Focal
brain ischemia leads to a slow type of neuronal death in the penumbra that starts several hours after
ischemia and continues to mature for days. During this maturation period, blood flow, cellular
ATP and ionic homeostasis are gradually recovered in the penumbral region. In striking contrast,
protein synthesis is irreversibly inhibited. This study used a rat focal
brain ischemia model to investigate whether or not irreversible translational inhibition is due to abnormal aggregation of translational complex components, i.e. the ribosomes and their associated nascent
polypeptides,
protein synthesis
initiation factors and co-translational chaperones. Under electron microscopy, most rosette-shaped polyribosomes were relatively evenly distributed in the cytoplasm of
sham-operated control neurons, but clumped into large abnormal aggregates in penumbral neurons subjected to 2 h of focal
ischemia followed by 4 h of reperfusion. The abnormal
ribosomal protein aggregation lasted until the onset of delayed neuronal death at 24-48 h of reperfusion after
ischemia. Biochemical study further suggested that translational complex components, including small ribosomal subunit
protein 6 (S6), large
subunit protein 28 (L28),
eukaryotic initiation factors 2alpha, 4E and 3eta, and co-translational chaperone heat-shock cognate
protein 70 (HSC70) and co-chaperone Hdj1, were all irreversibly clumped into large abnormal
protein aggregates after
ischemia. Translational complex components were also highly ubiquitinated. This study clearly demonstrates that focal
ischemia leads to irreversible aggregation of
protein synthesis machinery that contributes to neuronal death after focal
brain ischemia.