Scientists fromUniversity of Utah Healthsaythat removing the gene encoding Lef1 in mice and zebrafish disrupts the development of nerve cells in the hypothalamus that affect stress and anxiety, causing the animals to exhibit increased anxiety. Their study ("Lef1-Dependent Hypothalamic Neurogenesis Inhibits Anxiety"), which appears in PLOS Biology, suggeststhat Lef1 functions in the hypothalamus to mediate behavior. The team believes this knowledge could prove useful for diagnosing and treating human brain disorders.
"...we demonstrate that the Wnt/-catenin effector Lef1 is required for the differentiation of anxiolytic hypothalamic neurons in zebrafish and mice, although the identity of Lef1-dependent genes and neurons differ between these 2 species. We further show that zebrafish andDrosophilahave common Lef1-dependent gene expression in their respective neuroendocrine organs, consistent with a conserved pathway that has diverged in the mouse," write the investigators.
"Finally, orthologs of Lef1-dependent genes from both zebrafish and mouse show highly correlated hypothalamic expression in marmosets and humans, suggesting co-regulation of 2 parallel anxiolytic pathways in primates. These findings demonstrate that during evolution, a transcription factor can act through multiple mechanisms to generate a common behavioral output, and that Lef1 regulates circuit development that is fundamentally important for mediating anxiety in a wide variety of animal species."
"Anxiety is an essential behavior that is much more complex than we thought," says first author Yuanyuan Xie, Ph.D., who led the research in collaboration with senior authorRichard Dorsky, Ph.D., professor of neurobiology and anatomy at University of Urah Health. "This work is making us think about how brain structures control behavior in a different way."
Anxiety happens in humans, mice, fish, and flies. It's not always a bad thing. Anxiety in zebrafish causes them to stop moving so they can hide in plain sight from predators. But being anxious at inappropriate times is counterproductive and can be a sign of unnecessary stress, a characterization that holds true not only for fish but also for people, say the researchers.
When Drs. Xie and Dorsky began their investigation, nothing was known about a role for Lef1 in anxiety. Brains of fish missing the gene were relatively normal except there were cells missing from the hypothalamus. "Before we did the experiments we had no idea that the neurons impacted by Lef1 would preferentially impact one type of behavior," says Dr. Dorsky.
Studying the genes that were most perturbed by loss of Lef1 in this brain region revealed that over 20 were involved in mood disorders like depression and anxiety. The scientists then noticed that the fish had telltale signs consistent with these disorders. The animals were reluctant to explore their environment when placed into a new tank, preferred to remain immobile at the bottom. And they grew slowly, another condition often related to elevated stress.
Lef1 appears to mediate anxiety across species, although it uses diverse mechanisms to do so.Mice in which Lef1 had been removed from the hypothalamus showed signs of anxiety, including being smaller and a reluctance to explore. They also had fewer brain cells in the region where Lef1 is normally present. However, the missing cells make pro-melanin-concentrating hormone (Pmch), a brain signal that was not perturbed in zebrafish. By contrast, zebrafish and Drosophila fruit flies lacking their versions of Lef1 are missing cells that make corticotropin-releasing hormone binding protein (Crhbp), and these cells were unaffected in mice.
These results suggested that Lef1 could regulate anxiety through two different nerve cell signals. Support for this scenario was unexpectedly found in humans, where expression of Crhbp and Pmch are extremely closely linked in the hypothalamus, indicating they may actually be present in the same cells and together act downstream of Lef1 to regulate behavior.
"When you think about genes with a conserved function you think everything that gene does must be the same in all animals. But our study shows that that isn't necessarily true," says Dr. Dorsky, who adds that the team's workcould explain how a gene that specifies a particular behavior can adapt to accommodate changes in brain circuitry that happen over evolutionary time. "Our results suggest that during evolution, the brain can innovate different ways to get to the same outcome."
The study reveals information about specific sets of genes and the brain cells they affect as being involved in regulating anxiety. Future work will focus on determining whether these pathways may define a subset of human behavioral and mood disorders.