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The limitation of available islets for transplantation is a major obstacle for the treatment of diabetes through islet therapy. However, islet monolayers expanded ex vivo may provide a source of progenitor cells and a model to help understand islet development from precursor cell types. The existence of progenitor cells within the islets is highly likely, yet, to date, no fully defined or characterized postnatal stem cell has been isolated, expanded or marked. Our study evaluates the expression of progenitor markers, including the haematopoietic stem cell marker c-Kit, in epithelial monolayers derived from postnatal rat islets through immunofluorescence and RT-PCR, and the ability of precursor-rich monolayers to reform islet-like structures. Islets formed confluent monolayers when cultured on a type I collagen gel which lacked endocrine phenotypes but were positive for cytokeratin 20 and contained an increased proportion of proliferating c-Kit-expressing cells, with the proportion reaching a maximum of 45+/-6% at 8 weeks of culture. Evaluation of transcription factors at the mRNA level revealed constant PDX-1, ngn3 and Pax4 expression, while undifferentiated cell markers, such as Oct4 and alpha-fetoprotein, were also detected frequently after 4 weeks of culture. Changing the extracellular matrix protein to laminin-rich Matrigel, the monolayers re-formed islet-like clusters that secreted insulin in a glucose-responsive fashion. Our data show that islets can be expanded ex vivo to form epithelial monolayers with rich undifferentiating cell populations that are characterized by cells expressing the progenitor markers. These monolayers are capable of extensive proliferation and retain plasticity to form new islet cells, and c-Kit-expressing cells may play an important role in new islet cluster formation.

There has over the last several years been renewed interest in developing a system for generating new islets and a search for a self-renewing population in the pancreas. In particular, the neural stem cell marker nestin has been implicated as an islet precursor marker and its immunoreactivity has been localized in the islets of Langerhans. This study examines islet-derived epithelial monolayers expanded ex vivo to provide a source of nestin-expressing progenitor cells – a model that will help us understand the role of nestin-expressing cells in islet cell development. When cultured on a type I collagen gel, islets formed confluent monolayers which lacked endocrine phenotypes but were positive for cytokeratin 20 and contained an increased proportion of proliferating nestin-expressing cells, reaching a maximum of 54±10%. Co-expression studies demonstrated that the nestin-positive cells are heterogeneous, with some nestin-expressing cells co-localizing with the transcription factor PDX-1 and glucose transporter type 2 or lack of co-expression with vimentin. When clonal populations of nestin-positive cells were expanded and subjected to a differentiation protocol, only a population that expressed the transcription factor PDX-1 at the mRNA level was capable of re-expressing insulin at the mRNA and protein level. In conclusion, these studies demonstrate that expanded nestin-expressing cells in vitro from islet-derived epithelial monolayers are heterogeneous; clonal analysis of nestin-positive cells reveals that a distinct subpopulation of nestin/PDX-1-expressing cells is capable of forming insulin-producing cells.

Hepatocyte growth factor (HGF) has been suggested to be a potent regulator of β-cell function and proliferation. The purpose of this study was to investigate whether HGF could regulate the proliferation and differentiation of islet-derived epithelial monolayers into insulin-producing cells. We have generated islet-derived epithelial monolayers that are enriched with cells expressing c-Kit, a tyrosine kinase receptor and putative marker, from isolated postnatal rat islets. Monolayers were cultured on type I collagen gel and treated in defined differentiation medium with or without HGF (50 ng/ml) for 7 days. Subsequently, the expression of transcription factors and pancreatic endocrine cell markers as well as c-Kit expression were compared between the HGF (HGF⁺), no HGF treatment (HGF⁻) and monolayers without differentiation medium (control) groups, using immunocytochemical and RT-PCR approaches. We observed that the number of c-Kit-, glucose transport type 2 (Glut2)- and the transcription factor pancreatic duodenal homeobox-1 (PDX-1)-expressing cells were significantly increased in the HGF⁺ group. The expression of insulin at the mRNA and protein level was also increased in this treatment group with a 1.7-fold increase in basal insulin release and a 2.3-fold increase in insulin content in comparison with the HGF⁻ group. A high proliferative capacity was also found in the HGF⁺ group. Co-localization of insulin and PDX-1 or Glut2 was revealed frequently in cells treated with HGF⁺ with occasional co-staining of c-Kit and insulin observed. This study showed that HGF can activate the proliferation and differentiation of islet-derived epithelial monolayer into insulin-producing cells. However, no formation of islet-like clusters was observed. Taken together, this study implies that HGF mediates differentiation of immature cell types into insulin-expressing cells; however, HGF supplementation alone is insuffcient in restoring full β-cell function.