Alternative Splicing of Toll-Like Receptor 9 Transcript in Teleost Fish Grouper Is Regulated by NF-κB Signaling via Phosphorylation of the C-Terminal Domain of the RPB1 Subunit of RNA Polymerase II

Similar to its mammalian counterparts, teleost Toll-like receptor 9 (TLR9) recognizes unmethylated CpG DNA presented in the genome of bacteria or DNA viruses and initiates signaling pathway(s) for immune responses. We have previously shown that the TLR9 pathway in grouper, an economically important teleost, can be debilitated by an inhibitory gTLR9B isoform, whose production is mediated by RNA alternative splicing. However, how does grouper TLR9 (gTLR9) signaling impinge on the RNA splicing machinery to produce gTlr9B is unknown. Here we show that the gTlr9 alternative splicing is regulated through ligand-induced phosphorylation of the C-terminal domain (CTD) of the largest subunit of RNA polymerase II (Pol II). We first observed that ligand-activated NF- κB pathway biased the production of the gTlr9B isoform. Because NF- κB is known to recruit p-TEFb kinase, which phosphorylates the Pol II CTD at Ser2 residues, we examined p-TEFb's role in alternative splicing. We found that promoting p-TEFb kinase activity significantly favored the production of the gTlr9B isoform, whereas inhibiting p-TEFb yielded an opposite result. We further showed that p-TEFb-mediated production of the gTlr9B isoform down-regulates its own immune responses, suggesting a self-limiting mechanism. Taken together, our data indicate a feedback mechanism of the gTLR9 signaling pathway to regulate the alternative splicing machinery, which in turn produces an inhibitor to the pathway.