Defining the transcriptome and the genetic pathways of pancreatic endocrine and exocrine cell types is crucial for elucidating the molecular attributes of disorders such as diabetes and pancreatic cancer. The comparison of these transcriptomes amongst distant vertebrate species highlight the genes under strong evolutionary constraints which have maintained their selective expression in pancreatic cells due to their crucial function in these cells.
We took advantage of zebrafish transgenic lines to isolate by FACS the major pancreatic cell types and we obtained highly purified preparations of endocrine α-, β- and δ cells as well as the exocrine acinar and ductal cells. Transcriptomic profiling by RNA-seq identified the transcriptomic signature of each cell type and highlighted novel cell-specific markers including transcription factors, signaling pathways components and lincRNAs. By performing interspecies comparisons, we identified hundreds of genes with conserved enriched expression in endocrine or in exocrine cells amongst human, mouse and zebrafish. This list includes many regulatory genes known as crucial for the differentiation and the function of endocrine and exocrine cells, but also pinpoints previously unrecognized regulators. While the transcriptomic signature of pancreatic endocrine and exocrine cells is well conserved amongst vertebrates, the signatures distinguishing the endocrine cell subtypes, and notably alpha (glucagon+) and beta (insulin+) cells, are much less conserved between zebrafish, mouse and human, with only a few genes displaying conserved cell subtype specific expression. This suggests that endocrine cell subtype identity may be determined by one or a few regulatory genes. The function of some identified endocrine regulators is presently addressed through CRISPR mutagenesis.
In conclusions, this study provides the molecular signature of the major zebrafish pancreatic cell types and identifies sets of genes with selective and evolutionary conserved expression in pancreatic endocrine or exocrine cells from fish to mammals, likely important in pancreas physiology and relevant to pancreatic diseases such as diabetes and cancer.