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Formin 2 Regulates Lysosomal Degradation of Wnt-Associated β-Catenin in Neural Progenitors.

ABSTRACT: Although neural progenitor proliferation along the ventricular zone is regulated by β-catenin through Wnt signaling, the cytoskeletal mechanisms that regulate expression and localization of these proteins are not well understood. Our prior studies have shown that loss of the actin-binding Filamin A (FlnA) and actin-nucleating protein Formin 2 (Fmn2) impairs endocytosis of low-density-lipoprotein-receptor-related protein 6 (Lrp6), thereby disrupting β-catenin activation, resulting in decreased brain size. Here, we report that activated RhoA-GTPase disengages Fmn2 N- to C-terminal binding to promote Fmn2 activation and redistribution into lysosomal vesicles. Fmn2 colocalizes with β-catenin in lysosomes and promotes its degradation. Further, Fmn2 binds the E3 ligase Smurf2, enhances Smurf2-dependent ubiquitination, and degradation of Dishevelled-2 (Dvl2), thereby initiates β-catenin degradation. Finally, Fmn2 overexpression disrupts neuroepithelial integrity, neuronal migration, and proliferation-phenotypes in E13 mouse embryos, as seen with loss of Fmn2+FlnA function. Conversely, co-expression of Dvl2 with Fmn2 rescues the proliferation defect due to Fmn2 overexpression in mouse embryos. These findings suggest that there is a homeostatic feedback mechanism in the cytoskeletal-dependent regulation of neural proliferation within the cerebral cortex. Upstream, Fmn2 promotes proliferation by stabilizing the Lrp6 receptor, leading to β-catenin activation. Downstream, RhoA-activated Fmn2 promotes lysosomal degradation of Dvl2, leading to β-catenin degradation.

SUBMITTER: Lian G

PROVIDER: S-EPMC6458913 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Improving the neuronal yield from in vitro cultivated neural progenitor cells (NPCs) is an essential challenge in transplantation therapy in neurological disorders. In this regard, Ascorbic acid (AA) is widely used to expand neurogenesis from NPCs in cultures although the mechanisms of its action remain unclear. Neurogenesis from NPCs is regulated by the redox-sensitive WNT/β-catenin signaling pathway. We therefore aimed to investigate how AA interacts with this pathway and potentiates neurogenesis.Effects of 200 μM AA were compared with the pro-neurogenic reagent and WNT/β-catenin signaling agonist lithium chloride (LiCl), and molecules with antioxidant activities i.e. N-acetyl-L-cysteine (NAC) and ruthenium red (RuR), in differentiating neural progenitor ReNcell VM cells. Cells were supplemented with reagents for two periods of treatment: a full period encompassing the whole differentiation process versus an early short period that is restricted to the cell fate commitment stage. Intracellular redox balance and reactive oxygen species (ROS) metabolism were examined by flow cytometry using redox and ROS sensors. Confocal microscopy was performed to assess cell viability, neuronal yield, and levels of two proteins: Nucleoredoxin (NXN) and the WNT/β-catenin signaling component Dishevelled 2 (DVL2). TUBB3 and MYC gene responses were evaluated by quantitative real-time PCR. DVL2-NXN complex dissociation was measured by fluorescence resonance energy transfer (FRET).In contrast to NAC which predictably exhibited an antioxidant effect, AA treatment enhanced ROS metabolism with no cytotoxic induction. Both drugs altered ROS levels only at the early stage of the differentiation as no changes were held beyond the neuronal fate commitment stage. FRET studies showed that AA treatment accelerated the redox-dependent release of the initial pool of DVL2 from its sequestration by NXN, while RuR treatment hampered the dissociation of the two proteins. Accordingly, AA increased WNT/β-catenin signaling output i.e. MYC mRNA level, whereas RuR attenuated it. Moreover, AA improved neurogenesis as much as LiCl as both TUBB3-positive cell yield and TUBB3 mRNA level increased, while NAC or RuR attenuated neurogenesis. Markedly, the neurogenesis outputs between the short and the full treatment with either NAC or AA were found unchanged, supporting our model that neuronal yield is altered by events taking place at the early phase of differentiation.Our findings demonstrate that AA treatment elevates ROS metabolism in a non-lethal manner prior to the NPCs commitment to their neuronal fate. Such effect stimulates the redox-sensitive DVL2 activation and WNT/β-catenin signaling response that would enhance the ensuing neuronal cell differentiation.

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Formin 2 Regulates Lysosomal Degradation of Wnt-Associated β-Catenin in Neural Progenitors.

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