Contribution of solvent drag through intercellular junctions to absorption of nutrients by the small intestine of the rat

Role of Stress on Driving the Intestinal Paracellular Permeability

The gut epithelium is a polarized monolayer that exhibits apical and basolateral membrane surfaces. Monolayer cell components are joined side by side via protein complexes known as tight junction proteins (TJPs), expressed at the most apical extreme of the basolateral membrane. The gut epithelium is a physical barrier that determinates intestinal permeability, referred to as the measurement of the transit of molecules from the intestinal lumen to the bloodstream or, conversely, from the blood to the gut lumen. TJPs play a role in the control of intestinal permeability that can be disrupted by stress through signal pathways triggered by the ligation of receptors with stress hormones like glucocorticoids. Preclinical studies conducted under in vitro and/or in vivo conditions have addressed underlying mechanisms that account for the impact of stress on gut permeability. These mechanisms may provide insights for novel therapeutic interventions in diseases in which stress is a risk factor, like irritable bowel syndrome. The focus of this study was to review, in an integrative context, the neuroendocrine effects of stress, with special emphasis on TJPs along with intestinal permeability.

Oleuropein Promotes Neural Plasticity and Neuroprotection via PPARα-Dependent and Independent Pathways

Oleuropein (OLE), a main constituent of olives, displays a pleiotropic beneficial dynamic in health and disease; the effects are based mainly on its antioxidant and hypolipidemic properties, and its capacity to protect the myocardium during ischemia. Furthermore, OLE activates the peroxisome proliferator-activated receptor (PPARα) in neurons and astrocytes, providing neuroprotection against noxious biological reactions that are induced following cerebral ischemia. The current study investigated the effect of OLE in the regulation of various neural plasticity indices, emphasizing the role of PPARα. For this purpose, 129/Sv wild-type (WT) and Pparα-null mice were treated with OLE for three weeks. The findings revealed that chronic treatment with OLE up-regulated the brain-derived neurotrophic factor (BDNF) and its receptor TrkB in the prefrontal cortex (PFC) of mice via activation of the ERK1/2, AKT and PKA/CREB signaling pathways. No similar effects were observed in the hippocampus. The OLE-induced effects on BDNF and TrkB appear to be mediated by PPARα, because no similar alterations were observed in the PFC of Pparα-null mice. Notably, OLE did not affect the neurotrophic factors NT3 and NT4/5 in both brain tissues. However, fenofibrate, a selective PPARα agonist, up-regulated BDNF and NT3 in the PFC of mice, whereas the drug induced NT4/5 in both brain sites tested. Interestingly, OLE provided neuroprotection in differentiated human SH-SY5Y cells against β-amyloid and H2O2 toxicity independently from PPARα activation. In conclusion, OLE and similar drugs, acting either as PPARα agonists or via PPARα independent mechanisms, could improve synaptic function/plasticity mainly in the PFC and to a lesser extent in the hippocampus, thus beneficially affecting cognitive functions.

Regulatory mechanisms of glucose absorption in the mouse proximal small intestine during fasting and feeding

Fasting is known to alter the function of various organs and the mechanisms of glucose metabolism, which affect health outcomes and slow aging. However, it remains unclear how fasting and feeding affects glucose absorption function in the small intestine. We studied the effects of the fasting and feeding on glucose-induced short-circuit current (I_sc) in vitro using an Ussing chamber technique. Glucose-induced I_sc by SGLT1 was observed in the ileum, but little or no I_sc was observed in the jejunum in ad libitum-fed mice. However, in mice fasted for 24-48 h, in addition to the ileum, robust glucose-induced I_sc was observed over time in the jejunum. The expression of SGLT1 in the brush border membranes was significantly decreased in the jejunum under fed conditions compared to 48 h fasting, as analyzed by western blotting. Additionally, when mice were fed a 60% high glucose diet for 3 days, the increase in glucose-induced I_sc was observed only in the ileum, and totally suppressed in the jejunum. An increase in Na⁺ permeability between epithelial cells was concomitantly observed in the jejunum of fasted mice. Transepithelial glucose flux was assessed using a non-metabolizable glucose analog, ¹⁴C-methyl α-D-glucopyranoside glucose (MGP). Regardless of whether fed or fasted, no glucose diffusion mechanism was observed. Fasting increased the SGLT1-mediated MGP flux in the jejunum. In conclusion, segment-dependent up- and down-regulation mechanisms during fasting and feeding are important for efficient glucose absorption once the fast is broken. Additionally, these mechanisms may play a crucial role in the small intestine's ability to autoregulate glucose absorption, preventing acute hyperglycemia when large amounts of glucose are ingested.

Review on the effect of chemotherapy on the intestinal barrier: Epithelial permeability, mucus and bacterial translocation

Chemotherapy kills fast-growing cells including gut stem cells. This affects all components of the physical and functional intestinal barrier, i.e., the mucus layer, epithelium, and immune system. This results in an altered intestinal permeability of toxic compounds (e.g., endotoxins) as well as luminal bacterial translocation into the mucosa and central circulation. However, there is uncertainty regarding the relative contributions of the different barrier components for the development of chemotherapy-induced gut toxicity. This review present an overview of the intestinal mucosal barrier determined with various types of molecular probes and methods, and how they are affected by chemotherapy based on reported rodent and human data. We conclude that there is overwhelming evidence that chemotherapy increases bacterial translocation, and that it affects the mucosal barrier by rendering the mucosa more permeable to large permeability probes. Chemotherapy also seems to impede the intestinal mucus barrier, even though this has been less clearly evaluated from a functional standpoint but certainly plays a role in bacteria translocation. Combined, it is however difficult to outline a clear temporal or succession between the different gastrointestinal events and barrier functions, especially as chemotherapy-induced neutropenia is also involved in intestinal immunological homeostasis and bacterial translocation. A thorough characterization of this would need to include a time dependent development of neutropenia, intestinal permeability, and bacterial translocation, ideally after a range of chemotherapeutics and dosing regimens.

Paracellular permeability and tight junction regulation in gut health and disease

Epithelial tight junctions define the paracellular permeability of the intestinal barrier. Molecules can cross the tight junctions via two distinct size-selective and charge-selective paracellular pathways: the pore pathway and the leak pathway. These can be distinguished by their selectivities and differential regulation by immune cells. However, permeability increases measured in most studies are secondary to epithelial damage, which allows non-selective flux via the unrestricted pathway. Restoration of increased unrestricted pathway permeability requires mucosal healing. By contrast, tight junction barrier loss can be reversed by targeted interventions. Specific approaches are needed to restore pore pathway or leak pathway permeability increases. Recent studies have used preclinical disease models to demonstrate the potential of pore pathway or leak pathway barrier restoration in disease. In this Review, we focus on the two paracellular flux pathways that are dependent on the tight junction. We discuss the latest evidence that highlights tight junction components, structures and regulatory mechanisms, their impact on gut health and disease, and opportunities for therapeutic intervention.

Tight junction proteins occludin and ZO-1 as regulators of epithelial proliferation and survival

Epithelial cells are the first line of mucosal defense. In the intestine, a single layer of epithelial cells must establish a selectively permeable barrier that supports nutrient absorption and waste secretion while preventing the leakage of potentially harmful luminal materials. Key to this is the tight junction, which seals the paracellular space and prevents unrestricted leakage. The tight junction is a protein complex established by interactions between members of the claudin, zonula occludens, and tight junction-associated MARVEL protein (TAMP) families. Claudins form the characteristic tight junction strands seen by freeze-fracture microscopy and create paracellular channels, but the functions of ZO-1 and occludin, founding members of the zonula occludens and TAMP families, respectively, are less well defined. Recent studies have revealed that these proteins have essential noncanonical (nonbarrier) functions that allow them to regulate epithelial apoptosis and proliferation, facilitate viral entry, and organize specialized epithelial structures. Surprisingly, neither is required for intestinal barrier function or overall health in the absence of exogenous stressors. Here, we provide a brief overview of ZO-1 and occludin canonical (barrier-related) functions, and a more detailed examination of their noncanonical functions.

Local Mucosal CO2 but Not O2 Insufflation Improves Gastric and Oral Microcirculatory Oxygenation in a Canine Model of Mild Hemorrhagic Shock

Introduction

Acute hemorrhage results in perfusion deficit and regional hypoxia. Since failure of intestinal integrity seem to be the linking element between hemorrhage, delayed multi organ failure, and mortality, it is crucial to maintain intestinal microcirculation in acute hemorrhage. During critical bleeding physicians increase FiO₂ to raise total blood oxygen content. Likewise, a systemic hypercapnia was reported to maintain microvascular oxygenation (μHbO₂). Both, O₂ and CO₂, may have adverse effects when applied systemically that might be prevented by local application. Therefore, we investigated the effects of local hyperoxia and hypercapnia on the gastric and oral microcirculation.

Methods

Six female foxhounds were anaesthetized, randomized into eight groups and tested in a cross-over design. The dogs received a local CO₂-, O₂-, or N₂-administration to their oral and gastric mucosa. Hemorrhagic shock was induced through a withdrawal of 20% of estimated blood volume followed by retransfusion 60 min later. In control groups no shock was induced. Reflectance spectrophotometry and laser Doppler were performed at the gastric and oral surface. Oral microcirculation was visualized by incident dark field imaging. Systemic hemodynamic parameters were recorded continuously. Statistics were performed using a two-way-ANOVA for repeated measurements and post hoc analysis was conducted by Bonferroni testing (p < 0.05).

Results

The gastric μHbO₂ decreased from 76 ± 3% to 38 ± 4% during hemorrhage in normocapnic animals. Local hypercapnia ameliorated the decrease of μHbO₂ from 78 ± 4% to 51 ± 8%. Similarly, the oral μHbO₂ decreased from 81 ± 1% to 36 ± 4% under hemorrhagic conditions and was diminished by local hypercapnia (54 ± 4%). The oral microvascular flow quality but not the total microvascular blood flow was significantly improved by local hypercapnia. Local O₂-application failed to change microvascular oxygenation, perfusion or flow quality. Neither CO₂ nor O₂ changed microcirculatory parameters and macrocirculatory hemodynamics under physiological conditions.

Discussion

Local hypercapnia improved microvascular oxygenation and was associated with a continuous blood flow in hypercapnic individuals undergoing hemorrhagic shock. Local O₂ application did not change microvascular oxygenation, perfusion and blood flow profiles in hemorrhage. Local gas application and change of microcirculation has no side effects on macrocirculatory parameters.

Establishment of Intestinal Epithelial Cell Monolayers and Their Use in Calcium Switch Assay for Assessment of Intestinal Tight Junction Assembly

Intestinal barrier function relies primarily on the assembly and integrity of tight junctions, which forms a size-selective barrier. This barrier restricts paracellular movement of solutes in various types of epithelia. Of note, extracellular Ca²⁺ concentration affects tight junction assembly. Therefore, the removal and re-addition of Ca²⁺ into cell culture medium of cultured intestinal epithelial cells causes destabilization and reassembly of tight junction to membrane periphery near apical surface, respectively. Based on this principle, the Ca²⁺-switch assay was established to investigate tight junction assembly in fully differentiated intestinal epithelial cells. This chapter provides a stepwise protocol for culture of intestinal epithelial cell monolayers using T84 cell line as an in vitro model and the Ca²⁺-switch assay for evaluating tight junction assembly.

Computational Modelling of Glucose Uptake by SGLT1 and Apical GLUT2 in the Enterocyte

It has been suggested that glucose absorption in the small intestine depends on both constitutively expressed SGLT1 and translocated GLUT2 in the brush border membrane, especially in the presence of high levels of luminal glucose. Here, we present a computational model of non-isotonic glucose uptake by small intestinal epithelial cells. The model incorporates apical uptake via SGLT1 and GLUT2, basolateral efflux into the blood via GLUT2, and cellular volume changes in response to non-isotonic conditions. The dependence of glucose absorption on luminal glucose, blood flow rate, and inlet blood glucose concentration is studied. Uptake via apical GLUT2 is found to be sensitive to all these factors. Under a range of conditions, the maximum apical GLUT2 flux is about half of the SGLT1 flux and is achieved at high luminal glucose (> 50 mM), high blood flow rates, and low inlet blood concentrations. In contrast, SGLT1 flux is less sensitive to these factors. When luminal glucose concentration is less than 10 mM, apical GLUT2 serves as an efflux pathway for glucose to move from the blood to the lumen. The model results indicate that translocation of GLUT2 from the basolateral to the apical membrane increases glucose uptake into the cell; however, the reduction of efflux capacity results in a decrease in net absorption. Recruitment of GLUT2 from a cytosolic pool elicits a 10–20% increase in absorption for luminal glucose levels in the a 20–100 mM range. Increased SGLT1 activity also leads to a roughly 20% increase in absorption. A concomitant increase in blood supply results in a larger increase in absorption. Increases in apical glucose transporter activity help to minimise cell volume changes by reducing the osmotic gradient between the cell and the lumen.

The Hydrating Effects of Hypertonic, Isotonic and Hypotonic Sports Drinks and Waters on Central Hydration During Continuous Exercise: A Systematic Meta-Analysis and Perspective

Background

Body-fluid loss during prolonged continuous exercise can impair cardiovascular function, harming performance. Delta percent plasma volume (dPV) represents the change in central and circulatory body-water volume and therefore hydration during exercise; however, the effect of carbohydrate–electrolyte drinks and water on the dPV response is unclear.

Objective

To determine by meta-analysis the effects of ingested hypertonic (> 300 mOsmol kg⁻¹), isotonic (275–300 mOsmol kg⁻¹) and hypotonic (< 275 mOsmol kg⁻¹) drinks containing carbohydrate and electrolyte ([Na⁺] < 50 mmol L⁻¹), and non-carbohydrate drinks/water (< 40 mOsmol kg⁻¹) on dPV during continuous exercise.

Methods

A systematic review produced 28 qualifying studies and 68 drink treatment effects. Random-effects meta-analyses with repeated measures provided estimates of effects and probability of superiority (p₊) during 0–180 min of exercise, adjusted for drink osmolality, ingestion rate, metabolic rate and a weakly informative Bayesian prior.

Results

Mean drink effects on dPV were: hypertonic − 7.4% [90% compatibility limits (CL) − 8.5, − 6.3], isotonic − 8.7% (90% CL − 10.1, − 7.4), hypotonic − 6.3% (90% CL − 7.4, − 5.3) and water − 7.5% (90% CL − 8.5, − 6.4). Posterior contrast estimates relative to the smallest important effect (dPV = 0.75%) were: hypertonic-isotonic 1.2% (90% CL − 0.1, 2.6; p₊ = 0.74), hypotonic-isotonic 2.3% (90% CL 1.1, 3.5; p₊ = 0.984), water-isotonic 1.3% (90% CL 0.0, 2.5; p₊ = 0.76), hypotonic-hypertonic 1.1% (90% CL 0.1, 2.1; p₊ = 0.71), hypertonic-water 0.1% (90% CL − 0.8, 1.0; p₊ = 0.12) and hypotonic-water 1.1% (90% CL 0.1, 2.0; p₊ = 0.72). Thus, hypotonic drinks were very likely superior to isotonic and likely superior to hypertonic and water. Metabolic rate, ingestion rate, carbohydrate characteristics and electrolyte concentration were generally substantial modifiers of dPV.

Conclusion

Hypotonic carbohydrate–electrolyte drinks ingested continuously during exercise provide the greatest benefit to hydration.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s40279-021-01558-y.

Higher doses of ascorbic acid may have the potential to promote nutrient delivery via intestinal paracellular absorption

The significance of plasma ascorbic acid (AA) is underscored by its enzymatic and antioxidant properties as well as involvement in many aspects of health including the synthesis of biomolecules during acute illness, trauma and chronic health conditions. Dietary intake supports maintenance of optimal levels with supplementation at higher doses more likely pursued. Transient increased intestinal paracellular permeability following high dose AA may be utilised to enhance delivery of other micronutrients across the intestinal lumen. The potential mechanism following dietary intake however needs further study but may provide an avenue to increase small intestinal nutrient co transport and absorption, including in acute and chronic illness.

Oleuropein-Induced Acceleration of Cytochrome P450-Catalyzed Drug Metabolism: Central Role for Nuclear Receptor Peroxisome Proliferator-Activated Receptor α

Oleuropein (OLE), the main constituent of Olea europaea, displays pleiotropic beneficial effects in health and disease, which are mainly attributed to its anti-inflammatory and cardioprotective properties. Several food supplements and herbal medicines contain OLE and are available without a prescription. This study investigated the effects of OLE on the main cytochrome P450s (P450s) catalyzing the metabolism of many prescribed drugs. Emphasis was given to the role of peroxisome proliferator-activated receptor α (PPARα), a nuclear transcription factor regulating numerous genes including P450s. 129/Sv wild-type and Ppara-null mice were treated with OLE for 6 weeks. OLE induced Cyp1a1, Cyp1a2, Cyp1b1, Cyp3a14, Cyp3a25, Cyp2c29, Cyp2c44, Cyp2d22, and Cyp2e1 mRNAs in liver of wild-type mice, whereas no similar effects were observed in Ppara-null mice, indicating that the OLE-induced effect on these P450s is mediated by PPARα. Activation of the pathways related to phosphoinositide 3-kinase/protein kinase B (AKT)/forkhead box protein O1, c-Jun N-terminal kinase, AKT/p70, and extracellular signal-regulated kinase participates in P450 induction by OLE. These data indicate that consumption of herbal medicines and food supplements containing OLE could accelerate the metabolism of drug substrates of the above-mentioned P450s, thus reducing their efficacy and the outcome of pharmacotherapy. Therefore, OLE-induced activation of PPARα could modify the effects of drugs due to their increased metabolism and clearance, which should be taken into account when consuming OLE-containing products with certain drugs, in particular those of narrow therapeutic window. SIGNIFICANCE STATEMENT: This study indicated that oleuropein, which belongs to the main constituents of the leaves and olive drupes of Olea europaea, induces the synthesis of the major cytochrome P450s (P450s) metabolizing the majority of prescribed drugs via activation of peroxisome proliferator-activated receptor α. This effect could modify the pharmacokinetic profile of co-administered drug substrates of the P450s, thus altering their therapeutic efficacy and toxicity.

Drug Screening, Oral Bioavailability and Regulatory Aspects: A Need for Human Organoids

The intestinal epithelium critically contributes to oral bioavailability of drugs by constituting an important site for drug absorption and metabolism. In particular, intestinal epithelial cells (IEC) actively serve as gatekeepers of drug and nutrient availability. IECs' transport processes and metabolism are interrelated to the whole-body metabolic state and represent potential points of origin as well as therapeutic targets for a variety of diseases. Human intestinal organoids represent a superior model of the intestinal epithelium, overcoming limitations of currently used in vitro models. Caco-2 cells or rodent explant models face drawbacks such as their cancer and non-human origin, respectively, but are commonly used to study intestinal nutrient absorption, enterocyte metabolism and oral drug bioavailability, despite poorly correlative data. In contrast, intestinal organoids allow investigating distinct aspects of bioavailability including spatial resolution of transport, inter-individual differences and high-throughput screenings. As several countries have already developed strategic roadmaps to phase out animal experiments for regulatory purposes, intestinal organoid culture and organ-on-a-chip technology in combination with in silico approaches are roads to go in the preclinical and regulatory setup and will aid implementing the 3Rs (reduction, refinement and replacement) principle in basic science.

Intestinal epithelial tight junctions and permeability can be rescued through the regulation of endoplasmic reticulum stress by amniotic fluid stem cells during necrotizing enterocolitis

Necrotizing enterocolitis (NEC) is one of the most severe gastrointestinal diseases affecting premature infants. It has been shown that NEC is associated with disrupted intestinal barrier and dysregulated endoplasmic reticulum (ER)-stress response. It has also been shown that stem cells derived from amniotic fluid (AFSC) rescued intestinal injury in experimental NEC. Herein, we hypothesized that the beneficial effects of AFSC in the injured intestine are due to the restoration of intestinal barrier function. We evaluated intestinal barrier function using an ex vivo intestinal organoid model of NEC. We found that AFSC restored the expression and localization of tight junction proteins in intestinal organoids, and subsequently decreased epithelial permeability. AFSC rescued tight junction expression by inducing a protective ER stress response that prevents epithelial cell apoptosis in injured intestinal organoids. Finally, we validated these results in our experimental mouse model of NEC and confirmed that AFSC induced sustained ER stress and prevented intestinal apoptosis. This response led to the restoration of tight junction expression and localization, which subsequently reduced intestinal permeability in NEC pups. These findings confirm that intestinal barrier function is disrupted during NEC intestinal injury, and further demonstrate the disruption can be reversed by the administration of AFSC through the activation of the ER stress pathway. This study provides insight into the pathogenesis of NEC and highlights potential therapeutic targets for the treatment of NEC.

Implication of actin in the uptake of sucrose and valine in the tap root and leaf of sugar beet

Actin microfilaments (F-actin) are major components of the cytoskeleton essential for many cellular dynamic processes (vesicle trafficking, cytoplasmic streaming, organelle movements). The aim of this study was to examine whether cortical actin microfilaments might be implicated in the regulation of nutrient uptake in root and leaf cells of Beta vulgaris. Using antibodies raised against actin and the AtSUC1 sucrose transporter, immunochemical assays demonstrated that the expression of actin and a sucrose transporter showed different characteristics, when detected on plasma membrane vesicles (PMVs) purified from roots and from leaves. The in situ immunolabeling of actin and AtSUC1 sites in PMVs and tissues showed their close proximity to the plasma membrane. Using co-labeling in protoplasts, actin and sucrose transporters were localized along the internal border and in the outermost part of the plasma membrane, respectively. This respective membrane co-localization was confirmed on PMVs and in tissues using transmission electronic microscopy. The possible functional role of actin in sucrose uptake (and valine uptake, comparatively) by PMVs and tissues from roots and leaves was examined using the pharmacological inhibitors, cytochalasin B (CB), cytochalasin D (CD), and phalloidin (PH). CB and CD inhibited the sucrose and valine uptake by root tissues in a concentration-dependent manner above 1 μM, whereas PH had no such effect. Comparatively, the toxins inhibited the sucrose and valine uptake in leaf discs to a lesser extent. The inhibition was not due to a hindering of the proton pumping and H⁺ -ATPase catalytic activity determined in PMVs incubated in presence of these toxins.

Digestibility and metabolism of copper in diets for pigs and influence of dietary copper on growth performance, intestinal health, and overall immune status: a review

The current contribution reviews absorption and metabolism of copper (Cu), Cu deficiency, Cu toxicity, Cu bioavailability, and effects of pharmacological levels of Cu on growth performance and intestinal health of pigs. Copper is a micro mineral involved in metabolic reactions including cellular respiration, tissue pigmentation, hemoglobin formation, and connective tissue development. Copper is mostly absorbed in the upper gastrointestinal tract, particularly in the duodenum, but some Cu is absorbed in the stomach. One way to evaluate the efficacy of sources of Cu is to measure relative bioavailability where responses include tissue concentrations of Cu, concentrations of metalloproteins, and enzymatic activity of animals fed test diets containing graded levels of Cu. The requirement for Cu by pigs is 5 to 10 mg/kg diet, however, Cu can be included at growth-promoting levels (i.e., 75 to 250 mg/kg diet) in diets for weanling and growing pigs to reduce post-weaning diarrhea and improve growth performance. The consistently observed improvement in growth performance upon Cu supplementation is likely a result of increases in lipase activity, growth hormone secretion, and expression of genes involved in post-absorptive metabolism of lipids. The growth-promoting effects of dietary Cu have also been attributed to its bacteriostatic and bactericidal properties because Cu may change bacterial populations in the intestine, and thereby reduce inflammation caused by pathogens. However, further research is needed to determine potential interactions between Cu and non-nutritive feed additives (e.g., enzymes, probiotics, phytobiotics), and the optimum quantity of Cu as well as the optimum duration of feeding supplemental Cu in diets for pigs should be further investigated. These gaps needs to be addressed to maximize inclusion of Cu in diets to improve growth performance while minimizing diseases and mortality.

Evaluation of the digestion and transport profiles and potential immunocompetence of puerarin and its acylated derivatives

Acylation has become one of the most widely used methods to improve the lipid solubility and bioavailability of flavonoids. In this study, puerarin acid esters (PAES) with different chain lengths were synthesized via biocatalytic acylation. This was the first study to evaluate the digestion and transport profiles and immunocompetence of PAES. The relationship between the digestion and transport profiles and potential immunocompetence of the acylated derivatives in Caco-2 cell monolayers was also explored. Puerarin and PAES remained stable in gastric phases, whereas different degrees of hydrolysis of PAES were found in the intestine. PAES with less than 12 carbon chains were positively correlated with the degree of hydrolysis, while those with more than 12 carbon chains showed higher resistance to hydrolysis by the artificial human digestive juice. The apparent permeability coefficients of puerarin, puerarin acetate, puerarin propanoate, puerarin butyrate, puerarin hexanoate, puerarin octanate and puerarin laurate were 1.62 ± 0.09, 1.70 ± 0.15, 1.89 ± 0.19, 1.86 ± 0.18, 2.29 ± 0.12, 4.06 ± 1.01 and 2.32 ± 0.88 × 10-6 cm s-1, respectively, in Caco-2 cell monolayers. The results of the immune factor assays indicated that puerarin propanoate, puerarin hexanoate and puerarin myristate could significantly promote the secretion of IL-6, TNF-α and IL-10. These findings suggested that a better absorption could be predicted after oral intake using PAES. Meanwhile, the concentration of esters and their metabolites (puerarin) found in the digestion and transport profiles directly affected their potential immunocompetence.

Impaired Intestinal Barrier and Tissue Bacteria: Pathomechanisms for Metabolic Diseases

An intact intestinal barrier, representing the interface between inner and outer environments, is an integral regulator of health. Among several factors, bacteria and their products have been evidenced to contribute to gut barrier impairment and its increased permeability. Alterations of tight junction integrity - caused by both external factors and host metabolic state - are important for gut barrier, since they can lead to increased influx of bacteria or bacterial components (endotoxin, bacterial DNA, metabolites) into the host circulation. Increased systemic levels of bacterial endotoxins and DNA have been associated with an impaired metabolic host status, manifested in obesity, insulin resistance, and associated cardiovascular complications. Bacterial components and cells are distributed to peripheral tissues via the blood stream, possibly contributing to metabolic diseases by increasing chronic pro-inflammatory signals at both tissue and systemic levels. This response is, along with other yet unknown mechanisms, mediated by toll like receptor (TLR) transduction and increased expression of pro-inflammatory cytokines, which in turn can further increase intestinal permeability leading to a detrimental positive feedback loop. The modulation of gut barrier function through nutritional and other interventions, including manipulation of gut microbiota, may represent a potential prevention and treatment target for metabolic diseases.

An Update on Pharmaceutical Strategies for Oral Delivery of Therapeutic Peptides and Proteins in Adults and Pediatrics

While each route of therapeutic drug delivery has its own advantages and limitations, oral delivery is often favored because it offers convenient painless administration, sustained delivery, prolonged shelf life, and often lower manufacturing cost. Its limitations include mucus and epithelial cell barriers in the gastrointestinal (GI) tract that can block access of larger molecules including Therapeutic protein or peptide-based drugs (TPPs), resulting in reduced bioavailability. This review describes these barriers and discusses different strategies used to modify TPPs to enhance their oral bioavailability and/or to increase their absorption. Some seek to stabilize the TTPs to prevent their degradation by proteolytic enzymes in the GI tract by administering them together with protease inhibitors, while others modify TPPs with mucoadhesive polymers like polyethylene glycol (PEG) to allow them to interact with the mucus layer, thereby delaying their clearance. The further barrier provided by the epithelial cell membrane can be overcome by the addition of a cell-penetrating peptide (CPP) and the use of a carrier molecule such as a liposome, microsphere, or nanosphere to transport the TPP-CPP chimera. Enteric coatings have also been used to help TPPs reach the small intestine. Key efficacious TPP formulations that have been approved for clinical use will be discussed.

Impact of the Escherichia coli Heat-Stable Enterotoxin b (STb) on Gut Health and Function

Enterotoxigenic Escherichia coli (ETEC) produces the heat-stable enterotoxin b (STb), which is responsible for secretory diarrhea in humans and animals. This toxin is secreted within the intestinal lumen of animals and humans following ETEC colonization, becoming active on enterocytes and altering fluid homeostasis. Several studies have outlined the nature of this toxin and its effects on gut health and the integrity of the intestinal epithelium. This review summarizes the mechanisms of how STb alters the gastrointestinal tract. These include the manipulation of mucosal tight junction protein integrity, the formation of enterocyte cellular pores and toxin internalization and the stimulation of programmed cell death. We conclude with insights into the potential link between STb intoxication and altered gut hormone regulation, and downstream physiology.

The ontogeny of Na+ balance during rapid smoltification in pink salmon (Oncorhynchus gorbuscha)

Pink salmon hatch in fresh water, but their highly anadromous life history requires them to migrate into the ocean immediately after gravel-emergence, at a very small size. During their down-river migration these larvae undergo rapid smoltification that completely remodels their osmoregulatory physiology. At this time, the larvae reportedly have high whole-body Na⁺ contents and we hypothesised that the active accumulation of internal Na⁺ occurs in preparation for ocean entry. Using a comparative approach, the present study characterised the ontogeny of Na⁺ regulation in larvae of the anadromous pink salmon and the fresh-water rainbow trout. Our results indicate that larvae from both species actively accumulated Na⁺; however, whole-body Na⁺ content was higher in rainbow trout larvae compared to pink salmon. The time-course of this response was similar in the two species, with highest Na⁺-uptake rates ([Formula: see text]) shortly after yolk sac absorption, but the mechanism of Na⁺ accumulation differed between the species. Rainbow trout larvae greatly increased [Formula: see text] to overcompensate for a large simultaneous increase in Na⁺-efflux rate ([Formula: see text]), whereas pink salmon mounted a smaller increase in [Formula: see text] while maintaining tight control over [Formula: see text], which is supported by a significantly lower paracellular permeability. Our results indicate that the transient accumulation of internal Na⁺ is not a unique feature of the highly anadromous life history in pink salmon and may be a common ontogenetic pattern during larval development in salmonids; and perhaps it is associated with the development of the cardiovascular system during the larvae's transition to a more active lifestyle.

Contributions of Myosin Light Chain Kinase to Regulation of Epithelial Paracellular Permeability and Mucosal Homeostasis

Intestinal barrier function is required for the maintenance of mucosal homeostasis. Barrier dysfunction is thought to promote progression of both intestinal and systemic diseases. In many cases, this barrier loss reflects increased permeability of the paracellular tight junction as a consequence of myosin light chain kinase (MLCK) activation and myosin II regulatory light chain (MLC) phosphorylation. Although some details about MLCK activation remain to be defined, it is clear that this triggers perijunctional actomyosin ring (PAMR) contraction that leads to molecular reorganization of tight junction structure and composition, including occludin endocytosis. In disease states, this process can be triggered by pro-inflammatory cytokines including tumor necrosis factor-α (TNF), interleukin-1β (IL-1β), and several related molecules. Of these, TNF has been studied in the greatest detail and is known to activate long MLCK transcription, expression, enzymatic activity, and recruitment to the PAMR. Unfortunately, toxicities associated with inhibition of MLCK expression or enzymatic activity make these unsuitable as therapeutic targets. Recent work has, however, identified a small molecule that prevents MLCK1 recruitment to the PAMR without inhibiting enzymatic function. This small molecule, termed Divertin, restores barrier function after TNF-induced barrier loss and prevents disease progression in experimental chronic inflammatory bowel disease.

Na+-Coupled Nutrient Cotransport Induced Luminal Negative Potential and Claudin-15 Play an Important Role in Paracellular Na+ Recycling in Mouse Small Intestine

Many nutrients are absorbed via Na⁺ cotransport systems, and therefore it is predicted that nutrient absorption mechanisms require a large amount of luminal Na⁺. It is thought that Na⁺ diffuses back into the lumen via paracellular pathways to support Na⁺ cotransport absorption. However, direct experimental evidence in support of this mechanism has not been shown. To elucidate this, we took advantage of claudin-15 deficient (cldn15^-/-) mice, which have been shown to have decreased paracellular Na⁺ permeability. We measured glucose-induced currents (ΔI_sc) under open- and short-circuit conditions and simultaneously measured changes in unidirectional ²²Na⁺ fluxes (ΔJ^Na) in Ussing chambers. Under short-circuit conditions, application of glucose resulted in an increase in ΔI_sc and unidirectional mucosal to serosal ²²Na⁺ (∆J^Na_MS) flux in both wild-type and cldn15^-/- mice. However, under open-circuit conditions, ΔI_sc was observed but ∆J^Na_MS was strongly inhibited in wild-type but not in cldn15^-/- mice. In addition, in the duodenum of mice treated with cholera toxin, paracellular Na⁺ conductance was decreased and glucose-induced ∆J^Na_MS increment was observed under open-circuit conditions. We concluded that the Na⁺ which is absorbed by Na⁺-dependent glucose cotransport is recycled back into the lumen via paracellular Na⁺ conductance through claudin-15, which is driven by Na⁺ cotransport induced luminal negativity.

Exogenous vasopressin dose-dependently modulates gastric microcirculatory oxygenation in dogs via V1A receptor

BACKGROUND

Hypercapnia improves gastric microcirculatory oxygenation (μHbO₂) and increases vasopressin plasma levels, whereas V1A receptor blockade abolishes the increase of μHbO₂. The aim of this study was to evaluate the effect of exogenous vasopressin (AVP) in increasing doses on microcirculatory perfusion and oxygenation and systemic hemodynamic variables. Furthermore, we evaluated the role of the vasopressin V1A receptor in mediating the effects.

METHODS

In repetitive experiments, six anesthetized dogs received a selective vasopressin V1A receptor inhibitor ([Pmp¹, Tyr (Me)²]-Arg⁸-Vasopressin) or sodium chloride (control groups). Thereafter, a continuous infusion of AVP was started with dose escalation every 30 min (0.001 ng/kg/min-1 ng/kg/min). Microcirculatory variables of the oral and gastric mucosa were measured with reflectance spectrometry, laser Doppler flowmetry, and incident dark field imaging. Transpulmonary thermodilution was used to measure systemic hemodynamic variables. AVP plasma concentrations were measured during baseline conditions and 30 min after each dose escalation.

RESULTS

During control conditions, gastric μHbO₂ did not change during the course of experiments. Infusion of 0.001 ng/kg/min and 0.01 ng/kg/min AVP increased gastric μHbO₂ to 87 ± 4% and 87 ± 6%, respectively, compared to baseline values (80 ± 7%), whereas application of 1 ng/kg/min AVP strongly reduced gastric μHbO₂ (59 ± 16%). V1A receptor blockade prior to AVP treatment abolished these effects on μHbO₂. AVP dose-dependently enhanced systemic vascular resistance (SVR) and decreased cardiac output (CO). After prior V1A receptor blockade, SVR was reduced and CO increased (0.1 ng/kg/min + 1 ng/kg/min AVP).

CONCLUSIONS

Exogenous AVP dose-dependently modulates gastric μHbO₂, with an increased μHbO₂ with ultra-low dose AVP. The effects of AVP on μHbO₂ are abolished by V1A receptor inhibition. These effects are independent of a modulation of systemic hemodynamic variables.

Difference in glucose tolerance between phytophagous and insectivorous bats

Bats are mostly insectivorous or phytophagous. It is hypothesized that bats are evolved from small insectivorous mammals. Therefore, the digestive and metabolic systems of phytophagous and insectivorous bats must have evolved differently to adapt to their dietary habits. To investigate the difference in sugar tolerance in bats, we determined changes in blood glucose levels after intraperitoneal (i.p.) injection of glucose in three species of phytophagous and four species of insectivorous bats under resting conditions. Results showed that phytophagous bats eliminated blood glucose faster than insectivorous bats. All three species of fruit bats reduced blood glucose to fasting levels within 30-45 min, whereas all insectivorous bats failed to lower blood glucose to fasting levels even 120 min after i.p. glucose injection. Taken together, results of this study suggest that bats have undergone adaptations and become diversified in dietary habits.

The osmorespiratory compromise in the euryhaline killifish: water regulation during hypoxia

Freshwater- and seawater-acclimated Fundulus heteroclitus were exposed to acute hypoxia (10% air saturation, 3 h), followed by normoxic recovery (3 h). In both salinities, ventilation increased and heart rate fell in the classic manner, while Ṁ _O2  initially declined by ∼50%, with partial restoration by 3 h of hypoxia, and no O₂ debt repayment during recovery. Gill paracellular permeability (measured with [¹⁴C] PEG-4000) was 1.4-fold higher in seawater, and declined by 50% during hypoxia with post-exposure overshoot to 188%. A similar pattern with smaller changes occurred in freshwater. Drinking rate (also measured with [¹⁴C] PEG-4000) was 8-fold higher in seawater fish, but declined by ∼90% during hypoxia in both groups, with post-exposure overshoots to ∼270%. Gill diffusive water flux (measured with ³H₂O) was 1.9-fold higher in freshwater fish, and exhibited a ∼35% decrease during hypoxia, which persisted throughout recovery, but was unchanged during hypoxia in seawater fish. Nevertheless, freshwater killifish gained mass while seawater fish lost mass during hypoxia, and these changes were not corrected during normoxic recovery. We conclude that this hypoxia-tolerant teleost beneficially reduces gill water permeability in a salinity-dependent fashion during hypoxia, despite attempting to simultaneously improve Ṁ _O2 , but nevertheless incurs a net water balance penalty in both freshwater and seawater.

Tests of intestinal mucosal hyperpermeability: Many diseases, many biomarkers and a bright future

The number of disorders now linked to increased intestinal mucosal permeability implies that a substantial percent of the population is affected. Drug interventions targeting reduced tight junctional permeability are being pursued. Although hyper-permeability in itself is not a clinically recognized disease entity, its relationship to disease processes has driven interest in measuring, and even monitoring mucosal permeability in vivo. Along with improved knowledge of gut barrier physiology, advances have been made in tests and biomarkers of barrier function. Drawing from our experiences in the past decade, considerations and challenges faced in assessing in vivo intestinal permeability are discussed herein, along with indications of what the future might hold.

Morphological bases for intestinal paracellular absorption in bats and rodents

Flying mammals present unique intestinal adaptations, such as lower intestinal surface area than nonflying mammals, and they compensate for this with higher paracellular absorption of glucose. There is no consensus about the mechanistic bases for this physiological phenomenon. The surface area of the small intestine is a key determinant of the absorptive capacity by both the transcellular and the paracellular pathways; thus, information about intestinal surface area and micro-anatomical structure can help explain differences among species in absorptive capacity. In order to elucidate a possible mechanism for the high paracellular nutrient absorption in bats, we performed a comparative analysis of intestinal villi architecture and enterocyte size and number in microchiropterans and rodents. We collected data from intestines of six bat species and five rodent species using hematoxylin and eosin staining and histological measurements. For the analysis we added measurements from published studies employing similar methodology, making in total a comparison of nine species each of rodents and bats. Bats presented shorter intestines than rodents. After correction for body size differences, bats had ~41% less nominal surface area (NSA) than rodents. Villous enhancement of surface area (SEF) was ~64% greater in bats than in rodents, mainly because of longer villi and a greater density of villi in bat intestines. Both taxa exhibited similar enterocyte diameter. Bats exceeded rodents by ~103% in enterocyte density per cm² NSA, but they do not significantly differ in total number of enterocytes per whole animal. In addition, there is a correlation between SEF and clearance per cm² NSA of L-arabinose, a nonactively transported paracellular probe. We infer that an increased enterocyte density per cm² NSA corresponds to increased density of tight junctions per cm² NSA, which provides a partial mechanistic explanation for understanding the high paracellular absorption observed in bats compared to nonflying mammals.

Local gastric RAAS inhibition improves gastric microvascular perfusion in dogs

During circulatory shock, gastrointestinal microcirculation is impaired, especially via activation of the renin-angiotensin-aldosterone system. Therefore, inhibition of the renin-angiotensin-aldosterone system might be beneficial in maintaining splanchnic microcirculation. The aim of this study was to analyze whether locally applied losartan influences gastric mucosal perfusion (µflow, µvelo) and oxygenation (µHbO2) without systemic hemodynamic changes. In repetitive experiments six anesthetized dogs received 30 mg losartan topically on the oral and gastric mucosa during normovolemia and hemorrhage (-20% blood volume). Microcirculatory variables were measured with reflectance spectrometry, laser Doppler flowmetry and incident dark field imaging. Transpulmonary thermodilution and pulse contour analysis were used to measure systemic hemodynamic variables. Gastric barrier function was assessed via differential absorption of inert sugars. During normovolemia, losartan increased gastric µflow from 99 ± 6 aU to 147 ± 17 aU and µvelo from 17 ± 1 aU to 19 ± 1 aU. During hemorrhage, losartan did not improve µflow. µvelo decreased from 17 ± 1 aU to 14 ± 1 aU in the control group. Application of losartan did not significantly alter µvelo (16 ± 1 aU) compared to the control group and to baseline levels (17 ± 1 aU). No effects of topical losartan on macrohemodynamic variables or microcirculatory oxygenation were detected. Gastric microcirculatory perfusion is at least partly regulated by local angiotensin receptors. Topical application of losartan improves local perfusion via vasodilation without significant effects on systemic hemodynamics. During mild hemorrhage losartan had minor effects on regional perfusion, probably because of a pronounced upstream vasoconstriction.

Degree of SGLT1 phosphorylation is associated with but does not determine segment-specific glucose transport features in the porcine small intestines

Glucose‐induced electrogenic ion transport is higher in the porcine ileum compared with the jejunum despite equal apical abundance of SGLT1. The objective of this study was a detailed determination of SGLT1 and GLUT2 expressions at mRNA and protein levels along the porcine small intestinal axis. Phosphorylation of SGLT1 at serine 418 was assessed as a potential modulator of activity. Porcine intestinal tissues taken along the intestinal axis 1 h or 3 h after feeding were analyzed for relative mRNA (RT‐PCR) and protein levels (immunoblot) of SGLT1, pSGLT1, GLUT2, (p)AMPK, β₂‐receptor, and PKA substrates. Functional studies on electrogenic glucose transport were done (Ussing chambers: short circuit currents (I_sc)). Additionally, effects of epinephrine (Epi) administration on segment‐specific glucose transport and pSGLT1 content were examined. SGLT1 and GLUT2 expression was similar throughout the small intestines but lower in the duodenum and distal ileum. pSGLT1 abundance was significantly lower in the ileum compared with the jejunum associated with significantly higher glucose‐induced I_sc. SGLT1 phosphorylation was not inducible by Epi. Epi treatment decreased glucose‐induced I_sc and glucose flux rates in the jejunum but increased basal I_sc in the ileum. Epi‐induced PKA activation was detectable in jejunal tissue. These results may indicate that SGLT1 phosphorylation at Ser418 represents a structural change to compensate for certain conditions that may decrease glucose transport (unfavorable driving forces/changed apical membrane potential) rather than being the cause for the overall differences in glucose transport characteristics between the jejunum and ileum.

Effect of dietary additives on intestinal permeability in both Drosophila and a human cell co-culture

Increased intestinal barrier permeability has been correlated with aging and disease, including type 2 diabetes, Crohn's disease, celiac disease, multiple sclerosis and irritable bowel syndrome. The prevalence of these ailments has risen together with an increase in industrial food processing and food additive consumption. Additives, including sugar, metal oxide nanoparticles, surfactants and sodium chloride, have all been suggested to increase intestinal permeability. We used two complementary model systems to examine the effects of food additives on gut barrier function: a Drosophila in vivo model and an in vitro human cell co-culture model. Of the additives tested, intestinal permeability was increased most dramatically by high sugar. High sugar also increased feeding but reduced gut and overall animal size. We also examined how food additives affected the activity of a gut mucosal defense factor, intestinal alkaline phosphatase (IAP), which fluctuates with bacterial load and affects intestinal permeability. We found that high sugar reduced IAP activity in both models. Artificial manipulation of the microbiome influenced gut permeability in both models, revealing a complex relationship between the two. This study extends previous work in flies and humans showing that diet can play a role in the health of the gut barrier. Moreover, simple models can be used to study mechanisms underlying the effects of diet on gut permeability and function.This article has an associated First Person interview with the first author of the paper.

An iso-osmolar oral supplement increases natriuresis and does not increase stomal output in patients with an ileostomy: A randomised, double-blinded, active comparator, crossover intervention study

BACKGROUND

Patients with an ileostomy often experience fluid and electrolyte depletion because of gastrointestinal loss. This study aimed to compare how an iso-osmolar and a hyperosmolar oral supplement affect ileostomy output, urine production, and natriuresis as proxy measurements of water-electrolyte balance.

METHODS

In a randomised, double-blinded, active comparator, crossover intervention study, we included eight adult ileostomy patients who were independent of parenteral support. We investigated how an iso-osmolar (279 mOsm/kg) and a hyperosmolar (681 mOsm/kg) oral supplement affected ileostomy output mass, urine volume, and natriuresis. In addition to their habitual diet, each participant ingested 800 mL/day of either the iso-osmolar or hyperosmolar supplement in each of two study periods. Each period started with 24-hour baseline measurements, and the supplements were ingested during the following 48 h. All measurements were repeated in the last 24 h.

RESULTS

No statistically significant changes in ileostomy output were detected following the intake of either oral supplement (median (range) 67 (-728 to 290) g/day, p = 0.25) despite increased fluid intake. Compared with the hyperosmolar supplement, the iso-osmolar supplement induced a statistically significant increase in urine volume (470 (0-780) mL/day, p = 0.02) and natriuresis (36 (0-66) mmol/day, p = 0.02).

CONCLUSION

Intake of the two oral supplements did not affect ileostomy output during this short intervention. Natriuresis increased following intake of the iso-osmolar supplement compared to that after ingesting the hyperosmolar supplement, indicating that patients with an ileostomy may benefit from increasing their ingestion of iso-osmolar fluids. ClinicalTrials.gov identifier:NCT03348709.

Egg perivitelline fluid of the invasive snail Pomacea canaliculata affects mice gastrointestinal function and morphology

Background

Species beloging to the genus Pomacea (Ampullariidae), often referred as apple snails, are freshwater, amphibious snails native to South, Central and North America. Some species such as P. canaliculata have become a driver of ecosystem changes in wetlands and an important rice and taro pest after its introduction to Asia and other parts of the world. Females deposit colored egg clutches above the waterline, a reproductive strategy that exposes the eggs to harsh conditions and terrestrial predation. However, eggs have no reported predators in their native range, probably because of the acquisition of unparalleled biochemical defenses provided by a set of proteins (perivitellins) that nourish embryos and protect them from predators and abiotic factors. Notably, ingestion of egg perivitelline fluid (PVF) decreases rat growth rate and alters their gastrointestinal morphology. The aim of the study is to determine the effect of apple snail egg PVF on mice gut digestive activity, morphology and nutrient absorption.

Methods

Carbohydrate digestion by intestinal disaccharidases (sucrase-isomaltase and maltase-glucoamylase) was evaluated ex vivo in mice gavaged with 1 or 4 doses of PVF. Changes in gut morphological and absorptive surface were measured. In addition, alteration on nutrient absorption rates, transport pathways and intestinal permeability was evaluated by luminal perfusions of small intestine with radiolabeled L-proline (absorbed by paracellular and transcellular pathways) and L-arabinose (absorbed exclusively by paracellular pathway).

Results

Perivitelline fluid affected mice displayed significant morphological changes in the small intestine epithelium inducing the appearance of shorter and wider villi as well as fused villi. This resulted in a diminished absorptive surface, notably in the proximal portion. Likewise, the activity of disaccharidases diminished in the proximal portion of the intestine. Total absorption of L-proline increased in treated mice in a dose-dependent manner. There were no differences neither in the ratio of paracellular-to-transcellular absorption of L-proline nor in gut permeability as revealed by the clearance of L-arabinose.

Discussion

Oral administration of apple snail PVF to mice adversely alters gut morphophysiology by reducing the intestinal absorptive surface, affecting enzymes of sugar metabolism and increasing the absorption rate of nutrients without affecting the relative contribution of the absorption pathways or gut permeability. These results further support the role of PVF in passive anti-predator defenses in Pomacea snail eggs that target the digestive system.

Integrative physiology of transcellular and paracellular intestinal absorption

Glucose absorption by the small intestine has been studied for nearly a century. Despite extensive knowledge about the identity, functioning and regulation of the relevant transporters, there has been and there remains controversy about how these transporters work in concert to determine the overall epithelial absorption of key nutrients (e.g. sugars, amino acids) over a wide range of dietary and/or luminal concentrations. Our broader, integrative understanding of intestinal absorption requires more than the reductionist dissection of all the components and their elaboration at molecular and genetic levels. This Commentary emphasizes the integration of discrete molecular players and processes (including paracellular absorption) that, in combination, determine the overall epithelial absorption of key nutrients (e.g. sugars, amino acids) and putative anti-nutrients (water-soluble toxins), and the integration of that absorption with other downstream processes related to metabolic demands. It identifies historic key advances, controversies and future research ideas, as well as important perspectives that arise through comparative as well as biomedical physiological research.

An intestinal paracellular pathway biased toward positively-charged macromolecules

Lacking an effective mechanism to safely and consistently enhance macromolecular uptake across the intestinal epithelium, prospects for successful development of oral therapeutic peptide drugs remain unlikely. We previously addressed this challenge by identifying an endogenous mechanism that controls intestinal paracellular permeability that can be activated by a peptide, termed PIP 640, which can increase cellular levels of phosphorylated myosin light chain at position S¹⁹ (MLC-pS¹⁹). Apical application in vitro or luminal application in vivo was shown to increase macromolecular solute transport within minutes that recovered completely within a few hours after removal. We now examine the nature of PIP 640-mediated permeability changes. Confluent Caco-2 cell monolayers treated with PIP 640 enhanced apical-to-basolateral (AB) transport of 4-kDa, but not 10-kDa, dextran. Expression and/or cellular distribution changes of tight junction (TJ) proteins were restricted to increased claudin-2 over a time course that correlated with an apparent shift in its distribution from the nucleus to the membrane fraction of the cell. PIP 640-mediated epithelial changes were distinct from the combined actions of the pro-inflammatory cytokines tumor necrosis factor alpha (TNF-α) and interferon gamma (IFN-γ). While TNF-α/IFN-γ treatment also increased MLC-pS¹⁹ levels, these cytokines enhanced AB transport for 70-kDa dextran and decreased occludin expression at TJs. Claudin-2-dependent changes induced by PIP 640 resulted in an AB transport bias for positively-charged macromolecules demonstrated in vitro using charge variants of 4-kDa dextrans and by comparing transport of salmon calcitonin to exenatide. Comparable outcomes of increased TJ localization of claudin-2 and enhanced transport of these therapeutic peptides that biased toward cationic characteristics was demonstrated in vivo following after intra-luminal injection into rat jejunum. Together, these data have shown a potential mechanism for PIP 640 to enhance paracellular permeability of solutes in the size range of small therapeutic peptides that is biased toward positively-charged solutes.

Small intestinal epithelial permeability to water-soluble nutrients higher in passerine birds than in rodents

In the small intestine transcellular and paracellular pathways are implicated in water-soluble nutrient absorption. In small birds the paracellular pathway is quantitatively important while transcellular pathway is much more important in terrestrial mammals. However, there is not a clear understanding of the mechanistic underpinnings of the differences among taxa. This study was aimed to test the hypothesis that paracellular permeability in perfused intestinal segments is higher in passerine birds than rodents. We performed in situ intestinal perfusions on individuals of three species of passerine birds (Passer domesticus, Taeniopygia guttata and Furnarius rufus) and two species of rodents (Mus musculus and Meriones ungiculatus). Using radio-labelled molecules, we measured the uptake of two nutrients absorbed by paracellular and transcellular pathways (L-proline and 3-O-methyl-D-glucose) and one carbohydrate that has no mediated transport (L-arabinose). Birds exhibited ~2 to ~3 times higher L-arabinose clearance per cm² epithelium than rodents. Moreover, paracellular absorption accounted for proportionally more of 3-O-methyl-D-glucose and L-proline absorption in birds than in rodents. These differences could be explained by differences in intestinal permeability and not by other factors such as increased retention time or higher intestinal nominal surface area. Furthermore, analysis of our results and all other existing data on birds, bats and rodents shows that insectivorous species (one bird, two bats and a rodent) had only 30% of the clearance of L-arabinose of non-insectivorous species. This result may be explained by weaker natural selection for high paracellular permeability in animal- than in plant-consumers. Animal-consumers absorb less sugar and more amino acids, whose smaller molecular size allow them to traverse the paracellular pathway more extensively and faster than glucose.

Enhancing Oral Bioavailability of Cyclic RGD Hexa-peptides by the Lipophilic Prodrug Charge Masking Approach: Redirection of Peptide Intestinal Permeability from a Paracellular to Transcellular Pathway

Hydrophilic peptides constitute most of the active peptides. They mostly permeate via tight junctions (paracellular pathway) in the intestine. This permeability mechanism restricts the magnitude of their oral absorption and bioavailability. We hypothesized that concealing the hydrophilic residues of the peptide using the lipophilic prodrug charge masking approach (LPCM) can improve the bioavailability of hydrophilic peptides. To test this hypothesis, a cyclic N-methylated hexapeptide containing Arg-Gly-Asp (RGD) and its prodrug derivatives, masking the Arg and Asp charged side chains, were synthesized. The library was evaluated for intestinal permeability in vitro using the Caco-2 model. Further investigation of metabolic stability ex vivo models in rat plasma, brush border membrane vesicles (BBMVs), and isolated CYP3A4 microsomes and pharmacokinetic studies was performed on a selected peptide and its prodrug (peptide 12). The parent drug analogues were found to have a low permeability rate in vitro, corresponding to atenolol, a marker for paracellular permeability. Moreover, palmitoyl carnitine increased the P_app of peptide 12 by 4-fold, indicating paracellular permeability. The P_app of the prodrug derivatives was much higher than that of their parent peptides. For instance, the P_app of the prodrug 12P was 20-fold higher than the P_app of peptide 12 in the apical to basolateral (AB) direction. Whereas the permeability in the opposite direction (BA of the Caco-2 model) was significantly faster than the P_app AB, indicating the involvement of an efflux system. These results were corroborated when verapamil, a P-gp inhibitor, was added to the Caco-2 model and increased the P_app AB of prodrug 12P by 3-fold. The prodrug 12P was stable in the BBMVs environment, yet degraded quickly (less than 5 min) in the plasma into the parent peptide 12. Pharmacokinetic studies in rats showed an increase in the bioavailability of peptide 12 > 70-fold (from 0.58 ± 0.11% to 43.8 ± 14.9%) after applying the LPCM method to peptide 12 and converting it to the prodrug 12P. To conclude, the LPCM approach converted the absorption mechanism of the polar peptides from a paracellular to transcellular pathway that tremendously affects their oral bioavailability. The LPCM method provides a solution for the poor bioavailability of RGD cyclohexapeptides and paves the way for other active hydrophilic and charged peptides with poor oral bioavailability.

In vitro effect of uremic serum on barrier function and inflammation in human colonocytes

Introduction:

In chronic kidney disease (CKD), it has been suggested that alterations within the gut are associated with an inflammatory state and uremic toxicity. Studies suggest that uremia may impair the function of the intestinal barrier via the promotion of increased intestinal permeability. To understand the mechanisms that are involved in intestinal barrier damage in the setting of uremia, we evaluated the in vitro effect of uremic serum on transepithelial electrical resistance (TER), inflammation, and apoptosis in intestinal epithelial cells (T84).

Methods:

Pools of serum from healthy individuals, patients not on dialysis, and patients on hemodialysis (Pre-HD and Post-HD) were prepared. T84 cells were incubated for 24 h in medium, of which 10% consisted of the pooled serum from each group. After incubation, the TER was measured and the following parameters were determined by flow cytometry: expression of toll-like receptors (TLRs), production of reactive oxygen species (ROS), and apoptosis. The level of IL-6 in the culture supernatant was determined by ELISA.

Results:

No difference was observed among the groups with respect to TER, apoptosis, and ROS or the expression of TLR-2, TLR-4, and TLR-9. IL-6 secretion was higher (p < 0.001) in cells that were incubated with pre- and post-HD serum.

Conclusion:

The results that were obtained from this model suggest that uremic serum per se does not seem to impair the integrity of intestinal epithelial cells. The increased IL-6 secretion by cells that were incubated with HD serum suggests a potential effect of uremia in the intestinal inflammatory response.

The olive constituent oleuropein, as a PPARα agonist, markedly reduces serum triglycerides

Oleuropein (OLE), a main constituent of olive, exhibits antioxidant and hypolipidemic effects, while it reduces the infarct size in chow- and cholesterol-fed rabbits. Peroxisome proliferator-activated receptor α (PPARα) has essential roles in the control of lipid metabolism and energy homeostasis. This study focused on the mechanisms underlying the hypolipidemic activity of OLE and, specifically, on the role of PPARα activation in the OLE-induced effect. Theoretical approach using Molecular Docking Simulations and luciferase reporter gene assay indicated that OLE is a ligand of PPARα. The effect of OLE (100 mg/kg, p.o., per day, ×6 weeks) on serum triglyceride (TG) and cholesterol levels was also assessed in adult male wild-type and Ppara-null mice. Molecular Docking Simulations, Luciferase reporter gene assay and gene expression analysis indicated that OLE is a PPARα agonist that up-regulates several PPARα target genes in the liver. This effect was associated with a significant reduction of serum TG and cholesterol levels. In contrast, OLE had no effect in Ppara-null mice, indicating a direct involvement of PPARα in the OLE-induced serum TG and cholesterol reduction. Activation of hormone-sensitive lipase in the white adipose tissue (WAT) and the liver of wild-type mice and up-regulation of several hepatic factors involved in TG uptake, transport, metabolism and clearance may also contribute in the OLE-induced TG reduction. In summary, OLE has a beneficial effect on TG homeostasis via PPARα activation. OLE also activates the hormone sensitive lipase in the WAT and liver and up-regulates several hepatic genes with essential roles in TG homeostasis.

Potent Sodium/Glucose Cotransporter SGLT1/2 Dual Inhibition Improves Glycemic Control Without Marked Gastrointestinal Adaptation or Colonic Microbiota Changes in Rodents

The sodium/glucose cotransporters (SGLT1 and SGLT2) transport glucose across the intestinal brush border and kidney tubule. Dual SGLT1/2 inhibition could reduce hyperglycemia more than SGLT2-selective inhibition in patients with type 2 diabetes. However, questions remain about altered gastrointestinal (GI) luminal glucose and tolerability, and this was evaluated in slc5a1^-/- mice or with a potent dual inhibitor (compound 8; SGLT1 K_i = 1.5 ± 0.5 nM 100-fold greater potency than phlorizin; SGLT2 K_i = 0.4 ± 0.2 nM). ¹³C₆-glucose uptake was quantified in slc5a1^-/- mice and in isolated rat jejunum. Urinary glucose excretion (UGE), blood glucose (Sprague-Dawley rats), glucagon-like peptide 1 (GLP-1), and hemoglobin A1c (HbA1c) levels (Zucker diabetic fatty rats) were measured. Intestinal adaptation and rRNA gene sequencing was analyzed in C57Bl/6 mice. The blood ¹³C₆-glucose area under the curve (AUC) was reduced in the absence of SGLT1 by 75% (245 ± 6 vs. 64 ± 6 mg/dl⋅h in wild-type vs. slc5a1^-/- mice) and compound 8 inhibited its transport up to 50% in isolated rat jejunum. Compound 8 reduced glucose excursion more than SGLT2-selective inhibition (e.g., AUC = 129 ± 3 vs. 249 ± 5 mg/dl⋅h for 1 mg/kg compound 8 vs. dapagliflozin) with similar UGE but a lower renal glucose excretion threshold. In Zucker diabetic fatty rats, compound 8 decreased HbA1c and increased total GLP-1 without changes in jejunum SGLT1 expression, mucosal weight, or villus length. Overall, compound 8 (1 mg/kg for 6 days) did not increase cecal glucose concentrations or bacterial diversity in C57BL/6 mice. In conclusion, potent dual SGLT1/2 inhibition lowers blood glucose by reducing intestinal glucose absorption and the renal glucose threshold but minimally impacts the intestinal mucosa or luminal microbiota in chow-fed rodents.