Molecular map of neuron maturation in the adult brain

Although it has been known for almost six decades that the hippocampus of the adult brain of many mammals generates new neurons throughout life, the genes involved in neuronal maturation, the process that allows the functions required for the functioning of brain circuits, were unknown. Now, scientists have revealed the molecular mechanisms involved in each stage, knowledge that could prove decisive for possible interventions against neurodegenerative diseases or brain injuries.

The team that made these discoveries is made up of specialists from the National Scientific and Technical Research Council (CONICET) in Argentina and Harvard University in Boston, United States.

“We identified and created a detailed map of the transcription factors (genes that regulate the expression of other genes) that are turned on and off during the maturation process of a stem cell into a mature neuron in the dentate gyrus of the hippocampus, which is one of the regions where new neurons are generated in the adult brain,” explains Damiana Giacomini, one of the lead authors of the work and a CONICET researcher at the Institute of Biochemical Research of Buenos Aires (IIBBA, of CONICET and the Leloir Institute Foundation) and at the Neuronal Plasticity Laboratory of the Leloir Institute Foundation (FIL).

“This provides a new scenario because the hippocampus is one of the areas affected by neurodegenerative diseases such as Alzheimer’s. Therefore, understanding what happens in normal physiological situations will allow us to think about how to prevent or minimize pathological situations in the long term,” adds the scientist.

Currently, various laboratories around the world are researching neuronal reprogramming, an experimental technique aimed at changing the identity of a cell to convert it into a neuron as a way of “repairing” or “replacing” the functioning of other cells that are altered. “To do this, for example, it is necessary to know which transcription factors or genes give not only the identity to a neuron but also the distinctive characteristics of each stage of development, which is what we are contributing with this work,” explains Giacomini.

For many years, the Neuronal Plasticity Laboratory at the FIL, led by CONICET researcher Alejandro Schinder, has been studying the development of neurons that are born in the adult brain, which has allowed them to describe, among other things, how a new neuron connects to the circuit of an already formed brain. “Our laboratory established that a new neuron in the adult brain needs eight weeks to complete its development and integrate. However, we did not know which genes were responsible for the changes that occur during this process,” says Natalí Rasetto, a CONICET doctoral fellow at the IIBBA and also co-author of the new study. “So we set out to study the sets of genes that guide the development of neurons in the hippocampus of an adult brain,” she adds.

All cells in an organism possess the same genes. However, for each cell to fulfil a specific role, a set of particular genes must be activated (expressed). In this project, the researchers studied the genes of almost forty thousand neurons of different ages in mice. To do so, they used a precise birth dating strategy, which allowed them to follow the development of newly generated neurons by means of single-nucleus RNA sequencing (sn-RNAseq), a novel technique that allows the elucidation of the expression of each of the genes throughout the life of each individual neuron.

The scientists determined that, depending on the genes expressed over eight weeks, the new neurons pass through four main states: quiescent neural stem cells, proliferative cells, post-mitotic immature granule cells, and mature granule cells.

“We used neurons of different ages, from week 1 to week 8, which we labeled with fluorescent proteins. Knowing the ‘age’, and using RNA sequencing, we were able to evaluate the transcriptome (the genes that are expressed) of particular neurons and compare it with that of other neurons over time to order them according to their similarity and, thus, obtain a trajectory of neuronal maturation,” Giacomini described.

Microscopy image of the dentate gyrus of the hippocampus. The colors show the expression of genes in some of the different cellular states described in the study. (Photo: FIL / CONICET. CC BY 2.5 AR)

The work was directed by Alejandro Schinder and also included the participation of Ariel Berardino, Maximiliano Beckel and Ariel Chernomoretz, from the IIBBA and the Laboratory of Integrative Systems Biology of the FIL, and Paola Arlotta and her team, from the Department of Stem Cells and Regenerative Biology of Harvard University.

“Unlike previous studies, which focused on the early stages of neuronal development in adult animals, this work describes for the first time the complete genetic development program of a neuron born in the adult brain,” explains Rasetto. He concludes: “This work constitutes a genetic atlas with an unprecedented temporal resolution, which establishes the molecular bases that will allow us to study the process of adult neurogenesis in different animal species, including humans.”

The study is titled “Transcriptional dynamics orchestrating the development and integration of neurons born in the adult hippocampus.” It has been published in the academic journal Science Advances. (Source: FIL / CONICET. CC BY 2.5 AR)