A newly discovered vascular brain barrier that blocks the passage of inflammatory molecules triggered by gut bacteria may be why patients with inflammatory bowel disease (IBD) are at increased risk for certain mental health disorders, including anxiety and depression, early research suggests.
The discovery, which was based on a preclinical model, could lead to new therapeutic targets that could have applications for both gastrointestinal and psychiatric conditions, investigators note.
The research team, which was led by immunologist Maria Rescigno, PhD, and neuroscientist Simona Lodato, PhD, both from Humanitas University, Milan, notes that the barrier resides in the choroid plexus, a region of the brain that is involved in filtering cerebrospinal fluid. The researchers found that the region closes in response to inflammatory molecules produced in reaction to the presence of intestinal bacteria in patients with gut disorders.
Dr. Lodato said in an interview that the brain’s choroid plexus vascular barrier, along with another barrier between the gut and liver, known as the gut vascular barrier, appear to control the movement of molecules along the gut-brain axis.
“We show that in addition to the epithelial barrier in the choroid plexus, there is a functional vascular barrier that only becomes evident in blocking entry of various inflammatory molecules under conditions of systemic inflammation,” Dr. Lodato said.
“This interruption of the gut-brain interaction has developed to protect the brain from inflammation. Why this happens is not yet known, but it is likely to prevent epileptic seizures and imbalanced neuronal activity,” added Dr. Rescigno.
The study was published online October 22 in Science.
The gut a root cause of mental illness?
Nearly 40% of patients with IBD also experience depression and anxiety. It was once thought that these conditions arose because of patients’ difficulties in coping with their disease, said Dr. Rescigno.
“People with these disorders conventionally thought to be caused by an imbalance in the brain may actually find the root cause is located in the intestine. This is the first time these symptoms have been associated with the choroid plexus vascular brain barrier and its closure,” she noted.
Dr. Rescigno added that subtle, rather than overt, inflammation may be all that’s required for closure of the choroid plexus and the subsequent effects on mental health.
In 2015, Dr. Rescigno’s group first described the gut vascular barrier that protects the systemic circulation from gut bacteria or associated bacteria-derived molecules. During intestinal inflammation, such as occurs in IBD, this barrier is compromised and becomes more permeable. This allows microbes to pass across the epithelium of the gut barrier and enter the systemic circulation, including the liver and spleen, explained Dr. Rescigno.
Dr. Rescigno and Dr. Lodato then explored whether this systemic inflammatory condition was connected to the brain along a gut-brain axis and found that it was.
The researchers tested the hypothesis that central nervous system symptoms may be due to vascular changes at the interface between the gut or the brain and elsewhere in the body.
“We set out to test whether opening of the gut vascular barrier would allow gut bacteria to trigger the release of inflammatory molecules that spread to more distant areas, possibly leading to a deficiency of certain nutrients and precipitating mental disorders,” they said.
An experimental preclinical model of the choroid plexus vascular barrier closure led to anxiety-like behavior, as well as short-term memory loss. That this behavior occurred independently of inflammation suggested that it was likely a response to closure itself, they note.
In the noninflammatory state, the epithelium of the choroid plexus filters molecules. Those that are ≤70 kDa are allowed to pass through to the brain. However, the investigators found that during systemic inflammation, this filtration stops, and the blood capillaries of the choroid plexus prevent entry of inflammatory molecules such as cytokines.
Dr. Lodato speculated that when the vascular barrier of the choroid plexus shuts off during the systemic inflammatory state, it responds by bathing the brain in cerebrospinal fluid.
“When the choroid plexus closes, like a door slamming shut, then communication between the brain and the rest of the body is halted. This means that the brain is deprived of certain nutrients and other beneficial molecules that usually enter via the cerebrospinal fluid or enriched of potentially dangerous ones, as drainage could also be affected,” she said.
If confirmed in further studies, these results may open the way to new interventions.