Written by Nick Trompeter, Ph.D., Cole Toohey, Payton Olson, Peter Lee, and Ken Dorko
Figure 1: Immunofluorescence of intestinal epithelium.
Postbiotics: Nurturing Gut Health and Harbingers of Disease
Beyond live bacteria, microbial metabolites known as postbiotics wield profound effects on intestinal barrier function. Short-chain fatty acids (SCFAs), biosurfactants, and vitamins exert immunomodulatory activities, fortifying the intestinal barrier and promoting metabolic health. Short-chain fatty acids produced by bacteria appear to protect individuals from the effects of a high fat diet (HFD), as supplementation of butyrate, propionate, and specifically acetate in a high-fat-diet mouse model stems weight gain Short Chain Fatty Acids Prevent High-fat-diet-induced Obesity in Mice by Regulating G Protein-coupled Receptors and Gut Microbiota.(10) This study indicates that admixtures of SCFAs inhibit production of certain inflammatory markers such as interleukin-1β (IL-1β), IL-6, and monocyte chemoattractant protein-1 in the plasma of the experimental mice model used. Furthermore, in obese mice, SCFAs reduce the expression of glucagon-like peptide 1 mRNA within colonocytes, a key target of incretin-based weight loss drugs.(14)
While SCFAs appear to promote weight reduction when given as a supplementation, paradoxically, obese individuals have increased levels of SCFAs.(11) This increase in SCFAs may be due to alterations in the ratio of commensal bacteria or the appearance of Archae bacteria within the gut microbiome in obese individuals. Methanogenic strains of Archae were found in only in obese individuals, including one individual after gastric bypass surgery, but not within normal weight individuals.(12) Methanogenic bacteria increase caloric intake by enhancing utilization of normally non-digestible carbohydrates into SCFAs. This increase in SCFAs stimulates storage into energy deposits to induce obesity.
Dysbiosis of the gut-microbiome, hallmarked by the appearance of gram-negative bacteria and other aberrant compositions of the intestinal microflora, activates the innate immune systems and an inflammatory response via the production of lipopolysaccharide (LPS). Excess LPS increases epithelial permeability within the colon, culminating in the appearance of LPS within the liver and systemic circulation.(13) The role of LPS in fatty liver disorders and NASH is underscored by the reduction in fibrosis and inflammation in antibiotic treated mice.(15) Antibiotic treatment appears to inhibit fibrosis through the maintenance of the intestinal epithelium via IL-18-mediated mechanisms, while also decreasing blood plasma LPS concentrations.(16)
In addition to LPS, other metabolites from commensal bacteria influence intestinal inflammation and epithelial integrity. Taurine, histamine, and spermine appear to modulate the intestinal epithelial barrier through modulation of the NOD‐like receptor family pyrin domain containing 6 (NLRP6) inflammasome, downstream of IL-18 signaling. Taurine, a bacterial metabolite that is decreased during dysbiosis, stimulates the secretion of IL-18, which activates NLRP6 and in turn increases production of anti-microbial peptides.(16,17) Furthermore, supplementation of Taurine promotes epithelial integrity, enhances survivals, and improves colitis scoring metrics.(16) Conversely, spermine and histamine appear to inhibit the protective effect of IL-18 mediated signaling and anti-microbial peptide production. The authors observed that histamine supplementation in the above colitis model exacerbated disease progression, with antibiotic treatment abrogating this effect.(16)
Additional gut microbiota metabolites, such as Trimethylamine (TMA) and the its oxidized form of TMA, trimethylamine N-oxide (TMAO), may induce cardiovascular disease, metabolic syndrome, and liver disease. Excessive amounts of precursors to TMA, including choline and L-carnitine from dietary consumption, leads to increased production of TMA. In healthy subjects, TMA and TMAO excretion occurs through the urine and feces. However, in excessive amounts, of TMA will enter the plasma, translocate to the liver where are oxidizations convers TMA to TMAO.(18,19) Studies suggest that elevated levels in plasma TMAO correlates with cardiovascular, liver, kidney disease, as well as type II diabetes.(18,19,24) The correlation of TMA/TMAO and disease is controversial, as additional studies observed little correlation between plasma TMA/TMAO and cardiovascular disease, with one study showing an improved metabolic profile in obese women whose TMAO plasma levels increase with histidine supplementation.(19,20) Thus, this observation suggests, this suggests that the specific dietary precursory and the accompanying nutrients consumed may exert differential effect of TMA and TMAO on human physiology.
Understanding the complex interplay of bacterial metabolites on intestinal pathology, especially during the study of hepatic diseases and IBD, may help to identify novel therapeutic strategies that can be implemented in current treatment regimens.
The vasculature: The Gateway of disease
At the heart of the gut-liver axis lies the gut-vascular barrier (GVB) and the hepatic portal vein, a conduit that facilitates the exchange of signals from the gut to the liver. Through this vital pathway molecules, nutrients, and microbial byproducts traverse, influencing both local and systemic physiology. Under normal conditions, the intestine has a robust defense system of mucus and anti-microbial peptides, that limits the direct interaction of the gut microbiome with intestinal epithelial cells. However, a high fat diet (HFD), implicated in the obesity epidemic across the globe, alters the gut microbiome and the mucosal layer protecting the intestinal epithelial barrier. These alterations cause significant impact on the gut-liver axis through enhancing both intestinal epithelial and gut vascular barrier permeability, as well as the permeability of the gut vascular barrier. Leakiness of these barriers allows for increased LPS serum levels and subsequent endotoxemia, which may disrupt homeostatic physiological function.
The role of the gut-vascular barrier in disease was first observed by Spadoni et al., when they discovered that Salmonella typhimurium can penetrate the gut endothelium to allow for molecules larger than 4 KDa to extravasate into the intestinal capillaries.(21) Furthermore, they determined that the GVB was modified in patients with celiac disease and was correlated with liver damage.(21) The research group then sought to characterize the cellular machinery responsible for maintenance of a GVB and how protein signaling and expression change during disease. The GVB utilizes both tight junctions and adherenss junctions to form the endothelial barrier. Similar to the intestinal epithelial barrier, the GVB utilizes JAM-A, claudins, and ZO-1 to interact and create a semi-permeable barrier. In addition, vascular endothelial cadherin (VE-cadherin) and junctional β-catenin form complexes with tight junction networks to inhibit paracellular transport of molecules larger than 4 KDa.(22) The WNT/ β-catenin pathway is a known regulator of the brain-blood barrier and Spadoni et al., show similar regulation of the GVB by WNT/β-catenin activation. Their finding that constitutive activation of β-catenin in mice abolishes penetration of the GVB by Salmonella, highlighting the importance of WNT signaling in GVB homeostasis. The studies by Spadoni and colleagues were the basis for future studies that now implicate permeability of the GVB to non-alcoholic fatty liver disease (NAFLD), alcoholic liver disease, cirrhosis, and additional conditions.
The incidence of NAFLD across the globe has steadily increased in the past few decades. It is estimated that approximately 40% of adults in the United States suffer from NAFLD, with a fifth of this population suspected to have further disease progression to non-alcoholic steatohepatitis. The cause for the rapid rise in NAFLD occurrence is caused by the western diet that is associated with high amounts of unhealthy fats and sugar. The In mice, a a HFD can increase the permeability of the intestinal epithelial barrier within 48 hours.(23) After 1 week of feeding mice a HFD, alterations to the gut vascular barrier are observed, namely increased permeability and increased plasmalemma vesicle-associated protein 1 (PV-1) expression, membrane protein integral to endothelial fenestrations.(23) GVB dysfunction elicited by a HFD has been linked into hepatic steatosis, which can further be exacerbated by colitis.(21,25) The gut-microbiota appears to influence the maintenance of the GVB. Germ-free mice and mice treated with antibiotics are protected from the effects of a HFD, with no loss of endothelial integrity.(15,26)
The physiologic effects of an impaired GVB go beyond NAFLD in the liver, underscored by the recent terminology change of NAFLD to metabolic dysfunction-associated steatotic liver disease (MASLD). In mice with impaired GVB from a HFD, there was a concomitant elevation in glucose and insulin levels were observed with steatosis.(23) These results implicate GVB dysfunction with metabolic syndrome, which includes co-morbidities such as type II diabetes, cardiovascular disease, and even arthritis.(26)
Searching for primary human intestinal and liver cells for your experiments? Contact Nick Trompeter (ntrompeter@mosaiccellsci.com) to learn how we can support and advance your assays.
References:
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