Investigating the negative regulatory role of neurogranin in IgE-mediated mast cell activation

Abstract

Mast cells are granulocytic immune sentinels that drive the chronic inflammatory state characteristic of allergic pathologies, through the release of a plethora of pro-inflammatory mediators following IgE-mediated activation. Calcium mobilization following mast cell activation is essential for degranulation and pro-inflammatory cytokine/chemokine production, processes that require the Ca2+/calmodulin (CaM)-dependent phosphatase, calcineurin. Neurogranin is a protein that regulates Ca2+-mediated signaling pathways by binding and sequestering of CaM, subsequently acting as a critical suppressor of calcineurin phosphatase activity. Neurogranin was previously thought to exist exclusively in the brain; however, this is the first study to demonstrate the presence of neurogranin in mast cells. Since Ca2+/CaM signaling is so critical to the inflammatory response, the objective of this study was to characterize the role of neurogranin in allergen activated mast cells using bone marrow-derived mast cells (BMMCs) from Nrgn+/- mice. In this study, BMMCs from Nrgn+/- and Nrgn+/+ mice were sensitized with IgE overnight and stimulated the next day with TNP-BSA under SCF potentiation. Early phase mast cell degranulation, measured through a -hexosaminidase assay, was demonstrated to be uninterrupted in neurogranin deficient BMMCs. In the late phase, neurogranin was found to function as a negative regulator of the transcription of IL-6 and IL-13 genes, as well as the secretion of IL-6 and TNF. These observed functional effects of neurogranin deficiency were found to be independent of modulation of the MAPK signaling pathway. Inhibitory signals that terminate inappropriate mast cell responses and limit undesirable inflammation remain incompletely defined, and these findings are the first to suggest that neurogranin has an integral role in IgE-mediated mast cell inhibitory signaling.