Recent studies suggest that new brain cells are being formed every day in response to injury, physical exercise, and mental stimulation. Glial cells, and in particular the ones called oligodendrocyte progenitors, are highly responsive to external signals and injuries.
They can detect changes in the nervous system and form new myelin, which wraps around nerves and provides metabolic support and accurate transmission of electrical signals. As we age, however, less myelin is formed in response to external signals, and this progressive decline has been linked to the age-related cognitive and motor deficits detected in older people in the general population.
Impaired myelin formation also has been reported in older individuals with neurodegenerative diseases such as Multiple Sclerosis or Alzheimer’s and identified as one of the causes of their progressive clinical deterioration.
A new study from the Neuroscience Initiative team at the Advanced Science Research Center at The Graduate Center, CUNY (CUNY ASRC) has identified a molecule called ten-eleven-translocation 1 (TET1) as a necessary component of myelin repair. The research, published today in Nature Communications, shows that TET1 modifies the DNA in specific glial cells in adult brains so they can form new myelin in response to injury.
“We designed experiments to identify molecules that could affect brain rejuvenation,” said Sarah Moyon, Ph.D., a research assistant professor with the CUNY ASRC Neuroscience Initiative and the study’s lead author.
“We found that TET1 levels progressively decline in older mice, and with that, DNA can no longer be properly modified to guarantee the formation of functional myelin.”
Combining whole-genome sequencing bioinformatics, the authors showed that the DNA modifications induced by TET1 in young adult mice were essential to promote a healthy dialogue among cells in the central nervous system and for guaranteeing proper function.
The authors also demonstrated that young adult mice with a genetic modification of TET1 in the myelin-forming glial cells were not capable of producing functional myelin, and therefore behaved like older mice.
“This newly identified age-related decline in TET1 may account for the inability of older individuals to form new myelin,” said Patrizia Casaccia, founding director of the CUNY ASRC Neuroscience Initiative, a professor of Biology and Biochemistry at The Graduate Center, CUNY, and the study’s primary investigator.
“I believe that studying the effect of aging in glial cells in normal conditions and in individuals with neurodegenerative diseases will ultimately help us design better therapeutic strategies to slow the progression of devastating diseases like multiple sclerosis and Alzheimer’s.”
The discovery also could have important implications for molecular rejuvenation of aging brains in healthy individuals, said the researchers. Future studies aimed at increasing TET1 levels in older mice are underway to define whether the molecule could rescue new myelin formation and favor proper neuro-glial communication.
The research team’s long-term goal is to promote recovery of cognitive and motor functions in older people and in patients with neurodegenerative diseases.
Myelination by oligodendrocytes (OLs) enables saltatory conduction of action potentials and provides long-term trophic support for axons, maintaining integrity throughout the central nervous system (CNS) 1. The formation of mature myelinating OLs is a complex process that is tightly coordinated spatially and temporally by genetic and epigenetic events 2, 3. Epigenetic regulation by DNA methylation, histone modification, and chromatin remodeling is critical for multiple aspects of OL development, function, and regeneration 4-6.
For instance, proper maintenance of genomic 5-methyl cytosine (5mC) is essential for normal development, homeostasis, and function of mammalian cells 7, 8. Genetic ablation of Dnmt1, which encodes the DNA methyltransferase that maintains DNA methylation after replication, results in impaired OL precursor cell (OPC) expansion and differentiation during early development 9.
The modified nucleotide 5-hydroxymethylcytosine (5hmC) has been shown to be an intermediate product generated during cytosine demethylation 10, 11. DNA demethylation, like methylation, is a highly regulated process. DNA demethylation is mediated by the Ten-Eleven Translocation (TET) family of dioxygenases.
The TET enzymes oxidize 5mC into 5hmC to initiate the DNA demethylation process 11, 12. Dynamic regulation of cytosine methylation or demethylation has been established as common epigenetic modification regulating various processes from development to diseases in a cell-type and context-dependent manner 13-15. TET enzymes are present in OL lineage cells 16, and here we interrogated how DNA demethylation contributes to OL lineage development, myelination, and remyelination after injury.
In this study, we demonstrate that there is a genome-wide shift in 5hmC landscape during OL specification and identify an age-dependent function of TET1 in OL lineage development and homeostasis. The mice with Tet1 deletion in OL lineage develop schizophrenia-like behaviors. In addition, we show that TET1-regulated epigenetic program is required for efficient remyelination as depletion of Tet1 in OPCs impairs myelin recovery after demyelinating injury in adult animals.
Moreover, Tet1 depletion resulted in genome-wide alterations in 5hmC and transcriptomic profiles that are associated with OPC differentiation and myelination, as well as calcium transport. Ablation of Itpr2, one of the TET1-5hmC targets that responsible for calcium release from endoplasmic reticulum in the OL lineage significantly impairs oligodendrocyte differentiation. These data suggest that TET1 and DNA hydroxymethylation mediated transcriptional and epigenetic programming regulate oligodendrocyte homeostasis and are required for proper myelination and animal behaviors.
reference link : https://www.biorxiv.org/content/10.1101/821496v2.full
More information: Nature Communications (2021). DOI: 10.1038/s41467-021-23735-3