A novel intramuscular Interstitial Cell of Cajal is a candidate for generating pacemaker activity in the mouse internal anal sphincter

TMEM16A in smooth muscle cells acts as a pacemaker channel in the internal anal sphincter

Maintenance of fecal continence requires a continuous or basal tone of the internal anal sphincter (IAS). Paradoxically, the basal tone results largely from high-frequency rhythmic contractions of the IAS smooth muscle. However, the cellular and molecular mechanisms that initiate these contractions remain elusive. Here we show that the IAS contains multiple pacemakers. These pacemakers spontaneously generate propagating calcium waves that drive rhythmic contractions and establish the basal tone. These waves are myogenic and act independently of nerve, paracrine or autocrine signals. Using cell-specific gene knockout mice, we further found that TMEM16A Cl- channels in smooth muscle cells (but not in the interstitial cells of Cajal) are indispensable for pacemaking, rhythmic contractions, and basal tone. Our results identify TMEM16A in smooth muscle cells as a critical pacemaker channel that enables the IAS to contract rhythmically and continuously. This study provides cellular and molecular insights into fecal continence.

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.

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.

Ca2+ signalling in interstitial cells of Cajal contributes to generation and maintenance of tone in mouse and monkey lower esophageal sphincters

KEY POINTS

The lower esophageal sphincter (LES) generates contractile tone preventing reflux of gastric contents into the esophagus. LES smooth muscle cells (SMCs) display depolarized membrane potentials facilitating activation of L-type Ca²⁺ channels. Interstitial cells of Cajal (ICC) express Ca²⁺ -activated Cl^- channels encoded by Ano1 in mouse and monkey LES. Ca²⁺ signaling in ICC activates ANO1 currents in ICC. ICC displayed spontaneous Ca²⁺ transients in mice from multiple firing sites in each cell and no entrainment of Ca²⁺ firing between sites or between cells. Inhibition of ANO1 channels with a specific antagonist caused hyperpolarization of mouse LES and inhibition of tone in monkey and mouse LES muscles. Our data suggest a novel mechanism for LES tone in which Ca²⁺ transient activation of ANO1 channels in ICC generates depolarizing inward currents that conduct to SMCs to activate L-type Ca²⁺ currents, Ca²⁺ entry and contractile tone.

ABSTRACT

The lower esophageal sphincter (LES) generates tone and prevents reflux of gastric contents. LES smooth muscle cells (SMCs) are relatively depolarized, facilitating activation of Ca_v 1.2 channels to sustain contractile tone. We hypothesised that intramuscular interstitial cells of Cajal (ICC-IM), through activation of Ca²⁺ -activated-Cl^- channels (ANO1), set membrane potentials of SMCs favorable for activation of Ca_v 1.2 channels. In some gastrointestinal muscles, ANO1 channels in ICC-IM are activated by Ca²⁺ transients, but no studies have examined Ca²⁺ dynamics in ICC-IM within the LES. Immunohistochemistry and qPCR were used to determine expression of key proteins and genes in ICC-IM and SMCs. These studies revealed that Ano1 and its gene product, ANO1 are expressed in c-Kit⁺ cells (ICC-IM) in mouse and monkey LES clasp muscles. Ca²⁺ signaling was imaged in situ, using mice expressing GCaMP6f specifically in ICC (Kit-KI-GCaMP6f). ICC-IM exhibited spontaneous Ca²⁺ transients from multiple firing sites. Ca²⁺ transients were abolished by CPA or caffeine but were unaffected by tetracaine or nifedipine. Maintenance of Ca²⁺ transients depended on Ca²⁺ influx and store reloading, as Ca²⁺ transient frequency was reduced in Ca²⁺ free solution or by Orai antagonist. Spontaneous tone of LES muscles from mouse and monkey was reduced ∼80% either by Ani9, an ANO1 antagonist or by the Ca_v 1.2 channel antagonist nifedipine. Membrane hyperpolarisation occurred in the presence of Ani9. These data suggest that intracellular Ca²⁺ activates ANO1 channels in ICC-IM in the LES. Coupling of ICC-IM to SMCs drives depolarization, activation of Ca_v 1.2 channels, Ca²⁺ entry and contractile tone. Abstract figure legend Proposed mechanism for generation of contractile tone in the lower esophageal sphincter (LES). Interstitial cells of Cajal (ICC) in the LES generate spontaneous, stochastic Ca²⁺ transients via Ca²⁺ release from the endoplasmic reticulum (ER). The Ca²⁺ transients activate ANO1 Cl- channels causing Cl- efflux (inward current). ANO1 currents have a depolarizing effect on ICC (+++s inside membrane) and this conducts through gap junctions (GJ) to smooth muscle cells (SMCs). Input from thousands of ICC results in depolarized membrane potentials (-40 to -50 mV) which is within the window current range for L-type Ca²⁺ channels. Activation of these channels causes Ca²⁺ influx, activation of contractile elements (CE) and development of tonic contraction. This article is protected by copyright. All rights reserved.

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

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

Interstitial cells of Cajal and human colon motility in health and disease

Our understanding of human colonic motility, and autonomic reflexes that generate motor patterns, has increased markedly through high-resolution manometry. Details of the motor patterns are emerging related to frequency and propagation characteristics that allow linkage to interstitial cells of Cajal (ICC) networks. In studies on colonic motor dysfunction requiring surgery, ICC are almost always abnormal or significantly reduced. However, there are still gaps in our knowledge about the role of ICC in the control of colonic motility and there is little understanding of a mechanistic link between ICC abnormalities and colonic motor dysfunction. This review will outline the various ICC networks in the human colon and their proven and likely associations with the enteric and extrinsic autonomic nervous systems. Based on our extensive knowledge of the role of ICC in the control of gastrointestinal motility of animal models and the human stomach and small intestine, we propose how ICC networks are underlying the motor patterns of the human colon. The role of ICC will be reviewed in the autonomic neural reflexes that evoke essential motor patterns for transit and defecation. Mechanisms underlying ICC injury, maintenance, and repair will be discussed. Hypotheses are formulated as to how ICC dysfunction can lead to motor abnormalities in slow transit constipation, chronic idiopathic pseudo-obstruction, Hirschsprung's disease, fecal incontinence, diverticular disease, and inflammatory conditions. Recent studies on ICC repair after injury hold promise for future therapies.

Oscillating calcium signals in smooth muscle cells underlie the persistent basal tone of internal anal sphincter

A persistent basal tone in the internal anal sphincter (IAS) is essential for keeping the anal canal closed and fecal continence; its inhibition via the rectoanal inhibitory reflex (RAIR) is required for successful defecation. However, cellular signals underlying the IAS basal tone remain enigmatic. Here we report the origin and molecular mechanisms of calcium signals that control the IAS basal tone, using a combination approach including a novel IAS slice preparation that retains cell arrangement and architecture as in vivo, 2-photon imaging, and cell-specific gene-modified mice. We found that IAS smooth muscle cells generate two forms of contractions (i.e., phasic and sustained contraction) and Ca2+ signals (i.e., synchronized Ca2+ oscillations [SCaOs] and asynchronized Ca2+ oscillations [ACaOs]) that last for hours. RyRs, TMEM16A, L-type Ca2+ channels, and gap junctions are required for SCaOs, which account for phasic contraction and 75% of sustained contraction. Nevertheless, only RyRs are required for ACaOs, which contribute 25% of sustained contraction. Nitric oxide, the primary neurotransmitter mediating the RAIR, blocks both types of Ca2+ signals, leading to IAS's full relaxation. Our results show that the oscillating nature of Ca2+ signals generates and maintains the basal tone without causing cytotoxicity to IAS. Our study provides insight into fecal continence and normal defecation.

Bioengineering of the digestive tract: approaching the clinic

The field of regenerative medicine is developing technologies that, in the near future, will offer alternative approaches to either cure diseases affecting the gastrointestinal tract or slow their progression by leveraging the intrinsic ability of our tissues and organs to repair after damage. This article will succinctly illustrate the three technologies that are closer to clinical translation-namely, human intestinal organoids, sphincter bioengineering and decellularization, whereby the cellular compartment of a given segment of the digestive tract is removed to obtain a scaffold consisting of the extracellular matrix. The latter will be used as a template for the regeneration of a functional organ, whereby the newly generated cellular compartment will be obtained from the patient's own cells. Although clinical application of this technology is approaching, product development challenges are being tackled to warrant safety and efficacy.