Bovine PAS-6/7 binds alpha v beta 5 integrins and anionic phospholipids through two domains

MFG-E8: a model of multiple binding modes associated with ps-binding proteins

Membrane-binding proteins often associate with lipid membranes through a singular binding interface which is generally modeled as a two-state system: bound or unbound. However, even a single interface can engage with more than one mode of binding since a variety of interactions can contribute to the binding event. Unfortunately, the ability to clearly delineate the different binding modes of a singular binding interface has been elusive with existing models. Here, we present a study on milk fat globule EGF factor 8 (MFG-E8), which belongs to a class of proteins that identifies and binds phosphatidylserine (PS). These proteins detect membrane dysregulation implicated in exposed PS in apoptosis and malignant cells. In order to elucidate the factors affecting the binding of MFG-E8, we used a model system consisting of a series of lipid vesicles with varying PS mole fraction to identify the sensitivity of MFG-E8's binding affinity to changes in electrostatics using a tryptophan fluorescence spectral shift assay. Using a newly developed model, we experimentally identified three binding modes, each associated with a different number of PS lipids, with its cooperativity for binding being enhanced by the availability of negatively charged lipids. X-ray reflectivity experiments additionally suggest that MFG-E8's binding modes are influenced by membrane packing. The protocols established for elucidating MFG-E8's interaction with lipid membranes under different membrane conditions can be applied to the study of other membrane-binding proteins that target specific membrane attributes, such as fluidity and electrostatics, and help elucidate these membrane targeting mechanisms and their subsequent binding events.

Lactadherin's Multistate Binding Predicts Stable Membrane-Bound Conformations of Factors V and VIII's C Domains

Protein binding to negatively charged lipids is essential for maintaining numerous vital cellular processes where its dysfunction can lead to various diseases. One such protein that plays a crucial role in this process is lactadherin, which competes with coagulation factors for membrane binding sites to regulate blood clotting. Despite identifying key binding regions of these proteins through structural and biochemical studies, models incorporating membrane dynamics are still lacking. In this study, we report on the multimodal binding of lactadherin and use it to gain insight into the binding mechanisms of its C domain homologs, factor V and factor VIII. Molecular dynamics simulations enhanced with the highly mobile mimetic model enabled the determination of lactadherin's multimodal binding on membranes that revealed critical interacting residues consistent with prior NMR and mutagenesis data. The binding occurred primarily via two dynamic structural ensembles: an inserted state and an unreported, highly conserved side-lying state driven by a cationic patch. We utilized these findings to analyze the membrane binding domains of coagulation factors V and VIII and identified their preferred membrane-bound conformations. Specifically, factor V's C domains maintained an inserted state, while factor VIII preferred a tilted, side-lying state that permitted antibody binding. Insight into lactadherin's atomistically resolved membrane interactions from a multistate perspective can guide new therapeutic opportunities in treating diseases related to blood coagulation.

Macrophages in intestinal fibrosis and regression

Intestinal macrophages are heterogenous cell populations with different developmental ontogeny and tissue anatomy. The concerted actions of intestinal macrophage subsets are critical to maintaining tissue homeostasis. However, the dysregulation of macrophages following tissue injury or chronic inflammation could also lead to intestinal fibrosis, with few treatment options in the clinic. In this review, we will characterize the features of intestinal macrophages in light of the latest advances in lineage tracing and single-cell sequencing technology. The roles of macrophages in distinct stages of intestinal fibrosis would be also elaborated. Finally, based on the reciprocal interaction between macrophages and intestinal fibrosis, we will propose the potential macrophage targeting anti-intestinal fibrosis therapies.

Milk polar lipids composition and functionality: a systematic review

Polar lipids including glycerophospholipids and sphingophospholipids are important nutrients and milk is a major source, particularly for infants. This systematic review describes the human and bovine milk polar lipid composition, structural organization, sources for formulation, and physiological functionality. A total of 2840 records were retrieved through Scopus, 378 were included. Bovine milk is a good source of polar lipids, where yield and composition are highly dependent on the choice of dairy streams and processing. In milk, polar lipids are organized in the milk fat globule membrane as a tri-layer encapsulating triglyceride. The overall polar lipid concentration in human milk is dependent on many factors including lactational stage and maternal diet. Here, reasonable ranges were determined where possible. Similar for bovine milk, where differences in milk lipid concentration proved the largest factor determining variation. The role of milk polar lipids in human health has been demonstrated in several areas and critical review indicated that brain, immune and effects on lipid metabolism are best substantiated areas. Moreover, insights related to the milk fat globule membrane structure-function relation as well as superior activity of milk derived polar lipid compared to plant-derived sources are emerging areas of interest regarding future research and food innovations.

StaR-related lipid transfer-like domain-containing protein CLDP43 affects cardiolipin synthesis and mitochondrial function in Trypanosoma brucei

Cardiolipin is known to interact with bacterial and mitochondrial proteins and protein complexes. Unlike in Escherichia coli and Saccharomyces cerevisiae, the synthesis of cardiolipin is essential for growth of Trypanosoma brucei parasites in culture. Inhibition of cardiolipin production has been shown to result in major changes in the T. brucei proteome and energy metabolism, with CLDP43, a mitochondrial protein containing a StaR-related lipid transfer (START)-like domain, being depleted in a cardiolipin-dependent way. We now show that in T. brucei procyclic forms lacking CLDP43, cardiolipin metabolism and mitochondrial function are affected. Using quantitative and qualitative lipid analyses, we found that while steady-state levels of cardiolipin were elevated in CLDP43 knock-out parasites compared to parental cells, de novo formation of cardiolipin was down-regulated. In addition, depletion of CLDP43 resulted in partial loss of mitochondrial membrane potential and decreased ATP production via substrate level phosphorylation. Recombinant CLDP43 was found to bind cardiolipin and phosphatidic acid in lipid overlay experiments, suggesting that it may be involved in transport or synthesis of cardiolipin or its precursors in T. brucei.

How Do Red Blood Cells Die?

Normal human red blood cells have an average life span of about 120 days in the circulation after which they are engulfed by macrophages. This is an extremely efficient process as macrophages phagocytose about 5 million erythrocytes every second without any significant release of hemoglobin in the circulation. Despite large number of investigations, the precise molecular mechanism by which macrophages recognize senescent red blood cells for clearance remains elusive. Red cells undergo several physicochemical changes as they age in the circulation. Several of these changes have been proposed as a recognition tag for macrophages. Most prevalent hypotheses for red cell clearance mechanism(s) are expression of neoantigens on red cell surface, exposure phosphatidylserine and decreased deformability. While there is some correlation between these changes with aging their causal role for red cell clearance has not been established. Despite plethora of investigations, we still have incomplete understanding of the molecular details of red cell clearance. In this review, we have reviewed the recent data on clearance of senescent red cells. We anticipate recent progresses in in vivo red cell labeling and the explosion of modern proteomic techniques will, in near future, facilitate our understanding of red cell senescence and their destruction.

Novel blood coagulation molecules: Skeletal muscle myosin and cardiac myosin

Essentials Striated muscle myosins can promote prothrombin activation by FXa or FVa inactivation by APC. Cardiac myosin and skeletal muscle myosin are pro-hemostatic in murine tail cut bleeding models. Infused cardiac myosin exacerbates myocardial injury caused by myocardial ischemia reperfusion. Skeletal muscle myosin isoforms that circulate in human plasma can be grouped into 3 phenotypes. ABSTRACT: Two striated muscle myosins, namely skeletal muscle myosin (SkM) and cardiac myosin (CM), may potentially contribute to physiologic mechanisms for regulation of thrombosis and hemostasis. Thrombin is generated from activation of prothrombin by the prothrombinase (IIase) complex comprising factor Xa, factor Va, and Ca++ ions located on surfaces where these factors are assembled. We discovered that SkM and CM, which are abundant motor proteins in skeletal and cardiac muscles, can provide a surface for thrombin generation by the prothrombinase complex without any apparent requirement for phosphatidylserine or lipids. These myosins can also provide a surface that supports the inactivation of factor Va by activated protein C/protein S, resulting in negative feedback downregulation of thrombin generation. Although the physiologic significance of these reactions remains to be established for humans, substantive insights may be gleaned from murine studies. In mice, exogenously infused SkM and CM can promote hemostasis as they are capable of reducing tail cut bleeding. In a murine myocardial ischemia-reperfusion injury model, exogenously infused CM exacerbates myocardial infarction damage. Studies of human plasmas show that SkM antigen isoforms of different MWs circulate in human plasma, and they can be used to identify three plasma SkM phenotypes. A pilot clinical study showed that one SkM isoform pattern appeared to be linked to isolated pulmonary embolism. These discoveries enable multiple preclinical and clinical studies of SkM and CM, which should provide novel mechanistic insights with potential translational relevance for the roles of CM and SkM in the pathobiology of hemostasis and thrombosis.

Acidocalcisomes and Polyphosphate Granules Are Different Subcellular Structures in Agrobacterium tumefaciens

Acidocalcisomes are membrane-enclosed, polyphosphate-containing acidic organelles in lower Eukaryota but have also been described for Agrobacterium tumefaciens (M. Seufferheld, M. Vieira, A. Ruiz, C. O. Rodrigues, S. Moreno, and R. Docampo, J Biol Chem 278:29971-29978, 2003, https://doi.org/10.1074/jbc.M304548200). This study aimed at the characterization of polyphosphate-containing acidocalcisomes in this alphaproteobacterium. Unexpectedly, fluorescence microscopic investigation of A. tumefaciens cells using fluorescent dyes and localization of constructed fusions of polyphosphate kinases (PPKs) and of vacuolar H+-translocating pyrophosphatase (HppA) with enhanced yellow fluorescent protein (eYFP) suggested that acidocalcisomes and polyphosphate are different subcellular structures. Acidocalcisomes and polyphosphate granules were frequently located close together, near the cell poles. However, they never shared the same position. Mutant strains of A. tumefaciens with deletions of both ppk genes (Δppk1 Δppk2) were unable to form polyphosphate but still showed cell pole-located eYFP-HppA foci and could be stained with MitoTracker. In conclusion, A. tumefaciens forms polyP granules that are free of a surrounding membrane and thus resemble polyP granules of Ralstonia eutropha and other bacteria. The composition, contents, and function of the subcellular structures that are stainable with MitoTracker and harbor eYFP-HppA remain unclear.IMPORTANCE The uptake of alphaproteobacterium-like cells by ancestors of eukaryotic cells and subsequent conversion of these alphaproteobacterium-like cells to mitochondria are thought to be key steps in the evolution of the first eukaryotic cells. The identification of acidocalcisomes in two alphaproteobacterial species some years ago and the presence of homologs of the vacuolar proton-translocating pyrophosphatase HppA, a marker protein of the acidocalcisome membrane in eukaryotes, in virtually all species within the alphaproteobacteria suggest that eukaryotic acidocalcisomes might also originate from related structures in ancestors of alphaproteobacterial species. Accordingly, alphaproteobacterial acidocalcisomes and eukaryotic acidocalcisomes should have similar features. Since hardly any information is available on bacterial acidocalcisomes, this study aimed at the characterization of organelle-like structures in alphaproteobacterial cells, with A. tumefaciens as an example.

Intravital Vascular Phototheranostics and Real-Time Circulation Dynamics of Micro- and Nanosized Erythrocyte-Derived Carriers

Erythrocyte-based carriers can serve as theranostic platforms for delivery of imaging and therapeutic payloads. Engineering these carriers at micro- or nanoscales makes them potentially useful for broad clinical applications ranging from vascular diseases to tumor theranostics. Longevity of these carriers in circulation is important in delivering a sufficient amount of their payloads to the target. We have investigated the circulation dynamics of micro (∼4.95 μm diameter) and nano (∼91 nm diameter) erythrocyte-derived carriers in real time using near-infrared fluorescence imaging, and evaluated the effectiveness of such carrier systems in mediating photothermolysis of cutaneous vasculature in mice. Fluorescence emission half-lives of micro- and nanosized carriers in response to a single intravenous injection were ∼49 and ∼15 min, respectively. A single injection of microsized carriers resulted in a 3-fold increase in signal-to-noise ratio that remained nearly persistent over 1 h of imaging time. Our results also suggest that a second injection of the carriers 7 days later can induce a transient inflammatory response, as manifested by the apparent leakage of the carriers into the perivascular tissue. The administration of the carriers into the mice vasculature reduced the threshold laser fluence to induce photothermolysis of blood vessels from >65 to 20 J/cm². We discuss the importance of membrane physicochemical and mechanical characteristics in engineering erythrocyte-derived carriers and considerations for their clinical translation.

Phagocytosis by endothelial cells inhibits procoagulant activity of platelets of essential thrombocythemia in vitro

BACKGROUND

Essential thrombocythemia (ET) is characterized by thrombocytosis with increased platelet number and persistent activation. The mechanisms of thrombosis and the fate of these platelets are not clear. The aim of the present study is to explore the phagocytosis of platelets of ET patients by endothelial cells (ECs) in vitro and its relevance to the procoagulant activity (PCA).

METHODS

Phosphatidylserine (PS) exposure on platelets was detected by flow cytometry. Phagocytosis of the platelets by ECs was performed using flow cytometry, confocal microscopy, and electron microscopy. The PCA of platelets was evaluated by coagulation time and purified coagulation complex assays.

RESULTS

The PS exposure on platelets in ET patients is higher than that in healthy controls. The PS-exposed platelets are highly procoagulant and lactadherin reduced 80% of the PCA by blockade of PS. When cocultured, the platelets of ET patients were sequestered by ECs in a time-dependent fashion. Lactadherin enhanced phagocytosis by bridging the PS on activated platelets and the integrin αvβ3 on ECs, and P-selectin played at least a partial role in this process. Furthermore, factor Xa and prothrombinase activity of PS-exposed platelets were decreased after incubation with ECs.

CONCLUSION

Our results suggest that phagocytic clearance of platelets by ECs occurs in ET patients, thus representing a novel mechanism to remove activated platelets from the circulation; lactadherin and phagocytosis could cooperatively limit the thrombophilia in ET patients.

Serum milk fat globule-EGF factor 8 (MFG-E8) as a diagnostic and prognostic biomarker in patients with hepatocellular carcinoma

Current serum hepatocellular carcinoma (HCC) biomarkers are insufficient for early diagnosis. We aimed to clarify whether serum MFG-E8 can serve as a diagnostic or prognostic biomarker of HCC. Serum MFG-E8 levels of 282 HCC patients, who underwent primary hepatectomy, were examined by ELISA. We also quantified serum MFG-E8 levels in patients with chronic hepatitis (CH), liver cirrhosis (LC), as well as in healthy volunteers (HVs). Serum MFG-E8 levels were significantly lower in HCC patients than in HVs regardless of the etiology of liver disease (3.6 ± 0.1 vs 5.8 ± 0.2 ng/mL, p < 0.0001), and recovered after treatment of HCC. Serum MFG-E8 levels in CH and LC patients were comparable to those in HVs. Serum MFG-E8 could detect HCCs, even α-fetoprotein (AFP)-negative or des-γ-carboxy prothrombin (DCP)-negative HCCs, in CH and LC patients. Our new HCC prediction model using MFG-E8 and DCP (Logit(p) = 2.619 − 0.809 × serum MFG-E8 + 0.0226 × serum DCP) distinguished HCC patients from CH and LC patients with an area under the curve of 0.923, a sensitivity of 81.1%, and a specificity of 89.8%. Futhermore, low preoperative serum MFG-E8 was an independent predictor of poor overall survival. Thus, serum MFG-E8 could serve as a feasible diagnostic and prognostic biomarker for HCC.

The significance of tumor cells-derived MFG-E8 in tumor growth of angiosarcoma

BACKGROUND

Previous studies have indicated that MFG-E8 enhances tumor cell survival, invasion and angiogenesis. However, the role of MFG-E8 in angiosarcoma (AS) has not been clarified.

OBJECTIVE

Objective was to elucidate the mechanism of the regulation by MFG-E8 in AS and the association between MFG-E8 and clinicopathological features of AS.

METHODS

The effects of the depletion of MFG-E8 by siRNA on tube formation, migration and proliferation in murine AS cells were examined. The effect of administration of anti-MFG-E8 antibody (Ab) on tumor growth of AS in mice was examined. The associations of MFG-E8 expression and clinicopathological features of human AS were assessed.

RESULTS

The expressions of MFG-E8 in murine and human AS cells were significantly higher than those in melanoma cells, macrophages and endothelial cells. Depletion of MFG-E8 in murine AS cells by siRNA significantly inhibited the formation of capillary-like structures and migration, but not proliferation. Administration of anti-MFG-E8 Ab significantly inhibited tumor growth and decreased the number of tumor-associated macrophages (TAMs) in AS tumors. Tumor size and the number of TAMs in human AS with high expression of MFG-E8 were significantly increased compared to those of AS with low expression of MFG-E8. Progression-free survival and overall survival time of the patients of AS with high expression of MFG-E8 were significantly shorter than those of AS with low expression of MFG-E8.

CONCLUSIONS

AS-derived MFG-E8 might enhance tumor growth via angiogenesis and the induction of TAMs in autocrine/paracrine manner, and administration of anti-MFG-E8 Ab could be a therapeutic potential for AS.

The glycoproteins EDIL3 and MFGE8 regulate vesicle-mediated eggshell calcification in a new model for avian biomineralization

The avian eggshell is a critical physical barrier, which permits extra-uterine development of the embryo. Its formation involves the fastest known biomineralization process in vertebrates. The eggshell consists of proteins and proteoglycans that interact with the mineral phase to impart its specific microstructure and mechanical properties. In this study, we investigated the role of epidermal growth factor (EGF)-like repeats and discoidin-like domains 3 (EDIL3) and milk fat globule-EGF factor 8 (MFGE8), two glycoproteins that are consistently detected in eggshell proteomes. We verified their common evolutionary history and identified the timing of the duplication event giving rise to these two distinct proteins. Edil3/mfge8 chromosomal locations revealed a nested syntenous relationship with other genes (hapln1/hapln3 and vcan/acan) that are also involved in vertebrate calcification. EDIL3 and MFGE8 proteins possess EGF-like and coagulation factor 5/8 (F5/8C) domains, and their 3D structures predicted that they bind calcium and extracellular vesicles. In chicken, we confirmed the presence of EDIL3 and MFGE8 proteins in eggshell, uterine fluid, and uterus. We observed that only edil3 is overexpressed in tissues in which eggshell mineralization takes place and that this overexpression occurs only at the onset of shell calcification. We therefore propose a model in which EDIL3 and, to a lesser extent, MFGE8 proteins guide vesicles containing amorphous calcium carbonate to the mineralization site. This model was supported by the observation that extracellular vesicles accumulate in uterine fluid during eggshell calcification and that they contain high levels of calcium, carbon, and oxygen that correspond to calcium carbonate.

Lactadherin promotes microvesicle clearance to prevent coagulopathy and improves survival of severe TBI mice

Coagulopathy is common in patients with traumatic brain injury (TBI) and predicts poor clinical outcomes. We have shown that brain-derived extracellular microvesicles, including extracellular mitochondria, play a key role in the development of TBI-induced coagulopathy. Here, we further show in mouse models that the apoptotic cell-scavenging factor lactadherin, given at a single dose of 400 μg/kg 30 minutes before (preconditioning) or 30 minutes after cerebral fluid percussion injury, prevented coagulopathy as defined by clotting time, fibrinolysis, intravascular fibrin deposition, and microvascular bleeding of the lungs. Lactadherin also reduced cerebral edema, improved neurological function, and increased survival. It achieved these protective effects by enhancing the clearance of circulating microvesicles through phosphatidylserine-mediated phagocytosis. Together, these results identify the scavenging system for apoptotic cells as a potential therapeutic target to prevent TBI-induced coagulopathy and improve the outcome of TBI.

Clearance of Apoptotic Cells by Tissue Epithelia: A Putative Role for Hepatocytes in Liver Efferocytosis

Toxic substances and microbial or food-derived antigens continuously challenge the liver, which is tasked with their safe neutralization. This vital organ is also important for the removal of apoptotic immune cells during inflammation and has been previously described as a “graveyard” for dying lymphocytes. The clearance of apoptotic and necrotic cells is known as efferocytosis and is a critical liver function to maintain tissue homeostasis. Much of the research into this form of immunological control has focused on Kupffer cells, the liver-resident macrophages. However, hepatocytes (and other liver resident cells) are competent efferocytes and comprise 80% of the liver mass. Little is known regarding the mechanisms of apoptotic and necrotic cell capture by epithelia, which lack key receptors that mediate phagocytosis in macrophages. Herein, we discuss recent developments that increased our understanding of efferocytosis in tissues, with a special focus on the liver parenchyma. We discuss the impact of efferocytosis in health and in inflammation, highlighting the role of phagocytic epithelia.

Determination of lactadherin concentration in dairy by-products by ELISA: Effect of heat treatment and hydrolysis

Lactadherin is a peripheral glycoprotein of the milk fat globule membrane with several attributed biological activities. In this study, we developed an indirect competitive ELISA to determine lactadherin concentration by using a rabbit polyclonal antiserum. The ELISA was applied to quantify lactadherin in several dairy by-products. Of the products tested, raw and commercial buttermilk had the highest concentrations of lactadherin (6.79 and 5.27 mg/g of product, respectively), followed by commercial butter serum (4.86 mg/g), commercial skim milk (4.84 mg/g), and raw whey (1.20 mg/g). The concentration of immunoreactive lactadherin was also determined in dairy by-products after they were subjected to different technological treatments. Thus, raw products were heat treated at combinations of temperature and time typically used in the dairy industry, and commercial products were hydrolyzed using 3 proteolytic enzyme preparations. Heat treatments of whey and buttermilk resulted in a smaller decrease in lactadherin concentration than did hydrolysis as determined by ELISA and electrophoresis. At high temperatures for long durations, the loss of lactadherin was higher in whey than in buttermilk, with the maximal reduction of around 48% found after treating whey at 72°C for 60 min. Hydrolysis of commercial products with proteolytic enzymes resulted in a marked decrease of immunoreactivity within the first 5 min of treatment, which thereafter was constant throughout 4 h of hydrolysis. These results demonstrate that dairy by-products from milk fat processing are good natural sources of lactadherin, although technological processes have to be considered, because they have different effects on lactadherin content.

Lactadherin: An unappreciated haemostasis regulator and potential therapeutic agent

Lactadherin is a small (53-66kDa) multifunctional glycoprotein belonging to the secreted extracellular matrix protein family. It has a multi-domain structure and is involved in many biological and physiological processes, including phagocytosis, angiogenesis, atherosclerosis, tissue remodeling, and haemostasis regulation. Lactadherin binds phosphatidylserine (PS)-enriched cell surfaces in a receptor-independent manner. Interaction between lactadherin and PS is crucial for regulation of blood coagulation processes. This review summarizes recent knowledge on the possible role of lactadherin in haemostasis control, emphasizing the great significance of the interaction between lactadherin and PS expressed on activated platelets and extracellular vesicles. The possible role of lactadherin as a therapeutic target and biomarker is also discussed.

Lactadherin orthologs inhibit migration of human, porcine and murine intestinal epithelial cells

Lactadherin was originally described due to its appearance in milk, but is abundantly expressed especially by professional and nonprofessional phagocytes. The proteins has been shown to have a multitude of bioactive effects, including inhibition of inflammatory phospholipases, induction of effero‐ and phagocytosis, prevent rotavirus induced gastroenteritis, and modulate intestinal homeostasis by regulating epithelial cell migration. The level of expression seems to be important in a row of serious pathologies linked to the intestinal epithelial barrier function, vascular‐ and autoimmune disease. This study examines the ability of lactadherin to modulate migration of intestinal epithelium. A cell exclusion assay is used to quantify the ability of human, bovine and murine lactadherin orthologs to affect migration of primary small intestine epithelium cells. Previous reports show that recombinant murine lactadherin stimulate rat small intestine cell migration. The present study could not confirm this. Conversely, 10 μg/ml lactadherin inhibits migration. Therefore, as lactadherins enteroprotective properties is well established using in vivo models we conclude that the protective effects are linked to lactadherins ability operate as an opsonin, or other modulating effects, and not a direct lactadherin‐cell induction of migration. Thus, the molecular mechanism behind the enteroprotective role of lactadherin remains to be established.

Mesenchymal stem cells-derived MFG-E8 accelerates diabetic cutaneous wound healing

BACKGROUND

Diabetic wounds are intractable due to complex factors, such as the inhibition of angiogenesis, dysfunction of phagocytosis by macrophages and abnormal inflammatory responses. It is recognized that mesenchymal stem cells (MSCs) promote wound healing in diabetic mice. We previously demonstrated that MSCs produce large amounts of MFG-E8.

OBJECT

The objective was to ascertain the role of MSCs-derived MFG-E8 in murine diabetic wounds.

METHODS

MFG-E8 WT/KO MSCs or rMFG-E8 were subcutaneously injected around the wound in diabetic db/db mice, and wound areas were analyzed. Quantification of angiogenesis, infiltrating inflammatory cells, apoptotic cells at the wound area was performed by immunofluorescence staining and real-time PCR. Phagocytosis assay was performed using peritoneal macrophages from WT or db/db mice.

RESULTS

MFG-E8 expression in granulation tissue in diabetic mice was significantly reduced compared with that in non-diabetic mice. We next examined the effect of subcutaneous injection of MFG-E8 WT/KO MSCs around the wound. Diabetic wound healing was significantly accelerated by the injection of MSCs. Diabetic wound healing in MFG-E8 KO MSCs-injected wounds was significantly delayed compared to that in WT MSCs-injected wounds. The numbers of CD31⁺ EC and NG2⁺ pericytes, as well as M2 macrophages in wounds in KO MSCs-injected mice were significantly decreased. MFG-E8 WT MSCs treatment suppressed the number of apoptotic cells and TNF-α⁺ cells in wounds. In an in vitro assay, MFG-E8 WT MSCs-conditioned medium enhanced phagocytosis of apoptotic cells by peritoneal macrophages from diabetic mice.

CONCLUSION

MSCs-derived MFG-E8 might accelerate diabetic wound healing by promoting angiogenesis, the clearance of apoptotic cells, and the infiltration of M2 macrophages, and by suppressing inflammatory cytokines in wound area.

Apoptotic cell recognition receptors and scavenger receptors

Phosphatidylserine recognition receptors are a highly diverse set of receptors grouped by their ability to recognize the 'eat-me' signal phosphatidylserine on apoptotic cells. Most of the phosphatidylserine recognition receptors dampen inflammation by inducing the production of anti-inflammatory mediators during the phagocytosis of apoptotic corpses. However, many phosphatidylserine receptors are also capable of recognizing other ligands, with some receptors being categorized as scavenger receptors. It is now appreciated that these receptors can elicit different downstream events for particular ligands. Therefore, how phosphatidylserine recognition receptors mediate specific signals during recognition of apoptotic cells versus other ligands, and how this might help regulate the inflammatory state of a tissue is an important question that is not fully understood. Here, we revisit the work on signaling downstream of the phosphatidylserine recognition receptor BAI1, and evaluate how these and other signaling modules mediate signaling downstream from other receptors, including Stabilin-2, MerTK, and αvβ5. We also propose the concept that phosphatidylserine recognition receptors could be viewed as a subset of scavenger receptors that are capable of eliciting anti-inflammatory responses to apoptotic cells.

Polyhydroxyalkanoate (PHA) Granules Have no Phospholipids

Polyhydroxybutyrate (PHB) granules, also designated as carbonosomes, are supra-molecular complexes in prokaryotes consisting of a PHB polymer core and a surface layer of structural and functional proteins. The presence of suspected phospholipids in the surface layer is based on in vitro data of isolated PHB granules and is often shown in cartoons of the PHB granule structure in reviews on PHB metabolism. However, the in vivo presence of a phospholipid layer has never been demonstrated. We addressed this topic by the expression of fusion proteins of DsRed2EC and other fluorescent proteins with the phospholipid-binding domain (LactC2) of lactadherin in three model organisms. The fusion proteins specifically localized at the cell membrane of Ralstonia eutropha but did not co-localize with PHB granules. The same result was obtained for Pseudomonas putida, a species that accumulates another type of polyhydroxyalkanoate (PHA) granules related to PHB. Notably, DsRed2EC-LactC2 expressed in Magnetospirillum gryphiswaldense was detected at the position of membrane-enclosed magnetosome chains and at the cytoplasmic membrane but not at PHB granules. In conclusion, the carbonosomes of representatives of α-proteobacteria, β-proteobacteria and γ-proteobacteria have no phospholipids in vivo and we postulate that the PHB/PHA granule surface layers in natural producers generally are free of phospholipids and consist of proteins only.

Molecular Mechanisms Underlying the Regulation of the MFG-E8 Gene Promoter Activity in Physiological and Inflammatory Conditions

Milk fat globule-EGF factor 8 (MFG-E8) is expressed by macrophages and plays an important role in attenuating inflammation and maintaining tissue homeostasis. Previously, we and others found that lipopolysaccharide (LPS) inhibits MFG-E8 gene expression in macrophages. Here, we characterized the 5'-flanking region of the mouse MFG-E8 gene. To functionally analyze the upstream regulatory region of the MFG-E8 gene, a series of luciferase reporter gene constructs containing deleted or mutated regulatory elements were prepared. Using the luciferase assay, we revealed that Sp1 binding motifs within the proximal promoter region were necessary for full activity of the MFG-E8 promoter, whereas AP-1 like binding sequence at -372 played a role in governing the promoter activity at a homeostatic level. With chromatin immunoprecipitation assay, we showed that Sp1 and c-Jun physically interact with the MFG-E8 promoter region in vivo. In addition, Sp1 was found to regulate the MFG-E8 promoter activity positively and c-Jun negatively. Furthermore, we demonstrated that LPS inhibited MFG-E8 promoter activity via targeting Sp1 and AP-1-like motifs in the 5'-flanking region. Collectively, our data indicate that Sp1 and AP-1-related factors are involved in the regulation of MFG-E8 gene transcription by targeting their binding sites in the 5'-flanking region under physiological and inflammatory states.

Expression, purification, and characterization of recombinant human and murine milk fat globule-epidermal growth factor-factor 8

Milk fat globule-epidermal growth factor-factor 8 (MFG-E8), as its name suggests, is a major glycoprotein component of milk fat globules secreted by the mammary epithelium. Although its role in milk fat production is unclear, MFG-E8 has been shown to act as a bridge linking apoptotic cells to phagocytes for removal of these dying cells. MFG-E8 is capable of bridging these two very different cell types via interactions through both its epidermal growth factor (EGF)-like domain(s) and its lectin-type C domains. The EGF-like domain interacts with αVβ3 and αVβ5 integrins on the surface of phagocytes, whereas the C domains bind phosphatidylserine found on the surface of apoptotic cells. In an attempt to purify full-length, recombinant MFG-E8 expressed in either insect cells or CHO cells, we find that it is highly aggregated. Systematic truncation of the domain architecture of MFG-E8 indicates that the C domains are mainly responsible for the aggregation propensity. Addition of Triton X-100 to the conditioned cell culture media allowed partial recovery of non-aggregated, full-length MFG-E8. A more comprehensive detergent screen identified CHAPS as a stabilizer of MFG-E8 and allowed purification of a significant portion of non-aggregated, full-length protein. The CHAPS-stabilized recombinant MFG-E8 retained its natural ability to bind both αVβ3 and αVβ5 integrins and phosphatidylserine suggesting that it is properly folded and active. Herein we describe an efficient purification method for production of non-aggregated, full-length MFG-E8.

Identification of bovine sperm acrosomal proteins that interact with a 32-kDa acrosomal matrix protein

Mammalian fertilization is accomplished by the interaction between sperm and egg. Previous studies from this laboratory have identified a stable acrosomal matrix assembly from the bovine sperm acrosome termed the outer acrosomal membrane-matrix complex (OMC). This stable matrix assembly exhibits precise binding activity for acrosin and N-acetylglucosaminidase. A highly purified OMC fraction comprises three major (54, 50, and 45 kDa) and several minor (38-19 kDa) polypeptides. The set of minor polypeptides (38-19 kDa) termed "OMCrpf polypeptides" is selectively solubilized by high-pH extraction (pH 10.5), while the three major polypeptides (55, 50, and 45 kDa) remain insoluble. Proteomic identification of the OMC32 polypeptide (32 kDa polypeptide isolated from high-pH soluble fraction of OMC) yielded two peptides that matched the NCBI database sequence of acrosin-binding protein. Anti-OMC32 recognized an antigenically related family of polypeptides (OMCrpf polypeptides) in the 38-19-kDa range with isoelectric points ranging between 4.0 and 5.1. Other than glycohydrolases, OMC32 may also be complexed to other acrosomal proteins. The present study was undertaken to identify and localize the OMC32 binding polypeptides and to elucidate the potential role of the acrosomal protein complex in sperm function. OMC32 affinity chromatography of a detergent-soluble fraction of bovine cauda sperm acrosome followed by mass spectrometry-based identification of bound proteins identified acrosin, lactadherin, SPACA3, and IZUMO1. Co-immunoprecipitation analysis also demonstrated the interaction of OMC32 with acrosin, lactadherin, SPACA3, and IZUMO1. Our immunofluorescence studies revealed the presence of SPACA3 and lactadherin over the apical segment, whereas IZUMO1 is localized over the equatorial segment of Triton X-100 permeabilized cauda sperm. Immunoblot analysis showed that a significant portion of SPACA3 was released after the lysophosphatidylcholine (LPC)-induced acrosome reaction, whereas the IZUMO1 and lactadherin polypeptides remain associated to the particulate fraction. Almost entire population of bovine sperm IZUMO1 relocates to the equatorial segment during the LPC-induced acrosome reaction. We propose that the interaction of OMC32 matrix polypeptide with detergent-soluble acrosomal proteins regulates the release of hydrolases/other acrosomal protein(s) during the acrosome reaction.

Fetal hematopoietic stem cells express MFG-E8 during mouse embryogenesis

The milk fat globule-EGF-factor 8 protein (MFG-E8) has been identified in various tissues, where it has an important role in intercellular interactions, cellular migration, and neovascularization. Previous studies showed that MFG-E8 is expressed in different cell types under normal and pathophysiological conditions, but its expression in hematopoietic stem cells (HSCs) during hematopoiesis has not been reported. In the present study, we investigated MFG-E8 expression in multiple hematopoietic tissues at different stages of mouse embryogenesis. Using immunohistochemistry, we showed that MFG-E8 was specifically expressed in CD34⁺ HSCs at all hematopoietic sites, including the yolk sac, aorta-gonad-mesonephros region, placenta and fetal liver, during embryogenesis. Fluorescence-activated cell sorting and polymerase chain reaction analyses demonstrated that CD34⁺ cells, purified from the fetal liver, expressed additional HSC markers, c-Kit and Sca-1, and that these CD34⁺ cells, but not CD34⁻ cells, highly expressed MFG-E8. We also found that MFG-E8 was not expressed in HSCs in adult mouse bone marrow, and that its expression was confined to F4/80⁺ macrophages. Together, this study demonstrates, for the first time, that MFG-8 is expressed in fetal HSC populations, and that MFG-E8 may have a role in embryonic hematopoiesis.

MFG-E8 regulates angiogenesis in cutaneous wound healing

Our research group recently demonstrated that pericytes are major sources of the secreted glycoprotein and integrin ligand lactadherin (MFG-E8) in B16 melanoma tumors, and that MFG-E8 promotes angiogenesis via enhanced PDGF-PDGFRβ signaling mediated by integrin-growth factor receptor crosstalk. However, sources of MFG-E8 and its possible roles in skin physiology are not well characterized. The objective of this study was to characterize the involvement of MFG-E8 in skin wound healing. In the dermis of normal murine and human skin, accumulations of MFG-E8 were found around CD31(+) blood vessels, and MFG-E8 colocalized with PDGFRβ(+), αSMA(+), and NG2(+) pericytes. MFG-E8 protein and mRNA levels were elevated in the dermis during full-thickness wound healing in mice. MFG-E8 was diffusely present in granulation tissue and was localized around blood vessels. Wound healing was delayed in MFG-E8 knockout mice, compared with the wild type, and myofibroblast and vessel numbers in wound areas were significantly reduced in knockout mice. Inhibition of MFG-E8 production with siRNA attenuated the formation of capillary-like structures in vitro. Expression of MFG-E8 in fibrous human granulation tissue with scant blood vessels was less than that in granulation tissue with many blood vessels. These findings suggest that MFG-E8 promotes cutaneous wound healing by enhancing angiogenesis.

Lactadherin inhibits secretory phospholipase A2 activity on pre-apoptotic leukemia cells

Secretory phospholipase A₂ (sPLA₂) is a critical component of insect and snake venoms and is secreted by mammalian leukocytes during inflammation. Elevated secretory PLA₂ concentrations are associated with autoimmune diseases and septic shock. Many sPLA₂’s do not bind to plasma membranes of quiescent cells but bind and digest phospholipids on the membranes of stimulated or apoptotic cells. The capacity of these phospholipases to digest membranes of stimulated or apoptotic cells correlates to the exposure of phosphatidylserine. In the present study, the ability of the phosphatidyl-L-serine-binding protein, lactadherin to inhibit phospholipase enzyme activity has been assessed. Inhibition of human secretory phospholipase A₂-V on phospholipid vesicles exceeded 90%, whereas inhibition of Naja mossambica sPLA₂ plateaued at 50–60%. Lactadherin inhibited 45% of activity of Naja mossambica sPLA₂ and >70% of human secretory phospholipase A₂-V on the membranes of human NB4 leukemia cells treated with calcium ionophore A23187. The data indicate that lactadherin may decrease inflammation by inhibiting sPLA₂.

Endothelial cell phagocytosis of senescent neutrophils decreases procoagulant activity

Abundant senescent neutrophils traverse the vascular compartment and may contribute to pathologic conditions. For example, they become procoagulant when undergoing apoptosis and may contribute to thrombosis or inflammation. Our previous studies demonstrated a dominant clearance pathway in which the neutrophils can be phagocytosed by liver macrophages. The aim of this study was to explore an alternate pathway of neutrophil clearance by endothelial cells. Phagocytosis of the neutrophils by endothelial cells was performed using various experimental approaches includingflow cytometry, confocal microscopy and electron microscopy assays in vitro and in vivo. Procoagulant activity of cultured neutrophils was evaluated by coagulation time, factor Xase and prothrombinase assays. Lactadherin functioned as a novel probe for the detection of phosphatidylserine on apoptotic cells, an opsonin (bridge) between apoptotic cell and phagocyte for promoting phagocytosis, and an efficient anticoagulant for inhibition of factor Xase and thrombin formation. When cultured, purified human neutrophils spontaneously entered apoptosis and developed procoagulant activity that was directly related to the degree of phosphatidylserine exposure. Co-culture of aged neutrophils and endothelial cells resulted in phagocytosis of the neutrophils and prolonged coagulation time. Lactadherin diminished the procoagulant activity and increased the rate of neutrophil clearance. In vivo, neutrophils were sequestered by endothelial cells after blockade of Kupffer cells, a process that was dependent upon both phosphatidylserine exposure and P-selectin expression. Thus, the ability of endothelial cells to clear senescent neutrophils may limit the procoagulant and/or inflammatory impact of these cells.

The impact of MFG-E8 in chronic pancreatitis: potential for future immunotherapy?

Background

The glycoprotein MFG-E8 mediates phagocytic clearance of apoptotic cells and influences the pathogenesis and progression of inflammatory diseases. MFG-E8 was shown to attenuate the progression of inflammation and to improve survival in septic rats. Accumulating evidence suggests an immunomodulatory link between MFG-E8 and the pro-inflammatory chemokine fractalkine, which may determine the severity of pain, fibrosis, and inflammation in chronic pancreatitis (CP).

Methods

The expression and localization of MFG-E8 was investigated in CP (n = 62), and normal pancreas (NP; n = 34) by QRT-PCR, Western-blot and immunohistochemistry analyses. Results were correlated with mRNA expression of fractalkine, CX3CR1, and with the presence and degree of pain and fibrosis. Human pancreatic stellate cells (hPSCs) were isolated from CP tissues and evaluated for MFG-E8 mRNA expression after fractalkine stimulation.

Results

MFG-E8-mRNA was significantly overexpressed in CP and isolated hPSCs when compared to NP. Western-blot and immunohistochemistry analysis confirmed accumulation of MFG-E8 in CP, with noticeably increased MFG-E8 immunoreactivity in tubular complexes. MFG-E8 expression correlated significantly with fractalkine expression, severe fibrosis, and the presence of pain in CP patients. Stimulation of hPSCs with fractalkine led to a significant increase in MFG-E8 expression.

Conclusions

In the present study, we demonstrated for the first time that MFG-E8 is significantly up-regulated in CP patients and together with fractalkine correlated noticeably with severe fibrosis and the presence of pain. hPSCs overexpress MFG-E8 upon fractalkine stimulation in vitro, which underlines the suggested immunmodulatory link in CP and may be a key mechanism in CP fibrogenesis and pain generation. Taken together, these novel findings suggest that MFG-E8 blockade may be a promising tool for future immunotherapy in CP to attenuate both fibrosis and pain sensation.

Re-evaluation of milk-fat globule EGF-factor VIII (MFG-E8) as an intrinsic component of the mouse milk-fat globule membrane

In lactating mammary glands, milk fat is secreted as fat globules surrounded by a cell plasma membrane containing characteristic membrane-associated proteins. Among these, butyrophilin has been shown to be specific and intrinsic to the fat globule membrane, whereas milk-fat globule EGF-factor VIII (MFG-E8) is uncertain. We characterized in the present study MFG-E8 in milk fat globules and in the culture medium of HC11 mammary epithelial cells. MFG-E8 was immunologically detected in the mammary tissues of both pregnant and lactating mice. Double-immunofluorescence staining for MFG-E8 and butyrophilin showed diversity in the MFG-E8-staining intensity among different fat globules in milk. HC11 cells secreted monomeric MFG-E8 with phosphatidylserine-binding activity, despite no fat globules being detected in the cells. This secretion was upregulated by not only prolactin but also by insulin or EGF. These results suggest that milk MFG-E8 was not equally present among fat globules and not necessarily intrinsic to the fat globules.

Recombinant human MFG-E8 attenuates intestinal injury and mortality in severe whole body irradiation in rats

The gastrointestinal (GI) syndrome component of acute radiation syndrome (ARS) results from depletion of immature parenchymal stem cells after high dose irradiation and contributes significantly to early mortality. It is associated with severe, irreparable damage in the GI tract and extremely low survival. There is a need for the development of viable mitigators of whole body irradiation (WBI) due to the possibility of unexpected high level radiation exposure from nuclear accidents or attacks. We therefore examined the effect of recombinant human milk fat globule-EGF factor 8 (rhMFG-E8) in mitigating damage after WBI. Male Sprague-Dawley rats were exposed to 10 Gy WBI using Cesium-137 as the radiation source. The animals in the treatment group received rhMFG-E8 (166 µg/kg BW) subcutaneously once a day with the first dose given 6 h after WBI. Blood and tissue samples from the ileum were collected after 3 days of treatment. A separate cohort of animals was treated for 7 days and the 21 day mortality rate was determined. Treatment with rhMFG-E8 significantly improved the survival from 31% to 75% over 21 days. Furthermore, rhMFG-E8 treatment resulted in a 36% reduction in the radiation injury intestinal mucosal damage score, corresponding to visible histological changes. MFG-E8 gene expression was significantly decreased in WBI-induced animals as compared to sham controls. Treatment with rhMFG-E8 increased p53 and p21 expression by 207% and 84% compared to untreated controls. This was accompanied by an 80% increase in the expression of anti-apoptotic cell regulator Bcl-2. p53 and p21 levels correlate with improved survival after radiation injury. These cell regulators arrest the cell after DNA damage and enable DNA repair as well as optimize cell survival. Taken together, these results indicate that rhMFG-E8 ameliorates the GI syndrome and improves survival after WBI by minimizing intestinal cell damage and optimizing recovery.

Stability of milk fat globule membrane proteins toward human enzymatic gastrointestinal digestion

The milk fat globule membrane (MFGM) fraction refers to the thin film of polar lipids and membrane proteins that surrounds fat globules in milk. It is its unique biochemical composition that renders MFGM with some beneficial biological activities, such as anti-adhesive effects toward pathogens. However, a prerequisite for the putative bioactivity of MFGM is its stability during gastrointestinal digestion. We, therefore, subjected MFGM material, isolated from raw milk, to an in vitro enzymatic gastrointestinal digestion. Sodium dodecyl sulfate PAGE, in combination with 2 staining methods, Coomassie Blue and periodic acid Schiff staining, was used to evaluate polypeptide patterns of the digest, whereas mass spectrometry was used to confirm the presence of specific MFGM proteins. Generally, it was observed that glycoproteins showed higher resistance to endogenous proteases compared with non-glycosylated proteins. Mucin 1 displayed the highest resistance to digestion and a considerable part of this protein was still detected at its original molecular weight after gastric and small intestine digestion. Cluster of differentiation 36 was also quite resistant to pepsin. A significant part of periodic acid Schiff 6/7 survived the gastric digestion, provided that the lipid moiety was not removed from the MFGM material. Overall, MFGM glycoproteins are generally more resistant to gastrointestinal digestion than serum milk proteins and the presence of lipids, besides glycosylation, may protect MFGM glycoproteins from gastrointestinal digestion. This gastrointestinal stability makes MFGM glycoproteins amenable to further studies in which their putative health-promoting effects can be explored.

Exosomes as biomarker enriched microvesicles: characterization of exosomal proteins derived from a panel of prostate cell lines with distinct AR phenotypes

Prostate cancer is the leading type of cancer diagnosed in men. In 2010, ~217,730 new cases of prostate cancer were reported in the United States. Prompt diagnosis of the disease can substantially improve its clinical outcome. Improving capability for early detection, as well as developing new therapeutic targets in advanced disease are research priorities that will ultimately lead to better patient survival. Eukaryotic cells secrete proteins via distinct regulated mechanisms which are either ER/Golgi dependent or microvesicle mediated. The release of microvesicles has been shown to provide a novel mechanism for intercellular communication. Exosomes are nanometer sized cup-shaped membrane vesicles which are secreted from normal and cancerous cells. They are present in various biological fluids and are rich in characteristic proteins. Exosomes may thus have potential both in facilitating early diagnosis via less invasive procedures or be candidates for novel therapeutic approaches for castration resistance prostate cancer. Because exosomes have been shown previously to have a role in cell-cell communication in the local tumor microenvironment, conferring activation of numerous survival mechanisms, we characterized constitutive lipids, cholesterol and proteins from exosomes derived from six prostate cell lines and tracked their uptake in both cancerous and benign prostate cell lines respectively. Our comprehensive proteomic and lipidomic analysis of prostate derived exosomes could provide insight for future work on both biomarker and therapeutic targets for the treatment of prostate cancer.

Potentiation of platelet-derived growth factor receptor-β signaling mediated by integrin-associated MFG-E8

OBJECTIVE

Pericytes/pericyte precursors produce milk fat globule-associated protein with epidermal growth factor and factor VIII-like domains (MFG-E8) in vivo, and this α(v) integrin ligand enhances angiogenesis in tumors and in oxygen-induced retinopathy in mice. Inhibition of MFG-E8 production or function attenuates platelet-derived growth factor-BB (PDGF-BB)-induced migration of pericyte/pericyte precursor-like 10T1/2 cells in vitro. Herein, we describe mechanisms by which MFG-E8 modulates PDGF-BB:PDGF receptor β (PDGFRβ) signaling in 10T1/2 cells.

METHODS AND RESULTS

Small interfering RNA depletion of MFG-E8 from 10T1/2 cells or antibody inhibition of MFG-E8 action enhanced PDGF-BB-dependent degradation of PDGFRβ and attenuated signaling. Coimmunoprecipitation revealed transient association of MFG-E8 with PDGFRβ in PDGF-BB-treated 10T1/2 cells and reduced PDGFRβ-focal adhesion kinase association in MFG-E8-depleted cells. Confocal microscopy demonstrated that MFG-E8 binding to 10T1/2 cells was RGD motif and α(v) dependent but PDGF-BB treatment independent, whereas colocalization of MFG-E8 with PDGFRβ was enhanced by PDGF-BB. Ubiquitination of PDGFRβ was also increased in MFG-E8 small interfering RNA-transfected cells.

CONCLUSION

Integrin α(v)-bound MFG-E8 associates with PDGFRβ and focal adhesion kinase after PDGF-BB treatment, results in cell surface retention of PDGFRβ, delays receptor degradation, potentiates downstream signaling, and enhances migration of 10T1/2 cells. MFG-E8 may promote angiogenesis, in part, via cell autonomous actions on pericytes or pericyte precursors that result in enhanced PDGF-BB:PDGFRβ signaling mediated via integrin-growth factor receptor cross-talk.

A role for activated endothelial cells in red blood cell clearance: implications for vasopathology

BACKGROUND

Phosphatidylserine exposure by red blood cells is acknowledged as a signal that initiates phagocytic removal of the cells from the circulation. Several disorders and conditions are known to induce phosphatidylserine exposure. Removal of phosphatidylserine-exposing red blood cells generally occurs by macrophages in the spleen and liver. Previously, however, we have shown that endothelial cells are also capable of erythrophagocytosis. Key players in the erythrophagocytosis by endothelial cells appeared to be lactadherin and α(v)-integrin. Phagocytosis via the phosphatidylserine-lactadherin-α(v)-integrin pathway is the acknowledged route for removal of apoptotic innate cells by phagocytes.

DESIGN AND METHODS

Endothelial cell phagocytosis of red blood cells was further explored using a more (patho)physiological approach. Red blood cells were exposed to oxidative stress, induced by tert-butyl hydroperoxide. After opsonization with lactadherin, red blood cells were incubated with endothelial cells to study erythrophagocytosis and examine cytotoxicity.

RESULTS

Red blood cells exposed to oxidative stress show alterations such as phosphatidylserine exposure and loss of deformability. When incubated with endothelial cells, marked erythrophagocytosis occurred in the presence of lactadherin under both static and flow conditions. As a consequence, intracellular organization was disturbed and endothelial cells were seen to change shape ('rounding up'). Increased expression of apoptotic markers indicated that marked erythrophagocytosis has cytotoxic effects.

CONCLUSIONS

Activated endothelial cells show significant phagocytosis of phosphatidylserine-exposing and rigid red blood cells under both static and flow conditions. This results in a certain degree of cytotoxicity. We postulate that activated endothelial cells play a role in red blood cell clearance in vivo. Significant erythrophagocytosis can induce endothelial cell loss, which may contribute to vasopathological effects as seen, for instance, in sickle cell disease.

Airway tissue factor-dependent coagulation activity in response to sulfur mustard analog 2-chloroethyl ethyl sulfide

Acute lung injury is a principal cause of morbidity and mortality in response to mustard gas (SM) inhalation. Obstructive, fibrin-containing airway casts have recently been reported in a rat inhalation model employing the SM analog 2-chloroethyl ethyl sulfide (CEES). The present study was designed to identify the mechanism(s) causing activation of the coagulation cascade after CEES-induced airway injury. Here we report that CEES inhalation elevates tissue factor (TF) activity and numbers of detached epithelial cells present in lavage fluid (BALF) from rats after exposure (18 h). In vitro studies using 16HBE cells, or with rat BALF, indicated that detached epithelial cells could convert factor X (FX) to the active form FXa when incubated with factor VII and could elicit rapid clotting of plasma. In addition, immunocytochemical analysis demonstrated elevated cell surface (TF) expression on CEES-exposed 16HBE cells as a function of time. However, total cell TF expression did not increase. Since membrane surfaces bearing TF are important determinants of clot initiation, anticoagulants directed against these entities were tested for ability to limit plasma clotting or FX activation capacity of BALF or culture media. Addition of tifacogin, a TF pathway inhibitor, effectively blocked either activity, demonstrating that the procoagulant actions of CEES were TF pathway dependent. Lactadherin, a protein capable of competing with clotting factors for phospholipid-binding sites, was partially effective in limiting these procoagulant actions. These findings indicate that TF pathway inhibition could be an effective strategy to prevent airway obstruction after SM or CEES inhalation.

Recombinant human MFG-E8 attenuates cerebral ischemic injury: its role in anti-inflammation and anti-apoptosis

Excessive inflammation and apoptosis contribute to the pathogenesis of ischemic stroke. MFG-E8 is a 66-kDa glycoprotein that has shown tissue protection in various models of organ injury. However, the potential role of MFG-E8 in cerebral ischemia has not been investigated. We found that levels of MFG-E8 protein in the brain were reduced at 24 h after cerebral ischemia. To assess the potential role of MFG-E8 in cerebral ischemia, adult male Sprague-Dawley rats were subjected to permanent middle cerebral artery occlusion (MCAO). At 1 h post-stroke onset, an intravenous administration of 1 ml saline as vehicle or 160 μg/kg BW recombinant human MFG-E8 (rhMFG-E8) as treatment was given. The optimal dose of rhMFG-E8 was obtained from previous dose-response organ protection in rat sepsis studies. Neurological scores were determined at 24 h and 48 h post-MCAO. Rats were sacrificed thereafter and brains rapidly removed and analyzed for infarct size, histopathology, and markers of inflammation and apoptosis. Compared with saline vehicle, rhMFG-E8 treatment led to significant decreases in sensorimotor and vestibulomotor deficits, and infarct size at 24 h and 48 h post-MCAO. Measures associated with improved outcome included reduced microglial inflammatory cytokine secretion, adhesion molecules and neutrophil influx, cleaved caspase-3, and upregulation of peroxisome proliferator activated receptor-γ (PPAR-γ), and Bcl-2/Bax ratio leading to decreased apoptosis. Thus, rhMFG-E8 treatment is neuroprotective against cerebral ischemia through suppression of inflammation and apoptosis. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Lactadherin binds to phosphatidylserine-containing vesicles in a two-step mechanism sensitive to vesicle size and composition

Lactadherin binds to phosphatidylserine (PS) in a stereospecific and calcium independent manner that is promoted by vesicle curvature. Because membrane binding of lactadherin is supported by a PS content of as little as 0.5%, lactadherin is a useful marker for cell stress where limited PS is exposed, as well as for apoptosis where PS freely traverses the plasma membrane. To gain further insight into the membrane-binding mechanism, we have utilized intrinsic lactadherin fluorescence. Our results indicate that intrinsic fluorescence increases and is blue-shifted upon membrane binding. Stopped-flow kinetic experiments confirm the specificity for PS and that the C2 domain contains a PS recognition motif. The stopped-flow kinetic data are consistent with a two-step binding mechanism, in which initial binding is followed by a slower step that involves either a conformational change or an altered degree of membrane insertion. Binding is detected at concentrations down to 0.03% PS and the capacity of binding reaches saturation around 1% PS (midpoint 0.15% PS). Higher concentrations of PS (and also to some extent PE) increase the association kinetics and the affinity. Increasing vesicle curvature promotes association. Remarkably, replacement of vesicles with micelles destroys the specificity for PS lipids. We conclude that the vesicular environment provides optimal conditions for presentation and recognition of PS by lactadherin in a simple binding mechanism. This article is part of a Special Issue entitled: Protein Folding in Membranes.

MFG-E8/lactadherin regulates cyclins D1/D3 expression and enhances the tumorigenic potential of mammary epithelial cells

Milk fat globule-EGF factor 8 (MFG-E8) is a glycoprotein highly expressed in breast cancer that contributes to tumor progression through largely undefined mechanisms. By analyzing publicly available gene expression profiles of breast carcinomas, we found that MFG-E8 is highly expressed in primary and metastatic breast carcinomas, associated with absent estrogen receptor expression. Immunohistochemistry analysis of breast cancer biopsies revealed that MFG-E8 is expressed on the cell membrane as well as in the cytoplasm and nucleus. We also show that increased expression of MFG-E8 in mammary carcinoma cells increases their tumorigenicity in immunodeficient mice, and conversely, its downregulation reduces their in vivo growth. Moreover, expression of MFG-E8 in immortalized mammary epithelial cells promotes their growth and branching in three-dimensional collagen matrices and induces the expression of cyclins D1/D3 and N-cadherin. A mutant protein unable to bind integrins can in part exert these effects, indicating that MFG-E8 function is only partially dependent on integrin activation. We conclude that MFG-E8-dependent signaling stimulates cell proliferation and the acquisition of mesenchymal properties and contributes to mammary carcinoma development.

Pericyte-derived MFG-E8 regulates pathologic angiogenesis

OBJECTIVE

MFG-E8 (also called lactadherin and SED1) is a secreted glycoprotein that has been previously implicated in enhancement of vascular endothelial growth factor-dependent angiogenesis. Major sources of MFG-E8 in vivo and precise mechanisms of MFG-E8 action remain undetermined. The objective of this study was to identify important sources of MFG-E8 in vivo and further elucidate the role(s) of MFG-E8 in the regulation of angiogenesis.

METHODS AND RESULTS

We used knockout mice and anti-MFG-E8 antibodies to study MFG-E8 function in vivo. In melanomas and in retinas of mice with oxygen-induced retinopathy, MFG-E8 colocalized with pericytes rather than endothelial cells, and platelet-derived growth factor receptor β+ pericytes/pericyte precursors purified from tumors contained large amounts of MFG-E8 mRNA. Tumor- and retinopathy-associated angiogenesis was diminished in MFG-E8 knockout mice, and pericyte coverage of neovessels was reduced. Inhibition of MFG-E8 production by 10T1/2 cells (surrogate pericyte/pericyte precursors) using small interfering RNAs and short hairpin RNAs, or inhibition of MFG-E8 action with some anti-MFG-E8 antibodies, selectively attenuated migration in vitro. Significantly, the anti-MFG-E8 antibodies that inhibited 10T1/2 cell migration in vitro also inhibited pathological angiogenesis in vivo.

CONCLUSIONS

These studies strongly implicate MFG-E8 in pericyte/pericyte precursor function and indicate that MFG-E8-directed therapeutics may merit further development.

Sensing phosphatidylserine in cellular membranes

Phosphatidylserine, a phospholipid with a negatively charged head-group, is an important constituent of eukaryotic cellular membranes. On the plasma membrane, rather than being evenly distributed, phosphatidylserine is found preferentially in the inner leaflet. Disruption of this asymmetry, leading to the appearance of phosphatidylserine on the surface of the cell, is known to play a central role in both apoptosis and blood clotting. Despite its importance, comparatively little is known about phosphatidylserine in cells: its precise subcellular localization, transmembrane topology and intracellular dynamics are poorly characterized. The recent development of new, genetically-encoded probes able to detect phosphatidylserine within live cells, however, is leading to a more in-depth understanding of the biology of this phospholipid. This review aims to give an overview of the current methods for phosphatidylserine detection within cells, and some of the recent realizations derived from their use.