Inducible Mosaic Cell Labeling Provides Insights Into Pancreatic Islet Morphogenesis.

Julia Freudenblum,Robin A Kimmel,Dirk Meyer

doi:10.3389/fcell.2020.586651

Julia Freudenblum, Robin A Kimmel + Show 1 more

Open Access

https://doi.org/10.3389/fcell.2020.586651

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Abstract

Pancreatic islets, discrete microorgans embedded within the exocrine pancreas, contain beta cells which are critical for glucose homeostasis. Loss or dysfunction of beta cells leads to diabetes, a disease with expanding global prevalence, and for which regenerative therapies are actively being pursued. Recent efforts have focused on producing mature beta cells in vitro, but it is increasingly recognized that achieving a faithful three-dimensional islet structure is crucial for generating fully functional beta cells. Our current understanding of islet morphogenesis is far from complete, due to the deep internal location of the pancreas in mammalian models, which hampers direct visualization. Zebrafish is a model system well suited for studies of pancreas morphogenesis due to its transparency and the accessible location of the larval pancreas. In order to further clarify the cellular mechanisms of islet formation, we have developed new tools for in vivo visualization of single-cell dynamics. Our results show that clustering islet cells make contact and interconnect through dynamic actin-rich processes, move together while remaining in close proximity to the duct, and maintain high protrusive motility after forming clusters. Quantitative analyses of cell morphology and motility in 3-dimensions lays the groundwork to define therapeutically applicable factors responsible for orchestrating the morphogenic behaviors of coalescing endocrine cells.

Highlights

During organ formation, differentiating cells organize into complex structures that correspond with their physiologic roles
In this study we have developed a genetic system for mosaic labeling of the actin cytoskeleton in emerging endocrine cells, to enable detailed examination of cell-cell interactions and directed translocations during islet formation
We demonstrate that the previously reported 5 kb neurod promotor (Mo and Nicolson, 2011) is active in pancreatic endocrine cells, and we incorporated this promoter into transgenic lines for studying endocrine cell morphology

Summary

Introduction

During organ formation, differentiating cells organize into complex structures that correspond with their physiologic roles. Intricate forms emerge through mechanisms that include differential adhesion, dynamic motility, and cell displacements. The vertebrate pancreas consists of digestive hormone-secreting acinar cells, a ductal system and the endocrine islets. The islets, clustered endocrine cells enveloped within the exocrine tissue, develop from progressive coalescence of differentiating progenitor cells that emerge from the ductal epithelium. While previous work suggested that progenitors actively migrate through the pancreatic mesenchyme during islet morphogenesis (Puri and Hebrok, 2007; Kesavan et al, 2014; Pauerstein et al, 2017), recent evidence reveals that clustering occurs in proximity to the pancreatic duct, either concurrent with cell exit (Sharon et al, 2019), or following movements along the ductal epithelium (Nyeng et al, 2019). Imaging studies in vivo and in explants have reported formation of dynamic protrusions in

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