Microbiota-Liver Diseases Interactions

Abstract

Gut microbiota regulates essential processes of host metabolism and physiology: synthesis of vitamins, digestion of foods non-digestible by the host (such as fibers), and-most important-protects the digestive tract from pathogens. In this study, we focus on the CRISPR/Cas9 technology, which is extensively used to correct multiple diseases, including liver diseases. Then, we discuss the non-alcoholic fatty liver disease (NAFLD), affecting more than 25% of the global population; colorectal cancer (CRC) is second in mortality. We give space to rarely discussed topics, such as pathobionts and multiple mutations. Pathobionts help to understand the origin and complexity of the microbiota. Since several types of cancers have as target the gut, it is vital extending the research of multiple mutations to the type of cancers affecting the gut-liver axis.

Keywords: gut–microbiota; liver diseases; pathobionts.

Figure 1

CRISPR/Cas system in bacteria. When a bacteriophage infects a bacterium for the first time (1), its DNA is scanned by Caspase proteins 1 and 2, which recognize specific viral sequences located near the protospacer adjacent motif (PAM) (2). Once a PAM is recognized, DNA is cleaved upstream the PAM sequence and protospacer is integrated into the CRISPR-array, which includes hexogen DNA (spacers) intercalated between bacterial DNA palindromic repeated sequences (3). The CRISPR-array transcribed by bacterium (pre-crRNA) complements with trans-activating RNA (tracr-RNA) encoded in proximity of the Cas genes (4). The RNaseIII recognizes this complex and cleaves it into smaller RNAs named CRISPR-RNA (crRNA). The crRNA is bound by Caspase9 (Cas9) forming the ribonucleoprotein complex named CRISPR/Cas9 I (5). Once the same bacteriophage tries to infect the host cell for the second time (6), the CRISPR/Cas9 complex recognizes PAM and cleaves the viral DNA (7).

Figure 2

In vivo CRISPR applications. The (A) part of the figure represents a scheme of CRISPR/Cas9 complex incapsulated into Lipid Nanoparticles (LNPs). The LNPs are coated with apolipoprotein E (apoE) which will lead the particle into the liver. After intravenous administration, the complex reaches the liver and is taken by the hepatocytes through the interaction with apolipoprotein E. Then CRISPR/Cas9 is released and leads to double strands breaks in the gene which will lose function. In the right side of figure (B) is depicted the method for the knockout of the BCLA11A promoter in β-thalassemia. After isolation of hematopoietic stem and progenitor cells (HSPc) from the patient, CRISPR/Cas9 complex has been inserted in these cells ex vivo through electroporation. The cells in which the gene loses its function are reimplanted into the patient. These cells will colonize the bone-narrow without producing BCLA11A which causes the disease.

Figure 3

The gut microbiota dysbiosis induces NAFLD progression. The gut microbiota alterations associated with metabolic, genetic or environmental factors play key roles in liver disorders. Dysbiosis increases gut permeability, favors LPS translocation and promotes pro-inflammatory cytokines production, thus participating to liver fibrosis and NAFLD occurrence.

Figure 4

The gut microbiota regulates fat storage. The gut microbiota favors lipoprotein lipase (LPL) activation through inhibition of the fasting-induced adipose factor (Fiaf). This enhances triglycerides storage in adipose tissue and decreases fatty acid oxidation via suppression of AMP-activated protein kinase (AMPK) activity, thus leading to heightened adiposity and liver fibrosis. On the contrary, in germ-free mice, the absence of the gut microbiota favors resistance to diet-induced adiposity through inhibition of LPL and activation of AMPK.