In neuroscience, epigenetic regulation of neurogenesis refers to the role of epigenetic mechanisms in controlling the production of neurons from neural stem cells during brain development and adulthood. Processes such as DNA methylation, histone modification, and microRNA expression regulate neural stem-cell proliferation, differentiation, maturation, and integration into neural circuits. Dysregulation of these mechanisms has been associated with neurological and psychiatric disorders, including Alzheimer's disease, Parkinson's disease, Huntington's disease, and Bipolar disorder.
Mechanisms
The three primary mechanisms of epigenetic regulation are histone modifications, DNA methylation, and microRNA (miRNA) expression.
Histones Histones keep the DNA of the eukaryotic cell tightly packaged with charge interactions (like molecular Velcro) between the positive charge of the histone tail and the negative charge of the DNA and between the histone tails of nearby nucleosomes. Although there are many types of histone modification, in neural epigenetics there are two primary mechanisms: histone methylation and histone acetylation.
Histone methylation Histone methylation occurs when methyl groups are added or removed from the histone, altering its structure to expose chromatin and leading to gene activation (or deactivation). Trimethylation of histone H3 at lysine 4 (H3K4me3) is associated with gene promoters in neurons, and trimethylation at lysine 27 (H3K27me3) represses genes that maintain stem-cell identity. A 2020 study indicated that polycomb repressive complex 2 (PRC2), which deposits H3K27me3, silences genes involved in differentiation such as Neurog2 to preserve neural stem-cell pools in the developing brain.
Histone acetylation Histone acetylation is the addition of acetyl groups to histones by histone acetyltransferases (HATs). Acetylation neutralizes the positive charge of histones, loosening their grip on DNA and allowing transcription factors to access genes. This process is regulated by histone deacetylases (HDACs). HDACs remove acetyl groups from histones, condensing chromatin to silence genes. A 2020 study indicated that HDAC3 maintains adult hippocampal neural stem cells by balancing quiescence and activation; by deleting HDAC3, neurogenesis is disrupted and memory formation is impaired.
DNA methylation/demethylation DNA methylation is the addition of methyl groups to cytosine or adenosine residues on DNA, forming 5-methylcytosine (typically at CpG sites); demethylation is the removal of these methyl groups. Both are crucial epigenetic processes that influence gene expression. DNA methylation, a more lasting method of gene inactivation than histone modification, is sometimes reversible. Active demethylation is mediated by Ten-Eleven Translocation (TET) enzymes, which oxidize 5-Methylcytosine to promote DNA repair. A 2024 study indicated that TET3 oxidizes methylated DNA in adult neural stem cells, activating Prox1 to drive hippocampal neurogenesis. DNA methylation by DNMT3A in adult neurons is critical for their survival and integration into memory circuits, as shown in a 2020 Nature Neuroscience study.
MicroRNAs (miRNAs) MicroRNAs are small forms of non-coding RNA (ncRNA) that often act as "fine-tuning" mechanisms for gene expression by repressing or degrading messenger RNA (mRNA) in neural cells. A 2019 study indicated that miR-9 coordinates the switch from neural progenitor proliferation to differentiation by repressing stem-cell genes like Hes1 and Tlx. MiRNAs can also act with transcription factors to guide neurogenesis. A paper from 2020 revealed that miR-124, the most abundant brain miRNA, promotes adult neurogenesis by targeting mRNAs like PTBP1 and SCP1 to enhance neuronal maturation and synaptic integration.
Embryonic neurogenesis
Histone modifications Embryonic neurogenesis is the process in which neurons are generated during the development of an embryo. In cortical development, neural stem cells follow a precise "inside-out" sequence. Early in development, these stem cells generate the first neurons that settle in the deeper cortical layers. As development progresses, later neurons migrate past the earlier ones to form the upper layers of the cortex. This mechanism, observable in vitro and in vivo, ensures that the cortex is organized; older neurons establish foundational deep layers, and newer neurons refine the upper layers.
Histone methylation directs cortical layering Mutant analysis indicated that disruption of the PRC2 complex, which includes the histone methyltransferase Ezh2, reduces the number of upper-layer neurons (those expressing markers such as POU3F2/BRN2 and SATB2) by half, leaving deeper layers unaffected. One study demonstrated that PRC2-mediated H3K27me3 silences stem-cell genes such as Sox11, enabling progenitors to transition to upper-layer neurons.
Experiments using mouse embryonic stem cell-derived neural progenitors showed that increased histone deacetylase, induced by the HDAC inhibitor valproic acid, promoted neuronal differentiation and selectively enriched upper-layer neurons. It is hypothesized that HDAC inhibition may drive a switch from progenitors which produce deep-layer neurons to those that generate upper-layer neurons. The mechanisms behind this selective differentiation and the timing control associated with HDAC inhibition are not fully understood.
DNA methylation DNA methylation plays a crucial role in corticogenesis, as demonstrated by experiments in mice. Ablation of DNMT3b and DNMT1 in embryos is fatal due to neural tube defects, highlighting their importance during early development. Deletion of DNMT3a does not result in embryonic death but severely impairs postnatal neurogenesis, indicating that it is not essential for embryonic survival but is vital for the proper formation of the cerebral cortex after birth.
Timing of DNMT activity DNMT3b is expressed primarily in early neural progenitors, and its expression declines as development proceeds. DNMT3a is barely detectable until embryonic day 10 (E10), after which its levels surge around E13.5 and remain elevated in adult neurogenic regions such as the subventricular zone (SVZ) and the hippocampal dentate gyrus.
DNMT3a regulates neuron-glia fate switching DNMT3a plays an important role in regulating neuron-glia fate switching. In postnatal neural progenitors, loss of DNMT3a leads to the downregulation of neuronal genes such as Dlx2, Neurog2 and Sp8, and upregulates genes associated with astrocytes and oligodendrocytes. A study published in 2021 confirmed that DNMT3a is essential for maintaining neuronal identity by methylating the promoters of gliogenic genes, including SOX9.
DNA methylation in key genes DNA methylation plays a pivotal role in regulating key genes during neurogenesis. HES5 is hypermethylated at embryonic day 7.5 (E7.5), but becomes demethylated by E9.5 through the activity of TET enzymes rather than GCM1 or GCM2. A 2019 study indicated that TET2 oxidizes 5-Methylcytosine at HES5, which enables notch signaling to drive the expansion of neural stem cells. The GFAP gene is repressed during early neurogenesis due to methylation of its STAT3 binding sites. Research published in 2001 demonstrated that DNMT3a maintains GFAP methylation until E14.5, delaying the differentiation of astrocytes.
miRNAs Mechanisms
Conditional knockout of Dicer (an enzyme essential for miRNA synthesis) in the mouse neocortex reduces cortical size, increases neuronal apoptosis, and disrupts cortical layering. Neuroepithelial and progenitor cells remain unaffected until embryonic day 14 (E14), after which they undergo apoptosis. A 2013 study confirmed that this stage-specific requirement for miRNAs ensures proper cortical development, although the miRNAs involved are unclear.
Key miRNAs in neurogenesis MiRNAs play essential roles in neurogenesis by finely tuning gene expression to control neuron differentiation and migration. MiR-124, the most abundant CNS miRNA, promotes the transition of subventricular zone progenitors into neuroblasts by suppressing SOX9. A 2009 study indicated that miR-124 silences SOX9 via a conserved binding site in its 3'UTR, enabling progression along the neuronal lineage. Another example is miR-9, which regulates neuronal differentiation and self-renewal. A 2016 study found that ectopic expression of miR-9 in the mouse cortex prematurely activates NEUROD1 (a pro-neuronal gene) and disrupts neuronal migration by targeting FOXG1.
Beyond fine-tuning Contrary to the idea that miRNAs fine-tune gene expression, miR-9 and miR-124 can reprogram human fibroblasts into neurons without the need for transcription factors like NeuroD1. A 2018 study demonstrated that these miRNAs can remodel chromatin accessibility at key neuronal loci such as MAP2 and SYN1, enabling the conversion of fibroblasts to neurons (although this method is less efficient than approaches based on transcription factors).
These epigenetic mechanisms are not confined to embryonic development. In the adult brain, dynamic DNA methylation, histone modifications, and miRNAs continue to orchestrate neurogenesis in niches such as the hippocampal dentate gyrus and subventricular zone. DNMT3a-dependent methylation and of miR-124 activity play a crucial role in maintaining the balance between neuronal stem-cell quiescence and activation. This regulation is essential for lifelong neural plasticity and cognitive function.
Adult neurogenesis
Adult DNA methylation
Neurogenesis that persists through adulthood is known as adult neurogenesis. An important animal gene involved in the epigenetic regulation of adult neurogenesis is the growth arrest and DNA-damage-inducible, beta (GADD45B) gene, often studied in rodents. Through the demethylation of promoters, GADD45B activates genes such as brain-derived neurotrophic factor (Bdnf) and basic fibroblast growth factor (FGF-2 or bFGF); both are essential for neural progenitor-cell development. There is increased expression of Bdnf and FGF2 with upregulation of GADD45B, resulting in more neural progenitor cells in adulthood.
Histone acetylation in neural stem-cell regulation
DNA is packaged into chromatin, a structure made up of nucleosome subunits consisting of four histone proteins: H2A, H2B, H3 and H4. Histone acetylation and deacetylation play a role in the proliferation and renewal of post-embryonic neural stem cells. H4 histones retain their basic charge in the absence of acetylation, allowing them to interact with the acidic pockets of H2A-H2B dimers in neighboring nucleosomes. This interaction reinforces nucleosome-nucleosome association, resulting in more tightly-packed chromatin. Acetylation neutralizes the basic charge of H4 histones, disrupting these interactions and preventing chromatin compaction. This process allows greater expression of target genes, such as brain-derived neurotrophic factor, which are involved in adult neurogenesis. Acetylation is made possible by histone acetyltransferases (HATs), which add acetyl groups to histones and promote gene expression by loosening chromatin structure. Histone deacetylases (HDACs) remove acetyl groups, leading to chromatin condensation and gene repression. Histone acetylation plays a key role in the differentiation of neural stem cells into specific cell types. A notable example is the chromatin regulator BRPF1, which is expressed in the developing central nervous system. It is crucial for the formation of brain regions such as the neocortex and dentate gyrus of the hippocampus. BRPF1 functions by activating histone acetyltransferases such as MOZ, MORF, and HBO1.
Role of HDAC Inhibitors in Promoting Neurogenesis