THE HETEROGENEOUS RIBONUCLEOPROTEIN L-LIKE: ROLE IN ALTERNATIVE SPLICING REGULATION DURING IMMUNE CELL DEVELOPMENT

Two closely related hnRNPs coexist in several cell types: the general splicing regulator hnRNP L and the cell type-specific hnRNP L-like (LL)¹. Searching for cell types with high expression of hnRNP LL, we found it highly expressed in various plasma cell lines, in contrast to cell lines derived from earlier stages of B cell development (in collaboration with H.-M. Jäck). In addition, we studied the role of hnRNP LL in the monocyte cell line THP1: Here we observed that hnRNP LL levels strongly increased after differentiation to a macrophage-like state. These data were supported by assaying primary monocytes vs. lung macrophages. In sum, in both B cells and monocytes the alternative splicing pattern of CD45 exons 4/5/6 correlated directly with the expression levels of hnRNP LL protein, in agreement with results obtained from T cell activation².

To identify hnRNP LL-dependent alternative splicing events by an RNA sequencing approach (Solexa®), we established hnRNP LL overexpression in a human B lymphocyte cell line. Target validation using RT-PCR verified several cases of single exon-skipping regulated by hnRNP LL.

To address the differential activities of the hnRNP L/LL proteins, we studied their modification by phosphorylation. In contrast to hnRNP L, we found that hnRNP LL was phosphorylated in B cells as well as in monocytes. In in vitro splicing assays the phosphorylated form of hnRNP LL was a much stronger repressor, compared to unphosphorylated hnRNP LL.