Neutrophil transmigration, focal adhesion kinase and endothelial barrier function

Abstract

Neutrophil activation is an essential component of innate immune defense against infection and injury. In response to inflammatory stimulation, circulating neutrophils undergo a series of dynamic and metabolic changes characterized by β2-intergrin mediated adhesion to microvascular endothelium and subsequent transendothelial migration. During this process, neutrophils release granular contents containing digestive enzymes and produce cytotoxic agents such as reactive oxygen species and cytokines. These products target endothelial barriers inducing phosphorylation-triggered junction dissociation, actin stress fiber formation, and actomyosin contraction, manifest as paracellular hyperpermeability. Endothelial cell-matrix focal adhesions play an integral role in this process by providing structural support for endothelial conformational changes that facilitate neutrophil transmigration, as well as by recruiting intracellular molecules that constitute the hyperpermeability signaling cascades. As a central connector of the complex signaling network, focal adhesion kinase (FAK) is activated following neutrophil adhesion, and further mediates the reorganization of endothelial integrin-matrix attachments in a pattern coordinating with cytoskeleton contraction and junction opening. In this review, we present recent experimental evidence supporting the importance of FAK in neutrophil-dependent regulation of endothelial permeability. The discussion focuses on the mechanisms by which neutrophils activate FAK and its downstream effects on endothelial barriers.

Figure 1

Schematic process of neutrophil adhesion and transendothelial migration. In response to inflammatory stimuli, adhesion molecules such as selectins are upregulated on endothelial cells and neutrophils, and neutrophils roll along the vascular endothelial wall through selectin-mediated weak interactions (I). This is followed by firm adhesion of neutrophils to endothelium through binding of integrins on the neutrophil surface to ICAM-1 or VCAM-1 on the endothelial cell surface (II). Subsequently, neutrophils transmigrate through the microvascular endothelium via a process involving complex interactions with endothelial cell-cell junction molecules, including VE-cadherin, JAMs, and PECAM-1 (III).

Figure 2

Signaling mechanisms involved in neutrophil transendothelial migration and endothelial hyperpermeability. Activated neutrophils release vasoactive or cytotoxic agents, such as ROS, cytokines, proteases, and leukotrienes, which in turn act on endothelial cells causing barrier injury. In parallel, neutrophil adhesion to the endothelium via β2 integrin ligand (e.g., LFA-1, MAC-1) binding to endothelial receptors (e.g. ICAM-1 or VCAM-1) triggers multiple endothelial signaling cascades causing elevated intracellular calcium, cytoskeleton contraction, and junction protein phosphorylation and dissociation. These responses act in concert leading to widened intercellular space and facilitating plasma leakage and neutrophil transmigration.