Interference in xbp1 gene expression induces defective cell differentiation and sensory organ development in Drosophila

Abstract

The X-box binding protein 1 (Xbp1) is a transcription factor that is important in the unfolded protein response to protect cells from endoplasmic reticulum (ER) stress and is implicated in several other biological processes, including liver growth and B lymphocyte differentiation. Although the cellular function of Xbp1 has been well studied, its developmental role remains to be elucidated. In the present study, we investigated the developmental role of Drosophila xbp1 using an RNAi-mediated gene silencing system. When xbp1 gene expression was specifically interfered with in developing tissues, such as wing and eye during larval growth, the tissues were strongly deformed. Cell death was not observed in these tissues, and this phenotype was not rescued by the overexpression of Drosophila inhibitor of apoptosis protein 1 (DIAP1), a caspase inhibitor, indicating that these developmental defects are not associated with apoptosis. Furthermore, changes were not observed in cell cycle progression and Jun-N-terminal kinase activation in tissues whose xbp1 gene expression was interfered with. Rather, the xbp1 gene silencing resulted in defective cell differentiation and supernumerary of sensory organ precursors. These results suggest the inability of xbp1 gene silencing to induce strong ER stress and the importance of Drosophila Xbp1 in cell fate determination and sensory organ development.

Integration of UPRER and Oxidative Stress Signaling in the Control of Intestinal Stem Cell Proliferation

“…Clones generated by ISCs homozygous for the Xbp1 loss-of-function allele Xbp1 k13803 , the Hrd1 loss of function allele hrd1 Delta (a deletion that deletes Hrd1 and CG2126, see methods), or clones expressing Xbp1 RNAi or Hrd1 RNAi grew significantly faster than wild-type controls ( Figure 2G , Figure 2H , Figure S1B –E). Accordingly, clones derived from ISCs over-expressing endogenous Xbp1 (using Xbp1 d08698 , a line in which Xbp1 transcription is induced downstream of a transgenic UAS [37] , [41] ), a transgene encoding a constitutively active, spliced version of Xbp1 [11] , or transgenic Hrd1 [42] , grew significantly slower than clones derived from wild-type ISCs ( Figure 2G , Figure 2H , Figure S1F ). While maintaining ER homeostasis through the UPR ER is thus essential to limit ISC proliferation and prevent dysplasia, a functional UPR ER is also required for normal homeostatic regeneration.…”

“…Feeding tunicamycin very robustly induced ISC proliferation, supporting a role for activation of the UPR ER in promoting ISC proliferation ( Figure 3A , Figure 3B , Figure 3D , and Figure 3E ). Increasing ER stress tolerance by over-expressing endogenous Xbp1, spliced Xbp1, Hrd1, or Hsc3/Bip in ISCs and EBs (using esg::Gal4 and esg ts ; spliced Xbp1 was expressed only in adults using esg ts ) is sufficient to significantly reduce tunicamycin-induced ISC proliferation ( Figure 3A , Figure 3B , Figure 3D , note that expressing spliced Xbp1, endogenous Xbp1 (using Xbp1 d08698 or Xbp1 EP2112 [37] , [41] ), as well as Hsc3/Bip also inhibited proliferation induced by oxidative stress inducer paraquat, Figure 3C , Figure 3D ,). This inhibition was also observed when spliced Xbp1 was over-expressed selectively only in ISCs (using esg ts ; Su(H)Gbe::Gal80; Figure 3E ), but not when spliced Xbp1 or Hrd1 were expressed in ECs or EBs only (using the EC-specific NP1::Gal4 or the EB-specific Su(H)Gbe::Gal4, both rendered heat-inducible by combination with tub::Gal80ts; Figure 3F , Figure 3G ).…”