Linking Endoplasmic Reticular Stress, ELL2, and Alternative Splicing

Endo-reticular stress induces the unfolded protein response including a highly conserved set of genes crucial for cell survival against a variety of onslaughts. Among the activated stress response genes is Ire1, which undergoes auto-phosphorylation and acquires a regulated Ire1-dependent mRNA decay activity. Ire1P non-canonically splices the mRNA for Xbp1 in the cytoplasm. This spliced Xbp1 serves as mRNA encoding a transcription factor for unfolded protein response genes. Meanwhile the mRNA decay function of Ire1P degrades other cellular mRNAs and can cause changes to the translation machinery by altering regulators in a cell specific manner. Naïve splenic B cells differentiate into Antibody Secreting Cells and activate Ire1 phosphorylation early on after LPS stimulation, within 18 hrs. When Ire1 is activated in B cells, in addition to Xbp1 splicing, there are large-scale changes in mRNA; inhibition of the mRNA degradation function of Ire1 both reduces the number and changes the type of genes involved in altered splicing patterns, including factors for snRNA transcription. Some of the splicing changes seen at 18 hrs after LPS persist into the late stages of antibody secretion, up to 72 hrs, while others are supplanted by new splicing changes introduced by the induction of ELL2, a transcription elongation factor. ELL2 changes mRNA processing patterns and is necessary for Immunoglobulin secretion. RNA splicing patterns in antibody secreting cells are thus shaped by endo-reticular stress, ELL2 induction, and are associated with changes in the levels of snRNAs.