Neurog3 misexpression unravels mouse pancreatic ductal cell plasticity

Ductal Ngn3-expressing progenitors contribute to adult β cell neogenesis in the pancreas

Ductal cells have been proposed as a source of adult β cell neogenesis, but this has remained controversial. By combining lineage tracing, 3D imaging, and single-cell RNA sequencing (scRNA-seq) approaches, we show that ductal cells contribute to the β cell population over time. Lineage tracing using the Neurogenin3 (Ngn3)-CreERT line identified ductal cells expressing the endocrine master transcription factor Ngn3 that were positive for the δ cell marker somatostatin and occasionally co-expressed insulin. The number of hormone-expressing ductal cells was increased in Akita^+/- diabetic mice, and ngn3 heterozygosity accelerated diabetes onset. scRNA-seq of Ngn3 lineage-traced islet cells indicated that duct-derived somatostatin-expressing cells, some of which retained expression of ductal markers, gave rise to β cells. This study identified Ngn3-expressing ductal cells as a source of adult β cell neogenesis in homeostasis and diabetes, suggesting that this mechanism, in addition to β cell proliferation, maintains the adult islet β cell population.

Hnf1b-CreER causes efficient recombination of a Rosa26-RFP reporter in duct and islet δ cells

The Hnf1b-CreER^T2 BAC transgenic (Tg(Hnf1b-cre/ERT2)1Jfer) has been used extensively to trace the progeny of pancreatic ducts in developmental, regeneration, or cancer models. Hnf1b-CreER^T2 transgenics have been used to show that the cells that form the embryonic pancreas duct-like plexus are bipotent duct-endocrine progenitors, whereas adult mouse duct cells are not a common source of β cells in various regenerative settings. The interpretation of such genetic lineage tracing studies is critically dependent on a correct understanding of the cell type specificity of recombinase activity with each reporter system. We have reexamined the performance of Hnf1b-CreER^T2 with a Rosa26-RFP reporter transgene. This showed inducible recombination of up to 96% adult duct cells, a much higher efficiency than previously used reporter transgenes. Despite this high duct-cell excision, recombination in α and β cells remained very low, similar to previously used reporters. However, nearly half of somatostatin-expressing δ cells showed reporter activation, which was due to Cre expression in δ cells rather than to duct to δ cell conversions. The high recombination efficiency in duct cells indicates that the Hnf1b-CreER^T2 model can be useful for both ductal fate mapping and genetic inactivation studies. The recombination in δ cells does not modify the interpretation of studies that failed to show duct conversions to other cell types, but needs to be considered if this model is used in studies that aim to modify the plasticity of pancreatic duct cells.

Regeneration of the pancreas: proliferation and cellular conversion of surviving cells

The most common pancreas-related disorders are diabetes, pancreatitis and different types of pancreatic cancers. Diabetes is a chronic condition which results from insufficient functional β-cell mass, either as a result of an autoimmune destruction of insulin producing β-cells, or as their death or de-differentiation following years of hyperactivity to compensate for insulin resistance. Chronic pancreatitis leads to cell death and can develop into diabetes or pancreatic cancer. To stimulate regeneration in such pathologies, it is of high importance to evaluate the endogenous regeneration capacity of the pancreas, to understand the conditions needed to trigger it, and to investigate the cellular and molecular regenerative responses. This short review focuses on observations made in the last 2 years on the mechanisms enhancing pancreatic cell proliferation, notably new combinations of pharmacological agents, as well as those triggering cellular conversion.