Characterization of phospholipase A2 activity in reticulocyte endocytic vesicles

Emerging Potential of Exosomes on Adipogenic Differentiation of Mesenchymal Stem Cells

The mesenchymal stem cells have multidirectional differentiation potential and can differentiate into adipocytes, osteoblasts, cartilage tissue, muscle cells and so on. The adipogenic differentiation of mesenchymal stem cells is of great significance for the construction of tissue-engineered fat and the treatment of soft tissue defects. Exosomes are nanoscale vesicles secreted by cells and widely exist in body fluids. They are mainly involved in cell communication processes and transferring cargo contents to recipient cells. In addition, exosomes can also promote tissue and organ regeneration. Recent studies have shown that various exosomes can influence the adipogenic differentiation of stem cells. In this review, the effects of exosomes on stem cell differentiation, especially on adipogenic differentiation, will be discussed, and the mechanisms and conclusions will be drawn. The main purpose of studying the role of these exosomes is to understand more comprehensively the influencing factors existing in the process of stem cell differentiation into adipocytes and provide a new idea in adipose tissue engineering research.

Exosomes

Exosomes are small, single-membrane, secreted organelles of ∼30 to ∼200 nm in diameter that have the same topology as the cell and are enriched in selected proteins, lipids, nucleic acids, and glycoconjugates. Exosomes contain an array of membrane-associated, high-order oligomeric protein complexes, display pronounced molecular heterogeneity, and are created by budding at both plasma and endosome membranes. Exosome biogenesis is a mechanism of protein quality control, and once released, exosomes have activities as diverse as remodeling the extracellular matrix and transmitting signals and molecules to other cells. This pathway of intercellular vesicle traffic plays important roles in many aspects of human health and disease, including development, immunity, tissue homeostasis, cancer, and neurodegenerative diseases. In addition, viruses co-opt exosome biogenesis pathways both for assembling infectious particles and for establishing host permissiveness. On the basis of these and other properties, exosomes are being developed as therapeutic agents in multiple disease models.

Increased T-helper 17 cell differentiation mediated by exosome-mediated microRNA-451 redistribution in gastric cancer infiltrated T cells

MicroRNA (miR)‐451 is a cell metabolism‐related miRNA that can mediate cell energy‐consuming models by several targets. As miR‐451 can promote mechanistic target of rapamycin (mTOR) activity, and increased mTOR activity is related to increased differentiation of T‐helper 17 (Th17) cells, we sought to investigate whether miR‐451 can redistribute from cancer cells to infiltrated T cells and enhance the distribution of Th17 cells through mTOR. Real‐time PCR was used for detecting expression of miR‐451 in gastric cancer, tumor infiltrated T cells and exosomes, and distribution of Th17 was evaluated by both flow cytometry and immunohistochemistry (IHC). Immunofluorescence staining was used in monitoring the exosome‐enveloped miR‐451 from cancer cells to T cells with different treatments, and signaling pathway change was analyzed by western blot. miR‐451 decreased significantly in gastric cancer (GC) tissues but increased in infiltrated T cells and exosomes; tumor miR‐451 was negatively related to infiltrated T cells and exosome miR‐451. Exosome miR‐451 can not only serve as an indicator for poor prognosis of post‐operation GC patients but is also related to increased Th17 distribution in gastric cancer. miR‐451 can redistribute from cancer cells to T cells with low glucose treatment. Decreased 5′ AMP‐activated protein kinase (AMPK) and increased mTOR activity was investigated in miR‐451 redistributed T cells and the Th17 polarized differentiation of these T cells were also increased. Exosome miR‐451 derived from tumor tissues can serve as an indicator for poor prognosis and redistribution of miR‐451 from cancer cells to infiltrated T cells in low glucose treatment can enhance Th17 differentiation by enhancing mTOR activity.

Exosomes in erythropoiesis

Multivesicular endosomes contain membrane vesicles which can be released into the extracellular environment as exosomes. This review describes the role of exosome secretion in the remodeling of the red cell plasma membrane during the last stage of erythropoietic differentiation. Herein, we propose that the exosome biogenesis involves several mechanisms of protein sorting and leads to partial or complete loss of membrane activities, in some cases in a regulated way.

Biophysical characterization of a new phospholipid analogue with a spin-labeled unsaturated fatty acyl chain

Spin-labeled analogs of phospholipids have been used widely to characterize the biophysical properties of membranes. We describe synthesis and application of a new spin-labeled phospholipid analog, SL-POPC. The advantage of this molecule is that the EPR active doxyl group is linked to an unsaturated fatty acyl chain different to saturated phospholipid analogs used so far. The need for those analogs arises from the fact that biological membranes contain unsaturated phospholipids to a large extent. The biophysical properties of SL-POPC in membranes were characterized using EPR and NMR spectroscopy and compared with those of the saturated spin-labeled phospholipid, SL-PSPC. To this end, POPC membranes were labeled with either analog to assess whether the spin-labeled counterpart SL-POPC mimics the membrane properties better than the often used SL-PSPC. The results show that SL-POPC and SL-PSPC explore different molecular environments of the bilayer, and that the type and degree of perturbation of bilayer caused by the label moiety also differs between both analogs. We found that SL-POPC is more appropriate to assess the versatile dynamics of POPC membranes than SL-PSPC.

Reticulocyte-secreted exosomes bind natural IgM antibodies: involvement of a ROS-activatable endosomal phospholipase iPLA2

Reticulocytes release small membrane vesicles termed exosomes during their maturation into erythrocytes. It has been suggested that reticulocytes remodel the plasma membrane of the immature red cell during erythropoiesis by specifically eliminating various proteins. We report here that exosome release is associated with a physiologic cascade induced by the expression of a 15-lipoxygenase at the reticulocyte stage. We found that the phospholipase iPLA2 specifically associated with the endosomal and exosomal membranes could be activated by reactive oxygen species (ROSs) produced during mitochondria degeneration induced by 15-lipoxygenase. Since iPLA2 has recently been demonstrated to participate in the clearance of apoptotic cells, we investigated its role in vesicle removal. We found that exosomes isolated directly from the blood of an anemic rat or released during in vitro maturation of rat reticulocytes bind IgM antibodies on their surface, in contrast to immature and mature red cells. These natural IgM antibodies recognize lysophosphatidylcholine and are able to specifically bind to apoptotic cells. Finally, evidence of C3 deposition on the exosome surface leads us to hypothesize that this cascade may favor the clearance of exosomes by cells once released into the bloodstream, via a mechanism similar to that involved in the elimination of apoptotic cells.

Exosome lipidomics unravels lipid sorting at the level of multivesicular bodies

Exosomes are part of the family of "bioactive vesicles" and appear to be involved in distal communications between cells. They vehiculate bioactive lipids and lipolytic enzymes and their biogenesis require specific lipids and a membrane reorganisation. Their biogenesis pathway could be a way to secrete enzymes involved in lipid signalling and to generate "particulate agonists". However, this pathway seems also to be used by pathogens such as HIV. This review will consider several aspects of lipidomics studies which might help to understand the fate and role of these fascinating vesicles.

Exosome release by reticulocytes—An integral part of the red blood cell differentiation system

Reticulocyte maturation into erythrocytes is the final step of erythropoiesis that occurs in the blood circulation. This terminal differentiation period corresponds to a cellular remodeling phase following expulsion of the nucleus into the bone marrow. Among other events, this remodeling leads to the disappearance of intracellular organelles and acquisition of the typical cellular biconcave form. Here, we propose that exosome biogenesis and secretion, which contributes to net loss of the cell surface membrane via selective vesicular membrane secretion, is also closely interconnected with upstream (nucleus expulsion), accompanying (mitoptosis) and downstream (vesicle clearance) events.

Phospholipase A(2)s and lipid peroxidation

Lipid peroxidation of membrane phospholipids can proceed both enzymatically via the mammalian 15-lipoxygenase-1 or the NADPH-cytochrome P-450 reductase system and non-enzymatically. In some cells, such as reticulocytes, this process is biologically programmed, whereas in the majority of biological systems lipid peroxidation is a deleterious process that has to be repaired via a deacylation-reacylation cycle of phospholipid metabolism. Several reports in the literature pinpoint a stimulation by lipid peroxidation of the activity of secretory phospholipase A(2)s (mainly pancreatic and snake venom enzymes) which was originally interpreted as a repair function. However, recent experiments from our laboratory have demonstrated that in mixtures of lipoxygenated and native phospholipids the former are not preferably cleaved by either secretory or cytosolic phospholipase A(2)s. We propose that the platelet activating factor (PAF) acetylhydrolases of type II, which cleave preferentially peroxidised or lipoxygenated phospholipids, are competent for the phospholipid repair, irrespective of their role in PAF metabolism. A corresponding role of Ca(2+)-independent phospholipase A(2), which has been proposed to be involved in phospholipid remodelling in biomembranes, has not been addressed so far. Direct and indirect 15-lipoxygenation of phospholipids in biomembranes modulates cell signalling by several ways. The stimulation of phospholipase A(2)-mediated arachidonic acid release may constitute an alternative route of the arachidonic acid cascade. Thus, 15-lipoxygenase-mediated oxygenation of membrane phospholipids and its interaction with phospholipase A(2)s may play a crucial role in the pathogenesis of diseases, such as bronchial asthma and atherosclerosis.

Inhibition of endosome fusion in primary hepatocytes prevents asialoglycoprotein degradation but not uptake of transferrin iron demonstrating that intracellular iron release occurs from early endosomes

A comparison of the effects of inhibitors of membrane fusion on the uptake of asialoglycoprotein and transferrin by primary rat hepatocytes was made. This showed that while high potassium medium inhibited the degradation but not the uptake of asialoorosomucoid, both transferrin endocytosis and iron delivery to the cells were unaffected. This difference between the two pathways was also observed with an inhibitor of phospholipase A2, bromophenacyl bromide. With the latter, it was found that the asialoglycoproteins failed to traverse from a low-density to a high-density intracellular compartment, implying a role for phospholipase A2 in the trafficking of asialoglycoprotein receptor but not that for transferrin or iron. This demonstrates that, after its release from transferrin, iron is transported to the cytoplasm directly from the early endosome without the need for fusion of the iron-containing vesicle with a lysosome.

Exoenzyme S from P. aeruginosa ADP ribosylates rab4 and inhibits transferrin recycling in SLO-permeabilized reticulocytes

ADP-ribosylation of rab proteins by exoenzyme S (Exo S) of P. aeruginosa was studied using reticulocytes. 14-3-3 protein, the eukaryotic cofactor that is obligatory for Exo S activity, was found in association with reticulocyte endocytic vesicles and exosomes, vesicles previously shown to be enriched with rab4. Incubation of purified endocytic vesicles with Exo S triggered rab4 ADP-ribosylation. Transferrin recycling in SLO-permeabilized reticulocytes was highly impaired when Exo S was added to the cells, suggesting that ADP-ribosylation affected rab4 function. Moreover, in vitro ADP-ribosylation of different rab proteins was studied using the cofactor activity extracted from reticulocytes.

Apical ANG II-stimulated PLA2 activity and Na+ flux: a potential role for Ca2+-independent PLA2

Type 1 angiotensin II (ANG II) receptors (AT1R), which mediate proximal tubule (PT) salt and water reabsorption, undergo endocytosis and recycling. Prior studies in a PT-like model (LLC-PKcl4 cells expressing rabbit AT1R) (LLC-PK-AT1R cells) determined that quinacrine, a nonspecific phospholipase A2 (PLA2) inhibitor, and the haloenol lactone suicide substrate (HELSS), a Ca2+-independent PLA2 inhibitor, attenuated apical (AP) AT1R recycling. Further studies were undertaken to examine the association between AT1R endocytotic movement and PLA2 activity in this model. AP ANG II (100 nM) increased [3H]arachidonic acid ([3H]AA) release 4.4 +/- 0.38-fold in LLC-PK-AT1R cells cultured on permeable supports. Basolateral (BL) ANG II had no significant effect. Reversed-phase high-performance liquid chromatography confirmed that AP ANG II stimulated free [3H]AA release. Quinacrine, HELSS, and palmitoyl trifluoromethyl ketone, another Ca2+-independent PLA2 inhibitor, inhibited AP ANG II-stimulated [3H]AA release, as did inhibiting AP AT1R internalization with phenylarsine oxide. The role of HELSS-inhibitable AA release in ANG II-mediated 22Na flux was examined, given the effects of AT1R-mediated PLA2 activity on salt and water reabsorption. AP ANG II (100 nM) stimulated 22Na flux (AP--> BL), a response inhibited by HELSS. Thus, in this model, AP AT1R activated PLA2 with concomitant 22Na flux (AP --> BL), suggesting a link between AP AT1R endocytotic movement, AT1R-stimulated PLA2 activity, and 22Na flux in this model. The effects of HELSS suggest that Ca2+-independent PLA2 activity may be involved in this AP ANG II response.

In vitro fusion of reticulocyte endocytic vesicles with liposomes

Since reticulocytes have a high demand for iron, which is required for heme biosynthesis, these cells are highly specialized in the endocytosis of the iron carrier transferrin (Tf). From the resulting endocytic vesicles (EVs), iron is released and the vesicles rapidly return to the cell membrane where they fuse, causing the release of the apotransferrin. Due to a lack of other intracellular compartments, the endocytic vesicles can be readily isolated. In this study, we have investigated the fusogenic properties of EVs, using liposomes as target membranes. Membrane fusion was monitored by a lipid mixing assay based on the relief of fluorescence self-quenching, using octadecylrhodamine B-chloride (R18). Application of this procedure was verified and solidified by analysis of the fusion event by an independent lipid mixing assay, after in situ labeling of EVs, and by determination of the mixing of aqueous contents. We demonstrate that the endocytic vesicles are particularly prone to fuse with target membranes that contain dioleoylphosphatidylethanolamine (DOPE). Relative to DOPE, bilayers composed of phosphatidylserine or phosphatidylcholine show a reduced fusion activity with EV. The specific and strong inhibition of fusion by cyclosporin A and a peptide known to interfere with the propensity of DOPE to adopt the hexagonal HII phase suggests that the mechanism of fusion involves the ability of this lipid to readily adopt non-bilayer phases. ATP, GTP, and/or cytosol are not necessary to obtain fusion. However, trypsin treatment of the endocytic vesicles inhibits fusion, indicating the involvement of (a) protein(s) in the fusion event.