IInfections are often associated with symptoms not directly related to the pathogen, such as lethargy and loss of appetite. Scientists have long sought to understand where these so-called ‘sickness behaviors’ are ultimately controlled, as this information could shed light on the brain’s influence on the immune system and potentially lead to new treatments to speed recovery from disease. a myriad of diseases. Now research on mice published earlier this month in Nature traced much of this control down to a set of neurons deep within the brainstem.
“I think that’s really a significant step forward,” says Keith Kelleyprofessor emeritus of immunophysiology at the University of Illinois and former longtime editor of the journal Brain, behavior and immunity, who did not participate in the work. “It actually shows a population of cells in the brainstem that are responsible for connecting what’s going on in the body to what’s going on in the brain.”
Focus on the neurons that make us sick
Our body is constantly trying to maintain some sort of balance, controlling things like our body temperature, how often we are hungry, and how long we sleep. This careful balance, known as homeostasis, is how we manage to stay alive and healthy in the world. “Usually these things are really well controlled, and the body really prioritizes that,” says the study’s co-author. Anoj Ilangesbiologist at the Janelia Research Campus in Ashburn, Va., who conducted the study at Rockefeller University in New York.
This balance changes when we get sick, triggering a constellation of symptoms and physiological changes collectively called sickness behaviors that can help us recover.
Previous research had suggested at least some of the signals that lead to sickness behaviors come from the brainstem but did not identify exactly where in the structure. So, Ilanges and his colleagues decided to investigate. First, they exposed lab mice to lipopolysaccharide (LPS), a toxin made up of pieces of dead bacteria that is known cause an immune response similar to that triggered by live bacteria. As expected, the LPS made the animals sick: they became lethargic and lost their appetites, even though they were not infected with a pathogen. And that effect was strong, says study co-author and molecular biologist from Rockefeller University. Jeff Friedman: Mice exposed to LPS even refused to eat after being subjected to a long fast that would normally cause them to eat.
Next, the scientists examined neuronal activity by looking for a protein called FOS in the brains of mice euthanized after injection of LPS. FOS is involved in long-term changes in the brain and is often expressed after neurons fire, and therefore can act as a proxy for neuronal activity. Higher concentrations of FOS indicated a burst of activity in two areas: the nucleus tractus solitarius (NTS) and the area postrema (AP), which lie side by side in the brainstem.
But to determine if the neurons in these areas are really responsible for the pathological behaviors, they had to activate them without using LPS, because the toxin is known to cause other changes in the body and brain.
To do this, they injected a virus that delivers a molecular switch sensitive to the antipsychotic clozapine directly into the NTS-AP region of the brainstem of special mice. These mice had been genetically engineered so that when exposed to the cancer drug tamoxifen, active-firing neurons — and only active-firing neurons — incorporated this switch into the gene encoding FOS. This meant that if the mice were later exposed to clozapine, the neurons that fired in the NTS-AP region where the virus was injected while the mice received a priming dose of tamoxifen would become active again. This, in essence, gave the researchers a way to take a snapshot of neural activity as well as a way to recreate that snapshot later.
The researchers then injected the modified mice with LPS along with switch-priming and snapshot-taking tamoxifen. After a few weeks of recovery, the researchers gave the mice an injection of clozapine, and once again the NTS-AP neurons produced FOS and the mice displayed sickness behaviors, even without any LPS in their system. For the team, this confirmed that neurons in the NTS-AP region contribute to feeling bad. Further experiments using single-nucleus RNA sequencing further reduced the specificity of LPS-activated neurons to those in these regions that also express a protein called ADCYAP1.
There’s a lot going on, in terms of the immune system communicating with the brain and the brain controlling our physiology during infection. And I think this is just the beginning of a real exploration of this axis.
—Anoj Ilanges, Janelia Research Campus
Ilanges’ team also found that inhibiting ADCYAP1-expressing neurons reduced sickness behaviors in response to LPS injection, although it did not completely eliminate them.
Kelley noted that he thought the mouse model the team developed to reactivate a specific population of neurons “was really smart.” He also said he would be interested to see further work on some of the sickness behaviors not included in Ilanges’ work, such as sleep disturbances or the assortment of general aches and pains known collectively as myalgia name.
Patricia C.Lopes, a biologist at Chapman University in California who studies sickness behaviors but did not work on the study, points out that NTS-AP neurons may not be the only neurons in the brain that contribute to sickness behaviors. sickness. In June, another group of scientists, also publishing in Nature, identified neurons located in the hypothalamus that act as a kind of control center to coordinate fever, loss of appetite and heat-seeking behavior. Seeing the two papers come out so close to each other — and in the same newspaper — “was exciting, but also surprising,” Lopes says. The brainstem and hypothalamus had previously been identified as important for disease behaviors, but being able to identify cell populations is remarkable, she says. “The specificity they are arriving at is unprecedented.”
Lopes noted an interesting wrinkle in both articles: All of the animals used were male. This is not uncommon in mouse studies, as female mice exhibit large fluctuations in body temperature related to estrus (a potentially confounding factor that scientists may wish to avoid), but it does mean that any potential difference due to gender is unknown.
See “Not-so-sweet yellow: Pregnant mouse urine stresses males”
Ilanges’ team were also unable to investigate the specific bodily signals to which these neurons were responding, although they note that the NTS is known to relay signals from the vagus nerve – a line of communication important between the brain and internal organs – while PA is known to detect humoral signals, such as proteins released into the bloodstream. They were also unable to determine whether the neurons were active during viral infections or other non-bacterial infections.
Nonetheless, they hope that others can use the data and methods they defined to continue exploring how the brain and immune system interact, and Ilanges plans to continue this research at Janelia.
Ilanges says understanding how the brain controls sickness behaviors could also open the door to potential methods for adjusting these mechanisms. For example, one could imagine a drug designed to help people with chronic diseases regain their appetite.
More broadly, this work shows that the brain plays an essential role and actively participates in the fight against infections, explains Friedman. Ilanges expresses similar sentiments. “There’s a lot going on, in terms of communication between the immune system and the brain and the brain controlling our physiology during infection. And I think this is just the beginning of a real exploration of this axis.