Understanding the language of brain-muscle crosstalk

IAs the COVID-19 pandemic escalated in 2020, many universities closed or reduced research laboratory capacity in an attempt to limit the spread of the virus. At Washington University School of Medicine in St. Louis, developmental biologist Aaron Johnson is allowed to have one person in his lab to keep things running. At that time, Yang Shuo, a postdoctoral researcher engaged in muscle development biology research, stood up.

Yang, now an immunologist at Fudan University, wanted to learn more about the virus that was wreaking havoc around the world. Because his model organism, Drosophila melanogaster, is naturally insusceptible to the SARS-CoV-2 virus, Yang generated a line of fruit flies that expresses a gene called open reading frame 3a, or ORF3a, in its brain. of coronavirus proteins to simulate infection.¹ He observed that this resulted in motor dysfunction; the fly lost its ability to climb upward against gravity.

The researchers note that this behavior is similar to some common symptoms of human disease. “[When] You’re sick and your muscles are really tired and sore,” Johnson said. “We’ve all been there.” This prompted them to explore the phenomenon in more detail.

Nearly four years later, their research showed that cytokines induced by viral and bacterial infections interfere with mitochondrial activity in skeletal muscle in flies and mice, providing insight into why brain infections cause motor dysfunction.² The results were published in Scientific Immunologyhighlights the brain-muscle signaling axis and reveals a potential therapeutic target for alleviating muscle discomfort associated with neuroinflammation.

After observing ORF3a-induced motor dysfunction in fruit flies, Johnson’s team explored whether bacterial infection, which produces a similar immune response to viral infection, would have the same impact on fruit fly movement. When researchers infected fruit fly brains with viruses E. coli, Drosophila flies cleared the infection within 24 hours but showed motor dysfunction for up to 9 days.

To determine the mechanism by which short-term central nervous system (CNS) infections lead to long-term movement problems, the researchers evaluated the structure and appearance of cells in skeletal muscle, including mitochondria, an important hub for energy production. ORF3a and E. coli Infection leads to reduced mitochondrial activity in skeletal muscle.

Yang knew that infections could alter the chemicals secreted by affected tissues, including signaling molecules such as cytokines, so he dug into the literature to search for possible candidates.³ Unpaired-3 (Upd3), the Drosophila counterpart of mammalian interleukin-6 (IL-6), is an important cytokine that stresses secretion in Drosophila tissues.⁴

When they analyzed gene expression in the brains of infected flies, they found Update 3 level. Success in testing the first candidate cytokine “required a little luck and a lot of hard work,” Johnson said. “As they say, a day in the library can save you a week on the bench.”

In Drosophila, Upd3 activates the Janus kinase (JAK)-signal transducer and activator of transcription (STAT) pathway, which plays an important role in Drosophila development and immune responses.⁵ However, overactivity of this pathway impairs normal mitochondrial activity in skeletal muscle.⁶ The researchers investigated whether this pathway is involved in disrupting muscle mitochondrial activity in response to Upd3 secretion. They observed that infection increased the expression of a gene in the JAK-STAT pathway in skeletal muscle, suggesting that this pathway is involved in mediating brain-muscle crosstalk.

Once they established the link between Upd3 and muscle dysfunction, they began investigating the mechanisms by which infection upregulates Upd3 production. Infection can trigger the production of reactive oxygen species (ROS), which induces the expression of cytokines in many cell types.⁷ When the researchers expressed reactive oxygen reductase in the central nervous system of flies, the infected flies showed increased muscle mitochondrial activity.

These experiments led the researchers to conclude that ROS induced by CNS infection triggers the production of cytokines that leave the brain and travel to skeletal muscle, where they inhibit mitochondrial activity.

Next, the researchers investigated whether similar pathways operate in mammals. They injected ORF3a into the central nervous system of mice, which resulted in increased ROS, increased levels of cytokines (including IL-6), and induced cell death in the brain. The mice showed fatigue during regular treadmill running experiments, and a closer look at their muscle cells revealed mitochondrial dysfunction.

Johnson said the results are very interesting and exciting. “When the mouse data starts to replay [observations in the] Flying is even more exciting.

Using data from flies and mice, Johnson’s team wondered whether a similar process might be at work in humans, and indeed, histological analysis showed the presence of ORF3a in the brains of deceased SARS-CoV-2 patients.

Finally, the team explored whether similar players modulate crosstalk between the brain and muscles in non-communicable diseases such as Alzheimer’s disease (AD), which cause nerve inflammation and muscle weakness. The researchers conducted a meta-analysis of previously published studies showing that AD patients had higher serum IL-6 levels than control participants.

To test whether a fly model of AD triggers similar pathways, the researchers expressed beta-amyloid, a neurotoxic protein associated with AD pathology, in the brain. This leads to motor dysfunction with increased reactive oxygen species and Update 3 expression in the brain.

“This is a very elegant paper using a variety of methods and, even more impressively, a variety of model organisms,” said Fabio Demo, a scientist at St. Jude Children’s Research Hospital who studies intertissue signaling. Fabio Demontis, who was not involved in the study, said he was not involved in the study. Because other research groups have found that people with COVID-19 have dysfunction of the diaphragm, a skeletal muscle involved in breathing, Demontis said studying whether similar inflammatory pathways are involved in this damage could help scientists identify respiratory diseases of potential treatments.⁸

Overall, de Montes said, these results add a novel component to the field of inter-organ communication. They also suggest IL-6 and other players in this pathway as potential targets for preventing muscle fatigue caused by nerve inflammation.

“Hopefully this paper will serve as a call to arms to start considering IL-6 as a target for treating patients with severe chronic inflammation,” Johnson agreed. Next, his team plans to design and conduct clinical trials to test the impact of the IL-6 antibody, which has been approved by the U.S. Food and Drug Administration, on motor function in patients with AD or long-term COVID-19.