Golgi-associated cPLA2alpha regulates endothelial cell-cell junction integrity by controlling the trafficking of transmembrane junction proteins

Signaling mechanisms underlying lymphatic vessel dysfunction in skin aging and possible anti-aging strategies

Aging-related skin diseases are gradually increasing due to the imbalance of cutaneous homeostasis in the aging population. Skin aging-induced inflammation promotes systemic inflammation and may lead to whole-body aging. Lymphatic vessels play an important role in maintaining fluid and homeostasis balance. In intrinsically aged skin, the number of lymphatic vessels decrease and their functions decline, which is related to the reduced adhesion junctions between lymphatic endothelial cells, particularly VE-cadherin. VEGFC/VEGFR-3 signal pathway plays an important role in remodeling and expansion of lymphatic vessels; the downregulation of this pathway contributes to the dysfunction of lymphatic vessels. Meanwhile, we proposed some additional mechanisms. Decline of the pumping activity of lymphatic vessels might be related to age-related changes in extracellular matrix, ROS increase, and eNOS/iNOS disturbances. In extrinsically aged skin, the hyperpermeability of lymphatic vessels results from a decrease in endothelial-specific tight junction molecules, upregulation of VEGF-A, and downregulation of the VEGFC/VEGFR-3 signaling pathway. Furthermore, some of the Phyto therapeutics could attenuate skin aging by modulating the lymphatic vessels. This review summarized the lymphatic vessel dysfunction in skin aging and anti-aging strategies based on lymphatic vessel modulation.

Toxoplasma gondii Dysregulates Barrier Function and Mechanotransduction Signaling in Human Endothelial Cells

Toxoplasma gondii can infect and replicate in vascular endothelial cells prior to entering host tissues. However, little is known about the molecular interactions at the parasite-endothelial cell interface. We demonstrate that T. gondii infection of primary human umbilical vein endothelial cells (HUVEC) altered cell morphology and dysregulated barrier function, increasing permeability to low-molecular-weight polymers. T. gondii disrupted vascular endothelial cadherin (VE-cadherin) and β-catenin localization to the cell periphery and reduced VE-cadherin protein expression. Notably, T. gondii infection led to reorganization of the host cytoskeleton by reducing filamentous actin (F-actin) stress fiber abundance under static and microfluidic shear stress conditions and by reducing planar cell polarity. RNA sequencing (RNA-Seq) comparing genome-wide transcriptional profiles of infected to uninfected endothelial cells revealed changes in gene expression associated with cell-cell adhesion, extracellular matrix reorganization, and cytokine-mediated signaling. In particular, genes downstream of Hippo signaling and the biomechanical sensor and transcriptional coactivator Yes-associated protein (YAP) were downregulated in infected endothelial cells. Interestingly, T. gondii infection activated Hippo signaling by increasing phosphorylation of LATS1, leading to cytoplasmic retention of YAP, and reducing YAP target gene expression. These findings suggest that T. gondii infection triggers Hippo signaling and YAP nuclear export, leading to an altered transcriptional profile of infected endothelial cells.IMPORTANCE Toxoplasma gondii is a foodborne parasite that infects virtually all warm-blooded animals and can cause severe disease in individuals with compromised or weakened immune systems. During dissemination in its infected hosts, T. gondii breaches endothelial barriers to enter tissues and establish the chronic infections underlying the most severe manifestations of toxoplasmosis. The research presented here examines how T. gondii infection of primary human endothelial cells induces changes in cell morphology, barrier function, gene expression, and mechanotransduction signaling under static conditions and under the physiological conditions of shear stress found in the bloodstream. Understanding the molecular interactions occurring at the interface between endothelial cells and T. gondii may provide insights into processes linked to parasite dissemination and pathogenesis.

Herpes Simplex Virus 1-Induced Blood-Brain Barrier Damage Involves Apoptosis Associated With GM130-Mediated Golgi Stress

Herpes simplex encephalitis (HSE) caused by herpes simplex virus 1 (HSV-1) infection can lead to a high mortality rate and severe neurological sequelae. The destruction of the blood-brain barrier (BBB) is an important pathological mechanism for the development of HSE. However, the specific mechanism underlying the BBB destruction remains unclear. Our previous study found that the Golgi apparatus (GA) plays a crucial role in maintaining the integrity of the BBB. Therefore, this present study aimed to investigate the role of the GA in the destruction of the BBB and its underlying mechanisms. Mouse brain endothelial cells (Bend.3) were cultured to establish a BBB model in vitro, and then infected with HSV-1. The results showed that HSV-1 infection caused downregulation of the Golgi-associated protein GM130, accompanied by Golgi fragmentation, cell apoptosis, and downregulation of tight junction proteins occludin and claudin 5. Knockdown of GM130 with small interfering RNA in uninfected Bend.3 cells triggered Golgi fragmentation, apoptosis, and downregulation of occludin and claudin 5. However, overexpression of GM130 in HSV-1 infected Bend.3 cells by transient transfection partially attenuated the aforementioned damage caused by HSV-1 infection. When the pan-caspase inhibitor Z-VAD-fmk was used after HSV-1 infection to inhibit apoptosis, the protein levels of GM130, occludin and claudin 5 were partially restored. Taken together, these observations indicate that HSV-1 infection of Bend.3 cells triggers a GM130-mediated Golgi stress response that is involved in apoptosis, which in turn results in downregulation of occludin and claudin 5 protein levels. Meanwhile, GM130 downregulation is partially due to apoptosis triggered by HSV-1 infection. Our findings reveal an association between the GA and the BBB during HSV-1 infection and identify potentially novel targets for protecting the BBB and therapeutic approaches for patients with HSE.

Pathophysiology of aged lymphatic vessels

Lymphatic vessels maintain body homeostasis by recirculation of fluid and cells. Cell senescence induces lymphatic dysfunction. Impaired contractile function is caused by low muscle cell investiture and decrease of nitric oxide in aged lymphatic collectors, leading to poor drainage of lymph. Aging-induced loss of endothelial glycocalyx and production of inflammatory cytokines increases permeability of lymphatic vessels. In addition, aging-associated basal activation of mast cells delays immune response. In this review, we summarize the structural and pathological changes of aged lymphatic vessels, and discuss the underlying molecular mechanisms.

VE-cadherin regulates migration inhibitory factor synthesis and release

OBJECTIVE

Vascular endothelial (VE)-cadherin-mediated adherens junction is critical to maintain endothelial integrity. Besides its role of homophilic intercellular adhesion, VE-cadherin also has a role of outside-in signaling with functional consequences for vascular physiology. However, the nature of these signals remains not completely understood.

MATERIALS AND METHODS

Human umbilical vein endothelial cells (HUVECs) were used in cell culture experiments. Confluent HUVECs were treated with VE-cadherin function-blocking antibodies BV9 (50 μg/ml) or IgG control. Antibody array was used to screen for cytokine/chemokine in supernatant. For VE-cadherin knockdown, siRNA transfection was used. ELISA, Western blot, and qRT-PCR were used to confirm the expression of screened cytokine/chemokine. To explore the possible mechanisms, Scr phosphorylation was detected and Scr inhibitor PP2 (1 μM) was used. To investigate in vivo relevance of the findings, BV9 and the indicated neutralizing antibodies were injected into mice and then lung vascular leak and inflammation were examined by Evans blue assay and lung tissue H&E, respectively.

RESULTS

Using a non-biased, high-throughout human cytokine/chemokine antibody array, we first found that disruption of VE-cadherin-mediated adhesion by function-blocking antibody BV9 triggered the release of migration inhibitory factor (MIF). This VE-cadherin-mediated release of MIF further confirmed by ELISA with both VE-cadherin blocking antibody and siRNA technique was due to enhanced expression of MIF mRNA, which was mediated by Src kinase activation. In addition, in vivo lung vascular leak induced by VE-cadherin function-blocking antibody was partly alleviated by neutralizing MIF.

CONCLUSIONS

VE-cadherin regulates MIF synthesis and release via Src kinase. Our data provide additional evidence to the concept that VE-cadherin transfers intracellular signals to coordinate the state of cell-cell adhesion with gene expression.

Possible Involvement of Intracellular Calcium-Independent Phospholipase A2 in the Release of Secretory Phospholipases from Mast Cells-Increased Expression in Ileal Mast Cells of Crohn's Disease

Increased activity of secretory phospholipases A₂ (sPLA₂) type-II was previously observed in ileum of Crohn’s disease (CD). Our aims were to explore the involvement of calcium-independent (i)PLA₂β in the release of sPLA₂s from the human mast cell (MC) line (HMC-1) and investigate expressions of cytosolic (c)PLA₂α, iPLA₂β, sPLA₂-IIA and sPLA₂-V in MCs of CD ileum. The release of sPLA₂ was investigated in HMC-1 by immunocytochemistry and ELISA. The expression intensities of PLA₂s in mucosal MCs, and the proportion of PLA₂-positive MCs, were investigated in normal ileum and in ileum from patients with CD by immunohistochemistry. The calcium ionophore-stimulated release of sPLA₂-IIA and sPLA₂-V from HMC-1 was reduced by the iPLA₂-inhibitor bromoenol lactone. All four PLA₂s were detectable in mucosal MCs, both in normal ileum and in CD, but the proportion of iPLA₂β-containing mucosal MCs and the expression intensity of sPLA₂-IIA was increased in CD. Results indicate that iPLA₂β is involved in the secretion of sPLA₂s from HMC-1, and suggest that iPLA₂β-mediated release of sPLA₂ from intestinal MCs may contribute to CD pathophysiology. Ex vivo studies on isolated mucosal mast cells are however needed to clarify the precise role of MC PLA₂s in the inflammatory processes of CD.

Expression of Cytosolic Phospholipase A2 Alpha in Glioblastoma Is Associated With Resistance to Chemotherapy

BACKGROUND

The clinical management of glioblastoma is still challenging despite aggressive surgery and radio-chemotherapy approaches. Better understanding of the molecules involved in glioblastoma chemoresistance is necessary to improve the treatment and predict prognosis.

MATERIALS AND METHODS

We analyzed the expression and possible roles of cytosolic phospholipase A2 alpha (cPLA2α) in human glioblastoma cell lines and patient samples using immunohistochemistry and cellular assays. We analyzed the signaling pathways that cPLA2α regulates in glioblastoma cells using western blot analysis.

RESULTS

Our work demonstrated that cPLA2α is upregulated in glioblastoma compared with normal neuron cells. The expression of cPLA2α varies in multiple glioblastoma cell lines and is associated with chemoresistance rather than tumor development. cPLA2α depletion moderately inhibits glioblastoma growth and survival but remarkably sensitizes chemo-resistant glioblastoma cells to several chemotherapeutic agents. Mechanistically, cPLA2α knockdown significantly suppresses the PI3K/Akt/mTOR pathway in glioblastoma cells.

CONCLUSIONS

We are the first to identify the important role of cPLA2α in glioblastoma in response to chemotherapy. Our data also suggest that cPLA2α may serve as a biomarker to indicate prognosis of glioblastoma patients with high level of cPLA2α to chemotherapy.

Env7p Associates with the Golgin Protein Imh1 at the trans-Golgi Network in Candida albicans

A multitier regulation exists at the trans-Golgi network in all higher organisms. We report a palmitoylated protein kinase, Env7, that functions at the TGN interface by interacting with two more TGN-resident proteins, namely, Imh1 and Arl1. Palmitoylation seems to be important for the specific localization. This study focuses on the involvement of a ubiquitous protein kinase, whose substrates had not yet been reported from any organism, as an upstream signaling component that modulates the activity of the Imh1-Arl1 complex crucial for maintaining membrane asymmetry. Virulence is significantly diminished in an Env7 mutant. The functioning of this protein in C. albicans seems to be quite different from its nearest homologue in S. cerervisiae, which reflects the evolutionary divergence between these two organisms.

Vesicular dynamics is one of the very important aspects of cellular physiology, an imbalance of which leads to the disorders or diseases in higher eukaryotes. We report the functional characterization of a palmitoylated protein kinase from Candida albicans whose homologue in Saccharomyces cerevisiae has been reported to be involved in negative regulation of membrane fusion and was named Env7. However, the downstream target of this protein remains to be identified. Env7 in C. albicans (CaEnv7) could be isolated from the membrane fraction and localized to vesicular structures associated with the Golgi apparatus. Our work reports Env7 in C. albicans as a new player involved in maintaining the functional dynamics at the trans-Golgi network (TGN) by interacting with two other TGN-resident proteins, namely, Imh1p and Arl1p. Direct interaction could be detected between Env7p and the golgin protein Imh1p. Env7 is itself phosphorylated (Env7p) and phosphorylates Imh1 in vivo. An interaction between Env7 and Imh1 is required for the targeted localization of Imh1. CaEnv7 has a putative palmitoylation site toward both N and C termini. An N-terminal palmitoylation-defective strain retains its ability to phosphorylate Imh1 in vitro. An ENV7 homozygous mutant showed compromised filamentation in solid media and attenuated virulence, whereas an overexpressed strain affected cell wall integrity. Thus, Env7 plays a subtle but important role at the level of multitier regulation that exists at the TGN.

IMPORTANCE A multitier regulation exists at the trans-Golgi network in all higher organisms. We report a palmitoylated protein kinase, Env7, that functions at the TGN interface by interacting with two more TGN-resident proteins, namely, Imh1 and Arl1. Palmitoylation seems to be important for the specific localization. This study focuses on the involvement of a ubiquitous protein kinase, whose substrates had not yet been reported from any organism, as an upstream signaling component that modulates the activity of the Imh1-Arl1 complex crucial for maintaining membrane asymmetry. Virulence is significantly diminished in an Env7 mutant. The functioning of this protein in C. albicans seems to be quite different from its nearest homologue in S. cerervisiae, which reflects the evolutionary divergence between these two organisms.

Rab5-mediated VE-cadherin internalization regulates the barrier function of the lung microvascular endothelium

The small GTPase Rab5 has been well defined to control the vesicle-mediated plasma membrane protein transport to the endosomal compartment. However, its function in the internalization of vascular endothelial (VE)-cadherin, an important component of adherens junctions, and as a result regulating the endothelial cell polarity and barrier function remain unknown. Here, we demonstrated that lipopolysaccharide (LPS) simulation markedly enhanced the activation and expression of Rab5 in human pulmonary microvascular endothelial cells (HPMECs), which is accompanied by VE-cadherin internalization. In parallel, LPS challenge also induced abnormal cell polarity and dysfunction of the endothelial barrier in HPMECs. LPS stimulation promoted the translocation of VE-cadherin from the plasma membrane to intracellular compartments, and intracellularly expressed VE-cadherin was extensively colocalized with Rab5. Small interfering RNA (siRNA)-mediated depletion of Rab5a expression attenuated the disruption of LPS-induced internalization of VE-cadherin and the disorder of cell polarity. Furthermore, knockdown of Rab5 inhibited the vascular endothelial hyperpermeability and protected endothelial barrier function from LPS injury, both in vitro and in vivo. These results suggest that Rab5 is a critical mediator of LPS-induced endothelial barrier dysfunction, which is likely mediated through regulating VE-cadherin internalization. These findings provide evidence, implicating that Rab5a is a potential therapeutic target for preventing endothelial barrier disruption and vascular inflammation.

Cytosolic phospholipase A₂: physiological function and role in disease

The group IV phospholipase A2 (PLA2) family is comprised of six intracellular enzymes (GIVA, -B, -C, -D, -E, and -F) commonly referred to as cytosolic PLA2 (cPLA2)α, -β, -γ, -δ, -ε, and -ζ. They contain a Ser-Asp catalytic dyad and all except cPLA2γ have a C2 domain, but differences in their catalytic activities and subcellular localization suggest unique regulation and function. With the exception of cPLA2α, the focus of this review, little is known about the in vivo function of group IV enzymes. cPLA2α catalyzes the hydrolysis of phospholipids to arachidonic acid and lysophospholipids that are precursors of numerous bioactive lipids. The regulation of cPLA2α is complex, involving transcriptional and posttranslational processes, particularly increases in calcium and phosphorylation. cPLA2α is a highly conserved widely expressed enzyme that promotes lipid mediator production in human and rodent cells from a variety of tissues. The diverse bioactive lipids produced as a result of cPLA2α activation regulate normal physiological processes and disease pathogenesis in many organ systems, as shown using cPLA2α KO mice. However, humans recently identified with cPLA2α deficiency exhibit more pronounced effects on health than observed in mice lacking cPLA2α, indicating that much remains to be learned about this interesting enzyme.

Scaffolding protein GOPC regulates tight junction structure

GOPC (FIG/PIST/CAL) is a PDZ-domain scaffolding protein that regulates the trafficking of a wide array of proteins, including small GTPases, receptors and cell surface molecules such as cadherin 23 and cystic fibrosis transmembrane regulator. In Madin-Darby canine kidney (MDCK) cells, we find that GOPC localizes to the trans-Golgi network (TGN) but not to the cis- or trans-Golgi cisternae. Colocalization occurs with the early endosome Rab GTPase Rab5 and a TGN/endosome marker Rab14 but not with Rab11, a marker of recycling endosomes. No localization of GOPC was detected to the lateral membranes or tight junctions. Knockdown of GOPC in MDCK cells results in decreased transepithelial resistance and increased paracellular flux. This might be attributable to the compromised trafficking of tight junction components from the TGN, as GOPC-knockdown cells have decreased lateral labeling of the tight junction protein claudin-1 and decreased protein levels of claudin-2. GOPC might mediate the trafficking of newly synthesized tight junction proteins from the TGN to the cell surface or the recycling of these proteins from specialized endosomal compartments.

Gβ1γ2 activates phospholipase A2-dependent Golgi membrane tubule formation

Heterotrimeric G proteins transduce the ligand binding of transmembrane G protein coupled receptors into a variety of intracellular signaling pathways. Recently, heterotrimeric Gβγ subunit signaling at the Golgi complex has been shown to regulate the formation of vesicular transport carriers that deliver cargo from the Golgi to the plasma membrane. In addition to vesicles, membrane tubules have also been shown to mediate export from the Golgi complex, which requires the activity of cytoplasmic phospholipase A₂ (PLA₂) enzyme activity. Through the use of an in vitro reconstitution assay with isolated Golgi complexes, we provide evidence that Gβ1γ2 signaling also stimulates Golgi membrane tubule formation. In addition, we show that an inhibitor of Gβγ activation of PLA₂ enzymes inhibits in vitro Golgi membrane tubule formation. Additionally, purified Gβγ protein stimulates membrane tubules in the presence of low (sub-threshold) cytosol concentrations. Importantly, this Gβγ stimulation of Golgi membrane tubule formation was inhibited by treatment with the PLA₂ antagonist ONO-RS-082. These studies indicate that Gβ1γ2 signaling activates PLA₂ enzymes required for Golgi membrane tubule formation, thus establishing a new layer of regulation for this process.

PAFAH Ib phospholipase A2 subunits have distinct roles in maintaining Golgi structure and function

Recent studies showed that the phospholipase subunits of Platelet Activating Factor Acetylhydrolase (PAFAH) Ib, α1 and α2 partially localize to the Golgi complex and regulate its structure and function. Using siRNA knockdown of individual subunits, we find that α1 and α2 perform overlapping and unique roles in regulating Golgi morphology, assembly, and secretory cargo trafficking. Knockdown of either α1 or α2 reduced secretion of soluble proteins, but neither single knockdown reduced secretion to the same degree as knockdown of both. Knockdown of α1 or α2 inhibited reassembly of an intact Golgi complex to the same extent as knockdown of both. Transport of VSV-G was slowed but at different steps in the secretory pathway: reduction of α1 slowed trans Golgi network to plasma membrane transport, whereas α2 loss reduced endoplasmic reticulum to Golgi trafficking. Similarly, knockdown of either subunit alone disrupted the Golgi complex but with markedly different morphologies. Finally, knockdown of α1, or double knockdown of α1 and α2, resulted in a significant redistribution of kinase dead protein kinase D from the Golgi to the plasma membrane, whereas loss of α2 alone had no such effect. These studies reveal an unexpected complexity in the regulation of Golgi structure and function by PAFAH Ib. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

Endothelial cell senescence is associated with disrupted cell-cell junctions and increased monolayer permeability

Background

Cellular senescence is associated with cellular dysfunction and has been shown to occur in vivo in age-related cardiovascular diseases such as atherosclerosis. Atherogenesis is accompanied by intimal accumulation of LDL and increased extravasation of monocytes towards accumulated and oxidized LDL, suggesting an affected barrier function of vascular endothelial cells. Our objective was to study the effect of cellular senescence on the barrier function of non-senescent endothelial cells.

Methods

Human umbilical vein endothelial cells were cultured until senescence. Senescent cells were compared with non-senescent cells and with co-cultures of non-senescent and senescent cells. Adherens junctions and tight junctions were studied. To assess the barrier function of various monolayers, assays to measure permeability for Lucifer Yellow (LY) and horseradish peroxidase (PO) were performed.

Results

The barrier function of monolayers comprising of senescent cells was compromised and coincided with a change in the distribution of junction proteins and a down-regulation of occludin and claudin-5 expression. Furthermore, a decreased expression of occludin and claudin-5 was observed in co-cultures of non-senescent and senescent cells, not only between senescent cells but also along the entire periphery of non-senescent cells lining a senescent cell.

Conclusions

Our findings show that the presence of senescent endothelial cells in a non-senescent monolayer disrupts tight junction morphology of surrounding young cells and increases the permeability of the monolayer for LY and PO.

Regulation of the Golgi complex by phospholipid remodeling enzymes

The mammalian Golgi complex is a highly dynamic organelle consisting of stacks of flattened cisternae with associated coated vesicles and membrane tubules that contribute to cargo import and export, intra-cisternal trafficking, and overall Golgi architecture. At the morphological level, all of these structures are continuously remodeled to carry out these trafficking functions. Recent advances have shown that continual phospholipid remodeling by phospholipase A (PLA) and lysophospholipid acyltransferase (LPAT) enzymes, which deacylate and reacylate Golgi phospholipids, respectively, contributes to this morphological remodeling. Here we review the identification and characterization of four cytoplasmic PLA enzymes and one integral membrane LPAT that participate in the dynamic functional organization of the Golgi complex, and how some of these enzymes are integrated to determine the relative abundance of COPI vesicle and membrane tubule formation. This article is part of a Special Issue entitled Lipids and Vesicular Transport.

A VE-cadherin-PAR3-α-catenin complex regulates the Golgi localization and activity of cytosolic phospholipase A(2)α in endothelial cells

The rapid regulation of phospholipase A₂ activity is essential for vascular function. Evidence is found for a VE-cadherin–α-catenin–PAR3 complex regulating the reversible association of cPLA₂α with the Golgi apparatus in confluent endothelial cells. This regulation is important for controlling both cPLA₂α activity and angiogenesis.

Phospholipase A₂ enzymes hydrolyze phospholipids to liberate arachidonic acid for the biosynthesis of prostaglandins and leukotrienes. In the vascular endothelium, group IV phospholipase A₂α (cPLA₂α) enzyme activity is regulated by reversible association with the Golgi apparatus. Here we provide evidence for a plasma membrane cell adhesion complex that regulates endothelial cell confluence and simultaneously controls cPLA₂α localization and enzymatic activity. Confluent endothelial cells display pronounced accumulation of vascular endothelial cadherin (VE-cadherin) at cell–cell junctions, and mechanical wounding of the monolayer stimulates VE-cadherin complex disassembly and cPLA₂α release from the Golgi apparatus. VE-cadherin depletion inhibits both recruitment of cPLA₂α to the Golgi and formation of tubules by endothelial cells. Perturbing VE-cadherin and increasing the soluble cPLA₂α fraction also stimulated arachidonic acid and prostaglandin production. Of importance, reverse genetics shows that α-catenin and δ-catenin, but not β-catenin, regulates cPLA₂α Golgi localization linked to cell confluence. Furthermore, cPLA₂α Golgi localization also required partitioning defective protein 3 (PAR3) and annexin A1. Disruption of F-actin internalizes VE-cadherin and releases cPLA₂α from the adhesion complex and Golgi apparatus. Finally, depletion of either PAR3 or α-catenin promotes cPLA₂α-dependent endothelial tubule formation. Thus a VE-cadherin–PAR3–α-catenin adhesion complex regulates cPLA₂α recruitment to the Golgi apparatus, with functional consequences for vascular physiology.

Critical role of cPLA2 in Aβ oligomer-induced neurodegeneration and memory deficit

Soluble beta-amyloid (Aβ) oligomers are considered to putatively play a critical role in the early synapse loss and cognitive impairment observed in Alzheimer's disease. We previously demonstrated that Aβ oligomers activate cytosolic phospholipase A(2) (cPLA(2)), which specifically releases arachidonic acid from membrane phospholipids. We here observed that cPLA(2) gene inactivation prevented the alterations of cognitive abilities and the reduction of hippocampal synaptic markers levels noticed upon a single intracerebroventricular injection of Aβ oligomers in wild type mice. We further demonstrated that the Aβ oligomer-induced sphingomyelinase activation was suppressed and that phosphorylation of Akt/protein kinase B (PKB) was preserved in neuronal cells isolated from cPLA(2)(-/-) mice. Interestingly, expression of the Aβ precursor protein (APP) was reduced in hippocampus homogenates and neuronal cells from cPLA(2)(-/-) mice, but the relationship with the resistance of these mice to the Aβ oligomer toxicity requires further investigation. These results therefore show that cPLA(2) plays a key role in the Aβ oligomer-associated neurodegeneration, and as such represents a potential therapeutic target for the treatment of Alzheimer's disease.

A PLA1-2 punch regulates the Golgi complex

The mammalian Golgi complex, trans Golgi network (TGN) and ER-Golgi intermediate compartment (ERGIC) are comprised of membrane cisternae, coated vesicles and membrane tubules, all of which contribute to membrane trafficking and maintenance of their unique architectures. Recently, a new cast of players was discovered to regulate the Golgi and ERGIC: four unrelated cytoplasmic phospholipase A (PLA) enzymes, cPLA(2)α (GIVA cPLA(2)), PAFAH Ib (GVIII PLA(2)), iPLA(2)-β (GVIA-2 iPLA(2)) and iPLA(1)γ. These ubiquitously expressed enzymes regulate membrane trafficking from specific Golgi subcompartments, although there is evidence for some functional redundancy between PAFAH Ib and cPLA(2)α. Three of these enzymes, PAFAH Ib, cPLA(2)α and iPLA(2)-β, exert effects on Golgi structure and function by inducing the formation of membrane tubules. We review our current understanding of how PLA enzymes regulate Golgi and ERGIC morphology and function.

The phospholipase A₂ enzyme complex PAFAH Ib mediates endosomal membrane tubule formation and trafficking

For the first time, a cytoplasmic phospholipase A2 enzyme, platelet-activating factor acetylhydrolase (I)b, is described that is directly involved in the formation of membrane tubules from endosomes and trafficking through the endocytic recycling pathway.

Previous studies have shown that membrane tubule–mediated export from endosomal compartments requires a cytoplasmic phospholipase A₂ (PLA₂) activity. Here we report that the cytoplasmic PLA₂ enzyme complex platelet-activating factor acetylhydrolase (PAFAH) Ib, which consists of α1, α2, and LIS1 subunits, regulates the distribution and function of endosomes. The catalytic subunits α1 and α2 are located on early-sorting endosomes and the central endocytic recycling compartment (ERC) and their overexpression, but not overexpression of their catalytically inactive counterparts, induced endosome membrane tubules. In addition, overexpression α1 and α2 altered normal endocytic trafficking; transferrin was recycled back to the plasma membrane directly from peripheral early-sorting endosomes instead of making an intermediate stop in the ERC. Consistent with these results, small interfering RNA–mediated knockdown of α1 and α2 significantly inhibited the formation of endosome membrane tubules and delayed the recycling of transferrin. In addition, the results agree with previous reports that PAFAH Ib α1 and α2 expression levels affect the distribution of endosomes within the cell through interactions with the dynein regulator LIS1. These studies show that PAFAH Ib regulates endocytic membrane trafficking through novel mechanisms involving both PLA₂ activity and LIS1-dependent dynein function.

Recent progress in phospholipase A₂ research: from cells to animals to humans

Mammalian genomes encode genes for more than 30 phospholipase A₂s (PLA₂s) or related enzymes, which are subdivided into several classes including low-molecular-weight secreted PLA₂s (sPLA₂s), Ca²+-dependent cytosolic PLA₂s (cPLA₂s), Ca²+-independent PLA₂s (iPLA₂s), platelet-activating factor acetylhydrolases (PAF-AHs), lysosomal PLA₂s, and a recently identified adipose-specific PLA. Of these, the intracellular cPLA₂ and iPLA₂ families and the extracellular sPLA₂ family are recognized as the "big three". From a general viewpoint, cPLA₂α (the prototypic cPLA₂ plays a major role in the initiation of arachidonic acid metabolism, the iPLA₂ family contributes to membrane homeostasis and energy metabolism, and the sPLA₂ family affects various biological events by modulating the extracellular phospholipid milieus. The cPLA₂ family evolved along with eicosanoid receptors when vertebrates first appeared, whereas the diverse branching of the iPLA₂ and sPLA₂ families during earlier eukaryote development suggests that they play fundamental roles in life-related processes. During the past decade, data concerning the unexplored roles of various PLA₂ enzymes in pathophysiology have emerged on the basis of studies using knockout and transgenic mice, the use of specific inhibitors, and information obtained from analysis of human diseases caused by mutations in PLA₂ genes. This review focuses on current understanding of the emerging biological functions of PLA₂s and related enzymes.

A role for phospholipase A2 activity in membrane tubule formation and TGN trafficking

We have investigated the role of phospholipase A(2) (PLA(2) ) enzymes in generating membrane tubules at the trans-Golgi network (TGN). Constitutive TGN membrane tubules and those induced by over-expressing kinase dead protein kinase D were inhibited by the PLA(2) inhibitors ONO-RS-082 (ONO) and bromoenol lactone. These antagonists also inhibited secretory delivery of both soluble and transmembrane cargoes. Finally, use of the reversible antagonist ONO and time-lapse imaging revealed for the first time that PLA(2) antagonists inhibit the initiation of membrane tubule formation at the TGN. Thus, PLA(2) enzymes appear to have an important role in the earliest steps of membrane tubule formation at the TGN, which are utilized for membrane trafficking.

VE-cadherin: at the front, center, and sides of endothelial cell organization and function

Endothelial cells form cell-cell adhesive structures, called adherens and tight junctions, which maintain tissue integrity, but must be dynamic for leukocyte transmigration during the inflammatory response and cellular remodeling during angiogenesis. This review will focus on Vascular Endothelial (VE)-cadherin, an endothelial-specific cell-cell adhesion protein of the adherens junction complex. VE-cadherin plays a key role in endothelial barrier function and angiogenesis, and consequently VE-cadherin availability and function are tightly regulated. VE-cadherin also participates directly and indirectly in intracellular signaling pathways that control cell dynamics and cell cycle progression. Here we highlight recent work that has advanced our understanding of multiple regulatory and signaling mechanisms that converge on VE-cadherin and have consequences for endothelial barrier function and angiogenic remodeling.

The phospholipase complex PAFAH Ib regulates the functional organization of the Golgi complex

We report that platelet-activating factor acetylhydrolase (PAFAH) Ib, comprised of two phospholipase A(2) (PLA(2)) subunits, alpha1 and alpha2, and a third subunit, the dynein regulator lissencephaly 1 (LIS1), mediates the structure and function of the Golgi complex. Both alpha1 and alpha2 partially localize on Golgi membranes, and purified catalytically active, but not inactive alpha1 and alpha2 induce Golgi membrane tubule formation in a reconstitution system. Overexpression of wild-type or mutant alpha1 or alpha2 revealed that both PLA(2) activity and LIS1 are important for maintaining Golgi structure. Knockdown of PAFAH Ib subunits fragments the Golgi complex, inhibits tubule-mediated reassembly of intact Golgi ribbons, and slows secretion of cargo. Our results demonstrate a cooperative interplay between the PLA(2) activity of alpha1 and alpha2 with LIS1 to facilitate the functional organization of the Golgi complex, thereby suggesting a model that links phospholipid remodeling and membrane tubulation to dynein-dependent transport.

Localization and function of cytosolic phospholipase A2alpha at the Golgi

Cytosolic phospholipase A(2)alpha (cPLA(2)alpha, Group IVA phospholipase A(2)) is a central mediator of arachidonate release from cellular phospholipids for the biosynthesis of eicosanoids. cPLA(2)alpha translocates to intracellular membranes including the Golgi in response to a rise in intracellular calcium level. The enzyme's calcium-dependent phospholipid-binding C2 domain provides the targeting specificity for cPLA(2)alpha translocation to the Golgi. However, other features of cPLA(2)alpha regulation are incompletely understood such as the role of phosphorylation of serine residues in the catalytic domain and the function of basic residues in the cPLA(2)alpha C2 and catalytic domains that are proposed to interact with anionic phospholipids in the membrane to which cPLA(2)alpha is targeted. Increasing evidence strongly suggests that cPLA(2)alpha plays a role in regulating Golgi structure, tubule formation and intra-Golgi transport. For example, recent data suggests that cPLA(2)alpha regulates the transport of tight junction and adherens junction proteins through the Golgi to cell-cell contacts in confluent endothelial cells. However, there are now examples where data based on knockdown using siRNA or pharmacological inhibition of enzymatic activity of cPLA(2)alpha affects fundamental cellular processes yet these phenotypes are not observed in cells from cPLA(2)alpha deficient mice. These results suggest that in some cases there may be compensation for the lack of cPLA(2)alpha. Thus, there is continued need for studies employing highly specific cPLA(2)alpha antagonists in addition to genetic deletion of cPLA(2)alpha in mice.

Vascular integrity mediated by vascular endothelial cadherin and regulated by sphingosine 1-phosphate and angiopoietin-1

Development of blood vessels is coordinated by angiogenesis and stabilization of vascular endothelial cells (ECs). The vascular network is established during embryogenesis to supply oxygen and nutrients to the tissues and organs. However, after cardiac or peripheral ischemia is caused by occlusion of the vessels, new vessels must be formed to rescue the ischemic tissues. Many angiogenic growth factors and chemokines are produced in the ischemic tissue to induce angiogenic sprouting of preexisting vessels. Branched vessels must be again restabilized to form mature vessels that deliver blood to the tissues. To this end, vascular EC-cell adhesion is tightly regulated by cell-cell adhesion molecules and extracellular stimuli that activate G protein-coupled receptors and receptor tyrosine kinases exclusively expressed on vascular ECs. This review spotlights the recent studies of vascular endothelial cadherin and of sphingosine 1-phosphate signaling and angiopoietin-Tie signaling.