Transient ischemia induces massive nuclear accumulation of SUMO2/3-conjugated proteins in spinal cord neurons

Abstract

The objective of this study is to determine whether transient spinal cord ischemia activates small ubiquitin-like modifier (SUMO1-3) conjugation, a post-translational protein modification that protects neurons from ischemia-like conditions. Mice were subjected to 8-12 min of spinal cord ischemia and 3-24 h of recovery using a newly developed experimental model. To characterize the model, activation of stress response pathways induced after spinal cord ischemia, previously observed in other experimental models, was verified by western blot analysis. Levels and subcellular localization of SUMO-conjugated proteins in spinal cords were evaluated by western blot analysis and immunohistochemistry, respectively. Following transient spinal cord ischemia, stress responses were activated as indicated by increased phosphorylation of eukaryotic initiation factor 2 (eIF2α), extracellular signal-regulated kinases (ERK1/2) and Akt. SUMO1 conjugation was not altered, but a selective rise in levels of SUMO2/3-conjugated proteins occurred, peaking at 6 h reperfusion. The marked activation of SUMO2/3 conjugation was a neuronal response to ischemia, as indicated by co-localization with the neuronal marker NeuN, and was associated with nuclear accumulation of SUMO2/3-conjugated proteins. Our study suggests that spinal cord neurons respond to ischemic stress by activation of SUMO2/3 conjugation. Many of the identified SUMO target proteins are transcription factors and other nuclear proteins involved in gene expression and genome stability. It is therefore concluded that the post-ischemic activation of SUMO2/3 conjugation may define the fate of neurons exposed to a transient interruption of blood supply, and that this pathway could be a therapeutic target to increase the resistance of spinal cord neurons to transient ischemia.