Cleavage of the Wnt Receptor Ryk Regulates Neuronal Differentiation during Cortical Neurogenesis

Eli and Edythe Broad Center for Regenerative Medicine and Stem Cell Research, Department of Biochemistry and Molecular Biology, Keck School of Medicine, University of Southern California, Los Angeles, CA 90033.
Developmental Cell (Impact Factor: 9.71). 12/2008; 15(5):773-80. DOI: 10.1016/j.devcel.2008.10.004
Source: PubMed


Ryk is a transmembrane receptor tyrosine kinase (RTK). It functions as a receptor of Wnt proteins required for cell-fate determination, axon guidance, and neurite outgrowth in different organisms; however, the molecular mechanism of Ryk signaling is unknown. Here, we show that Ryk is cleaved, permitting the intracellular C-terminal fragment of Ryk to translocate to the nucleus in response to Wnt3 stimulation. We also show that the cleaved intracellular domain of Ryk is required for Wnt3-induced neuronal differentiation in vitro and in vivo. These results demonstrate an unexpected mechanism of signal transduction for Ryk as a Wnt receptor, in which the intracellular domain itself functions as the transducing molecule to bring extracellular signals from the cell surface into the nucleus, to regulate neural progenitor cell differentiation.

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    • "It remains, however, controversial whether this signaling pathway promotes or inhibits neuronal differentiation of NPCs. Previous studies have shown that expression of a Wnt ligand or active β-catenin promotes neuronal differentiation of NPCs and/or INPs in various regions at different stages [7], [11]–[13], [15], [49]–[54]. Expression of a VP16-fused (transcriptionally active) form of Lef1 or Tcf4 was also found to drive neuronal differentiation [15], [55]. "
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    ABSTRACT: During mouse neocortical development, the Wnt-β-catenin signaling pathway plays essential roles in various phenomena including neuronal differentiation and proliferation of neural precursor cells (NPCs). Production of the appropriate number of neurons without depletion of the NPC population requires precise regulation of the balance between differentiation and maintenance of NPCs. However, the mechanism that suppresses Wnt signaling to prevent premature neuronal differentiation of NPCs is poorly understood. We now show that the HMG box transcription factor Tcf3 (also known as Tcf7l1) contributes to this mechanism. Tcf3 is highly expressed in undifferentiated NPCs in the mouse neocortex, and its expression is reduced in intermediate neuronal progenitors (INPs) committed to the neuronal fate. We found Tcf3 to be a repressor of Wnt signaling in neocortical NPCs in a reporter gene assay. Tcf3 bound to the promoter of the proneural bHLH gene Neurogenin1 (Neurog1) and repressed its expression. Consistent with this, Tcf3 repressed neuronal differentiation and increased the self-renewal activity of NPCs. We also found that Wnt signal stimulation reduces the level of Tcf3, and increases those of Tcf1 (also known as Tcf7) and Lef1, positive mediators of Wnt signaling, in NPCs. Together, these results suggest that Tcf3 antagonizes Wnt signaling in NPCs, thereby maintaining their undifferentiated state in the neocortex and that Wnt signaling promotes the transition from Tcf3-mediated repression to Tcf1/Lef1-mediated enhancement of Wnt signaling, constituting a positive feedback loop that facilitates neuronal differentiation.
    Full-text · Article · May 2014 · PLoS ONE
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    • "NPCs were cultured as described (Lyu et al., 2008). Briefly, neocortices were mechanically dissociated into single cells using a flame-polished Pasteur pipet. "
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    ABSTRACT: Neural progenitor cells (NPCs) are multipotent cells that can self-renew and differentiate into neurons and glial cells. However, mechanisms that control their fate decisions are poorly understood. Here, we show that Smek1, a regulatory subunit of the serine/threonine protein phosphatase PP4, promotes neuronal differentiation and suppresses the proliferative capacity of NPCs. We identify the cell polarity protein Par3, a negative regulator of neuronal differentiation, as a Smek1 substrate and demonstrate that Smek1 suppresses its activity. We also show that Smek1, which is predominantly nuclear in NPCs, is excluded from the nucleus during mitosis, allowing it to interact with cortical/cytoplasmic Par3 and mediate its dephosphorylation by the catalytic subunit PP4c. These results identify the PP4/Smek1 complex as a key regulator of neurogenesis.
    Full-text · Article · Oct 2013 · Cell Reports
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    • "Subsequently, a C-terminal cleavage product, DFz2C, is imported into the nucleus (Mathew et al., 2005) via canonical nuclear import machinery (Mosca and Schwarz, 2010) where it localizes to discrete foci (Ataman et al., 2008; Mathew et al., 2005). A similar transduction pathway has been reported for the Wnt receptor Ryk during mammalian cortical neuron development (Lyu et al., 2008). However, the nuclear function of these DFz2C/Ryk C-terminal fragments remains unexplored. "
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    ABSTRACT: Localized protein synthesis requires assembly and transport of translationally silenced ribonucleoprotein particles (RNPs), some of which are exceptionally large. Where in the cell such large RNP granules first assemble was heretofore unknown. We previously reported that during synapse development, a fragment of the Wnt-1 receptor, DFrizzled2, enters postsynaptic nuclei where it forms prominent foci. Here we show that these foci constitute large RNP granules harboring synaptic protein transcripts. These granules exit the nucleus by budding through the inner and the outer nuclear membranes in a nuclear egress mechanism akin to that of herpes viruses. This budding involves phosphorylation of A-type lamin, a protein linked to muscular dystrophies. Thus nuclear envelope budding is an endogenous nuclear export pathway for large RNP granules.
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