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Selective proteasomal dysfunction in the hippocampal CA1 region after transient forebrain ischemia
Akio Asai1, Nobuyuki Tanahashi, Jian-Hua Qiu
1Laboratory for Neuroscience and Neurooncology, Department of Neurosurgery, Faculty of Medicine, University of Tokyo, Tokyo, Japan. asaisan-tky@umin.ac.jp
Insights
Impaired proteasome function, specifically the 26S proteasome, in the hippocampus CA1 region after ischemia leads to delayed neuronal death. This impairment involves ATP-dependent reassociation of proteasome subunits and is crucial for understanding neurodegeneration.
Area of Science:
- Neuroscience
- Biochemistry
Background:
- Delayed neuronal death in the hippocampus CA1 region following transient forebrain ischemia is a significant concern.
- The exact mechanisms underlying this neuronal death are not fully understood.
- Impaired proteasome function, indicated by ubiquitin accumulation and depletion, is implicated.
Purpose of the Study:
- To investigate the role of proteasome activity in delayed neuronal death after transient forebrain ischemia.
- To determine the regional and temporal changes in proteasome activity in the hippocampus and frontal cortex.
- To elucidate the ATP-dependent mechanisms affecting proteasome function post-ischemia.
Main Methods:
- Transient forebrain ischemia was induced in male Mongolian gerbils.
- Regional proteasome activity was measured at various time points post-ischemia and reperfusion.
- Proteasome activities (20S and 26S) were assessed using glycerol gradient centrifugation with and without ATP.
Main Results:
- Proteasome activity was globally reduced in the forebrain during ischemia.
- Following reperfusion, proteasome activity recovered in most regions except the hippocampal CA1, where it remained low for up to 48 hours.
- ATP-dependent reassociation of 20S and PA700 subunits to form the 26S proteasome was severely impaired specifically in the hippocampus.
Conclusions:
- Transient forebrain ischemia causes global proteasome impairment.
- The failure of 26S proteasome recovery in the hippocampal CA1 region is specific and linked to ATP-dependent subunit reassociation.
- Irreversible proteasome dysfunction in the CA1 region is a key factor in delayed neuronal death post-ischemia.
Abstract:
Delayed neuronal death in the hippocampal CA1 region after transient forebrain ischemia may share its underlying mechanism with neurodegeneration and other modes of neuronal death. The precise mechanism, however, remains unknown. In the postischemic hippocampus, conjugated ubiquitin accumulates and free ubiquitin is depleted, suggesting impaired proteasome function. The authors measured regional proteasome activity after transient forebrain ischemia in male Mongolian gerbils. At 30 minutes after ischemia, proteasome activity was 40% of normal in the frontal cortex and hippocampus. After 2 hours of reperfusion, it had returned to normal levels in the frontal cortex, CA3 region, and dentate gyrus, but remained low for up to 48 hours in the CA1 region. Thus, the 26S proteasome was globally impaired in the forebrain during transient ischemia and failed to recover only in the CA1 region after reperfusion. The authors also measured 20S and 26S proteasome activities directly after decapitation ischemia (at 5 and 20 minutes) by fractionating the extracts with glycerol gradient centrifugation. Without adenosine triphosphate (ATP), only 20S proteasome activity was detected in extracts from both the hippocampus and frontal cortex. When the extracts were incubated with ATP in an ATP-regenerating system, 26S proteasome activity recovered almost fully in the frontal cortex but only partially in the hippocampus. Thus, after transient forebrain ischemia, ATP-dependent reassociation of the 20S catalytic and PA700 regulatory subunits to form the active 26S proteasome is severely and specifically impaired in the hippocampus. The irreversible loss of proteasome function underlies the delayed neuronal death induced by transient forebrain ischemia in the hippocampal CA1 region.