ROCK-nmMyoll, Notch and Neurog3 gene-dosage link epithelial morphogenesis with cell fate in the pancreatic endocrine-progenitor niche

During mouse pancreatogenesis, endocrine cells are born from progenitors residing in an epithelial plexus niche called the ‘plexus state’, which comprises of a web-like epithelium arising around embryonic day E12.5 from within an anlage of multipotent progenitors (MPC). The progressive replacement of the plexus by an arbor-like ductal epithelium is associated with an abrupt reduction in the ability to produce new endocrine cells. Pancreatic endocrine cells are derived from a Notch-responsive Sox9+ progenitor pool via activation of the transcription factor and endocrine-lineage determinant Neurogenin 3 (Neurog3). Neurog3 is necessary and sufficient for endocrine-cell birth. Cells with high-level Neurog3 (Neurog3^HI) exit the cell cycle, egress from the epithelium and cluster tightly with each other to form the nascent islets of Langerhans. Prevention of the acquisition of the Neurog3^HI state causes cells to remain intra-epithelial and differentiate into non-endocrine acinar and ductal fates. Several studies have considered that Neurog3-expressing cells coordinate progenitor maintenance, cell fate allocation, and plexus morphogenesis, to mediate the appropriate growth and assembly of the mature ductal network and dispersed endocrine islets.

The molecular motor nmMyoll acts on filamentous-actin (F-actin) substrates in apical constriction, tissue folding, cell delamination, and migration, and generation of inter and intra-cellular tension. Rho-associated kinase (ROCK) positively regulates nmMyoll activity but also functions as a modulator of actin remodeling, affecting tissue and cell change shape, cell polarization and migration. Together, nmMyoII and ROCK are important regulators of epithelial cell shape and morphogenesis.

In recent research published in Development, Vanderbilt University scientists led by Professor Christopher Wright from the Vanderbilt Center for Stem Biology investigated in elegant and carefully designed studies the mechanistic interconnections between epithelial morphogenesis and cell-fate acquisition in the pancreatic plexus niche. They used three-dimensional tissue reconstructions, gene reporter analyses, and genetic and pharmacological interventions to study the interdependencies between Neurog3 gene activity, Notch signaling and ROCK/nmMyoll-controlled epithelial-cell morphogenesis. They proposed a model in which a circuit comprising ROCK-nmMyoll, Neurog3 gene dosage, and Notch signaling balances apportionment of endocrine cells from the plexus while still ensuring proper growth and morphogenesis of the pancreatic epithelium.

The authors observed that the morphological transitions of the F-actin apical cortex are associated with cell-fate determination, and that apical-surface narrowing, further focalization, and complementary basalward cell displacement (with intensive filopodial protrusive activity), mark the Neurog3^HI endocrine-committed state. They also observed that nmMyoll-dependent pathways mediate plexus morphogenesis and endocrine-cell birth, and that nmMyoll activity maintains the plexus state and promotes endocrine-cell differentiation.

The research team focused at both cell and tissue-level resolution to understand the functional integration between programs of Neurog3-driven endocrine-fate determination, Notch-mediated progenitor maintenance, and ROCK/nmMyoll–mediated epithelial morphogenesis. Their study describes how the ROCK/nmMyoll pathway mediates a dissociable sequence of epithelial egression events linked to the acquisition of pre-endocrine–committed and endocrine-committed Neurog3-expressing states within the remodeling epithelial plexus. The authors propose that adaptive morphogenetic processes cooperate with Neurog3-dependent processes to together confer a robust and prolonged allocation of new endocrine cells from the epithelial progenitors in the plexus niche.

Professor Christopher Wright and his colleagues, through their work, have been able to support and extend previous studies that postulate that differentiating endocrine-cells perform a ‘feedback control’ role mediating multiple developmental processes in the epithelial plexus niche, and extends the range of functions for Neurog3 during pancreatogenesis. The report will motivate further studies investigating the functions of transcription-factor determinants.