Regulators and Effectors of Arf GTPases in Neutrophils

Abstract

Polymorphonuclear neutrophils (PMNs) are key innate immune cells that represent the first line of defence against infection. They are the first leukocytes to migrate from the blood to injured or infected sites. This process involves molecular mechanisms that coordinate cell polarization, delivery of receptors, and activation of integrins at the leading edge of migrating PMNs. These phagocytes actively engulf microorganisms or form neutrophil extracellular traps (NETs) to trap and kill pathogens with bactericidal compounds. Association of the NADPH oxidase complex at the phagosomal membrane for production of reactive oxygen species (ROS) and delivery of proteolytic enzymes into the phagosome initiate pathogen killing and removal. G protein-dependent signalling pathways tightly control PMN functions. In this review, we will focus on the small monomeric GTPases of the Arf family and their guanine exchange factors (GEFs) and GTPase activating proteins (GAPs) as components of signalling cascades regulating PMN responses. GEFs and GAPs are multidomain proteins that control cellular events in time and space through interaction with other proteins and lipids inside the cells. The number of Arf GAPs identified in PMNs is expanding, and dissecting their functions will provide important insights into the role of these proteins in PMN physiology.

Small GTPase ARF6 Is a Coincidence-Detection Code for RPH3A Polarization in Neutrophil Polarization

“…Additionally, ARF6 plays a direct role in the transendothelial chemotaxis of leukocytes triggered by chemokines in both monocytic Mono Mac 6 cells and lymphocytic SK-b2.7 cells (29). ARF6 also activates lipidmodifying enzymes phospholipase D (PLD) and PtdIns 4-phosphate 5-kinase (PIP5-kinase), which promotes the production of PtdIns-4,5-bisphosphate (PIP2) (28,(35)(36)(37). Through this, ARF6 regulates multiple neutrophil functions, including superoxide production, degranulation, and chemotaxis (35)(36)(37).…”

“…ARF6 also activates lipidmodifying enzymes phospholipase D (PLD) and PtdIns 4-phosphate 5-kinase (PIP5-kinase), which promotes the production of PtdIns-4,5-bisphosphate (PIP2) (28,(35)(36)(37). Through this, ARF6 regulates multiple neutrophil functions, including superoxide production, degranulation, and chemotaxis (35)(36)(37). Of note, the direct role of ARF6 in neutrophil adhesion and polarization during neutrophil recruitment remains unknown.…”

Assessment of Arf6 Deletion in PLB-985 Differentiated in Neutrophil-Like Cells and in Mouse Neutrophils: Impact on Adhesion and Migration

“…The correct timing of events leading to PMN activation is regulated through the integration of external signals picked up by transmembrane receptors that initiate numerous intracellular signalling pathways controlled by various G proteins including small GTPases [10]. Small GTPases switch into the active and inactive conformations when bound to guanosine triphosphate (GTP) and to guanosine diphosphate (GDP), respectively.…”

“…GTPase-Activating Proteins (GAPs) and Guanine Nucleotide Exchange Factors (GEFs) specific for their small GTPases regulate this cycle [11]. These small GTPases, including those of the Arf family, such as Arf6, play roles in mediating integrin outside-in and inside-out signals and are critical components regulating PMN adhesion dynamics and migration [10,12]. Arf6 differs from other Arfs in that it localizes specifically to the plasma membrane and endosomal compartments, which is thought to stem from the individual protein environment [13].…”

“…Arf6 plays roles in N-formyl-Methionyl-Leucyl-Phenylalanine (fMLP) receptor-mediated downstream signalling pathways leading to phospholipase D (PLD) activation and superoxide production in PMNs or differentiated PLB-985 cells [19,20]. Several members of the cytohesin (CTH) family of Arf-GEFs are expressed by PMNs [10,[20][21][22][23]. Pharmacological inhibition of CTH-1 with SecinH3 or silencing of CTH-1 inhibited fMLP-mediated membrane translocation of Arf6 and Arf1 and prevented Arf6 activation in PMNs [20].…”