John Svaren, PhD – Slide of the Week

John Svaren, PhD - Slide of the Week

Title: Injury-induced enhancers are important for induction of the Sonic Hedgehog gene in Schwann cells

Legend: SC enhancers of Shh. A, map of known Shh enhancers (color coded) that drive Shh expression in various embryo regions. Numbered enhancers are Shh brain enhancers (SBE). Green arrows depict the newly identified SC injury induced enhancers identified by H3K27ac and JUN binding (shown below). B, Generation of mouse enhancer deletions using CRISPR guides (red arrowheads). In injured Enh1/2/3 nerve (1dpi), Shh induction is virtually abolished, whereas Gdnf and Mbp are not significantly different than controls. n=5 mice each, *** p<0.001. C, The heat map shows binding of the JUN transcription factor preferentially to injury-induced enhancers, compared to enhancers that are decommissioned after nerve injury.

Citation: Ramesh, R., Manurung, Y., Ma, K. H., Blakely, T., Jr, Won, S., Moreno-Ramos, O. A., Wyatt, E., Awatramani, R., & Svaren, J. (2022). JUN Regulation of Injury-Induced Enhancers in Schwann Cells. The Journal of  Neuroscience, 42(34), 6506–6517.

Abstract: Schwann cells play a critical role after peripheral nerve injury by clearing myelin debris, forming axon-guiding bands of Büngner, and remyelinating regenerating axons. Schwann cells undergo epigenomic remodeling to differentiate into a repair state that expresses unique genes, some of which are not expressed at other stages of Schwann cell development. We previously identified a set of enhancers that are activated in Schwann cells after nerve injury, and we determined whether these enhancers are preprogrammed into the Schwann cell epigenome as poised enhancers before injury. Poised enhancers share many attributes of active enhancers, such as open chromatin, but are marked by repressive histone H3 lysine 27 (H3K27) trimethylation rather than H3K27 acetylation. We find that most injury-induced enhancers are not marked as poised enhancers before injury indicating that injury-induced enhancers are not preprogrammed in the Schwann cell epigenome. Injury-induced enhancers are enriched with AP-1 binding motifs, and the c-JUN subunit of AP-1 had been shown to be critical to drive the transcriptional response of Schwann cells after injury. Using in vivo chromatin immunoprecipitation sequencing analysis in rat, we find that c-JUN binds to a subset of injury-induced enhancers. To test the role of specific injury-induced enhancers, we focused on c-JUN-binding enhancers upstream of the Sonic hedgehog (Shh) gene, which is only upregulated in repair Schwann cells compared with other stages of Schwann cell development. We used targeted deletions in male/female mice to show that the enhancers are required for robust induction of the Shh gene after injury. SIGNIFICANCE STATEMENT The proregenerative actions of Schwann cells after nerve injury depends on profound reprogramming of the epigenome. The repair state is directed by injury-induced transcription factors, like JUN, which is uniquely required after nerve injury. In this study, we test whether the injury program is preprogrammed into the epigenome as poised enhancers and define which enhancers bind JUN. Finally, we test the roles of these enhancers by performing clustered regularly interspaced short palindromic repeat (CRISPR)-mediated deletion of JUN-bound injury enhancers in the Sonic hedgehog gene. Although many long-range enhancers drive expression of Sonic hedgehog at different developmental stages of specific tissues, these studies identify an entirely new set of enhancers that are required for Sonic hedgehog induction in Schwann cells after injury.

About the Lab: The Svaren laboratory is focused on the transcriptional and epigenetic regulation of myelination. Myelin is a vital constituent of the nervous system that increases the speed of action potentials and also provides trophic support for the long axons that project from neurons. Their studies are centered on the myelin-producing cells of the peripheral nervous system, called Schwann cells. The Svaren lab has focused on elucidating regulation of gene networks during Schwann cell development and response to injury. They have also recently found a novel role of the polycomb repressive complex 2, an epigenetic regulator, in controlling the regenerative responses of Schwann cells after peripheral nerve injury.

Investigator: John Svaren, PhD

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