The gut-based tango of microbial and host genes
December 5, 2016 § Leave a comment
When you think ‘oscillations in your gut,’ you might think of motion sickness or food poisoning. But there is another type of oscillations in both the gut and other organs. These oscillations are an interplay of genetic switches for protein expression that turn on and off throughout the day as we transition through eating, working, exercising and sleeping. In a paper published in Cell on Dec. 1, researchers in Israel have investigated the links between the day-and-night circadian rhythm in mice and the microbes that thrive in their gastrointestinal tracts. They’ve found that the daily fluctuations that occur between the two systems are more intimately linked than previously expected.
The systems close to one another but tightly coordinated, like a “tango between two partners,” says Eran Elinav at the Weizmann Institute of Science in Rehovot, Israel. Elinav and colleagues had previously linked shifts in our day-and-night circadian rhythm, such as jet lag, to disruptions of microbial gut communities in humans and mice that can lead to metabolic conditions, such as obesity and diabetes.
In their Cell paper, Elinav and colleagues, including Eran Segal from the same institute, homed in on the microbes that adhere to the epithelial cells of the gastrointestinal tract in mice with imaging and sequencing. They targeted the microbes’ entire genome. The targeting allowed the researchers to determine both the composition and function of the microbes.
They found that the bacteria residing in close proximity to the host lining epithelial cells displayed highly circadian behavior, with composition, function and microbial numbers differing at throughout a 24-hour cycle. Moreover, the thickness of the mucosal layer separating the gut bacteria from the mice’s epithelial layers fluctuated with the mice’s circadian rhythm and feeding patterns.
“Our findings also add to the increasing body of evidence that strongly suggests that disruption of proper circadian activity, such as that present in shift workers and frequent travelers, may drive metabolic derangements through a mechanism that is partly mediated by disruption of proper diurnal microbiome activity,” says Segal.
The investigators then wiped out these microbial communities with antibiotics to see how the mice’s transcriptome, the aggregate of genes being expressed via messenger RNA, adapted to the loss.
“There were a few hundred genes — the genes encoding the host clock itself — which did not care about the disruption to the microbiome,” says Elinav. “But there was another group of genes which normally oscillate in the host. Once we disrupted the gut microbes, these oscillations were completely lost.”
The investigators also noted that a subset of mouse genes that normally operate independently of the mice’s circadian oscillations began to follow the oscillations after the microbial communities were wiped out. These genes were picking up functions that had previously been performed by genes expressed by the microbiome. “This brings the option that this “superorganism” shifts the tasks from one partner to the other once it is disrupted,” says Elinav.
The researchers were most shocked when they checked up on the mice’s livers. Despite the liver’s relative distance from the gastrointestinal tract, about 15 to 20 percent of its genes displayed circadian activity. “Surprisingly, when we disrupted the gut microbiome, the genetic program in the liver was severely disrupted,” says Elinav.
The researchers found that the metabolites, small molecules that are extensively modified by the microbiome and make up 80 percent of all the small molecules in peripheral blood, also displayed strong circadian activity. These molecules allow the gut microbiome to regulate the circadian activity in the liver.
When this was brought into the context of drug metabolism, the researchers found that liver toxicity induced by administration of high doses of the painkiller acetaminophen also displayed circadian activity. Interestingly, the researchers found that disrupting the gut microbiome reduced the toxicity of acetaminophen and stabilized it throughout a 24-hour period.
Elinav and colleagues are currently planning to continue investigating the intimacy of the gut microbiota and the effects of its diurnal activity in humans in order to elucidate potential systemic effects of antibiotics. They want to develop rational and safe intervention methods in the microbiome, potentially impacting human disease and drug metabolism.
This post was written by John Arnst, ASBMB Today’s science writer.