Progenitor cell domains in the developing and adult pancreas

Abstract

Unlike organs with defined stem cell compartments, such as the intestine, the pancreas has limited capacity to regenerate. The question of whether the adult pancreas harbors facultative stem/progenitor cells has been a prime subject of debate. Cumulative evidence from recent genetic lineage tracing studies, in which specific cell populations were marked and traced in adult mice, suggests that endocrine and acinar cells are no longer generated from progenitors in the adult pancreas. These studies further indicate that adult pancreatic ductal cells are not a source for endocrine cells following pancreatic injury, as previously suggested. Our own studies have shown that adult ductal cells reinitiate expression of some endocrine progenitor markers, including Ngn3, after injury by partial duct ligation (PDL), but that these cells do not undergo endocrine cell differentiation. Here, we present additional evidence that endocrine cells do not arise from ducts following b-cell ablation by streptozotocin or by a diphtheria toxin-expressing transgene or when b-cell ablation is combined with PDL. In this review, we discuss findings from recent lineage tracing studies of embryonic and adult pancreatic ductal cells. Based upon the combined evidence from these studies, we propose that multipotency is associated with a specific transcriptional signature.

Figure 1

Multipotent cell identity in the embryonic pancreas. (A) The Venn diagram represents the expression domains of Ptf1a, Sox9 and Nkx6.1 from early embryonic stages until adulthood. While most early pancreatic epithelial cells coexpress all three factors (white color), their expression domains gradually diverge as pancreatic development progresses. In the adult pancreas, each transcription factor is restricted to a particular cell compartment: Ptf1a, acinar cells; Sox9, ductal cells; Nkx6.1, β cells. We propose that multipotency is associated with the concomitant expression of all three transcription factors and that the developmental potential of cells becomes restricted as expression domains diverge. Ptf1a, Sox9 and Nkx6.1 were chosen to illustrate the idea. However, multipotent pancreatic progenitors express numerous transcription factors, and it is currently unclear which combination of these factors is critical to maintain multipotency. (B) As the expression domains of transcription factors diverge, cells progressively lose their ability to contribute to all pancreatic lineages and become bi- or unipotent. Acinar-committed cells are confined to the tip domain, while duct- and endocrine-committed cells are localized in the trunk. It should be noted that aspects of this model need to be experimentally confirmed; as an example, the bipotent or multipotent state of cells at the border between tip and trunk at e14.5. e, embryonic day.

Figure 2

Pancreatic injury does not induce β-cell neogenesis from Sox9⁺ ductal cells. Three-week-old Sox9CreER^T2; R26R^YFP mice were injected with tamoxifen (5 mg/40 g body weight) for three consecutive days. Individual groups of mice were then subjected to one of three different treatments. (A) At eight weeks of age, Sox9CreER^T2; R26R^YFP mice were injected for five consecutive days with 50 mg/kg of streptozotocin (STZ) or vehicle (control). One month later, the number of glucagon⁺/insulin⁺/YFP⁺ (Ins-Glu⁺YFP⁺) cells was quantified as a percentage of the total number of glucagon and insulin-positive cells. A minimum of 60 insulin⁺/glucagon⁺ cell clusters were scored per mouse (n = 3). (B) Sox9CreER^T2; R26R^YFP mice also carrying the RIP-DTR transgene were given three intraperitoneal injections of diphtheria toxin (DT) over five days (126 ng of DT per injection on days 0, 3 and 4) at eight weeks of age. Two days (control) and three months post-DT injection, the number of Ins-Glu⁺YFP⁺ cells was quantified as a percentage of the total number of glucagon and insulin-positive cells. A minimum of 1,300 insulin⁺/glucagon⁺ cells were scored per mouse (n = 3). (C) At eight weeks of age, Sox9CreER^T2; R26R^YFP mice were administered 90 mg/kg of alloxan intravenously. Partial duct ligation (PDL) was performed immediately thereafter. Only mice displaying blood glucose levels exceeding 500 mg/dL within two days post-alloxan administration were examined. Two weeks post-alloxan + PDL or post-alloxan (control), the number of Ins-Glu⁺YFP⁺ cells was quantified as a percentage of the total number of glucagon and insulin-positive cells. A minimum of 700 insulin⁺/glucagon⁺ cells were analyzed per mouse (n = 3), representing approximately 100 islets. None of the pancreatic injury models examined resulted in a significant increase in the percentage of insulin⁺/glucagon⁺ cells la beled by YFP. d, day; DT, diphtheria toxin; glu, glucagon; ins, insulin; STZ, streptozotocin; PDL, partial duct ligation; TM, tamoxifen; wk, week. Values are shown as mean ± standard error of the mean.