Microglia, the missing link in the brain-gut-hypertension axis

The pathophysiological mechanisms of hypertension are complex and involve multiple interacting systems (cardiac, vascular, renal, endocrine, neural, immune) acting, on a genetic background influenced by environmental factors (epigenetics, epistasis). The complexity was highlighted over 70 years ago in the mosaic theory of hypertension by Irvine Page (1). More recently this theory has been revisited by David Harrison where molecular and cellular events, specifically oxidative stress and inflammation, have been identified as common key processes underlying the altered systems of the mosaic theory of hypertension (2). Oxidative stress and inflammatory processes increase neuronal firing in the cardioregulatory centres of the brain (neuroinflammation) and stimulate sympathetic nervous system activity, leading to vascular dysfunction and renal sodium retention, processes that cause blood pressure elevation (3). The importance of neuroinflammation in the pathogenesis of hypertension is based on increased levels of inflammatory mediators and cytokines produced by neurons and microglial cells in the brains of experimental models of hypertension, processes driven by activation of the renin angiotensin aldosterone system and other pro-hypertensive factors, such as salt.

To add to the complexity of the mosaic theory of hypertension, emerging experimental evidence indicates that the gut microbiota plays an important role in blood pressure regulation and metabolism. This was first demonstrated in the 1980s when modification of bacterial flora with antibiotics in rats was associated with steroid-related hypertension (4). Short chain fatty acids (SCFA), which are end products of fermentation by the gut microbiota, were found to modulate blood pressure in mice (5). They are absorbed into the bloodstream where they exert effects on target tissues, including vessels, kidney, heart and brain. In mice exposed to an acute bolus of SCFA, blood pressure decreased rapidly through effects mediated primarily via the G protein-coupled receptor, Gpr41, localized in endothelial cells (6). Alterations in SCFA receptors in the small intestine have also been associated with elevated blood pressure in experimental models (7). Further demonstrating a role for gut microbiota in cardiovascular disease, propionate, a SCFA, significantly attenuated cardiac hypertrophy, fibrosis, vascular dysfunction, and hypertension in models of hypertension and atherosclerosis (8). Using new research strategies including metagenomics, which provides high resolution and culture-independent sequencing of bacterial DNA, bioinformatics analysis for microbial identification and taxonomy, fecal transplantation approaches and antibiotic-induced microbial depletion in experimental models, there is now growing evidence that gut microbial dysbiosis is linked to hypertension pathophysiology in both experimental models and humans. In experimental models of hypertension, administration of gastrointestinal-cleansing antibiotics causes a transient reduction in blood pressure and fecal transfer from a hypertensive rat to a normotensive rat resulted in hypertension (9). In addition, fecal transplants from hypertensive patients to germ-free mice caused hypertension in the recipient mice (9). These studies strongly suggest that the intestinal microbiome influences pathophysiological processes that regulate blood pressure.

Specific mechanisms triggering the gut dysbiosis-induced hypertension are unclear but the gut enteric nervous system and extrinsic neural inputs seem to be important (10). This notion of neural-gut communication was further developed by the Raizada group (11) and more recently was found to be driven by the brain and sympathetic nerves and accordingly has been termed the brain-gut axis in hypertension as highlighted in the current issue (12). Antibiotics, especially tetracyclines, such as minocycline, reduced blood pressure in spontaneously hypertensive rats (13) and in Ang II-induced hypertension (14). In a case report, an antibiotic cocktail (vancomycin, rifampin and ciprofloxacin) given for post-surgical infection, significantly altered the gut microbiota and reduced blood pressure in a patient with resistant hypertension (15), further highlighting an association between immune-modulatory and gut microbiota-related effects in hypertension. A comprehensive transcriptomic biomarker analysis in experimental hypertensive models using the Comparative Toxicogenomics Database demonstrated that transcriptomic data in the rodent central nervous system converge on processes associated with gastrointestinal function (transit, motility, inflammation) supporting interplay between the brain and gut in neurogenic hypertension. (16) However the central elements driving the system remain elusive.

In the current issue Sharma et al (12) provide new insights on how the central nervous system influences hypertension through the gastrointestinal system. In particular they develop the theory that pro-hypertensive factors enhance sympathetic outflow and neuroinflammation leading to sympathetic activation of the gut, altered gut microbiota and increased mucosal permeability, processes leading to release of microbial toxins, pro-inflammatory mediators and SCFAs into the circulation to perpetuate systemic inflammation and blood pressure elevation. The concept of the ‘leaky gut’ characterised by an increase in permeability of the gastrointestinal mucosa allowing bacterial toxins, metabolites, reactive oxygen species, pro-inflammatory molecules and cytokines to leak into the blood stream was originally associated with coeliac disease and other inflammatory bowel diseases (17). However, as evidenced in the study in this issue (12), these processes are more widespread causing systemic inflammation implicated in a multitude of diseases including diabetes, chronic kidney disease, aneurysms, stroke and hypertension (12,18,19). The novelty of the Sharma study is that it clearly identifies microglial activation in the paraventricular nuclear region of the hypothalamus and neuroinflammation as being key drivers for the brain-gut axis in hypertension pathophysiology (12). These findings were based on elegant experiments that investigated effects of a chemically modified tetracycline-3 (CMT-3,) that has primarily anti-inflammatory rather than antibiotic effects, on microglia activity, gut microbiota and blood pressure pathology in hypertensive rats. CMT-3 was administered centrally to interrogate the communication central neurogenic effects on the gut but the brain-gut communication appears to be bidirectional where gut microbiota and their products are also implicated in sympathetic activation. Bacterial SCFAs have been shown to influence microglia homeostasis and neural control mechanisms in hypertension. Accordingly, the communication between the brain and the gut is circuitous where pro-hypertensive factors stimulate brain-gut communication causing hypertension through systemic inflammatory processes and oxidative stress, which in turn promotes sympathetic nervous system activation and perpetuation of the damaging processes underlying hypertension (figure).

Figure.

Schematic demonstrating the circuitous relationship between pro-hypertensive factors, microglial activation, sympathetic outflow and gut microbiota in the pathophysiology of hypertension. Neurogenic inflammation and oxidative stress are key molecular processes driving the brain-gut axis. SNS, sympathetic nervous system; PVN, paraventricular nucleus

Targeting the dysfunctional brain-gut connection with antibiotics, such as CMT-3, may be a novel approach in the management of hypertension. However there are a number of concerns and limitations that need to be addressed when such strategies are considered. Firstly, antibiotics, and especially tetracyclines such as minocycline, have been associated with severe intracranial hypertension in both pediatric and adult patients (20–22). Hence antibiotics may actually increase rather than decrease blood pressure, a phenomenon also observed in salt-sensitive hypertensive rats (13). Secondly, although CMT-3 was described as an anti-inflammatory tetracycline derivative in the paper in this issue (12), it does have some antibiotic and antifungal actions and therefore the changes observed in the CMT-3-treated rats may not only be due to reduced neuroinflammation. Thirdly if antibiotics are indeed effective antihypertensive agents, acting in part through modulation of the gut microbiota, epidemiological and clinical studies would have already shown a relationship between antibiotic use and blood pressure, especially considering the large population of patients who are hypertensive and on antibiotics for unrelated diseases. Finally while research on experimental models has defined cross-talk between the brain and microbiota/gut in the pathophysiology of hypertension, evidence for such a system in humans still awaits confirmation. Nevertheless, based on pre-clinical data and from a theoretical viewpoint, the brain-gut axis in hypertension is certainly conceivable and could constitute a new axis to the framework of the Mosaic Theory of hypertension. However to date robust clinical evidence for this is still lacking. Before any consideration can be given for microglia-microbiota-gut targeted therapies to treat hypertension, unambiguous proof of clinical efficacy and safety without unwanted secondary effects are needed.