Osteopontin-Deficient Mice Exhibit Less Inflammation, Greater Tissue Damage, and Impaired Locomotor Recovery from Spinal Cord Injury Compared with Wild-Type Controls

Abstract

Osteopontin (OPN) is expressed in many tissues during inflammatory responses. After spinal cord injury, microglia expresses OPN at the site of injury during the early to subacute stages. However, the function of OPN in spinal cord injury is not well understood. This study examines the responses of OPN knock-out (KO) and wild-type (WT) mice to spinal cord contusion injury. KO and WT mice were injured with a modified New York University impactor. Weights of 10 or 5.6 g were dropped 6.25 mm onto the T13 spinal cord under isoflurane anesthesia. At 24 h, homogenized spinal cords were analyzed for total potassium concentration to estimate lesion volumes. Expression of apoptotic genes, proinflammatory cytokines, and nerve growth factors was measured by reverse transcription (RT)-PCR and Western blot. In a series of animals, locomotor recovery was assessed with the Basso mouse scale (BMS) for 6 weeks, and histological analyses was performed to determine tissue preservation. Lesion volume showed no significant differences between KO and WT mice at 24 h. RT-PCR indicated that KO mice had significantly less Bcl-2, tumor necrosis factor-alpha, interleukin-1beta, and interleukin-6 mRNA compared with WT controls. Western blot also showed that KO had significantly less Bcl-2 7 d after spinal cord injury. KO mice had significantly worse BMS locomotor scores than WT at 6 weeks. KO mice also had a significantly reduced area of spared white matter and fewer neuronal-specific nuclear protein-positive neurons in the spinal cord surrounding the impact site. This result supports a potential neuroprotective role for OPN in the inflammatory response to spinal cord injury.