The IRE1 Pathway Regulates Honey Bee Unfolded Protein Response Gene Expression

Our molecular understanding of honey bee cellular stress responses is incomplete. Previously, we sought to identify and began functional characterization of the components of the UPR in honey bees. We observed that UPR stimulation resulted in induction of target genes upon and IRE1 pathway activation, as assessed by splicing of Xbp1 mRNA. However, were not able to determine the relative role of the various UPR pathways in gene activation. Our understanding of honey bee signal transduction and transcriptional regulation has been hampered by a lack of tools. After using RNAseq to expand the known UPR targets in the bee, we use the Drosophila melanogaster S2 cell line and honey bee trans and cis elements to investigate the role of the IRE-1 pathway in the transcriptional activation of one of these targets, the honey bee Hsc70-3 gene. Using a luciferase reporter, we show that honey bee hsc70 promoter activity is inducible by UPR activation. In addition, we show that this activation is IRE1-dependent and relies on specific cis regulatory elements. Experiments using exogenous honey bee or fruit fly XBP1S proteins demonstrate that both factors can activate the Hsc70-3 promoter and further support a role for the IRE-1 pathway in control of its expression in the honey bee. By providing foundational knowledge about the UPR in the honey bee and demonstrating the usefulness of a heterologous cell line for molecular characterization of honey bee pathways, this work stands to improve our understanding of this critical species. Overall design: Six Samples, 3 controls and 3 fed solution containing Tunicamycin