Brain-derived neurotrophic factor in astrocytes, oligodendrocytes, and microglia/macrophages after spinal cord injury

Recent studies suggest that the injured adult spinal cord responds to brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT3) with enhanced neuron survival and axon regeneration. Potential neurotrophin sources and cellular localization in spinal cord are largely undefined. We examined glial BDNF localization in normal cord and its temporospatial distribution after injury in vivo. We used dual immunolabeling for BDNF and glial fibrillary acidic protein (GFAP) in astrocytes, adenomatous polyposis coli tumor suppressor protein (APC) for oligodendrocytes or type III CDH receptor (OX42) for microglia/macrophages. In normal cord, small subsets of astrocytes and microglia/macrophages and most oligodendrocytes exhibited BDNF-immunoreactivity. Following injury, the number of BDNF-immunopositive astrocytes and microglia/macrophages increased dramatically at the injury site over time. Most oligodendrocytes contained BDNF 1 day and 1 week following injury, but APC-positive cells were largely absent at the injury site 6 weeks postinjury. Glial BDNF-immunolabeling was also examined 10 and 20 mm from the wound. Ten millimeters from the lesion, astrocyte and microglia/macrophage BDNF-immunolabeling resembled that at the injury at all times examined. Twenty millimeters from injury, BDNF localization in all three glial subtypes resembled controls, regardless of time postlesion. Our findings suggest that in normal adult cord, astrocytes, oligodendrocytes, and microglia/macrophages play roles in local trophin availability and in trophin-mediated injury and healing responses directly within and surrounding the wound site.