Activation of PAD4 in NET formation

Abstract

Peptidylarginine deiminases, or PADs, convert arginine residues to the non-ribosomally encoded amino acid citrulline in a variety of protein substrates. PAD4 is expressed in granulocytes and is essential for the formation of neutrophil extracellular traps (NETs) via PAD4-mediated histone citrullination. Citrullination of histones is thought to promote NET formation by inducing chromatin decondensation and facilitating the expulsion of chromosomal DNA that is coated with antimicrobial molecules. Numerous stimuli have been reported to lead to PAD4 activation and NET formation. However, how this signaling process proceeds and how PAD4 becomes activated in cells is largely unknown. Herein, we describe the various stimuli and signaling pathways that have been implicated in PAD4 activation and NET formation, including the role of reactive oxygen species generation. To provide a foundation for the above discussion, we first describe PAD4 structure and function, and how these studies led to the development of PAD-specific inhibitors. A comprehensive survey of the receptors and signaling pathways that regulate PAD4 activation will be important for our understanding of innate immunity, and the identification of signaling intermediates in PAD4 activation may also lead to the generation of pharmaceuticals to target NET-related pathogenesis.

Peptidylarginine deiminase inhibition is immunomodulatory and vasculoprotective in murine lupus

“…Our study therefore differs in at least 2 important ways from the Nox2-deficient MRL/lpr model (68), in that we are inhibiting NET formation in a type I IFN-dependent model and Cl-amidine does not interfere with ROS formation (which may play a protective role in lupus). Indeed, data to date points toward ROS formation being upstream of PAD4 activation in neutrophils (69), supporting the idea that PAD inhibition is a more targeted strategy for inhibiting NET formation. Given the ability of Cl-amidine to also improve other inflammatory disease phenotypes (31,32), it will be of interest to revisit those inflammatory murine models in the context of what we now know about Cl-amidine's effectiveness in inhibiting NET formation in mice.…”

Renal participation of myeloperoxidase in antineutrophil cytoplasmic antibody (ANCA)-associated glomerulonephritis

“…Peptidyl arginine 4 deiminase (PAD4) is a nuclear enzyme that can prevent the induction of neutrophil apoptosis 24 and is essential for the production of NETs. [25][26][27] Citrullinated histones (histone H3Cit) correlate with extensive chromatin decondensation, as occurs in NET formation. [28][29][30] Figure 7a demonstrates that the percentage of glomeruli with NETs, identified with H2A-H2B, MPO, and DAPI, was identical to the proportion of glomeruli with the (Figures 7b-d).…”

Peptidyl arginine deiminase-4 activation exacerbates kidney ischemia-reperfusion injury

“…PAD4 requires dimerization and Ca 2ϩ -mediated conformational changes to promote the deamination process (4,15,42). PAD4 is known to be activated by the rise in intracellular Ca 2ϩ as well as by ROS and proinflammatory cytokines (e.g., TNF-␣), chemokines (e.g., IL-8), and Toll-like receptor ligands (e.g., lipopolysaccharides) (37). All of these stimuli that result in PAD4 activation occur during and after renal I/R injury (5,18,35).…”