Endothelial PI 3-kinase activity regulates lymphocyte diapedesis

Malaria Elimination in Africa: Rethinking Strategies for Plasmodium vivax and Lessons from Botswana

The global malaria community has picked up the theme of malaria elimination in more than 90% of the world's population in the next decade. Recent reports of Plasmodium vivax (P. vivax) in sub-Saharan Africa, including in Duffy-negative individuals, threaten the efforts aimed at achieving elimination. This is not only in view of strategies that are tailored only to P. falciparum elimination but also due to currently revealed biological characteristics of P. vivax concerning the relapse patterns of hypnozoites and conservation of large biomasses in cryptic sites in the bone marrow and spleen. A typical scenario was observed in Botswana between 2008 and 2018, which palpably projects how P. vivax could endanger malaria elimination efforts where the two parasites co-exist. The need for the global malaria community, national malaria programs (NMPs), funding agencies and relevant stakeholders to engage in a forum to discuss and recommend clear pathways for elimination of malaria, including P. vivax, in sub-Saharan Africa is warranted.

Platelets directly enhance neutrophil transmigration in response to oxidised low-density lipoprotein

Beyond their primary role in haemostasis and tissue repair, platelets are causally involved in the onset of inflammatory reactions, cell proliferation and immune response. Platelet activation and platelet binding to the endothelium result in release of chemokines and increased expression of adhesion molecules, which promote the recruitment of leukocytes that will eventually migrate across the endothelium into the tissue. Here, we provide the first evidence that platelets stimulated with oxidised low-density lipoprotein (oxLDL) directly enhance recruitment and transmigration of neutrophils, via cell-cell interaction. OxLDL immediately activates platelets, which then rapidly bind to neutrophils, foster their activation and facilitate transmigration through an endothelial monolayer. The observed effects of oxLDL on platelet-neutrophil aggregate (PNA) formation depend on incubation time, lipoprotein concentration and the degree of oxidative modification of LDL. PNA form within minutes following stimulation by oxLDL and remain for up to 1 h post stimulation, while native LDL is unable to induce platelet-neutrophil interactions. In the presence of acetylsalicylic acid the formation of PNA in response to oxLDL is virtually absent, and platelets fail to further enhance oxLDL-induced neutrophil transmigration. P2Y1 and P2Y12 inhibitors have less pronounced effects on PNA formation in response to oxLDL. Furthermore, we demonstrate that the PI3K pathway is essential for efficient neutrophil transmigration induced by oxLDL. Consequently, platelets enhance neutrophil transmigration in response to oxLDL and might thereby contribute essentially to the amplification of inflammatory processes within the vessel wall, which fosters the development of atherosclerosis.

Recent insights into endothelial control of leukocyte extravasation

In the process of leukocyte migration from the circulation across the vascular wall, the crosstalk with endothelial cells that line the blood vessels is essential. It is now firmly established that in endothelial cells important signaling events are initiated upon leukocyte adhesion that impinge on the regulation of cell-cell contact and control the efficiency of transendothelial migration. In addition, several external factors such as shear force and vascular stiffness were recently identified as important regulators of endothelial signaling and, consequently, leukocyte transmigration. Here, I review recent insights into endothelial signaling events that are linked to leukocyte migration across the vessel wall. In this field, protein phosphorylation and Rho-mediated cytoskeletal dynamics are still widely studied using increasingly sophisticated mouse models. In addition, activation of tyrosine phosphatases, changes in endothelial cell stiffness as well as different vascular beds have all been established as important factors in endothelial signaling and leukocyte transmigration. Finally, I address less-well-studied but interesting components in the endothelium that also control transendothelial migration, such as the ephrins and their Eph receptors, that provide novel insights in the complexity associated with this process.

Heparanase induces inflammatory cell recruitment in vivo by promoting adhesion to vascular endothelium

Heparanase (HPSE1) is known to be involved in mechanisms of metastatic tumor cell migration. This enzyme selectively cleaves heparan sulfate proteoglycans (HSPG), which are ubiquitously expressed in mammals and are known to be involved in regulating the activity of an array of inflammatory mediators. In the present study, we have investigated the effects of human recombinant heparanase, the inactive precursor of this enzyme (proheparanase) and enzymatically inactivated heparanase, on inflammatory cell recruitment in the rat and on human leukocyte-endothelial adhesion in vitro. Intraperitoneal injection of heparanase (500 μg) induced a significant inflammatory cell infiltrate in the rat, as assessed by peritoneal lavage 4 h later. Intravital microscopy of the mesenteric microcirculation of anesthetized rats showed an increase in rolling and adherent cells in postcapillary venules that was sensitive to heparin, a nonselective inhibitor of heparanase activity. In vitro, heparanase augmented the adhesion of human neutrophils and mononuclear cells to human umbilical vein endothelial cells in a concentration-dependent manner. Proheparanase had similar effects to the active enzyme both with respect to leukocyte accumulation in the peritoneal cavity and adhesion in vitro. However, heat-inactivated heparanase induced cell adhesion in vitro but was without effect in vivo. Together, these data indicate a role for heparanase in inflammatory cell trafficking in vivo that appears to require enzymatic activity.

Adipose tissue insulin sensitivity and macrophage recruitment: Does PI3K pick the pathway?

In the United States, obesity is a burgeoning health crisis, with over 30% of adults and nearly 20% of children classified as obese. Insulin resistance, a common metabolic complication associated with obesity, significantly increases the risk of developing metabolic diseases such as hypertension, coronary heart disease, stroke, type 2 diabetes, and certain cancers. With the seminal finding that obese adipose tissue harbors cytokine secreting immune cells, obesity-related research over the past decade has focused on understanding adipocyte–macrophage crosstalk and its impact on systemic insulin sensitivity. Indeed, adipose tissue has emerged as a central mediator of obesity- and diet-induced insulin resistance. In this mini-review, we focus on a potential role of adipose tissue phosphoinositide 3-kinase (PI3K) as a point of convergence of cellular signaling pathways that integrates nutrient sensing and inflammatory signaling to regulate tissue insulin sensitivity.

Phosphoinositides in chemotaxis

Phosphatidylinositol lipids generated through the action of phosphinositide 3-kinase (PI3K) are key mediators of a wide array of biological responses. In particular, their role in the regulation of cell migration has been extensively studied and extends to amoeboid as well as mesenchymal migration. Through the emergence of fluorescent probes that target PI3K products as well as the use of specific inhibitors and knockout technologies, the spatio-temporal distribution of PI3K products in chemotaxing cells has been shown to represent a key anterior polarity signal that targets downstream effectors to actin polymerization. In addition, through intricate cross-talk networks PI3K products have been shown to regulate signals that control posterior effectors. Yet, in more complex environments or in conditions where chemoattractant gradients are steep, a variety of cell types can still chemotax in the absence of PI3K signals. Indeed, parallel signal transduction pathways have been shown to coordinately regulate cell polarity and directed movement. In this chapter, we will review the current role PI3K products play in the regulation of directed cell migration in various cell types, highlight the importance of mathematical modeling in the study of chemotaxis, and end with a brief overview of other signaling cascades known to also regulate chemotaxis.

Endothelial f-actin depolymerization enables leukocyte transmigration

A demanding task of medicine is to understand and control the immune system. Central players in the cellular immune response are the leukocytes that leave the blood stream for host defense. Endothelial cells limit the emigration rate of leukocytes. Being located between blood and tissues, they permit or deny the passage. The exact mechanism of this process called diapedesis is not solved yet. Leukocytes can principally traverse either between cells (paracellularly) or directly through an individual endothelial cell (transcellularly). The transcellular way has recently gained experimental support, but it is not clear how the endothelial cytoskeleton manages to open and close a transmigratory channel. Atomic force microscopy was used to investigate the endothelial cytoskeleton. In order to directly access the leukocyte-endothelial interaction site, we applied a special protocol ("nanosurgery"). As a result, the endothelial cell turned out to become softer in a confined region strictly underneath the leukocyte. Fluorescence microscopy confirmed a depolymerization of the f-actin strands at the invasion site. Leukocytes dramatically rearrange the endothelial cytoskeleton to form transmigratory channels.

The PI3K p110alpha isoform regulates endothelial adherens junctions via Pyk2 and Rac1

Only the p110α isoform of PI3K mediates the association of VE-cadherin with Pyk2, a Rac GEF and the p85 PI3K regulatory subunit, to reduce junctional integrity in response to TNF.

Endothelial cell–cell junctions control efflux of small molecules and leukocyte transendothelial migration (TEM) between blood and tissues. Inhibitors of phosphoinositide 3-kinases (PI3Ks) increase endothelial barrier function, but the roles of different PI3K isoforms have not been addressed. In this study, we determine the contribution of each of the four class I PI3K isoforms (p110α, -β, -γ, and -δ) to endothelial permeability and leukocyte TEM. We find that depletion of p110α but not other p110 isoforms decreases TNF-induced endothelial permeability, Tyr phosphorylation of the adherens junction protein vascular endothelial cadherin (VE-cadherin), and leukocyte TEM. p110α selectively mediates activation of the Tyr kinase Pyk2 and GTPase Rac1 to regulate barrier function. Additionally, p110α mediates the association of VE-cadherin with Pyk2, the Rac guanine nucleotide exchange factor Tiam-1 and the p85 regulatory subunit of PI3K. We propose that p110α regulates endothelial barrier function by inducing the formation of a VE-cadherin–associated protein complex that coordinates changes to adherens junctions with the actin cytoskeleton.

Endothelial IQGAP1 regulates efficient lymphocyte transendothelial migration

Leukocyte movement from the blood to the tissues is a fundamental process in acute and chronic inflammatory diseases. While the role of endothelial cells (EC) to recruit leukocytes to sites of inflammation is well known, the mechanisms that control remodeling of EC shape and adhesive contacts during leukocyte transendothelial migration (TEM) are not completely understood. We studied the role of IQGAP1, an adaptor protein that binds to filamentous-actin and microtubules (MT) at interendothelial junctions, during lymphocyte TEM. EC IQGAP1 knockdown decreases MT tethered to the adherens junction, and decreases lymphocyte TEM to approximately 70% (p<0.05) versus control. Similarly, loss of adherens junction-associated MT induced by brief nocodazole (ND) treatment decreases lymphocyte TEM to approximately 65% of control (p<0.01). Confocal microscopy imaging indicates that EC IQGAP1 knockdown and MT depolymerization both result in lymphocyte accumulation above the vascular endothelial cadherin (VE-cadherin) junctions and reduces the fraction of adherent lymphocytes that complete diapedesis across interendothelial cell junctions. However, we observe no change in VE-cadherin gap formation underlying adherent lymphocytes among control, IQGAP1 knockdown, or MT depolymerised EC monolayers. These data indicate that IQGAP1 contributes to MT stability at endothelial junctions. Further, IQGAP1 is involved in junction remodeling required for efficient lymphocyte diapedesis, independent of VE-cadherin gap formation.

Phosphoinositide 3-kinases in cell migration

Cell migration is essential for many biological processes in animals and is a complex highly co-ordinated process that involves cell polarization, actin-driven protrusion and formation and turnover of cell adhesions. The PI3K (phosphoinositide 3-kinase) family of lipid kinases regulate cell migration in many different cell types, both through direct binding of proteins to their lipid products and indirectly through crosstalk with other pathways, such as Rho GTPase signalling. Emerging evidence suggests that the involvement of PI3Ks at different stages of migration varies even within one cell type, and is dependent on the combination of external stimuli, as well as on the signalling status of the cell. In addition, it appears that different PI3K isoforms have distinct roles in cell polarization and migration. This review describes how PI3K signalling is regulated by pro-migratory stimuli, and the diverse ways in which PI3K-mediated signal transduction contributes to different aspects of cell migration.