Non-technical summary 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.Abstract Morphologists have described 'fibroblast-like cells' (FLCs) in smooth muscles. In the gastrointestinal tract, FLCs are distributed along processes of enteric motor neurons and between the circular and longitudinal muscle layers. They are close to nerve varicosities and make gap junctions with smooth muscle cells. They are labelled with antibodies for platelet derived growth factor receptor α (PDGFRα) and small conductance Ca 2+ -activated K + (SK3) channels. We used transgenic mice with constitutive expression of enhanced green fluorescent protein (eGFP) in PDGFRα + cells to isolate and study the function of PDGFRα + cells as possible mediators of purinergic neurotransmission. PDGFRα + cells expressed purine receptors (P2Y1) and SK3 channels abundantly. Under whole cell voltage clamp some PDGFRα + cells generated large amplitude spontaneous transient outward currents that were blocked by apamin (300 nM). Dialysis of cells with Ca 2+ (500 nM) activated large amplitude K + currents that were also blocked by apamin. Application of adenosine triphosphate (ATP), adenine diphosphate (ADP) or β-nicotinamide adenine dinucleotide (β-NAD) (1-1000 μM) activated large amplitude, apamin-sensitive K + currents in PDGFRα + cells that were blocked by the P2Y1 antagonist MRS2500 (1 μM). Responses to purines were not elicited in smooth muscle cells under equivalent conditions, and only very small outward currents were elicited under optimized conditions (e.g. permeabilized patches and high concentrations of ATP; 1 mM). These data show that PDGFRα + cells are a novel class of excitable cells with large current densities attributable to SK channels and the molecular and ionic apparatus to mediate enteric inhibitory responses to purines in GI muscles.This paper is dedicated to the memory of Professor Mollie Homan . Professor Holman was a pioneer in studies of the autonomic and enteric nervous systems and neural control of visceral smooth muscles. Her recordings were the first records of the postjunctional electr...

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 Ca2+-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.

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 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/Wv 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/Wv 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/Wv 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.

Sanders KM. A novel population of subepithelial platelet-derived growth factor receptor ␣-positive cells in the mouse and human colon.