Differential Temporal Expression of Members of the Transforming Growth Factor β Superfamily During Murine Fracture Healing

Abstract

Fracture healing is a unique postnatal repair process in which the events of endochondral and intramembranous bone formation follow a definable temporal sequence. The temporal patterns of messenger RNA (mRNA) expression for members of the transforming growth factor β (TGF-β) superfamily were examined over a 28-day period of fracture healing in mouse tibias. Bone morphogenetic protein 2 (BMP-2) and growth and differentiation factor 8 (GDF8) showed maximal expression on day 1 after fracture, suggesting their roles as early response genes in the cascade of healing events. Restricted expression of GDF8 to day 1, in light of its known actions as a negative regulator of skeletal muscle growth, suggests that it may similarly regulate cell differentiation early in the fracture healing process. GDF5, TGF-β2, and TGF-β3 showed maximal expression on day 7, when type II collagen expression peaked during cartilage formation. In contrast, BMP-3, BMP-4, BMP-7, and BMP-8 showed a restricted period of expression from day 14 through day 21, when the resorption of calcified cartilage and osteoblastic recruitment were most active. TGF-β1, BMP-5 and BMP-6, and GDF10 were constitutively expressed from day 3 to day 21. However, during the same time period, GDF3, GDF6, and GDF9 could not be detected, and GDF1 was expressed at extremely low levels. These findings suggest that several members of the TGF-β superfamily are actively involved in fracture healing and although they are closely related both structurally and functionally, each has a distinct temporal expression pattern and potentially unique role in fracture healing.