**2. Some molecules involved in the "dialogue"**

#### **2.1 Short-chain fatty acids**

SCFAs as byproducts of microbiota fermentations are widely studied. It is proven that microbial SCFAs (acetate, propionate, butyrate) are involved in the energy metabolism of the host [8, 9]. Attempts to cope with metabolic disorders in several diseases, including those of the brain, with the help of diets were unsuccessful. One study found different amounts of SCFAs were produced in the guts of subjects following the same diet (in terms of the amount and composition of fiber), since initially different gut microbiota can trigger different fermentation pathways of indigestible carbohydrates [6].

In their review, Dalile et al. [10] describe the effects of SCFAs on cellular systems and their interaction with gut–brain signaling pathways through immune, endocrine, neural, and humoral mechanisms. The researchers concluded that SCFAs can

**29**

*"Dialogue" between the Human Microbiome and the Brain*

penetrate the blood–brain barrier (BBB) to directly interact with brain tissues and even contribute to strengthening the integrity of the BBB. In addition, SCFAs promote serotonin biosynthesis and affect the levels of certain neurotrophic proteins, in particular, brain-derived neurotrophic factor (BDNF) and glial cell line-derived neurotrophic factor (GDNF) [10]. SCFAs also interfere with pathological mechanisms that are important for Alzheimer's disease. Thus, SCFAs are able to inhibit the formation of soluble beta-amyloid (Aβ) aggregates, which are associated with synaptic dysfunction and neurotoxicity. Another study examined the formation of neurotoxic amyloid aggregates (in vitro) and the dose-dependent effects of individual SCFAs on this process [11]. The authors call for the development of a new generation of probiotics that can metabolize individual dietary fibers to form valerian, butyric, and propionic acids and thus reduce the risk of developing neurodegenerative disorders. Unfortunately, animal and in vitro studies using pure fatty acid substances have several limitations. The source of SCFAs in vivo is the gut microbiota, and it remains unclear whether physiologically significant concentrations of SCFAs can be created

Tyrosine and tryptophan are two of the nine essential amino acids that cannot be synthesized in the human body. Various metabolic pathways of metabolism of aromatic amino acids, such as tyrosine and tryptophan, with different endogenous and microbial enzymes, have been previously described [1]. Most often, the products of microbial protein biodegradation are associated with negative or toxic effects [2, 12]. At the same time, results of various studies suggest that the products of anaerobic bacteria from a healthy human gut (metabolites of some *Clostridium* species, *Bacteroids*, *Bifidobacterium*, etc.) can be useful [8, 11–13], including for

Phenylcarboxylic acids (PhCAs) are metabolites of tyrosine that circulate in the blood of a healthy human in a constantly low concentration, normally not exceeding 5 μM [12]. Their microbial origin has been proven [12, 13], as have the causes of a significant increase in the number of certain PhCAs, such as *p*-HPhLA, PhLA, and p-HPhAA, in the blood serum of patients with sepsis and sepsis-associated encephalopathy [2]. Serum and fecal profiles of these aromatic microbial metabolites reflect gut microbiota disruption in critically ill patients, including those with brain pathology. It has been shown that the aromatic microbial metabolite profiles in the gut and serum are interlinked and reflect a disruption of the gut microbial community [14]. The taxonomic composition of microbiota and the profile of microbial metabolites of PhCAs were studied in critically ill patients with severe brain damage in comparison with other groups of patients, including healthy individuals. Using the 16S-ribosomal RNA (16S-*r*RNA) gene sequencing method, it was found that patients with positive dynamics were more characterized by a shift in the balance of the gut microbiota towards the predominance of *Clostridium* taxa [14]. The Glasgow Coma Scale (GCS), the National Institutes of Health Stroke Scale (NIHSS), the Rivermead Mobility Index Scale, and the Rankin scale were used to assess neurological status over time, while the monitoring of serum PhCAs levels was performed by gas chromatography–mass spectrometry (GC–MS). Results showed that the positive dynamics of neurological status in patients with brain damage was associated with serum level of phenylpropionic acid (PhPA) [15]. Based on studies that have established that PhPA is the end product of tyrosine metabolism by *Clostridia sporogenes*

*DOI: http://dx.doi.org/10.5772/intechopen.94431*

in the human brain [10].

**2.2 Metabolites of aromatic amino acids**

brain function, which we discuss later in the chapter.

*2.2.1 Phenolic metabolites of tyrosine*

*"Dialogue" between the Human Microbiome and the Brain DOI: http://dx.doi.org/10.5772/intechopen.94431*

*Human Microbiome*

**Figure 1.**

*microbiome and behaviour.*

The results of numerous studies show that the gut microbiota affects the development of diseases of the central nervous system (CNS), including motor and behavioral disorders, neurodegenerative diseases, and cardiovascular and neuroimmune-mediated disorders [4–6]. The existence of the microbiome–gut–brain axis is now generally recognized. There are several different mechanisms of gut bacteria action on the nervous system, including changes in the activity of the stress-related hypothalamic–pituitary–adrenal axis, vagus nerve stimulation, and the secretion of short-chain fatty acids (SCFAs), which can activate microglial cells and affect the permeability of the blood–brain barrier. Evolutionarily conserved signals that are involved in the communication between microbiota and the host, which include

*Graph showing a 20-fold increase from 2010 to 2019 in the number of publications on the relationship between the human microbiome and the brain, according to PubMed. Keyword search results: microbiome and brain,* 

This chapter focuses on several groups of low-molecular-weight compounds that originate primarily from the gut microbiota; their involvement in the interaction of the microbiota and the brain has been studied in various experimental and clinical studies.

SCFAs as byproducts of microbiota fermentations are widely studied. It is proven that microbial SCFAs (acetate, propionate, butyrate) are involved in the energy metabolism of the host [8, 9]. Attempts to cope with metabolic disorders in several diseases, including those of the brain, with the help of diets were unsuccessful. One study found different amounts of SCFAs were produced in the guts of subjects following the same diet (in terms of the amount and composition of fiber), since initially different gut microbiota can trigger different fermentation pathways

In their review, Dalile et al. [10] describe the effects of SCFAs on cellular systems and their interaction with gut–brain signaling pathways through immune, endocrine, neural, and humoral mechanisms. The researchers concluded that SCFAs can

different neuroactive substances, are known as neurochemicals [7].

**2. Some molecules involved in the "dialogue"**

**2.1 Short-chain fatty acids**

of indigestible carbohydrates [6].

**28**

penetrate the blood–brain barrier (BBB) to directly interact with brain tissues and even contribute to strengthening the integrity of the BBB. In addition, SCFAs promote serotonin biosynthesis and affect the levels of certain neurotrophic proteins, in particular, brain-derived neurotrophic factor (BDNF) and glial cell line-derived neurotrophic factor (GDNF) [10]. SCFAs also interfere with pathological mechanisms that are important for Alzheimer's disease. Thus, SCFAs are able to inhibit the formation of soluble beta-amyloid (Aβ) aggregates, which are associated with synaptic dysfunction and neurotoxicity. Another study examined the formation of neurotoxic amyloid aggregates (in vitro) and the dose-dependent effects of individual SCFAs on this process [11]. The authors call for the development of a new generation of probiotics that can metabolize individual dietary fibers to form valerian, butyric, and propionic acids and thus reduce the risk of developing neurodegenerative disorders. Unfortunately, animal and in vitro studies using pure fatty acid substances have several limitations. The source of SCFAs in vivo is the gut microbiota, and it remains unclear whether physiologically significant concentrations of SCFAs can be created in the human brain [10].

#### **2.2 Metabolites of aromatic amino acids**

Tyrosine and tryptophan are two of the nine essential amino acids that cannot be synthesized in the human body. Various metabolic pathways of metabolism of aromatic amino acids, such as tyrosine and tryptophan, with different endogenous and microbial enzymes, have been previously described [1]. Most often, the products of microbial protein biodegradation are associated with negative or toxic effects [2, 12]. At the same time, results of various studies suggest that the products of anaerobic bacteria from a healthy human gut (metabolites of some *Clostridium* species, *Bacteroids*, *Bifidobacterium*, etc.) can be useful [8, 11–13], including for brain function, which we discuss later in the chapter.

#### *2.2.1 Phenolic metabolites of tyrosine*

Phenylcarboxylic acids (PhCAs) are metabolites of tyrosine that circulate in the blood of a healthy human in a constantly low concentration, normally not exceeding 5 μM [12]. Their microbial origin has been proven [12, 13], as have the causes of a significant increase in the number of certain PhCAs, such as *p*-HPhLA, PhLA, and p-HPhAA, in the blood serum of patients with sepsis and sepsis-associated encephalopathy [2]. Serum and fecal profiles of these aromatic microbial metabolites reflect gut microbiota disruption in critically ill patients, including those with brain pathology. It has been shown that the aromatic microbial metabolite profiles in the gut and serum are interlinked and reflect a disruption of the gut microbial community [14].

The taxonomic composition of microbiota and the profile of microbial metabolites of PhCAs were studied in critically ill patients with severe brain damage in comparison with other groups of patients, including healthy individuals. Using the 16S-ribosomal RNA (16S-*r*RNA) gene sequencing method, it was found that patients with positive dynamics were more characterized by a shift in the balance of the gut microbiota towards the predominance of *Clostridium* taxa [14]. The Glasgow Coma Scale (GCS), the National Institutes of Health Stroke Scale (NIHSS), the Rivermead Mobility Index Scale, and the Rankin scale were used to assess neurological status over time, while the monitoring of serum PhCAs levels was performed by gas chromatography–mass spectrometry (GC–MS). Results showed that the positive dynamics of neurological status in patients with brain damage was associated with serum level of phenylpropionic acid (PhPA) [15]. Based on studies that have established that PhPA is the end product of tyrosine metabolism by *Clostridia sporogenes*

[16, 17], we believe that special attention should be paid to further confirmation of the involvement of *C. sporogenes* and studying the pathophysiological role of its metabolites in the process of neurorehabilitation.

## *2.2.2 Indolic metabolites of tryptophan*

The essential amino acid tryptophan is the only amino acid that contains the structure of an indole-bicyclic compound consisting of a six-membered benzene ring connected to a five-membered N-containing pyrrole ring, according to the Human Metabolome Database. Tryptophan is absorbed in the small intestine and metabolized to kynurenine, serotonin, and melatonin via the host's endogenous pathways. Manipulating heavily depleted tryptophan by way of diet has helped to identify patients who are prone to depression or other mood-lowering symptoms associated with dysfunctional monoaminergic systems, which can be attributed to serotonin deficiency [18]. The part of tryptophan that reaches the colon can be catabolized by the gut bacteria resulting in a variety of indole derivatives, such as indole, tryptamine, indoleethanol, indolepropionic acid (IPA), indolelactic acid (ILA), indoleacetic acid (IAA), skatole, indolealdehyde (IAld), and indoleacrylic acid [18, 19]. It is known that some products of bacterial biodegradation of tryptophan can be toxic, for example, indole, as well as indoxyl sulfate (IS), which is produced in the liver from indole and has a cytotoxic effect in high concentrations [19]. However, research shows that microbial tryptophan metabolites may also have a positive impact on host physiology. Tryptophan metabolites can modulate both the function of intestinal immune cells and astrocytes in the CNS via the aryl hydrocarbon receptor (AHR) [19, 20]. In experimental autoimmune encephalomyelitis, the effect of limiting inflammation of the CNS by affecting astrocytes in mice treated with antibiotics was shown by adding microbial metabolites of tryptophan from the gut microbiota (indole, indoxyl-3-sulfate, IPA, IAld) or the bacterial enzyme tryptophanase as AHR agonists [20].

Several studies have noted that IPA and IAA have anti-oxidative and antiinflammatory effects. A comparison of the varying data on the blood concentrations of IPA and IAA in patients with different diseases suggests that levels of both indole metabolites (IPA and IAA) are reduced in cancer [21]. Unfortunately, no studies to date have analyzed the behavior of these metabolites in patients with brain tumors, which could be extremely interesting.

There is information about the bacteria of the gut microbiota that is associated with the production of specific metabolites of indole. Interestingly, many species of anaerobes from different families are able to carry out biotransformation of tryptophan in vitro with the formation of IAA (nine species of *Clostridium*, four of *Bacteroides*, three of *Bifidobacterium*, and one of *Peptostreptococcus*). However, the ability to produce IPA was found only in three *species* of *Clostridiaceae*, and one of *Peptostreptococcus* [16, 21]. At the same time, the results obtained in vivo are more modest. In an experimental study of germ-free (GF) mice, production of IPA was completely dependent on gut colonization only by *C. sporogenes* [22].

The severity of stroke outcome in patients is associated with a stroke-induced inflammatory response, which in turn is linked with an increase in tryptophan catabolism [23, 24]. In Parkinson disease (PD) patients, CSF levels of tryptophan and kynurenic acid have been found to be significantly lower compared to healthy controls [25]. Future investigations are required to decipher how tryptophan metabolites derived from microbes are linked to inflammation in brain disorders [5]. The search and modification of methods for accurate measurement of microbial tryptophan metabolites continues. The availability of methods for determining

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*"Dialogue" between the Human Microbiome and the Brain*

microbial tryptophan metabolites are needed [26].

indicating its ability to penetrate the blood–brain barrier [28].

concentrations of microbial tryptophan metabolites in serum and CSF is currently limited and better quantitative analytical methods targeting a larger variety of

The formation of trimethylamine (TMA) occurs in the intestine via biotransformation of dietary lecithin, choline, or L-carnitine found in certain animal products (red meat, egg yolks) and is associated with bacteria of the genera *Anaerococcus*, *Clostridium*, *Escherichia*, *Proteus*, *Providencia*, and *Edwardsiella*. It is known that TMA is absorbed into the blood and oxidized in the liver by the flavin monooxygenase enzyme to form trimethylamine N-oxide (TMAO) [27]. TMAO is found in CSF,

The role of TMAO in neurodegenerative diseases, including AD, has been investigated extensively in the last five years. A study by Xu et al. [29] analyzed 20 metabolites that are significantly associated with cognitive decline in patients with AD. Potential genetic pathways underlying the strong association between TMAO and AD have been investigated. Employing an integrated computational approach, researchers identified nine main pathways and found that AD is closely related to TMAO. Thus, common genetic pathways underlying known biomarkers of AD were identified, with TMAO identified as the top-ranked microbial

Researchers studied TMAO as a biomarker of AD by comparing three groups of patients: those with AD clinical syndrome, those with mild cognitive impairment (MCI), and cognitively unimpaired individuals. All patient groups had undergone lumbar puncture with CSF collection (n = 410), as well as TMAO and other biomarkers of AD quantification. Metabolites of microbiota TMAO were significantly elevated in CSF and associated with other biomarkers of AD pathology (phosphorylated tau and phosphorylated tau/Aβ42) and neuronal degeneration (total tau and neurofilament light chain protein), which confirms gut microbial involvement in AD [30].

The gut microbiota can produce and/or consume numerous neurotransmitters, including dopamine, norepinephrine, serotonin, or gamma-aminobutyric acid (GABA) [4, 31]. Microbiota-dependent effects on gut serotonin significantly impact host physiology. For example, it is known that the gut contains the bulk of the body's serotonin (more than 85 percent 5-hydroxytryptamine (5-HT)), but the mechanisms that control the metabolism of 5-HT obtained from the gut are still unclear. A mammalian experiment showed that indigenous spore-forming bacteria from mouse and human microbiota promote 5-HT biosynthesis from colonic enterochromaffin cells, which supply 5-HT to the mucosa, lumen, and circulating

Disorders of the gut microbiome have been experimentally documented in some brain diseases and stroke. In animal models of AD, PD, and acute stroke, dysbiosis, intestinal motility disorders, and/or increased intestinal permeability were demonstrated. A pro-inflammatory immune response and increased microglia reactivity were recorded, compared with a non-diseased condition. Special experimental

*DOI: http://dx.doi.org/10.5772/intechopen.94431*

**2.3 Trimethylamines**

metabolite [29].

**2.4 Neurotransmitters**

platelets [32].

**3. Special experimental models**

concentrations of microbial tryptophan metabolites in serum and CSF is currently limited and better quantitative analytical methods targeting a larger variety of microbial tryptophan metabolites are needed [26].
