Retrograde transport mediated ER recycling of Wntsess
Date
2011
Authors
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Publisher
University of Delaware
Abstract
Wnts are secreted signaling molecules that have numerous functions during organismal development and are important in adult stem cell maintenance. While extensive study has elucidated many of the Wnt signaling mechanisms within signal-receiving cells, the process of Wnt maturation within signal producing cells remains poorly understood. Within the developing Drosophila wing imaginal disc, Wntless (Wls) and the retromer complex are required to sustain secretion of Wingless (Wg), the prototypical Drosophila Wnt. Wls directly binds Wg and escorts it through the secretory system for plasma membrane deployment. The retromer complex recycles Wls back to the secretory system to maintain Wls levels needed for continuous Wg secretion. The retromer complex is known to recycle cell surface proteins back to the Golgi, which has led to the broad view that Wls initially binds Wg within the Golgi. Here we present data showing Wls initially engages Wg within the endoplasmic reticulum (ER). This result leads to the hypothesis that Wls must be transported from the Golgi to the ER following retromer mediated Golgi targeting. To test this hypothesis, we performed ß-COP and Yip1A RNAi knockdowns to specifically block Golgi to ER retrograde transport in Wg expressing cells. ß-COP is a component of COPI coatomer vesicles shown to be important for retrograde transport. Whereas Yip1A is a protein that regulates the membrane association of Rab protein shown to regulate the retrograde transport of Shiga toxin from the Plasma membrane to the ER. Remarkably, the ß-COP and YipA1 knockdowns cells and adult wing nicks. RNAi induced Wg retention and wing nicks are rescued by over-expression of wls in knockdown cells. This suggests Wg accumulation results from insufficient ER levels of Wls since it is required for Wg transport and deployment. These data suggest a model where Wls cycles from the ER to the plasma membrane with its Wg cargo and then returns to the ER for additional rounds of Wg export via Golgi mediated retromer targeting followed by Golgi to ER retrograde transport.