Ionic permeation mechanisms in epithelia: biionic potentials, dilution potentials, conductances, and streaming potentials

Lipid-Based Nanoparticles as Oral Drug Delivery Systems: Overcoming Poor Gastrointestinal Absorption and Enhancing Bioavailability of Peptide and Protein Therapeutics

Delivery and formulation of oral peptide and protein therapeutics have always been a challenge for the pharmaceutical industry. The oral bioavailability of peptide and protein therapeutics mainly relies on their gastrointestinal solubility and permeability which are affected by their poor membrane penetration, high molecular weight and proteolytic (chemical and enzymatic) degradation resulting in limited delivery and therapeutic efficacy. The present review article highlights the challenges and limitations of oral delivery of peptide and protein therapeutics focusing on the application, potential and importance of solid lipid nanoparticles (SLNs) and nanostructured lipid carriers (NLCs) as lipid-based drug delivery systems (LBDDSs) and their advantages and drawbacks. LBDDSs, due to their lipid-based matrix can encapsulate both lipophilic and hydrophilic drugs, and by reducing the first-pass effect and avoiding proteolytic degradation offer improved drug stability, dissolution rate, absorption, bioavailability and controlled drug release. Furthermore, their small size, high surface area and surface modification increase their mucosal adhesion, tissue-targeted distribution, physiological function and half-life. Properties such as simple preparation, high-scale manufacturing, biodegradability, biocompatibility, prolonged half-life, lower toxicity, lower adverse effects, lipid-based structure, higher drug encapsulation rate and various drug release profile compared to other similar carrier systems makes LBDDSs a promising drug delivery system (DDS). Nevertheless, undesired physicochemical features of peptide and protein drug development and discovery such as plasma stability, membrane permeability and circulation half-life remain a serious challenge which should be addressed in future.

Electrophysiologic Analysis of Tight Junction Size and Charge Selectivity

Tight junctions form selectively permeable barriers that limit paracellular flux across epithelial-lined surfaces. Rather than being absolute barriers, tight junctions in many tissues allow ions, water, and other small molecules to cross on the basis of size and charge selectivity via the high-capacity pore pathway. Most probes currently used to assess tight junction permeability exceed the maximum size capacity of the pore pathway. As a result, available analytical tools have generally been limited to measurement of transepithelial electrical resistances. These provide no information regarding size selectivity and, therefore, cannot be used to distinguish between the pore pathway and the leak pathway, a low-capacity route that accommodates larger macromolecules. This article describes use of dilution potential and bi-ionic potential measurements for analysis of tight junction size and charge selectivity within monolayers of cultured epithelial cells. © 2021 Wiley Periodicals LLC. Basic Protocol 1: Culture of MDCK monolayers on semipermeable supports and induction of claudin-2 expression Basic Protocol 2: Configuring voltage/current clamp and other equipment Basic Protocol 3: Measuring dilution and bi-ionic potentials Basic Protocol 4: Calculating ion permeabilities and pore diameter Support Protocol: Preparation of agar bridges and electrophysiology rig setup.

Nanoparticles for oral delivery: targeted nanoparticles with peptidic ligands for oral protein delivery

As the field of biotechnology has advanced, oral protein delivery has also made significant progress. Oral delivery is the most common method of drug administration with high levels of patient acceptance. Despite the preference of oral delivery, administration of therapeutic proteins has been extremely difficult. Increasing the bioavailability of oral protein drugs to the therapeutically acceptable level is still a challenging goal. Poor membrane permeability, high molecular weight, and enzymatic degradation of protein drugs have remained unsolved issues. Among diverse strategies, nanotechnology has provided a glimpse of hope in oral delivery of protein drugs. Nanoparticles have advantages, such as small size, high surface area, and modification using functional groups for high capacity or selectivity. Nanoparticles with peptidic ligands are especially worthy of notice because they can be used for specific targeting in the gastrointestinal (GI) tract. This article reviews the transport mechanism of the GI tract, barriers to protein absorption, current status and limitations of nanotechnology for oral protein delivery system.

Single-channel measurements of an N-acetylneuraminic acid-inducible outer membrane channel in Escherichia coli

NanC is an Escherichia coli outer membrane protein involved in sialic acid (Neu5Ac, i.e., N-acetylneuraminic acid) uptake. Expression of the NanC gene is induced and controlled by Neu5Ac. The transport mechanism of Neu5Ac is not known. The structure of NanC was recently solved (PDB code: 2WJQ) and includes a unique arrangement of positively charged (basic) side chains consistent with a role in acidic sugar transport. However, initial functional measurements of NanC failed to find its role in the transport of sialic acids, perhaps because of the ionic conditions used in the experiments. We show here that the ionic conditions generally preferred for measuring the function of outer-membrane porins are not appropriate for NanC. Single channels of NanC at pH 7.0 have: (1) conductance 100 pS to 800 pS in 100 mM: KCl to 3 M: KCl), (2) anion over cation selectivity (V (reversal) = +16 mV in 250 mM: KCl || 1 M: KCl), and (3) two forms of voltage-dependent gating (channel closures above ± 200 mV). Single-channel conductance decreases by 50% when HEPES concentration is increased from 100 μM: to 100 mM: in 250 mM: KCl at pH 7.4, consistent with the two HEPES binding sites observed in the crystal structure. Studying alternative buffers, we find that phosphate interferes with the channel conductance. Single-channel conductance decreases by 19% when phosphate concentration is increased from 0 mM: to 5 mM: in 250 mM: KCl at pH 8.0. Surprisingly, TRIS in the baths reacts with Ag|AgCl electrodes, producing artifacts even when the electrodes are on the far side of agar-KCl bridges. A suitable baseline solution for NanC is 250 mM: KCl adjusted to pH 7.0 without buffer.

Effects of methylenedianiline on tight junction permeability of biliary epithelial cells in vivo and in vitro

Methylenedianiline (DAPM) is considered a cholangiodestructive toxicant in vivo. Increases in biliary inorganic phosphate (P(i)) and glucose occur prior to biliary epithelial cell (BEC) injury, which could be due to increased paracellular permeability and/or impairment of P(i) and glucose uptake by BEC. To evaluate these possibilities, we induced mild injury [loss of BEC from major bile ducts (6 h), ultrastructural alterations in BEC mitochondria and Golgi cisternae (3 h), and striking increases in biliary P(i) and glucose (3-6 h)] with 25 mg DAPM/kg and then assessed temporal alterations in tight junction (TJ) permeability by measuring bile to plasma (B:P) ratios of [(3)H]-inulin. Parameters maintained by hepatocytes in bile were unchanged (bile flow, bile salts, bilirubin) or only transiently perturbed (protein, glutathione). Minimal elevations in B:P ratios of inulin occurred temporally later (4 h) in DAPM-treated rats than increases in biliary P(i) and glucose. To confirm a direct effect of DAPM on BEC TJs, we measured transepithelial resistance (TER) and bi-ionic potentials of BEC monolayers prior to and after exposure to pooled (4-6) bile samples collected from untreated rats (Basal Bile) or rats treated with 50 mg DAPM/kg (DAPM-Bile). BEC TJs were found to be cation selective. Exposure to DAPM-Bile for 1 h decreased TERs by approximately 35% and decreased charge selectivity of BEC TJs while exposure to Basal Bile had no effects. These observations indicate that DAPM-Bile impairs paracellular permeability of BEC in vitro. Further, our in vivo model suggests that increases in paracellular permeability induced by DAPM are localized to BEC because bile flow and constituents excreted by hepatocytes were unchanged, BEC damage was temporally correlated with increases in biliary P(i) and glucose, and elevations in B:P ratios of inulin were delayed and minimal.

Modelling of epithelial tissue impedance measured using three different designs of probe

Impedance measurement is a promising technique for detecting pre-malignant changes in epithelial tissue. This paper considers how the design of the impedance probe affects the ability to discriminate between tissue types. To do this, finite element models of the electrical properties of squamous and glandular columnar epithelia have been used. The glandular tissue model is described here for the first time. Glandular mucosa is found in many regions of the gastrointestinal tract, such as the stomach and intestine, and has a large effective surface area. Firstly, the electrical properties of a small section of gland, with epithelial cells and supportive tissue, are determined. These properties are then used to build up a three-dimensional model of a whole section of mucosa containing many thousands of glands. Measurements using different types of impedance probe were simulated by applying different boundary conditions to the models. Transepithelial impedance, and tetrapolar measurement with a probe placed on the tissue surface have been modelled. In the latter case, the impedance can be affected by conductive fluid, such as mucus, on the tissue surface. This effect has been investigated, and a new design of probe, which uses a guard electrode to counteract this potential source of variability, is proposed.

Molecular structure of tight junctions and their role in epithelial transport

Tight junctions create a paracellular barrier with physiological properties that differ among epithelia. Among these differences are electrical resistance and discrimination for solute size and charge. Emerging evidence suggests that a large family of transmembrane proteins called the claudins create these variable properties.

Extracellular K+ activates a K(+)- and H(+)-permeable conductance in frog taste receptor cells

1. The effect of extracellular K+ on membrane currents of bull frog (Rana catesbeiana) taste receptor cells (TRCs) was investigated by the patch clamp and fast perfusion techniques. Extracellular K+ (2.5-90 mM) increased a TRC resting conductance and enhanced both inward and outward whole-cell currents. 2. To isolate the inward current activated by external potassium (PA current), TRCs were dialysed with 110 mM NMGCl while extracellular NaCl was replaced with NMGCl. Under these conditions, the PA current displayed an S-shaped current-voltage (I-V) curve in the -100 to 100 mV range. Extracellular Rb+ and NH4+, but not Li+, Na+ or Cs+, evoked similar currents. 3. The PA current reversal potential (Vr) did not follow the equilibrium K+ potential under experimental conditions. Therefore, K+ ions were not the only current carriers. The influence of other ions on the PA current Vr indicated that the channels involved are permeable to K+ and H+ and much less so to Na+, Ca2+ and Mg2+. Relative permeabilities were estimated on the basis of the Goldman-Hodgkin-Katz equation as follows: PH:PK:PNa = 4000:1:0.04. 4. All I-V curves of the PA current were nearly linear at low negative potentials. The slope conductance at these voltages was used to characterize the dependence of the PA current on external K+ and H+. The slope conductance versus K+ concentration was fitted by the Hill equation. The data yielded a half-maximal concentration, K1/2 = 19 +/- 3 mM and a Hill coefficient, nH = 1.53 +/- 0.36 (means +/- S.E.M.). 5. The dependence of the mean PA current and the current variance on the K+ concentration indicated a rise in the open probability of the corresponding channels as extracellular K+ was increased. With 110 mM KCl in the bath, the single channel conductance was estimated at about 6 pS. Taken together, the data suggest that extracellular K+ may serve as a ligand to activate specific small-conductance cation channels (PA channels). The mean number of the PA channels per TRC was estimated as at least 2000. 6. Extracellular Ba2+, Cd2+, Co2+, Ni2+ and Cs+ blocked the PA current in a potential-dependent manner. The PA current was blocked by Cs+ as quickly as the blocker could be applied (approximately 15 ms). The time course of the divalent cation block was well fitted by a single exponential function. The time constants were estimated at 26.5 +/- 1.9, 41.7 +/- 3.1, 56.1 +/- 4.2 and 370 +/- 18 ms at 1 mM Cd2+, Co2+, Ni2+ and Ba2+, respectively. The blocker efficiency at negative voltages followed the sequence: Cs+ > Cd2+ > Ba2+ > Ni2+ > Co2+. 7. The data indicate that protons and divalent blockers act within the PA channel pore and that H+ and the divalent ions probably act via similar mechanisms to affect the PA current. These observations and the strong pH dependence of the PA current Vr suggest that H+ occupation of the PA channel pore leading to interruption of K+ flux is the main mechanism of the pH dependence of the PA current. 8. Extracellular K+ enhanced the sensitivity of isolated TRCs to bath solution acidification due to activation of the PA channels. With 10 mM K+ in the bath, half-maximal depolarization of the TRCs was observed at pH values of 6.4-6.8. The possible role of the PA channels in sour transduction is discussed.

JPCalc, a software package for calculating liquid junction potential corrections in patch-clamp, intracellular, epithelial and bilayer measurements and for correcting junction potential measurements

The JPCalc program has been designed to graphically illustrate how junction potential contributions arise in various electrophysiological situations, to enable the magnitude and direction of those values to be readily calculated and to show clearly how the resultant appropriate corrections need to be applied to experimental measurements. In particular, the program covers the analysis of junction potential contributions and corrections in all the various different patch-clamp configurations (intact patches, inside-out patches, outside-out patches, whole-cell recording and perforated patches), intracellular recording, epithelial and bilayer measurements. It can also be used to correct experimental measurements designed to try to directly measure junction potentials and to compare both the magnitude and direction of such corrected measurements with their theoretical values. Although it is recommended that all of the above measurements be done with salt-filled electrodes or salt bridges, the program will allow the use of Ag/AgCl reference electrodes in direct contact with bathing solution in most situations. JPCalc also makes use of a library file of mobility and valency data for about 30 ions which can be extended by the user. In addition to its value in correcting electrical measurements, the program with its graphical displays may also be of value in focusing discussion about the contribution of junction potentials to such measurements and in helping to design experiments to either minimize their contribution or to measure their values. The JPCalc program, designed for IBM-compatible computers with color or monochrome monitors, can also be run on Macintosh computers with SoftPC emulation software.