Associate editor: P.S. Foster
Mast cell function: Regulation of degranulation by serine/threonine phosphatases

https://doi.org/10.1016/j.pharmthera.2006.04.011Get rights and content

Abstract

Mast cells play both effector and modulatory roles in a range of allergic and immune responses. The principal function of these cells is the release of inflammatory mediators from mast cells by degranulation, which involves a complex interplay of signalling molecules. Understanding the molecular architecture underlying mast cell signalling has attracted renewed interest as the capacity for therapeutic intervention through controlling mast cell degranulation is now accepted as a viable proposition. The dynamic regulation of signalling by protein phosphorylation is a well-established phenomenon and many of the early events involved in mast cell activation are well understood. Less well understood however are the events further downstream of receptor activation that allow movement of granules through the cytoskeletal barrier and docking and fusion of granules with the plasma membrane. Whilst a potential role for the protein phosphatase family of signalling enzymes in mast cell function has been accepted for some time, the evidence has largely been derived from the use of broad specificity pharmacological inhibitors and results often depend upon the experimental conditions, leading to conflicting views. In this review, we present and discuss the pharmacological and recent molecular evidence that protein phosphatases, and in particular the protein phosphatase serine/threonine phosphatase type 2A (PP2A), have major regulatory roles to play and may be potential targets for the design of new therapeutic agents.

Introduction

Mast cells, which are derived from haematopoetic stem cells, are key effector cells in allergic reactions and IgE associated immune responses. They are also implicated as regulators of adaptive immune responses and this immunoregulatory role has been suggested to be potentially more important than the better established effector role. Attention has therefore begun to re-focus on the mast cell as a potential therapeutic target for asthma and related disorders (Bradding, 2003, Brightling et al., 2003). Mast cells secrete a large number of mediators in response to a diverse range of stimuli with the majority being secreted by regulated exocytosis degranulation (Blank & Rivera, 2004). While significant advances have been made in understanding the basic mechanisms of exocytosis, the key regulatory steps in mast cell degranulation remain unknown. In this review the molecular mechanisms of mast cell degranulation will be reviewed with emphasis on the role of serine/threonine protein phosphatases which we suggest have the potential to be novel targets for the design of new drugs that regulate the degranulation process.

Section snippets

Mast cells and asthma

Asthma is a major cause of morbidity and mortality and its prevalence continues to rise (Cantani & Micera, 2005, Butland et al., 2006). At the cellular level, asthma involves a complex interplay between a variety of cell types including mast cells, basophils, eosinophils, neutrophils and lymphocytes with associated increased production of chemokines and cytokines. The precise role(s) played by each cell type and their relative importance in the pleiotropic presentation of asthma and related

General mechanisms of exocytosis

Mast cells secrete a wide range of intragranular mediators by the process of exocytosis, which utilises much of the same conserved basic molecular machinery that drives membrane trafficking in most cells (Blank & Rivera, 2004). Common proteins that are involved in the basic exocytotic machinery include the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) family of proteins which are found on both vesicular and plasma membranes and form stable fusion (‘core’)

Classification of protein phosphatases

Whilst the human genome encodes for approximately 500 protein kinases, only 4 distinct families of protein phosphatase with around 150 individual members account for the dephosphorylation process (Cohen, 2002). Of these 150, more than 2/3 of the members, belong to the protein tyrosine phosphatase (PTP) family (Tautz et al., 2006) with only 40 members being responsible for the specific dephosphorylation of serine and threonine residues (Cohen, 2002). The PTP family of protein phosphatase also

Regulation of the phosphoprotein phosphatase family

Clearly, a relatively small number of serine/threonine phosphatases is responsible for dephosphorylating a vast array of proteins that have been phosphorylated by any one or several of a much larger number of protein kinases. Although historically the view of protein phosphatases as relatively non-specific and uncontrolled has been held, it is now clear that specific changes in substrate phosphorylation levels are regulated at both kinase and phosphatase levels. To specifically regulate cell

Serine/threonine phosphatases in mast cell degranulation

The majority of evidence suggesting a role for serine/threonine phosphatases in regulating mast cell degranulation has arisen from the use of the marine toxin, okadaic acid. Okadaic acid inhibits all PPP family phosphatases, with a degree of selectivity for the PPP2/4/6 family (Hastie & Cohen, 1998). However, differential sensitivity in vitro is of limited value in vivo where the concentrations of PPP family members is in the high nanomolar to low micromolar range and therefore requires

Protein phosphatases in mediator synthesis

Whilst it is clear that protein phosphatases regulate exocytosis in a variety of cell types (Sim et al., 2003), measurements of exocytosis must also take into account the rate of synthesis and accumulation of granule contents prior to release. Indeed in adrenal chromaffin cells, okadaic acid appears to increase exocytosis but this is due to increased uptake of catecholamine into secretory granules (Machado et al., 2001). Activation of mast cells also stimulates the production of several

Conclusions and future perspectives

Renewed interest in mast cells as key effector and modulator cells in a variety of physiological processes and diseased states has heightened the importance of understanding the molecular events underlying mast cell degranulation. A dynamic molecular architecture is required to promote the action of a vast array of signals that all result in the movement of secretory granules through the cytoskeletal barrier to fuse with the plasma membrane and release their contents. A range of protein–protein

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