Serotonergic Control of Gastrointestinal Development, Motility, and Inflammation.

Although it is most well-known for its roles in central nervous system (CNS) function, the vast majority of serotonin, or 5-hydroxytryptamine (5-HT), is produced in the gastrointestinal (GI) tract. 5-HT is synthesized mostly by enterochromaffin (EC) cells of the GI epithelium and, in small part, by neurons of the enteric nervous system (ENS). The GI tract contains an array of broadly distributed 5-HT receptors, which participate in functions such as motility, sensation, inflammation, and neurogenesis. The roles of 5-HT in these functions are reviewed, as well as its role in the pathophysiology of disorders of gut-brain interaction (DGBIs) and inflammatory bowel diseases (IBD). © 2023 American Physiological Society. Compr Physiol 13:4851-4868, 2023.

Phenylboronic Acid-Functionalized Polyplexes Tailored to Oral CRISPR Delivery.

Effective delivery of the CRISPR-Cas9 components is crucial to realizing the therapeutic potential. Although many delivery approaches have been developed for this application, oral delivery has not been explored due to the degradative nature of the gastrointestinal tract. For this issue, we developed a series of novel phenylboronic acid (PBA)-functionalized chitosan-polyethylenimine (CS-PEI) polymers for oral CRISPR delivery. PBA functionalization equipped the polyplex with higher stability, smooth transport across the mucus, and efficient endosomal escape and cytosolic unpackaging in the cells. From a library of 12 PBA-functionalized CS-PEI polyplexes, we identified a formulation that showed the most effective penetration in the intestinal mucosa after oral gavage to mice. The optimized formulation performed feasible CRISPR-mediated downregulation of the target protein and reduction in the downstream cholesterol. As the first oral CRISPR carrier, this study suggests the potential of addressing the needs of both local and systemic editing in a patient-compliant manner.

The tactile sensors of the gut.

In a recent study, Treichel, Finholm et al. showed that the mechanoreceptor Piezo2 enables enteroendocrine cells in the intestinal epithelium to sense luminal contents. Through neuroepithelial signaling, these cells modulate intestinal motility and transit of digestive products.

Pain in Inflammatory Bowel Disease: Optogenetic Strategies for Study of Neural-Epithelial Signaling.

Abdominal pain is common in patients with active inflammation of the colon but can persist even in its absence, suggesting other mechanisms of pain signaling. Recent findings suggest colon epithelial cells are direct regulators of pain-sensing neurons. Optogenetic activation of epithelial cells evoked nerve firing and pain-like behaviors. Inhibition of epithelial cells caused the opposite effect, reducing responses to colon distension and inflammatory hypersensitivity. Thus, epithelial cells alone can regulate the activation of pain circuits. Future goals are to define the anatomical and cellular mechanisms that underlie epithelial-neural pain signaling and how it is altered in response to colon inflammation.

Optogenetic activation of the distal colon epithelium engages enteric nervous system circuits to initiate motility patterns.

Digestive functions of the colon depend on sensory-motor reflexes in the enteric nervous system (ENS), initiated by intrinsic primary afferent neurons (IPANs). IPAN terminals project to the mucosal layer of the colon, allowing communication with epithelial cells comprising the colon lining. The chemical nature and functional significance of this epithelial-neural communication in regard to secretion and colon motility are of high interest. Colon epithelial cells can produce and release neuroactive substances such as ATP and 5-hydroxytryptamine (5-HT), which can activate receptors on adjacent nerve fibers, including IPAN subtypes. In this study, we examined if stimulation of epithelial cells alone is sufficient to activate neural circuits that control colon motility. Optogenetics and calcium imaging were used in ex vivo preparations of the mouse colon to selectively stimulate the colon epithelium, measure changes in motility, and record activity of neurons within the myenteric plexus. Light-mediated activation of epithelial cells lining the distal, but not proximal, colon caused local contractions and increased the rate of colonic migrating motor complexes. Epithelial-evoked local contractions in the distal colon were reduced by both ATP and 5-HT receptor antagonists. Our findings indicate that colon epithelial cells likely use purinergic and serotonergic signaling to initiate activity in myenteric neurons, produce local contractions, and facilitate large-scale coordination of ENS activity responsible for whole colon motility patterns.NEW & NOTEWORTHY Using an all-optical approach to measure real-time cell-to-cell communication responsible for colon functions, we show that selective optogenetic stimulation of distal colon epithelium produced activity in myenteric neurons, as measured with red genetically encoded calcium indicators. The epithelial-induced neural response led to local contractions, mediated by both purinergic and serotonergic signaling, and facilitated colonic motor complexes that propagate from proximal to distal colon.

Optogenetic inhibition of the colon epithelium reduces hypersensitivity in a mouse model of inflammatory bowel disease.

ABSTRACT: Visceral pain is a prevalent symptom of inflammatory bowel disease that can be difficult to treat. Pain and hypersensitivity are mediated by extrinsic primary afferent neurons (ExPANs) that innervate the colon. Recent studies indicate that the colon epithelium contributes to initiating ExPAN firing and nociceptive responses. Based on these findings, we hypothesized that the epithelium contributes to inflammation-induced hypersensitivity. A key prediction of this hypothesis is that inhibition of the epithelium would attenuate nociceptive signaling and inflammatory hypersensitivity. To test this hypothesis, the inhibitory yellow light-activated protein archaerhodopsin was targeted to the intestinal epithelium (villin-Arch) or the ExPANs (TRPV1-Arch) that innervate the colon. Visceral sensitivity was assessed by measuring the visceromotor response (VMR) to colorectal distension (CRD), with and without yellow light illumination of the colon lumen. Inhibition of the colon epithelium in healthy villin-Arch mice significantly diminished the CRD-induced VMR. Direct inhibition of ExPANs during CRD using TRPV1-Arch mice showed that ExPAN and epithelial inhibition were similarly effective in reducing the VMR to CRD. We then investigated the effect of epithelial and ExPAN inhibition in the dextran sulfate sodium model of inflammatory bowel disease. Inhibition of the colon epithelium significantly decreased dextran sulfate sodium-induced hypersensitivity and was comparable with the inhibition of ExPANs. Together, these results reveal the potential of targeting the colon epithelium for the treatment of pain.

Sympathetic Input to Multiple Cell Types in Mouse and Human Colon Produces Region-Specific Responses.

BACKGROUND & AIMS: The colon is innervated by intrinsic and extrinsic neurons that coordinate functions necessary for digestive health. Sympathetic input suppresses colon motility by acting on intrinsic myenteric neurons, but the extent of sympathetic-induced changes on large-scale network activity in myenteric circuits has not been determined. Compounding the complexity of sympathetic function, there is evidence that sympathetic transmitters can regulate activity in non-neuronal cells (such as enteric glia and innate immune cells). METHODS: We performed anatomical tracing, immunohistochemistry, optogenetic (GCaMP calcium imaging, channelrhodopsin), and colon motility studies in mice and single-cell RNA sequencing in human colon to investigate how sympathetic postganglionic neurons modulate colon function. RESULTS: Individual neurons in each sympathetic prevertebral ganglion innervated the proximal or distal colon, with processes closely opposed to multiple cell types. Calcium imaging in semi-intact mouse colon preparations revealed changes in spontaneous and evoked neural activity, as well as activation of non-neuronal cells, induced by sympathetic nerve stimulation. The overall pattern of response to sympathetic stimulation was unique to the proximal or distal colon. Region-specific changes in cellular activity correlated with motility patterns produced by electrical and optogenetic stimulation of sympathetic pathways. Pharmacology experiments (mouse) and RNA sequencing (human) indicated that appropriate receptors were expressed on different cell types to account for the responses to sympathetic stimulation. Regional differences in expression of α-1 adrenoceptors in human colon emphasize the translational relevance of our mouse findings. CONCLUSIONS: Sympathetic neurons differentially regulate activity of neurons and non-neuronal cells in proximal and distal colon to promote distinct changes in motility patterns, likely reflecting the distinct roles played by these 2 regions.

Epithelial-Neuronal Communication in the Colon: Implications for Visceral Pain.

Visceral hypersensitivity and pain result, at least in part, from increased excitability of primary afferents that innervate the colon. In addition to intrinsic changes in these neurons, emerging evidence indicates that changes in lining epithelial cells may also contribute to increased excitability. Here we review recent studies on how colon epithelial cells communicate directly with colon afferents. Specifically, anatomical studies revealed specialized synaptic connections between epithelial cells and nerve fibers and studies using optogenetic activation of the epithelium showed initiation of pain-like responses. We review the possible mechanisms of epithelial-neuronal communication and provide an overview of the possible neurotransmitters and receptors involved. Understanding the biology of this interface and how it changes in pathological conditions may provide new treatments for visceral pain conditions.

A multiplexed in vitro assay system for evaluating human skeletal muscle functionality in response to drug treatment.

In vitro systems that mimic organ functionality have become increasingly important tools in drug development studies. Systems that measure the functional properties of skeletal muscle are beneficial to compound screening studies and also for integration into multiorgan devices. To date, no studies have investigated human skeletal muscle responses to drug treatments at the single myotube level in vitro. This report details a microscale cantilever chip-based assay system for culturing individual human myotubes. The cantilevers, along with a laser and photo-detector system, enable measurement of myotube contractions in response to broad-field electrical stimulation. This system was used to obtain baseline functional parameters for untreated human myotubes, including peak contractile force and time-to-fatigue data. The cultured myotubes were then treated with known myotoxic compounds and the resulting functional changes were compared to baseline measurements as well as known physiological responses in vivo. The collected data demonstrate the system's capacity for screening direct effects of compound action on individual human skeletal myotubes in a reliable, reproducible, and noninvasive manner. Furthermore, it has the potential to be utilized for high-content screening, disease modeling, and exercise studies of human skeletal muscle performance utilizing iPSCs derived from specific patient populations such as the muscular dystrophies.

Extrinsic Primary Afferent Neurons Link Visceral Pain to Colon Motility Through a Spinal Reflex in Mice.

BACKGROUND & AIMS: Proper colon function requires signals from extrinsic primary afferent neurons (ExPANs) located in spinal ganglia. Most ExPANs express the vanilloid receptor TRPV1, and a dense plexus of TRPV1-positive fibers is found around myenteric neurons. Capsaicin, a TRPV1 agonist, can initiate activity in myenteric neurons and produce muscle contraction. ExPANs might therefore form motility-regulating synapses onto myenteric neurons. ExPANs mediate visceral pain, and myenteric neurons mediate colon motility, so we investigated communication between ExPANs and myenteric neurons and the circuits by which ExPANs modulate colon function. METHODS: In live mice and colon tissues that express a transgene encoding the calcium indicator GCaMP, we visualized levels of activity in myenteric neurons during smooth muscle contractions induced by application of capsaicin, direct colon stimulation, stimulation of ExPANs, or stimulation of preganglionic parasympathetic neuron (PPN) axons. To localize central targets of ExPANs, we optogenetically activated TRPV1-expressing ExPANs in live mice and then quantified Fos immunoreactivity to identify activated spinal neurons. RESULTS: Focal electrical stimulation of mouse colon produced phased-locked calcium signals in myenteric neurons and produced colon contractions. Stimulation of the L6 ventral root, which contains PPN axons, also produced myenteric activation and contractions that were comparable to those of direct colon stimulation. Surprisingly, capsaicin application to the isolated L6 dorsal root ganglia, which produced robust calcium signals in neurons throughout the ganglion, did not activate myenteric neurons. Electrical activation of the ganglia, which activated even more neurons than capsaicin, did not produce myenteric activation or contractions unless the spinal cord was intact, indicating that a complete afferent-to-efferent (PPN) circuit was necessary for ExPANs to regulate myenteric neurons. In TRPV1-channel rhodopsin-2 mice, light activation of ExPANs induced a pain-like visceromotor response and expression of Fos in spinal PPN neurons. CONCLUSIONS: In mice, ExPANs regulate myenteric neuron activity and smooth muscle contraction via a parasympathetic spinal circuit, linking sensation and pain to motility.

Optogenetic Activation of Colon Epithelium of the Mouse Produces High-Frequency Bursting in Extrinsic Colon Afferents and Engages Visceromotor Responses.

Epithelial cells of the colon provide a vital interface between the internal environment (lumen of the colon) and colon parenchyma. To examine epithelial-neuronal signaling at this interface, we analyzed mice in which channelrhodopsin (ChR2) was targeted to either TRPV1-positive afferents or to villin-expressing colon epithelial cells. Expression of a ChR2-EYFP fusion protein was directed to either primary sensory neurons or to colon epithelial cells by crossing Ai32 mice with TRPV1-Cre or villin-Cre mice, respectively. An ex vivo preparation of the colon was used for single-fiber analysis of colon sensory afferents of the pelvic nerve. Afferents were characterized using previously described criteria as mucosal, muscular, muscular-mucosal, or serosal and then tested for blue light-induced activation. Light activation of colon epithelial cells produced robust firing of action potentials, similar to that elicited by physiologic stimulation (e.g., circumferential stretch), in 50.5% of colon afferents of mice homozygous for ChR2 expression. Light-induced activity could be reduced or abolished in most fibers using a cocktail of purinergic receptor blockers suggesting ATP release by the epithelium contributed to generation of sensory neuron action potentials. Using electromyographic recording of visceromotor responses we found that light stimulation of the colon epithelium evoked behavioral responses in Vil-ChR2 mice that was similar to that seen with balloon distension of the colon. These ex vivo and in vivo data indicate that light stimulation of colon epithelial cells alone, without added mechanical or chemical stimuli, can directly activate colon afferents and elicit behavioral responses.SIGNIFICANCE STATEMENT Abdominal pain that accompanies inflammatory diseases of the bowel is particularly vexing because it can occur without obvious changes in the structure or inflammatory condition of the colon. Pain reflects abnormal sensory neuron activity that may be controlled in part by release of substances from lining epithelial cells. In support of this mechanism we determined that blue-light stimulation of channelrhodopsin-expressing colon epithelial cells could evoke action potential firing in sensory neurons and produce changes in measures of behavioral sensitivity. Thus, activity of colon epithelial cells alone, without added mechanical or chemical stimuli, is sufficient to activate pain-sensing neurons.