Changes in NG2 cells and oligodendrocytes in a new model of intraspinal hemorrhage

Spinal cord injury (SCI) evokes rapid deleterious and reparative glial reactions. Understanding the triggers for these responses is necessary for designing strategies to maximize repair. This study examined lesion formation and glial responses to vascular disruption and hemorrhage, a prominent feature of acute SCI. The specific role of hemorrhage is difficult to evaluate in trauma-induced lesions, because mechanical injury initiates many downstream responses. To isolate vascular disruption from trauma-induced effects, we created a novel and reproducible model of collagenase-induced intraspinal hemorrhage (ISH) and compared glial reactions between unilateral ISH and a hemi-contusion injury. Similar to contusion injuries, ISH lesions caused loss of myelin and axons and became filled with iron-laden macrophages. We hypothesized that intraspinal hemorrhage would also initiate reparative cellular responses including NG2+ oligodendrocyte progenitor cell (OPC) proliferation and oligodendrocyte genesis. Indeed, ISH induced OPC proliferation within 1 d post-injury (dpi), which continued throughout the first week and resulted in a sustained elevation of NG2+ OPCs. ISH also caused oligodendrocyte loss within 4 h that was sustained through 3 d post-ISH. However, oligodendrogenesis, as determined by bromo-deoxyuridine (BrdU) positive oligodendrocytes, restored oligodendrocyte numbers by 7 dpi, revealing that proliferating OPCs differentiated into new oligodendrocytes after ISH. The signaling molecules pERK1/2 and pSTAT3 were robustly increased acutely after ISH, with pSTAT3 being expressed in a portion of OPCs, suggesting that activators of this signaling cascade may initiate OPC responses. Aside from subtle differences in timing of OPC responses, changes in ISH tissue closely mimicked those in hemi-contusion tissue. These results are important for elucidating the contribution of hemorrhage to lesion formation and endogenous cell-mediated repair, and will provide the foundation for future studies geared toward identifying the role of specific blood components on injury and repair mechanisms. This understanding may provide new clinical targets for SCI and other devastating conditions such as intracerebral hemorrhage.