A novel role for Gab2 in bFGF-mediated cell survival during retinoic acid-induced neuronal differentiation.

Gab proteins amplify and integrate signals stimulated by many growth factors. In culture and animals, retinoic acid (RA) induces neuronal differentiation. We show that Gab2 expression is detected in neurons in three models of neuronal differentiation: embryonic carcinoma (EC) stem cells, embryonic stem cells, and primary neural stem cells (NSCs). RA treatment induces apoptosis, countered by basic FGF (bFGF). In EC cells, Gab2 silencing results in hypersensitivity to RA-induced apoptosis and abrogates the protection by bFGF. Gab2 suppression reduces bFGF-dependent activation of AKT but not ERK, and constitutively active AKT, but not constitutively active MEK1, reverses the hypersensitization. Thus, Gab2-mediated AKT activation is required for bFGF's protection. Moreover, Gab2 silencing impairs the differentiation of EC cells to neurons. Similarly, in NSCs, Gab2 suppression reduces bFGF-dependent proliferation as well as neuronal survival and production upon differentiation. Our findings provide the first evidence that Gab2 is an important player in neural differentiation, partly by acting downstream of bFGF to mediate survival through phosphoinositide 3 kinase-AKT.

Measurement of dissociation rate of biomolecular complexes using CE.

Fluorescence anisotropy (FA), non-equilibrium CE of equilibrium mixtures (NECEEM) and high-speed CE were evaluated for measuring dissociation kinetics of peptide-protein binding systems. Fyn-SH3-SH2, a protein construct consisting of the src homology 2 (SH2) and 3 (SH3) domain of the protein Fyn, and a fluorescein-labeled phosphopeptide were used as a model system. All three methods gave comparable half-life of approximately 53 s for Fyn-SH3-SH2:peptide complex. Achieving satisfactory results by NECEEM required columns over 30 cm long. When using Fyn-SH2-SH3 tagged with glutathione S-transferase (GST) as the binding protein, both FA and NECEEM assays gave evidence of two complexes forming with the peptide, yet neither method allowed accurate measurement of dissociation rates for both complexes because of a lack of resolution. High-speed CE, with a 7 s separation time, enabled separation of both complexes and allowed determination of dissociation rate of both complexes independently. The two complexes had half-lives of 22.0+/-2.7 and 58.8+/-6.1 s, respectively. Concentration studies revealed that the GST-Fyn-SH3-SH2 protein formed a dimer so that complexes had binding ratios of 2:1 (protein-to-peptide ratio) and 2:2. Our results demonstrate that although all methods are suitable for 1:1 binding systems, high-speed CE is unique in allowing multiple complexes to be resolved simultaneously. This property allows determination of binding kinetics of complicated systems and makes the technique useful for discovering novel affinity interactions.

Multiplexed detection of protein-peptide interaction and inhibition using capillary electrophoresis.

High-speed capillary electrophoresis (CE) was employed to detect binding and inhibition of SH2 domain proteins using fluorescently labeled phosphopeptides as affinity probes. Single SH2 protein-phosphopeptide complexes were detected and confirmed by competition and fluorescence anisotropy. The assay was then extended to a multiplexed system involving separation of three SH2 domain proteins: Src, SH2-Bbeta, and Fyn. The selectivity of the separation was improved by altering the charge of the peptide binding partners used, thus demonstrating a convenient way to control resolution for the multiplexed assay. The separation was completed within 6 s, allowing rapidly dissociating complexes to be detected. Two low molecular weight inhibitors were tested for inhibition selectivity and efficacy. One inhibitor interrupted binding interaction of all three proteins, while the other selectively inhibited Src only leaving SH2-Bbeta and Fyn complex barely affected. IC(50) of both selective and nonselective inhibitors were determined and compared for different proteins. The IC(50) of the nonselective inhibitor was 49 +/- 9, 323 +/- 42, and 228 +/- 19 microM (n = 3) for Src, SH2-Bbeta, and Fyn, respectively, indicating different efficacy of the nonselective inhibitor for different SH2 domain protein. It is concluded that high-speed CE has the potential for multiplexed screening of drugs that disrupt protein-protein interactions.

A juxtamembrane tyrosine in the colony stimulating factor-1 receptor regulates ligand-induced Src association, receptor kinase function, and down-regulation.

Recent literature implicates a regulatory function of the juxtamembrane domain (JMD) in receptor tyrosine kinases. Mutations in the JMD of c-Kit and Flt3 are associated with gastrointestinal stromal tumors and acute myeloid leukemias, respectively. Additionally, autophosphorylated Tyr559 in the JMD of the colony stimulating factor-1 (CSF-1) receptor (CSF-1R) binds to Src family kinases (SFKs). To investigate SFK function in CSF-1 signaling we established stable 32D myeloid cell lines expressing CSF-1Rs with mutated SFK binding sites (Tyr559-TFI). Whereas binding to I562S was not significantly perturbed, Y559F and Y559D exhibited markedly decreased CSF-1-dependent SFK association. All JMD mutants retained intrinsic kinase activity, but Y559F, and less so Y559D, showed dramatically reduced CSF-1-induced autophosphorylation. CSF-1-mediated wild-type (WT)-CSF-1R phosphorylation was not markedly affected by SFK inhibition, indicating that lack of SFK binding is not responsible for diminished Y559F phosphorylation. Unexpectedly, cells expressing Y559F were hyperproliferative in response to CSF-1. Hyperproliferation correlated with prolonged activation of Akt, ERK, and Stat5 in the Y559F mutant. Consistent with a defect in receptor negative regulation, c-Cbl tyrosine phosphorylation and CSF-1R/c-Cbl co-association were almost undetectable in the Y559F mutant. Furthermore, Y559F underwent reduced multiubiquitination and delayed receptor internalization and degradation. In conclusion, we propose that Tyr559 is a switch residue that functions in kinase regulation, signal transduction and, indirectly, receptor down-regulation. These findings may have implications for the oncogenic conversion of c-Kit and Flt3 with JMD mutations.