Date of Award
Doctor of Philosophy
Molecular Biology, Microbiology and Biochemistry
In multicellular organisms, cell growth and differentiation is controlled in part by apoptosis. One major apoptotic pathway is triggered by Fas receptor (Fas)-Fas ligand (FasL) interaction. Resistance to Fas-mediated apoptosis is regulated through the production of a soluble Fas isoform (sFas), created by exclusion of transmembrane spanning sequences encoded within exon 6 (FasΔ6) that can bind FasL and block apoptosis. Long noncoding RNAs (lncRNAs) are >200-nucleotide sequences that are important regulators of cellular programs. However, their role in erythropoiesis is just beginning to be appreciated with studies limited to murine systems. I studied the potential role of lncRNAs during human red blood cell development using RNA from cultured CD34+ purified from fetal liver (FL), cord blood (CB), or adult bone marrow (BM) to screen 82 documented lncRNAs. This screen revealed that the Fas Anti-sense (Saf) was consistently increased during maturation and levels high for BM compared to FL or CB. Next, I characterized the regulatory sequence of Saf and by In silico analysis identified canonical binding sites for the erythroid-specific transcription factors KLF1 and GATA1. Chromatin immunoprecipitation (ChIP) assays confirmed binding of both factors to their target sequences and luciferase reporter constructs revealed synergistic activity evidenced by increase in luciferase expression relative to controls. Genome wide expression analysis using cells with overexpression of Saf showed no effect on global gene transcription, suggesting Saf function at post transcriptional levels. Saf was shown to participate in alternative splicing of Fas pre-mRNA through unknown mechanisms. Using a combination of biochemical assays, overexpression and knockdown studies, I showed how this occurs. Cell fractionation and RT-PCR demonstrated that Saf is localized in the nucleus. I found that Saf directly interacted with Fas pre-mRNA to form RNaseA-resistant double-stranded RNA intermediates at regions that flank Exon6. Post-transcriptional function of Saf was confirmed by qRT-PCR demonstrating significantly increased levels of sFas for Saf overexpressing cells. Enrichment for sFas RNA coincided with reduced Fas on the cell surface and increased sFas protein levels when conditioned supernatants were assayed by ELISA. Conversely, siRNA-mediated knockdown of Saf significantly reduced sFas production compared to non-targeting siRNA controls. Saf-interacting proteins were identified by mixing in vitro transcribed and biotin-labeled Saf RNA with nuclear lysates followed by mass spectrometry analysis. This screen identified human splicing factor 45 (SPF45) which has a known role in Fas pre-mRNA alternative splicing. Specific SPF45/Saf interaction was confirmed by RNA pulldown and western blot with SPF45-specific antibodies and the ability to detect sequences for Saf, Fas and sFas by RT-PCR of RNA that immunoprecipitated with SPF45. SPF45 knockdown decreased sFas transcripts and this reduction corresponded to limited production of sFas and increased sensitivity to Fas-mediated apoptosis when cells were exposed to the Fas-activating antibody CH11. Importantly, overexpression of Saf in SPF45 knockdown cells failed to rescue production of sFas supporting the hypothesis that Saf and SPF45 co-participate in modulating Fas pre-mRNA splicing. Protein phosphorylation modulates the interaction of the splicing factors with RNA. The effect of phosphorylation on the Saf-SPF45 interaction was evaluated using stable cell lines expressing a myc-tagged SPF45 protein or versions modified to introduce alanine in place of threonine 71 (T-71-A) or serine 222 (S-222-A) to prevent phosphorylation. Mutation in S-222-A reduced the interaction of SPF45 with Saf. I conclude that Saf interacts with Fas pre-mRNA at sequences that flank exon 6 and recruits phosphorylated SPF45 as mechanism to recognize exon 6 and allow for alternative splicing of Fas Pre-mRNA. Collectively, these studies reveal a novel mechanism to regulate apoptosis that may be responsible for cell proliferation and drug resistance.
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