H OH metabolites OH Review Exploring the Gut Microbiota and Cardiovascular Disease Kiera Murphy 1,2, Aoife N. O’Donovan 1,2,3, Noel M. Caplice 2,4, R. Paul Ross 2 and Catherine Stanton 1,2,3,* 1 Teagasc Food Research Centre, Moorepark, Co. Cork P61 C996, Ireland; [email protected] (K.M.); [email protected] (A.N.O.) 2 APC Microbiome Ireland, Biosciences Institute, University College Cork, Cork T12 YT20, Ireland; [email protected] (N.M.C.); [email protected] (R.P.R.) 3 VistaMilk SFI Research Centre, Teagasc, Moorepark, Co. Cork P61 C996, Ireland 4 Centre for Research in Vascular Biology, Biosciences Institute, University College Cork, Cork T12 YT20, Ireland * Correspondence: [email protected]; Tel.: +353-25-42606 Abstract: Cardiovascular disease (CVD) has been classified as one of the leading causes of mor- bidity and mortality worldwide. CVD risk factors include smoking, hypertension, dyslipidaemia, obesity, inflammation and diabetes. The gut microbiota can influence human health through mul- tiple interactions and community changes are associated with the development and progression of numerous disease states, including CVD. The gut microbiota are involved in the production of several metabolites, such as short-chain fatty acids (SCFAs), bile acids and trimethylamine-N-oxide (TMAO). These products of microbial metabolism are important modulatory factors and have been associated with an increased risk of CVD. Due to its association with CVD development, the gut microbiota has emerged as a target for therapeutic approaches. In this review, we summarise the Citation: Murphy, K.; O’Donovan, current knowledge on the role of the gut microbiome in CVD development, and associated microbial A.N.; Caplice, N.M.; Ross, R.P.; communities, functions, and metabolic profiles. We also discuss CVD therapeutic interventions that Stanton, C. Exploring the Gut target the gut microbiota such as probiotics and faecal microbiota transplantation. Microbiota and Cardiovascular Disease. Metabolites 2021, 11, 493. Keywords: cardiovascular disease; gut microbiota; metabolites; probiotics; faecal microbiota trans- https://doi.org/10.3390/ plantation metabo11080493 Academic Editors: Lluis Arola, Manuel Suárez Recio and 1. Introduction Cristina Torres-Fuentes Cardiovascular disease (CVD) is a general term for a number of pathologies including coronary heart disease (CHD), cerebrovascular disease, peripheral artery disease, con- Received: 27 May 2021 genital and rheumatic heart disease and venous thromboembolism [1]. CVD is a chronic Accepted: 27 July 2021 Published: 29 July 2021 progressive condition, often leading to irreversible damage to vascular structures in the form of atherosclerosis and detrimental clinical outcomes such as arterial thrombosis, Publisher’s Note: MDPI stays neutral myocardial infarction (MI) and stroke. Elevated levels of serum cholesterol (hypercholes- with regard to jurisdictional claims in terolemia), particularly low-density lipoprotein cholesterol (LDL-C) is a well-documented published maps and institutional affil- risk factor for CVD. LDL-C deposits cholesterol into artery walls and this plaque build-up iations. can lead to atherosclerosis [2]. CVD has been classified as one of the leading causes of morbidity and mortality worldwide, which has led to extensive health and economic burdens accounting for approximately one in every three deaths in the United States and one in every four deaths in Europe [3]. The World Health Organisation (WHO) has estimated that over 75% of premature Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. CVD is avoidable and that risk factor improvement can aid in the reduction in the growing This article is an open access article incidence of CVD [4]. The INTERHEART study established a case–control study of acute distributed under the terms and MI from 52 countries worldwide. The study identified nine easily assessed risk factors conditions of the Creative Commons which were highly and significantly associated with an increased risk of acute MI [5]. Attribution (CC BY) license (https:// Smoking, dyslipidaemia, hypertension, diabetes, abdominal obesity, psychosocial factors, creativecommons.org/licenses/by/ daily fruits and vegetables, exercise, alcohol intake and apolipoprotein (Apo)B/ApoA1 4.0/). ratio accounted for over 90% of the risk of acute MI. Metabolites 2021, 11, 493. https://doi.org/10.3390/metabo11080493 https://www.mdpi.com/journal/metabolites Metabolites 2021, 11, 493 2 of 23 The human body harbours trillions of microbial cells that form a complex microbial community in the intestine known as the gut microbiota. The gut is predominantly in- habited by two phyla, Bacteriodetes and Firmicutes, which account for 90% of the total inhabiting microbes. The remaining 10% consists of Actinobacteria, Cyanobacteria, Fu- sobacteria, Proteobacteria and Verrucomicrobia [6,7]. Beginning at birth, there are multiple factors that can influence how the gut is colonised, such as mode of birth (vaginal vs. caesarean), feeding method (breast feeding vs. infant formula), exposure to antibiotics, hy- giene standards and geographical location [8,9]. It has been shown that the gut microbiota of infants who are vaginally delivered predominantly consist of Bifidobacterium in the weeks and months after birth, whereas those infants born via caesarean section predominantly consist initially of maternal skin microbiota, mainly Staphylococcus [10]. As the infant ages, the initial dominant aerobic gut environment matures and evolves to form an anaerobic environment, resulting in greater abundances of Bifidobacterium and Clostridia [7]. The gut microbiota begin to stabilise and evolve toward an adult-like composition by the second year of life [10]. The gut microbiota perform essential metabolic functions, serve as a source of essential nutrients and vitamins and assist in energy and nutrient extraction from food [11,12]. The gut microbiota also work in conjunction with the host’s defence and immune systems to protect against pathogen infiltration and colonisation. An imbalance in gut microbial com- position can result in these mechanisms becoming disrupted and can contribute to increased intestinal permeability or a ‘leaky gut’ [13]. In the leaky gut, the tight junctions which support the epithelial lining barrier are disrupted, allowing infiltration of microbes, toxins or antigens into the tissues beneath. This may trigger inflammation through the activation of local and systemic immune responses. Shifts in the composition of the gut microbiota disrupt homeostasis and have been associated with long-term consequences, leading to disorders including CVD. 16S rRNA gene sequencing and whole-metagenome shotgun sequencing have added to the growing body of evidence that suggests that alterations of the gut microbiome composition affect the pathogenesis of CVD [7,14–16]. In addition to the composition of the gut microbiota, metabolites produced by these microbes can enter into the circulation and act as a modifiers of gut microbial effects on the host [17,18].A number of microbiome studies have shown a potential link between microbial metabolites including short-chain fatty acids (SCFAs), bile acids (BA) and trimethylamine N-oxide (TMAO) and CVD [19–21]. The gut microbiota are involved in the digestion of food through two catabolic path- ways, the saccharolytic pathway and the proteolytic pathway [22]. The saccharolytic pathway uses the gut microbiota to help break down sugars in the ingested foods, leading to SCFA production. The proteolytic pathway is characterised by the fermentation of ingested proteins, leading to SCFA production as well as formation of other metabolites including ammonia, thiols, amines, phenols and indoles. Diet is an important risk factor for CVD and the complex interactions between food and gut microbiota, and resulting metabolites play a key role in cardiovascular health. Given the growing body of evidence linking the gut microbiota with development of CVD, therapies aimed at modulating the gut microbiota have been suggested among the most promising strategies to prevent CVD. 2. Gut Microbiota and CVD Many metabolites in the human body originate from intestinal microbes and there is growing evidence supporting the relationship between these metabolites and the de- velopment of CVD [18,23]. Through these metabolites, the gut microbiota influence host metabolic pathways such as cholesterol metabolism, in addition to inflammatory reac- tions and oxidative stress. Gut microbe-dependent metabolites may be absorbed into the bloodstream through the intestinal epithelium, which in turn influences the function of several organs and bodily systems [24]. Production of TMAO by the gut microbiota is a key mechanism of CVD. Dietary choline, phosphatidylcholine and carnitine are metabolised by gut microbes to produce trimethylamine (TMA). TMA is transported to the liver and fur- Metabolites 2021, 11, 493 3 of 23 ther converted to TMAO via hepatic enzymes, flavin-containing monooxygenases [25,26]. TMAO has been shown to modulate cholesterol and sterol metabolism, cholesterol trans- port and bile acids levels [19–21,27]. Elevated serum levels of TMAO are associated with early atherosclerosis, severity of peripheral artery disease and with high risk of CVD mortality [28,29]. In a study of 4007 patients undergoing elective coronary angiography, increased plasma levels of TMAO were associated with an increased risk of death, MI, and stroke during three years of follow-up [21]. Dietary supplementation