Gut microbiota exert a pivotal influence on various functions including gastrointestinal (GI) motility, metabolism, nutrition, immunity, and the neuroendocrine system in the host. These effects are mediated by not only short-chain fatty acids produced by microbiota but also gut hormones and inflammatory signaling by enteroendocrine and immune cells under the influence of the microbiota. GI motility is orchestrated by the enteric nervous system and hormonal networks, and disturbance of GI motility plays an important role in the pathophysiology of functional gastrointestinal disorders (FGIDs). In this context, microbiota-associated mediators are considered to act on specific receptors, thus affecting the enteric nervous system and, subsequently, GI motility. Thus, the pathophysiology of FGIDs is based on alterations of the gut microbiota/gut hormone axis, which have crucial effects on GI motility.
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).
Motilin, which is produced mainly by M cells in the duodenum during the interdigestive period, promotes phase III activity of the migrating motor complex and gastric contraction. Erythromycin, a motilin receptor agonist, increases gastric emptying and not only alleviates gastroparesis but also mediates blood glucose control in diabetic patients.4 However, clinical use of erythromycin for dysmotility is thought to be difficult, as its continuous use causes dysbiosis of the gut flora.
In 1999, ghrelin was isolated from the stomach as the endogenous ligand of growth hormone secretagogue receptor 1a.5 Ghrelin is produced and secreted by X/A-like cells in the stomach (especially the fundus)5 and stimulates appetite and food intake.6 In addition, accumulating evidence has clarified that ghrelin stimulates both gastric motility and gastric acid secretion.7,8 Ghrelin stimulates GI motility by acting on not only neuropeptide Y, the preganglionic dorsal vagal complex and vagal afferent neurons, but also intrinsic cholinergic neurons in the GI tract,9–12 and these effects are remarkably abolished by bilateral vagotomy.7
Cholecystokinin (CCK) was discovered in jejunal extracts as a gallbladder contraction factor.13 CCK is abundantly synthesized in small-intestinal I-cells and cerebral neurons. In addition, CCK is expressed in various endocrine glands (pituitary, thyroid, pancreatic islets, adrenal, and testis), peripheral nerves and kidney.14 Indeed, CCK plays roles in not only digestive function (pancreatic enzyme secretion and gut motility) but also neurotransmission in the cerebral and peripheral neuron systems.14 In the context of gut motility, it has been reported that exogenous CCK suppresses antral and duodenal motility,15 whereas CCK receptor antagonist accelerates gastric emptying.16 CCK is able to excite mucosal vagal afferent fibers in the stomach and regulate postprandial gastric emptying and satiation largely via the vagal pathway.17–19 On the other hand, small-intestinal transit time is shortened by stimulation with exogenous CCK, and prolonged by CCK receptor antagonist.20 In the colon, CCK administration does not affect human rectal motor function.21
Glucose-dependent insulinotropic polypeptide (GIP; also known as gastric inhibitory polypeptide) is produced mainly by K cells in the duodenum22 and stimulates insulin secretion as an in-cretin hormone in a glucose-dependent manner. In addition, GIP is likely to inhibit gastric acid secretion and gastric emptying in animals, whereas these inhibitory effects remain unclear in humans.22 On the other hand, triglyceride disposal and adipose uptake of fatty acids may be important functions of GIP in humans.23
Glucagon-like peptide 1 (GLP-1) is secreted predominantly from L cells in the ileum and colon and released as an incretin hormone in response to enteral nutrient exposure.24 GLP-1 as well as GIP stimulates insulin secretion and is rapidly degraded by dipeptidylpeptidase-4 (DPP4).25 In this context, not only GLP-1 agonist but also the DPP4 inhibitor was developed as a therapeutic medicine for diabetes. Of note, GLP-1 also plays a role in postprandial GI motility. Studies using endogenous/exogenous GLP-1 and/or DPP4 inhibitors have revealed that GLP-1 slows gastric emptying and intestinal motility.26 Moreover, recent evidence has suggested that GLP-1 inhibits postprandial GI motility through the GLP-1 receptor at myenteric neurons, involving nitrergic and cAMP-dependent mechanisms.27,28
Peptide YY (PYY) is secreted mainly from L cells in the ileum and colon29 and degraded by DPP4,30 similarly to GLP-1. Furthermore, the function of PYY resembles that of GLP-1; thus, PYY is likely to suppress appetite, slow gastric emptying and inhibit small-intestinal motility.31 Endogenous PYY acts via neuronal Y2 receptors to inhibit colonic transit.32
Serotonin, also termed 5-hydroxytryptamine (5-HT), functions as both a neurotransmitter in the CNS and a local hormone in the GI tract. More than 90% of the body’s 5-HT is synthesized in the gut (approximately 90% of 5-HT originates from enterochromaffin (EC) cells and 10% from enteric neuron cells).33 Tryptophan hydroxylase-1 is the rate-limiting enzyme for biosynthesis of 5-HT, and serotonin reuptake transporter (SERT) terminates the actions of 5-HT by removing it from the interstitial space.34 5-HT has motor function through interaction with neurons within the myenteric and submucosal plexuses, intrinsic and extrinsic sensory neurons, and EC cells. Among the 7 subtypes of serotonin receptors, 5-HT3 and 5-HT4 receptors have been most studied in the context of GI motility.35 5-HT released by mucosal mechanical and chemical stimuli is capable of inducing the mucosal peristaltic reflex, and hence propulsive peristalsis, and also affects the colonic migrating motor complexes.35
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.
Functional dyspepsia (FD) is a heterogeneous disorder associated with abnormalities of gut motor function, including initially accelerated or delayed gastric emptying, impaired proximal gastric relaxation, increased perception of gastric distension, and disordered antro-duodenal motility.36
Motilin and ghrelin, which may accelerate gastric emptying, are potential pathogenetic factors targets for clinical management of FD. For instance, although fasting motilin levels in FD patients do not differ from those in healthy subjects, exogenous motilin stimulation produces greater inhibition of proximal gastric accommodation in FD patients.36 In connection with gastric emptying, mitemcinal (a motilin receptor agonist) and ABT-229 (a motilin agonist) have been administered to patients with gastroparesis or FD, but it is still unclear whether they relieve symptoms such as abdominal satiety and pain in patients with FD.37,38 Another motilin receptor agonist (camicinal; GSK962040) has been developed and used in a phase II trial for critically ill patients with food intolerance.39 Camicinal has been shown to accelerate gastric emptying by 35–60% in patients with gastroparesis,40 and therefore further clinical studies of camicinal may be justified in patients with FD.
As for ghrelin, a few studies have reported that the level of ghrelin in plasma is decreased in FD patients,41,42 whereas others have indicated that it is elevated in such patients and related to the severity of their symptoms,43,44 and thus opinion is still divided. The plasma ghrelin concentration may decrease in accordance with the progression of gastric atrophy due to
In patients with FD, both fasting and postprandial plasma CCK concentrations are higher. Interestingly, intake of a high-fat diet increases the CCK level significantly and is related to the severity of nausea, suggesting that fat diet-associated CCK is involved in the development of FD symptoms.47 Furthermore, Chua et al48 have reported that FD patients stimulated with CCK-8 showed more severe symptoms of dyspepsia than healthy controls, suggesting that FD patients are hypersensitive to CCK stimulation. Also, as CCK promotes serotonin secretion in the hypothalamus,49 FD patients likely have central nervous system hypersensitivity to serotonin.50 Thus, postprandial CCK may affect serotonin signaling in the central nervous system in FD patients and participate in the development of their symptoms.
The hormone incretin plays a role in not only postprandial glucose metabolism but also GI motility, strongly suggesting significant involvement of incretin in the food-intake-associated pathophysiology of FD. Although the fasting plasma GIP and GLP-1 concentrations do not differ between FD patients and healthy controls, FD patients show hypersensitive responses to lipid infusion into the duodenum.51 Moreover, FD patients with severe symptoms show higher GIP and GLP-1 levels in response to lipid stimulation, supporting the contention that incretin mediates increased intestinal sensitivity to nutrients in FD. Witte et al52 have also reported that although the GLP-1 concentration is altered in FD, postprandial GLP-1 secretion correlates with nausea in affected patients. GIP and GLP-1 may be important targets for the treatment of not only diabetes/metabolic syndrome but also FD, and therefore further clinical studies should be encouraged.
PYY as well as GLP-1 is known to act as an “ileal brake” by suppressing GI motility, implying its pathophysiologic involvement in FD. In this connection, it is tempting to speculate that plasma PYY might be increased in FD patients. However, Pilichiewicz et al47 have reported that both the fasting and postprandial PYY levels are lower in FD patients than in healthy subjects, and Bharucha et al51 have found no difference between the two. Thus, although plasma PYY is not increased in patients with FD, this issue requires further investigation.
Data on 5-HT abnormalities are relatively fewer for FD than for IBS patients. It has been reported that 5-HT4 receptor agonists may improve symptoms of dyspepsia, particularly in patients with delayed gastric emptying.53 Previous studies have shown that
Irritable bowel syndrome (IBS) is characterized by symptoms such as abdominal pain or discomfort, bloating, and stool irregularities, without any structural or organic lesions.57 Many factors are involved in the pathogenesis of IBS, and indeed gut hormones are key players, as described below.
The levels of motilin reported in IBS patients have been conflicting, various studies indicating that they are higher,58 similar to,59 or reduced60 in comparison with healthy controls. Although dysmotility and visceral hypersensitivity are thought to play a crucial role in the pathophysiology of IBS, a high level of motilin may not reflect alteration of GI motility in IBS patients.61 In addition, exogenous motilin does not affect rectal sensation, at least in healthy volunteers.62 These findings suggest that alterations of the motilin level may be a consequence rather than a cause of IBS. The effect of erythromycin (a motilin receptor agonist) on colonic motility is also controversial; some studies have demonstrated that erythromycin accelerates the intestinal and/or colonic transit time,63,64 whereas others have not confirmed this.64,65
It is interesting that the ghrelin cell density in the gastric oxyntic mucosa is altered in patients with IBS. On the other hand, since IBS and FD frequently overlap, it is not unlikely that endocrine cells may be altered in the upper GI in IBS. Although evidence is still insufficient, it has been reported that ghrelin cell density is decreased in patients with IBS with constipation (IBS-C).66 This finding may not be surprising because the delay in intestinal transit time can be explained by inhibition of motility-promoting ghrelin. In contrast, ghrelin-positive cells are increased in patients with IBS with diarrhea (IBS-D),66 supporting the fact that such patients show rapid gastric emptying.67 However, gastric emptying in IBS is still controversial.68
El-Salhy M et al69 have shown that the densities of CCK are reduced in the duodenum of IBS-D patients, whereas they are not altered in IBS-C patients. In IBS-D patients, reduction of CCK may be involved in the rapid gastric emptying.67 A subgroup of IBS patients are known to have a history of infectious colitis, and show an increased number of CCK-positive cells.70 Furthermore, the plasma level of CCK is likely increased in the fasting and postprandial periods in IBS patients,70 similarly to the situation in FD patients.47 In contrast to the stomach, CCK stimulates intestinal motility, and therefore high CCK levels may be associated with the diarrhea seen in post-infectious IBS.
The density of duodenal endocrine cells differs between healthy controls and IBS patients. Indeed, GIP cell density is significantly decreased in both IBS-C and IBS-D,69 but other information on GIP in IBS is sparse. Clinical trials of a GLP-1 analog have been performed in IBS patients. Li et al71 have shown that the serum GLP-1 level was significantly decreased in IBS-C patients and negatively correlated with the severity of abdominal pain/discomfort. However, this seems paradoxical because GLP-1 inhibits intestinal motility and reduction of GLP-1 would lead to acceleration of GI motility. In this connection, opinion regarding the action of GLP-1 on colonic motility is still divided, both inhibitory and stimulatory effects having been reported.72–74 In a placebo-controlled trial, Camilleri et al75 showed that a GLP-1 receptor agonist, ROSE-010, improved colonic transit time in IBS-C patients, and Hellström et al76 found that ROSE-010 administration relieved acute pain in patients with IBS. Additionally, the GLP-1 receptor agonist also decreases visceral sensitivity and accelerates colonic transit via the central corticotropin-releasing factor and peripheral vagal pathways, although these findings were obtained in animal experiments.77,78
The expression of PYY is increased in the ileum of patients with IBS-C.79 As PYY inhibits colonic transit, it seems logical that IBS-C patients with delayed colonic transit show increased expression of PYY.80 However, the number of PYY-positive endocrine cells is smaller in the colon and rectum in both IBS-D and IBS-C patients,79,81 and PYY expression is high in the colon of patients with post-infectious IBS.82 These discrepancies may reflect the complicated pathophysiology of IBS. In fact, it is known that IBS patients switch from one subtype to another at least once yearly,83,84 probably as a result of the complex behavior of their enteroendocrine cells.
Data on 5-HT in IBS patients have been conflicting (Table 2). Some studies have reported that in both fasting and postprandial states, the plasma 5-HT level is increased in IBS patients relative to healthy volunteers,85–87 whereas others have found no differences in the plasma 5-HT level between the 2 groups.88 When IBS patients are subdivided, the plasma serotonin level is decreased in those with IBS-C89,90 and increased in those with IBS-D.90,91 A few histological studies have demonstrated that the number of 5-HT-positive cells is increased in IBS (especially post-infectious IBS),89,92 although conflicting data have also been reported.93,94 Genetic analyses of SERT polymorphism have also yielded mixed results.95–97 However, such discrepancies are not surprising because 5-HT is not the only factor operating in the development of IBS. On the other hand, the receptors for 5-HT are widely expressed in various types of cells including neurons and smooth muscle cells,35 and indeed 5-HT signaling is involved in GI motility and the development of IBS-related symptoms. In this context, it is noteworthy that release of 5-HT from the colonic mucosa correlated with the severity of abdominal pain in IBS patients.92
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.
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
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.
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
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
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
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).
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
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.
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