisomes, as a model for selective autophagy. Peroxisomes are dynamic organelles whose abundance is rapidly modulated in response to metabolic demands. Peroxisomes are routinely turned over by pexophagy for organelle quality control yet can also be degraded by pexophagy in response to external stimuli such as amino acid starvation or hypoxia. This review discusses the molecular machinery and regulatory mechanisms governing substrate selectivity during both quality-control pexophagy and pexophagy in response to external stimuli, in yeast and mammalian systems. We draw lessons from pexophagy to infer how the cell may coordinate the degradation of individual substrates by selective autophagy across different cellular cues.

Granulovacuolar degeneration bodies are neuron-selective lysosomal structures induced by intracellular tau pathology

Granulovacuolar degeneration bodies (GVBs) are membrane-bound vacuolar structures harboring a dense core that accumulate in the brains of patients with neurodegenerative disorders, including Alzheimer's disease and other tauopathies. Insight into the origin of GVBs and their connection to tau pathology has been limited by the lack of suitable experimental models for GVB formation. Here, we used confocal, automated, super-resolution and electron microscopy to demonstrate that the seeding of tau pathology triggers the formation of GVBs in different mouse models in vivo and in primary mouse neurons in vitro. Seeding-induced intracellular tau aggregation, but not seed exposure alone, causes GVB formation in cultured neurons, but not in astrocytes. The extent of tau pathology strongly correlates with the GVB load. Tau-induced GVBs are immunoreactive for the established GVB markers CK1δ, CK1ɛ, CHMP2B, pPERK, peIF2α and pIRE1α and contain a LAMP1- and LIMP2-positive single membrane that surrounds the dense core and vacuole. The proteolysis reporter DQ-BSA is detected in the majority of GVBs, demonstrating that GVBs contain degraded endocytic cargo. GFP-tagged CK1δ accumulates in the GVB core, whereas GFP-tagged tau or GFP alone does not, indicating selective targeting of cytosolic proteins to GVBs. Taken together, we established the first in vitro model for GVB formation by seeding tau pathology in primary neurons. The tau-induced GVBs have the marker signature and morphological characteristics of GVBs in the human brain. We show that GVBs are lysosomal structures distinguished by the accumulation of a characteristic subset of proteins in a dense core.

Staying in Healthy Contact: How Peroxisomes Interact with Other Cell Organelles

Peroxisomes share extensive metabolic connections with other cell organelles. Membrane contact sites (MCSs) establish and maintain such interactions, and they are vital for organelle positioning and motility. In the past few years peroxisome interactions and MCSs with other cellular organelles have been explored extensively, resulting in the identification of new MCSs, the tethering molecules involved, and their functional characterization. Defective tethering and compartmental communication can lead to pathological conditions that can be termed 'organelle interaction diseases'. We review peroxisome-organelle interactions in mammals and summarize the most recent knowledge of mammalian peroxisomal organelle contacts in health and disease.

Selective Autophagy of Peroxisomes in Plants: From Housekeeping to Development and Stress Responses

Peroxisomes are dynamic organelles involved in multiple functions, including oxygen and nitrogen reactive species metabolism. In plants, these organelles have a close relationship with chloroplasts and mitochondria, characterized by intense metabolic activity and signal transduction. Peroxisomes undergo rapid changes in size, morphology, and abundance depending on the plant development stage and environmental conditions. As peroxisomes are essential not only for redox homeostasis but also for sensing stress, signaling transduction, and cell survival, their formation and degradation need to be rigorously regulated. In this review, new insights into the regulation of plant peroxisomes are briefly described, with a particular emphasis on pexophagy components and their regulation.

Mechanistic Insights into the Role of Atg11 in Selective Autophagy

Macroautophagy (referred to hereafter as autophagy) is an intracellular degradation pathway in which the formation of a double-membrane vesicle called the autophagosome is a key event in the transport of multiple cytoplasmic cargo (e.g., proteins, protein aggregates, lipid droplets or organelles) to the vacuole (lysosome in mammals) for degradation and recycling. During this process, autophagosomes are formed de novo by membrane fusion events leading to phagophore formation initiated at the phagophore assembly site. In yeast, Atg11 and Atg17 function as protein scaffolds, essential for selective and non-selective types of autophagy, respectively. While Atg17 functions in non-selective autophagy are well-defined in the literature, less attention is concentrated on recent findings regarding the roles of Atg11 in selective autophagy. Here, we summarize current knowledge about the Atg11 scaffold protein and review recent findings in the context of its role in selective autophagy initiation and autophagosome formation.

A facile forward-genetic screen for Arabidopsis autophagy mutants reveals twenty-one loss-of-function mutations disrupting six ATG genes

Macroautophagy is a process through which eukaryotic cells degrade large substrates including organelles, protein aggregates, and invading pathogens. Over 40 autophagy-related (ATG) genes have been identified through forward-genetic screens in yeast. Although homology-based analyses have identified conserved ATG genes in plants, only a few atg mutants have emerged from forward-genetic screens in Arabidopsis thaliana. We developed a screen that consistently recovers Arabidopsis atg mutations by exploiting mutants with defective LON2/At5g47040, a protease implicated in peroxisomal quality control. Arabidopsis lon2 mutants exhibit reduced responsiveness to the peroxisomally-metabolized auxin precursor indole-3-butyric acid (IBA), heightened degradation of several peroxisomal matrix proteins, and impaired processing of proteins harboring N-terminal peroxisomal targeting signals; these defects are ameliorated by preventing autophagy. We optimized a lon2 suppressor screen to expedite recovery of additional atg mutants. After screening mutagenized lon2-2 seedlings for restored IBA responsiveness, we evaluated stabilization and processing of peroxisomal proteins, levels of several ATG proteins, and levels of the selective autophagy receptor NBR1/At4g24690, which accumulates when autophagy is impaired. We recovered 21 alleles disrupting 6 ATG genes: ATG2/At3g19190, ATG3/At5g61500, ATG5/At5g17290, ATG7/At5g45900, ATG16/At5g50230, and ATG18a/At3g62770. Twenty alleles were novel, and 3 of the mutated genes lack T-DNA insertional alleles in publicly available repositories. We also demonstrate that an insertional atg11/At4g30790 allele incompletely suppresses lon2 defects. Finally, we show that NBR1 is not necessary for autophagy of lon2 peroxisomes and that NBR1 overexpression is not sufficient to trigger autophagy of seedling peroxisomes, indicating that Arabidopsis can use an NBR1-independent mechanism to target peroxisomes for autophagic degradation.

Abbreviations: ATG: autophagy-related; ATI: ATG8-interacting protein; Col-0: Columbia-0; DSK2: dominant suppressor of KAR2; EMS: ethyl methanesulfonate; GFP: green fluorescent protein; IAA: indole-3-acetic acid; IBA: indole-3-butyric acid; ICL: isocitrate lyase; MLS: malate synthase; NBR1: Next to BRCA1 gene 1; PEX: peroxin; PMDH: peroxisomal malate dehydrogenase; PTS: peroxisomal targeting signal; thiolase: 3-ketoacyl-CoA thiolase; UBA: ubiquitin-associated; WT: wild type

Pexophagy in yeast and mammals: an update on mysteries

Peroxisomes are ubiquitous and highly dynamic organelles that play a central role in the metabolism of lipids and reactive oxygen species. The importance of peroxisomal metabolism is illustrated by severe peroxisome biogenesis disorders in which functional peroxisomes are absent or disorders caused by single peroxisomal enzyme deficiencies. These multisystemic diseases manifest specific clinical and biochemical disturbances that originate from the affected peroxisomal pathways. An emerging role of the peroxisome has been identified in many types of diseases, including cancer, neurodegenerative disorders, aging, obesity, and diabetes. Peroxisome homeostasis is achieved via a tightly regulated interplay between peroxisome biogenesis and degradation via selective autophagy, which is commonly known as "pexophagy". Dysregulation of either peroxisome biogenesis or pexophagy may be detrimental to the health of cells and contribute to the pathophysiology of these diseases. Autophagy is an evolutionary conserved catabolic process for non-selective degradation of macromolecules and organelles in response to various stressors. In selective autophagy, specific cargo-recognizing receptors connect the cargo to the core autophagic machinery, and additional posttranslational modifications such as ubiquitination and phosphorylation regulate this process. Several stress conditions have been shown to stimulate pexophagy and decrease peroxisome abundance. However, our understanding of the mechanisms that particularly regulate mammalian pexophagy has been limited. In recent years considerable progress has been made uncovering signaling pathways, autophagy receptors and adaptors as well as posttranslational modifications involved in pexophagy. In this review, which is published back-to-back with a peroxisome review by Islinger et al. [(Histochem Cell Biol 137:547-574, 2018). The peroxisome: an update on mysteries 2.0], we focus on recent novel findings on the underlying molecular mechanisms of pexophagy in yeast and mammalian cells and highlight concerns and gaps in our knowledge.