Role of the gut microbiota in vascular health

Abstract

Background. Hypertension is a major cause of cardiovascular disease and is the leading modifiable risk factor for all-cause mortality and morbidity. The gut microbiota has been increasingly implicated in hypertension, with studies demonstrating reduced microbiome diversity and specific alterations in gut microbial composition. These studies were conducted in predominantly Caucasian cohorts, with little systemic effort to characterise the gut microbiota in hypertensive subjects in the local Hong Kong Chinese population, or in the Asia- Pacific region. Much of the literature only considered mean clinic blood pressure measurement and 16s ribosomal RNA (rRNA) gene sequencing to study gut bacterial taxonomy. Unadjusted covariables such as dietary sodium intake and the use of anti-hypertensive medications were major limitations present in much of the literature. Objectives. Using both next-generation sequencing (NGS) and bioinformatics approaches for gut microbiome profiling, this thesis aimed to study its associations with 24-hour blood pressure measurement in the local Hong Kong population. Using analytical chemistry techniques from metabolomics, this thesis also aimed to investigate associations between gutmediated short chain fatty acids (SCFAs) and hypertension. Methods. To achieve the above objectives, I conducted a cross-sectional study during the period 1/7/2018 – 30/6/2020. 241 individuals satisfying the inclusion and exclusion criteria of the study protocol were recruited from the general public. Vascular assessments including clinic and 24-hour blood pressure, carotid intima-media thickness (IMT), flow-mediated dilatation (FMD), and arterial stiffness reflected by brachial-ankle pulse wave velocity (baPWV) were performed on all subjects. Blood samples were drawn from all subjects to measure serum SCFAs, fasting glucose, and lipids. Stool samples were collected from all subjects for gut microbiome profiling and measurement of stool SCFAs concentration. Results. The mean 24-hour systolic and diastolic blood pressure of the study population was 122}13/76}10mmHg, with 38% of subjects classified as being hypertensive according to the 2021 European Society of Cardiology Guidelines. Hypertension was more prevalent in men compared to women. Hypertension in either sex was associated with a higher body-mass index (BMI), greater waist hip ratio, higher mean baPWV, and 10-year projected stroke risk. The mean carotid IMT was also higher in hypertensive women compared to normotensive women. BMI and smoking status were strong independent predictors of 24-hour systolic and diastolic blood pressure. Gut microbiome beta diversity was significantly reduced in hypertensive women compared to normotensive women. Beta diversity was not significantly different between hypertensive and normotensive men. Butyrate-producing species Ruminococcus gnavus, Clostridium bolteae, Bacteroides ovatus were consistently associated with hypertension in women, but not in men. Fusobacterium mortiferum positively correlated with hypertension in both men and women. Conversely, Oscillibacter sp CAG 241, Oscillibacter sp 57 20, Gemmiger formicillis, Alistipes indistinctus, Dorea formicigenerans, Asaccharobacter celatus, and Adlercreutzia equolifaciens all had negative associations with hypertension in women, but not in men. Most statistically significant associations were robust to different models of covariate adjustments. To examine the role of gut-mediated SCFAs in hypertension, correlation models and linear regression analysis were performed. Interestingly, there was no significant correlation between stool SCFAs and 24-hour systolic and diastolic blood pressure in the whole cohort, and when stratified by sex. Significant correlations between plasma total SCFAs and 24-hour systolic and diastolic blood pressure were found in the overall cohort, mainly driven by significant positive correlations in women, but not in men. Plasma propionic acid had significant positive correlations with 24-hour systolic and diastolic blood pressure in women but not in men, whereas plasma butyric acid had significant positive correlation with 24-hour systolic and diastolic blood pressure in the overall cohort, but not when analysed separately in men and women. Conclusions. These findings revealed important changes in the gut microbiome associated with hypertension, and was possibly mediated via gut-derived SCFAs. Gut microbial profiling was performed with NGS, providing high taxonomic resolution with identification of individual bacterial species associated with hypertension. These findings were consistent with plasma levels of SCFAs in hypertensive subjects. Future research should focus on (i) studying the effect of diet and lifestyle factors such as exercise and sleep on the gut microbiome and include these factors as covariate adjustments in future studies; (ii) exploring the biology underlying sex differences in hypertension with a focus on the gut microbiome and gut-mediated SCFAs; (iii) performing mechanistic studies to understand the pathways of gut microbiome and SCFAmediated hypertension, and to integrate knowledge in this field thus far.