Age-related accumulation of high-molecular-weight ubiquitin protein conjugates in mouse brains

It has been demonstrated that the amount of ubiquitin (Ub)-protein conjugates increases in the whole brain homogenates of old mice. To determine the specificity of the accumulation of the conjugates, we further investigated age-related changes in Ub-protein conjugates in other tissues (heart, liver, and kidney), and also in various regions (cerebrum, cerebellum, brain stem, and medulla oblongata) of the brain. The amount of high-molecular-weight Ub-protein conjugates (HMW, > 60KDa) increased remarkably in all regions of the brain examined, especially in the cerebellum, of senescent mice, but not much in other tissues. No appreciable difference in the size of free Ub pool was found between young and old mice except in the medulla oblongata. This suggests that the increase in Ub-protein conjugates is not simply due to a rise in the free Ub pool. The activity of isopeptidases in homogenates was estimated by measuring rates of deubiquitination of endogenous Ub-protein conjugates under ATP-depleted conditions. In the brain stem and medulla oblongata, they decreased significantly with age. In the cerebrum and cerebellum, however, no significant difference was observed between young and old animals. These results partially explain the increases observed in medulla and brain stem, but cannot explain the increases in cerebrum and cerebellum.