Autophagy Proteins in Phagocyte Endocytosis and Exocytosis

Deciphering the interplay between autophagy and polarity in epithelial tubulogenesis

The Metazoan complexity arises from a primary building block, the epithelium, which comprises a layer of polarized cells that divide the organism into compartments. Most of these body compartments are organs formed by epithelial tubes that enclose an internal hollow space or lumen. Over the last decades, multiple studies have unmasked the paramount events required to form this lumen de novo. In epithelial cells, these events mainly involve recognizing external clues, establishing and maintaining apicobasal polarity, endo-lysosomal trafficking, and expanding the created lumen. Although canonical autophagy has been classically considered a catabolic process needed for cell survival, multiple studies have also emphasized its crucial role in epithelial polarity, morphogenesis and cellular homeostasis. Furthermore, non-canonical autophagy pathways have been recently discovered as atypical secretory routes. Both canonical and non-canonical pathways play essential roles in epithelial polarity and lumen formation. This review addresses how the molecular machinery for epithelial polarity and autophagy interplay in different processes and how autophagy functions influence lumenogenesis, emphasizing its role in the lumen formation key events.

Association of Glial Activation and α-Synuclein Pathology in Parkinson's Disease

The accumulation of pathological α-synuclein (α-syn) in the central nervous system and the progressive loss of dopaminergic neurons in the substantia nigra pars compacta are the neuropathological features of Parkinson's disease (PD). Recently, the findings of prion-like transmission of α-syn pathology have expanded our understanding of the region-specific distribution of α-syn in PD patients. Accumulating evidence suggests that α-syn aggregates are released from neurons and endocytosed by glial cells, which contributes to the clearance of α-syn. However, the activation of glial cells by α-syn species produces pro-inflammatory factors that decrease the uptake of α-syn aggregates by glial cells and promote the transmission of α-syn between neurons, which promotes the spread of α-syn pathology. In this article, we provide an overview of current knowledge on the role of glia and α-syn pathology in PD pathogenesis, highlighting the relationships between glial responses and the spread of α-syn pathology.

RUFY4 exists as two translationally regulated isoforms, that localize to the mitochondrion in activated macrophages

We report here that RUFY4, a newly characterized member of the 'RUN and FYVE domain-containing' family of proteins previously associated with autophagy enhancement, is highly expressed in alveolar macrophages (AM). We show that RUFY4 interacts with mitochondria upon stimulation by microbial-associated molecular patterns of AM and dendritic cells. RUFY4 interaction with mitochondria and other organelles is dependent on a previously uncharacterized OmpH domain located immediately upstream of its C-terminal FYVE domain. Further, we demonstrate that rufy4 messenger RNA can be translated from an alternative translation initiation codon, giving rise to a N-terminally truncated form of the molecule lacking most of its RUN domain and with enhanced potential for its interaction with mitochondria. Our observations point towards a role of RUFY4 in selective mitochondria clearance in activated phagocytes.

Broad and Complex Roles of NBR1-Mediated Selective Autophagy in Plant Stress Responses

Selective autophagy is a highly regulated degradation pathway for the removal of specific damaged or unwanted cellular components and organelles such as protein aggregates. Cargo selectivity in selective autophagy relies on the action of cargo receptors and adaptors. In mammalian cells, two structurally related proteins p62 and NBR1 act as cargo receptors for selective autophagy of ubiquitinated proteins including aggregation-prone proteins in aggrephagy. Plant NBR1 is the structural and functional homolog of mammalian p62 and NBR1. Since its first reports almost ten years ago, plant NBR1 has been well established to function as a cargo receptor for selective autophagy of stress-induced protein aggregates and play an important role in plant responses to a broad spectrum of stress conditions including heat, salt and drought. Over the past several years, important progress has been made in the discovery of specific cargo proteins of plant NBR1 and their roles in the regulation of plant heat stress memory, plant-viral interaction and special protein secretion. There is also new evidence for a possible role of NBR1 in stress-induced pexophagy, sulfur nutrient responses and abscisic acid signaling. In this review, we summarize these progresses and discuss the potential significance of NBR1-mediated selective autophagy in broad plant responses to both biotic and abiotic stresses.

The Interplay of Host Lysosomes and Intracellular Pathogens

Lysosomes are an integral part of the intracellular defense system against microbes. Lysosomal homeostasis in the host is adaptable and responds to conditions such as infection or nutritional deprivation. Pathogens such as Mycobacterium tuberculosis (Mtb) and Salmonella avoid lysosomal targeting by actively manipulating the host vesicular trafficking and reside in a vacuole altered from the default lysosomal trafficking. In this review, the mechanisms by which the respective pathogen containing vacuoles (PCVs) intersect with lysosomal trafficking pathways and maintain their distinctness are discussed. Despite such active inhibition of lysosomal targeting, emerging literature shows that different pathogens or pathogen derived products exhibit a global influence on the host lysosomal system. Pathogen mediated lysosomal enrichment promotes the trafficking of a sub-set of pathogens to lysosomes, indicating heterogeneity in the host-pathogen encounter. This review integrates recent advancements on the global lysosomal alterations upon infections and the host protective role of the lysosomes against these pathogens. The review also briefly discusses the heterogeneity in the lysosomal targeting of these pathogens and the possible mechanisms and consequences.

Sublethal Levels of Antibiotics Promote Bacterial Persistence in Epithelial Cells

Antibiotic therapy and host cells frequently fail to eliminate invasive bacterial pathogens due to the emergence of antibiotic resistance, resulting in the relapse and recurrence of infections. Bacteria evolve various strategies to persist and survive in epithelial cells, a front-line barrier of host tissues counteracting invasion; however, it remains unclear how bacteria hijack cellular responses to promote cytoplasmic survival under antibiotic therapy. Here, it is demonstrated that extracellular bacteria show invasive behavior and survive in epithelial cells in both in vivo and in vitro models, to increase antibiotic tolerance. In turn, sublethal levels of antibiotics increase bacterial invasion through promoting the production of bacterial virulence factors. Furthermore, antibiotic treatments interrupt lysosomal acidification in autophagy due to the internalized bacteria, using Bacillus cereus and ciprofloxacin as a model. In addition, it is found that sublethal levels of ciprofloxacin cause mitochondrial dysfunction and reactive oxygen species (ROS) accumulation to impair lysosomal vascular tape ATPase (V-ATPase) to further promote bacterial persistence. Collectively, these results highlight the potential of host cells mediated antibiotic tolerance, which markedly compromises antibiotic efficacy and worsens the outcomes of infection.

Atg7 Knockdown Reduces Chemerin Secretion in Murine Adipocytes

CONTEXT

In individuals with obesity, adipocyte endocrine function is affected by altered autophagy. Genetic variants in autophagy-related gene 7 (ATG7) correlated with serum chemerin (RARRES2) concentrations.

OBJECTIVES

To investigate a functional interplay between chemerin and ATG7, how it may relate to autophagy-mediated adipocyte dysfunction in obesity, and the relevance of genetic ATG7 variants in chemerin physiology.

DESIGN

Adipose ATG7 mRNA expression and adiposity measures were available in two human study cohorts. The effect of a high-calorie diet on adipose Rarres2 and Atg7 expression was investigated in mice. In 3T3-L1 adipocytes, the effect of Atg7 knockdown on chemerin expression and secretion was studied. The influence of single nucleotide polymorphisms on ATG7 transcription and chemerin physiology was investigated using a luciferase assay.

SETTING

Mouse model, clinical trials, in vitro studies.

PARTICIPANTS

Native American (n = 83) and white (n = 100) cohorts.

MAIN OUTCOME MEASURE

Adipocyte chemerin expression and secretion.

RESULTS

In mice fed a high-calorie diet, adipose Atg7 mRNA expression did not parallel an increase in Rarres2 mRNA expression. ATG7 mRNA expression in human subcutaneous adipose tissue correlated with body mass index, fat mass (r > 0.27; P < 0.01), and adipocyte cell size (r > 0.24; P < 0.02). Atg7 knockdown in 3T3-L1 adipocytes decreased chemerin secretion by 22% (P < 0.04). Rs2606729 in ATG7 was predicted to alter ATG7 transcription and induced higher luciferase activity in vitro (P < 0.0001).

CONCLUSIONS

Human adipose ATG7 mRNA expression relates to measures of adiposity. Atg7 knockdown reduces chemerin secretion from adipocytes in vitro, supportive of a functional interplay between ATG7 and chemerin in autophagy-mediated adipocyte dysfunction.

Glial phagocytic clearance in Parkinson's disease

An emerging picture suggests that glial cells' loss of beneficial roles or gain of toxic functions can contribute to neurodegenerative conditions. Among glial cells, microglia and astrocytes have been shown to play phagocytic roles by engulfing synapses, apoptotic cells, cell debris, and released toxic proteins. As pathogenic protein accumulation is a key feature in Parkinson's disease (PD), compromised phagocytic clearance might participate in PD pathogenesis. In contrast, enhanced, uncontrolled and potentially toxic glial clearance capacity could contribute to synaptic degeneration. Here, we summarize the current knowledge of the molecular mechanisms underlying microglial and astrocytic phagocytosis, focusing on the possible implication of phagocytic dysfunction in neuronal degeneration. Several endo-lysosomal proteins displaying genetic variants in PD are highly expressed by microglia and astrocytes. We also present the evidence that lysosomal defects can affect phagocytic clearance and discuss the therapeutic relevance of restoring or enhancing lysosomal function in PD.

LC3-associated phagocytosis at a glance

Classically, canonical autophagy has been considered a survival mechanism initiated in response to nutrient insufficiency. We now understand that autophagy functions in multiple scenarios where it is necessary to maintain homeostasis. Recent evidence has established that a variety of non-canonical functions for autophagy proteins are mechanistically and functionally distinct from autophagy. LC3-associated phagocytosis (LAP) is one such novel function for autophagy proteins and is a contributor to immune regulation and inflammatory responses across various cell and tissue types. Characterized by the conjugation of LC3 family proteins to phagosome membranes, LAP uses a portion of the canonical autophagy machinery, following ligation of surface receptors that recognize a variety of cargos including pathogens, dying cells, soluble ligands and protein aggregates. However, instead of affecting canonical autophagy, manipulation of the LAP pathway in vivo alters immune activation and inflammatory responses. In this Cell Science at a Glance article and the accompanying poster, we detail the divergence of this distinctive mechanism from that of canonical autophagy by comparing and contrasting shared and unique components of each pathway.

Coordination of Autophagy and Other Cellular Activities

Conventionally, autophagy (=self-eating) is thought to be a catabolic cellular process that is responsible for regulating cell homeostasis. However, the newly evidence have expanded the range of the impact of autophagy in biology. Autophagy interplays with endocytosis through shared factors such as phosphatidylinositol 3 kinase complex (PI(3)K complex), autophagy associated gene (Atg), and lysosome. Autophagy and phagocytosis orchestrate in maintaining homeostasis, in MHC class II antigen processing, in the removal of pathogens, in cell death, immunity, and inflammation. There are numerous cross talks of autophagy with biosynthetic processes such as conventional and unconventional secretion of biologically active cargo and trafficking of integral membrane proteins, as well as the exosome secretion. There are also links between autophagy and trafficking events from plasma membrane, including lateral plasma membrane proteins connexins, cell connections, and ciliogenesis.

Lipopolysaccharide-Induced Dephosphorylation of AMPK-Activated Protein Kinase Potentiates Inflammatory Injury via Repression of ULK1-Dependent Autophagy

AMP-activated protein kinase (AMPK) is a crucial metabolic regulator with profound modulatory activities on inflammation. Although the anti-inflammatory benefits of AMPK activators were well documented in experimental studies, the pathological significance of endogenous AMPK in inflammatory disorders largely remains unknown. This study investigated the phosphorylation status of endogenous AMPK and the potential roles of AMPK in mice with lipopolysaccharide (LPS)-induced lethal inflammation. The results indicated that LPS dose-dependently decreased the phosphorylation level of AMPK and its target protein acetyl-CoA carboxylase (ACC). Reactivation of AMPK with the AMPK activator A-769662 suppressed LPS-induced elevation of interleukin 6, alleviated histological abnormalities in lung and improved the survival of LPS-challenged mice. Treatment with A-769662 restored LPS-induced suppression of autophagy, inhibition of autophagy by 3-MA reversed the beneficial effects of A-769662. Treatment with A-769662 suppressed LPS-induced activation of mammalian target of rapamycin (mTOR), co-administration of mTOR activator abolished the beneficial effects of A-769662, and the suppressive effects of A-769662 on uncoordinated-51-like kinase 1 (ULK1) phosphorylation. Inhibition of ULK1 removed the beneficial effects of A-769662. These data indicated that LPS-induced dephosphorylation of AMPK could result in weakened inhibition of mTOR and repression of ULK1-dependent autophagy, which might potentiate the development of LPS-induced inflammatory injury. These data suggest that pharmacological restoration of AMPK activation might be a beneficial approach for the intervention of inflammatory disorders.

Ginkgolic acids induce HepG2 cell death via a combination of apoptosis, autophagy and the mitochondrial pathway

Ginkgolic acids may induce malignant cell death via the B-cell lymphoma 2 (Bcl-2)-associated X protein (Bax)/Bcl-2 apoptosis pathway. Concurrently, apoptosis, autophagy and mitochondrial dysfunction may also be involved in bringing about this endpoint. The anticancer effect of Ginkgolic acids (GAs) was investigated using the HepG2 cell line. The median lethal dose of the GAs of the HepG2 was measured via an MTT assay, the dose-response curves were evaluated and changes in cell morphology were monitored by microscopy. Autophagy in HepG2 cells was down regulated using 3-methyladenine (3-MA) or Beclin-1-specific small interfering RNA (siRNA) and the expression of apoptosis associated proteins caspase-3, Bax/Bcl-2, and the autophagy-associated protein 5 and microtubule-associated protein 1A/1B-light chain 3 in the GA-treated HepG2 cells were all measured by western blot analysis. The level of apoptosis in the GA-treated cells was also assessed using terminal deoxynucleotidyl-transferase-mediated dUTP nick-end labeling (TUNEL) assay, and the mitochondrial membrane potential (Δψ_m) was detected by immunofluorescence. The results of the MTT and TUNEL assays indicated that the proliferation of HepG2 cells treated with GAs was significantly reduced compared with the control group, and the rate of the inhibition was dose-dependent. Western blot analysis indicated that treatment with the Gas induced apoptosis and autophagy in the HepG2 cells. The Δψ_m of the GA-treated HepG2 cells was decreased compared with the control, as monitored by immunofluorescence. However, upon the administration of 3-MA or Beclin-1-specific siRNAs (inhibitors of the autophagy), the expression levels of the apoptosis- and autophagy-associated proteins were decreased. In conclusion, the results of the present study indicated that GAs are potent anticancer agents that function through a combination of the apoptosis, autophagy and mitochondrial pathways.

Endocytosis regulation by autophagy proteins in MHC restricted antigen presentation

The macroautophagy machinery supports membrane remodeling and fusion events that lead to the engulfment of cytoplasmic constituents in autophagosomes and their degradation in lysosomes. The capacity of this machinery to regulate membrane adaptors and influence vesicle fusion with lysosomes seems to be used not only for autophagosomes, but also for endosomes. We summarize recent evidence that two aspects of endocytosis are regulated by parts of the macroautophagy machinery. These are recruitment of adaptors for the internalization of surface receptors and the fusion of phagosomes with lysosomes. Antigen processing for MHC presentation is affected by these alternative functions of the macroautophagy machinery. Primarily extracellular antigen presentation by MHC class II molecules after phagocytosis benefits from this regulation of phagosome maturation. Furthermore, MHC class I molecules are more efficiently internalized in the presence of the core macroautophagy machinery. The identification of these alternative functions of macroautophagy proteins not only complicates the interpretation of their deficiencies in biological processes, but could also be harnessed for the regulation of antigen presentation to T cells.