Recent research published in circulation research We provide evidence that specific molecules produced by gut bacteria can protect the heart from stiffness and dysfunction by communicating directly with the brain. This study suggests that restoring this bacterial byproduct may offer a new way to approach high blood pressure and related heart disease.
Hypertension and related cardiovascular diseases involve complex interactions between the gastrointestinal, nervous, and cardiovascular systems. High blood pressure tends to make the heart muscle stiff and lose its ability to relax properly between beats, a condition known as diastolic dysfunction. Although this stiffness represents a major physiological cause of heart failure, the biological signals that drive this structural change are still poorly understood.
To understand this process, researchers aimed to identify the chemical messengers that link these physiological systems. “Hypertension is a systemic disease caused by complex interactions between the gut, brain, kidneys, and cardiovascular system,” said study author Supantha Sawamipak, principal investigator at the Max Delbrück Center for Molecular Medicine of the Helmholtz Society in Berlin, Germany.
“We knew that high blood pressure is associated with dysbiosis, which often impairs the heart’s ability to relax, but we lacked the precise molecular signals that connect these systems. We wanted to fill this gap and identify the specific microbial metabolites that mediate this interorgan communication during hypertensive stress.”
To study this biological relationship, scientists used a special zebrafish model. Zebrafish larvae are largely transparent, allowing researchers to use high-speed microscopes to observe their beating hearts and circulating blood in real time. The research team raised the larvae in water with gradually decreasing salinity over five days to induce high blood pressure. This low ionic environment caused the fish to activate internal hormonal mechanisms to retain sodium, resulting in increased blood pressure and stiffening of the heart muscle.
The researchers first analyzed the zebrafish’s gut bacteria after a five-day hypertension challenge. Sequencing the genetic material of bacteria in the gastrointestinal tracts of 10 treatment groups and 11 control groups revealed a significant reduction in overall bacterial diversity. Stressed fish lost certain bacteria responsible for breaking down tryptophan, an amino acid found in food, into indole molecules.
The researchers then tested whether the presence of gut bacteria was necessary to protect the heart. They raised groups of eight to 12 germ-free zebrafish. In other words, the fish had no intestinal bacteria at all. When exposed to the same low-salt stress, these germ-free fish showed more severe blood pressure spikes and worsened heart stiffness compared to fish with normal gut bacteria. This finding provides evidence that a healthy microbial community helps protect the cardiovascular system from damage.
The researchers then looked at specific chemical byproducts produced by gut bacteria. They analyzed the fish intestines using mass spectrometry, a specialized laboratory technique that measures the mass and concentration of different molecules. They found that stressed fish had significantly lower levels of indole-3 acetic acid, a particular byproduct of tryptophan metabolism, compared to healthy fish.
Depletion of this beneficial molecule has a cascading effect on the body’s stress response. “Our gut microbiome actively protects the heart during hypertension by producing certain molecules derived from dietary tryptophan, particularly indole-3 acetic acid (IAA),” Sawami-Pak explained. “When the microbiome is disrupted by high blood pressure, the resulting loss of IAA releases the brakes on stress signaling in the brain, particularly within hypocretin-producing neurons. The lack of this brake leads to sympathetic overdrive, impairing the ability of the heart muscle to relax properly between heartbeats (diastolic dysfunction).”
To see if replacing this missing molecule would help, the scientists administered indole-3-acetic acid directly into the fish’s digestive tracts. Fish that received this supplement maintained normal blood pressure and healthy heart function even when exposed to low-salt stress. The treatment prevented individual heart muscle cells from expanding and kept the heart’s main pumping chambers normally relaxed between beats.
The researchers then looked into the brain to understand how molecules in the gut can protect the heart. They focused on hypocretin neurons, a specialized group of brain cells in the hypothalamus that help regulate involuntary functions such as heart rate and vasoconstriction. Using a special fluorescent marker that lights up when the neurons are active, they observed that hypocretin neurons become highly hyperactive during hypertensive stress. When the fish were given indole-3 acetic acid, these brain cells quieted down and returned to normal baseline levels.
Further experiments revealed exactly how the molecule affects the brain. Scientists have discovered that hypocretin neurons have specific chemical sensors called aryl hydrocarbon receptors. Injecting indole-3 acetic acid directly into the brain cavity of the fish activated this receptor and prevented heart stiffening. Blocking the receptor with chemical inhibitors completely abolishes the protective effect.
By preventing hypocretin neurons from becoming overactive, indole-3 acetate stopped an excessive cascade of nervous system signals from reaching the heart. Using a technique called calcium imaging, which monitors neural activity in live fish, the researchers confirmed that the treatment calmed the sympathetic nervous system, the network responsible for the body’s physical response to stress. The treatment also lowered systemic levels of hormones that constrict blood vessels, acting pleiotropically to protect the cardiovascular system.
To determine whether these findings apply to humans, the researchers analyzed blood samples from a cohort of 194 people under the age of 50. This group included 97 hypertensive patients and 97 healthy individuals matched for age, sex, and BMI. Scientists found that patients with high blood pressure had significantly lower levels of indole-3 acetic acid in their blood.
This clinical data strongly reflects the physiological changes observed in animal models. “We were struck by the ability of a single microbial metabolite, IAA, to act centrally in the brain to simultaneously prevent both neurogenic (sympathetic overdrive) and hormonal (renin-angiotensin system) causes of hypertension,” Sawami-Pak said. “Furthermore, finding that this specific reduction in circulating IAA is strongly conserved in human hypertensive cohorts, with a particularly pronounced sex-specific reduction in female patients, is a remarkable validation of our zebrafish model.”
Although this study provides substantial evidence for the relationship between the gut, brain, and heart, it also has some limitations. The zebrafish model provides a simplified view of biology and does not fully capture the complexity of human aging and the metabolic diseases that often accompany cardiac problems. The human data used in this study was observational and, while it shows an association between low indole triacetic acid and high blood pressure, it does not prove that one causes the other in humans.
The authors caution against taking these results as immediate clinical treatment. “It is important not to misinterpret these findings as evidence that taking over-the-counter IAA or tryptophan supplements is the sole treatment for hypertension,” Sawami-Pak noted. “Although we have established a direct causal mechanism in animal models, the human data we have analyzed are correlational at this time. Hypertension is a highly complex multifactorial disease, and IAA deficiency represents one component of a broader systemic dysregulation.”
Future research is needed to determine whether restoring this molecule can safely and effectively treat or prevent heart disease in human patients. “Our immediate next step is to understand precisely how microbial metabolites like IAA modulate neural activity at the molecular level,” Sawami-Pak said. “Besides IAA, we are also looking at a wide range of microbial metabolites that change during disease states, especially those known to modulate the immune system.”
The long-term goal is to unravel these complex biological interactions in detail and pave the way for medical advances. “Ultimately, our most important goal is to decipher the communication networks throughout this complex system between the gut, brain, immune system, and heart,” Sawamipak explained.
“Although our laboratory focuses on basic biological discovery rather than conducting human clinical trials, identifying these precise disease mechanisms and molecular targets provides an important foundation. This will ultimately allow clinical researchers to develop targeted therapies, such as postbiotics, that precisely deliver missing beneficial molecules to restore balance in cardiovascular and metabolic diseases.”
The study, “Indole-3 acetate limits dysdiastolic dysdialysis via Hcrt neurons,” was authored by Bhakti I. Zakarauskas-Seth, Giovanni Forcari, Harithaa Anandakumar, Ilan Kotlar-Goldaper, Clara M. Barraud, Nina Jovanovic, Ulrike Brüning, Jennifer A. Kirwan, and Nicola. Wilk, Sofia K. Forslund, Dominik N. Müller, Alessandro Filosa, and Supansa Sawamipak.
