The annexin 2/S100A10 complex controls the distribution of transferrin receptor-containing recycling endosomes

EndoMAP.v1, a Structural Protein Complex Landscape of Human Endosomes

Early/sorting endosomes are dynamic organelles that play key roles in proteome control by triaging plasma membrane proteins for either recycling or degradation in the lysosome1,2,3. These events are coordinated by numerous transiently-associated regulatory complexes and integral membrane components that contribute to organelle identity during endosome maturation4. While a subset of the several hundred protein components and cargoes known to associate with endosomes have been studied at the biochemical and/or structural level, interaction partners and higher order molecular assemblies for many endosomal components remain unknown. Here, we combine cross-linking and native gel mass spectrometry5-8 of purified early endosomes with AlphaFold9,10 and computational analysis to create a systematic human endosomal structural interactome. We present dozens of structural models for endosomal protein pairs and higher order assemblies supported by experimental cross-links from their native subcellular context, suggesting structural mechanisms for previously reported regulatory processes. Using induced neurons, we validate two candidate complexes whose interactions are supported by crosslinks and structural predictions: TMEM230 as a subunit of ATP8/11 lipid flippases11 and TMEM9/9B as subunits of CLCN3/4/5 chloride-proton antiporters12. This resource and its accompanying structural network viewer provide an experimental framework for understanding organellar structural interactomes and large-scale validation of structural predictions.

Analysis of gene expression in microglial apoptotic cell clearance following spinal cord injury based on machine learning algorithms

Spinal cord injury (SCI) is a severe neurological complication following spinal fracture, which has long posed a challenge for clinicians. Microglia play a dual role in the pathophysiological process after SCI, both beneficial and detrimental. The underlying mechanisms of microglial actions following SCI require further exploration. The present study combined three different machine learning algorithms, namely weighted gene co-expression network analysis, random forest analysis and least absolute shrinkage and selection operator analysis, to screen for differentially expressed genes in the GSE96055 microglia dataset after SCI. It then used protein-protein interaction networks and gene set enrichment analysis with single genes to investigate the key genes and signaling pathways involved in microglial function following SCI. The results indicated that microglia not only participate in neuroinflammation but also serve a significant role in the clearance mechanism of apoptotic cells following SCI. Notably, bioinformatics analysis and lipopolysaccharide + UNC569 (a MerTK-specific inhibitor) stimulation of BV2 cell experiments showed that the expression levels of Anxa2, Myo1e and Spp1 in microglia were significantly upregulated following SCI, thus potentially involved in regulating the clearance mechanism of apoptotic cells. The present study suggested that Anxa2, Myo1e and Spp1 may serve as potential targets for the future treatment of SCI and provided a theoretical basis for the development of new methods and drugs for treating SCI.

Cortical miR-709 links glutamatergic signaling to NREM sleep EEG slow waves in an activity-dependent manner

MicroRNAs (miRNAs) are key post-transcriptional regulators of gene expression that have been implicated in a plethora of neuronal processes. Nevertheless, their role in regulating brain activity in the context of sleep has so far received little attention. To test their involvement, we deleted mature miRNAs in post-mitotic neurons at two developmental ages, i.e., in early adulthood using conditional Dicer knockout (cKO) mice and in adult mice using an inducible conditional Dicer cKO (icKO) line. In both models, electroencephalographic (EEG) activity was affected and the response to sleep deprivation (SD) altered; while the rapid-eye-movement sleep (REMS) rebound was compromised in both, the increase in EEG delta (1 to 4 Hz) power during non-REMS (NREMS) was smaller in cKO mice and larger in icKO mice compared to controls. We subsequently investigated the effects of SD on the forebrain miRNA transcriptome and found that the expression of 48 miRNAs was affected, and in particular that of the activity-dependent miR-709. In vivo inhibition of miR-709 in the brain increased EEG power during NREMS in the slow-delta (0.75 to 1.75 Hz) range, particularly after periods of prolonged wakefulness. Transcriptome analysis of primary cortical neurons in vitro revealed that miR-709 regulates genes involved in glutamatergic neurotransmission. A subset of these genes was also affected in the cortices of sleep-deprived, miR-709-inhibited mice. Our data implicate miRNAs in the regulation of EEG activity and indicate that miR-709 links neuronal activity during wakefulness to brain synchrony during sleep through the regulation of glutamatergic signaling.

Recent Advances in Molecular and Cellular Functions of S100A10

S100A10 (p11, annexin II light chain, calpactin light chain) is a multifunctional protein with a wide range of physiological activity. S100A10 is unique among the S100 family members of proteins since it does not bind to Ca2+, despite its sequence and structural similarity. This review focuses on studies highlighting the structure, regulation, and binding partners of S100A10. The binding partners of S100A10 were collated and summarized.

Fungal mechanisms of intracellular survival: what can we learn from bacterial pathogens?

Fungal infections represent a major, albeit neglected, public health threat with serious medical and economic burdens globally. With unacceptably high mortality rates, invasive fungal pathogens are responsible for millions of deaths each year, with a steadily increasing incidence primarily in immunocompromised individuals. The poor therapeutic options and rise of antifungal drug resistance pose further challenges in controlling these infections. These fungal pathogens have adapted to survive within mammalian hosts and can establish intracellular niches to promote survival within host immune cells. To do that, they have developed diverse methods to circumvent the innate immune system attack. This includes strategies such as altering their morphology, counteracting macrophage antimicrobial action, and metabolic adaptation. This is reminiscent of how bacterial pathogens have adapted to survive within host cells and cause disease. However, relative to the great deal of information available concerning intracellular bacterial pathogenesis, less is known about the mechanisms fungal pathogens employ. Therefore, here we review our current knowledge and recent advances in our understanding of how fungi can evade and persist within host immune cells. This review will focus on the major fungal pathogens, including Cryptococcus neoformans, Candida albicans, and Aspergillus fumigatus, among others. As we discover and understand the strategies used by these fungi, similarities with their bacterial counterparts are becoming apparent, hence we can use the abundant information from bacteria to guide our studies in fungi. By understanding these strategies, new lines of research will open that can improve the treatments of these devastating fungal diseases.

Getaway car: Fungal HscA steers human phagosomal p11 into an escape route

In this issue of Cell Host & Microbe, Jia and colleagues discover how the human p11 (s100A10)-Anxa2 heterodimer drives sorting of microbial phagosomes into recycling versus degradative pathways. In a remarkable evolutionary arms race, the Aspergillus fumigatus protein HscA latches to p11 to steer its phagosome away from fungal killing.

Aspergillus fumigatus hijacks human p11 to redirect fungal-containing phagosomes to non-degradative pathway

The decision whether endosomes enter the degradative or recycling pathway in mammalian cells is of fundamental importance for pathogen killing, and its malfunctioning has pathological consequences. We discovered that human p11 is a critical factor for this decision. The HscA protein present on the conidial surface of the human-pathogenic fungus Aspergillus fumigatus anchors p11 on conidia-containing phagosomes (PSs), excludes the PS maturation mediator Rab7, and triggers binding of exocytosis mediators Rab11 and Sec15. This reprogramming redirects PSs to the non-degradative pathway, allowing A. fumigatus to escape cells by outgrowth and expulsion as well as transfer of conidia between cells. The clinical relevance is supported by the identification of a single nucleotide polymorphism in the non-coding region of the S100A10 (p11) gene that affects mRNA and protein expression in response to A. fumigatus and is associated with protection against invasive pulmonary aspergillosis. These findings reveal the role of p11 in mediating fungal PS evasion.

Ca2+ and Annexins - Emerging Players for Sensing and Transferring Cholesterol and Phosphoinositides via Membrane Contact Sites

Maintaining lipid composition diversity in membranes from different organelles is critical for numerous cellular processes. However, many lipids are synthesized in the endoplasmic reticulum (ER) and require delivery to other organelles. In this scenario, formation of membrane contact sites (MCS) between neighbouring organelles has emerged as a novel non-vesicular lipid transport mechanism. Dissecting the molecular composition of MCS identified phosphoinositides (PIs), cholesterol, scaffolding/tethering proteins as well as Ca²⁺ and Ca²⁺-binding proteins contributing to MCS functioning. Compelling evidence now exists for the shuttling of PIs and cholesterol across MCS, affecting their concentrations in distinct membrane domains and diverse roles in membrane trafficking. Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) at the plasma membrane (PM) not only controls endo-/exocytic membrane dynamics but is also critical in autophagy. Cholesterol is highly concentrated at the PM and enriched in recycling endosomes and Golgi membranes. MCS-mediated cholesterol transfer is intensely researched, identifying MCS dysfunction or altered MCS partnerships to correlate with de-regulated cellular cholesterol homeostasis and pathologies. Annexins, a conserved family of Ca²⁺-dependent phospholipid binding proteins, contribute to tethering and untethering events at MCS. In this chapter, we will discuss how Ca²⁺ homeostasis and annexins in the endocytic compartment affect the sensing and transfer of cholesterol and PIs across MCS.

Pathobiological functions and clinical implications of annexin dysregulation in human cancers

Annexins are an extensive superfamily of structurally related calcium- and phospholipid-binding proteins, largely conserved and widely distributed among species. Twelve human annexins have been identified, referred to as Annexin A1-13 (A12 remains as of yet unassigned), whose genes are spread throughout the genome on eight different chromosomes. According to their distinct tissue distribution and subcellular localization, annexins have been functionally implicated in a variety of biological processes relevant to both physiological and pathological conditions. Dysregulation of annexin expression patterns and functions has been revealed as a common feature in multiple cancers, thereby emerging as potential biomarkers and molecular targets for clinical application. Nevertheless, translation of this knowledge to the clinic requires in-depth functional and mechanistic characterization of dysregulated annexins for each individual cancer type, since each protein exhibits varying expression levels and phenotypic specificity depending on the tumor types. This review specifically and thoroughly examines the current knowledge on annexin dysfunctions in carcinogenesis. Hence, available data on expression levels, mechanism of action and pathophysiological effects of Annexin A1-13 among different cancers will be dissected, also further discussing future perspectives for potential applications as biomarkers for early diagnosis, prognosis and molecular-targeted therapies. Special attention is devoted to head and neck cancers (HNC), a complex and heterogeneous group of aggressive malignancies, often lately diagnosed, with high mortality, and scarce therapeutic options.

Molecular analysis of Annexin expression in cancer

Background

Uptake of apoptotic cells induces a tolerogenic phenotype in phagocytes and promotes peripheral tolerance. The highly conserved Annexin core domain, present in all members of the Annexin family, becomes exposed on the apoptotic cell-surface and triggers tolerogenic signalling in phagocytes via the Dectin-1 receptor. Consequently, Annexins exposed on tumour cells upon cell death are expected to induce tolerance towards tumour antigens, inhibiting tumour rejection.

Methods

Expression analysis for all Annexin family members was conducted in cancer cell lines of diverse origins. Presentation of Annexins on the cell surface during apoptosis of cancer cell lines was investigated using surface washes and immunoblotting. Expression data from the GEO database was analysed to compare Annexin levels between malignant and healthy tissue.

Results

Six Annexins at least were consistently detected on mRNA and protein level for each investigated cell line. AnxA1, AnxA2 and AnxA5 constituted the major part of total Annexin expression. All expressed Annexins translocated to the cell surface upon apoptosis induction in all cell lines. Human expression data indicate a correlation between immune infiltration and overall Annexin expression in malignant compared to healthy tissue.

Conclusions

This study is the first comprehensive analysis of expression, distribution and presentation of Annexins in cancer.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12885-022-10075-8.

Annexin A2 and Ahnak control cortical NuMA-dynein localization and mitotic spindle orientation

Proper mitotic spindle orientation depends on the correct anchorage of astral microtubules to the cortex. It relies on the remodeling of the cell cortex, a process not fully understood. Annexin A2 (Anx2) is a protein known to be involved in cortical domain remodeling. Here, we report that in early mitosis, Anx2 recruits the scaffold protein Ahnak at the cell cortex facing spindle poles, and the distribution of both proteins is controlled by cell adhesion. Depletion of either protein or impaired cortical Ahnak localization result in delayed anaphase onset and unstable spindle anchoring, which leads to altered spindle orientation. We find that Ahnak is present in a complex with dynein-dynactin. Furthermore, Ahnak and Anx2 are required for dynein and NuMA proper cortical localization and dynamics. We propose that the Ahnak/Anx2 complex influences the cortical organization of the astral microtubule anchoring complex, and thereby mitotic spindle positioning in human cells.

The Annexin A2/S100A10 Complex: The Mutualistic Symbiosis of Two Distinct Proteins

Mutualistic symbiosis refers to the symbiotic relationship between individuals of different species in which both individuals benefit from the association. S100A10, a member of the S100 family of Ca²⁺-binding proteins, exists as a tight dimer and binds two annexin A2 molecules. This association forms the annexin A2/S100A10 complex known as AIIt, and modifies the distinct functions of both proteins. Annexin A2 is a Ca²⁺-binding protein that binds F-actin, phospholipid, RNA, and specific polysaccharides such as heparin. S100A10 does not bind Ca²⁺, but binds tPA, plasminogen, certain plasma membrane ion channels, neurotransmitter receptors, and the structural scaffold protein, AHNAK. S100A10 relies on annexin A2 for its intracellular survival: in the absence of annexin A2, it is rapidly destroyed by ubiquitin-dependent and independent proteasomal degradation. Annexin A2 requires S100A10 to increase its affinity for Ca²⁺, facilitating its participation in Ca²⁺-dependent processes such as membrane binding. S100A10 binds tissue plasminogen activator and plasminogen, and promotes plasminogen activation to plasmin, which is a process stimulated by annexin A2. In contrast, annexin A2 acts as a plasmin reductase and facilitates the autoproteolytic destruction of plasmin. This review examines the relationship between annexin A2 and S100A10, and how their mutualistic symbiosis affects the function of both proteins.

Establishment of an Endocytosis-Related Prognostic Signature for Patients With Low-Grade Glioma

Background

Low-grade glioma (LGG) is a heterogeneous tumor that might develop into high-grade malignant glioma, which markedly reduces patient survival time. Endocytosis is a cellular process responsible for the internalization of cell surface proteins or external materials into the cytosol. Dysregulated endocytic pathways have been linked to all steps of oncogenesis, from initial transformation to late invasion and metastasis. However, endocytosis-related gene (ERG) signatures have not been used to study the correlations between endocytosis and prognosis in cancer. Therefore, it is essential to develop a prognostic model for LGG based on the expression profiles of ERGs.

Methods

The Cancer Genome Atlas and the Genotype-Tissue Expression database were used to identify differentially expressed ERGs in LGG patients. Gene ontology, Kyoto Encyclopedia of Genes and Genomes, and Gene set enrichment analysis methodologies were adopted for functional analysis. A protein-protein interaction (PPI) network was constructed and hub genes were identified based on the Search Tool for the Retrieval of Interacting Proteins database. Univariate and multivariate Cox regression analyses were used to develop an ERG signature to predict the overall survival (OS) of LGG patients. Finally, the association between the ERG signature and gene mutation status was further analyzed.

Results

Sixty-two ERGs showed distinct mRNA expression patterns between normal brain tissues and LGG tissues. Functional analysis indicated that these ERGs were strikingly enriched in endosomal trafficking pathways. The PPI network indicated that EGFR was the most central protein. We then built a 29-gene signature, dividing patients into high-risk and low-risk groups with significantly different OS times. The prognostic performance of the 29-gene signature was validated in another LGG cohort. Additionally, we found that the mutation scores calculated based on the TTN, PIK3CA, NF1, and IDH1 mutation status were significantly correlated with the endocytosis-related prognostic signature. Finally, a clinical nomogram with a concordance index of 0.881 predicted the survival probability of LGG patients by integrating clinicopathologic features and ERG signatures.

Conclusion

Our ERG-based prediction models could serve as an independent prognostic tool to accurately predict the outcomes of LGG.

Plasminogen Receptors and Fibrinolysis

The ability of cells to promote plasminogen activation on their surfaces is now well recognized, and several distinct cell surface proteins have been demonstrated to function as plasminogen receptors. Here, we review studies demonstrating that plasminogen bound to cells, in addition to plasminogen directly bound to fibrin, plays a major role in regulating fibrin surveillance. We focus on the ability of specific plasminogen receptors on eukaryotic cells to promote fibrinolysis in the in vivo setting by reviewing data obtained predominantly in murine models. Roles for distinct plasminogen receptors in fibrin surveillance in intravascular fibrinolysis, immune cell recruitment in the inflammatory response, wound healing, and lactational development are discussed.

The Essential Role of anxA2 in Langerhans Cell Birbeck Granules Formation

Langerhans cells (LC) are the resident antigen presenting cells of the mucosal epithelium and play an essential role in initiating immune responses. LC are the only cells in the body to contain Birbeck granules (BG), which are unique cytoplasmic organelles comprised of c-type lectin langerin. Studies of BG have historically focused on morphological characterizations, but BG have also been implicated in viral antigen processing which suggests that they can serve a function in antiviral immunity. This study focused on investigating proteins that could be involved in BG formation to further characterize their structure using transmission electron microscopy (TEM). Here, we report a critical role for the protein annexin A2 (anxA2) in the proper formation of BG structures. When anxA2 expression is downregulated, langerin expression decreases, cytoplasmic BG are nearly ablated, and the presence of malformed BG-like structures increases. Furthermore, in the absence of anxA2, we found langerin was no longer localized to BG or BG-like structures. Taken together, these results indicate an essential role for anxA2 in facilitating the proper formation of BG.

Annexins-Coordinators of Cholesterol Homeostasis in Endocytic Pathways

The spatiotemporal regulation of calcium (Ca2+) storage in late endosomes (LE) and lysosomes (Lys) is increasingly recognized to influence a variety of membrane trafficking events, including endocytosis, exocytosis, and autophagy. Alterations in Ca2+ homeostasis within the LE/Lys compartment are implicated in human diseases, ranging from lysosomal storage diseases (LSDs) to neurodegeneration and cancer, and they correlate with changes in the membrane binding behaviour of Ca2+-binding proteins. This also includes Annexins (AnxA), which is a family of Ca2+-binding proteins participating in membrane traffic and tethering, microdomain organization, cytoskeleton interactions, Ca2+ signalling, and LE/Lys positioning. Although our knowledge regarding the way Annexins contribute to LE/Lys functions is still incomplete, recruitment of Annexins to LE/Lys is greatly influenced by the availability of Annexin bindings sites, including acidic phospholipids, such as phosphatidylserine (PS) and phosphatidic acid (PA), cholesterol, and phosphatidylinositol (4,5)-bisphosphate (PIP2). Moreover, the cytosolic portion of LE/Lys membrane proteins may also, directly or indirectly, determine the recruitment of Annexins to LE. Strikingly, within LE/Lys, AnxA1, A2, A6, and A8 differentially contribute to cholesterol transport along the endocytic route, in particular, cholesterol transfer between LE and other compartments, positioning Annexins at the centre of major pathways mediating cellular cholesterol homeostasis. Underlying mechanisms include the formation of membrane contact sites (MCS) and intraluminal vesicles (ILV), as well as the modulation of LE-cholesterol transporter activity. In this review, we will summarize the current understanding how Annexins contribute to influence LE/Lys membrane transport and associated functions.

MYO6 Regulates Spatial Organization of Signaling Endosomes Driving AKT Activation and Actin Dynamics

APPL1- and RAB5-positive signaling endosomes play a crucial role in the activation of AKT in response to extracellular stimuli. Myosin VI (MYO6) and two of its cargo adaptor proteins, GIPC and TOM1/TOM1L2, localize to these peripheral endosomes and mediate endosome association with cortical actin filaments. Loss of MYO6 leads to the displacement of these endosomes from the cell cortex and accumulation in the perinuclear space. Depletion of this myosin not only affects endosome positioning, but also induces actin and lipid remodeling consistent with endosome maturation, including accumulation of F-actin and the endosomal lipid PI(3)P. These processes acutely perturb endosome function, as both AKT phosphorylation and RAC-dependent membrane ruffling were markedly reduced by depletion of either APPL1 or MYO6. These results place MYO6 and its binding partners at a central nexus in cellular signaling linking actin dynamics at the cell surface and endosomal signaling in the cell cortex.

The Metalloprotease, Mpr1, Engages AnnexinA2 to Promote the Transcytosis of Fungal Cells across the Blood-Brain Barrier

Eukaryotic pathogens display multiple mechanisms for breaching the blood-brain barrier (BBB) and invading the central nervous system (CNS). Of the fungal spp., that cause disease in mammals, only some cross brain microvascular endothelial cells which constitute the BBB, and invade the brain. Cryptococcus neoformans, the leading cause of fungal meningoencephalitis, crosses the BBB directly by transcytosis or by co-opting monocytes. We previously determined that Mpr1, a secreted fungal metalloprotease, facilitates association of fungal cells to brain microvascular endothelial cells and we confirmed that the sole expression of CnMPR1 endowed S. cerevisiae with an ability to cross the BBB. Here, the gain of function conferred onto S. cerevisiae by CnMPR1 (i.e., Sc<CnMPR1> strain) was used to identify targets of Mpr1 that might reside on the surface of the BBB. Following biotin-labeling of BBB surface proteins, Sc<CnMPR1>-associated proteins were identified by LC-MS/MS. Of the 62 proteins identified several were cytoskeleton-endocytosis-associated including AnnexinA2 (AnxA2). Using an in vitro model of the human BBB where AnxA2 activity was blocked, we found that the lack of AnxA2 activity prevented the movement of S. cerevisiae across the BBB (i.e., transcytosis of Sc<CnMPR1> strain) but unexpectedly, TEM analysis revealed that AnxA2 was not required for the association or the internalization of Sc<CnMPR1>. Additionally, the co-localization of AnxA2 and Sc<CnMPR1> suggest that successful crossing of the BBB is dependent on an AxnA2-Mpr1-mediated interaction. Collectively the data suggest that AnxA2 plays a central role in fungal transcytosis in human brain microvascular endothelial cells. The movement and exocytosis of Sc<CnMPR1> is dependent on membrane trafficking events that involve AnxA2 but these events appear to be independent from the actions of AnxA2 at the host cell surface. We propose that Mpr1 activity promotes cytoskeleton remodeling in brain microvascular endothelial cells and thereby engages AnxA2 in order to facilitate fungal transcytosis of the BBB.

Annexins as Overlooked Regulators of Membrane Trafficking in Plant Cells

Annexins are an evolutionary conserved superfamily of proteins able to bind membrane phospholipids in a calcium-dependent manner. Their physiological roles are still being intensively examined and it seems that, despite their general structural similarity, individual proteins are specialized toward specific functions. However, due to their general ability to coordinate membranes in a calcium-sensitive fashion they are thought to participate in membrane flow. In this review, we present a summary of the current understanding of cellular transport in plant cells and consider the possible roles of annexins in different stages of vesicular transport.

Annexins: Ca2+ Effectors Determining Membrane Trafficking in the Late Endocytic Compartment

Despite the discovery of annexins 40 years ago, we are just beginning to understand some of the functions of these still enigmatic proteins. Defined and characterized by their ability to bind anionic membrane lipids in a Ca²⁺-dependent manner, each annexin has to be considered a multifunctional protein, with a multitude of cellular locations and diverse activities. Underlying causes for this considerable functional diversity include their capability to associate with multiple cytosolic and membrane proteins. In recent years, the increasingly recognized establishment of membrane contact sites between subcellular compartments opens a new scenario for annexins as instrumental players to link Ca²⁺ signalling with the integration of membrane trafficking in many facets of cell physiology. In this chapter, we review and discuss current knowledge on the contribution of annexins in the biogenesis and functioning of the late endocytic compartment, affecting endo- and exocytic pathways in a variety of physiological consequences ranging from membrane repair, lysosomal exocytosis, to cell migration.

Annexin A2 is critical for blood-testis barrier integrity and spermatid disengagement in the mammalian testis

Throughout spermatogenesis, two important processes occur at late stage VIII of the seminiferous epithelial cycle in the rat testis: preleptotene spermatocytes commence entry into the adluminal compartment and step 19 spermatids release from the seminiferous epithelium. Presently, it is not clear how these processes, which involve extensive restructuring of unique Sertoli-Sertoli and Sertoli-germ cell junctions, are mediated. We aimed to determine whether annexin A2 (ANXA2), a Ca²⁺-dependent and phospholipid-binding protein, participates in cell junction dynamics. To address this, in vitro and in vivo RNA interference studies were performed on prepubertal Sertoli cells and adult rat testes. The endpoints of Anxa2 knockdown were determined by immunoblotting, morphological analyses, fluorescent immunostaining, and barrier integrity assays. In the testis, ANXA2 localized to the Sertoli cell stalk, with specific staining at the blood-testis barrier and the concave (ventral) surface of elongated spermatids. ANXA2 also bound actin when testis lysates were used for immunoprecipitation. Anxa2 knockdown was found to disrupt the Sertoli cell/blood-testis barrier in vitro and in vivo. The disruption in barrier function was substantiated by changes in the localization of claudin-11, zona occludens-1, N-cadherin, and β-catenin. Furthermore, Anxa2 knockdown resulted in spermiation defects caused by a dysfunction of tubulobulbar complexes, testis-specific actin-rich ultrastructures that internalize remnant cell junction components prior to spermiation. Additionally, there were changes in the localization of several tubulobulbar complex component proteins, including actin-related protein 3, cortactin, and dynamin I/II. Our results indicate that ANXA2 is critical for the integrity of the blood-testis barrier and the timely release of spermatids.

S100A10 Regulates ULK1 Localization to ER-Mitochondria Contact Sites in IFN-γ-Triggered Autophagy

During the process of autophagy, the autophagy-related proteins are translocated to autophagosome formation sites. Here, we demonstrate that S100A10 is required for ULK1 localization to autophagosome formation sites. Silencing of S100A10 reduces IFN-γ-induced autophagosome formation. We also determined the role of annexin A2 (ANXA2), a binding partner of S100A10, which has been reported to promote phagophore assembly. Silencing of ANXA2 reduced S100A10 expression. However, overexpression of S100A10 in ANXA2-silenced cells was still able to enhance autophagosome formation, suggesting that ANXA2 regulates IFN-γ-induced autophagy through S100A10. We also observed that S100A10 interacted with ULK1 after IFN-γ stimulation, and S100A10 knockdown prevented ULK1 localization to autophagosome formation sites. Finally, the release of high mobility group protein B1, one of the functions mediated by IFN-γ-induced autophagy, was inhibited in S100A10 knockdown cells. These results elucidate the importance of S100A10 in autophagosome formation and reveal the relationship between S100A10 and ULK1 in IFN-γ-induced autophagy.

Cell- and region-specific expression of depression-related protein p11 (S100a10) in the brain

P11 (S100a10), a member of the S100 family of proteins, has widespread distribution in the vertebrate body, including in the brain, where it has a key role in membrane trafficking, vesicle secretion, and endocytosis. Recently, our laboratory has shown that a constitutive knockout of p11 (p11-KO) in mice results in a depressive-like phenotype. Furthermore, p11 has been implicated in major depressive disorder (MDD) and in the actions of antidepressants. Since depression affects multiple brain regions, and the role of p11 has only been determined in a few of these areas, a detailed analysis of p11 expression in the brain is warranted. Here we demonstrate that, although widespread in the brain, p11 expression is restricted to distinct regions, and specific neuronal and nonneuronal cell types. Furthermore, we provide comprehensive mapping of p11 expression using in situ hybridization, immunocytochemistry, and whole-tissue volume imaging. Overall, expression spans multiple brain regions, structures, and cell types, suggesting a complex role of p11 in depression. J. Comp. Neurol. 525:955-975, 2017. © 2016 Wiley Periodicals, Inc.

Annexins in plasma membrane repair

Disruption of the plasma membrane poses deadly threat to eukaryotic cells and survival requires a rapid membrane repair system. Recent evidence reveal various plasma membrane repair mechanisms, which are required for cells to cope with membrane lesions including membrane fusion and replacement strategies, remodeling of cortical actin cytoskeleton and vesicle wound patching. Members of the annexin protein family, which are Ca2+-triggered phospholipid-binding proteins emerge as important components of the plasma membrane repair system. Here, we discuss the mechanisms of plasma membrane repair involving annexins spanning from yeast to human cancer cells.

The progressive ankylosis protein ANK facilitates clathrin- and adaptor-mediated membrane traffic at the trans-Golgi network-to-endosome interface

Dominant or recessive mutations in the progressive ankylosis gene ANKH have been linked to familial chondrocalcinosis (CCAL2), craniometaphyseal dysplasia (CMD), mental retardation, deafness and ankylosis syndrome (MRDA). The function of the encoded membrane protein ANK in cellular compartments other than the plasma membrane is unknown. Here, we show that ANK localizes to the trans-Golgi network (TGN), clathrin-coated vesicles and the plasma membrane. ANK functionally interacts with clathrin and clathrin associated adaptor protein (AP) complexes as loss of either protein causes ANK dispersion from the TGN to cytoplasmic endosome-like puncta. Consistent with its subcellular localization, loss of ANK results in reduced formation of tubular membrane carriers from the TGN, perinuclear accumulation of early endosomes and impaired transferrin endocytosis. Our data indicate that clathrin/AP-mediated cycling of ANK between the TGN, endosomes, and the cell surface regulates membrane traffic at the TGN/endosomal interface. These findings suggest that dysfunction of Golgi-endosomal membrane traffic may contribute to ANKH-associated pathologies.

Annexin A2 facilitates endocytic trafficking of antisense oligonucleotides

Chemically modified antisense oligonucleotides (ASOs) designed to mediate site-specific cleavage of RNA by RNase H1 are used as research tools and as therapeutics. ASOs modified with phosphorothioate (PS) linkages enter cells via endocytotic pathways. The mechanisms by which PS-ASOs are released from membrane-enclosed endocytotic organelles to reach target RNAs remain largely unknown. We recently found that annexin A2 (ANXA2) co-localizes with PS-ASOs in late endosomes (LEs) and enhances ASO activity. Here, we show that co-localization of ANXA2 with PS-ASO is not dependent on their direct interactions or mediated by ANXA2 partner protein S100A10. Instead, ANXA2 accompanies the transport of PS-ASOs to LEs, as ANXA2/PS-ASO co-localization was observed inside LEs. Although ANXA2 appears not to affect levels of PS-ASO internalization, ANXA2 reduction caused significant accumulation of ASOs in early endosomes (EEs) and reduced localization in LEs and decreased PS-ASO activity. Importantly, the kinetics of PS-ASO activity upon free uptake show that target mRNA reduction occurs at least 4 hrs after PS-ASOs exit from EEs and is coincident with release from LEs. Taken together, our results indicate that ANXA2 facilitates PS-ASO trafficking from early to late endosomes where it may also contribute to PS-ASO release.

Annexin A2, autoimmunity, anxiety and depression

OBJECTIVES

Antiphospholipid syndrome (APS) is associated with neurological manifestations and one of the novel autoantigens associated with this disease is Annexin A2 (ANXA2). In this work we have examined the effect of high levels of autoantibodies to ANXA2 on the brain in a mouse model.

METHODS

Recombinant ANXA2 emulsified in adjuvant was used to immunize mice while mice immunized with adjuvant only served as controls. At peak antibody levels the animal underwent behavioral and cognitive tests and their brains were examined for ANXA2 immunoglobulin G (IgG) and expression of ANXA2 and the closely linked protein p11.

RESULTS

Very high levels of anti-ANXA2 antibodies (Abs) were associated with reduced anxiety in the open field 13.14% ± 0.89% of the time in the center compared to 8.64% ± 0.91% observed in the control mice (p < 0.001 by t-test). A forced swim test found significantly less depression manifested by immobility in the ANXA2 group. The changes in behavior were accompanied by a significant reduction in serum corticosteroid levels of ANXA2 group compared to controls. Moreover, higher levels of total IgG and p11 expression were found in ANXA2 group brains. Lower levels of circulating anti-ANXA2 Abs were not associated with behavioral changes.

CONCLUSIONS

We have established an animal model with high levels of anti-ANXA2 Abs which induced IgG accumulation in the brain and specific anxiolytic and anti-depressive effects. This model promises to further our understanding of autoimmune disease such as APS and to provide better understanding of the role of the ANXA2-p11 complex in the brain.

Long-term efficacy and downstream mechanism of anti-annexinA2 monoclonal antibody (anti-ANX A2 mAb) in a pre-clinical model of aggressive human breast cancer

There is considerable direct evidence that calcium binding protein ANX A2 is a potential target for treating aggressive breast cancer. The most compelling data are based on the finding of ANX A2 overexpression in aggressive triple negative human breast cancer (TNBC) cell lines and in human breast cancer tissues. Previously, we and others reported a unique role of ANX A2 in cancer invasion, including breast cancer. Moreover, we demonstrated that anti-ANX A2 mAb-mediated immunoneutralization of ANX A2 inhibited invasive human breast cancer growth in a xenograft model. We further evaluated the long-term effects of multiple treatments with anti-ANX A2 mAb and its mechanism of inhibition on human breast tumor growth. We now demonstrate that three treatments with anti-ANX A2 mAb led to significant inhibition of breast tumor growth in immunodeficient mice, and that the anti-tumor response was demonstrable from day 94. After treatment, we followed tumor growth for 172 days and demonstrated 67% inhibition of tumor growth without detectable adverse effects. Biochemical analysis demonstrated that anti-ANX A2 mAb treatment caused significant inhibition of conversion of tissue plasminogen activator (tPA) in the tumor microenvironment. This led to disruption of plasmin generation that consequently inhibited activation of MMP-9 and MMP-2. These results suggest that ANX A2 plays an important role in aggressive breast tumor growth by regulating proteolytic pathways in the tumor microenvironment. ANX A2 may represent a new target for the development of therapeutics for treatment of aggressive breast cancer.

BLOC-1 Brings Together the Actin and Microtubule Cytoskeletons to Generate Recycling Endosomes

Recycling endosomes consist of a tubular network that emerges from vacuolar sorting endosomes and diverts cargoes toward the cell surface, the Golgi, or lysosome-related organelles. How recycling tubules are formed remains unknown. We show that recycling endosome biogenesis requires the protein complex BLOC-1. Mutations in BLOC-1 subunits underlie an inherited disorder characterized by albinism, the Hermansky-Pudlak Syndrome, and are associated with schizophrenia risk. We show here that BLOC-1 coordinates the kinesin KIF13A-dependent pulling of endosomal tubules along microtubules to the Annexin A2/actin-dependent stabilization and detachment of recycling tubules. These components cooperate to extend, stabilize and form tubular endosomal carriers that function in cargo recycling and in the biogenesis of pigment granules in melanocytic cells. By shaping recycling endosomal tubules, our data reveal that dysfunction of the BLOC-1-KIF13A-Annexin A2 molecular network underlies the pathophysiology of neurological and pigmentary disorders.

Calcium signaling in membrane repair

Resealing allows cells to mend damaged membranes rapidly when plasma membrane (PM) disruptions occur. Models of PM repair mechanisms include the "lipid-patch", "endocytic removal", and "macro-vesicle shedding" models, all of which postulate a dependence on local increases in intracellular Ca(2+) at injury sites. Multiple calcium sensors, including synaptotagmin (Syt) VII, dysferlin, and apoptosis-linked gene-2 (ALG-2), are involved in PM resealing, suggesting that Ca(2+) may regulate multiple steps of the repair process. Although earlier studies focused exclusively on external Ca(2+), recent studies suggest that Ca(2+) release from intracellular stores may also be important for PM resealing. Hence, depending on injury size and the type of injury, multiple sources of Ca(2+) may be recruited to trigger and orchestrate repair processes. In this review, we discuss the mechanisms by which the resealing process is promoted by vesicular Ca(2+) channels and Ca(2+) sensors that accumulate at damage sites.

Acute Administration of Branched-Chain Amino Acids Increases the Pro-BDNF/Total-BDNF Ratio in the Rat Brain

Maple syrup urine disease (MSUD) is caused by an inborn error in metabolism resulting from a deficiency in the branched-chain α-keto acid dehydrogenase complex activity. This blockage leads to accumulation of the branched-chain amino acids (BCAA) leucine, isoleucine and valine, as well as their corresponding α-keto acids and α-hydroxy acids. High levels of BCAAs are associated with neurological dysfunction and the role of pro- and mature brain-derived neurotrophic factor (BDNF) in the neurological dysfunction of MSUD is still unclear. Thus, in the present study we investigated the effect of an acute BCAA pool administration on BDNF levels and on the pro-BDNF cleavage-related proteins S100A10 and tissue plasminogen activator (tPA) in rat brains. Our results demonstrated that acute Hyper-BCAA (H-BCAA) exposure during the early postnatal period increases pro-BDNF and total-BDNF levels in the hippocampus and striatum. Moreover, tPA levels were significantly decreased, without modifications in the tPA transcript levels in the hippocampus and striatum. On the other hand, the S100A10 mRNA and S100A10 protein levels were not changed in the hippocampus and striatum. In the 30-day-old rats, we observed increased pro-BDNF, total-BDNF and tPA levels only in the striatum, whereas the tPA and S100A10 mRNA expression and the immunocontent of S100A10 were not altered. In conclusion, we demonstrated that acute H-BCAA administration increases the pro-BDNF/total-BDNF ratio and decreases the tPA levels in animals, suggesting that the BCAA effect may depend, at least in part, on changes in BDNF post-translational processing.

Evidence for annexin A6-dependent plasma membrane remodelling of lipid domains

BACKGROUND AND PURPOSE

Annexin A6 (AnxA6) is a calcium-dependent phospholipid-binding protein that can be recruited to the plasma membrane to function as a scaffolding protein to regulate signal complex formation, endo- and exocytic pathways as well as distribution of cellular cholesterol. Here, we have investigated how AnxA6 influences the membrane order.

EXPERIMENTAL APPROACH

We used Laurdan and di-4-ANEPPDHQ staining in (i) artificial membranes; (ii) live cells to investigate membrane packing and ordered lipid phases; and (iii) a super-resolution imaging (photoactivated localization microscopy, PALM) and Ripley's K second-order point pattern analysis approach to assess how AnxA6 regulates plasma membrane order domains and protein clustering.

KEY RESULTS

In artificial membranes, purified AnxA6 induced a global increase in membrane order. However, confocal microscopy using di-4-ANEPPDHQ in live cells showed that cells expressing AnxA6, which reduces plasma membrane cholesterol levels and modifies the actin cytoskeleton meshwork, displayed a decrease in membrane order (∼15 and 30% in A431 and MEF cells respectively). PALM data from Lck10 and Src15 membrane raft/non-raft markers revealed that AnxA6 expression induced clustering of both raft and non-raft markers. Altered clustering of Lck10 and Src15 in cells expressing AnxA6 was also observed after cholesterol extraction with methyl-β-cyclodextrin or actin cytoskeleton disruption with latrunculin B.

CONCLUSIONS AND IMPLICATIONS

AnxA6-induced plasma membrane remodelling indicated that elevated AnxA6 expression decreased membrane order through the regulation of cellular cholesterol homeostasis and the actin cytoskeleton. This study provides the first evidence from live cells that support current models of annexins as membrane organizers.

Annexin A2 autoantibodies in thrombosis and autoimmune diseases

Antiphospholipid syndrome (APS) is an autoimmune disease characterized by arterial, venous or small-vessel thrombotic events, and recurrent miscarriages or fetal loss. APS diagnosis is based on the repeated detection of anti-phospholipid (PL) antibodies (Ab), typically associated with anti-β2 glycoprotein I (β2GPI)-Ab. Recent studies suggest that anti-β2GPI Ab activity involves a protein complex including β2GPI and annexin A2 (ANXA2). Anti-ANXA2 Ab recognizes this complex, and these Ab can effectively promote thrombosis by inhibiting plasmin generation, and by activating endothelial cells. Therefore, anti-ANXA2 Ab represent a new biomarker, which can be detected in up to 25% of APS patients. Moreover, anti-ANXA2 Ab have been detected, in thrombotic associated diseases including pre-eclampsia, in other autoimmune diseases, and in cancer.

G protein-coupled receptors: what a difference a 'partner' makes

G protein-coupled receptors (GPCRs) are important cell signaling mediators, involved in essential physiological processes. GPCRs respond to a wide variety of ligands from light to large macromolecules, including hormones and small peptides. Unfortunately, mutations and dysregulation of GPCRs that induce a loss of function or alter expression can lead to disorders that are sometimes lethal. Therefore, the expression, trafficking, signaling and desensitization of GPCRs must be tightly regulated by different cellular systems to prevent disease. Although there is substantial knowledge regarding the mechanisms that regulate the desensitization and down-regulation of GPCRs, less is known about the mechanisms that regulate the trafficking and cell-surface expression of newly synthesized GPCRs. More recently, there is accumulating evidence that suggests certain GPCRs are able to interact with specific proteins that can completely change their fate and function. These interactions add on another level of regulation and flexibility between different tissue/cell-types. Here, we review some of the main interacting proteins of GPCRs. A greater understanding of the mechanisms regulating their interactions may lead to the discovery of new drug targets for therapy.

Lipid segregation and membrane budding induced by the peripheral membrane binding protein annexin A2

The formation of dynamic membrane microdomains is an important phenomenon in many signal transduction and membrane trafficking events. It is driven by intrinsic properties of membrane lipids and integral as well as membrane-associated proteins. Here we analyzed the ability of one peripherally associated membrane protein, annexin A2 (AnxA2), to induce the formation of phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2)-rich domains in giant unilamellar vesicles (GUVs) of complex lipid composition. AnxA2 is a cytosolic protein that can bind PI(4,5)P2 and other acidic phospholipids in a Ca(2+)-dependent manner and that has been implicated in cellular membrane dynamics in endocytosis and exocytosis. We show that AnxA2 binding to GUVs induces lipid phase separation and the recruitment of PI(4,5)P2, cholesterol and glycosphingolipids into larger clusters. This property is observed for the full-length monomeric protein, a mutant derivative comprising the C-terminal protein core domain and for AnxA2 residing in a heterotetrameric complex with its intracellular binding partner S100A10. All AnxA2 derivatives inducing PI(4,5)P2 clustering are also capable of forming interconnections between PI(4,5)P2-rich microdomains of adjacent GUVs. Furthermore, they can induce membrane indentations rich in PI(4,5)P2 and inward budding of these membrane domains into the lumen of GUVs. This inward vesiculation is specific for AnxA2 and not shared with other PI(4,5)P2-binding proteins such as the pleckstrin homology (PH) domain of phospholipase Cδ1. Together our results indicate that annexins such as AnxA2 can efficiently induce membrane deformations after lipid segregation, a mechanism possibly underlying annexin functions in membrane trafficking.

cAMP-induced secretion of endothelial von Willebrand factor is regulated by a phosphorylation/dephosphorylation switch in annexin A2

The large multimeric glyocoprotein von Willebrand factor (VWF) is a crucial component of both primary and secondary hemostasis. It is stored in secretory granules of vascular endothelial cells, the Weibel-Palade bodies (WPBs), and is released following stimulation by agonists that raise intracellular Ca(2+) or cyclic adenosine monophosphate (cAMP) levels. cAMP-induced exocytosis of WPBs requires protein kinase A activity, but downstream factors that are regulated by phosphorylation/dephosphorylation are not known. Here we identify the complex consisting of the lipid-binding protein annexin A2 (AnxA2) and S100A10 as such a factor. Knockdown and specific rescue approaches reveal that a functional AnxA2-S100A10 complex is required for the forskolin-induced, cAMP-dependent release of VWF. Forskolin triggers dephosphorylation of AnxA2 that is mediated by a calcineurin-like phosphatase and stabilizes the AnxA2-S100A10 complex, thereby promoting VWF release. Serine 11 of AnxA2 was identified as the target residue of this phosphorylation switch because a phosphomimicking mutation at this site prevents complex formation with S100A10 and, in contrast to wild-type or S11A-AnxA2, is unable to restore cAMP-dependent VWF secretion in AnxA2-depleted cells. Thus, complex formation of AnxA2 with S100A10 is a central regulatory mechanism in the acute release of VWF in response to cAMP-elevating agonists.

Annexin A2 system in human biology: cell surface and beyond

Annexin A2 (A2) is a multicompartmental, multifunctional protein that orchestrates a growing spectrum of biologic processes. At the endothelial cell surface, A2 and S100A10 (p11) form a heterotetramer, which accelerates tissue plasminogen activator-dependent activation of the fibrinolytic protease, plasmin. In antiphospholipid syndrome, anti-A2 antibodies are associated with clinical thrombosis, whereas overexpression of A2 in acute promyelocytic leukemia promotes hyperfibrinolytic bleeding. A2 is upregulated in hypoxia, and mice deficient in A2 are resistant to oxygen-induced retinal neovascularization, suggesting a role for A2 in human retinal vascular proliferation. In solid malignancies, the (A2•p11)(2) tetramer may promote cancer cell invasion, whereas in multiple myeloma A2 enables malignant plasmacyte growth and predicts prognosis. In the central nervous system, the p11 enables membrane insertion of serotonin receptors that govern mood. In the peripheral nervous system, p11 directs sodium channels to the plasma membrane, enabling pain perception. In cerebral cortex neurons, A2 stabilizes the microtubule-associated tau protein, which, when mutated, is associated with frontotemporal dementia. In inflammatory dendritic cells, A2 maintains late endosomal/lysosomal membrane integrity, thus modulating inflammasome activation and cytokine secretion in a model of aseptic arthritis. Together, these findings suggest an emerging, multifaceted role for A2 in human health and disease.

Ceramide 1-phosphate mediates endothelial cell invasion via the annexin a2-p11 heterotetrameric protein complex

The bioactive sphingolipid, ceramide 1-phosphate (C-1-P), has been implicated as an extracellular chemotactic agent directing cellular migration in hematopoietic stem/progenitor cells and macrophages. However, interacting proteins that could mediate these actions of C-1-P have, thus far, eluded identification. We have now identified and characterized interactions between ceramide 1-phosphate and the annexin a2-p11 heterotetramer constituents. This C-1-P-receptor complex is capable of facilitating cellular invasion. Herein, we demonstrate in both coronary artery macrovascular endothelial cells and retinal microvascular endothelial cells that C-1-P induces invasion through an extracellular matrix barrier. By employing surface plasmon resonance, lipid-binding ELISA, and mass spectrometry technologies, we have demonstrated that the heterotetramer constituents bind to C-1-P. Although the annexin a2-p11 heterotetramer constituents do not bind the lipid C-1-P exclusively, other structurally similar lipids, such as phosphatidylserine, sphingosine 1-phosphate, and phosphatidic acid, could not elicit the potent chemotactic stimulation observed with C-1-P. Further, we show that siRNA-mediated knockdown of either annexin a2 or p11 protein significantly inhibits C-1-P-directed invasion, indicating that the heterotetrameric complex is required for C-1-P-mediated chemotaxis. These results imply that extracellular C-1-P, acting through the extracellular annexin a2-p11 heterotetrameric protein, can mediate vascular endothelial cell invasion.

Annexin A2 regulates β1 integrin internalization and intestinal epithelial cell migration

The gastrointestinal epithelium functions as an important barrier that separates luminal contents from the underlying tissue compartment and is vital in maintaining mucosal homeostasis. Mucosal wounds in inflammatory disorders compromise the critical epithelial barrier. In response to injury, intestinal epithelial cells (IECs) rapidly migrate to reseal wounds. We have previously observed that a membrane-associated, actin binding protein, annexin A2 (AnxA2), is up-regulated in migrating IECs and plays an important role in promoting wound closure. To identify the mechanisms by which AnxA2 promotes IEC movement and wound closure, we used a loss of function approach. AnxA2-specific shRNA was utilized to generate IECs with stable down-regulation of AnxA2. Loss of AnxA2 inhibited IEC migration while promoting enhanced cell-matrix adhesion. These functional effects were associated with increased levels of β1 integrin protein, which is reported to play an important role in mediating the cell-matrix adhesive properties of epithelial cells. Because cell migration requires dynamic turnover of integrin-based adhesions, we tested whether AnxA2 modulates internalization of cell surface β1 integrin required for forward cell movement. Indeed, pulse-chase biotinylation experiments in IECs lacking AnxA2 demonstrated a significant increase in cell surface β1 integrin that was accompanied by decreased β1 integrin internalization and degradation. These findings support an important role of AnxA2 in controlling dynamics of β1 integrin at the cell surface that in turn is required for the active turnover of cell-matrix associations, cell migration, and wound closure.

Annexin A2 heterotetramer: structure and function

Annexin A2 is a pleiotropic calcium- and anionic phospholipid-binding protein that exists as a monomer and as a heterotetrameric complex with the plasminogen receptor protein, S100A10. Annexin A2 has been proposed to play a key role in many processes including exocytosis, endocytosis, membrane organization, ion channel conductance, and also to link F-actin cytoskeleton to the plasma membrane. Despite an impressive list of potential binding partners and regulatory activities, it was somewhat unexpected that the annexin A2-null mouse should show a relatively benign phenotype. Studies with the annexin A2-null mouse have suggested important functions for annexin A2 and the heterotetramer in fibrinolysis, in the regulation of the LDL receptor and in cellular redox regulation. However, the demonstration that depletion of annexin A2 causes the depletion of several other proteins including S100A10, fascin and affects the expression of at least sixty-one genes has confounded the reports of its function. In this review we will discuss the annexin A2 structure and function and its proposed physiological and pathological roles.

Extracellular activities of aminoacyl-tRNA synthetases: new mediators for cell-cell communication

Over the last decade, many reports have discussed aminoacyl-tRNA synthetases (ARSs) in extracellular space. Now that so many of them are known to be secreted with distinct activities in the broad range of target cells including endothelial, various immune cells, and fibroblasts, they need to be classified as a new family of extracellular signal mediators. In this chapter the identity of the secreted ARSs, receptors, and their physiological and pathological implications will be described.

Screening and identification of lung cancer metastasis-related genes by suppression subtractive hybridization

BACKGROUND AND OBJECTIVE

  Lung cancer metastasis is a complicated process in which multiple stages and multiple genes are involved. There is an urgent need to use new molecular biology techniques to get more systematic information and have a general idea of the molecular events that take place in lung cancer metastasis. The object of this study was to construct the subtracted cDNA libraries of different metastatic potential lung cancer cell lines, NL9980 and L9981, which were established and screened from human lung large cell carcinoma cell line, WCQH-9801.

METHOD

  The forward and reverse subtracted cDNA libraries were constructed in the large cell lung cancer cell lines NL9980 and L9981 with the same heredity background but different metastatic potential, by suppression subtractive hybridization (SSH). The positive clones were preliminarily screened by blue-white colony and precisely identified by PCR. The forward and reverse subtracted libraries were screened and identified by dot blot so as to obtain the clones corresponding to gene segments with differential expression. DNA sequencing was performed to analyze the sequences of differential expression segments, which were then searched and compared using the Basic Local Alignment Search Tool from The National Center for Biotechnology Information NCBI BLAST tools. Quantitative real-time reverse transcription polymerase chain reaction (RT-PCR) and western blotting were performed to confirm the differential expressed genes both on RNA and protein levels.

RESULTS

  The forward and reverse subtracted cDNA libraries of the different large cell lung cancer cell lines with metastatic potential were successfully constructed. With blue-white colony and dot blot, 307 positive clones in the forward subtracted library and 78 positive clones in the reverse subtracted library were obtained. Fifty-five clones were successfully sequenced in the forward subtracted library while 31 clones were successfully sequenced in the reverse subtracted library. One new expressed sequence tag (EST) segment was identified from the reverse subtracted cDNA library and was successfully submitted to GenBank and embodied by GenBank. For the differentially expressed genes between L9981 and NL9980 screened by SSH, four genes, ANXA2, KRT18, ACTG1 was upregulated in L9981 cells compared to NL9980 cells. Annexin A2 (which was encoded by ANXA2), γ-actin (which was encoded by ACTG1), and aldose reductase (which was encoded by AKR1B1) proteins were upregulated in L9981 cells compared to NL9980 cells by western blotting.

CONCLUSION

  The forward and reverse subtracted cDNA libraries of different metastatic potential large cell lung cancer cell lines were successfully constructed by SSH. A series of genes have been screened out to have significantly different expression levels between lung cancer cell lines NL9980 and L9981. A new EST segment that may represent a new metastasis-related gene has been identified. Consistent with the result of SSH, both quantitative real-time RT-PCR and western Blotting confirmed the upregulation of ANXA2, ACTG1 and AKR1B1 in lung cancer cell line L9981 compared with NL9980. These three genes may play important roles in lung cancer metastasis.

Annexin A1 and A2: roles in retrograde trafficking of Shiga toxin

Annexins constitute a family of calcium and membrane binding proteins. As annexin A1 and A2 have previously been linked to various membrane trafficking events, we initiated this study to investigate the role of these annexins in the uptake and intracellular transport of the bacterial Shiga toxin (Stx) and the plant toxin ricin. Once endocytosed, both toxins are retrogradely transported from endosomes to the Golgi apparatus and the endoplasmic reticulum before being targeted to the cytosol where they inhibit protein synthesis. This study was performed to obtain new information both about toxin transport and the function of annexin A1 and annexin A2. Our data show that depletion of annexin A1 or A2 alters the retrograde transport of Stx but not ricin, without affecting toxin binding or internalization. Knockdown of annexin A1 increases Golgi transport of Stx, whereas knockdown of annexin A2 slightly decreases the same transport step. Interestingly, annexin A1 was found in proximity to cytoplasmic phospholipase A2 (cPLA₂), and the basal as well as the increased Golgi transport of Stx upon annexin A1 knockdown is dependent on cPLA₂ activity. In conclusion, annexin A1 and A2 have different roles in Stx transport to the trans-Golgi network. The most prominent role is played by annexin A1 which normally works as a negative regulator of retrograde transport from the endosomes to the Golgi network, most likely by complex formation and inhibition of cPLA₂.

Do annexins participate in lipid messenger mediated intracellular signaling? A question revisited

Annexins are physiologically important proteins that play a role in calcium buffering but also influence membrane structure, participate in Ca²⁺-dependent membrane repair events and in remodelling of the cytoskeleton. Thirty years ago several peptides isolated from lung perfusates, peritoneal leukocytes, neutrophiles and renal cells were proven inhibitory to the activity of phospholipase A₂. Those peptides were found to derive from structurally related proteins: annexins AnxA1 and AnxA2. These findings raised the question whether annexins may participate in regulation of the production of lipid second messengers and, therefore, modulate numerous lipid mediated signaling pathways in the cell. Recent advances in the field of annexins made also with the use of knock-out animal models revealed that these proteins are indeed important constituents of specific signaling pathways. In this review we provide evidence supporting the hypothesis that annexins, as membrane-binding proteins and organizers of the membrane lateral heterogeneity, may participate in lipid mediated signaling pathways by affecting the distribution and activity of lipid metabolizing enzymes (most of the reports point to phospholipase A₂) and of protein kinases regulating activity of these enzymes. Moreover, some experimental data suggest that annexins may directly interact with lipid metabolizing enzymes and, in a calcium-dependent or independent manner, with some of their substrates and products. On the basis of these observations, many investigators suggest that annexins are capable of linking Ca²⁺, redox and lipid signaling to coordinate vital cellular responses to the environmental stimuli.

Clathrin mediates endocytosis of progastrin and activates MAPKs: role of cell surface annexin A2

Cell-surface-associated annexin A2 (CS-ANXA2) is a nonconventional "receptor" for progastrin; expression levels of both are elevated in colon cancers, and downregulation of either reduces tumorigenic potential of cells. We recently reported internalization of progastrin in target cells. Here, mechanisms mediating internalization of progastrin were examined. Initially, we confirmed that cell-surface ANXA2 mediates binding and internalization of progastrin in intestinal cells. Progastrin, covalently linked to sepharose beads, failed to activate p38MAPK/ERKs, suggesting internalization of progastrin was required for eliciting biological effects; importantly annexin A2 expression and availability of CS-ANXA2 were required for internalization of progastrin. Clathrin expression and formation of clathrin-coated pits were critically required for endocytotic internalization of progastrin; in the absence of clathrin, progastrin failed to activate p38MAPK/ERKs. Downregulation of caveolin had no effect on binding or internalization of progastrin. We therefore demonstrate for the first time that progastrin binds CS-ANXA2 and is rapidly internalized via clathrin-mediated endocytotic pathway, resulting in activation of MAPKinases. Targeting clathrin-mediated endocytosis of progastrin may thus inhibit previously reported co-carcinogenic/tumorigenic effects of progastrin on intestinal cells.

Annexin A2 at the interface of actin and membrane dynamics: a focus on its roles in endocytosis and cell polarization

Annexins are a family of calcium- and phospholipid-binding proteins found in nearly all eukaryotes. They are structurally highly conserved and have been implicated in a wide range of cellular activities. In this paper, we focus on Annexin A2 (AnxA2). Altered expression of this protein has been identified in a wide variety of cancers, has also been found on the HIV particle, and has been implicated in the maturation of the virus. Recently, it has also been shown to have an important role in the establishment of normal apical polarity in epithelial cells. We synthesize here the known biochemical properties of this protein and the extensive literature concerning its involvement in the endocytic pathway. We stress the importance of AnxA2 as a platform for actin remodeling in the vicinity of dynamic cellular membranes, in the hope that this may shed light on the normal functions of the protein and its contribution to disease.