Bone morphogenetic protein-2 used in spinal fusion with spinal cord injury penetrates intrathecally and elicits a functional signaling cascade

Background context: The use of recombinant human bone morphogenetic protein-2 (rhBMP-2) and its indications for spinal fusion continue to be expanded with recent reports citing spinal trauma application. However, there are no data establishing the effects of rhBMP-2 on the injured spinal cord. Purpose: The purpose of this study was to evaluate the extent of bone morphogenetic protein (BMP)-specific intrathecal signaling after application to the spine at various time points after a spinal cord injury (SCI). Study design: This is an in vivo rat study using a combination of the dorsal hemisection SCI and the posterolateral arthrodesis animal models. Methods: Sixty-five female Sprague-Dawley rats underwent either a T9-T10 dorsal hemisection SCI (n=52) or laminectomy only (n=13). Spinal cord injury animals were further subdivided into four follow-up groups (n=13/group): 30 minutes, 24 hours, 7 days, and 21 days, at which time one of two secondary surgeries were performed: Eight rats per time point received either 43 μg of rhBMP-2 per side or sterile water control over T9-T11 on absorbable collagen sponges (ACSs). Animals were perfused after 24 hours, and spinal cords were immunohistochemically analyzed. Sections of the lesion were stained with BMP-specific pSmad 1, 5, 8 antibody and costained with cell-specific markers. pSmad-positive cells were then counted around the lesion. The remaining five rats (n=5/time point) had luciferase (blood spinal cord barrier [BSCB] permeability marker) injected through the jugular vein. Subsequently, spinal cords were collected and luciferase activity was quantified around the lesion and in the cervical samples (controls) using a luminometer. Results: After injury, a significant increase in the number of pSmad-positive cells was observed when rhBMP-2 was implanted at the 30-minute, 24-hour, and 7-day time points (p<.05). Costaining revealed BMP-specific signaling activation in neurons, glial cells, macrophages, and fibroblasts. Spinal cord permeability to luciferase was significantly increased at 30 minutes, 24 hours, and 7 days post lesion (p<.05). A significant linear regression was established between the extent of BSCB permeability and pSmad signaling (r²=0.66, p=.000). Conclusions: Our results indicate that rhBMP-2 use around a spinal cord lesion elicits a robust signaling response within the spinal cord parenchyma. All CNS cell types and the invading fibroblasts are activated to the extent dependent on the integrity of the meningeal and BSCB barriers. Therefore, in the presence of a SCI and/or dural tear, rhBMP-2 diffuses intrathecally and activates a signaling cascade in all major CNS cell types, which may increase glial scarring and impact neurologic recovery.