A new mouse model
by Holly Korschun


The way to a man’s heart may be through his stomach, if the old adage is true, but the way to everyone’s stomach may be through a particular protein concentrated in the brain. Emory geneticist Xiao-Jiang Li has discovered that the protein Hap1 is a critical cog in the brain machinery that regulates eating behavior and appetite.

     Li first discovered Hap1 (huntingtin-associated protein) in 1995 while studying Huntington’s disease as a postdoctoral fellow at Johns Hopkins. Huntington’s is a progressive neurologic disease caused by a mutation in the huntingtin gene. Under normal conditions, the huntingtin protein interacts congenially with the Hap1 protein, but in Huntington’s disease, this interaction turns toxic and helps fuel the degenerative effects of Huntington’s. Li’s discovery was published in the journal Nature, and today he and his Emory colleagues are leading the field in new findings about the roles of this important protein.
     In addition to the protein’s part in Huntington’s disease, Li has found that Hap1 is critically important during embryonic development and also has important links to eating behavior and appetite control. Li and other scientists have discovered that Hap1 conducts its business by transporting and trafficking with other molecules and proteins within cells.
     After first identifying Hap1, Li wanted to find out more about its specific role in the brain apart from Huntington’s as well as what would happen to animals lacking the protein. He used a transgenic mouse model to knock out the Hap1 gene and discovered a striking phenotype: the mice without Hap1 did not eat after birth. They survived only two or three days and died of starvation.

     Hap1’s connection to this eating phenomenon in newborn mice made sense when Li discovered that Hap1 was concentrated in the hypothalamus—the area of the brain that serves as the central switching control for neural signals that regulate food intake, feeding behavior, and energy balance. Without Hap1, important neurons in the hypothalamus degenerate and die of apoptosis, a cell self-destruction program that activates when cells fail to receive proper growth signals.
     When Li examined the mouse model of Huntington’s disease, he found that neurons in the hypothalamus were dying of apoptosis. The mutated huntingtin protein was having a toxic effect on Hap1. This finding fit with the physical manifestations of Huntington’s, which in its late stages often causes patients to lose their appetites and experience extreme weight loss.
     To find out more about Hap1 and the neural pathways involved in appetite and eating, Li needed to study adult mice. However, the knockout mice lacking the Hap1 protein died almost immediately and fell far short of adulthood. Along with scientists from other universities, Li began a series of experiments using a technique that allowed the reduction of gene expression in adult animals by selectively interfering with RNA. RNA is the molecule that translates genetic instructions from DNA into proteins, and this technique is known as sRNA interference (selective RNA interference).
     With the reduced Hap1 expression, the mice grew to adults, enabling the scientists to expand their research. Li was now able to manipulate the level of Hap1 and explore its interaction with other elements of the eating pathway. What he found was that adult mice with reduced Hap1 expression ate less food and lost body weight—just as the newborn mice had done when they were born without Hap1. When the adult mice were forced to fast, however, the level of Hap1 increased in the hypothalamus.

     Next, Li wanted to know how Hap1 interacts with the neural circuitry already known to be involved with eating behavior, but not well understood. Hormones such as insulin and leptin, for instance, circulate in the blood plasma and regulate the brain’s feeding signals to reduce appetite. Neurotransmitters in the hypothalamus such as GABA (gamma aminubutyric acid) are an important part of the pathway that regulates feeding behavior.
     Using their selective interference with Hap1 expression, Li and his colleagues discovered that by reducing Hap1, they could decrease the level and activity of GABA receptors. They also found that by administering insulin, they could reduce the level of Hap1. In the process, they discovered that Hap1 is important for transporting growth factor receptors to the cell surface—a necessary step for brain cells to differentiate and become mature neurons.
     “All this work led us to several conclusions,” says Li. “First, it told us that increasing Hap1 leads to increased feeding behavior. Also, it helped explain how Hap1 is linked to feeding-related molecules such as insulin and to the function of neurons in the hypothalamus. GABA is known to have a stimulating effect on feeding behavior. Because insulin decreases Hap1 levels and reducing Hap1 decreases GABA receptor activity, we think Hap1 is the link between circulating insulin and the GABA receptors in the hypothalamus that regulate eating behavior.”
     Scientists already know that diabetes and obesity are related to abnormalities of hypothalamic function, Li says. However, this complicated pathway is not fully understood. That’s where Hap1 can help. Recently, Li developed a “conditional” knockout mouse, in which Hap1 gene expression can be turned off in adult mice and specifically in the hypothalamus. He will be able to isolate neurons, culture them in the laboratory, and manipulate them to find out how they respond to growth factors or drugs. By changing Hap1, either through deletions or over-expressions of the protein’s genes, he can regulate eating behavior in the mice. This work, he hopes, eventually will lead to treatments for eating disorders and obesity.
     “We now know that Hap1 has multiple functions,” says Li. One is in the development of the brain itself. Another is for regulating neuronal function in the hypothalamus, which is involved in eating behavior. We hope that our model can help us provide a good target to find drugs to regulate Hap1.”

Holly Korschun is director of Research Communications. Illustration by Penny Carter.



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