Heterogeneous nuclear ribonucleoprotein L-like (hnRNPLL) and elongation factor, RNA polymerase II, 2 (ELL2) are regulators of mRNA processing in plasma cells

Abstract

B cells and plasma cells possess distinct RNA processing environments that respectively promote the expression of membrane-associated Ig by B cells versus the secretion of Ig by plasma cells. Through a combination of transcriptional profiling and screening using a lentiviral short-hairpin RNA interference library, we show that both the splicing factor hnRNPLL and the transcription elongation factor ELL2 modulate the ratio of secreted versus membrane-encodingIghg2btranscripts in MPC11 plasmacytoma cell lines. hnRNPLL and ELL2 are both highly expressed in primary plasma cells relative to B cells, but hnRNPLL bindsIghg2bmRNA transcripts and promotes an increase in levels of the membrane-encodingIghg2bisoform at the expense of the secretedIghg2bisoform, whereas ELL2 counteracts this effect and drives Ig secretion by increasing the frequency of the secretedIghg2bisoform. As in T cells, hnRNPLL also alters the splicing pattern of mRNA encoding the adhesion receptor CD44, promoting exon inclusion, and decreasing the overall level of CD44 expression. Further characterization of ELL2-dependent transcription by RNA-Seq revealed that ∼12% of transcripts expressed by plasma cells were differentially processed because of the activities of ELL2, including B-cell maturation antigen BCMA, a receptor with a defined role in plasma cell survival. Taken together, our data identify hnRNPLL and ELL2 as regulators of pre-mRNA processing in plasma cells.