Novel strategies in diabetes therapy would obviously benefit from the use of beta (beta) cell stem/progenitor cells. However, whether or not adult beta cell progenitors exist is one of the most controversial issues in today's diabetes research. Guided by the expression of Neurogenin 3 (Ngn3), the earliest islet cell-specific transcription factor in embryonic development, we show that beta cell progenitors can be activated in injured adult mouse pancreas and are located in the ductal lining. Differentiation of the adult progenitors is Ngn3 dependent and gives rise to all islet cell types, including glucose responsive beta cells that subsequently proliferate, both in situ and when cultured in embryonic pancreas explants. Multipotent progenitor cells thus exist in the pancreas of adult mice and can be activated cell autonomously to increase the functional beta cell mass by differentiation and proliferation rather than by self-duplication of pre-existing beta cells only.

Neurogenin 3 (Ngn3) is key for endocrine cell specification in the embryonic pancreas and induction of a neuroendocrine cell differentiation program by misexpression in adult pancreatic duct cells. We identify the gene encoding IA1, a zinc-finger transcription factor, as a direct target of Ngn3 and show that it forms a novel branch in the Ngn3-dependent endocrinogenic transcription factor network. During embryonic development of the pancreas, IA1 and Ngn3 exhibit nearly identical spatio-temporal expression patterns. However, embryos lacking Ngn3 fail to express IA1 in the pancreas. Upon ectopic expression in adult pancreatic duct cells Ngn3 binds to chromatin in the IA1 promoter region and activates transcription. Consistent with this direct effect, IA1 expression is normal in embryos mutant for NeuroD1, Arx, Pax4 and Pax6, regulators operating downstream of Ngn3. IA1 is an effector of Ngn3 function as inhibition of IA1 expression in embryonic pancreas decreases the formation of insulin-and glucagon-positive cells by 40%, while its ectopic expression amplifies neuroendocrine cell differentiation by Ngn3 in adult duct cells. IA1 is therefore a novel Ngn3-regulated factor required for normal differentiation of pancreatic endocrine cells.

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Plakophilin 3 (PKP3) is a recently described armadillo protein of the desmosomal plaque, which is synthesized in simple and stratified epithelia. We investigated the localization pattern of endogenous and exogenous PKP3 and fragments thereof. The desmosomal binding properties of PKP3 were determined using yeast two-hybrid, coimmunoprecipitation and colocalization experiments. To this end, novel mouse anti-PKP3 mAbs were generated. We found that PKP3 binds all three desmogleins, desmocollin (Dsc) 3a and -3b, and possibly also Dsc1a and -2a. As such, this is the first protein interaction ever observed with a Dsc-b isoform. Moreover, we determined that PKP3 interacts with plakoglobin, desmoplakin (DP) and the epithelial keratin 18. Evidence was found for the presence of at least two DP–PKP3 interaction sites. This finding might explain how lateral DP–PKP interactions are established in the upper layers of stratified epithelia, increasing the size of the desmosome and the number of anchoring points available for keratins. Together, these results show that PKP3, whose epithelial and epidermal desmosomal expression pattern and protein interaction repertoire are broader than those of PKP1 and -2, is a unique multiprotein binding element in the basic architecture of a vast majority of epithelial desmosomes.