Video-Rate Bioluminescence Imaging of Degranulation of Mast Cells Attached to the Extracellular Matrix

A convenient fluorimetry-based degranulation assay using RBL-2H3 cells

Type I hypersensitivity is triggered by mast cell degranulation, a stimulus-induced exocytosis of preformed secretory granules (SGs) containing various inflammatory mediators. The degree of degranulation is generally expressed as a percentage of secretory granule markers (such as β-hexosaminidase and histamine) released into the external solution, and considerable time and labor are required for the quantification of markers in both the supernatants and cell lysates. In this study, we developed a simple fluorimetry-based degranulation assay using rat basophilic leukemia (RBL-2H3) mast cells. During degranulation, the styryl dye FM1-43 in the external solution fluorescently labeled the newly exocytosed SGs, whose increase in intensity was successively measured using a fluorescence microplate reader. In addition to the rate of β-hexosaminidase secretion, the cellular FM1-43 intensity successfully represented the degree and kinetics of degranulation under various conditions, suggesting that this method facilitates multi-sample and/or multi-time-point analyses required for screening substances regulating mast cell degranulation.

Ephedra Herb, Mao, Inhibits Antigen-Induced Mast Cell Degranulation by Induction of the Affinity Receptor for IgE Internalization

PURPOSE

Ephedra herb (Mao) exerts potent anti-allergic effects. This study aimed to examine the underlying mechanisms of Mao on allergic inflammation using in vitro cultured mast cells (MCs) and an in vivo model of MC-dependent anaphylaxis.

METHODS

Bone marrow-derived MCs (BMMCs) were presensitized with anti-2,4-dinitrophenol (DNP) immunoglobulin E (IgE) and challenged with antigens (Ag; DNP-human serum albumin). Degranulation responses and cell surface high-affinity receptor for IgE (FcεRI) expression were assessed with/without Mao treatment. Passive systemic anaphylaxis (PSA)-treated mice were administered Mao and the pathophysiological responses were evaluated.

RESULTS

Mao inhibited Ag-induced BMMC degranulation, but not polyclonal activation with phorbol 12-myristate 13-acetate (PMA) and ionomycin, indicating that Mao inhibits IgE-dependent activation of BMMCs. Mao-treated BMMCs exhibited significant reductions in expression of surface IgE and its receptor FcεRI. Analysis of subcellular localization revealed that Mao induces FcεRI internalization in BMMCs without degranulation. In the PSA mouse model, Mao administration prevented antigen-induced hypothermia. Mao administration significantly reduced cell surface expression of IgE-bound FcεRI on peritoneal MCs.

CONCLUSIONS

Mao induced FcεRI internalization in MCs, thereby inhibiting Ag-induced IgE-dependent degranulation. The inhibitory effects of Mao on MC degranulation may offer a novel therapeutic approach for allergic diseases.

[Novel Technology for Studying Insulin Secretion: Imaging and Quantitative Analysis by a Bioluminescence Method]

We developed a method of video-rate bioluminescence imaging to visualize proteins secreted from living cells. A protein of interest was fused to Gaussia luciferase (GLase), and the luminescence signals of secreted GLase with coelenterazine (luciferin) were visualized at a video-rate of 30-500 ms/frame by using a water-cooled EM-CCD camera. We established a subclonal rat INS-1E cell line, named iGL cells, stably expressing the fusion protein of insulin and GLase (Insulin-GLase). By stimulation with high glucose, 3D-cultured iGL cells showed synchronized oscillatory secretion of insulin for over 1 h, as similarly observed in an isolated rat pancreatic islet. In 2D-cultured iGL cells, the luminescence images indicated that synchronized insulin secretion was localized in intercellular spaces between cells. Further, the relative amount of insulin secretion from iGL cells was easily determined with a luminometer, and we demonstrated that cell-cell interaction of beta cells is fundamental to increase glucose-stimulated insulin secretion by synchronization. Thus, iGL cells would be valuable for studying oscillatory insulin secretion and evaluating anti-diabetic drugs. Our bioluminescence imaging method with GLase could be generally used for investigating protein secretion in 2D and 3D cell culture systems.