Macrophage Recruitment and Activation in Early Fracture Healing

David Gazzaniga, MD
Advisor: Samy Ashkar, MD
Advisor: Peter Hauschaka, MD

Dr. David Gazzaniga presented his hypothesis that osteopontin (OPN), a protein found in fracture hematoma, may be chemotactic for macrophages and with other chemokines, may serve to induce the expression of multiple growth factors pivotal in fracture healing. Using a tibial fracture model in both normal and OPN deficient mice, Dr. Gazzaniga and his advisor Dr. Samy Ashkar performed histological, immunohistochemical, and protein analysis of fracture callus 12 and 28 days after injury. The presence of OPN and chemokines were associated with improved callus formation and fracture healing, as well as elevated levels of local monocytes and growth factors. Dr. Gazzaniga concluded that these proteins may play a pivotal role in fracture healing through a pathway mediated by macrophage recruitment and stimulation. 

In his discussion of this thesis, Dr. Peter Hauschka highlighted the novel hypothesis that macrophages may play a stimulatory as well as phagocytic role in secondary fracture healing. He encouraged further investigation into the role of both monocytic cells as well as chemokines in this fundamental biological process.

Studies of the Role of Nitric Oxide Synthase Isoforms Using a Mice Gene Knock-out Model

Saechin Kim, MD
Advisor: Peter Hauschka, MD
Discussor: Christopher Evans, MD

Dr. Kim found that several isoforms of nitric oxide synthase (NOS), an enzyme responsible for the production of the signaling molecule nitric oxide, are present in bone. He then used normal and NOS deficient mice to study the role of this enzyme in bone formation and resorption. Using a murine tibial fracture model, Dr. Kim confirmed that both cytokines and estrogen result in decreased bony resorption through a pathway mediated by increased nitric oxide production. Animals treated with NOS inhibitors and animals genetically deficient in NOS, however, demonstrated increased bony resorp-tion. This suggests a critical role for nitric oxide in cytokine- and hormone-mediated bone formation and resorption.

Dr. Christopher Evans led the discussion of this thesis, providing a historical and biological background for the role of nitric oxide in the muscu-loskeletal and other systems. Encouraged by their preliminary findings, he challenged Dr. Kim and Dr. Hauschka to continue to distinguish between the roles of the various NOS isoforms in bone biology in both the tibial torsion fracture model and other models of bony remodeling.