Investigation of the relocation of cytosolic phospholipase A2 and annexin V in activated platelets

Annexins and cardiovascular diseases: Beyond membrane trafficking and repair

Cardiovascular diseases (CVD) remain the leading cause of mortality worldwide. The main cause underlying CVD is associated with the pathological remodeling of the vascular wall, involving several cell types, including endothelial cells, vascular smooth muscle cells, and leukocytes. Vascular remodeling is often related with the development of atherosclerotic plaques leading to narrowing of the arteries and reduced blood flow. Atherosclerosis is known to be triggered by high blood cholesterol levels, which in the presence of a dysfunctional endothelium, results in the retention of lipoproteins in the artery wall, leading to an immune-inflammatory response. Continued hypercholesterolemia and inflammation aggravate the progression of atherosclerotic plaque over time, which is often complicated by thrombus development, leading to the possibility of CV events such as myocardial infarction or stroke. Annexins are a family of proteins with high structural homology that bind phospholipids in a calcium-dependent manner. These proteins are involved in several biological functions, from cell structural organization to growth regulation and vesicle trafficking. In vitro gain- or loss-of-function experiments have demonstrated the implication of annexins with a wide variety of cellular processes independent of calcium signaling such as immune-inflammatory response, cell proliferation, migration, differentiation, apoptosis, and membrane repair. In the last years, the use of mice deficient for different annexins has provided insight into additional functions of these proteins in vivo, and their involvement in different pathologies. This review will focus in the role of annexins in CVD, highlighting the mechanisms involved and the potential therapeutic effects of these proteins.

Therapeutic Potential of Annexins in Sepsis and COVID-19

Sepsis is a continuing problem in modern healthcare, with a relatively high prevalence, and a significant mortality rate worldwide. Currently, no specific anti-sepsis treatment exists despite decades of research on developing potential therapies. Annexins are molecules that show efficacy in preclinical models of sepsis but have not been investigated as a potential therapy in patients with sepsis. Human annexins play important roles in cell membrane dynamics, as well as mediation of systemic effects. Most notably, annexins are highly involved in anti-inflammatory processes, adaptive immunity, modulation of coagulation and fibrinolysis, as well as protective shielding of cells from phagocytosis. These discoveries led to the development of analogous peptides which mimic their physiological function, and investigation into the potential of using the annexins and their analogous peptides as therapeutic agents in conditions where inflammation and coagulation play a large role in the pathophysiology. In numerous studies, treatment with recombinant human annexins and annexin analogue peptides have consistently found positive outcomes in animal models of sepsis, myocardial infarction, and ischemia reperfusion injury. Annexins A1 and A5 improve organ function and reduce mortality in animal sepsis models, inhibit inflammatory processes, reduce inflammatory mediator release, and protect against ischemic injury. The mechanisms of action and demonstrated efficacy of annexins in animal models support development of annexins and their analogues for the treatment of sepsis. The effects of annexin A5 on inflammation and platelet activation may be particularly beneficial in disease caused by SARS-CoV-2 infection. Safety and efficacy of recombinant human annexin A5 are currently being studied in clinical trials in sepsis and severe COVID-19 patients.

Cytosolic phospholipase A2-alpha and cyclooxygenase-2 localize to intracellular membranes of EA.hy.926 endothelial cells that are distinct from the endoplasmic reticulum and the Golgi apparatus

Cytosolic phospholipase A2-alpha (cPLA2-alpha) is a calcium-activated enzyme that plays an important role in agonist-induced arachidonic acid release. In endothelial cells, free arachidonic acid can be converted subsequently into prostacyclin, a potent vasodilator and inhibitor of platelet activation, through the action of cyclooxygenase (COX) enzymes. Here we study the relocation of cPLA2-alpha in human EA.hy.926 endothelial cells following stimulation with the calcium-mobilizing agonist, A23187. Relocation of cPLA2-alpha was seen to be highly cell specific, and in EA.hy.926 cells occurred primarily to intracellular structures resembling the endoplasmic reticulum (ER) and Golgi. In addition, relocation to both the inner and outer surfaces of the nuclear membrane was observed. Colocalization studies with markers for these subcellular organelles, however, showed colocalization of cPLA2-alpha with nuclear membrane markers but not with ER or Golgi markers, suggesting that the relocation of cPLA2-alpha occurs to sites that are separate from these organelles. Colocalization with annexin V was also observed at the nuclear envelope, however, little overlap with staining patterns for the potential cPLA2-alpha interacting proteins, annexin I, vimentin, p11 or actin, was seen in this cell type. In contrast, cPLA2-alpha was seen to partially colocalize specifically with the COX-2 isoform at the ER-resembling structures, but not with COX-1. These studies suggest that cPLA2-alpha and COX-2 may function together at a distinct and novel compartment for eicosanoid signalling.

Proteomic approach to identify acute phase response-related proteins with low molecular weight in loach skin following injury

Proteome analysis by two-dimensional gel electrophoresis (2-DE) together with mass spectrometry was applied to screen acute phase response (APR)-related proteins with low molecular weight in loach skin following injury. Furthermore, Western blotting and function tests were applied to confirm the results obtained from the proteomic study. Fifteen APR-related proteins with sixteen spots (PLA with two spots) on a 2-DE map were identified in this study. Furthermore, six were known acute phase proteins including galactose-binding lectin (GBL), lysozyme, C3, CD59, double PLA and 50s ribosomal protein; while ATP kinase, zinc finger protein 183, alpha-neurotoxin homology, angiostatin, serine/threonine kinase, metalloproteinase inhibitor, regulator of G-protein 4, cryptdin-9 and disintegrin trigranin were found by our lab to be APR-related proteins. In addition, our results suggest that proteomes with low molecular weight can be characterized by 2-DE with a Tris-tricine system followed by mass spectrometry.

Cellular biomechanics in the lung

Mechanical forces affect both the function and phenotype of cells in the lung. In this symposium, recent studies were presented that examined several aspects of biomechanics in lung cells and their relationship to disease. Wound healing and recovery from injury in the airways involve epithelial cell spreading and migration on a substrate that undergoes cyclic mechanical deformation; enhanced green fluorescent protein-actin was used in a stable cell line to examine cytoskeletal changes in airway epithelial cells during wound healing. Eosinophils migrate into the airways during asthmatic attacks and can also be exposed to cyclic mechanical deformation; cyclic mechanical stretch caused a decrease in leukotriene C(4) synthesis that may be dependent on mechanotransduction mechanisms involving the production of reactive oxygen species. Recent studies have suggested that proinflammatory cytokines are increased in ventilator-induced lung injury and may be elevated by overdistention of the lung tissue; microarray analysis of human lung epithelial cells demonstrated that cyclic mechanical stretch alone profoundly affects gene expression. Finally, airway hyperresponsiveness is a basic feature of asthma, but the relationship between airway hyperresponsiveness and changes in airway smooth muscle (ASM) function remain unclear. New analysis of the behavior of the ASM cytoskeleton (CSK) suggests, however, that the CSK may behave as a glassy material and that glassy behavior may account for the extensive ASM plasticity and remodeling that contribute to airway hyperresponsiveness. Together, the presentations at this symposium demonstrated the remarkable and varied roles that mechanical forces may play in both normal lung physiology as well as pathophysiology.

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.