Before birth, the intricate architecture of the human brain is meticulously constructed through an astonishing cascade of cellular decisions. At the very heart of this complex process are radial glia, a specialized class of neural stem cells that play a pivotal role in shaping many of the unique characteristics that distinguish the human brain from those of other species. These remarkable cells are responsible for generating the vast majority of neurons and supporting cells that constitute the cerebral cortex, the brain’s sophisticated command center for cognition, memory, and language. Furthermore, radial glia are strongly implicated in the significant expansion of the human cortex, a feature that sets our species apart. While the vast majority of these crucial cells vanish before birth, a disquieting echo of their existence can be observed later in life, appearing in brain cancers for reasons that continue to elude scientific understanding.
"Radial glia are the coolest cells that have ever existed," remarked Aparna Bhaduri, an assistant professor of biological chemistry at the David Geffen School of Medicine at UCLA. "They’re really key to making us human. But they’re also at the center of many neurodevelopmental and neuropsychiatric disorders, as well as cancer — so understanding how they make their decisions is one way to start understanding how those conditions arise."
Two groundbreaking studies, published concurrently in the prestigious journals Cell and Science, are now shedding unprecedented light on the intricate mechanisms by which radial glia navigate these critical developmental choices. The research, spearheaded by Bhaduri and her colleagues, reveals that these versatile cells respond to a dual stream of information: the way they metabolize nutrients and direct physical signals originating from distinct regions of the developing brain. Collectively, these findings offer profound new insights into the processes that enable the human cortex to produce its extraordinary diversity of cell types.
Metabolism: A Guiding Force for Brain Stem Cells
The study published in Cell represents a monumental effort to construct a comprehensive atlas of metabolism within the developing human cortex. This ambitious project was a synergistic collaboration between Bhaduri’s laboratory and that of Heather Christofk, with Jessenya Mil and Jose Soto sharing the role of co-first authors.
To achieve this detailed mapping, the research team meticulously analyzed donated human brain tissue alongside brain organoids—miniature, self-organizing three-dimensional brain structures grown from stem cells in the laboratory. The results yielded a striking and unexpected conclusion: metabolism is not merely a passive supporting player in brain development. Instead, it actively influences and directs the types of cells that are ultimately produced.
The researchers observed that radial glia exhibit a pronounced dependence on the pentose phosphate pathway, a fundamental metabolic process that utilizes glucose to synthesize essential molecular building blocks required by cells that are undergoing rapid division. This pathway is crucial for providing the necessary precursors for DNA synthesis and for generating reducing equivalents like NADPH, which are vital for cellular growth and protection against oxidative stress.
When the scientists experimentally reduced the availability of glucose or interfered with the function of this critical metabolic pathway, the behavior of the stem cells demonstrably shifted. They began to generate a greater proportion of inhibitory neurons and other cell types that are typically observed later in the developmental timeline. This suggests a direct regulatory role for metabolic state in determining the fate of these progenitor cells.
"What was surprising is that metabolism isn’t just a passive thing that happens in the background," stated Bhaduri, who is also an integral member of the UCLA Broad Stem Cell Research Center and the UCLA Health Jonsson Comprehensive Cancer Center. "It can really control how stem cells make decisions."
The implications of these findings are far-reaching. They offer a potential avenue for scientists to investigate how factors such as maternal nutrition, metabolic disorders during pregnancy, and other environmental influences can impact the developing human brain. Furthermore, the metabolic atlas generated by this research provides an invaluable and highly detailed resource for the broader scientific community studying the complexities of metabolism during human brain development. This could pave the way for understanding how metabolic dysregulation might contribute to neurodevelopmental disorders.
Early Signals from the Thalamus Shape Cortical Development
Complementing the metabolic insights, the second study, published in Science and led by first author Claudia Nguyen, explored an entirely different source of developmental information. This research focused on signals originating from the thalamus, a deep-seated brain structure that serves as a critical relay station for sensory and motor information throughout the nervous system.
For decades, neuroscientists have known that neurons within the thalamus extend long projections, known as axons, towards the developing cerebral cortex. These projections eventually establish synaptic connections with specific cortical neurons, forming intricate neural circuits. However, anatomical studies had revealed a puzzling observation: in humans, these thalamic projections reach the cortex significantly earlier in development than the final synaptic connections are established. This temporal discrepancy raised a fundamental question: why do these axons arrive so prematurely?
Employing human stem cell-derived brain "assembloids"—sophisticated laboratory models that recapitulate key aspects of early human brain development—the UCLA researchers have uncovered a significant part of the answer. Their findings indicate that these nascent thalamic projections physically interact with radial glia during the crucial period of brain development.
This direct physical contact between thalamic axons and radial glia was found to profoundly alter the behavior of the neural stem cells. Specifically, it stimulated them to produce a greater number of excitatory neurons, which are the primary neurons responsible for transmitting signals within the cortex. This effect was particularly pronounced for upper-layer neurons, a population that is notably expanded in the human brain, contributing to its increased processing power.
"We already knew that these projections influence how the cortex develops," Bhaduri explained. "What we specifically found is that this influence comes through an actual physical connection between the projections and the radial glia—a point of contact that just hasn’t been identified before, and one that very likely does not exist in rodents." This suggests a species-specific mechanism contributing to the unique development of the human cortex.
A Link to Autism: The Role of NRXN1
Intriguingly, the researchers were able to connect this physical interaction between thalamic projections and radial glia to the function of NRXN1, a gene already recognized for its critical role in facilitating the formation of neuronal connections. Mutations within the NRXN1 gene have been previously associated with autism spectrum disorder (ASD), a neurodevelopmental condition characterized by challenges in social interaction and communication.
To further investigate the role of NRXN1 in this developmental process, the team engineered assembloids derived from patient cells carrying a known NRXN1 mutation. In these experimental models, the altered thalamic signals, influenced by the genetic mutation, exhibited different behaviors compared to signals originating from unaffected cells.
These observed changes in signal transmission led to a disruption in the delicate balance between the number of radial glia and the neurons they generate. This finding offers a compelling new avenue for researchers to study how early disruptions in brain development, potentially stemming from genetic predispositions like NRXN1 mutations, can influence the intricate formation of the cerebral cortex and contribute to neurodevelopmental disorders.
The Developing Brain: A Symphony of Constant Communication
While the two studies delved into distinct yet complementary mechanisms—one focusing on cellular metabolism and the other on physical neural connections—they converge on a unifying principle: radial glia do not make their critical developmental decisions in isolation. Their behavior is dynamically and continuously shaped by the complex interplay of signals emanating from their surrounding environment.
These studies also underscore the transformative impact of organoid technology on the scientific investigation of human brain development. Just a decade ago, direct experimental approaches to study the behavior of uniquely human neural stem cells were severely limited. Today, the advent of brain organoids and related in vitro models allows researchers to meticulously recreate and study key features of human brain development in a controlled laboratory setting. Crucially, these advanced models enable scientists to probe questions that are intractable using animal models alone, offering a more direct window into human-specific developmental processes.
Bhaduri expresses hope that these findings will encourage the scientific community to reframe their understanding of metabolism and physical cellular interactions, viewing them not as passive background processes but as active, driving forces in development.
"Ultimately, these studies give us a glimpse under the hood of how these cells make decisions," she concluded. "Understanding those decisions is a first step toward understanding normal brain development, disease vulnerability and, potentially, how similar stem-cell programs operate in brain cancer." The research was generously supported by numerous prestigious institutions, including the National Institutes of Health, the National Science Foundation, the Brain & Behavior Research Foundation, the Alfred P. Sloan Foundation, the Rose Hills Foundation, the Esther A. & Joseph Klingenstein Fund, the Simons Foundation, the Chan Zuckerberg Initiative, the NIH BRAIN Initiative Cell Atlas Network, the International Foundation for Ethical Research, the UCLA Broad Stem Cell Research Center’s Stem Cell Research Training Program, and the UCLA Health Jonsson Comprehensive Cancer Center and UCLA Broad Stem Cell Research Center Ablon Scholars Program.
