Protein aggregation in neurons following OGD: a role for Na+ and Ca2+ ionic dysregulation

In this study, we investigated whether disruption of Na(+) and Ca(2+) homeostasis via activation of Na(+)-K(+)-Cl(-) cotransporter isoform 1 (NKCC1) and reversal of Na(+)/Ca(2+) exchange (NCX(rev)) affects protein aggregation and degradation following oxygen-glucose deprivation (OGD). Cultured cortical neurons were subjected to 2 h OGD and 1-24 h reoxygenation (REOX). Redistribution of ubiquitin and formation of ubiquitin-conjugated protein aggregates occurred in neurons as early as 2 h REOX. The protein aggregation progressed further by 8 h REOX. There was no significant recovery at 24 h REOX. Moreover, the proteasome activity in neurons was inhibited by 80-90% during 2-8 h REOX and recovered partially at 24 h REOX. Interestingly, pharmacological inhibition or genetic ablation of NKCC1 activity significantly decreased accumulation of ubiquitin-conjugated protein aggregates and improved proteasome activity. A similar protective effect was obtained by blocking NCX(rev) activity. Inhibition of NKCC1 activity also preserved intracellular ATP and Na(+) homeostasis during 0-24 h REOX. In a positive control study, disruption of endoplasmic reticulum Ca(2+) with thapsigargin triggered redistribution of free ubiquitin and protein aggregation. We conclude that overstimulation of NKCC1 and NCX(rev) following OGD/REOX partially contributes to protein aggregation and proteasome dysfunction as a result of ionic dysregulation.