The cycles of light and dark that make up the typical day turn off and on the genes that control an organism’s internal clock. These genes affect our brains, our sleep patterns and even how we metabolize food into energy.
In a previous study, Dr. Kjersti Aagaard, associate professor of obstetrics and gynecology at Baylor College of Medicine, and her colleagues identified one of these circadian genes Npas2. When it is altered, there are persistent alterations in metabolism in the livers of primate fetuses, resulting in a condition called nonalcoholic fatty liver, resulting in insulin resistance and metabolic syndrome - both conditions related to type 2 diabetes and obesity.
"Circadian rhythms are important for linking appetite and metabolism," said Aagaard. "We know, for example, that night shift workers have aberrant metabolism. What about a fetus? There is no light and dark in the womb, yet babies are born with circadian rhythms. How are these established in absence of light and dark cycles?"
To answer that question, Derek O’Neil, a graduate student in the Translational Biology and Molecular Medicine program working in Aagaard’s laboratory, used mice engineered with a disruption in the DNA binding domain of the Npas2 gene to study the effects at two time points - day 2 after birth and week 25, when they are adults. At day 2, infant mice have not yet opened their eyes and, thus, receive no light and dark signals. A report on the work appears online in the journal Molecular Genetics and Metabolism.
When they looked at the levels of gene expression in the livers of infant mice, they found a significant difference between those who lacked theNpas2 gene and normal mice.
"In fact, we found more than 3,000 genes that were differentially regulated," said Aagaard. "We asked, ‘What is specifically going on? What pathways are disrupted? What kinds of genes are affected?’"
"We discovered that the major pathway affected was the lipid (fat) metabolism pathway," said O’Neil.
"It is interesting that even in the absence of light and dark, fetal disruption of this circadian gene can totally disrupt a large percentage of the gene repertoire," said Aagaard.
"Once we had established this change at day 2, we wanted to know what effect it has on adult life," said O¹Neil. "What we found was that in terms of its whole body, the mouse trends toward obesity. However, if you restrict its feeding to the light cycle, which mirrors humans’ night time, they fail to adapt to restricted eating. They waste and die." (Because mice of nocturnal animals that are active in the dark, feeding them in the light is equivalent to feeding humans in the night.)
"They cannot figure out they are hungry," said Aagard. "Their satiety signal is disrupted, and their lipid metabolism pathways are out of whack."
"Our hypothesis is that there is a compensatory mechanism for Npas2, a gene called Clock, another circadian gene. At day 2, Clock is downregulated but it’s normal by adulthood," said O’Neil. In fact, he said, there are compensatory mechanisms for most but not all genes that are disrupted when Npas2 is knocked out.
One in particular is Ppargc1-alpha (or PGC) a gene that controls the energy potential in a cell. It remains disrupted into adulthood, he said. The gene is upregulated in infancy and adulthood in mice that lack Npas2.
The disruption of Npas2, a key regulator of energy metabolism, in early life removes a safeguard against metabolic insult, said O¹Neil.
"We think it orchestrates a lot of the responses and sets up early circadian pathways that are related to satiety. While some could be compensated for, at least some never return to normal," said Aagaard. "We find this interesting given that a high fat maternal diet disrupts the same gene. Those kids never return to normal."
"Could this serve as a clinical target?" she said.
Others who took part in this work include Hector Mendez-Figueroa; Toni-Ann Mistretta and Chunliu Su, all of BCM, and Dr. Robert H. Lane of the University of Utah in Salt Lake. Aagaard, Mendez-Figueroa and Su are also with Texas Children’s Hospital.
Funding for this work came from the National Institutes of Health Director New Innovator Award to Aagaard (DP2120OD001500-01), the NIH Reproductive Scientist Development Program (5K12HD00849), NIH R01 (R01DK079194) and the National Institute of General Medical Sciences (T32 GM088129-01).