At the RNA level: We have used microarray analysis of preconditioned brains and show that preconditioning alters the genomic response to stroke: a genomic reprogramming of the response to injury. The signature of tolerance at the transcriptional level is that of transcriptional suppression. Thus, the genomic response to stress (ischemic stress) is different in the setting of preconditioning. The preconditioned brain responds to the ischemic stimulus via a novel transcriptome the read out of which is neuroprotection rather than ischemic injury. Accordingly, tolerance is encoded at the genomic level. Tolerance “reprograms” the transcriptional response to an injurious stimulus, which would ordinarily produce injury, but the transcriptional reprogramming results in a novel transcriptome inducing neuroprotection / tolerance. Ischemic tolerance comprises 1) suppression of transcription; 2) requires synthesis of new proteins; 3) reduces expression of genes related to metabolism and transport; and 4) reduces activity of ion channels.
At the protein level: Ischemic tolerance is a global repressive response of transcription that confers endogenous neuroprotection in stroke. Using a new broad based proteomic analysis, we discovered that repressive epigenetic regulator proteins, namely the Polycomb group (PcG) proteins and histone proteins, are enriched in rodent brain during ischemic tolerance . Subsequent studies reveled that PcG proteins are essential for the induction of tolerance. Further, we now know the opposing changes of Trithorax group (TrxG) proteins, the gene activator and antagonist proteins of PcG proteins, in ischemic tolerance. Further the roles of PcG proteins in ischemic tolerance indicate PcG proteins as the epigenetic mediators of such repressive mechanism. Thus there is an epigenetic mechanism of ischemic tolerance – PcG/TrxG bivalent complex in control.
At the DNA level: In Drosophila, where the PcG and TrxG proteins act were originally identified as cell-cycle regulators, these proteins function as part of complex protein assemblies to alter gene expression by epigenetic means. Epigenetic mechanisms control gene expression by remodeling the architecture of chromatin in ways that allow or deny transcription factors access to genes. PcG proteins are epigenetic regulators that silence the expression of a wide range of genes. PcG proteins assemble into several different complexes that can modify histones in distinct ways and leads to transcription repression. Thus, the regulatory role of PcG proteins in the development of brain ischemic tolerance occurs through multi-target epigenetic regulations. Defining these epigenetic regulations is the labs current focus.
Roger Simon, M.D.
An Zhou, Ph.D.