A hyperpolarizing neuron recruits undocked innexin hemichannels to transmit neural information in Caenorhabditis elegans

While depolarization of the neuronal membrane is known to evoke the neurotransmitter release from synaptic vesicles, hyperpolarization is regarded as a resting state of chemical neurotransmission. Here, we report that hyperpolarizing neurons can actively signal neural information by employing undocked hemichannels. We show that UNC-7, a member of the innexin family in Caenorhabditis elegans, functions as a hemichannel in thermosensory neurons and transmits temperature information from the thermosensory neurons to their postsynaptic interneurons. By monitoring neural activities in freely behaving animals, we find that hyperpolarizing thermosensory neurons inhibit the activity of the interneurons and that UNC-7 hemichannels regulate this process. UNC-7 is required to control thermotaxis behavior and functions independently of synaptic vesicle exocytosis. Our findings suggest that innexin hemichannels mediate neurotransmission from hyperpolarizing neurons in a manner that is distinct from the synaptic transmission, expanding the way of neural circuitry operations.

Evidence of a Hardening in the Cosmic Ray Proton Spectrum at around 166 TeV Observed by the GRAPES-3 Experiment

We present the measurement of the cosmic ray proton spectrum from 50 TeV to 1.3 PeV using 7.81×10^{6} extensive air shower events recorded by the ground-based GRAPES-3 experiment between 1 January 2014 and 26 October 2015 with a live time of 460 day. Our measurements provide an overlap with direct observations by satellite and balloon-based experiments. The electromagnetic and muon components in the shower were measured by a dense array of plastic scintillator detectors and a tracking muon telescope, respectively. The relative composition of the proton primary from the air shower data containing all primary particles was extracted using the multiplicity distribution of muons which is a sensitive observable for mass composition. The observed proton spectrum suggests a spectral hardening at ∼166  TeV and disfavors a single power law description of the spectrum up to the Knee energy (∼3  PeV).

Structural basis of hydroxycarboxylic acid receptor signaling mechanisms through ligand binding

Hydroxycarboxylic acid receptors (HCA) are expressed in various tissues and immune cells. HCA2 and its agonist are thus important targets for treating inflammatory and metabolic disorders. Only limited information is available, however, on the active-state binding of HCAs with agonists. Here, we present cryo-EM structures of human HCA2-Gi and HCA3-Gi signaling complexes binding with multiple compounds bound. Agonists were revealed to form a salt bridge with arginine, which is conserved in the HCA family, to activate these receptors. Extracellular regions of the receptors form a lid-like structure that covers the ligand-binding pocket. Although transmembrane (TM) 6 in HCAs undergoes dynamic conformational changes, ligands do not directly interact with amino acids in TM6, suggesting that indirect signaling induces a slight shift in TM6 to activate Gi proteins. Structural analyses of agonist-bound HCA2 and HCA3 together with mutagenesis and molecular dynamics simulation provide molecular insights into HCA ligand recognition and activation mechanisms.

Supermicrovascular anastomosis training using chicken wings and colored water

Plastic surgeons require experience in supermicroscopic vascular anastomosis. Herein, we report a simple, rapid, and cost-effective training method using chicken wings and colored water. The avian ventral metacarpal artery was selected for dissection and anastomosis to mimic supermicrosurgery. Over 14 weeks (one anastomosis per day), the ulnar artery in 100 chicken wings was exposed by dissection, cut proximally, and injected with blue food dye-colored water by an inexperienced surgeon. After ligating the artery branches, it was cut and subjected to end-to-end anastomosis. Next, colored water was injected into the ulnar artery to check for suture sufficiency. The vessel was re-dissected to inspect the lumen and sutures qualitatively. Of the 100 wings, the first and last 20 wings' ventral metacarpal artery dissection, anastomosis times, and leakage frequency were compared. Avian ventral metacarpal artery diameter was recorded, and the cumulative anastomosis time where individual anastomosis times started decreasing was determined. Leakage rates before and after this point were compared. The avian ventral metacarpal artery diameter was 0.7-0.8 mm. The last 20 wings had significantly shorter median dissection times (12:27 vs. 17:45 min), anastomosis times (9:02 vs. 12:29 min), and leakage rates (15% vs. 70%); more even stitching and parallel ligature points; and less vessel layer inversion than the first 20 wings. After a cumulative anastomosis time of 10 h 26 min, individual times sharply decreased, and the leakage rate decreased significantly (58.3% vs. 23.8%). The proposed method significantly improved supermicrosurgical anastomosis. Thus, we believe that this method will help surgeons improve their supermicrosurgical skills.

The structural basis of divalent cation block in a tetrameric prokaryotic sodium channel

Divalent cation block is observed in various tetrameric ion channels. For blocking, a divalent cation is thought to bind in the ion pathway of the channel, but such block has not yet been directly observed. So, the behaviour of these blocking divalent cations remains still uncertain. Here, we elucidated the mechanism of the divalent cation block by reproducing the blocking effect into NavAb, a well-studied tetrameric sodium channel. Our crystal structures of NavAb mutants show that the mutations increasing the hydrophilicity of the inner vestibule of the pore domain enable a divalent cation to stack on the ion pathway. Furthermore, non-equilibrium molecular dynamics simulation showed that the stacking calcium ion repel sodium ion at the bottom of the selectivity filter. These results suggest the primary process of the divalent cation block mechanism in tetrameric cation channels.

Recognition mechanism of a novel gabapentinoid drug, mirogabalin, for recombinant human α2δ1, a voltage-gated calcium channel subunit

Mirogabalin is a novel gabapentinoid drug with a hydrophobic bicyclo substituent on the γ-aminobutyric acid moiety that targets the voltage-gated calcium channel subunit α₂δ1. Here, to reveal the mirogabalin recognition mechanisms of α₂δ1, we present structures of recombinant human α₂δ1 with and without mirogabalin analyzed by cryo-electron microscopy. These structures show the binding of mirogabalin to the previously reported gabapentinoid binding site, which is the extracellular dCache_1 domain containing a conserved amino acid binding motif. A slight conformational change occurs around the residues positioned close to the hydrophobic group of mirogabalin. Mutagenesis binding assays identified that residues in the hydrophobic interaction region, in addition to several amino acid binding motif residues around the amino and carboxyl groups of mirogabalin, are critical for mirogabalin binding. The A215L mutation introduced to decrease the hydrophobic pocket volume predictably suppressed mirogabalin binding and promoted the binding of another ligand, L-Leu, with a smaller hydrophobic substituent than mirogabalin. Alterations of residues in the hydrophobic interaction region of α₂δ1 to those of the α₂δ2, α₂δ3, and α₂δ4 isoforms, of which α₂δ3 and α₂δ4 are gabapentin-insensitive, suppressed the binding of mirogabalin. These results support the importance of hydrophobic interactions in α₂δ1 ligand recognition.

Structural Basis for Binding of Potassium-Competitive Acid Blockers to the Gastric Proton Pump

As specific inhibitors of the gastric proton pump, responsible for gastric acidification, K⁺-competitive acid blockers (P-CABs) have recently been utilized in the clinical treatment of gastric acid-related diseases in Asia. However, as these compounds have been developed based on phenotypic screening, their detailed binding poses are unknown. We show crystal and cryo-EM structures of the gastric proton pump in complex with four different P-CABs, tegoprazan, soraprazan, PF-03716556 and revaprazan, at resolutions reaching 2.8 Å. The structures describe molecular details of their interactions and are supported by functional analyses of mutations and molecular dynamics simulations. We reveal that revaprazan has a novel binding mode in which its tetrahydroisoquinoline moiety binds deep in the cation transport conduit. The mechanism of action of these P-CABs can now be evaluated at the molecular level, which will facilitate the rational development and improvement of currently available P-CABs to provide better treatment of acid-related gastrointestinal diseases.

A cryptic phosphate-binding pocket on the SPFH domain of human stomatin that regulates a novel fibril-like self-assembly

Human stomatin (hSTOM) is a component of the membrane skeleton of erythrocytes that maintains the membrane's shape and stiffness through interconnecting spectrin and actin. hSTOM is a member of the protein family that possesses a single stomatin/prohibitin/flotillin/HflK (SPFH) domain at the center of the molecule. Although SPFH domain proteins are widely distributed from archaea to mammals, the detailed function of the domain remains unclear. In this study, we first determined the solution structure of the SPFH domain of hSTOM (hSTOM(SPFH)) via NMR. The solution structure of hSTOM(SPFH) is essentially identical to the already reported crystal structure of the STOM SPFH domain (mSTOM(SPFH)) of mice, except for the existence of a small hydrophilic pocket on the surface. We identified this pocket as a phosphate-binding site by comparing its NMR spectra with and without phosphate ions. Meanwhile, during the conventional process of protein NMR analysis, we eventually discovered that hSTOM(SPFH) formed a unique solid material after lyophilization. This lyophilized hSTOM(SPFH) sample was moderately slowly dissolved in a physiological buffer. Interestingly, it was resistant to dissolution against the phosphate buffer. We then found that the lyophilized hSTOM(SPFH) formed a fibril-like assembly under electron microscopy. Finally, we succeeded in reproducing this fibril-like assembly of hSTOM(SPFH) using a centrifugal ultrafiltration device, thus demonstrating that the increased protein concentration may promote self-assembly of hSTOM(SPFH) into fibril forms. Our observations may help understand the molecular function of the SPFH domain and its involvement in protein oligomerization as a component of the membrane skeleton. (245 words).

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Solution structure of human stomatin SPFH domain is determined.

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A cryptic phosphate-binding pocket was identified.

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Stomatin SPFH domain can form a fibril-like assembly at a high concentration.

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Phosphate ions promote formation of the fibril-like assembly.

Structure and dynamics of Odinarchaeota tubulin and the implications for eukaryotic microtubule evolution

Tubulins are critical for the internal organization of eukaryotic cells, and understanding their emergence is an important question in eukaryogenesis. Asgard archaea are the closest known prokaryotic relatives to eukaryotes. Here, we elucidated the apo and nucleotide-bound x-ray structures of an Asgard tubulin from hydrothermal living Odinarchaeota (OdinTubulin). The guanosine 5'-triphosphate (GTP)-bound structure resembles a microtubule protofilament, with GTP bound between subunits, coordinating the "+" end subunit through a network of water molecules and unexpectedly by two cations. A water molecule is located suitable for GTP hydrolysis. Time course crystallography and electron microscopy revealed conformational changes on GTP hydrolysis. OdinTubulin forms tubules at high temperatures, with short curved protofilaments coiling around the tubule circumference, more similar to FtsZ, rather than running parallel to its length, as in microtubules. Thus, OdinTubulin represents an evolutionary stage intermediate between prokaryotic FtsZ and eukaryotic microtubule-forming tubulins.

Structures of human pannexin-1 in nanodiscs reveal gating mediated by dynamic movement of the N terminus and phospholipids

Pannexin (PANX) family proteins form large-pore channels that mediate purinergic signaling. We analyzed the cryo-EM structures of human PANX1 in lipid nanodiscs to elucidate the gating mechanism and its regulation by the amino terminus in phospholipids. The wild-type channel has an amino-terminal funnel in the pore, but in the presence of the inhibitor probenecid, a cytoplasmically oriented amino terminus and phospholipids obstruct the pore. Functional analysis using whole-cell patch-clamp and oocyte voltage clamp showed that PANX1 lacking the amino terminus did not open and had a dominant negative effect on channel activity, thus confirming that the amino-terminal domain played an essential role in channel opening. These observations suggest that dynamic conformational changes in the amino terminus of human PANX1 are associated with lipid movement in and out of the pore. Moreover, the data provide insight into the gating mechanism of PANX1 and, more broadly, other large-pore channels.

Clinical implications of troponin-T elevations following TAVR

Funding Acknowledgements

Type of funding sources: None.

Background

Baseline and post-procedural elevations in serum troponin-T levels are associated with increased morbidity and mortality following transcatheter aortic valve replacement (TAVR). However, the prognostic impact of change in serum troponin-T level following TAVR remains unknown.

Methods

Among the patients with severe aortic stenosis who underwent TAVR, those with baseline serum troponin-T level ≥51.5 ng/L were excluded. The impact of increases in serum troponin-T level to an abnormally high range (≥51.5 ng/L) following TAVR on 2-year cardiovascular death or heart failure readmissions was investigated.

Results

Among 189 included patients (median 86 years old, 28% men), serum troponin-T level increased in 79 patients following TAVR. An increase in serum troponin-T was associated with a higher rate of 30-day adverse events, predominantly due to pacemaker implantation for complete atrio-ventricular block, and a higher 2-year cumulative incidence of the primary endpoint (hazard ratio 3.97, 95% confidence interval 1.51-10.4, p = 0.005) adjusted for the use of balloon-expandable valve and post-TAVR pacemaker implantation (Figure 1).

Conclusion

Post-procedural increase in serum troponin-T level was associated with adverse clinical outcomes following TAVR. Abstract Figure. Cumulative incidence of endpoint

Cryo-EM of the ATP11C flippase reconstituted in Nanodiscs shows a distended phospholipid bilayer inner membrane around transmembrane helix 2

ATP11C is a member of the P4-ATPase flippase family that mediates translocation of phosphatidylserine (PtdSer) across the lipid bilayer. In order to characterize the structure and function of ATP11C in a model natural lipid environment, we revisited and optimized a quick procedure for reconstituting ATP11C into Nanodiscs using methyl-β-cyclodextrin as a reagent for the detergent removal. ATP11C was efficiently reconstituted with the endogenous lipid, or the mixture of endogenous lipid and synthetic dioleoylphosphatidylcholine (DOPC)/dioleoylphosphatidylserine (DOPS), all of which retained the ATPase activity. We obtained 3.4 Å and 3.9 Å structures using single-particle cryo-electron microscopy (cryo-EM) of AlF- and BeF-stabilized ATP11C transport intermediates, respectively, in a bilayer containing DOPS. We show that the latter exhibited a distended inner membrane around ATP11C transmembrane helix 2, possibly reflecting the perturbation needed for phospholipid release to the lipid bilayer. Our structures of ATP11C in the lipid membrane indicate that the membrane boundary varies upon conformational changes of the enzyme and is no longer flat around the protein, a change that likely contributes to phospholipid translocation across the membrane leaflets.

Gastric proton pump with two occluded K+ engineered with sodium pump-mimetic mutations

The gastric H⁺,K⁺-ATPase mediates electroneutral exchange of 1H⁺/1K⁺ per ATP hydrolysed across the membrane. Previous structural analysis of the K⁺-occluded E2-P transition state of H⁺,K⁺-ATPase showed a single bound K⁺ at cation-binding site II, in marked contrast to the two K⁺ ions occluded at sites I and II of the closely-related Na⁺,K⁺-ATPase which mediates electrogenic 3Na⁺/2K⁺ translocation across the membrane. The molecular basis of the different K⁺ stoichiometry between these K⁺-counter-transporting pumps is elusive. We show a series of crystal structures and a cryo-EM structure of H⁺,K⁺-ATPase mutants with changes in the vicinity of site I, based on the structure of the sodium pump. Our step-wise and tailored construction of the mutants finally gave a two-K⁺ bound H⁺,K⁺-ATPase, achieved by five mutations, including amino acids directly coordinating K⁺ (Lys791Ser, Glu820Asp), indirectly contributing to cation-binding site formation (Tyr340Asn, Glu936Val), and allosterically stabilizing K⁺-occluded conformation (Tyr799Trp). This quintuple mutant in the K⁺-occluded E2-P state unambiguously shows two separate densities at the cation-binding site in its 2.6 Å resolution cryo-EM structure. These results offer new insights into how two closely-related cation pumps specify the number of K⁺ accommodated at their cation-binding site.

The gastric H⁺,K⁺-ATPase is a proton pump that creates the acidic environment of the stomach lumen, maintaining high proton gradient across the gastric mucosa cell membrane. Here, structural analysis of rationally designed H⁺,K⁺-ATPase mutants provides insight into this and other P-type ATPases cation binding stoichiometry and mechanisms.

Cryo-EM structures of undocked innexin-6 hemichannels in phospholipids

Gap junctions form intercellular conduits with a large pore size whose closed and open states regulate communication between adjacent cells. The structural basis of the mechanism by which gap junctions close, however, remains uncertain. Here, we show the cryo-electron microscopy structures of Caenorhabditis elegans innexin-6 (INX-6) gap junction proteins in an undocked hemichannel form. In the nanodisc-reconstituted structure of the wild-type INX-6 hemichannel, flat double-layer densities obstruct the channel pore. Comparison of the hemichannel structures of a wild-type INX-6 in detergent and nanodisc-reconstituted amino-terminal deletion mutant reveals that lipid-mediated amino-terminal rearrangement and pore obstruction occur upon nanodisc reconstitution. Together with molecular dynamics simulations and electrophysiology functional assays, our results provide insight into the closure of the INX-6 hemichannel in a lipid bilayer before docking of two hemichannels.

Measurement of the Electrical Properties of a Thundercloud Through Muon Imaging by the GRAPES-3 Experiment

The GRAPES-3 muon telescope located in Ooty, India records rapid (∼10  min) variations in the muon intensity during major thunderstorms. Out of a total of 184 thunderstorms recorded during the interval of April 2011-December 2014, the one on December 1, 2014 produced a massive potential of 1.3 GV. The electric field measured by four well-separated (up to 6 km) monitors on the ground was used to help estimate some of the properties of this thundercloud, including its altitude and area that were found to be 11.4 km above mean sea level and ≥380  km^{2}, respectively. A charging time of 6 min to reach 1.3 GV implied the delivery of a power of ≥2  GW by this thundercloud that was moving at a speed of ∼60  km h^{-1}. This work possibly provides the first direct evidence for the generation of gigavolt potentials in thunderclouds that could also possibly explain the production of highest-energy (100 MeV) gamma rays in the terrestrial gamma-ray flashes.

Structure of an innexin gap junction channel and cryo-EM sample preparation

Gap junction channels are essential for mediating intercellular communication in most multicellular organisms. Two gene families encode gap junction channels, innexin and connexin. Although the sequence similarity between these two families based on bioinformatics is not conclusively determined, the gap junction channels encoded by these two gene families are structurally and functionally analogous. We recently reported an atomic structure of an invertebrate innexin gap junction channel using single-particle cryo-electron microscopy. Our findings revealed that connexin and innexin families share several structural properties with regard to their monomeric and oligomeric structures, while simultaneously suggesting a diversity of gap junction channels in nature. This review summarizes cutting-edge progress toward determining an innexin gap junction channel structure, as well as essential tips for preparing cryo-electron microscopy samples for high-resolution structural analysis of an innexin gap junction channel.

Atomic structure of the innexin-6 gap junction channel determined by cryo-EM

Innexins, a large protein family comprising invertebrate gap junction channels, play an essential role in nervous system development and electrical synapse formation. Here we report the cryo-electron microscopy structures of Caenorhabditis elegans innexin-6 (INX-6) gap junction channels at atomic resolution. We find that the arrangements of the transmembrane helices and extracellular loops of the INX-6 monomeric structure are highly similar to those of connexin-26 (Cx26), despite the lack of significant sequence similarity. The INX-6 gap junction channel comprises hexadecameric subunits but reveals the N-terminal pore funnel, consistent with Cx26. The helix-rich cytoplasmic loop and C-terminus are intercalated one-by-one through an octameric hemichannel, forming a dome-like entrance that interacts with N-terminal loops in the pore. These observations suggest that the INX-6 cytoplasmic domains are cooperatively associated with the N-terminal funnel conformation, and an essential linkage of the N-terminal with channel activity is presumably preserved across gap junction families.

Transient Weakening of Earth's Magnetic Shield Probed by a Cosmic Ray Burst

The GRAPES-3 tracking muon telescope in Ooty, India measures muon intensity at high cutoff rigidities (15-24 GV) along nine independent directions covering 2.3 sr. The arrival of a coronal mass ejection on 22 June 2015 18:40 UT had triggered a severe G4-class geomagnetic storm (storm). Starting 19:00 UT, the GRAPES-3 muon telescope recorded a 2 h high-energy (∼20  GeV) burst of galactic cosmic rays (GCRs) that was strongly correlated with a 40 nT surge in the interplanetary magnetic field (IMF). Simulations have shown that a large (17×) compression of the IMF to 680 nT, followed by reconnection with the geomagnetic field (GMF) leading to lower cutoff rigidities could generate this burst. Here, 680 nT represents a short-term change in GMF around Earth, averaged over 7 times its volume. The GCRs, due to lowering of cutoff rigidities, were deflected from Earth's day side by ∼210° in longitude, offering a natural explanation of its night-time detection by the GRAPES-3. The simultaneous occurrence of the burst in all nine directions suggests its origin close to Earth. It also indicates a transient weakening of Earth's magnetic shield, and may hold clues for a better understanding of future superstorms that could cripple modern technological infrastructure on Earth, and endanger the lives of the astronauts in space.

Intraocular Ossification in the GSP/pe Chicken With Imperfect Albinism

The eyes of 2 male and 2 female GSP/pe chickens, the imperfect albino strain, were investigated at 52 weeks of age. Aged chickens of the GSP/pe colony became blind with bilateral ocular enlargement and opaque lenses. Affected eyes (bilateral in 2 males and unilateral in 2 females) were hard and difficult to section; histologic specimens were processed after decalcification. A large portion of the posterior chamber was occupied by cancellous bone containing fibrous and cartilaginous foci. Osseous tissues developed adjacent to the choroid, and no retinal pigment epithelium (RPE) was detected between osseous tissues and the choroid. Small segments of degenerate neuronal retina were scattered in the osseous tissue. The irises and ciliary bodies were deformed by osseous tissue, and the lenses had severe cataracts. These observations suggest that the intraocular osseous tissue may be derived from RPE in the hereditary incomplete-albino strain of chickens.

Prognosis of arterial aneurysm after surgery in patients with Behçet's disease

AIM

Despite improvements in therapeutic modalities, the treatment of arterial aneurysms complicating Behçet's disease (BD) is still challenging. This study examined the long-term prognosis after surgery for arterial aneurysms in BD.

METHODS

This study included 9 patients with BD (8 men and 1 woman) who underwent surgery for arterial aneurysms between 1989 and 2008. The outcomes after the surgical intervention were assessed, including procedure-related complications and survival.

RESULTS

The initial surgical procedures were performed for aortic or iliac aneurysms in 5 patients and for lower-extremity aneurysms in 4 patients. There was no operative mortality. The mean follow-up period was 135±69 months, ranging from 53 to 259 months. Patients with aortic or iliac aneurysms underwent graft interposition with Dacron prostheses. Their postoperative courses were uneventful, and all patients were alive during the follow-up with no procedure-related complications. Those treated for lower-extremity aneurysms tended to show perioperative and postoperative complications, including aneurysmal degeneration of the autogenous vein graft in 2 patients. One patient who initially underwent surgery for a popliteal artery aneurysm died due to the rupture of a dissecting aortic aneurysm after serial surgical interventions for multiple aneurysms. Concomitant aortic or iliac aneurysms in 2 patients were followed up without any change in size under medical treatment using colchicine and corticosteroids.

CONCLUSION

Although we cannot draw a firm conclusion because of the small number of cases in the present series, graft interposition can lead to a favorable prognosis in BD patients with aortic or iliac aneurysms, whereas surgical treatment of BD-related lower-extremity aneurysms is frequently associated with short- and long-term postoperative complications. Immunosuppressive therapy might possibly improve treatment outcomes.

Electron microscopy and functional analysis of recombinant innexin gap junctions

Innexin is a molecular component of invertebrate gap junctions, which have an important role in neural and muscular electrical activity in invertebrates. Although the structure of vertebrate connexin26 was revealed by X-ray crystallography [1], the structure of innexin channels remains poorly understood. To study the structure of innexin gap junction channels, we expressed and purified Caenorhabditis elegans innexin-6 (INX-6) gap junction channels, and characterized their molecular dimensions and channel permeability using electron microscopy (EM) and a fluorescent dye transfer assay, respectively [2]. Negative-staining and thin-section EM of isolated INX-6 gap junction plaques revealed a loosely packed hexagonal lattice. We performed single particle analysis of purified INX-6 channels with negative-staining and cryo EM. Based on the negative-stain EM images, the class average of the junction form had a longitudinal height of 220 Å, a channel diameter of 110 Å in the absence of detergent micelles, and an extracellular gap space of 60 Å, whereas the class average of the hemichannels had diameters of up to 140 Å in the presence of detergent micelles. Cryo EM images revealed rotational peaks that could be related to the INX-6 subunits. Structural analysis of the reconstituted INX-6 channels with single particle analysis and electron tomography suggested that the oligomeric number of the INX-6 channel was distinct from that of the dodecameric connexin channel. Dye transfer experiments indicated that the INX-6-GFP-His channels were permeable to 3-kDa and 10-kDa dextran-conjugated tracers. These findings indicate that INX-6 channels have a characteristic oligomer component that differs from that in connexin gap junction channels.

Structure and closure of connexin gap junction channels

Connexin gap junctions comprise assembled channels penetrating two plasma membranes for which gating regulation is associated with a variety of factors, including voltage, pH, Ca(2+), and phosphorylation. Functional studies have established that various parts of the connexin peptides are related to channel closure and electrophysiology studies have provided several working models for channel gating. The corresponding structural models supporting these findings, however, are not sufficient because only small numbers of closed connexin structures have been reported. To fully understand the gating mechanisms, the channels should be visualized in both the open and closed states. Electron crystallography and X-ray crystallography studies recently revealed three-dimensional structures of connexin channels in a couple of states in which the main difference is the conformation of the N-terminal domain, which have helped to clarify the structure in regard to channel closure. Here the closure models for connexin gap junction channels inferred from structural and functional studies are described in the context of each domain of the connexin protein associated with gating modulation.

Oligomeric structure and functional characterization of Caenorhabditis elegans Innexin-6 gap junction protein

Innexin is the molecular component of invertebrate gap junctions. Here we successfully expressed and purified Caenorhabditis elegans innexin-6 (INX-6) gap junction channels and characterized the molecular dimensions and channel permeability using electron microscopy (EM) and microinjection of fluorescent dye tracers, respectively. Negative staining and thin-section EM of isolated INX-6 gap junction membranes revealed a loosely packed hexagonal lattice and a greater cross-sectional width than that of connexin26 and connexin43 (Cx43)-GFP. In gel filtration analysis, the elution profile of purified INX-6 channels in dodecyl maltoside solution exhibited a peak at ∼400 kDa that was shifted to ∼800 kDa in octyl glucose neopentyl glycol. We also obtained the class averages of purified INX-6 channels from these peak fractions by single particle analysis. The class average from the ∼800-kDa fraction showed features of the junction form with a longitudinal height of 220 Å, a channel diameter of 110 Å in the absence of detergent micelles, and an extracellular gap space of 60 Å, whereas the class averages from the ∼400-kDa fraction showed diameters of up to 140 Å in the presence of detergent micelles. These findings indicate that the purified INX-6 channels are predominantly hemichannels in dodecyl maltoside and docked junction channels in octyl glucose neopentyl glycol. Dye transfer experiments revealed that the INX-6-GFP-His channels are permeable to 3- and 10-kDa tracers, whereas no significant amounts of these tracers passed through the Cx43-GFP channels. Based on these findings, INX-6 channels have a larger overall structure and greater permeability than connexin channels.

GATA3 inhibits lysyl oxidase-mediated metastases of human basal triple-negative breast cancer cells

Discovery of mechanisms that impede the aggressive and metastatic phenotype of human basal triple-negative type breast cancers (BTNBC) could provide novel targets for therapy for this form of breast cancer that has a relatively poor prognosis. Previous studies have demonstrated that the expression of GATA3, the master transcriptional regulator of mammary luminal differentiation, can reduce the tumorigenicity and metastatic propensity of the human BTNBC MDA-MB-231 cell line (MB231), although the mechanism for reduced metastases was not elucidated. We demonstrate through gene expression profiling that GATA3 expression in 231 cells resulted in the dramatic reduction in the expression of Lysyl oxidase (LOX), a metastasis-promoting matrix remodeling protein, in part, through methylation of the LOX promoter. Suppression of LOX expression by GATA3 was further confirmed in the BTNBC Hs578T cell line. Conversely, reduction of GATA3 expression by siRNA in luminal BT474 cells increased LOX expression. Reconstitution of LOX expression in 231-GATA3 cells restored metastatic propensity. A strong inverse association between high LOX and low GATA3 expression was confirmed in a panel of 51 human breast cancer cell lines. Similarly, human breast cancer microarray data demonstrated that high LOX/low GATA3 expression is associated with the BTNBC subtype of breast cancer and poor patient prognosis. Expression of GATA3 reprograms BTNBC to a less aggressive phenotype and inhibits a major mechanism of metastasis through inhibition of LOX. Induction of GATA3 in BTNBC cells or novel approaches that inhibit LOX expression or activity could be important strategies for treating BTNBC.

Asymmetric configurations and N-terminal rearrangements in connexin26 gap junction channels

Gap junction channels are unique in that they possess multiple mechanisms for channel closure, several of which involve the N terminus as a key component in gating, and possibly assembly. Here, we present electron crystallographic structures of a mutant human connexin26 (Cx26M34A) and an N-terminal deletion of this mutant (Cx26M34Adel2-7) at 6-Å and 10-Å resolutions, respectively. The three-dimensional map of Cx26M34A was improved by data from 60° tilt images and revealed a breakdown of the hexagonal symmetry in a connexin hemichannel, particularly in the cytoplasmic domain regions at the ends of the transmembrane helices. The Cx26M34A structure contained an asymmetric density in the channel vestibule ("plug") that was decreased in the Cx26M34Adel2-7 structure, indicating that the N terminus significantly contributes to form this plug feature. Functional analysis of the Cx26M34A channels revealed that these channels are predominantly closed, with the residual electrical conductance showing normal voltage gating. N-terminal deletion mutants with and without the M34A mutation showed no electrical activity in paired Xenopus oocytes and significantly decreased dye permeability in HeLa cells. Comparing this closed structure with the recently published X-ray structure of wild-type Cx26, which is proposed to be in an open state, revealed a radial outward shift in the transmembrane helices in the closed state, presumably to accommodate the N-terminal plug occluding the pore. Because both Cx26del2-7 and Cx26M34Adel2-7 channels are closed, the N terminus appears to have a prominent role in stabilizing the open configuration.

Risk factors for intrahepatic cholangiocarcinoma: a possible role of hepatitis B virus

There are several established risk factors for intrahepatic cholangiocarcinoma (ICC), namely primary sclerosing cholangitis, fibropolycystic liver disease, parasitic infection, intrahepatic biliary stones and chemical carcinogen exposure. However, the majority of patients with ICC do not have any of these risk factors. Therefore, identification of other risk factors is warranted for the prevention and early detection of ICC. We evaluated the risk factors for ICC in a large-scale cohort study in the province of Osaka, Japan. This retrospective cohort study included 154,814 apparently healthy individual blood donors, aged 40-64 years at the time of blood donation in the period 1991-1993. The average observation period was 7.6 years, resulting in 1.25 million person-years of observation. Incident ICC cases were identified by linking the blood-donor database to the records in the population-based cancer registry for the province. There were 11 incident ICC cases during follow-up, with an incidence rate of 0.88 per 100,000 person-years. Compared with subjects aged 40-49 years, the subjects aged 50-54 years and 55-59 years had a significantly higher risk for ICC (hazard ratio [HR] = 5.90; 95%CI:1.08-32.31 and 11.07; 95%CI:1.98-61.79, respectively). Compared with those with ALT level of 19 Karmen Units (KU) or less, subjects with ALT level of 40 KU or higher had a significantly higher risk for ICC (HR: 8.30; 95%CI:1.47-46.83). Compared with those who tested negative for both HBsAg and anti-HCV, those who tested HBsAg-positive had a significantly higher risk for ICC (HR: 8.56; 95%CI: 1.33-55.20). Our results suggest that HBV infection and liver inflammation are independently associated with ICC development. These findings need to be verified by further large cohort studies.

Analysis of four connexin26 mutant gap junctions and hemichannels reveals variations in hexamer stability

Connexin26 is a ubiquitous gap junction protein that serves critical homeostatic functions. Four single-site mutations found in the transmembrane helices (M1-M4) cause different types of dysfunctional channels: 1), Cx26T135A in M3 produces a closed channel; 2), Cx26M34A in M1 severely decreases channel activity; 3), Cx26P87L in M2 has been implicated in defective channel gating; and 4), Cx26V84L in M2, a nonsyndromic deafness mutant, retains normal dye coupling and electrophysiological properties but is deficient in IP(3) transfer. These mutations do not affect Cx26 trafficking in mammalian cells, and make normal-appearing channels in baculovirus-infected Sf9 membranes when imaged by negative stain electron microscopy. Upon dodecylmaltoside solubilization of the membrane fraction, Cx26M34A and Cx26V84L are stable as hexamers or dodecamers, but Cx26T135A and Cx26P87L oligomers are not. This instability is also found in Cx26T135A and Cx26P87L hemichannels isolated from mammalian cells. In this work, coexpression of both wild-type Cx26 and Cx26P87L in Sf9 cells rescued P87L hexamer stability. Similarly, in paired Xenopus oocytes, coexpression with wild-type restored function. In contrast, the stability of Cx26T135A hemichannels could not be rescued by coexpression with WT. Thus, T135 and P87 residues are in positions that are important for oligomer stability and can affect gap junction gating.

The M34A mutant of Connexin26 reveals active conductance states in pore-suspending membranes

Connexin26 (Cx26) is a member of the connexin family, the building blocks for gap junction intercellular channels. These dodecameric assemblies are involved in gap junction-mediated cell-cell communication allowing the passage of ions and small molecules between two neighboring cells. Mutations in Cx26 lead to the disruption of gap junction-mediated intercellular communication with consequences such as hearing loss and skin disorders. We show here that a mutant of Cx26, M34A, forms an active hemichannel in lipid bilayer experiments. A comparison with the Cx26 wild-type is presented. Two different techniques using micro/nano-structured substrates for the formation of pore-suspending lipid membranes are used. We reconstituted the Cx26 wild-type and Cx26M34A into artificial lipid bilayers and observed single channel activity for each technique, with conductance levels of around 35, 70 and 165 pS for the wild-type. The conductance levels of Cx26M34A were found at around 45 and 70 pS.

Gene expression signatures to predict the response of gastric cancer to cisplatin and fluorouracil

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Background: A prospective high-throughput gene expression study was conducted to identify transcriptional profiles predictive of a clinical response to cisplatin and fluorouracil (CF) combination chemotherapy and to identify dysregulated genes associated with acquired resistance to CF. Methods: Endoscopic biopsy samples were collected from CF-treated metastatic gastric cancer (MGC) patients (pts) prior to CF (n = 123) and following the development of resistance (n = 22) at the National Cancer Center of Korea from 2001 to 2006, and analyzed using CGH and expression microarrays. We developed 2 survival risk predictors. The first predictor was constructed using genes in DNA amplicons and identified in the expression signature that correlates with survival (intrinsic resistance signature). The second predictor was based on the acquired resistance signature, which was identified by comparing matched expression array data from initially responsive patients prior to treatment with profiles obtained at progressive disease. Results: Array CGH revealed the gene amplification of MYC, EGFR, and FGFR2 whose Affymetrix U133A signals significantly correlated with a poor prognosis (P values, 0.0154, 0.0096, and 0.0057) of training set pts (n = 96). Three-gene-predicted high-risk group of the validation cohort (n = 10) demonstrated a shorter median survival than low-risk (n = 17) group (7.4 vs 16.8 months; p = 0.047). The 3-gene signature, as a continuous variable, was the independent predictor for overall survival (OS) and time to progression (TTP) (adjusted P, 0.021, and 0.012). Importantly, the acquired resistance signature strongly overlapped the intrinsic resistance signature (LS P<10-5), and was highly enriched for MYC target genes (LS p = 2x10-5). A predictor based on MYC target genes within the acquired resistance signature was the independent predictor for OS and TTP of 101 separate pts (adjusted p, 0.015, and 0.011). Conclusions: Combined overexpression of MYC, EGFR, and FGFR2 predicts a poor response of MGC pts to CF. There is significant overlap between intrinsic and acquired resistance signatures of MGC, where the MYC gene network plays a central role. This is the first demonstration that the acquired resistance signature predicts the initial response to chemotherapy.

No significant financial relationships to disclose.

Projection structure of a N-terminal deletion mutant of connexin 26 channel with decreased central pore density

Gated gap junction channels are important cellular conduits for establishing and maintaining intercellular communication. The three-dimensional structure of a mutant human connexin 26 (Cx26M34A) by electron cryocrystallography revealed a plug-like density in the channel pore suggesting that physical blockage of the pore may be one mechanism of closure (Oshima et al. 2007, Proc Natl Acad Sci USA 104: 10034-10039). However, it remains to be determined what part of the sequence contributes to the plug. Here, we present the projection structure of an N-terminus deletion of Cx26M34A missing amino acids 2 to 7 (Cx26M34Adel2-7) crystallized in the same two-dimensional crystal form. A 10 A resolution projection map of Cx26M34Adel2-7 revealed that the plug density was dramatically reduced in comparison with that found in full-length Cx26 channel. The difference map between the deletion and full-length Cx26M34A channels strongly suggests that the N-terminus of connexin contributes to the plug for the physical closure of gap junction channels.

Mutation profile of the CDH23 gene in 56 probands with Usher syndrome type I

Mutations in the human gene encoding cadherin23 (CDH23) cause Usher syndrome type 1D (USH1D) and nonsyndromic hearing loss. Individuals with Usher syndrome type I have profound congenital deafness, vestibular areflexia and usually begin to exhibit signs of RP in early adolescence. In the present study, we carried out the mutation analysis in all 69 exons of the CDH23 gene in 56 Usher type 1 probands already screened for mutations in MYO7A. A total of 18 of 56 subjects (32.1%) were observed to have one or two CDH23 variants that are presumed to be pathologic. Twenty one different pathologic genome variants were observed of which 15 were novel. Out of a total of 112 alleles, 31 (27.7%) were considered pathologic. Based on our results it is estimated that about 20% of patients with Usher syndrome type I have CDH23 mutations.

The localization of proteins encoded by CRYM, KIAA1199, UBA52, COL9A3, and COL9A1, genes highly expressed in the cochlea

Genes that are highly expressed in the inner ear, as revealed by cDNA microarray analysis, may have a crucial functional role there. Those that are expressed specifically in auditory tissues are likely to be good candidates to screen for genetic alterations in patients with deafness, and several genes have been successfully identified as responsible for hereditary hearing loss. To understand the detailed mechanisms of the hearing loss caused by the mutations in these genes, the present study examined the immunocytochemical localization of the proteins encoded by Crym, KIAA1199 homolog, Uba52, Col9a3, and Col9a1 in the cochlea of rats and mice. Confocal microscopic immunocytochemistry was performed on cryostat sections. Ultrastructurally, postembedding immunogold cytochemistry was applied using Lowicryl sections. Crym protein was predominantly distributed in the fibrocytes in the spiral ligament, as well as the stria vascularis in rats. KIAA1199 protein homolog was localized in various supporting cells, including inner phalangeal, border, inner and outer pillar, and Deiters' cells. Uba52 protein was restrictedly localized within the surface of the marginal cells of the stria vascularis. Collagen type IX was found within the tectorial membrane as well as fibrocytes in the spiral ligament. The present results showed cell-specific localization of the encoded proteins of these highly expressed genes, indicating that the coordinated actions of various molecules distributed in different parts of the cochlea are essential for maintenance of auditory processing in the cochlea.

Relationship between age at onset and magnetic resonance image-defined hyperintensities in mood disorders

OBJECTIVES

To examine in patients with mood disorders the relationship of age at onset with the location and degree of MRI-defined brain hyperintensities.

METHOD

Fifty-two patients diagnosed as having mood disorders and 14 controls participated in the study. Brain MR images were analyzed according to semiquantitative ratings for the anatomical distribution and severity of T2-weighted hyperintensities. We compared these hyperintensities among the three age- and sex-matched groups of late-onset mood disorder patients (LOM), early-onset mood disorder patients (EOM), and controls. The time since the onset of disorder was significantly longer in the EOM than in the LOM group. We also conducted linear multiple regression analysis using the severity of hyperintensities as dependent variable to determine whether the clinical features correlate with vascular pathology.

RESULTS

As for deep white matter hyperintensity (DWMH), LOM exhibited higher ratings than EOM; as for brain areas, significant between-group differences were detected in the bilateral frontal areas and in the left parieto-occipital area. No significant difference was observed between EOM and controls. As for periventricular hyperintensity, there was no difference among the three groups. We obtained a significant regression model to predict DWMH ratings; age, number of ECTs, and LOM were selected as significant variables.

CONCLUSION

The present study suggests that the time since the onset of disorder does not affect the development of white matter lesions, but that white matter lesions are associated with late-onset mood disorders. The frontal areas and the left parieto-occipital area would be important for the development of late-onset mood disorders.