Researchers from the Okinawa Institute of Science and Technology Graduate University (OIST) have identified a protein that plays a key role in how the brain regulates appetite and metabolism. Their studies in rodents demonstrated that loss of the protein, XRN1, from the forebrain, leads to extreme appetite and obesity in mice. Mice without XRN1 were resistant to the appetite-suppressing hormone, leptin, and to insulin, and demonstrated increased activity of a type of neuron in the hypothalamus that releases the appetite stimulant, AgRP.
The team said their results could have implications for future obesity treatment strategies. “Identifying which neurons and proteins in the brain are involved in regulating appetite, and fully determining how resistance to leptin is caused, could eventually lead to a targeted treatment for obesity,” said Akiko Yanagiya, PhD, a researcher in the Cell Signal Unit at OIST, headed by Tadashi Yamamoto, PhD.
The researchers reported their findings in iScience, in a paper titled, “Neuronal XRN1 is required for maintenance of whole-body metabolic homeostasis.”
Obesity is a growing public health concern, with over 650 million adults worldwide designated as obese. The condition is linked with disorders including cardiovascular disease, type 2 diabetes, and cancer. “Fundamentally, obesity is caused by an imbalance between food intake and energy expenditure,” said Yanagiya, “But we still understand very little about how appetite or metabolism is regulated by communication between the brain and parts of the body, such as the pancreas, liver, and adipose tissues.”
When a gene is active, DNA is transcribed into a molecule of mRNA, which can then be used to build a specific protein. Cells have many ways of regulating the activity of genes, one of which is by degrading mRNA more slowly or more quickly, which results in either more or less protein, respectively, being produced. XRN1 plays a crucial role in gene activity, as it is involved in the last step of degrading mRNA.
For their reported study, the scientists created mice that were unable to produce XRN1 in a subset of neurons in the forebrain (Xrn1-cKO mice). This brain region includes the hypothalamus, an almond-sized structure that releases hormones into the body, helping to regulate body temperature, sleep, thirst, and hunger. The hypothalamus integrates inputs from various peripheral tissues and regulates feeding and energy expenditure, the authors pointed out. “Therefore, various studies have sought to identify hypothalamic nuclei and neuropeptides involved in eating and regulation of energy metabolism.”
The scientists noticed that at six weeks old, the Xrn1-cKO mice that lacked forebrain XRN1 rapidly began to gain weight and they became obese by 12 weeks of age, with fat accumulating in the animals’ adipose tissue and liver. And when the team monitored feeding behaviour in the mice, they noticed that those lacking forebrain XRN1 ate almost twice as much each day as the control mice.
“This finding was really surprising,” said Shohei Takaoka, PhD, a former student from the OIST Cell Signal Unit. “When we first knocked out XRN1 in the brain, we didn’t know exactly what we would find, but this drastic increase in appetite was very unexpected.”
The video shows two mice, one of which is normal weight. The other mouse is obese, due to a loss of XRN1 in the forebrain. These mice were part of a recent iScience study, with the video appearing in the associated press release, “Key protein linked to appetite and obesity in mice.” [OIST]
To investigate what might be causing the mice to overeat, the scientists measured the blood levels of leptin—a hormone that suppresses hunger. Compared to the controls, the level of leptin in the blood of the knockout mice was abnormally high, which would normally stop the mice from feeling hungry. “Serum leptin was already significantly increased in 5-week-old Xrn1-cKO mice compared to control mice, suggesting that alteration of peripheral metabolic signals had already developed before the onset of obesity,” the team noted. But unlike the control mice, the mice without XRN1 didn’t respond to the presence of leptin—a condition known as leptin resistance.
The scientists also found that 5-week-old mice were resistant to insulin, which is released by beta cells in the pancreas in response to the high levels of blood glucose that occur after eating. This type of failure in how the body responds to glucose and insulin can ultimately lead to diabetes. And as Xrn1-cKO got older, levels of glucose and insulin in their blood rose significantly, alongside the increased leptin levels.
“Moreover, glucose tolerance was impaired in 5-week-old Xrn1-cKO mice compared to control mice, indicating that glucose metabolism is dysregulated in Xrn1-cKO mice,” the investigators confirmed. “Furthermore, 5-week-old Xrn1-cKO mice exhibited insulin resistance, suggesting that insulin sensitivity is impaired in Xrn1-cKO mice. Weight gain, food intake, and metabolic dysregulation such as impaired glucose tolerance and insulin resistance developed as early as 5 weeks old in Xrn1-cKO mice, before the onset of increased body weight.”
“We think that the levels of glucose and insulin rose due to the lack of response to leptin,” explained Yanagiya. “Leptin resistance meant that the mice kept eating, keeping the level of glucose in the blood high, and therefore increasing insulin in the blood.”
The team in addition checked whether the obesity was also driven by the mice using less energy. They placed each mouse in a special cage that measured how much oxygen the mice used, which allowed them to indirectly work out the animal’s metabolic rate. They found no overall difference in energy expenditure between the 6-week-old control and XRN1-deficient mice. However, the animals lacking XRN1 in the forebrain were mainly using carbohydrates as an energy source, while the control mice were able to switch between burning carbohydrates at night, when they were most active, and fat during the day, when they were less active. “Xrn1-cKO mice do not exhibit the diurnal switch from carbohydrate to fat usage …” they wrote. “For some reason, this means that without XRN1, the mice cannot use fat as a fuel effectively,” said Yanagiya. “Why this occurs though, we still don’t know.”
At 12 weeks of age the XRN1-deficient animals did start to exhibit comparatively decreased energy expenditure but, the scientists believed, this was an effect of obesity, due to the mice being less active, rather than a cause. “Overall, we think overeating due to leptin resistance was the driving cause behind why these mice became obese,” noted Yanagiya.
To further investigate how loss of XRN1 results in leptin resistance and an increased appetite, the scientists then looked for any change in activity of appetite-regulating genes in the hypothalamus. They found that the mRNA used to make the protein Agouti-related peptide (AgRP)—one of the most potent stimulators of appetite—was elevated in the obese mice, leading to higher amounts of AgRP protein. “Because leptin acts primarily via AgRP neurons to regulate body weight and food intake, it is feasible that leptin resistance in Xrn1-cKO mice might originate from activated AgRP neuron,” they suggested.
The exact mechanism by which loss of XRN1 leads to increased activation of AgRP neurons remains unclear. Even so, Yanagiya noted, “It’s still only speculation, but we think that an increase of this protein, and abnormal activation of the neuron that produces it, might be the cause of leptin resistance in these mice. Leptin normally suppresses activity of the AgRP neuron, but if loss of XRN1 results in this neuron remaining highly active, it could override the leptin signal.”
XRN1 was removed only in a specific subset of neurons in the forebrain, and these don’t include AgRP neurons. This suggests that another neuron that did lose XRN1 may be involved and could be signaling incorrectly to the AgRP neurons and keeping them active. Moving forward, the lab hopes to collaborate with neuroscience research units, in order to pinpoint exactly how XRN1 impacts the activity of neurons in the hypothalamus to regulate appetite. And as the authors concluded, “understanding the molecular mechanisms of obesity and energy homeostasis regulated by XRN1 may enable development of novel therapeutic strategies for metabolic disorders such as obesity and diabetes.”
