Atrial fibrillation and electrophysiology in transgenic mice with cardiac-restricted overexpression of FKBP12.

Cardiomyocyte-restricted overexpression of FK506-binding protein 12 transgenic (αMyHC-FKBP12) mice develop spontaneous atrial fibrillation (AF). The aim of the present study is to explore the mechanisms underlying the occurrence of AF in αMyHC-FKBP12 mice. Spontaneous AF was documented by telemetry in vivo and Langendorff-perfused hearts of αMyHC-FKBP12 and littermate control mice in vitro. Atrial conduction velocity was evaluated by optical mapping. The patch-clamp technique was applied to determine the potentially altered electrophysiology in atrial myocytes. Channel protein expression levels were evaluated by Western blot analyses. Spontaneous AF was recorded in four of seven αMyHC-FKBP12 mice but in none of eight nontransgenic (NTG) controls. Atrial conduction velocity was significantly reduced in αMyHC-FKBP12 hearts compared with NTG hearts. Interestingly, the mean action potential duration at 50% but not 90% was significantly prolonged in αMyHC-FKBP12 atrial myocytes compared with their NTG counterparts. Consistent with decreased conduction velocity, average peak Na+ current ( INa) density was dramatically reduced and the INa inactivation curve was shifted by approximately +7 mV in αMyHC-FKBP12 atrial myocytes, whereas the activation and recovery curves were unaltered. The Nav1.5 expression level was significantly reduced in αMyHC-FKBP12 atria. Furthermore, we found increases in atrial Cav1.2 protein levels and peak L-type Ca2+ current density and increased levels of fibrosis in αMyHC-FKBP12 atria. In summary, cardiomyocyte-restricted overexpression of FKBP12 reduces the atrial Nav1.5 expression level and mean peak INa, which is associated with increased peak L-type Ca2+ current and interstitial fibrosis in atria. The combined electrophysiological and structural changes facilitated the development of local conduction block and altered action potential duration and spontaneous AF. NEW & NOTEWORTHY This study addresses a long-standing riddle regarding the role of FK506-binding protein 12 in cardiac physiology. The work provides further evidence that FK506-binding protein 12 is a critical component for regulating voltage-gated sodium current and in so doing has an important role in arrhythmogenic physiology, such as atrial fibrillation.

An RNAi-mediated screen identifies novel targets for next-generation antiepileptic drugs based on increased expression of the homeostatic regulator pumilio.

Despite availability of a diverse range of anti-epileptic drugs (AEDs), only about two-thirds of epilepsy patients respond well to drug treatment. Thus, novel targets are required to catalyse the design of next-generation AEDs. Manipulation of neuron firing-rate homoeostasis, through enhancing Pumilio (Pum) activity, has been shown to be potently anticonvulsant in Drosophila. In this study, we performed a genome-wide RNAi screen in S2R + cells, using a luciferase-based dPum activity reporter and identified 1166 genes involved in dPum regulation. Of these genes, we focused on 699 genes that, on knock-down, potentiate dPum activity/expression. Of this subgroup, 101 genes are activity-dependent based on comparison with genes previously identified as activity-dependent by RNA-sequencing. Functional cluster analysis shows these genes are enriched in pathways involved in DNA damage, regulation of cell cycle and proteasomal protein catabolism. To test for anticonvulsant activity, we utilised an RNA-interference approach in vivo. RNAi-mediated knockdown showed that 57/101 genes (61%) are sufficient to significantly reduce seizure duration in the characterized seizure mutant, parabss. We further show that chemical inhibitors of protein products of some of the genes targeted are similarly anticonvulsant. Finally, to establish whether the anticonvulsant activity of identified compounds results from increased dpum transcription, we performed a luciferase-based assay to monitor dpum promoter activity. Third instar larvae exposed to sodium fluoride, gemcitabine, metformin, bestatin, WP1066 or valproic acid all showed increased dpum promoter activity. Thus, this study validates Pum as a favourable target for AED design and, moreover, identifies a number of lead compounds capable of increasing the expression of this homeostatic regulator.

Effective suppression of Ih and INa caused by capsazepine, known to be a blocker of TRPV1 receptor.

A synthetic inhibitor of capsaicin-induced TRPV1 channel activation is called capsazepine (CPZ). In this study, we aimed to explore the effects of CPZ on hyperpolarization-activated cationic current (Ih) and voltage-gated Na + current (INa) in pituitary tumor (GH3) cells. Through patch-clamp recordings, we found that CPZ concentration-dependently inhibited Ih amplitude and slowed its activation time course. The IC50 and KD values were 3.1 and 3.16 µM, respectively. CPZ also shifted the steady-state activation curve of Ih towards a more hyperpolarized potential. However, there was no change in the gating charge of the curve. A modified Markovian model predicted the CPZ-induced decrease in the voltage-dependent hysteresis of Ih. CPZ suppressed INa in GH3 cells, without altering its activation or inactivation time course. Additionally, exposure to CPZ reduced spontaneous firing. These findings suggest that CPZ's inhibitory effects on Ih and INa are direct and not dependent on vanilloid receptor binding. This could provide light on an unidentified ionic mechanism influencing the membrane excitability of neurons and endocrine or neuroendocrine cells in vivo.

Safinamide, an inhibitor of monoamine oxidase, modulates the magnitude, gating, and hysteresis of sodium ion current.

BACKGROUND: Safinamide (SAF), an α-aminoamide derivative and a selective, reversible monoamine oxidase (MAO)-B inhibitor, has both dopaminergic and nondopaminergic (glutamatergic) properties. Several studies have explored the potential of SAF against various neurological disorders; however, to what extent SAF modulates the magnitude, gating, and voltage-dependent hysteresis [Hys(V)] of ionic currents remains unknown. METHODS: With the aid of patch-clamp technology, we investigated the effects of SAF on voltage-gated sodium ion (NaV) channels in pituitary GH3 cells. RESULTS: SAF concentration-dependently stimulated the transient (peak) and late (sustained) components of voltage-gated sodium ion current (INa) in pituitary GH3 cells. The conductance-voltage relationship of transient INa [INa(T)] was shifted to more negative potentials with the SAF presence; however, the steady-state inactivation curve of INa(T) was shifted in a rightward direction in its existence. SAF increased the decaying time constant of INa(T) induced by a train of depolarizing stimuli. Notably, subsequent addition of ranolazine or mirogabalin reversed the SAF-induced increase in the decaying time constant. SAF also increased the magnitude of window INa induced by an ascending ramp voltage Vramp. Furthermore, SAF enhanced the Hys(V) behavior of persistent INa induced by an upright isosceles-triangular Vramp. Single-channel cell-attached recordings indicated SAF effectively increased the open-state probability of NaV channels. Molecular docking revealed SAF interacts with both MAO and NaV channels. CONCLUSION: SAF may interact directly with NaV channels in pituitary neuroendocrine cells, modulating membrane excitability.

Highly Efficient Conversion of Motor Neuron-Like NSC-34 Cells into Functional Motor Neurons by Prostaglandin E2.

Motor neuron diseases are a group of progressive neurological disorders that degenerate motor neurons. The neuroblastoma × spinal cord hybrid cell line NSC-34 is widely used as an experimental model in studies of motor neuron diseases. However, the differentiation efficiency of NSC-34 cells to neurons is not always sufficient. We have found that prostaglandin E2 (PGE2) induces morphological differentiation in NSC-34 cells. The present study investigated the functional properties of PGE2-differentiated NSC-34 cells. Retinoic acid (RA), a widely-used agent inducing cell differentiation, facilitated neuritogenesis, which peaked on day 7, whereas PGE2-induced neuritogenesis took only 2 days to reach the same level. Whole-cell patch-clamp recordings showed that the current threshold of PGE2-treated cell action potentials was lower than that of RA-treated cells. PGE2 and RA increased the protein expression levels of neuronal differentiation markers, microtubule-associated protein 2c and synaptophysin, and to the same extent, motor neuron-specific markers HB9 and Islet-1. On the other hand, protein levels of choline acetyltransferase and basal release of acetylcholine in PGE2-treated cells were higher than in RA-treated cells. These results suggest that PGE2 is a rapid and efficient differentiation-inducing factor for the preparation of functionally mature motor neurons from NSC-34 cells.

Distinctive property and pharmacology of voltage-gated sodium current in rat atrial vs ventricular myocytes.

BACKGROUND: Several mammalian species display distinct biophysical properties between atrial and ventricular voltage-gated sodium current (INa); however, the potential mechanism behind this phenomenon is unknown. OBJECTIVE: The purpose of this study was to investigate the potential molecular identities of the different INa in atrial and ventricular myocytes of rat hearts. METHODS: Whole-cell patch voltage-clamp and molecular biology techniques were used in the study. RESULTS: Ventricular INa exhibited a slower inactivation, more positive potential of inactivation, and quicker recovery from inactivation compared to atrial INa. Real-time polymerase chain reaction and western blot analysis revealed that mRNA and protein levels of NaVß2 and NaVß4 subunits, but not NaV1.5, were greater in ventricular myocytes than in atrial myocytes. INa in heterologous HEK 293 cell expression system with coexpressing hNaV1.5 and hNaVß2/hNaVß4 showed similar biophysical properties to ventricular INa. Greater protein expression of NaVß2 and NaVß4 subunits was also observed in human ventricles. Interestingly, pharmacologic study revealed that the antiarrhythmic drug dronedarone (10 µM) inhibited atrial INa more (by 73%) than ventricular INa (by 42%), and shifted its inactivation to more negative voltages (-4.6 mV) compared to ventricular INa. CONCLUSION: The results of this study demonstrate the novel information that the distinctive biophysical properties of INa in atrial and ventricular myocytes can be attributed to inhomogeneous expression of NaVß2 and NaVß4 subunits, and that atrial INa is more sensitive to inhibition by dronedarone.