Genes discovered linking circadian clock with eating schedule

For most people, the urge to eat a meal or snack comes at a few, predictable times during the waking part of the day. But for those with a rare syndrome, hunger comes at unwanted hours, interrupts sleep and causes overeating.

Now, Salk scientists have discovered a pair of genes that normally keeps eating schedules in sync with daily sleep rhythms, and, when mutated, may play a role in so-called night eating syndrome. In mice with mutations in one of the genes, eating patterns are shifted, leading to unusual mealtimes and weight gain. The results were published in Cell Reports today.

"We really never expected that we would be able to decouple the sleep-wake cycle and the eating cycle, especially with a simple mutation," says senior study author Satchidananda Panda, an associate professor in Salk’s Regulatory Biology Laboratory. "It opens up a whole lot of future questions about how these cycles are regulated."

More than a decade ago, researchers discovered that individuals with an inherited sleep disorder often carry a particular mutation in a protein called PER2. The mutation is in an area of the protein that can be phosphorylated—the ability to bond with a phosphate chemical that changes the protein’s function. Humans have three PER, or period, genes, all thought to play a role in the daily circadian clock and all containing the same phosphorylation spot.

The Salk scientists joined forces with a Chinese team led by Ying Xu of Nanjing University to test whether mutations in the equivalent area of PER1 would have the same effect as those in PER2 that caused the sleep disorder. So they bred mice to lack the mouse period genes, and added in a human PER1 or PER2 with a mutation in the phosphorylation site. As expected, mice with a mutated PER2 had sleep defects, dozing off earlier than usual. The same wasn’t true for PER1 mutations though.

"In the mice without PER1, there was no obvious defect in their sleep-wake cycles," says Panda. "Instead, when we looked at their metabolism, we suddenly saw drastic changes."

Mice with the PER1 phosphorylation defects ate earlier than other mice—causing them to wake up and snack before their sleep cycle was over—and ate more food throughout their normal waking period. When the researchers looked at the molecular details of the PER1 protein, they found that the mutated PER1 led to lower protein levels during the sleeping period, higher levels during the waking period, and a faster degradation of protein whenever it was produced by cells.

Panda and his colleagues hypothesize that normally, PER1 and PER2 are kept synchronized since they have identical phosphorylation sites—they are turned on and off at the same times, keeping sleep and eating cycles aligned. But a mutation in one of the genes could break this link, and cause off-cycle eating or sleeping.

"For a long time, people discounted night eating syndrome as not real," says Panda. "These results in mice suggest that it could actually be a genetic basis for the syndrome." The researchers haven’t yet tested, however, whether any humans with night eating syndrome have mutations in PER1.

When Panda and Xu’s team restricted access to food, providing it only at the mice’s normal meal times, they found that even with a genetic mutation in PER1, mice could maintain a normal weight. Over a 10-week follow-up, these mice—with a PER1 mutation but timed access to food—showed no differences to control animals. This tells the researchers that the weight gain caused by PER1 is entirely caused by meal mistiming, not other metabolic defects.

Next, they hope to study exactly how PER1 controls appetite and eating behavior—whether its molecular actions work through the liver, fat cells, brain or other organs.

Blood marrow derived cells regulate appetite

Bone marrow cells that produce brain-derived eurotrophic factor (BDNF), known to affect regulation of food intake, travel to part of the hypothalamus in the brain where they “fine-tune” appetite, said researchers from Baylor College of Medicine and Shiga University of Medical Science in Otsu, Shiga, Japan, in a report that appears online in the journal Nature Communications.

"We knew that blood cells produced BDNF," said Dr. Lawrence Chan, professor of molecular and cellular biology and professor and chief of the division of diabetes, endocrinology & metabolism in the department of medicine and director of the federally funded Diabetes Research Center, all at BCM. The factor is produced in the brain and in nerve cells as well. "We didn’t know why it was produced in blood cells."

Fluorescent marker reveals surprise

Dr. Hiroshi Urabe and Dr. Hideto Kojima, current and former postdoctoral fellows in Chan’s laboratory respectively, looked for BDNF in the brains of mice who had not been fed for about 24 hours. The bone marrow-derived cells had been marked with a fluorescent protein that showed up on microscopy. To their surprise, they found cells producing BDNF in a part of the brain’s hypothalamus called the paraventricular nucleus.

"We knew that in embryonic development, some blood cells do go to the brain and become microglial cells," said Chan. (Microglial cells form part of the supporting structure of the central nervous system. They are characterized by a nucleus from which "branches" expand in all directions.) "This is the first time we have shown that this happens in adulthood. Blood cells can go to one part of the brain and become physically changed to become microglial-like cells."

However, these bone marrow cells produce a bone marrow-specific variant of BDNF, one that is different from that produced by the regular microglial cells already in the hypothalamus.

Only a few of these blood-derived cells actually reach the hypothalamus, said Chan.

"It’s not very impressive if you look casually under the microscope," he said. However, a careful scrutiny showed that the branching nature of these cells allow them to come into contact with a whole host of brain cells.

"Their effects are amplified," said Chan.

Curbing the urge

Mice that are born lacking the ability to produce blood cells that make BDNF overeat, become obese and develop insulin resistance (a lack of response to insulin that affects the ability to metabolize glucose). A bone marrow transplant that restores the gene for making the cells that produce BDNF can normalize appetite, said Chan. However, a transplant of bone marrow that does not contain this gene does not reverse overeating, obesity or insulin resistance.

When normal bone marrow cells that produce BDNF are injected into the third ventricle (a fluid-filled cavity in the brain) of mice that lack BDNF, they no longer have the urge to overeat, said Chan.

All in all, the studies represent a new mechanism by which these bone-marrow derived cells control feeding through BDNF and could provide a new avenue to attack obesity, said Chan.

He and his colleagues hypothesize that the bone marrow cells that produce BDNF fine tune the appetite response, although a host of different appetite-controlling hormones produced by the regular nerve cells in the hypothalamus do the lion’s share of the work.

"Bone marrow cells are so accessible," said Chan. “If these cells play a regulatory role, we could draw some blood, modify something in it or add something that binds to blood cells and give it back. We may even be able to deliver medication that goes to the brain," crossing the blood-brain barrier. Even a few of these cells can have an effect because their geometry means that they have contact with many different neurons or nerve cells.