Disrupted WNT signaling in mouse embryonic stem cells in the absence of calreticulin.

The role of endoplasmic reticulum (ER) homeostasis and protein quality control in the regulation of WNT signaling is not understood. Here we provide evidence for a role of calreticulin in the regulation of WNT signaling. We show that a deficiency in calreticulin disrupted WNT signaling, and prevented cell cycle progression via the miR-302 microRNA family. These effects were dependent on the Ca(2+) buffering capacity of calreticulin, as the protein is important in regulating ER Ca(2+) release and activation of Ca(2+)-dependent kinase and phosphatase cascades (including c-Src, Akt, and PTP1B). We also show that calreticulin plays a role in the secretion and ER retention of WNT3a, thereby affecting downstream WNT signaling. In calreticulin-deficient ES cells, the WNT and miR-302 dependent maintenance of the naïve ES cell state and the transition to primed pluripotency transition were lost, preventing cells from undergoing accurate differentiation. Together, these findings demonstrate unexpected roles of calreticulin and ER Ca(2+) homeostasis/signaling in the canonical WNT signaling pathway.