1. Chapter 38 Gut Microbial Metabolism in Health and Disease

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FIGURE 38.1: Gut microbiota shape the immune system to promote microbiotahost mutualism. Abbreviations: IL, interleukin; TH, T-helper cell; Treg, T-regulatory cell; PSA, polysaccharide A (see text also).
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FIGURE 38.2: (A) The TRP - 5-HT - Melatonin metabolic pathway. (B) The TRP - KYN metabolic pathway. Abbreviations: NAS, N-acetylserotonin; 5-HTR2B,3A, 5-HT receptor 2B,3A; β-AR, β-adrenergic receptor; Ucp-1, uncoupling protein-1; BAT, brown adipose tissue; GPR35, G-protein coupled receptor-35; NAD, nicotinamide adenine dinucleotide; iNOS, inducible nitric oxide synthase; NO, nitric oxide; PLA2, phospholipase A2; AA, arachidonic acid; COX-2, cyclooxygenase-2; PGE, prostaglandin; 5-LOX, arachidonate 5-lipoxygenase; AAND, age-associated neuroendocrine disorders; IFN, interferon; TNFα, tumor necrosis factor-α.
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FIGURE 43.4: The pathway for inactivation of vitamin D involving conversion of either 25-hydroxyvitamin D [25(OH)D] or 1,25-dihydroxyvitamin D [1,25(OH)2D] by the enzyme CYP24. From W.L. Miller, Genetic disorders of vitamin D biosynthesis and degradation, J. Steroid Biochem. Mol. Biol. 165 (2017)
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FIGURE 38.3: (A) The effect of SCFAs in IEC, (B) liver, and (C) WAT (see text for details). Abbreviations: HDAC, histone deacetylase; ACC, acetyl-CoA decarboxylase.
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FIGURE 38.4 TLR4 activation. LPS binds to LPB and then to the CD14 accessory protein. LPS is then transferred to the next TLR4 accessory molecule, MD-2. Two TLR4/MD-2/LPS complexes must then dimerize to initiate the internal cell signaling pathways. There are two pathways for TLR4, MyD88 dependent and MyD88 independent, which lead to production of inflammatory cytokines/ROS and IFNs, respectively. Abbreviations: CD, cluster of differentiation; LPB, liposaccharide-binding protein; MD-2, myeloid differentiation factor; MyD88, myeloid differentiating primary response gene 88; TIRAP, toll-interleukin 1 receptor domain containing adaptor protein; TRIF, TIR-domain-containing adaptorinducing
interferon-B; TRAM, toll-like receptor adaptor molecule 2.
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FIGURE 38.5: (A) The gut-brain axis. (B) The antiinflammatory effect of the VN via the cholinergic pathway. ACh is released at the distal end of VN efferent fibers which activates the α7nAChR receptor in macrophages. The activation of α7nAChR causes inhibition of TNFR receptor activation by TNFα thus preventing downstream activation of NF-κB signaling. Additionally, activation of α7nAChR causes activation of a JAK-STAT mediated antiinflammatory pathway [81]. Abbreviations: TNFR, tumor necrosis factor receptor; CRF, corticotrophin-releasing factor; JAK, Janus kinase;
STAT, signal transducer and activator of transcription. ACTH, adrenocorticotropic hormone; DMN, dorsal motor nucleus of the vagus (origin of VN efferents); EN, epinephrine; LC, locus ceruleus (the primary brain noradrenergic nucleus located in the pons and involved in the stress response); NE, norepinephrine; PVN, paraventricular nucleus of the hypothalamus (source of corticotrophin-releasing factor, CRF); RVM, rostral ventrolateral medulla; NTS, nucleus tractus solitarii (termination point for vagus efferents).
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FIGURE 38.6 (A) Structure of the TJ between adjacent IECs. Transmembrane proteins occludin, claudin, and JAM family proteins seal the paracellular space between IECs. Plaque proteins such as ZO family proteins bind to one another to form a stabilizing scaffold and a direct link to the cytoskeleton via ZO-1 binding to F-actin. (B) The effect of PICs that have been found to be increased in healthy, normal weight seniors on the TJ. Activated macrophages (Fig. 38.1) produce TNFα, IL-1β, and IL-6 as well as IL-12. IL-12 goes on to stimulate production of TH1 cells that secrete IFNγ. TNFα binds to the TNFR1 receptor on IECs and causes activation of NF-κB and its subsequent translocation to the nucleus where it acts as a
transcription factor for the MLCK promoter resulting in production of MLCK protein. MLCK then phosphorylates MLC to MLC-P which then acts on F-actin to cause cytoskeletal contraction and opening of the TJ. IL-1β has been shown to downregulate the expression of occludin and also acts via NF-κB - MLC-P to cause TJ opening. IL-6 has been shown to decrease expression of ZO-1 causing disruption of the TJ stabilizing scaffold. IFNγ, also via MLC-P, has been shown to increase IEC macropinocytosis of occludin and JAM proteins, thus removing them from the paracellular space.
The culmination of all of these effects is a “leaky” mucosal membrane. Abbreviations: MLCK, myosin light chain kinase; MLC, myosin light chain protein; TNFR1, tumor necrosis factor receptor-1.
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