The IRE1/Xbp1 axis restores ER and tissue homeostasis perturbed by excess Notch in Drosophila.

Notch signaling controls numerous key cellular processes including cell fate determination and cell proliferation. Its malfunction has been linked to many developmental abnormalities and human disorders. Overactivation of Notch signaling is shown to be oncogenic. Retention of excess Notch protein in the endoplasmic reticulum (ER) can lead to altered Notch signaling and cell fate, but the mechanism is not well understood. In this study, we show that V5-tagged or untagged exogenous Notch is retained in the ER when overexpressed in fly tissues. Furthermore, we show that Notch retention in the ER leads to robust ER enlargement and elicits a rough eye phenotype. Gain-of-function of unfolded protein response (UPR) factors IRE1 or spliced Xbp1 (Xbp1-s) alleviates Notch accumulation in the ER, restores ER morphology and ameliorates the rough eye phenotype. Our results uncover a pivotal role of the IRE1/Xbp1 axis in regulating the detrimental effect of ER-localized excess Notch protein during development and tissue homeostasis. Schematic representation of Notch protein distribution in the ER. When IRE1 activity is low and ER overloaded with excess Notch protein, the ER membrane is remodeled to form enlarged structures. On the other hand, high IRE1 activity would lead to the clearance of ER-localized excess Notch protein and maintenance of ER homeostasis. EGF repeats: epidermal growth factor (EGF) repeats, NRR: negative regulatory region, NICD: Notch intracellular domain. [Display omitted] • Excess Notch causes developmental defects in Drosophila. • Exogenous Notch proteins accumulate in the ER. • The level of excess Notch in the ER is modulated by the IRE1/Xbp1 axis. • Gain-of-function of IRE1 or Xbp1-s ameliorates the rough eye phenotype caused by excess Notch. [ABSTRACT FROM AUTHOR]