AIMS/HYPOTHESIS - The Cre/loxP system, which enables tissue-specific manipulation of genes, is widely used in mice for diabetes research. Our aim was to develop a new Cre-driver mouse line for the specific and efficient manipulation of genes in pancreatic alpha cells.
METHODS - A Gcg knockin mouse, which expresses a tamoxifen-inducible form of Cre from the endogenous preproglucagon (Gcg) gene locus, was generated by homologous recombination. The new Gcg mouse line was crossed to the Rosa26 (R26 ) Cre reporter mouse line in order to evaluate the tissue specificity, efficiency and tamoxifen dependency of Gcg -mediated recombination. Cell types of pancreatic islets were identified using immunohistochemistry. Biochemical and physiological data, including blood glucose levels, plasma glucagon and glucagon-like peptide (GLP)-1 levels, and pancreatic glucagon content, were collected and used to assess the overall effect of Gcg gene targeting on Gcg heterozygous mice.
RESULTS - Tamoxifen-treated Gcg ;R26 mice displayed Cre reporter activity, i.e. expression of tdTomato red fluorescent protein (RFP) in all known cells that produce proglucagon-derived peptides. In the adult pancreas, RFP was detected in 94-97% of alpha cells, whereas it was detected in a negligible (~ 0.2%) proportion of beta cells. While more than 98% of cells labelled with tamoxifen-induced RFP were glucagon-positive cells, 14-25% of pancreatic polypeptide (PP)-positive cells were also positive for RFP, indicating the presence of glucagon/PP bihormonal cell population. Tamoxifen-independent expression of RFP occurred in approximately 6% of alpha cells. In contrast to alpha cells and GLP-1-producing neurons, in which RFP expression persisted for at least 5 months after tamoxifen administration (presumably due to rare neogenesis in these cell types in adulthood), nearly half of RFP-positive intestinal L cells were replaced with RFP-negative L cells over the first 2 weeks after tamoxifen administration. Heterozygous Gcg mice showed reduced Gcg mRNA levels in islets, but maintained normal levels of pancreatic and plasma glucagon. The mice did not exhibit any detectable baseline physiological abnormalities, at least in young adulthood.
CONCLUSIONS/INTERPRETATION - The newly developed Gcg knockin mouse shows faithful expression of CreER in pancreatic alpha cells, intestinal L cells and GLP-1-producing neurons. This mouse line will be particularly useful for manipulating genes in alpha cells, due to highly specific and efficient CreER-mediated recombination in this cell type in the pancreas.