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

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.

Interstitial cells of Cajal and telocytes in the gut: twins, related or simply neighbor cells?

In the interstitium of the connective tissue several types of cells occur. The fibroblasts, responsible for matrix formation, the mast cells, involved in local response to inflammatory stimuli, resident macrophages, plasma cells, lymphocytes, granulocytes and monocytes, all engaged in immunity responses. Recently, another type of interstitial cell, found in all organs so far examined, has been added to the previous ones, the telocytes (TC). In the gut, in addition to the cells listed above, there are also the interstitial cells of Cajal (ICC), a peculiar type of cell exclusively detected in the alimentary tract with multiple functions including pace-maker activity. The possibility that TC and ICC could correspond to a unique cell type, where the former would represent an ICC variant outside the gut, was initially considered, however, further studies have clearly shown that ICC and TC are two distinct types of cells. In the gut, while the features and the roles of the ICC are established, part of the scientific community is still disputing these 'new' interstitial cells to which several names such as fibroblast-like cells (FLCs), interstitial Cajal-like cells or, most recently, PDGFRα+ cells have been attributed. This review will detail the main features and roles of the TC and ICC with the aim to establish their relationships and hopefully define the identity of the TC in the gut.

Convergence of inhibitory neural inputs regulate motor activity in the murine and monkey stomach

Inhibitory motor neurons regulate several gastric motility patterns including receptive relaxation, gastric peristaltic motor patterns, and pyloric sphincter opening. Nitric oxide (NO) and purines have been identified as likely candidates that mediate inhibitory neural responses. However, the contribution from each neurotransmitter has received little attention in the distal stomach. The aims of this study were to identify the roles played by NO and purines in inhibitory motor responses in the antrums of mice and monkeys. By using wild-type mice and mutants with genetically deleted neural nitric oxide synthase (Nos1^-/-) and P2Y1 receptors (P2ry1^-/-) we examined the roles of NO and purines in postjunctional inhibitory responses in the distal stomach and compared these responses to those in primate stomach. Activation of inhibitory motor nerves using electrical field stimulation (EFS) produced frequency-dependent inhibitory junction potentials (IJPs) that produced muscle relaxations in both species. Stimulation of inhibitory nerves during slow waves terminated pacemaker events and associated contractions. In Nos1^-/- mice IJPs and relaxations persisted whereas in P2ry1^-/- mice IJPs were absent but relaxations persisted. In the gastric antrum of the non-human primate model Macaca fascicularis, similar NO and purine neural components contributed to inhibition of gastric motor activity. These data support a role of convergent inhibitory neural responses in the regulation of gastric motor activity across diverse species.

Overexpression of PDGFRA cooperates with loss of NF1 and p53 to accelerate the molecular pathogenesis of malignant peripheral nerve sheath tumors

Malignant peripheral nerve sheath tumors (MPNSTs) are aggressive, frequently metastatic sarcomas that are associated with neurofibromatosis type 1 (NF1), a prominent inherited genetic disease in humans. Although loss of the NF1 gene predisposes to MPNST induction, relatively long tumor latency in NF1 patients suggests that additional genetic or epigenetic abnormalities are needed for the development of these nerve sheath malignancies. To study the molecular pathways contributing to the formation of MPNSTs in NF1 patients, we used a zebrafish tumor model defined by nf1 loss in a p53-deficient background together with the overexpression of either wild-type or constitutively activated PDGFRA (platelet-derived growth factor receptor-α) under control of the sox10 neural crest-specific promoter. Here we demonstrate the accelerated onset and increased penetrance of MPNST formation in fish overexpressing both the wild-type and the mutant PDGFRA transgenes in cells of neural crest origin. Interestingly, overexpression of the wild-type PDGFRA was even more potent in promoting transformation than the mutant PDGFRA, which is important because ~78% of human MPNSTs have expression of wild-type PDGFRA, whereas only 5% harbor activating mutations of the gene encoding this receptor. Further analysis revealed the induction of cellular senescence in zebrafish embryos overexpressing mutant, but not wild-type, PDGFRA, suggesting a mechanism through which the oncogenic activity of the mutant receptor is tempered by the activation of premature cellular senescence in an NF1-deficient background. Taken together, our study suggests a model in which overexpression of wild-type PDGFRA associated with NF1 deficiency leads to aberrant activation of downstream RAS signaling and thus contributes importantly to MPNST development-a prediction supported by the ability of the kinase inhibitor sunitinib alone and in combination with the MEK inhibitor trametinib to retard MPNST progression in transgenic fish overexpressing the wild-type receptor.

Deficiency of platelet-derived growth factor receptor-α-positive cells in Hirschsprung's disease colon

AIM: To investigate whether the expression of platelet-derived growth factor receptor-α-positive (PDGFRα⁺)-cells is altered in Hirschsprung’s disease (HD).

METHODS: HD tissue specimens (n = 10) were collected at the time of pull-through surgery, while colonic control samples were obtained at the time of colostomy closure in patients with imperforate anus (n = 10). Immunolabelling of PDGFRα⁺-cells was visualized using confocal microscopy to assess the distribution of these cells, while Western blot analysis was undertaken to quantify PDGFRα protein expression.

RESULTS: Confocal microscopy revealed PDGFRα⁺-cells within the mucosa, myenteric plexus and smooth muscle in normal controls, with a marked reduction in PDGFRα⁺-cells in the HD specimens. Western blotting revealed high levels of PDGFRα protein expression in normal controls, while there was a striking decrease in PDGFRα protein expression in the HD colon.

CONCLUSION: These findings suggest that the altered distribution of PDGFRα⁺-cells in both the aganglionic and ganglionic HD bowel may contribute to the motility dysfunction in HD.

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.

Enteric Inhibitory Neurotransmission, Starting Down Under

The idea of an inhibitory innervation in the gut came from Geoff Burnstock's group at the University of Melbourne in the 1960s. Being resistant to antagonists of norepinephrine and acetylcholine, enteric inhibitory neurotransmission became known as non-cholinergic, non-adrenergic (NANC) neurotransmission. ATP (or a closely related nucleotide) was proposed as the inhibitory neurotransmitter based on release of purines during nerve stimulation and similarities between responses to ATP and transmural nerve stimulation in several gut preparations. Apamin was found to block purinergic responses, providing evidence that small-conductance Ca(2+)-activated K(+) (SK) channels were responsible for inhibitory junction potentials (IJPs). Actually the IJPs in GI muscles are composed of multiple components, and later studies discovered nitric oxide (NO) to be the other major mediator of NANC inhibitory neurotransmission. The purinergic component of enteric inhibitory neurotransmission is mediated by P2Y1 receptors, and this component is absent in P2Y1(-/-) mice. The criteria for a neurotransmitter are better met by β-nicotinamide adenine dinucleotide (β-NAD) or its immediate metabolite ADP-ribose (ADPR) than by ATP. The cells mediating post-junctional responses have been identified. In addition to smooth muscle cells, two classes of interstitial cells express receptors and effectors for NANC neurotransmitters and are electrically coupled to smooth muscle cells. This integrated structure has been named the SIP syncytium. Interstitial cells of Cajal are involved in transduction of cholinergic and nitrergic inputs to GI muscles, and PDGFRα(+) cells mediate purinergic effects. This short symposium report summarizes major historical points of interest and some of the more recent findings related to enteric inhibitory neurotransmission.

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.

A novel class of interstitial cells in the mouse and monkey female reproductive tracts

Growing evidence suggests important roles for specialized platelet-derived growth factor receptor alpha-positive (PDGFRalpha(+)) cells in regulating the behaviors of visceral smooth muscle organs. Examination of the female reproductive tracts of mice and monkeys showed that PDGFRalpha(+) cells form extensive networks in ovary, oviduct, and uterus. PDGFRalpha(+) cells were located in discrete locations within these organs, and their distribution and density were similar in rodents and primates. PDGFRalpha(+) cells were distinct from smooth muscle cells and interstitial cells of Cajal (ICC). This was demonstrated with immunohistochemical techniques and by performing molecular expression studies on PDGFRalpha(+) cells from mice with enhanced green fluorescent protein driven off of the endogenous promoter for Pdgfralpha. Significant differences in gene expression were found in PDGFRalpha(+) cells from ovary, oviduct, and uterus. Differences in gene expression were also detected in cells from different tissue regions within the same organ (e.g., uterine myometrium vs. endometrium). PDGFRalpha(+) cells are unlikely to provide pacemaker activity because they lack significant expression of key pacemaker genes found in ICC (Kit and Ano1). Gja1 encoding connexin 43 was expressed at relatively high levels in PDGFRalpha(+) cells (except in the ovary), suggesting these cells can form gap junctions to one another and neighboring smooth muscle cells. PDGFRalpha(+) cells also expressed the early response transcription factor and proto-oncogene Fos, particularly in the ovary. These data demonstrate extensive distribution of PDGFRalpha(+) cells throughout the female reproductive tract. These cells are a heterogeneous population of cells that are likely to contribute to different aspects of physiological regulation in the various anatomical niches they occupy.

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.

Intracellular Ca(2+) release from endoplasmic reticulum regulates slow wave currents and pacemaker activity of interstitial cells of Cajal

Interstitial cells of Cajal (ICC) provide pacemaker activity in gastrointestinal muscles that underlies segmental and peristaltic contractions. ICC generate electrical slow waves that are due to large-amplitude inward currents resulting from anoctamin 1 (ANO1) channels, which are Ca(2+)-activated Cl(-) channels. We investigated the hypothesis that the Ca(2+) responsible for the stochastic activation of ANO1 channels during spontaneous transient inward currents (STICs) and synchronized activation of ANO1 channels during slow wave currents comes from intracellular Ca(2+) stores. ICC, obtained from the small intestine of Kit(+/copGFP) mice, were studied under voltage and current clamp to determine the effects of blocking Ca(2+) uptake into stores and release of Ca(2+) via inositol 1,4,5-trisphosphate (IP3)-dependent and ryanodine-sensitive channels. Cyclocpiazonic acid, thapsigargin, 2-APB, and xestospongin C inhibited STICs and slow wave currents. Ryanodine and tetracaine also inhibited STICs and slow wave currents. Store-active compounds had no direct effects on ANO1 channels expressed in human embryonic kidney-293 cells. Under current clamp, store-active drugs caused significant depolarization of ICC and reduced spontaneous transient depolarizations (STDs). After block of ryanodine receptors with ryanodine and tetracaine, repolarization did not restore STDs. ANO1 expressed in ICC has limited access to cytoplasmic Ca(2+) concentration, suggesting that pacemaker activity depends on Ca(2+) dynamics in restricted microdomains. Our data from studies of isolated ICC differ somewhat from studies on intact muscles and suggest that release of Ca(2+) from both IP3 and ryanodine receptors is important in generating pacemaker activity in ICC.

Differential functional role of purinergic and nitrergic inhibitory cotransmitters in human colonic relaxation

AIM

ATP and nitric oxide (NO) are released from enteric inhibitory motor neurones and are responsible for colonic smooth muscle relaxation. However, how frequency of neural stimulation affects this cotransmission process and the post-junctional responses has not been systematically characterized in the human colon.

METHODS

The dynamics of inhibitory cotransmission were studied using different protocols of electrical field stimulation (EFS) to characterize the inhibitory junction potentials (IJP) and the corresponding relaxation in colonic strips obtained from 36 patients.

RESULTS

Single pulses elicited a fast IJP (IJPf(MAX) = -27.6 ± 1.6 mV), sensitive to the P2Y1 antagonist MRS2500 1 μm, that ran down with frequency increase leaving a residual hyperpolarization at high frequencies (IJPf∞ = -3.7 ± 0.6 mV). Accordingly, low frequencies of EFS caused purinergic transient relaxations that cannot be maintained at high frequencies. Addition of the P2Y1 agonist MRS2365 10 μm during the purinergic rundown did not cause any hyperpolarization. Protein kinase C (PKC), a putative P2Y1 desensitizator, was able to reduce the amplitude of the IJPf when activated, but the rundown was not modified by PKC inhibitors. Frequencies higher than 0.60 ± 0.15 Hz were needed to evoke a sustained nitrergic hyperpolarization that progressively increased reaching IJPs∞ = -13 ± 0.4 mV at high frequencies and leading to a sustained inhibition of spontaneous motility.

CONCLUSION

Changes in frequency of stimulation possibly mimicking neuronal firing will post-junctionally determine purinergic vs. nitrergic responses underlying different functional roles. NO will be responsible for sustained relaxations needed in physiological processes such as storage, while purinergic neurotransmission evoking sharp transient relaxations will be dominant in processes such as propulsion.

Gastrointestinal motility and its enteric actors in mechanosensitivity: past and present

Coordinated contractions of the smooth muscle layers of the gastrointestinal (GI) tract are required to produce motor patterns that ensure normal GI motility. The crucial role of the enteric nervous system (ENS), the intrinsic ganglionated network located within the GI wall, has long been recognized in the generation of the main motor patterns. However, devising an appropriate motility requires the integration of informations emanating from the lumen of the GI tract. As already found more than half a century ago, the ability of the GI tract to respond to mechanical forces such as stretch is not restricted to neuronal mechanisms. Instead, mechanosensitivity is now recognized as a property of several non-neuronal cell types, the excitability of which is probably involved in shaping the motor patterns. This brief review gives an overview on how mechanosensitivity of different cell types in the GI tract has been established and, whenever available, on what ionic conductances are involved in mechanotransduction and their potential impact on normal GI motility.

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.

Calcium signalling in Cajal-like interstitial cells of the lower urinary tract

Interstitial cells of Cajal (ICC) serve several critical physiological roles in visceral smooth muscle organs, including acting as electrical pacemakers to modulate phasic contractile activity and as intermediaries in motor neurotransmission. The major roles of ICC have been described in the gastrointestinal tract, however, ICC-like cells (ICC-LC) can also be found in other visceral organs, including those of the lower urinary tract (LUT), where they provide similar functions, acting as electrical pacemakers and as intermediary cells involved in the modulation of neurotransmission to adjacent smooth muscle cells. The physiological functions of ICC-LC, in particular their role as pacemakers, relies on their ability to generate transient and propagating intracellular Ca(2+) events. The role of ICC-LC as pacemakers and neuromodulators in the LUT is increasingly apparent and the study of their intracellular Ca(2+) dynamics will provide a better understanding of their role in LUT excitability.

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.

Platelet-derived growth factor receptor-α-positive cells and not smooth muscle cells mediate purinergic hyperpolarization in murine colonic muscles

Enteric inhibitory neurotransmission is an important feature of the neural regulation of gastrointestinal motility. Purinergic neurotransmission, via P2Y1 receptors, mediates one phase of inhibitory neural control. For decades, ATP has been assumed to be the purinergic neurotransmitter and smooth muscle cells (SMCs) have been considered the primary targets for inhibitory neurotransmission. Recent experiments have cast doubt on both of these assumptions and suggested that another cell type, platelet-derived growth factor receptor-α-positive (PDGFRα(+)) cells, is the target for purinergic neurotransmission. We compared responses of PDGFRα(+) cells and SMCs to several purine compounds to determine if these cells responded in a manner consistent with enteric inhibitory neurotransmission. ATP hyperpolarized PDGFRα(+) cells but depolarized SMCs. Only part of the ATP response in PDGFRα(+) cells was blocked by MRS 2500, a P2Y1 antagonist. ADP, MRS 2365, β-NAD, and adenosine 5-diphosphate-ribose, P2Y1 agonists, hyperpolarized PDGFRα(+) cells, and these responses were blocked by MRS 2500. Adenosine 5-diphosphate-ribose was more potent in eliciting hyperpolarization responses than β-NAD. P2Y1 agonists failed to elicit responses in SMCs. Small hyperpolarization responses were elicited in SMCs by a small-conductance Ca(2+)-activated K(+) channel agonist, cyclohexyl-[2-(3,5-dimethyl-pyrazol-1-yl)-6-methyl-pyrimidin-4-yl]-amine, consistent with the low expression and current density of small-conductance Ca(2+)-activated K(+) channels in these cells. Large-amplitude hyperpolarization responses, elicited in PDGFRα(+) cells, but not SMCs, by P2Y1 agonists are consistent with the generation of inhibitory junction potentials in intact muscles in response to purinergic neurotransmission. The responses of PDGFRα(+) cells and SMCs to purines suggest that SMCs are unlikely targets for purinergic neurotransmission in colonic muscles.

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.

Interstitial cells of Cajal mediate nitrergic inhibitory neurotransmission in the murine gastrointestinal tract

Nitric oxide (NO) is a major inhibitory neurotransmitter in the gastrointestinal (GI) tract. Its main effector, NO-sensitive guanylyl cyclase (NO-GC), is expressed in several GI cell types, including smooth muscle cells (SMC), interstitial cells of Cajal (ICC), and fibroblast-like cells. Up to date, the interplay between neurons and these cells to initiate a nitrergic inhibitory junction potential (IJP) is unclear. Here, we investigate the origin of the nitrergic IJP in murine fundus and colon. IJPs were determined in fundus and colon SMC of mice lacking NO-GC globally (GCKO) and specifically in SMC (SM-GCKO), ICC (ICC-GCKO), and both SMC/ICC (SM/ICC-GCKO). Nitrergic IJP was abolished in ICC-GCKO fundus and reduced in SM-GCKO fundus. In the colon, the amplitude of nitrergic IJP was reduced in ICC-GCKO, whereas nitrergic IJP in SM-GCKO was reduced in duration. These results were corroborated by loss of the nitrergic IJP in global GCKO. In conclusion, our results prove the obligatory role of NO-GC in ICC for the initiation of an IJP. NO-GC in SMC appears to enhance the nitrergic IJP, resulting in a stronger and prolonged hyperpolarization in fundus and colon SMC, respectively. Thus NO-GC in both cell types is mandatory to induce a full nitrergic IJP. Our data from the colon clearly reveal the nitrergic IJP to be biphasic, resulting from individual inputs of ICC and SMC.

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.

Reprint of: Role of enteric neurotransmission in host defense and protection of the gastrointestinal tract

Host defense is a vital role played by the gastrointestinal tract. As host to an enormous and diverse microbiome, the gut has evolved an elaborate array of chemical and physicals barriers that allow the digestion and absorption of nutrients without compromising the mammalian host. The control of such barrier functions requires the integration of neural, humoral, paracrine and immune signaling, involving redundant and overlapping mechanisms to ensure, under most circumstances, the integrity of the gastrointestinal epithelial barrier. Here we focus on selected recent developments in the autonomic neural control of host defense functions used in the protection of the gut from luminal agents, and discuss how the microbiota may potentially play a role in enteric neurotransmission. Key recent findings include: the important role played by subepithelial enteric glia in modulating intestinal barrier function, identification of stress-induced mechanisms evoking barrier breakdown, neural regulation of epithelial cell proliferation, the role of afferent and efferent vagal pathways in regulating barrier function, direct evidence for bacterial communication to the enteric nervous system, and microbial sources of enteric neurotransmitters. We discuss these new and interesting developments in our understanding of the role of the autonomic nervous system in gastrointestinal host defense.

Purinergic inhibitory regulation of murine detrusor muscles mediated by PDGFRα+ interstitial cells

Purines induce transient contraction and prolonged relaxation of detrusor muscles. Transient contraction could be due to activation of inward currents in smooth muscle cells, but the mechanism of purinergic relaxation has not been determined. We recently reported a new class of interstitial cells in detrusor muscles and showed that these cells could be identified with antibodies against platelet-derived growth factor receptor-α (PDGFRα(+) cells). The current density of small conductance Ca(2+)-activated K(+) (SK) channels in these cells is far higher (∼100 times) than in smooth muscle cells. Thus, we examined purinergic receptor (P2Y) mediated SK channel activation as a mechanism for purinergic relaxation. P2Y receptors (mainly P2ry1 gene) were highly expressed in PDGFRα(+) cells. Under voltage clamp conditions, ATP activated large outward currents in PDGFRα(+) cells that were inhibited by blockers of SK channels. ATP also induced significant hyperpolarization under current clamp conditions. A P2Y1 agonist, MRS2365, mimicked the effects of ATP, and a P2Y1 antagonist, MRS2500, inhibited ATP-activated SK currents. Responses to ATP were largely abolished in PDGFRα(+) cells of P2ry1(-/-) mice, and no response was elicited by MRS2365 in these cells. A P2X receptor agonist had no effect on PDGFRα(+) cells but, like ATP, activated transient inward currents in smooth muscle cells (SMCs). A P2Y1 antagonist decreased nerve-evoked relaxation. These data suggest that purines activate SK currents via mainly P2Y1 receptors in PDGFRα(+) cells. Our findings provide an explanation for purinergic relaxation in detrusor muscles and show that there are no discrete inhibitory nerve fibres. A dual receptive field for purines provides the basis for inhibitory neural regulation of excitability.

Role of enteric neurotransmission in host defense and protection of the gastrointestinal tract

Host defense is a vital role played by the gastrointestinal tract. As host to an enormous and diverse microbiome, the gut has evolved an elaborate array of chemical and physicals barriers that allow the digestion and absorption of nutrients without compromising the mammalian host. The control of such barrier functions requires the integration of neural, humoral, paracrine and immune signaling, involving redundant and overlapping mechanisms to ensure, under most circumstances, the integrity of the gastrointestinal epithelial barrier. Here we focus on selected recent developments in the autonomic neural control of host defense functions used in the protection of the gut from luminal agents, and discuss how the microbiota may potentially play a role in enteric neurotransmission. Key recent findings include: the important role played by subepithelial enteric glia in modulating intestinal barrier function, identification of stress-induced mechanisms evoking barrier breakdown, neural regulation of epithelial cell proliferation, the role of afferent and efferent vagal pathways in regulating barrier function, direct evidence for bacterial communication to the enteric nervous system, and microbial sources of enteric neurotransmitters. We discuss these new and interesting developments in our understanding of the role of the autonomic nervous system in gastrointestinal host defense.