Casein kinase 2 phosphorylates and induces the SALL2 tumor suppressor degradation in colon cancer cells

Spalt-like proteins are Zinc finger transcription factors from Caenorhabditis elegans to vertebrates, with critical roles in development. In vertebrates, four paralogues have been identified (SALL1-4), and SALL2 is the family's most dissimilar member. SALL2 is required during brain and eye development. It is downregulated in cancer and acts as a tumor suppressor, promoting cell cycle arrest and cell death. Despite its critical functions, information about SALL2 regulation is scarce. Public data indicate that SALL2 is ubiquitinated and phosphorylated in several residues along the protein, but the mechanisms, biological consequences, and enzymes responsible for these modifications remain unknown. Bioinformatic analyses identified several putative phosphorylation sites for Casein Kinase II (CK2) located within a highly conserved C-terminal PEST degradation motif of SALL2. CK2 is a serine/threonine kinase that promotes cell proliferation and survival and is often hyperactivated in cancer. We demonstrated that CK2 phosphorylates SALL2 residues S763, T778, S802, and S806 and promotes SALL2 degradation by the proteasome. Accordingly, pharmacological inhibition of CK2 with Silmitasertib (CX-4945) restored endogenous SALL2 protein levels in SALL2-deficient breast MDA-MB-231, lung H1299, and colon SW480 cancer cells. Silmitasertib induced a methuosis-like phenotype and cell death in SW480 cells. However, the phenotype was significantly attenuated in CRISPr/Cas9-mediated SALL2 knockout SW480 cells. Similarly, Sall2-deficient tumor organoids were more resistant to Silmitasertib-induced cell death, confirming that SALL2 sensitizes cancer cells to CK2 inhibition. We identified a novel CK2-dependent mechanism for SALL2 regulation and provided new insights into the interplay between these two proteins and their role in cell survival and proliferation.

Pycnoporus cinnabarinus glyoxal oxidases display differential catalytic efficiencies on 5-hydroxymethylfurfural and its oxidized derivatives

Background

5-Hydroxymethylfurfural (HMF), a major residual component of a lignocellulosic bio-refinery process, can be transformed into fundamental building blocks for green chemistry via oxidation. While chemical methods are well established, interest is also being directed into the enzymatic oxidation of HMF into the bio-plastic precursor 2,5-furandicarboxylic acid (FDCA).

Results

We demonstrate that three glyoxal oxidases (PciGLOX) isoenzymes from the Basidiomycete fungus Pycnoporus cinnabarinus were able to oxidize HMF, with PciGLOX2 and PciGLOX3 being the most efficient. The major reaction product obtained with the three isoenzymes was 5-hydroxymethyl-2-furancarboxylic (HMFCA), a precursor in polyesters and pharmaceuticals production, and very little subsequent conversion of this compound was observed. However, small concentrations of FDCA, a substitute for terephthalic acid in the production of polyesters, were also obtained. The oxidation of HMF was significantly boosted in the presence of catalase for PciGLOX2, leading to 70% HMFCA yield. The highest conversion percentages were observed on 2,5-furandicarboxaldehyde (DFF), a minor product from the reaction of PciGLOX on HMF. To bypass HMFCA accumulation and exploit the efficiency of PciGLOX in oxidizing DFF and 5-formyl-2-furan carboxylic acid (FFCA) towards FDCA production, HMF was oxidized in a cascade reaction with an aryl alcohol oxidase (UmaAAO). After 2 h of reaction, UmaAAO completely oxidized HMF to DFF and further to FFCA, with FDCA only being detected when PciGLOX3 was added to the reaction. The maximum yield of 16% FDCA was obtained 24 h after the addition of PciGLOX3 in the presence of catalase.

Conclusions

At least two conversion pathways for HMF oxidation can be considered for PciGLOX; however, the highest selectivity was seen towards the production of the valuable polyester precursor HMFCA. The three isoenzymes showed differences in their catalytic efficiencies and substrate specificities when reacted with HMF derivatives.

Electronic supplementary material

The online version of this article (10.1186/s40694-019-0067-8) contains supplementary material, which is available to authorized users.

Pancreatic two P domain K+ channels TALK-1 and TALK-2 are activated by nitric oxide and reactive oxygen species

This study firstly shows with in situ hybridization on human pancreas that TALK-1 and TALK-2, two members of the 2P domain potassium channel (K(2P)) family, are highly and specifically expressed in the exocrine pancreas and absent in Langherans islets. On the contrary, expression of TASK-2 in mouse pancreas is found both in the exocrine pancreas and in the Langherans islets. This study also shows that TALK-1 and TALK-2 channels, expressed in Xenopus oocytes, are strongly and specifically activated by nitric oxide (obtained with a mixture of sodium nitroprussate (SNP) and dithiothreitol (DTT)), superoxide anion (obtained with xanthine and xanthine oxidase) and singlet oxygen (obtained upon photoactivation of rose bengal, and with chloramine T). Other nitric oxide and reactive oxygen species (NOS and ROS) donors, as well as reducing conditions were found to be ineffective on TALK-1, TALK-2 and TASK-2 (sin-1, angeli's salt, SNP alone, tBHP, H(2)O(2), and DTT). These results suggest that, in the exocrine pancreas, specific members of the NOS and ROS families could act as endogenous modulators of TALK channels with a role in normal secretion as well as in disease states such as acute pancreatitis and apoptosis.

TREK-1, a K+ channel involved in neuroprotection and general anesthesia

TREK-1 is a two-pore-domain background potassium channel expressed throughout the central nervous system. It is opened by polyunsaturated fatty acids and lysophospholipids. It is inhibited by neurotransmitters that produce an increase in intracellular cAMP and by those that activate the Gq protein pathway. TREK-1 is also activated by volatile anesthetics and has been suggested to be an important target in the action of these drugs. Using mice with a disrupted TREK-1 gene, we now show that TREK-1 has an important role in neuroprotection against epilepsy and brain and spinal chord ischemia. Trek1-/- mice display an increased sensitivity to ischemia and epilepsy. Neuroprotection by polyunsaturated fatty acids, which is impressive in Trek1+/+ mice, disappears in Trek1-/- mice indicating a central role of TREK-1 in this process. Trek1-/- mice are also resistant to anesthesia by volatile anesthetics. TREK-1 emerges as a potential innovative target for developing new therapeutic agents for neurology and anesthesiology.

Genomic and functional characteristics of novel human pancreatic 2P domain K(+) channels

We isolated three novel 2P domain K(+) channel subunits from human. The first two subunits, TALK-1 and TALK-2, are distantly related to TASK-2. Their genes form a tight cluster of 25 kb on chromosome 6p21.1-p21.2. The corresponding channels produce quasi-instantaneous and non-inactivating currents that are activated at alkaline pHs. These currents are sensitive to Ba(2+), quinine, quinidine, chloroform, halothane, and isoflurane but are not affected by TEA, 4-AP, Cs(+), arachidonic acid, hypertonic solutions, agents activating protein kinases C and A, changes of internal Ca(2+) concentrations, and by activation of G(i) and G(q) proteins. TALK-1 is exclusively expressed in the pancreas. TALK-2 is mainly expressed in the pancreas, but is also expressed at a lower level in liver, placenta, heart, and lung. We also cloned a third subunit, named hTHIK-2 which is present in many tissues with high levels again in the pancreas but which could not be functionally expressed.

PDZ proteins interacting with C-terminal GluR2/3 are involved in a PKC-dependent regulation of AMPA receptors at hippocampal synapses

We investigated the role of PDZ proteins (GRIP, ABP, and PICK1) interacting with the C-terminal GluR2 by infusing a ct-GluR2 peptide ("pep2-SVKI") into CA1 pyramidal neurons in hippocampal slices using whole-cell recordings. Pep2-SVKI, but not a control or PICK1 selective peptide, caused AMPAR-mediated EPSC amplitude to increase in approximately one-third of control neurons and in most neurons following the prior induction of LTD. Pep2-SVKI also blocked LTD; however, this occurred in all neurons. A PKC inhibitor prevented these effects of pep2-SVKI on synaptic transmission and LTD. We propose a model in which the maintenance of LTD involves the binding of AMPARs to PDZ proteins to prevent their reinsertion. We also present evidence that PKC regulates AMPAR reinsertion during dedepression.

The neuroprotective agent riluzole activates the two P domain K(+) channels TREK-1 and TRAAK

Riluzole (RP 54274) is a potent neuroprotective agent with anticonvulsant, sedative, and anti-ischemic properties. It is currently used in the treatment of amyotrophic lateral sclerosis. This article reports that riluzole is an activator of TREK-1 and TRAAK, two important members of a new structural family of mammalian background K(+) channels with four transmembrane domains and two pore regions. Whereas riluzole activation of TRAAK is sustained, activation of TREK-1 is transient and is followed by an inhibition. The inhibitory process is attributable to an increase of the intracellular cAMP concentration by riluzole that produces a protein kinase A-dependent inhibition of TREK-1. Mutants of TREK-1 lacking the Ser residue where the kinase A phosphorylation takes place are activated in a sustained manner by riluzole. TRAAK is permanently activated by riluzole because, unlike TREK-1, it lacks the negative regulation by cAMP.

Hippocampal LTD expression involves a pool of AMPARs regulated by the NSF-GluR2 interaction

We investigated whether the interaction between the N-ethyl-maleimide-sensitive fusion protein (NSF) and the AMPA receptor (AMPAR) subunit GluR2 is involved in synaptic plasticity in the CA1 region of the hippocampus. Blockade of the NSF-GluR2 interaction by a specific peptide (pep2m) introduced into neurons prevented homosynaptic, de novo long-term depression (LTD). Moreover, saturation of LTD prevented the pep2m-induced reduction in AMPAR-mediated excitatory postsynaptic currents (EPSCs). Minimal stimulation experiments indicated that both pep2m action and LTD were due to changes in quantal size and quantal content but were not associated with changes in AMPAR single-channel conductance or EPSC kinetics. These results suggest that there is a pool of AMPARs dependent on the NSF-GluR2 interaction and that LTD expression involves the removal of these receptors from synapses.

Cloning and expression of a novel pH-sensitive two pore domain K+ channel from human kidney

A complementary DNA encoding a novel K+ channel, called TASK-2, was isolated from human kidney and its gene was mapped to chromosome 6p21. TASK-2 has a low sequence similarity to other two pore domain K+ channels, such as TWIK-1, TREK-1, TASK-1, and TRAAK (18-22% of amino acid identity), but a similar topology consisting of four potential membrane-spanning domains. In transfected cells, TASK-2 produces noninactivating, outwardly rectifying K+ currents with activation potential thresholds that closely follow the K+ equilibrium potential. As for the related TASK-1 and TRAAK channels, the outward rectification is lost at high external K+ concentration. The conductance of TASK-2 was estimated to be 14.5 picosiemens in physiological conditions and 59.9 picosiemens in symmetrical conditions with 155 mM K+. TASK-2 currents are blocked by quinine (IC50 = 22 microM) and quinidine (65% of inhibition at 100 microM) but not by the other classical K+ channel blockers tetraethylammonium, 4-aminopyridine, and Cs+. They are only slightly sensitive to Ba2+, with less than 17% of inhibition at 1 mM. As TASK-1, TASK-2 is highly sensitive to external pH in the physiological range. 10% of the maximum current was recorded at pH 6. 5 and 90% at pH 8.8. Unlike all other cloned channels with two pore-forming domains, TASK-2 is essentially absent in the brain. In human and mouse, TASK-2 is mainly expressed in the kidney, where in situ hybridization shows that it is localized in cortical distal tubules and collecting ducts. This localization, as well as its functional properties, suggest that TASK-2 could play an important role in renal K+ transport.

A mammalian two pore domain mechano-gated S-like K+ channel

Aplysia S-type K+ channels of sensory neurons play a dominant role in presynaptic facilitation and behavioural sensitization. They are closed by serotonin via cAMP-dependent phosphorylation, whereas they are opened by arachidonic acid, volatile general anaesthetics and mechanical stimulation. We have identified a cloned mammalian two P domain K+ channel sharing the properties of the S channel. In addition, the recombinant channel is opened by lipid bilayer amphipathic crenators, while it is closed by cup-formers. The cytoplasmic C-terminus contains a charged region critical for chemical and mechanical activation, as well as a phosphorylation site required for cAMP inhibition.

[Birdshot chorioretinopathy: a case with delayed detection]

We report a case of birdshot retinochoroidopathy following a cataract surgery in a 78-year-old man. Low-grade forms of the affection may be discovered lately on a recurrence of the inflammation following any event as an ocular surgical procedure. Medical management of cataract surgery in uveitis is then discussed.

A neuronal two P domain K+ channel stimulated by arachidonic acid and polyunsaturated fatty acids

TWIK-1, TREK-1 and TASK K+ channels comprise a class of pore-forming subunits with four membrane-spanning segments and two P domains. Here we report the cloning of TRAAK, a 398 amino acid protein which is a new member of this mammalian class of K+ channels. Unlike TWIK-1, TREK-1 and TASK which are widely distributed in many different mouse tissues, TRAAK is present exclusively in brain, spinal cord and retina. Expression of TRAAK in Xenopus oocytes and COS cells induces instantaneous and non-inactivating currents that are not gated by voltage. These currents are only partially inhibited by Ba2+ at high concentrations and are insensitive to the other classical K+ channel blockers tetraethylammonium, 4-aminopyridine and Cs+. A particularly salient feature of TRAAK is that they can be stimulated by arachidonic acid (AA) and other unsaturated fatty acids but not by saturated fatty acids. These channels probably correspond to the functional class of fatty acid-stimulated K+ currents that recently were identified in native neuronal cells but have not yet been cloned. These TRAAK channels might be essential in normal physiological processes in which AA is known to play an important role, such as synaptic transmission, and also in pathophysiological processes such as brain ischemia. TRAAK channels are stimulated by the neuroprotective drug riluzole.

New modulatory alpha subunits for mammalian Shab K+ channels

Two novel K+ channel alpha subunits, named Kv9.1 and Kv9.2, have been cloned. The Kv9.2 gene is situated in the 8q22 region of the chromosome. mRNAs for these two subunits are highly and selectively expressed in the nervous system. High levels of expressions are found in the olfactory bulb, cerebral cortex, hippocampal formation, habenula, basolateral amygdaloid nuclei, and cerebellum. Interestingly Kv9.1 and Kv9.2 colocalized with Kv2.1 and/or Kv2.2 alpha subunits in several regions of the brain. Neither Kv9.1 nor Kv9.2 have K+ channel activity by themselves, but both modulate the activity of Kv2.1 and Kv2.2 channels by changing kinetics and levels of expression and by shifting the half-inactivation potential to more polarized values. This report also analyzes the changes in electrophysiological properties of Kv2 subunits induced by Kv5.1 and Kv6.1, two other modulatory subunits. Each modulatory subunit has its own specific properties of regulation of the functional Kv2 subunits, and they can lead to extensive inhibitions, to large changes in kinetics, and/or to large shifts in the voltage dependencies of the inactivation process. The increasing number of modulatory subunits for Kv2.1 and Kv2.2 provides an amazingly new capacity of functional diversity.

TASK, a human background K+ channel to sense external pH variations near physiological pH

TASK is a new member of the recently recognized TWIK K+ channel family. This 395 amino acid polypeptide has four transmembrane segments and two P domains. In adult human, TASK transcripts are found in pancreas<placenta<brain<lung, prostate<heart, kidney<uterus, small intestine and colon. Electrophysiological properties of TASK were determined after expression in Xenopus oocytes and COS cells. TASK currents are K+-selective, instantaneous and non-inactivating. They show an outward rectification when external [K+] is low ([K+]out = 2 mM) which is not observed for high [K+]out (98 mM). The rectification can be approximated by the Goldman-Hodgkin-Katz current equation that predicts a curvature of the current-voltage plot in asymmetric K+ conditions. This strongly suggests that TASK lacks intrinsic voltage sensitivity. The absence of activation and inactivation kinetics as well as voltage independence are characteristic of conductances referred to as leak or background conductances. For this reason, TASK is designated as a background K+ channel. TASK is very sensitive to variations of extracellular pH in a narrow physiological range; as much as 90% of the maximum current is recorded at pH 7.7 and only 10% at pH 6.7. This property is probably essential for its physiological function, and suggests that small pH variations may serve a communication role in the nervous system.

The structure, function and distribution of the mouse TWIK-1 K+ channel

The two P domain K+ channel mTWIK-1 has been cloned from mouse brain. In Xenopus oocytes, mTWIK-1 currents are K+-selective, instantaneous, and weakly inward rectifying. These currents are blocked by Ba2+ and quinine, decreased by protein kinase C and increased by internal acidification. The apparent molecular weight of mTWIK-1 in brain is 81 kDa. A 40 kDa form is revealed after treatment with a reducing agent, strongly suggesting that native mTWIK-1 subunits dimerize via a disulfide bridge. TWIK-1 mRNA is expressed abundantly in brain and at lower levels in lung, kidney, and skeletal muscle. In situ hybridization shows that mTWIK-1 expression is restricted to a few brain regions, with the highest levels in cerebellar granule cells, brainstem, hippocampus and cerebral cortex.

Cloning, functional expression and brain localization of a novel unconventional outward rectifier K+ channel

Human TWIK-1, which has been cloned recently, is a new structural type of weak inward rectifier K+ channel. Here we report the structural and functional properties of TREK-1, a mammalian TWIK-1-related K+ channel. Despite a low amino acid identity between TWIK-1 and TREK-1 (approximately 28%), both channel proteins share the same overall structural arrangement consisting of two pore-forming domains and four transmembrane segments (TMS). This structural similarity does not give rise to a functional analogy. K+ currents generated by TWIK-1 are inwardly rectifying while K+ currents generated by TREK-1 are outwardly rectifying. These channels have a conductance of 14 pS. TREK-1 currents are insensitive to pharmacological agents that block TWIK-1 activity such as quinine and quinidine. Extensive inhibitions of TREK-1 activity are observed after activation of protein kinases A and C. TREK-1 currents are sensitive to extracellular K+ and Na+. TREK-1 mRNA is expressed in most tissues and is particularly abundant in the lung and in the brain. Its localization in this latter tissue has been studied by in situ hybridization. TREK-1 expression is high in the olfactory bulb, hippocampus and cerebellum. These results provide the first evidence for the existence of a K+ channel family with four TMS and two pore domains in the nervous system of mammals. They also show that different members in this structural family can have totally different functional properties.

Dimerization of TWIK-1 K+ channel subunits via a disulfide bridge

TWIK-1 is a new type of K+ channel with two P domains and is abundantly expressed in human heart and brain. Here we show that TWIK-1 subunits can self-associate to give dimers containing an interchain disulfide bridge. This assembly involves a 34 amino acid domain that is localized to the extracellular M1P1 linker loop. Cysteine 69 which is part of this interacting domain is implicated in the formation of the disulfide bond. Replacing this cysteine with a serine residue results in the loss of functional K+ channel expression. This is the first example of a covalent association of functional subunits in voltage-sensitive channels via a disulfide bridge.

A new K+ channel beta subunit to specifically enhance Kv2.2 (CDRK) expression

Cloned K+ channel beta subunits are hydrophilic proteins which associate to pore-forming alpha subunits of the Shaker subfamily. The resulting alphabeta heteromultimers K+ channels have inactivation kinetics significantly more rapid than those of the corresponding alpha homomultimers. This paper reports the cloning and the brain localization of mKvbeta4 (m for mouse), a new beta subunit. This new beta subunit is highly expressed in the nervous system but is also present in other tissues such as kidney. In contrast with other beta subunits, coexpression of the mKvbeta4 subunit with alpha subunits of Shaker-type K+ channel does not modify the kinetic properties or voltage-dependence of these channels in Xenopus oocytes. Instead, mKvbeta4 associates to Kv2.2 (CDRK), a Shab K+ channel, to specifically enhance (a factor of up to 6) its expression level without changing its elementary conductance or kinetics. It is without effect on another closely related Shab K+ channel Kv2.1 (DRK1). Chimeras between Kv2.1 and Kv2. 2 indicate that the COOH-terminal end of the Kv2.2 protein is essential for its mKvbeta4 sensitivity. The functional results associated with the observation of the co-localization of mKvbeta4 and Kv2.2 transcripts in most brain areas strongly suggest that both subunits interact in vivo to form a slowly-inactivating K+ channel. A chaperone-like effect of mKvbeta4 seems to permit the integration of a larger number of Kv2.2 channels at the plasma membrane.

TWIK-1, a ubiquitous human weakly inward rectifying K+ channel with a novel structure

A new human weakly inward rectifying K+ channel, TWIK-1, has been isolated. This channel is 336 amino acids long and has four transmembrane domains. Unlike other mammalian K+ channels, it contains two pore-forming regions called P domains. Genes encoding structural homologues are present in the genome of Caenorhabditis elegans. TWIK-1 currents expressed in Xenopus oocytes are time-independent and present a nearly linear I-V relationship that saturated for depolarizations positive to O mV in the presence of internal Mg2+. This inward rectification is abolished in the absence of internal Mg2+. TWIK-1 has a unitary conductance of 34 pS and a kinetic behaviour that is dependent on the membrane potential. In the presence of internal Mg2+, the mean open times are 0.3 and 1.9 ms at -80 and +80 mV, respectively. The channel activity is up-regulated by activation of protein kinase C and down-regulated by internal acidification. Both types of regulation are indirect. TWIK-1 channel activity is blocked by Ba2+(IC50=100 microM), quinine (IC50=50 microM) and quinidine (IC50=95 microM). This channel is of particular interest because its mRNA is widely distributed in human tissues, and is particularly abundant in brain and heart. TWIK-1 channels are probably involved in the control of background K+ membrane conductances.

A pH-sensitive yeast outward rectifier K+ channel with two pore domains and novel gating properties

YORK is a newly cloned K+ channel from yeast. Unlike all other cloned K+ channels, it has two pore domains instead of one. It displays eight transmembrane segments arranged like a covalent assembly of a Shaker-type voltage-dependent K+ channel (without S4 transmembrane segments) with an inward rectifier K+ channel. When expressed in Xenopus oocytes, YORK does not pass inward currents; it conducts only K+-selective outward currents. However, the mechanism responsible for this strict outward rectification is unusual. Like inward rectifiers, its activation potential threshold closely follows the K+ equilibrium potential. Unlike inward rectifiers, the rectification is not due to a voltage-dependent Mg2+ block. The blocking element is probably intrinsic to the YORK protein itself. YORK activity is decreased at acidic internal pH, with a pKa of 6.5. Pharmacological and regulation properties were analyzed. Ba2+ ions and quinine block YORK currents through high and low affinity sites, while tetraethylammonium displays only one affinity for blocking. Activation of protein kinase C indirectly produces an increase of the current, while protein kinase A activation has no effect.

Dominant negative chimeras provide evidence for homo and heteromultimeric assembly of inward rectifier K+ channel proteins via their N-terminal end

Chimeras have been constructed using three different fragments (N-terminal, central and C-terminal) of IRK3, a constitutive inward rectifier K+ channel subunit, and GIRK2, a G-protein activated inward rectifier K+ channel subunit and have been coinjected into Xenopus oocytes together with IRK3 or IRK1 (another constitutive inward rectifier) cRNA. Both IRK1 and IRK3 expression was inhibited by coinjection with chimeras containing a N-terminal fragment of IRK3 suggesting that subunits of K+ channels in the IRK family form a functional multimeric assembly where the N-terminal end has an important role. In situ hybridization shows that IRK1 and IRK3 are coexpressed in the same areas of the brain and probably in the same cells. Taken together both the localization and the oocyte expression results suggest that not only homomultimeric IRK1 or homomultimeric IRK3 assemblies take place but that heteromultimeric IRK1/IRK3 assemblies are also formed.

Susceptibility of cloned K+ channels to reactive oxygen species

Free radical-induced oxidant stress has been implicated in a number of physiological and pathophysiological states including ischemia and reperfusion-induced dysrhythmia in the heart, apoptosis of T lymphocytes, phagocytosis, and neurodegeneration. We have studied the effects of oxidant stress on the native K+ channel from T lymphocytes and on K+ channels cloned from cardiac, brain, and T-lymphocyte cells and expressed in Xenopus oocytes. The activity of three Shaker K+ channels (Kv1.3, Kv1.4, and Kv1.5), one Shaw channel (Kv3.4), and one inward rectifier K+ channel (IRK3) was drastically inhibited by photoactivation of rose bengal, a classical generator of reactive oxygen species. Other channel types (such as Shaker K+ channel Kv1.2, Shab channels Kv2.1 and Kv2.2, Shal channel Kv4.1, inward rectifiers IRK1 and ROMK1, and hIsK) were completely resistant to this treatment. On the other hand tert-butyl hydroperoxide, another generator of reactive oxygen species, removed the fast inactivation processes of Kv1.4 and Kv3.4 but did not alter other channels. Xanthine/xanthine oxidase system had no effect on all channels studied. Thus, we show that different types of K+ channels are differently modified by reactive oxygen species, an observation that might be of importance in disease states.

Molecular properties of neuronal G-protein-activated inwardly rectifying K+ channels

Four cDNA-encoding G-activated inwardly rectifying K+ channels have been cloned recently (Kubo, Y., Reuveny, E., Slesinger, P. A., Jan, Y. N., and Jan, L. Y. (1993) Nature 364, 802-806; Lesage, F., Duprat, F., Fink, M., Guillemare, E., Coppola, T., Lazdunski, M., and Hugnot, J. P. (1994) FEBS Lett. 353, 37-42; Krapivinsky, G., Gordon, E. A., Wickman, K., Velimirovic, B., Krapivinsky, L., and Clapham, D. E. (1995) Nature 374, 135-141). We report the cloning of a mouse GIRK2 splice variant, noted mGIRK2A. Both channel proteins are functionally expressed in Xenopus oocytes upon injection of their cRNA, alone or in combination with the GIRK1 cRNA. Three GIRK channels, mGIRK1-3, are shown to be present in the brain. Colocalization in the same neurons of mGIRK1 and mGIRK2 supports the hypothesis that native channels are made by an heteromeric subunit assembly. GIRK3 channels have not been expressed successfully, even in the presence of the other types of subunits. However, GIRK3 chimeras with the amino- and carboxyl-terminal of GIRK2 are functionally expressed in the presence of GIRK1. The expressed mGIRK2 and mGIRK1, -2 currents are blocked by Ba2+ and Cs+ ions. They are not regulated by protein kinase A and protein kinase C. Channel activity runs down in inside-out excised patches, and ATP is required to prevent this rundown. Since the nonhydrolyzable ATP analog AMP-PCP is also active and since addition of kinases A and C as well as alkaline phosphatase does not modify the ATP effect, it is concluded that ATP hydrolysis is not required. An ATP binding process appears to be essential for maintaining a functional state of the neuronal inward rectifier K+ channel. A Na+ binding site on the cytoplasmic face of the membrane acts in synergy with the ATP binding site to stabilize channel activity.

Heterologous multimeric assembly is essential for K+ channel activity of neuronal and cardiac G-protein-activated inward rectifiers

The family of G-protein-activated inward-rectifiers K+ channels presently comprise at least 3 cloned members called GIRK1, GIRK2 and GIRK3. A close structural parent of GIRK channels has recently been described as being an ATP-sensitive K+ channel. This paper shows that Xenopus expression of this new channel that we call GIRK4 does not produce an ATP-inhibitable activity with a pharmacological activation by pinacidil as previously described but instead a G-protein activated inward-rectifier. While oocyte expression of single subunits is infrequent and relatively modest in intensity, expression of GIRK1,2, GIRK1,4 and GIRK2,4 mixtures leads to routine and robust expression of K+ channels indicating that heterologous subunit assembly is necessary for activity. Activity of GIRK1,2, GIRK1,4 and GIRK2,4 channels required the presence of ATP acting as an activator at the cytoplasmic face and is further activated by the beta gamma subunits.

Cloning provides evidence for a family of inward rectifier and G-protein coupled K+ channels in the brain

MbIRK3, mbGIRK2 and mbGIRK3 K+ channels cDNAs have been cloned from adult mouse brain. These cDNAs encode polypeptides of 445, 414 and 376 amino acids, respectively, which display the hallmarks of inward rectifier K+ channels, i.e. two hydrophobic membrane-spanning domains M1 and M2 and a pore-forming domain H5. MbIRK3 shows around 65% amino acid identity with IRK1 and rbIRK2 and only 50% with ROMK1 and GIRK1. On the other hand, mbGIRK2 and mbGIRK3 are more similar to GIRK1 (60%) than to ROMK1 and IRK1 (50%). Northern blot analysis reveals that these three novel clones are mainly expressed in the brain. Xenopus oocytes injected with mbIRK3 and mbGIRK2 cRNAs display inward rectifier K(+)-selective currents very similar to IRK1 and GIRK1, respectively. As expected from the sequence homology, mbGIRK2 cRNA directs the expression of G-protein coupled inward rectifier K+ channels which has been observed through their functional coupling with co-expressed delta-opioid receptors. These results provide the first evidence that the GIRK family, as the IRK family, is composed of multiple genes with members specifically expressed in the nervous system.

[Surgical prevention of retinal detachment]

Prophylaxis of rhegmatogenous retinal detachment consists of measures both non surgical and/or surgical that are effective in preventing some risk-factors that are part of the disease. The procedure must avoid any types of danger. This paper describes a new reliable surgical procedure of circular buckling used for prophylaxis on predisposed fellow-eye of giant retinal tear.

[Experimental and clinical study of conjunctivo-scleral tolerance of PTFE (Goretex) sutures compared to polyglactine (Vicryl)]

The authors report a study of the modifications observed with sutures of PTFE (Goretex) and polyglactin (Vicryl) at the level of the sclera and the conjunctival. Inflammatory cells are fewer with PTFE both in the rabbit eyes (14 cases) from 3 to 110 days and in the human eye in a clinical study of 28 cases.