The potential impact of a probiotic: Akkermansia muciniphila in the regulation of blood pressure—the current facts and evidence

The SCFAs are produced from the diet mainly via saccharolytic fermentation of undigested carbohydrates, by anaerobic gut bacteria [43], and highly glycosylated mucin-degrading microbes, including A. muciniphila [5]. Humans produce approximately 500–600 mmol of SCFAs every day. Still, it strictly depends upon the amount of diet, especially the macronutrient content, such as carbohydrates, fibers, protein, and fat, that we consume [44]. The primary role of A. muciniphila is to degrade the mucin in the intestinal epithelial cells and supply energy sources (carbon and nitrogen) to goblet cells to the secretion of mucin again. This cycle should be in homeostasis [43]. In the secreted mucin, SCFAs such as acetate, propionate, butyrate, formate, isobutyrate, valerate, and isovalerate are produced in different concentrations, with acetate, propionate, and butyrate being the most abundant [45]. A. muciniphila mainly secretes acetate and propionate, which strengthen the gut barrier integrity through the regulation of tight junction proteins, such as occludin, claudins, and zona occludens [46]. Furthermore, the various clinical and preclinical studies using the 16s rRNA sequencing data have suggested that a decrease in the abundance of A. muciniphila could lead to the development of HTN [25, 28, 47]. Thus, all these results are based on association or correlation analysis. Still, to date, no direct studies (in-vivo, in-vitro, and clinical) use A. muciniphila to control BP. One of the major mechanisms of A. muciniphila in controlling BP could be the release of SCFAs from mucin [1, 7]. In the early days, SCFAs were thought to regulate BP through G-protein coupled receptors (GPR41 or FFAR3 and GPR43 or FFAR2) in humans, and Olfr78 in mice. In addition to this, presently, SCFAs regulate BP through GPR109a (also called HCA2 or NIACR1 or PUMA-G or HM47a) and OR51E2 in humans. In humans and mice, all three dominant SCFAs—propionate, acetate, butyrate, and subdominant SCFA—lactate regulate BP through these receptors, with propionate being the strongest ligand [48‐53] to influence BP in humans via OR51E2.

Additionally, β-ionone—a volatile compound present in fruits, vegetables, and plants and cleaved from beta-carotene by an enzyme called carotenoid cleavage dioxygenase [54] influences BP in humans via OR51E2 [55, 56], but not through Olfr78 in mice [57]. The SCFAs – acetate and propionate [58‐60], bind with the GPR41/GPR43 receptors, and promote the secretion of GLP-1/GLP-2 in the enteroendocrine L-cells of ileum and large intestine [1, 61]. In different diabetic and non-diabetic animal models, activation of GLP-1R (both exogenous and endogenous) has been demonstrated to lower BP by reducing sympathetic activity in the carotid body [62, 63]. Also, propionate involves in the release of a fat hormone–leptin, which alters the normotensive to hypertensive conditions. Zhao et al. have found that supplementation of A. muciniphila in chow diet-fed mice, reduced chronic low-grade inflammation by reducing plasma leptin and LPS [64], indicating that A. muciniphila might be playing more complex roles than what we knew so far. Besides, previous studies using GPR41⁻/⁻ mice, have demonstrated that the acetate and propionate-promoted release of leptin is mediated with the activation of the GPR43 receptor only, not through the GPR41 receptor [65, 66]. These data suggest that A. muciniphila supplementation might have profound effects on BP regulation through the GPR41 receptor rather than the GPR43 receptor, but this hypothesis needs to be further studied.

Gut barrier integrity plays an important role in preventing the leaking of synthesized lipopolysaccharides (LPS) and inflammatory markers into circulation, thereby regulating BP [67]. LPS is a group of lipopolysaccharide chemical complexes produced by the bacteria, especially from the outer membrane of the gram-negative bacteria, and it acts as a permeability barrier for the bacteria [68]. Also, LPS is a pathogen-associated molecular pattern (PAMP) molecule known to stimulate the Toll-Like Receptors (TLRs)—a class of pattern recognition receptors that potentiate the innate immune response. In particular, the TLR4 induces myddosome, which consists of adaptor proteins, such as MyD88, TIRAP, and serine-threonine kinases from the IRAK family, which receive the signals from TLR4, leading to the activation of inflammatory gene expression through NF-kB and AP-1 activation [69‐71]. Disruption in the gut barrier integrity is closely associated with HTN due to increased intestinal inflammation and permeability [67, 72]. Kim et al. have demonstrated that hypertensive subjects had increased plasma LPS, intestinal fatty acid-binding protein (I-FABP), proinflammatory Th17 cells, and zonulin levels [72]. Supplementation of A. muciniphila improves the gut epithelial barrier integrity possibly through the reduction of circulatory LPS, inflammatory markers such as TNF-α, IL-6, CD36, SR-A1), IL-2, IFN-γ, IL-12p40, and MCP-1, and potentiating the expression of tight junction proteins (occludin, zonal occludens, and claudin-3, claudin-5) [73‐76]. In addition, Chelakkot et al. have shown that Akkermansia-derived extracellular vesicles–Amuc_1100 reduce circulatory LPS levels [76] and activation of the TLR2 pathway [77], thereby promoting an anti-inflammatory effect. On the contrary, few studies have reported a higher level of A. muciniphila during disease progression [78, 79]. In our previous two studies, we found that a higher abundance of A. muciniphila was linked with the activation of LPS in CKD mice than in the control mice [80], and insufficient glycemic control in pediatric subjects with T1DM is linked to the higher level of A. muciniphila [81]. Thus, the level of A. muciniphila is critical for the pathophysiology of certain diseases where there is activation of an inflammatory-mediated pathway. Increased abundance of A. muciniphila degrades more mucus in the gut which damages the mucosal barrier, leading to the leak of inflammatory markers, LPS, and the activation of an inflammatory response [82]. Ganesh et al. have reported that A. muciniphila could differentially affect the gut ecosystem in relevance to the level of inflammation. In brief, they showed the presence of A. muciniphila in Salmonella enterica Typhimurium-infected gnotobiotic mice exacerbates the gut inflammatory response by increasing the mRNA levels of IFN-γ, IP-10, TNF-α, IL-2, IL-17, and IL-6 in the cecal and colonic tissue compared to the gnotobiotic controls mice [83], which explains A. muciniphila’s ability to disturb the host mucus homeostasis through the excessive mucin degradation process and eventually disturbs the gut ecosystem. Li et al. have demonstrated that the supplementation of A. muciniphila prevents the severity of atherosclerotic lesions by reducing metabolic endotoxemia [10]. To support this hypothesis, a recent study by Dan et al. on the Chinese population has demonstrated that the abundance of A. muciniphila is greatly elevated in hypertensive subjects [84]. Also, Cekanaviciute et al. have reported that A. muciniphila promotes Th1 lymphocyte differentiation [85]. Further, a study on the Brazilian population conducted by Silveira-Nunes et al. found a higher abundance of Akkermansia along with the increased TNF-α/IFN-γ ratio in the hypertensive subjects [86], suggesting the differential role of Akkermansia in mediating HTN.

So, activation of the LPS pathway might be detrimental in inciting hypertension through various mechanisms, such as endothelial dysfunction through the release of inducible nitric oxide, activating TLR4, and vasculature inflammation through the release of NADPH oxidase-dependent free radicals [87‐89]. On the other hand, Akkermansia supplementation reduced low-grade inflammation by suppressing circulatory LPS in HFD-diet mice [64].

Several studies demonstrated that trimethylamine-N-oxide (TMAO) is a pathogenic metabolite formed by the gut microbiota and act as an independent risk factor for cardiovascular diseases, including HTN [90‐93]. TMAO is formed by the oxidation of the intermediate metabolite TMA from the dietary L-carnitine, phosphatidylcholine, lecithin, and γ-butyrobetaine (γ-BB) via flavin monooxygenase 3 (FMO3) enzyme from gut microbes [94]. TMAO is involved in the development of hypertension through the elevation of vascular oxidative stress and endothelial dysfunction [95]. Xu et al. reported that the supplementation of A. muciniphila reverses the high expression of FMO3 in obesity conditions [76]. Also, recently Zhou et al. studied the relationship between circulating plasma TMAO levels and HTN. They found that TMAO levels are higher in patients with HTN than those without HTN [96]. Hsu et al. reported that a higher TMA level is associated with high BP load and abnormal 24-h ambulatory BP monitoring profile, along with the reduced abundance of genus Akkermansia [92]. Plovier et al. demonstrated that the supplementation of A. muciniphila significantly decreased plasma levels of TMAO and TMA and a two-fold reduction of FMO3 expression in mice [97]. Luo et al. demonstrated that cold exposure modulated the A. muciniphila abundance level in rats, thereby causing excessive secretion of TMAO which elevated the susceptibility to atrial fibrillation (AF) condition mainly through the enhanced infiltration of M1 macrophages in atria and increased protein expression of Casp1-p20 and cleaved GSDMD. The oral supplementation of A. muciniphila in rats significantly ameliorated the pro-AF property induced by cold exposure [98]. These results indicate a positive correlation between a higher level of TMAO or TMA with the HTN and a negative association with the abundance of the genus Akkermansia.

Another potential mechanism by which the A. muciniphila regulates BP could be the release of endogenous hydrogen sulfide (H₂S), as it is involved in the sulfate reduction process [99]. Rosario et al. have demonstrated that A. muciniphila produced H₂S together with Escherichia spp., which has been considered a potential regulator of vascular homeostasis, possibly through the regulation of vascular tone and inflammation, antioxidant mechanism, vascular cell proliferation, and apoptosis [100, 101]. Several studies have pointed out that the H₂S levels are inversely associated with hypertensive disease severity [102‐104]. Supplementation of H₂S reduces BP in experimental animal models [105, 106]. On the contrary, H₂S has been implied in activating proinflammatory response [107, 108], and its higher level could cause inflammation in the gut [99]. Interestingly, A. muciniphila has been proposed to use the H₂S for the production of cysteine [43], and cysteine is well-known for its anti-hypertensive effect [109], thereby it could control BP regulation. In addition, when there is an activation of the inflammatory response, the relation between A. muciniphila and H₂S might turn on pathologic response rather than the protective response, because both entities have been found to have a differential role. There is a possibility that a high level of A. muciniphila can produce a high amount of H₂S, which might elevate the BP due to the activation of an inflammatory response [99]. This is a mere hypothesis and must be proved experimentally, but the evidence strongly encourages this concept. Recently, pangenomic analysis of A. muciniphila revealed that the phylotype AmI contains genes required for the assimilatory sulfate reduction (ASR) process and thereby possibly increasing H₂S level.

Diet is an essential modulator of gut microbiota [143‐145], and the specific dietary pattern called the dietary approach to stop hypertension (DASH) has been proposed to control or lower BP [146]. The DASH diet promotes a diet that is adjusted to be lower in sodium and rich in potassium, magnesium, and calcium nutrients [146], which causes a reduction in high BP [147]. Maifeld et al. demonstrated that fasting followed by the modified DASH diet reduced the BP in patients with metabolic syndrome (MetS) through an altered gut microbiome, especially increasing the abundance of A. muciniphila, which in turn, increases SCFAs either locally or systemically [148]. Also, beta-glucan supplementation for four weeks in patients with MetS had a higher outcome in patients with a higher baseline A. muciniphila abundance [149]. Another weight-loss intervention study in patients with MetS had improved BP by enhancing the abundance of A. muciniphila and F. parusnitzii [150]. American Heart Association (AHA) recommends the Mediterranean diet (MD) as a healthy eating style; it improves both systolic and diastolic BP, and arterial stiffness [151‐154]. Also, MD has been reported to increase the relative abundance of A. muciniphila in the stool samples from participants with different body weight compositions [155]. Thus, A. muciniphila could be a potential entity in lowering the BP in patients with MetS.

Most hypertensive drugs, such as ARB, ACE inhibitors, beta-blockers (BBs), and calcium channel blockers (CCBs), are administered orally. After the administration by oral route, it is metabolized into pharmacologically active metabolites or toxic based on the molecule's structure [156]. These active molecules can be further metabolized by the rich enzyme repositories of the gut microbiome [157, 158]. The interaction between the drug and gut microbiome is bidirectional. Thus, identifying gut bacteria and their communities, which produce drug-metabolizing microbial proteins, can contribute to improving the drug’s effect and can be converted into new drugs and reduce the adverse effect. Understanding the relationship between the gut microbiome and HTN drug bioavailability can help us better understand individual variation, which can be useful in precision and personalized medicine. Unfortunately, the literature review indicates the need to study these two entities (anti-hypertensive drugs vs A. muciniphila) as we have very little evidence. For example, Li et al. reported that amlodipine aspartate and amlodipine besylate, a CCBs, improved the NFALD condition in mice with HTN through the enhancement of A. muciniphila, which potentiates the intestinal expression of antimicrobial peptide Reg3g [159]. As mentioned earlier, losartan treatment significantly improved the hypertensive condition in SHR rats possibly through the restoration of gut dysbiosis and modulation of immune response, along with the increased the abundance of A. muciniphila [123]. Furthermore, minocycline, an age-old antibiotic, is demonstrated to lower BP, and administration of minocycline in an SHR animal model decreased BP with an increase of A. muciniphila abundance [28]. Also, A. muciniphila secreted proteins induces the uptake of Ca⁺⁺ through Ryanodine Receptors (RYr), a family of Calcium channel, in an enteroendocrine cell line and increase the synthesis of reactive oxygen species (ROS) [160]. Nonetheless, more research is needed to fully understand the possible bidirectional role of A. muciniphila and anti-hypertensive drugs.