engleski

Long-term dysfunction in brain insulin receptor signalling pathway in a rat model of sporadic Alzheimer's disease

Introduction: Neurofibrillar tangles associated with tau protein hyperphosphorylation mostly resulting from increased glycogen synthase kinase 3 (GSK3) activity are one of the major hallmarks of sporadic Alzheimer’s disease (sAD). GSK3 is an important enzyme in the brain insulin receptor (IR) signalling pathway, the functioning of which has been found impaired in sAD. Brain insulin dysfunction was found also in streptozotocin (STZ)-intracerebroventricularly (icv) treated rats, which represent an experimental model of sAD. In order to continue our previous follow up studies (≤6 months), we aimed to investigate hippocampal insulin signalling pathway nine months following the experimental sAD induction. Material and methods: Three month-old male Wistar rats were injected with STZ (3mg/kg) bilaterally into the lateral ventricles. Control animals received vehicle only. Cognitive functions were tested by Morris Water Maze Swimming Test nine months after STZ-icv treatment, followed by sacrifice and brain removal. Protein expression of insulin-degrading enzyme (IDE), IR, protein kinase B (Akt/PKB), total tau (t-tau), phospho-tau (p-tau), phospho- GSK3α and β (pGSK3α and pGSK3β) and total GSK3 (tGSK3) in hippocampus (HPC) was measured by SDS- PAGE electrophoresis, followed by Western blot analysis. Data were analysed by Mann-Whitney U test (p<0, 05). Results: Nine months following the STZ-icv treatment, deficit in learning and memory functions (-38, 4%) was still present in STZ-icv rats in comparison to the controls. Protein expression of IDE, pGSK3β and t-tau was found significantly decreased (- 37, 5%, -45, 1% and 31, 8%, respectively), while tGSK3β expression was found significantly increased (49, 9%). These changes were associated with increased p/total tau ratio (51, 3%) and decreased p/total GSK3β (-62, 6%) ratio which indirectly suggested an increase in GSK3β activity. No change in protein expression of IR, Akt/PKB, pGSK3α, tGSK3α and p-tau was observed in comparison to the control animals. Conclusion: In line with our previous follow-up studies, increased p/total tau protein and decreased p/total GSK3β ratio, respectively, found 9 months following the STZ-icv administration suggest that changes of HPC tau protein phosphorylation are persistently mediated by GSK3β-induced phosphorylation of tau protein. However, finding of unaltered IR and Akt/PKB protein levels indicate that some other signalling pathway might also be involved in regulation of GSK3β phosphorylation status after STZ-icv administration at this time point when IDE levels were found persistently decreased since three- month post STZ-icv period. Acknowledgement: Supported by MZOŠ (108-1080003-0020) and DAAD projects.

streptozotocin; insulin signalling pathway; Alzheimer

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