Temporal sequence of activation of cells involved in purinergic neurotransmission in the colon

Immunohistochemical characterization of interstitial cells and their relationship to motor neurons within the mouse esophagus

Interstitial cells of Cajal (ICC) and PDGFRα+ cells regulate smooth muscle motility in the gastrointestinal (GI) tract. However, their role(s) in esophageal motility are still unclear. The mouse esophagus has traditionally been described as almost entirely skeletal muscle in nature though ICC have been identified along its entire length. The current study evaluated the distribution of skeletal and smooth muscle within the esophagus using a mouse selectively expressing eGFP in smooth muscle cells (SMCs). The relationship of SMCs to ICC and PDGFRα+ cells was also examined. SMCs declined in density in the oral direction however SMCs represented ~ 25% of the area in the distal esophagus suggesting a likeness to the transition zone observed in humans. ANO1+ intramuscular ICC (ICC-IM) were distributed along the length of the esophagus though like SMCs, declined proximally. ICC-IM were closely associated with SMCs but were also found in regions devoid of SMCs. Intramuscular and submucosal PDGFRα+ cells were densely distributed throughout the esophagus though only intramuscular PDGFRα+ cells within the LES and distal esophagus highly expressed SK3. ICC-IM and PDGFRα+ cells were closely associated with nNOS+, VIP+, VAChT+ and TH+ neurons throughout the LES and distal esophagus. GFAP+ cells resembling intramuscular enteric glia were observed within the muscle and were closely associated with ICC-IM and PDGFRα+ cells, occupying a similar location to c. These data suggest that the mouse esophagus is more similar to the human than thought previously and thus set the foundation for future functional and molecular studies using transgenic mice.

Ca2+ dynamics in interstitial cells: foundational mechanisms for the motor patterns in the gastrointestinal tract

The gastrointestinal (GI) tract displays multiple motor patterns that move nutrients and wastes through the body. Smooth muscle cells (SMCs) provide the forces necessary for GI motility, but interstitial cells, electrically coupled to SMCs, tune SMC excitability, transduce inputs from enteric motor neurons, and generate pacemaker activity that underlies major motor patterns, such as peristalsis and segmentation. The interstitial cells regulating SMCs are interstitial cells of Cajal (ICC) and PDGF receptor (PDGFR)α+ cells. Together these cells form the SIP syncytium. ICC and PDGFRα+ cells express signature Ca2+-dependent conductances: ICC express Ca2+-activated Cl- channels, encoded by Ano1, that generate inward current, and PDGFRα+ cells express Ca2+-activated K+ channels, encoded by Kcnn3, that generate outward current. The open probabilities of interstitial cell conductances are controlled by Ca2+ release from the endoplasmic reticulum. The resulting Ca2+ transients occur spontaneously in a stochastic manner. Ca2+ transients in ICC induce spontaneous transient inward currents and spontaneous transient depolarizations (STDs). Neurotransmission increases or decreases Ca2+ transients, and the resulting depolarizing or hyperpolarizing responses conduct to other cells in the SIP syncytium. In pacemaker ICC, STDs activate voltage-dependent Ca2+ influx, which initiates a cluster of Ca2+ transients and sustains activation of ANO1 channels and depolarization during slow waves. Regulation of GI motility has traditionally been described as neurogenic and myogenic. Recent advances in understanding Ca2+ handling mechanisms in interstitial cells and how these mechanisms influence motor patterns of the GI tract suggest that the term "myogenic" should be replaced by the term "SIPgenic," as this review discusses.

Complementary mechanisms of modulation of spontaneous phasic contractions by the gaseous signalling molecules NO, H2S, HNO and the polysulfide Na2S3 in the rat colon

OBJECTIVES

Reactive oxygen and nitrogen species may be produced during inflammation leading to the formation of NO, H2S or HNO. Enzymes such as iNOS, CSE and CBS might also be responsible for polysulfide production. Since these signalling molecules might have an impact on colonic motility, the aim of this study was to compare their effect on rat colonic slow phasic contractions (SPC).

METHODS

Organ bath measurements with strips obtained from rat proximal colon were performed using the polysulfide Na2S3, sodium nitroprusside (NaNP), sodium hydrogen sulfide (NaHS), Angeli's salt as NO, H2S, and HNO donors, respectively. TTX (1 µM) was used to block neuronal activity.

RESULTS

All four molecules, concentration-dependently, inhibited the amplitude and frequency of SPC both in the circular and longitudinal muscle layer. The relative potency was NaNP>Angeli's salt>NaHS>Na2S3. The inhibitory response induced by NaNP (1 µM) and Angeli's salt (50 µM) was reversed by ODQ (10 µM) whereas the inhibitory effect of NaHS (1 mM) was reversed by apamin (1 µM) and glibenclamide (10 µM). Na2S3 (1 mM) response was partially reversed by apamin (1 µM) and glibenclamide (10 µM). High concentrations of Na2S3 caused an increase in tone. Low concentrations of NaHS or Na2S3 did not potentiate NaNP responses.

CONCLUSIONS

All signalling molecules inhibit SPC in both muscle layers. The effect is independent of neural activity and involves guanylyl cyclase (NO and HNO) and SKCa and KATP channels (NaHS or Na2S3). Other pathways might also be involved in Na2S3 responses. Accordingly, complementary mechanisms of inhibition might be attributable to these signalling molecules.

The enteric nervous system

Of all the organ systems in the body, the gastrointestinal tract is the most complicated in terms of the numbers of structures involved, each with different functions, and the numbers and types of signaling molecules utilized. The digestion of food and absorption of nutrients, electrolytes, and water occurs in a hostile luminal environment that contains a large and diverse microbiota. At the core of regulatory control of the digestive and defensive functions of the gastrointestinal tract is the enteric nervous system (ENS), a complex system of neurons and glia in the gut wall. In this review, we discuss 1) the intrinsic neural control of gut functions involved in digestion and 2) how the ENS interacts with the immune system, gut microbiota, and epithelium to maintain mucosal defense and barrier function. We highlight developments that have revolutionized our understanding of the physiology and pathophysiology of enteric neural control. These include a new understanding of the molecular architecture of the ENS, the organization and function of enteric motor circuits, and the roles of enteric glia. We explore the transduction of luminal stimuli by enteroendocrine cells, the regulation of intestinal barrier function by enteric neurons and glia, local immune control by the ENS, and the role of the gut microbiota in regulating the structure and function of the ENS. Multifunctional enteric neurons work together with enteric glial cells, macrophages, interstitial cells, and enteroendocrine cells integrating an array of signals to initiate outputs that are precisely regulated in space and time to control digestion and intestinal homeostasis.

Single Nucleus Sequencing of Human Colon Myenteric Plexus-Associated Visceral Smooth Muscle Cells, Platelet Derived Growth Factor Receptor Alpha Cells, and Interstitial Cells of Cajal

BACKGROUND AND AIMS

Smooth muscle cells (SMCs), interstitial cells of Cajal (ICCs), and platelet-derived growth factor receptor alpha (PDGFRα+) cells (PαCs) form a functional syncytium in the bowel known as the "SIP syncytium." The SIP syncytium works in concert with the enteric nervous system (ENS) to coordinate bowel motility. However, our understanding of individual cell types that form this syncytium and how they interact with each other remains limited, with no prior single-cell RNAseq analyses focused on human SIP syncytium cells.

METHODS

We analyzed single-nucleus RNA sequencing data from 10,749 human colon SIP syncytium cells (5572 SMC, 372 ICC, and 4805 PαC nuclei) derived from 15 individuals.

RESULTS

Consistent with critical contractile and pacemaker functions and with known enteric nervous system interactions, SIP syncytium cell types express many ion channels, including mechanosensitive channels in ICCs and PαCs. PαCs also prominently express extracellular matrix-associated genes and the inhibitory neurotransmitter receptor for vasoactive intestinal peptide (VIPR2), a novel finding. We identified 2 PαC clusters that differ in the expression of many ion channels and transcriptional regulators. Interestingly, SIP syncytium cells co-express 6 transcription factors (FOS, MEIS1, MEIS2, PBX1, SCMH1, and ZBTB16) that may be part of a combinatorial signature that specifies these cells. Bowel region-specific differences in SIP syncytium gene expression may correlate with regional differences in function, with right (ascending) colon SMCs and PαCs expressing more transcriptional regulators and ion channels than SMCs and PαCs in left (sigmoid) colon.

CONCLUSION

These studies provide new insights into SIP syncytium biology that may be valuable for understanding bowel motility disorders and lead to future investigation of highlighted genes and pathways.

Distribution and Function of Platelet-derived Growth Factor Receptor Alpha-positive Cells and Purinergic Neurotransmission in the Human Colon: Is It Different Between the Right and Left Colon?

Background/Aims

Platelet-derived growth factor receptor alpha-positive (PDGFRα⁺) cells function in the purinergic regulation of gastrointestinal motility, and purines are reportedly inhibitory neurotransmitters in the enteric nervous system. We explore the distribution and function of PDGFRα⁺ cells related to purinergic inhibitory neurotransmission in human right and left colons.

Methods

Human colonic segments were prepared with mucosa and submucosa intact, and the circular muscle tension and longitudinal muscle tension were recorded. Purinergic neurotransmitters were administered after recording the regular contractions. Immunohistochemistry was performed on the circular muscle layers. Intracellular recording was performed on the colonic muscular layer. SK3, P2RY1, and PDGFR-α mRNA expression was tested by quantitative real-time polymerase chain reaction (qPCR).

Results

Adenosine triphosphate (ATP) treatment significantly decreased the frequency and area under the curve (AUC) of the segmental contraction in right and left colons. Beta-nicotinamide adenine dinucleotide (β-NAD) decreased the frequency in the right colon and the amplitude, frequency and AUC in the left colon. Apamin significantly increased frequency and AUC in the left colon, and after apamin pretreatment, ATP and β-NAD did not change segmental contractility. Through intracellular recordings, a resting membrane potential decrease occurred after ATP administration; however, the degree of decrease between the right and left colon was not different. PDGFRα⁺ cells were distributed evenly in the circular muscle layers of right and left colons. SK3, P2RY1, and PDGFRα expression was not different between the right and left colon.

Conclusion

Purines reduce right and left colon contractility similarly, and purinergic inhibitory neurotransmission can be regulated by PDGFRα⁺ cells in the human colon.

Insights on gastrointestinal motility through the use of optogenetic sensors and actuators

The muscularis of the gastrointestinal (GI) tract consists of smooth muscle cells (SMCs) and various populations of interstitial cells of Cajal (ICC), platelet-derived growth factor receptor α⁺ (PDGFRα⁺ ) cells, as well as excitatory and inhibitory enteric motor nerves. SMCs, ICC and PDGFRα⁺ cells form an electrically coupled syncytium, which together with inputs from the enteric nervous system (ENS) regulate GI motility. Early studies evaluating Ca²⁺ signalling behaviours in the GI tract relied upon indiscriminate loading of tissues with Ca²⁺ dyes. These methods lacked the means to study activity in specific cells of interest without encountering contamination from other cells within the preparation. Development of mice expressing optogenetic sensors (GCaMP, RCaMP) has allowed visualization of Ca²⁺ signalling behaviours in a cell specific manner. Additionally, availability of mice expressing optogenetic modulators (channelrhodopsins or halorhodospins) has allowed manipulation of specific signalling pathways using light. GCaMP expressing animals have been used to characterize Ca²⁺ signalling behaviours of distinct classes of ICC and SMCs throughout the GI musculature. These findings illustrate how Ca²⁺ signalling in ICC is fundamental in GI muscles, contributing to tone in sphincters, pacemaker activity in rhythmic muscles and relaying enteric signals to SMCs. Animals that express channelrhodopsin in specific neuronal populations have been used to map neural circuitry and to examine post junctional neural effects on GI motility. Thus, optogenetic approaches provide a novel means to examine the contribution of specific cell types to the regulation of motility patterns within complex multi-cellular systems. Abstract Figure Legends Optogenetic activators and sensors can be used to investigate the complex multi-cellular nature of the gastrointestinal (GI tract). Optogenetic activators that are activated by light such as channelrhodopsins (ChR2), OptoXR and halorhodopsinss (HR) proteins can be genetically encoded into specific cell types. This can be used to directly activate or silence specific GI cells such as various classes of enteric neurons, smooth muscle cells (SMC) or interstitial cells, such as interstitial cells of Cajal (ICC). Optogenetic sensors that are activated by different wavelengths of light such as green calmodulin fusion protein (GCaMP) and red CaMP (RCaMP) make high resolution of sub-cellular Ca²⁺ signalling possible within intact tissues of specific cell types. These tools can provide unparalleled insight into mechanisms underlying GI motility and innervation. This article is protected by copyright. All rights reserved.

PDGFRα+ Interstitial Cells are Effector Cells of PACAP Signaling in Mouse and Human Colon

BACKGROUND & AIMS

Platelet-derived growth factor receptor α (PDGFRα)-positive interstitial cells (PIC) are interposed between enteric nerve fibers and smooth muscle cells (SMCs) in the tunica muscularis of the gastrointestinal tract. PIC have robust expression of small conductance Ca²⁺ activated K⁺ channels 3 (SK3 channels) and transduce inhibitory inputs from purinergic and sympathetic nerves in mouse and human colon. We investigated whether PIC also express pituitary adenylate cyclase-activating polypeptide (PACAP) receptors, PAC1 (PAC₁R), and are involved in mediating inhibitory regulation of colonic contractions by PACAP in mouse and human colons.

METHODS

Gene expression analysis, Ca²⁺ imaging, and contractile experiments were performed on mouse colonic muscles. Ca²⁺ imaging, intracellular electrical recordings, and contractile experiments were performed on human colonic muscles.

RESULTS

Adcyap1r1 (encoding PAC₁R) is highly expressed in mouse PIC. Interstitial cells of Cajal (ICC) and SMCs expressed far lower levels of Adcyap1r. Vipr1 and Vipr2 were expressed at low levels in PIC, ICC, and SMCs. PACAP elicited Ca²⁺ transients in mouse PIC and inhibited spontaneous phasic contractions via SK channels. In human colonic muscles, PAC₁R agonists elicited Ca²⁺ transients in PIC, hyperpolarized SMCs through SK channels and inhibited spontaneous phasic contractions.

CONCLUSIONS

PIC of mouse and human colon utilize PAC₁R-SK channel signal pathway to inhibit colonic contractions in response to PACAP. Effects of PACAP are in addition to the previously described purinergic and sympathetic inputs to PIC. Thus, PIC integrate inhibitory inputs from at least 3 neurotransmitters and utilize several types of receptors to activate SK channels and regulate colonic contractile behaviors.

Ca2+ Signaling Is the Basis for Pacemaker Activity and Neurotransduction in Interstitial Cells of the GI Tract

Years ago gastrointestinal motility was thought to be due to interactions between enteric nerves and smooth muscle cells (SMCs) in the tunica muscularis. Thus, regulatory mechanisms controlling motility were either myogenic or neurogenic. Now we know that populations of interstitial cells, c-Kit⁺ (interstitial cells of Cajal or ICC), and PDGFRα⁺ cells (formerly "fibroblast-like" cells) are electrically coupled to SMCs, forming the SIP syncytium. Pacemaker and neurotransduction functions are provided by interstitial cells through Ca²⁺ release from the endoplasmic reticulum (ER) and activation of Ca²⁺-activated ion channels in the plasma membrane (PM). ICC express Ca²⁺-activated Cl^- channels encoded by Ano1. When activated, Ano1 channels produce inward current and, therefore, depolarizing or excitatory effects in the SIP syncytium. PDGFRα⁺ cells express Ca²⁺-activated K⁺ channels encoded by Kcnn3. These channels generate outward current when activated and hyperpolarizing or membrane-stabilizing effects in the SIP syncytium. Inputs from enteric and sympathetic neurons regulate Ca²⁺ transients in ICC and PDGFRα⁺ cells, and currents activated in these cells conduct to SMCs and regulate contractile behaviors. ICC also serve as pacemakers, generating slow waves that are the electrophysiological basis for gastric peristalsis and intestinal segmentation. Pacemaker types of ICC express voltage-dependent Ca²⁺ conductances that organize Ca²⁺ transients, and therefore Ano1 channel openings, into clusters that define the amplitude and duration of slow waves. Ca²⁺ handling mechanisms are at the heart of interstitial cell function, yet little is known about what happens to Ca²⁺ dynamics in these cells in GI motility disorders.

Morphological analysis of interstitial cells in murine epididymis using light microscopy and transmission electron microscopy

Smooth muscle contraction of the epididymis plays an important role in sperm transport. Although PDGFRα-positive interstitial cells (PDGFRα (+) ICs) are thought to be involved in controlling smooth muscle movement via intercellular signaling, they have not yet been reported to date in the epididymis. Therefore, we aimed to investigate the morphological characteristics of PDGFRα (+) ICs in the interstitial space of the murine epididymis. Immunohistochemistry showed that PDGFRα (+) ICs co-labeled with CD34 (PDGFRα (+) CD34 (+) ICs were distributed in the interstitial space of the murine epididymis from the initial segment (IS) to the cauda of the epididymis. PDGFRα (+) ICs that were not co-labeled with CD34 (PDGFRα (+) CD34 (-) ICs) were observed just beneath the epithelium from the corpus to the cauda but not in the IS. Both types of PDGFRα (+) ICs were in close proximity to each other as well as the surrounding nerves and macrophages. In addition, PDGFRα (+) CD34 (-) ICs beneath the epithelium were also in close proximity to the basal cells. Using transmission electron microscopy, we identified ICs that possessed elongated and woven cellular processes and were in close proximity to each other, surrounding the cells in the interstitial space. In the murine epididymis, it is suggested that there are two subtypes of ICs that show different distribution patterns depending on the segment, which may reflect segmental differences in mechanisms of sperm transport, forming a cellular network by physical interactions in the murine epididymis.

P2X3 receptors participate in purinergic inhibition of gastrointestinal smooth muscle

The ATP analogue α,β-meATP is a potent relaxant of gastrointestinal smooth muscle, but its molecular target is uncertain inside the gut. α,β-meATP relaxed the carbachol-precontracted guinea-pig taenia coli in a concentration-dependent manner (EC₅₀, 2.0 ± 0.1 μM). A luciferase-based assay confirmed that α,β-meATP solutions were minimally contaminated with ATP. α,β-meATP-evoked relaxations were inhibited by the competitive P2Y1 antagonist MRS2179 (pA₂ = 5.36), but also by the competitive P2X3 antagonist, A-317491 (pA₂ = 5.51). When MRS2179 and A-317491 were applied together, residual α,β-meATP responses converted from brief to prolonged relaxations. Sodium nitroprusside (a nitric oxide donor) also caused prolonged relaxations. Immunohistochemistry revealed that P2X3 receptors were present in myenteric ganglion cells and their varicose nerve terminals. The amplitude of α,β-meATP responses was not inhibited by TTX (NaV channel blocker) and ωCgTx (N-type CaV channel blocker). However, responses to α,β-meATP were inhibited by TEA (non-selective K⁺-channel blocker), indicating that relaxations involved opening K⁺-channels. The findings of this study are consistent with the conclusion that α,β-meATP stimulates Ca²⁺-permeable P2X3 receptors on varicose nerve terminals to release inhibitory nucleotides: 1) ATP and β-NAD release results in P2Y1-mediated brief relaxations; 2) another released transmitter (possibly NO) results in prolonged relaxations. Prejunctional P2X3 receptors represent a purinergic feed-forward mechanism to augment the action of inhibitory nerves on gut motility. This positive feed-forward mechanism may counter-balance the known negative feedback mechanism caused by adenosine and prejunctional A1 receptors on inhibitory motor nerves.

Neurotransmitters responsible for purinergic motor neurotransmission and regulation of GI motility

Classical concepts of peripheral neurotransmission were insufficient to explain enteric inhibitory neurotransmission. Geoffrey Burnstock and colleagues developed the idea that ATP or a related purine satisfies the criteria for a neurotransmitter and serves as an enteric inhibitory neurotransmitter in GI muscles. Cloning of purinergic receptors and development of specific drugs and transgenic mice have shown that enteric inhibitory responses depend upon P2Y₁ receptors in post-junctional cells. The post-junctional cells that transduce purinergic neurotransmitters in the GI tract are PDGFRα⁺ cells and not smooth muscle cells (SMCs). PDGFRα⁺ cells express P2Y₁ receptors, are activated by enteric inhibitory nerve stimulation and generate Ca²⁺ oscillations, express small-conductance Ca²⁺-activated K⁺ channels (SK3), and generate outward currents when exposed to P2Y₁ agonists. These properties are consistent with post-junctional purinergic responses, and similar responses and effectors are not functional in SMCs. Refinements in methodologies to measure purines in tissue superfusates, such as high-performance liquid chromatography (HPLC) coupled with etheno-derivatization of purines and fluorescence detection, revealed that multiple purines are released during stimulation of intrinsic nerves. β-NAD⁺ and other purines, better satisfy criteria for the purinergic neurotransmitter than ATP. HPLC has also allowed better detection of purine metabolites, and coupled with isolation of specific types of post-junctional cells, has provided new concepts about deactivation of purine neurotransmitters. In spite of steady progress, many unknowns about purinergic neurotransmission remain and require additional investigation to understand this important regulatory mechanism in GI motility.

Identification of PDGFRα-positive interstitial cells in the distal segment of the murine vas deferens

Platelet-derived growth factor receptor-α (PDGFRα)-positive interstitial cells (ICs) are widely distributed in various organs and may be involved in the motility of various tubular organs. We, for the first time, aimed to investigate the distribution, immunohistochemical characteristics, and ultrastructure of PDGFRα-positive ICs in murine vas deferens, using confocal laser scanning microscopy, transmission electron microscopy (TEM), and immuno-electron microscopy (immuno-EM). For immunofluorescence, we used antibodies against PDGFRα and other markers of ICs. PDGFRα-positive ICs were distributed widely in the lamina propria, smooth muscles, and serosal layers. Although most PDGFRα-positive ICs labeled CD34, they did not label CD34 in the subepithelial layers. Additionally, PDGFRα-positive ICs were in close proximity to each other, as also to the surrounding cells. TEM and immuno-EM findings revealed that PDGFRα-positive ICs established close physical interactions with adjacent ICs. Extracellular vesicles were also detected around the PDGFRα-positive ICs. Our morphological findings suggest that PDGFRα-positive ICs may have several subpopulations, which can play an important role in intercellular signaling via direct contact with the IC network and the extracellular vesicles in the murine vas deferens. Further investigation on PDGFRα-positive ICs in the vas deferens may lead to understanding the vas deferens mortility.

Ca2+ signaling driving pacemaker activity in submucosal interstitial cells of Cajal in the murine colon

Interstitial cells of Cajal (ICC) generate pacemaker activity responsible for phasic contractions in colonic segmentation and peristalsis. ICC along the submucosal border (ICC-SM) contribute to mixing and more complex patterns of colonic motility. We show the complex patterns of Ca²⁺ signaling in ICC-SM and the relationship between ICC-SM Ca²⁺ transients and activation of smooth muscle cells (SMCs) using optogenetic tools. ICC-SM displayed rhythmic firing of Ca²⁺transients ~ 15 cpm and paced adjacent SMCs. The majority of spontaneous activity occurred in regular Ca²⁺ transients clusters (CTCs) that propagated through the network. CTCs were organized and dependent upon Ca²⁺ entry through voltage-dependent Ca²⁺ conductances, L- and T-type Ca²⁺ channels. Removal of Ca²⁺ from the external solution abolished CTCs. Ca²⁺ release mechanisms reduced the duration and amplitude of Ca²⁺ transients but did not block CTCs. These data reveal how colonic pacemaker ICC-SM exhibit complex Ca^2+-firing patterns and drive smooth muscle activity and overall colonic contractions.

Extracellular metabolism of the enteric inhibitory neurotransmitter β-nicotinamide adenine dinucleotide (β-NAD) in the murine colon

KEY POINTS

β-Nicotinamide adenine dinucleotide (β-NAD) is a key inhibitory neurotransmitter in the colon. The neuroeffector junction in the gut consists of enteric motor neurons and SIP syncytium, including smooth muscle cells (SMCs), interstitial cells of Cajal (ICC), and cells expressing platelet-derived growth factor receptor α (PDGFRα⁺ cells). Measuring metabolism of 1,N⁶ -etheno-NAD (eNAD) in colonic tunica muscularis and in SMCs, ICC and PDGFRα⁺ cells with HPLC-FLD, we report that (1) in tissues, eNAD is degraded to eADP-ribose, eAMP and e-adenosine (eADO) by CD38, ENPP1 and NT5E, (2) with SMCs and PDGFRα⁺ cells, eNAD is metabolized to eADO by ENPP1 and NT5E, (3) eNAD is not metabolized by ICC, (4) NT5E is expressed chiefly by SMCs and moderately by PDGFRα⁺ cells, (5) SIP cells are not the primary location of CD38. These data argue that the duration and strength of purinergic neurotransmission can be modulated by targeting multiple enzymes with specialized cellular distribution in the colon.

ABSTRACT

Prior studies suggest that β-nicotinamide adenine dinucleotide (β-NAD) is an important inhibitory motor neurotransmitter in the enteric nervous system. Metabolism of β-NAD at the neuroeffector junction (NEJ) is likely to be necessary for terminating inhibitory neurotransmission and may also produce bioactive metabolites. The enteric NEJ consists of enteric neurons and postjunctional cells of the SIP syncytium, including smooth muscle cells (SMCs), interstitial cells of Cajal (ICC), and cells expressing platelet-derived growth factor receptor α (PDGFRα⁺ cells). We examined possible specialized functions of the NEJ in β-NAD metabolism by determining the degradation of 1,N⁶ -etheno-NAD (eNAD) in colonic tunica muscularis of wild-type, Cd38^-/- , Nt5e^-/- , Enpp1^-/- and Cd38^-/- /Nt5e^-/- mice and in SIP cells from mice expressing cell-specific fluorescent reporters purified by fluorescence activated cell sorting (FACS). We measured eNAD and its metabolites eADP-ribose (eADPR), eAMP and e-adenosine (eADO) from tissues and sorted SIP cells using liquid chromatography. eNAD exposed to colonic muscularis of wild-type mice produced eADPR, eAMP and eADO. CD38 mediated the conversion of eNAD to eADPR, whereas ENPP1 mediated degradation of eNAD and eADPR to eAMP. NT5E (aka CD73) was the primary enzyme forming eADO from eAMP. PDGFRα⁺ cells and SMCs were involved in production of eADO from eNAD, and ICC were not involved in extracellular metabolism of eNAD. CD38 mediated the eNAD metabolism in whole tissues, but CD38 did not appear to be functionally expressed by SMCs or ICC. NT5E was expressed in SMCs > PDGFRα⁺ cells. Our data show that extracellular metabolism of β-NAD in the colon is mediated by multiple enzymes with cell-specific expression.

Mechanisms Associated to Nitroxyl (HNO)-Induced Relaxation in the Intestinal Smooth Muscle

The pharmacological properties of nitroxyl (HNO) donors in the gastrointestinal tract are unknown. We investigated the properties of this molecule in the regulation of gastrointestinal contractility focusing on its possible interaction with other gaseous signaling molecules such as NO and H₂S. Organ bath, Ca²⁺ imaging, and microelectrode recordings were performed on rat intestinal samples, using Angeli’s salt as HNO donor. Angeli’s salt caused a concentration-dependent relaxation of longitudinal or circular muscle strips of the ileum and the proximal colon. This relaxation was strongly inhibited by the Rho-kinase inhibitor Y-27632 (10 μM), by the reducing agent DTT or by the inhibitor of soluble guanylate cyclase (sGC) ODQ (10 μM) alone or in combination with the inhibitors of the endogenous synthesis of H₂S β-cyano-L-alanine (5 mM) and amino-oxyacetate (5 mM). Preventing endogenous synthesis of NO by the NO synthase inhibitor L-NAME (200 μM) did not affect the relaxation induced by HNO. HNO induced an increase in cytosolic Ca²⁺ concentration in colonic myocytes. It also elicited myocyte membrane hyperpolarization that amounted to −10.6 ± 1.1 mV. ODQ (10 μM) and Apamin (1 μM), a selective inhibitor of small conductance Ca²⁺-activated K⁺ channels (SKca), strongly antagonized this effect. We conclude that HNO relaxes the gastrointestinal tract musculature by hyperpolarizing myocytes via activation of the sGC/cGMP pathway similarly to NO, not only inhibiting the RhoK and activating MLCP as do both NO and H₂S but also increasing cytosolic Ca²⁺ for activation of SK_Ca contributing to hyperpolarization.

Norepinephrine Has Dual Effects on Human Colonic Contractions Through Distinct Subtypes of Alpha 1 Adrenoceptors

BACKGROUND & AIMS

Colonic musculature contain smooth muscle cells (SMC), interstitial cells of Cajal (ICC), and platelet-derived growth factor receptor α+ cells (PDGFRα+ cells), which are electrically coupled and operate together as the SIP syncytium. PDGFRα+ cells have enriched expression of small conductance Ca2+-activated K+ (SK) channels. Purinergic enteric neural input activates SK channels in PDGFRα+ cells, hyperpolarizes SMC, and inhibits colonic contractions. Recently we discovered that PDGFRα+ cells in mouse colon have enriched expression of α1A adrenoceptors (ARs), which coupled to activation of SK channels and inhibited colonic motility, and α1A ARs were principal targets for sympathetic regulation of colonic motility. Here we investigated whether PDGFRα+ cells in human colon express α1A ARs and share the roles as targets for sympathetic regulation of colonic motility.

METHODS

Isometric tension recording, intracellular recording, and Ca2+ imaging were performed on muscles of the human colon. Responses to α1 ARs agonists or electric field stimulation with AR antagonists and neuroleptic reagents were studied.

RESULTS

Exogenous or endogenous norepinephrine released from nerve fibers inhibited colonic contractions through binding to α1A ARs or enhanced colonic contractions by acting on α1D ARs. Inhibitory responses were blocked by apamin, an antagonist of SK channels. Phenylephrine, α1 AR agonists, or norepinephrine increased intracellular [Ca2+] in PDGFRα+ cells, but not in ICC, and hyperpolarized SMCs by binding to α1 ARs expressed by PDGFRα+ cells.

CONCLUSIONS

Human colonic contractions are inhibited by α1A ARs expressed in PDGFRα+ cells and activated by α1D ARs expressed in SMC.

The asymmetric innervation of the circular and longitudinal muscle of the mouse colon differently modulates myogenic slow phasic contractions

BACKGROUND

Neuromuscular transmission has been extensively studied in the circular layer of the mouse colon where a co-transmission of purines acting on P2Y₁ receptors and NO has been previously described. However, the corresponding mechanisms in the longitudinal layer are less known.

METHODS

Electrophysiological and myography techniques were used to evaluate spontaneous phasic contractions (SPC) and neural-mediated responses in the proximal, mid, and distal colon devoid of CD1 mice. Immunohistochemistry against c-kit and PDGFRα was performed in each colonic segment.

KEY RESULTS

SPC were recorded in both muscle layers at a similar frequency being about four contractions per minute (c.p.m.) in the proximal and distal colon compared to the mid colon (2 c.p.m.). In non-adrenergic, non-cholinergic conditions, L-NNA (1 mmol/L) increased contractility in the circular but not in the longitudinal layer. In the longitudinal muscle, both electrophysiological and mechanical neural-mediated inhibitory responses were L-NNA and ODQ (10 µmol/L) sensitive. NaNP (1 µmol/L) caused cessation of SPC and the response was blocked by ODQ. Neither ADPßS (10 µmol/L) nor CYPPA (10 µmol/L), which both targeted the purinergic pathway, altered longitudinal contractions. PDGFRα + cells were located in both muscle layers and were more numerous compared with cKit + cells, which both formed a heterologous cellular network. A decreasing gradient of the PDGFRα labeling was observed along the colon.

CONCLUSION

An inhibitory neural tone was absent in the longitudinal layer and neuronal inhibitory responses were mainly nitrergic. Despite the presence of PDGFRα + cells, purinergic responses were absent. Post-junctional pathways located in different cell types might be responsible for neurotransmitter transduction.

A novel postsynaptic signal pathway of sympathetic neural regulation of murine colonic motility

Transcriptome data revealed α1 adrenoceptors (ARs) expression in platelet-derived growth factor receptor α⁺ cells (PDGFRα⁺ cells) in murine colonic musculature. The role of PDGFRα⁺ cells in sympathetic neural regulation of murine colonic motility was investigated. Norepinephrine (NE), via α1A ARs, activated a small conductance Ca²⁺ -activated K⁺ (SK) conductance, evoked outward currents and hyperpolarized PDGFRα⁺ cells (the α1A AR-SK channel signal pathway). α1 AR agonists increased intracellular Ca²⁺ transients in PDGFRα⁺ cells and inhibited spontaneous phasic contractions (SPCs) of colonic muscle through activation of a SK conductance. Sympathetic nerve stimulation inhibited both contractions of distal colon and propulsive contractions represented by the colonic migrating motor complexes (CMMCs) via the α1A AR-SK channel signal pathway. Postsynaptic signaling through α1A ARs in PDGFRα⁺ cells is a novel mechanism that conveys part of stress responses in the colon. PDGFRα⁺ cells appear to be a primary effector of sympathetic neural regulation of murine colonic motility.

A myogenic motor pattern in mice lacking myenteric interstitial cells of Cajal explained by a second coupled oscillator network

The interstitial cells of Cajal associated with the myenteric plexus (ICC-MP) are a network of coupled oscillators in the small intestine that generate rhythmic electrical phase waves leading to corresponding waves of contraction, yet rhythmic action potentials and intercellular calcium waves have been recorded from c-kit-mutant mice that lack the ICC-MP, suggesting that there may be a second pacemaker network. The gap junction blocker carbenoxolone induced a "pinstripe" motor pattern consisting of rhythmic "stripes" of contraction that appeared simultaneously across the intestine with a period of ~4 s. The infinite velocity of these stripes suggested they were generated by a coupled oscillator network, which we call X. In c-kit mutants rhythmic contraction waves with the period of X traveled the length of the intestine, before the induction of the pinstripe pattern by carbenoxolone. Thus X is not the ICC-MP and appears to operate under physiological conditions, a fact that could explain the viability of these mice. Individual stripes consisted of a complex pattern of bands of contraction and distension, and between stripes there could be slide waves and v waves of contraction. We hypothesized that these phenomena result from an interaction between X and the circular muscle that acts as a damped oscillator. A mathematical model of two chains of coupled Fitzhugh-Nagumo systems, representing X and circular muscle, supported this hypothesis. The presence of a second coupled oscillator network in the small intestine underlines the complexity of motor pattern generation in the gut.NEW & NOTEWORTHY Physiological experiments and a mathematical model indicate a coupled oscillator network in the small intestine in addition to the c-kit-expressing myenteric interstitial cells of Cajal. This network interacts with the circular muscle, which itself acts as a system of damped oscillators, to generate physiological contraction waves in c-kit (W) mutant mice.

Recommendations for evaluation of bladder and bowel function in pre-clinical spinal cord injury research

Objective: In order to encourage the inclusion of bladder and bowel outcome measures in preclinical spinal cord injury (SCI) research, this paper identifies and categorizes 1) fundamental, 2) recommended, 3) supplemental and 4) exploratory sets of outcome measures for pre-clinical assessment of bladder and bowel function with broad applicability to animal models of SCI.Methods: Drawing upon the collective research experience of autonomic physiologists and informed in consultation with clinical experts, a critical assessment of currently available bladder and bowel outcome measures (histological, biochemical, in vivo functional, ex vivo physiological and electrophysiological tests) was made to identify the strengths, deficiencies and ease of inclusion for future studies of experimental SCI.Results: Based upon pre-established criteria generated by the Neurogenic Bladder and Bowel Working Group that included history of use in experimental settings, citations in the literature by multiple independent groups, ease of general use, reproducibility and sensitivity to change, three fundamental measures each for bladder and bowel assessments were identified. Briefly defined, these assessments centered upon tissue morphology, voiding efficiency/volume and smooth muscle-mediated pressure studies. Additional assessment measures were categorized as recommended, supplemental or exploratory based upon the balance between technical requirements and potential mechanistic insights to be gained by the study.Conclusion: Several fundamental assessments share reasonable levels of technical and material investment, including some that could assess bladder and bowel function non-invasively and simultaneously. Such measures used more inclusively across SCI studies would advance progress in this high priority area. When complemented with a few additional investigator-selected study-relevant supplemental measures, they are highly recommended for research programs investigating the efficacy of therapeutic interventions in preclinical animal models of SCI that have a bladder and/or bowel focus.

Diadenosine tetraphosphate activates P2Y1 receptors that cause smooth muscle relaxation in the mouse colon

P2Y₁ receptors play an essential role in inhibitory neuromuscular transmission in the gastrointestinal tract. The signalling pathway involves the opening of small conductance calcium activated potassium-channels (K_ca2 family) that results in smooth muscle hyperpolarization and relaxation. Inorganic polyphosphates and dinucleotidic polyphosphates are putative neurotransmitters that potentially act on P2Y₁ receptors. A pharmacological approach using both orthosteric (MRS2500) and allosteric (BPTU) blockers of the P2Y₁ receptor and openers (CyPPA) and blockers (apamin) of K_ca2 channels was used to pharmacologically characterise the effect of these neurotransmitters. Organ bath and microelectrodes were used to evaluate the effect of P1,P4-Di (adenosine-5') tetraphosphate ammonium salt (Ap₄A), inorganic polyphosphates (PolyP) and CyPPA on spontaneous contractions and membrane potential of mouse colonic smooth muscle cells. PolyP neither modified contractions nor membrane potential. In contrast, Ap₄A caused a concentration-dependent inhibition of spontaneous contractions reaching a maximum effect at 100 μM Ap₄A response was antagonised by MRS2500 (1 μM), BPTU (3 μM) and apamin (1 μM). CyPPA (10 μM) inhibited spontaneous contractions and this response was antagonised by apamin but it was not affected by MRS2500 or BPTU. Both CyPPA and Ap₄A caused smooth muscle hyperpolarization that was blocked by apamin and MRS2500 respectively. We conclude that Ap₄A but not PolyP activates P2Y₁ receptors causing smooth muscle hyperpolarization and relaxation. Ap₄A signalling causes activation of K_ca2 channels through activation of P2Y₁ receptors. In contrast, CyPPA acts directly on K_ca2 channels. Further studies are needed to evaluate if dinucleotidic polyphosphates are released from inhibitory motor neurons.

The roles of PDGFRα signaling in the postnatal development and functional maintenance of the SMC-ICC-PDGFRα+ cell (SIP) syncytium in the colon

BACKGROUND

The SIP syncytium in the gut consists of smooth muscle cells, interstitial cells of Cajal, and PDGFRα+ cells. We studied the fate of SIP cells after blocking PDGFRα receptor to explore the roles of PDGFRα signaling in the postnatal development and functional maintenance of the SIP syncytium.

METHODS

Crenolanib was administered to mice from P0, P10, or P50. The morphological changes in SIP cells were examined by immunofluorescence. Protein expression in SIP cells was detected by Western blotting. Moreover, colonic transit was analyzed by testing the colonic bead expulsion time.

KEY RESULTS

A dose of 5 mg(kg•day)^-1 crenolanib administered for 10 days beginning on P0 apparently hindered the development of PDGFRα+ cells in the colonic longitudinal muscularis and myenteric plexus without influencing their proliferative activity and apoptosis, but this result was not seen in the colonic circular muscularis. SMCs were also inhibited by crenolanib. A dose of 7.5 mg(kg•day)^-1 crenolanib administered for 15 days beginning on P0 caused reductions in both PDGFRα+ cells and ICC in the longitudinal muscularis, myenteric plexus, and circular muscularis. However, when crenolanib was administered at a dose of 5 mg(kg•day)^-1 beginning on P10 or P50, it only noticeably decreased the number of PDGFRα+ cells in the colonic longitudinal muscularis. Crenolanib also caused PDGFRα+ cells to transdifferentiate into SMC in adult mice. Colonic transit was delayed after administration of crenolanib.

CONCLUSIONS & INFERENCES

Therefore, PDGFRα signaling is essential for the development and functional maintenance of the SIP cells, especially PDGFRα+ cells.

Tonic inhibition of murine proximal colon is due to nitrergic suppression of Ca2+ signaling in interstitial cells of Cajal

Spontaneous excitability and contractions of colonic smooth muscle cells (SMCs) are normally suppressed by inputs from inhibitory motor neurons, a behavior known as tonic inhibition. The post-junctional cell(s) mediating tonic inhibition have not been elucidated. We investigated the post-junctional cells mediating tonic inhibition in the proximal colon and whether tonic inhibition results from suppression of the activity of Ano1 channels, which are expressed exclusively in interstitial cells of Cajal (ICC). We found that tetrodotoxin (TTX), an inhibitor of nitric oxide (NO) synthesis, L-NNA, and an inhibitor of soluble guanylyl cyclase, ODQ, greatly enhanced colonic contractions. Ano1 antagonists, benzbromarone and Ani9 inhibited the effects of TTX, L-NNA and ODQ. Ano1 channels are activated by Ca²⁺ release from the endoplasmic reticulum (ER) in ICC, and blocking Ca²⁺ release with a SERCA inhibitor (thapsigargin) or a store-operated Ca²⁺ entry blocker (GSK 7975 A) reversed the effects of TTX, L-NNA and ODQ. Ca²⁺ imaging revealed that TTX, L-NNA and ODQ increased Ca²⁺ transient firing in colonic ICC. Our results suggest that tonic inhibition in the proximal colon occurs through suppression of Ca²⁺ release events in ICC. Suppression of Ca²⁺ release in ICC limits the open probability of Ano1 channels, reducing the excitability of electrically-coupled SMCs.

Smooth Muscle Transcriptome Browser: offering genome-wide references and expression profiles of transcripts expressed in intestinal SMC, ICC, and PDGFRα+ cells

Transcriptome data on the quantitative numbers of transcriptional variants expressed in primary cells offer essential clues into specific cellular functions and biological processes. We have previously collected transcriptomes from primary smooth muscle cells (SMC), interstitial cells of Cajal (ICC), and PDGFRα+ cells (fibroblast-like cells) isolated from murine jejunal and colonic smooth muscle and/or mucosal tissues as well as transcriptomes from the associated tissues (jejunal smooth muscle, colonic smooth muscle, and colonic mucosa). In this study, we have built the Smooth Muscle Transcriptome Browser (SMTB), https://med.unr.edu/physio/transcriptome , a web-based, graphical user interface that offers genetic references and expression profiles of all transcripts expressed at both the cellular (SMC, ICC, and PDGFRα+ cells) and tissue level (smooth muscle and mucosal tissue). This browser brings new insights into the cellular and biological functions of the cell types in gastrointestinal smooth muscle biology.

Inhibitory Neural Regulation of the Ca 2+ Transients in Intramuscular Interstitial Cells of Cajal in the Small Intestine

Gastrointestinal motility is coordinated by enteric neurons. Both inhibitory and excitatory motor neurons innervate the syncytium consisting of smooth muscle cells (SMCs) interstitial cells of Cajal (ICC) and PDGFRα+ cells (SIP syncytium). Confocal imaging of mouse small intestines from animals expressing GCaMP3 in ICC were used to investigate inhibitory neural regulation of ICC in the deep muscular plexus (ICC-DMP). We hypothesized that Ca2+ signaling in ICC-DMP can be modulated by inhibitory enteric neural input. ICC-DMP lie in close proximity to the varicosities of motor neurons and generate ongoing Ca2+ transients that underlie activation of Ca2+-dependent Cl- channels and regulate the excitability of SMCs in the SIP syncytium. Electrical field stimulation (EFS) caused inhibition of Ca2+ for the first 2-3 s of stimulation, and then Ca2+ transients escaped from inhibition. The NO donor (DEA-NONOate) inhibited Ca2+ transients and Nω-Nitro-L-arginine (L-NNA) or a guanylate cyclase inhibitor (ODQ) blocked inhibition induced by EFS. Purinergic neurotransmission did not affect Ca2+ transients in ICC-DMP. Purinergic neurotransmission elicits hyperpolarization of the SIP syncytium by activation of K+ channels in PDGFRα+ cells. Generalized hyperpolarization of SIP cells by pinacidil (KATP agonist) or MRS2365 (P2Y1 agonist) also had no effect on Ca2+ transients in ICC-DMP. Peptidergic transmitter receptors (VIP and PACAP) are expressed in ICC and can modulate ICC-DMP Ca2+ transients. In summary Ca2+ transients in ICC-DMP are blocked by enteric inhibitory neurotransmission. ICC-DMP lack a voltage-dependent mechanism for regulating Ca2+ release, and this protects Ca2+ handling in ICC-DMP from membrane potential changes in other SIP cells.

Loss of nitric oxide-mediated inhibition of purine neurotransmitter release in the colon in the absence of interstitial cells of Cajal

Regulation of colonic motility depends on the integrity of enteric inhibitory neurotransmission mediated by nitric oxide (NO), purine neurotransmitters, and neuropeptides. Intramuscular interstitial cells of Cajal (ICC-IM) and platelet-derived growth factor receptor-α-positive (PDGFRα⁺) cells are involved in generating responses to NO and purine neurotransmitters, respectively. Previous studies have suggested a decreased nitrergic and increased purinergic neurotransmission in Kit^W/Kit^W-v (W/W^v ) mice that display lesions in ICC-IM along the gastrointestinal tract. However, contributions of NO to these phenotypes have not been evaluated. We used small-chamber superfusion assays and HPLC to measure the spontaneous and electrical field stimulation (EFS)-evoked release of nicotinamide adenine dinucleotide (NAD⁺)/ADP-ribose, uridine adenosine tetraphosphate (Up4A), adenosine 5'-triphosphate (ATP), and metabolites from the tunica muscularis of human, monkey, and murine colons and circular muscle of monkey colon, and we tested drugs that modulate NO levels or blocked NO receptors. NO inhibited EFS-evoked release of purines in the colon via presynaptic neuromodulation. Colons from W/W^v, Nos1^-/- , and Prkg1^-/- mice displayed augmented neural release of purines that was likely due to altered nitrergic neuromodulation. Colons from W/W^v mice demonstrated decreased nitrergic and increased purinergic relaxations in response to nerve stimulation. W/W^v mouse colons demonstrated reduced Nos1 expression and reduced NO release. Our results suggest that enhanced purinergic neurotransmission may compensate for the loss of nitrergic neurotransmission in muscles with partial loss of ICC. The interactions between nitrergic and purinergic neurotransmission in the colon provide novel insight into the role of neurotransmitters and effector cells in the neural regulation of gastrointestinal motility.NEW & NOTEWORTHY This is the first study investigating the role of nitric oxide (NO) and intramuscular interstitial cells of Cajal (ICC-IM) in modulating neural release of purines in colon. We found that NO inhibited release of purines in human, monkey, and murine colons and that colons from Kit^W/Kit^W-v (W/W^v ) mice, which present with partial loss of ICC-IM, demonstrated augmented neural release of purines. Interactions between nitrergic and purinergic neurotransmission may affect motility in disease conditions with ICC-IM deficiencies.

Transcriptome analysis of PDGFRα+ cells identifies T-type Ca2+ channel CACNA1G as a new pathological marker for PDGFRα+ cell hyperplasia

Platelet-derived growth factor receptor alpha (PDGFRα)+ cells are distributed into distinct morphological groups within the serosal, muscular, and submucosal layers as well as the myenteric and deep muscular plexi. PDGFRα+ cells directly interact with interstitial cells of Cajal (ICC) and smooth muscle cells (SMC) in gastrointestinal smooth muscle tissue. These three cell types, SMC, ICC, and PDGFRα+ cells (SIP cells), form an electrical syncytium, which dynamically regulates gastrointestinal motility. We have previously reported the transcriptomes of SMC and ICC. To complete the SIP cell transcriptome project, we obtained transcriptome data from jejunal and colonic PDGFRα+ cells. The PDGFRα+ cell transcriptome data were added to the Smooth Muscle Genome Browser that we previously built for the genome-scale gene expression data of ICC and SMC. This browser provides a comprehensive reference for all transcripts expressed in SIP cells. By analyzing the transcriptomes, we have identified a unique set of PDGFRα+ cell signature genes, growth factors, transcription factors, epigenetic enzymes/regulators, receptors, protein kinases/phosphatases, and ion channels/transporters. We demonstrated that the low voltage-dependent T-type Ca2+ channel Cacna1g gene was particularly expressed in PDGFRα+ cells in the intestinal serosal layer in mice. Expression of this gene was significantly induced in the hyperplasic PDGFRα+ cells of obstructed small intestine in mice. This gene was also over-expressed in colorectal cancer, Crohn's disease, and diverticulitis in human patients. Taken together, our data suggest that Cacna1g exclusively expressed in serosal PDGFRα+ cells is a new pathological marker for gastrointestinal diseases.

Clustering of Ca2+ transients in interstitial cells of Cajal defines slow wave duration

Electrical slow waves in the small intestine are generated by pacemaker cells called interstitial cells of Cajal. Drumm et al. record clusters of Ca²⁺ transients in these cells that are entrained by voltage-dependent Ca²⁺ entry and which define the duration of the electrical slow waves.

Interstitial cells of Cajal (ICC) in the myenteric plexus region (ICC-MY) of the small intestine are pacemakers that generate rhythmic depolarizations known as slow waves. Slow waves depend on activation of Ca²⁺-activated Cl⁻ channels (ANO1) in ICC, propagate actively within networks of ICC-MY, and conduct to smooth muscle cells where they generate action potentials and phasic contractions. Thus, mechanisms of Ca²⁺ regulation in ICC are fundamental to the motor patterns of the bowel. Here, we characterize the nature of Ca²⁺ transients in ICC-MY within intact muscles, using mice expressing a genetically encoded Ca²⁺ sensor, GCaMP3, in ICC. Ca²⁺ transients in ICC-MY display a complex firing pattern caused by localized Ca²⁺ release events arising from multiple sites in cell somata and processes. Ca²⁺ transients are clustered within the time course of slow waves but fire asynchronously during these clusters. The durations of Ca²⁺ transient clusters (CTCs) correspond to slow wave durations (plateau phase). Simultaneous imaging and intracellular electrical recordings revealed that the upstroke depolarization of slow waves precedes clusters of Ca²⁺ transients. Summation of CTCs results in relatively uniform Ca²⁺ responses from one slow wave to another. These Ca²⁺ transients are caused by Ca²⁺ release from intracellular stores and depend on ryanodine receptors as well as amplification from IP₃ receptors. Reduced extracellular Ca²⁺ concentrations and T-type Ca²⁺ channel blockers decreased the number of firing sites and firing probability of Ca²⁺ transients. In summary, the fundamental electrical events of small intestinal muscles generated by ICC-MY depend on asynchronous firing of Ca²⁺ transients from multiple intracellular release sites. These events are organized into clusters by Ca²⁺ influx through T-type Ca²⁺ channels to sustain activation of ANO1 channels and generate the plateau phase of slow waves.

Regulation of Gastrointestinal Smooth Muscle Function by Interstitial Cells

Interstitial cells of mesenchymal origin form gap junctions with smooth muscle cells in visceral smooth muscles and provide important regulatory functions. In gastrointestinal (GI) muscles, there are two distinct classes of interstitial cells, c-Kit(+) interstitial cells of Cajal and PDGFRα(+) cells, that regulate motility patterns. Loss of these cells may contribute to symptoms in GI motility disorders.

Teaching a changing paradigm in physiology: a historical perspective on gut interstitial cells

The study and teaching of gastrointestinal (GI) physiology necessitates an understanding of the cellular basis of contractile and electrical coupling behaviors in the muscle layers that comprise the gut wall. Our knowledge of the cellular origin of GI motility has drastically changed over the last 100 yr. While the pacing and coordination of GI contraction was once thought to be solely attributable to smooth muscle cells, it is now widely accepted that the motility patterns observed in the GI tract exist as a result of a multicellular system, consisting of not only smooth muscle cells but also enteric neurons and distinct populations of specialized interstitial cells that all work in concert to ensure proper GI functions. In this historical perspective, we focus on the emerging role of interstitial cells in GI motility and examine the key discoveries and experiments that led to a major shift in a paradigm of GI physiology regarding the role of interstitial cells in modulating GI contractile patterns. A review of these now classic experiments and papers will enable students and educators to fully appreciate the complex, multicellular nature of GI muscles as well as impart lessons on how shifting paradigms in physiology are fueled by new technologies that lead to new emerging discoveries.

ANO1 in intramuscular interstitial cells of Cajal plays a key role in the generation of slow waves and tone in the internal anal sphincter

KEY POINTS

The internal anal sphincter develops tone important for maintaining high anal pressure and continence. Controversy exists regarding the mechanisms underlying tone development. We examined the hypothesis that tone depends upon electrical slow waves (SWs) initiated in intramuscular interstitial cells of Cajal (ICC-IM) by activation of Ca²⁺ -activated Cl^- channels (ANO1, encoded by Ano1) and voltage-dependent L-type Ca²⁺ channels (Cav_L , encoded by Cacna1c). Measurement of membrane potential and contraction indicated that ANO1 and Cav_L have a central role in SW generation, phasic contractions and tone, independent of stretch. ANO1 expression was examined in wildtype and Ano1^/+egfp mice with immunohistochemical techniques. Ano1 and Cacna1c expression levels were examined by quantitative PCR in fluorescence-activated cell sorting. ICC-IM were the predominant cell type expressing ANO1 and the most likely candidate for SW generation. SWs in ICC-IM are proposed to conduct to smooth muscle where Ca²⁺ entry via Cav_L results in phasic activity that sums to produce tone.

ABSTRACT

The mechanism underlying tone generation in the internal anal sphincter (IAS) is controversial. We examined the hypothesis that tone depends upon generation of electrical slow waves (SWs) initiated in intramuscular interstitial cells of Cajal (ICC-IM) by activation of Ca²⁺ -activated Cl^- channels (encoded by Ano1) and voltage-dependent L-type Ca²⁺ channels (encoded by Cacna1c). Phasic contractions and tone in the IAS were nearly abolished by ANO1 and Cav_L antagonists. ANO1 antagonists also abolished SWs as well as transient depolarizations that persisted after addition of Cav_L antagonists. Tone development in the IAS did not require stretch of muscles, and the sensitivity of contraction to ANO1 antagonists was the same in stretched versus un-stretched muscles. ANO1 expression was examined in wildtype and Ano1^/+egfp mice with immunohistochemical techniques. Dual labelling revealed that ANO1 expression could be resolved in ICC but not smooth muscle cells (SMCs) in the IAS and rectum. Ano1, Cacna1c and Kit gene expression were the same in extracts of IAS and rectum muscles. In IAS cells isolated with fluorescence-activated cell sorting, Ano1 expression was 26.5-fold greater in ICC than in SMCs while Cacna1c expression was only 2-fold greater in SMCs than in ICC. These data support a central role for ANO1 and Cav_L in the generation of SWs and tone in the IAS. ICC-IM are the probable cellular candidate for ANO1 currents and SW generation. We propose that ANO1 and Cav_L collaborate to generate SWs in ICC-IM followed by conduction to adjacent SMCs where phasic calcium entry through Cav_L sums to produce tone.

BPTU, an allosteric antagonist of P2Y1 receptor, blocks nerve mediated inhibitory neuromuscular responses in the gastrointestinal tract of rodents

P2Y1 receptors mediate nerve mediated purinergic inhibitory junction potentials (IJP) and relaxations in the gastrointestinal (GI) tract in a wide range of species including rodents and humans. A new P2Y1 antagonist, with a non-nucleotide structure, BPTU, has recently been described using X-ray crystallography as the first allosteric G-protein-coupled receptor antagonist located entirely outside of the helical bundle. In this study, we tested its effect on purinergic responses in the gastrointestinal tract of rodents using electrophysiological and myographic techniques. BPTU concentration dependently inhibited purinergic inhibitory junction potentials and inhibition of spontaneous motility induced by electrical field stimulation in the colon of rats (EC50 = 0.3 μM) and mice (EC50 = 0.06 μM). Mechanical inhibitory responses were also concentration-dependently blocked in the stomach of both species. Compared to MRS2500, BPTU displays a lower potency. In the rat colon nicotine induced relaxation was also blocked by BPTU. BPTU also blocked the cessation of spontaneous contractility elicited by ADPβS and the P2Y1 agonist MRS2365. We conclude that BPTU is a novel antagonist with different structural and functional properties than nucleotidic antagonists that is able to block the P2Y1 receptor located at the neuromuscular junction of the GI tract.

P2Y receptor-mediated transient relaxation of rat longitudinal ileum preparations involves phospholipase C activation, intracellular Ca(2+) release and SK channel activation

AIM

Purinergic signaling plays a major role in the enteric nervous system, where it governs gut motility through a number of P2X and P2Y receptors. The aim of this study was to investigate the P2Y receptor-mediated motility in rat longitudinal ileum preparations.

METHODS

Ileum smooth muscle strips were prepared from rats, and fixed in an organ bath. Isometric contraction and relaxation responses of the muscle strips were measured with force transducers. Drugs were applied by adding of stock solutions to the organ bath to yield the individual final concentrations.

RESULTS

Application of the non-hydrolyzable P2 receptor agonists α,β-Me-ATP or 2-Me-S-ADP (10, 100 μmol/L) dose-dependently elicited a transient relaxation response followed by a sustained contraction. The relaxation response was largely blocked by SK channel blockers apamin (500 nmol/L) and UCL1684 (10 μmol/L), PLC inhibitor U73122 (100 μmol/L), IP3 receptor blocker 2-APB (100 μmol/L) or sarcoendoplasmic Ca(2+) ATPase inhibitor thapsigargin (1 μmol/L), but not affected by atropine, NO synthase blocker L-NAME or tetrodotoxin. Furthermore, α,β-Me-ATP-induced relaxation was suppressed by P2Y1 receptor antagonist MRS2179 (50 μmol/L) or P2Y13 receptor antagonist MRS2211 (100 μmol/L), and was abolished by co-application of the two antagonists, whereas 2-Me-S-ADP-induced relaxation was abolished by P2Y6 receptor antagonist MRS2578 (50 μmol/L). In addition, P2Y1 receptor antagonist MRS2500 (1 μmol/L) not only abolished α,β-Me-ATP-induced relaxation, but also suppressed 2-Me-S-ADP-induced relaxation.

CONCLUSION

P2Y receptor agonist-induced transient relaxation of rat ileum smooth muscle strips is mediated predominantly by P2Y1 receptor, but also by P2Y6 and P2Y13 receptors, and involves PLC, IP3, Ca(2+) release and SK channel activation, but is independent of acetylcholine and NO release.

Spontaneous Ca(2+) transients in interstitial cells of Cajal located within the deep muscular plexus of the murine small intestine

KEY POINTS

Interstitial cells of Cajal at the level of the deep muscular plexus (ICC-DMP) in the small intestine generate spontaneous Ca(2+) transients that consist of localized Ca(2+) events and limited propagating Ca(2+) waves. Ca(2+) transients in ICC-DMP display variable characteristics: from discrete, highly localized Ca(2+) transients to regionalized Ca(2+) waves with variable rates of occurrence, amplitude, duration and spatial spread. Ca(2+) transients fired stochastically, with no cellular or multicellular rhythmic activity being observed. No correlation was found between the firing sites in adjacent cells. Ca(2+) transients in ICC-DMP are suppressed by the ongoing release of inhibitory neurotransmitter(s). Functional intracellular Ca(2+) stores are essential for spontaneous Ca(2+) transients, and the sarco/endoplasmic reticulum Ca(2+) -ATPase (SERCA) pump is necessary for maintenance of spontaneity. Ca(2+) release mechanisms involve both ryanodine receptors (RyRs) and inositol triphosphate receptors (InsP3 Rs). Release from these channels is interdependent. ICC express transcripts of multiple RyRs and InsP3 Rs, with Itpr1 and Ryr2 subtypes displaying the highest expression.

ABSTRACT

Interstitial cells of Cajal in the deep muscular plexus of the small intestine (ICC-DMP) are closely associated with varicosities of enteric motor neurons and generate responses contributing to neural regulation of intestinal motility. Responses of ICC-DMP are mediated by activation of Ca(2+) -activated Cl(-) channels; thus, Ca(2+) signalling is central to the behaviours of these cells. Confocal imaging was used to characterize the nature and mechanisms of Ca(2+) transients in ICC-DMP within intact jejunal muscles expressing a genetically encoded Ca(2+) indicator (GCaMP3) selectively in ICC. ICC-DMP displayed spontaneous Ca(2+) transients that ranged from discrete, localized events to waves that propagated over variable distances. The occurrence of Ca(2+) transients was highly variable, and it was determined that firing was stochastic in nature. Ca(2+) transients were tabulated in multiple cells within fields of view, and no correlation was found between the events in adjacent cells. TTX (1 μm) significantly increased the occurrence of Ca(2+) transients, suggesting that ICC-DMP contributes to the tonic inhibition conveyed by ongoing activity of inhibitory motor neurons. Ca(2+) transients were minimally affected after 12 min in Ca(2+) free solution, indicating these events do not depend immediately upon Ca(2+) influx. However, inhibitors of sarco/endoplasmic reticulum Ca(2+) -ATPase (SERCA) pump and blockers of inositol triphosphate receptor (InsP3 R) and ryanodine receptor (RyR) channels blocked ICC Ca(2+) transients. These data suggest an interdependence between RyR and InsP3 R in the generation of Ca(2+) transients. Itpr1 and Ryr2 were the dominant transcripts expressed by ICC. These findings provide the first high-resolution recording of the subcellular Ca(2+) dynamics that control the behaviour of ICC-DMP in situ.

P2Y(1) receptors mediate purinergic relaxation in the equine pelvic flexure

In the equine large intestine, the knowledge of the basic mechanisms underlying motility function is crucial to properly treat motility disorders. P2Y1 receptors are responsible for mediating purinergic colonic relaxation in several species. In vitro experimental studies of the circular muscle from the equine pelvic flexure (n = 6) were performed to characterize inhibitory and excitatory neuromuscular transmission. Electrophysiological studies showed that electrical field stimulation (EFS) evoked biphasic inhibitory junction potentials (IJPs) in smooth muscle cells: a fast IJP (IJPf) followed by a sustained IJP (IJPs). IJPs was sensitive to L-NNA 1 mM (a nitric oxide synthase inhibitor) (P <0.01), while IJPf was abolished by MRS2500 1 µM (a P2Y1 receptor antagonist) (P <0.001). EFS (5 Hz for 2 min) in the organ bath inhibited rhythmic contractions to 3.0 ± 2.5% of basal area under the curve (P <0.0001). EFS under MRS2500 1 µM or L-NNA 1 mM incubation inhibited contractions to 6.0 ± 2.8% (P <0.05) and 24.4 ± 11.3% respectively (P <0.05). Combination of MRS2500 1 µM and L-NNA 1 mM completely reversed the EFS-induced inhibition of colonic motility. Non-nitrergic, non-purinergic conditions were used to reveal voltage-dependent EFS-induced contractions sensitive to atropine 1 µM (P <0.001) and, therefore, cholinergic. In conclusion, nerve-mediated relaxation and contraction in the equine pelvic flexure involve the same mechanisms as those observed in the human colon. P2Y1 receptors mediate purinergic relaxations and are potential targets for the treatment of equine colonic motor disorders.

Reduced expression of voltage-gated Kv11.1 (hERG) K(+) channels in aganglionic colon in Hirschsprung's disease

PURPOSE

The pathophysiology of Hirschsprung's disease (HSCR) is not entirely understood. There is no clear explanation for the occurrence of the spastic or tonically contracted aganglionic segment of bowel. Kv11.1 (hERG) channels play a critical role in the regulation of the resting membrane potential as well as affecting either the force or frequency of contraction of smooth muscles. We designed this study to investigate the expression and distribution of hERG channels in the normal colon and the colon of patients with HSCR.

METHODS

We investigated hERG protein expression in both the ganglionic and aganglionic regions of HSCR patients (n = 10) versus normal control colon (n = 10). Protein distribution was assessed using immunofluorescence and confocal microscopy. Gene and protein expressions were quantified using real-time polymerase chain reaction, western blot analysis and densitometry.

RESULTS

Confocal microscopy of the normal colon revealed strong hERG channel expression in interstitial cells of Cajal, platelet-derived growth factor-alpha receptor- (PDGFRα(+)) positive cells and enteric neurons. hERG expression was markedly decreased in aganglionic bowel, whereas colonic hERG gene expression levels were significantly decreased in aganglionic compared to ganglionic bowel and controls (p < 0.05). Western blotting revealed decreased colonic hERG protein expression in aganglionic HSCR specimens compared to controls.

CONCLUSIONS

We demonstrate, for the first time, the expression and distribution of hERG channels in the human colon. The decreased expression of hERG in the aganglionic colon may be responsible for the increased tone in the aganglionic narrow spastic segment of bowel.

Inverse gradient of nitrergic and purinergic inhibitory cotransmission in the mouse colon

AIM

Gastrointestinal smooth muscle relaxation is accomplished by the neural corelease of ATP or a related purine and nitric oxide. Contractions are triggered by acetylcholine and tachykinins. The aim of this work was to study whether regional differences in neurotransmission could partially explain the varied physiological roles of each colonic area.

METHODS

We used electrophysiological and myography techniques to evaluate purinergic (L-NNA 1 mm incubated tissue), nitrergic (MRS2500 0.3 μm incubated tissue) and cholinergic neurotransmission (L-NNA 1 mm and MRS2500 0.3 μm incubated tissue) in the proximal, mid and distal colon of CD1 mice (n = 42).

RESULTS

Purinergic electrophysiological responses elicited by single pulses (28 V) were greater in the distal (IJPfMAX = -35.3 ± 2.2 mV), followed by the mid (IJPfMAX = -30.6 ± 1.0 mV) and proximal (IJPfMAX = -11.7 ± 1.1 mV) colon. In contrast, nitrergic responses decreased from the proximal colon (IJPsMAX = -11.4 ± 1.1 mV) to the mid (IJPsMAX = -9.1 ± 0.4 mV), followed by the distal colon (IJPsMAX = -1.8 ± 0.3 mV). A similar rank of order was observed in neural mediated inhibitory mechanical responses including electrical field stimulation-mediated responses and neural tone. ADPβs concentration-response curve was shifted to the left in the distal colon. In contrast, NaNP responses did not differ between regions. Cholinergic neurotransmission elicited contractions of a similar amplitude throughout the colon.

CONCLUSION

An inverse gradient of purinergic and nitrergic neurotransmission exists through the mouse colon. The proximal and mid colon have a predominant nitrergic neurotransmission probably due to the fact that their storage function requires sustained relaxations. The distal colon, in contrast, has mainly purinergic neurotransmission responsible for the phasic relaxations needed to propel dehydrated faeces.

UTP activates small-conductance Ca2+-activated K+ channels in murine detrusor PDGFRα+ cells

Purines induce transient contraction and prolonged relaxation of detrusor muscles. Transient contraction is likely due to activation of inward currents in smooth muscle cells, and prolonged relaxation may be due to activation of small-conductance Ca(2+)-activated K(+) (SK) channels via P2Y1 receptors expressed by detrusor PDGF receptor (PDGFR)α(+) cells. We investigated whether other subtypes of P2Y receptors are involved in the activation of SK channels in PDGFRα(+) cells of detrusor muscles. Quantitative analysis of transcripts revealed that P2ry2, P2ry4, and P2ry14 are expressed in PDGFRα(+) cells of P2ry1-deficient/enhanced green fluorescent protein (P2ry1(-/-)/eGFP) mice at similar levels as in wild-type mice. UTP, a P2Y2/P2Y4 agonist, activated large outward currents in detrusor PDGFRα(+) cells. SK channel blockers and an inhibitor of phospholipase C completely abolished currents activated by UTP. In contrast, UTP activated nonselective cation currents in smooth muscle cells. Under current-clamp (current = 0), UTP induced significant hyperpolarization of PDGFRα(+) cells. MRS2500, a selective P2Y1 antagonist, did not affect UTP-activated outward currents in PDGFRα(+) cells from wild-type mice, and activation of outward currents by UTP was retained in P2ry1(-/-)/eGFP mice. As a negative control, we tested the effect of MRS2693, a selective P2Y6 agonist. This compound did not activate outward currents in PDGFRα(+) cells, and currents activated by UTP were unaffected by MRS2578, a selective P2Y6 antagonist. The nonselective P2Y receptor blocker suramin inhibited UTP-activated outward currents in PDGFRα(+) cells. Our data demonstrate that P2Y2 and/or P2Y4 receptors function, in addition to P2Y1 receptors, in activating SK currents in PDGFRα(+) cells and possibly in mediating purinergic relaxation responses in detrusor muscles.

Optical Tools to Investigate Cellular Activity in the Intestinal Wall

Live imaging has become an essential tool to investigate the coordinated activity and output of cellular networks. Within the last decade, 2 Nobel prizes have been awarded to recognize innovations in the field of imaging: one for the discovery, use, and optimization of the green fluorescent protein (2008) and the second for the development of super-resolved fluorescence microscopy (2014). New advances in both optogenetics and microscopy now enable researchers to record and manipulate activity from specific populations of cells with better contrast and resolution, at higher speeds, and deeper into live tissues. In this review, we will discuss some of the recent developments in microscope technology and in the synthesis of fluorescent probes, both synthetic and genetically encoded. We focus on how live imaging of cellular physiology has progressed our understanding of the control of gastrointestinal motility, and we discuss the hurdles to overcome in order to apply the novel tools in the field of neurogastroenterology and motility.