**4. Microbiota products and their local and systemic effects**

The main physiological effects observed in the host by gut microbiota could be explained by their metabolite production. There are different products identified and the most studied are short-chain fatty acids (SCFA), where acetate, butyrate, and propionate are the most common and with the most known effects [36]. Other metabolites include trimethylamine-N-oxide (TMAO), obtained from compounds containing choline [37, 38]; secondary bile acids [39]; free anthocyanidins and protocatechuic acid, derived from flavonoid anthocyanins [38], and indolepropionic acid, produced from tryptophan [40]. The ones with beneficial effects on host health are SCFA, anthocyanidins, and indole compounds, and we are going to focus on the first ones.

SCFAs are produced in the bowel lumen by fermentation of dietary fiber [41] by anaerobic bacteria such as *Eubacterium*, *Roseburia*, *Faecalibacterium*, *Coprococcus*, and *Bifidobacterium* [38, 39]. Acetate is predominant, representing 60–75% of the SCFA generated [36] and it is produced via acetyl-CoA and the Wood-Ljungdahl pathway [38]. Propionate can be synthesized by succinate, acrylate, and propanediol pathways, and butyrate by the phosphotransbutyrylase/butyrate kinase, accounting for 25% and 15%, respectively [36, 38].

These molecules exercise their effects by direct or indirect pathways [37]. Direct mechanisms include local or systemic effects, where the microbiota-gut-brain axis is the most studied systemic example [42]; and indirect ways include the effects of these metabolites in other microbes that could modify their function [37].

#### **4.1 Local effects**

SCFAs are associated with the maintenance of gut epithelium integrity and protection of the intestinal barrier [36, 37]. Their principal mechanism is as an energy source for enterocytes, but also butyrate and indole derivatives have been associated with aryl hydrocarbon receptor (AhR) ligands, a nuclear receptor whose activation is reported to modulate cell proliferation, immune response, gene expression, and epithelial barrier function [43]. This association with a healthy intestinal epithelium had been explained by the "Warburg effect" or "butyrate paradox." Briefly, fiber-rich diets, associated with an increase in SCFA-producing bacteria, induce normal colonocyte proliferation and apoptosis in neoplastic cells, when metabolism is promoted by glucose [37, 38].

Furthermore, butyrate is important for the maintenance of intestinal barrier integrity because increases the expression of tight junction proteins, such as claudin-1, claudin-7, zonula occludens-1 (ZO-1), and ZO-2 [36, 38, 44]. Also, SCFAs can modulate mucin glycoprotein in the mucus layer [45], induce epithelial cell production of RegIIIγ and β-defensins, antimicrobial peptides [46], and reduce luminal pH [36]. All these functions help to avoid the proliferation of pathogenic bacteria and reduce the translocation of molecules to the systemic circulation.

#### **4.2 Systemic effects**

Besides local effects, microbiota metabolites can travel across the intestinal epithelium to systemic circulation or the central nervous system. This can impact different cells via extracellular receptors previously known as G protein-coupled receptors (GPRs) 43, 41, 81, 109A, and 91 [37]. For instance, propionate has a high affinity to

*Could Alterations in the Infant Gut Microbiota Explain the Development of Noncommunicable… DOI: http://dx.doi.org/10.5772/intechopen.105168*

GPR41, now called free fatty acid receptor 3 (Ffar3), which modulates cyclic adenosine monophosphate (cAMP); and to GPR43, now Ffar2, which increases the activity of calcium/protein kinase C (PKC) [36]. Butyrate also has activity on GPR 41 and is the only ligand of GPR109A, now hydrocarboxylic acid receptor 2 (HCA2), which also increases cAMP. Depending on the stimulated cells, effects can be seen in the endocrine, immune, and neurologic systems. For example, activation of the HCA2 receptor in dendritic cells and macrophages is associated with stimulation of T cells into the Treg phenotype [47, 48].

SCFAs also act as inhibitors of histone deacetylases (HDACs). When N-acetyl lysine on DNA histones loses its acetyl group, a more tightly wrapped double chain is formed. HDACs are enzymes that remove this acetyl group, altering DNA transcription by limiting access to transcriptional factors [37]. SCFAs can modify the transcription of a broad range of genes by inhibiting HDACs. Besides, butyrate can act as a ligand of nuclear receptor peroxisome proliferator-activated receptor γ (PPARγ) to modulate the transcription of genes associated with lipolysis and adipogenesis [38, 49]. These different pathways help understand the systemic effects that SCFA can have in several organs, depending on which receptor is activated and the dominant SCFA.

#### **4.3 Role of SCFA in inflammation and immune response**

The most beneficial effects of SCFAs are associated with an anti-inflammatory profile. They help to regulate cytokine expression, promoting the production of IL-10, and subsequently, differentiation of Treg cells by the Ffar2 mechanism [36, 37]. Besides, due to their capacity for inhibiting HDACs, SCFAs can impede the activation of nuclear factor-kappa β (NF-κB) [38], a protein complex mainly associated with inflammation. When its RelA/p65 subunit is acetylated, NF-κB can increase gene expression of pro-inflammatory cytokines, such as IL17, IL-1b, IL-6, and IL-12 [50], and enhance transcription of growth factors, adhesion molecules, and immune receptors [36]. Altogether, when the production of pro-inflammatory cytokines is reduced and Treg cells are predominant, the immune response is more regulated, and the risk of inflammatory pathologies is decreased.

SCFAs can suppress the NLRP3 inflammasome and promote an adequate immunologic response by directing T cell differentiation in appropriate phenotypes [36]. For example, reducing systemic inflammation in allergic reactions by modification of T helper type 2 cell numbers [37]. Besides, SCFAs are associated with decreased IL-8 in macrophages and neutrophils, TNF-α in mononuclear cells, and nitric oxide synthase in monocytes [51]. Similarly, butyrate can reduce prostaglandin synthesis by inhibiting COX-2 transcription [50]. All these effects help support the anti-inflammatory profile associated with a fiber-rich diet.

Moreover, SCFAs can influence humoral response. In plasmatic cells, acetate can increase retinoic acid conversion from vitamin A, facilitating response to CD4+ T cell and IgA production [47, 52]. Besides, butyrate and propionate favor antigen affinity inhibiting somatic hypermutation and enhancing class-switch DNA recombination in B cells [53]. SCFAs also influence the proliferation and migration of immune cells, not only as energy sources but through MPAK signal transduction and cascades associated with Ffar2 and Ffar3 receptors [51]. HDACs inhibition activity modulates lymphocyte function, increasing Th1, Th17, and innate lymphoid cells2 (ILC2) and ILC3 [47]. In summary, SCFAs not only allow a more balanced immune response but a more efficient and effective one.

SCFAs have proved to impact immune system development in early life. Exposure to SCFAs during the weaning period is associated with a tolerogenic phenotype and lower risk of inflammatory pathologies later in life, improving CD25+ Treg cells, humoral response, and gut epithelium integrity; confirming microbiota's role in immune system development [54].

#### **4.4 Microbiota-gut-brain axis**

Microbiota and their metabolites participate in the bidirectional communication between gut and brain, called the microbiota-gut-brain axis [42]. When SCFAs translocate from intestinal epithelium, they can travel by system circulation, immune system, or enteric-cerebral nervous pathway to provoke changes in distal organs [37, 49].

In the nervous system, butyrate is associated with an increase in cholinergic neurons in the gastrointestinal tract to facilitate motility, propionate with sympathetic activation to greater energy expenditure, and acetate with satiety by hypothalamic stimulation [55]. Similarly, along the gastrointestinal tract, there are enteroendocrine cells (EECs) that sense luminal content and release hormones in the systemic circulation. SCFAs can increase the release of glucagon-like peptide 1 (GLP-1) and peptide YY (PYY), affecting appetite signals and influencing weight control [49]. Therefore, SCFAs can modify autonomic functions and behavior, separately from CNS influence [56].

Another mechanism by which SCFAs alter neurological functions is by direct communication through the vagus nerve and enteric nervous system. SCFAs can alter the expression of GABA receptors [49], production of endothelial nitric oxide, anti-inflammatory and pro-inflammatory components in cerebral microcirculation [55], and increase neurogenesis [56]. Likewise, microbiota's metabolites are associated especially with microglia maturation and function, involving Toll-like receptors (TLRs) [49] and blood–brain barrier integrity [55]. These effects on CNS immune cells explain why SCFAs are associated with less risk of neuroinflammatory disorders.

There are still many mechanisms to be elucidated that could explain all the beneficial effects that microbiota's metabolites in eubiosis could have on host health. However, so far, our diet and early life events are one of the most important interventions to secure a healthy immune and neurologic system, through microbiota modulation.
