Sterical hindrance promotes selectivity of the autophagy cargo receptor NDP52 for the danger receptor galectin-8 in antibacterial autophagy

Galectin-8N-Selective 4-Halophenylphthalazinone-Galactals Double π-Stack in a Unique Pocket

Galectin-8 contains two different carbohydrate recognition domains (CRDs). Selective inhibitors for at least one CRD are desirable for galectin-8 biology studies and potentially for pharmacological purposes. Structure-guided design led to the discovery of potent and selective glycomimetic-heterocycle hybrid ligands, with a 4-(p-bromophenyl)phthalazinone derivative displaying a 34 μM K d for galectin-8N (N-terminal CRD), no binding to galectin-8C (C-terminal CRD), -1, -3, -4N, -7, -9C, or -9N, and >40-fold selectivity over galectin-4C. Selectivity was achieved with the halogenated 4-phenylphthalazinone moiety occupying a galectin-8N-specific sub-pocket. A 1.30 Å resolution X-ray structure revealed the phthalazinone moiety stacking with Arg45 and the 4-bromophenyl moiety stacking both Arg59 and Tyr141 of galectin-8N. Physicochemical and in vitro ADME studies revealed a desirable LogD, which also translated to good passive permeability. The chemical, microsome, and plasma stability support these compounds as promising tool compounds and candidates for hit-to-lead optimization.

The role of galectins in the regulation of autophagy and inflammasome in host immunity

Galectins, a family of glycan-binding proteins have been shown to bind a wide range of glycans. In the cytoplasm, these glycans can be endogenous (or "self"), originating from damaged endocytic vesicles, or exogenous (or "non-self"), found on the surface of invading microbial pathogens. Galectins can detect these unusual cytosolic exposures to glycans and serve as critical regulators in orchestrating immune responses in innate and adaptive immunity. This review provides an overview of how galectins modulate host cellular responses, such as autophagy, xenophagy, and inflammasome-dependent cell death program, to infection.

Dysregulation of pancreatic β-cell autophagy and the risk of type 2 diabetes

Macroautophagy/autophagy is an essential degradation process that removes abnormal cellular components, maintains homeostasis within cells, and provides nutrition during starvation. Activated autophagy enhances cell survival during stressful conditions, although overactivation of autophagy triggers induction of autophagic cell death. Therefore, early-onset autophagy promotes cell survival whereas late-onset autophagy provokes programmed cell death, which can prevent disease progression. Moreover, autophagy regulates pancreatic β-cell functions by different mechanisms, although the precise role of autophagy in type 2 diabetes (T2D) is not completely understood. Consequently, this mini-review discusses the protective and harmful roles of autophagy in the pancreatic β cell and in the pathophysiology of T2D.

A brief history of galectin evolution

Galectins are a family of carbohydrate-binding proteins found in vertebrates in great abundance and diversity in terms of both structure and ligand-binding properties as well as physiological function. Proteins with clear relationships to vertebrate galectins are already found in primitive Bilateria. The increasing amount of accessible well-annotated bilaterian genomes has allowed us to reveal, through synteny analyses, a new hypothesis about the phylogenetic history of the galectin family in this animal group. Thus, we can trace the genomic localization of the putative ancestral Bilateria galectin back to the scallops as a still very primitive slow-evolving bilaterian lineage. Intriguingly, our analyses show that the primordial galectin of the Deuterostomata most likely exhibited galectin-8-like characteristics. This basal standing galectin is characterized by a tandem-repeat type with two carbohydrate recognition domains as well as by a sialic acid binding property of the N-terminal domain, which is typical for galectin-8. With the help of synteny, the amplification of this potential primordial galectin to the broad galectin cosmos of modern jawed vertebrates can be reconstructed. Therefore, it is possible to distinguish between the paralogs resulting from small-scale duplication and the ohnologues generated by whole-genome duplication. Our findings support a substantially new hypothesis about the origin of the various members of the galectin family in vertebrates. This allows us to reveal new theories on the kinship relationships of the galectins of Gnatostomata. In addition, we focus for the first time on the galectines of the Cyclostomata, which as a sister group of jawed vertebrates providing important insights into the evolutionary history of the entire subphylum. Our studies also highlight a previously neglected member of the galectin family, galectin-related protein 2. This protein appears to be a widespread ohnologue of the original tandem-repeat ancestor within Gnathostomata that has not been the focus of galectin research due to its nonclassical galactose binding sequence motif and the fact that it was lost during mammalian evolution.

The interrelation of galectins and autophagy

Autophagy is a vital physiological process that maintains intracellular homeostasis by removing damaged organelles and senescent or misfolded molecules. However, excessive autophagy results in cell death and apoptosis, which will lead to a variety of diseases. Galectins are a type of animal lectin that binds to β-galactosides and can bind to the cell surface or extracellular matrix glycans, affecting a variety of immune processes in vivo and being linked to the development of many diseases. In many cases, galectins and autophagy both play important regulatory roles in the cellular life course, yet our understanding of the relationship between them is still incomplete. Galectins and autophagy may share common etiological cofactors for some diseases. Hence, we summarize the relationship between galectins and autophagy, aiming to draw attention to the existence of multiple associations between galectins and autophagy in a variety of physiological and pathological processes, which provide new ideas for etiological diagnosis, drug development, and therapeutic targets for related diseases.

The role of galectins in immunity and infection

The galectin family consists of carbohydrate (glycan) binding proteins that are expressed by a wide variety of cells and bind to galactose-containing glycans. Galectins can be located in the nucleus or the cytoplasm, or can be secreted into the extracellular space. They can modulate innate and adaptive immune cells by binding to glycans on the surface of immune cells or intracellularly via carbohydrate-dependent or carbohydrate-independent interactions. Galectins expressed by immune cells can also participate in host responses to infection by directly binding to microorganisms or by modulating antimicrobial functions such as autophagy. Here we explore the diverse ways in which galectins have been shown to impact immunity and discuss the opportunities and challenges in the field.

Secreted filarial nematode galectins modulate host immune cells

Lymphatic filariasis (LF) is a mosquito-borne disease caused by filarial nematodes including Brugia malayi. Over 860 million people worldwide are infected or at risk of infection in 72 endemic countries. The absence of a protective vaccine means that current control strategies rely on mass drug administration programs that utilize inadequate drugs that cannot effectively kill adult parasites, thus established infections are incurable. Progress to address deficiencies in the approach to LF control is hindered by a poor mechanistic understanding of host-parasite interactions, including mechanisms of host immunomodulation by the parasite, a critical adaptation for establishing and maintaining infections. The canonical type 2 host response to helminth infection characterized by anti-inflammatory and regulatory immune phenotypes is modified by filarial nematodes during chronic LF. Current efforts at identifying parasite-derived factors driving this modification focus on parasite excretory-secretory products (ESP), including extracellular vesicles (EVs). We have previously profiled the cargo of B. malayi EVs and identified B. malayi galectin-1 and galectin-2 as among the most abundant EV proteins. In this study we further investigated the function of these proteins. Sequence analysis of the parasite galectins revealed highest homology to mammalian galectin-9 and functional characterization identified similar substrate affinities consistent with this designation. Immunological assays showed that Bma-LEC-2 is a bioactive protein that can polarize macrophages to an alternatively activated phenotype and selectively induce apoptosis in Th1 cells. Our data shows that an abundantly secreted parasite galectin is immunomodulatory and induces phenotypes consistent with the modified type 2 response characteristic of chronic LF infection.

Interaction Between Autophagy and Porphyromonas gingivalis-Induced Inflammation

Autophagy is an immune homeostasis process induced by multiple intracellular and extracellular signals. Inflammation is a protective response to harmful stimuli such as pathogen microbial infection and body tissue damage. Porphyromonas gingivalis infection elicits both autophagy and inflammation, and dysregulation of autophagy and inflammation promotes pathology. This review focuses on the interaction between autophagy and inflammation caused by Porphyromonas gingivalis infection, aiming to elaborate on the possible mechanism involved in the interaction.

The marriage of chemokines and galectins as functional heterodimers

Trafficking of leukocytes and their local activity profile are of pivotal importance for many (patho)physiological processes. Fittingly, microenvironments are complex by nature, with multiple mediators originating from diverse cell types and playing roles in an intimately regulated manner. To dissect aspects of this complexity, effectors are initially identified and structurally characterized, thus prompting familial classification and establishing foci of research activity. In this regard, chemokines present themselves as role models to illustrate the diversification and fine-tuning of inflammatory processes. This in turn discloses the interplay among chemokines, their cell receptors and cognate glycosaminoglycans, as well as their capacity to engage in new molecular interactions that form hetero-oligomers between themselves and other classes of effector molecules. The growing realization of versatility of adhesion/growth-regulatory galectins that bind to glycans and proteins and their presence at sites of inflammation led to testing the hypothesis that chemokines and galectins can interact with each other by protein-protein interactions. In this review, we present some background on chemokines and galectins, as well as experimental validation of this chemokine-galectin heterodimer concept exemplified with CXCL12 and galectin-3 as proof-of-principle, as well as sketch out some emerging perspectives in this arena.

Mechanistic insights into COVID-19 by global analysis of the SARS-CoV-2 3CLpro substrate degradome

The main viral protease (3CLpro) is indispensable for SARS-CoV-2 replication. We delineate the human protein substrate landscape of 3CLpro by TAILS substrate-targeted N-terminomics. We identify more than 100 substrates in human lung and kidney cells supported by analyses of SARS-CoV-2-infected cells. Enzyme kinetics and molecular docking simulations of 3CLpro engaging substrates reveal how noncanonical cleavage sites, which diverge from SARS-CoV, guide substrate specificity. Cleaving the interactors of essential effector proteins, effectively stranding them from their binding partners, amplifies the consequences of proteolysis. We show that 3CLpro targets the Hippo pathway, including inactivation of MAP4K5, and key effectors of transcription, mRNA processing, and translation. We demonstrate that Spike glycoprotein directly binds galectin-8, with galectin-8 cleavage disengaging CALCOCO2/NDP52 to decouple antiviral-autophagy. Indeed, in post-mortem COVID-19 lung samples, NDP52 rarely colocalizes with galectin-8, unlike in healthy lungs. The 3CLpro substrate degradome establishes a foundational substrate atlas to accelerate exploration of SARS-CoV-2 pathology and drug design.

What is the Sugar Code?

A code is defined by the nature of the symbols, which are used to generate information‐storing combinations (e. g. oligo‐ and polymers). Like nucleic acids and proteins, oligo‐ and polysaccharides are ubiquitous, and they are a biochemical platform for establishing molecular messages. Of note, the letters of the sugar code system (third alphabet of life) excel in coding capacity by making an unsurpassed versatility for isomer (code word) formation possible by variability in anomery and linkage position of the glycosidic bond, ring size and branching. The enzymatic machinery for glycan biosynthesis (writers) realizes this enormous potential for building a large vocabulary. It includes possibilities for dynamic editing/erasing as known from nucleic acids and proteins. Matching the glycome diversity, a large panel of sugar receptors (lectins) has developed based on more than a dozen folds. Lectins ‘read’ the glycan‐encoded information. Hydrogen/coordination bonding and ionic pairing together with stacking and C−H/π‐interactions as well as modes of spatial glycan presentation underlie the selectivity and specificity of glycan‐lectin recognition. Modular design of lectins together with glycan display and the nature of the cognate glycoconjugate account for the large number of post‐binding events. They give an entry to the glycan vocabulary its functional, often context‐dependent meaning(s), hereby building the dictionary of the sugar code.

Galectin-8 Senses Phagosomal Damage and Recruits Selective Autophagy Adapter TAX1BP1 To Control Mycobacterium tuberculosis Infection in Macrophages

Mycobacterium tuberculosis (Mtb) causes one of the deadliest infectious diseases worldwide. Upon infection, Mtb is phagocytosed by macrophages and uses its virulence-associated ESX-1 secretion system to modulate the host cell. We showed previously that the ESX-1 secretion system perturbs the Mtb-containing phagosome, and a population (∼30%) of intracellular Mtb is tagged with ubiquitin and targeted to selective autophagy. However, our understanding of how macrophages sense and respond to damaged Mtb-containing phagosomes remains incomplete. Here, we demonstrate that several cytosolic glycan-binding proteins called galectins recognize Mtb-containing phagosomes; in macrophage cell lines and in primary macrophages, galectin-3, -8, and -9 are all recruited to the same Mtb population that colocalizes with selective autophagy markers (ubiquitin, p62, and LC3). To test whether galectins are required for controlling Mtb replication in macrophages, we generated CRISPR/Cas9 knockouts and found that galectin-8-/- and galectin-3/8/9-/- macrophages were similarly defective in targeting Mtb to selective autophagy and controlling replication. This suggests galectin-8 plays a unique role in anti-Mtb autophagy. In investigating galectin-8's role, we identified a novel and specific interaction between galectin-8 and the selective autophagy adapter TAX1BP1 and found that this galectin-8/TAX1BP1 interaction was necessary for macrophages to efficiently target Mtb to selective autophagy. Remarkably, overexpressing galectin-8 increased targeting of Mtb to autophagy and limited Mtb replication. Taken together, these data demonstrate that while several galectins are capable of recognizing damaged Mtb-containing phagosomes, galectin-8 plays a privileged role in recruiting downstream autophagy machinery and may represent a promising target for host-directed tuberculosis therapies. IMPORTANCE Mycobacterium tuberculosis (Mtb) infects one-quarter of the global population and causes one of the deadliest infectious diseases worldwide. Macrophages are the first line of defense against Mtb infection and are typically incredibly efficient at destroying intracellular pathogens, but Mtb has evolved to survive and replicate in this harsh environment. Previous work has found that a portion of intracellular Mtb bacilli damage their phagosomes, leaving them vulnerable to detection by the host and delivery to an antibacterial pathway called selective autophagy. Here, we show that in macrophages, galectin-8 recognizes damaged Mtb-containing phagosomes and targets Mtb to selective autophagy; we found that galectin-8, unlike other highly similar and closely related galectins, is required for targeting and controlling Mtb in macrophages. The specific role for galectin-8 appears to stem from its interaction with TAX1BP1, a selective autophagy adapter protein. Interestingly, overexpressing galectin-8 helps macrophages target and control Mtb, highlighting the importance of galectin-8 in the innate immune response to Mtb.

Glycans in autophagy, endocytosis and lysosomal functions

Glycans have been shown to function as versatile molecular signals in cells. This prompted us to look at their roles in endocytosis, endolysosomal system and autophagy. We start by introducing the cell biological aspects of these pathways, the concept of the sugar code, and provide an overview on the role of glycans in the targeting of lysosomal proteins and in lysosomal functions. Moreover, we review evidence on the regulation of endocytosis and autophagy by glycans. Finally, we discuss the emerging concept that cytosolic exposure of luminal glycans, and their detection by endogenous lectins, provides a mechanism for the surveillance of the integrity of the endolysosomal compartments, and serves their eventual repair or disposal.

Galectin-3: A factotum in carcinogenesis bestowing an archery for prevention

Cancer metastasis and therapy resistance are the foremost hurdles in oncology at the moment. This review aims to pinpoint the functional aspects of a unique multifaceted glycosylated molecule in both intracellular and extracellular compartments of a cell namely galectin-3 along with its metastatic potential in different types of cancer. All materials reviewed here were collected through the search engines PubMed, Scopus, and Google scholar. Among the 15 galectins identified, the chimeric gal-3 plays an indispensable role in the differentiation, transformation, and multi-step process of tumor metastasis. It has been implicated in the molecular mechanisms that allow the cancer cells to survive in the intravascular milieu and promote tumor cell extravasation, ultimately leading to metastasis. Gal-3 has also been found to have a pivotal role in immune surveillance and pro-angiogenesis and several studies have pointed out the importance of gal-3 in establishing a resistant phenotype, particularly through the epithelial-mesenchymal transition process. Additionally, some recent findings suggest the use of gal-3 inhibitors in overcoming therapeutic resistance. All these reports suggest that the deregulation of these specific lectins at the cellular level could inhibit cancer progression and metastasis. A more systematic study of glycosylation in clinical samples along with the development of selective gal-3 antagonists inhibiting the activity of these molecules at the cellular level offers an innovative strategy for primary cancer prevention.

Galectin-3 in Inflammasome Activation and Primary Biliary Cholangitis Development

Primary biliary cholangitis (PBC) is a chronic inflammatory autoimmune liver disease characterized by inflammation and damage of small bile ducts. The NLRP3 inflammasome is a multimeric complex of proteins that after activation with various stimuli initiates an inflammatory process. Increasing data obtained from animal studies implicate the role of NLRP3 inflammasome in the pathogenesis of various diseases. Galectin-3 is a β-galactoside-binding lectin that plays important roles in various biological processes including cell proliferation, differentiation, transformation and apoptosis, pre-mRNA splicing, inflammation, fibrosis and host defense. The multilineage immune response at various stages of PBC development includes the involvement of Gal-3 in the pathogenesis of this disease. The role of Galectin-3 in the specific binding to NLRP3, and inflammasome activation in models of primary biliary cholangitis has been recently described. This review provides a brief pathogenesis of PBC and discusses the current knowledge about the role of Gal-3 in NLRP3 activation and PBC development.

Salmonella Interacts With Autophagy to Offense or Defense

Autophagy is an important component of the innate immune system in mammals. Low levels of basic autophagy are sustained in normal cells, to help with the clearance of aging organelles and misfolded proteins, thus maintaining their structural and functional stability. However, when cells are faced with challenges, such as starvation or pathogenic infection, their level of autophagy increases significantly. Salmonella is a facultative intracellular pathogen, which imposes an economic burden on the poultry farming industry and human public health. Previous studies have shown that Salmonella can induce the autophagy of cells following invasion, which to a certain extent helps to protect the cells from bacterial colonization. This review summarizes the latest research in the field of Salmonella-induced autophagy, including: (i) the autophagy induction and escape mechanisms employed by Salmonella during the infection of host cells; (ii) the effect of autophagy on intracellular Salmonella; (iii) the important autophagy adaptors that recognize intracellular Salmonella in host cells; and (iv) the effect of autophagy-modulating drugs on Salmonella infection.

Salmonella Enteritidis Effector AvrA Suppresses Autophagy by Reducing Beclin-1 Protein

Autophagy is a cellular process to clear pathogens. Salmonella enterica serovar Enteritidis (S.E) has emerged as one of the most important food-borne pathogens. However, major studies still focus on Salmonella enterica serovar Typhimurium. Here, we reported that AvrA, a S. Enteritidis effector, inhibited autophagy to promote bacterial survival in the host. We found that AvrA regulates the conversion of LC3 I into LC3 II and the enrichment of lysosomes. Beclin-1, a key molecular regulator of autophagy, was decreased after AvrA expressed strain colonization. In S.E-AvrA--infected cells, we found the increases of protein levels of p-JNK and p-c-Jun and the transcription level of AP-1. AvrA-reduction of Beclin-1 protein expression is through the JNK pathway. The JNK inhibitor abolished the AvrA-reduced Beclin-1 protein expression. Moreover, we identified that the AvrA mutation C186A abolished its regulation of Beclin-1 expression. In addition AvrA protein was found interacted with Beclin-1. In organoids and infected mice, we explored the physiologically related effects and mechanism of AvrA in reducing Beclin-1 through the JNK pathway, thus attenuating autophagic responses. This finding not only indicates an important role of S. Enteritidis effector in reducing host protein as a strategy to suppress autophagy, but also suggests manipulating autophagy as a new strategy to treat infectious diseases.

AMPK Promotes Xenophagy through Priming of Autophagic Kinases upon Detection of Bacterial Outer Membrane Vesicles

The autophagy pathway is an essential facet of the innate immune response, capable of rapidly targeting intracellular bacteria. However, the initial signaling regulating autophagy induction in response to pathogens remains largely unclear. Here, we report that AMPK, an upstream activator of the autophagy pathway, is stimulated upon detection of pathogenic bacteria, before bacterial invasion. Bacterial recognition occurs through the detection of outer membrane vesicles. We found that AMPK signaling relieves mTORC1-mediated repression of the autophagy pathway in response to infection, positioning the cell for a rapid induction of autophagy. Moreover, activation of AMPK and inhibition of mTORC1 in response to bacteria is not accompanied by an induction of bulk autophagy. However, AMPK signaling is required for the selective targeting of bacteria-containing vesicles by the autophagy pathway through the activation of pro-autophagic kinase complexes. These results demonstrate a key role for AMPK signaling in coordinating the rapid autophagic response to bacteria.

A MicroRNA Network Controls Legionella pneumophila Replication in Human Macrophages via LGALS8 and MX1

Cases of Legionella pneumophila pneumonia occur worldwide, with potentially fatal outcome. When causing human disease, Legionella injects a plethora of virulence factors to reprogram macrophages to circumvent immune defense and create a replication niche. By analyzing Legionella-induced changes in miRNA expression and genomewide chromatin modifications in primary human macrophages, we identified a cell-autonomous immune network restricting Legionella growth. This network comprises three miRNAs governing expression of the cytosolic RNA receptor DDX58/RIG-I, the tumor suppressor TP53, the antibacterial effector LGALS8, and MX1, which has been described as an antiviral factor. Our findings for the first time link TP53, LGALS8, DDX58, and MX1 in one miRNA-regulated network and integrate them into a functional node in the defense against L. pneumophila.

Legionella pneumophila is an important cause of pneumonia. It invades alveolar macrophages and manipulates the immune response by interfering with signaling pathways and gene transcription to support its own replication. MicroRNAs (miRNAs) are critical posttranscriptional regulators of gene expression and are involved in defense against bacterial infections. Several pathogens have been shown to exploit the host miRNA machinery to their advantage. We therefore hypothesize that macrophage miRNAs exert positive or negative control over Legionella intracellular replication. We found significant regulation of 85 miRNAs in human macrophages upon L. pneumophila infection. Chromatin immunoprecipitation and sequencing revealed concordant changes of histone acetylation at the putative promoters. Interestingly, a trio of miRNAs (miR-125b, miR-221, and miR-579) was found to significantly affect intracellular L. pneumophila replication in a cooperative manner. Using proteome-analysis, we pinpointed this effect to a concerted downregulation of galectin-8 (LGALS8), DExD/H-box helicase 58 (DDX58), tumor protein P53 (TP53), and then MX dynamin-like GTPase 1 (MX1) by the three miRNAs. In summary, our results demonstrate a new miRNA-controlled immune network restricting Legionella replication in human macrophages.

Galectin-8 in the onset of the immune response and inflammation

Galectins (Gals), a family of mammalian lectins, have emerged as key regulators of the immune response, being implicated in several physiologic and pathologic conditions. Lately, there is increasing data regarding the participation of Galectin-8 (Gal-8) in both the adaptive and innate immune responses, as well as its high expression in inflammatory disorders. Here, we focus on the pro- and anti-inflammatory properties of Gal-8 and discuss the potential use of this lectin in order to shape the immune response, according to the context.

Galectin 13 (PP13) Facilitates Remodeling and Structural Stabilization of Maternal Vessels during Pregnancy

Galectins regulate cell growth, proliferation, differentiation, apoptosis, signal transduction, mRNA splicing, and interactions with the extracellular matrix. Here we focus on the galectins in the reproductive system, particularly on a group of six galectins that first appears in anthropoid primates in conjunction with the evolution of highly invasive placentation and long gestation. Of these six, placental protein 13 (PP13, galectin 13) interacts with glycoproteins and glycolipids to enable successful pregnancy. PP13 is related to the development of a major obstetric syndrome, preeclampsia, a life-threatening complication of pregnancy which affects ten million pregnant women globally. Preeclampsia is characterized by hypertension, proteinuria, and organ failure, and is often accompanied by fetal loss and major newborn disabilities. PP13 facilitates the expansion of uterine arteries and veins during pregnancy in an endothelial cell-dependent manner, via the eNOS and prostaglandin signaling pathways. PP13 acts through its carbohydrate recognition domain that binds to sugar residues of extracellular and connective tissue molecules, thus inducing structural stabilization of vessel expansion. Further, decidual PP13 aggregates may serve as a decoy that induces white blood cell apoptosis, contributing to the mother's immune tolerance to pregnancy. Lower first trimester PP13 level is one of the biomarkers to predict the subsequent risk to develop preeclampsia, while its molecular mutations/polymorphisms that are associated with reduced PP13 expression are accompanied by higher rates of preeclampsia We propose a targeted PP13 replenishing therapy to fight preeclampsia in carriers of these mutations.

Gal8 Visualization of Endosome Disruption Predicts Carrier-Mediated Biologic Drug Intracellular Bioavailability

Endolysosome entrapment is one of the key barriers to the therapeutic use of biologic drugs that act intracellularly. The screening of prospective nanoscale endosome-disrupting delivery technologies is currently limited by methods that are indirect and cumbersome. Here, we statistically validate Galectin 8 (Gal8) intracellular tracking as a superior approach that is direct, quantitative, and predictive of therapeutic cargo intracellular bioactivity through in vitro high-throughput screening and in vivo validation. Gal8 is a cytosolically dispersed protein that, when endosomes are disrupted, redistributes by binding to glycosylation moieties selectively located on the inner face of endosomal membranes. The quantitative redistribution of a Gal8 fluorescent fusion protein from the cytosol into endosomes is demonstrated as a real-time, live-cell assessment of endosomal integrity that does not require labeling or modification of either the carrier or the biologic drug and that allows quantitative distinction between closely related, endosome-disruptive drug carriers. Through screening two families of siRNA polymeric carrier compositions at varying dosages, we show that Gal8 endosomal recruitment correlates strongly ( r = 0.95 and p < 10-4) with intracellular siRNA bioactivity. Through this screen, we gathered insights into how composition and molecular weight affect endosome disruption activity of poly[(ethylene glycol)- b-[(2-(dimethylamino)ethyl methacrylate)- co-(butyl methacrylate)]] [PEG-(DMAEMA- co-BMA)] siRNA delivery systems. Additional studies showed that Gal8 recruitment predicts intracellular bioactivity better than current standard methods such as Lysotracker colocalization ( r = 0.35, not significant), pH-dependent hemolysis (not significant), or cellular uptake ( r = 0.73 and p < 10-3). Importantly, the Gal8 recruitment method is also amenable to fully objective high-throughput screening using automated image acquisition and quantitative image analysis, with a robust estimated Z' of 0.6 (whereas assays with Z' > 0 have high-throughput screening utility). Finally, we also provide measurements of in vivo endosomal disruption based on Gal8 visualization ( p < 0.03) of a nanocarrier formulation confirmed to produce significant cytosolic delivery and bioactivity of siRNA within tumors ( p < 0.02). In sum, this report establishes the utility of Gal8 subcellular tracking for the rapid optimization and high-throughput screening of the endosome disruption potency of intracellular delivery technologies.

Sugar-Recognizing Ubiquitin Ligases: Action Mechanisms and Physiology

F-box proteins, the substrate recognition subunits of SKP1–CUL1–F-box protein (SCF) E3 ubiquitin ligase complexes, play crucial roles in various cellular events mediated by ubiquitination. Several sugar-recognizing F-box proteins exist in both mammalian and plant cells. Although glycoproteins generally reside outside of cells, or in organelles of the secretory pathway, these lectin-type F-box proteins reside in the nucleocytoplasmic compartment. Mammalian sugar-recognizing F-box proteins commonly bind to the innermost position of N-glycans through a unique small hydrophobic pocket in their loops. Two cytosolic F-box proteins, Fbs1 and Fbs2, recognize high-mannose glycans synthesized in the ER, and SCF^Fbs1 and SCF^Fbs2 ubiquitinate excess unassembled or misfolded glycoproteins in the ERAD pathway by recognizing the innermost glycans, which serve as signals for aberrant proteins. On the other hand, endomembrane-bound Fbs3 recognizes complex glycans as well as high-mannose glycans, and SCF^Fbs3 ubiquitinates exposed glycoproteins in damaged lysosomes fated for elimination by selective autophagy. Plants express stress-inducible lectin-type F-box proteins recognizing a wider range of N- and O-glycans, suggesting that the roles of mammalian and plant lectin-type F-box proteins have diverged over the course of evolution to recognize species-specific targets with distinct functions. These sugar-recognizing F-box proteins interpret glycans in the cytosol as markers of unwanted proteins and organelles, and degrade them via the proteasome or autophagy.

Extracellular and intracellular small-molecule galectin-3 inhibitors

Galectin-3 is a carbohydrate binding protein which has important roles in cancer and immunity. Potent galectin-3 inhibitors have been synthesized, for experimental purposes and potential clinical use. As galectin-3 is implicated in both intra- and extracellular activities, permeability of galectin-3 inhibitors is an important parameter determining biological effects. We compared the cellular uptake of galectin-3 inhibitors and their potency in the intracellular or extracellular space. The inhibitors differed in their polar surface area (PSA), but had similar affinities for galectin-3. Using a well-established permeability assay, we confirmed that the uptake was significantly higher for the inhibitor with the lowest PSA, as expected. To analyze intracellular activity of the inhibitors, we developed a novel assay based on galectin-3 accumulation around damaged intracellular vesicles. The results show striking differences between the inhibitors intracellular potency, correlating with their PSAs. To test extracellular activity of the inhibitors, we analyzed their potency to block binding of galectin-3 to cell surfaces. All inhibitors were equally able to block galectin-3 binding to cells and this was proportional to their affinity for galectin-3. These inhibitors may serve as useful tools in exploring biological roles of galectin-3 and may further our understanding of intracellular versus extracellular roles of galectin-3.

Methods to Detect Loss of Lysosomal Membrane Integrity

Loss of lysosomal membrane integrity, often referred to as lysosomal membrane permeabilization (LMP), occurs in many instances of cell death either as an initiating or as an amplifying event. Currently, the best method for detecting LMP is the galectin puncta formation assay which can be used for a broad range of sample types, both fixed and live, is easy to perform, and highly sensitive. This method, which is similar to the widely used LC3 puncta formation assay for autophagy, is based on the translocation of galectins to damaged lysosomes resulting in a change from uniform to punctate staining pattern. Here, we provide protocols for the galectin puncta formation assay in fixed and live cells and for an alternative assay based on fluorescent dextran release from damaged lysosomes, which can be performed in parallel.

Mechanistic insights into the interactions of NAP1 with the SKICH domains of NDP52 and TAX1BP1

NDP52 and TAX1BP1, two SKIP carboxyl homology (SKICH) domain-containing autophagy receptors, play crucial roles in selective autophagy. The autophagic functions of NDP52 and TAX1BP1 are regulated by TANK-binding kinase 1 (TBK1), which may associate with them through the adaptor NAP1. However, the molecular mechanism governing the interactions of NAP1 with NDP52 and TAX1BP1, as well as the effects induced by TBK1-mediated phosphorylation of NDP52 and TAX1BP1, remains elusive. Here, we report the atomic structures of the SKICH regions of NDP52 and TAX1BP1 in complex with NAP1, which not only uncover the mechanistic bases underpinning the specific interactions of NAP1 with the SKICH domains of NDP52 and TAX1BP1 but also reveal the binding mode of a SKICH domain. Moreover, we uncovered that the SKICH domains of NDP52 and TAX1BP1 share a general binding mode to interact with NAP1. Finally, we also evaluated the currently known TBK1-mediated phosphorylation sites in the SKICH domains of NDP52 and TAX1BP1 on the basis of their interactions with NAP1. In all, our findings provide mechanistic insights into the interactions of NAP1 with NDP52 and TAX1BP1, and are valuable for further understanding the functions of these proteins in selective autophagy.

Role of Optineurin in the Mitochondrial Dysfunction: Potential Implications in Neurodegenerative Diseases and Cancer

Optineurin (Optn) is a 577 aa protein encoded by the Optn gene. Mutations of Optn are associated with normal tension glaucoma and amyotrophic lateral sclerosis, and its gene has also been linked to the development of Paget’s disease of bone and Crohn’s disease. Optn is involved in diverse cellular functions, including NF-κB regulation, membrane trafficking, exocytosis, vesicle transport, reorganization of actin and microtubules, cell cycle control, and autophagy. Besides its role in xenophagy and autophagy of aggregates, Optn has been identified as a primary autophagy receptor, among the five adaptors that translocate to mitochondria during mitophagy. Mitophagy is a selective macroautophagy process during which irreparable mitochondria are degraded, preventing accumulation of defective mitochondria and limiting the release of reactive oxygen species and proapoptotic factors. Mitochondrial quality control via mitophagy is central to the health of cells. One of the important surveillance pathways of mitochondrial health is the recently defined signal transduction pathway involving the mitochondrial PTEN-induced putative kinase 1 (PINK1) protein and the cytosolic RING-between-RING ubiquitin ligase Parkin. Both of these proteins, when mutated, have been identified in certain forms of Parkinson’s disease. By targeting ubiquitinated mitochondria to autophagosomes through its association with autophagy related proteins, Optn is responsible for a critical step in mitophagy. This review reports recent discoveries on the role of Optn in mitophagy and provides insight into its link with neurodegenerative diseases. We will also discuss the involvement of Optn in other pathologies in which mitophagy dysfunctions are involved including cancer.

A Structural View of Xenophagy, a Battle between Host and Microbes

The cytoplasm in mammalian cells is a battlefield between the host and invading microbes. Both the living organisms have evolved unique strategies for their survival. The host utilizes a specialized autophagy system, xenophagy, for the clearance of invading pathogens, whereas bacteria secrete proteins to defend and escape from the host xenophagy. Several molecules have been identified and their structural investigation has enabled the comprehension of these mechanisms at the molecular level. In this review, we focus on one example of host autophagy and the other of bacterial defense: the autophagy receptor, NDP52, in conjunction with the sugar receptor, galectin-8, plays a critical role in targeting the autophagy machinery against Salmonella; and the cysteine protease, RavZ secreted by Legionella pneumophila cleaves the LC3-PE on the phagophore membrane. The structure-function relationships of these two examples and the directions of future research will be discussed.

Galectin-13, a different prototype galectin, does not bind β-galacto-sides and forms dimers via intermolecular disulfide bridges between Cys-136 and Cys-138

During pregnancy, placental protein-13 (galectin-13) is highly expressed in the placenta and fetal tissue, and less so in maternal serum that is related to pre-eclampsia. To understand galectin-13 function at the molecular level, we solved its crystal structure and discovered that its dimer is stabilized by two disulfide bridges between Cys136 and Cys138 and six hydrogen bonds involving Val135, Val137, and Gln139. Native PAGE and gel filtration demonstrate that this is not a crystallization artifact because dimers also form in solution. Our biochemical studies indicate that galectin-13 ligand binding specificity is different from that of other galectins in that it does not bind β-galactosides. This is partly explained by the presence of Arg53 rather than His53 at the bottom of the carbohydrate binding site in a position that is crucial for interactions with β-galactosides. Mutating Arg53 to histidine does not re-establish normal β-galactoside binding, but rather traps cryoprotectant glycerol molecules within the ligand binding site in crystals of the R53H mutant. Moreover, unlike most other galectins, we also found that GFP-tagged galectin-13 is localized within the nucleus of HeLa and 293 T cells. Overall, galectin-13 appears to be a new type of prototype galectin with distinct properties.

Host-pathogen interactions and subversion of autophagy

Macroautophagy (‘autophagy’), is the process by which cells can form a double-membraned vesicle that encapsulates material to be degraded by the lysosome. This can include complex structures such as damaged mitochondria, peroxisomes, protein aggregates and large swathes of cytoplasm that can not be processed efficiently by other means of degradation. Recycling of amino acids and lipids through autophagy allows the cell to form intracellular pools that aid survival during periods of stress, including growth factor deprivation, amino acid starvation or a depleted oxygen supply. One of the major functions of autophagy that has emerged over the last decade is its importance as a safeguard against infection. The ability of autophagy to selectively target intracellular pathogens for destruction is now regarded as a key aspect of the innate immune response. However, pathogens have evolved mechanisms to either evade or reconfigure the autophagy pathway for their own survival. Understanding how pathogens interact with and manipulate the host autophagy pathway will hopefully provide a basis for combating infection and increase our understanding of the role and regulation of autophagy. Herein, we will discuss how the host cell can identify and target invading pathogens and how pathogens have adapted in order to evade destruction by the host cell. In particular, we will focus on interactions between the mammalian autophagy gene 8 (ATG8) proteins and the host and pathogen effector proteins.

Regulation of Salmonella-host cell interactions via the ubiquitin system

Salmonella infections cause acute intestinal inflammatory responses through the action of bacterial effector proteins secreted into the host cytosol. These proteins promote Salmonella survival, amongst others, by deregulating the host innate immune system and interfering with host cell ubiquitylation signaling. This review describes the recent findings of dynamic changes of the host ubiquitinome during pathogen infection, how bacterial effector proteins modulate the host ubiquitin system and how the host innate immune system counteracts Salmonella invasion by using these pathogens as signaling platforms to initiate immune responses.

Molecular definitions of autophagy and related processes

Over the past two decades, the molecular machinery that underlies autophagic responses has been characterized with ever increasing precision in multiple model organisms. Moreover, it has become clear that autophagy and autophagy-related processes have profound implications for human pathophysiology. However, considerable confusion persists about the use of appropriate terms to indicate specific types of autophagy and some components of the autophagy machinery, which may have detrimental effects on the expansion of the field. Driven by the overt recognition of such a potential obstacle, a panel of leading experts in the field attempts here to define several autophagy-related terms based on specific biochemical features. The ultimate objective of this collaborative exchange is to formulate recommendations that facilitate the dissemination of knowledge within and outside the field of autophagy research.

LUBAC-synthesized linear ubiquitin chains restrict cytosol-invading bacteria by activating autophagy and NF-κB

Cell-autonomous immunity relies on the ubiquitin coat surrounding cytosol-invading bacteria functioning as an 'eat-me' signal for xenophagy. The origin, composition and precise mode of action of the ubiquitin coat remain incompletely understood. Here, by studying Salmonella Typhimurium, we show that the E3 ligase LUBAC generates linear (M1-linked) polyubiquitin patches in the ubiquitin coat, which serve as antibacterial and pro-inflammatory signalling platforms. LUBAC is recruited via its subunit HOIP to bacterial surfaces that are no longer shielded by host membranes and are already displaying ubiquitin, suggesting that LUBAC amplifies and refashions the ubiquitin coat. LUBAC-synthesized polyubiquitin recruits Optineurin and Nemo for xenophagy and local activation of NF-κB, respectively, which independently restrict bacterial proliferation. In contrast, the professional cytosol-dwelling Shigella flexneri escapes from LUBAC-mediated restriction through the antagonizing effects of the effector E3 ligase IpaH1.4 on deposition of M1-linked polyubiquitin and subsequent recruitment of Nemo and Optineurin. We conclude that LUBAC-synthesized M1-linked ubiquitin transforms bacterial surfaces into signalling platforms for antibacterial immunity reminiscent of antiviral assemblies on mitochondria.

Structure-based rationale for differential recognition of lacto- and neolacto- series glycosphingolipids by the N-terminal domain of human galectin-8

Glycosphingolipids are ubiquitous cell surface molecules undertaking fundamental cellular processes. Lacto-N-tetraose (LNT) and lacto-N-neotetraose (LNnT) are the representative core structures for lacto- and neolacto-series glycosphingolipids. These glycolipids are the carriers to the blood group antigen and human natural killer antigens mainly found on blood cells, and are also principal components in human milk, contributing to infant health. The β-galactoside recognising galectins mediate various cellular functions of these glycosphingolipids. We report crystallographic structures of the galectin-8 N-terminal domain (galectin-8N) in complex with LNT and LNnT. We reveal the first example in which the non-reducing end of LNT binds to the primary binding site of a galectin, and provide a structure-based rationale for the significant ten-fold difference in binding affinities of galectin-8N toward LNT compared to LNnT, such a magnitude of difference not being observed for any other galectin. In addition, the LNnT complex showed that the unique Arg59 has ability to adopt a new orientation, and comparison of glycerol- and lactose-bound galectin-8N structures reveals a minimum atomic framework for ligand recognition. Overall, these results enhance our understanding of glycosphingolipids interactions with galectin-8N, and highlight a structure-based rationale for its significantly different affinity for components of biologically relevant glycosphingolipids.

In Silico Knockout Studies of Xenophagic Capturing of Salmonella

The degradation of cytosol-invading pathogens by autophagy, a process known as xenophagy, is an important mechanism of the innate immune system. Inside the host, Salmonella Typhimurium invades epithelial cells and resides within a specialized intracellular compartment, the Salmonella-containing vacuole. A fraction of these bacteria does not persist inside the vacuole and enters the host cytosol. Salmonella Typhimurium that invades the host cytosol becomes a target of the autophagy machinery for degradation. The xenophagy pathway has recently been discovered, and the exact molecular processes are not entirely characterized. Complete kinetic data for each molecular process is not available, so far. We developed a mathematical model of the xenophagy pathway to investigate this key defense mechanism. In this paper, we present a Petri net model of Salmonella xenophagy in epithelial cells. The model is based on functional information derived from literature data. It comprises the molecular mechanism of galectin-8-dependent and ubiquitin-dependent autophagy, including regulatory processes, like nutrient-dependent regulation of autophagy and TBK1-dependent activation of the autophagy receptor, OPTN. To model the activation of TBK1, we proposed a new mechanism of TBK1 activation, suggesting a spatial and temporal regulation of this process. Using standard Petri net analysis techniques, we found basic functional modules, which describe different pathways of the autophagic capture of Salmonella and reflect the basic dynamics of the system. To verify the model, we performed in silico knockout experiments. We introduced a new concept of knockout analysis to systematically compute and visualize the results, using an in silico knockout matrix. The results of the in silico knockout analyses were consistent with published experimental results and provide a basis for future investigations of the Salmonella xenophagy pathway.

Structure biology of selective autophagy receptors

Autophagy is a process tightly regulated by various autophagy-related proteins. It is generally classified into non-selective and selective autophagy. Whereas non-selective autophagy is triggered when the cell is under starvation, selective autophagy is involved in eliminating dysfunctional organelles, misfolded and/or ubiquitylated proteins, and intracellular pathogens. These components are recognized by autophagy receptors and delivered to phagophores. Several selective autophagy receptors have been identified and characterized. They usually have some common domains, such as LC3-interacting- region (LIR) motif, a specific cargo interacting (ubiquitin-dependent or ubiquitin-independent) domain. Recently, structural data of these autophagy receptors has been described, which provides an insight of their function in the selective autophagic process. In this review, we summarize the most up-to-date findings about the structure-function of autophagy receptors that regulates selective autophagy.

Molecular basis of ubiquitin recognition by the autophagy receptor CALCOCO2

The autophagy receptor CALCOCO2/NDP52 functions as a bridging adaptor and plays an essential role in the selective autophagic degradation of invading pathogens by specifically recognizing ubiquitin-coated intracellular pathogens and subsequently targeting them to the autophagic machinery; thereby it is required for innate immune defense against a range of infectious pathogens in mammals. However, the mechanistic basis underlying CALCOCO2-mediated specific recognition of ubiqutinated pathogens is still unknown. Here, using biochemical and structural analyses, we demonstrated that the cargo-binding region of CALCOCO2 contains a dynamic unconventional zinc finger as well as a C2H2-type zinc-finger, and only the C2H2-type zinc finger specifically recognizes mono-ubiquitin or poly-ubiquitin chains. In addition to elucidating the specific ubiquitin recognition mechanism of CALCOCO2, the structure of the CALCOCO2 C2H2-type zinc finger in complex with mono-ubiquitin also uncovers a unique zinc finger-binding mode for ubiquitin. Our findings provide mechanistic insight into how CALCOCO2 targets ubiquitin-decorated pathogens for autophagic degradations.

Critical role of bacterial isochorismatase in the autophagic process induced by Acinetobacter baumannii in mammalian cells

A recent study reported that Acinetobacter baumannii could induce autophagy, but the recognition and clearance mechanism of intracytosolic A. baumannii in the autophagic process and the molecular mechanism of autophagy induced by the pathogen remains unknown. In this study, we first demonstrated that invading A. baumannii induced a complete, ubiquitin-mediated autophagic response that is dependent upon septins SEPT2 and SEPT9 in mammalian cells. We also demonstrated that autophagy induced by A. baumannii was Beclin-1 dependent via the AMPK/ERK/mammalian target of rapamycin pathway. Of interest, we found that the isochorismatase mutant strain had significantly decreased siderophore-mediated ferric iron acquisition ability and had a reduced the ability to induce autophagy. We verified that isochorismatase was required for the recognition of intracytosolic A. baumannii mediated by septin cages, ubiquitinated proteins, and ubiquitin-binding adaptor proteins p62 and NDP52 in autophagic response. We also confirmed that isochorismatase was required for the clearance of invading A. baumannii by autophagy in vitro and in the mouse model of infection. Together, these findings provide insight into the distinctive recognition and clearance of intracytosolic A. baumannii by autophagy in host cells, and that isochorismatase plays a critical role in the A. baumannii–induced autophagic process.—Wang, Y., Zhang, K., Shi, X., Wang, C., Wang, F., Fan, J., Shen, F., Xu, J., Bao, W., Liu, M., Yu, L. Critical role of bacterial isochorismatase in the autophagic process induced by Acinetobacter baumannii in mammalian cells.

Recruitment of TBK1 to cytosol-invading Salmonella induces WIPI2-dependent antibacterial autophagy

Mammalian cells deploy autophagy to defend their cytosol against bacterial invaders. Anti‐bacterial autophagy relies on the core autophagy machinery, cargo receptors, and “eat‐me” signals such as galectin‐8 and ubiquitin that label bacteria as autophagy cargo. Anti‐bacterial autophagy also requires the kinase TBK1, whose role in autophagy has remained enigmatic. Here we show that recruitment of WIPI2, itself essential for anti‐bacterial autophagy, is dependent on the localization of catalytically active TBK1 to the vicinity of cytosolic bacteria. Experimental manipulation of TBK1 recruitment revealed that engagement of TBK1 with any of a variety of Salmonella‐associated “eat‐me” signals, including host‐derived glycans and K48‐ and K63‐linked ubiquitin chains, suffices to restrict bacterial proliferation. Promiscuity in recruiting TBK1 via independent signals may buffer TBK1 functionality from potential bacterial antagonism and thus be of evolutionary advantage to the host.

Molecular Functions of Glycoconjugates in Autophagy

Glycoconjugates, glycans, carbohydrates, and sugars: these terms encompass a class of biomolecules that are diverse in both form and function ranging from free oligosaccharides, glycoproteins, and proteoglycans, to glycolipids that make up a complex glycan code that impacts normal physiology and disease. Recent data suggest that one mechanism by which glycoconjugates impact physiology is through the regulation of the process of autophagy. Autophagy is a degradative pathway necessary for differentiation, organism development, and the maintenance of cell and tissue homeostasis. In this review, we will highlight what is known about the regulation of autophagy by glycoconjugates focusing on signaling mechanisms from the extracellular surface and the regulatory roles of intracellular glycans. Glycan signaling from the extracellular matrix converges on "master" regulators of autophagy including AMPK and mTORC1, thus impacting their localization, activity, and/or expression. Within the intracellular milieu, gangliosides are constituents of the autophagosome membrane, a subset of proteins composing the autophagic machinery are regulated by glycosylation, and oligosaccharide exposure in the cytosol triggers an autophagic response. The examples discussed provide some mechanistic insights into glycan regulation of autophagy and reveal areas for future investigation.

Myosins, Actin and Autophagy

Myosin motor proteins working together with the actin cytoskeleton drive a wide range of cellular processes. In this review, we focus on their roles in autophagy – the pathway the cell uses to ensure homeostasis by targeting pathogens, misfolded proteins and damaged organelles for degradation. The actin cytoskeleton regulated by a host of nucleating, anchoring and stabilizing proteins provides the filament network for the delivery of essential membrane vesicles from different cellular compartments to the autophagosome. Actin networks have also been implicated in structurally supporting the expanding phagophore, moving autophagosomes and enabling efficient fusion with the lysosome. Only a few myosins have so far been shown to play a role in autophagy. Non‐muscle myosin IIA functions in the early stages delivering membrane for the initial formation of the autophagosome, whereas myosin IC and myosin VI are involved in the final stages providing specific membranes for autophagosome maturation and its fusion with the lysosome.

Low or No Inhibitory Potency of the Canonical Galectin Carbohydrate-binding Site by Pectins and Galactomannans

Some complex plant-derived polysaccharides, such as modified citrus pectins and galactomannans, have been shown to have promising anti-inflammatory and anti-cancer effects. Most reports propose or claim that these effects are due to interaction of the polysaccharides with galectins because the polysaccharides contain galactose-containing side chains that might bind this class of lectin. However, their direct binding to and/or inhibition of the evolutionarily conserved galactoside-binding site of galectins has not been demonstrated. Using a well established fluorescence anisotropy assay, we tested the direct interaction of several such polysaccharides with physiological concentrations of a panel of galectins. The bioactive pectic samples tested were very poor inhibitors of the canonical galactoside-binding site for the tested galectins, with IC₅₀ values >10 mg/ml for a few or in most cases no inhibitory activity at all. The galactomannan Davanat® was more active, albeit not a strong inhibitor (IC₅₀ values ranging from 3 to 20 mg/ml depending on the galectin). Pure synthetic oligosaccharide fragments found in the side chains and backbone of pectins and galactomannans were additionally tested. The most commonly found galactan configuration in pectins had no inhibition of the galectins tested. Galactosylated tri- and pentamannosides, representing the structure of Davanat®, had an inhibitory effect of galectins comparable with that of free galactose. Further evaluation using cell-based assays, indirectly linked to galectin-3 inhibition, showed no inhibition of galectin-3 by the polysaccharides. These data suggest that the physiological effects of these plant polysaccharides are not due to inhibition of the canonical galectin carbohydrate-binding site.

Structural characterisation of human galectin-4 N-terminal carbohydrate recognition domain in complex with glycerol, lactose, 3'-sulfo-lactose, and 2'-fucosyllactose

Galectin-4 is a tandem-repeat galectin with two distinct carbohydrate recognition domains (CRD). Galectin-4 is expressed mainly in the alimentary tract and is proposed to function as a lipid raft and adherens junction stabilizer by its glycan cross-linking capacity. Galectin-4 plays divergent roles in cancer and inflammatory conditions, either promoting or inhibiting each disease progression, depending on the specific pathological condition. The study of galectin-4's ligand-binding profile may help decipher its roles under specific conditions. Here we present the X-ray structures of human galectin-4 N-terminal CRD (galectin-4N) bound to different saccharide ligands. Galectin-4's overall fold and its core interactions to lactose are similar to other galectin CRDs. Galectin-4N recognises the sulfate cap of 3'-sulfated glycans by a weak interaction through Arg45 and two water-mediated hydrogen bonds via Trp84 and Asn49. When galectin-4N interacts with the H-antigen mimic, 2'-fucosyllactose, an interaction is formed between the ring oxygen of fucose and Arg45. The extended binding site of galectin-4N may not be well suited to the A/B-antigen determinants, α-GalNAc/α-Gal, specifically due to clashes with residue Phe47. Overall, galectin-4N favours sulfated glycans whilst galectin-4C prefers blood group determinants. However, the two CRDs of galectin-4 can, to a less extent, recognise each other's ligands.

Dining in: intracellular bacterial pathogen interplay with autophagy

Intracellular bacterial pathogens have evolved many ways to manipulate host cells for successful infection. Many of these pathogens use specialized secretion systems to inject bacterial proteins into the host cytosol that manipulate cellular processes to favor infection. Autophagy is a eukaryotic cellular remodeling process with a critical role in many diseases, including bacterial clearance. A growing field of research highlights mechanisms used by intracellular bacteria to manipulate autophagy as a pro-survival strategy. This review focuses on a select group of bacterial pathogens with diverse intracellular lifestyles that exploit autophagy-derived nutrients and membrane for survival. This group of pathogens uses secretion systems and specific effectors to subvert distinct components of autophagy. By understanding how intracellular pathogens manipulate autophagy, we gain insight not only into bacterial pathogenesis but also host cell signaling and autophagolysosome maturation.

NMR assignments of the C-terminal domain of human galectin-8

Galectins recognize β-galectosides to promote a variety of cellular functions. Despite their sequence variations, all galectins share the same carbohydrate recognition domains (CRD) and their modes of ligand recognition at a structural level are essentially identical. Human galectin 8 plays an important role in numerous cancer and immune responses. It consists of two CRDs that are connected via a flexible linker. The substrate affinities and specificities of the N- and C-terminal domains are quite different. In order to investigate the structural basis of their substrate specificities, we complete the NMR (1)H, (13)C, and (15)N chemical shift assignments of C-terminal domain of human galectin-8 (hG8C).

Sugar recognition and protein-protein interaction of mammalian lectins conferring diverse functions

Recent advances in structural analyses of mammalian lectins reveal atomic-level details of their fine specificities toward diverse endogenous and exogenous glycans. Local variations on a common scaffold can enable certain lectins to recognize complex carbohydrate ligands including branched glycans and O-glycosylated peptides. Simultaneous recognition of both glycan and the aglycon moieties enhances the affinity and specificity of lectins such as CLEC-2 and PILRα. Attention has been paid to the roles of galectin and RegIII family of proteins in protein-protein interactions involved in critical biological functions including signal transduction and bactericidal pore formation.

Self and nonself: how autophagy targets mitochondria and bacteria

Autophagy is an evolutionarily conserved pathway that transports cytoplasmic components for degradation into lysosomes. Selective autophagy can capture physically large objects, including cell-invading pathogens and damaged or superfluous organelles. Selectivity is achieved by cargo receptors that detect substrate-associated "eat-me" signals. In this Review, we discuss basic principles of selective autophagy and compare the "eat-me" signals and cargo receptors that mediate autophagy of bacteria and bacteria-derived endosymbionts-i.e., mitochondria.

Immunologic manifestations of autophagy

The broad immunologic roles of autophagy span innate and adaptive immunity and are often manifested in inflammatory diseases. The immune effects of autophagy partially overlap with its roles in metabolism and cytoplasmic quality control but typically expand further afield to encompass unique immunologic adaptations. One of the best-appreciated manifestations of autophagy is protection against microbial invasion, but this is by no means limited to direct elimination of intracellular pathogens and includes a stratified array of nearly all principal immunologic processes. This Review summarizes the broad immunologic roles of autophagy. Furthermore, it uses the autophagic control of Mycobacterium tuberculosis as a paradigm to illustrate the breadth and complexity of the immune effects of autophagy.