Transposons, stress and the functions of the deep genome.

The brain is responsible for both recognition and adaptation to stressful stimuli. Many molecular mechanisms have been implicated in this response including those governing neuronal plasticity, neurogenesis and, changes gene expression. Far less is known regarding effects of stress on the deep genome. In the hippocampus, stress appears to regulate expression of non-coding elements of the genome as well as the chromatin permissive for their transcription. Specifically, hippocampal retrotransposon (RT) elements are regulated by acute stress via the accumulation of the repressive H3K9me3 mark at RT loci. Further, corticosteroids appear to induce changes in heterochromatin status as well as RT expression in both adrenalectomized animal and rat cell culture models. Dysregulation of RT expression is predicted to result in functional deficits in affected brain areas. More broadly, however, transposons may have a variety of adaptive functions. As techniques improve to probe the deep genome, this approach to understanding stress neurobiology has the potential to yield insights into environment and genome interactions that may contribute to the physiology underlying a number of stress-related mental health disorders.