Identification of two novel activities of the Wnt signaling regulator Dickkopf 3 and characterization of its expression in the mouse retina.
BACKGROUND
The Wnt signaling pathway is a cellular communication pathway that plays critical roles in development and disease. A major class of Wnt signaling regulators is the Dickkopf (Dkk) family of secreted glycoproteins. Although the biological properties of Dickkopf 1 (Dkk1) and Dickkopf 2 (Dkk2) are well characterized, little is known about the function of the related Dickkopf 3 (Dkk3) protein in vivo or in cell lines. We recently demonstrated that Dkk3 transcripts are upregulated during photoreceptor death in a mouse model of retinal degeneration. In this study, we characterized the activity of Dkk3 in Wnt signaling and cell death.


RESULTS
Dkk3 was localized to Müller glia and retinal ganglion cells in developing and adult mouse retina. Western blotting confirmed that Dkk3 is secreted from Müller glia cells in culture. We demonstrated that Dkk3 potentiated Wnt signaling in Müller glia and HEK293 cells but not in COS7 cells, indicating that it is a cell-type specific regulator of Wnt signaling. This unique Dkk3 activity was blocked by co-expression of Dkk1. Additionally, Dkk3 displayed pro-survival properties by decreasing caspase activation and increasing viability in HEK293 cells exposed to staurosporine and H2O2. In contrast, Dkk3 did not protect COS7 cells from apoptosis.


CONCLUSIONS
These data demonstrate that Dkk3 is a positive regulator of Wnt signaling, in contrast to its family member Dkk1. Furthermore, Dkk3 protects against apoptosis by reducing caspase activity, suggesting that Dkk3 may play a cytoprotective role in the retina.
Nakamura
Rei E I
RE
Bascom Palmer Eye Institute, University of Miami Miller School of Medicine, Miami, FL, USA. rnakamura@med.miami.edu
Hunter
Dale D
DD
Yi
Hyun
H
Brunken
William J
WJ
Hackam
Abigail S
AS
D048868:Adaptor Proteins, Signal Transducing; D000328:Adult; D000818:Animals; D017209:Apoptosis; D020169:Caspases / Q000378:metabolism; D016923:Cell Death / Q000235:genetics; D002470:Cell Survival / Q000235:genetics; D002478:Cells, Cultured; D018925:Chemokines; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006801:Humans; D036341:Intercellular Signaling Peptides and Proteins / Q000378:metabolism*; D051379:Mice; D008810:Mice, Inbred C57BL; D008856:Microscopy, Fluorescence; D009457:Neuroglia / Q000378:metabolism / Q000473:pathology; D020419:Photoreceptor Cells, Vertebrate / Q000378:metabolism / Q000473:pathology; D051381:Rats; D012160:Retina / Q000166:cytology / Q000378:metabolism*; D012162:Retinal Degeneration / Q000235:genetics / Q000378:metabolism / Q000473:pathology; D012165:Retinal Ganglion Cells / Q000378:metabolism / Q000473:pathology; D015398:Signal Transduction / Q000235:genetics*; D051153:Wnt Proteins / Q000378:metabolism*
D048868:Adaptor Proteins, Signal Transducing; D018925:Chemokines; C501126:DKK1 protein, human; C498624:DKK3 protein, human; C501127:Dkk1 protein, mouse; C405061:Dkk3 protein, mouse; D004791:Enzyme Inhibitors; D036341:Intercellular Signaling Peptides and Proteins; D051153:Wnt Proteins; D020169:Caspases