Investigating Srsf3-mediated alternative RNA splicing in craniofacial development

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Project Summary Craniofacial development involves complex signaling to coordinate tissue organization to form the head and face, and disruptions in this process result in common congenital malformations. A key question in this field is how external stimuli lead to gene expression changes required to form fully developed craniofacial structures. Signaling through the Platelet-derived growth factor receptor alpha (PDGFRα) plays a critical role in this process, as mutations in PDGFRΑ are associated with cleft lip/palate in humans. Relatedly, Pdgfra mutant mouse models develop a range of phenotypes from cleft palate to complete facial clefting. Phosphatidylinositol 3-kinase (PI3K) is the primary effector of PDGFRα signaling during skeletal development in the mouse, leading to the activation of the kinase Akt. A previous phosphoproteomic screen demonstrated that Akt phosphorylates the RNA-binding protein (RBP) Serine/arginine-rich splicing factor 3 (Srsf3) downstream of PI3K-mediated PDGFRα signaling in mouse embryonic palatal mesenchyme (MEPM) cells, leading to translocation of phosphorylated Srsf3 into the nucleus. Srsf3 is ubiquitously expressed with enhanced expression in the neural crest-derived mesenchyme and overlying ectoderm of mouse facial processes at mid-gestation. Additionally, ablation of Srsf3 in the murine neural crest cell lineage (cKO) results in a severe midline facial clefting phenotype due to defects in proliferation and survival of cranial neural crest cells. Further, RNA-sequencing of Srsf3 cKO facial process mesenchyme identified alternative RNA splicing events that were enriched for transcripts encoding protein serine/threonine kinases, suggesting that alternative splicing may serve as a novel feedback mechanism for intracellular kinase signaling. The goal of this proposal is to test the hypothesis that PI3K/Akt-mediated PDGFRα signaling regulates Srsf3 protein and RNA interactions to affect the alternative RNA splicing of transcripts necessary for craniofacial development. First, Srsf3 will be immunoprecipitated from MEPM cells in the absence or presence of PDGF-AA ligand and analyzed by mass spectrometry to comprehensively map phosphorylation changes in response to PDGFRα signaling. Further, craniofacial phenotypes will be analyzed in a Srsf3 phosphomutant knock-in mouse model to determine the role of Akt- mediated phosphorylation of Srsf3 in craniofacial development. Next, BioID2 proximity labeling and mass spectrometry will be used to identify Srsf3 protein interacting partners in response to PDGFRα signaling in MEPM cells. Finally, Srsf3-RNA interactions will be purified and sequenced in response to PDGFRα signaling in MEPM cells through enhanced crosslinking and immunoprecipitation analysis to identify direct targets of Srsf3 and determine if RNA binding and/or sequence specificity changes upon Srsf3 phosphorylation. This project will determine the molecular mechanisms by which Srsf3 activity is controlled in response to PDGFRa signaling in the facial mesenchyme, thus providing considerable insight into mechanisms underlying gene expression regulation during mammalian craniofacial development.