A new research in mice demonstrates how a specific chemical generated by gut bacteria impacts brain function and induces anxiety-like behaviour.
A team of Caltech researchers found that a small-molecule metabolite generated by bacteria in the mouse stomach may go to the brain and change the activity of brain cells, causing anxiety in mice. The research contributes to the discovery of a biological basis for recent findings that gut microbiome alterations are linked to complex emotional behaviour.
Sarkis Mazmanian, the Luis B. and Nelly Soux Professor of Microbiology at Caltech and an associated faculty member of the Tianqiao and Chrissy Chen Institute for Neuroscience, led the study. The study’s findings will be published in the journal Nature on February 14th.
The immune system and metabolism are influenced by the colonies of bacteria that live in animals’ guts (the microbiome), according to decades of study; recent studies have connected the microbiome to brain function and mood. The gut bacteria populations of people with specific neurological disorders vary significantly. Furthermore, research in mice have demonstrated that modifying these communities may affect neurodevelopmental and neurodegenerative states, either improving or worsening symptoms.
“It’s been really difficult to show causation between something that’s happening in the gut and the brain, rather than just associations between disease states and the presence or absence of certain microbes,” says Brittany Needham, a postdoctoral scholar in the Mazmanian lab and the study’s first author. “We wanted to learn more about the chemical communications that go between the stomach and the brain, and how these signals might lead to behavioral changes.”
This research focuses on 4-ethylphenyl sulfate, or 4EPS, a bacterial metabolite (a by-product of microorganisms). In both humans and mice, 4EPS is created by bacteria in the intestines and then taken into the circulation, where it circulates throughout the body. The Mazmanian group discovered that this molecule was present in increased amounts in animals with abnormal neurological development, especially a mouse model of autism and schizophrenia, in 2013. 4EPS levels were by far the most different from the healthy microbiome, despite the fact that other characteristics of the changed microbiome varied. Furthermore, 4EPS levels were roughly seven times higher in children on the autism spectrum than in neurotypical children in a screen of 231 human blood samples.
The scientists focused on the impact of 4EPS on anxiety mice models in this study. While human anxiety disorders are complicated, animal models allow researchers to investigate the specific alterations in the brain and body that rise to anxious behaviors. The inclination to investigate or hide in a new place, as well as the amount of time spent in a dangerous situation, are used to quantify “anxiety” in mice. Instead of investigating a new location, brave mice will hide, as if confronted with a predator.
The researchers studied two groups of laboratory mice: one was colonized with bacteria that were genetically altered to make 4EPS, while the other was infected with bacteria that were similar but lacked the capacity to produce 4EPS. The mice were then moved to a separate arena, where researchers observed their behavior.
When compared to their non-4EPS counterparts, the mice with 4EPS spent substantially less time investigating the surroundings and much more time hiding, showing greater levels of fear. In addition to overall alterations in brain activity and functional connectivity, brain scans of the 4EPS mice revealed that certain of the brain areas linked with fear and anxiety were more active.
The scientists discovered that oligodendrocytes, a kind of brain cell, were changed inside these altered locations when they looked closer. These cells are crucial in part because they generate a protein called myelin, which functions like insulation around neurons and nerve fibers called axons. The researchers discovered that when 4EPS is present, oligodendrocytes become less mature and make less myelin, resulting in poorer insulation surrounding axons.
When the 4EPS mice were given a treatment that increases myelin synthesis in oligodendrocytes, the therapy was able to overcome the detrimental effects of 4EPS, resulting in normal myelin production and a reduction in nervous behaviors.
Needham found that giving mice an oral medication to soak up and eliminate 4EPS from their systems reduced nervous behaviors in a separate research published in the journal Nature Medicine at the same time. This outcome allowed for a limited clinical trial in which the medicine was given to people in an open-label experiment (no placebo or control group). Sequestering 4EPS in the human gut resulted in lower levels of 4EPS in the blood and urine, as well as a reduction in anxiety in many of the 26 research participants.
“It’s an intriguing proof-of-concept discovery that a particular microbial metabolite modifies the activity of brain cells and complicated behaviors in mice, but how this is occurring remains unclear,” adds Mazmanian. “Integration of sensory and molecular inputs from the periphery, as well as the environment, is part of the fundamental foundation for brain function. In concept, what we present here is comparable, but with the addition of the revelation that the neuroactive chemical is microbial in origin. This research, I think, has ramifications for human anxiety and other mood disorders.”
The next stage in the research will be to look at the methods by which 4EPS impacts oligodendrocytes, such as which proteins it interacts with and if 4EPS influences changes directly in the brain or whether it affects another region of the body and the consequences go up to the brain. In addition, demonstrating that the human data have an impact in a well-powered and controlled clinical study, which is now happening, would be crucial.
Former research technician Mark Adame; research technician Joseph Boktor; former postdoctoral scholar Wei-Li Wu (now of National Cheng Kung University in Taiwan); postdoctoral scholar Claire Rabut; EM scientist Mark Ladinsky; lecturer in chemistry Son-Jong Hwang; graduate student Jessica Griffiths; Pamela Bjorkman, David Baltimore Professor of Biology and Bioengineering, Merkin Institute Professor, and executiv professor Pamela Bjorkman, David Baltimore Professor of
Masanori Funabashi of Stanford University and Daiichi Sankyo RD Novare Co.; Zhuo Wang, Yumei Guo, and Daniel Holschneider of USC; Jillian Haney and Daniel Geschwind of UCLA; Qiyun Zhu of UC San Diego and Arizona State University; Rob Knight of UC San Diego; and Michael Fischbach of Stanford University are among the additional co-authors.
The National Science Foundation, the Human Frontier Science Program, the National Institutes of Health, the Ministry of Science and Technology in Taiwan, the Heritage Medical Research Institute, and Lynda and Blaine Fetter all contributed money to the study. Axial Therapeutics, which performed the clinical research, is co-founded by Sarkis Mazmanian.