Dysregulated FcεRI Signaling and Altered Fyn and SHIP Activities in Lyn-Deficient Mast Cells

Abstract

Studies in B cells from Lyn-deficient mice have identified Lyn as both a kinetic accelerator and negative regulator of signaling through the BCR. The signaling properties of bone marrow-derived mast cells from Lyn−/− mice (Lyn−/− BMMCs) have also been explored, but their signaling phenotype remains controversial. We confirm that Lyn−/− BMMCs release more β-hexosaminidase than wild-type BMMCs following FcεRI cross-linking and show that multiple mast cell responses to FcεRI cross-linking (the phosphorylation of receptor subunits and other proteins, the activation of phospholipase Cγ isoforms, the mobilization of Ca2+, the synthesis of phosphatidylinositol 3,4,5-trisphosphate, the activation of the α4β1 integrin, VLA-4) are slow to initiate in Lyn−/− BMMCs, but persist far longer than in wild-type cells. Mechanistic studies revealed increased basal as well as stimulated phosphorylation of the Src kinase, Fyn, in Lyn−/− BMMCs. Conversely, there was very little basal or stimulated tyrosine phosphorylation or activity of the inositol phosphatase, SHIP, in Lyn−/− BMMCs. We speculate that Fyn may substitute (inefficiently) for Lyn in signal initiation in Lyn−/− BMMCs. The loss of SHIP phosphorylation and activity very likely contributes to the increased levels of phosphatidylinositol 3,4,5-trisphosphate and the excess FcεRI signaling in Lyn−/− BMMCs. The unexpected absence of the transient receptor potential channel, Trpc4, from Lyn−/− BMMCs may additionally contribute to their altered signaling properties.

Deficiency of Src family kinases compromises the repopulating ability of hematopoietic stem cells

“…Intriguingly, Lyn-deficient B cells are also hyperresponsive to anti-IgMinduced proliferation and demonstrate enhanced activation of mitogen-activated protein kinase and sustained calcium mobilization [21][22][23]26]. Hernandez-Hansen et al [32] have shown that dysregulated signaling in Lyn-deficient mast cells is due in part to enhanced Fyn activation, and a recent study by Hong et al [33] suggested that the Src kinase Hck regulates mast cell activation by suppressing activation of Lyn. These results suggest that regulation of SFKs in HSC/Ps is likely to be complex and may depend on the stimulus and development stage of the cells under investigation.…”

Positive and Negative Regulation of High Affinity IgE Receptor Signaling by Src Homology Region 2 Domain-Containing Phosphatase 1

“…However, the Ca 2ϩ response in Lyndeficient mast cells was impaired at early transient stage of the response and was almost intact at the later prolonged phase (48,50,51) with one exception in which Lyn deficiency caused a marked decrease in overall Ca 2ϩ response (49). It should be noted that Stim1-deficient mast cells showed severely impaired sustained Ca 2ϩ increase and degranulation (64).…”

Regulation of allergy by Fc receptors

“…IgE-induced ITAM phosphorylation, PLC-γ activation and Ca 2+ mobilization were reduced but persisted longer [28]; PI(3,4,5)P3 levels, and Fyn activity were however markedly enhanced in Lyn -/-mast cells. Increased PI(3,4,5)P3 levels were accounted for by an impaired activity of SHIP1 [28] and Fyn hyperactivity by an abrogation of Lyndependent phosphorylation of Csk-Binding Protein (Cbp) which mediates the recruitment of Csk. Csk phosphorylates regulatory tyrosines in Fyn and thereby inhibits its catalytic activity.…”