P-Rex family Rac-GEFs are important activators of the small G protein Rac, regulating cell shape, migration, reactive oxygen species (ROS) formation, gene expression and cell growth. Neutrophils are leukocytes of the innate immune system that have a wide range of Rac-dependent responses, including adhesion, chemotaxis, degranulation, phagocytosis and ROS formation. Some of these neutrophil responses, especially those elicited by GPCR signalling, are regulated by Prex1.
We recently identified a new regulator of Prex1, the neuronal GPCR adaptor protein Norbin1. This initial study had revealed that Norbin can bind Prex1, stimulate its Rac-GEF activity and promote its plasma membrane localization. It showed furthermore that Norbin is expressed in neutrophils. In order to assess the functional importance of the Prex1/Norbin interaction in neutrophils, we generated two new genetically-modified mouse strains: a strain with a conditional Norbin deletion in myeloid cells and a strain with combined Norbin and Prex1 deficiency.
Unexpectedly, we found that isolated Norbin-deficient neutrophils show increased adhesion and spreading, as well as increased ROS production upon stimulation of GPCRs, and an increased capacity to kill S. aureus bacteria. Norbin deficient-mice show increased recruitment of neutrophils to the inflamed peritoneum during aseptic peritonitis. Recent results suggest that Norbin deficiency provides immunity against pulmonary infection with S. pneumoniae, even in immune-deficient (Prex-/-) mice. Under most conditions, we found that Norbin deficiency overrides the functional impairments caused by the Prex1 deficiency, whereas some effects (e.g. cell spreading and tissue-recruitment) were Prex1-dependent. Mechanistically, the Norbin-deficiency seems to cause a constitutive upregulation of GPCRs on the neutrophil surface and to promote GPCR-dependent Rac1 and Rac2 activity independently of Prex1.
These data indicate that the GPCR adaptor and Prex1 regulator Norbin plays an important functional role suppressing host defence functions of mouse neutrophils. A subset of Norbin functions are Prex1 dependent, whereas others seem to be regulated through control of GPCR trafficking. Further experiments to characterize neutrophil responses in isolated cells and in vivo are required to understand the importance of Norbin and its interaction with Prex1 for the ability of neutrophils to fight bacterial and fungal infections.