Mast cell function: regulation of degranulation by serine/threonine phosphatases

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.