Although the pathogenesis of functional gastrointestinal disorders (FGIDs) is multifactorial, gastrointestinal (GI) dysmotility, and visceral hypersensitivity play a central role.¹ Recently, much attention has been focused on gut microbiota, as it has become clear that alterations in the gut microenvironment are significantly involved in the pathophysiology of various diseases such as GI disease, metabolic syndrome, autoimmune disease, and nervous/endocrine system disorders.² Gut microbiota have a complex influence on metabolism, nutrition and immune function in the host, and therefore disruption or alteration of the microbiota plays a pivotal role in GI inflammatory and/or FGIDs. Gut hormones also have central mediating roles in GI motility, appetite and body energy metabolism via the brain-gut axis, and thus also have pivotal involvement in FGIDs whose characteristic symptoms are closely associated with food intake. The present review provides a broad outline of interactions between the gut microbiota and gut hormone axis in relation to GI motility in the pathophysiology of FGIDs.

The motility of the GI tract is mediated by both neural and hormonal networks. Gut hormones are released from enteroendocrine cells scattered along the GI tract (comprising fewer than 1% of all GI epithelial cells),³ which play prominent roles in the hormonal networks during the interdigestive and postprandial periods. At present, more than 30 gut hormones have been isolated. Interestingly, the gut hormones that function in GI motility also affect appetite and body energy metabolism, suggesting that feeding behavior and GI motility are cooperatively regulated by gut hormone secretion (Table 1).

Gut Hormones Affecting Interdigestive Motility

Gut Hormones Affecting Postprandial Motility

Dysregulation of Gut Hormone in Functional Gastrointestinal Disorders

FGIDs are defined by symptom-based diagnostic criteria that combine chronic or recurrent symptoms attributable to the GI tract in the absence of other pathologically based disorders. A number of factors are involved in the pathophysiology of FGIDs, including visceral sensitivity, GI motility, GI mucosal immunity, gut microbiota, and psychosocial stress in brain-gut interaction.¹ Gut hormones have been proposed as key mediators of these factors in the brain-gut axis and are indeed involved in the development and/or exacerbation of FGID symptoms (Tables 2 and 3), as described below.

The human microbiota comprises approximately 1200 different bacterial species whose number (10¹³–10¹⁴) in the gut is 10 times greater than the total number of cells in the human body.² Gut microbiota exert a marked influence on host physiology including metabolism, nutrition, and immune function, and therefore, its disruption or alteration has been linked with GI inflammatory and functional disease.⁹⁸ It has become increasingly evident that host-gut microbial interactions have an extremely important role in the pathogenesis of FGIDs (Table 4), especially IBS.⁹⁹ Recent advances in sequencing technology have revealed that the profile of gut microbiota differs between patients with IBS and healthy subjects, and this has promoted researchers to investigate the role of the gut microbiota in IBS.

Gut Microbiota Production of Serotonin and SCFA

Evolutionarily-oriented studies have shown that many enzymes involved in human hormone metabolism have evolved through gene transfer from bacteria.¹⁰⁰ Interestingly, serotonin and other hormones such as epinephrine, norepinephrine and dopamine are produced and/or secreted from specific bacterial strains.¹⁰¹ Furthermore, it has been revealed that microorganisms harbor hormone receptors,¹⁰² suggesting that gut hormones act as possible mediators of communication between the host and gut microbiota.

Depending on the substrates (amino acids, lipids or carbohydrates) present in the intestinal lumen, gut microbiota can generate specific metabolites. Short-chain fatty acids (SCFAs) are produced by microbiota in the ileum and colon from carbohydrates with low digestibility (resistant starch and soluble oligo- and polysaccharides). The main components of SCFAs in the human colon are acetate (2-carbon), propionate (3-carbon), and butyrate (4-carbon), at a ratio of about 3:1:1.^103,104 Butyrate is considered a major energy source for the colonic epithelium. Propionate, entering the portal circle, is primarily utilized for gluconeogenesis in the liver. Consequently, SCFAs in plasma are largely acetate.^105,106 Approximately 95% of the produced SCFAs are rapidly absorbed by colonocytes in the large intestine while the remaining 5% are secreted in the stools.¹⁰⁷

Short-chain Fatty Acids as a Mediator between Gut Microbiota and Endocrine Cells

SCFAs serve as not only an important energy source but also chemical messengers or signaling molecules for various types of cells. GPR41 and GPR43, which are classified as G protein-coupled receptors and known as free fatty acid receptor (FFAR)3 and FFAR2, respectively, have recently been identified as receptors for SCFAs. Since these receptors are expressed in a variety of cell types, including colonic endocrine L cells, mucosal mast cells, adipose tissue, neutrophils, and monocytes, activation of these receptors may elicit distinct functions in various fields. Both GPR41 and GPR43 exhibit coupling to the Gi/o family and inhibit cAMP production, whereas GPR43, but not GPR41, also couples efficiently through the Gq protein family.¹⁰⁸

In the ileum and colon, GPR41 and GPR43 are expressed in enteroendocrine L-cells that secrete GLP-1 and PYY.¹⁰⁹ Therefore, it had been expected that SCFA would promote GLP-1 and PYY secretion by activating these GPRs, and indeed several in vivo studies have demonstrated that intraluminal injection of SCFAs induced the release of PYY and GLP-1 into the circulating blood.¹¹⁰ Furthermore, in vitro studies have shown that activation of GPR43 by SCFAs promotes GLP-1 secretion¹¹¹ and PYY expression¹¹² in enteroendocrine cells. On the other hand, individual GPR41- or GPR43-deficient mice show low GLP-1 release in response to SCFA stimulation, indicating that both GPR41 and GPR43 are involved in SCFA-evoked GLP-1 release.¹¹¹ These findings strongly suggest that SCFA signaling to GPR41 and GPR43 on L cells is crucial for controlling the levels of GLP-1 and PYY, thus playing a pivotal role in the “ileal brake.” However, in GF mice, Wichmann et al¹¹³ have demonstrated that total SCFA in the cecal content is decreased whereas the plasma GLP-1 level is increased. They showed that colonization of GF mice with microbiota or treatment with SCFAs reduced the expression of GLP-1 in the colon.¹¹³ Although these findings are difficult to reconcile, the explanation proposed is that GF mice are deprived of SCFAs produced by gut microbiota and serve as an important energy source for colonocytes. Subsequently, in response to this insufficiency of available energy in the colon, the GLP-1 level is increased to slow intestinal transit. In this context, GPRs may act as sensors of the amount of SCFA rather than being receivers of SCFA signaling.

SCFAs may be involved in the production of not only GLP-1 and PYY, but also other gut hormones such as 5-HT, GIP, ghrelin, and CKK. Histological studies have clarified that GPR43 is co-expressed with 5-HT,¹¹⁴ and that SCFAs stimulate the release of 5-HT by EC cells in the colonic mucosa.¹¹⁵ On the other hand, GPR40 and/or GPR120 are co-localized in cells expressing GIP, GLP-1, PYY, CCK, ghrelin, or 5HT,^116–118 although their ligands are predominantly medium- to long-chain fatty acids.¹¹⁹ Taken together, the available data suggest that GPRs are key tools involved in the interaction between endocrine cells and fatty acids produced from food and gut microbiota.

Alterations of Gut Microbiota in the Pathophysiology of Functional Gastrointestinal Disorders

The Rome Foundation has recently provided an excellent overview of the importance of microbiota in health and disease, especially functional bowel diseases.⁹⁹ It is widely accepted that GI infections caused by bacteria, viruses, or parasites are strong risk factors for the development of FGIDs.¹²⁰ With regard to the association between bacterial infection and FD, almost all current knowledge is based on H. pylori infection. H. pylori causes chronic atrophic gastritis, gastroduodenal ulcers and gastric cancers, suggesting a close involvement in the development of organic disease. In this context, whether or not patients with H. pylori infection should be excluded from the category of FD has been discussed. However, possible pathogenetic concepts to explain the link between H. pylori infection and dyspeptic symptoms have been positively incorporated into the recent Rome IV classification¹²¹ and management guidelines for H. pylori infection.¹²² In response to the recent focus on gut microbiota and FGIDs, several studies have begun to investigate the gastric microbiota in FD patients. For instance, as dysbiosis has been observed in the gastric fluid of patients with FD,¹²³ it has been suggested that normalization of the gastric microbiota by probiotics might be an effective treatment.¹²⁴ However, studies of the gastric microbiota in FGIDs are still at an early stage.

The link between GI infection and FGIDs has been studied mainly in IBS. In 1994, a group in the United Kingdom reported that IBS frequently occurs in patients after Salmonella-induced enteritis.¹²⁵ Thereafter, similar observations were reported worldwide, and a recent meta-analysis has clearly shown that IBS occurs frequently after bacterial enteritis caused by pathogenic strains of Escherichia coli, Salmonella, and Campylobacter jejuni.¹²⁶ Moreover, it is interesting that FD also occurs frequently in patients after bacterial enteritis, being compatible with the fact that IBS and FD often overlap in a single patient.¹²⁶ After remission of bacterial enteritis, no endoscopic and/or microscopic abnormalities are detectable in the GI tract of such FGID patients. Why, then, do patients with post-infectious IBS show visceral hypersensitivity and GI dysmotility? Accumulating evidence has led to a hypothesis that pathogens disrupt the mucosal barrier and that subsequently mucosal immune cells are persistently activated as a result of increased exposure to luminal antigens.¹²⁷ This low-grade mucosal inflammation is able to affect the immune system, endocrine cell behavior, and subsequently visceral sensitivity and GI motility. This modification of the gut environment may be central to the development of post-infectious IBS.

As only about 10% of IBS patients have a history of infectious colitis,¹²⁸ what is the role of gut microbiota in IBS patients without such a history? Recent advances in sequencing technology have made it easier to obtain information on gut microbiota at the genus level. Comprehensive analyses of fecal samples have demonstrated that the gut microbiota profile in IBS patients is largely different from that in healthy subjects, and has reduced diversity.^99,129 Specifically, several studies have demonstrated increased Firmicutes to Bacteroidetes ratios at the phylum level in IBS patients.^99,129,130 At the genus level, Lactobacillus, Bifidobacterium and uncultured Clostridium are decreased in patients with IBS, whereas Ruminococcus species are enriched.¹³⁰ Although it seems impossible to determine which bacterial strain is responsible for the development of IBS, dysbiosis of the microbiota is definitely a contributing factor. This is supported by experimental evidence that germ-free (GF) animals given microbiota transplants from IBS patients show visceral hypersensitivity and GI dysmotility.^131,132 On the other hand, although stress events in early life are closely associated with IBS development, the gut microbiota profile may be possibly modified by such events.^133,134 Moreover, antibiotic-induced dysbiosis (especially in childhood) has also been shown to be involved in the development of IBS.^133,135 These findings suggest that events in early life may be crucial in determining the IBS-related profile of gut microbiota. This invites speculation that manipulation of the gut microbiota may be useful for the treatment of IBS. Interestingly, the antibiotic rifaximin has been demonstrated to relieve the symptoms of IBS,¹³⁶ although the mechanism responsible remains unclear. A meta-analysis has also indicated that probiotics may reduce IBS symptoms, although there was significant heterogeneity among the studies investigated,^129,137,138 and the mechanism through which such probiotics act remains to be elucidated.

FGIDs are diagnosed on the basis of characteristic symptoms that are closely associated with food intake. The pathogenesis of FGIDs is multifactorial, and includes the neuroendocrine system, gut microbiota, neuroimmune reactions, interactions, psychological factors, and dietary factors.¹²⁹ The complexity of FGID pathophysiology may be due to the fact that through their interaction, these factors may become a cause and/or a consequence of the pathophysiology. The physiological alterations in FGIDs are also complicated, but there is little doubt that GI dysmotility and visceral hypersensitivity are critical. Here, we would like to focus on the interrelationships existing among gut microbiota, gut hormones and GI dysmotility in FGIDs (Figure).

Enteric Nervous System Receptors

The enteric nervous system (ENS) in the GI tract comprises a network of 200–600 million neurons and is involved in multiple aspects of host physiology including motility, metabolism, and behavior. This neural network is arranged in distinct units between the longitudinal and circular muscle layers of the intestine or in the submucosa as ganglionated plexi.¹³⁹ Gut microbiota may interact with the ENS directly or via afferent nerves (vagal sensory neurons, spinal sensory neurons and intrinsic primary afferent neurons) and the CNS indirectly through neurotransmitters. The link between microbiota and the ENS has been demonstrated in GF mice, which show a reduced number of enteric neurons and associated deficits of gut motility,¹⁴⁰ whereas reconstitution of GF mice with conventional microbiota normalizes the density of the ENS network and, subsequently, gut physiology.¹⁴¹

How, then, does the gut microbiota affect the ENS? Importantly, the ENS expresses pattern recognition receptors known as Toll-like receptors (TLRs) by which the microbiota communicate with the host.¹⁴² Anitha et al¹⁴⁰ have demonstrated that TLR4-deficient mice show a significant decrease of both the ENS network and GI transit time, and similar findings have been observed in TLR2-deficient mice.¹⁴³ Another important ENS receptor type are the GPRs such as GPR41 and GPR43, which respond to SCFA signaling. Since the gut microbiota produce SCFAs, they can use them as chemical messengers or signaling molecules to communicate with the host ENS. At present, although little is known about a molecular alteration of the ENS by SCFA stimulation, treatment with butyrate enhances histone H3 acetylation in enteric neurons and increases the contraction of cholinergic-mediated colonic circular muscle.¹⁴⁴

Gut Microbiota, Immune System Activation, and Gastrointestinal Motility

Not only the direct but also the indirect action of gut micro-biota on the ENS is important, and this interaction is complicated. Microbiota-derived SCFAs play roles in the maintenance of intestinal barrier function,^145,146 and indeed, butyrate, a microbiota-derived SCFA, prevents bacterial translocation by increasing the expression of tight junction proteins such as claudin, occludin, and the zonula occludens.¹⁴⁷ In this context, it is interesting to note that in IBS patients butyrate-producing bacteria may be reduced¹⁴⁸ and intestinal permeability is increased.^149,150 Thus, leakage of pathogens into the lamina propria of the intestinal mucosa is a significant trigger for activation of the mucosal immune system. In IBS patients, increased amounts of pro-inflammatory cytokines (TNF-α, IL-1β, IL-6, and IL-8) and decreased amounts of the anti-inflammatory IL-10 have been reported.¹⁵¹ Th1 cytokines such as TNFα, IL-1β, and IL-6 may act on the ENS and smooth muscle, resulting in suppression of GI motility.^152,153 On the other hand, Th2 cytokines including IL-10 may inhibit the expression of the above cytokines, thus accelerating of GI motility.^152,154 In IBS, it is still unclear whether these cytokines simply show this pattern of interaction. However, imbalance of not only gut microbiota but also the cytokine profile must be associated with intestinal low-grade inflammation, which is a significant pathophysiologic alteration in IBS.

Among the various immune cells, mast cells have been highlighted in the pathophysiology of IBS as their numbers are increased in colonic tissues of affected patients, and this increase is correlated with the severity of the clinical symptoms.^155,156 Mast cells are able to release histamine, serotonin, tryptase, and prostaglandins, and these mediators may act on their specific receptors on myenteric neural cells, leading to altered motor function.¹⁵⁵ Recent evidence has emphasized the pivotal roles of macrophages in GI motility through their action on myenteric neural cells,^{152,153,157,158} and indeed increased infiltration of macrophages into colonic tissues is observed in IBS patients,^156,159 implying that macrophages, like mast cells, may affect the ENS and smooth muscle through various mediators.^{152,153,157,158} Recently, macrophages have been classified into the M1 and M2 types that produce mainly Th1 and Th2 cytokines, respectively.^160,161 Thus, M1 macrophages release proinflammatory cytokines such as TNFα, IL-1β, and IL-6, whereas M2 macrophages may suppress M1 macrophages by releasing anti-inflammatory cytokines. Interestingly, M2 macrophages are known to infiltrate into the muscle layer of the intestine and may accelerate GI motility through stimulation with Th2 cytokines.¹⁵⁵ Furthermore, it has been reported that serotonin plays a role in polarization of macrophages toward an M2 phenotype.^160,162 Supporting these findings, we have clarified that 5-HT expression is increased in the colon of GF mice after fecal transplantation, and that moreover M2 macrophages in the colonic muscular layer are increased in those mice.¹⁶³ Furthermore, it is noteworthy that the numbers of muscularis M2 macrophages and 5-HT-positive endocrine cells are significantly correlated throughout the GI tract, and that their increase is associated with acceleration of GI motility. Thus, the gut microbiota plays a role in the association between accelerated GI motility and induction of the 5 HT/muscularis mannose receptor positive macrophage axis in the GI tract.¹⁶³ Specifically, Muller et al¹⁶⁴ have demonstrated that M2 macrophages migrating adjacent to the ENS may be involved in the control of GI motility through cross-talk with enteric neurons via bone morphogenetic protein 2 signaling.

Gut Microbiota and Gastrointestinal Motility-associated Hormones

The gut microbiota is able to affect GI motility via gut hormones. As shown in Tables 2 and 3, many investigators have intensively studied the gut microbiota and 5-HT in patients with IBS. However, to clarify the role of gut microbiota in the gut hormone/GI motility axis in IBS, experimental animal studies are needed. In particular, much evidence has been obtained from experiments using GF animals. For example, Wikoff et al¹⁶⁵ first demonstrated that GF mice display lower levels of 5-HT than conventionally raised animals, suggesting that the presence of gut microbiota is essential for the production and release of 5-HT.¹⁶⁵ In addition, Kashyap et al¹⁶⁶ have clarified that colonization of GF mice with gut micro-biota from humans or mice can significantly shorten the GI transit time and that this effect is partially inhibited by 5-HT receptor antagonist.¹⁶⁶ These findings strongly indicate that 5-HT induced by microbiota stimuli play a role in the acceleration of GI motility. Moreover, Yano et al¹⁶⁷ have clarified that indigenous spore-forming bacteria from the gut microbiota promote tryptophan hydroxylase 1 expression and 5-HT biosynthesis in EC cells, thus modulating GI motility. Although the mechanism by which microbiota induce 5-HT expression is still unclear, SCFA and cytokines in minimal inflammation may be candidate stimuli for 5-HT-producing EC cells,¹⁶⁸ and the microbiota themselves may also produce 5-HT.¹⁰¹ On the other hand, various types of 5-HT receptors are present on not only central and peripheral neural cells but also immune cells, smooth muscle, and enterocytes.^168,169 Additionally, the SERT, which terminates the action of 5-HT, is expressed in epithelial cells, neural cells, and platelets.¹⁶⁹ Accordingly, it is extremely difficult to determine how the gut microbiota/5-HT axis operates in GI motility and the development of symptoms in FGIDs.

Recently, not only 5-HT but also other gut hormones have been highlighted in relation to the gut microbiota/gut hormone axis in GI motility. As a result of their elegant work, Wichmann et al¹¹³ have proposed that the microbiota/GLP-1 axis is important for regulation of energy availability and GI motility. Specifically, enteroendocrine L cells sense the amount of bacteria-producing SCFA in the colon and secrete GLP-1 to allow greater nutrient absorption by inhibiting intestinal motility. Accordingly, it is tempting to speculate that GPR41 and GPR43 may play some roles in this process because these receptors are responsive to SCFA ligands. In support of this hypothesis, in vitro studies have shown that the SCFA affects GLP-1 secretion via GPR43 and/or GPR41 in L cells.^111,170 Enteroendocrine cells are significant intermediates in facilitating communication between microbes and the ENS.¹⁷¹ By using GF with fecal transplantation animal models, we have recently demonstrated that gut microbiota accelerate GI motility while suppressing the expression of the GLP-1 receptor in myenteric neural cells throughout the GI tract.¹⁷² These findings suggest that the gut microbiota affects the expression of gut hormone receptors on the ENS in the GI muscle layer.

Except for 5-HT and GLP-1, there are few data on the role of the gut microbiota/gut hormone axis in GI motility. As PYY is produced in L cells, it may––like GLP-1––be a sensor for SCFA and inhibit GI motility. Interestingly, GPRs are widely expressed in enteroendocrine cells such as those producing ghrelin, CCK, GIP, PYY, or GLP-1.¹⁷⁰ Moreover, since enteric neurons harbor the receptors for all of these gut hormones, a mechanism similar to that for GLP-1 or 5-HT may exist in the microbiota/gut hormone axis to mediate GI motility.

Gut microbiota have a pivotal influence on various functions including GI motility, metabolism, nutrition, immunity, and the neuroendocrine system in the host. This variety of bacterial actions is due to the production of SCFA and various mediators such as gut hormones and/or cytokines, which gut microbiota induce in enteroendocrine and immune cells. Moreover, the receptors for SCFA and gut hormones are widely distributed in neural cells, endocrine cells, immune cells and smooth muscle, further making it difficult to understand the role of gut microbiota in the pathophysiology of FGIDs. GI motility is well orchestrated by the ENS and hormonal networks, and its disturbance is closely associated with not only GI dysmotility but also visceral hypersensitivity and energy imbalance, that are significant abnormalities in the pathophysiology of FGIDs. In this article, we have mentioned that the gut microbiota may affect the ENS directly via the SCFA, or indirectly via gut hormones. Furthermore, we have described that gut microbiota elicit minimal inflammation by activation of the immune system and that mast cells and macrophages modify GI motility by acting on the ENS and smooth muscle. In this context, alteration of the gut microbiota/gut hormone axis significantly affects GI motility in the pathophysiology of FGIDs.

Although we have focused on GI motility in FGIDs, both the gut microbiota and gut hormones also play pivotal roles in metabolism, the brain-gut axis, and systemic immunity. Interestingly, the gut microbiota and gut hormones have been highlighted as targets of new therapeutic approaches for FGIDs, and indeed some trials using probiotics, antibiotics or gut hormone agonist/antagonists have been undertaken. Also, fecal microbiota transplantation, as applied to Clostridium difficile infection, may be attempted for FGIDs in the future. The research field focusing on interactions between gut microbiota and gut hormones may be referred to as “microbial endocrinology,” and is expected to become a focus of intense interest in the near future.