Pancreatic cell fate specification: insights into developmental mechanisms and their application for lineage reprogramming

A 3D atlas of the human developing pancreas to explore progenitor proliferation and differentiation

AIMS/HYPOTHESIS

Rodent pancreas development has been described in great detail. On the other hand, there are still gaps in our understanding of the developmental trajectories of pancreatic cells during human ontogenesis. Here, our aim was to map the spatial and chronological dynamics of human pancreatic cell differentiation and proliferation by using 3D imaging of cleared human embryonic and fetal pancreases.

METHODS

We combined tissue clearing with light-sheet fluorescence imaging in human embryonic and fetal pancreases during the first trimester of pregnancy. In addition, we validated an explant culture system enabling in vitro proliferation of pancreatic progenitors to determine the mitogenic effect of candidate molecules.

RESULTS

We detected the first insulin-positive cells as early as five post-conceptional weeks, two weeks earlier than previously observed. We observed few insulin-positive clusters at five post-conceptional weeks (mean ± SD 9.25±5.65) with a sharp increase to 11 post-conceptional weeks (4307±152.34). We identified a central niche as the location of onset of the earliest insulin cell production and detected extra-pancreatic loci within the adjacent developing gut. Conversely, proliferating pancreatic progenitors were located in the periphery of the epithelium, suggesting the existence of two separated pancreatic niches for differentiation and proliferation. Additionally, we observed that the proliferation ratio of progenitors ranged between 20% and 30%, while for insulin-positive cells it was 1%. We next unveiled a mitogenic effect of the platelet-derived growth factor AA isoform (PDGFAA) in progenitors acting through the pancreatic mesenchyme by increasing threefold the number of proliferating progenitors.

CONCLUSIONS/INTERPRETATION

This work presents a first 3D atlas of the human developing pancreas, charting both endocrine and proliferating cells across early development.

Multiple Cell Lineages Give Rise to a Cell Type

It has long been assumed that a specific cell type arises following stepwise specification of cells corresponding to the branching of cell lineages. However, accumulating evidence indicates that multiple and even remote cell lineages can lead to the development of the same cells. Four examples giving different yet new insights will be discussed: skeletal muscle development from precursors with distinct initial histories of transcriptional regulation, lens cell development from remote lineages yet sharing basic transcription factors, blood cell development under intersectional pathways, and neural tissue development from cardiac precursors through the manipulation of just one component of epigenetic regulation. These examples provide flexible and nondogmatic perspectives on developmental cell regulation, fundamentally revising the old model relying on cell lineages.

NEUROD1 reinforces endocrine cell fate acquisition in pancreatic development

NEUROD1 is a transcription factor that helps maintain a mature phenotype of pancreatic β cells. Disruption of Neurod1 during pancreatic development causes severe neonatal diabetes; however, the exact role of NEUROD1 in the differentiation programs of endocrine cells is unknown. Here, we report a crucial role of the NEUROD1 regulatory network in endocrine lineage commitment and differentiation. Mechanistically, transcriptome and chromatin landscape analyses demonstrate that Neurod1 inactivation triggers a downregulation of endocrine differentiation transcription factors and upregulation of non-endocrine genes within the Neurod1-deficient endocrine cell population, disturbing endocrine identity acquisition. Neurod1 deficiency altered the H3K27me3 histone modification pattern in promoter regions of differentially expressed genes, which resulted in gene regulatory network changes in the differentiation pathway of endocrine cells, compromising endocrine cell potential, differentiation, and functional properties.

USP7 controls NGN3 stability and pancreatic endocrine lineage development

Understanding the factors and mechanisms involved in beta-cell development will guide therapeutic efforts to generate fully functional beta cells for diabetes. Neurogenin 3 (NGN3) is the key transcription factor that marks endocrine progenitors and drives beta-cell differentiation. Here we screen for binding partners of NGN3 and identify the deubiquitylating enzyme USP7 as a key regulator of NGN3 stability. Mechanistically, USP7 interacts with, deubiquitinates and stabilizes NGN3. In vivo, conditional knockout of Usp7 in the mouse embryonic pancreas causes a dramatic reduction in islet formation and hyperglycemia in adult mice, due to impaired NGN3-mediated endocrine specification during pancreatic development. Furthermore, pharmacological inhibition of USP7 during endocrine specification in human iPSC models of beta-cell differentiation decreases NGN3 expressing progenitor cell numbers and impairs beta cell differentiation. Thus, the USP7-NGN3 axis is an essential mechanism for driving endocrine development and beta-cell differentiation, which can be therapeutically exploited.

Heart Failure and Pancreas Exocrine Insufficiency: Pathophysiological Mechanisms and Clinical Point of View

Heart failure is associated with decreased tissue perfusion and increased venous congestion that may result in organ dysfunction. This dysfunction has been investigated extensively for many organs, but data regarding pancreatic (exocrine) dysfunction are scarce. In the present review we will discuss the available data on the mechanisms of pancreatic damage, how heart failure can lead to exocrine dysfunction, and its clinical consequences. We will show that heart failure causes significant impairment of pancreatic exocrine function, particularly in the elderly, which may exacerbate the clinical syndrome of heart failure. In addition, pancreatic exocrine insufficiency may lead to further deterioration of cardiovascular disease and heart failure, thus constituting a true vicious circle. We aim to provide insight into the pathophysiological mechanisms that constitute this reciprocal relation. Finally, novel treatment options for pancreatic dysfunction in heart failure are discussed.