Faecal Transplant Therapy: A Promising Treatment Modality for Cardiovascular Diseases

Cardiovascular diseases (CVD) are considered as “lifestyle” diseases and so far “No uniﬁ ed procedure” or medicines are eﬀ ective in the management of this group of diseases. Researchers and clinicians have indicated that no safe therapeutic window is available in therapeutics at present. Recent research showed that gut microbiota are eﬀ ective in managing lifestyle diseases therefore we introduced the inﬂ uence of gut microbiota in the prognosis of the CVDs. Faecal transplant therapy(FMT) has been anticipated to treat many diseases similar to recurrent bacterial Clostridioides di ﬃ cile infection which has been used worldwide. Recently, FMT was tried on an animal model to treat CVDs, and recent human trials that were tried to manage CVDs in humans by FMT showed encouraging results. The mechanism of action of transplanted bacteria to manage CVDs in the human population is also discussed. In-depth knowledge on the pros and cons of FMT will pave the way to standardize the procedure once the lacuna existing at present in treating CVDs, is paved, this technology will be useful for the masses.


Introduction
Cardiovascular Diseases (CVDs), especially coronary atherosclerosis, arteriosclerosis, Hypertension (HTN), and Heart Failure (HF), are the main causes of death, accounting for a huge health and economic burden on the global population. In lammation, diabetes, diet, nutritional status, and lifestyle are identi ied as causal factors for CVDs [1,2].
In arteriosclerosis thickening and stiffening of arteries take place, due to which low of oxygen and nutrients are restricted [3], whereas in atherosclerosis plaque formation in the arteries takes place which results in high blood pressure, because of high blood pressure there is a possibility for the plaque to burst, resulting a blood clot formation". Hypertension (HTN) is the predominant and most common risk factor associated with stroke and Coronary Heart Disease (CHD). Pregnancy can also increase the risk of developing high blood pressure. Uncared prolonged high blood pressure increases the risk of developing a number of serious long-term health conditions such as damaging the blood vessels in the kidneys or eyes and coronary heart disease.
Decreased blood low to the heart can cause angina. Heart attack, which happens when the lack of blood supply results in the death of the heart muscles without enough oxygen. Stroke can end up in serious disabilities in speech, movement, and our attention on the relationship between gut microbiota and CVDs. It has also been revealed that intestinal microbiotarelated metabolites, such as Trimethylamine-N-oxide (TMAO), Short-Chain Fatty Acids (SCFA), and Bile Acids (BAs), are also related to the development, prevention, treatment, and prognosis of CVDs. Animal models as well as human trials are in vogue to standardize the modality to utilize FMT as an alternative in therapeutics due to the serious side effects of the drugs currently used for the CVDs. The human gut is a huge microbial habitat with hundreds of species of bacteria. The role of biologically active metabolites produced by the gut microbiota in various aspects of host physiology is indispensable to the extent that gut microbiota is given the status of the ninth system of the human body [17]. They are responsible for maintaining the integrity of the intestinal epithelial barrier, regulating immune function [18;10], digesting nutrients, producing vitamins, and preventing the invasion of pathogenic bacteria, which is essential for human health [2]. The dysbiosis of the gut microbiota due to dietary habits, environmental factors, intestinal infections, and other factors leads to intestinal malnutrition, triggers in lammation, and abnormal metabolism a causal factor for the prognosis of CVDs [18].
Gut microbiota and coronary atherosclerosis The connection between gut microbiota and atherosclerosis was already described in 1999, when endotoxin levels, following bacterial translocation, were found to be independently correlated with carotid atherosclerosis measured by duplex ultrasound [19]. Traditional risk factors contribute to about half of the atherosclerotic burden in linear regression and genetics are believed to explain another 10 percent. Microbiota and their many metabolic products may largely account for the rest [20]. For example, DNA of oral microbiota Veillonella and Streptococcus were found in the plaques of individuals with atherosclerosis, and their abundance correlated with the increased number of these species in the oral cavity [20]. As for gut microbiota, Karlsson, et al. [21] found in 2012 that atherosclerosis is associated with a different gut metagenome [22]. Metagenomic sequencing technique gave evidence that gut microbiota in patients with atherosclerosis differed from healthy individuals, dominated by higher levels of Streptococcus and Enterobacteriaceae [20]. In addition, the Roseburia, Ruminococcaceae, and Clostridium may regulate the metabolic activity of Bile Acids (BAs) and aromatic compounds, which will further speed up the progression of coronary atherosclerosis [23]. The dysbiosis may aggrevate pro-atherosclerotic effects through metabolism-dependent pathways by altering the production of various metabolites, including TMAO, BAs, serum indoxylate, protocatechuic acid, and Lipopolysaccharide (LPS) [2].
One mechanism of microbiota-mediated atherosclerosis induction is through L-carnitine and phosphatidylcholine (from red meat, cheese, and eggs). These food components are irst converted by the microbiota to trimethylamine (TMA), then by the liver into Trimethylamine-N-oxide (TMAO), which increases atherosclerotic burden [24] and promotes a prothrombotic phenotype [25]. Studies have given evidence for the role of TAMO in immune system regulation, cholesterol metabolism, oxidative stress, and in lammatory responses to a certain extent, thereby increasing the risk of coronary atherosclerosis [2]. Faecal transplantation (FMT) of TMAO-rich gut microbiota into germ-free mice was suggested to promote platelet function and arterial thrombosis giving a clue for the role of TMAO in the prognosis of arterial thrombosis [26]. Microbiota can also protect from atherosclerosis, as recently shown when Akkermansia municiphila reversed Western dietinduced atherosclerosis and endotoxemia in ApoE-knockout mice [27]. Another recent study in ApoEKO mice showed that the probiotic mixture VSL#3 can protect from atherosclerosis [28]

Gut microbiota and hypertension
Scienti ic evidence is available for the in luence of gut microbiota on the regulation of blood pressure and abnormal bacterial populations may be one of the causal factors for the development of HTN. In fact, compared with healthy individuals, the abundance and diversity of gut microbes in hypertensive patients decreased, instead the genus Prevotella was signi icantly increased [29]. In addition, an FMT study con irmed that the faecal microbiota of patients with HTN can increase the blood pressure in germ-free mice, revealing a close link between gut microbiota and the regulation of blood pressure [30].
The excessive formation of gut microbiota metabolites is also considered to be a key factor in the occurrence of HTN more than their composition, for example, neurotransmitters produced within the autonomic nervous system by genera Bi idobacterium, Lactobacillus, Streptococcus, and Escherichia coli will alter vascular tone, leading to HTN [31]. Higher levels of circulating TMAO are positively associated with a high risk of blood pressure [31]. In turn Liu, et al. [32] found that the use of the Lactobacillus rhamnosus G strain can prevent HTN deterioration by reducing the levels of TMAO [31]. HTN is the most common risk factor associated with CVDs, and as the main risk factor for stroke and CHD morbidity and mortality, it has always been a hot topic. Recently, studies have shown that the gut microbiota is involved in blood pressure regulation and that abnormal bacterial populations are associated with HTN [31]. Hence, there exists a link between gut microbiota and HTN.

Gut microbiota and heart failure
Heart Failure (HF) is an irreversible end-stage disease with high mortality, characterized by edema and dyspnoea. Studies have found that patients with HF presented increased levels of pathogenic bacteria such as Candida and decreased levels of anti-in lammatory bacteria such as Faecalibacterium, therefore contributing to the development of HF by participating in the regulation of the mucosal immune system [2]. This indicated that there exists a correlation between gut microbiota and HF. Gut microbiota metabolites such as SCFAs, TMAO, indoxyl sulfate, and LPS also play an important role in the development of HF as in atherosclerosis.
Microorganism-targeted therapies Studies on animal models as well as human trials suggest a strong in luence of the gut microbiota on CVDs, the relationship between pathophysiology and gut microbiota is still unveri ied. A standardized alternative approach in therapeutic is wanting to escape from the side effects of antibiotics. Worldwide though there existed several microorganism-targeted therapies used in CVDs earlier, now the focus is to accumulate in-depth knowledge from human trials as well as animal models. FMT refers to the replacement of enteric pathogens by introducing the fecal contents of healthy subjects into the gastrointestinal tract of patients [35]. To elucidate the in luence of the gut microbiota on atherosclerosis pathogenesis caused by genetic de iciency an atherosclerosis-prone mouse model (C1q/TNFrelated protein 9-knockout (CTRP9-KO) mice) was generated. Kim, et al. [36] used mice model FMT to eliminate the increased Bacteroides/Firmicutes ratio ultimately reducing in lammation in cardiomyocytes and myocarditis [36;37]. In a previous study of oral and gut samples from patients with atherosclerosis, the abundance of Veillonella and Streptococcus in atherosclerotic plaques correlated with their abundance in the oral cavity, suggesting that the plaque microbiota may correlate with disease markers of atherosclerosis [18]. Furthermore, patients with symptomatic atherosclerosis had a higher relative abundance of Anaeroglobus in the oral microbiota than asymptomatic atherosclerosis controls [38]. In a previous study, Kim, et al. [37] found patients with asymptomatic atherosclerosis enriched with Collinsella genus but Roseburia and Eubacterium were more in healthy controls [38;39]. The probiotic bacterium Akkermansia muciniphila attenuates atherosclerotic lesions by improving metabolic endotoxemia-induced in lammation by restoring the gut barrier [25]. Two other dominant species of the genus Bacteroides vulgatus and Bacteroides dorei, have also been observed to be bene icial since they are capable of impairing the formation of atherosclerotic lesions in atherosclerosisprone mice, markedly ameliorating endotoxemia, decreasing gut microbial lipopolysaccharide production, and effectively suppressing proin lammatory immune responses [40] CTRP9-KO mice protected against the progression of atherosclerosis. In turn, the transplantation of CTRP9-KO microbiota into WT mice promoted the progression of atherosclerosis. Kim, et al. [37] proved in into CTRP9-KO mice CTRP9 gene de iciency is related to the distribution of the gut microbiota in subjects with atherosclerosis. Transplantation of WT microbiota into CTRP9-KO mice protected against the progression of atherosclerosis. Conversely, the transplantation of CTRP9-KO microbiota into WT mice promoted the progression of atherosclerosis. In other words, the effect is two-way since genetic variations that affect atherosclerosis alter the composition of the gut microbiota and altered gut microbial composition affects the progression of atherosclerosis, giving a clue to suggesting that fecal microbiota transplantation may help to prevent atherosclerosis. In this study, Kim, et al. [37] also showed that mutations in the genetic background can alter the composition of the gut microbiome and result in atherosclerosis. In such a situation, FMT from healthy donor stool can protect against this disease in CTRP9-de icient mice. These observations from experimental studies indicate the possibility of controlling gut microbial composition to treat arteriosclerosis caused by genetic de iciency. Akkermansia muciniphila, B. vulgatus, and B. dorei did not show any differences between KO and WT control mice in this study. This is probably due to differences in the relative abundances of the dominant gut microbiota in mice and humans. This result may also be due to a de iciency of the CTRP9 gene [37].
From clinical trials, promising results were observed for FMT to restore the gut microbiota of healthy people after the use of antibiotics quickly [41]. For the fear of transferring endotoxins or infectious agents resulting in the development of new gastrointestinal complications during FMT therapy, currently, this technique is not encouraged for treating CVDs [2]. Dietary intervention to regulate the treatment of CVDs has broad prospects since ibre-rich diets have been proven to improve the growth of bene icial symbiotic bacteria and inhibit the growth of opportunistic pathogens [42]. Xiao, et al. [43] suggested that whole grains and traditional Chinese medicine foods can reduce Enterobacteriaceae pathogenic bacteria and increase intestinal protective bacteria such as Bi idobacterium [42]. In addition, acetic acid-producing microbiota thrives well in high-iber diets which in turn lowers blood pressure [44]. The ibre-rich diet gives an additive value to the enhancement of bene icial bacteria in the host gut [45]. It was found that Bi idobacterium breve and Lactobacillus fermentum may have antihypertensive effects by restoring gut microbiota balance and preventing endothelial dysfunction [46]. Lam et al. [47] were surprised to ind that Lactobacillus plantarum improves ventricular function and reduces myocardial infarction size [46]. In the myocardial ischemia rat model also similar results were obtained when treated with Lactobacillus rhamnosus GR-1 [48]. Saccharomyces boulardii reduces the level of in lammatory markers and serum creatinine, with promising results in patients with HF [49]. Furthermore, resveratrol from Polygonum cuspidatum can alleviate Trimethylamine-N-Oxide (TMAO) induced atherosclerosis by remodeling the microbiota and reducing TMAO levels [50]. Besides, exercise proves to be a booster from Firmicutes to Bacteroides, increasing the number of bacterial metabolites preventing myocardial infarction. However, the effects of exercise on the gut microbiome are transient and reversible [2].

Conclusion
The involvement of gut microbiota in the occurrence and development of CHD, HTN, and HF has been proved in a large number of studies. Gut microbiota in luences CVDs through immune regulation, the in lammatory response, gut barrier integrity, and metabolic homeostasis. CVDs, in turn, also affect the structure and function of the gut microbiota. The two-way effect between Microbiota and CVDs, and the mechanism of action through metabolic pathways has been well studied. At present, most studies are based on animal experiments to correlate the involvement of gut microbiota in the prognosis of CVDs. In-depth studies based more on human trials and clinical studies alone will be helpful to standardize procedures like FMT on recurring Clostridioides dif icile infection. Some approaches based on gut microbiota for the treatment of CVDs are still in clinical trials and have potential advantages as well as limitations. Therapeutic strategies to improve the gut microbiota are potential avenues for the treatment of CVDs.