Phosphorylated Tyr142 β-Catenin signaling in axon morphogenesis and centrosomal functions
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β-catenin is a multifunctional protein, key component of adherent junctions and effector of the Wnt canonical pathway, was recently implicated in centrosomal functions. In the canonical Wnt pathway, when Wnt is present in the system, β-catenin escapes degradation, accumulates in the cytosol and translocates to the nucleus where, together with T-cell Factor (TCF) transcription factors, it regulates transcription of Wnt targets. Switching from adhesive to signaling functions (independent of Wnt) is achieved in part through phosphorylation of β-catenin at Tyr142 that promotes detachment of β-catenin from the adhesion complex and promotes migration by transcriptional regulation of target genes. Met receptor tyrosine kinase (the receptor for Hepatocyte Growth Factor (HGF)), is one of the kinases regulating β-catenin phosphoryation at Tyr142 during cell migration and axon outgrowth stimulated by HGF. On the other hand, β-catenin phosphorylation at Ser/Thr regulates β-catenin degradation and has been demonstrated to affect centrosomal cohesion/separation and spindle formation. Here we focus on PhosphoTyrosine142 β-catenin (PTyr142 β-cat) signaling. First, we demonstrate that chemokines of CC and CXC families promote axon outgrowth. Furthermore, chemokine signaling acts downstream to HGF/Met/β-catenin/TCF signaling to regulate axon morphogenesis in developing hippocampal neurons. We also show that CXCL2 promotes axon branching and is involved in sensory axon outgrowth from dorsal root ganglia. In the second part of the work, we find for the first time that phosphorylated Tyr142 β-catenin localizes to centrosomes in primary astrocytes and glioma cells, and that centrosomal levels drop in mitosis. We also demonstrate the novel centrosomal localization of Met phosphorylated at Tyr1234/35. Aiming at identifying which is the kinase(s) regulating centrosomal PTyr142 β-cat, we show that a Met inhibitor does not affect it. However, an inhibitor of Spleen Tyrosine Kinase (Syk) decreases centrosomal PTyr142 β-cat, suggesting that Syk regulates the phosphorylation of Tyr142 β-catenin at centrosome. In addition, β-catenin is involved in the correct positioning of centrosomes during astrocyte migration and phosphorylation of β-catenin at Tyr142 is needed for HGF-stimulated cell migration. Collectively, this work demonstrates the multiple roles of PTyr142 β-cat signaling, influencing axon morphogenesis (via regulation of chemokines expression) as well as centrosomal functions, cell polarity and migration.