Alternative splicing regulates vesicular trafficking genes in cardiomyocytes during postnatal heart development

During postnatal development the heart undergoes a rapid and dramatic transition to adult function through transcriptional and post-transcriptional mechanisms, including alternative splicing (AS). Here we perform deep RNA-sequencing on RNA from cardiomyocytes and cardiac fibroblasts to conduct a high-resolution analysis of transcriptome changes during postnatal mouse heart development. We reveal extensive changes in gene expression and AS that occur primarily between postnatal days 1 and 28. Cardiomyocytes and cardiac fibroblasts show reciprocal regulation of gene expression reflecting differences in proliferative capacity, cell adhesion functions and mitochondrial metabolism. We further demonstrate that AS plays a role in vesicular trafficking and membrane organization. These AS transitions are enriched among targets of two RNA-binding proteins, Celf1 and Mbnl1, which undergo developmentally regulated changes in expression. Vesicular trafficking genes affected by AS during normal development (when Celf1 is downregulated) show a reversion to neonatal splicing patterns after Celf1 re-expression in adults. Short-term Celf1 induction in adult animals results in disrupted transverse tubule organization and calcium handling. These results identify potential roles for AS in multiple aspects of postnatal heart maturation, including vesicular trafficking and intracellular membrane dynamics.Myotonic Dystrophy Foundation (Postdoctoral Fellowship

Pancreatic Cell Fate Determination Relies on Notch Ligand Trafficking by NFIA

Summary: Notch is activated globally in pancreatic progenitors; however, for progenitors to differentiate into endocrine cells, they must escape Notch activation to express Neurogenin-3. Here, we find that the transcription factor nuclear factor I/A (NFIA) promotes endocrine development by regulating Notch ligand Dll1 trafficking. Pancreatic deletion of NFIA leads to cell fate defects, with increased duct and decreased endocrine formation, while ectopic expression promotes endocrine formation in mice and human pancreatic progenitors. NFIA-deficient mice exhibit dysregulation of trafficking-related genes including increased expression of Mib1, which acts to target Dll1 for endocytosis. We find that NFIA binds to the Mib1 promoter, with loss of NFIA leading to an increase in Dll1 internalization and enhanced Notch activation with rescue of the cell fate defects after Mib1 knockdown. This study reveals NFIA as a pro-endocrine factor in the pancreas, acting to repress Mib1, inhibit Dll1 endocytosis and thus promote escape from Notch activation. : Scavuzzo et al. show NFIA is expressed at the onset of the secondary transition in pancreatic progenitors to regulate endocrine fate induction. NFIA binds the Mib1 promoter, with increased Mib1 in NFIA-deficient cells, more Dll1 endocytosis, and higher Notch activation, leading to formation of more ducts and fewer endocrine cells. Keywords: development, pancreas, cell fate, endocrine cell, Notch signaling, NFIA, Mib1, endocytosi