IntroductionMast cells play important protective roles during immune responses, particularly to helminth and bacterial infections. 1 Immature mast cells are released from bone marrow, then migrate to target tissues to undergo terminal differentiation and perform their biologic functions. 2 The White-spotting (W) locus in mice encodes Kit, a type III receptor protein tyrosine kinase (PTK) 3,4 Kit is required for development of erythrocytes, melanocytes, germ cells, mast cells, and interstitial cells of Cajal (ICCs). Stem cell factor (SCF) is the ligand for Kit and is produced in both soluble and membrane-bound forms. 5 In the absence of SCF, the juxtamembrane domain and activation loop region of Kit repress kinase activity. 6 Mutations within the extracellular, juxtamembrane, and kinase domains of Kit that lead to SCF-independent activation have been identified in human cancers derived from mast cells and germ cells, and in gastrointestinal stromal tumors (GISTs) derived from ICCs. 7 Mutations in Kit are frequent in GISTs (85%) and mostly involve juxtamembrane mutations that correlate with poor prognosis. 8 Imatinib mesylate blocks Kit-mediated growth of GIST cell lines and has recently been approved for treatment of unresectable and metastatic GISTs. 9 Activation of Kit by SCF results in recruitment of many SH2 domain-containing proteins (for a review, see Roskoski 10 ). Mutational analyses of individual docking sites on Kit have demonstrated the critical involvement of Src family kinases (SFKs) and phosphatidylinositol 3-kinase (PI3K) for growth and migration responses. [11][12][13] Juxtamembrane tyrosines (Y567/Y569 in mouse; Y568/Y570 in human) of Kit are potential recruitment sites for SFKs, Csk-homology kinase (Chk), and Shc adaptor protein. 14,15 In mast cells, there is evidence for roles of 2 SFKs, Fyn and Lyn, in signaling from juxtamembrane tyrosines of Kit. 14,16 Knock-in of Y567F/Y569F mutations of the kit allele in mice causes postnatal lethality and failure to produce melanocytes and mast cells in vivo. 17 This occurs despite the detection of normal numbers of mast cell precursors in the bone marrow (BM) and the ability to generate ex vivo cultures of BM-derived mast cells (BMMCs). Because individual SFK knock-out mouse lines do not have defects in mast cell production, 18 it is likely that redundancy exists between SFKs for Kit-mediated signals for migration or survival of mast cells in vivo.The p85 subunit of class I A PI3K can be recruited directly to phosphorylated Y719 of Kit or indirectly via the adaptor protein Grb2-associated binding-2 (Gab2). Gab2 is phosphorylated following SCF-induced recruitment of Shc adaptor protein to pY567 of Kit. Shc binds a Grb2-Gab2 complex, a process that leads to phosphorylation of Gab2 on multiple sites that bind the SH2 domains of p85 and Shp2 PTP (PTPN11). 19,20 SCF-induced Gab2 phosphorylation requires Y567 and, therefore, may involve Fyn PTK, as previously shown downstream of the IgE receptor (Fc⑀RI) on mast cells. 21,22 However, another study indicates t...

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Mast cells express the high affinity IgE receptor Fc⑀RI, which is composed of an IgE-binding ␣-chain, a tetramembrane spanning ␤ chain, and a dimeric ␥ chain (1). Signals are transduced via immunoreceptor tyrosine-based activation motifs (ITAMs) 3 that are present in both the ␤ and ␥ subunits and serve as docking sites for the recruitment of signaling molecules (2). Fc⑀RI signaling is initiated by binding of IgE, which is sufficient for induction of survival pathways as well as cytokine production (3, 4). Some highly cytokinergic IgEs can induce antigen-independent degranulation, survival, adhesion, and chemotaxis of mast cells (5). However, in most cases, aggregation of Fc⑀RI by multivalent antigens is required for a full mast cell response including degranulation, lipid mediator release, increased cell adhesion, and increased motility (6). The Src family protein-tyrosine kinase (PTK) Lyn is constitutively associated with Fc⑀RI (7), and upon antigen-mediated clustering of receptor chains, Lyn phosphorylates ITAMs on ␤-and ␥-chains. The ␤-chain ITAMs are thought to recruit additional Lyn and Fyn kinases, the p85 subunit of phosphatidylinositol 3-kinase, SH2-containing inositol 5Ј-phosphatase, and Shp2 phosphatase (8). Phosphorylated ITAMs on the ␥-chains recruit the dual SH2 domain-containing PTK Syk (9). Syk activity is essential for signal transduction downstream of Fc⑀RI, because Syk-deficient mast cells fail to degranulate, synthesize leukotrienes, and secrete cytokines following antigen challenge (10). Although Fc⑀RI-induced tyrosine phosphorylation is greatly reduced in Syk-deficient mast cells, phosphorylation of the receptor ITAMs and Lyn are maintained (10). Lyn-deficient mast cells, despite severely reduced tyrosine phosphorylation and delayed calcium flux, are able to degranulate and secrete cytokines (11). In fact, Lyn-deficient mast cells release more of the granule constituent ␤-hexosaminidase than do wild type mast cells. Further studies have shown that multiple responses to Fc⑀RI aggregation are delayed in Lyn-deficient mast cells, including tyrosine phosphorylation of receptor subunits, calcium flux, and phosphatidylinositol 3,4,5-trisphosphate production but persist far longer than in wild type mast cells (12). Other notable characteristics of Lyn-deficient mast cells are that Fyn kinase is hyperactivate, whereas SH2-containing inositol 5-phosphatase is completely inactive (12). Thus, in addition to initiating signaling downstream of Fc⑀RI, Lyn is also involved in signal termination at least partly through activation of SHZ-containing inositol 5-phosphatase, which hydrolyzes phosphatidylinositol 3,4,5-trisphosphate and thereby reduces the plasma membrane localization of pleckstrin homology domain-containing proteins.

PTP␣ 2 is a receptor-type protein-tyrosine phosphatase that is widely expressed in many cells and tissues including those of hematopoietic origin. It has a short, glycosylated extracellular region with no known ligand binding specificity, a single transmembrane spanning region, and an intracellular region containing two tandem catalytic domains (1). The generation of PTP␣-deficient mice confirmed findings from PTP␣ overexpression studies that this phosphatase functions as a physiological regulator of Src family kinases (SFKs), catalyzing the dephosphorylation of the inhibitory C-terminal tyrosine residue of SFKs and activating them (2-5). Despite the widespread actions of SFKs in multiple cellular and biological processes, PTP␣-null mice with reduced SFK activity are viable and have an overall normal appearance, suggesting that PTP␣ is likely one of several SFK regulators with tissue-, cell-, and/or signaling pathway-specific functions.In keeping with PTP␣ being highly expressed in brain, several studies, many utilizing PTP␣-null mice and cells, have revealed multiple cellular and/or physiological roles of this PTP linked to nervous system development and function. These include regulating N-methyl-D-aspartate receptor phosphorylation and activity, long-term potentiation, and spatial memory (6 -9); sciatic nerve myelination and voltage-gated potassium channel activation and phosphorylation (10, 11), migration of pyramidal neurons (6), and neuronal outgrowth and differentiation (3, 12-15). In addition, PTP␣ regulates integrin-stimulated cell migration (4, 16), T cell activation (17), and mitosis (18). In the absence of an identified ligand for PTP␣, it appears that PTP␣-mediated SFK activation is often linked to activation of certain receptors by their ligands. Indeed, PTP␣ physically associates with receptors that themselves lack enzymatic activity, including the neural cell adhesion molecules contactin and NCAM140 (12,19), and integrin ␣ v (20), and in this way may mediate ligand-induced SFK activation.Cells of the immune system have multiple well defined signaling pathways that are initiated by receptor-mediated SFKdependent events. A role for PTP␣ has been recently demonstrated in T cell receptor-mediated activation and CD44-mediated spreading of T cells (17,21). To further our understanding of PTP␣ functions in immune cells, and also to query whether PTP␣ regulates SFK-dependent signaling that is initiated by a receptor with intrinsic enzymatic activity, we have investigated the role of PTP␣ in mast cells, and specifically in regulating signaling by the receptor-tyrosine kinase c-Kit.Mast cells play key roles in innate and adaptive immune responses, extending from their actions as regulators of allergic inflammation to roles in host defense, immunological tolerance and autoimmune diseases, atherosclerosis, and cancer (22-24). Mast cell progenitors leave the bone marrow and migrate to various connective and mucosal locations where they complete their development and give rise to mature populations with tissue...