Are gap junctions truly involved in inhibitory neuromuscular interaction in mouse proximal colon?

Expression of connexin 26 and connexin 43 is reduced in Hirschsprung's disease

BACKGROUND

Despite having an optimal pull-through operation, some children with Hirschsprung's disease (HSCR) continue to experience persistent bowel symptoms. Coordinated colonic electrical activity depends on intercellular communication between the enteric nerves, interstitial cells of Cajal (ICCs), smooth muscle cells, and fibroblast-like (platelet-derived growth factor receptor-alpha-positive) cells. Gap junctions are low-resistance channels composed of connexin (Cx) proteins which couple cells electrically and chemically. We aimed to evaluate the expression patterns of Cx26, Cx36, and Cx43 in the colon of children with HSCR and in healthy control colon.

MATERIALS AND METHODS

Entire resected colonic specimens were collected from children undergoing pull-through surgery for HSCR (n = 10). Colonic controls were collected at colostomy closure in children with imperforate anus (n = 8). Distribution of Cx26, Cx36, and Cx43 was assessed using double-labeled immunofluorescence and confocal microscopy. Protein expression was quantified using Western blot analysis.

RESULTS

Sparse punctate Cx36 expression was seen in the myenteric plexus in nerve trunks and some platelet-derived growth factor receptor-alpha-positive cell and ICC fibers. Cx26 was similarly distributed, although it was not coexpressed in ICCs. Cx43 was only coexpressed with ICCs in the myenteric plexus. Expression of Cx26 and Cx43 was markedly reduced in the aganglionic colon in HSCR compared to controls, while Cx26 expression was also moderately reduced in the ganglionic bowel in HSCR.

CONCLUSIONS

Reduced expression of Cx26 and Cx43 is implicated in the pathophysiology of colonic dysmotility in the aganglionic bowel as well as, in the case of Cx26, the ganglionic bowel in HSCR.

Roles of connexins and pannexins in digestive homeostasis

Connexin proteins are abundantly present in the digestive system. They primarily form gap junctions, which control the intercellular exchange of critical homeostasis regulators. By doing so, gap junctions drive a plethora of gastrointestinal and hepatic functional features, including gastric and gut motility, gastric acid secretion, intestinal innate immune defense, xenobiotic biotransformation, glycogenolysis, bile secretion, ammonia detoxification and plasma protein synthesis. In the last decade, it has become clear that connexin hemichannels, which are the structural precursors of gap junctions, also provide a pathway for cellular communication, namely between the cytosol and the extracellular environment. Although merely pathological functions have been described, some physiological roles have been attributed to connexin hemichannels, in particular in the modulation of colonic motility. This equally holds true for cellular channels composed of pannexins, connexin-like proteins recently identified in the intestine and the liver, which have become acknowledged key players in inflammatory processes and that have been proposed to control colonic motility, secretion and blood flow.

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.

Loss of responsiveness of circular smooth muscle cells from the guinea pig ileum is associated with changes in gap junction coupling

Gap junction coupling and neuromuscular transmission to smooth muscle were studied in the first 4 h after preparations were set up in vitro. Intracellular recordings were made from smooth muscle cells of guinea pig ileum. Fast inhibitory junction potentials (IJPs) were small (1.3 ± 1.0 mV) in the first 30 min but increased significantly over the first 120 min to 15.8 ± 0.9 mV (n = 12, P < 0.001). Comparable increases in slow IJPs and excitatory junction potentials were also observed. During the same period, resting membrane potential depolarized from -58.8 ± 1.4 to -47.2 ± 0.4 mV (n = 12, P < 0.001). Input resistance, estimated by intracellular current injection, decreased in parallel (P < 0.05), and dye coupling, measured by intracellular injection of carboxyfluorescein, increased (P < 0.001). Input resistance was higher and dye coupling was less in longitudinal than circular smooth muscle cells. Gap junction blockers [carbenoxolone (100 μM), 18β-glycyrrhetinic acid (10 μM), and 2-aminoethoxydiphenyl borate (50 μM)] hyperpolarized coupled circular smooth muscle cells, reduced the amplitude of fast and slow IJPs and excitatory junction potentials, increased input resistance, and reduced dye coupling. Local application of ATP (10 mM) mimicked IJPs and showed comparable increases in amplitude over the first 120 min; carbenoxolone and 2-aminoethoxydiphenyl borate significantly reduced ATP-evoked hyperpolarizations in coupled cells. In contrast, synaptic transmission between myenteric neurons was not suppressed during the first 30 min. Gap junction coupling between circular smooth muscle cells in isolated preparations was initially disrupted but recovered over the next 120 min to a steady level. This was associated with potent effects on neuromuscular transmission and responses to exogenous ATP.

Mounting evidence against the role of ICC in neurotransmission to smooth muscle in the gut

How nerves transmit their signals to regulate activity of smooth muscle is of fundamental importance to autonomic and enteric physiology, clinical medicine, and therapeutics. A traditional view of neurotransmission to smooth muscles has been that motor nerve varicosities release neurotransmitters that act on receptors on smooth muscles to cause their contraction or relaxation via electromechanical and pharmacomechanical signaling pathways in the smooth muscle. In recent years, an old hypothesis that certain interstitial cells of Cajal (ICC) may transduce neural signals to smooth muscle cells has been resurrected. This later hypothesis is based on indirect evidence of closer proximity and presence of synapses between the nerve varicosities and ICC, gap junctions between ICC and smooth muscles, and presence of receptors and signaling pathways for the neurotransmitters and ICC. This indirect evidence is at best circumstantial. The direct evidence is based on the reports of loss of neurotransmission in mutant animals lacking ICC due to c-Kit receptor deficiency. However, a critical analysis of the recent data show that animals lacking ICC have normal cholinergic and purinergic neurotransmission and tachykinergic neurotransmission is actually increased. The status of nitrergic neurotransmission in c-Kit deficient animals has been controversial. However, reports suggest that 1) nitrergic neurotransmission in the internal anal sphincter does not require ICC and 2) the in vivo phenotype of ICC deficiency does not resemble nNOS deficiency. 3) The most recent report, in this issue of the Journal, concludes that impaired nitrergic neurotransmission may be due to smooth muscle defects associated with c-Kit receptor deficiency.

Deficiency of intramuscular ICC increases fundic muscle excitability but does not impede nitrergic innervation

The motility of the gastrointestinal tract is generated by smooth muscle cells and is controlled to a large extent by an intrinsic neural network. A gap of approximately 200 nm usually separates nerve varicosities from smooth muscle cells, which suggests that direct innervation of the smooth muscle by synapses does not occur. Enteric nerves do make synapse-like contact with proposed regulatory cells, the interstitial cells of Cajal (ICC), which in turn may be in gap junction contact with smooth muscle cells. The role played by ICC in enteric innervation is controversial. Experimental evidence has been presented in vitro for the hypothesis that nitrergic inhibitory innervation is strongly reduced in the absence of ICC. However, in vivo data appear to dispute that. The present report provides evidence that explains the discrepancy between in vivo and in vitro data and provides evidence that inhibitory neurotransmitters can reach smooth muscle cells without hindrance when ICC are absent. The fundic musculature shows increased responses to substance P-mediated innervation and shows marked spontaneous activity, which is consistent with increased muscle excitability.

Do gap junctions play a role in nerve transmissions as well as pacing in mouse intestine?

Varicosities of nitrergic and other nerves end on deep muscular plexus interstitial cells of Cajal or on CD34-positive, c-kit-negative fibroblast-like cells. Both cell types connect to outer circular muscle by gap junctions, which may transmit nerve messages to muscle. We tested the hypotheses that gap junctions transmit pacing messages from interstitial cells of Cajal of the myenteric plexus. Effects of inhibitors of gap junction conductance were studied on paced contractions and nerve transmissions in small segments of circular muscle of mouse intestine. Using electrical field stimulation parameters (50 V/cm, 5 pps, and 0.5 ms) which evoke near maximal responses to nitrergic, cholinergic, and apamin-sensitive nerve stimulation, we isolated inhibitory responses to nitrergic nerves, inhibitory responses to apamin-sensitive nerves and excitatory responses to cholinergic nerves. 18beta-Glycyrrhetinic acid (10, 30, and 100 microM), octanol (0.1, 0.3, and 1 mM) and gap peptides (300 microM of (40)Gap27, (43)Gap26, (37,43)Gap27) all failed to abolish neurotransmission. 18beta-Glycyrrhetinic acid inhibited frequencies of paced contractions, likely owing to inhibition of l-type Ca(2+) channels in smooth muscle, but octanol or gap peptides did not. 18beta-Glycyrrhetinic acid and octanol, but not gap peptides, reduced the amplitudes of spontaneous and nerve-induced contractions. These reductions paralleled reductions in contractions to exogenous carbachol. Additional experiments with gap peptides in both longitudinal and circular muscle segments after N(G)-nitro-l-arginine and TTX revealed no effects on pacing frequencies. We conclude that gap junction coupling may not be necessary for pacing or nerve transmission to the circular muscle of the mouse intestine.