Accumulating evidence supports a relationship between the complexity and diversity of the gut microbiota and host diseases. In addition to alterations in the gut microbial composition, the metabolic potential of gut microbiota has been identified as a contributing factor in the development of diseases. Recent technological developments of molecular and biochemical analyses enable us to detect and characterize the gut microbiota via assessment and classification of its genomes and corresponding metabolites. These advances have provided emerging data supporting the role of gut microbiota in various physiological activities including host metabolism, neurological development, energy homeostasis, and immune regulation. Although few human studies have looked into the causative associations and underlying pathophysiology of the gut microbiota and host disease, a growing body of preclinical and clinical evidence supports the theory that the gut microbiota and its metabolites have the potential to be a novel therapeutic and preventative target for cardiovascular and metabolic diseases. In this review, we highlight the interplay between the gut microbiota and its metabolites, and the development and progression of hypertension, heart failure, and chronic kidney disease.
The human gut harbors more than 100 trillion microbial cells, and majority of them being classified into the phyla Firmicutes and Bacteroidetes. Additionally, these microbes are responsible for various physiological activities including host metabolism, neurological development, energy homeostasis and immune regulation, and vitamin synthesis and digestion , and are capable of regulating the normal function of intestinal epithelial mucosal barriers [2,3]. Disruption of intestinal epithelial barrier function can lead to increased gut permeability, increased bacterial translocation, and increased circulating endotoxins that can contribute to the underlying inflammation related to the progression of cardiovascular disease. Furthermore, accumulating evidence suggests that quantitative and qualitative alterations of the gut microbiota (so-called ‘dysbiosis’) can also play an important role in the pathogenesis of various cardiovascular and metabolic diseases [4–7]. Gut dysbiosis can induce significant changes in the gut immune system, leading to altered signals derived from the gut to the systemic immune system. Indeed, direct modulation of the gut microbiota has been associated with alterations in host metabolism, mainly through immune systems and hormone secretion [8–10]. Furthermore, the metabolic potential of the gut microbiota has been identified as a contributing factor in the development of diseases, with metabolites acting on distant target organs in a manner similar to the human host’s endocrine organ [11–15]. In addition to gut microbiota, many bacteriophages have been found in human gut . Prokaryotic viruses, which are currently thought to be the most abundant in the gut, are associated with human health by affecting bacterial community structure and function [17–19]. However, the precise mechanisms of these pathways are yet to be fully clarified. In this review, we highlight the interplay between the gut microbiota and its metabolites, and cardiovascular diseases, including hypertension and heart failure, and chronic kidney disease (CKD) (Figure 1).
Interplay between the gut and the brain, the heart and the kidneys
Gut–brain axis and blood pressure regulation
The homeostatic regulation of blood pressure is a complex process, which is regulated by several factors such as genes, environment, and endocrine hormone secretion. Recent studies have revealed that the human gut has a significant connection with the central nervous system via complex signaling pathways, including bidirectional neuroendocrine signals and immunological factors . This, so-called ‘gut–brain axis’ consists of the gut microbiota, central nervous system, enteric nervous system, and parasympathetic and sympathetic nervous systems . It has also been recognized that altered gut microbial composition is associated with the development of neurodegeneration [22,23]. Besides regulating the microglia, manipulation of the gut microbiota has been thought to be able to modulate neuroimmune activation through the production of microbial derived bioactive metabolites, such as short-chain fatty acids (SCFAs) [23–25].
Several studies have suggested a potential role of the gut microbiota in blood pressure regulation and the pathogenesis of hypertension [26–29]. A recent case report demonstrated a potent antihypertensive effect of the gut microbiota in a patient with resistant hypertension that was treated with minocycline . This effect was thought to be mainly due to the production of SCFAs, which are important signals generated by the gut microbiota [31–34]. Gut dysbiosis was observed in patients with hypertension [31–34], which was characterized by a reduction in SCFA production and a change in the Firmicutes to Bacteroidetes ratio [26,27,29]. An animal study of spontaneous hypertensive rats also demonstrated an increase in the Firmicutes to Bacteroidetes ratio, while chronic administration of SCFA reversed this ratio and attenuated the hypertension . Mechanistically, SCFAs can function to stimulate G-protein-coupled receptors 41 (GPR41) and 43 (GPR43), and these are expressed in the renal vasculature . The SCFA olfactory receptor 78 (Olf78) is also expressed in the kidneys, where it regulates blood pressure . However, the balance across these G-protein-coupled receptor activities is complex and likely to be dynamic. In a study using GPR41-deficient mice, administration of the SCFA propionate was associated with a significant increase in blood pressure, which suggested that GPR41 can negate a pressor response to SCFA .
Gut microbial composition was found to be significantly different between spontaneously hypertensive rats and Wistar–Kyoto control rats, with hypertensive rats having reduced taxa richness and altered microbial composition compared with the control rats . At the phylum level, the Firmicutes to Bacteroidetes ratio was significantly higher in the hypertensive rats than in the control rats. In addition, angiotensin II-treated rats showed a reduction in microbial species richness and an increased Firmicutes to Bacteroidetes ratio compared with control rats . Another study has reported that the microbiota of Dahl S (salt sensitive) rats was significantly different from that of Dahl R (salt resistant) rats. The phylum Bacteroidetes and family Veillonellaceae were reported to be more abundant in Dahl S rats than in Dahl R rats . Moreover, Dahl S rats showed a significant increase in blood pressure with a high-salt diet, whereas the Dahl R rats did not respond to a high-salt diet [22,29]. Furthermore, administration of an antibiotic to Dahl S rats did not affect their hypertensive responses to the high-salt diet. Although fecal transplantation from Dahl R rats to Dahl S rats exacerbated the hypertensive responses of Dahl S rats and was associated with significantly elevated plasma levels of the SCFAs, fecal transplantation from Dahl S rats to Dahl R rats could not change the hypertensive phenotype of the Dahl R rats . These findings imply that the physiologic differences reside in the differences of gut microbial compositions and corresponding functions between the two rats.
Gut microbial interaction with heart failure and atherosclerosis
Heart failure is a growing health problem and a main cause of mortality and morbidity worldwide. The gut has also been implicated in the pathophysiology and progression of heart failure, largely attributed to impaired perfusion to the intestines leading to intestinal barrier dysfunction. The intestinal endothelial barrier is maintained by several mechanisms of a well-balanced gut microbiota [36,37]. In patients with heart failure, reduced blood flow into the intestinal endothelium due to reduced cardiac output leads to intestinal wall ischemia, leading to increased permeability due to structural disruption of the intestinal epithelial barrier function . Furthermore, systemic congestion in patients with heart failure can cause intestinal wall edema, which also results in increased intestinal permeability. This ‘leaky gut’ is associated with translocation of endotoxins, microbial components, and microbial metabolites, such as lipopolysaccharides (LPS) produced by Gram-negative bacteria, to enter the systemic circulation [39,40]. This process can further activate cytokines and generate systemic inflammation that, in turn, contributes to the progression of heart failure [41–43]. Indeed, gut microbial DNA has been detectable in the peripheral blood of patients with advanced kidney diseases, suggesting the leaky gut may serve as the primary source .
In addition to hemodynamic deterioration in patients with heart failure, evidence showed that gut dysbiosis was associated with the production of several gut microbiota derived metabolites, as well as promoting the disruption of gut endothelial barrier function . In patients with heart failure, a significant interaction was observed between the amount of fecal gut microbiota and the intensity of intestinal permeability . Furthermore, patients who had bacterial DNA in the peripheral blood had significantly higher plasma levels of inflammatory markers, including high-sensitive C-reactive protein and interleukin-6 levels, than those who did not have bacterial DNA in their peripheral blood . A recent study has shown that heart failure patients had a significantly decreased diversity of gut microbiota and a depletion of core gut microbial groups . These observations suggested that better understanding of the regulation of intestinal barrier function has the potential to develop intestinal barrier directed heart failure therapy.
Trimethylamine N-oxide (TMAO), which is derived from metabolites of gut microbiota from specific dietary nutrients, is clearly linked to atherosclerotic cardiovascular risk [44,48–50]. Circulating TMAO levels are elevated and associated with disease severity in patients with atherosclerosis, CKD, and peripheral artery diseases [50–52]. We found that elevated TMAO concentrations in animal models were associated with corresponding increases in renal tubulointerstitial fibrosis . A previous study has shown that microbial taxa belonging to the Clostridiaceae and Peptostreptococcaceae families were positively associated with blood levels of TMAO in humans . Interestingly, patients with heart failure had significantly higher levels of TMAO than age-matched and sex-matched subjects without heart failure, irrespective of heart failure etiology . Moreover, plasma TMAO levels were associated with adverse outcomes, and elevated plasma TMAO levels had a strong adverse prognostic value in addition to the traditional risk factors, cardiorenal indices, and markers of systemic inflammation . In a recent animal study using a transaortic constriction model of heart failure, mice fed on a high-choline diet had higher TMAO levels than those with standard choline diet . In addition, high-choline diet fed mice showed adverse ventricular dilatation and wall thinning with a marked increase in fibrosis . The profibrotic transforming growth factor (TGF)-β p-Smad3 pathway has been shown to be enhanced in the choline diet fed mice . However, the causative effects of TMAO and the underlying mechanistic link that explains how TMAO might directly or indirectly promote heart failure are not well understood. In fact, although elevated TMAO levels have shown equivalent adverse prognostic value in patients with ischemic and non-ischemic etiologies , a recent report has reported a conflicting result that TMAO levels were elevated in ischemic heart failure compared with non-ischemic dilated cardiomyopathy . Furthermore, there is a lack of understanding of the relationship between TMAO or other gut microbial metabolites in altered intestinal permeability and hemodynamic status in the context of heart failure. Additional studies are needed to investigate whether manipulation of the gut microbial TMAO pathway can attenuate the progression of heart failure, and improve outcomes, even though early animal studies have shown promise with direct microbial enzyme inhibitors of TMA/TMAO synthesis in attenuating cardiac dysfunction .
Gut renal axis and uremic toxins
A complex interplay between the kidney and the gut microbiota has long been observed . Several mechanisms were suggested to underlie the interaction between the gut microbiota and pathogenesis of CKD . Gut microbiota has been shown to contribute to the generation of several uremic toxins such as advanced glycation end products (AGEs), phenols, and indoles . These are absorbed into the systemic circulation and are normally cleared by the kidneys. However, in states of impaired renal function, they accumulate and become toxic and induce an inflammatory response in the host, resulting in the progression of CKD [38,56]. The presence of uremic toxins has justified a wide range of innovative and/or obscure colonic cleansing practices or resection procedures to remove colonic uremic toxins over the past century. Nevertheless, several contemporary studies have suggested that blood levels of uremic toxins have been associated with mortality and cardiovascular risks [58–60]. Circulating LPS levels increase with the stages of CKD and were an independent predictor of mortality . Elevated indoxyl sulphate levels (microbial product of tryptophan metabolism) were associated with aortic calcification, increased vascular stiffness, and the risk of cardiovascular mortality in patients with CKD . In uremic rats, the administration of indoxyl sulphate mediates the renal gene expression of TGF-β1, which is related to tubulointerstitial fibrosis . Indoxyl sulphate has been suggested to be responsible for vascular disease through an induction of oxidative stress. Indoxyl sulphate induced the production of reactive oxygen species in human umbilical vein endothelial cells . Treatment with the oral adsorbent AST-120 resulted in a significant decrease in indoxyl sulphate with a corresponding increase in flow-mediated, endothelium-dependent vasodilatation . Meanwhile, another tryptophan metabolite, p-cresyl sulphate, may cause inflammation in blood vessels in experimental models . Elevated p-cresyl sulphate levels were also associated with all-cause mortality . Furthermore, in a partial nephrectomized mice model, indoxyl sulphate or p-cresyl sulphate activated the intrarenal renin–angiotensin–aldosterone system and interstitial fibrosis and glomerulosclerosis .
In humans and animals with advanced CKD, extensive change in the structure and function of the gut microbiota has been reported . Gut dysbiosis in patients with CKD has been implicated in an increased production of indoxyl sulphate and p-cresyl sulphate, and as a promoter for increasing inflammation and contributing to the progression of their disease . Patients with CKD also showed significant changes in SCFA-producing microbiota, which are known to have beneficial effects. These observations suggested that gut dysbiosis and uremic toxins can cause endothelial dysfunction, aggravate inflammation and oxidative stress, and impair the normal physiological repair process of the endothelium .
Patients with CKD have consistently high TMAO levels that are associated with higher mortality, accelerated atherosclerosis, and progressive loss of kidney function in patients with CKD [50,51,67,68]. Dietary induced elevation of TMAO concentrations in mouse models has been associated with greater degree of tubulointerstitial fibrosis . Animals with increased TMAO levels also had increased fibrosis and phosphorylation of Smad3, an important regulator of fibrosis . However, there was only a modest correlation between TMAO levels and the estimated glomerular filtration ratio [51,69]. Since TMAO is predominately excreted in urine and its clearance is largely dependent on renal function [49,70], decreased renal function in patients with CKD can confound the association of high TMAO levels in CKD . A recent meta-analysis clearly showed that TMAO and its precursors were associated with an increased risk of mortality and major adverse cardiac event independently of traditional risk factors such as diabetes mellitus, obesity, and CKD . Further studies exploring the physiology of TMAO generation and metabolism are warranted to more thoroughly define the etiology of TMAO elevations in CKD.
Several studies have shown that dietary interventions were effective strategies in reducing cardiovascular risks [73,74]. It has been reported that adherence to the Mediterranean diet leads to a decrease in all the causes of mortality and in the incidence of cardiovascular diseases . Moreover, dietary interventions can modify the microbiota composition by inducing rapid changes in certain nutrients [76,77]. A high level of variability in the microbiota significantly correlated with dietary habits, confirming the shaping effect of long-term dietary patterns on the gut microbiota . A previous study has shown that fiber-rich diets promote the growth of beneficial commensal bacteria and limit the growth of known opportunistic pathogens . Thus, modulation of the gut microbiota composition through dietary intervention represents a promising therapeutic target. However, little is known about the mechanistic interplay between the diet intervention and the relevant gut microbial metabolism.
Conclusion and future perspective
The gut microbiota is an exciting new research field with enormous possibilities for human health, and holds great promises for novel diagnostic and therapeutic approaches with lasting and preventive impact. Nevertheless, few human studies have looked into causative associations of the gut microbiota and its metabolites to host disease, and their putative mechanisms. Recent novel molecular biochemical analyses are expected to enable the detection and classification of the diverse microorganisms, and to assess all genomes of these microbiota and their metabolites. In particular, since the diversity of the gut microbiota in different individuals may lead to different responses to treatment and ultimately different outcomes, genome-scale metabolic models have the potential to be used as a key in understanding the role of individual species in the gut microbiota as well as the role of the microbiota as a whole. Genome-scale metabolic models are mathematical representations of the knowledge of a microbiota’s metabolic capacity and can predict how metabolic system functions differently in different species by integrating knowledge of the metabolism of the gut microbiota. These technological developments are expected to facilitate the move from correlation studies to a gain in mechanistic insights, and to result in new diagnostic tests and therapeutics in the near future. Additional studies are needed to investigate the exact mechanisms underlying interactions between the host and microbiota to better understand the impact of direct or indirect manipulations of the gut microbiota on their biological functions.
The authors declare that there are no competing interests associated with the manuscript.
This work was supported by the National Institute of Health (NIH)/ Office of Dietary Supplements [grant numbers P20HL113452, R01DK106000, R01HL126827 (to W.H.W.T)]. W.H.W.T. and T.K. have no relationships to disclose.