Endoplasmic reticulum signaling in neuron adaptation to hypoxia: role of unfolded protein response genes

Abstract

The endoplasmic reticulum (ER) plays a key role in neuron response to hypoxia by coupling cytosolic NADH changes to ER Ca2+ release, survival kinase phosphorylation and gene expression. We propose that the ER also senses and responds to mild hypoxia via a subset of unfolded protein response (UPR) genes, which normally senses misfolded proteins. In rat hippocampal slice cultures (postnatal days 6–9), the UPR translation regulator eif2alpha is downregulated by phosphorylation after just 15 min of hypoxia, before unfolded proteins accumulate. Bcl‐2 protein, a pro‐survival factor associated with the ER and the UPR, is increased within 30 min of hypoxia. We used quantitative PCR to examine the expression of genes associated with the UPR: Slice cultures were exposed to 30 min hypoxia and total mRNA was collected 1 and 6 hrs later. Relative to control (GAPDH), we found no significant changes in Eifak3/PERK, Ern1(IRE1), Ddit3 (Gadd153), Xbp1, Hsp1a (BiP), Hsp90b1, or Hsp1a mRNA. However our previous gene profiling studies found significant up‐regulation of several Hsp's at 24 hr post‐hypoxia. To further clarify the role of the ER in hypoxia response, we are assaying for cytoplasmic splicing of the Xbp1 mRNA and using whole genome microarray studies. This experimental approach will confirm whether ER stress adaptation is important in neuronal adaptation to hypoxia. Support NIH GM52212.