Metaplasia of Cartilage to Bone During Fracture Healing Is Regulated in Part by mTOR

Wednesday, October 18, 2023

11:17 AM – 11:23 AM PST

Location: 606-607

Presenting Author(s)

Kazuhito Morioka, MD, PhD (he/him/his)

Assistant Professor
Orthopaedic Trauma Institute, Department of Orthopaedic Surgery, University of California, San Francisco (UCSF), CA, United States
San Francisco, California, United States

Disclosure(s): No financial relationships to disclose

Corresponding Author(s)

RM

Ralph Marcucio, PhD

Professor
Orthopaedic Trauma Institute, Department of Orthopaedic Surgery, University of California, San Francisco (UCSF), CA, United States
San Francisco, California, United States

Disclosure(s): No financial relationships to disclose

Co-Author(s)

CL

Charles Lam, BS (he/him/his)

Staff Research Associate
Orthopaedic Trauma Institute, Department of Orthopaedic Surgery, University of California, San Francisco (UCSF), CA, United States
San Francisco, California, United States
KC

Ko Chiba, MD, PhD (he/him/his)

Assistant Professor
Department of Orthopaedic Surgery, Nagasaki University Graduate School of Biomedical Sciences, Nagasaki, Japan
Nagasaki, Japan
LB

Leila Bagheri, PhD (she/her/hers)

Postdoctoral Scholar
Orthopaedic Trauma Institute, Department of Orthopaedic Surgery, University of California, San Francisco (UCSF), CA, United States
San Francisco, California, United States
DH

Diane Hu, MD (she/her/hers)

Professional Researcher
Orthopaedic Trauma Institute, Department of Orthopaedic Surgery, University of California, San Francisco (UCSF), CA, United States
San Francisco, California, United States
Theodore Miclau, III, MD, FIOTA (he/him/his)

Professor and Vice Chairman
Orthopaedic Trauma Institute, Department of Orthopaedic Surgery, University of California, San Francisco (UCSF), CA, United States
San Francisco, California, United States

Purpose: Bone has a remarkable ability to regenerate after injury, and metaplasia is a prominent feature of the fracture callus. Previous studies have demonstrated that chondrocytes could differentiate into osteoblasts during fracture healing, resulting in the formation of the transition zone in the callus (Fig. 1). However, the mechanism of endochondral ossification through metaplasia is incompletely understood. Metaplasia, in which differentiated cells revert to an immature state and then redifferentiate into tissue-specific cell types, occurs in various tissues and is regulated, in part, by mammalian Target of Rapamycin (mTOR) activity. Here we investigated the impact of mTOR activity on the conversion of chondrocytes to osteoblasts during fracture healing using a well-established murine tibia fracture model.

Methods: A non-stabilized tibial fracture was created in the mid-diaphysis of adult male C57BL/6J mice using a 3-point bending device. Subjects were treated with rapamycin (8 mg/kg, intraperitoneal) or DMSO (control) to block mTOR activity beginning on day 8 after fracture and continuing until euthanasia. Tissues were collected after administration of BrdU (Bromodeoxyuridine) at 10, 14, and 21 days post-fracture. Morphometric evaluation was performed to assess non-calcified callus, calcified callus, and bony callus in tibial fracture. The chondro-osseous junction in the fracture callus was assessed using an unbiased histological analysis.

Results: Treatment with rapamycin significantly altered fracture healing. There was a significant retention of cartilage and reduction in bone formation at day 21 post-fracture (Fig. 2) . This was accompanied by a marked decrease of cell proliferation by hypertrophic chondrocytes that comprise the transition zone (Fig. 3). Our preliminary micro-CT data show an increase in the non-calcified callus at day 14 after fracture, but the sample size is small for this analysis.

Conclusion: Metaplasia requires a decrease in mTOR activity that stimulates autophagy, followed by an increase in mTOR activity that stimulates proliferation. These data indicated that blocking mTOR reduces the conversion of cartilage to bone possibly through the impact on cell proliferation.

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