The xbp-1 gene is essential for development in Drosophila

Abstract

We report in this paper the characterization of Dxbp-1, the Drosophila homologue of the xpb-1 gene that encodes a "bZIP"-containing transcription factor that plays a key role in the unfolded protein response (UPR), an evolutionarily conserved signalling pathway activated by an overload of misfolded proteins in the endoplasmic reticulum (ER). Dxbp-1 is ubiquitously transcribed, and high levels are found in embryonic salivary glands and in the ovarian follicle cells committed to the synthesis of the respiratory appendages. Loss of function of Dxbp-1 induced a recessive larval lethality, thus, revealing an essential requirement for this gene. The Dxbp-1 transcript was submitted to an "unconventional" splicing that generated a processed Dxbp-1s transcript encoding a DXbp-1 protein isoform, as is the case for yeast, Caenorhabditis elegans and vertebrate hac1/xbp-1 transcripts after UPR activation. However, in the absence of exogenously induced ER stress, the Dxbp-1s transcript was also detectable not only throughout embryonic and larval development but also in adults with a high level of accumulation in the male sexual apparatus and, to a lesser extent, in the salivary glands of the third-instar larvae. Using a Dxbp-1:GFP transgene as an in vivo reporter for Dxbp-1 mRNA unconventional splicing, we confirmed that Dxbp-1 processing took place in the salivary glands of the third-instar larvae. The Dxbp-1 gene appears, thus, to play an essential role during the development of Drosophila, hypothetically by stimulating the folding capacities of the ER in cells committed to intense secretory activities.

Wollknäuel is required for embryo patterning and encodes theDrosophilaALG5 UDP-glucose:dolichyl-phosphate glucosyltransferase

“…Reduced amounts of these transcription factors result in disruption of posterior segmentation and of dorsal-ventral patterning. Although the UPR plays important developmental and physiological roles in C. elegans, Drosophila and mammals (Ryoo et al, 2007;Shen et al, 2001;Shen et al, 2005;Souid et al, 2007;Wu and Kaufman, 2006), this is the first report to indicate that inappropriate UPR activation may disrupt embryonic patterning.…”

“…Accumulation of unfolded proteins in the ER induces the UPR that activates the IRE1 endoribonuclease. In Drosophila, this leads to the removal of a 23 bp intron from xbp1 mRNA in the cytoplasm (Plongthongkum et al, 2007;Ryoo et al, 2007;Souid et al, 2007), which generates a translational frameshift that gives rise to transcriptionally active Xbp1 protein (Plongthongkum et al, 2007).…”

Theire-1ER Stress-Response Pathway is Required for Normal Secretory- Protein Metabolism inC. elegans

“…Under these conditions, ire-1 and xbp-1 deficiency reduces basic ER functions, including translation, folding, secretion and degradation of proteins passing through the secretory pathway. These findings may explain a wide range of physiological defects associated with UPR deficiencies (Bicknell et al, 2007;Fonseca et al, 2012;Henis-Korenblit et al, 2010;Hu et al, 2009;Lee et al, 2008;Reimold et al, 2000;Reimold et al, 2001;Richardson et al, 2010;Souid et al, 2007;Todd et al, 2008;Zhao et al, 2003).…”

“…The ire-1/xbp-1 UPR pathway fulfills crucial functions required for embryonic development and viability in Drosophila, Xenopus and mice (Reimold et al, 2000;Souid et al, 2007;Zhao et al, 2003). A functional UPR is also required for the development of hepatic, bone marrow and B cells, as well as for yeast cytokinesis (Bicknell et al, 2007;Hu et al, 2009;Lee et al, 2008;Reimold et al, 2001;Todd et al, 2008).…”

Host and Viral Translational Mechanisms during Cricket Paralysis Virus Infection

“…The spliced version of Xbp-1 mRNA produces a frameshift in the translational reading frame resulting in the synthesis of a potent transcription factor that activates downstream ER stress-response genes (4, 84). To begin distinguish- ing which eIF2␣ kinase may be responsible for eIF2␣ phosphorylation in CrPV-infected cells, we reasoned that if dPERK was activated via ER stress, then dIRE1 would also be activated and thereby lead to splicing of the 23 nucleotide intron of dXbp-1 mRNA (59,71). To detect the spliced dXbp-1 mRNA, we designed primers that flank the intron, from which we could distinguish the spliced and unspliced versions by RT-PCR amplification.…”