Potentiometric-scanning ion conductance microscopy for measurement at tight junctions

Single-Cell Analysis with Spatiotemporal Control of Local pH

This work presents an experimental platform combining scanning ion conductance microscopy (SICM) with confocal laser scanning microscopy (CLSM), using intra- and extracellular pH indicator dyes to study the impact of acid delivery on individual HeLa cells within a population. The proton gradient generated by the SICM delivery is highly confined by the action of the media buffer, making the challenge local. Temporal and spatial aspects of the delivery are modeled by simulations, allowing for pH gradients across individual cells, even within a group, to be calculated. We find a strong dependency between the intracellular pH and the extracellular pH gradient imposed by local acid delivery. Postdelivery intracellular pH recovery depends on the extent of the acid challenge, with cells exposed to lower pH not returning to basal intracellular pH values after the extracellular pH recovers. This is a unique method for concentration-gradient challenge studies of cell populations that will have broad applications in cell biology. SICM can be used to deliver different chemicals and enables a wide range of local conditions to be applied across a cell population, for which the effects can be investigated at the single-cell level.

Spatiotemporal dynamics of microscopic biological barrier visualized by electric-double-layer modulation imaging

Live-cell label-free imaging of a microscopic biological barrier, generally referred to as 'tight junction', was realized by a recently developed electric-double-layer modulation imaging (EDLMI). The method allowed quantitative imaging of barrier integrity in real time, thus being an upper compatible of transepithelial electrical resistance (TEER) which is a conventional standard technique to evaluate spatially averaged barrier integrity. We demonstrate that the quantitative and real-time imaging capability of EDLMI unveils fundamental dynamics of biological barrier, some of which are totally different from conventional understandings.

Probing and Visualizing Interfacial Charge at Surfaces in Aqueous Solution

Surface charge density and distribution play an important role in almost all interfacial processes, influencing, for example, adsorption, colloidal stability, functional material activity, electrochemical processes, corrosion, nanoparticle toxicity, and cellular processes such as signaling, absorption, and adhesion. Understanding the heterogeneity in, and distribution of, surface and interfacial charge is key to elucidating the mechanisms underlying reactivity, the stability of materials, and biophysical processes. Atomic force microscopy (AFM) and scanning ion conductance microscopy (SICM) are highly suitable for probing the material/electrolyte interface at the nanoscale through recent advances in probe design, significant instrumental (hardware and software) developments, and the evolution of multifunctional imaging protocols. Here, we assess the capability of AFM and SICM for surface charge mapping, covering the basic underpinning principles alongside experimental considerations. We illustrate and compare the use of AFM and SICM for visualizing surface and interfacial charge with examples from materials science, geochemistry, and the life sciences.

Nanoscale Mapping of the Double Layer Potential at the Graphene-Electrolyte Interface

The electrical double layer (EDL) governs the operation of multiple electrochemical devices, determines reaction potentials, and conditions ion transport through cellular membranes in living organisms. The few existing methods of EDL probing have low spatial resolution, usually only providing spatially averaged information. On the other hand, traditional Kelvin probe force microscopy (KPFM) is capable of mapping potential with nanoscale lateral resolution but cannot be used in electrolytes with concentrations higher than several mmol/L. Here, we resolve this experimental impediment by combining KPFM with graphene-capped electrolytic cells to quantitatively measure the potential drop across the EDL in aqueous electrolytes of decimolar and molar concentrations with a high lateral resolution. The surface potential of graphene in contact with deionized water and 0.1 mol/L solutions of CuSO₄ and MgSO₄ as a function of counter electrode voltage is reported. The measurements are supported by numerical modeling to reveal the role of the graphene membrane in potential screening and to determine the EDL potential drop. The proposed approach proves to be especially useful for imaging spatially inhomogeneous systems, such as nanoparticles submerged in an electrolyte solution. It could be suitable for in operando and in vivo measurements of the potential drop in the EDL on the surfaces of nanocatalysts and biological cells in equilibrium with liquid solutions.

Structural dynamics of tight junctions modulate the properties of the epithelial barrier

Tight junctions are dynamic structures that are crucial in establishing the diffusion and electrical barrier of epithelial monolayers. Dysfunctions in the tight junctions can impede this barrier function and lead to many pathological conditions. Unfortunately, detailed understanding of the non-specific permeation pathway through the tight junctions, the so-called leak pathway, is lacking. We created computational models of the leak pathway to describe the two main barrier measures, molecular permeability and transepithelial electric resistance while using common structural dynamics. Our results showed that the proposed alternatives for the leak pathway, the bicellular strand opening dynamics and the tricellular pores, contribute together with distinct degrees, depending on the epithelium. The models can also capture changes in the tight junction barrier caused by changes in tight junction protein composition. In addition, we observed that the molecular permeability was markedly more sensitive to changes in the tight junction structure and strand dynamics compared with transepithelial electric resistance. The results highlight that our model creates a good methodological framework to integrate knowledge on the tight junction structure as well as to provide insights and tools to advance tight junction research.

Quantifying microbial chatter: scanning electrochemical microscopy as a tool to study interactions in biofilms

Bacteria are often found in their natural habitats as spatially organized biofilm communities. While it is clear from recent work that the ability to organize into precise spatial structures is important for fitness of microbial communities, a significant gap exists in our understanding regarding the mechanisms bacteria use to adopt such physical distributions. Bacteria are highly social organisms that interact, and it is these interactions that have been proposed to be critical for establishing spatially structured communities. A primary means by which bacteria interact is via small, diffusible molecules including dedicated signals and metabolic by-products; however, quantitatively monitoring the production of these molecules in time and space with the micron-scale resolution required has been challenging. In this perspective, scanning electrochemical microscopy (SECM) is discussed as a powerful tool to study microbe-microbe interactions through the detection of small redox-active molecules. We highlight SECM as a means to quantify and spatially resolve the chemical mediators of bacterial interactions and begin to elucidate the mechanisms used by bacteria to regulate the emergent properties of biofilms.

Claudins in barrier and transport function-the kidney

Claudins are discovered to be key players in renal epithelial physiology. They are involved in developmental, physiological, and pathophysiological differentiation. In the glomerular podocytes, claudin-1 is an important determinant of cell junction fate. In the proximal tubule, claudin-2 plays important roles in paracellular salt reabsorption. In the thick ascending limb, claudin-14, -16, and -19 regulate the paracellular reabsorption of calcium and magnesium. Recessive mutations in claudin-16 or -19 cause an inherited calcium and magnesium losing disease. Synonymous variants in claudin-14 have been associated with hypercalciuric nephrolithiasis by genome-wide association studies (GWASs). More importantly, claudin-14 gene expression can be regulated by extracellular calcium levels via the calcium sensing receptor. In the distal tubules, claudin-4 and -8 form paracellular chloride pathway to facilitate electrogenic sodium reabsorption. Aldosterone, WNK4, Cap1, and KLHL3 are powerful regulators of claudin and the paracellular chloride permeability. The lessons learned on claudins from the kidney will have a broader impact on tight junction biology in other epithelia and endothelia.

Viral interactions with the blood-brain barrier: old dog, new tricks

Brain endothelial cells form a unique cellular structure known as the tight junction to regulate the exchanges between the blood and the parenchyma by limiting the paracellular diffusion of blood-borne substance. Together with the restricted pathway of transcytosis, the tight junction in the brain endothelial cells provides the central nervous system (CNS) with effective protection against both the foreign pathogens and the host immune cells, which is also termed the "blood-brain barrier." The blood-brain barrier is particularly important for defending against neurotropic viral infections that have become a major source of diseases worldwide. Many neurotropic viruses are able to cross the BBB and infect the CNS through very poorly understood processes. This review focuses upon the structural and functional changes of the brain endothelial tight junction in response to viral infections in the CNS and how the tight junction changes may be studied with advanced imaging and recording approaches to reveal novel processes used by the viruses to cross the barrier system. Additional emphasis is placed upon new countermeasures that can act directly upon the tight junction to improve the pathogen clearance and minimize the inflammatory damage.

Biochemical and biophysical analyses of tight junction permeability made of claudin-16 and claudin-19 dimerization

Comprehensive biochemical, biophysical, genetic, and electron microscopic analyses of claudin-16 and -19 interactions show how claudin interaction can influence tight junction permeability and tight junction architecture.

The molecular nature of tight junction architecture and permeability is a long-standing mystery. Here, by comprehensive biochemical, biophysical, genetic, and electron microscopic analyses of claudin-16 and -19 interactions—two claudins that play key polygenic roles in fatal human renal disease, FHHNC—we found that 1) claudin-16 and -19 form a stable dimer through cis association of transmembrane domains 3 and 4; 2) mutations disrupting the claudin-16 and -19 cis interaction increase tight junction ultrastructural complexity but reduce tight junction permeability; and 3) no claudin hemichannel or heterotypic channel made of claudin-16 and -19 trans interaction can exist. These principles can be used to artificially alter tight junction permeabilities in various epithelia by manipulating selective claudin interactions. Our study also emphasizes the use of a novel recording approach based on scanning ion conductance microscopy to resolve tight junction permeabilities with submicrometer precision.

Nanophysiology: Bridging synapse ultrastructure, biology, and physiology using scanning ion conductance microscopy

Synaptic communication is at the core of neural circuit function, and its plasticity allows the nervous system to adapt to the changes in its environment. Understanding the mechanisms of this synaptic (re)organization will benefit from novel methodologies that enable simultaneous study of synaptic ultrastructure, biology, and physiology in identified circuits. Here, we describe one of these methodologies, i.e., scanning ion conductance microscopy (SICM), for electrical mapping of the membrane anatomy in tens of nanometers resolution in living neurons. When combined with traditional patch-clamp and fluorescence microscopy techniques, and the newly emerging nanointerference methodologies, SICM has the potential to mechanistically bridge the synaptic structure and function longitudinally throughout the life of a synapse.

The kidney tight junction (Review)

The tight junction is an important subcellular organelle which plays a vital role in epithelial barrier function. Claudin, as the integral membrane component of tight junctions, creates a paracellular transport pathway for various ions to be reabsorbed by the kidneys. This review summarizes advances in claudin structure, function and pathophysiology in kidney diseases. Different claudin species confer selective paracellular permeability to each of three major renal tubular segments: the proximal tubule, the thick ascending limb of Henle’s loop and the distal nephron. Defects in claudin function can cause a wide spectrum of kidney diseases, such as hypomagnesemia, hypercalciuria, kidney stones and hypertension. Studies using transgenic mouse models with claudin mutations have recapitulated several of these renal disease phenotypes and have elucidated the underlying biological mechanisms. Modern recording approaches based upon scanning ion conductance microscopy may resolve the biophysical nature of claudin transport function and provide novel insight into tight junction architecture.

Potentiometric-scanning ion conductance microscopy

We detail the operation mechanism and instrumental limitations for potentiometric-scanning ion conductance microscopy (P-SICM). P-SICM makes use of a dual-barrel probe, where probe position is controlled by the current measured in one barrel and the potential is measured in a second barrel. Here we determine the interaction of these two barrels and resultant effects in quantitation of signals. Effects due to the size difference in pipet tip opening are examined and compared to model calculations. These results provide a basis for quantitation and image interpretation for P-SICM.