Decreased L-type calcium current in antral smooth muscle cells of STZ-induced diabetic rats

Direct Stimulation of Gastric Smooth Muscle Cells via Gq Proteins With Light

BACKGROUND

Optogenetics is a cutting-edge approach that can enable direct stimulation of gastric smooth muscle cells (SMC) by combining cell-specific overexpression of light-sensitive proteins with light stimulation. We previously demonstrated that direct optogenetic stimulation of gastric SMC via depolarization can restore contractility and food propulsion and could become a new treatment strategy for gastroparesis. The human receptor Neuropsin (hOPN5) enables activation of Gq signaling with UV light. Herein, we explore this new strategy for direct optogenetic stimulation of gastric SMC.

METHODS

We used a transgenic mouse model expressing hOPN5 in fusion with eYFP. Antral longitudinal smooth muscle strips were used for isometric force measurements and whole stomachs for intragastric pressure measurements, comparing light stimulation to other stimuli. Adeno-associated virus (AAV) serotypes were screened for efficiency in transducing cultured gastric SMC, and transduced cells were tested by Ca2+ imaging.

RESULTS

hOPN5 expression was restricted to and found in ~1/3 of SMC in the stomach. UV light induced isometric force and increased intragastric pressure only in transgenic mice similarly to electrical field stimulation and reached approximately 1/3 of the force induced by global depolarization and muscarinic receptor activation. Importantly, optical stimulation remained effective in an ex vivo gastroparesis model. AAV 2.5 was by far the most effective serotype for SMC transduction, and UV light triggered Ca2+ transients in SMC expressing hOPN5.

CONCLUSION

hOPN5 is a new and effective tool to directly stimulate gastric SMC to control contractility with light. Thus, it is an additional and complementary approach to light-induced membrane depolarization to restore gastric motility.

Critical Evaluation of Animal Models of Gastrointestinal Disorders

Preclinical research remains an important tool for discovery and validation of novel therapeutics for gastrointestinal disorders. While in vitro assays can be used to verify receptor-ligand interactions and test for structural activity of new compounds, only whole-animal studies can demonstrate drug efficacy within the gastrointestinal system. Most major gastrointestinal disorders have been modeled in animals; however the translational relevance of each model is not equal. The purpose of this chapter is to provide a critical evaluation of common animal models that are being used to develop pharmaceuticals for gastrointestinal disorders. For brevity, the models are presented for upper gastrointestinal disorders involving the esophagus, stomach, and small intestine and lower gastrointestinal disorders that focus on the colon. Particular emphasis is used to explain the face and construct validity of each model, and the limitations of each model, including data interpretation, are highlighted. This chapter does not evaluate models that rely on surgical or other non-pharmacological interventions for treatment.

Advanced glycation end products interfere with gastric smooth muscle contractile marker expression via the AGE/RAGE/NF-κB pathway

Excessive production of advanced glycation end products (AGE) has been implicated in the pathogenesis of diabetic complications. Smooth muscle (SM) phenotype transition is involved in diabetes-associated gastric motility dysfunction. We investigated whether AGE interfere with gastric antral SM contractile marker expression. Sixteen Sprague-Dawley rats were randomly divided into control and streptozotocin-induced diabetic groups. Sixteen weeks after streptozotocin administration, gastric antral SM strip contractility in the groups were measured. The gastric tissue expression of AGE was tested. Primary cultured gastric smooth muscle cells (SMCs) were used in complementary in vitro studies. In the presence and absence of AGE, SMCs were transfected with myocardin plasmid or treated with nuclear factor-κB (NF-κB) inhibitor or anti-RAGE antibody. Diabetic rats showed weakness of SM strip contractility and decreased expression of SM contractile marker genes (myosin heavy chains [MHC], α-actin, calponin) as compared with the control group. Gastric antral SM layer Nε-(carboxymethyl) lysine (CML) level, the major AGE compound, were increased in the diabetic rats. AGE downregulated SM contractile markers and myocardin expression in a concentration-dependent manner. Myocardin overexpression prevented these results. AGE treatment activated NF-κB in SMCs. The NF-κB inhibitor BAY 11-7082 and anti-RAGE antibody blocked the effects of AGE on myocardin downregulation. AGE may induce the development of gastric dysmotility by downregulating SM contractile proteins and myocardin expression via the AGE/RAGE/NF-κB pathway.

Disorders of gastrointestinal hypomotility

Ingestion and digestion of food as well as expulsion of residual material from our gastrointestinal tract requires normal propulsive, i.e. motor, function. Hypomotility refers to inherited or acquired changes that come with decreased contractile forces or slower transit. It not only often causes symptoms but also may compromise nutritional status or lead to other complications. While severe forms, such as pseudo-obstruction or ileus, may have a tremendous functional impact, the less severe forms of hypomotility may well be more relevant, as they contribute to common disorders, such as functional dyspepsia, gastroparesis, chronic constipation, and irritable bowel syndrome (IBS). Clinical testing can identify changes in contractile activity, defined by lower amplitudes or abnormal patterns, and the related effects on transit. However, such biomarkers show a limited correlation with overall symptom severity as experienced by patients. Similarly, targeting hypomotility with pharmacological interventions often alters gut motor function but does not consistently improve symptoms. Novel diagnostic approaches may change this apparent paradox and enable us to obtain more comprehensive information by integrating data on electrical activity, mechanical forces, patterns, wall stiffness, and motions with information of the flow of luminal contents. New drugs with more selective effects or more specific delivery may improve benefits and limit adverse effects. Lastly, the complex regulation of gastrointestinal motility involves the brain-gut axis as a reciprocal pathway for afferent and efferent signaling. Considering the role of visceral input in emotion and the effects of emotion on visceral activity, understanding and managing hypomotility disorders requires an integrative approach based on the mind-body continuum or biopsychosocial model of diseases.

Role of estrogen in diabetic gastroparesis

Gastroparesis is a highly prevalent chronic disorder of the stomach, which is characterized by delayed gastric emptying and accompanied by a series of upper gastrointestinal symptoms. Diabetic gastroparesis (DGP) is one of the severe complications of diabetes, seriously affecting the patient's quality of life. At present, the pathogenesis of DGP is still unclear. The majority of DGP patients are women, and women's symptoms change with the fluctuation of the level of estrogen. Therefore, we speculate that estrogen may play an vital role in the stomach motility. Gastric emptying is an objective index for the diagnosis of gastroparesis. This article reviews the role of estrogen in DGP and the possible mechanisms.

Gastric emptying and glycaemia in health and diabetes mellitus

The rate of gastric emptying is a critical determinant of postprandial glycaemia and, accordingly, is fundamental to maintaining blood glucose homeostasis. Disordered gastric emptying occurs frequently in patients with longstanding type 1 diabetes mellitus and type 2 diabetes mellitus (T2DM). A complex bidirectional relationship exists between gastric emptying and glycaemia--gastric emptying accounts for ∼35% of the variance in peak postprandial blood glucose concentrations in healthy individuals and in patients with diabetes mellitus, and the rate of emptying is itself modulated by acute changes in glycaemia. Clinical implementation of incretin-based therapies for the management of T2DM, which diminish postprandial glycaemia, in part by slowing gastric emptying, is widespread. Other therapies for patients with T2DM, which specifically target gastric emptying include pramlintide and dietary-based treatment approaches. A weak association exists between upper gastrointestinal symptoms and the rate of gastric emptying. In patients with severe diabetic gastroparesis, pathological changes are highly variable and are characterized by loss of interstitial cells of Cajal and an immune infiltrate. Management options for patients with symptomatic gastroparesis remain limited in their efficacy, which probably reflects the heterogeneous nature of the underlying pathophysiology.