AimsAnecdotal observations suggest that sub-clinical electrophysiological manifestations of arrhythmogenic right ventricular cardiomyopathy (ARVC) develop before detectable structural changes ensue on cardiac imaging. To test this hypothesis, we investigated a murine model with conditional cardiac genetic deletion of one desmoplakin allele (DSP ±) and compared the findings to patients with non-diagnostic features of ARVC who carried mutations in desmoplakin.Methods and resultsMurine: the DSP (±) mice underwent electrophysiological, echocardiographic, and immunohistochemical studies. They had normal echocardiograms but delayed conduction and inducible ventricular tachycardia associated with mislocalization and reduced intercalated disc expression of Cx43. Sodium current density and myocardial histology were normal at 2 months of age. Human: ten patients with heterozygous mutations in DSP without overt structural heart disease (DSP+) and 12 controls with supraventricular tachycardia were studied by high-density electrophysiological mapping of the right ventricle. Using a standard S1–S2 protocol, restitution curves of local conduction and repolarization parameters were constructed. Significantly greater mean increases in delay were identified particularly in the outflow tract vs. controls (P< 0.01) coupled with more uniform wavefront progression. The odds of a segment with a maximal activation–repolarization interval restitution slope >1 was 99% higher (95% CI: 13%; 351%, P= 0.017) in DSP+ vs. controls. Immunostaining revealed Cx43 mislocalization and variable Na channel distribution.ConclusionDesmoplakin disease causes connexin mislocalization in the mouse and man preceding any overt histological abnormalities resulting in significant alterations in conduction–repolarization kinetics prior to morphological changes detectable on conventional cardiac imaging. Haploinsufficiency of desmoplakin is sufficient to cause significant Cx43 mislocalization. Changes in sodium current density and histological abnormalities may contribute to a worsening phenotype or disease but are not necessary to generate an arrhythmogenic substrate. This has important implications for the earlier diagnosis of ARVC and risk stratification.

A TP-sensitive potassium channels (K ATP ) are widely expressed in a range of tissues, including brain, heart, pancreas, and smooth muscle (SM), where they are involved in the regulation of biological processes such as insulin release, vascular tone, and adaptation to stresses such as ischemia and hypoxia. They are activated by either declining ATP or increasing ADP concentrations or both, thus coupling intracellular metabolism to membrane excitability. 1 K ATP channels are composed of 4 pore-forming Kir6.x subunits (Kir6.1 or Kir6.2) and 4 large regulatory sulphonylurea receptor subunits (SUR1, SUR2A or SUR2B) to form a functional hetero-octomeric complex.1 The vascular SM K ATP channel is thought to be composed of the Kir6.1 and SUR2B subunits.2,3 These SM K ATP channels have been implicated in the regulation of vascular tone through their proposed involvement in the actions of vasoconstrictors and vasodilators. [4][5][6][7] The integrative physiological role of these channels has been investigated in mice with global genetic deletion of either Kir6.1 or SUR2. 8,9 The mice were hypertensive and prone to sudden death, which was attributed to coronary artery vasospasm because of the absence of K ATP currents in the SM of the coronary arteries. However, when SUR2B was specifically expressed in SM in SUR2 global knockout mice resulting in reconstitution of the K ATP current, the lethal phenotype persisted.10 Furthermore, transgenic expression of SUR2A in cardiomyocytes in SUR2 null mice led to a dramatic reduction in the degree and frequency of episodes of ST elevation on the ECG measured using telemetry.11 The implication was that reconstitution of K ATP in cardiac myocytes led to a reduction of coronary artery SM spasm and it was proposed that K ATP channels outside the SM cell (SMC) are critical in driving the vascular phenotype in the global knockout mice and that the vascular SM K ATP channel contributes modestly to vascular control. 10 A global genetic deletion of Kir6.1 or SUR2 is not selective for the SM channel and potentially channels in the endothelium, nervous system, and heart might all be affected. Here, using a new mouse model, we show that Kir6.1 is indeed the pore-forming subunit of the K ATP channel in vascular SM and that it has a central role in the regulation of blood pressure (BP).Abstract-ATP-sensitive potassium channels (K ATP ) regulate a range of biological activities by coupling membrane excitability to the cellular metabolic state. In particular, it has been proposed that K ATP channels and specifically, the channel subunits Kir6.1 and SUR2B, play an important role in the regulation of vascular tone. However, recent experiments have suggested that K ATP channels outside the vascular smooth muscle compartment are the key determinant of the observed behavior. Thus, we address the importance of the vascular smooth muscle K ATP channel, using a novel murine model in which it is possible to conditionally delete the Kir6.1 subunit.

The sympathetic nervous system (SNS) arises from neural crest (NC) cells during embryonic development and innervates the internal organs of vertebrates to modulate their stress response. NRP1 and NRP2 are receptors for guidance cues of the class 3 semaphorin (SEMA) family and are expressed in partially overlapping patterns in sympathetic NC cells and their progeny. By comparing the phenotypes of mice lacking NRP1 or its ligand SEMA3A with mice lacking NRP1 in the sympathetic versus vascular endothelial cell lineages, we demonstrate that SEMA3A signalling through NRP1 has multiple cell-autonomous roles in SNS development. These roles include neuronal cell body positioning, neuronal aggregation and axon guidance, first during sympathetic chain assembly and then to regulate the innervation of the heart and aorta. Loss of NRP2 or its ligand SEMA3F impaired sympathetic gangliogenesis more mildly than loss of SEMA3A/NRP1 signalling, but caused ectopic neurite extension along the embryonic aorta. The analysis of compound mutants lacking SEMA3A and SEMA3F or NRP1 and NRP2 in the SNS demonstrated that both signalling pathways cooperate to organise the SNS. We further show that abnormal sympathetic development in mice lacking NRP1 in the sympathetic lineage has functional consequences, as it causes sinus bradycardia, similar to mice lacking SEMA3A.

AMPK is a conserved serine/threonine kinase whose activity maintains cellular energy homeostasis. Eukaryotic AMPK exists as αβγ complexes, whose regulatory γ subunit confers energy sensor function by binding adenine nucleotides. Humans bearing activating mutations in the γ2 subunit exhibit a phenotype including unexplained slowing of heart rate (bradycardia). Here, we show that γ2 AMPK activation downregulates fundamental sinoatrial cell pacemaker mechanisms to lower heart rate, including sarcolemmal hyperpolarization-activated current (I f) and ryanodine receptor-derived diastolic local subsarcolemmal Ca2+ release. In contrast, loss of γ2 AMPK induces a reciprocal phenotype of increased heart rate, and prevents the adaptive intrinsic bradycardia of endurance training. Our results reveal that in mammals, for which heart rate is a key determinant of cardiac energy demand, AMPK functions in an organ-specific manner to maintain cardiac energy homeostasis and determines cardiac physiological adaptation to exercise by modulating intrinsic sinoatrial cell behavior.