Exploring pathways to generating beta cells: Transdifferentiation of alpha cells to beta cells following beta cell ablation in adult mice
Clues to the origin of potential cellular sources of regenerated beta cells came from our understanding of T1D and T2D in humans. Increased glucagon production and secretion often is associated with these disorders and so prompted the authors to look at glucagon content in the RIP-DTR mice. They found an increase in glucagon gene expression and content and noted an initial increase in alpha cell numbers. However, overall alpha cell proliferation rate and mass did not increase throughout the study. Within days of DT treatment, cells expressing both glucagon and insulin, termed “bihormonal” cells were frequently observed and remained throughout the entire investigation. Moreover, in YFP-expressing RIP-DTR mice, none of these cells co-expressed YFP demonstrating they were not from non ablated beta cells. The investigators hypothesized the cells were generated from pre-existing alpha cells that start to produce insulin and transdifferentiate into beta cells. In support of this idea, the beta cell markers Pdx1 and Nkx6.1 were noted in a portion of the glucagon expressing cells.
To further define the origin of the bihormonal cells, the investigators generated a RIP-DTR mouse line in which alpha cells could be selectively and irreversibly labeled using the glucagon-rtTA, the TetO-Cre responder transgene, and the R26-YFP reporter gene. Following DOX exposure 90% of the alpha cells were YFP positive. When DOX treatment was followed by ablation, the islets contained mostly YFP positive alpha cells and 90% of these co expressed glucagon and insulin demonstrating they were preexisting alpha cells that now expressed insulin. In support of these results a significant number of YFP positive cells were also insulin positive after beta cell ablation in a mouse strain designed to trace glucagon cell lineages. A final confirmation used a glucagon-DTR mouse strain in which both alpha and beta cells were ablated. Analysis of these animals demonstrated that no bihormonal or beta cells were generated following DT exposure. In support of these results, studies of beta cell gene expression profiles demonstrated a substantial loss of beta cell specific transcripts following ablation. As bihormonal cells began to appear, beta cell expression patterns increased suggesting adoption of the beta cell phenotype. Analysis of protein levels of these markers confirmed expression supporting a role for the transdifferentiation of alpha cells to beta cells.
While beta cells have been generated from a variety of cellular sources within several experimental paradigms, this report is the first definitive evidence of beta cell regeneration from a heterologous cell source. Utilizing genetically programmed cellular ablation in concert with cell lineage tracing, the authors demonstrate that total beta cell loss triggers unknown cellular events that promote the transdifferentiation of existing alpha cells to beta cells. These studies open many avenues for continued exploration of the cellular and molecular mechanisms controlling the generation of beta cells in the pancreas.