# Cell Transitions during Bone Fracture Healing

> **NIH NIH R01** · UNIVERSITY OF CALIFORNIA, SAN FRANCISCO · 2023 · $581,449

## Abstract

Summary
Our understanding of cell lineages is currently being challenged. Cell plasticity appears to be more prevalent
than previously thought and cell fate switching, even among fully differentiated cells is being more fully
uncovered and understood. For example, ‘paligenosis’ is an emerging concept whereby fully differentiated cells
revert to a stem cell like state and give rise to a multitude of cell types in part due to mTOR signaling. These
observations have important implications for bone fracture healing. Multiple differentiation events occur for the
bone to heal. Initially, the mechanical environment directs cell fate decisions within the periosteum. Mechanical
stability directs differentiation of osteoblasts and intramembranous ossification, while instability directs
differentiation of chondrocytes and endochondral ossification. Concomitantly, the stem cell compartment is
maintained, and a renewed stem cell pool will eventually populate the periosteum that covers the new bone. At
later stages of endochondral ossification chondrocytes become osteoblasts as the cartilage transforms into
bone. Disruptions to these distinct events can lead to delayed or failed healing, which is often associated with
increased fibrosis of the fracture site. In this application we propose to examine the process of differentiation of
periosteal cells in response to the mechanical environment (Aim1), to assess transformation of chondrocytes
into osteoblasts (Aim 2), and maintenance of the stem cell pool and population of the newly formed periosteum
by stem cells (Aim 3). This work utilizes a systems biology approach to examine molecular mechanisms that
underlie these cell fate decisions, and in parallel a more standard hypothesis-based approach. We focus on
the role of Nf1 and Sox2 during differentiation of periosteal cells and the transformation of chondrocytes into
osteoblasts. Our preliminary data show that deletion of Nf1 from the developing periosteum leads to a fibrous
non-union after fracture, and we focus on the role of mTOR in mediating these outcomes. Our data also show
that Sox2 is necessary for endochondral ossification, and we test the requirement of Sox2 in hypertrophic
chondrocytes for transformation to osteoblasts. Finally, we examine a role for Sox2 in maintaining the stem cell
compartment in the periosteum using a set of loss-of-function experiments in serial fracture repair. In summary,
combining a systems biology approach with hypothesis testing is a powerful way to develop deep
understanding of the processes regulating cell differentiation during fracture healing.

## Key facts

- **NIH application ID:** 10754205
- **Project number:** 1R01AR081671-01A1
- **Recipient organization:** UNIVERSITY OF CALIFORNIA, SAN FRANCISCO
- **Principal Investigator:** Ralph S Marcucio
- **Activity code:** R01 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2023
- **Award amount:** $581,449
- **Award type:** 1
- **Project period:** 2023-09-18 → 2028-08-31

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/10754205

## Citation

> US National Institutes of Health, RePORTER application 10754205, Cell Transitions during Bone Fracture Healing (1R01AR081671-01A1). Retrieved via AI Analytics 2026-05-23 from https://api.ai-analytics.org/grant/nih/10754205. Licensed CC0.

---

*[NIH grants dataset](/datasets/nih-grants) · CC0 1.0*