E3 ubiquitin ligase Siah-1 downregulates synaptophysin expression under high glucose and hypoxia

Background: Diabetes is proved to be one of the independent risk factors for cognitive dysfunction. The pathophysiologic changes caused by diabetes including hyperglycemia and tissue hypoxia may contribute greatly to cognitive decline. In the present study, we demonstrate E3 Ubiquitin Ligase Siah-1 downregulates the key synaptic protein Synaptophysin expression under high glucose and hypoxia condition which may be the underlying factor leading to cognitive dysfunction in diabetic patients. Methods: In this study, hypoxia (2% oxygen) and high glucose (50 mM) were used to treat primary neuronal culture. By using quantitative PCR and western blotting we determined the influence of hypoxia and high glucose on the expression of synaptophysin and Siah-1 and the phosphorylated forms of extracellular signal-regulated kinase (ERK). Knockdown of Siah-1, inhibitors for proteasome, lysosome and ERK kinase was employed to evaluate the role of Siah-1 and ERK activity on the expression of synaptophysin. By immunoprecipitation we also examined the role of Siah-1 in the ubiquitination of synaptophysin under hypoxic and hyperglycemic condition. Results: We demonstrated that hypoxia and high glucose together but not hypoxia or high glucose along mediated posttranscriptional reduction of synaptophysin with increased ERK phosphorylation and Siah-1 expression. The downregulation of synaptophysin was reversed by inhibition of ERK and Siah-1 knockdown. Overexpression of Siah-1 accelerated the degradation of synaptophysin under hypoxia and high glucose conditions and promoted the ubiquitination of synaptophysin. Conclusions: The present results demonstrate that Siah-1 is the key factor that contributes to hypoxia and high glucose mediated synaptophysin degradation.