For a long time, it was a truism in science that the human brain is born with all the neurons it will ever have. But platitudes can crumble, and this one seems unstable. The brain struggles to produce new neurons in people with depression A tantalizing body of evidence suggests that the human brain produces new nerve cells throughout a person’s life. If that’s true, then our brains might have a lasting ability to repair and regenerate. That concept sounds very attractive to neuroscientist Arturo Álvarez-Buylla of the University of California, San Francisco, who has spent years researching this topic. “If someone shows it clearly, I’ll be the first to be super happy and say, well, I didn’t waste my life studying a mechanism that’s not present in humans.” The problem is, when you look at the data, you just don’t see it. The question of whether the human brain can generate nerve cells after childhood has been controversial for decades. Over the years, influential articles have appeared on both sides of the argument, and the concept has captured public interest far beyond academia. Part of the reason for the debate’s prominence is that it involves the giants of the field, says Hongjun Song, a neuroscientist at the University of Pennsylvania in Philadelphia. “People who have positive and negative data are the best researchers,” he says, “and they are all doing good science.” Some researchers think that the evidence for neurogenesis is strong enough to focus on understanding what these neurons do and, ultimately, whether this process can be used to treat diseases that involve neurodegeneration. But at the same time, doubts remain about the methods used to detect neurogenesis, which today are mainly based on genetic markers associated with it and only provide indirect evidence of its existence. Gerd Kempermann, a neuroscientist at the German Center for Neurodegenerative Diseases in Dresden, says that despite continued debate, this field is now moving into what he calls a consolidation phase. “Yes, there are some loopholes to close,” he says. “But at the same time, there is an explosion of opportunity.” Proving the axiom incorrect Santiago Ramón y Cajal, considered one of the founding fathers of modern neuroscience, wrote in the 1910s that, in the adult brains of mammals, “everything can die, nothing can regenerate.” For many years, this was a central dogma in this field. The first to question this was neuroscientist Joseph Altman, then at the Massachusetts Institute of Technology (MIT) in Cambridge, who found signs of adult neurogenesis in the brains of rats and cats1 in the 1960s. Some think that the discovery cost him his job. “The fact is that he didn’t really get a permanent position at MIT as a result of this,” says Rusty Gage, a neuroscientist at the Salk Institute for Biological Studies in La Jolla, California. “People found it difficult to believe that new cells could be born in the adult nervous system.” But in the 1980s, neurogenesis was discovered in adult birds2, where it was thought to play a role in song learning. Tests in many other animal species followed, and the idea became more widely accepted. New neurons (red) are produced in the hippocampus of middle-aged mice. Credit: Michael Fatt A typical method of detecting newly generated neurons in animals is to inject them with a chemical called bromodeoxyuridine (BrdU). Because of its structure, BrdU is incorporated into DNA when a cell divides, and researchers can then know which neurons are newly generated by using a marker that recognizes the molecule. In the 1990s, Gage and his colleagues wondered whether this process occurred in humans. They were specifically interested in the hippocampus, a brain region involved in memory and learning, because neurogenesis had been observed there in other adult animals. At that time, BrdU was sometimes used as a diagnostic tool in people with cancer to check how quickly their tumors were dividing. The researchers theorized that if they investigated the brains of these individuals shortly after their death, they could see if neurogenesis had occurred in the time since BrdU was administered. The brains of the “super-aged” are strong producers of new neurons. “So we were able to do it,” Gage says. The researchers were able to see cells that had incorporated BrdU, meaning they had originated from dividing cells. Some were non-neuronal cells, but the team identified others as neurons. “We knew this was important,” Gage says. So the group brought in scientists from other labs to look at the samples and see if they agreed with the team’s interpretation. The researchers also sent tissue to collaborators in Sweden, who analyzed the samples and came to the same conclusion. The findings, based on brain tissue from five people, were published3 in Nature Medicine in 1998. The article was accompanied by a light-hearted editorial4 declaring that there was no longer any reason to lose sleep over the thought of “the inevitable and gradual loss of our precious, predetermined 100 billion neural quota.” The case had apparently been resolved. Twenty years later, an article5 would cause confusion in this field. The Burden of Proof One of the authors of that controversial article was Shawn Sorrells, a neuroscientist now working at the University of Pittsburgh in Pennsylvania. In the 2010s, as a postdoc in Álvarez-Buylla’s lab, he was studying the amygdala, a brain region involved in processing emotions. Because Gage and other groups had established neurogenesis in the hippocampus, Sorrells’ project was to probe the amygdala for signs of proteins that are indicative of cell division and immature neurons, to verify whether these two stages of neurogenesis could also be found throughout life in that area. When a lab technician was sectioning a sample of human brain tissue, Sorrells asked him to take sections of the hippocampus as well. That way, the tissue could be used as a positive control; Given the findings of other groups, Sorrells thought that newly generated neurons would definitely be found there. He was confused when none appeared. The goal of Sorrells’ project focused on investigating why. She collaborated with another neuroscientist in her group, Mercedes Paredes, who now has her own laboratory at the University of California, San Francisco, and with researchers from laboratories in Spain and China. In total, they analyzed 59 brain samples from people of different ages and tested various methods to detect new neurons. Their findings suggested that the number of young neurons in the hippocampus drops sharply during the first year of a person’s life, to a small number later in childhood, and almost none appear in samples from people older than 13 years. “We did this independently in three different labs, because I really wanted to be convinced,” says Alvarez-Buylla. Many studies looking at the growth of new neurons in the human brain have included the hippocampus (cell nuclei shown in blue), a hotspot for neurogenesis in animals. Credit: Cell Applications Inc./Science Photo Library. All the checking and rechecking took time. “You start to realize that it’s not just you, it’s not just the antibodies, it’s not just your protocol, it’s not just that sample,” Sorrells says. “This took years and years of us working together to decide that it was clear enough to say something.” When the results were finally published in Nature in 2018, pushback was immediate, with some scientists saying the evidence was not strong enough to support the researchers’ conclusion. Kempermann believes the controversy sparked by the article sapped a lot of energy from the field. “It was really inconvenient, it’s killed careers and we know that people in the field had a hard time getting grants” after that, he says. In the years between the 1998 and 2018 papers, other influential work had supported the existence of neurogenesis in human adults, with the paper by Sorrells and colleagues being an outlier. “It was not clear to us why everyone was rushing to the negative finding.” The hype led researchers to investigate why some scientists were seeing these young neurons and others were not. The following year, María Llorens-Martín, a neuroscientist at the Autonomous University of Madrid, argued that the difference in findings could be due to how human brain tissue was preserved and processed. Using their own preservation methods and some of the same markers that Sorrells and his colleagues had used, his team found “abundant” neurogenesis in healthy people between 43 and 87 years of age6. Álvarez-Buylla says he tried his technique, but that didn’t change his conclusion. He says that although some cells show markers of immature neurons, they are not shaped like a young neuron and there is no convincing evidence that they are generated in the adult, rather than being cells that have not matured since childhood. Evolving Methods The latest studies in the field identify adult neurogenesis using gene expression techniques. The idea is that if scientists know which genes are expressed in cells at different stages of neuronal development – from neural stem cells to intermediate progenitor cells and neuroblasts (the precursors of neurons) to immature neurons – they will be able to look for them in adult human brain cells using single-nucleus RNA sequencing. Most studies use postmortem samples; If they find cells carrying these genetic signatures, it suggests that neurogenesis was ongoing at the time of death. When the technique was first used, it initially seemed to suggest that adult neurogenesis was not present in humans. The research led by scientists at Yale University in New Haven, Connecticut, used gene expression signatures that had been identified in newly generated mouse neurons and looked for the same signatures in brain tissue from several species. Although they detected cells with these signatures in pigs and monkeys, they were unable to find them in people. 7. Brain cell growth keeps mood disorders at bay. The problem, Song says, was that different species appear to use different genes to control the process of neurogenesis. To address this, he and his team used data from human childhood neurogenesis as a reference. His lab also pioneered an approach involving an artificial intelligence model to identify the molecular signature of young neurons in babies and search for them in adults. With this method, the team was able to find cells with the characteristics of immature neurons in people of different ages8. But critics argued that they could not say for sure whether the immature neurons had actually been produced in the adult brain, or whether they had existed in their immature state for decades. “There was a possibility that these immature cells were generated immediately after birth, or even before birth, and simply remained immature for a long period of time,” Song says. Last year, an article in Science addressed that gap. Using a similar technique involving single-nucleus sequencing and machine learning, stem cell researcher Jonas Frisén from the Karolinska Institute in Stockholm and his team found dividing cells (neuronal progenitors) in adult human brains9. “This is another step,” says Song. Now, it is clear that “in the human adult brain there are actually cells that can divide with the potential to give rise to neurons.” If they are there, what do they do?
