CO-ADMINISTRATION OF RESVERATROL RESCUED LEAD-INDUCED TOXICITY IN DROSOPHILA MELANOGASTER

Lead toxicity poses a significant environmental concern linked to diverse health issues. This study explores the potential mitigating effects of resveratrol on lead-induced toxicity in Drosophila melanogaster. Adult fruit flies, aged three days, were orally exposed to lead (60mg/L), Succimer (10mg), and varying concentrations of resveratrol (50, 100, and 150mg). The investigation encompassed the assessment of selected biological parameters, biochemical markers, oxidative stress indicators, and antioxidant enzymes. Resveratrol exhibited a dose-dependent enhancement of egg-laying, eclosion rate, filial generation output, locomotor activity, and life span in D. melanogaster, significantly to 150mg of diet. Most of the investigated biochemical parameters were significantly rescued in lead-exposed fruit flies when co-treated with resveratrol (p < 0.05). However, oxidative stress remained unaffected by resveratrol. The findings suggest that resveratrol effectively protects against lead toxicity in Drosophila melanogaster and may hold therapeutic potential as an agent for managing lead poisoning in humans.

Eviscerated liver: an extremely rare complication of abdominal wound dehiscense through a midline incision

Abdominal wound dehiscense, or burst abdomen, is a critical postoperative complication necessitating immediate intervention. We present an extremely rare case of left hepatic lobe evisceration through wound dehiscense in a 65-year-old female receiving palliative care for hypopharyngeal squamous cell carcinoma. The patient's midline incision that was performed for feeding jejunostomy tube displayed liver protrusion on Day 14 postoperatively. Surgical exploration revealed a healthy liver, prompting reduction and secondary sutures to prevent complications. Abdominal wound dehiscense risk factors, including advanced age, poor nutrition, and medical illness, contribute to its occurrence. Although guidelines for liver evisceration management are lacking, our case emphasizes proper technique, wound care, and nutritional support to aid the healing process and to ensure a better outcome for the patients.

Facile synthesis and dyeing performance of some disperse monomeric and polymeric dyes on nylon and polyester fabrics

ABSTRACT. Monoazo disperse polymeric dyes consisting of polyarylazocarboxybenzene-formaldehyde (PACB-F) and polyarylazohydroxynaphthalene-formaldehyde (PAHN-F) were synthesized through diazotisation, coupling and polycondensation reactions in the presence of formaldehyde and aqueous oxalic acid. The structure of the as synthesised dyes was acquired using UV-visible absorption maxima and FT-IR spectroscopy and their color, yield, melting point, solubility, and viscosity determined via standard methods. UV-visible and FTIR results showed successful formation of the polymeric dyes due to shift of wavelength of maximum absorption (λmax) (420-470 nm, 460-510 nm) and new absorption peak at around (2800-2995 cm-1) for methylene bridge, respectively. The yield of the monomeric dyes was 67.1-85.7% and polymeric 45.1-59.3%, melting point was 104.1-131.2 oC, and 136.0-143.5 oC, respectively. They are soluble in acetone, ethanol, and methanol and insoluble in n-hexane and water. The dyeing process was achieved via high temperature and carrier dyeing techniques on nylon and polyester fibers. The dyeing characteristics of the synthesised dyes were analyzed in comparison with commercial disperse dyes (terasil brilliant violet and terasil scarlet brown). The dyeing produced a very attractive hue brown shades with good to excellent washing, light, hot pressing, and rubbing fastness.                     KEY WORDS: Carbocyclic, Dyeing activity, Monoazo, Monomeric, Polymeric   Bull. Chem. Soc. Ethiop. 2021, 35(3), 485-497. DOI: https://dx.doi.org/10.4314/bcse.v35i3.2

Giant ankyrin-G regulates cardiac function

Cardiovascular disease (CVD) remains the most common cause of adult morbidity and mortality in developed nations. As a result, predisposition for CVD is increasingly important to understand. Ankyrins are intracellular proteins required for the maintenance of membrane domains. Canonical ankyrin-G (AnkG) has been shown to be vital for normal cardiac function, specifically cardiac excitability, via targeting and regulation of the cardiac voltage-gated sodium channel. Noncanonical (giant) AnkG isoforms play a key role in neuronal membrane biogenesis and excitability, with evidence for human neurologic disease when aberrant. However, the role of giant AnkG in cardiovascular tissue has yet to be explored. Here, we identify giant AnkG in the myocardium and identify that it is enriched in 1-week-old mice. Using a new mouse model lacking giant AnkG expression in myocytes, we identify that young mice displayed a dilated cardiomyopathy phenotype with aberrant electrical conduction and enhanced arrhythmogenicity. Structural and electrical dysfunction occurred at 1 week of age, when giant AnkG was highly expressed and did not appreciably change in adulthood until advanced age. At a cellular level, loss of giant AnkG results in delayed and early afterdepolarizations. However, surprisingly, giant AnkG cKO myocytes display normal I_Na, but abnormal myocyte contractility, suggesting unique roles of the large isoform in the heart. Finally, transcript analysis provided evidence for unique pathways that may contribute to the structural and electrical findings shown in giant AnkG cKO animals. In summary, we identify a critical role for giant AnkG that adds to the diversity of ankyrin function in the heart.

Abnormal myocardial expression of SAP97 is associated with arrhythmogenic risk

Synapse-associated protein 97 (SAP97) is a scaffolding protein crucial for the functional expression of several cardiac ion channels and therefore proper cardiac excitability. Alterations in the functional expression of SAP97 can modify the ionic currents underlying the cardiac action potential and consequently confer susceptibility for arrhythmogenesis. In this study, we generated a murine model for inducible, cardiac-targeted Sap97 ablation to investigate arrhythmia susceptibility and the underlying molecular mechanisms. Furthermore, we sought to identify human SAP97 (DLG1) variants that were associated with inherited arrhythmogenic disease. The murine model of cardiac-specific Sap97 ablation demonstrated several ECG abnormalities, pronounced action potential prolongation subject to high incidence of arrhythmogenic afterdepolarizations and notable alterations in the activity of the main cardiac ion channels. However, no DLG1 mutations were found in 40 unrelated cases of genetically elusive long QT syndrome (LQTS). Instead, we provide the first evidence implicating a gain of function in human DLG1 mutation resulting in an increase in Kv4.3 current (I_to) as a novel, potentially pathogenic substrate for Brugada syndrome (BrS). In conclusion, DLG1 joins a growing list of genes encoding ion channel interacting proteins (ChIPs) identified as potential channelopathy-susceptibility genes because of their ability to regulate the trafficking, targeting, and modulation of ion channels that are critical for the generation and propagation of the cardiac electrical impulse. Dysfunction in these critical components of cardiac excitability can potentially result in fatal cardiac disease.NEW & NOTEWORTHY The gene encoding SAP97 (DLG1) joins a growing list of genes encoding ion channel-interacting proteins (ChIPs) identified as potential channelopathy-susceptibility genes because of their ability to regulate the trafficking, targeting, and modulation of ion channels that are critical for the generation and propagation of the cardiac electrical impulse. In this study we provide the first data supporting DLG1-encoded SAP97's candidacy as a minor Brugada syndrome susceptibility gene.

Protein Phosphatase 2A Regulates Cardiac Na+ Channels

RATIONALE

Voltage-gated Na⁺ channel ( I_Na) function is critical for normal cardiac excitability. However, the Na⁺ channel late component ( I_Na,L) is directly associated with potentially fatal forms of congenital and acquired human arrhythmia. CaMKII (Ca²⁺/calmodulin-dependent kinase II) enhances I_Na,L in response to increased adrenergic tone. However, the pathways that negatively regulate the CaMKII/Na_v1.5 axis are unknown and essential for the design of new therapies to regulate the pathogenic I_Na,L.

OBJECTIVE

To define phosphatase pathways that regulate I_Na,L in vivo.

METHODS AND RESULTS

A mouse model lacking a key regulatory subunit (B56α) of the PP (protein phosphatase) 2A holoenzyme displayed aberrant action potentials after adrenergic stimulation. Unbiased computational modeling of B56α KO (knockout) mouse myocyte action potentials revealed an unexpected role of PP2A in I_Na,L regulation that was confirmed by direct I_Na,L recordings from B56α KO myocytes. Further, B56α KO myocytes display decreased sensitivity to isoproterenol-induced induction of arrhythmogenic I_Na,L, and reduced CaMKII-dependent phosphorylation of Na_v1.5. At the molecular level, PP2A/B56α complex was found to localize and coimmunoprecipitate with the primary cardiac Na_v channel, Na_v1.5.

CONCLUSIONS

PP2A regulates Na_v1.5 activity in mouse cardiomyocytes. This regulation is critical for pathogenic Na_v1.5 late current and requires PP2A-B56α. Our study supports B56α as a novel target for the treatment of arrhythmia.

Calmodulin kinase II regulates atrial myocyte late sodium current, calcium handling, and atrial arrhythmia

BACKGROUND

Atrial fibrillation (AF) is the most common type of arrhythmia. Abnormal atrial myocyte Ca²⁺ handling promotes aberrant membrane excitability and remodeling that are important for atrial arrhythmogenesis. The sequence of molecular events leading to loss of normal atrial myocyte Ca²⁺ homeostasis is not established. Late Na⁺ current (I_Na,L) is increased in atrial myocytes from AF patients together with an increase in activity of Ca²⁺/calmodulin-dependent kinase II (CaMKII).

OBJECTIVE

The purpose of this study was to determine whether CaMKII-dependent phosphorylation at Ser571 on Na_V1.5 increases atrial I_Na,L, leading to aberrant atrial Ca²⁺ cycling, altered electrophysiology, and increased AF risk.

METHODS

Atrial myocyte electrophysiology, Ca²⁺ handling, and arrhythmia susceptibility were studied in wild-type and Scn5a knock-in mice expressing phosphomimetic (S571E) or phosphoresistant (S571A) Na_V1.5 at Ser571.

RESULTS

Atrial myocytes from S571E but not S571A mice displayed an increase in I_Na,L and action potential duration, and with adrenergic stress have increased delayed afterdepolarizations. Frequency of Ca²⁺ sparks and waves was increased in S571E atrial myocytes compared to wild type. S571E mice showed an increase in atrial events induced by adrenergic stress and AF inducibility in vivo. Isolated S571E atria were more susceptible to spontaneous atrial events, which were abrogated by inhibiting sarcoplasmic reticulum Ca²⁺ release, CaMKII, or the Na⁺/Ca²⁺ exchanger. Expression of phospho-Na_V1.5 at Ser571 and autophosphorylated CaMKII were increased in atrial samples from human AF patients.

CONCLUSION

This study identified CaMKII-dependent regulation of Na_V1.5 as an important upstream event in Ca²⁺ handling defects and abnormal impulse generation in the setting of AF.

Ankyrin-B dysfunction predisposes to arrhythmogenic cardiomyopathy and is amenable to therapy

Arrhythmogenic cardiomyopathy (ACM) is an inherited arrhythmia syndrome characterized by severe structural and electrical cardiac phenotypes, including myocardial fibrofatty replacement and sudden cardiac death. Clinical management of ACM is largely palliative, owing to an absence of therapies that target its underlying pathophysiology, which stems partially from our limited insight into the condition. Following identification of deceased ACM probands possessing ANK2 rare variants and evidence of ankyrin-B loss of function on cardiac tissue analysis, an ANK2 mouse model was found to develop dramatic structural abnormalities reflective of human ACM, including biventricular dilation, reduced ejection fraction, cardiac fibrosis, and premature death. Desmosomal structure and function appeared preserved in diseased human and murine specimens in the presence of markedly abnormal β-catenin expression and patterning, leading to identification of a previously unknown interaction between ankyrin-B and β-catenin. A pharmacological activator of the WNT/β-catenin pathway, SB-216763, successfully prevented and partially reversed the murine ACM phenotypes. Our findings introduce what we believe to be a new pathway for ACM, a role of ankyrin-B in cardiac structure and signaling, a molecular link between ankyrin-B and β-catenin, and evidence for targeted activation of the WNT/β-catenin pathway as a potential treatment for this disease.

Defining new mechanistic roles for αII spectrin in cardiac function

Spectrins are cytoskeletal proteins essential for membrane biogenesis and regulation and serve critical roles in protein targeting and cellular signaling. αII spectrin (SPTAN1) is one of two α spectrin genes and αII spectrin dysfunction is linked to alterations in axon initial segment formation, cortical lamination, and neuronal excitability. Furthermore, human αII spectrin loss-of-function variants cause neurological disease. As global αII spectrin knockout mice are embryonic lethal, the in vivo roles of αII spectrin in adult heart are unknown and untested. Here, based on pronounced alterations in αII spectrin regulation in human heart failure we tested the in vivo roles of αII spectrin in the vertebrate heart. We created a mouse model of cardiomyocyte-selective αII spectrin-deficiency (cKO) and used this model to define the roles of αII spectrin in cardiac function. αII spectrin cKO mice displayed significant structural, cellular, and electrical phenotypes that resulted in accelerated structural remodeling, fibrosis, arrhythmia, and mortality in response to stress. At the molecular level, we demonstrate that αII spectrin plays a nodal role for global cardiac spectrin regulation, as αII spectrin cKO hearts exhibited remodeling of αI spectrin and altered β-spectrin expression and localization. At the cellular level, αII spectrin deficiency resulted in altered expression, targeting, and regulation of cardiac ion channels NaV1.5 and KV4.3. In summary, our findings define critical and unexpected roles for the multifunctional αII spectrin protein in the heart. Furthermore, our work provides a new in vivo animal model to study the roles of αII spectrin in the cardiomyocyte.

βIV-Spectrin regulates STAT3 targeting to tune cardiac response to pressure overload

Heart failure (HF) remains a major source of morbidity and mortality in the US. The multifunctional Ca2+/calmodulin-dependent kinase II (CaMKII) has emerged as a critical regulator of cardiac hypertrophy and failure, although the mechanisms remain unclear. Previous studies have established that the cytoskeletal protein βIV-spectrin coordinates local CaMKII signaling. Here, we sought to determine the role of a spectrin-CaMKII complex in maladaptive remodeling in HF. Chronic pressure overload (6 weeks of transaortic constriction [TAC]) induced a decrease in cardiac function in WT mice but not in animals expressing truncated βIV-spectrin lacking spectrin-CaMKII interaction (qv3J mice). Underlying the observed differences in function was an unexpected differential regulation of STAT3-related genes in qv3J TAC hearts. In vitro experiments demonstrated that βIV-spectrin serves as a target for CaMKII phosphorylation, which regulates its stability. Cardiac-specific βIV-spectrin-KO (βIV-cKO) mice showed STAT3 dysregulation, fibrosis, and decreased cardiac function at baseline, similar to what was observed with TAC in WT mice. STAT3 inhibition restored normal cardiac structure and function in βIV-cKO and WT TAC hearts. Our studies identify a spectrin-based complex essential for regulation of the cardiac response to chronic pressure overload. We anticipate that strategies targeting the new spectrin-based "statosome" will be effective at suppressing maladaptive remodeling in response to chronic stress.

Extracellular Matrix-Mediated Maturation of Human Pluripotent Stem Cell-Derived Cardiac Monolayer Structure and Electrophysiological Function

BACKGROUND

Human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) monolayers generated to date display an immature embryonic-like functional and structural phenotype that limits their utility for research and cardiac regeneration. In particular, the electrophysiological function of hPSC-CM monolayers and bioengineered constructs used to date are characterized by slow electric impulse propagation velocity and immature action potential profiles.

METHODS AND RESULTS

Here, we have identified an optimal extracellular matrix for significant electrophysiological and structural maturation of hPSC-CM monolayers. hPSC-CM plated in the optimal extracellular matrix combination have impulse propagation velocities ≈2× faster than previously reported (43.6±7.0 cm/s; n=9) and have mature cardiomyocyte action potential profiles, including hyperpolarized diastolic potential and rapid action potential upstroke velocity (146.5±17.7 V/s; n=5 monolayers). In addition, the optimal extracellular matrix promoted hypertrophic growth of cardiomyocytes and the expression of key mature sarcolemmal (SCN5A, Kir2.1, and connexin43) and myofilament markers (cardiac troponin I). The maturation process reported here relies on activation of integrin signaling pathways: neutralization of β1 integrin receptors via blocking antibodies and pharmacological blockade of focal adhesion kinase activation prevented structural maturation.

CONCLUSIONS

Maturation of human stem cell-derived cardiomyocyte monolayers is achieved in a 1-week period by plating cardiomyocytes on PDMS (polydimethylsiloxane) coverslips rather than on conventional 2-dimensional cell culture formats, such as glass coverslips or plastic dishes. Activation of integrin signaling and focal adhesion kinase is essential for significant maturation of human cardiac monolayers.

Common human ANK2 variant confers in vivo arrhythmia phenotypes

BACKGROUND

Human ANK2 (ankyrin-B) loss-of-function variants are directly linked with arrhythmia phenotypes. However, in atypical non-ion channel arrhythmia genes such as ANK2 that lack the same degree of robust structure/function and clinical data, it may be more difficult to assign variant disease risk based simply on variant location, minor allele frequency, and/or predictive structural algorithms. The human ankyrin-B p.L1622I variant found in arrhythmia probands displays significant diversity in minor allele frequency across populations.

OBJECTIVE

The objective of this study was to directly test the in vivo impact of ankyrin-B p.L1622I on cardiac electrical phenotypes and arrhythmia risk using a new animal model.

METHODS

We tested arrhythmia phenotypes in a new "knock-in" animal model harboring the human ankyrin-B p.L1622I variant.

RESULTS

Ankyrin-B p.L1622I displays reduced posttranslational expression in vivo, resulting in reduced cardiac ankyrin-B expression and reduced association with binding-partner Na/Ca exchanger. Ankyrin-B(L1622I/L1622I) mice display changes in heart rate, atrioventricular and intraventricular conduction, and alterations in repolarization. Furthermore, ankyrin-B(L1622I/L1622I) mice display catecholamine-dependent arrhythmias. At the cellular level, ankyrin-B(L1622I/L1622I) myocytes display increased action potential duration and severe arrhythmogenic afterdepolarizations that provide a mechanistic rationale for the arrhythmias.

CONCLUSION

Our findings support in vivo arrhythmogenic phenotypes of an ANK2 variant with unusual frequency in select populations. On the basis of our findings and current clinical data, we support classification of p.L1622I as a "mild" loss-of-function variant that may confer arrhythmia susceptibility in the context of secondary risk factors including environment, medication, and/or additional genetic variation.

Galectin-3 Regulates Atrial Fibrillation Remodeling and Predicts Catheter Ablation Outcomes

Atrial fibrillation (AF) usually starts as paroxysmal but can evolve relentlessly to the persistent and permanent forms. However, the mechanisms governing such a transition are unknown. The authors show that intracardiac serum levels of galectin (Gal)-3 are greater in patients with persistent than paroxysmal AF and that Gal-3 independently predicts atrial tachyarrhythmia recurrences after a single ablation procedure. Using a sheep model of persistent AF the authors further demonstrate that upstream therapy targeting Gal-3 diminishes both electrical remodeling and fibrosis by impairing transforming growth factor beta–mediated signaling and reducing myofibroblast activation. Accordingly, Gal-3 inhibition therapy increases the probability of AF termination and reduces the overall burden of AF. Therefore the authors postulate that Gal-3 inhibition is a potential new upstream therapy to prevent AF progression.

•

Intracardiac serum galectin (Gal)-3 levels are shown to be greater in patients with persistent than paroxysmal atrial fibrillation (AF), and the Gal-3 level was an independent predictor of AF recurrences after a single ablation procedure.

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In a sheep model, the Gal-3 inhibitor GM-CT-01 (GMCT) reduced atrial fibroblast proliferation in vitro.

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GMCT mitigated atrial dilation, myocyte hypertrophy, fibrosis, and the expected increase in DF during transition to persistent AF.

•

GMCT-treated sheep hearts had longer action potential durations, and fewer rotors and wavebreaks during AF than control.

•

GMCT increased the number of spontaneous AF terminations and reduced overall AF burden.

Protein phosphatase 2A regulatory subunit B56α limits phosphatase activity in the heart

Protein phosphatase 2A (PP2A) is a serine/threonine-selective holoenzyme composed of a catalytic, scaffolding, and regulatory subunit. In the heart, PP2A activity is requisite for cardiac excitation-contraction coupling and central in adrenergic signaling. We found that mice deficient in the PP2A regulatory subunit B56α (1 of 13 regulatory subunits) had altered PP2A signaling in the heart that was associated with changes in cardiac physiology, suggesting that the B56α regulatory subunit had an autoinhibitory role that suppressed excess PP2A activity. The increase in PP2A activity in the mice with reduced B56α expression resulted in slower heart rates and increased heart rate variability, conduction defects, and increased sensitivity of heart rate to parasympathetic agonists. Increased PP2A activity in B56α(+/-) myocytes resulted in reduced Ca(2+) waves and sparks, which was associated with decreased phosphorylation (and thus decreased activation) of the ryanodine receptor RyR2, an ion channel on intracellular membranes that is involved in Ca(2+) regulation in cardiomyocytes. In line with an autoinhibitory role for B56α, in vivo expression of B56α in the absence of altered abundance of other PP2A subunits decreased basal phosphatase activity. Consequently, in vivo expression of B56α suppressed parasympathetic regulation of heart rate and increased RyR2 phosphorylation in cardiomyocytes. These data show that an integral component of the PP2A holoenzyme has an important inhibitory role in controlling PP2A enzyme activity in the heart.

Voltage-Gated Sodium Channel Phosphorylation at Ser571 Regulates Late Current, Arrhythmia, and Cardiac Function In Vivo

BACKGROUND

Voltage-gated Na(+) channels (Nav) are essential for myocyte membrane excitability and cardiac function. Nav current (INa) is a large-amplitude, short-duration spike generated by rapid channel activation followed immediately by inactivation. However, even under normal conditions, a small late component of INa (INa,L) persists because of incomplete/failed inactivation of a subpopulation of channels. Notably, INa,L is directly linked with both congenital and acquired disease states. The multifunctional Ca(2+)/calmodulin-dependent kinase II (CaMKII) has been identified as an important activator of INa,L in disease. Several potential CaMKII phosphorylation sites have been discovered, including Ser571 in the Nav1.5 DI-DII linker, but the molecular mechanism underlying CaMKII-dependent regulation of INa,L in vivo remains unknown.

METHODS AND RESULTS

To determine the in vivo role of Ser571, 2 Scn5a knock-in mouse models were generated expressing either: (1) Nav1.5 with a phosphomimetic mutation at Ser571 (S571E), or (2) Nav1.5 with the phosphorylation site ablated (S571A). Electrophysiology studies revealed that Ser571 regulates INa,L but not other channel properties previously linked to CaMKII. Ser571-mediated increases in INa,L promote abnormal repolarization and intracellular Ca(2+) handling and increase susceptibility to arrhythmia at the cellular and animal level. Importantly, Ser571 is required for maladaptive remodeling and arrhythmias in response to pressure overload.

CONCLUSIONS

Our data provide the first in vivo evidence for the molecular mechanism underlying CaMKII activation of the pathogenic INa,L. Relevant for improved rational design of potential therapies, our findings demonstrate that Ser571-dependent regulation of Nav1.5 specifically tunes INa,L without altering critical physiological components of the current.

Eps15 Homology Domain-containing Protein 3 Regulates Cardiac T-type Ca2+ Channel Targeting and Function in the Atria

Proper trafficking of membrane-bound ion channels and transporters is requisite for normal cardiac function. Endosome-based protein trafficking of membrane-bound ion channels and transporters in the heart is poorly understood, particularly in vivo. In fact, for select cardiac cell types such as atrial myocytes, virtually nothing is known regarding endosomal transport. We previously linked the C-terminal Eps15 homology domain-containing protein 3 (EHD3) with endosome-based protein trafficking in ventricular cardiomyocytes. Here we sought to define the roles and membrane protein targets for EHD3 in atria. We identify the voltage-gated T-type Ca(2+) channels (CaV3.1, CaV3.2) as substrates for EHD3-dependent trafficking in atria. Mice selectively lacking EHD3 in heart display reduced expression and targeting of both Cav3.1 and CaV3.2 in the atria. Furthermore, functional experiments identify a significant loss of T-type-mediated Ca(2+) current in EHD3-deficient atrial myocytes. Moreover, EHD3 associates with both CaV3.1 and CaV3.2 in co-immunoprecipitation experiments. T-type Ca(2+) channel function is critical for proper electrical conduction through the atria. Consistent with these roles, EHD3-deficient mice demonstrate heart rate variability, sinus pause, and atrioventricular conduction block. In summary, our findings identify CaV3.1 and CaV3.2 as substrates for EHD3-dependent protein trafficking in heart, provide in vivo data on endosome-based trafficking pathways in atria, and implicate EHD3 as a key player in the regulation of atrial myocyte excitability and cardiac conduction.

Ankyrin-G coordinates intercalated disc signaling platform to regulate cardiac excitability in vivo

RATIONALE

Nav1.5 (SCN5A) is the primary cardiac voltage-gated Nav channel. Nav1.5 is critical for cardiac excitability and conduction, and human SCN5A mutations cause sinus node dysfunction, atrial fibrillation, conductional abnormalities, and ventricular arrhythmias. Further, defects in Nav1.5 regulation are linked with malignant arrhythmias associated with human heart failure. Consequently, therapies to target select Nav1.5 properties have remained at the forefront of cardiovascular medicine. However, despite years of investigation, the fundamental pathways governing Nav1.5 membrane targeting, assembly, and regulation are still largely undefined.

OBJECTIVE

Define the in vivo mechanisms underlying Nav1.5 membrane regulation.

METHODS AND RESULTS

Here, we define the molecular basis of an Nav channel regulatory platform in heart. Using new cardiac-selective ankyrin-G(-/-) mice (conditional knock-out mouse), we report that ankyrin-G targets Nav1.5 and its regulatory protein calcium/calmodulin-dependent kinase II to the intercalated disc. Mechanistically, βIV-spectrin is requisite for ankyrin-dependent targeting of calcium/calmodulin-dependent kinase II-δ; however, βIV-spectrin is not essential for ankyrin-G expression. Ankyrin-G conditional knock-out mouse myocytes display decreased Nav1.5 expression/membrane localization and reduced INa associated with pronounced bradycardia, conduction abnormalities, and ventricular arrhythmia in response to Nav channel antagonists. Moreover, we report that ankyrin-G links Nav channels with broader intercalated disc signaling/structural nodes, as ankyrin-G loss results in reorganization of plakophilin-2 and lethal arrhythmias in response to β-adrenergic stimulation.

CONCLUSIONS

Our findings provide the first in vivo data for the molecular pathway required for intercalated disc Nav1.5 targeting/regulation in heart. Further, these new data identify the basis of an in vivo cellular platform critical for membrane recruitment and regulation of Nav1.5.

EHD3-dependent endosome pathway regulates cardiac membrane excitability and physiology

RATIONALE

Cardiac function is dependent on the coordinate activities of membrane ion channels, transporters, pumps, and hormone receptors to tune the membrane electrochemical gradient dynamically in response to acute and chronic stress. Although our knowledge of membrane proteins has rapidly advanced during the past decade, our understanding of the subcellular pathways governing the trafficking and localization of integral membrane proteins is limited and essentially unstudied in vivo. In the heart, to our knowledge, there are no in vivo mechanistic studies that directly link endosome-based machinery with cardiac physiology.

OBJECTIVE

To define the in vivo roles of endosome-based cellular machinery for cardiac membrane protein trafficking, myocyte excitability, and cardiac physiology.

METHODS AND RESULTS

We identify the endosome-based Eps15 homology domain 3 (EHD3) pathway as essential for cardiac physiology. EHD3-deficient hearts display structural and functional defects including bradycardia and rate variability, conduction block, and blunted response to adrenergic stimulation. Mechanistically, EHD3 is critical for membrane protein trafficking, because EHD3-deficient myocytes display reduced expression/localization of Na/Ca exchanger and L-type Ca channel type 1.2 with a parallel reduction in Na/Ca exchanger-mediated membrane current and Cav1.2-mediated membrane current. Functionally, EHD3-deficient myocytes show increased sarcoplasmic reticulum [Ca], increased spark frequency, and reduced expression/localization of ankyrin-B, a binding partner for EHD3 and Na/Ca exchanger. Finally, we show that in vivo EHD3-deficient defects are attributable to cardiac-specific roles of EHD3 because mice with cardiac-selective EHD3 deficiency demonstrate both structural and electric phenotypes.

CONCLUSIONS

These data provide new insight into the critical role of endosome-based pathways in membrane protein targeting and cardiac physiology. EHD3 is a critical component of protein trafficking in heart and is essential for the proper membrane targeting of select cellular proteins that maintain excitability.

Dominant frequency increase rate predicts transition from paroxysmal to long-term persistent atrial fibrillation

BACKGROUND

Little is known about the mechanisms underlying the transition from paroxysmal to persistent atrial fibrillation (AF). In an ovine model of long-standing persistent AF we tested the hypothesis that the rate of electric and structural remodeling, assessed by dominant frequency (DF) changes, determines the time at which AF becomes persistent.

METHODS AND RESULTS

Self-sustained AF was induced by atrial tachypacing. Seven sheep were euthanized 11.5±2.3 days after the transition to persistent AF and without reversal to sinus rhythm; 7 sheep were euthanized after 341.3±16.7 days of long-standing persistent AF. Seven sham-operated animals were in sinus rhythm for 1 year. DF was monitored continuously in each group. Real-time polymerase chain reaction, Western blotting, patch clamping, and histological analyses were used to determine the changes in functional ion channel expression and structural remodeling. Atrial dilatation, mitral valve regurgitation, myocyte hypertrophy, and atrial fibrosis occurred progressively and became statistically significant after the transition to persistent AF, with no evidence for left ventricular dysfunction. DF increased progressively during the paroxysmal-to-persistent AF transition and stabilized when AF became persistent. Importantly, the rate of DF increase correlated strongly with the time to persistent AF. Significant action potential duration abbreviation, secondary to functional ion channel protein expression changes (CaV1.2, NaV1.5, and KV4.2 decrease; Kir2.3 increase), was already present at the transition and persisted for 1 year of follow up.

CONCLUSIONS

In the sheep model of long-standing persistent AF, the rate of DF increase predicts the time at which AF stabilizes and becomes persistent, reflecting changes in action potential duration and densities of sodium, L-type calcium, and inward rectifier currents.

Reduced Na⁺ current density underlies impaired propagation in the diabetic rabbit ventricle

Diabetes is associated with an increased risk of sudden cardiac death, but the underlying mechanisms remain unclear. Our goal was to investigate changes occurring in the action potential duration (APD) and conduction velocity (CV) in the diabetic rabbit ventricle, and delineate the principal ionic determinants. A rabbit model of alloxan-induced diabetes was utilized. Optical imaging was used to record electrical activity in isolated Langendorff-perfused hearts in normo-, hypo- and hyper-kalemia ([K(+)]o=4, 2, 12 mM respectively). Patch clamp experiments were conducted to record Na(+) current (I(Na)) in isolated ventricular myocytes. The mRNA/protein expression levels for Nav1.5 (the α-subunit of I(Na)) and connexin-43 (Cx43), as well as fibrosis levels were examined. Computer simulations were performed to interpret experimental data. We found that the APD was not different, but the CV was significantly reduced in diabetic hearts in normo-, hypo-, and, hyper-kalemic conditions (13%, 17% and 33% reduction in diabetic vs. control, respectively). The cell capacitance (Cm) was increased (by ~14%), and the density of INa was reduced by ~32% in diabetic compared to control hearts, but the other biophysical properties of I(Na) were unaltered. The mRNA/protein expression levels for Cx43 were unaltered. For Nav1.5, the mRNA expression was not changed, and though the protein level tended to be less in diabetic hearts, this reduction was not statistically significant. Staining showed no difference in fibrosis levels between the control and diabetic ventricles. Computer simulations showed that the reduced magnitude of I(Na) was a key determinant of impaired propagation in the diabetic ventricle, which may have important implications for arrhythmogenesis.

Pattern of paediatric solid cancers seen in radiotherapy and oncology department, Ahmadu Bello University Teaching Hospital, Zaria - Nigeria

Summary

AIMS AND OBJECTIVES

To evaluate the Pattern of Paediatric solid cancers seen over 5 years in the Radiotherapy and Oncology Department of Ahmadu Bello University Teaching Hospital, Zaria-Nigeria.

MATERIALS AND METHODS

The study involved patients aged 16 years and below diagnosed with solid cancers referred to the Radiotherapy and Oncology Centre for further management. Between January 2006 and December 2010, a total of 136 new patients with paediatric solid cancers were seen and evaluated. Only patients with histological confirmation were included in the study (136 out of 141 patients) irrespective of cancer type, sex, co-morbidity and performance status. Patients' folders were reviewed retrospectively with a structured pro forma. Information retrieved from patients folder included age, sex, histological type, stage and types of treatments received. Results were presented in tables.

RESULTS

A total of 136 patients were evaluated with a mean age of 6.9 years (median of 5 years, age range 1.5 - 16 years). 74 patients (54.4%) were 5 years and below and 98 patients (72%) were 10 years and below. The sex ratio M:F was 1.3:1. At presentation at the Radiotherapy and Oncology centre, only 4 (3%) patients had early stage disease, 100 (73.5%) with locally advanced disease and 32 (23.5%) with metastatic disease. Retinoblastoma 48 (35.3%) was the commonest cancer seen followed by soft tissue sarcoma 20 (14.7%, all rhabdomyosarcoma), lymphomas 15 (11%) (9 Burkitt's and 6 Hodgkin's lymphoma), nephroblastoma 13 (9.6%), bone sarcoma 12 (8.8%) (8 Osteosarcoma and 4 Ewing's sarcoma), nasopharyngeal cancers 8 (5.9%), brain cancer 6 (4.4%), sacrococcygeal tumours 6 (4.4%) and other cancers accounted for 8 (5.9%). 116 (85.3%) patients received chemotherapy, 86 (63.2%) patients were scheduled for radiotherapy but only 21 (15.4%) received local field irradiation due to financial constraints and 43 (31.6%) had definitive surgery. Palliative care with medications was done in 22 (16.2%) patients. Of 48 patients with retinoblastoma, 30 affected left eye, 16 affected right eye and 2 patients were with bilateral retinoblastoma. The mean age for retinoblastoma was 3.8 years (median age of 3 years and mode is 3 years). The mean age for rhabdomyosarcoma was 8.7 yr (median age of 6.5 years).

CONCLUSION

Paediatric solid cancers were found to be more common in males than females with more than half being diagnosed in children aged 5 years and below. Retinoblastoma and rhabdomyosarcoma were the most common neoplasms. Most patients presented with locally advanced and metastatic cancers. Geographic variation exists with histological types.

Inhibition of platelet-derived growth factor-AB signaling prevents electromechanical remodeling of adult atrial myocytes that contact myofibroblasts

BACKGROUND

Persistent atrial fibrillation (PAF) results in electromechanical and structural remodeling by mechanisms that are poorly understood. Myofibroblast proliferation and fibrosis are major sources of structural remodeling in PAF. Myofibroblasts also interact with atrial myocytes via direct physical contact and release of signaling molecules, which may contribute to remodeling.

OBJECTIVE

To determine whether myofibroblasts contribute to atrial myocyte electromechanical remodeling via direct physical contact and platelet-derived growth factor (PDGF) signaling.

METHODS

Myofibroblasts and myocytes from adult sheep atria were co-cultured for 24 hours. Alternatively adult sheep atrial myocytes were exposed to 1 ng/mL recombitant PDGF AB peptide for 24 hours.

RESULTS

Myocytes making contact with myofibroblasts demonstrated significant reduction (P ≤ .05) in peak L-type calcium current density, shortening of action potential duration (APD), and reduction in calcium transients. These effects were blocked by pretreatment with a PDGF-AB neutralizing anti-body. Heterocellular contact also severely disturbed the localization of the L-type calcium channel. Myocytes exposed to recombinant PDGF-AB peptide for 24 hours demonstrated reduced APD50, APD80 and Peak L-type calcium current. Pretreatment with a PDGF-AB neutralizing antibody prevented these effects. Finally, while control atrial myocytes did not respond in a 1:1 manner to pacing frequencies of 3 Hz or higher, atrial myocytes from hearts that were tachypaced for 2 months and normal myocytes treated with PDGF-AB for 24 hours could be paced up to 10 Hz.

CONCLUSIONS

In addition to leading to fibrosis, atrial myofibroblasts contribute to electromechanical remodeling of myocytes via direct physical contact and release of PDGF-AB, which may be a factor in PAF-induced remodeling.

TGF-β1, released by myofibroblasts, differentially regulates transcription and function of sodium and potassium channels in adult rat ventricular myocytes

Cardiac injury promotes fibroblasts activation and differentiation into myofibroblasts, which are hypersecretory of multiple cytokines. It is unknown whether any of such cytokines are involved in the electrophysiological remodeling of adult cardiomyocytes. We cultured adult cardiomyocytes for 3 days in cardiac fibroblast conditioned medium (FCM) from adult rats. In whole-cell voltage-clamp experiments, FCM-treated myocytes had 41% more peak inward sodium current (I_Na) density at −40 mV than myocytes in control medium (p<0.01). In contrast, peak transient outward current (I_to) was decreased by ∼55% at 60 mV (p<0.001). Protein analysis of FCM demonstrated that the concentration of TGF-β1 was >3 fold greater in FCM than control, which suggested that FCM effects could be mediated by TGF-β1. This was confirmed by pre-treatment with TGF-β1 neutralizing antibody, which abolished the FCM-induced changes in both I_Na and I_to. In current-clamp experiments TGF-β1 (10 ng/ml) prolonged the action potential duration at 30, 50, and 90 repolarization (p<0.05); at 50 ng/ml it gave rise to early afterdepolarizations. In voltage-clamp experiments, TGF-β1 increased I_Na density in a dose-dependent manner without affecting voltage dependence of activation or inactivation. I_Na density was −36.25±2.8 pA/pF in control, −59.17±6.2 pA/pF at 0.1 ng/ml (p<0.01), and −58.22±6.6 pA/pF at 1 ng/ml (p<0.01). In sharp contrast, I_to density decreased from 22.2±1.2 pA/pF to 12.7±0.98 pA/pF (p<0.001) at 10 ng/ml. At 1 ng/ml TGF-β1 significantly increased SCN5A (Na_V1.5) (+73%; p<0.01), while reducing KCNIP2 (Kchip2; −77%; p<0.01) and KCND2 (K_V4.2; −50% p<0.05) mRNA levels. Further, the TGF-β1-induced increase in I_Na was mediated through activation of the PI3K-AKT pathway via phosphorylation of FOXO1 (a negative regulator of SCN5A). TGF-β1 released by myofibroblasts differentially regulates transcription and function of the main cardiac sodium channel and of the channel responsible for the transient outward current. The results provide new mechanistic insight into the electrical remodeling associated with myocardial injury.

Breast-cancer screening with trained volunteers in a rural area of Sudan: a pilot study

BACKGROUND

Breast cancer has a low cure rate in low-income and middle-income countries because patients often present with late-stage disease that has metastasised to other organs. We assessed whether the implementation of a cancer awareness and breast examination programme that uses local, volunteer women could increase the early detection of breast cancer in a rural area of sub-Saharan Africa.

METHODS

We did this pilot study in two counties in Gezira State, Sudan. We chose Keremet (56 villages) as the experimental county and Abugota (79 villages) as the control county. Female volunteers from villages in Keremet were trained in the detection of breast abnormalities. When trained, volunteers visited households in their village and screened women aged 18 years or older for breast abnormalities, referring women with suspected breast cancer for medical diagnosis and, if necessary, treatment at the district hospital. We also ran a cancer awareness programme for both men and women in study villages. Villages in the control population received no intervention. This study is ongoing.

FINDINGS

Between Jan 1, 2010, and Oct 10, 2012, 10 309 (70%) of 14 788 women in Keremet were screened. 138 women were identified as having breast abnormalities and were referred to the district hospital for diagnosis and treatment. 20 of these women did not report to the hospital. Of the 118 women who did report, 101 were diagnosed with benign lesions, eight with carcinoma in situ, and nine had malignant disease. After treatment, 12 of the 17 women with either carcinoma in situ or malignant disease (four had early breast cancer and eight had ductal carcinoma in situ) were disease-free and had a good prognosis. In the control villages, only four women reported to the centre: one was found to have a benign lesion while three were diagnosed with advanced disease.

INTERPRETATION

Our findings show that a screening programme using local volunteers can increase the detection of breast cancer in asymptomatic women in low-income rural communities. These findings can inform policy-makers' decisions in the design of cancer control programmes in Sudan and other similar areas in sub-Saharan Africa.

FUNDING

Sudan National Cancer Institute.

The ionic bases of the action potential in isolated mouse cardiac Purkinje cell

BACKGROUND

Collecting electrophysiological and molecular data from the murine conduction system presents technical challenges. Thus, only little advantage has been taken of numerous genetically engineered murine models to study excitation through the cardiac conduction system of the mouse.

OBJECTIVE

To develop an approach for isolating murine cardiac Purkinje cells (PCs), to characterize major ionic currents and to use the data to simulate action potentials (APs) recorded from PCs.

METHODS

Light microscopy was used to isolate and identify PCs from apical and septal cells. Current and voltage clamp techniques were used to record APs and whole cell currents. We then simulated a PC AP on the basis of our experimental data.

RESULTS

APs recorded from PCs were significantly longer than those recorded from ventricular cells. The prominent plateau phase of the PC AP was very negative (≈-40 mV). Spontaneous activity was observed only in PCs. The inward rectifier current demonstrated no significant differences compared to ventricular myocytes (VMs). However, sodium current density was larger, and the voltage-gated potassium current density was significantly less in PCs compared with myocytes. T-type Ca(2+) currents (I(Ca,T)) were present in PCs but not VMs. Computer simulations suggest that I(Ca,T) and cytosolic calcium diffusion significantly modulate AP profile recorded in PCs, as compared to VMs.

CONCLUSIONS

Our study provides the first comprehensive ionic profile of murine PCs. The data show unique features of PC ionic mechanisms that govern its excitation process. Experimental data and numerical modeling results suggest that a smaller voltage-gated potassium current and the presence of I(Ca,T) are important determinants of the longer and relatively negative plateau phase of the APs.

Remodeling of mechanical junctions and of microtubule-associated proteins accompany cardiac connexin43 lateralization

BACKGROUND

Desmosomes and adherens junctions provide mechanical continuity between cardiac cells, whereas gap junctions allow for cell-cell electrical/metabolic coupling. These structures reside at the cardiac intercalated disc (ID). Also at the ID is the voltage-gated sodium channel (VGSC) complex. Functional interactions between desmosomes, gap junctions, and VGSC have been demonstrated. Separate studies show, under various conditions, reduced presence of gap junctions at the ID and redistribution of connexin43 (Cx43) to plaques oriented parallel to fiber direction (gap junction "lateralization").

OBJECTIVE

To determine the mechanisms of Cx43 lateralization, and the fate of desmosomal and sodium channel molecules in the setting of Cx43 remodeling.

METHODS

Adult sheep were subjected to right ventricular pressure overload (pulmonary hypertension). Tissue was analyzed by quantitative confocal microscopy and by transmission electron microscopy. Ionic currents were measured using conventional patch clamp.

RESULT

Quantitative confocal microscopy demonstrated lateralization of immunoreactive junctional molecules. Desmosomes and gap junctions in lateral membranes were demonstrable by electron microscopy. Cx43/desmosomal remodeling was accompanied by lateralization of 2 microtubule-associated proteins relevant for Cx43 trafficking: EB1 and kinesin protein Kif5b. In contrast, molecules of the VGSC failed to reorganize in plaques discernable by confocal microscopy. Patch-clamp studies demonstrated change in amplitude and kinetics of sodium current and a small reduction in electrical coupling between cells.

CONCLUSIONS

Cx43 lateralization is part of a complex remodeling that includes mechanical and gap junctions but may exclude components of the VGSC. We speculate that lateralization results from redirectionality of microtubule-mediated forward trafficking. Remodeling of junctional complexes may preserve electrical synchrony under conditions that disrupt ID integrity.

Loss of plakophilin-2 expression leads to decreased sodium current and slower conduction velocity in cultured cardiac myocytes

RATIONALE

Plakophilin-2 (PKP2) is an essential component of the cardiac desmosome. Recent data show that it interacts with other molecules of the intercalated disc. Separate studies show preferential localization of the voltage-gated sodium channel (Na(V)1.5) to this region.

OBJECTIVE

To establish the association of PKP2 with sodium channels and its role on action potential propagation.

METHODS AND RESULTS

Biochemical, patch clamp, and optical mapping experiments demonstrate that PKP2 associates with Na(V)1.5, and that knockdown of PKP2 expression alters the properties of the sodium current, and the velocity of action potential propagation in cultured cardiomyocytes.

CONCLUSIONS

These results emphasize the importance of intermolecular interactions between proteins relevant to mechanical junctions, and those involved in electric synchrony. Possible relevance to the pathogenesis of arrhythmogenic right ventricular cardiomyopathy is discussed.

Characterization of the molecular phenotype of two arrhythmogenic right ventricular cardiomyopathy (ARVC)-related plakophilin-2 (PKP2) mutations

BACKGROUND

Arrhythmogenic right ventricular cardiomyopathy (ARVC) has been linked to mutations in desmosomal proteins, including plakophilin-2 (PKP2). Little is known about the changes in cellular function and structure that follow expression of ARVC-relevant PKP2 mutations.

OBJECTIVE

The purpose of this study was to investigate the function and distribution of an ARVC-relevant PKP2 mutant where arginine at position 79 was replaced by a stop codon (R79x).

METHODS

Results were compared with those obtained with mutation 179fs (frameshift at position 179). Mutant constructs were introduced by adenoviral infection into neonatal rat ventricular myocytes in culture.

RESULTS

Both mutant proteins failed to preferentially localize to sites of cell-cell apposition. Their expression did not disrupt localization of endogenous PKP2, connexin-43 (Cx43), or desmoplakin (DP). However, we observed reduced abundance of Cx43 after R79x expression. Early truncation of PKP2 at position 79 also prevented its physical interaction with both DP and Cx43. Finally, R79x expression correlated with loss of expression of HSP90, a protein relevant to cardiomyocyte apoptosis.

CONCLUSION

These results provide the first observations of the cellular/molecular phenotype consequent to these PKP2 mutations and give insight into the possible cellular substrates that lead to ARVC.

Efficacy and safety of misoprostol in induction of labour in a Nigerian tertiary hospital

BACKGROUND

Misoprostol - a stable prostaglandin E1 analogue- is effective and safe in the induction of labour. There is paucity of information about the use of misoprostol for labour induction in Nigeria.

OBJECTIVE

To evaluate the efficacy of misoprostol in the induction of labour in the third trimester. METHODS. Consecutive patients for induction of labour were randomized into misoprostol or oxytocin study groups. The misoprostol group received intravaginal 50 microg 6- hourly to a maximum of four doses. Those in the oxytocin group received a maximum of 48 iu/min. Outcome measures included induction-delivery interval, mode of delivery, Apgar score, perinatal death and maternal complications.

RESULTS

Sixty-two patients were recruited into the study-34 received misoprostol while 28 received oxytocin. The modal gestational age and Bishop score prior at induction were >36 weeks and 5-7 respectively. Hypertension in pregnancy was the commonest indication for induction of labour followed by prolonged pregnancy. The overall induction-delivery interval was 12.2 +/- 5.2 hours; Misoprostol v oxytocin, mean(range): 12.1(7-27) vs 12.3(4-27) hours, p = 0.88). There were no significant differences in the mean Apgar score and perinatal mortality rate in the two study groups. There were two cases of primary postpartum haemorrhage in the oxytocin group but none in the misoprostol group. One case of ruptured uterus was encountered in the misoprostol group. No case of maternal mortality was recorded. Four patients in the misoprostol group had minor side effects mainly nausea and vomiting.

CONCLUSION

The efficacy of misoprostol in the induction of third trimester labour is comparable to oxytocin. The risk of ruptured uterus associated with misoprostol appears higher than that of oxytocin in the induction of labour. Further studies are needed to verify this observation in our setting.

Connexin43 remodeling caused by inhibition of plakophilin-2 expression in cardiac cells

Desmosomes and gap junctions are distinct structural components of the cardiac intercalated disc. Here, we asked whether the presence of plakophilin (PKP)2, a component of the desmosome, is essential for the proper function and distribution of the gap junction protein connexin (Cx)43. We used RNA silencing technology to decrease the expression of PKP2 in cardiac cells (ventricular myocytes, as well as epicardium-derived cells) obtained from neonatal rat hearts. We evaluated the content, distribution, and function of Cx43 gap junctions. Our results show that loss of PKP2 expression led to a decrease in total Cx43 content, a significant redistribution of Cx43 to the intracellular space, and a decrease in dye coupling between cells. Separate experiments showed that Cx43 and PKP2 can coexist in the same macromolecular complex. Our results support the notion of a molecular crosstalk between desmosomal and gap junction proteins. The results are discussed in the context of arrhythmogenic right ventricular cardiomyopathy, an inherited disease involving mutations in desmosomal proteins, including PKP2.

Functional characterization of connexin43 mutations found in patients with oculodentodigital dysplasia

Specific mutations in GJA1, the gene encoding the gap junction protein connexin43 (Cx43), cause an autosomal dominant disorder called oculodentodigital dysplasia (ODDD). Here, we characterize the effects of 8 of these mutations on Cx43 function. Immunochemical studies have shown that most of the mutant proteins formed gap junction plaques at the sites of cell-cell apposition. However, 2 of the mutations (a codon duplication in the first extracellular loop, F52dup, and a missense mutation in the second extracellular loop, R202H, produced full-length connexins that failed to properly form gap junction plaques. Cx43 proteins containing ODDD mutations found in the N-terminus (Y17S), first transmembrane domain (G21R, A40V), second transmembrane domain (L90V), and cytoplasmic loop (I130T, K134E) do form gap junction plaques but show compromised channel function. L90V, I130T, and K134E demonstrated a significant decrease in junctional conductance relative to Cx43WT. Mutations Y17S, G21R, and A40V demonstrated a complete lack of functional electrical coupling even in the presence of significant plaque formation between paired cells. Heterologous channels formed by coexpression of Cx43WT and mutation R202H resulted in electrically functional gap junctions that were not permeable to Lucifer yellow. Therefore, the mutations found in ODDD not only cause phenotypic variability, but also result in various functional consequences. Overall, our data show an extensive range of molecular phenotypes, consistent with the pleiotropic nature of the clinical syndrome as a whole.

Amino terminal glutamate residues confer spermine sensitivity and affect voltage gating and channel conductance of rat connexin40 gap junctions

Connexin40 (Cx40) contains a specific binding site for spermine (affinity approximately 100 microm) whereas connexin43 (Cx43) is unaffected by identical concentrations of intracellular spermine. Replacement of two unique glutamate residues, E9 and E13, from the cytoplasmic amino terminal domain of Cx40 with the corresponding lysine residues from Cx43 eliminated the block by 2 mm spermine, reduced the transjunctional voltage (V(j)) gating sensitivity, and reduced the unitary conductance of this Cx40E9,13K gap junction channel protein. The single point mutations, Cx40E9K and Cx40E13K, predominantly affected the residual conductance state (G(min)) and V(j) gating properties, respectively. Heterotypic pairing of Cx40E9,13K with wild-type Cx40 in murine neuro2A (N2A) cells produced a strongly rectifying gap junction reminiscent of the inward rectification properties of the Kir (e.g. Kir2.x) family of potassium channels. The reciprocal Cx43K9,13E mutant protein exhibited reduced V(j) sensitivity, but displayed much less rectification in heterotypic pairings with wtCx43, negligible changes in the unitary channel conductance, and remained insensitive to spermine block. These data indicate that the connexin40 amino terminus may form a critical cytoplasmic pore-forming domain that serves as the receptor for V(j)-dependent closure and block by intracellular polyamines. Functional reciprocity between Cx40 and Cx43 gap junctions involves other amino acid residues in addition to the E or K 9 and 13 loci located on the amino terminal domain of these two connexins.

Microtubule assembly in cultured myoblasts and myotubes following nocodazole induced microtubule depolymerisation

When myoblasts fuse into myotubes, the organisation of the cytoskeleton changes dramatically. For example, microtubules emanate in a radial array form the centrosome in myoblasts, but form linear arrays not linked to a centrosome in myotubes. It is not clear how these linear arrays are formed and nucleated. They could arise in a number of ways: by nucleation and release from centrosomal like structures, cytoplasmic assembly, breakage/severing or nucleation from non-centrosomal sites. To test which of the above mechanisms or combination of mechanisms are responsible we investigated the re-formation of microtubules after depolymerisation by nocodazole, using antibodies against pericentrin, gamma-tubulin, EB1, and tyrosinated alpha-tubulin. In myoblasts, we found that when microtubules were allowed to recover after complete depolymerisation with nocodazole, microtubule recovery began within 1 min and was complete after 5 min. Microtubules grew out from the centrosome, which was positively stained for gamma-tubulin or pericentrin. In untreated myotubes, microtubules were arranged in linear arrays, with EB1 at their ends. The pericentriolar protein, pericentrin was arranged in a band around the nucleus as well as discrete spots in the cytoplasm. In contrast, the microtubule nucleating protein gamma-tubulin was not found in a band around the nucleus, but was found in several punctuate spots throughout the cytoplasm. Further, when microtubules were allowed to recover, after complete depolymerisation with nocodazole, recovery was not as rapid as that seen in myoblasts, and we found that regrowth began with the formation of short microtubule fragments throughout the cytoplasm. Gamma-tubulin was associated with these fragments. These results suggest that in myotubes, nucleation of microtubules can be non-centrosomal.