An epigenetic barrier sets the timing of human neuronal maturation.

The pace of human brain development is highly protracted compared with most other species1-7. The maturation of cortical neurons is particularly slow, taking months to years to develop adult functions3-5. Remarkably, such protracted timing is retained in cortical neurons derived from human pluripotent stem cells (hPSCs) during in vitro differentiation or upon transplantation into the mouse brain4,8,9. Those findings suggest the presence of a cell-intrinsic clock setting the pace of neuronal maturation, although the molecular nature of this clock remains unknown. Here we identify an epigenetic developmental programme that sets the timing of human neuronal maturation. First, we developed a hPSC-based approach to synchronize the birth of cortical neurons in vitro which enabled us to define an atlas of morphological, functional and molecular maturation. We observed a slow unfolding of maturation programmes, limited by the retention of specific epigenetic factors. Loss of function of several of those factors in cortical neurons enables precocious maturation. Transient inhibition of EZH2, EHMT1 and EHMT2 or DOT1L, at progenitor stage primes newly born neurons to rapidly acquire mature properties upon differentiation. Thus our findings reveal that the rate at which human neurons mature is set well before neurogenesis through the establishment of an epigenetic barrier in progenitor cells. Mechanistically, this barrier holds transcriptional maturation programmes in a poised state that is gradually released to ensure the prolonged timeline of human cortical neuron maturation.

Macrophage-Dependent Interleukin-6-Production and Inhibition of IK Contributes to Acquired QT Prolongation in Lipotoxic Guinea Pig Heart.

In the heart, the delayed rectifier K current, IK, composed of the rapid (IKr) and slow (IKs) components contributes prominently to normal cardiac repolarization. In lipotoxicity, chronic elevation of pro-inflammatory cytokines may remodel IK, elevating the risk for ventricular arrythmias and sudden cardiac death. We investigated whether and how the pro-inflammatory interleukin-6 altered IK in the heart, using electrophysiology to evaluate changes in IK in adult guinea pig ventricular myocytes. We found that palmitic acid (a potent inducer of lipotoxicity), induced a rapid (~24 h) and significant increase in IL-6 in RAW264.7 cells. PA-diet fed guinea pigs displayed a severely prolonged QT interval when compared to low-fat diet fed controls. Exposure to isoproterenol induced torsade de pointes, and ventricular fibrillation in lipotoxic guinea pigs. Pre-exposure to IL-6 with the soluble IL-6 receptor produced a profound depression of IKr and IKs densities, prolonged action potential duration, and impaired mitochondrial ATP production. Only with the inhibition of IKr did a proarrhythmic phenotype of IKs depression emerge, manifested as a further prolongation of action potential duration and QT interval. Our data offer unique mechanistic insights with implications for pathological QT interval in patients and vulnerability to fatal arrhythmias.

Neurological Disorders and Risk of Arrhythmia.

Neurological disorders including depression, anxiety, post-traumatic stress disorder (PTSD), schizophrenia, autism and epilepsy are associated with an increased incidence of cardiovascular disorders and susceptibility to heart failure. The underlying molecular mechanisms that link neurological disorders and adverse cardiac function are poorly understood. Further, a lack of progress is likely due to a paucity of studies that investigate the relationship between neurological disorders and cardiac electrical activity in health and disease. Therefore, there is an important need to understand the spatiotemporal behavior of neurocardiac mechanisms. This can be advanced through the identification and validation of neurological and cardiac signaling pathways that may be adversely regulated. In this review we highlight how dysfunction of the hypothalamic-pituitary-adrenal (HPA) axis, autonomic nervous system (ANS) activity and inflammation, predispose to psychiatric disorders and cardiac dysfunction. Moreover, antipsychotic and antidepressant medications increase the risk for adverse cardiac events, mostly through the block of the human ether-a-go-go-related gene (hERG), which plays a critical role in cardiac repolarization. Therefore, understanding how neurological disorders lead to adverse cardiac ion channel remodeling is likely to have significant implications for the development of effective therapeutic interventions and helps improve the rational development of targeted therapeutics with significant clinical implications.

hERG 1a LQT2 C-terminus truncation mutants display hERG 1b-dependent dominant negative mechanisms.

BACKGROUND: The human ether-à-go-go-related gene (hERG 1a) potassium channel is critical for cardiac repolarization. hERG 1b, another variant subunit, co-assembles with hERG 1a, modulates channel biophysical properties and plays an important role in repolarization. Mutations of hERG 1a lead to type 2 long QT syndrome (LQT2), and increased risk for fatal arrhythmias. The functional consequences of these mutations in the presence of hERG 1b are not known. OBJECTIVE: To investigate whether hERG 1a mutants exert dominant negative gating and trafficking defects when co-expressed with hERG 1b. METHODS: Electrophysiology, co-immunoprecipitation, and fluorescence resonance energy transfer (FRET) experiments in HEK293 cells and guinea pig cardiomyocytes were used to assess the mutants on gating and trafficking. Mutations of 1a-G965X and 1a-R1014X, relevant to gating and trafficking were introduced in the C-terminus region. RESULTS: The hERG 1a mutants when expressed alone did not result in decreased current amplitude. Compared to wild-type hERG 1a currents, 1a-G965X currents were significantly larger, whereas those produced by the 1a-R1014X mutant were similar in magnitude. Only when co-expressed with wild-type hERG 1a and 1b did a mutant phenotype emerge, with a marked reduction in surface expression, current amplitude, and a corresponding positive shift in the V1/2 of the activation curve. Co-immunoprecipitation and FRET assays confirmed association of mutant and wild-type subunits. CONCLUSION: Heterologously expressed hERG 1a C-terminus truncation mutants, exert a dominant negative gating and trafficking effect only when co-expressed with hERG 1b. These findings may have potentially profound implications for LQT2 therapy.