Translocation of annexin XI to neutrophil subcellular organelles

Persistent serum protein signatures define an inflammatory subcategory of long COVID

Long COVID or post-acute sequelae of SARS-CoV-2 (PASC) is a clinical syndrome featuring diverse symptoms that can persist for months following acute SARS-CoV-2 infection. The aetiologies may include persistent inflammation, unresolved tissue damage or delayed clearance of viral protein or RNA, but the biological differences they represent are not fully understood. Here we evaluate the serum proteome in samples, longitudinally collected from 55 PASC individuals with symptoms lasting ≥60 days after onset of acute infection, in comparison to samples from symptomatically recovered SARS-CoV-2 infected and uninfected individuals. Our analysis indicates heterogeneity in PASC and identified subsets with distinct signatures of persistent inflammation. Type II interferon signaling and canonical NF-κB signaling (particularly associated with TNF), appear to be the most differentially enriched signaling pathways, distinguishing a group of patients characterized also by a persistent neutrophil activation signature. These findings help to clarify biological diversity within PASC, identify participants with molecular evidence of persistent inflammation, and highlight dominant pathways that may have diagnostic or therapeutic relevance, including a protein panel that we propose as having diagnostic utility for differentiating inflammatory and non-inflammatory PASC.

Annexin A3 in sepsis: novel perspectives from an exploration of public transcriptome data

According to publicly available transcriptome datasets, the abundance of Annexin A3 (ANXA3) is robustly increased during the course of sepsis; however, no studies have examined the biological significance or clinical relevance of ANXA3 in this pathology. Here we explored this interpretation gap and identified possible directions for future research. Based on reference transcriptome datasets, we found that ANXA3 expression is restricted to neutrophils, is upregulated in vitro after exposure to plasma obtained from septic patients, and is associated with adverse clinical outcomes. Secondly, an increase in ANXA3 transcript abundance was also observed in vivo, in the blood of septic patients in multiple independent studies. ANXA3 is known to mediate calcium-dependent granules-phagosome fusion in support of microbicidal activity in neutrophils. More recent work has also shown that ANXA3 enhances proliferation and survival of tumour cells via a Caspase-3-dependent mechanism. And this same molecule is also known to play a critical role in regulation of apoptotic events in neutrophils. Thus, we posit that during sepsis ANXA3 might either play a beneficial role, by facilitating microbial clearance and resolution of the infection; or a detrimental role, by prolonging neutrophil survival, which is known to contribute to sepsis-mediated organ damage.

Structure of the ALS Mutation Target Annexin A11 Reveals a Stabilising N-Terminal Segment

The functions of the annexin family of proteins involve binding to Ca²⁺, lipid membranes, other proteins, and RNA, and the annexins share a common folded core structure at the C terminus. Annexin A11 (AnxA11) has a long N-terminal region, which is predicted to be disordered, binds RNA, and forms membraneless organelles involved in neuronal transport. Mutations in AnxA11 have been linked to amyotrophic lateral sclerosis (ALS). We studied the structure and stability of AnxA11 and identified a short stabilising segment in the N-terminal end of the folded core, which links domains I and IV. The crystal structure of the AnxA11 core highlights main-chain hydrogen bonding interactions formed through this bridging segment, which are likely conserved in most annexins. The structure was also used to study the currently known ALS mutations in AnxA11. Three of these mutations correspond to buried Arg residues highly conserved in the annexin family, indicating central roles in annexin folding. The structural data provide starting points for detailed structure-function studies of both full-length AnxA11 and the disease variants being identified in ALS.

Annexin A11 gene polymorphism (R230C variant) and sarcoidosis in a Portuguese population

A recent genome-wide association study detected a protective effect for the annexin A11 rs1049550*T allele (R230Cvariant) in susceptibility to sarcoidosis. We evaluated the association between rs1049550 C/T and sarcoidosis susceptibility, distinct disease phenotypes and evolution in a Portuguese population. We performed a case-control study of 208 patients and 197 healthy controls. Samples were genotyped for rs1049550 C/T using real-time polymerase chain reaction. The frequency of the annexin A11 rs1049550*T allele was significantly lower in patients than in controls (33.2 vs 44.9%, P < 0.001). Odds ratio of 0.52 and 0.44 were obtained, respectively for carriers of one (CT) and two (TT) copies normalized to the CC wild-type genotype (P < 0.001). There were no significant differences in patients with and without Löfgren syndrome. A significant increase in the frequency of the T allele was observed in patients with bronchoalveolar lavage (BAL) fluid neutrophilia (P = 0.04). No significant associations were seen for lung function pattern, radiological stages or different forms of disease evolution. Our study confirms that rs1049550*T allele exerts a significant protective effect on sarcoidosis susceptibility. Given the role of annexin A11 in cell division, apoptosis and neutrophil function, this polymorphism may affect key elements of granulomatous and interstitial inflammation in sarcoidosis.

Annexin A11 in disease

Ubiquitously expressed in many cell types, annexin A11 (Anxa11) is a member of the multigene family of Ca(2+)-regulated phospholipid-dependent and membrane-binding annexin proteins. Studies have shown that Anxa11 plays an important role in cell division, Ca(2+) signaling, vesicle trafficking and apoptosis. The deregulation and mutation of Anxa11 are involved in systemic autoimmune diseases, sarcoidosis and the development, chemoresistance and recurrence of cancers. Malfunction of Anxa11 may lead to or enhance the metastasis, invasion and drug resistance of cancers through the platelet-derived growth factor receptor (PDGFR) pathway and/or the mitogen-activated protein kinase (MAPK)/p53 pathway. In a variety of diseases, Anxa11 is most commonly reported to function through interactions with apoptosis-linked gene-2 protein (ALG-2) and/or calcyclin (S100A6). Although it has been little studied, Anxa11 is a promising biomarker for the diagnosis, treatment and prognosis of certain diseases. In this review, the associations of Anxa11 with Ca(2+)-regulated exocytosis, cytokinesis, sex differentiation, autoimmune diseases, thrombolysis and cancers are summarized and interpreted.

The role of calcium signaling in phagocytosis

Immune cells kill microbes by engulfing them in a membrane-enclosed compartment, the phagosome. Phagocytosis is initiated when foreign particles bind to receptors on the membrane of phagocytes. The best-studied phagocytic receptors, those for Igs (FcgammaR) and for complement proteins (CR), activate PLC and PLD, resulting in the intracellular production of the Ca(2+)-mobilizing second messengers InsP3 and S1P, respectively. The ensuing release of Ca(2+) from the ER activates SOCE channels in the plasma and/or phagosomal membrane, leading to sustained or oscillatory elevations in cytosolic Ca(2+) concentration. Cytosolic Ca(2+) elevations are required for efficient ingestion of foreign particles by some, but not all, phagocytic receptors and stringently control the subsequent steps involved in the maturation of phagosomes. Ca(2+) is required for the solubilization of the actin meshwork that surrounds nascent phagosomes, for the fusion of phagosomes with granules containing lytic enzymes, and for the assembly and activation of the superoxide-generating NADPH oxidase complex. Furthermore, Ca(2+) entry only occurs at physiological voltages and therefore, requires the activity of proton channels that counteract the depolarizing action of the phagocytic oxidase. The molecules that mediate Ca(2+) ion flux across the phagosomal membrane are still unknown but likely include the ubiquitous SOCE channels and possibly other types of Ca(2+) channels such as LGCC and VGCC. Understanding the molecular basis of the Ca(2+) signals that control phagocytosis might provide new, therapeutic tools against pathogens that subvert phagocytic killing.

Impaired phagocytic mechanism in annexin 1 null macrophages

The role of the anti-inflammatory protein annexin-A1 (Anx-A1) in the phagocytic process has been investigated using a murine bone marrow culture-derived macrophage model from Anx-A1(+/+) and Anx-A1(-/-) mice. Macrophages prepared from Anx-A1(-/-) mice exhibited a reduced ingestion of zymosan, Neisseria meningitidis or sheep red blood cells, when compared to Anx-A1(+/+) cells and in the case of zymosan this effect was also mirrored by a reduced clearance in vivo when particles were injected into the peritoneal cavity of Anx-A1(-/-) mice. The ablation of the Anx-A1 gene did not cause any apparent cytoskeletal defects associated with particle ingestion but the cell surface expression of the key adhesion molecule CD11b was depressed in the Anx-A1(-/-) cells providing a possible explanation for the attenuated phagocytic potential of these cells. The production of the cytokines TNFalpha and IL-6 was increased in Anx-A1(-/-) macrophages following phagocytosis of all types of particle.

Neutrophil granules and secretory vesicles in inflammation

The neutrophil is a major effector cell of innate immunity. Exocytosis of granules and secretory vesicles plays a pivotal role in most neutrophil functions from early activation to the destruction of phagocytosed microorganisms. Neutrophil granules contain a multitude of antimicrobial and potentially cytotoxic substances that are delivered to the phagosome or to the exterior of the cell following degranulation. This review summarises current knowledge of granule biology and highlights the effects of neutrophil degranulation in the acute inflammatory response.

Dissecting the cellular functions of annexin XI using recombinant human annexin XI-specific autoantibodies cloned by phage display

Functional studies of cellular proteins are often complicated by the lack of well-defined monoclonal antibodies, the production of which is hampered by the highly conserved nature of these cellular proteins across species. Annexin XI, a member of the Ca2+-dependent, phospholipid-binding protein family, is an example of such a protein and was used in studies to devise a strategy using human autoimmune phage display libraries to generate reagents for biological studies of conserved cellular proteins. An IgG phage display library was generated from bone marrow of an autoimmune patient with high serum antibody titer against annexin XI, which was identified recently as an autoantigen targeted by autoantibodies in several systemic autoimmune diseases. From this phage library, a panel of human monoclonal annexin XI-specific Fabs were isolated and applied to studies of the cellular functions of annexin XI. Confocal microscopy showed a cell cycle-specific redistribution of annexin XI from the cytoplasm to the mitotic spindle. In metaphase, annexin XI was up-regulated and costained with alpha-tubulin. The subcellular distribution of annexin XI in COS-7 cells was shown to be Ca2+-dependent, and exhibited a predominantly nuclear pattern at low concentrations and a cytoplasmic pattern at high Ca2+ concentrations. Calcyclin, found previously to bind annexin XI in vitro, in vivo coated the nuclear membrane of cultured cell lines and did not colocalize with annexin XI. Ultrastructural analysis by immunoelectron microscopy revealed that annexin XI associated with specific granules in both neutrophils and eosinophils, suggesting a role in the exocytotic pathway. Our results illuminate the multifunctional nature of human annexin XI, provide the first evidence that annexin XI associates with the mitotic spindles and might play a role in cell division, and clearly illustrate the potential of phage display-derived human autoantibodies in broader analyses of the function of highly conserved cellular proteins.

Annexins: from structure to function

Annexins are Ca2+ and phospholipid binding proteins forming an evolutionary conserved multigene family with members of the family being expressed throughout animal and plant kingdoms. Structurally, annexins are characterized by a highly alpha-helical and tightly packed protein core domain considered to represent a Ca2+-regulated membrane binding module. Many of the annexin cores have been crystallized, and their molecular structures reveal interesting features that include the architecture of the annexin-type Ca2+ binding sites and a central hydrophilic pore proposed to function as a Ca2+ channel. In addition to the conserved core, all annexins contain a second principal domain. This domain, which NH2-terminally precedes the core, is unique for a given member of the family and most likely specifies individual annexin properties in vivo. Cellular and animal knock-out models as well as dominant-negative mutants have recently been established for a number of annexins, and the effects of such manipulations are strikingly different for different members of the family. At least for some annexins, it appears that they participate in the regulation of membrane organization and membrane traffic and the regulation of ion (Ca2+) currents across membranes or Ca2+ concentrations within cells. Although annexins lack signal sequences for secretion, some members of the family have also been identified extracellularly where they can act as receptors for serum proteases on the endothelium as well as inhibitors of neutrophil migration and blood coagulation. Finally, deregulations in annexin expression and activity have been correlated with human diseases, e.g., in acute promyelocytic leukemia and the antiphospholipid antibody syndrome, and the term annexinopathies has been coined.

Annexin A11 (ANXA11) gene structure as the progenitor of paralogous annexins and source of orthologous cDNA isoforms

The genomic organization of the annexin A11 gene was determined in mouse and human to assess its congruity with other family members and to examine the species variation in alternative splicing patterns. Mouse annexin A11 genomic clones were characterized by restriction analysis, Southern blotting, and DNA sequencing, and the homologous human gene (HGMW-approved gene symbol ANXA11) was deciphered from high-throughput genomic sequence with coanalysis of expressed sequence tags. Exons 6-15 of the tetrad core repeat region differ from annexins A7 and A13 but are spliced identically to other phylogenetic descendents, making annexin A11 the putative primary progenitor of up to nine paralogous human annexins. The 5' regions consist of untranslated exon 1, followed by an extensive intron 1 comprising almost half the total gene length of >40 kb, and additional GC-rich exons 2-5 encoding the proline- and glycine-rich amino-terminus. Distinct cDNA isoforms in cow and human were determined to be unique to each species and hence of dubious general significance for this gene's function. Multiple transcription start sites were revealed by primer extension analysis of the mouse gene, and transfection constructs containing the prospective promoter generated transcriptional activity comparable to that of the SV40 promoter. Internal repetitive elements and vicinal gene markers were mapped for the complete human annexin A11 gene sequence to characterize the surrounding genomic environment.

Dynamic ca(2+)changes in neutrophil phagosomes A source for intracellular ca(2+)during phagolysosome formation?

An increase in cytosolic Ca(2+)concentration periphagosomally is critical for phagolysosomal formation and neutrophil elimination of microbes. The Ca(2+)increase could be achieved through release of Ca(2+)from mobilized intracellular stores. Alternatively, Ca(2+)that passively enter the phagosome during phagocytosis could be provided by the phagosome. Intraphagosomal Ca(2+)changes in single human neutrophils was measured during phagocytosis of serum opsonized Fura-2-conjugated zymosan particles, using a digital image processing system for microspectrofluorometry. A decrease in phagosomal Ca(2+)down to nanomolar concentrations was seen within minutes following phagosomal closure. Blockage of plasma membrane Ca(2+)channels by econazole abolished this decrease. The fluorescence properties of Fura-2 zymosan were retained after phagocytosis and stable to pH changes, reactive oxygen species, and proteolytic enzymes. We suggest that Ca(2+)ions present in the phagosome enter the cell cytosol through Ca(2+)channels in the phagosomal membrane, achieving a localized Ca(2+)rise that is important for phagosome processing.

Calcium-dependent secretion in human neutrophils: a proteomic approach

Bactericidal, proteolytic and signal proteins released by activated neutrophils play a major role in infection fighting and inflammatory processes. These proteins are mainly stored in organelles called granules until induction of their controlled exocytosis. The present work deals with the characterization of the proteins which are secreted upon activation of human neutrophils by ionomycin and calcium. Proteins were separated by two-dimensional gel electrophoresis and identified by peptide mass fingerprinting. Almost all the previously described soluble components of neutrophil granules could be identified. Moreover, several additional proteins were shown to be secreted by activated neutrophils, namely calgranulins, human cartilage glycoprotein of 39 kDa (HC gp-39), chitotriosidase, and annexin XI. Their subcellular localization and possible functions are discussed.

Subcellular fractionation of human neutrophils on Percoll density gradients

Subcellular fractionation has been an important tool in the investigation of neutrophil structural organization including granule heterogeneity, composition and mobilization. The resolution of organelles obtained by subcellular fractionation was improved considerably after the introduction of nitrogen cavitation as an efficient but gentle means of disrupting neutrophils and with Percoll as a density medium. This paper describes in detail the methodology of subcellular fractionation of nitrogen cavitated neutrophils on one-, two-, and three-layer Percoll density gradients. Appropriate marker proteins are presented for neutrophil organelles including azurophil, specific and gelatinase granules, in addition to secretory vesicles and plasma membranes. The dynamics of granule and secretory vesicle exocytosis is demonstrated by subcellular fractionation of resting and activated human neutrophils. Finally, the paper describes the applications of subcellular fractionation in the investigation of the localization of neutrophil constituents, in protein purification schemes and in the study of translocation of cytosolic proteins to isolated neutrophil organelles.