The class II histone deacetylases, HDAC4 and HDAC5, directly bind to and repress myogenic transcription factors of the myocyte enhancer factor-2 (MEF-2) family thereby inhibiting skeletal myogenesis. During muscle differentiation, repression of gene transcription by MEF-2/HDAC complexes is relieved due to calcium/calmodulin-dependent (CaM) kinase-induced translocation of HDAC4 and HDAC5 to the cytoplasm. MEF-2 proteins and HDACs are also highly expressed in the nervous system and have been implicated in neuronal survival and differentiation. Here we investigated the possibility that the subcellular localization of HDACs, and thus their ability to repress target genes, is controlled by synaptic activity in neurones. We found that, in cultured hippocampal neurones, the localization of HDAC4 and HDAC5 is dynamic and signal-regulated. Spontaneous electrical activity was sufficient for nuclear export of HDAC4 but not of HDAC5. HDAC5 translocation to the cytoplasm was induced following stimulation of calcium flux through synaptic NMDA receptors or L-type calcium channels; glutamate bath application (stimulating synaptic and extrasynaptic NMDA receptors) antagonized nuclear export. Activity-induced nucleocytoplasmic shuttling of both HDACs was partially blocked by the CaM kinase inhibitor KN-62 with HDAC5 nuclear export being more sensitive to CaM kinase inhibition than that of HDAC4. Thus, the subcellular localization of HDACs in neurones is specified by neuronal activity; differences in the activation thresholds for HDAC4 and HDAC5 nuclear export provides a mechanism for input-specific gene expression.
Cardiac arrhythmias can follow disruption of the normal cellular electrophysiological processes underlying excitable activity and their tissue propagation as coherent wavefronts from the primary sinoatrial node pacemaker, through the atria, conducting structures and ventricular myocardium. These physiological events are driven by interacting, voltage-dependent, processes of activation, inactivation, and recovery in the ion channels present in cardiomyocyte membranes. Generation and conduction of these events are further modulated by intracellular Ca homeostasis, and metabolic and structural change. This review describes experimental studies on murine models for known clinical arrhythmic conditions in which these mechanisms were modified by genetic, physiological, or pharmacological manipulation. These exemplars yielded molecular, physiological, and structural phenotypes often directly translatable to their corresponding clinical conditions, which could be investigated at the molecular, cellular, tissue, organ, and whole animal levels. Arrhythmogenesis could be explored during normal pacing activity, regular stimulation, following imposed extra-stimuli, or during progressively incremented steady pacing frequencies. Arrhythmic substrate was identified with temporal and spatial functional heterogeneities predisposing to reentrant excitation phenomena. These could arise from abnormalities in cardiac pacing function, tissue electrical connectivity, and cellular excitation and recovery. Triggering events during or following recovery from action potential excitation could thereby lead to sustained arrhythmia. These surface membrane processes were modified by alterations in cellular Ca homeostasis and energetics, as well as cellular and tissue structural change. Study of murine systems thus offers major insights into both our understanding of normal cardiac activity and its propagation, and their relationship to mechanisms generating clinical arrhythmias.
] o respectively. Endocardial APD 90 s correspondingly increased from 51.6 AE 1.9 ms (n ¼ 7) to 62.8 AE 2.8 ms (n ¼ 7) and 62.9 AE 5.9 ms (n ¼ 11) giving reductions in endocardial-epicardial differences, DAPD 90 , from 14.4 AE 2.6 to 4.4 AE 5.0 and )3.4 AE 6.0 ms respectively. Early afterdepolarizations (EADs) occurred in epicardia in three of seven spontaneously beating hearts at 4 mm [K + ] o with triggered beats followed by episodes of non-sustained VT in nine of 11 preparations at 3 mm. Programmed electrical stimulation never induced arrhythmic events in preparations perfused with normokalemic solutions yet induced VT in two of seven and nine of 11 preparations at 4 and 3 mm
Rationale: Mutations in glycerol-3-phosphate dehydrogenase 1-like (GPD1-L) protein reduce cardiac Na ؉ current (I Na ) and cause Brugada Syndrome (BrS). GPD1-L has >80% amino acid homology with glycerol-3-phosphate dehydrogenase, which is involved in NAD-dependent energy metabolism.Objective: Therefore, we tested whether NAD(H) could regulate human cardiac sodium channels (Na v 1.5).
Over the past decade, advances in molecular tools, stem cell differentiation, osteoclast and osteoblast signaling mechanisms, and genetically manipulated mice models have resulted in major breakthroughs in understanding osteoclast biology. This review focuses on key advances in our understanding of molecular mechanisms underlying the formation, function, and survival of osteoclasts. These include key signals mediating osteoclast differentiation, including PU.1, RANK, CSF-1/c-fms, and src, and key specializations of the osteoclast including HCl secretion driven by H ؉ -ATPase and the secretion of collagenolytic enzymes including cathepsin K and matrix metalloproteinases (MMPs). These pathways and highly expressed proteins provide targets for specific therapies to modify bone degradation. The main outstanding issues, basic and translational, will be considered in relation to the osteoclast as a target for antiresorptive therapies.
We have examined sino-atrial node (SAN) function in hearts from adult mice with heterozygous targeted disruption of the Scn5a gene to clarify the role of Scn5a-encoded cardiac Na + channels in normal SAN function and the mechanism(s) by which reduced Na + channel function might cause sinus node dysfunction. Scn5a +/− mice showed depressed heart rates and occasional sino-atrial (SA) block. Their isolated peripheral SAN pacemaker cells showed a reduced Na + channel expression and slowed intrinsic pacemaker rates. Wild-type (WT) and Scn5a +/− SAN preparations exhibited similar activation patterns but with significantly slower SA conduction and frequent sino-atrial conduction block in Scn5a +/− SAN preparations. Furthermore, isolated WT and Scn5a +/− SAN cells demonstrated differing correlations between cycle length, maximum upstroke velocity and action potential amplitude, and cell size. Small myocytes showed similar, but large myocytes reduced pacemaker rates, implicating the larger peripheral SAN cells in the reduced pacemaker rate that was observed in Scn5a +/− myocytes. These findings were successfully reproduced in a model that implicated i Na directly in action potential propagation through the SAN and from SAN to atria, and in modifying heart rate through a coupling of SAN and atrial cells. Functional alterations in the SAN following heterozygous-targeted disruption of Scn5a thus closely resemble those observed in clinical sinus node dysfunction. The findings accordingly provide a basis for understanding of the role of cardiac-type Na + channels in normal SAN function and the pathophysiology of sinus node dysfunction and suggest new potential targets for its clinical management.
Mutations in KCNE1, the gene encoding the beta subunit of the slowly activating delayed rectifier potassium current (IKs) channel protein, may lead to the long QT syndrome (LQTS), a condition associated with enhanced arrhythmogenesis. Mice with homozygous deletion of the coding sequence of KCNE1 have inner ear defects strikingly similar to those seen in the corresponding human condition. The present study demonstrated and assessed the mechanism of ventricular arrhythmias in Langendorff-perfused whole heart preparations from homozygous KCNE1-/- mice compared to wild-type mice of the same age. The effects of programmed electrical stimulation with decremental pacing from the basal right ventricular epicardial surface upon electrogram waveforms recorded from the basal left ventricle were assessed and quantified using techniques of paced electrogram fractionation analysis for the first time in an experimental system. All KCNE1-/-(n = 10) but not wild-type (n = 14) mouse hearts empirically demonstrated marked pacing-induced ventricular arrhythmogenicity. This correlated with significant increases in electrogram dispersion, consistent with a wider spread in conduction velocities, in parallel with clinical findings from LQTS patients with potassium channel mutations. In contrast, introduction of 100 nM isoprenaline induced arrhythmogenicity in both KCNE1-/- (n = 7) and wild-type (n = 6) hearts during pacing. Furthermore, pretreatment with 1 muM nifedipine exerted a strong anti-arrhythmic effect in the KCNE1-/- hearts (n = 12) that persisted even in the presence of 100 nM isoprenaline (n = 6). Our findings associate KCNE1-/- with an arrhythmogenic phenotype that shows an increased dispersion of conduction velocities, and whose initiation is prevented by nifedipine, a finding that in turn may have therapeutic applications in conditions such as LQTS.
The effects of the ryanodine receptor (RyR) antagonists ryanodine and daunorubicin on the kinetic and steady-state properties of intramembrane charge were investigated in intact voltage-clamped frog skeletal muscle fibers under conditions that minimized timedependent ionic currents. A hypothesis that RyR gating is allosterically coupled to configurational changes in dihydropyridine receptors (DHPRs) would predict that such interactions are reciprocal and that RyR modification should influence intramembrane charge. Both agents indeed modified the time course of charging transients at 100-200-p.M concentrations. They independently abolished the delayed charging phases shown by q~ currents, even in fibers held at fully polarized, -90-mV holding potentials; such waveforms are especially prominent in extracellular solutions containing gluconate. Charge movements consistently became exponential decays to stable baselines in the absence of intervening inward or other time-dependent currents. The steady-state charge transfers nevertheless remained equal through the ON and the OFF parts of test voltage steps. The charge-voltage function, Q(VT), shifted by ~+ 10 mV, particularly through those test potentials at which delayed q~ currents normally took place but retained steepness factors (k ~ 8.0 to 10.6 mV) that indicated persistent, steeply voltage-dependent q~ contributions. Furthermore, both RyR antagonists preserved the total charge, and its variation with holding potential, Qma~(VH), which also retained similarly high voltage sensitivities (k ~ 7.0 to 9.0 mV). RyR antagonists also preserved the separate identities of qv and qa species, whether defined by their steady-state voltage dependence or inactivation or pharmacological properties. Thus, tetracaine (2 mM) reduced the available steady-state charge movement and gave shallow Q(VT) (k --~ 14 to 16 mV) and Qma~(VH) (k 14 to 17 mV) curves characteristic of q~ charge. These features persisted with exposure to test agent. Finally, q~ charge movements showed steep voltage dependences with both activation (k ~ 4.0 to 6.5 mV) and inactivation characteristics (k ~ 4.3 to 6.6 mV) distinct from those shown by the remaining q~ charge, whether isolated through differential tetracaine sensitivities, or the full approximation of charge-voltage data to the sum of two Boltzmann distributions. RyR modification thus specifically alters qx kinetics while preserving the separate identities of steady-state qf~ and q~ charge. These findings permit a mechanism by which transverse tubular voltage provides the primary driving force for configurafional changes in DHPRs, which might produce q~ charge movement. However, they attribute its kinetic complexities to the reciprocal allosteric coupling by which DHPR voltage sensors and RyR-Ca 2 § release channels might interact even though these receptors reside in electrically distinct membranes. RyR modification then would still permit tubular voltage change to drive net q~ charge transfer but would transform its complex waveforms into s...
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