Under the microscope, it can be difficult to distinguish between two neurons, the brain cells that store and process information. So scientists have turned to molecular methods to try to identify groups of neurons with different functions.
Scientists from the Salk Institute and the University of California, San Diego have described for the first time the chemical modifications of DNA molecules in individual neurons , giving the most detailed information about what makes a brain cell different from its neighbor. This is a critical step in starting to identify how many types of neurons there are , which has always been a hidden mystery to neuroscientists, and which could lead to a better understanding of brain development and dysfunction. Each cell’s methylome – a pattern of chemical markers – gave a different reading, which helped Salk’s team classify neurons into subtypes.
“We think it is quite surprising that we can separate a brain into individual cells, sequence its methylomas and identify many new types of cells along with their gene regulatory elements, the genetic switches that make these neurons different from each other,” according to one of the study’s lead authors, Joseph Ecker, professor and director of the Salk Genomic Analysis Laboratory and a Howard Hughes Medical Institute investigator.
So far, to identify what sets different types of neurons apart from each other, the researchers studied the levels of RNA molecules within individual brain cells. But RNA levels can change rapidly when a cell is exposed to new conditions, or even within the same day. So Salk’s team focused on cell methylomas, which are generally stable throughout adulthood.
“Our research shows that we can clearly define neuronal types based on their methylomes, ” says Margarita Behrens of the Salk Institute and other lead authors of the new paper. “The finding opens the possibility of understanding what makes two neurons – sitting in the same brain region and otherwise similar – behave differently.”
The team began their work on both the mouse and the human brain, focusing on the frontal cortex, the area of the brain responsible for complex thinking, personality, social behaviors, and decision-making , among other things. 3,377 neurons were isolated from the frontal cortex of mice and 2,784 neurons from the frontal cortex of a deceased 25-year-old human.
The researchers then used a new method they recently developed called snmC-seq to sequence the methylomas in each cell . Unlike other cells in the body, neurons have two types of methylation.
Neurons in the mouse frontal cortex were found to be grouped into 16 subtypes based on methylation patterns, while neurons in the human frontal cortex were more diverse and formed 21 subtypes. The study also identified unique subtypes of human neurons that had never been defined before. These results open the door to a deeper understanding of what distinguishes human brains from those of other animals.
