Gut Microbe-Dependent TMAO Promotes Platelet Hyperreactivity, Raises Thrombosis Risk

Add two more entries to the fast-growing list of cardiometabolic effects of elevated levels of the gut microbial metabolite TMAO: platelet hyperreactivity and increased risk for thrombotic events.

In a paper just published in Cell, researchers in the Cleveland Clinic lab of Stanley Hazen, MD, PhD, report in vivo studies that collectively show that gut microbes, through generation of TMAO (trimethylamine-N-oxide), directly contribute to platelet hyperreactivity and increased thrombosis potential.

“Our findings reveal a previously unrecognized mechanistic link between specific dietary nutrients, gut microbes, platelet function and thrombosis risk,” says Dr. Hazen, Chair of the Department of Cellular and Molecular Medicine and Section Head of Preventive Cardiology and Rehabilitation at Cleveland Clinic.

Hypothesis-generating data from a large human cohort

The new findings build on multiple clinical studies — many by Dr. Hazen’s research team and summarized here — linking elevated blood levels of TMAO with heightened risk of cardiometabolic diseases in humans, including atherosclerosis, heart failure and chronic kidney disease. Central to this link are the dietary nutrients choline, lecithin and carnitine (all abundant in animal products), which are ultimately metabolized into TMAO after they come in contact with microbes in the gut.

Yet while these prior studies showed mechanistic links between TMAO and atherosclerosis development through changes in tissue sterol metabolism as well as arterial macrophage and endothelial cell activation, no studies had explored the strong association between TMAO and acute cardiovascular events or vulnerable plaque, triggered by acute thrombotic events. Neither had there been exploration of the potential involvement of gut microbes and TMAO in the modulation of platelet function or in any resulting effects on risk for thrombotic events.

Suspecting such involvement, Dr. Hazen’s team reanalyzed data from its landmark 2013 New England Journal of Medicine study of the relation between TMAO and cardiovascular events in more than 4,000 patients. Their new analysis, reported in the new Cell paper, tested specifically for an association between plasma levels of TMAO and incident risk for thrombotic events (myocardial infarction [MI] or stroke). The analysis revealed such an association and that it remained significant even after adjustment for cardiovascular disease history, traditional cardiovascular risk factors, renal function and medication use. Specifically, it found a 1.64-fold increase in thrombotic events in patients whose TMAO levels were in the highest quartile versus those whose levels were in the lowest quartile, even after adjustments (P < .001).

“We found that a high TMAO tracks with heightened risk for thrombotic MI and stroke in a large human sample,” says Dr. Hazen. “The effect was seen across a large, dynamic range of TMAO levels.”

Testing the hypothesis in animal models

These findings prompted the Hazen lab to undertake a series of animal model studies to test the notion that TMAO directly modulates platelet function. These studies used dietary choline or TMAO, germ-free mice and microbial transplantation to collectively demonstrate the following:

• TMAO directly promotes human platelet responsiveness by enhancing stimulus-dependent release of Ca2+ from intracellular Ca2+ stores and activation in the setting of submaximal agonist stimulation.
• This effect is not stimulus-specific but applies to several distinct agonists, including ADP, thrombin, collagen and arachidonic acid.
• The effect of TMAO on platelet reactivity and in vivo thrombosis is both rapid and reversible.
• Dietary choline enhances platelet responsiveness and in vivo thrombosis potential.
• Gut microbes play an essential role in choline diet-dependent enhancement in thrombosis potential.
• Specific gut microbial taxa are associated with TMAO levels and thrombosis potential.
• Thrombosis potential is a transmissible trait, as shown in gut microbial transplantation studies conducted with microbes producing low versus high levels of the TMAO precursor TMA.

“These studies show that gut microbes modulate thrombosis potential in vivo,” says Dr. Hazen. “Importantly, they show that TMAO’s effect on platelet function is dose-dependent across the physiological range of TMAO concentrations in humans and animal models alike.”

Remaining questions — and potential clinical payoffs

At the same time, he notes, the existence and identity of molecular receptors that serve to sense TMAO within platelets remain uncertain and a focus of intense research interest.

Despite these remaining questions, the new study holds considerable clinical relevance. “While antiplatelet medications are a cornerstone of current treatment and prevention of acute complications of cardiovascular disease, they are hampered by the potential to cause bleeding,” Dr. Hazen observes. “Our new paper suggests that targeting the gut microbial TMAO pathway as a treatment strategy has the potential to temper the platelet hyperresponsiveness associated with elevated TMAO, and to do so without enhanced bleeding complications.”

And with the recent commercial availability of an assay for plasma TMAO through Cleveland HeartLab (and potentially other labs soon), the new findings also underscore the potential utility of TMAO levels as a clinical tool for better identifying subjects who may benefit from antiplatelet therapy to prevent thrombotic events. “This is an area of considerable interest,” Dr. Hazen notes.