Global ischemia induces immediate-early genes encoding zinc finger transcription factors

Abstract

Ischemia induces immediate-early genes (IEGs) in brain. Since prolonged expression of some lEGs may precede neuronal death, some researchers have suggested that these lEGs mediate neuronal death. We therefore examined the effect of 5 and 10 min of global ischemia on the expression of the IEGs NGFI- A, NGFI-B, NGFI-C, egr-2, egr-3, and Nurr1 in gerbil brain. All of the IEGs were induced after 30 min of reperfusion in the hippocampus. Most of them were induced in several other regions as well, including cortex, hypothalamus, thalamus, and amygdala. The acute IEG induction decreased in most brain areas by 2-6 h. However, at 24 h following 5 min of ischemia NGFI- A continued to be expressed in the CAI region and dentate gyrus. In the dentate gyrus, NGFI-C continued to be expressed for 24 h and egr-3 for as long as 72 h. In other brain areas, all of the lEGs returned to control levels by 72 h except in CA1, where most messenger RNA (mRNA) levels were decreased; this decrease correlated with marked neuronal loss. The persistent expression of NGFI-A in CA1 neurons destined to die and the persistent expression of NGFI-A, NGFI-C, and egr-3 genes in dentate granule cell neurons that survive may indicate that some transcription factors modulate cell death whereas others support cell survival when expressed for prolonged periods. The protein products of several transcription factors, including c-fos, are known to downregulate their own expression. The persistent expression of NGFI-A in the CA1 neurons destined to die could therefore be due to ischemia- induced transcriptional activation caused by, e.g., increased intracellular calcium levels plus a lack of negative feedback caused by the blockade of the translation of NGFI-A mRNA into protein.