-
Biochimica Et Biophysica Acta.... Jan 2018This review focuses on the biophysical properties and structure of the pore and vestibule of homotypic gap junction channels as they relate to channel permeability and... (Review)
Review
This review focuses on the biophysical properties and structure of the pore and vestibule of homotypic gap junction channels as they relate to channel permeability and selectivity. Gap junction channels are unique in their sole role to connect the cytoplasm of two adjacent cells. In general, these channels are considered to be poorly selective, possess open probabilities approximating unity, and exhibit mean open times ranging from milliseconds to seconds. These properties suggest that such channels can function as delivery pathways from cell to cell for solutes that are significantly larger than monovalent ions. We have taken quantitative data from published works concerning unitary conductance, ion flux, and permeability for homotypic connexin 43 (Cx43), Cx40, Cx26, Cx50, and Cx37, and performed a comparative analysis of conductance and/or ion/solute flux versus diffusion coefficient. The analysis of monovalent cation flux portrays the pore as equivalent to an aqueous space where hydrogen bonding and weak interactions with binding sites dominate. For larger solutes, size, shape and charge are also significant components in determining the permeation rate. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.
Topics: Animals; Cell Membrane Permeability; Connexins; Gap Junctions; Humans; Ion Channels; Ion Transport
PubMed: 28690048
DOI: 10.1016/j.bbamem.2017.07.002 -
Cells Oct 2023Epithelial transport is a multifaceted process crucial for maintaining normal physiological functions in the human body. This comprehensive review delves into the... (Review)
Review
Epithelial transport is a multifaceted process crucial for maintaining normal physiological functions in the human body. This comprehensive review delves into the pathophysiological mechanisms underlying epithelial transport and its significance in disease pathogenesis. Beginning with an introduction to epithelial transport, it covers various forms, including ion, water, and nutrient transfer, followed by an exploration of the processes governing ion transport and hormonal regulation. The review then addresses genetic disorders, like cystic fibrosis and Bartter syndrome, that affect epithelial transport. Furthermore, it investigates the involvement of epithelial transport in the pathophysiology of conditions such as diarrhea, hypertension, and edema. Finally, the review analyzes the impact of renal disease on epithelial transport and highlights the potential for future research to uncover novel therapeutic interventions for conditions like cystic fibrosis, hypertension, and renal failure.
Topics: Humans; Cystic Fibrosis; Cystic Fibrosis Transmembrane Conductance Regulator; Epithelial Cells; Ion Transport; Hypertension
PubMed: 37887299
DOI: 10.3390/cells12202455 -
Nature Communications Aug 2023The hinged-lid model was long accepted as the canonical model for fast inactivation in Nav channels. It predicts that the hydrophobic IFM motif acts intracellularly as...
The hinged-lid model was long accepted as the canonical model for fast inactivation in Nav channels. It predicts that the hydrophobic IFM motif acts intracellularly as the gating particle that binds and occludes the pore during fast inactivation. However, the observation in recent high-resolution structures that the bound IFM motif is located far from the pore, contradicts this preconception. Here, we provide a mechanistic reinterpretation of fast inactivation based on structural analysis and ionic/gating current measurements. We demonstrate that in Nav1.4 the final inactivation gate is comprised of two hydrophobic rings at the bottom of S6 helices. These rings function in series and close downstream of IFM binding. Reducing the volume of the sidechain in both rings leads to a partially conductive, leaky inactivated state and decreases the selectivity for Na ion. Altogether, we present an alternative molecular framework to describe fast inactivation.
Topics: Ear Auricle; Electric Conductivity; Ion Transport; Ions
PubMed: 37604801
DOI: 10.1038/s41467-023-40514-4 -
Journal of the American Chemical Society Jun 2022Ion transport across lipid membranes in biology is controlled by stimuli-responsive membrane channels and molecular machine ion pumps such as ATPases. Here, we report a...
Ion transport across lipid membranes in biology is controlled by stimuli-responsive membrane channels and molecular machine ion pumps such as ATPases. Here, we report a synthetic molecular machine-like ion transport relay, in which transporters on opposite sides of a lipid bilayer membrane facilitate transport by passing ions between them. By incorporating a photo-responsive telescopic arm into the relay design, this process is reversibly controlled in response to irradiation with blue and green light. Transport occurs only in the extended state when the length of the arm is sufficient to pass the anion between transporters located on opposite sides of the membrane. In contrast, the contracted state of the telescopic arm is too short to mediate effective transport. The system acts as a stimuli-responsive ensemble of machine-like components, reminiscent of robotic arms in a factory assembly line, working cooperatively to mediate ion transport. This work points to new prospects for using lipid bilayer membranes as scaffolds for confining, orientating, and controlling the relative positions of molecular machines, thus enabling multiple components to work in concert and opening up new applications in biological contexts.
Topics: Anions; Biological Transport; Ion Channels; Ion Transport; Lipid Bilayers
PubMed: 35652660
DOI: 10.1021/jacs.2c02612 -
International Journal of Molecular... Jul 2022Membrane transport proteins are widely present in all living organisms, however, their function, transported substrate, and mechanism of action are unknown. Here we use...
Membrane transport proteins are widely present in all living organisms, however, their function, transported substrate, and mechanism of action are unknown. Here we use diverse bioinformatics tools to investigate the evolution of MTPs, analyse domain organisation and loop topology, and study the comparative alignment of modelled 3D structures. Our results suggest a high level of conservancy between MTPs from different taxa on both amino acids and structural levels, which imply some degree of functional similarities. The presence of loop/s of different lengths in various positions suggests tax-on-specific adaptation to transported substrates, intracellular localisation, accessibility for post-translation modifications, and interaction with other proteins. The comparison of modelled structures proposes close relations and a common origin for MTP and Na/H exchanger. Further, a high level of amino acid similarity and identity between archaeal and bacterial MTPs and Na/H exchangers imply conservancy of ion transporting function at least for archaeal and bacterial MTPs.
Topics: Amino Acid Sequence; Biological Transport; Ion Transport; Membrane Transport Proteins; Sodium-Hydrogen Exchangers
PubMed: 35897663
DOI: 10.3390/ijms23158094 -
Chemistry (Weinheim An Der Bergstrasse,... May 2021Anion receptors can be used to transport ions across lipid bilayers, which has potential for therapeutic applications. Synthetic bicarbonate transporters are of...
Anion receptors can be used to transport ions across lipid bilayers, which has potential for therapeutic applications. Synthetic bicarbonate transporters are of particular interest, as defects in transmembrane transport of bicarbonate are associated with various diseases. However, no convenient method exists to directly observe bicarbonate transport and study the mechanisms involved. Here, an assay is presented that allows the kinetics of bicarbonate transport into liposomes to be monitored directly and with great sensitivity. The assay utilises an encapsulated europium(III) complex, which exhibits a large increase in emission intensity upon binding bicarbonate. Mechanisms involving CO diffusion and the dissipation of a pH gradient are shown to be able to lead to an increase in bicarbonate concentration within liposomes, without transport of the anion occurring at all. By distinguishing these alternative mechanisms from actual bicarbonate transport, this assay will inform the future development of bicarbonate transporters.
Topics: Bicarbonates; Biological Transport; Hydrogen-Ion Concentration; Ion Transport; Kinetics; Lipid Bilayers
PubMed: 33932059
DOI: 10.1002/chem.202100491 -
International Journal of Molecular... Mar 2021Intra- and extracellular pH regulation is a pivotal function of all cells and tissues. Net outward transport of H is a prerequisite for normal physiological function,... (Review)
Review
Intra- and extracellular pH regulation is a pivotal function of all cells and tissues. Net outward transport of H is a prerequisite for normal physiological function, since a number of intracellular processes, such as metabolism and energy supply, produce acid. In tumor tissues, distorted pH regulation results in extracellular acidification and the formation of a hostile environment in which cancer cells can outcompete healthy local host cells. Cancer cells employ a variety of H/HCO-coupled transporters in combination with intra- and extracellular carbonic anhydrase (CA) isoforms, to alter intra- and extracellular pH to values that promote tumor progression. Many of the transporters could closely associate to CAs, to form a protein complex coined "transport metabolon". While transport metabolons built with HCO-coupled transporters require CA catalytic activity, transport metabolons with monocarboxylate transporters (MCTs) operate independently from CA catalytic function. In this article, we assess some of the processes and functions of CAs for tumor pH regulation and discuss the role of intra- and extracellular pH regulation for cancer pathogenesis and therapeutic intervention.
Topics: Animals; Biomarkers; Carbonic Anhydrases; Disease Susceptibility; Drug Discovery; Energy Metabolism; Humans; Hydrogen-Ion Concentration; Intracellular Space; Ion Pumps; Ion Transport; Molecular Targeted Therapy; Neoplasms; Protons
PubMed: 33804674
DOI: 10.3390/ijms22063171 -
The Journal of General Physiology Oct 2016Ion channels are membrane proteins that mediate efficient ion transport across the hydrophobic core of cell membranes, an unlikely process in their absence. K(+)... (Review)
Review
Ion channels are membrane proteins that mediate efficient ion transport across the hydrophobic core of cell membranes, an unlikely process in their absence. K(+) channels discriminate K(+) over cations with similar radii with extraordinary selectivity and display a wide diversity of ion transport rates, covering differences of two orders of magnitude in unitary conductance. The pore domains of large- and small-conductance K(+) channels share a general architectural design comprising a conserved narrow selectivity filter, which forms intimate interactions with permeant ions, flanked by two wider vestibules toward the internal and external openings. In large-conductance K(+) channels, the inner vestibule is wide, whereas in small-conductance channels it is narrow. Here we raise the idea that the physical dimensions of the hydrophobic internal vestibule limit ion transport in K(+) channels, accounting for their diversity in unitary conductance.
Topics: Ion Transport; Models, Molecular; Potassium; Potassium Channels; Protein Conformation
PubMed: 27619418
DOI: 10.1085/jgp.201611625 -
International Journal of Molecular... Dec 2022The plasma-membrane homeostasis Na/Ca exchangers (NCXs) mediate Ca extrusion/entry to dynamically shape Ca signaling/in biological systems ranging from bacteria to... (Review)
Review
The plasma-membrane homeostasis Na/Ca exchangers (NCXs) mediate Ca extrusion/entry to dynamically shape Ca signaling/in biological systems ranging from bacteria to humans. The NCX gene orthologs, isoforms, and their splice variants are expressed in a tissue-specific manner and exhibit nearly 10-fold differences in the transport rates and regulatory specificities to match the cell-specific requirements. Selective pharmacological targeting of NCX variants could benefit many clinical applications, although this intervention remains challenging, mainly because a full-size structure of eukaryotic NCX is unavailable. The crystal structure of the archaeal NCX_Mj, in conjunction with biophysical, computational, and functional analyses, provided a breakthrough in resolving the ion transport mechanisms. However, NCX_Mj (whose size is nearly three times smaller than that of mammalian NCXs) cannot serve as a structure-dynamic model for imitating high transport rates and regulatory modules possessed by eukaryotic NCXs. The crystal structures of isolated regulatory domains (obtained from eukaryotic NCXs) and their biophysical analyses by SAXS, NMR, FRET, and HDX-MS approaches revealed structure-based variances of regulatory modules. Despite these achievements, it remains unclear how multi-domain interactions can decode and integrate diverse allosteric signals, thereby yielding distinct regulatory outcomes in a given ortholog/isoform/splice variant. This article summarizes the relevant issues from the perspective of future developments.
Topics: Animals; Humans; Scattering, Small Angle; X-Ray Diffraction; Protein Isoforms; Ion Transport; Eukaryotic Cells; Sodium-Calcium Exchanger; Calcium; Mammals
PubMed: 36613523
DOI: 10.3390/ijms24010061 -
Current Opinion in Nephrology and... Sep 2019Studies of the genetic model organism, Drosophila melanogaster, have unraveled molecular pathways relevant to human physiology and disease. The Malpighian tubule, the... (Review)
Review
PURPOSE OF REVIEW
Studies of the genetic model organism, Drosophila melanogaster, have unraveled molecular pathways relevant to human physiology and disease. The Malpighian tubule, the Drosophila renal epithelium, is described here, including tools available to study transport; conserved transporters, channels, and the signaling pathways regulating them; and fly models of kidney stone disease.
RECENT FINDINGS
Tools to measure Malpighian tubule transport continue to advance, including use of a transgenic sensor to quantify intracellular pH and proton fluxes. A recent study generated an RNA-sequencing-based atlas of tissue-specific gene expression, with resulting insights into Malpighian tubule gene expression of transporters and channels. Advances have been made in understanding the molecular physiology of the With No Lysine kinase-Ste20-related proline/alanine rich kinase/oxidative stress response kinase cascade that regulates epithelial ion transport in flies and mammals. New studies in Drosophila kidney stone models have characterized zinc transporters and used Malpighian tubules to study the efficacy of a plant metabolite in decreasing stone burden.
SUMMARY
Study of the Drosophila Malpighian tubule affords opportunities to better characterize the molecular physiology of epithelial transport mechanisms relevant to mammalian renal physiology.
Topics: Animals; Disease Models, Animal; Drosophila melanogaster; Humans; Ion Transport; Kidney Calculi; Kidney Diseases; Malpighian Tubules; Signal Transduction
PubMed: 31268918
DOI: 10.1097/MNH.0000000000000521