Epigenetic Regulation Of Axon Regeneration and Glial Activation in Injury Responses

Abstract

Injury to the nervous system triggers a multicellular response in which epigenetic mechanisms play an important role in regulating cell type-specific transcriptional changes. Here, we summarize recent progress in characterizing neuronal intrinsic and extrinsic chromatin reconfigurations and epigenetic changes triggered by axonal injury that shape neuroplasticity and glial functions. We specifically discuss regeneration-associated transcriptional modules comprised of transcription factors and epigenetic regulators that control axon growth competence. We also review epigenetic regulation of neuroinflammation and astroglial responses that impact neural repair. These advances provide a framework for developing epigenetic strategies to maximize adaptive alterations while minimizing maladaptive stress responses in order to enhance axon regeneration and achieve functional recovery after injury.

Keywords: CNS injury; axon regeneration; chromatin accessibility; epigenetic regulation; neural repair; neuroepigenetics; neuroinflammation; spinal cord injury.

Figure 1

Schematic diagram of epigenetic mechanisms. (A) Cytosine methylation and demethylation process. DNMT, DNA methyltransferase; Tet, Ten-eleven translocation methylcytosine dioxygenase; BER, base excision repair enzymes; MBD, Methyl-CpG-binding domain (MBD) proteins. (B) Histone modifications. HAT, Histone acetyltransferases; HDAC, histone deacetylases; KMT, lysine methyltransferases; KDM, lysine demethylases; PRC, polycomb repressor complex. (C) Micro RNAs are embedded in a multi-protein complex termed RNA-induced silencing complex (RISC) and they repress gene expression by complementary binding to 3’ untranslated region (3’ UTR). (D) mRNA modifications include N⁶-methyladenosine (m⁶A), controlled by methyltransferase complex comprised of Mettl3, Mettl14 to install m⁶A, and demethylases Fto and Alkbh5 to remove m⁶A. m⁶A-binding proteins such as YTHDF regulates RNA metabolism.

Figure 2

Transcription modules in regulating axon growth potential of DRG neurons after peripheral axotomy. (A) Schematic depiction of transcriptional module comprised of Smad1 and p300 to enhance histone acetylation in conditioned DRG at target loci (Atf3, Sprr1a, Galanin and Npy). Promoter occupancy of Smad1 helps recruitment of p300 and displacement of HDAC1. Peripheral axotomy also triggers nuclear exit of HDAC5 in response to retropropagation of calcium wave, leading to induction of RAGs (Jun, Fos, and Klf). (B) Transcriptional module comprised of p53 and p300/PCAF regulates H3K9ac and H3K18ac at target RAGs (Galanin, Bdnf, Gap43 and Coronin1b) in conditioned DRG. Acetylation of p53 is also increased by HATs in cortical neurons. Retrograde signaling of pERK results in threonine phosphorylation and nuclear localization of PCAF. In conditioned DRG, H3K9me2 is decreased regulated by a yet unknown KDM. (C) Epigenetic factors in mediating DNA hydroxy- and demethylation in conditioned DRG. Peripheral axotomy triggers Tet3 upregulation, which may be dependent on Calcium wave. Tet3 catalyzes conversion of 5mC to 5hmC, while TDG mediates conversion back to C, resulting in RAG induction of Atf3, Smad1, Stat3, and C-Myc. Folate may influence DNA methylation through DNMT3a/3b. Tet3 may partner with HIF, STAT, and IRF for 5hmC reconfigurations. Hypoxia stabilizes HIF complex to induce target genes, e.g. VEGFA, NFGR and HMOX1.

Figure 3

Histone acetylation in regulating glial response after CNS injury. Top, depiction of timelines of activation of different immune cells and astroglia at the injury site after CNS injury. Bottom, in microglia/macrophages, HDAC inhibition reduces inflammation by enhancing anti-inflammatory/pro-repair phenotype and by inducing apoptosis through p53 and caspase. Different micro-RNAs also regulate inflammatory phenotypes of microglia/macrophage. HDAC inhibition by CI-994 suppresses neutrophil accumulation and reduces inflammatory cytokine expression. BETs are epigenetic readers of acetylated histones and promote transcription of inflammatory genes. BET inhibitor JQ1 reduced inflammatory cytokine expression and leukocyte recruitment to the injury site. In astrocytes, transcriptional module consisting of STAT3, p300 and Smad1 induces GFAP expression. HDAC inhibition reduces secretion of inflammatory cytokines, and increases neurotrophic cytokines from reactive astrocytes.