Lgr5⁺ intestinal stem cells reside in an unlicensed G₁ phase

Area of Science:

• Cell Biology
• Molecular Biology
• Gastrointestinal Biology

Background:

• Replication origin licensing, mediated by MCM2-7 hexamers, occurs during late mitosis and early G1 phase.
• Understanding the coupling of licensing and proliferative commitment is crucial for tissue homeostasis, particularly in the rapidly renewing intestinal epithelium.

Purpose of the Study:

• To investigate the coupling of replication licensing and proliferative commitment in the small intestine epithelium.
• To determine the licensing status of intestinal stem cells and their role in cell cycle regulation.

Main Methods:

• Development of a novel method to identify cells with DNA-bound MCM2-7 in intact intestinal tissue.
• Analysis of MCM2-7 protein expression and DNA binding in different cell populations within the intestinal epithelium.

Main Results:

• Interphase cells above the transit-amplifying compartment showed no DNA-bound MCM2-7, indicating licensing inhibition upon differentiation.
• Most proliferative Lgr5+ stem cells were found in an unlicensed state, suggesting a prolonged G1 phase with dormant origins.
• Apc-mutant epithelium, lacking a functional restriction point, lost the unlicensed state, leading to immediate G1 licensing.

Conclusions:

• The unlicensed G1 phase in intestinal stem cells contributes to their elongated cell cycle and creates a window for proliferative fate decisions.
• Dysregulation of licensing, as seen in Apc-mutant cells, highlights the importance of controlled origin licensing in preventing uncontrolled proliferation.