Localization of Ca2+-activated K+ channel, SK3, in fibroblast-like cells forming gap junctions with smooth muscle cells in the mouse small intestine

The Imperative for Innovative Enteric Nervous System-Intestinal Organoid Co-Culture Models: Transforming GI Disease Modeling and Treatment

This review addresses the need for innovative co-culture systems integrating the enteric nervous system (ENS) with intestinal organoids. The breakthroughs achieved through these techniques will pave the way for a transformative era in gastrointestinal (GI) disease modeling and treatment strategies. This review serves as an introduction to the companion protocol paper featured in this journal. The protocol outlines the isolation and co-culture of myenteric and submucosal neurons with small intestinal organoids. This review provides an overview of the intestinal organoid culture field to establish a solid foundation for effective protocol application. Remarkably, the ENS surpasses the number of neurons in the spinal cord. Referred to as the "second brain", the ENS orchestrates pivotal roles in GI functions, including motility, blood flow, and secretion. The ENS is organized into myenteric and submucosal plexuses. These plexuses house diverse subtypes of neurons. Due to its proximity to the gut musculature and its cell type complexity, there are methodological intricacies in studying the ENS. Diverse approaches such as primary cell cultures, three-dimensional (3D) neurospheres, and induced ENS cells offer diverse insights into the multifaceted functionality of the ENS. The ENS exhibits dynamic interactions with the intestinal epithelium, the muscle layer, and the immune system, influencing epithelial physiology, motility, immune responses, and the microbiome. Neurotransmitters, including acetylcholine (ACh), serotonin (5-HT), and vasoactive intestinal peptide (VIP), play pivotal roles in these intricate interactions. Understanding these dynamics is imperative, as the ENS is implicated in various diseases, ranging from neuropathies to GI disorders and neurodegenerative diseases. The emergence of organoid technology presents an unprecedented opportunity to study ENS interactions within the complex milieu of the small and large intestines. This manuscript underscores the urgent need for standardized protocols and advanced techniques to unravel the complexities of the ENS and its dynamic relationship with the gut ecosystem. The insights gleaned from such endeavors hold the potential to revolutionize GI disease modeling and treatment paradigms.

Neural regulation of slow waves and phasic contractions in the distal stomach: a mathematical model

Objective.Neural regulation of gastric motility occurs partly through the regulation of gastric bioelectrical slow waves (SWs) and phasic contractions. The interaction of the tissues and organs involved in this regulatory process is complex. We sought to infer the relative importance of cellular mechanisms in inhibitory neural regulation of the stomach by enteric neurons and the interaction of inhibitory and excitatory electrical field stimulation.Approach.A novel mathematical model of gastric motility regulation by enteric neurons was developed and scenarios were simulated to determine the mechanisms through which enteric neural influence is exerted. This model was coupled to revised and extended electrophysiological models of gastric SWs and smooth muscle cells (SMCs).Main results.The mathematical model predicted that regulation of contractile apparatus sensitivity to intracellular calcium in the SMC was the major inhibition mechanism of active tension development, and that the effect on SW amplitude depended on the inhibition of non-specific cation currents more than the inhibition of calcium-activated chloride current (kiNSCC= 0.77 vs kiAno1= 0.33). The model predicted that the interaction between inhibitory and excitatory neural regulation, when applied with simultaneous and equal intensity, resulted in an inhibition of contraction amplitude almost equivalent to that of inhibitory stimulation (79% vs 77% decrease), while the effect on frequency was overall excitatory, though less than excitatory stimulation alone (66% vs 47% increase).Significance.The mathematical model predicts the effects of inhibitory and excitatory enteric neural stimulation on gastric motility function, as well as the effects when inhibitory and excitatory enteric neural stimulation interact. Incorporation of the model into organ-level simulations will provide insights regarding pathological mechanisms that underpin gastric functional disorders, and allow forin silicotesting of the effects of clinical neuromodulation protocols for the treatment of these disorders.

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.

A Spatiotemporal and Machine-Learning Platform Accelerates the Manufacturing of hPSC-derived Esophageal Mucosa

Human pluripotent stem cell-derived tissue engineering offers great promise in designer cell-based personalized therapeutics. To harness such potential, a broader approach requires a deeper understanding of tissue-level interactions. We previously developed a manufacturing system for the ectoderm-derived skin epithelium for cell replacement therapy. However, it remains challenging to manufacture the endoderm-derived esophageal epithelium, despite both possessing similar stratified structure. Here we employ single cell and spatial technologies to generate a spatiotemporal multi-omics cell atlas for human esophageal development. We illuminate the cellular diversity, dynamics and signal communications for the developing esophageal epithelium and stroma. Using the machine-learning based Manatee, we prioritize the combinations of candidate human developmental signals for in vitro derivation of esophageal basal cells. Functional validation of the Manatee predictions leads to a clinically-compatible system for manufacturing human esophageal mucosa. Our approach creates a versatile platform to accelerate human tissue manufacturing for future cell replacement therapies to treat human genetic defects and wounds.

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.

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.

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.

Muscularis macrophages establish cell-to-cell contacts with telocytes/PDGFRα-positive cells and smooth muscle cells in the human and mouse gastrointestinal tract

BACKGROUND AND AIM

Muscularis macrophages (MMs) not only mediate the innate immunity, but also functionally interact with cells important for gastrointestinal motility. The aim of this study was to determine the spatial relationship and types of contacts between the MMs and neighboring cells in the muscularis propria of human and mouse stomach, small intestine, and large intestine.

METHODS

The distribution and morphology of MMs and their contacts with other cells were investigated by immunohistochemistry and transmission electron microscopy.

KEY RESULTS

Immunohistochemistry showed variable shape and number of MMs according to their location in different portions of the muscle coat. By double labeling, a close association between MMs and neighboring cells, that is, neurons, smooth muscle cells, interstitial cells of Cajal (ICCs), telocytes (TCs)/PDGFRα-positive cells, was seen. Electron microscopy demonstrated that in the muscle layers of both animal species, MMs have similar ultrastructural features and have specialized cell-to-cell contacts with smooth muscle cells and TCs/PDGFRα-positive cells but not with ICCs and enteric neurons.

CONCLUSION & INFERENCES

This study describes varying patterns of distribution of MMs between different regions of the gut, and reports the presence of distinct and extended cell-to-cell contacts between MMs and smooth muscle cells and between MMs and TCs/PDGFRα-positive cells. In contrast, MMs, although close to ICCs and nerve elements, did not make contact with them. These findings indicate specialized and variable roles for MMs in the modulation of gastrointestinal motility whose significance should be more closely investigated in normal and pathological conditions.

Rhythmic calcium transients in smooth muscle cells of the mouse internal anal sphincter

BACKGROUND

The internal anal sphincter (IAS) exhibits slow waves (SWs) and tone that are dependent upon L-type Ca²⁺ channels (Cav_L ) suggesting that phasic events (ie, SWs) play a fundamental role in tone generation. The present study further examined phasic activity in the IAS by measuring the spatiotemporal properties of Ca²⁺ transients (CTs) in IAS smooth muscle cells (SMCs).

METHODS

Ca²⁺ transients were recorded with spinning disk confocal microscopy from the IAS of SM-GCaMP mice. Muscles were pinned submucosal surface up at two different lengths. Drugs were applied by inclusion in the superfusate.

KEY RESULTS

Ca²⁺ transients displayed ongoing rhythmic firings at both lengths and were abolished by nifedipine and the K_ATP channel activator pinacidil indicating their dependence upon Cav_L . Like SWs, CTs were greatest in frequency (average 70.6 cpm) and amplitude at the distal extremity and conducted proximally. Removal of the distal IAS reduced but did not abolish CTs. The time constant for clearing cytoplasmic Ca²⁺ averaged 0.46 seconds and basal Ca²⁺ levels were significantly elevated.

CONCLUSIONS & INFERENCES

The similarities in spatiotemporal and pharmacological properties of CTs and SWs suggest that SW gives rise to CTs while muscle stretch is not required. Elevated relative basal Ca²⁺ in the IAS is likely due to the inability of cells to clear or sequester Ca²⁺ between rapid frequency voltage-dependent Ca²⁺ entry events, that is, conditions that will lead to tone development. The conduction of CTs from distal to proximal IAS will lead to orally directed contractions and likely contribute to the maintenance of fecal continence.

Post-inflammatory Ileitis Induces Non-neuronal Purinergic Signaling Adjustments of Cholinergic Neurotransmission in the Myenteric Plexus

Uncoupling between ATP overflow and extracellular adenosine formation changes purinergic signaling in post-inflammatory ileitis. Adenosine neuromodulation deficits were ascribed to feed-forward inhibition of ecto-5′-nucleotidase/CD73 by high extracellular adenine nucleotides in the inflamed ileum. Here, we hypothesized that inflammation-induced changes in cellular density may also account to unbalance the release of purines and their influence on [³H]acetylcholine release from longitudinal muscle-myenteric plexus preparations of the ileum of 2,4,6-trinitrobenzenesulfonic acid (TNBS)-treated rats. The population of S100β-positive glial cells increase, whereas Ano-1-positive interstitial cells of Cajal (ICCs) diminished, in the ileum 7-days after the inflammatory insult. In the absence of changes in the density of VAChT-positive cholinergic nerves detected by immunofluorescence confocal microscopy, the inflamed myenteric plexus released smaller amounts of [³H]acetylcholine which also became less sensitive to neuronal blockade by tetrodotoxin (1 μM). Instead, [³H]acetylcholine release was attenuated by sodium fluoroacetate (5 mM), carbenoxolone (10 μM) and A438079 (3 μM), which prevent activation of glial cells, pannexin-1 hemichannels and P2X7 receptors, respectively. Sodium fluoroacetate also decreased ATP overflow without significantly affecting the extracellular adenosine levels, thus indicating that surplus ATP release parallels reactive gliosis in post-inflammatory ileitis. Conversely, loss of ICCs may explain the lower amounts of adenosine detected in TNBS-treated preparations, since blockade of Ca_v3 (T-type) channels existing in ICCs with mibefradil (3 μM) or inhibition of the equilibrative nucleoside transporter 1 with dipyridamole (0.5 μM), both decreased extracellular adenosine. Data indicate that post-inflammatory ileitis operates a shift on purinergic neuromodulation reflecting the upregulation of ATP-releasing enteric glial cells and the depletion of ICCs accounting for decreased adenosine overflow via equilibrative nucleoside transporters.

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.

A mechanistic model of a PDGFRα(+) cell

A novel platelet-derived growth factor receptor alpha-positive cell (PDGFRα(+)) has recently been identified as part of the purinergic inhibitory neural control mechanism in the gastrointestinal (GI) tract. The mechanism through which PDGFRα(+) cells mediate GI muscle relaxation has been found to be associated with the purine receptors P2Y1 and apamin-sensitive SK3 channels that are highly expressed in these cells. This study aims to develop a mechanistic model elucidating a proposed mechanism through which PDGFRα(+) cells contribute to purinergic inhibitory neuromuscular transmission. In accordance with recent experimental findings, the model describes how the binding of neurotransmitters, released from enteric neurons, triggers the release of Ca(2+) from the endoplasmic reticulum in the PDGFRα(+) cells, and how this subsequently leads to large amplitude transient outward currents, which in turn hyperpolarize the cell. The model has been validated against experimental recordings and good agreement was found under normal and pharmacologically-altered conditions. This model demonstrates the feasibility of the proposed mechanism and provides a basis for understanding the mechanism underlying purinergic control of colonic motility.

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.

Estrogen regulates the expression of small-conductance Ca-activated K+ channels in colonic smooth muscle cells

AIM

This study aimed to determine the effects of small-conductance Ca(2+)-activated K(+) (SK) channels in colonic relaxation and the regulation of SK channels by estrogen.

METHODS

The contractile activity of muscle strips from male rats was estimated, and drugs including vehicle (DMSO), 17β-estradiol (E2), or apamin (SK blocker) were added, respectively. In a further experiment, muscle strips were preincubated with apamin before exposure to E2. The levels of the SK2 and SK3 protein expression in the colonic smooth muscle cells (SMCs) were detected. SMCs were treated with ICI 182780 (estrogen receptor [ER] antagonist) plus E2, BSA-E2, PPT (ERα agonist), or DPN (ERβ agonist). SK3 mRNA and protein expression levels were detected.

RESULTS

The muscle strips responded to E2 with a decrease and to apamin with a transient increase in tension. Preincubation with apamin partially prevented E2-induced relaxation. Two SK channel subtypes, SK2 and SK3, were coexpressed with α-actin in colonic SMCs. The quantitative ratio of the SK transcriptional expression in colonic SMCs was SK3 > SK2. The SK3 expression was upregulated by E2, and was downregulated by ICI 182780, but was not influenced by BSA-E2. Furthermore, the effect of PPT on the expression of SK3 was almost the same as that of E2, while DPN did not influence the protein expression of SK3.

CONCLUSION

These findings indicate that SK3 is involved in the E2-induced relaxing effect on rat colonic smooth muscle. Furthermore, E2 upregulates the expression of SK3 in rat SMCs, and that this effect is mediated via the ERα receptor.

Altered distribution of small-conductance calcium-activated potassium channel SK3 in Hirschsprung's disease

PURPOSE

SK3 channels are voltage-independent Ca(2+)-dependent K(+) channels that play a key role in regulating smooth muscle membrane potential during purinergic inhibitory neurotransmission in the colon. Dysmotility problems are common after a properly performed pull-through operation for Hirschsprung's disease (HSCR). We hypothesised that ganglionic bowel just proximal to the transition zone is abnormal and designed this study to investigate SK3 channel expression in HSCR.

METHODS

Entire resected bowel specimens were collected at the time of pull-through surgery for HSCR (n=6). Control colonic specimens were obtained at the time of colostomy closure in patients following anorectoplasty (n=6). SK3 protein expression was assessed qualitatively using immunofluorescence with confocal microscopy and quantitatively using western blot (WB) analysis.

RESULTS

Positive SK3 immunofluorescence was seen in the mucosa and in all three smooth muscle layers and the myenteric plexus in control specimens. SK3 immunofluorescence co-localised with PDGFRα. A band was detected at ~70kDa on WB. SK3 protein expression was barely detectable in aganglionic tissue and markedly reduced in the ganglionic bowel of 3 patients with HSCR compared to controls.

CONCLUSION

Decreased SK3 expression in ganglionic bowel may explain the basis of persistent bowel symptoms in some patients following a properly performed pull-through operation for HSCR.

Histologic changes in diabetic gastroparesis

The cellular abnormalities that lead to diabetic gastroparesis are increasingly being understood. Several key cell types are affected by diabetes, leading to gastroparesis. These changes include abnormalities in the extrinsic innervation to the stomach, loss of key neurotransmitters at the level of the enteric nervous system, smooth muscle abnormalities, loss of interstitial cells of Cajal, and changes in the macrophage population resident in the muscle wall. This article reviews the current understanding with a focus on data from human studies when available.

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

KEY POINTS

Platelet derived growth factor receptor α (PDGFRα(+) ) cells in colonic muscles are innervated by enteric inhibitory motor neurons. PDGFRα(+) cells generate Ca(2+) transients in response to exogenous purines and these responses were blocked by MRS-2500. Stimulation of enteric neurons, with cholinergic and nitrergic components blocked, evoked Ca(2+) transients in PDGFRα(+) and smooth muscle cells (SMCs). Responses to nerve stimulation were abolished by MRS-2500 and not observed in muscles with genetic deactivation of P2Y1 receptors. Ca(2+) transients evoked by nerve stimulation in PDGFRα(+) cells showed the same temporal characteristics as electrophysiological responses. PDGFRα(+) cells express gap junction genes, and drugs that inhibit gap junctions blocked neural responses in SMCs, but not in nerve processes or PDGFRα(+) cells. PDGFRα(+) cells are directly innervated by inhibitory motor neurons and purinergic responses are conducted to SMCs via gap junctions.

ABSTRACT

Interstitial cells, known as platelet derived growth factor receptor α (PDGFRα(+) ) cells, are closely associated with varicosities of enteric motor neurons and suggested to mediate purinergic hyperpolarization responses in smooth muscles of the gastrointestinal tract (GI), but this concept has not been demonstrated directly in intact muscles. We used confocal microscopy to monitor Ca(2+) transients in neurons and post-junctional cells of the murine colon evoked by exogenous purines or electrical field stimulation (EFS) of enteric neurons. EFS (1-20 Hz) caused Ca(2+) transients in enteric motor nerve processes and then in PDGFRα(+) cells shortly after the onset of stimulation (latency from EFS was 280 ms at 10 Hz). Responses in smooth muscle cells (SMCs) were typically a small decrease in Ca(2+) fluorescence just after the initiation of Ca(2+) transients in PDGFRα(+) cells. Upon cessation of EFS, several fast Ca(2+) transients were noted in SMCs (rebound excitation). Strong correlation was noted in the temporal characteristics of Ca(2+) transients evoked in PDGFRα(+) cells by EFS and inhibitory junction potentials (IJPs) recorded with intracellular microelectrodes. Ca(2+) transients and IJPs elicited by EFS were blocked by MRS-2500, a P2Y1 antagonist, and absent in P2ry1((-/-)) mice. PDGFRα(+) cells expressed gap junction genes, and gap junction uncouplers, 18β-glycyrrhetinic acid (18β-GA) and octanol blocked Ca(2+) transients in SMCs but not in neurons or PDGFRα(+) cells. IJPs recorded from SMCs were also blocked. These findings demonstrate direct innervation of PDGFRα(+) cells by motor neurons. PDGFRα(+) cells are primary targets for purinergic neurotransmitter(s) in enteric inhibitory neurotransmission. Hyperpolarization responses are conducted to SMCs via gap junctions.

Nitrergic neuromuscular transmission in the mouse internal anal sphincter is accomplished by multiple pathways and postjunctional effector cells

The effector cells and second messengers participating in nitrergic neuromuscular transmission (NMT) were investigated in the mouse internal anal sphincter (IAS). Protein expression of guanylate cyclase (GCα, GCβ) and cyclic GMP-dependent protein kinase I (cGKI) were examined in cryostat sections with dual-labeling immunohistochemical techniques in PDGFRα(+) cells, interstitial cells of Cajal (ICC), and smooth muscle cells (SMC). Gene expression levels were determined with quantitative PCR of dispersed cells from Pdgfrα(egfp/+), Kit(copGFP/+), and smMHC(Cre-egfp) mice sorted with FACS. The relative gene and protein expression levels of GCα and GCβ were PDGFRα(+) cells > ICC ≫ SMC. In contrast, cGKI gene expression sequence was SMC = ICC > PDGFRα(+) cells whereas cGKI protein expression sequence was neurons > SMC ≫ ICC = PDGFRα(+) cells. The functional role of cGKI was investigated in cGKI(-/-) mice. Relaxation with 8-bromo (8-Br)-cGMP was greatly reduced in cGKI(-/-) mice whereas responses to sodium nitroprusside (SNP) were partially reduced and forskolin responses were unchanged. A nitrergic relaxation occurred with nerve stimulation (NS, 5 Hz, 60 s) in cGKI(+/+) and cGKI(-/-) mice although there was a small reduction in the cGKI(-/-) mouse. N(ω)-nitro-l-arginine (l-NNA) abolished responses during the first 20-30 s of NS in both animals. The GC inhibitor ODQ greatly reduced or abolished SNP and nitrergic NS responses in both animals. These data confirm an essential role for GC in NO-induced relaxation in the IAS. However, the expression of GC and cGKI by all three cell types suggests that each may participate in coordinating muscular responses to NO. The persistence of nitrergic NMT in the cGKI(-/-) mouse suggests the presence of a significant GC-dependent, cGKI-independent pathway.

The purinergic neurotransmitter revisited: a single substance or multiple players?

The past half century has witnessed tremendous advances in our understanding of extracellular purinergic signaling pathways. Purinergic neurotransmission, in particular, has emerged as a key contributor in the efficient control mechanisms in the nervous system. The identity of the purine neurotransmitter, however, remains controversial. Identifying it is difficult because purines are present in all cell types, have a large variety of cell sources, and are released via numerous pathways. Moreover, studies on purinergic neurotransmission have relied heavily on indirect measurements of integrated postjunctional responses that do not provide direct information for neurotransmitter identity. This paper discusses experimental support for adenosine 5'-triphosphate (ATP) as a neurotransmitter and recent evidence for possible contribution of other purines, in addition to or instead of ATP, in chemical neurotransmission in the peripheral, enteric and central nervous systems. Sites of release and action of purines in model systems such as vas deferens, blood vessels, urinary bladder and chromaffin cells are discussed. This is preceded by a brief discussion of studies demonstrating storage of purines in synaptic vesicles. We examine recent evidence for cell type targets (e.g., smooth muscle cells, interstitial cells, neurons and glia) for purine neurotransmitters in different systems. This is followed by brief discussion of mechanisms of terminating the action of purine neurotransmitters, including extracellular nucleotide hydrolysis and possible salvage and reuptake in the cell. The significance of direct neurotransmitter release measurements is highlighted. Possibilities for involvement of multiple purines (e.g., ATP, ADP, NAD(+), ADP-ribose, adenosine, and diadenosine polyphosphates) in neurotransmission are considered throughout.

Spontaneous transient hyperpolarizations in the rabbit small intestine

Four types of electrical activity were recorded and related to cell structure by intracellular recording and dye injection into impaled cells in muscles of rabbit small intestine. The specific cell types from which recordings were made were longitudinal smooth muscle cells (LSMCs), circular smooth muscle cells (CSMCs), interstitial cells of Cajal distributed in the myenteric region (ICC-MY) and fibroblast-like cells (FLCs). Slow waves (slow wavesSMC) were recorded from LSMCs and CSMCs. Slow waves (slow wavesICC) were of greatest amplitude (>50 mV) and highest maximum rate of rise (>10 V s(-1)) in ICC-MY. The dominant activity in FLCs was spontaneous transient hyperpolarizations (STHs), with maximum amplitudes above 30 mV. STHs were often superimposed upon small amplitude slow waves (slow wavesFLC). STHs displayed a cyclical pattern of discharge irrespective of background slow wave activity. STHs were inhibited by MRS2500 (3 μm), a P2Y1 antagonist, and abolished by apamin (0.3 μm), a blocker of small conductance Ca(2+)-activated K(+) channels. Small amplitude STHs (<15 mV) were detected in smooth muscle layers, whereas STHs were not resolved in cells identified as ICC-MY. Electrical field stimulation evoked purinergic inhibitory junction potentials (IJPs) in CSMCs. Purinergic IJPs were not recorded from ICC-MY. These results suggest that FLCs may regulate smooth muscle excitability in the rabbit small intestine via generation of rhythmic apamin-sensitive STHs. Stimulation of P2Y1 receptors modulates the amplitudes of STHs. Our results also suggest that purinergic inhibitory motor neurons regulate the motility of the rabbit small intestine by causing IJPs in FLCs that conduct to CSMCs.

Purinergic neuromuscular transmission in the gastrointestinal tract; functional basis for future clinical and pharmacological studies

Nerve-mediated relaxation is necessary for the correct accomplishment of gastrointestinal (GI) motility. In the GI tract, NO and a purine are probably released by the same inhibitory motor neuron as inhibitory co-transmitters. The P2Y1 receptor has been recently identified as the receptor responsible for purinergic smooth muscle hyperpolarization and relaxation in the human gut. This finding has been confirmed in P2Y1 -deficient mice where purinergic neurotransmission is absent and transit time impaired. However, the mechanisms responsible for nerve-mediated relaxation, including the identification of the purinergic neurotransmitter(s) itself, are still debatable. Possibly different mechanisms of nerve-mediated relaxation are present in the GI tract. Functional demonstration of purinergic neuromuscular transmission has not been correlated with structural studies. Labelling of purinergic neurons is still experimental and is not performed in routine pathology studies from human samples, even when possible neuromuscular impairment is suspected. Accordingly, the contribution of purinergic neurotransmission in neuromuscular diseases affecting GI motility is not known. In this review, we have focused on the physiological mechanisms responsible for nerve-mediated purinergic relaxation providing the functional basis for possible future clinical and pharmacological studies on GI motility targeting purine receptors.

Interstitial cells: regulators of smooth muscle function

Smooth muscles are complex tissues containing a variety of cells in addition to muscle cells. Interstitial cells of mesenchymal origin interact with and form electrical connectivity with smooth muscle cells in many organs, and these cells provide important regulatory functions. For example, in the gastrointestinal tract, interstitial cells of Cajal (ICC) and PDGFRα(+) cells have been described, in detail, and represent distinct classes of cells with unique ultrastructure, molecular phenotypes, and functions. Smooth muscle cells are electrically coupled to ICC and PDGFRα(+) cells, forming an integrated unit called the SIP syncytium. SIP cells express a variety of receptors and ion channels, and conductance changes in any type of SIP cell affect the excitability and responses of the syncytium. SIP cells are known to provide pacemaker activity, propagation pathways for slow waves, transduction of inputs from motor neurons, and mechanosensitivity. Loss of interstitial cells has been associated with motor disorders of the gut. Interstitial cells are also found in a variety of other smooth muscles; however, in most cases, the physiological and pathophysiological roles for these cells have not been clearly defined. This review describes structural, functional, and molecular features of interstitial cells and discusses their contributions in determining the behaviors of smooth muscle tissues.

The significance of interstitial cells in neurogastroenterology

Smooth muscle layers of the gastrointestinal tract consist of a heterogeneous population of cells that include enteric neurons, several classes of interstitial cells of mesenchymal origin, a variety of immune cells and smooth muscle cells (SMCs). Over the last number of years the complexity of the interactions between these cell types has begun to emerge. For example, interstitial cells, consisting of both interstitial cells of Cajal (ICC) and platelet-derived growth factor receptor alpha-positive (PDGFRα(+)) cells generate pacemaker activity throughout the gastrointestinal (GI) tract and also transduce enteric motor nerve signals and mechanosensitivity to adjacent SMCs. ICC and PDGFRα(+) cells are electrically coupled to SMCs possibly via gap junctions forming a multicellular functional syncytium termed the SIP syncytium. Cells that make up the SIP syncytium are highly specialized containing unique receptors, ion channels and intracellular signaling pathways that regulate the excitability of GI muscles. The unique role of these cells in coordinating GI motility is evident by the altered motility patterns in animal models where interstitial cell networks are disrupted. Although considerable advances have been made in recent years on our understanding of the roles of these cells within the SIP syncytium, the full physiological functions of these cells and the consequences of their disruption in GI muscles have not been clearly defined. This review gives a synopsis of the history of interstitial cell discovery and highlights recent advances in structural, molecular expression and functional roles of these cells in the GI tract.

The significance of interstitial cells in neurogastroenterology

Smooth muscle layers of the gastrointestinal tract consist of a heterogeneous population of cells that include enteric neurons, several classes of interstitial cells of mesenchymal origin, a variety of immune cells and smooth muscle cells (SMCs). Over the last number of years the complexity of the interactions between these cell types has begun to emerge. For example, interstitial cells, consisting of both interstitial cells of Cajal (ICC) and platelet-derived growth factor receptor alpha-positive (PDGFRα(+)) cells generate pacemaker activity throughout the gastrointestinal (GI) tract and also transduce enteric motor nerve signals and mechanosensitivity to adjacent SMCs. ICC and PDGFRα(+) cells are electrically coupled to SMCs possibly via gap junctions forming a multicellular functional syncytium termed the SIP syncytium. Cells that make up the SIP syncytium are highly specialized containing unique receptors, ion channels and intracellular signaling pathways that regulate the excitability of GI muscles. The unique role of these cells in coordinating GI motility is evident by the altered motility patterns in animal models where interstitial cell networks are disrupted. Although considerable advances have been made in recent years on our understanding of the roles of these cells within the SIP syncytium, the full physiological functions of these cells and the consequences of their disruption in GI muscles have not been clearly defined. This review gives a synopsis of the history of interstitial cell discovery and highlights recent advances in structural, molecular expression and functional roles of these cells in the GI tract.

Toward a better understanding of gastrointestinal nitrergic neuromuscular transmission

BACKGROUND

Nitric oxide (NO) is an important inhibitory neurotransmitter in the gastrointestinal (GI) tract. The majority of nitrergic effects are transduced by NO-sensitive guanylyl cyclase (NO-GC) as the receptor for NO, and, thus, mediated by cGMP-dependent mechanisms. Work carried out during the past years has demonstrated NO to be largely involved in GI smooth muscle relaxation and motility. However, detailed investigation of nitrergic signaling has turned out to be complicated as NO-GC was identified in several different GI cell types such as smooth muscle cells, interstitial cells of Cajal and fibroblast-like cells. With regards to nitrergic neurotransmission, special focus has been placed on the role of interstitial cells of Cajal using mutant mice with reduced populations of ICC. Recently, global and cell-specific knockout mice for enzymes participating in nitrergic signaling have been generated providing a suitable approach to further examine the role of NO-mediated signaling in GI smooth muscle.

PURPOSE

This review discusses the current knowledge on nitrergic mechanisms in gastrointestinal neuromuscular transmission with a focus on genetic models and outlines possible further investigations to gain better understanding on NO-mediated effects in the GI tract.

Telocytes express PDGFRα in the human gastrointestinal tract

Telocytes (TC), a cell population located in the connective tissue of many organs of humans and laboratory mammals, are characterized by a small cell body and extremely long and thin processes. Different TC subpopulations share unique ultrastructural features, but express different markers. In the gastrointestinal (GI) tract, cells with features of TC were seen to be CD34-positive/c-kit-negative and several roles have been proposed for them. Other interstitial cell types with regulatory roles described in the gut are the c-kit-positive/CD34-negative/platelet-derived growth factor receptor α (PDGFRα)-negative interstitial cells of Cajal (ICC) and the PDGFRα-positive/c-kit-negative fibroblast-like cells (FLC). As TC display the same features and locations of the PDGFRα-positive cells, we investigated whether TC and PDGFRα-positive cells could be the same cell type. PDGFRα/CD34, PDGFRα/c-kit and CD34/c-kit double immunolabelling was performed in full-thickness specimens from human oesophagus, stomach and small and large intestines. All TC in the mucosa, submucosa and muscle coat were PDGFRα/CD34-positive. TC formed a three-dimensional network in the submucosa and in the interstitium between muscle layers, and an almost continuous layer at the submucosal borders of muscularis mucosae and circular muscle layer. Moreover, TC encircled muscle bundles, nerve structures, blood vessels, funds of gastric glands and intestinal crypts. Some TC were located within the muscle bundles, displaying the same location of ICC and running intermingled with them. ICC were c-kit-positive and CD34/PDGFRα-negative. In conclusion, in the human GI tract the TC are PDGFRα-positive and, therefore, might correspond to the FLC. We also hypothesize that in human gut, there are different TC subpopulations probably playing region-specific roles.

Immunolocalization and expression of small-conductance calcium-activated potassium channels in human myometrium

Small-conductance calcium-activated potassium (SK3) channels have been detected in human myometrium and we have previously shown a functional role of SK channels in human myometrium in vitro. The aims of this study were to identify the precise localization of SK3 channels and to quantify SK3 mRNA expression in myometrium from pregnant and non-pregnant women. Myometrial biopsies were obtained from pregnant (n = 11) and non-pregnant (n = 11) women. The expression of SK3 channels was assessed using immunohistochemistry and SK3 mRNA was determined by qRT-PCR. In non-pregnant myometrium SK3 immunoreactivity was observed in CD34 positive (CD34(+)) interstitial Cajal-like cells (ICLC), now called telocytes. Although CD34(+) cells were also present in pregnant myometrium, they lacked SK3 immunoreactivity. Furthermore, the immunohistochemical results showed that SK3 expression in vascular endothelium was similar between the two groups. CD117 immunoreactivity was only detected in small round cells that resemble mast cells. Compared to non-pregnant myometrium we found significantly less SK3 mRNA in pregnant myometrium. We demonstrate that SK3 channels are localized solely in CD34(+) cells and not in smooth muscle cells, and that the molecular expression of SK3 channels is higher in non-pregnant compared to pregnant myometrium. On the basis of our previous study and the present findings, we propose that SK3 activators reduce contractility in human myometrium by modulating telocyte function. This is the first report to provide evidence for a possible role of SK3 channels in human uterine telocytes.

Calcium responses in subserosal interstitial cells of the guinea-pig proximal colon

BACKGROUND

In the subserosal layer between the longitudinal muscle layer and mesothelium, heterogeneous populations of interstitial cells are distributed. As the distribution of nerve elements in this layer is sparse as compared with the nerve plexus layer or tunica muscularis, there may be unique communication among subserosal interstitial cells (SSICs). This study aimed to explore functional properties of SSICs.

METHODS

In subserosal preparations of the guinea-pig proximal colon, changes in intracellular Ca(2+) ([Ca(2+) ]i ) were visualized using Fluo-4 Ca(2+) imaging. Immunohistochemistry was also performed to identify the SSICs exhibiting Ca(2+) transients.

KEY RESULTS

A majority of SSICs responded to adenosine triphosphate (ATP, 10 μM) by increasing [Ca(2+) ]i , but remained quiescent during the application of acetylcholine (10 μM). ATP-induced Ca(2+) responses were mimicked by adenosine 5'-diphosphate (10 μM), MRS2365 (10 nM) but not α, β-methylene ATP (10 μM) or uridine triphosphate (10 μM), and could be reproduced in Ca(2+) -free solution, suggesting that ATP acts via P2Y receptors, most likely P2Y1 subtype, but not P2X receptors. Live staining of the same preparations after Ca(2+) imaging indicated the ATP-sensitive SSICs were not positive for c-Kit antibody, a specific marker for gastrointestinal interstitial cells of Cajal (ICC). Immunohistochemistry identified vimentin (mesenchymal cell marker)+/Kit- and SK3 (fibroblast-like cell (FLC) marker)+/Kit- cells that had a similar morphology to the ATP-sensitive SSICs in Ca(2+) imaging.

CONCLUSIONS & INFERENCES

A majority of the SSICs in the guinea-pig proximal colon, presumably FLC, are capable of responding to ATP and thus may contribute to smooth muscle relaxation upon stimulation with ATP released from non-neuronal cells.

Distribution and Ca(2+) signalling of fibroblast-like (PDGFR(+)) cells in the murine gastric fundus

Platelet-derived growth factor receptor α positive (PDGFRα(+)) cells are suggested to mediate purinergic inputs in GI muscles, but the responsiveness of these cells to purines in situ has not been evaluated. We developed techniques to label and visualize PDGFRα(+) cells in murine gastric fundus, load cells with Ca(2+) indicators, and follow their activity via digital imaging. Immunolabelling demonstrated a high density of PDGFRα(+) cells in the fundus. Cells were isolated and purified by fluorescence-activated cell sorting (FACS) using endogenous expression of enhanced green fluorescent protein (eGFP) driven off the Pdgfra promoter. Quantitative PCR showed high levels of expression of purinergic P2Y1 receptors and SK3 K(+) channels in PDGFRα(+) cells. Ca(2+) imaging was used to characterize spontaneous Ca(2+) transients and responses to purines in PDGFRα(+) cells in situ. ATP, ADP, UTP and β-NAD elicited robust Ca(2+) transients in PDGFRα(+) cells. Ca(2+) transients were also elicited by the P2Y1-specific agonist (N)-methanocarba-2MeSADP (MRS-2365), and inhibited by MRS-2500, a P2Y1-specific antagonist. Responses to ADP, MRS-2365 and β-NAD were absent in PDGFRα(+) cells from P2ry1((-/-)) mice, but responses to ATP were retained. Purine-evoked Ca(2+) transients were mediated through Ca(2+) release mechanisms. Inhibitors of phospholipase C (U-73122), IP3 (2-APB), ryanodine receptors (Ryanodine) and SERCA pump (cyclopiazonic acid and thapsigargin) abolished Ca(2+) transients elicited by purines. This study provides a link between purine binding to P2Y1 receptors and activation of SK3 channels in PDGFRα(+) cells. Activation of Ca(2+) release is likely to be the signalling mechanism in PDGFRα(+) cells responsible for the transduction of purinergic enteric inhibitory input in gastric fundus muscles.

Platelet-derived growth factor receptor α-positive cells in the tunica muscularis of human colon

An obstacle to understanding motor pathologies of the gastrointestinal (GI) tract is that the physiology of some of the cellular components of the gut wall is not understood. Morphologists identified fibroblast-like cells in the tunica muscularis many years ago, but little is known about these interstitial cells because of inadequate techniques to identify these cells. Recent findings have shown that fibroblast-like cells express platelet-derived growth factor receptor α (PDGFRα) in mice and that antibodies for these receptors can be used to label the cells. We used immunohistochemical techniques to study the phenotype and intercellular relationships of fibroblast-like cells in the human colon. Fibroblast-like cells are labelled specifically with antibodies to PDGFRα and widely distributed through the tunica muscularis of human colon. These cells form discrete networks in the region of the myenteric plexus and within the circular and longitudinal muscle layers. Platelet-derived growth factor receptor α(+) cells are distinct from c-Kit(+) interstitial cells of Cajal and closely associated with varicose processes of neurons expressing substance P (excitatory motor neurons) or neuronal nitric oxide synthase (nNOS) (inhibitory motor neurons). Platelet-derived growth factor receptor α(+) cells express small conductance Ca(2+)-activated K(+) channels (SK3), which are likely to mediate purinergic neural regulation of colonic muscles. Our data suggest that PDGFRα(+) cells may have an important role in transducing inputs from enteric motor neurons. This study identifies reagents and techniques that will allow investigation of this class of interstitial cells and help develop an understanding of the role of PDGFRα(+) cells in the human GI tract in health and disease.

Platelet-derived growth factor receptor α (PDGFRα)-expressing "fibroblast-like cells" in diabetic and idiopathic gastroparesis of humans

BACKGROUND

Emerging evidence suggests that "fibroblast-like cells" (FLC) may play a role in the regulation of gastrointestinal (GI) motor function. FLC are ultrastructurally distinct from other interstitial cells, including interstitial cells of Cajal (ICC), and express small-conductance Ca(2+) -activated K(+) channels (SK3). In mice, platelet-derived growth factor receptor α (PDGFRα) antibody has also been shown to label FLC. The aims of this study were to determine the morphology and distribution of PDGFRα-immunoreactive (ir) FLC in human gastric muscle and to determine if FLC are altered in gastroparesis, where ICC are reduced.

METHODS

Full thickness gastric body biopsies from five healthy subjects, 10 diabetic, and 10 idiopathic gastroparesis patients were immunolabeled using SK3 and PDGFRα staining for FLC and Kit staining for ICC. Intramuscular FLC and ICC were quantified.

KEY RESULTS

Intramuscular PDGFRα-ir cells had slender cell bodies and long, thin processes and were more abundant in the longitudinal compared with the circular muscle. In the region of myenteric plexus, FLC had smaller, rounder cell bodies with 3-4 processes and formed networks, often around ganglia. All SK3-ir cell structures showed complete overlap with PDGFRα-ir. FLC were in close proximity to ICC, but their cell bodies did not overlap. No differences were seen in the distribution, morphology, or overall numbers of FLC in gastroparesis patients.

CONCLUSIONS & INFERENCES

In conclusion, PDGFRα identifies FLC in human gastric smooth muscle. FLC were not altered in distribution or overall numbers in gastroparesis. Additional studies are required to determine their role in human GI function.

Relative contribution of SKCa and TREK1 channels in purinergic and nitrergic neuromuscular transmission in the rat colon

Purinergic and nitrergic neurotransmission predominantly mediate inhibitory neuromuscular transmission in the rat colon. We studied the sensitivity of both purinergic and nitrergic pathways to spadin, a TWIK-related potassium channel 1 (TREK1) inhibitor, apamin, a small-conductance calcium-activated potassium channel blocker and 1H-[1,2,4]oxadiazolo[4,3-α]quinoxalin-1-one (ODQ), a specific inhibitor of soluble guanylate cyclase. TREK1 expression was detected by RT-PCR in the rat colon. Patch-clamp experiments were performed on cells expressing hTREK1 channels. Spadin (1 μM) reduced currents 1) in basal conditions 2) activated by stretch, and 3) with arachidonic acid (AA; 10 μM). l-Methionine (1 mM) or l-cysteine (1 mM) did not modify currents activated by AA. Microelectrode and muscle bath studies were performed on rat colon samples. l-Methionine (2 mM), apamin (1 μM), ODQ (10 μM), and N(ω)-nitro-l-arginine (l-NNA; 1 mM) depolarized smooth muscle cells and increased motility. These effects were not observed with spadin (1 μM). Purinergic and nitrergic inhibitory junction potentials (IJP) were studied by incubating the tissue with l-NNA (1 mM) or MRS2500 (1 μM). Both purinergic and nitrergic IJP were unaffected by spadin. Apamin reduced both IJP with a different potency and maximal effect for each. ODQ concentration dependently abolished nitrergic IJP without affecting purinergic IJP. Similar effects were observed in hyperpolarizations induced by sodium nitroprusside (1 μM) and nitrergic relaxations induced by electrical stimulation. We propose a pharmacological approach to characterize the pathways and function of purinergic and nitrergic neurotransmission. Nitrergic neurotransmission, which is mediated by cyclic guanosine monophosphate, is insensitive to spadin, an effective TREK1 channel inhibitor. Both purinergic and nitrergic neurotransmission are inhibited by apamin but with different relative sensitivity.

Purinergic neuromuscular transmission is absent in the colon of P2Y(1) knocked out mice

Purinergic and nitrergic co-transmission is the dominant mechanism responsible for neural-mediated smooth muscle relaxation in the gastrointestinal tract. The aim of the present paper was to test whether or not P2Y(1) receptors are involved in purinergic neurotransmission using P2Y(1)(−/−) knock-out mice. Tension and microelectrode recordings were performed on colonic strips. In wild type (WT) animals, electrical field stimulation (EFS) caused an inhibitory junction potential (IJP) that consisted of a fast IJP (MRS2500 sensitive, 1 μm) followed by a sustained IJP (N(ω)-nitro-L-arginine (L-NNA) sensitive, 1 mm). The fast component of the IJP was absent in P2Y(1)(−/−) mice whereas the sustained IJP (L-NNA sensitive) was recorded. In WT animals, EFS-induced inhibition of spontaneous motility was blocked by the consecutive addition of L-NNA and MRS2500. In P2Y(1)(−/−) mice, EFS responses were completely blocked by L-NNA. In WT and P2Y(1)(−/−) animals, L-NNA induced a smooth muscle depolarization but ‘spontaneous' IJP (MRS2500 sensitive) could be recorded in WT but not in P2Y(1)(−/−) animals. Finally, in WT animals, 1 μm MRS2365 caused a smooth muscle hyperpolarization that was blocked by 1 μm MRS2500. In contrast, 1 μm MRS2365 did not modify smooth muscle resting membrane potential in P2Y(1)(−/−) mice. β-Nicotinamide adenine dinucleotide (β-NAD, 1 mm) partially mimicked the effect of MRS2365. We conclude that P2Y(1) receptors mediate purinergic neurotransmission in the gastrointestinal tract and β-NAD partially fulfils the criteria to participate in rodent purinergic neurotransmission. The P2Y(1)(−/−) mouse is a useful animal model to study the selective loss of purinergic neurotransmission.

Changes in neuromuscular transmission in the W/W(v) mouse internal anal sphincter

BACKGROUND

Intramuscular interstitial cells of Cajal (ICC-IM) have been shown to participate in nitrergic neuromuscular transmission (NMT) in various regions of the gastrointestinal (GI) tract, but their role in the internal anal sphincter (IAS) is still uncertain. Contractile studies of the IAS in the W/W(v) mouse (a model in which ICC-IM numbers are markedly reduced) have reported that nitrergic NMT persists and that ICC-IM are not required. However, neither the changes in electrical events underlying NMT nor the contributions of other non-nitrergic neural pathways have been examined in this model.

METHODS

The role of ICC-IM in NMT was examined by recording the contractile and electrical events associated with electrical field stimulation (EFS) of motor neurons in the IAS of wildtype and W/W(v) mice. Nitrergic, purinergic, and cholinergic components were identified using inhibitors of these pathways.

KEY RESULTS

Under NANC conditions, purinergic and nitrergic pathways both contribute to EFS-induced inhibitory junction potentials (IJPs) and relaxation. Purinergic IJPs and relaxation were intact in the W/W(v) mouse IAS, whereas nitrergic IJPs were reduced by 50-60% while relaxation persisted. In the presence of L-NNA (NOS inhibitor) and MRS2500 (P2Y1 receptor antagonist), EFS gave rise to cholinergic depolarization and contractions that were abolished by atropine. Cholinergic depolarization was absent in the W/W(v) mouse IAS while contraction persisted.

CONCLUSIONS & INFERENCES

ICC-IM significantly contributes to the electrical events underlying nitrergic and cholinergic NMT, whereas contractile events persist in the absence of ICC-IM. The purinergic inhibitory neural pathway appears to be independent of ICC-IM.

Targeting ion channels for the treatment of gastrointestinal motility disorders

Gastrointestinal (GI) functional and motility disorders are highly prevalent and responsible for long-term morbidity and sometimes mortality in the affected patients. It is estimated that one in three persons has a GI functional or motility disorder. However, diagnosis and treatment of these widespread conditions remains challenging. This partly stems from the multisystem pathophysiology, including processing abnormalities in the central and peripheral (enteric) nervous systems and motor dysfunction in the GI wall. Interstitial cells of Cajal (ICCs) are central to the generation and propagation of the cyclical electrical activity and smooth muscle cells (SMCs) are responsible for electromechanical coupling. In these and other excitable cells voltage-sensitive ion channels (VSICs) are the main molecular units that generate and regulate electrical activity. Thus, VSICs are potential targets for intervention in GI motility disorders. Research in this area has flourished with advances in the experimental methods in molecular and structural biology and electrophysiology. However, our understanding of the molecular mechanisms responsible for the complex and variable electrical behavior of ICCs and SMCs remains incomplete. In this review, we focus on the slow waves and action potentials in ICCs and SMCs. We describe the constituent VSICs, which include voltage-gated sodium (Na(V)), calcium (Ca(V)), potassium (K(V), K(Ca)), chloride (Cl(-)) and nonselective ion channels (transient receptor potentials [TRPs]). VSICs have significant structural homology and common functional mechanisms. We outline the approaches and limitations and provide examples of targeting VSICs at the pores, voltage sensors and alternatively spliced sites. Rational drug design can come from an integrated view of the structure and mechanisms of gating and activation by voltage or mechanical stress.

Crohn's disease: ultrastructure of interstitial cells in colonic myenteric plexus

The role of the interstitial cells of Cajal (ICC) in chronic inflammatory bowel disease, i.e., ulcerative colitis (UC) and Crohn's disease (CD), remains unclear. Ultrastructural alterations in ICC in the colonic myenteric plexus (ICC-MP) have been reported previously in UC, but descriptions of ICC-MP and other interstitial cells in the myenteric region of the colon are lacking for CD. In the present study, we characterized the ultrastructure of interstitial cells, nerves, and glial cells in the myenteric region in Crohn's colitis (CC). In comparison with controls, varicosities of the myenteric bundles were dilated and appeared to be empty. Lipid droplets and lipofuscin-bodies were prominent in glial cells and neurons. ICC-MP were scanty but, as in controls, had caveolae, prominent intermediate filaments, cytoplasmic dense bodies, and membrane-associated dense bands with a patchy basal lamina. ICC-MP were similar in the various colonic regions. ICC-MP in CC showed no signs of degeneration or cytological changes. As in controls, fibroblast-like cells had abundant coated vesicles but lacked prominent intermediate filaments and caveolae. Macrophages also appeared as in controls. In comparison with ICC-MP in UC, the cytology of ICC-MP in CC were thus undisturbed. The ultrastructural differences between UC and CC might reflect pathophysiological differences of importance for understanding pathogenetic differences between CD and UC.

Relationship between interstitial cells of Cajal, fibroblast-like cells and inhibitory motor nerves in the internal anal sphincter

Interstitial cells of Cajal (ICC) have been shown to participate in nitrergic neurotransmission in various regions of the gastrointestinal (GI) tract. Recently, fibroblast-like cells, which are positive for platelet-derived growth factor receptor α (PDGFRα(+)), have been suggested to participate additionally in inhibitory neurotransmission in the GI tract. The distribution of ICC and PDGFRα(+) cell populations and their relationship to inhibitory nerves within the mouse internal anal sphincter (IAS) are unknown. Immunohistochemical techniques and confocal microscopy were therefore used to examine the density and arrangement of ICC, PDGFRα(+) cells and neuronal nitric-oxide-synthase-positive (nNOS(+)) nerve fibers in the IAS of wild-type (WT) and W/W ( v ) mice. Of the total tissue volume within the IAS circular muscle layer, 18% consisted in highly branched PDGFRα(+) cells (PDGFRα(+)-IM). Other populations of PDGFRα(+) cells were observed within the submucosa and along the serosal and myenteric surfaces. Spindle-shaped intramuscular ICC (ICC-IM) were present in the WT mouse IAS but were largely absent from the W/W ( v ) IAS. The ICC-IM volume (5% of tissue volume) in the WT mouse IAS was significantly smaller than that of PDGFRα(+)-IM. Stellate-shaped submucosal ICC (ICC-SM) were observed in the WT and W/W ( v ) IAS. Minimum surface distance analysis revealed that nNOS(+) nerve fibers were closely aligned with both ICC-IM and PDGFRα(+)-IM. An even closer association was seen between ICC-IM and PDGFRα(+)-IM. Thus, a close morphological arrangement exists between inhibitory motor neurons, ICC-IM and PDGFRα(+)-IM suggesting that some functional interaction occurs between them contributing to inhibitory neurotransmission in the IAS.

β-nicotinamide adenine dinucleotide is an enteric inhibitory neurotransmitter in human and nonhuman primate colons

BACKGROUND & AIMS

An important component of enteric inhibitory neurotransmission is mediated by a purine neurotransmitter, such as adenosine 5'-triphosphate (ATP), binding to P2Y1 receptors and activating small conductance K(+) channels. In murine colon β-nicotinamide adenine dinucleotide (β-NAD) is released with ATP and mimics the pharmacology of inhibitory neurotransmission better than ATP. Here β-NAD and ATP were compared as possible inhibitory neurotransmitters in human and monkey colons.

METHODS

A small-volume superfusion assay and high-pressure liquid chromatography with fluorescence detection were used to evaluate spontaneous and nerve-evoked overflow of β-NAD, ATP, and metabolites. Postjunctional responses to nerve stimulation, β-NAD and ATP were compared using intracellular membrane potential and force measurements. Effects of β-NAD on smooth muscle cells (SMCs) were recorded by patch clamp. P2Y receptor transcripts were assayed by reverse transcription polymerase chain reaction.

RESULTS

In contrast to ATP, overflow of β-NAD evoked by electrical field stimulation correlated with stimulation frequency and was diminished by the neurotoxins, tetrodotoxin, and ω-conotoxin GVIA. Inhibitory junction potentials and responses to exogenous β-NAD, but not ATP, were blocked by P2Y receptor antagonists suramin, pyridoxal-phosphate-6-azophenyl-2',4'-disulfonate (PPADS), 2'-deoxy-N6-methyladenosine 3',5'-bisphosphate (MRS 2179), and (1R,2S,4S,5S)-4-[2-Iodo-6-(methylamino)-9H-purin-9-yl]-2-(phosphonooxy)bicyclo[3.1.0]hexane-1-methanol dihydrogen phosphate ester tetraammonium salt (MRS 2500). β-NAD activated nonselective cation currents in SMCs, but failed to activate outward currents.

CONCLUSIONS

β-NAD meets the criteria for a neurotransmitter better than ATP in human and monkey colons and therefore may contribute to neural regulation of colonic motility. SMCs are unlikely targets for inhibitory purine neurotransmitters because dominant responses of SMCs were activation of net inward, rather than outward, current.

A functional role for the 'fibroblast-like cells' in gastrointestinal smooth muscles

Smooth muscles, as in the gastrointestinal tract, are composed of several types of cells. Gastrointestinal muscles contain smooth muscle cells, enteric neurons, glial cells, immune cells, and various classes of interstitial cells. One type of interstitial cell, referred to as 'fibroblast-like cells' by morphologists, are common, but their function is unknown. These cells are found near the terminals of enteric motor neurons, suggesting they could have a role in generating neural responses that help control gastrointestinal movements. We used a novel mouse with bright green fluorescent protein expressed specifically in the fibroblast-like cells to help us identify these cells in the mixture of cells obtained when whole muscles are dispersed with enzymes. We isolated these cells and found they respond to a major class of inhibitory neurotransmitters - purines. We characterized these responses, and our results provide a new hypothesis about the role of fibroblast-like cells in smooth muscle tissues.

Neuroeffector apparatus in gastrointestinal smooth muscle organs

Control of gastrointestinal (GI) movements by enteric motoneurons is critical for orderly processing of food, absorption of nutrients and elimination of wastes. Work over the past several years has suggested that motor neurotransmission is more complicated than simple release of transmitter from nerve terminals and binding of receptors on smooth muscle cells. In fact the 'neuro-effector' junction in the tunica muscularis might consist of synaptic-like connectivity with specialized cells, and contributions from multiple cell types in integrated post-junctional responses. Interstitial cells of Cajal (ICC) were proposed as potential mediators in motor neurotransmission based on reduced post-junctional responses observed in W mutants that have reduced populations of ICC. More recent studies on W mutants have contradicted the original findings, and suggested that ICC may not be significant players in motor neurotransmission. This review examines the evidence for and against the role of ICC in motor neurotransmission and outlines areas for additional investigation that would help further resolve this controversy.

Distribution and possible role of PDGF-AA and PDGFR-alpha in the gastrointestinal tract of adult guinea pigs

It was reported that a signaling pathway of platelet-derived growth factors (PDGFs) and their receptors (PDGFRs) played a critical role in the developing gut of mice. Overexpression of the PDGFR-alpha gene in gastrointestinal stromal tumors (GISTs) indicated that parts of tumor cells originated from PDGFR-alpha-positive cells, but a more detailed distribution of PDGFR-alpha and possible role in the adult mammalian gut are still unclear. In the present study, we examined the expression of both PDGF-AA and its receptor PDGFR-alpha in the gastrointestinal (GI) tract of adult guinea pigs using western blotting and immunohistochemistry. PDGF-AA-immunoreactive cells were mainly distributed in the mucosal epithelium of the stomach, small intestine, and large intestine. Only a few PDGF-AA-positive cells were seen in the longitudinal muscle layer of the large intestine. In contrast, PDGFR-alpha-positive cells were widely distributed throughout the GI tract, including the lamina propria, muscular layer, and subserosa. Double staining showed that the distribution of the PDGFR-alpha-positive cells in the muscular layer were similar to those of the interstitial cells of Cajal (ICCs), and they were associated with ICCs and enteric nerves, but no double-labeled cells were observed by anti-PDGFR or Kit antibody. It was noted that PDGFR-alpha-positive cells were also stained with a vimentin monoclonal antibody. Based on the double staining and morphological features, we consider the PDGFR-alpha-positive cells belong to a subtype of fibroblast. Our results not only provide a roadmap for understanding the function of the PDGF/PDGFR signaling pathway in both normal adult mammals and during gut injury and repair but also might help in understanding the growth and development of GISTs in the clinic.

Purinergic and nitrergic neuromuscular transmission mediates spontaneous neuronal activity in the rat colon

Nitric oxide (NO) and ATP mediate smooth muscle relaxation in the gastrointestinal tract. However, the involvement of these neurotransmitters in spontaneous neuronal activity is unknown. The aim of the present work was to study spontaneous neuromuscular transmission in the rat midcolon. Microelectrode experiments were performed under constant stretch both in circular and longitudinal directions. Spontaneous inhibitory junction potentials (sIJP) were recorded. Tetrodotoxin (1 microM) and apamin (1 microM) depolarized smooth muscle cells and inhibited sIJP. N(omega)-nitro-l-arginine (l-NNA, 1 mM) depolarized smooth muscle cells but did not modify sIJP. In contrast, the P2Y(1) antagonist MRS-2500 (1 microM) did not modify the resting membrane potential (RMP) but reduced sIJP (IC(50) = 3.1 nM). Hexamethonium (200 microM), NF-023 (10 microM), and ondansetron (1 microM) did not modify RMP and sIJP. These results correlate with in vitro (muscle bath) and in vivo (strain gauges) data where l-NNA but not MRS-2500 induced a sustained increase of spontaneous motility. We concluded that, in the rat colon, inhibitory neurons regulate smooth muscle RMP and cause sIJP. In vitro, the release of inhibitory neurotransmitters is independent of nicotinic, P2X, and 5-hydroxytryptamine type 3 receptors. Neuronal NO causes a sustained smooth muscle hyperpolarization that is responsible for a constant inhibition of spontaneous motility. In contrast, ATP acting on P2Y(1) receptors is responsible for sIJP but does not mediate inhibitory neural tone. ATP and NO have complementary physiological functions in the regulation of gastrointestinal motility.

Properties of Rikkunshi-to (TJ-43)-induced relaxation of rat gastric fundus smooth muscles

The relaxant effects of Rikkunshi-to (TJ-43), a gastroprotective herbal medicine, on rat gastric fundus were investigated. Experiments were carried out using standard tension and intracellular microelectrode recording techniques. During contraction induced by enprostil (0.5 microM), a prostaglandin E(2) analog, TJ-43, produced relaxation dose dependently (0.1-5.0 mg/ml) in the rat fundic circular smooth muscle (CSM) strips. The relaxant effects of TJ-43 were not affected by tetrodotoxin or 1 H[1, 2, 4] oxadiazolo [4, 3-a] quinoxalin-1-one (10 microM), an inhibitor of soluble guanylate cyclase. TJ-43 inhibited enprostil-induced membrane depolarization. Apamin (1 microM), a blocker of small-conductance Ca(2+)-activated K(+) (SK) channel, inhibited T-43-induced membrane repolarization. TJ-43-induced relaxation was biphasic, comprising of an initial fast followed by a second slow relaxation. The fast relaxation was abolished by apamin. Application of high K(+) (29.4 mM [K(+)](o)) also abolished the fast relaxation induced by TJ-43. In diabetic Goto-Kakizaki (GK) rat fundic CSM strips, the relaxant responses of TJ-43 during enprostil-induced contraction were increased compared with control rat strips. These results indicate that TJ-43 elicited fast muscle relaxation through membrane hyperpolarization induced by the activation of SK channels; the time-dependent slow relaxation reflects an additional direct of TJ-43 on CSM in the rat gastric fundus. Because TJ-43-evoked relaxation of fundic CSM strips was more potent in diabetic GK rat than in control rat, further analysis of this herb could lead to better treatments of diabetic gastroparesis.

Immunohistochemical demonstration of c-Kit-negative fibroblast-like cells in murine gastrointestinal musculature

In the gastrointestinal musculature, interstitial cells of Cajal (ICC) distribute and regulate the gastrointestinal motility. Another type of mesenchymal cell, known as the fibroblast-like cell (FLC), has also been reported to be juxtaposed to the ICC. In this study, we examined the immunohistochemical properties of FLC in the murine gastrointestinal musculature using antibodies to small conductance Ca(2+)-activated K(+) channel 3 (SK3), platelet-derived growth factor receptor alpha (PDGFRalpha), and CD34. SK3-immunopositive (SK3-ip) cells were observed in the musculature throughout the gastrointestinal tract. These SK3-ip cells were distinct from the ICC that were identified by c-Kit immunoreactivity. In the muscular layers, SK3-ip cells were bipolar in shape and were associated with the intramuscular ICC and nerve fiber bundles. In the myenteric layer multipolar-shaped SK3-ip cells encompassed the myenteric ganglia. SK3-ip cells in the subserosal plane formed a cellular network with their ramified processes. The distribution pattern of the SK3-ip cells in the ICC-deficient W(v)/W(v) mutant mice was similar to that in normal mice. We also demonstrated that SK3-ip cells showed the intense PDGFRalpha immunoreactivity that was previously examined in FLC. However, CD34 immunoreactivity, one of the markers of human FLC, was not observed in SK3-ip cells with the exception of subserosal FLC. Thus, our observations indicate that SK3- and PDGFRalpha-double immunopositive cells are FLC in the murine gastrointestinal musculature and behave as a basic cellular element throughout the gastrointestinal musculature.

Distribution, expression and functional effects of small conductance Ca-activated potassium (SK) channels in rat myometrium

Calcium-activated potassium channels are important in a variety of smooth muscles, contributing to excitability and contractility. In the myometrium previous work has focussed on the large conductance channels (BK), and the role of small conductance channels (SK) has received scant attention, despite the finding that over-expression of an SK channel isoform (SK3) results in uterine dysfunction and delayed parturition. This study therefore characterises the expression of the three SK channel isoforms (SK1-3) in rat myometrium throughout pregnancy and investigates their effect on cytosolic [Ca] and force and compares this with that of BK channels. Consistent expression of all SK isoform transcripts and clear immunostaining of SK1-3 was found. Inhibition of SK1-3 channels (apamin, scyllatoxin) significantly inhibited outward current, caused membrane depolarisation and elicited action potentials in previously quiescent cells. Apamin or scyllatoxin increased the amplitude of [Ca] and force in spontaneously contracting myometrial strips throughout gestation. The functional effect of SK inhibition was larger than that of BK channel inhibition. Thus we show for the first time that SK1-3 channels are expressed and translated throughout pregnancy and contribute to outward current, regulate membrane potential and hence Ca signals in pregnant rat myometrium. They contribute more to quiescence that BK channels.

Igf1r+/CD34+ immature ICC are putative adult progenitor cells, identified ultrastructurally as fibroblast-like ICC in Ws/Ws rat colon

The colon of Ws/Ws mutant rats shows impairment of pacemaker activity and altered inhibitory neurotransmission. The present study set out to find structural correlates to these findings to resolve mechanisms. In the colon of Ws/Ws rats, interstitial cells of Cajal associated with Auerbach's plexus (ICC-AP) were significantly decreased and ICC located at the submuscular plexus and intramuscular ICC were rarely observed based on immunohistochemistry and electron microscopy. Ultrastructural investigations revealed that there was no overall loss of all types of interstitial cells combined. Where loss of ICC was observed, a marked increase in fibroblast-like ICC (FL-ICC) was found at the level of AP. Immunoelectron microscopy proved FL-ICC to be c-Kit(-) but gap junction coupled to each other and to c-Kit(+) ICC; they were associated with enteric nerves and occupied space normally occupied by ICC in the wild-type rat colon, suggesting them to be immature ICC. In addition, a marked increase in immunoreactivity for insulin-like growth factor 1 receptor (Igf1r) occurred, co-localized with CD34 but not with c-Kit. A significantly higher number of Igf1r(+)/CD34(+) cells were found in Ws/Ws compared to wild-type rat colons. These CD34(+)/Igf1r(+) cells in the Ws/Ws colon occupied the same space as FL-ICC. Hence we propose that a subset of immature ICC (FL-ICC) consists of adult progenitor cells. Immunohistochemistry revealed a reduction of neurons positive for neuronal nitric oxide synthase. The functional capabilities of the immature ICC and the regenerative capabilities of the adult progenitor cells need further study. The morphological features described here show that the loss of pacemaker activity is not associated with failure to develop a network of interstitial cells around AP but a failure to develop this network into fully functional pacemaker cells. The reduction in nitrergic innervation associated with the Ws mutation may be the result of a reduction in nitrergic neurons.

c-Kit-negative fibroblast-like cells express platelet-derived growth factor receptor alpha in the murine gastrointestinal musculature

Platelet-derived growth factor receptors (PDGFRs) belong to the same kinase group as c-Kit receptor tyrosine kinase that is specifically expressed in the interstitial cells of Cajal (ICC) in the gastrointestinal tract. In this study, we examined PDGFRalpha immunoreactivity in the murine gastrointestinal tract. PDGFRalpha-immunopositive (PDGFRalpha-ip) cells were observed in the musculature in all parts of the gastrointestinal tract. Although PDGFRalpha-ip cells were distinct from ICC and neurons, these cells were closely associated with intramuscular ICC and enteric nerve fibers. In the myenteric layer, PDGFRalpha-ip cells formed a cellular network with their ramified processes and encompassed myenteric ganglia. Numerous PDGFRalpha-ip cells were observed in the subserosal plane and showed a multipolar shape. The distribution pattern of the PDGFRalpha-ip cells in the ICC-deficient W(v)/W(v) mutant mice was the same as that in normal mice. PDGFRalpha-ip cells that showed intense immunoreactivity of SK3 potassium channel were considered to correspond to fibroblast-like cells or non-Cajal interstitial cells. Our observations suggest that PDGFRalpha-ip cells are basic cellular elements throughout the gastrointestinal musculature and are involved in the gastrointestinal functions.

Interstitial cells of Cajal are innervated by nitrergic nerves and express nitric oxide-sensitive guanylate cyclase in the guinea-pig gastrointestinal tract

Nitric oxide (NO) is a major signaling molecule in the gastrointestinal tract, and released NO inhibits muscular contraction. The actions of NO are mediated by stimulation of soluble guanylate cyclase (sGC, NO-sensitive GC) and a subsequent increase in cGMP concentration. To elucidate NO targets in the gastrointestinal musculature, we investigated the immunohistochemical localization of the beta1 and alpha1 subunits of sGC and the distribution of neuronal NO synthase (nNOS) -containing nerves in the guinea-pig gastrointestinal tract. Distinct immunoreactivity for sGCbeta1 and sGCalpha1 was observed in the interstitial cells of Cajal (ICC), fibroblast-like cells (FLC) and enteric neurons in the musculature. Double immunohistochemistry using anti-c-Kit antibody and anti-sGCbeta1 antibody revealed sGCbeta1 immunoreactivity in almost all intramuscular ICC throughout the entire gastrointestinal tract. Immunoelectron microscopy revealed that sGCbeta1-immunopositive cells possessed some of the criteria for intramuscular ICC: presence of caveolae; frequently associated with nerve bundles; and close contact with smooth muscle cells. sGCbeta1-immunopositive ICC were closely apposed to nNOS-containing nerve fibers in the muscle layers. Immunohistochemical and immunoelectron microscopical observations revealed that FLC in the musculature also showed sGCbeta1 immunoreactivity. FLC were often associated with nNOS-immunopositive nerve fibers. In the myenteric layer, almost all myenteric ganglia contained nNOS-immunopositive nerve cells and were surrounded by myenteric ICC and FLC. Myenteric ICC in the large intestine and FLC in the entire gastrointestinal tract showed sGCbeta1 immunoreactivity in the myenteric layer. Smooth muscle cells in the stomach and colon showed weak sGCbeta1 immunoreactivity, and those in the muscularis mucosae and vasculature also showed evident immunoreactivity. These data suggest that ICC are primary targets for NO released from nNOS-containing enteric neurons, and that some NO signals are received by FLC and smooth muscle cells in the gastrointestinal tract.