Mitogen-Activated Protein Kinase Signaling Regulates Proteoglycan Composition of Mast Cell Secretory Granules.

Jun Mei Hu Frisk,Gunnar Pejler,Fabio R Melo,Helena Öhrvik,Lena Kjellén

doi:10.3389/fimmu.2018.01670

Abstract

Mast cells (MCs) are characterized by an abundance of lysosome-like secretory granules filled with immunomodulatory compounds including histamine, cytokines, lysosomal hydrolases, MC-restricted proteases, and serglycin proteoglycans. The latter are essential for promoting the storage of other granule compounds and are built up of the serglycin core protein to which highly sulfated and thereby negatively charged glycosaminoglycan (GAG) side chains of heparin or chondroitin sulfate type are attached. In the search for mechanisms operating in regulating MC granule homeostasis, we here investigated the role of mitogen-activated protein kinase (MAPK) signaling. We show that inhibition of MEK1/2 (a MAPK kinase) leads to increased metachromatic staining of MC granules, indicative of increased proteoglycan content. Indeed, MEK1/2 inhibition caused a profound increase in the expression of the gene coding for the serglycin core protein and of genes coding for various enzymes involved in the biosynthesis/sulfation of the GAGs attached to the serglycin core protein. This was accompanied by corresponding increases in the levels of the respective GAGs. Deletion of the serglycin core protein abrogated the induction of enzymes operative in proteoglycan synthesis, indicating that availability of the serglycin proteoglycan core protein has a regulatory function impacting on the expression of the various serglycin-modifying enzymes. MEK1/2 inhibition also caused a substantial increase in the expression of granule-localized, proteoglycan-binding proteases. Altogether, this study identifies a novel role for MAPK signaling in regulating the content of secretory granules in MCs.

Highlights

Mast cells (MCs) are crucial effector cells of the immune system, contributing to both the adaptive and innate arms of immune defense against external insults, such as pathogens and noxious substances including toxins from various venoms [1,2,3,4]
To address the possible role of mitogen-activated protein kinase (MAPK) signaling in regulating secretory granule homeostasis in MCs, we developed bone marrow-derived mast cell (BMMC) from mice and exposed these populations to PD98059, a highly selective inhibitor of MEK1/2 [26], which are MAPK kinases that act by phosphorylation of ERK1/2
Increased metachromatic staining of BMMCs was seen after incubation of the cells with an alternative MEK1/2 kinase inhibitor, U0126 (Figure S1 in Supplementary Material), supporting a role for MAPK signaling in regulating MC granule homeostasis

Summary

Introduction

Mast cells (MCs) are crucial effector cells of the immune system, contributing to both the adaptive and innate arms of immune defense against external insults, such as pathogens and noxious substances including toxins from various venoms [1,2,3,4]. Mast cells are derived from the bone marrow, from which they egress as immature progenitors and home to various tissues where they mature under the influence of local growth factors such as stem cell factor and IL-3 [7] In this process, they acquire an abundance of lysosome-like secretory granules, densely packed with numerous preformed bioactive substances, including histamine, cytokines/growth factors, lysosomal hydrolases, proteases [tryptase, chymase, carboxypeptidase A3 (CPA3)], and proteoglycans of serglycin type [8]. When the MCs degranulate, a process that can be triggered by various stimuli including IgE receptor crosslinking, the preformed granule compounds are released to the external milieu where they can cause a powerful inflammatory reaction as exemplified by anaphylactic shock

Methods

Results

Conclusion