The Role of Gut Microbiota in Atherosclerosis and Hypertension

1Gut microbiota is the collection of bacteria that inhabit in the gastrointestinal tract producing a diverse ecosystem about 1014 microorganisms. The majority of the gut microbiota is composed of five phyla, namely Bacteroidetes, Firmicutes, Actinobacteria, Proteobacteria, and Cerrucomicrobia, in which the relative abundance of Bacteroidetes and Firmicutes phyla is >90%. The homeostasis of gut microbiota is critical for maintaining human health, with gut dysbiosis contributing to the development of various diseases including cardiovascular disease (CVD;; ), obesity, type 2 diabetes mellitus (T2DM;;; ), non-alcoholic fatty liver disease (NAFLD;; ), and even some types of cancer.

2Cardiovascular disease is the leading cause of death worldwide, especially in developed countries, and encompasses multiple disorders including atherosclerosis, hypertension, stroke, and heart failure. Although genetic contributions are intimately involved, other factors such as nutrition and gut microbiota have also been implicated as the main risk factors for developing CVD. reported the gut microbiota-dependent mechanism of CVD, highlighting the intricate relationship between gut microbiota and CVD. Recently, gut dysbiosis has been recognized as an important factor contributing to the development of atherosclerosis and hypertension, two major risk factors for CVD. Consequently, gut microbiota-targeted therapy is a promising strategy to treat CVD.

3In this review, we extensively retrieved the publications on the topics of gut microbiota and CVD mainly published within the past 10 years through PubMed. We discuss the roles of gut microbiota implicated in the development of CVD, especially focusing on atherosclerosis and hypertension, and briefly summarize the recent advances of gut microbiota-targeted therapies for CVD.

4Atherosclerosis is the major risk factor for CVD, which is characterized by accumulation of cholesterol and recruitment of macrophages into artery walls, contributing to the formation of atherosclerotic plaques. Interestingly, recent studies have suggested that gut dysbiosis can also contribute to the development of atherosclerosis. Using shotgun sequencing of the gut metagenome in patients with or without symptomatic atherosclerosis, scientists found that the relative abundance of Roseburia and Eubacterium was lower, while Collinsella was higher in atherosclerosis patients compared to healthy controls. In addition, Akkermansia muciniphila was found to improve gut barrier functions and exert protective effects against atherosclerosis. Although meta-analysis showed no significant benefit in coronary artery disease patients treated with antibiotics, nevertheless, evidence is accumulating which indicates that gut microbiota play a causative role in atherosclerotic by modulating inflammation and the production of microbial metabolites.

5Inflammation is commonly involved in a number of diseases, including atherosclerosis, which is a classical chronic inflammatory disease. Gut epithelium is the first barrier of the host, which protects against the invasion of pathogens. Given its critical role in preventing the translocation of intestinal content, mainly bacterial components, the integrity of the gut barrier is essential for maintaining the health of the host. Intestinal permeability is associated with reduced expression of tight junction proteins, including zonula occludens-1 (ZO-1), claudin-1, and occludin, and an imbalance between intestinal epithelial cell death and regeneration. If the intestinal epithelial barrier is impaired, the invasion of pathogen associated molecular patterns (PAMPs) drives an immune response and results in systemic and tissue-specific inflammation. Accordingly, impairments to the gut barrier integrity induced by gut dysbiosis have been suggested as risk factor for chronic inflammation in various diseases. It is noteworthy that lipopolysaccharide (LPS) and peptidoglycan are microbial components that are recognized as risk factors for CVD.

6Lipopolysaccharide is a cell wall component of Gram-negative [G] bacteria, which has been extensively studied as it is one of the PAMPs involved in CVD risk. The association between LPS and CVD was first proposed in 1999 by measuring plasma endotoxin levels in the clinic. Subsequently, the relationship was gradually confirmed by multiple experiments by different research groups. For example, in one study, it was concluded that the level of circulating endotoxemia was most notable in patients with the highest CVD burden. found that gut dysbiosis suppressed the expression of tight junction proteins, leading to an increase in intestinal permeability and subsequently the translocation of LPS into the blood. Gut dysbiosis-derived LPS may play important roles by modulation of Toll-like receptors (TLRs) and their downstream targets. As part of the pattern-recognition receptors family, TLRs can recognize bacterial products and modulate the host immune system. Circulating LPS can bind to cell-surface-receptor complexes composed of TLR4 and its co-receptors cluster of differentiation 14 (CD14; ). Using TLR4 and LDL receptors double knockout mice, found that a TLR4 deficiency reduced atherosclerosis without effect on inflammation. Consistently, clinical investigations have revealed that upregulation of TLRs was associated with inflammatory activation in human atherosclerosis, and promoted the development of atherosclerosis. However, a meta-analysis in 2012 indicated that Asp299Gly, a TLR4 polymorphism, did not play an obvious role in the development of atherosclerosis. Moreover, the binding of LPS to TLR4 activated its downstream pathways including MYD88 and nuclear factor kappa B (NF-κB), contributing to the increased production of pro-inflammatory cytokines such as IL-6, IL-1, IL-27, and tumor necrosis factor-alpha (TNF-α), leading to an increased risk of developing CVD. showed that a deficiency of MyD88 reduced atherosclerosis by decreasing macrophage recruitment. The main interactions between gut microbiota and inflammation are shown schematically in Figure 1.

7In addition, another bacterial PAMP, peptidoglycan (PG), was also found to be associated with CVD risk by impairing the intestinal epithelial barrier. PG is a minor cell wall component of [G] bacteria; however, it is also a major component of Gram-positive [G(+)] bacteria. Using metagenomic sequencing, scientists found that patients with atherosclerosis had enrichment of genes that encoded PG synthesis. Indeed, pro-inflammatory bacterial PG was observed in atherosclerotic arteries and associated with vulnerable plaques. Through PG recognition, the nucleotide-binding oligomerization domain (NOD) proteins NOD1 and NOD2 promote intracellular bacteria clearance through a program involving NF-κB and mitogen-activated protein kinase (MAPK) signaling pathways. Studies in Nod2-deficient mice revealed that NOD2 was a critical regulator of intestinal bacterial immunity and helps to maintain the integrity of the gut barrier. In recent years, scientists have investigated the potential role of NOD1 in atherosclerosis using Nod1 knockout mice. Data showed that knockout of apolipoprotein E and Nod1 in mice significantly reduced the development of atherosclerotic lesions. Additionally, there are other PAMPs that can promote inflammatory processes through the engagement of host pattern recognition receptors (PRRs), such as CpG oligodeoxynucleotides flagellin, lipopeptides, and so on. Collectively, all of the evidence suggests that functional changes in the gut microbiota might be involved in the atherosclerosis risk. Although the vast majority of studies revealed that pathogenic bacteria contributed to atherosclerosis pathogenesis, two antibiotic trials reported controversial benefits of antibiotic therapy in CVD.

8In addition to gut dysbiosis-related inflammation, increasing evidence has revealed that gut microbiota-derived metabolites play essential roles in the development of CVD. A variety of metabolites are derived from the gut microbiota, as well as co-metabolism of gut microbiota such as amines methylamines, polyamines, short-chain fatty acids (SCFAs), trimethylamine N-oxide (TMAO), and secondary bile acids (BAs). SCFAs are a group of well-established gut microbial metabolites that are critically involved in metabolic diseases. Recent advances detailing their involvement in atherosclerosis in both human and animal models have been extensively reviewed. Therefore, in the current review, we mainly focused on the roles of TMAO and secondary BAs in atherosclerosis.

9Dietary phosphatidylcholine or L-carnitine is metabolized by gut microbiota into trimethylamine (TMA) in the intestine. It is a precursor of TMAO, which is transported to liver and oxidized by flavin monooxygenase 3 (FMO3), one member of the hepatic FMO enzymes family, leading to the production of TMAO. Hepatic knockdown of FMO3 in mice using an antisense oligonucleotide decreased circulating TMAO levels and attenuated atherosclerosis through stimulating basal metabolism and activating macrophage reverse cholesterol transport (RCT;;; ). It was also found that plasma levels of gut microbial dietary phosphatidylcholine metabolites and TMAO that produced related molecules (L-carnitine and γ-butyrobetaine) were associated with the risk of CVD. The higher level of plasma TMAO was correlated with atherosclerosis formation and the extent of the atherosclerotic plaque area. Consistently, a prospective and observational clinical study on patients with or without chronic heart failure has shown that plasma levels of TMAO were positively correlated with the risk of chronic heart failure. These findings suggest that circulating levels of TMAO are important risk factors for the pathogenesis of CVD.

10Given the roles of TMAO in the pathogenesis of CVD, the underlying mechanisms have been extensively investigated. To explore potential mechanisms by which TMAO might promote atherosclerosis, a dietary choline supplement was administered to ApoE-/- mice, in which the expression of CD36 and steroid receptor RNA activator 1 (SR-A1), two macrophage scavenger receptors implicated in atherosclerosis, was measured. The results revealed elevated levels of CD36 and SR-A1 in the macrophages of TMAO-treated mice compared to normal controls, and antibiotic intervention reduced the formation of foam cells by decreasing TMA production. However, no significant impact of TMAO on foam cell formation was observed in mouse macrophages. In contrast, TMAO can lead to atherosclerosis by suppressing RCT and modulating the activity of cholesterol transporters in macrophages. In addition, TMAO administration could suppress levels of liver BA synthetase (Cyp7a1 and Cyp27a1) and BA transporters (Oatp1, Oatp4, Mrp2, and Ntcp), leading to a disorder of BA-related pathways and atherosclerosis, suggesting that the atherosclerotic promoting effect of TMAO is also associated with the variation in BA metabolism. Farnesoid X receptor (FXR) is an important nuclear receptor that controls BA metabolism, which can also regulate the expression of hepatic FMO3, resulting in an alteration in TMAO production. An FXR agonist inhibited the expression of CYP7A1 and CYP8B1 in ApoE-/- mice and protected mice against atherosclerosis. Recently, found that TMAO upregulated the expression of vascular cell adhesion molecule-1 (VCAM-1) and activated protein kinase C (PKC) and NF-κB, highlighting that TMAO may speed up the development of atherosclerosis by inducing endothelial cell dysfunction and by increasing monocyte adhesion. Additionally, the direct exposure of platelets to TMAO increased stimulus-dependent platelet activation by elevating Ca2+ release from intracellular stores, contributing to the increased risks of thrombosis and plaque instability. Generally, TMAO accelerates the development of atherosclerosis by promoting cholesterol influx, inhibiting cholesterol efflux, blocking the BA pathway, and/or causing excessive activation of platelets. All of these findings confirmed TMAO as a biomarker for CVD risk and a promoter of atherosclerotic diseases. TMAO is regarded as one of the most promising metabolites that may not only be an independent risk factor for CVD, but also a potential therapeutic target for CVD on the basis of a large amount of experimental and clinical data. However, inconsistent results were also observed, especially in large population observations. Choline is generally regarded as a dietary source of TMAO; however, in a cohort study, there was no clear evidence of significant associations between choline intake and the risk of developing CVD. Likewise, in ApoE(-/-)mice, L-carnitine administration resulted in a significant increase in circulating TMAO levels, which surprisingly was inversely correlated with aortic lesion size. Unfortunately, several large population studies conducted by different countries have demonstrated that dietary choline and betaine intake was not associated with the pathogenesis of CVD. Consequently, more studies are needed to confirm the exact roles of TMAO in atherosclerosis, as well as the validation of its therapeutic potential by targeting TMAO-producing bacteria or enzymes.

11Bile acids are another group of gut microbiota-derived metabolites involved in various metabolic diseases, which are stored in the gallbladder and released into the intestine to facilitate the absorption of dietary lipids and fat-soluble vitamins. Primary BAs are synthesized from cholesterol in the liver and mainly include cholic acid (CA) and chenodeoxycholic acid (CDCA). Primary BAs are usually metabolized into secondary BAs including deoxycholic acid (DCA) and lithocholic acid (LCA), hyodeoxycholic acid, and ursodeoxycholic acid through gut microbiota-derived enzymes. Previous studies reported that germ-free mice had higher levels of primary BAs, but non-detectable secondary BAs in the enterohepatic system. It was found that suppression of hepatic BA biosynthesis could inhibit the HFD-induced gut microbiome alterations, which highlights the liver–BA–gut microbiome metabolic axis. Thus, there is a bidirectional relationship between gut microbiota and BA metabolism.

12Bile acids are also important signaling molecules that modulate host metabolism and energy expenditure processes. Bile salts can be diversified into biologically active species by gut microbiota that can survive in the bile salt-rich microenvironment. Gut microbiota-mediated BA metabolism in CVD has been well reviewed recently. Nevertheless, to date, the role of BAs in CVD development is still poorly understood so far. It is well recognized that BAs can promote the development of atherosclerosis mainly through bile-salt hydrolase (BSH) and BA receptors. The C24 N-acyl bond of glycine-conjugated or taurine-conjugated bile salts can be hydrolyzed into free BAs by BSH. In addition to deconjugation, the BA pool can also be chemically diversified by bacteria-derived 7α-dehydroxylase and 7β-dehydroxylase. The produced secondary BAs enter the portal circulation to function as signaling molecules with profound effects on host physiology and pathology. Bacteria-mediated BSH activity can affect the processes underlying the pathogenesis of atherosclerosis by increasing cholesterol accumulation, foam cell formation, and the size of the atherosclerotic plaque. BSH is present in a wide range of bacteria such as Methanobrevibacter smithii, Clostridium, Enterococcus, and so on.

13In addition to BA itself, BA receptors are indispensable in mediating their biological functions. Farnesoid X-activated receptor (FXR) is one of the most important and well-studied BA receptors that regulates glucose and lipid metabolism by affecting transcription of genes that are involved in primary BA synthesis. The critical role of FXR in mediating cholesterol metabolism was elucidated by using FXR-/- mice which have increased plasma high density lipoprotein (HDL) cholesterol, non-HDL cholesterol and triglyceride levels compared to wild-type mice. In a previous study, loss of functional FXR in apolipoprotein E-deficient (ApoE-/-) mice, a mouse model of atherosclerosis, resulted in more severe lipid metabolism defects and enhanced aortic plaque formation. Furthermore, FXR deficiency can result in a decrease of plasma low-density lipoprotein cholesterol and CD36 expression in macrophages, leading to a reduced risk of atherosclerosis in LDLR knockout (LDLR-/-) mice. On the other hand, research indicates that activation of FXR with an agonist can protect against atherosclerosis in LDLR-/- and ApoE-/- mice, which may be associated with suppression of genes involved in BAs synthesis. The G protein-coupled BA receptor, also known as TGR5, is another important host BA receptor that is responsive to BAs. Recent investigations have indicated that activation of TGR5 can inhibit atherosclerosis formation, an effect associated with a reduction of macrophage inflammation and lipid loading. Moreover, activation of TGR5 also contributes to enhanced energy expenditure and improved glycemic control. Pregnane X receptor (PXR) is another type of nuclear hormone receptor that regulates the expression of genes involved in the biosynthesis, transport, and metabolism of BAs, and can also be activated by secondary BAs such as LCA. Deletion of PXR attenuates the development of atherosclerosis in PXR and apoE double knockout (PXR-/- and ApoE-/-) mice, which may be associated with the reduction of CD36 expression and lipid uptake in macrophages. It has been reported that activation of PXR by a PXR agonist increases the levels of atherogenic lipoproteins VLDL and LDL, and that PXR activation accelerates atherosclerosis in ApoE-/- mice. In addition, the vitamin D3 receptor (VDR) is a sensor for bacteria-induced BA that is much more sensitive to LCA and its metabolite (3-oxo-LCA) than other nuclear receptors. It has been found that macrophage VDR signaling attenuates atherosclerosis in mice in part by inhibiting the local renin-angiotensin system. Finally, sphingosine-1-phosphate receptor 2 (S1PR2) can be activated by various conjugated BAs and then promotes atherosclerosis by regulating macrophage retention and inflammatory cytokine secretion, whereas S1PR2 knockdown attenuates atherosclerosis in ApoE-/- mice.

14In summary, gut microbiota-derived secondary BAs play important roles in the development of atherosclerosis through the modulation of various BA receptors such as FXR, PXR, TGR5, VDR, and S1PR2. This finding highlights the great potential for novel atherosclerosis therapy by targeting gut microbiota. The main mechanisms associated with gut microbiota-derived metabolites and atherosclerosis is shown schematically in Figure 2.

15Hypertension is another important risk for CVD that is induced by both genetic susceptibility and environmental factors. Given the increasing recognition of the role of gut microbiota in metabolic diseases, the relationship between gut microbiota and hypertension has also been evaluated in recent years. In 1982, it was demonstrated that antibiotic treatment could produce a higher blood pressure, which implicated the probable involvement of gut microbiota in regulating blood pressure. In spontaneously hypertensive rats, observed a significant decrease in microbial richness and diversity, and an increase in the ratio of Firmicutes/Bacteroidetes. In another study, compared with conventionally raised (CONV-R) mice, GF mice infused with AngII showed attenuation of the blood pressure increase in response to AngII, indicating that gut microbiota promotes AngII-induced vascular dysfunction and hypertension. Accordingly, the gut microbiota is probably involved in the development of hypertension. Although the relationship and mechanism underlying gut microbiota and hypertension have not yet been fully elucidated, the existing evidence has highlighted the critical roles of SCFAs and oxidized low-density lipoprotein (ox-LDL) in hypertension.

16Short-chain fatty acids (such as acetate, proprionate, and butyrate), which are derived from dietary fiber (mainly polysaccharides), play crucial roles in maintaining the homeostasis of the gut microbiome and host immunity. Interestingly, bacteria that metabolize polysaccharides into different types of SCFAs are specific. For instance, the major acetate-producing bacteria are Streptococcus spp., Prevotella spp., Bifidobacterium spp., Clostridiums pp., A. muciniphila, and so on. Propionate is generated from carbohydrate fermentation by Bacteroides spp., Salmonella spp., Dialister spp., Veillonella spp., Roseburia inulinivorans, Coprococcus catus, Blautia obeum, etc., while butyrate is derived from Lachnospiraceae, Ruminococcaceae, and Acidaminococcaceae families. Clinical evidence has shown that the abundance of butyrate-producing bacteria is associated with a lower blood pressure in obese pregnant women. A recent study found that fiber and acetate supplementation improved gut dysbiosis, associated with an increase in Bacteroides acidifaciens, which may play a protective role in hypertension and heart failure in hypertensive mice.

17The role of host G-protein-coupled receptors (GPCRs) in the development of hypertension has been well reviewed. To date, there are at least three GPCRs that are regulated by SCFAs including GPR41, GPR43, and GPR109A. SCFAs can stimulate host GPCRs-regulated pathways to affect renin secretion and therefore blood pressure. One study has reported that GPR41 knockout mice exhibited systolic hypertension compared with wild-type mice, and that SCFAs lowered blood pressure by regulating endothelial GPR41. Olfactory receptor 78 (Olfr78) is another type of GPCR expressed in the kidney, which can also be modulated by SCFAs such as acetate and propionate. In addition, both Olfr78 and GPR41 are expressed in smooth muscle cells of small resistance blood vessels. Propionate can induce vasodilation and produce an acute hypotensive response in mice through modulation of Olfr78 and GPR41 activity. On the other hand, it was found that stimulation of GPR41 resulted in a reduction of the hypotensive response, and this effect could be opposed by stimulating Olfr78. Interestingly, antibiotic treatment not only altered the composition of gut microbiota, but also increased blood pressure in Olfr78 knockout mice. In recent years, conducted a randomized double-blind placebo-controlled trial, in which SCFAs and a number of metabolic parameters were measured. The inconsistent outcome was reported that the levels of SCFAs had no significant effects on energy or glucose homeostasis in humans. Overall, although all these findings revealed that gut microbiota may play important roles in modulating the host blood pressure through production of microbial SCFAs, the potential for SCFAs to be a therapeutic target for CVD needs to be confirmed by additional investigations in the future.

18Generally, the regulation of blood pressure depends on the magnitude of blood vessel vasoconstriction and vasodilation. In addition to the regulation of various receptors, gut dysbiosis also contributes to hypertension through vasoconstriction mediated by oxidation of LDL.

19Dysbiosis can promote the expression of pro-inflammatory cytokines and induce oxidative stress, which can stimulate Ox-LDL. Previous studies have shown that higher levels of oxLDL contribute to hypertension by inhibiting the production of nitric oxide (NO) and endothelin-1. NO is a well-established vasodilator that is produced through oxidation of L-arginine by NO synthase. Ox-LDL decreases the production of NO and reduces the degree of vasodilation. Moreover, endothelin-1 plays crucial roles in maintaining basic vascular tension and cardiovascular system homeostasis. Interestingly, the activity of endothelin-1 on blood vessels is concentration-dependent, that is, endothelin-1 produces vasodilatory effects at low concentrations by activating the endothelial receptor B (ETB) and promoting NO production, but produces vasoconstriction at high concentrations by increasing ox-LDL production in plaques and activating the endothelial receptor A (ETA; ).

20Although a causative relationship between gut dysbiosis and hypertension has been acquired, the exact role of gut microbiota in mediating hypertension still requires further extensive investigation. The main mechanisms associated with gut microbiota and hypertension are shown schematically in Figure 3, together with a summary of microbial-derived metabolites and CVD development in Table 1.

21Given the contributions of gut microbiota to the development of CVD, they have emerged as a potentially important target for CVD therapy. The most frequently used approaches to manipulate the gut microbiota include probiotic, prebiotic, natural components, fecal transplantation, and so on.

22Probiotic is a collection of bacteria with a wide range of beneficial effects on host metabolism. The widely used probiotics are Lactobacillus, Bifidobacterium, and Satreptococcus. In a randomized double-blind clinical trial, found that a probiotic of Lactobacillus plantarum CECT 7527, 7528, and 7529 reduced circulating cholesterol levels and inhibited the formation of atherosclerotic plaques in hyper-cholesterol patients. In another randomized control study, subjects taking Lactobacillus reuteri NCIMB 30242 showed more significant reductions of LDL-C and total cholesterol levels compared to subjects given placebo capsules. In addition, the benefits of probiotics of different Lactobacillus bacteria (Lactobacillus fermentum CECT5716 (LC40), Lactobacillus coryniformis CECT5711 (K8) and Lactobacillus gasseri CECT5714 (LC9) in the regulation of blood pressure have been investigated in spontaneously hypertensive rats, and it was found that long-term administration of these probiotics could reduce systolic blood pressure. A recent study has reported that the probiotic L. plantarum ECGC13110402 was well tolerated and can be used as an alternative or supplement to reduce cardiovascular risk.

23Prebiotic is a class of indigestible food ingredients with benefits via selectively stimulating the growth of “good” and suppressing the growth of “bad” bacteria in the intestinal tract. Prebiotic can usually cause specific changes in the composition of gut microbiota and exert beneficial effects on host metabolism. Recent investigations have shown that an inulin-type fructans (ITFs) supplement improved endothelial function in ApoE-/- mice, while administration of ITFs promoted the production of butyrate and resulted in atheroprotective effects. A previous investigation reported that long-chain inulin could inhibit the formation of atherosclerotic plaque in ApoE-/- mice, an effect that may be associated with alterations in lipid metabolism. In a randomized, single-blind, controlled crossover clinical trial, consumption of β-glucan altered the composition of gut microbiota, an effect associated with a reduction of CVD risk markers. Additionally, mannan oligosaccharide (MOS) is another type of prebiotic. In a recent study, a MOS supplement modulated the composition of gut microbiota, lowered plasma cholesterol levels, and improved atherosclerotic plaques in high cholesterol diet-fed mice.

24In addition to probiotic and prebiotic, some natural active ingredients from herbs also have protective or therapeutic actions on CVD by modulating the gut microbiota. For example, berberine is a well-studied herbal-derived chemical with effective activity against atherosclerosis. It was found that the anti-atherosclerotic effect of berberine was associated with the stimulation of Akkermansia in ApoE-/- mice. Another example is resveratrol that may have protective effect against several cardiovascular risk factors such as hyper-cholesterol and TMAO by modulating the gut microbiota and expression of genes involved in maintaining the integrity of the gut barrier. Moreover, resveratrol was found to attenuate TMAO-induced atherosclerosis by decreasing gut microbiota-mediated TMAO synthesis and increasing BA metabolism.

25Fecal microbiota transplantation (FMT) is a promising method of introducing “healthy” bacteria from healthy subjects into the gastrointestinal tract of patients with dysfunctional guts, which has received much attention in recent years. In one study, the insulin sensitivity of recipients was significantly enhanced after 6 weeks transfer of microbiota from lean normal donors to male recipients with metabolic syndrome. FMT increased the abundance of butyrate-producing bacteria suggesting that FMT is a potential strategy for CVD therapy. Nevertheless, the use of FMT is also limited in the clinic due to the possible risk of transferring endotoxins or infectious diseases to recipients.

26Although gut microbiota-targeted therapy to treat CVD is promising in the context of increasing positive experimental and clinical evidence, discrepant results have also been reported in both experimental and clinical studies. For instance, recently, scientists evaluated the effects of probiotic intervention on plasma TMAO levels in CKD patients, but there no significant change was observed after 3 months supplementation. Similarly, FMT from vegans resulted in a slight alteration in the composition of the gut microbiota, but no improvement in TMAO production or vascular inflammation.

27Although many types of medicines are available in the clinic to treat CVD, currently, it is still the leading cause of death worldwide. In recent years, increasing evidence has suggested an important role for gut microbiota in the development of both metabolic diseases and CVD. The findings have shed light on the great potential of targeting the gut microbiota to aid the elucidation of the fundamental mechanisms underlying disease and/or to uncover novel preventative or therapeutic regimes. Currently, most of the research efforts have focused on paid on establishing the relationship between gut dysbiosis and the development of CVD. Although much progress has been made, there is some way to go before the unequivocal establishment of gut microbiota-targeted therapy for CVD in the clinic.

28Given the experimental and clinical advances with regard to the mechanisms of gut microbiota in the pathogenesis of CVD, there is great promise of finding new approaches to treat CVD by using gut microbial metabolites such as SCFAs and some types of BAs, or blocking the production of detrimental microbial metabolites such as TMAO with inhibitors. In addition, methods to alter the gut microbial composition with probiotic, prebiotic, natural components, and FMT should be further explored. In the future well-designed large-scale clinical studies will be needed to validate experimental and other small-scale preliminary clinical data. The integration of omics approaches (metabolomics, metagenomics, and metatranscriptomics) may be of critical significance to explore the exact roles of identified gut bacteria in the pathogenesis of many diseases.

29JM drafted the manuscript. HL revised the manuscript.

30The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.