STIM1 for stimulation of phagocyte NADPH oxidase

In this issue of Blood, Zhang et al show that mice lacking the stromal-interacting molecule 1 (STIM1) gene in bone marrow cells are more susceptible to bacterial infection but are resistant to ischemia/reperfusion injury because of defective activation of phagocyte nicotinamide adenine dinucleotide phosphate (NADPH) oxidase.¹ 

The concentration of cytosolic Ca²⁺ is crucial for important cellular functions such as neurotransmitter release, muscle contraction, and transcriptional regulation.²  A variety of cell surface receptors signal through phospholipase C activation for the generation of inositol 1,4,5-trisphosphate (IP₃) that triggers Ca²⁺ release from endoplasmic reticulum (ER), a major intracellular Ca²⁺ store. The resulting store-operated Ca²⁺ entry (SOCE) is responsible for sustained increase of cytosolic Ca²⁺ concentration resulting from the opening of membrane channels and influx of Ca²⁺, which is present extracellularly at higher concentrations. At the center of this regulatory process is STIM1, a protein that senses Ca²⁺ concentration changes in the ER and communicates with calcium channels on plasma membrane (see figure; reviewed in Baba and Kurosaki³ ).

Zhang et al provide experimental evidence for an in vivo function of STIM1 in neutrophil-mediated killing of bacteria and ischemia/reperfusion injury.¹  Through bone marrow transplantation with Stim1^−/− fetal liver cells, the authors successfully obtained mouse neutrophils lacking STIM1, thus overcoming lethality resulting from Stim1 deletion. The availability of these chimeras has allowed the authors to examine the functional changes resulting from STIM1 deficiency. One of their experiments shows that these mice are prone to infection by Listeria monocytogenes and Staphylococcus aureus, resulting in more pronounced pathologic changes in the lungs. The increased susceptibility can be attributed to diminished neutrophil superoxide production, because phagocytosis and neutrophil infiltration are minimally altered while neutrophil NADPH oxidase activation is abrogated. These findings are consistent with a previous report on the involvement of STIM1 in NADPH oxidase activation using neutrophil-like HL-60 cells.⁴  The study by Zhang et al further demonstrates that SOCE is defective in Stim1^−/− neutrophils activated through chemoattractant receptors as well as Fcγ receptors and integrins. These receptors are activated in different manners, but they all stimulate IP₃ production that leads to SOCE (see figure). Zhang et al also show that sustained increase in cytosolic Ca²⁺ concentration is required for neutrophil superoxide production primarily because of the role of calcium in the activation of the classical protein kinase C (PKC) isoforms (PKCα and PKCβ in neutrophils). These PKCs are highly important to the phosphorylation of the phagocyte NADPH oxidase components, p47^phox and p40^phox. Phosphorylation of p47^phox is of particular interest because this posttranslational modification changes the conformation of p47^phox, leading to its membrane translocation and binding to p22^phox, a subunit of the core NADPH oxidase termed flavocytochrome b₅₅₈.⁵  In STIM1-deficient neutrophils, PKC-dependent phosphorylation of these cytosolic factors of the NADPH oxidase is markedly reduced.

The work by Zhang et al provides molecular details for STIM1-dependent Ca²⁺ influx leading to phagocyte NADPH oxidase activation and also shows several interesting aspects of Ca²⁺ signaling in relation to other neutrophil functions. It is particularly interesting that neutrophil chemotaxis, known to rely on Ca²⁺ influx,⁶  is not affected by STIM1 deficiency, suggesting that a sustained rise in cytosolic Ca²⁺ may not be required. It is possible that other forms of Ca²⁺ signaling, including transient Ca²⁺ mobilization from intracellular stores,⁷  or localized calcium flickers as reported in other types of cells,⁸  may be sufficient for neutrophil migration. Chemotaxis mediated by different receptors may also have different requirement for Ca²⁺ influx.⁹ 

Production of oxygen radicals is a major bactericidal function of phagocytes. However, it is a double-edged sword because of its tissue-damaging property. Zhang et al show that the Stim1^−/− chimeras are resistant to ischemia/reperfusion injury to the liver, suggesting that neutrophil production of oxygen radicals might contribute to the pathological changes in wild-type subjects. Future work using granulocyte-specific deletion of Stim1 will be helpful to ascertain the respective contribution of different blood cells to the observed changes. These studies create new opportunities for therapeutic intervention that targets STIM1.