Type 1 diabetics and severe type 2 diabetics show reduction of insulin secretion as a result of destruction of the specific cells in the pancreas that produce insulin. These cells, the so-called beta cells, suffer destruction from the patient’s immune system (type 1 diabetes) or from overwork (type 2 diabetes). The holy grail of diabetes treatment is the regeneration of lost beta cells.
Several reports have marshaled evidence that the pancreatic beta cells do regenerate, but the constant assault by the immune system eventually destroys all the beta cells. Other reports have argued that a stem cell population in the ductal system of the pancreas can replenish the beta cells. Thus, augmenting beta cell regeneration seemed to be simply a matter of employing the already-present regenerative properties of the pancreas.
Unfortunately, a recent study seems to put the kibosh on any hope that the pancreatic beta cells regenerate. This new study was published in the Journal of Clinical Investigation. In this paper, researchers at Children’s Hospital of Pittsburgh report were unable to find signs of new beta cell production in several common models of pancreatic injury (see Xiao, X., et al. 2013. No evidence for beta-cell neogenesis in murine adult pancreas. J Clin Invest., 123(5):2207-17).
“Overall, the paper puts one more nail in what was already becoming an increasingly tight coffin for what had been the prevailing hypothesis about β-cell neogenesis in adult mice,” said Fred Levine, who studies β-cell regeneration at Sanford Burnham Medical Research Institute in La Jolla, California and was not involved in the study. Still, Levine cautions that this negative result does not completely rule out adult regeneration of β-cells in other injury models.
To detect the formation of new beta cells, George Gittes and his colleagues used an old cell tracking method, but applied it in a different manner. They used two fluorescent tags in transgenic mice: a red tag that targets a protein in the cell membrane of most cells in the body, except for insulin producing cells, and a green tag that only tagged pancreatic beta cells. Gittes team looked for cells that turned on their insulin genes for the first time during a 40–48 hour window. The cells, therefore, would express both tags and, as a result, appeared yellow.
The yellow transition was detected in embryonic mice, where neogenesis (new beta cell production) is expected to occur. But when the researchers examined adult cells, they saw no yellow cells—meaning no evidence of neogenesis. They repeated this experiment in several common models of pancreatic damage. For example, the pancreatic duct ligation model (PDL damages other pancreatic cell types but not β-cells. The absence of detectable neogenesis in these models “puts pressure on us to find models in which there is neogenesis,” said Gittes. But he remains “very confident” that there are other models in which neogenesis occurs.
In fact, several models not tested in this paper have shown evidence of neogenesis, including one of Gittes’ own. In 2011, in which his team found evidence of neogenesis in a mice who beta cells were engineered to express diphtheria toxin receptors, that led to their death (see Criscimanna, A., et al. 2011. Duct cells contribute to regeneration of endocrine and acinar cells following pancreatic damage in adult mice. Gastroenterology, 141(4):1451-62). In 2010, two other research groups, including one headed by Levine, also demonstrated neogenesis in adult mice through trandifferentiation of preexisting α-cells in pancreatic islets into β-cells (Thorel, F., et al. 2010. Conversion of adult pancreatic alpha-cells to beta-cells after extreme beta-cell loss. Nature,464(7292):1149-54 and Chung, C.H., et al. 2010. Pancreatic β-cell neogenesis by direct conversion from mature α-cells. Stem Cells, 28(9):1630-8).
“Overall, I believe that the pathway by which β-cell regeneration occurs…is likely to vary depending on the stimulus for regeneration,” said Levine. Therefore, the current work does not rule out neogenesis, even from duct cells, in other models. “I would argue that the old cliché, ‘Absence of evidence is not evidence of absence’ should be kept in mind when evaluating studies like this.”