Genotype–phenotype correlations in neonatal epilepsies caused by mutations in the voltage sensor of Kv7.2 potassium channel subunits

Mutations in the K_V7.2 gene encoding for voltage-dependent K⁺ channel subunits cause neonatal epilepsies with wide phenotypic heterogeneity. Two mutations affecting the same positively charged residue in the S₄ domain of K_V7.2 have been found in children affected with benign familial neonatal seizures (R213W mutation) or with neonatal epileptic encephalopathy with severe pharmacoresistant seizures and neurocognitive delay, suppression-burst pattern at EEG, and distinct neuroradiological features (R213Q mutation). To examine the molecular basis for this strikingly different phenotype, we studied the functional characteristics of mutant channels by using electrophysiological techniques, computational modeling, and homology modeling. Functional studies revealed that, in homomeric or heteromeric configuration with K_V7.2 and/or K_V7.3 subunits, both mutations markedly destabilized the open state, causing a dramatic decrease in channel voltage sensitivity. These functional changes were (i) more pronounced for channels incorporating R213Q- than R213W-carrying K_V7.2 subunits; (ii) proportional to the number of mutant subunits incorporated; and (iii) fully restored by the neuronal K_v7 activator retigabine. Homology modeling confirmed a critical role for the R213 residue in stabilizing the activated voltage sensor configuration. Modeling experiments in CA1 hippocampal pyramidal cells revealed that both mutations increased cell firing frequency, with the R213Q mutation prompting more dramatic functional changes compared with the R213W mutation. These results suggest that the clinical disease severity may be related to the extent of the mutation-induced functional K⁺ channel impairment, and set the preclinical basis for the potential use of K_v7 openers as a targeted anticonvulsant therapy to improve developmental outcome in neonates with K_V7.2 encephalopathy.     

Assessing the severity of the small inframe deletion mutation in the α-subunit of β-hexosaminidase A found in the Turkish population by reproducing it in the more stable β-subunit

GM(2) gangliosidoses are a group of panethnic lysosomal storage diseases in which GM(2) ganglioside accumulates in the lysosome due to a defect in one of three genes, two of which encode the alpha- or beta-subunits of beta- N -acetylhexosaminidase (Hex) A. A small inframe deletion mutation in the catalytic domain of the alpha-subunit of Hex has been found in five Turkish patients with infantile Tay-Sachs disease. To date it has not been detected in other populations and is the only mutation to be found in exon 10. It results in detectable levels of inactive alpha-protein in its precursor form. Because the alpha- and beta-subunits share 60% sequence identity, the Hex A and Hex B genes are believed to have arisen from a common ancestral gene. Thus the subunits must share very similar three-dimensional structures with conserved functional domains. Hex B, the beta-subunit homodimer is more stable than the heterodimeric Hex A, and much more stable than Hex S, the alpha homodimer. Thus, mutations that completely destabilize the alpha-subunit can often be partially rescued if expressed in the aligned positions in the beta-subunit. To better understand the severity of the Turkish HEXA mutation, we reproduced the 12 bp deletion mutation (1267-1278) in the beta-subunit cDNA. Western blot analysis of permanently transfected CHO cells expressing the mutant detected only the pro-form of the beta-subunit coupled with a total lack of detectable Hex B activity. These data indicate that the deletion of the four amino acids severely affects the folding of even the more stable beta-subunit, causing its retention in the endoplasmic reticulum and ultimate degradation.     

Functional protein expression of multiple sodium channel α- and β-subunit isoforms in neonatal cardiomyocytes

Voltage-gated sodium channels are composed of pore-forming α- and auxiliary β-subunits and are responsible for the rapid depolarization of cardiac action potentials. Recent evidence indicates that neuronal tetrodotoxin (TTX) sensitive sodium channel α-subunits are expressed in the heart in addition to the predominant cardiac TTX-resistant Na_v1.5 sodium channel α-subunit. These TTX-sensitive isoforms are preferentially localized in the transverse tubules of rodents. Since neonatal cardiomyocytes have yet to develop transverse tubules, we determined the complement of sodium channel subunits expressed in these cells. Neonatal rat ventricular cardiomyocytes were stained with antibodies specific for individual isoforms of sodium channel α- and β-subunits. α-actinin, a component of the z-line, was used as an intracellular marker of sarcomere boundaries. TTX-sensitive sodium channel α-subunit isoforms Na_v1.1, Na_v1.2, Na_v1.3, Na_v1.4 and Na_v1.6 were detected in neonatal rat heart but at levels reduced compared to the predominant cardiac α-subunit isoform, Na_v1.5. Each of the β-subunit isoforms (β1-β4) was also expressed in neonatal cardiac cells. In contrast to adult cardiomyocytes, the α-subunits are distributed in punctate clusters across the membrane surface of neonatal cardiomyocytes; no isoform-specific subcellular localization is observed. Voltage clamp recordings in the absence and presence of 20 nM TTX provided functional evidence for the presence of TTX-sensitive sodium current in neonatal ventricular myocardium which represents between 20 and 30% of the current, depending on membrane potential and experimental conditions. Thus, as in the adult heart, a range of sodium channel α-subunits are expressed in neonatal myocytes in addition to the predominant TTX-resistant Na_v1.5 α-subunit and they contribute to the total sodium current.     

Overexpression of potassium channel ether à go-go in human osteosarcoma

Human ether à go-go (hEAG) potassium channels are primarily expressed in brain but also frequently overexpressed in solid tumors, which could indicate their potential value for cancer diagnosis and therapy. hEAG1, one member of the hEAG subfamily, has been shown to play a role in neoplastic process. Here we report the expression of hEAG1 in human osteosarcoma detected by a new polyclonal antibody. The full-length hEAG1 cDNA was cloned from human osteosarcoma cell line MG63 by RT-PCR and expressed in Escherichia coli as His tagged protein. The 6His-hEAG1F protein was purified by nickel agarose and used as the antigen to immunize rabbits following standard protocols. The obtained antiserum could detect hEAG1 exogenously expressed in HEK 293 cells. Furthermore, the polyclonal antibody was used to evaluate hEAG1 expression in 42 human osteosarcoma specimens and 19 osteochondromas specimens by immunohistochemistry. hEAG1 was expressed in 71.4% (30/42) osteosarcoma, and 15.8% (3/19) osteochondromas. Moreover, statistical analysis revealed that hEAG1 expression was not dependent on age, sex, site, histology, grade and type in the osteosarcoma specimens. Our data provide evidence that hEAG1 is overexpressed in human osteosarcoma and the hEAG1 polyclonal antibody offers a good tool for further characterization of the oncogenic function of hEAG1 in osteosarcoma.     

Subetta increases phosphorylation of insulin receptor β-subunit alone and in the presence of insulin

It has been previously shown that Subetta (a drug containing released-active forms of antibodies to the insulin receptor β-subunit and antibodies to endothelial nitric oxide synthase) stimulated insulin-induced adiponectin production by mature human adipocytes in the absence of insulin. Therefore, it was assumed that Subetta could activate the insulin receptor. To confirm this hypothesis, the capacity of Subetta to activate the insulin receptor in mature human adipocytes in the absence or presence of the insulin was investigated. Cells were incubated either with Subetta or with vehicle, or with basal medium for 3 days. Then, adipocytes were treated with water or insulin (100 nm) for 15 min. Following treatment, lysates were prepared and phosphorylation of insulin receptor β-subunits was analyzed by western blot analysis. It was shown that Subetta significantly increased (P<0.001) the ‘phosphorylated-insulin receptor β-subunit/total insulin receptor β-subunit'' ratios in both the presence and the absence of insulin. These results support previously published data and indicate that Subetta could activate the insulin receptor through the effect on its β-subunits, whose conformational state is essential for insulin receptor activation. This action might serve as one of the primary mechanisms of the drug''s antidiabetic effect.     

Blocking antibody to the β-subunit of FSH prevents bone loss by inhibiting bone resorption and stimulating bone synthesis

Low estrogen levels undoubtedly underlie menopausal bone thinning. However, rapid and profuse bone loss begins 3 y before the last menstrual period, when serum estrogen is relatively normal. We have shown that the pituitary hormone FSH, the levels of which are high during late perimenopause, directly stimulates bone resorption by osteoclasts. Here, we generated and characterized a polyclonal antibody to a 13-amino-acid-long peptide sequence within the receptor-binding domain of the FSH β-subunit. We show that the FSH antibody binds FSH specifically and blocks its action on osteoclast formation in vitro. When injected into ovariectomized mice, the FSH antibody attenuates bone loss significantly not only by inhibiting bone resorption, but also by stimulating bone formation, a yet uncharacterized action of FSH that we report herein. Mesenchymal cells isolated from mice treated with the FSH antibody show greater osteoblast precursor colony counts, similarly to mesenchymal cells isolated from FSH receptor (FSHR)(-/-) mice. This suggests that FSH negatively regulates osteoblast number. We confirm that this action is mediated by signaling-efficient FSHRs present on mesenchymal stem cells. Overall, the data prompt the future development of an FSH-blocking agent as a means of uncoupling bone formation and bone resorption to a therapeutic advantage in humans.     

Optical detection of rate-determining ion-modulated conformational changes of the ether-à-go-go K+ channel voltage sensor

In voltage-dependent ether-à-go-go (eag) K+ channels, the process of activation is modulated by Mg2+ and other divalent cations, which bind to a site in the voltage sensor and slow channel opening. Previous analysis of eag ionic and gating currents indicated that Mg2+ has a much larger effect on ionic than gating current kinetics. From this, we hypothesized that ion binding modulates voltage sensor conformational changes that are poorly represented in gating current recordings. We have now tested this proposal by using a combined electrophysiological and optical approach. We find that a fluorescent probe attached near S4 in the voltage sensor reports on two phases of the activation process. One component of the optical signal corresponds to the main charge-moving conformational changes of the voltage sensor. This is the phase of activation that is well represented in gating current recordings. Another component of the optical signal reflects voltage sensor conformational changes that occur at more hyperpolarized potentials. These transitions, which are rate-determining for activation and highly modulated by Mg2+, have not been detected in gating current recordings. Our results demonstrate that the eag voltage sensor undergoes conformational changes that have gone undetected in electrical measurements. These transitions account for the time course of eag activation in the presence and absence of extracellular Mg2+.     

Reduced KCNQ4-encoded voltage-dependent potassium channel activity underlies impaired β-adrenoceptor-mediated relaxation of renal arteries in hypertension

KCNQ4-encoded voltage-dependent potassium (Kv7.4) channels are important regulators of vascular tone that are severely compromised in models of hypertension. However, there is no information as to the role of these channels in responses to endogenous vasodilators. We used a molecular knockdown strategy, as well as pharmacological tools, to examine the hypothesis that Kv7.4 channels contribute to β-adrenoceptor-mediated vasodilation in the renal vasculature and underlie the vascular deficit in spontaneously hypertensive rats. Quantitative PCR and immunohistochemistry confirmed gene and protein expression of KCNQ1, KCNQ3, KCNQ4, KCNQ5, and Kv7.1, Kv7.4, and Kv7.5 in rat renal artery. Isoproterenol produced concentration-dependent relaxation of precontracted renal arteries and increased Kv7 channel currents in isolated smooth muscle cells. Application of the Kv7 blocker linopirdine attenuated isoproterenol-induced relaxation and current. Isoproterenol-induced relaxations were also reduced in arteries incubated with small interference RNAs targeted to KCNQ4 that produced a ≈60% decrease in Kv7.4 protein level. Relaxation to isoproterenol and the Kv7 activator S-1 were abolished in arteries from spontaneously hypertensive rats, which was associated with ≈60% decrease in Kv7.4 abundance. This study provides the first evidence that Kv7 channels contribute to β-adrenoceptor-mediated vasodilation in the renal vasculature and that abrogation of Kv7.4 channels is strongly implicated in the impaired β-adrenoceptor pathway in spontaneously hypertensive rats. These findings may provide a novel pathogenic link between arterial dysfunction and hypertension.     

The NALCN ion channel is a new actor in pancreatic β-cell physiology

Ion channels are critical components of cell excitability involved in many physiological processes, including hormone secretion, and are thought be targets of choice in a pathological context. In the present paper, we summarize and discuss our recent findings on a four domain cation channel named NALCN which has been previously described as mediating a TTX-resistant leak sodium current in neurons. We recently reported that NALCN is also expressed in rodent islets of Langerhans as well as in the mouse MIN6 pancreatic β-cell line. This pancreatic NALCN channel encodes for a cation current triggered by acetylcholine activation of M3 muscarinic receptors. Importantly, the activation mechanism is independent of G protein action, but is dependent on a SFK-pathway, and involves the co-inclusion of M3 muscarinic receptors and NALCN in the same complex. Although additional work is now needed, considering the importance of the cholinergic control on the pancreatic β-cell function, this study has unravelled the molecular identity of a new actor in pancreatic β-cell excitability that could be a major target for new compounds modulating insulin secretion.     

The NALCN ion channel is a new actor in pancreatic β-cell physiology

Ion channels are critical components of cell excitability involved in many physiological processes, including hormone secretion, and are thought be targets of choice in a pathological context. In the present paper, we summarize and discuss our recent findings on a four domain cation channel named NALCN which has been previously described as mediating a TTX-resistant leak sodium current in neurons. We recently reported that NALCN is also expressed in rodent islets of Langerhans as well as in the mouse MIN6 pancreatic β-cell line. This pancreatic NALCN channel encodes for a cation current triggered by acetylcholine activation of M3 muscarinic receptors. Importantly, the activation mechanism is independent of G protein action, but is dependent on a SFK-pathway, and involves the co-inclusion of M3 muscarinic receptors and NALCN in the same complex. Although additional work is now needed, considering the importance of the cholinergic control on the pancreatic β-cell function, this study has unravelled the molecular identity of a new actor in pancreatic β-cell excitability that could be a major target for new compounds modulating insulin secretion.     

Genetic variations in the pancreatic ATP-sensitive potassium channel, β-cell dysfunction, and susceptibility to type 2 diabetes

The KCNJ11 and ABCC8 genes encode the components of the pancreatic ATP-sensitive potassium (KATP) channel, which regulates insulin secretion by β-cells and hence could be involved in the pathogenesis of type 2 diabetes (T2D). The KCNJ11 E23K and ABCC8 exon 31 variants have been studied in 127 Russian T2D patients and 117 controls using the polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) approach. The KCNJ11 E23 variant and the ABCC8 exon 31 allele A were associated with higher risk of T2D [Odds ratio (OR) of 1.53 (P = 0.023) and 2.41 (P = 1.95 × 10⁻⁵)], respectively. Diabetic carriers of the ABCC8 G/G variant had reduced 2 h glucose compared to A/A + A/G (P = 0.031). The G/G genotype of ABCC8 was also significantly associated with increased both fasting and 2 h serum insulin in diabetic and non-diabetic patients. A HOMA-β value characterizing the β-cell homeostasis was higher in the non-diabetic carriers homozygous for G/G (98.0 ± 46.9) then for other genotypes (HOMA-β = 85.6 ± 45.5 for A/A + A/G, P = 0.0015). The KCNJ11 E23K and ABCC8 exon 31 variants contribute to susceptibility to T2D diabetes, glucose intolerance and altered insulin secretion in a Russian population.     

Apoptosis of β-cell and KATP channel opener

The main treatment of diabetes is oral antidiabetic drugs, but none of them can prevent β cell apoptosis. Diazoxide is a non-selective potassium channel opener (KCOs) ,which is first used in clinic. It can reduce autoantigen expression of type 1 diabetes,inhibit insulin secretion of pancreatic β-cell effectively, give a period of time for β-cell to rest, reduce excessive apoptosis of β cells, then restore β cell function. It can improve the sensitivity of insulin in peripheral tissues of type 2 diabetes meilitus ,as well as glucose-stim-ulated insulin secretion (GSIS), and can delay and block the process of diabetes and its complications.     

Apoptosis of β-cell and KATP channel opener

The main treatment of diabetes is oral antidiabetic drugs, but none of them can prevent β cell apoptosis. Diazoxide is a non-selective potassium channel opener (KCOs) ,which is first used in clinic. It can reduce autoantigen expression of type 1 diabetes,inhibit insulin secretion of pancreatic β-cell effectively, give a period of time for β-cell to rest, reduce excessive apoptosis of β cells, then restore β cell function. It can improve the sensitivity of insulin in peripheral tissues of type 2 diabetes meilitus ,as well as glucose-stim-ulated insulin secretion (GSIS), and can delay and block the process of diabetes and its complications.     

Apoptosis of β-cell and KATP channel opener

The main treatment of diabetes is oral antidiabetic drugs, but none of them can prevent β cell apoptosis. Diazoxide is a non-selective potassium channel opener (KCOs) ,which is first used in clinic. It can reduce autoantigen expression of type 1 diabetes,inhibit insulin secretion of pancreatic β-cell effectively, give a period of time for β-cell to rest, reduce excessive apoptosis of β cells, then restore β cell function. It can improve the sensitivity of insulin in peripheral tissues of type 2 diabetes meilitus ,as well as glucose-stim-ulated insulin secretion (GSIS), and can delay and block the process of diabetes and its complications.     

Apoptosis of β-cell and KATP channel opener

The main treatment of diabetes is oral antidiabetic drugs, but none of them can prevent β cell apoptosis. Diazoxide is a non-selective potassium channel opener (KCOs) ,which is first used in clinic. It can reduce autoantigen expression of type 1 diabetes,inhibit insulin secretion of pancreatic β-cell effectively, give a period of time for β-cell to rest, reduce excessive apoptosis of β cells, then restore β cell function. It can improve the sensitivity of insulin in peripheral tissues of type 2 diabetes meilitus ,as well as glucose-stim-ulated insulin secretion (GSIS), and can delay and block the process of diabetes and its complications.     

Apoptosis of β-cell and KATP channel opener

The main treatment of diabetes is oral antidiabetic drugs, but none of them can prevent β cell apoptosis. Diazoxide is a non-selective potassium channel opener (KCOs) ,which is first used in clinic. It can reduce autoantigen expression of type 1 diabetes,inhibit insulin secretion of pancreatic β-cell effectively, give a period of time for β-cell to rest, reduce excessive apoptosis of β cells, then restore β cell function. It can improve the sensitivity of insulin in peripheral tissues of type 2 diabetes meilitus ,as well as glucose-stim-ulated insulin secretion (GSIS), and can delay and block the process of diabetes and its complications.     

Apoptosis of β-cell and KATP channel opener

新近研究显示,糖尿病是一种低水平的炎症性疾病。多种因素刺激下,环氧化酶(COX)2在胰岛及多种组织中高水平表达。它通过与炎症因子和炎症介质如白细胞介素1、一氧化氮、自由基、前列腺素E等相互作用,对胰岛β细胞产生毒性效应,抑制胰岛素分泌,在糖尿病发生、发展中起重要作用,而且COX2与糖尿病微血管、神经系统并发症也密切相关。对COX2的研究可进一步揭示糖尿病发生的分子机制,为预防和治疗糖尿病提供新的思路。     

Apoptosis of β-cell and KATP channel opener

The main treatment of diabetes is oral antidiabetic drugs, but none of them can prevent β cell apoptosis. Diazoxide is a non-selective potassium channel opener (KCOs) ,which is first used in clinic. It can reduce autoantigen expression of type 1 diabetes,inhibit insulin secretion of pancreatic β-cell effectively, give a period of time for β-cell to rest, reduce excessive apoptosis of β cells, then restore β cell function. It can improve the sensitivity of insulin in peripheral tissues of type 2 diabetes meilitus ,as well as glucose-stim-ulated insulin secretion (GSIS), and can delay and block the process of diabetes and its complications.     

Apoptosis of β-cell and KATP channel opener

介绍了血管紧张素Ⅱ受体及其拮抗剂的分型和作用 ,并比较了血管紧张素Ⅱ受体拮抗剂和血管紧张素转换酶抑制剂在糖尿病肾病中的作用 ,前者显示了较好的应用前景。