ResearchPad - 508 https://www.researchpad.co Default RSS Feed en-us © 2020 Newgen KnowledgeWorks <![CDATA[Structural and functional insights into transmembrane AMPA receptor regulatory protein complexes]]> https://www.researchpad.co/article/elastic_article_15288 Fast excitatory neurotransmission is mediated by the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) subtype of ionotropic glutamate receptor (AMPAR). AMPARs initiate depolarization of the postsynaptic neuron by allowing cations to enter through their ion channel pores in response to binding of the neurotransmitter glutamate. AMPAR function is dramatically affected by auxiliary subunits, which are regulatory proteins that form various complexes with AMPARs throughout the brain. The most well-studied auxiliary subunits are the transmembrane AMPAR regulatory proteins (TARPs), which alter the assembly, trafficking, localization, kinetics, and pharmacology of AMPARs. Recent structural and functional studies of TARPs and the TARP-fold germ cell-specific gene 1-like (GSG1L) subunit have provided important glimpses into how auxiliary subunits regulate the function of synaptic complexes. In this review, we put these recent structures in the context of new functional findings in order to gain insight into the determinants of AMPAR regulation by TARPs. We thus reveal why TARPs display a broad range of effects despite their conserved modular architecture.

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<![CDATA[Unique structural features in an Nramp metal transporter impart substrate-specific proton cotransport and a kinetic bias to favor import]]> https://www.researchpad.co/article/elastic_article_15285 Natural resistance-associated macrophage protein (Nramp) transporters enable uptake of essential transition metal micronutrients in numerous biological contexts. These proteins are believed to function as secondary transporters that harness the electrochemical energy of proton gradients by “coupling” proton and metal transport. Here we use the Deinococcus radiodurans (Dra) Nramp homologue, for which we have determined crystal structures in multiple conformations, to investigate mechanistic details of metal and proton transport. We untangle the proton-metal coupling behavior of DraNramp into two distinct phenomena: ΔpH stimulation of metal transport rates and metal stimulation of proton transport. Surprisingly, metal type influences substrate stoichiometry, leading to manganese-proton cotransport but cadmium uniport, while proton uniport also occurs. Additionally, a physiological negative membrane potential is required for high-affinity metal uptake. To begin to understand how Nramp’s structure imparts these properties, we target a conserved salt-bridge network that forms a proton-transport pathway from the metal-binding site to the cytosol. Mutations to this network diminish voltage and ΔpH dependence of metal transport rates, alter substrate selectivity, perturb or eliminate metal-stimulated proton transport, and erode the directional bias favoring outward-to-inward metal transport under physiological-like conditions. Thus, this unique salt-bridge network may help Nramp-family transporters maximize metal uptake and reduce deleterious back-transport of acquired metals. We provide a new mechanistic model for Nramp proton-metal cotransport and propose that functional advantages may arise from deviations from the traditional model of symport.

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<![CDATA[Spatiotemporal organization and protein dynamics involved in regulated exocytosis of MMP-9 in breast cancer cells]]> https://www.researchpad.co/article/elastic_article_15284 Altered regulation of exocytosis is an important mechanism controlling many diseases, including cancer. Defects in exocytosis have been implicated in many cancer cell types and are generally attributed to mutations in cellular transport, trafficking, and assembly of machinery necessary for exocytosis of secretory vesicle cargo. In these cancers, up-regulation of trafficking and secretion of matrix metalloproteinase-9 (MMP-9), a proteolytic enzyme, is responsible for degrading the extracellular matrix, a necessary step in tumor progression. Using TIRF microscopy, we identified proteins associated with secretory vesicles containing MMP-9 and imaged the local dynamics of these proteins at fusion sites during regulated exocytosis of MMP-9 from MCF-7 breast cancer cells. We found that many regulators of exocytosis, including several Rab GTPases, Rab effector proteins, and SNARE/SNARE modulator proteins, are stably assembled on docked secretory vesicles before exocytosis. At the moment of fusion, many of these components are quickly lost from the vesicle, while several endocytic proteins and lipids are simultaneously recruited to exocytic sites at precisely that moment. Our findings provide insight into the dynamic behavior of key core exocytic proteins, accessory proteins, lipids, and some endocytic proteins at single sites of secretory vesicle fusion in breast cancer cells.

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<![CDATA[A channel profile report of the unusual K<sup>+</sup> channel KtrB]]> https://www.researchpad.co/article/elastic_article_15282 KtrAB is a key player in bacterial K+ uptake required for K+ homeostasis and osmoadaptation. The system is unique in structure and function. It consists of the K+-translocating channel subunit KtrB, which forms a dimer in the membrane, and the soluble regulatory subunit KtrA, which attaches to the cytoplasmic side of the dimer as an octameric ring conferring Na+ and ATP dependency to the system. Unlike most K+ channels, KtrB lacks the highly conserved T(X)GYG selectivity filter sequence. Instead, only a single glycine residue is found in each pore loop, which raises the question of how selective the ion channel is. Here, we characterized the KtrB subunit from the Gram-negative pathogen Vibrio alginolyticus by isothermal titration calorimetry, solid-supported membrane–based electrophysiology, whole-cell K+ uptake, and ACMA-based transport assays. We found that, despite its simple selectivity filter, KtrB selectively binds K+ with micromolar affinity. Rb+ and Cs+ bind with millimolar affinities. However, only K+ and the poorly binding Na+ are efficiently translocated, based on size exclusion by the gating loop. Importantly, the physiologically required K+ over Na+ selectivity is provided by the channel’s high affinity for potassium, which interestingly results from the presence of the sodium ions themselves. In the presence of the KtrA subunit, sodium ions further decrease the Michaelis–Menten constant for K+ uptake from milli- to micromolar concentrations and increase the Vmax, suggesting that Na+ also facilitates channel gating. In conclusion, high binding affinity and facilitated K+ gating allow KtrAB to function as a selective K+ channel.

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<![CDATA[Tension pneumothorax in a patient with COVID-19]]> https://www.researchpad.co/article/elastic_article_12524 A 36-year-old man was brought to the emergency department with suspected COVID-19, following a 3-week history of cough, fevers and shortness of breath, worsening suddenly in the preceding 4 hours. On presentation he was hypoxaemic, with an SpO2 of 88% on 15 L/min oxygen, tachycardic and had no audible breath sounds on auscultation of the left hemithorax. Local guidelines recommended that the patient should be initiated on continuous positive airway pressure while investigations were awaited, however given the examination findings an emergency portable chest radiograph was performed. The chest radiograph demonstrated a left-sided tension pneumothorax. This was treated with emergency needle decompression, with good effect, followed by chest drain insertion. A repeat chest radiograph demonstrated lung re-expansion, and the patient was admitted to a COVID-19 specific ward for further observation. This case demonstrates tension pneumothorax as a possible complication of suspected COVID-19 and emphasises the importance of thorough history-taking and clinical examination.

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<![CDATA[Phosphoinositides modulate the voltage dependence of two-pore channel 3]]> https://www.researchpad.co/article/N20f2aa01-0a02-4cb4-81d8-0e377fcc0916

Among the three two-pore channels (TPCs), TPC1 and TPC2 are selectively activated by PI(3,5)P2, while TPC3 has been considered not to respond to any PIP2s. Shimomura and Kubo find that TPC3 responds to both PI(3,5)P2 and PI(3,4)P2, but not to PI(4,5)P2.

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<![CDATA[The voltage-gated sodium channel pore exhibits conformational flexibility during slow inactivation]]> https://www.researchpad.co/article/5c910777d5eed0c4841a4305

Voltage-gated sodium channels undergo slow inactivation during prolonged depolarization by means of a mechanism that is poorly understood. Chatterjee et al. study this process spectroscopically and reveal conformational flexibility of the pore region in the slow-inactivated state.

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<![CDATA[A forward genetic screen identifies chaperone CNX-1 as a conserved biogenesis regulator of ERG K+ channels]]> https://www.researchpad.co/article/5c69ec93d5eed0c48414df01

The mechanism underlying the biogenesis of hERG channels is not fully understood. Bai et al. identify CNX-1 as a novel regulator of ERG K+ channel biogenesis that is conserved from Caenorhabditis elegans to humans.

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<![CDATA[The N terminus of α-ENaC mediates ENaC cleavage and activation by furin]]> https://www.researchpad.co/article/5c69eca0d5eed0c48414e004

A natural splice deletion of rat α-ENaC was previously reported to produce reduced ENaC current that was not attributable to reduced surface expression. Kota et al. show that this ENaC variant resists furin cleavage, implicating α-ENaC residues 34–82 in ENaC posttranslational processing.

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<![CDATA[Duplex signaling by CaM and Stac3 enhances CaV1.1 function and provides insights into congenital myopathy]]> https://www.researchpad.co/article/5c69ec9cd5eed0c48414dfaf

CaV1.1 is essential for initiating skeletal muscle contraction. Niu et al. demonstrate that both CaM and stac3 enhance trafficking and gating of CaV1.1. Stac3 mutations associated with congenital myopathy, weaken its binding of CaV1.1, and thus reduce trafficking.

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<![CDATA[Biaryl sulfonamide motifs up- or down-regulate ion channel activity by activating voltage sensors]]> https://www.researchpad.co/article/5c69ec8ad5eed0c48414de3c

There exist many different ion channel modulators that inhibit channel function, but few that increase it. Liin et al. tested 18,000 compounds by high-throughput electrophysiology and found several channel openers with a biaryl-sulfonamide motif that acts on the voltage-sensing machinery.

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<![CDATA[Electrical recordings of the mitochondrial calcium uniporter in Xenopus oocytes]]> https://www.researchpad.co/article/5c368381d5eed0c4841f3fa3

The mitochondrial calcium uniporter is a Ca2+ channel that has been hard to characterize electrophysiologically. Tsai and Tsai establish a method that permits efficient electrophysiological recordings of the human uniporter in Xenopus oocytes and demonstrate characteristic uniporter behaviour.

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<![CDATA[Respiratory disease mortality in the United Kingdom compared with EU15+ countries in 1985-2015: observational study]]> https://www.researchpad.co/article/5c19b34ad5eed0c484c531b0

Abstract

Objective

To compare age standardised death rates for respiratory disease mortality between the United Kingdom and other countries with similar health system performance.

Design

Observational study.

Setting

World Health Organization Mortality Database, 1985-2015.

Participants

Residents of the UK, Austria, Belgium, Denmark, Finland, France, Germany, Greece, Ireland, Italy, Luxembourg, the Netherlands, Portugal, Spain, Sweden, Australia, Canada, the United States, and Norway (also known as EU15+ countries).

Main outcome measures

Mortality from all respiratory disease and infectious, neoplastic, interstitial, obstructive, and other respiratory disease. Differences between countries were tested over time by mixed effect regression models, and trends in subcategories of respiratory related diseases assessed by a locally weighted scatter plot smoother.

Results

Between 1985 and 2015, overall mortality from respiratory disease in the UK and EU15+ countries decreased for men and remained static for women. In the UK, the age standardised death rate (deaths per 100 000 people) for respiratory disease mortality in the UK fell from 151 to 89 for men and changed from 67 to 68 for women. In EU15+ countries, the corresponding changes were from 108 to 69 for men and from 35 to 37 in women. The UK had higher mortality than most EU15+ countries for obstructive, interstitial, and infectious subcategories of respiratory disease in both men and women.

Conclusion

Mortality from overall respiratory disease was higher in the UK than in EU15+ countries between 1985 and 2015. Mortality was reduced in men, but remained the same in women. Mortality from obstructive, interstitial, and infectious respiratory disease was higher in the UK than in EU15+ countries.

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<![CDATA[Analysis of the quality of crystallographic data and the limitations of structural models]]> https://www.researchpad.co/article/5c01f8b8d5eed0c4842be84a

Arkhipova et al. caution that the limitations of structural models be taken into account when interpreting crystallographic data.

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<![CDATA[A new model for an old friend]]> https://www.researchpad.co/article/5c01f8c0d5eed0c4842be9db

JGP study suggests the anion exchanger AE1 operates via an elevator-like mechanism.

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