Loss of Pnn expression results in mouse early embryonic lethality and cellular apoptosis through SRSF1-mediated alternative expression of Bcl-xS and ICA**

Pinin (Pnn), a serine/arginine-rich (SR)-related protein, has been shown to play multiple roles within eukaryotic cells including in cell-cell adhesion, cell migration, regulation of gene transcription, mRNA export, and alternative splicing. In this study, an attempt to generate mice homozygously deficient in Pnn failed due to early embryonic lethality. To evaluate the effects of loss of Pnn expression on cell survival, RNA interference experiments were performed in MCF-7 cells. With Pnn-depletion, cellular apoptosis and nuclear condensation were observed. In addition, nuclear speckles were disrupted, and expression levels of SR proteins were diminished. RT-PCR analysis showed that alternative splicing patterns of SRSF1 as well as of apoptosis-related genes Bcl-x and ICAD were altered and expression levels of Bim isoforms were modulated in Pnn-depleted cells. Cellular apoptosis induced by Pnn depletion was rescued by overexpression of SRSF1 which also restored generation of Bcl-xL and functionless ICAD. Pnn expression is, therefore, essential for survival of mouse embryos and the breast carcinoma cell line MCF-7. Moreover, Pnn-depletion, modulated by SRSF1, determines cellular apoptosis through activation of expression of pro-apoptotic Bcl-xS transcripts.