Host-pathogen interactions and subversion of autophagy

Shrimp autophagy receptor protein PvTAX1BP1 regulates autophagy and facilitates white spot syndrome virus replication in Penaeus vannamei

Tax1 binding protein 1 (TAX1BP1) plays a role in autophagy regulation and proteasomal pathway across different species, though its function in shrimp is still being explored. In Penaeus vannamei, the homolog PvTAX1BP1 was characterized by a CALCOCO1 domain, a LC3-interacting region (LIR), two coiled-coil domains, and two ubiquitin-binding zinc finger regions (UBZs). It was ubiquitously expressed in shrimp tissues while its expression in hemocytes was notably downregulated following white spot syndrome virus (WSSV) infection. Silencing PvTAX1BP1 reduced WSSV replication and prolonged shrimp survival, suggesting its involvement in viral pathogenesis. Immunofluorescence assay revealed co-localization of PvTAX1BP1 with the autophagy-related protein PvLC3, indicating a potential interaction and its role in LC3-mediated autophagy. Additionally, knockdown of PvTAX1BP1 resulted in downregulation of the autophagy marker PvLC3-II in WSSV-infected shrimp, reinforcing its role in autophagy regulation during infection. Both yeast two-hybrid (Y2H) and immunofluorescence assays confirmed that PvTAX1BP1 directly interacts with three WSSV proteins: WSSV325, WSSV458, and WSSV517. These findings suggest that PvTAX1BP1 facilitates WSSV replication by modulating host LC3-mediated autophagy, potentially through its interactions with WSSV proteins. This highlights a mechanism by which viruses can exploit cellular processes for their own benefit.

Conserved components of the macroautophagy machinery in Caenorhabditis elegans

Macroautophagy involves the sequestration of cytoplasmic contents in a double-membrane autophagosome and its subsequent delivery to lysosomes for degradation and recycling. In Caenorhabditis elegans, autophagy participates in diverse processes such as stress resistance, cell fate specification, tissue remodeling, aging, and adaptive immunity. Genetic screens in C. elegans have identified a set of metazoan-specific autophagy genes that form the basis for our molecular understanding of steps unique to the autophagy pathway in multicellular organisms. Suppressor screens have uncovered multiple mechanisms that modulate autophagy activity under physiological conditions. C. elegans also provides a model to investigate how autophagy activity is coordinately controlled at an organismal level. In this chapter, we will discuss the molecular machinery, regulation, and physiological functions of autophagy, and also methods utilized for monitoring autophagy during C. elegans development.

Xenophagy receptors Optn and p62 and autophagy modulator Dram1 independently promote the zebrafish host defense against Mycobacterium marinum

Anti-bacterial autophagy, also known as xenophagy, is a crucial innate immune process that helps maintain cellular homeostasis by targeting invading microbes. This defense pathway is widely studied in the context of infections with mycobacteria, the causative agents of human tuberculosis and tuberculosis-like disease in animal models. Our previous work in a zebrafish tuberculosis model showed that host defense against Mycobacterium marinum (Mm) is impaired by deficiencies in xenophagy receptors, optineurin (Optn) or sequestome 1 (p62), and Damage-regulated autophagy modulator 1 (Dram1). However, the interdependency of these receptors and their interaction with Dram1 remained unknown. In the present study, we used single and double knockout zebrafish lines in combination with overexpression experiments. We show that Optn and p62 can compensate for the loss of each other's function, as their overexpression restores the infection susceptibility of the mutant phenotypes. Similarly, Dram1 can compensate for deficiencies in Optn and p62, and, vice versa, Optn and p62 compensate for the loss of Dram1, indicating that these xenophagy receptors and Dram1 do not rely on each other for host defense against Mm. In agreement, Dram1 overexpression in optn/p62 double mutants restored the interaction of autophagosome marker Lc3 with Mm. Finally, optn/p62 double mutants displayed more severe infection susceptibility than the single mutants. Taken together, these results suggest that Optn and p62 do not function downstream of each other in the anti-mycobacterial xenophagy pathway, and that the Dram1-mediated defense against Mm infection does not rely on specific xenophagy receptors.

Cryptosporidium parvum regulates HCT-8 cell autophagy to facilitate survival via inhibiting miR-26a and promoting miR-30a expression

BACKGROUND

Cryptosporidium parvum is an important zoonotic parasite, which not only causes economic losses in animal husbandry but also harms human health. Due to the lack of effective measures for prevention and treatment, it is important to understand the pathogenesis and survival mechanism of C. parvum. Autophagy is an important mechanism of host cells against parasite infection through key regulatory factors such as microRNAs and MAPK pathways. However, the regulatory effect of C. parvum on autophagy has not been reported. Here, we demonstrated that C. parvum manipulated autophagy through host cellular miR-26a, miR-30a, ERK signaling and P38 signaling for parasite survival.

METHODS

The expression of Beclin1, p62, LC3, ERK and P38 was detected using western blotting in HCT-8 cells infected with C. parvum as well as treated with miR-26a-mimic, miR-30a-mimic, miR-26a-mimic or miR-30a-inhibitor post C. parvum infection. The qPCR was used to detect the expression of miR-26a and miR-30a and the number of C. parvum in HCT-8 cells. Besides, the accumulation of autophagosomes was examined using immunofluorescence.

RESULTS

The expression of Beclin1 and p62 was increased, whereas LC3 expression was increased initially at 0-8 h but decreased at 12 h and then increased again in C. parvum-infected cells. C. parvum inhibited miR-26a-mimic-induced miR-26a but promoted miR-30a-mimic-induced miR-30a expression. Suppressing miR-30a resulted in increased expression of LC3 and Beclin1. However, upregulation of miR-26a reduced ERK/P38 phosphorylation, and inhibiting ERK/P38 signaling promoted Beclin1 and LC3 while reducing p62 expression. Treatment with miR-26a-mimic, autophagy inducer or ERK/P38 signaling inhibitors reduced but treatment with autophagy inhibitor or miR-30a-mimic increased parasite number.

CONCLUSIONS

The study found that C. parvum could regulate autophagy by inhibiting miR-26a and promoting miR-30a expression to facilitate the proliferation of parasites. These results revealed a new mechanism for the interaction of C. parvum with host cells.

Brucella BtpB Manipulates Apoptosis and Autophagic Flux in RAW264.7 Cells

Brucella transfers effectors into host cells, manipulating cellular processes to its advantage; however, the mechanism by which effectors regulate cellular processes during infection is poorly understood. A growing number of studies have shown that apoptosis and autophagy are critical mechanisms for target cells to cope with pathogens and maintain cellular homeostasis. BtpB is a Brucella type IV secretion system effector with a complex mechanism for manipulating host infection. Here, we show that the ectopic expression of BtpB promoted DNA fragmentation. In contrast, an isogenic mutant strain, ΔbtpB, inhibited apoptosis compared to the wild-type strain B. suis S2 in RAW264.7 cells. In addition, BtpB inhibited autophagy, as determined by LC3-II protein levels, the number of LC3 puncta, and p62 degradation. We also found that BtpB reduced autophagolysosome formation and blocked the complete autophagic flux. Moreover, our results revealed that the autophagy inhibitor, chloroquine, reduces Brucella's intracellular survival. Overall, our data unveil new mechanisms of virulence implicating the effector BtpB in regulating host intracellular infection.

SMER28 Attenuates PI3K/mTOR Signaling by Direct Inhibition of PI3K p110 Delta

SMER28 (Small molecule enhancer of Rapamycin 28) is an autophagy-inducing compound functioning by a hitherto unknown mechanism. Here, we confirm its autophagy-inducing effect by assessing classical autophagy-related parameters. Interestingly, we also discovered several additional effects of SMER28, including growth retardation and reduced G1 to S phase progression. Most strikingly, SMER28 treatment led to a complete arrest of receptor tyrosine kinase signaling, and, consequently, growth factor-induced cell scattering and dorsal ruffle formation. This coincided with a dramatic reduction in phosphorylation patterns of PI3K downstream effectors. Consistently, SMER28 directly inhibited PI3Kδ and to a lesser extent p110γ. The biological relevance of our observations was underscored by SMER28 interfering with InlB-mediated host cell entry of Listeria monocytogenes, which requires signaling through the prominent receptor tyrosine kinase c-Met. This effect was signaling-specific, since entry of unrelated, gram-negative Salmonella Typhimurium was not inhibited. Lastly, in B cell lymphoma cells, which predominantly depend on tonic signaling through PI3Kδ, apoptosis upon SMER28 treatment is profound in comparison to non-hematopoietic cells. This indicates SMER28 as a possible drug candidate for the treatment of diseases that derive from aberrant PI3Kδ activity.

The infection characteristics and autophagy defect of dermal macrophages in STZ-induced diabetic rats skin wound Staphylococcus aureus infection model

INTRODUCTION

Diabetic foot ulcer infection (DFI) is an infectious disease of the skin and soft tissue in diabetics notorious for making rapid progress and being hard to cure. Staphylococcus aureus (S. aureus), most frequently detected in DFI, recently was suggested as an intracellular pathogen that can invade and survive within mammalian host cells. Autophagy in macrophages plays a vital immune role in combating intracellular pathogens through bacterial destruction, but there is a lack of empirical research about the infection characteristics and autophagy in diabetic skin infection.

METHODS

Here, we used streptozotocin-induced Sprague Dawley rats as a diabetic skin wound model to examine the S. aureus clearance ability and wound healing in vitro. Western blot and immunofluorescence staining were used to evaluate the autophagic flux of the macrophages in diabetic rats dermis, even with S. aureus infection.

RESULTS

We demonstrated that infections in diabetic rats appeared more severe and more invasive with weakened pathogen clearance ability of the host immune system, which coincided with the suppressed autophagic flux in dermal macrophages, featured by a significant increase in endogenous LC3II/I and in p62.

CONCLUSIONS

Our results first provided convincing evidence that autophagy of macrophages was dysfunctional in diabetes, especially after being infected by S. aureus, which weakens the intracellular killing of S. aureus, potentially worsens the infections, and accelerates the infection spread in the diabetic rat model. Further understanding of the special immune crosstalk between diabetes host and S. aureus infection through autophagic factors will help to explain the complex clinical phenomenon and guarantee the development of effective therapies for diabetic foot infections.

Distinct effects of different doses of kaempferol on D‑GalN/LPS‑induced ALF depend on the autophagy pathway

Kaempferol, a flavonoid compound, has various biological functions, such as anti-inflammatory and antitumor activities. Acute liver failure (ALF) is a lethal clinical syndrome that occurs due to severe damage of the liver function. In the present study, the mechanisms underlying the therapeutic effects of kaempferol in ALF were evaluated. An ALF mouse model was established using D-galactosamine (D-GalN; 700 mg/kg)/lipopolysaccharide (LPS; 10 µg/kg). A total of 2 h before the administration of D-GalN/LPS, mice were pretreated with different doses of kaempferol (2.5, 5, 10, 20 and 40 mg/kg), and 6 h after injection of D-GalN/LPS, mice were euthanized. The survival rate, liver function and levels of inflammatory cytokines were assessed. The results demonstrated that kaempferol pretreatment protected hepatocytes from ALF induced by D-GalN/LPS via regulation of the autophagy pathway, both in vivo and in vitro. Pretreatment with a high dose of kaempferol significantly decreased the survival rates and increased severe liver damage; however, pretreatment with a low dose of kaempferol had the opposite effect. Furthermore, pretreatment with a high dose of kaempferol enhanced the levels of proinflammatory cytokines [TNF-α, IL-6, IL-12p40, IL-1β, C-X-C motif chemokine ligand (CXCL)-2, CXCL-10] and markers of the MAPK signaling pathway [phosphorylated (p)-JNK, p-ERK, p-p38], whereas pretreatment with a low dose of kaempferol had the opposite effect. Pretreatment with a high dose of kaempferol decreased autophagy, whereas pretreatment with a low dose of kaempferol increased autophagy in vivo and in vitro. It was also shown that pretreatment with 3-methyadenine or autophagy related 7 small interfering RNA, to inhibit autophagy, partially abrogated the hepatoprotective effects of pretreatment with 5 mg/kg kaempferol in the ALF mouse model. These results demonstrate that the effects of different doses of kaempferol on D-GalN/LPS-induced ALF varies based on the dose, and that kaempferol exerted its effects via regulation of the autophagy pathway.

TSPO protein binding partners in bacteria, animals, and plants

The ancient membrane protein TSPO is phylogenetically widespread from archaea and bacteria to insects, vertebrates, plants, and fungi. TSPO’s primary amino acid sequence is only modestly conserved between diverse species, although its five transmembrane helical structure appears mainly conserved. Its cellular location and orientation in membranes have been reported to vary between species and tissues, with implications for potential diverse binding partners and function. Most TSPO functions relate to stress-induced changes in metabolism, but in many cases it is unclear how TSPO itself functions—whether as a receptor, a sensor, a transporter, or a translocator. Much evidence suggests that TSPO acts indirectly by association with various protein binding partners or with endogenous or exogenous ligands. In this review, we focus on proteins that have most commonly been invoked as TSPO binding partners. We suggest that TSPO was originally a bacterial receptor/stress sensor associated with porphyrin binding as its most ancestral function and that it later developed additional stress-related roles in eukaryotes as its ability to bind new partners evolved.

Cryptosporidium parvum infection induces autophagy in intestinal epithelial cells

Autophagy, a process of degradation and recycling of macromolecules and organelles to maintain cellular homeostasis, has also been shown to help eliminate invading pathogens. Conversely, various pathogens including parasites have been shown to modulate/exploit host autophagy facilitating their intracellular infectious cycle. In this regard, Cryptosporidium parvum (CP), a protozoan parasite of small intestine is emerging as a major global health challenge. However, the pathophysiology of cryptosporidiosis is mostly unknown. We have recently demonstrated CP-induced epithelial barrier disruption via decreasing the expression of specific tight junction (TJ) and adherens junction (AJ) proteins such as occludin, claudin-4 and E-cadherin. Therefore, we utilised confluent Caco-2 cell monolayers as in vitro model of intestinal epithelial cells (IECs) to investigate the potential role of autophagy in the pathophysiology of cryptosporidiosis. Autophagy was assessed by increase in the ratio of LC3II (microtubule associated protein 1 light chain 3) to LC3I protein and decrease in p62/SQSTM1 protein levels. CP treatment of Caco-2 cells for 24 hr induced autophagy with a maximum effect observed with 0.5 × 106 oocyst/well. CP decreased mTOR (mammalian target of rapamycin, a suppressor of autophagy) phosphorylation, suggesting autophagy induction via mTOR inactivation. Measurement of autophagic flux utilizing the lysosomal inhibitor chloroquine (CQ) showed more pronounced increase in LC3II level in cells co-treated with CP + CQ as compared to CP or CQ alone, suggesting that CP-induced increase in LC3II was due to enhanced autophagosome formation rather than impaired lysosomal clearance. CP infection did not alter ATG7, a key autophagy protein. However, the decrease in occludin, claudin-4 and E-cadherin by CP was partially blocked following siRNA silencing of ATG7, suggesting the role of autophagy in CP-induced decrease in these TJ/AJ proteins. Our results provide novel evidence of autophagy induction by CP in host IECs that could alter important host cell processes contributing to the pathophysiology of cryptosporidiosis.

Pivotal role of endothelial cell autophagy in sepsis

Sepsis is a fatal organ dysfunction resulting from a disordered host response to infection. Endothelial cells (ECs) are usually the primary targets of inflammatory mediators in sepsis; damage to ECs plays a pivotal part in vital organ failure. In recent studies, autophagy was suggested to play a critical role in the ECs injury although the mechanisms by which ECs are injured in sepsis are not well elucidated. Autophagy is a highly conserved catabolic process that includes sequestrating plasma contents and transporting cargo to lysosomes for recycling the vital substrates required for metabolism. This pathway also counteracts microbial invasion to balance and retain homeostasis, especially during sepsis. Increasing evidence indicates that autophagy is closely associated with endothelial function. The role of autophagy in sepsis may or may not be favorable depending upon conditions. In the present review, the current knowledge of autophagy in the process of sepsis and its influence on ECs was evaluated. In addition, the potential of targeting EC autophagy for clinical treatment of sepsis was discussed.

Mixed signals - how Trypanosoma cruzi exploits host-cell communication and signaling to establish infection

Chagas disease (American trypanosomiasis) is a 'neglected' pathology that affects millions of people worldwide, mainly in Latin America. Trypanosoma cruzi, the causative agent, is an obligate intracellular parasite with a complex and diverse biology that infects several mammalian species, including humans. Because of genetic variability among strains and the presence of four biochemically and morphologically distinct parasite forms, the outcome of T. cruzi infection varies considerably depending on host cell type and parasite strain. During the initial contact, cellular communication is established by host-recognition-mediated responses, followed by parasite adherence and penetration. For this purpose, T. cruzi expresses a variety of proteins that modify the host cell, enabling it to safely reach the cytoplasm. After entry into the host cell, T. cruzi forms a transitory structure termed 'parasitophorous vacuole' (PV), followed by its cytoplasmic replication and differentiation after PV rupture, and subsequent invasion of other cells. The success of infection, maintenance and survival inside host cells is facilitated by the ability of T. cruzi to subvert various host signaling mechanisms. We focus in this Review on the various mechanisms that induce host cytoskeletal rearrangements, activation of autophagy-related proteins and crosstalk among major immune response regulators, as well as recent studies on the JAK-STAT pathway.

Innate immunity in C. elegans

In its natural habitat, C. elegans encounters a wide variety of microbes, including food, commensals and pathogens. To be able to survive long enough to reproduce, C. elegans has developed a complex array of responses to pathogens. These activities are coordinated on scales that range from individual organelles to the entire organism. Often, the response is triggered within cells, by detection of infection-induced damage, mainly in the intestine or epidermis. C. elegans has, however, a capacity for cell non-autonomous regulation of these responses. This frequently involves the nervous system, integrating pathogen recognition, altering host biology and governing avoidance behavior. Although there are significant differences with the immune system of mammals, some mechanisms used to limit pathogenesis show remarkable phylogenetic conservation. The past 20 years have witnessed an explosion of host-pathogen interaction studies using C. elegans as a model. This review will discuss the broad themes that have emerged and highlight areas that remain to be fully explored.

Vibrio cholerae cytotoxin MakA induces noncanonical autophagy resulting in the spatial inhibition of canonical autophagy

Autophagy plays an essential role in the defense against many microbial pathogens as a regulator of both innate and adaptive immunity. Some pathogens have evolved sophisticated mechanisms that promote their ability to evade or subvert host autophagy. Here, we describe a novel mechanism of autophagy modulation mediated by the recently discovered Vibrio cholerae cytotoxin, motility-associated killing factor A (MakA). pH-dependent endocytosis of MakA by host cells resulted in the formation of a cholesterol-rich endolysosomal membrane aggregate in the perinuclear region. Aggregate formation induced the noncanonical autophagy pathway driving unconventional LC3 (herein referring to MAP1LC3B) lipidation on endolysosomal membranes. Subsequent sequestration of the ATG12-ATG5-ATG16L1 E3-like enzyme complex, required for LC3 lipidation at the membranous aggregate, resulted in an inhibition of both canonical autophagy and autophagy-related processes, including the unconventional secretion of interleukin-1β (IL-1β). These findings identify a novel mechanism of host autophagy modulation and immune modulation employed by V. cholerae during bacterial infection.

Human Genetic Variation Influences Enteric Fever Progression

In the 21st century, enteric fever is still causing a significant number of mortalities, especially in high-risk regions of the world. Genetic studies involving the genome and transcriptome have revealed a broad set of candidate genetic polymorphisms associated with susceptibility to and the severity of enteric fever. This review attempted to explain and discuss the past and the most recent findings on human genetic variants affecting the progression of Salmonella typhoidal species infection, particularly toll-like receptor (TLR) 4, TLR5, interleukin (IL-) 4, natural resistance-associated macrophage protein 1 (NRAMP1), VAC14, PARK2/PACRG, cystic fibrosis transmembrane conductance regulator (CFTR), major-histocompatibility-complex (MHC) class II and class III. These polymorphisms on disease susceptibility or progression in patients could be related to multiple mechanisms in eliminating both intracellular and extracellular Salmonella typhoidal species. Here, we also highlighted the limitations in the studies reported, which led to inconclusive results in association studies. Nevertheless, the knowledge obtained through this review may shed some light on the development of risk prediction tools, novel therapies as well as strategies towards developing a personalised typhoid vaccine.

Interfering with Autophagy: The Opposing Strategies Deployed by Legionella pneumophila and Coxiella burnetii Effector Proteins

Autophagy is a fundamental and highly conserved eukaryotic process, responsible for maintaining cellular homeostasis and releasing nutrients during times of starvation. An increasingly important function of autophagy is its role in the cell autonomous immune response; a process known as xenophagy. Intracellular pathogens are engulfed by autophagosomes and targeted to lysosomes to eliminate the threat to the host cell. To counteract this, many intracellular bacterial pathogens have developed unique approaches to overcome, evade, or co-opt host autophagy to facilitate a successful infection. The intracellular bacteria Legionella pneumophila and Coxiella burnetii are able to avoid destruction by the cell, causing Legionnaires' disease and Q fever, respectively. Despite being related and employing homologous Dot/Icm type 4 secretion systems (T4SS) to translocate effector proteins into the host cell, these pathogens have developed their own unique intracellular niches. L. pneumophila evades the host endocytic pathway and instead forms an ER-derived vacuole, while C. burnetii requires delivery to mature, acidified endosomes which it remodels into a large, replicative vacuole. Throughout infection, L. pneumophila effectors act at multiple points to inhibit recognition by xenophagy receptors and disrupt host autophagy, ensuring it avoids fusion with destructive lysosomes. In contrast, C. burnetii employs its effector cohort to control autophagy, hypothesized to facilitate the delivery of nutrients and membrane to support the growing vacuole and replicating bacteria. In this review we explore the effector proteins that these two organisms utilize to modulate the host autophagy pathway in order to survive and replicate. By better understanding how these pathogens manipulate this highly conserved pathway, we can not only develop better treatments for these important human diseases, but also better understand and control autophagy in the context of human health and disease.

Recent advances in studies of molecular hydrogen in the treatment of pancreatitis

Pancreatitis is an inflammatory disease of the pancreas characterized by acinar cell injury and is associated with the abnormal release of trypsin, which results in high mortality due to systemic inflammatory response syndrome (SIRS) and multiple organ dysfunction syndrome (MODS). The inflammatory response, impaired autophagic flux, endoplasmic reticulum stress (ERS) and their interactions are involved in the development of pancreatitis. Molecular hydrogen (H₂) is a novel antioxidant that possesses the features of selective scavenging of oxygen free radicals and nontoxic metabolites and has been shown to be efficacious for treating infection, injury, tumors, ischemia-reperfusion organ injury, metabolic disease and several other diseases. Recent studies have found that H₂ is also useful in the treatment of pancreatitis, which may be related to the mechanism of antioxidative stress, anti-inflammation, anti-apoptosis, regulation of immunity and regulation of molecular pathways. This review focuses on the pathogenesis of pancreatitis and the research progress and potential mechanisms of H₂ against pancreatitis to provide theoretical bases for future research and clinical application of H₂ therapy for pancreatitis.

Low-level contamination of deoxynivalenol: A threat from environmental toxins to porcine epidemic diarrhea virus infection

Mycotoxins are toxic metabolites produced by fungal species that commonly present in the global environment, especially in cereals and animal forages. The changing global environment may further increase the exposure to these toxins, posing a serious threat to humans and animals. Recently, coronavirus has become one of the most important pathogens threatening human and animal health. It is not clear whether environmental toxins, such as mycotoxins, will affect coronavirus infection. Given that pigs are among the animals most affected by coronavirus and highly homologous to humans, weaned piglets and IPEC-J2 cells were respectively chosen as in vivo and in vitro model to explore the impacts of deoxynivalenol (DON), the most abundant trichothecene mycotoxin in feed, on porcine epidemic diarrhea virus (PEDV) infection and the mechanisms involved. In vivo, twenty-seven piglets infected naturally with PEDV were randomly divided into three groups, receiving the basal diet containing 0, 750 and 1500 μg/kg DON, respectively. Significant increases in the diarrhea rates, gut barrier injury and PEDV proliferation of piglets' small intestine were observed in experimental groups compared with the control. Additionally, the autophagosome-like vesicles and the autophagy-related proteins expression were also increased in experimental groups. In vitro, we observed that 0.1, 0.5 and 1.0 μM DON significantly promoted the entry and replication of PEDV in IPEC-J2 cells, along with the induction of a complete autophagy. CRISPR-Cas9-mediated knockout of LC3B indicated a vital role of autophagy in the promotion. Pretreatment with p38 signaling inhibitor could significantly block the induction of autophagy, indicating that DON could promote the PEDV infection by triggering p38-mediated autophagy. Our findings suggest that mycotoxin could influence the prevalence of coronavirus and provide new ideas for the prevention and control of coronavirus.

Autophagy plays a protective role during Pseudomonas aeruginosa-induced apoptosis via ROS-MAPK pathway

Pseudomonas aeruginosa infection can induce alveolar macrophage apoptosis and autophagy, which play a vital role in eliminating pathogens. These two processes are usually not independent. Recently, autophagy has been found to interact with apoptosis during pathogen infections. Nevertheless, the role of autophagy in P. aeruginosa-infected cell apoptosis is unclear. In this study, we explored the impact of P. aeruginosa infection on autophagy and apoptosis in RAW264.7 cells. The autophagy activator rapamycin was used to stimulate autophagy and explore the role of autophagy on apoptosis in P. aeruginosa-infected RAW264.7 cells. The results indicated that P. aeruginosa infection induced autophagy and apoptosis in RAW264.7 cells, and that rapamycin could suppress P. aeruginosa-induced apoptosis by regulating the expression of apoptosis-related proteins. In addition, rapamycin scavenged the cellular reactive oxygen species (ROS) and diminished p-JNK, p-ERK1/2 and p-p38 expression of MAPK pathways in RAW264.7 cells infected with P. aeruginosa. In conclusion, the promotion of autophagy decreased P. aeruginosa-induced ROS accumulation and further attenuated the apoptosis of RAW264.7 cells through MAPK pathway. These results provide novel insights into host-pathogen interactions and highlight a potential role of autophagy in eliminating P. aeruginosa.

Eating the unknown: Xenophagy and ER-phagy are cytoprotective defenses against pathogens

Autophagy is an evolutionary conserved catabolic process devoted to the removal of unnecessary and harmful cellular components. In its general form, autophagy governs cellular lifecycle through the formation of double membrane vesicles, termed autophagosomes, that enwrap and deliver unwanted intracellular components to lysosomes. In addition to this omniscient role, forms of selective autophagy, relying on specialized receptors for cargo recognition, exert fine-tuned control over cellular homeostasis. In this regard, xenophagy plays a pivotal role in restricting the replication of intracellular pathogens, thus acting as an ancient innate defense system against infections. Recently, selective autophagy of the endoplasmic reticulum (ER), more simply ER-phagy, has been uncovered as a critical mechanism governing ER network shape and function. Six ER-resident proteins have been characterized as ER-phagy receptors and their orchestrated function enables ER homeostasis and turnover overtime. Unfortunately, ER is also the preferred site for viral replication and several viruses hijack ER machinery for their needs. Thus, it is not surprising that some ER-phagy receptors can act to counteract viral replication and minimize the spread of infection throughout the organism. On the other hand, evolutionary pressure has armed pathogens with strategies to evade and subvert xenophagy and ER-phagy. Although ER-phagy biology is still in its infancy, the present review aims to summarize recent ER-phagy literature, with a special focus on its role in counteracting viral infections. Moreover, we aim to offer some hints for future targeted approaches to counteract host-pathogen interactions by modulating xenophagy and ER-phagy pathways.

Advanced glycation end products reduce macrophage-mediated killing of Staphylococcus aureus by ARL8 upregulation and inhibition of autolysosome formation

Staphylococcus aureus, a pathogen most frequently found in diabetic foot ulcer infection, was recently suggested as an intracellular pathogen. Autophagy in professional phagocytes like macrophages allows selective destruction of intracellular pathogens, and its dysfunction can increase the survival of internalized pathogens, causing infections to worsen and spread. Previous works have shown that S. aureus infections in diabetes appeared more severe and invasive, and coincided with the suppressed autophagy in dermal tissues of diabetic rat, but the exact mechanisms are unclear. Here, we demonstrated that accumulation of advanced glycation end products (AGEs) contributed to the diminished autophagy-mediated clearance of S. aureus in the macrophages differentiated from PMA-treated human monocytic cell line THP-1. Importantly, infected macrophages showed increased S. aureus containing autophagosome, but the subsequent fusion of S. aureus containing autophagosome and lysosome was suppressed in AGEs-pretreated cells, suggesting AGEs blocked the autophagic flux and enabled S. aureus survival and escape. At the molecular level, elevated lysosomal ARL8 expression in AGEs-treated macrophages was required for AGEs-mediated inhibition of autophagosome-lysosome fusion. Silencing ARL8 in AGEs-treated macrophages restored autophagic flux and increased S. aureus clearance. Our results therefore demonstrate a new mechanism, in which AGEs accelerate S. aureus immune evasion in macrophages by ARL8-dependent suppression of autophagosome-lysosome fusion and bactericidal capability.

Regulation and repurposing of nutrient sensing and autophagy in innate immunity

Nutrients not only act as building blocks but also as signaling molecules. Nutrient-availability promotes cell growth and proliferation and suppresses catabolic processes, such as macroautophagy/autophagy. These effects are mediated by checkpoint kinases such as MTOR (mechanistic target of rapamycin kinase), which is activated by amino acids and growth factors, and AMP-activated protein kinase (AMPK), which is activated by low levels of glucose or ATP. These kinases have wide-ranging activities that can be co-opted by immune cells upon exposure to danger signals, cytokines or pathogens. Here, we discuss recent insight into the regulation and repurposing of nutrient-sensing responses by the innate immune system during infection. Moreover, we examine how natural mutations and pathogen-mediated interventions can alter the balance between anabolic and autophagic pathways leading to a breakdown in tissue homeostasis and/or host defense.Abbreviations: AKT1/PKB: AKT serine/threonine kinase 1; ATG: autophagy related; BECN1: beclin 1; CGAS: cyclic GMP-AMP synthase; EIF2AK4/GCN2: eukaryotic translation initiation factor 2 alpha kinase 4; ER: endoplasmic reticulum; FFAR: free fatty acid receptor; GABARAP: GABA type A receptor-associated protein; IFN: interferon; IL: interleukin; LAP: LC3-associated phagocytosis; MAP1LC3B/LC3B: microtubule associated protein 1 light chain 3 beta; MAP3K7/TAK1: mitogen-activated protein kinase kinase kinase 7; MAPK: mitogen-activated protein kinase; MTOR: mechanistic target of rapamycin kinase; NLR: NOD (nucleotide-binding oligomerization domain) and leucine-rich repeat containing proteins; PI3K, phosphoinositide 3-kinase; PRR: pattern-recognition receptor; PtdIns3K: phosphatidylinositol 3-kinase; RALB: RAS like proto-oncogene B; RHEB: Ras homolog, MTORC1 binding; RIPK1: receptor interacting serine/threonine kinase 1; RRAG: Ras related GTP binding; SQSTM1/p62: sequestosome 1; STING1/TMEM173: stimulator of interferon response cGAMP interactor 1; STK11/LKB1: serine/threonine kinase 11; TBK1: TANK binding kinase 1; TLR: toll like receptor; TNF: tumor necrosis factor; TRAF6: TNF receptor associated factor 6; TRIM: tripartite motif protein; ULK1: unc-51 like autophagy activating kinase 1; V-ATPase: vacuolar-type H⁺-proton-translocating ATPase.

Mammalian STE20‑like kinase 1 regulates pancreatic cancer cell survival and migration through Mfn2‑mediated mitophagy

Mammalian STE20-like kinase 1 (MST1) plays an important role in pancreatic cancer progression, but its downstream targets are still unknown. In the present study, our results indicated that MST1 expression was significantly downregulated in pancreatic cancer cell lines (PANC‑1, BxPC‑3 and HPAC) compared with that in the normal ductal epithelial cell line (hTERT‑HPNE). Moreover, MST1 overexpression in PANC‑1 cells led to increased apoptosis as determined by MTT and TUNEL assays and inhibited cellular migration. Mechanistically, upregulation of MST1 expression caused mitochondrial dysfunction, decreased ATP production, and activation of the mitochondrial‑dependent apoptotic pathway via inhibition of mitofusin 2 (Mfn2)‑mediated mitophagy, which ultimately resulted in increased cellular apoptosis and decreased cellular migration. Collectively, the present study demonstrated that MST1 may regulate pancreatic cancer PANC‑1 cell survival, invasion and migration through Mfn2‑mediated mitophagy, laying the foundation for the exploration of novel therapeutic targets for pancreatic cancer.

Microautophagy upregulation in cutaneous lymph nodes of dogs naturally infected by Leishmania infantum

This is the first study showing an in vivo microautophagy upregulation by Leishmania infantum in dogs. Both Leishmania amastigotes and promastigotes were detected in the cytoplasm of many professional and nonprofessional phagocytic cells of popliteal lymph node of three dogs suffering from chronic cutaneous leishmaniasis. Ultrastructurally, parasites appeared to be wrapped by lysosomes and/or multivesicular bodies. Neither phagophores nor double-membraned vacuoles consistent with autophagosomes were observed. Transcription factor EB (TFEB), a key factor involved in lysosome biogenesis, showed a statistically significant increase in the total component when examined by western blot in samples from leishmaniotic dogs compared with samples from healthy dogs. Instead, phosphorylated TFEB showed unmodified expression levels both in leishmaniotic and healthy dogs. Furthermore, Hsc70 and endosomal sorting complex required for transport (ESCRT)-I, which are known to play a role in microautophagy, showed no variation in expression levels both in diseased and healthy animals. Vps4A/B, an evolutionary conserved ATPase responsible for ESCRT-I complex disassembly and MVB maturation, was statistically significantly overexpressed in lymph nodal samples from leishmaniotic dogs. Bag3 was downregulated in diseased dogs whereas CHIP, p62, and LC3-II did not show any variation in expression levels. The altered expression profile of Bag3 suggested an altered interaction of Bag3 with Hsc70 and CHIP, which usually form a molecular complex involved in autophagosome-lysosome pathways. Ultrastructural and molecular findings suggested that the microautophagy pathway is upregulated in lymph nodes of dogs suffering from a chronic natural infection by Leishmania infantum.

Nutritional support in sepsis: when less may be more

Despite sound basis to suspect that aggressive and early administration of nutritional support may hold therapeutic benefits during sepsis, recommendations for nutritional support have been somewhat underwhelming. Current guidelines (ESPEN and ASPEN) recognise a lack of clear evidence demonstrating the beneficial effect of nutritional support during sepsis, raising the question: why, given the perceived low efficacy of nutritionals support, are there no high-quality clinical trials on the efficacy of permissive underfeeding in sepsis? Here, we review clinically relevant beneficial effects of permissive underfeeding, motivating the urgent need to investigate the clinical benefits of delaying nutritional support during sepsis.

Elucidating the Efficacy of the Bacille Calmette-Guérin Vaccination in Conjunction with First Line Antibiotics and Liposomal Glutathione

Mycobacterium tuberculosis (M. tb) is the etiological agent that is responsible for causing tuberculosis (TB). Although every year M. tb infection affects millions of people worldwide, the only vaccine that is currently available is the Bacille Calmette–Guérin (BCG) vaccine. However, the BCG vaccine has varying efficacy. Additionally, the first line antibiotics administered to patients with active TB often cause severe complications and side effects. To improve upon the host response mechanism in containing M. tb infection, our lab has previously shown that the addition of the biological antioxidant glutathione (GSH) has profound antimycobacterial effects. The aim of this study is to understand the additive effects of BCG vaccination and ex-vivo GSH enhancement in improving the immune responses against M. tb in both groups; specifically, their ability to mount an effective immune response against M. tb infection, maintain CD4⁺ and CD8⁺ T cells in the granulomas, their response to liposomal glutathione (L-GSH), with varying suboptimal levels of the first line antibiotics isoniazid (INH) and pyrazinamide (PZA), the expressions of programmed death receptor 1 (PD-1), and their ability to induce autophagy. Our results revealed that BCG vaccination, along with GSH enhancement, can prevent the loss of CD4⁺ and CD8⁺ T cells in the granulomas and improve the control of M. tb infection by decreasing the expressions of PD-1 and increasing autophagy and production of the cytokines interferon gamma IFN-γ and tumor necrosis factor-α (TNF-α).

Organization of the Skin Immune System and Compartmentalized Immune Responses in Infectious Diseases

The skin is an organ harboring several types of immune cells that participate in innate and adaptive immune responses. The immune system of the skin comprises both skin cells and professional immune cells that together constitute what is designated skin-associated lymphoid tissue (SALT). In this review, I extensively discuss the organization of SALT and the mechanisms involved in its responses to infectious diseases of the skin and mucosa. The nature of these SALT responses, and the cellular mediators involved, often determines the clinical course of such infections. I list and describe the components of innate immunity, such as the roles of the keratinocyte barrier and of inflammatory and natural killer cells. I also examine the mechanisms involved in adaptive immune responses, with emphasis on new cytokine profiles, and the role of cell death phenomena in host-pathogen interactions and control of the immune responses to infectious agents. Finally, I highlight the importance of studying SALT in order to better understand host-pathogen relationships involving the skin and detail future directions in the immunological investigation of this organ, especially in light of recent findings regarding the skin immune system.

N-terminal β-strand underpins biochemical specialization of an ATG8 isoform

Autophagy-related protein 8 (ATG8) is a highly conserved ubiquitin-like protein that modulates autophagy pathways by binding autophagic membranes and a number of proteins, including cargo receptors and core autophagy components. Throughout plant evolution, ATG8 has expanded from a single protein in algae to multiple isoforms in higher plants. However, the degree to which ATG8 isoforms have functionally specialized to bind distinct proteins remains unclear. Here, we describe a comprehensive protein-protein interaction resource, obtained using in planta immunoprecipitation (IP) followed by mass spectrometry (MS), to define the potato ATG8 interactome. We discovered that ATG8 isoforms bind distinct sets of plant proteins with varying degrees of overlap. This prompted us to define the biochemical basis of ATG8 specialization by comparing two potato ATG8 isoforms using both in vivo protein interaction assays and in vitro quantitative binding affinity analyses. These experiments revealed that the N-terminal β-strand-and, in particular, a single amino acid polymorphism-underpins binding specificity to the substrate PexRD54 by shaping the hydrophobic pocket that accommodates this protein's ATG8-interacting motif (AIM). Additional proteomics experiments indicated that the N-terminal β-strand shapes the broader ATG8 interactor profiles, defining interaction specificity with about 80 plant proteins. Our findings are consistent with the view that ATG8 isoforms comprise a layer of specificity in the regulation of selective autophagy pathways in plants.

Oncolytic paramyxoviruses-induced autophagy; a prudent weapon for cancer therapy

Oncolytic virotherapy has currently emerged as a promising approach upon which scientists have been able to induce tumor-specific cell death in a broad spectrum of malignancies. Paramyxoviruses represent intrinsic oncolytic capability, which makes them excellent candidates to be widely used in oncolytic virotherapy. The mechanisms through which these viruses destroy the cancerous cells involve triggering the autophagic machinery and apoptosis in target cells. Interestingly, oncolytic paramyxoviruses have been found to induce autophagy and lead to tumor cells death rather than their survival. Indeed, the induction of autophagy has been revealed to enhance the immunogenicity of tumor cells via the release of damage-associated molecular patterns (DAMPs) and the activation of autophagy-related immunogenic cell death (ICD). Subsequent cross-presentation of tumor-associated antigens (TAA) through the MHC-I complex to CD8+ T cells results in the productive priming of the tumor-specific immune response. In this review, we first briefly discuss autophagy and explain the process of viral xenophagy. Finally, we focus on the interactions between virus and autophagy proteins, elaborating on the global preclinical studies on oncolytic paramyxoviruses.

A Mycobacterium tuberculosis surface protein recruits ubiquitin to trigger host xenophagy

Ubiquitin-mediated xenophagy, a type of selective autophagy, plays crucial roles in host defense against intracellular pathogens including Mycobacterium tuberculosis (Mtb). However, the exact mechanism by which host ubiquitin targets invaded microbes to trigger xenophagy remains obscure. Here we show that ubiquitin could recognize Mtb surface protein Rv1468c, a previously unidentified ubiquitin-binding protein containing a eukaryotic-like ubiquitin-associated (UBA) domain. The UBA-mediated direct binding of ubiquitin to, but not E3 ubiquitin ligases-mediated ubiquitination of, Rv1468c recruits autophagy receptor p62 to deliver mycobacteria into LC3-associated autophagosomes. Disruption of Rv1468c-ubiquitin interaction attenuates xenophagic clearance of Mtb in macrophages, and increases bacterial loads in mice with elevated inflammatory responses. Together, our findings reveal a unique mechanism of host xenophagy triggered by direct binding of ubiquitin to the pathogen surface protein, and indicate a diplomatic strategy adopted by Mtb to benefit its persistent intracellular infection through controlling intracellular bacterial loads and restricting host inflammatory responses.

Ubiquitin (Ub)-mediated xenophagy is important in defense against Mycobacterium tuberculosis (Mtb). Here, Chai et al. describe autophagy triggering by Ub binding to the Mtb surface protein Rv1468c, and show that its deletion leads to increased bacterial loads and hyperinflammatory responses in mice.

Natural variation in the roles of C. elegans autophagy components during microsporidia infection

Natural genetic variation can determine the outcome of an infection, and often reflects the co-evolutionary battle between hosts and pathogens. We previously found that a natural variant of the nematode Caenorhabditis elegans from Hawaii (HW) has increased resistance against natural microsporidian pathogens in the Nematocida genus, when compared to the standard laboratory strain of N2. In particular, HW animals can clear infection, while N2 animals cannot. In addition, HW animals have lower levels of initial colonization of Nematocida inside intestinal cells, compared to N2. Here we investigate how this natural variation in resistance relates to autophagy. We found that there is much better targeting of autophagy-related machinery to parasites under conditions where they are cleared. In particular, ubiquitin targeting to Nematocida cells correlates very well with their subsequent clearance in terms of timing, host strain and age, as well as species of Nematocida. Furthermore, clearance correlates with targeting of the LGG-2/LC3 autophagy protein to parasite cells, with HW animals having much more efficient targeting of LGG-2 to parasite cells than N2 animals. Surprisingly, however, we found that LGG-2 is not required to clear infection. Instead, we found that LGG-2/LC3 regulates Nematocida colonization inside intestinal cells. Interestingly, LGG-2/LC3 regulates intracellular colonization only in the HW strain, and not in N2. Altogether these results demonstrate that there is natural genetic variation in an LGG-2-dependent process that regulates microsporidia colonization inside intestinal cells, although not microsporidia clearance.

[Crocetin promotes autophagy in injured rat hepatocytes induced by lipopolysaccharide and D-galactosamine in vitro]

OBJECTIVE

To observe the effect of crocetin on autophagy in rat hepatocytes exposed to lipopolysaccharide (LPS) and D-galactosamine (D-gal) and explore the mechanism.

METHODS

Cultured rat hepatocytes were exposed to LPS (1 mg/L) and Dgal (60 mg/L) to induce cell injury and treated with crocetin, 3MA, or crocetin+3MA. Twelve hours after the treatments, the cells were examined for levels of ALT, AST and LDH in the supernatant using ELISA. LC3 fluorescence in the cells following immunofluorescence staining was observed using fluorescence microscopy. Autophagosomes in the cells were observed by transmission electron microscopy, and the cellular expressions of LC3, p62 and SIRT1 were detected using Western blotting.

RESULTS

The levels of ALT, AST and LDH in the hepatocytes were elevated after LPS- and D-gal-induced injury, reached the highest levels after 3MA treatment, but were decreased significantly by crocetin treatment. LC3 fluorescence increased obviously in the injured hepatoctyes, and the increment was the most obvious in crocetin-treated cells; LC3 fluorescence was decreased significantly after 3MA treatment. Cell injury induced obvious increase in autophagy in the hepatocytes, and the number of autophagosomes increased significantly after crocetin treatment but was reduced significantly after 3MA treatment. The cell injury caused an obvious up-regulation of LC3 and SIRT1 expression and down-regulated p62 expression. LC3 and SIRT1 expression levels were the highest and the expression of p62 was the lowest in cells with crocetin treatment. 3MA treatment significantly reduced the expression of LC3 and SIRT1 and increased the expression of p62 in the injured cells.

CONCLUSIONS

Autophagy is increased in injured rat hepatocytes, and crocetin can promote autophagy in the injured cells to reduce further cell injury.

Signalling mechanisms in autophagy: an introduction to the issue

Essays in Biochemistry volume 61 (issue 6), entitled Signalling Mechanisms in Autophagy, covers a range of topics in autophagy signalling, touching on emerging new details on the mechanisms of autophagy regulation, novel aspects of selective autophagy and how autophagy functions in organelle homeostasis. It also looks at how autophagy research is leading to better understanding of human disease and plant biology that can be exploited for the benefit of society.