NETosis: Molecular Mechanisms, Role in Physiology and Pathology

Abstract

NETosis is a program for formation of neutrophil extracellular traps (NETs), which consist of modified chromatin decorated with bactericidal proteins from granules and cytoplasm. Various pathogens, antibodies and immune complexes, cytokines, microcrystals, and other physiological stimuli can cause NETosis. Induction of NETosis depends on reactive oxygen species (ROS), the main source of which is NADPH oxidase. Activation of NADPH oxidase depends on increase in the concentration of Ca2+ in the cytoplasm and in some cases on the generation of ROS in mitochondria. NETosis includes release of the granule components into the cytosol, modification of histones leading to chromatin decondensation, destruction of the nuclear envelope, as well as formation of pores in the plasma membrane. In this review, basic mechanisms of NETosis, as well as its role in the pathogenesis of some diseases including COVID-19 are discussed.

Fig. 1.

Electron microscopy of neutrophils in the early stages of NETosis. a, b) Intact human neutrophil; c and d) neutrophil stimulated with 2 µM A23187 for 30 min. Chromatin decondensation, characteristic of NETosis, and swelling of mitochondria were observed simultaneously indicating opening of the non-selective mitochondrial pore, mPTP. Scale bars: 2 µm (a, c) and 500 nm (b, d) [16].

Fig. 2.

Scheme illustrating the mechanisms of NETosis induced by various stimuli. NETosis caused by the calcium ionophore A23187 starts with the mobilization of Ca²⁺ from the endoplasmic reticulum (ER), which leads to the activation of CRAC channels located in the cytoplasmic membrane, and the entry of extracellular Ca²⁺ into the cytoplasm. In addition, A23187 catalyzes the transfer of Ca²⁺ through the plasma membrane into the cytoplasm. Subsequent accumulation of Ca²⁺ in the mitochondrial matrix leads to the activation of non-selective mitochondrial pores (mPTP) and the formation of mitochondrial reactive oxygen species (mtROS). Along with Ca^{2 +} overload, oxidative stress is also an inducer of the mPTP. mtROS released from the mitochondria into the cytosol activate NADPH oxidase apparently with the participation of protein kinase C (PKC). PKC can be activated with the phorbol ester (PMA), which induces NADPH oxidase and NETosis independently of mtROS and the mPTP. NADPH oxidase can be also activated by a chemoattractant N-formylmethionyl-leucyl-phenylalanine (fMLP), which by binding to a specific receptor activates phospholipase C (PLC) to stimulate the formation of diacylglycerol (DAG) and inositol triphosphate (IP3) from the phosphatidylinositol 4,5-bisphosphate. The activation of NADPH oxidase in this case depends on mtROS and on the opening of mPTP. For reasons not fully understood, fMLP does not induce NETosis [16]. (Color version of the figure is available in online version of the article and can be accessed at: https://www.springer.com/journal/10541)