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by Mary Silva
Green highlights show expression of the Gsx2 gene in early cells of the basal ganglia, a portion of the brain responsible for movement and learning.
Researchers at Cincinnati Children’s have revealed that the developing brain of an embryo and the brain of an adult have much in common.
Developmental neurobiologists Kenneth Campbell, PhD, and Masato Nakafuku, MD, PhD, in the Division of Developmental Biology, discovered this while studying neurogenesis — the development of new neurons — at different stages of life. Campbell is focused on embryonic brain development; Nakafuku studies the adult brain as it ages.
The researchers have joined forces to find clues in early brain development that could help repair and restore function as the brain ages. Their findings could be helpful in understanding and even treating developmental diseases such as autism and ADHD, neurodegenerative diseases like Alzheimer’s and Huntington’s, and acute brain injury like stroke and trauma.
A number of these disorders are believed to stem from altered function of the basal ganglia, a group of nuclei located deep in the brain beneath the cerebral cortex. Basal ganglia control movement and play a role in how we learn and how we form habits, both good and bad.
Campbell and his team focus on how the fate of neurons is determined as well as how neural circuitry develops in the basal ganglia. “If you believe that alterations in the establishment of the circuitry could underlie neuropsychiatric disorders, then you have multiple points in the process where changes could occur,” he says.
With a five-year, $3.3 million renewal of an NIH grant now entering its 12th year of funding, Campbell and co-principal investigator Brian Gebelein, PhD, a molecular biologist in the Division of Developmental Biology, are studying the roles of Gsx1 and Gsx2, two genes expressed only in neural progenitor cells Neural progenitors typically give rise to both types of brain cells, neurons and glia. But progenitors that express the Gsx factors appear to favor the creation of neurons rather than glial cells.
Campbell studies how Gsx1 and 2 control neurogenesis in basal ganglia progenitors. He believes that the Gsx proteins play different but complementary roles in this process.
“Gsx2 appears to keep neural progenitors in an undifferentiated state but ‘poised’ to become neuronal cells. On the other hand, Gsx1 seems to promote progenitor maturation toward neuronal differentiation,” Campbell says But questions remain about what prompts the genes to do what they do. “We’ve spent years using genetics and phenotyping to describe their roles, but that has left a hole in our understanding about how these factors work on the molecular level.”
Gebelein’s research helps fill that molecular gap He has particular expertise in studying cis-regulatory elements, sites within the DNA that control gene expression. He and Campbell have already identified several of the cis-regulatory elements for Gsx1 and 2, and hope to learn more about the signaling pathways and transcription factors that control their expression.
Using mouse and fly models, they have explored an activity called phosphorylation in the Gsx proteins. Phosphorylation causes changes in a protein’s shape and activity, altering its behavior — for example, whether it enters the cell nucleus or remains in the cytoplasm. Campbell and Gebelein are examining mitogen-activated protein (MAP) kinase phosphorylation as well as the interaction of other transcription factors to understand how they influence Gsx proteins in neurogenesis.
Research grants that combine the expertise of multiple principal investigators on research studies is a new direction in funding for the NIH Research Project Grant and a boon to discovery in basic science, Gebelein says.
“Allowing researchers with different areas of expertise to tackle a developmental question is crucial to moving research forward,” Gebelein says. “In this instance, we have been able to demonstrate similar results in different genetic model systems, which lets us know more quickly that we are heading in the right direction.”
The right direction is important, because the Gsx protein is also found in the adult brain, where it plays a key role in repairing neuron damage. Developmental biologist Masato Nakafuku, MD, PhD, is a co-investigator with Campbell on another multiple-PI study exploring the role of Gsx1 and Gsx2 in the adult brain.
“Stem cells in embryos and adult brains are similar in that they proliferate and differentiate in the same way,” Nakafuku says. “But in embryos, every stem cell divides rapidly and creates new cells. In the adult, only a very small proportion of them do that. Most sit there and do nothing. Why is that?”
At about age 11 or 12, Nakafuku says, human brains slow their active regeneration of new brain cells to a trickle. Our brains retain a pool of stem cells that keep producing a small number of new neurons and glia for “plastic” activities in the brain — activities such as learning and remembering new things. But the vast majority of stem cells are kept as a reservoir presumably for repair purposes, much like other animals. When there is injury to the brain, the Gsx2 gene activates and attempts to recruit those stem cells for repair, Nakafuku says. But nothing much happens.
“In adults, you have the same set of genes at work, but at a time when it is critical to get more cells, the genes counteract each other. When the stem cell needs to decide whether to go back to quiescence or to make more cells, Gsx2 keeps it in check and the cells go back (to quiescence).”
Nakafuku proposes that this is likely the result of a built-in regulatory mechanism that controls how many new brain cells an adult brain can generate. Making too many new brain cells could cause problems with brain function, he says. But he would like to know how to call a reasonable supply of cells into action when there is significant injury to the brain.
“We need to understand how to manipulate the process,” he says. “There must be a reason for this stopping mechanism. But we need a broader view of other genes and how they interact with the Gsx factors so that we can draw on these cells when damage occurs.”
Images above, second and third from left: Drs. Gebelein and Nakafuku; Drs. Campbell and Gebelein with postdoctoral fellow Kaushik Roychoudhury.
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