► Coexpression of β-catenin increased voltage-gated kV current in Kv1.5 expressing oocytes. ► The effect was not dependent on the stimulation of transcription. ► β-catenin enhances the membrane ...abundance of Kv1.5. ► β-catenin fosters insertion of Kv1.5 protein into the plasma membrane. ► The effect on Kv1.5 requires the same protein domains that are required for association of β-catenin with cadherin.
Voltage-gated Kv1.5 channels are expressed in a wide variety of tissues including cardiac myocytes, smooth muscle and tumor cells. Kv1.5 channel activity is modified by N-cadherin, which in turn binds the multifunctional oncogenic protein β-catenin. The present experiments explored the effect of β-catenin on Kv1.5 channel activity. To this end, Kv1.5 was expressed in Xenopus oocytes with or without β-catenin and the voltage-gated Kv current determined by dual electrode voltage clamp. As a result, expression of β-catenin significantly increased the voltage-gated Kv current at positive potentials. The stimulating effect of β-catenin on Kv1.5 was not dependent on the stimulation of transcription since it was observed even in the presence of the transcription inhibitor actinomycin D. Specific antibody binding to surface Kv1.5 in Xenopus oocytes revealed that β-catenin enhances the membrane abundance of Kv1.5. Further experiments with brefeldin A showed that β-catenin fosters the insertion of Kv1.5 into rather than delaying the retrieval from the plasma membrane. According to electrophysiological recordings with mutant β-catenin, the effect on Kv1.5 requires the same protein domains that are required for association of β-catenin with cadherin. The experiments disclose a completely novel function of β-catenin, i.e. the regulation of Kv1.5 channel activity.
Connexins provide intercellular connections that allow passage of ions and small organic molecules. They clamp the cell membrane potential and cellular ion composition to that of neighboring cells. ...The cell membrane potential and ion composition of an energy-depleted cell could thus be maintained despite its compromised Na
+
/K
+
activity. By the same token, however, the breakdown of ion gradients in that cell imposes an additional challenge to the neighboring cells, which may jeopardize their survival. Thus, timely closure of connexins may be critically important for the survival of those cells. Energy depletion stimulates the AMP-activated protein kinase (AMPK), a serine/threonine kinase that senses energy depletion and stimulates several cellular mechanisms to enhance energy production and to limit energy utilization. The present study explored whether AMPK regulates connexin 26. To this end, cRNA encoding connexin 26 was injected into
Xenopus
oocytes with and without additional injection of wild-type AMPK (α1β1γ1), of the constitutively active
γR70Q
AMPK (α1β1γ1R70Q) or of the inactive mutant
αK45R
AMPK (α1K45Rβ1γ1). Connexin 26 activity was determined in dual-electrode voltage-clamp experiments. Moreover, connexin 26 abundance was determined in the oocyte cell membrane by chemiluminescence and confocal microscopy. As a result, connexin 26-mediated current and connexin 26 protein abundance were significantly decreased by coexpression of
γR70Q
AMPK and, to a lower extent, by wild-type AMPK but not by
αK45R
AMPK. In conclusion, AMPK is a potent regulator of connexin 26.
Abstract
β-catenin, a multifunctional protein expressed in all tissues including the heart stimulates the expression of several genes important for cell proliferation. Signaling involving ß-catenin ...participates in directing cardiac development and in the pathophysiology of cardiac hypertrophy. Nothing is known, however, on the role of β-catenin in the regulation of cardiac ion channels. The present study explored the functional interaction of β-catenin and KCNE1/KCNQ1, the K+ channel complex underlying the slowly activating outwardly rectifying K+ current. To this end, KCNE1/KCNQ1 was expressed in Xenopus oocytes with and without β-catenin and the depolarization (up to + 80 mV) induced current (IKs) was determined using the two-electrode voltage clamp. As a result, β-catenin enhanced IKs by 30%. The effect of β-catenin on IKs was not affected by actinomycin D (10 μM), an inhibitor of transcription, indicating that β-catenin was not effective as transcription factor. Confocal microscopy revealed that β-catenin enhanced the KCNE1/KCNQ1 protein abundance in the cell membrane. Exposure of the oocytes to brefeldin A (5 μM), an inhibitor of vesicle insertion, was followed by a decline of IKs, which was then similar in oocytes expressing KCNE1/KCNQ1 together with β-catenin and in oocytes expressing KCNE1/KCNQ1 alone. In conclusion, β-catenin enhances IKs by increasing the KCNE1/KCNQ1 protein abundance in the cell membrane, an effect requiring vesicle insertion into the cell membrane.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Mutations in GJB2, a gene encoding the gap junction protein connexin 26 (Cx26), are a major cause for inherited and sporadic non-syndromic hearing loss, albeit with highly variable clinical effects. ...To determine new mutations and their frequencies in a Southern Egyptian population restriction fragment length polymorphism, gene sequencing, and single strand conformational polymorphism revealed only 2 mutations for GJB2: c.35delG and p.I71N. The allelic frequency of the c.35delG mutation was 8.7% (found in 27 out of 310 investigated alleles) resulting in a relatively low carrier frequency (1.6%) in Upper Egypt. The new mutation, a substitution of isoleucin (I) (a non-polar amino acid) by the polar amino acid asparagin (N), was localized within the conserved Cx26 structure. The functional significance of p.I71N was tested by injection of cRNA into Xenopus laevis oocytes. Cx26 hemi-channel activity was measured by depolarization activated conductance in non-coupled oocytes. As a result, the p.I71N mutated channel was non-functional. The study discloses a novel, functionally relevant GJB2 mutation and defines the contribution of Cx26 alterations to the hearing loss in the Southern Egyptian population.
Human ether-a-go-go related-gene K+ channels (hERG) participate in the regulation of tumor cell proliferation and apoptosis. HERG channel activity is up-regulated by growth factors. Kinases sensitive ...to growth factor signaling include the serine/threonine protein kinase B-RAF. The present study thus explored whether B-RAF influences hERG channel expression and activity. To this end, hERG channels were expressed in Xenopus oocytes with or without wild-type B-RAF, hERG channel activity was determined utilizing dual-electrode voltage clamp and hERG protein abundance in the cell membrane was analyzed utilizing confocal microscopy as well as chemiluminescence. Moreover, in rhabdomyosarcoma RD cells the effect of B-RAF inhibitor PLX-4720 on hERG-mediated current was quantified by whole-cell patch clamp and hERG cell surface protein abundance by utilizing biotinylation of cell surface proteins as well as flow cytometry. As a result, co-expression of wild-type B-RAF in hERG-expressing Xenopus oocytes significantly increased hERG channel activity and hERG channel protein abundance in the cell membrane. Treatment for 24 hours of B-RAF and hERG-expressing Xenopus oocytes with B-RAF inhibitor PLX-4720 (10 mu M) significantly decreased hERG-mediated current and hERG cell surface expression. Similarly, in rhabdomyosarcoma RD cells, treatment for 24 hours with B-RAF inhibitor PLX-4720 significantly decreased hERG cell membrane protein abundance and hERG-mediated current. In conclusion, B-RAF is a powerful regulator of hERG channel activity and cell surface hERG protein abundance.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Sustained increase of cardiac workload is known to trigger cardiac remodeling with eventual development of cardiac failure. Compelling evidence points to a critical role of enhanced cardiac Na^sup ...+^/H^sup +^ exchanger (NHE1) activity in the underlying pathophysiology. The signaling triggering up-regulation of NHE1 remained, however, ill defined. The present study explored the involvement of the serum- and glucocorticoid-inducible kinase Sgk1 in cardiac remodeling due to transverse aortic constriction (TAC). To this end, experiments were performed in gene targeted mice lacking functional Sgk1 (sgk1 ^sup -/-^) and their wild-type controls (sgk1 ^sup +/+^). Transcript levels have been determined by RT-PCR, cytosolic pH (pH^sub i^) utilizing 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF) fluorescence, Na^sup +^/H^sup +^ exchanger activity by the Na^sup +^-dependent realkalinization after an ammonium pulse, ejection fraction (%) utilizing cardiac cine magnetic resonance imaging and cardiac glucose uptake by PET imaging. As a result, TAC increased the mRNA expression of Sgk1 in sgk1 ^sup +/+^ mice, paralleled by an increase in Nhe1 transcript levels as well as Na^sup +^/H^sup +^ exchanger activity, all effects virtually abrogated in sgk1 ^sup -/-^ mice. In sgk1 ^sup +/+^ mice, TAC induced a decrease in Pgc1a mRNA expression, while Spp1 mRNA expression was increased, both effects diminished in the sgk1 ^sup -/-^ mice. TAC was followed by a significant increase of heart and lung weight in sgk1 ^sup +/+^ mice, an effect significantly blunted in sgk1 ^sup -/-^ mice. TAC increased the transcript levels of Anp and Bnp, effects again significantly blunted in sgk1 ^sup -/-^ mice. TAC increased transcript levels of Collagen I and III as well as Ctgf mRNA and CTGF protein abundance, effects significantly blunted in sgk1 ^sup -/-^ mice. TAC further decreased the ejection fraction in sgk1 ^sup +/+^ mice, an effect again attenuated in sgk1 ^sup -/-^ mice. Also, cardiac FDG-glucose uptake was increased to a larger extent in sgk1 ^sup +/+^ mice than in sgk1 ^sup -/-^ mice after TAC. These observations point to an important role for SGK1 in cardiac remodeling and development of heart failure following an excessive work load.PUBLICATION ABSTRACT
Research highlights: {yields} The oncogenic transcription factor {beta}-catenin stimulates the Na{sup +}/K{sup +}-ATPase. {yields} {beta}-Catenin stimulates SGLT1 dependent Na{sup +}, glucose ...cotransport. {yields} The effects are independent of transcription. {yields} {beta}-Catenin sensitive transport may contribute to properties of proliferating cells. -- Abstract: {beta}-Catenin is a multifunctional protein stimulating as oncogenic transcription factor several genes important for cell proliferation. {beta}-Catenin-regulated genes include the serum- and glucocorticoid-inducible kinase SGK1, which is known to stimulate a variety of transport systems. The present study explored the possibility that {beta}-catenin influences membrane transport. To this end, {beta}-catenin was expressed in Xenopus oocytes with or without SGLT1 and electrogenic transport determined by dual electrode voltage clamp. As a result, expression of {beta}-catenin significantly enhanced the ouabain-sensitive current of the endogeneous Na{sup +}/K{sup +}-ATPase. Inhibition of vesicle trafficking by brefeldin A revealed that the stimulatory effect of {beta}-catenin on the endogenous Na{sup +}/K{sup +}-ATPase was not due to enhanced stability of the pump protein in the cell membrane. Expression of {beta}-catenin further enhanced glucose-induced current (Ig) in SGLT1-expressing oocytes. In the absence of SGLT1 Ig was negligible irrespective of {beta}-catenin expression. The stimulating effect of {beta}-catenin on both Na{sup +}/K{sup +} ATPase and SGLT1 activity was observed even in the presence of actinomycin D, an inhibitor of transcription. The experiments disclose a completely novel function of {beta}-catenin, i.e. the regulation of transport.
Klotho, a transmembrane protein, which can be cleaved off as beta -glucuronidase and hormone, is released in both, kidney and choroid plexus and encountered in blood and cerebrospinal fluid. Klotho ...deficiency leads to early appearance of age-related disorders and premature death. Klotho may modify transport by inhibiting 1,25(OH)2D3 formation or by directly affecting channel and carrier proteins. The present study explored whether Klotho influences the activity of the Na+-coupled excitatory amino acid transporters EAAT3 and EAAT4, which are expressed in kidney (EAAT3), intestine (EAAT3) and brain (EAAT3 and EAAT4). To this end, cRNA encoding EAAT3 or EAAT4 was injected into Xenopus oocytes with and without additional injection of cRNA encoding Klotho. EAAT expressing Xenopus oocytes were further treated with recombinant human beta -Klotho protein with or without beta -glucuronidase inhibitor D-saccharic acid 1,4-lactone monohydrate (DSAL). Electrogenic excitatory amino acid transport was determined as L-glutamate-induced current (Iglu) in two electrode voltage clamp experiments. EAAT3 and EAAT4 protein abundance in the Xenopus oocyte cell membrane was visualized by confocal microscopy and quantified utilizing chemiluminescence. As a result, coexpression of Klotho cRNA significantly increased Iglu in both, EAAT3 or EAAT4-expressing Xenopus oocytes. Klotho cRNA coexpression significantly increased the maximal current and cell membrane protein abundance of both EAAT3 and EAAT4. The effect of Klotho coexpression on EAAT3 and EAAT4 activity was mimicked by treating EAAT3 or EAAT4-expressing Xenopus oocytes with recombinant human beta -Klotho protein. The effects of Klotho coexpression and of treatment with recombinant human beta -Klotho protein were both abrogated in the presence of DSAL (10 mu M). In conclusion, Klotho is a novel, powerful regulator of the excitatory amino acid transporters EAAT3 and EAAT4.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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