In vivo validation of signaling pathways regulating human monocyte chemotaxis

Rac1 plays a crucial role in MCP-1-induced monocyte adhesion and migration

Monocyte migration is an important process in inflammation and atherogenesis. Identification of the key signalling pathways that regulate monocyte migration can provide prospective targets for prophylactic treatments in inflammatory diseases. Previous research showed that the focal adhesion kinase Pyk2, Src kinase and MAP kinases play an important role in MCP-1-induced monocyte migration. In this study, we demonstrate that MCP-1 induces iPLA2 activity, which is regulated by PKCβ and affects downstream activation of Rac1 and Pyk2. Rac1 interacts directly with iPLA2 and Pyk2, and plays a crucial role in MCP-1-mediated monocyte migration by modulating downstream Pyk2 and p38 MAPK activation. Furthermore, Rac1 is necessary for cell spreading and F-actin polymerization during monocyte adhesion to fibronectin. Finally, we provide evidence that Rac1 controls the secretion of inflammatory mediator vimentin from MCP-1-stimulated monocytes. Altogether, this study demonstrates that the PKCβ/iPLA2/Rac1/Pyk2/p38 MAPK signalling cascade is essential for MCP-1-induced monocyte adhesion and migration.

A PKCβ-LYN-PYK2 Signaling Axis Is Critical for MCP-1-Dependent Migration and Adhesion of Monocytes

MCP-1-induced monocyte chemotaxis is a crucial event in inflammation and atherogenesis. Identifying the important signal transduction pathways that control monocyte chemotaxis can unravel potential targets for preventive therapies in inflammatory disease conditions. Previous studies have shown that the focal adhesion kinase Pyk2 plays a critical role in monocyte motility. In this study, we investigated the MCP-1-mediated activation of Pyk2 (particularly by the phosphorylation of Tyr⁴⁰²) in primary human peripheral blood monocytes. We showed that MCP-1 induces Src phosphorylation in a similar time frame and that the MCP-1-induced Pyk2 tyrosine phosphorylation is controlled by the Src family kinase. We also report, in this study, that PKCβ, an isoform of PKC, is required for both Src and Pyk2 activation/phosphorylation in response to MCP-1 stimulation. We identified Lyn as the specific Src kinase isoform that is activated by MCP-1 and acts upstream of Pyk2 in primary monocytes. Furthermore, Lyn is found to be indispensable for monocyte migration in response to MCP-1 stimulation. Moreover, our coimmunoprecipitation studies in monocytes revealed that PKCβ, Pyk2, and Lyn exist constitutively in a molecular complex. To our knowledge, our study has uncovered a novel PKCβ-Lyn-Pyk2 signaling cascade in primary monocytes that regulates MCP-1-induced monocyte adhesion and migration.

Novel ex vivo culture method for human monocytes uses shear flow to prevent total loss of transendothelial diapedesis function

Monocyte recruitment to inflammatory sites and their transendothelial migration into tissues are critical to homeostasis and pathogenesis of chronic inflammatory diseases. However, even short-term suspension culture of primary human monocytes leads to phenotypic changes. In this study, we characterize the functional effects of ex vivo monocyte culture on the steps involved in monocyte transendothelial migration. Our data demonstrate that monocyte diapedesis is impaired by as little as 4 h culture, and the locomotion step is subsequently compromised. After 16 h in culture, monocyte diapedesis is irreversibly reduced by ∼90%. However, maintenance of monocytes under conditions mimicking physiological flow (5-7.5 dyn/cm²) is sufficient to reduce diapedesis impairment significantly. Thus, through the application of shear during ex vivo culture of monocytes, our study establishes a novel protocol, allowing functional analyses of monocytes not currently possible under static culture conditions. These data further suggest that monocyte-based therapeutic applications may be measurably improved by alteration of ex vivo conditions before their use in patients.

The generation of NGF-secreting primary rat monocytes: a comparison of different transfer methods

Nerve growth factor (NGF), a member of the neurotrophin family, is responsible for the maintenance and survival of cholinergic neurons in the basal forebrain. The degeneration of cholinergic neurons and reduced acetycholine levels are hallmarks of Alzheimer's disease (AD) as well as associated with learning and memory deficits. Thus far, NGF has proven the most potent neuroprotective molecule against cholinergic neurodegeneration. However, delivery of this factor into the brain remains difficult. Recent studies have begun to elucidate the potential use of monocytes as vehicles for therapeutic delivery into the brain. In this study, we employed different transfection and transduction methods to generate NGF-secreting primary rat monocytes. Specifically, we compared five methods for generating NGF-secreting monocytes: (1) cationic lipid-mediated transfection (Effectene and FuGene), (2) classical electroporation, (3) nucleofection, (4) protein delivery (Bioporter) and (5) lentiviral vectors. Here, we report that classical transfection methods (lipid-mediated transfection, electroporation, nucleofection) are inefficient tools for proper gene transfer into primary rat monocytes. We demonstrate that lentiviral infection and Bioporter can successfully transduce/load primary rat monocytes and produce effective NGF secretion. Furthermore, our results indicate that NGF is bioactive and that Bioporter-loaded monocytes do not appear to exhibit any functional disruptions (i.e. in their ability to differentiate and phagocytose beta-amyloid). Taken together, our results show that primary monocytes can be effectively loaded or transduced with NGF and provides information on the most effective method for generating NGF-secreting primary rat monocytes. This study also provides a basis for further development of primary monocytes as therapeutic delivery vehicles to the diseased AD brain.

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Monocytes can be easily transduced using Bioporter protein reagent

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Bioporter-loaded monocytes exhibit no functional disruptions

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Lentiviral vectors are by far the most potent tool for monocyte transduction

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Classical transfection methods are not sufficient for primary monocyte gene transfer

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NGF-secreting monocytes may serve as potential therapeutic vehicles in Alzheimer`s disease

Signal-activated phospholipase regulation of leukocyte chemotaxis

Signal-activated phospholipases are a recent focus of the rapidly growing field of lipid signaling. The extent of their impact on the pathways regulating diverse cell functions is beginning to be appreciated. A critical step in inflammation is the attraction of leukocytes to injured or diseased tissue. Chemotaxis of leukocytes, a requisite process for monocyte and neutrophil extravasation from the blood into tissues, is a critical step for initiating and maintaining inflammation in both acute and chronic settings. Recent studies have identified new important and required roles for two signal-activated phospholipases A2 (PLA2) in regulating chemotaxis. The two intracellular phospholipases, cPLA2alpha (Group IVA) and iPLA2beta (Group VIA), act in parallel to provide distinct lipid mediators at different intracellular sites that are both required for leukocytes to migrate toward the chemokine monocyte chemoattractant protein-1. This review will summarize the separate roles of these phospholipases as well as what is currently known about the influence of two other classes of intracellular signal-activated phospholipases, phospholipase C and phospholipase D, in regulating chemotaxis in eukaryotic cells, but particularly in human monocytes. The contributions of these phospholipases to chemotaxis both in vitro and in vivo will be highlighted.

iPLA2beta: front and center in human monocyte chemotaxis to MCP-1

Monocyte chemoattractant protein-1 (MCP-1) directs migration of blood monocytes to inflamed tissues. Despite the central role of chemotaxis in immune responses, the regulation of chemotaxis by signal transduction pathways and their in vivo significance remain to be thoroughly deciphered. In this study, we examined the intracellular location and functions of two recently identified regulators of chemotaxis, Ca²⁺-independent phospholipase (iPLA₂β) and cytosolic phospholipase (cPLA₂α), and substantiate their in vivo importance. These enzymes are cytoplasmic in unstimulated monocytes. Upon MCP-1 stimulation, iPLA₂β is recruited to the membrane-enriched pseudopod. In contrast, cPLA₂α is recruited to the endoplasmic reticulum. Although iPLA₂β or cPLA₂α antisense oligodeoxyribonucleotide (ODN)–treated monocytes display reduced speed, iPLA₂β also regulates directionality and actin polymerization. iPLA₂β or cPLA₂α antisense ODN–treated adoptively transferred mouse monocytes display a profound defect in migration to the peritoneum in vivo. These converging observations reveal that iPLA₂β and cPLA₂α regulate monocyte migration from different intracellular locations, with iPLA₂β acting as a critical regulator of the cellular compass, and identify them as potential targets for antiinflammatory strategies.