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R. Nakamura, D. D. Hunter, W. J. Brunken, A. S. Hackam; Dickkopf-3 (Dkk3) is an Anti-Apoptotic Protein That Regulates Wnt Signaling in Retinal Muller Glia. Invest. Ophthalmol. Vis. Sci. 2007;48(13):621.
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© ARVO (1962-2015); The Authors (2016-present)
We previously demonstrated that Wnt signaling activators induced complete rescue of retinal cultures exposed to oxidative stress (Yi, ARVO; 2005). In an effort to identify novel pro-survival genes in the degenerating retina, we found that the Wnt regulator Dickkopf 3 protein Dkk3 was increased during photoreceptor death in a mouse model of retinal degeneration (Hackam et al, 2004). Further, we found that Dkk3 was anti-apoptotic in HEK293 cells. The purpose of this study is to understand possible roles of Dkk3 in the retina by characterizing its expression, regulation of cell survival and Wnt activity.
Immunohistochemistry was performed on C57BL/6 retinal sections to localize Dkk3 expression. Western blotting was performed to confirm Dkk3 expression and secretion from Muller glia. The role of Dkk3 in cellular viability and apoptosis was measured using Wst-1 and caspase DEVD-AMC assays, respectively. Potentiation of Wnt signaling by Dkk3 was measured with the TOP-FLASH luciferase reporter assay.
Dkk3 is expressed in the adult and developing retina in the GCL and INL and is synthesized and released from Muller glia cells in culture. Overexpression of Dkk3 increased viability of HEK293 cells upon oxidative stress (p<0.001) and reduced caspase activity in cells exposed to staurosporine (p<0.001). Furthermore, Dkk3 potentiated Wnt signaling in the Muller glia cell line MIO-M1 and HEK293 cells, although not in Cos-7 (p<0.001). Together with the finding that Dkk3 is upregulated during retinal degeneration, these data suggest that Dkk3 and activation of the Wnt pathway play a neuroprotective role during photoreceptor death.
Dkk3 protects several cell lines from exogenous insults, and this protection may utilize the canonical Wnt signaling pathway. In future studies we will explore the protective effect of Wnt signaling in vivo by investigating potential therapeutic applications of Dkk3 in the retina.
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