Cell cycle control of wnt receptor activation

Area of Science:

• Cell Biology
• Molecular Biology
• Developmental Biology

Background:

• Low-density lipoprotein receptor related proteins 5 and 6 (LRP5/6) are key initiators of Wnt/beta-catenin signaling.
• Phosphorylation of specific motifs in LRP6 is critical for its function in signal transduction.

Purpose of the Study:

• To identify kinases regulating LRP6 phosphorylation.
• To elucidate the role of cell cycle regulators in Wnt/beta-catenin signaling.

Main Methods:

• Kinome-wide RNA interference (RNAi) screen in Drosophila.
• Western blotting to detect LRP6 phosphorylation.
• Cell cycle synchronization and arrest studies.
• In vivo studies using Xenopus embryos.

Main Results:

• Drosophila Cyclin-dependent kinase (CDK) L63 was identified as essential for LRP6 PPPSP motif phosphorylation.
• The G2/M Cyclin Y regulates L63 activity and mediates LRP6 phosphorylation.
• LRP6 phosphorylation and Wnt signaling peak at the G2/M phase of the cell cycle.
• Knockdown of CDC25/string, causing G2/M arrest, enhances Wnt signaling in a Cyclin Y-dependent manner.
• Cyclin Y is crucial in vivo for LRP6 phosphorylation and Wnt-dependent embryonic patterning in Xenopus.

Conclusions:

• Wnt/beta-catenin signaling is under cell cycle control, with G2/M phase priming of LRP6 by a Cyclin/CDK complex representing a novel regulatory mechanism.
• Cyclin Y and CDK L63 (and its vertebrate homolog PFTK) form a regulatory axis for LRP6 phosphorylation, linking cell cycle progression to Wnt pathway activation.