Nuclear Speckle-related Protein 70 Binds to Serine/Arginine-rich Splicing Factors 1 and 2 via an Arginine/Serine-like Region and Counteracts Their Alternative Splicing Activity.

Abstract

We have previously identified a novel gene named NSRP1, whose function was attributed to alternatively splice mRNA. NSrp70—a protein product of NSRP1 gene—was highly expressed in early developmental stages and knockout of NSRP1 resulted in embryonic lethality. In the present study, interestingly, we found that NSrp70 is also exclusively expressed in immune-related tissues such as thymus, spleen, and lymph node as well as in T and B lymphocytes. These results led us to investigate whether NSrp70 plays a role for the development of both T cells and B cells. In thymus, the expression of NSrp70 was high in CD4+CD8+ double positive (DP) thymocytes, but low in mature CD4+ and CD8+ single positive (SP) thymocytes, suggesting that NSrp70 may involve in the transition of DP thymocytes into both the CD4+ and CD8+ SP lineages. To prove this hypothesis, we used a CD4Cre/loxP system to generate conditional deletion of NSRP1 in DP stage. NSRP1f/fCD4Cre mice showed dramatic reduction of the number of mature thymic CD4+ and CD8+ SP T cells, while no significant changes were induced in double negative (DN) and DP populations. Strikingly, RNA-seq analysis revealed that knockout of NSRP1 increases the expression of gene clusters that regulate cell cycle progression. In accordance with this, DP thymocytes with NSRP1-deficiency showed significant proliferation with high expression of Ki-67, a marker of proliferating cells, as compared to wild-type DP thymocytes. These results suggest that NSrp70 is a key regulator at the stage when cells transit from DP thymocytes to SP lineage at which cells need to slow down their growth.