Under a microscope, it can be difficult to tell the difference between any two neurons, the brain cells that store and process information. The mammalian brain contains diverse neuronal types, yet the global medical community lacks single-cell epigenomic assays to identify and characterize them. Now, a study led by Salk Institute develops a method that profiles chemical modifications of DNA molecules in individual neurons, allowing them to identify previously unknown neuronal subtypes. The team states their new technique is a critical step in identifying how many types of neurons exist, which could lead to a dramatically better understanding of brain development and dysfunction The study is published in the journal Science.
Previous studies show that levels of RNA molecules inside individual brain cells were used to identify what sets different types of neurons apart from each other. However, levels of RNA can rapidly change when a cell is exposed to new conditions throughout the day. Therefore, the team turned instead to the cells’ methylomes, which are generally stable throughout adulthood. The current study shows each cell’s methylome, the pattern of chemical markers made up of methyl groups that serve as a marker on the cell’s DNA, gave a distinct readout which allowed them to sort neurons into previously unknown subtypes.
The current study isolates 3,377 neurons from the frontal cortex of mice and 2,784 neurons from the frontal cortex of a 25-year-old human. The lab then used a new method they recently developed called snmC-seq to sequence the methylomes of each cell. Results show neurons from the mouse frontal cortex clustered into 16 subtypes based on methylation patterns, while neurons from the human frontal cortex were more diverse and formed 21 subtypes.
Data findings show inhibitory neurons, which provide stop signals for messages in the brain, exhibited more conserved methylation patterns between mice and humans compared to excitatory neurons. The group states their study also identified unique human neuron subtypes never before defined, opening the door to a deeper understanding of what sets human brains apart from other animals.
The team surmises they have developed a technique that clearly defines neuronal types based on their methylomes, enabling them to identify previously unknown neuronal subtypes. They go on to add this opens up the possibility of understanding what makes two neurons, which sit in the same brain region and otherwise look similar, behave differently. For the future, the researchers state they now plan to expand their methylome study to look at more parts of the brain, as well as different species of brains.
Source: Salk Institute
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