University of Groningen
Targeting the endocannabinoid system with microbial interventions to improve gut integrity
Jansma, Jack; Brinkman, Fleur; van Hemert, Saskia; El Aidy, Sahar
Published in:
Progress in Neuro-Psychopharmacology and Biological Psychiatry
DOI:
10.1016/j.pnpbp.2020.110169
IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from
it. Please check the document version below.
Document Version
Version created as part of publication process; publisher's layout; not normally made publicly available
Publication date:
2021
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA):
Jansma, J., Brinkman, F., van Hemert, S., & El Aidy, S. (2021). Targeting the endocannabinoid system with
microbial interventions to improve gut integrity. Progress in Neuro-Psychopharmacology and Biological
Psychiatry, 106, [110169]. https://doi.org/10.1016/j.pnpbp.2020.110169
Copyright
Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).
Take-down policy
If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.
Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum.
Progress in Neuropsychopharmacology & Biological Psychiatry xxx (xxxx) xxx
Available online 11 November 2020 0278-5846/© 2020 Published by Elsevier Inc.
Targeting the endocannabinoid system with microbial interventions to
improve gut integrity
Jack Jansma
a, Fleur Brinkman
a, Saskia van Hemert
b, Sahar El Aidy
a,*aDepartment of Molecular Immunology and Microbiology, Groningen Biomolecular Sciences and Biotechnology Institute (GBB), University of Groningen, Nijenborgh 7, 9747 AG Groningen, the Netherlands
bResearch and Development, Winclove Probiotics, Hulstweg 11, 1032LB Amsterdam, the Netherlands
A R T I C L E I N F O Keywords: Endocannabinoid system Probiotic Microbiota Intestinal barrier Gut Anandamide 2-arachidonoylglycerol Gut brain axis
A B S T R A C T
The endocannabinoid system is a metabolic pathway involved in the communication between the gut microbiota and the host. In the gut, the endocannabinoid system regulates the integrity of the intestinal barrier. A compromised integrity of the intestinal barrier is associated with several disorders such as inflammatory bowel disorder, obesity and major depressive disorder. Decreasing the integrity of the intestinal barrier results in an increased translocation of bacterial metabolites, including lipopolysaccharides, across the epithelial layer of the gut, causing the subsequent inflammation. Targeting the endocannabinoid system in the gut can improve the integrity of the intestinal barrier. Currently, microbial interventions in the form of probiotics are under inves-tigation for the treatment of diseases related to a compromised integrity of the intestinal barrier. However, the role of the endocannabinoid system in the gut is ambiguous since activity of the endocannabinoid system is increased in obesity and decreased in inflammatory bowel disease, emphasizing the need for development of personalized microbial interventions. This review discusses the role of the endocannabinoid system in regulating the gut barrier integrity and highlights current efforts to develop new endocannabinoid-targeted microbial interventions.
List of abbreviations
IBS irritable bowel syndrome MDD major depressive disorder LPS lipopolysaccharides ECS endocannabinoid system 2-AG 2-arachidonoylglycerol AEA N-arachidonoylethanolamine
AA arachidonic acid
NAPE-PLD N-acylphosphatidylethanolamine-hydrolyzing phospholipase D
FAAH fatty acid amide hydrolase DAGL diacylglycerol lipase MAGL monoacylglycerol lipase 2-LG 2-linoleoyl glycerol 2-PG 2-palmitoyl glycerol
CB1 ESC-G-protein coupled receptors 1 CB2 ESC-G-protein coupled receptors 2 SEA N-stearoylethanolamine
PEA N-palmitoylethanolamine
OEA N-oleoyethanolamine
LEA N-linolenoylethanolamine
(continued on next column)
(continued)
DHEA N-docosahexaenoyl ethanolamine
2-OG 2-oleoyl glycerol
TRPV-1 transient receptor potential vanilloid-1 PPAR-γ peroxisome proliferator-activated receptor GF germ-free
DIO diet induced obesity HFHS high fat high sucrose MGBA microbiota gut brain axis TNBS trinitrobenzene sulfonic acid IBS-C irritable bowel syndrome constipation IBS-D irritable bowel syndrome diarrhea HF high-fat
NAPE N-acyl-phosphatidylethanolamines
TEER transepithelial electrical resistance ETA ethanolamine
1. Introduction
An enormous number of microorganisms, including bacteria reside
* Corresponding author.
E-mail address: sahar.elaidy@rug.nl (S. El Aidy).
Contents lists available at ScienceDirect
Progress in Neuropsychopharmacology & Biological
Psychiatry
journal homepage: www.elsevier.com/locate/pnp
https://doi.org/10.1016/j.pnpbp.2020.110169
Progress in Neuropsychopharmacology & Biological Psychiatry xxx (xxxx) xxx
2 in the human gut. Under normal circumstances, the gut microbiota is in a balanced and symbiotic relationship with the host (Li et al., 2008). Disruption of the balance between the host and its microbiota, which is called dysbiosis, can be detrimental for the host health. For example, dysbiosis is associated with gut related diseases such as, irritable bowel syndrome (IBS) (Wang et al., 2020), metabolic disorders such as obesity (Nagpal et al., 2018), as well as brain related diseases including major depressive disorder (MDD) (Cheung et al., 2019). A possible factor that links dysbiosis to host malfunction is the integrity of the intestinal barrier.
The intestinal barrier is comprised of the lamina propria, epithelial layer and the mucus layer (Vancamelbeke and Vermeire, 2017). When the integrity of the intestinal barrier is compromised, its permeability is increased, and in turn, translocation of bacteria and their metabolites across the intestinal barrier becomes more prevalent (Bischoff et al., 2014). Bacteria and their metabolites can translocate across the intes-tinal barrier either transcellularly or paracellularly. Paracellular trans-location is the movement of molecules in the space between adjacent epithelial cells. This space is not large enough to allow the translocation of a whole bacterium or large protein complexes, but translocation of ions and small bacterial metabolites, including lipopolysaccharides (LPS), does occur (Guerville and Boudry, 2016). Transcellular trans-location is the crossing of apical and basolateral membranes of an epithelial cell by bacteria or their metabolites, which is often mediated by receptors (Karasov, 2017). A compromised integrity of the intestinal barrier results in activation of the immune system, and consequently increased production of pro inflammatory compounds, which results in systemic inflammation (Camilleri, 2019)(Fig. 1).
A metabolic pathway at the crosslink of microbial dysbiosis and in-testinal barrier integrity is the endocannabinoid system (ECS) (Cani et al., 2016). The ECS is associated with several diseases including in-flammatory bowel disorders (Ahmad et al., 2017), neurological disor-ders (Cristino et al., 2020), and MDD (Ohlsson et al., 2019). Thus, microbial dysbiosis might interfere with the normal balance of the ECS, which alters the integrity of the intestinal barrier, and may lead to dis-ease development (Muccioli et al., 2010). However, the precise molec-ular mechanisms involved are not yet fully elucidated. This review summarizes the state-of-the-art knowledge on the role of the ECS in regulating the gut barrier integrity and highlights current efforts to develop new ECS-targeted microbial interventions.
2. The endocannabinoid system
The ECS is comprised of endocannabinoids, enzymes, and receptors. The endocannabinoids present in the ECS are 2-arachidonoylglycerol (2- AG), and N-arachidonoylethanolamine (also known as anandamide (AEA)) (Fig. 2). In human plasma, the concentration of 2-AG and AEA is in a nanomolar range, where AEA ranges between 0.6 and 18.7 nM (Balvers et al., 2009; Matias et al., 2006) and 2-AG between 0.4 and 4.6 nM (Di Marzo et al., 2009; Fern´andez-Rodriguez et al., 2004). Endo-cannabinoids are synthesized from arachidonic acid (AA) and mem-brane precursors; phosphatidylinositol for 2-AG and phosphatidylethanolamine for AEA, respectively (Piomelli, 2003). The levels of endocannabinoids are tightly controlled in the human body. For example, AEA can be synthesized via several metabolic pathways (Leung Donmienne et al., 2006; Liu Jie et al., 2008) including N-acylphospha-tidylethanolamine-hydrolyzing phospholipase D (NAPE-PLD) enzyme (Maccarrone Mauro, 2017) and degraded by fatty acid amide hydrolase (FAAH) enzyme. 2-AG is synthesized by diacylglycerol lipase (DAGL) and degraded by monoacylglycerol lipase (MAGL) enzyme (Chiurchiù et al., 2018; Ol´ah et al., 2017). Notably, MAGL is inhibited by 2-linoleoyl glycerol (2-LG) and 2-palmitoyl glycerol (2-PG), which could, in turn, enhance the effect of 2-AG (Ben-Shabat et al., 1998).
AEA and 2-AG function through the activation of the ESC-G-protein coupled receptors 1 and 2 (CB1 and CB2) (Cani et al., 2016; Chiurchiù et al., 2018). CB1 is expressed in the brain and on the terminus of
peripheral nerves, where it controls memory, motor activity and sensory perception (Cani et al., 2016). CB2 is expressed on myeloid cells, lymphoid tissue and peripheral organs (Leimuranta et al., 2018).
The ECS can be expanded with other metabolites, receptors and enzymes. The expanded ECS is called the endocannabinoidome (Di Marzo, 2018). Besides CB1 and CB2, AEA can also activate the transient receptor potential vanilloid-1 (TRPV-1), peroxisome proliferator- activated receptor (PPAR-γ), and inhibit T-type Ca2+channels. 2-AG can also activate TRPV1 (Di Marzo and Silvestri, 2019). TRPV1 is located at the nerve terminals of extrinsic primary afferents (Ol´ah et al., 2017), where it is involved in peripheral inflammation, the integration of pain stimuli and modulation of nociceptive inputs to the brainstem and spinal cord (Cortright et al., 2007). The activation of TRPV1 by 2-AG and AEA has been shown to play a role in reducing pain (Storozhuk and Zholos, 2017). PPAR-γ is involved in a variety of physiological pro-cesses, such as lipid and glucose homeostasis, fat cell differentiation, inflammation and aging (Han et al., 2017; Lefterova et al., 2014; Wang et al., 2016). Additionally, next to AEA and 2-AG the endocannabinoi-dome consists of other fatty acid derived metabolites such as N-stear-oylethanolamine (SEA), N-palmitN-stear-oylethanolamine (PEA), N- oleoyethanolamine (OEA), N-linolenoylethanolamine (LEA), N-docosa-hexaenoyl ethanolamine (DHEA), 2-oleoyl glycerol (2-OG) and 2-LG (Di Marzo, 2018). These endocannabinoidome ligands, also have different targets. For example, PEA can directly activate PPAR-α, GPR55 and
indirectly activate TRPV1 through activation of PPAR-α or via
enhancing the effect of AEA (De Petrocellis et al., 2001; Paolo et al., 2013), DHEA activates GPR110, LEA and OEA activate TRPV1 and GPR119, OEA activates PPAR-α, and 2-OG and 2-LG activate GPR119
and TRPV1 (Di Marzo, 2018).
3. The endocannabinoid system in the gut
In the gut, both CB1 and CB2 are expressed on enteric nerves, enterocytes, and immune cells. In addition, CB1 is highly expressed on the enteroendocrine cells of the intestinal mucosa (Izzo and Sharkey, 2010). Activation of CB1 induces mesenteric vasodilation, suppresses secretion of fluid and acid, and stimulates motility of the GI tract (Izzo and Sharkey, 2010). Under normal conditions, the actions of the ECS in the gut are mostly mediated through CB1. However, during inflamma-tion, activation of both CB1 and CB2 occurs resulting in the production of anti-inflammatory cytokines, which reduces intestinal inflammation and gastric damage (Fichna et al., 2013; Kinsey et al., 2011; Toczek and Malinowska, 2018).
Furthermore, Sykaras et al. showed that CB1 mRNA is highly expressed on I-cells, which produce cholecystokinin (Sykaras et al., 2012), and DiPatrizio et al. examined the role of the ECS in dietary fat intake. The authors showed increased levels of AEA and 2-AG in the small intestine when rats were sham fed. Administering a CB1 inhibitor to the duodenum reduced the amount of fat intake (DiPatrizio et al., 2011). Together, these results show that the ECS influences the energy balance through feeding behavior (Di Marzo, 2011; Piomelli, 2003). The ECS has also been described to be involved in regulation of the intestinal barrier integrity (Moludi et al., 2018). In obese mice, CB1 activation resulted in reduced levels of tight junction proteins. Importantly, germ- free (GF)-mice showed higher transcription of CB1 in the colon compared to conventionally raised mice, implying a key role for the microbiota in regulating the induced transcription of CB1 (Muccioli et al., 2010). Manca et al. confirmed the results of Muccioli et al. and showed that a fecal microbiota transplant (FMT) could reverse CB1 transcription in the colon. Furthermore, the authors showed decreased transcription of GPR55 in the whole GI tract of GF-mice compared to conventionally raised mice, which was upregulated in response to FMT (Manca et al., 2020), further confirming the significance of the micro-biota in regulating the intestinal ECS.
Since the microbiota is a key player in the intestinal tract, alterations in the gut microbiota can also affect the ECS balance and, in turn, the
Fig. 1. Intestinal barrier integrity in health and disease. The intestinal barrier is comprised of the mucus layer, epithelial cells and the lamina propria. During disease, expression of tight junction proteins is decreased. In turn, the space between epithelial cells is larger, resulting in increased paracellular translocation of metabolites produced by the microbiota in the lumen, which, in turn causes activation of the immune system and inflammation.
Progress in Neuropsychopharmacology & Biological Psychiatry xxx (xxxx) xxx
4 integrity of the intestinal barrier (Moludi et al., 2018). For example, administration of the CB1 receptor antagonist SR141716A via oral gavage to mice with diet-induced obesity (DIO) showed an increase in
Akkermansia muciniphila and a decrease in Lanchnospiraceae and Erysi-pelotrichaceae in the colon compared to DIO mice treated with a vehicle
and these changes were associated with an increase in mucus layer thickness (Mehrpouya-Bahrami et al., 2017). Moreover, Guida et al. showed a reduction in the levels of N-acyl-serotonins, which are lipids included in the endocannabinoidome, in homogenized tissue of the duodenum and jejunum of antibiotic treated mice, which was accom-panied by a reduction in levels of AEA. In addition, the authors showed an elevation of the expression levels of TRPV1 in the hippocampus. Subsequently, the antibiotic treated mice showed more depressive-like symptoms compared to the controls. Interestingly, administration of
Lactobacillus casei improved the depressive-like symptoms, and
normalized the levels of N-acyl-serotonins and, in turn, the levels of AEA in the GI tract (Guida et al., 2018). This effect is possibly mediated by altering the microbial composition in the gut. In another study, mice were fed a high fat high sucrose (HFHS) diet and showed a significantly increased level of AEA in the ileum compared to mice fed a low fat low sucrose diet. Furthermore, levels of PEA and OEA were decreased in the HFHS-mice. However, after 56 days of the HFHS diet, the levels of PEA were restored. Additionally, the levels of 2-AG, 2-OG and 2-LG after 56 days showed a decreasing trend in the ileum. The altered levels of these metabolites can be explained by altered expression levels of FAAH mRNA and MAGL in the ileum. Using Spearman correlation analysis, the authors showed an association of AEA with the relative abundance of
Barnesiella, Adlercreutzia, Parasutterlla, Porpionibacterium, Enterococcus
and Methylbacterium species in the ileum (Lacroix et al., 2019). The ECS in the gut is linked to the ECS in the brain via the microbiota-gut-brain axis (MGBA). The MGBA is a bidirectional communication network that includes the peripheral nerves, the enteric and the central nervous system, gut hormones, the gut microbiota and microbiota derived molecules (Farzi et al., 2018; Russo et al., 2017). One of the mechanisms involved in the MGBA is the regulation of the in-testinal barrier via expression of tight junction proteins and mucus metabolism (Carabotti et al., 2015; Julio-Pieper et al., 2014). Therefore,
a compromised integrity of the intestinal barrier is associated with brain-related disorders. For example, dysfunction of tight junction pro-teins is associated with Parkinson’s disease (Santos et al., 2019). Higher concentrations of immunoglobulin A and M are detected in the serum of depressed patients compared to healthy controls, indicating higher translocation of bacteria across the epithelial barrier (Maes et al., 2012). Furthermore, Ohlsson et al. found decreased levels of zonulin, a protein involved in assembling of tight junctions, in plasma of depressed pa-tients compared to healthy controls (Ohlsson et al., 2019). Since mi-crobial dysbiosis is associated with MDD and Parkinson’s disease and the ECS plays a central role in the integrity of the intestinal barrier, altering the ECS in the gut could be a mechanism by which microbial dysbiosis induces disease. For example, stress reduces the thickness of the mucus layer in the colon of rats compared to non-stressed rats, which was accompanied by microbial dysbiosis. Interestingly, these changes were also associated with upregulation of CB2 mRNA expression in the colon (Aguilera et al., 2013). Other endocannabinoidome ligands, in particular, PEA and OEA, have been reported as regulators of the in-testinal barrier integrity. Karwad et al. investigated the effect of OEA and PEA on the permeability of a Caco-2 monolayer. The authors showed an increase in permeability when OEA was applied to the apical membrane, and a decrease in permeability when OEA was applied to the basolateral membrane. This effect was mediated via TRPV1. Moreover, PEA was shown to decrease permeability in a PPAR-α dependent manner
when applied to both the apical and basolateral membrane (Karwad Mustafa et al., 2017; Karwad et al., 2019). Together, these results sug-gest that altered ECS metabolism induced by members of the gut microbiota could impair the health status.
4. The endocannabinoid system in health and disease
Alterations in the ECS are also associated with metabolic disorders. For example, in obese mice, the expression levels of the enzyme NAPE- PLD and CB1 receptor are elevated, whereas FAAH enzyme is reduced in adipose tissue compared to lean mice. Moreover, the concentration of AEA in obese mice is increased compared to lean mice, showing that an altered ECS balance is associated with obesity (Forte et al., 2020). The
Fig. 2. Overview of the compounds and enzymes in the endocannabinoid system. The two compounds of the ECS are arachidonoylethanolamide (AEA) and 2-arach-idonoylglycerol (2-AG). Membrane lipid precursors are the substrate for the synthesis of AEA by NAPE-phospholipase D (NAPE-PLD) and 2-AG by diacylglycerol lipase (DAGL). AEA is degraded into arachidonic acid (AA) and ethanolamine (ETA) by fatty acid amidehydrolase (FAAH), while 2-AG is degraded into AA and glycerol via monoacylglycerol lipase (MAGL) (Simon and Cota, 2017).
relationship between obesity and the ECS is mediated by lower expres-sion levels of tight junction proteins in the colon of obese mice and an elevated concentration of LPS in their plasma compared to their lean counterparts, highlighting the role of the intestinal barrier in obesity (Muccioli et al., 2010). Similarly, intraduodenal infusion of a CB1 re-ceptor antagonist to rats reduced fat intake and reduced body weight (DiPatrizio et al., 2011). Elevated plasma levels of 2-AG were accom-panied by an increase in the consumption of favorite food compared to normal food in humans (Monteleone et al., 2012), showing another link between the ECS and obesity. Moreover, when NAPE-PLD enzyme was selectively knocked out in hepatocytes in mice, the knockout mice developed an obese-like phenotype (Lefort et al., 2020). When NAPE- PLD was selectively knocked-out in adipose tissue of mice, the knockout mice developed an obese-like phenotype too (Geurts et al., 2015), highlighting the importance of NAPE-PLD in the development of obesity. When the microbiota of the mice with the adipose tissue-NAPE- PLD knockout was transferred to GF-mice the mice developed more fat mass, gained more weight and had a higher adipose index compared to GF-mice treated with WT microbiota (Geurts et al., 2015). When NAPE- PLD was selectively knocked-out in intestinal epithelial cells of mice, the mice accumulated more fat, showed higher liver mass, as well as altered microbiota composition when fed a high fat (HF) diet compared to wild type mice fed a HF-diet. However, treatment with A. muciniphila reduced body weight and fat mass of the NAPE-PLD knockout mice fed a HF-diet, indicating that intestinal epithelial NAPE-PLD is not required for the protective effects of A. muciniphila (Everard et al., 2019).
Similar to the development of obesity in combination with the ECS, more diseases are related to a malfunctioning ECS. Since the ECS plays a role in the functioning of the GI-tract, alteration in the ECS balance can be detrimental for host health. IBS, for example, is characterized by spasms, abdominal pain and an altered gut motility that can result in constipation (IBS-C) or diarrhea (IBS-D) (Saha, 2014). To link the role of the ECS in regulating gut motility directly to IBS, genetic variants of the CB1 and FAAH genes, as well as lower levels of OEA have been found in IBS-D, while higher levels of OEA were detected in patients with IBS-C (Fichna et al., 2013).
As mentioned above, the microbiota can affect mental health via alteration of the intestinal barrier, which is mediated by changes in in-testinal ECS (Maes et al., 2012; Ohlsson et al., 2019). Another mecha-nism by which the microbiota can affect the ECS in the brain is by altering lipid availability. Markey et al. showed that colonization of mice with Candida albicans increased anxiety like behavior and production of the stress hormone corticosterone compared to mice colonized with a mock community. While, the levels of AEA in the brain were reduced, the levels of LEA were increased in the cecum of mice colonized with
C. albicans (Markey et al., 2020). Additionally, Manca et al. showed decreased levels of 2-AG, 2-OG, and 2-LG in the whole brain of GF-mice compared to conventionally raised mice. However, these low levels of 2- AG were not significantly reversed after FMT (Manca et al., 2020). In the brain, alteration of the ECS can result in, among others, mood, anxiety and depressive disorders (Parolaro Daniela et al., 2010; Stampanoni Bassi et al., 2018). Indeed, Hill et al., showed that stimulation of the CB1 receptor improved behavior of rats during the forced swimming test, similar to administration of conventional anti-depressants (Hill and Gorzalka, 2005). In addition, antagonizing the CB1 receptor may induce anxiety and depression (Moreira and Crippa, 2009). The extensive research regarding the role of the ECS in the brain and in mental dis-orders is reviewed elsewhere (Ashton and Moore, 2011; Meyer et al., 2018). Nevertheless, the studies mentioned above emphasize the po-tential effect of the gut microbiota on the ECS in the brain. However, the precise mechanisms underlying this effect is not completely clear and needs to be investigated.
5. Improving gut permeability with microbial interventions
Since the microbiota can influence the integrity of the intestinal
barrier via the ECS, alteration in the microbiota might reduce disease symptoms caused by a compromised integrity of the intestinal barrier. Probiotic intervention can be used as an ECS-targeted intervention in the microbiota, which is already under investigation. For example, admin-istration of Lactobacillus acidophilus NCFM was found to induce CB2 expression in the colon of mice and rat, which showed analgesic effects against visceral pain (Rousseaux et al., 2007). Everard et al. showed an increase in 2-AG, 2-OG and 2-PG levels in the ileum after A. muciniphila administration to mice fed a HF-diet. Furthermore, A. muciniphila increased the inner mucus layer of the colon in HF-diet mice, whereby heat killed bacteria did not alter the mucus layer in HF-diet mice, indicating that the bacteria need to be alive to elicit an effect (Everard et al., 2013). The probiotic mixture VSL#3, containing a variety of
Bifidobacteria, Lactobacilli spp. and Streptococcus thermophilus, is
recog-nized to be potentially beneficial in chronic IBD (Chapman et al., 2006). Moreover, VSL#3 has been found to increase colonic CB1 expression in mice (Distrutti et al., 2014). In zebra fish, VSL#3 upregulates CB1 and CB2 expression and downregulates FAAH gene expression. Moreover, VSL#3 showed anti-inflammatory and immune stimulant capacities, which could be related to the upregulation of CB2 (Gioacchini et al., 2017). In search for probiotic treatment against obesity, Chen et al., introduced N-acyl-phosphatidylethanolamines (NAPE), the precursor for N-acylethanolamines such as AEA, in Escherichia coli Nissle 1917. After treatment with the genetically modified E. coli, plasma NAE levels were not affected in mice, which is likely due to the rapid conversion of NAE by FAAH. Although the NAE levels were not affected, changes were observed. Mice that were fed a HF-diet and treated with genetically modified E. coli gained less weight, and showed a reduction in food intake and meal size compared to control mice. These effects lasted for 12 weeks after bacterial administration. NAE levels were also elevated in the liver, suggesting a systemic effect of treatment with the genetically modified E. coli Nissle 1917 (Chen et al., 2014). Developing treatments targeting the ECS can help relieve symptoms associated with obesity (Fig. 3) and can lower the inflammatory response associated with a reduced integrity of the intestinal barrier (Fig. 4). For example, Alhouayek et al. showed that inhibition of MAGL enzymes reduced tri-nitrobenzene sulfonic acid (TNBS)-induced colitis in mice by lowering the inflammatory response in a CB1 and CB2 mediated manner (Alhouayek et al., 2011).
6. Conclusion and future perspectives
The ECS plays an ambiguous role in health and disease. CB1 inhi-bition reduces fat intake and obesity is associated with higher CB1 activation and a lower integrity of the intestinal barrier, indicating that a higher ECS activity is detrimental for host health (Muccioli et al., 2010) (Fig. 3). In contrast, lower ECS activity is associated with higher inflammation and lower integrity of the intestinal barrier in IBD. Acti-vation of the ECS in IBD mice reduces inflammation via production of anti-inflammatory compounds (Gyires and Z´adori, 2016; Leinwand et al., 2017) (Fig. 4). Since the gut microbiota is associated with alter-ations in the ECS and intestinal barrier integrity, a specific microbial dysbiosis can be the cause of the ambiguous role of the ECS in health in disease. To distinguish between cause and effect, more research is needed to elucidate the effect of endocannabinoids on bacteria and the metabolism of endocannabinoids by bacteria. Cohen et al. showed a wide distribution of a bacterial N-acyl synthase gene in stool samples. With liquid chromatography and mass spectrometry the authors showed production of several N-acyl amides in pure bacterial cultures and showed that these compounds could activate human GPR119 (Cohen et al., 2017). Friedman et al. incubated Vibrio harveyi with AEA and showed no effect on growth of the bacteria. However, incubation with 100 μg/mL AEA had an inhibitory effect on quorum sensing and motility
(Friedman et al., 2019). Although, the physiological concentration of AEA ranges between 0.6 and 18.7 nM and V. harveyi is a marine bac-terium it suggests unexplored effects of the ECS on the gut microbiota.
Progress in Neuropsychopharmacology & Biological Psychiatry xxx (xxxx) xxx
6 To better understand the role of the ECS in intestinal barrier func-tioning, a more detailed understanding of the integrity of the intestinal barrier is needed. Currently, the intestinal barrier integrity is often measured indirectly using biomarkers. Biomarkers can be measured in
plasma, urine or fecal samples and include claudine, calprotectin and endotoxins (Vancamelbeke and Vermeire, 2017). However, biomarkers cannot always distinguish between paracellular and transcellular translocation, which region of the intestine is affected and, between
Fig. 3. The role of the endocannabinoid system in obesity and potential treatment with probiotic intervention. In obesity, CB1 receptors expressed on epithelial cells of the gut are activated by 2-AG and AEA present in the lumen, resulting in lower production of tight junction proteins, thus widening the gap between epithelial cells, and in turn, causing more translocation of bacterial metabolites, activation of the immune system, and inflammation. Administration of bacterial species which is able to degrade AEA or 2-AG into AA may lower the activation of CB1 receptors, thereby treating inflammation.
Fig. 4. A proposed role of the endocannabinoid system in IBD and potential treatment with probiotic intervention. Lower activity of the ECS, microbial dysbiosis and decreased integrity of the intestinal barrier are associated with IBD, due to the increased translocation of bacterial metabolites, activation of the immune system, and inflammation. Administering bacterial species may result in increasing the synthesis of endocannabinoids via treating the microbial dysbiosis. In turn, activation of the ECS results in a lower immune response and alleviation of the symptoms of the disease.
bacterial origin of endotoxins. However, understanding the mechanism of translocation is essential for development of interventions. To answer some of these questions, transepithelial electrical resistance (TEER) has been used. TEER is an in vitro method which uses two compartments separated by a layer of cells or tissue and measures changes in ion current, whereby more intestinal permeability results in less electrical resistance due to higher ion flow between the compartments (Srinivasan et al., 2015). Impedance spectroscopy is a TEER method whereby para- and transcellular movement can be distinguished, but can be technically challenging (Günzel et al., 2012). Furthermore, choosing the correct type of cells or tissue can be challenging and does not capture the full complexity of the intestine. Nevertheless, in vitro methods are necessary to better understand the molecular mechanisms of intestinal barrier integrity and can be used to investigate the role the ECS plays. Alto-gether, accumulating evidence shows that intestinal barrier integrity, microbiota and host health are intertwined. At the crossroads, the ECS plays an important role and might be the molecular system linking the three together. Therefore, targeting the ECS with microbial in-terventions in the gut is a promising way to improve the integrity of the intestinal barrier, thereby improving host health.
Funding
The research of J.J. is funded by a research grant of Winclove B.V. S. E.A. is supported by Rosalind Franklin Fellowships, co-funded by the European Union and University of Groningen.
Declaration statement
The manuscript does not include any supporting data.
Declaration of Competing Interest
S van Hemert is employee in Winclove (Winclove manufactures and markets probiotics). The content of this study was neither influenced nor constrained by this fact. The other authors have no conflicts of interest to declare.
References
Aguilera, M., Vergara, P., Martínez, V., 2013. Environment-related adaptive changes of gut commensal microbiota do not alter colonic toll-like receptors but modulate the local expression of sensory-related systems in rats. Microb. Ecol. 66 (1), 232–243.
Ahmad, R., Sorrell, M.F., Batra, S.K., Dhawan, P., Singh, A.B., 2017. Gut permeability and mucosal inflammation: bad, good or context dependent. Mucosal Immunol. 10 (2), 307–317.
Alhouayek, Mireille, Lambert, Didier M., Delzenne, Nathalie M., Cani, Patrice D., Muccioli, Giulio G., 2011. Increasing endogenous 2-arachidonoylglycerol levels counteracts colitis and related systemic inflammation. FASEB J. 25 (8), 2711–2721.
Everard, Amandine, Plovier, Hubert, Rastelli, Marialetizia, Van Hul, Matthias, de Wouters d’Oplinter, Alice, 2019. Intestinal epithelial N-
Acylphosphatidylethanolamine phospholipase D links dietary fat to metabolic adaptations in obesity and steatosis. Nat. Commun. 10.
Ashton, C.H., Moore, P.B., 2011. Endocannabinoid system dysfunction in mood and related disorders. Acta Psychiatr. Scand. 124 (4), 250–261.
Balvers, Michiel G.J., Verhoeckx, Kitty C.M., Witkamp, Renger F., 2009. Development and validation of a quantitative method for the determination of 12
endocannabinoids and related compounds in human plasma using liquid chromatography-tandem mass spectrometry. J. Chromatog. B: Analytical Technol. Biomed. Life Sci. 877 (14–15), 1583–1590.
Ben-Shabat, Shimon, Fride, Ester, Sheskin, Tzviel, Tamiri, Tsippy, Rhee, Man Hee, Vogel, Zvi, Bisogno, Tiziana, De Petrocellis, Luciano, Di Marzo, Vincenzo, Mechoulam, Raphael, 1998. An entourage effect: inactive endogenous fatty acid glycerol esters enhance 2-arachidonoyl-glycerol cannabinoid activity. Eur. J. Pharmacol. 353 (1), 23–31.
Bischoff, Stephan C., Barbara, Giovanni, Buurman, Wim, Ockhuizen, Theo, Schulzke, J¨org Dieter, Serino, Matteo, Tilg, Herbert, Watson, Alastair, Wells, Jerry M., 2014. Intestinal permeability - a new target for disease prevention and therapy. BMC Gastroenterol. 14 (1), 189.
Camilleri, Michael, 2019. Leaky gut: mechanisms, measurement and clinical implications in humans. Gut. 68 (8), 1516–1526.
Cani, P.D., Plovier, Hubert, Van Hul, Matthias, Geurts, Lucie, Delzenne, Nathalie M., Druart, C´eline, Everard, Amandine, 2016. Endocannabinoids-at the crossroads
between the gut microbiota and host metabolism. Nature Rev. Endocrinol. 12 (3), 133–143.
Carabotti, Marilia, Scirocco, Annunziata, Maselli, Maria Antonietta, Severi, Carola, 2015. The gut-brain axis: interactions between enteric microbiota, central and enteric nervous systems. Annals Gastroenterol. 28 (2), 203–209.
Chapman, Therese M., Plosker, Greg L., Figgitt, David P., 2006. VSL#3 probiotic mixture: a review of its use in chronic inflammatory bowel diseases. Drugs 66 (10), 1371–1387.
Lefort, Charlotte, Roumain, Martin, Van Hul, Matthias, Rastelli, Marialetizia, Manco, Rita, 2020. Hepatic NAPE-PLD is a key regulator of liver lipid metabolism. Cells 9 (5).
Chen, Zhongyi, Guo, Lilu, Zhang, Yongqin, Walzem, Rosemary L., Pendergast, Julie S., Printz, Richard L., Lindsey C. Morris, et al., 2014. Incorporation of therapeutically modified bacteria into Gut microbiota inhibits obesity. J. Clin. Invest. 124 (8), 3391–3406.
Cheung, Stephanie G., Goldenthal, Ariel R., Uhlemann, Anne Catrin, Mann, J. John, Miller, Jeffrey M., Sublette, M. Elizabeth, 2019. Systematic review of gut microbiota and major depression. Front. Psychiatry 10 (FEB.).
Chiurchiù, Valerio, Leuti, Alessandro, Maccarrone, Mauro, 2018. Bioactive lipids and chronic inflammation: managing the fire within. Front. Immunol. 9 (JAN), 29.
Cohen, Louis J., Esterhazy, Daria, Kim, Seong Hwan, Lemetre, Christophe, Aguilar, Rhiannon R., Gordon, Emma A., Amanda J. Pickard, et al., 2017. Commensal bacteria make GPCR ligands that mimic human signalling molecules. Nature 549 (7670), 48–53.
Cortright, Daniel N., Krause, James E., Broom, Daniel C., 2007. TRP channels and pain. Biochim. Biophys. Acta 1772 (8), 978–988.
Cristino, Luigia, Bisogno, Tiziana, Di Marzo, Vincenzo, 2020. Cannabinoids and the expanded endocannabinoid system in neurological disorders. Nat. Rev. Neurol. 16 (1), 9–29.
De Petrocellis, L., Davis, J.B., Di Marzo, V., 2001. Palmitoylethanolamide enhances anandamide stimulation of human vanilloid VR1 receptors. FEBS Lett. 506 (3), 253–256.
Di Marzo, Vincenzo, 2011. Endocannabinoids: an appetite for fat. Proc. Natl. Acad. Sci. U. S. A. 108 (31), 12567–12568.
Di Marzo, Vincenzo, 2018. New approaches and challenges to targeting the endocannabinoid system. Nat. Rev. Drug Discov. 17 (9), 623–639.
Di Marzo, Vincenzo, Silvestri, Cristoforo, 2019. Lifestyle and metabolic syndrome: contribution of the endocannabinoidome. Nutrients 11 (8), 1–24.
Di Marzo, Vincenzo, Verrijken, An, Hakkarainen, Antti, Petrosino, Stefania, Mertens, Ilse, Lundbom, Nina, Piscitelli, Fabiana, et al., 2009. Role of insulin as a negative regulator of plasma endocannabinoid levels in obese and nonobese subjects. Eur. J. Endocrinol. 161 (5), 715–722.
DiPatrizio, Nicholas V., Astarita, Giuseppe, Schwartz, Gary, Li, Xiaosong, Piomelli, Daniele, 2011. Endocannabinoid signal in the gut controls dietary fat intake. Proc. Nat. Acad. Sci. U.S. Am. 108 (31), 12904–12908.
Distrutti, Eleonora, O’Reilly, Julie Ann, McDonald, Claire, Cipriani, Sabrina, Renga, Barbara, Lynch, Marina A., Fiorucci, Stefano, 2014. Modulation of intestinal microbiota by the probiotic VSL#3 resets brain gene expression and ameliorates the age-related deficit in LTP. PLoS ONE 9 (9).
Everard, Amandine, Belzer, Clara, Geurts, Lucie, Ouwerkerk, Janneke P., Druart, C´eline, Bindels, Laure B., Guiot, Yves, et al., 2013. Cross-talk between Akkermansia muciniphila and intestinal epithelium controls diet-induced obesity. Proc. Natl. Acad. Sci. U. S. A. 110 (22), 9066–9071.
Farzi, Aitak, Fr¨ohlich, Esther E., Holzer, Peter, 2018. Gut microbiota and the neuroendocrine system. Neurotherapeutics 15 (1), 5–22.
Fern´andez-Rodriguez, Conrado M., Romero, Julian, Petros, Timothy J.,
Bradshaw, Heather, Gasalla, Jos´e M., Guti´errez, Maria Luisa, Jos´e L. Lled´o, et al., 2004. Circulating endogenous cannabinoid anandamide and portal, systemic and renal hemodynamics in cirrhosis. Liver Int. 24 (5), 477–483.
Fichna, Jakub, Wood, JodiAnne T., Papanastasiou, Malvina, Vadivel, Subramanian K., Oprocha, Piotr, Sałaga, Maciej, Sobczak, Marta, et al., 2013. Endocannabinoid and cannabinoid-like fatty acid amide levels correlate with pain-related symptoms in patients with IBS-D and IBS-C: a pilot study. Edited by Michal Zmijewski PLoS ONE 8 (12), e85073.
Forte, Nicola, Fern´andez-rilo, Alba Clara, Palomba, Letizia, Di Marzo, Vincenzo, Cristino, Luigia, 2020. Obesity affects the microbiota–gut–brain axis and the regulation thereof by endocannabinoids and related mediators. Int. J. Mol. Sci. 21 (5).
Friedman, Liat, Smoum, Reem, Feldman, Mark, Mechoulam, Raphael, Steinberg, Doron, 2019. Does the endocannabinoid anandamide affect bacterial quorum sensing, vitality, and motility? Cannabis and Cannabinoid Res. 4 (2), 102–109.
Gioacchini, Giorgia, Rossi, Giacomo, Carnevali, Oliana, 2017. Host-probiotic interaction: new insight into the role of the endocannabinoid system by in vivo and ex vivo approaches. Sci. Rep. 7 (1), 1–12.
Guerville, Mathilde, Boudry, Ga¨elle, 2016. Gastrointestinal and hepatic mechanisms limiting entry and dissemination of lipopolysaccharide into the systemic circulation. Am. J. Physiol. Gastrointestinal Liver Physiol. 311 (1), G1–G15.
Guida, F., Turco, F., Iannotta, M., De Gregorio, D., Palumbo, I., Sarnelli, G., Furiano, A., et al., 2018. Antibiotic-induced microbiota perturbation causes gut
endocannabinoidome changes, hippocampal neuroglial reorganization and depression in mice. Brain Behav. Immun. 67, 230–245.
Günzel, Dorothee, Zakrzewski, Silke S., Schmid, Thomas, Pangalos, Maria,
Wiedenhoeft, John, Blasse, Corinna, Ozboda, Christopher, Krug, Susanne M., 2012. From TER to trans- and paracellular resistance: lessons from impedance spectroscopy. Ann. N. Y. Acad. Sci. 1257 (1), 142–151.
Progress in Neuropsychopharmacology & Biological Psychiatry xxx (xxxx) xxx
8
Gyires, Kl´ara, Z´adori, Zolt´an S., 2016. Role of cannabinoids in gastrointestinal mucosal defense and inflammation. Curr. Neuropharmacol. 14 (8), 935–951.
Han, Lu, Shen, Wen Jun, Bittner, Stefanie, Kraemer, Fredric B., Azhar, Salman, 2017. PPARs: Regulators of metabolism and as therapeutic targets in cardiovascular disease. Part II: PPAR-β/δ and PPAR-γ. Futur. Cardiol. 13 (3), 279–296.
Hill, M.N., Gorzalka, B.B., 2005. Pharmacological enhancement of cannabinoid CB1 receptor activity elicits an antidepressant-like response in the rat forced swim test. Eur. Neuropsychopharmacol. 15 (6), 593–599.
Izzo, Angelo A., Sharkey, Keith A., 2010. Cannabinoids and the gut: new developments and emerging concepts. Pharmacol. Ther. 126 (1), 21–38.
Julio-Pieper, M., Bravo, J.A., Aliaga, E., Gotteland, M., 2014. Review article: intestinal barrier dysfunction and central nervous system disorders - a controversial association. Aliment. Pharmacol. Ther. 40 (10), 1187–1201.
Karasov, William H., 2017. Integrative physiology of transcellular and paracellular intestinal absorption. J. Exp. Biol. 220 (14), 2495–2501.
Karwad, M.A.M.A., Couch, D.G., Wright, K.L., Tufarelli, C., Larvin, M., 2019. Endocannabinoids and endocannabinoid-like compounds modulate hypoxia-induced permeability in CaCo-2 cells via CB1, TRPV1, and PPARa. Biochem. Pharmacol. 168, 465–472.
Karwad Mustafa, A.M.A., Macpherson, Tara, Wang, Bo, Theophilidou, Elena, Sarmad, Sarir, 2017. Oleoylethanolamine and palmitoylethanolamine modulate intestinal permeability in vitro via TRPV1 and PPARa. FASEB J. 31 (2), 469–481.
Kinsey, Steven G., Nomura, Daniel K., O’Neal, Scott T., Long, Jonathan Z.,
Mahadevan, Anu, Cravatt, Benjamin F., Grider, John R., Lichtman, Aron H., 2011. Inhibition of monoacylglycerol lipase attenuates nonsteroidal anti-inflammatory drug-induced gastric hemorrhages in mice. J. Pharmacol. Exp. Ther. 338 (3), 795–802.
Lacroix, S´ebastien, Pechereau, Florent, Leblanc, Nadine, Boubertakh, Besma, Houde, Alain, Martin, Cyril, Flamand, Nicolas, et al., 2019. Rapid and concomitant gut microbiota and endocannabinoidome response to diet-induced obesity in mice. mSystems 4 (6), 1–14.
Lefterova, Martina I., Haakonsson, Anders K., Lazar, Mitchell A., Mandrup, Susanne, 2014. PPARγ and the global map of adipogenesis and beyond. Trends Endocrinol. Metab. 25 (6), 293–302.
Leimuranta, Pinja, Khiroug, Leonard, Giniatullin, Rashid, 2018. Emerging role of (endo) cannabinoids in migraine. Front. Pharmacol. 9 (4), 24.
Leinwand, Kristina L., Gerich, Mark E., Hoffenberg, Edward J., Collins, Colm B., 2017. Manipulation of the endocannabinoid system in colitis: a comprehensive review. Inflamm. Bowel Dis. 23 (2), 192–199.
Leung Donmienne, D., Saghatelian, A., Simon, G.M., Cravatt, B.F., 2006. Inactivation of N-acyl phosphatidylethanolamine phospholipase D reveals multiple mechanisms for the biosynthesis of endocannabinoids. Biochemistry 45 (15), 4720–4726.
Li, Min, Wang, Baohong, Zhang, Menghui, Rantalainen, Mattias, Wang, Shengyue, Zhou, Haokui, Zhang, Yan, et al., 2008. Symbiotic gut microbes modulate human metabolic phenotypes. Proc. Natl. Acad. Sci. U. S. A. 105 (6), 2117–2122.
Liu Jie, J., Wang, Lei, Harvey-White, Judith, Huang, Bill X., Kim, Hee-Yong, 2008. Multiple pathways involved in the biosynthesis of anandamide. Neuropharmacology 54 (1), 1–7.
Geurts, Lucie, Everard, Amandine, Van Hul, Matthias, Essaghir, Ahmed, Duparc, Thibaut, 2015. Adipose tissue NAPE-PLD controls fat mass development by altering the browning process and gut microbiota. Nat. Commun. 6.
Maccarrone Mauro, M., 2017. Metabolism of the endocannabinoid anandamide: open questions after 25 years. Front. Mol. Neurosci. 10.
Maes, Michael, Kubera, Marta, Leunis, Jean Claude, Berk, Michael, 2012. Increased IgA and IgM responses against gut commensals in chronic depression: further evidence for increased bacterial translocation or leaky gut. J. Affect. Disord. 141 (1), 55–62.
Manca, Claudia, Boubertakh, Besma, Leblanc, Nadine, Deschenes, Thomas, Lacroix, Sebastien, 2020. Germ-free mice exhibit profound gut microbiota- dependent alterations of intestinal endocannabinoidome signaling[S]. J. Lipid Res. 61 (1), 70–85.
Manca, Claudia, Shen, Melissa, Boubertakh, Besma, Martin, Cyril, Flamand, Nicolas, Silvestri, Cristoforo, Di Marzo, Vincenzo, 2020. Alterations of brain
endocannabinoidome signaling in germ-free mice. Biochimica Et Biophysica Acta (BBA) - Molecular Cell Biol. Lipids 1865 (12), 158786.
Markey, Laura, Hooper, Andrew, Melon, Laverne C., Baglot, Samantha, Hill, Matthew N., Maguire, Jamie, Kumamoto, Carol A., 2020. Colonization with the commensal fungus Candida albicans perturbs the gut-brain axis through dysregulation of endocannabinoid signaling. psychoneuroendocrinology 121, 104808.
Matias, Isabel, Gonthier, Marie Paule, Orlando, Pierangelo, Martiadis, Vassilis, De Petrocellis, Luciano, Cervino, Cristina, Petrosino, Stefania, et al., 2006. Regulation, function, and dysregulation of endocannabinoids in models of adipose and β-pancreatic cells and in obesity and hyperglycemia. J. Clin. Endocrinol. Metab. 91 (8), 3171–3180.
Mehrpouya-Bahrami, Pegah, Chitrala, Kumaraswamy Naidu, Ganewatta, Mitra S., Chuanbing, Tang, Angela Murphy, E., Enos, Reilly T., Velazquez, Kandy T.,
McCellan, Jamie, Nagarkatti, Mitzi, Nagarkatti, Prakash, 2017. Blockade of CB1 cannabinoid receptor alters gut microbiota and attenuates inflammation and diet- induced obesity. Sci. Rep. 7 (1).
Meyer, H.C., Lee, Francis S., Gee, Dylan G., 2018. The role of the endocannabinoid system and genetic variation in adolescent brain development.
Neuropsychopharmacology 43 (1), 21–33.
Moludi, Jalal, Alizadeh, Mohammad, Yagin, Ned Lotfi, Pasdar, Yahiya, Nachvak, Seyed Mostafa, Abdollahzad, Hadi, Tabaei, Ali Sadeghpour, 2018. New insights on atherosclerosis: a cross-talk between endocannabinoid systems with gut microbiota. J. Cardiovas. Thoracic Res. 10 (3), 129–137.
Monteleone, Palmiero, Piscitelli, Fabiana, Scognamiglio, Pasquale, Monteleone, Alessio Maria, Canestrelli, Benedetta, Di Marzo, Vincenzo, Maj, Mario, 2012. Hedonic eating is associated with increased peripheral levels of ghrelin and the endocannabinoid 2- arachidonoyl-glycerol in healthy humans: a pilot study. J. Clin. Endocrinol. Metabolism 97 (6), E917–E924.
Moreira, F.A., Crippa, Jose Alexandre S., 2009. The psychiatric side-effects of rimonabant. Brazilian J. Psychiatry 31 (2), 145–153.
Muccioli, G., Naslain, Damien, B¨ackhed, Fredrik, Reigstad, Christopher S., Lambert, Didier M., Delzenne, Nathalie M., Cani, Patrice D., 2010. The endocannabinoid system links gut microbiota to adipogenesis. Mol. Syst. Biol. 6.
Nagpal, Ravinder, Newman, Tiffany M., Wang, Shaohua, Jain, Shalini, Lovato, James F., Yadav, Hariom, 2018. Obesity-linked gut microbiome dysbiosis associated with derangements in gut permeability and intestinal cellular homeostasis independent of diet. J. Diabetes Res. 2018.
Ohlsson, L., Gustafsson, A., Lavant, E., Suneson, K., Brundin, L., Westrin, L. Ljunggren, Lindqvist, D., 2019. Leaky gut biomarkers in depression and suicidal behavior. Acta Psychiatr. Scand. 139 (2), 185–193.
Ol´ah, Attila, Szekanecz, Zolt´an, Bír´o, Tam´as, 2017. Targeting cannabinoid signaling in the immune system: “high”-ly exciting questions, possibilities, and challenges. Front. Immunol. 8, 10.
Paolo, Ambrosino, Soldovieri, Maria Virginia, Russo, Claudio, Taglialatela, Maurizio, 2013. Activation and desensitization of TRPV1 channels in sensory neurons by the PPAR alpha agonist palmitoylethanolamide. Br. J. Pharmacol. 168 (6), 1430–1444.
Parolaro Daniela, D., Realini, Natalia, Vigano, Daniela, Guidali, Cinzia, Rubino, Tiziana, 2010. The endocannabinoid system and psychiatric disorders. Exp. Neurol. 224 (1), 3–14.
Piomelli, Daniele, 2003. The molecular logic of endocannabinoid signalling. Nat. Rev. Neurosci. 4 (11), 873–884.
Rousseaux, Christel, Thuru, Xavier, Gelot, Agathe, Barnich, Nicolas, Neut, Christel, Dubuquoy, Laurent, Dubuquoy, Caroline, et al., 2007. Lactobacillus acidophilus modulates intestinal pain and induces opioid and cannabinoid receptors. Nat. Med. 13 (1), 35–37.
Russo, Roberto, Cristiano, Claudia, Avagliano, Carmen, De Caro, Carmen, La Rana, Giovanna, Raso, Giuseppina Mattace, Canani, Roberto Berni, Meli, Rosaria, Calignano, Antonio, 2017. Gut-brain Axis: role of lipids in the regulation of inflammation, pain and CNS diseases. Curr. Med. Chem. 25 (32), 3930–3952.
Saha, Lekha, 2014. Irritable bowel syndrome: pathogenesis, diagnosis, treatment, and evidence-based medicine. World J. Gastroenterol. 20 (22), 6759.
Santos, Susanne Fonseca, De Oliveira, Hadassa Loth, Yamada, Elizabeth Sumi, Neves, Bianca Cruz, Pereira, Antonio, 2019. The gut and Parkinson’s disease - a bidirectional pathway. Front. Neurol. 10 (JUN).
Simon, Vincent, Cota, Daniela, 2017. Mechanisms in endocrinology: endocannabinoids and metabolism: past, present and future. Eur. J. Endocrinol. 176 (6), R309–R324.
Srinivasan, Balaji, Kolli, Aditya Reddy, Esch, Mandy Brigitte, Abaci, Hasan Erbil, Shuler, Michael L., Hickman, James J., 2015. TEER measurement techniques for in vitro barrier model systems. J. Laboratory Automation 20 (2), 107–126.
Stampanoni Bassi, M., Gilio, Luana, Maffei, Pierpaolo, Dolcetti, Ettore, Bruno, Antonio, 2018. Exploiting the multifaceted effects of cannabinoids on mood to boost their therapeutic use against anxiety and depression. Front. Mol. Neurosci. 11.
Storozhuk, Maksim V., Zholos, Alexander V., 2017. TRP channels as novel targets for endogenous ligands: focus on endocannabinoids and nociceptive Signalling. Curr. Neuropharmacol. 16 (2).
Sykaras, Alexandros G., Claire, Demenis, Maynard Case, R., McLaughlin, John T., Smith, Craig P., 2012. Duodenal enteroendocrine I-cells contain mRNA transcripts encoding key endocannabinoid and fatty acid receptors. PLoS One 7 (8), e42373.
Toczek, Marek, Malinowska, Barbara, 2018. Enhanced endocannabinoid tone as a potential target of pharmacotherapy. Life Sci. 204, 20–45.
Vancamelbeke, Maaike, Vermeire, S´everine, 2017. The intestinal barrier: a fundamental role in health and disease. Expert Rev. Gastroenterol. Hepatol. 11 (9), 821–834.
Wang, Lin, Alammar, Nuha, Singh, Rajdeep, Nanavati, Julie, Song, Yiran,
Chaudhary, Rahul, Mullin, Gerard E., 2020. Gut microbial dysbiosis in the irritable bowel syndrome: a systematic review and meta-analysis of case-control studies. J. Acad. Nut. Dietetics 120 (4), 565–586.
Wang, Shuibang, Dougherty, Edward J., Danner, Robert L., 2016. PPARγ signaling and emerging opportunities for improved therapeutics. Pharmacol. Res. 111, 76–85.
