# Determining how the G1/S cell cycle transition regulates the homeostasis of adult intestinal stem cells

> **NIH NIH F32** · STANFORD UNIVERSITY · 2020 · $53,547

## Abstract

Project Summary
Regulation of the cell cycle of adult stem cells is crucial to maintaining tissue integrity. Cells
irreversibly commit to division at the G1/S transition; however, how G1/S transition regulates
adult stem cell cycles is currently unknown. I propose to study the molecular basis of how the
G1/S transition is regulated in intestinal stem cell (ISC), which have physiologically high rates of
proliferation. I will use intestinal organoid cultures as ex vivo models of ISC proliferation and
intestinal homeostasis. Organoids are especially suited to this question because they are
amenable to long-term live-cell imaging, while still retaining much of the stem cell physiology of
their in vivo counterparts. My preliminary studies suggest that ISCs are especially sensitive to
inhibition of Cyclin D/CDK4/6, a group of cyclin/CDK complexes that are key regulators of the
G1/S transition. I propose to use live three-dimensional imaging intestinal organoids to
investigate how the G1/S transition regulates ISC cell cycle and organoid homeostasis. (Aim 1)
In collaboration with Dr. Prisca Liberali in the Friedrich Miescher Institute in Basel, I will learn
and use light-sheet microscopy to perform long-term live-cell imaging on organoids expressing
cell cycle and cell stemness reporters. This allows me to directly measure cell cycling dynamics
in ICSs during CDK4/6 inhibition. For this aim, I will also develop computational image analysis
tools to analyze large-scale 4-dimensional image data. (Aim 2) I will investigate the molecular
mechanism that underlies G1/S transition in ISCs. To test that the cell cycle phenotype seen in
ISCs during CDK4/6 inhibition is mediated by its known substrate, retinoblastoma protein (RB), I
will specifically interfere with a motif in RB bound by Cyclin D, to see if RB phosphorylation
specifically by Cyclin D/CDK4/6 is necessary for proper ISC G1/S control. Furthermore, I will
test if expression levels of Cyclin D and RB also couple the G1/S transition to cell size in ISCs,
by perturbing the expression levels of CDK4/6, Cyclin D, and RB, I and examining how ISC cell
size distribution changes. Finally, I will investigate whether downstream cell fates are affected
by ISCs that experience cell cycle disruption due to CDK4/6 inhibition, to see how cell-type
homeostasis may also be affected by the G1/S transition. This project will elucidate the
mechanisms used by adult stem cells to regulate their G1/S transition, an important problem in
both developmental biology and cancer.

## Key facts

- **NIH application ID:** 9899107
- **Project number:** 5F32GM129878-02
- **Recipient organization:** STANFORD UNIVERSITY
- **Principal Investigator:** SHICONG XIE
- **Activity code:** F32 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2020
- **Award amount:** $53,547
- **Award type:** 5
- **Project period:** 2018-12-01 → 2020-08-31

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/9899107

## Citation

> US National Institutes of Health, RePORTER application 9899107, Determining how the G1/S cell cycle transition regulates the homeostasis of adult intestinal stem cells (5F32GM129878-02). Retrieved via AI Analytics 2026-05-24 from https://api.ai-analytics.org/grant/nih/9899107. Licensed CC0.

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