Increased cerebral protein ISGylation after focal ischemia is neuroprotective

Addition of a small peptide called ISG15 is known as ISGylation, which is an ubiquitin (ub)-like posttranslational modification. We currently show that focal ischemia induced by transient middle cerebral artery occlusion (MCAO) in adult mice significantly induces cortical protein ISGylation between 6 and 24 hours reperfusion. With two-dimensional western blotting, 45 proteins were observed to be significantly increased in ISGylation (by 1.8- to 9.7-fold) after focal ischemia compared with sham control. Immunochemistry showed that ISGylated proteins are localized in neurons within the ipsilateral striatum and in astroglia within the peri-infarct cortex of ischemic mice. When subjected to transient MCAO, ISG15(-/-) mice showed increased mortality, exacerbated infarction, and worsened neurologic recovery than did wild-type controls. In addition, mice lacking UBE1L (ub-activating enzyme E1-like protein, the first enzyme of the ISGylation cycle) also showed bigger infarcts when subjected to transient MCAO. Regional cerebral blood flow or other physiologic parameters were not significantly different in both knockouts compared with wild-type controls. These studies indicate that increased protein ISGylation might be an endogenous neuroprotective adaptation to minimize poststroke brain damage.