Nkx2.2 repressor complex regulates islet β-cell specification and prevents β-to-α-cell reprogramming

Lineage determinants in early endocrine development

Pancreatic endocrine cells are produced from a dynamic epithelium in a process that, as in any developing organ, is driven by interacting programs of spatiotemporally regulated intercellular signals and autonomous gene regulatory networks. These algorithms work to push progenitors and their transitional intermediates through a series of railroad-station-like switching decisions to regulate flux along specific differentiation tracks. Extensive research on pancreas organogenesis over the last 20 years, greatly spurred by the potential to restore functional β-cell mass in diabetic patients by transplantation therapy, is advancing our knowledge of how endocrine lineage bias is established and allocation is promoted. The field is working towards the goal of generating a detailed blueprint of how heterogeneous cell populations interact and respond to each other, and other influences such as the extracellular matrix, to move into progressively refined and mature cell states. Here, we highlight how signaling codes and transcriptional networks might determine endocrine lineage within a complex and dynamic architecture, based largely on studies in the mouse. The process begins with the designation of multipotent progenitor cells (MPC) to pancreatic buds that subsequently move through a newly proposed period involving epithelial plexus formation-remodeling, and ends with formation of clustered endocrine islets connected to the vascular and peripheral nervous systems. Developing this knowledge base, and increasing the emphasis on direct comparisons between mouse and human, will yield a more complete and focused picture of pancreas development, and thereby inform β-cell-directed differentiation from human embryonic stem or induced pluripotent stem cells (hESC, iPSC). Additionally, a deeper understanding may provide surprising therapeutic angles by defining conditions that allow the controllable reprogramming of endodermal or pancreatic cell populations.

Epigenetic regulation of pancreas development and function

Multiple signaling systems and transcription factor cascades control pancreas development and endocrine cell fate determination. Epigenetic processes contribute to the control of this transcriptional hierarchy, involving both histone modifications and DNA methylation. Here, we summarize recent advances in the field that demonstrate the importance of epigenetic regulation in pancreas development, β-cell proliferation, and cell fate choice. These breakthroughs were made using the phenotypic analysis of mice with mutations in genes that encode histone modifying enzymes and related proteins; by application of activators or inhibitors of the enzymes that acetylate or methylate histones to fetal pancreatic explants in culture; and by genomic approaches that determined the patterns of histone modifications and chromatin state genome-wide.

Epigenetics and diabetes treatment: an unrealized promise?

Epigenetic mechanisms may contribute to the pathogenesis of complex diseases. Early or late environmental influences such as intrauterine malnutrition or sedentary lifestyle have been shown to lead to an increased risk of diabetes. Recently, epigenetic mechanisms were shown to be involved in endocrine cell differentiation and islet function. Genomic profiling of pancreatic islets in non-diabetic and diabetic states is needed in order to dissect the contribution of epigenetic mechanisms to the declining proliferation potential of β cells that we see with aging or the β-cell failure observed in diabetes. In-depth understanding of epigenetic landscapes can help to improve protocols for in vitro differentiation towards the β-cell fate, enhance β-cell proliferation, and lead to the discovery of novel therapeutic targets.

Deconstructing pancreas developmental biology

The relentless nature and increasing prevalence of human pancreatic diseases, in particular, diabetes mellitus and adenocarcinoma, has motivated further understanding of pancreas organogenesis. The pancreas is a multifunctional organ whose epithelial cells govern a diversity of physiologically vital endocrine and exocrine functions. The mechanisms governing the birth, differentiation, morphogenesis, growth, maturation, and maintenance of the endocrine and exocrine components in the pancreas have been discovered recently with increasing tempo. This includes recent studies unveiling mechanisms permitting unexpected flexibility in the developmental potential of immature and mature pancreatic cell subsets, including the ability to interconvert fates. In this article, we describe how classical cell biology, genetic analysis, lineage tracing, and embryological investigations are being complemented by powerful modern methods including epigenetic analysis, time-lapse imaging, and flow cytometry-based cell purification to dissect fundamental processes of pancreas development.

The transcriptional co-repressor Grg3/Tle3 promotes pancreatic endocrine progenitor delamination and β-cell differentiation

Pancreatic β-cells arise from Ngn3(+) endocrine progenitors within the trunk epithelium of the embryonic pancreas. The emergence of endocrine cells requires E-cadherin downregulation, but the crucial steps that elicit such are not clear, yet probably important for ultimately being able to efficiently generate β-cells de novo from stem cells. Grg3 (groucho-related gene 3, also known as Tle3), encodes a member of the Groucho/TLE family of co-repressors and its function in various cell contexts is mediated by recruitment to target genes by different transcription factors. Grg proteins broadly regulate the progression of progenitor cells to differentiated cell types, but specific developmental mechanisms have not been clear. We find that Grg3 is expressed in most β-cells and a subset of other endocrine cell types in the pancreas. Grg3 is highly expressed in Ngn3(+) endocrine progenitor descendants just after transient Ngn3 expression. Grg3-null embryos die at E14.5, which is associated with placental defects, so we explanted E12.5 pancreata to allow endocrine differentiation to occur in culture. Grg3 knockout explants displayed a drastic decrease in the differentiation of all endocrine cell types owing to defects in the delamination of early endocrine progenitors from the trunk epithelium. We find that Grg3 normally suppresses E-cadherin gene expression, thereby allowing delamination of endocrine cells from the trunk epithelium and revealing how this transcriptional co-repressor modulates this crucial step of β-cell development.