Dual function for the adaptor MIST in IFN-γ production by NK and CD4+NKT cells regulated by the Src kinase Fgr

Natural killer (NK) cells and NKT cells play critical early roles in host defense. Here we show that MIST, an adaptor protein belonging to the SLP-76 family, functions negatively in NK cells but positively in CD4+NKT cells. NK-cell receptor-mediated IFN-γ production was enhanced in NK cells, whereas TCR- or NK-cell receptor-mediated cytokine production was reduced in CD4+NKT cells from MIST-deficient mice. These opposite effects of MIST paralleled the exclusive expression of the Src family kinase, Fgr, in NK cells between the 2 cell populations. We further demonstrated that interaction of MIST with Fgr, mediated by the C-terminal proline-rich region of MIST and the SH3 domain of Fgr, was required for the suppression of NK-cell receptor-induced IFN-γ production. This functional interdependence of signaling molecules demonstrates a new mechanism by which adaptor proteins can act as molecular switches to control diverse responses in different cell populations.

Transcriptional regulation of SLP-76 family hematopoietic cell adaptor MIST/Clnk by STAT5

SLP-76-related adaptor protein MIST (also called Clnk) is expressed in a variety of cytokine-dependent hematopoietic cell lines of myeloid and lymphoid origin as well as some cytokine-independent mast cell lines. To understand the molecular mechanisms underlying the MIST gene expression, we have characterized the 5'-flanking region of the mouse MIST gene. We have identified an enhancer region (-773 to -709), which is active in P815 mast cells expressing the endogenous MIST gene, but not in EL-4 T cells lacking MIST expression. Outside of this enhancer region, one STAT element present in the MIST promoter (-44 to -36) was found to bind STAT5A when IC-2 mast cells were stimulated with IL-3. Mutation of this STAT element did not affect basal MIST promoter activity in P815 mast cells, but was required for STAT5-mediated activation of the MIST promoter. Furthermore, endogenous MIST gene expression was induced in mast cells by a constitutively activated form of STAT5A, but not by an active mutant of c-Kit receptor. These findings suggest that STAT5 is involved in cytokine-mediated up-regulation of MIST gene expression, probably in collaboration with other lineage-specific transcription factors that promote basal MIST expression in mast cells.

Targeting of MIST to Src-family kinases via SKAP55-SLAP-130 adaptor complex in mast cells

MIST (mast cell immunoreceptor signal transducer; also termed Clnk) is an adaptor protein structurally related to SLP-76-family hematopoietic cell-specific adaptor proteins. We demonstrate here that two major MIST-associated phosphoproteins expressed in mast cell lines are SLAP-130 and SKAP55, adaptors known to interact with the Src-homology (SH) 2 domain of Src-family protein tyrosine kinases (PTKs). MIST directly associated with SLAP-130 via its SH2 domain, and collaboration of SLAP-130 with SKAP55 was required for the recruitment of MIST to Lyn. Furthermore, MIST was preferentially recruited to Fyn rather than Lyn, which is regulated by higher affinity binding of SLAP-130 and SKAP55 with the Fyn-SH2 domain than the Lyn-SH2 domain. Our results suggest that the MIST-SLAP-130-SKAP55 adaptor complex functions downstream of high-affinity IgE receptor-associated Src-PTKs in mast cells.

MIST functions through distinct domains in immunoreceptor signaling in the presence and absence of LAT

MIST (also termed Clnk) is an adaptor protein structurally related to SLP-76 and BLNK/BASH/SLP-65 hematopoietic cell-specific adaptor proteins. By using the BLNK-deficient DT40 chicken B cell system, we demonstrated MIST functions through distinct intramolecular domains in immunoreceptor signaling depending on the availability of linker for activation of T cells (LAT). MIST can partially restore the B cell antigen receptor (BCR) signaling in the BLNK-deficient cells, which requires phosphorylation of the two N-terminal tyrosine residues. Co-expression of LAT with MIST fully restored the BCR signaling and dispenses with the requirement of the two tyrosines in MIST for BCR signaling. However, some other tyrosine(s), as well as the Src homology (SH) 2 domain and the two proline-rich regions in MIST, is still required for full reconstitution of the BCR signaling, in cooperation with LAT. The C-terminal proline-rich region of MIST is dispensable for the LAT-aided full restoration of MAP kinase activation, although it is responsible for the interaction with LAT and for the localization in glycolipid-enriched microdomains. On the other hand, the N-terminal proline-rich region, which is a binding site of the SH3 domain of phospholipase Cgamma, is essential for BCR signaling. These results revealed a marked plasticity of MIST function as an adaptor in the cell contexts with or without LAT.

A BASH/SLP-76-related adaptor protein MIST/Clnk involved in IgE receptor-mediated mast cell degranulation

Cross-linking of the high-affinity IgE receptor (FcepsilonRI) on mast cells by IgE-antigen complex triggers signal transduction cascades leading to the release of inflammatory mediators and production of cytokines, which are critical for the development of allergic reactions. We have identified a novel member of the BASH/SLP-76 immunoreceptor-coupled adaptor family expressed in mast cells, termed MIST (for mast cell immunoreceptor signal transducer), which has later been found to be identical to a recently reported cytokine-dependent hemopoietic cell linker, Clnk. Upon FcepsilonRI cross-linking, MIST/Clnk is tyrosine phosphorylated and associates with signaling proteins, phospholipase Cgamma, Vav, Grb2 and linker for activation of T cells (LAT). Overexpression of a mutant form of MIST/Clnk inhibited FcepsilonRI-mediated degranulation, increase in intracellular Ca(2+), NF-AT activation and phosphorylation of LAT. As a crucial signaling component for FcepsilonRI-induced mast cell degranulation, MIST/Clnk might serve as a target for anti-allergic therapy.