OBESITY AND GUT-BRAIN AXIS

Abstract

Epidemic of obesity is ongoing and did not slow down. Causes of obesity are numerous and very complex. Among them, the concept of bidirectional signaling within the brain-gut-microbiome axis was recently proposed as possible pathophysiological mechanism and become a hot topic in the explanations for the control of food intake. Discoveries of new anti-obesity drugs that are analogs for the receptors for some hormones derived from gastrointestinal tract contribute to the investigations in this area. The human gut microbiota plays a fundamental role in human health and disease and it is considered that it represent an endocrine organ that participate in energy homeostasis and host immunity. Role of gut microbiome has been investigated in metabolic diseases such as obesity, type 2 diabetes and non-alcoholic fatty liver disease. Gut microbiome participate in regulation of various mechanisms inside the gastrointestinal tract due to its production of different bacterial metabolites. In our manuscript we present current knowledge about microbiota in the gut; the relation between gut microbiota and brain; neuroendocrine system and gut-brain axis; immune system and gut-brain axis; endocrine system and gut-brain axis; the role of gut microbiota in obesity development and possible use of gut microbiota for the treatment of obesity.

INTRODUCTION

Prevalence of obesity is worsening in last few years, especially during Covid-19 pandemic mainly due to the restrictions in physical activity. It is proposed that the main reason for the increase in prevalence of obesity is the inequilibrium between intake of energy and its output. Currently, efforts for the prevention of obesity through reduction in dietary intake and increase in physical activity intake did not stop its increased incidence worldwide. It seems that the causes of obesity are numerous and very complex. Scientist all over the world are trying to expand the knowledge about additional causes of obesity rise such as the contribution of obesogens, different behavioral factors, social factors, lifestyle, psychological factors, socioeconomic status, environmental factors, health factors and sociodemographic factors (1). Recently, concept of bidirectional signaling within the brain-gut-microbiome axis in the pathophysiology of obesity was proposed (2). The role of gut-brain axis and gut hormones become a hot topic in the field of regulation of food intake (3).

The aim of our review is to present current knowledge about microbiota in the gut. It will be addressed the relation between brain and gut microbiota, the role of gut microbiota in obesity development and possible use of gut microbiota for the treatment of obesity.

Gut microbiota

Gut microbiota represent the complex bacterial community together with their genes and their products that exist inside the gastrointestinal tract (4, 5). The human microbiota plays a fundamental role in human health and disease. Gut microbiota influence many aspects of the host, like regulation of intestinal function; activation of the immune responses; regulation of metabolism and appetite (6). It is believed that gut microbiota represent an endocrine organ that participate in energy homeostasis and host immunity (7, 8). Human gut microbiota consist of up to 100 trillion microbes which exist in symbiosis with their human hosts (9). Geographic location, age, lifestyle and mode of birth have a strong influence on gut microbiota (10). There are over 100 bacterial species in the content of gut microbiota and the total weight of gut microbiota is about 1 kg in the adult human. The gut flora is mainly composed of bacteria. Besides bacteria, protozoa, viruses, archaea and fungi are also components of gut flora (11).

Two phyla were isolated from healthy subjects: Bacteroidetes and Firmicutes. Gram-negative Phylum Bacteroidetes includes Bacteroides, Prevotella, Parabacteroides and Alistipes while gram-positive Firmicutes includes mainly Faecalibacterium prausnitzii, Eubacterium rectal and Eubacterium hallii (8). Healthy intestinal microbiota is characterized by low amounts of phylum Proteobacteria and high amount of Bacteroides, Prevotella and Ruminococcus (12). There is an increase in the Firmicutes: Bacteroidetes ratio in obesity (13, 14). These changes are reversible by bariatric surgery or dietary intervention. Different parts of gastrointestinal tract contain different number of microbial cells per gram of content (stomach: 101; duodenum: 103; jejunum: 104 cells; ileum 107 cells and colon 1014 cells) (15, 16).

Gut microbiome role were investigated in different diseases associated with gastrointestinal tract, such as intestinal bowel disease, coeliac disease, irritable bowel syndrome, colorectal cancer and pancreatic disorders (5). Role of gut microbiome has been also investigated in metabolic diseases such as obesity, type 2 diabetes and non-alcoholic fatty liver disease (17). Gut microbiome participate in regulation of various mechanisms inside the gastrointestinal tract due to its production of different bacterial metabolites. These metabolites consequently interact with cell receptors of the host by positive or negative effects on signaling pathways.

Preclinical studies demonstrated existence of bidirectional signaling channels among the gut-brain axis that participated in the pathogenesis of obesity. These bidirectional communications between brain and gut microbiota, forming a network, are defined as “gut-microbiota-brain axis” (18). Gut-microbiota-brain network through bidirectional signaling channels are functioning using metabolic, endocrine, neural and immune mechanisms. Neural mechanisms and pathways are rapid while humoral mechanisms are slower.

Connections between gut and brain

Primary communications between gut microbiome and central nervous system are achieved through microbial-derived intermediates such as short-chain fatty acids, secondary bile acids, tryptophan metabolites, glutamate, g-aminobuturic acid (GABA), dopamine, norepinephrine, serotonin and histamine (19; 20). On the other side, communications between brain and gastrointestinal tract are mediated through vagus nerve, autonomic nervous system and hypothalamic-pituitary-adrenal axis. These communications have effects in intestinal tract on transit and motility, mucus and fluid secretion, immune activation, intestinal permeability, gene expression of certain gut microbes as well as their abundance (18).

Communications between gut microbiota, neuroendocrine system and the brain are established through humoral pathways, immune pathways and neural pathways (21). Neural pathways are represented with afferent and efferent vagus nerve fibers. Afferent vagus nerve fibers transport signals from gastrointestinal tract and gut microbiota to the brain, while efferent vagus nerve transport signals from brain to the enterochromaffin cells and enteroendocrine cells in the gut wall and the mucosal immune system. 20 different hormones are synthesised and secreted from enteroendocrine cells in the gut (22). Neuroanatomical communications between brain and gut are performed through the vagus nerve and autonomic nervous system. Vagus nerve is responsible for the facilitation of the bidirectional signals between the gut and brain stem, while autonomous nervous system innervates the gut and enteric nervous system. It was described that vagus nerve has ability to sense the microbiota and to transfer information obtained from the gut to the CNS. Informations are integrated within the CNS with consequent generation of an adapted or inappropriate response (23). Vagus nerve is composed of 80 % of afferent fibers and 20 % of efferent fibers. Its sensory and motor fibers represent one of the key players in reciprocal gut-brain communication (24).

There are four-level of integrative organization of these neuroanatomical communications: first, including enteric nervous system; second, including prevertebral ganglia; third, including autonomous nervous system, together with dorsal motor nucleus of vagus nerve in brain stem and nucleus tractus solitarius with afferent and efferent fibers of vagus nerve; and fourth, which includes higher brain centers (18). Vagus nerve transmits signals from gut receptors, influenced by gut microbiota, to central nervous system (25, 26). These receptors belong to a group of chemoreceptors and mechanoreceptors. Downward signals from central nervous system via efferent fibers of vagus nerve influence on the gut environment and gut microbiota.

Neuroendocrine system and gut-brain axis

Interaction between the brain and gut microbiota is established through modulation of brain neuroendocrine system on one side, and through hormone-like metabolites that are produced by gut microbiota on the other side (27). There are many neurotransmitters that are produced by gut microbiota that may influence the hypothalamus. Hypothalamic-pituitary-adrenal axis in close association with gut microbiota represents one of neuroendocrine systems that are involved in stress situation reaction. The nature of this interaction between gut microbiota and hypothalamo-pituitary-adrenal axis is bidirectional and it is in close interrelation with other regulatory systems like immune system, blood-brain barrier, gut hormones, sensory and autonomic nervous system (28). Neural pathways are represented with afferent and efferent vagus nerve fibers. Afferent vagus nerve fibers transport signals from gastrointestinal tract and gut microbiota to the brain, while efferent vagus nerve transport signals from brain to the enterochromaffin cells and enteroendocrine cells in the gut wall and the mucosal immune system.

The enteroendocrine cells are located through the gastrointestinal tract representing ≤ 1 % of intestinal epithelial cell population (3). It is estimated that signals from the brain are transmitted through 100 to 500 milion neurons from enteric nervous system located in submucosa and myenteric plexus of the gut wall (20).

Gut microbiota has ability to alter the synthesis and degradation of neurotransmitters as well as to produce novel chemical mediators that have effects on chemosensory cells within the gastrointestinal tract. Gut bacteria digest ingested macronutrients in the food, producing active biomolecules. Bacterial fermentation of complex carbohydrates produce short-chain fatty acids and amino acid tryptophan. Tryptophan is further metabolized to indole and indole-derivatives (29). Bile acids are also modified by gut bacteria, leading to imrovement in solubility and facilitation of recycling of bile acids in the distal colon.

Gut hormone secretion is associated with multiple intracellular signaling pathways that are necessary to stimulate its release from enteroendocrine cells within the gastrointestinal tract (30). Short chain fatty acids act on cell surface G-protein coupled receptors, leading to activation of intracellular effector molecules that produce different physiological responses. The long-lasting effects of short chain fatty acids on enteroendocrine L-cells lead to the proposal that they may be key regulators of metabolic health, suggesting their promising role in diets for the management of type 2 diabaetes mellitus and obesity (29). Bile acids act as signalling mollecules to cells lining the gastrointestinal tract and have a functional role in lipid digestion and absorption. There are two main receptors in L-cells: the cell surface G-protein coupled bile acid receptor 1 and the nuclear transcription factor, Farnesoid-X receptor (31). Activation of intestinal Farnesoid-X receptor is inhibitory for GLP-1 release in L-cells. Inhibition of GLP-1 release takes place due to downregulation in glycolysis and reduction in glucose transport (32). Tryptophan may enhance GLP-1 release from enteroendocrine cells in vitro. However, in vivo studies there was a lack of GLP-1 release by tryptophan (33). Enteroendocrine cells sens the presence of tryptophan by extracellular calcium-sensing receptor and G-protein receptor 142. Enteroendocrine L-cells may be altered by indoles. There is dual and opposing effects of indoles: acute application increases GLP-1 release by increasing calcium mobilization in L-cells, while chronic indole exposure reduce GLP-1 secretion due to supression of mitochondrial adenosine triphospate production. Indoles also activate PXR, a nuclear receptor with DNA-binding and ligand binding domain (34).

Immune system and gut-brain axis

Alterations in the gut microbiota may induce abnormal immune response in lymphatic tissue of the gut, compromising the systemic immune response. The immune system of the gut mucosa is usually challenged by food antigens and by the intestinal microbiota, making a balanced equilibrium between tolerance and defense (35). Host immune response is regulated by two main mechanism: activation of the innate immune response via Toll-like receptor and/or activation of free fatty recptors (36). It was shown that alterations in gut microbiota may induce excessive activation of Toll-like receptors and low production of short-chain fatty acids that may be associated with the development of obesity (37). An inverse correlation between the abundance of Akkermansia muciniphila and obesity was demonstrated. The mechanisms behind the interaction between Akkermania muciniphila and obesity and glucose levels is still not completely clear. Akkermansia muciniphila is a gram-negative anaerobic microorganism that has special effects, such as adipose tissue inflamattion, metabolic endotoxemia, fat mass gain and insulin resistance of the host (38). Akkermansia muciniphila produce mucin-degrading enzyme and utilize mucins in the mucus layer of epithelium as nitrogen and carbon source.

Akkermansia muciniphila is able to increase thermogenesis and the secretion of GLP-1 and to reduce expression of proteins involved in adipose cell differentiation and the gene expression for transporters of glucose and fructose in jejunum, indicating its possible role in reduction of carbohydrate absorption (39). There is a negative correlation between the Akkermansia muciniphyla levels and diabetes, obesity and metabolic syndromes (40). It was shown that oral administration of Akkermansia muciniphila could make a reversion of high fat diet-induced obesity in mice interacting with adipocyte metabolism and gut barrier function without influence on food intake (41). Due to its multiple effects in the gut, Akkermansia muciniphila is a promising prebiotic for the improvement of metabolic syndrome and obesity (40).

Endocrine system and gut-brain axis

Various peptides and hormones that are secreted from the gut in response to food intake take part in functioning of gut-brain axis. Enteroendocrine cells, neuropod cells and enterochromafine cells secrete gut hormones or peptides like GLP-1, CCK, GIP and PYY on their basolateral side. They may be transferred through circulation and act directly on the nucleus tractus solitarius, or they may act via paracrine action on local vagal and spinal afferent neurons that participate in the gut innervation (42). The other way that vagal afferent neurons may be activated is by neurotransmitters such as 5-HT, originated in enterochromafine cells and intraganglionic laminar endings that sense intestinal strech.

Hormones from the gut has important role in glucose homeostasis, appetite control and adiposity. They may be secreted postprandially (GLP-1, GIP. PYY, 5-HT, CCK, OXM) or in fasting state (ghrelin, 5-HT) (22). They may act independently or in a synergistic manner on a glucose homeostasis or energy status. Gastrointestinal peptides are secreted from enteroendocrine cells in response to food and communicate metabolic and nutritient status to the centres in the brain. In response to this communications brain decides on food seeking, food intake and food choice in order to maintain energy homeostasis. Enteroendocrine cells are dispersed through the gastrointestinal tract: X/A, G, D, and EC cells are located in the stomach, G,D,I,K, L and EC cells are in the small intestines while L and EC cells are in the large intestines (43).

Incretin hormone release is in a relation with glucose disposal in gastro-intestinal system (44). The interaction between the gut microbiota and brain is functioning in two ways: through modulation of brain neuroendocrine system and through hormone-like metabolites that are produced by the gut microbiome (18). Cells in the gastrointestinal tract are in the direct contact with nutritiens or metabolites produced by the gut microbiota which activate receptors on distal enteroendocrine cells that may release GLP-1 and GLP-2. Both peptides binds to their G-protein-coupled transmembrane receptors. GLP-1 has significant role in metabolism of glucose. Increasing concentrations of GLP-1 through gut-brain axis or pharmacological activation of GLP-1 receptors reduces gastric emptying, lower food intake and regulate body weight (45). GLP-1 also increase satiety through central nervous system and vagal afferent signals. GLP-1 receptor agonist use may improve intestinal epithelial integrity, reduce increased gut permeability and activate the Brunner glands (46). Effects of GLP-1 on satiety and weight loss due to its participation in gut-brain axis maintains the ileocolonic feedback loop (“ileal brake”). Ileocolonic feedback loop regulates the optimal rates of transit and delivery of nutrients necessary for digestion and absorption (46).

Ghrelin secretion from gastrointestinal tract is under influence of gut microbiome. Gut microbiome may act through its metabolites at the level of ghrelin receptor, although the mechanisms of this interaction are largely unknown (43). Ghrelin is secreted in stomach from X/A-like oxyntic gland cells (P/D1 cells in humans) and from enteroendocrine cells of the small intestine. Its action is to stmulate appetite. Correlation was described between microbial composition and diversity with gut peptide secretion changes and it was demonstrated that some gut bacteria may modulate enteroendocrine cells. Positive associations were described between ghrelin and total bacteria, Clostridium and Ruminococcus (47) while negative associations were identified between ghrelin levels and increased Bacteroides/Firmicutes ratio, Faecalibacterium and Prevotellaceae (43). Negative association was described between ghrelin levels and microbial short-chain fatty acids (48). Endotoxins produced by gut microbiota may influence ghrelin signaling. Recently it was suggested that gut microbiome and enteropathogens may modulate secretion of gut peptides and ghrelin signalling via Toll-like receptors (TLRs) that are expressed on enteroendocrine cells (43). Functional brain imaging studies in healthy subject gave evidence that ghrelin and GLP-1 have opposing effects on palatable food cue reactivty (49). Gut microbiome can influence glucose and energy homeostasis through different mechanisms, including host gut-brain signalling and direct communications to the brain via microbe derived metabolites. PYY is secreted from L-cells at the distal small intestine and colon after a meal. PYY secretion leads to decreased gastric emptying and suppressed pancreatic secretion (50).

Possible use of gut microbiota for the treatment of obesity

There is evidence that the composition of the gut microbiota and its metabolites may influence the progression of obesity and obesity-related disease (type 2 diabetes, insulin resistance, nonalcoholic fatty liver disease, atherosclerosis and some form of cancers (51). Gut microbiota metabolites have important role in development of obesity and associated diseases. Based on this presumption, it seems logical that therapies that are directed toward gut microbiota, gut microbiota metabolites and metabolites produced by the host could be used for prevention and treating the obesity. It is possible to modulate gut microbiota in a negative or positive manner using different lifestyle and dietary factors. Changes in composition of gut microbiota may have effect on nutrients absorption and regulation of energy intake. It is possible to decrease the ratio of Firmicutes to Bacteroides bacteria in the gut with the use of calorie-restricted diets, while use of vegetarian diet upregulate Bacteroides bacteria. Both of these approaches may be useful in therapeutic management of obesity (52 ).

It was shown that gut microbiome of patient living with obesity may be more efficacious in harvesting energy from ingested food in comparison with gut microbiome of subjects with normal weight. At the moment, dietary intervention and drugs are used for balancing the gut microbiota. Different forms of diets are suggested such as carbohydrate-restricted or fat-restricted, low caloric diet.

Gut microbiota composition may be altered by use of probiotics (Biffidobacterium and Lactobacillus species) or prebiotics (lactulose, inulin, fructooligosaccharides and galactooligosaccharides) (51). Mediterranean diet is rich in prebiotics and acts positively on the stability of the gut microbiota (53). Anti-obesity effects of probiotics is achieved through the regulation of intestinal microflora, decrease of insulin resistance and improving satiety (54). Vancomycin therapy in human males with metabolic syndrome has been demonstrated to change the composition of intestinal microflora with consequent decrease in peripheral insulin sensitivity (51). Use of prebiotics may selectively stimulate the growth or activity of some bacteria in colon with subsequent positive effect on the health of the host. Positive effects of prebiotics are improvement of intestinal barrier function, significant decrease of total fat and body weight and improvement in insulin sensitivity. Fecal microbiota transplantation (FMT) is a method in which the fecal fluid of healthy people is transplanted into intestines of patients with dysbiosis-related diseases achieving remodeling of gut microflora. Use of FMT may produce reduction of BMI in some cases (55) and significant change in area under the curve for glucose and insulin in patients with the metabolic syndrome (56).

In conclusion, recent discoveries in the understanding the physiology of gut-brain axis and its derangements open new hot topic in the field of obesity research and its management. Gut microbiota represent an endocrine organ that participate in energy homeostasis and host immunity. Concept of bidirectional signaling within the brain-gut-microbiome axis in the pathophysiology of obesity was proposed. Primary communications between gut microbiome and central nervous system are achieved through microbial-derived intermediates while between brain and gastrointestinal tract are mediated through vagus nerve, autonomic nervous system and hypothalamic-pituitary-adrenal axis. The role of gut-brain axis and gut hormones become a hot topic in the field of regulation of food intake. Various peptides and hormones that are secreted from the gut in response to food intake take part in functioning of gut-brain axis. Gut microbiome can influence glucose and energy homeostasis through different mechanisms, including host gut-brain signalling and direct communications to the brain via microbe derived metabolites. Finally, new knowledge about possible use of gut microbiota for the treatment of obesity was developed, using prebiotics, probiotics, antibiotics and fetal microbiota transplantation. Discoveries of new anti-obesity medications that are analogs of the receptors for some hormones derived from the gastrointestinal tract with very promising results, brings new lights into complexity of gut-brain axis and its relationship with the obesity.

Conflict of interest

The authors declare that they have no conflict of interest.